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KOROZYON DERNEÐÝNDEN ÖNEMLÝ BÝR ADIM: SEVÝYE 1-2 KATODÝK KORUMA EÐÝTÝM KURSU Türkiye'nin elektrik enerji gereksiniminin yaklaþýk üçte biri, halen fosil yakýt kullanan enerji üretim tesislerinden karþýlanmaktadýr. Isýya dayanýklý ferritik çeliklerden imal edilen ýsý deðiþtirici borular buharýn üretildiði kazanlarýn en fazla zorlanan ve bu nedenle sýkça hasar görerek üretimin aksamasýna neden olan bölümleridir. Bu tür hasarlar " boru patlamalarý" olarak adlandýrýlýrlar. Boru patlamalarýna yol açan etmenleri (a) yüksek sýcaklýk deformasyonu (sürünme), (b) yüksek sýcaklýk oksitlenmesi ve (c) mekanik aþýnma olarak özetlemek mümkündür. Kazan sularýnýn iþleme basýncýna uygun olarak ileri düzeyde arýtýldýklarý dikkate alýndýðýnda, borular içinde dolaþan akýþkanýn (su veya buhar) iç korozyona neden olmasý zayýf bir olasýlýktýr. Bu nedenle, yukarýda sýralanan etmenlerin en önemlileri olan yüksek sýcaklýk oksidasyonu ve mekanik aþýnma, borularýn alev tarafý dýþ yüzeylerinde yoðunlaþýr. Ýç basýncýn boru cidarýnda oluþturduðu teðetsel gerilimler boru malzemesinin yüksek sýcaklýk deformasyonuna yol açabilir. Ancak, kazan iþletmelerinde rastlanan iç basýnç ve sýcaklýk dikkate alýndýðýnda, bu etmenin boru patlamalarýndaki payýnýn sýnýrlý olacaðý sonucuna varýlabilir. Gözlemler boru dýþ yüzeyinde baþlayan hasarlarýn kül ve içerilen aþýndýrýcý unsurlarýn yol açtýðý mekanik aþýnma ve bu oluþumu bir ölçüde desteklediði kabul edilen oksidasyondur. Ülkemizde termik enerji santrallerini iþleten kuruluþlar kazan borularýný bu alanda mevcut çok sayýda ürünler arasýndan seçmek zorundadýrlar. Malzeme ve ürün seçimi belli kriter ve standartlara uygun olarak yapýlmak zorundadýr. Borularýn seçiminde kullanýlacak kriterler nelerdir? Bu sorunun yanýtý zordur. Fevkalade karmaþýk çevresel koþullarýn sonucu olan boru patlamalarýna karþý ürün seçerken boru malzemesinin, mekanik, fiziksel ve kimyasal özelliklerini temel almak yeterli olabilir mi? Deneyimler, bu çerçeve içerisinde kalarak yapýlacak ürün seçiminin yeterli olmadýðýný göstermektedir. Bu koþullar altýnda seçim için, mevcut çevresel koþullarýn yeterli düzeyde simüle edildiði hýzlandýrýlmýþ testlerden yararlanmak gerekir. Bu yaklaþýmda borulardan alýnan örnekler iç basýnca eþdeðer mekanik gerilim ve boru dýþ yüzeyinde oluþmasý beklenen sýcaklýk altýnda test edilmelidir. Ayrýca, boru patlamarýnda birincil payý bulunan mekanik aþýnma için örnek yüzeyine aþýndýrýcý bir malzemenin püskürtülerek uygulanmasý önerilebilir. Borularýn seçiminde temel alýnacak kriterler koþullara göre (a) boru hasarý için gerekli süre veya (b) belli bir test süresi sonunda kaydedilen aðýrlýk kaybý olarak belirlenebilir. Yukarýdaki tarife uygun bir test düzeneðinin tasarýmý ODTÜ, Metalurji ve Malzeme Mühendisliði Bölümü'nde altý öðrenciden oluþan bir gruba görev olarak verilmiþtir. Bu çalýþma sonucunda ortaya konan tasarým genel hatlarý ile aþaðýda görülmektedir. Bu tasarýmý geliþtirme çabalarý sürmektedir. Nihai amaç bir prototip düzeneðin imalatý ve denenmesidir. En içten baþarý ve esenlik dileklerimizle,

AN IMPORTANT STEP FROM THE CORROSION ASSOCIATION: LEVEL 1-2 CATHODIC PROTECTION TRAINING COURSE One third of the energy requirement of Turkey is currently supplied by the power plants using fossil fuels. The heat exchanger tubes, produced from heat resistant ferritic steel, are the most strained parts of boilers and therefore cause downtimes due to frequent damages. Such kind of damages is called "tube bursts". It is possible to summarize the factors causing tube bursts as follows: (a) high temperature deformation (creep), (b) high temperature oxidation, and (c) mechanical wear. Considering the fact that boiler water is refined at a high level in accordance with the operating pressure, it is a small possibility that the fluid (water or vapor) passing through the tubes causes internal corrosion. For this reason, the most important ones of the abovementioned factors, namely high temperature oxidation and mechanical wear, intensify on the outer surfaces of pipes near to flame. Hoop stresses caused by the internal pressure in the tube may cause the high temperature deformation of the tube material. However, if the internal pressure and temperature found at the boiler facilities are considered, one may conclude that the share of this factor in tube bursts would be limited. Observations show that the damages starting on the outer surface of tubes are caused by the mechanical wear due to ash and other abrasive elements contained and by oxidation, which is accepted to support this formation to a certain extent. Organizations operating power plants in our country have to choose boiler tubes among many products present in the sector. Material and product selection should be carried out based on certain criteria and standards. What are the criteria applicable to the selection of heat exchanger tubes? Answering this question is fairly hard. Would it be sufficient to take the mechanical, physical and chemical properties of the tube material as basis when selecting product to prevent tube failures caused by extremely complicated peripheral conditions? Experience show that production selection made within this defined frame is far from being sufficient. Accelerated tests, in which the existing peripheral conditions are simulated to a sufficient level, should be used for selecting materials under such conditions. In this approach, the tube samples should be tested under mechanical tension equivalent to internal pressure and the expected temperature at the outer surface. Also spraying an abrasive material on the sample surface may be suggested for mechanical wear, which has the greatest influence on tube bursts. Depending on the conditions, criteria to be used in selecting pipes may be determined as (a) time required for complete failure or (b) loss of weight at a certain test period. Design of a test facility following the abovementioned explanation is given as a task to a group of six students in the Metallurgy and Material Engineers Department at METU. The design reached at the end of this study is given below in outlines. Studies to improve this design are being carried out. The final purpose is to manufacture and operate a prototype test equipment. With kind regards;

YAYIN KURULU PUBLISHING BOARD

Yves M.Günaltun TOTAL S.A, Paris/Fransa

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www.korozyondernegi.org.tr Dizgi ve Baský : Poyraz Ofset Tel: (312) 384 19 42 - 15 • Fax : (312) 384 18 77

BÝYOKOROZYONDA SÜLFAT ÝNDÝRGEYEN BAKTERÝLERÝN ROLÜ

ÖZET Mikrobiyel aktivite sonucu korozyon hýzýnýn artmasý biyokorozyon olarak adlandýrýlmaktadýr. Biyokorozyon petrol, doðal gaz, atýk arýtýmý, deniz taþýmacýlýðý gibi pek çok endüstri dalýnda önemli sorunlara yol açmaktadýr. Biyokorozyonda rol oynayan mikroorganizmalar arasýnda en etkili olanlar anaerobik sülfat indirgeyen bakteriler (SRB)'dir. Bu derlemede biyokorozyonu baþlatan biyofilmlerin oluþumu, SRB'lerin özellikleri, korozyondaki etkileri ve biyokorozyonu durdurmakta kullanýlan biyositlerle ilgili çalýþmalar özetlenmiþtir. ROLE OF SULFATE REDUCING BACTERIA IN BIOCORROSION Increase in corrosion rate due to microbial activities is called as biocorrosion. Biocorrosion cause serious problems in many industries such as petroleum, oil, natural gas, wastewater treatment, marine and shipping. Among the microorganisms participating in biocorrosion, the most effective ones are anaerobic sulphate reducing bacteria (SRB). In this review, researches about the formation of biofilms which initiate the biocorrosion, properties of SRB, their roles in corrosion, and biocides applied to inhibit biocorrosion are summarized.

1. GÝRÝÞ Korozyon hýzýnýn mikroorganizmalarca arttýrýlmasý mikrobiyolojik etkiden kaynaklanan korozyon (microbially induced veya influenced corrosion, MIC) veya biyokorozyon olarak adlandýrýlmaktadýr. Mikroorganizmalar ve metabolik ürünleri çeþitli metallerin, karbon ve pas-

lanmaz çeliklerin, alaþýmlarýn üzerinde etkili olduðu gibi taþ, beton, plastik ve ahþap malzemelerde de bozulmaya neden olabilmektedir1,2. Mikroorganizmalarýn korozyonu hýzlandýrmasýnda etkili olan mekanizmalar: • biyofilm oluþumu, • yapýþkan maddelerin (ekzopolisakkaritler) üretilmesi, • çatlak korozyonunun geliþmesini kolaylaþtýran birikintilerin üretilmesi, • metabolik ürünler ile korozif, asidik bir ortam oluþturulmasý, • gerilim korozyon kýrýlmasýna yol açan sülfür ve hidrojen gibi metabolitlerin üretimi, • ortama korozyonu engellemek için eklenen kimyasallarýn parçalanmasý, • katodik veya anodik reaksiyonlarýn hýzlandýrýlmasý ile korozyon reaksiyonlarýnýn direkt etkilenmesidir3-7. Biyofilm doðal koþullar altýnda mikroorganizmalarýn metal yüzeyine yapýþmasýyla oluþur. Biyofilm ekzopolisakkarit maddelerden (EPS) oluþan bir matriksi olan, içine hücreler ve inorganik maddelerin tutunduðu, %95 oranýnda su içeren bir yapý olarak tanýmlanabilir8-11. Biyofilm oluþumunun ilk aþa-

Demet ÇETÝN

masýnda yüzeyde yüksek molekül kütleli organik bileþikler ve inorganik bileþiklerin birikmesiyle 20-80 nm kalýnlýðýnda bir film oluþur. Bu baþlangýç filmi elektrostatik yükleri ve metal yüzeyin ýslanabilirliðini deðiþtirerek bakteri kolonizasyonunu kolaylaþtýrýr. Mikroorganizmanýn metalin iç veya dýþ yüzeyine tutunabilmesi için uygun bir bölge (besinlerin adsorblandýðý ve mikroorganizmanýn tutunabileceði metalurjik olarak belirgin özelliklere sahip) bulmasý gereklidir1,12. Borularýn kývrýldýðý bölgeler, kaynakla baðlandýðý bölgeler, eðrilikler, girintiler mikroorganizmalarýn yüzeye tutunabilmesini kolaylaþtýrýr10. Mikroorganizmalarýn ürettiði yapýþkan EPS'ler biyofilmin fiziksel özelliklerini belirleyen ana yapýsal bileþenidir. EPS'ler tutunmayý saðladýðý gibi, beslenme için kullanýlabilecek organik ve inorganik materyalleri içinde tutarlar. EPS'lerin metal iyonlarýný baðlama özelliði biyokorozyon için önemlidir11. Metal baðlanmasý metal iyonlarý ile EPS'lerin protein ve karbonhidrat bileþenlerinde bulunan karboksil, fosfat, sülfat, gliserat, pürüvat ve süksinat gibi anyonik fonksiyonel gruplar arasýndaki etkileþimi içerir. Farklý yükseltgenme durumundaki iyonlarýn biyofilmde tu-

KOROZYON, 15 (1-2), 2007 3

tulmasý, standart indirgenme potansiyellerinde deðiþimlere yol açabilir. EPS metal iyonlarýna baðlanarak elektron mekikleri gibi görev yapar ve metalden direk elektron transferi yaparak yeni redoks reaksiyonlarýna neden olabilir. Uygun bir elektron alýcý bulunduðunda bu redoks reaksiyonlarý katodun depolarizasyonunu saðlayarak korozyonu hýzlandýrýrlar. Biyokorozyon, sülfat indirgeyen bakteriler (SRB), sülfür yükseltgeyen bakteriler, demir indirgeyen/yükseltgeyen bakteriler, manganez okside eden bakteriler ve organik asit ve ekzopolisakkarit (EPS) üreten bakteriler gibi farklý bakteri türleri veya algler, mantarlar ve likenler tarafýndan gerçekleþtirilmektedir13. Biyokorozyonda en etkin bakteri grubu SRB'lerin özellikle Desulfovibrio cinsine ait türleridir3,9,11,14,15. Sülfat indirgeyen bakteriler (Þekil 1) laktat, asetat, pürüvat, etanol ya da yað asitlerini enerji ve karbon kaynaðý olarak kullanýrken sülfatý elektron alýcý olarak kullanýr ve bol miktarda toksik ve korozif hidrojen sülfür üretirler.

Sülfat indirgeyen bakteriler, çok çeþitlilik içeren bir canlý grubudur16. SRB'lerin çoðu mezofil, gram negatif, spor oluþturmayan bakteri cinsleridir17-19. Ayrýca mezofil ve termofil endospor oluþturan bakterileri cinsleri ve hipertermofilik arkeler de termal ortamlardan (sýcak su kaynaklarý, petrol, deniz altý hidrotermal kaynaklarý gibi) izole edilmiþtir16,17,20-22.

Þekil 1. Petrol sahalarýndan izole edilen iki SRB izolatýnýn elektron mikrograflarý Figure 1. Scanning electron micrographs of two SRB isolates obtained from crude oil fields.

Sedimentler, taban sularý gibi sulu çevrelerin yüzey altýndaki anoksik kýsýmlarý SRB'lerin tipik habitatlarýdýr. Bu bakteriler anoksik sedimentlerden ve çamurlardan petrol üretim tesislerine ve kaðýt üretim fabrikalarý gibi endüstriyel su sistemlerine bulaþýrlar. Ham petrolün içinde su, anaerobik ortam ve karbon rezervinin bol bulunmasý SRB lerin geliþimi için uygun ortam saðlar. SRB türleri ham petrolün bileþimindeki çeþitli alkanlarý, alkilbenzenleri okside ederek ve sülfatý indirgeyerek H2S üretmektedirler21,23. Petrolün taþýnma ve depolanmasý esnasýnda

4

KOROZYON, 15 (1-2), 2007

meydana gelen SRB kontaminasyonu, tank ve borularýn korozyonu baþta olmak üzere, filtre ve borularda týkanýklýk ve akaryakýt kalitesinde azalma gibi problemlere yol açmaktadýr. Miranda ve arkadaþlarý24, petrol-su ayrýþtýrýcýsýndan izole edilen Desulfovibrio capillatus ve tiyosülfat içeren besi ortamýnda korozyon hýzýnýn oldukça arttýðýný kütle kaybý ölçümleri ile göstermiþlerdir. Çelik kuponlarýndaki birikintiler üzerinde yapýlan SEM-EDAX (Taramalý elektron mikroskobu-enerji daðýtýcý X ýþýný) analizlerine göre korozyon ürünlerinde yüksek oranda Fe ve S bulunmuþtur. Korozyon ürünlerinin temizlenmesinden sonra metal yüzeyinde çukurcuklarýn oluþtuðu gözlenmiþtir. Biyofilm içinde deðiþik tipteki mikroorganizmalardan (Pseudomanas sp., Proteus sp. ve Bacillus sp. gibi) oluþan topluluðun etkileþimi sayesinde SRB'ler için mümkün olmayan çevrelerde de geliþim devam edebilir. Örneðin biyofilmde dýþ yüzeyde yer alan aerobik bakteriler, oksijeni tüketerek, oksijen konsantrasyonunu biyofilmin içine doðru azaltýr ve anaerobik SRB'lerin geliþebileceði bir ortam oluþturabilirler1,11. Ýndirgenmiþ bir ortam isteyen SRB'ler, suda ölçülebilir çözünmüþ oksijen olsa bile, biyofilm tabanýnda geliþebilirler Farklý SRB türleri farklý morfolojide biyofilm oluþturmaktadýr. Desulfovibrio desulfuricans ATCC 27774 ile deniz izolatý bir SRB'nin paslanmaz çelik üzerinde neden olduklarý korozyon ve biyofilm yapýlarý incelendiðinde türlerin birbirinden farklý biyofilm morfolojisi ve polarizasyon direnci gösterdiði tespit edilmiþtir25. D. desulfuricans gözenekli ve að yapýsý þeklinde biyofilm oluþturmuþ ve daha yüksek korozyon hýzý ölçülmüþtür. Deniz izolatý SRB ise kristal ve bütün yapýda bir biyofilm oluþturmuþ ve bu tür biyofilmin biyokorozyona karþý daha koruyucu olduðu ileri sürülmüþtür. D. desulfuricans'ýn oluþturduðu biyofilmin sürekli biriktiði, deniz izolatýnýn biyofilminin ise yüzeye yapýþýp, büyüyerek, yüzeyden ayrýldýðý ve tekrar yapýþtýðý gözlenmiþtir. Metal yüzeyinde biyofilmin düzensiz daðýlýmý nedeniyle oluþan farklý havalandýrma etkisi potansiyel farklýlýklara ve sonucunda korozyon akýmlarýnda farklýlýklara neden olmaktadýr. Solunum gerçekleþen alanlarýn altý anodik olmakta ve buralarda metal çözünmesi gerçekleþmektedir (Þekil 2). Buna karþýlýk, çevreleyen bölgeler katodik bölgeler olmaktadýr4. Korozyon iþlemindeki katodik veya anodik reaksiyonlardan birini engelleyen veya kolaylaþtýran, anodik ve katodik bölgeleri birbirinden tamamen ayýran biyolojik reaksiyonlar da korozyonu arttýrýr. Örneðin anodik reaksiyonlarýn mikroorganizmala-

olarak hýzlandýrýlýr. Oluþan korozyon ürünleri ferrosülfür (FeS) ve ferro hidroksittir (Fe (OH)2)7,29,30.

Þekil 2. Biyofilm etkisiyle oluþan havalandýrma bölgeleri26. Metalik katyonlar (M+2) anodik alandan salýnýr. Figure 2. Aeration zones created by a biofilm26. Metallic cations (M+2) are released from the anodic area.

rýn ürettiði asidik metabolitler (organik asitler) ile uyarýlmasý veya anodik reaksiyonlarýn SRB'lerin ürettiði hidrojen sülfür ile uyarýlmasý korozyonu arttýrýr4. Korozyon hýzýnýn metal yüzeyinde biriken metabolik ürünlerle arttýðý Gayosso ve arkadaþlarý27 tarafýndan gösterilmiþtir. Araþtýrmacýlar API XL52 çeliðinin korozyon hýzýna gaz taþýma borularýndan izole edilen sesil (tutunan) ve planktonik (yüzen) bakterilerin etkisini incelemiþlerdir. Bu incelemeleri sonucunda sývý ortamda 5 çeþit planktonik bakteri, metal yüzeyinde ise sadece bir SRB türü olan Desulfovibrio vietnamensis bulunduðu belirlenmiþtir. Korozyon hýzýnýn planktonik deðil sesil bakteri sayýsýndan etkilendiði ve sesil bakteri sayýsý sabit kalsa bile metal yüzeyinde biriken metabolik ürünlerin korozyonu arttýrdýðý görülmüþtür. Sülfat indirgeyen bakterilerin metal korozyonuna etki mekanizmasý Von Wolzogen Kuhr ve Vlugt van der28 tarafýndan katodik depolarizasyon teorisi (KDT) ile elektrokimyasal olarak açýklanmaya çalýþýlmýþtýr. Gerçekleþen reaksiyonlar aþaðýda gösterilmiþtir.

Bu teoriye göre, oksijen yokluðunda ortamdaki su, metal yüzeyinde hidrojen oluþturacak þekilde parçalanýr. Oluþan hidrojenin yüzeyi kapatmasý sebebiyle reaksiyon oldukça yavaþ ilerler. Bu durumda metal polarize olmuþtur. Polarize olmuþ metal yüzeyinden hidrojenin sülfat indirgeyen bakterilerce sülfat indirgenmesi sýrasýnda uzaklaþtýrýlmasý anottaki metal çözünmesi olayýný hýzlandýrmaktadýr. Hidrojenin demir yüzeydeki katodik alandan bakteri hidrojenazlarý ile uzaklaþtýrýlmasý sýrasýnda sülfat sülfite indirgenir (Þekil 3). Yani katodik reaksiyonun depolarizasyonu ile korozyon reaksiyonu indirek

Þekil 3. Hidrojenazlarýn mikrobiyel korozyondaki rolü31. Figure 3. Role of hydrogenases in microbial corrosion31.

Bryant ve arkadaþlarý32 korozyona uðramýþ ve uðramamýþ petrol borularýndan karýþýk SRB kültürleri izole etmiþler ve hidrojenaz aktivitesinin çeliðin korozyonuna etkisini araþtýrmýþlardýr. Çelik kuponlarda geliþen biyofilmlerde hidrojenaz aktivitesi gösterenlerde korozyon hýzý 7.79 mm/yýl olarak, hidrojenaz aktivitesi göstermeyenlerde ise 0.48 mm/yýl olarak belirlenmiþtir. Boru sisteminin korozyona duyarlýlýðýnda karýþýk kültürün mikrobiyel yapýsý yanýnda hidrojenaz enzim aktivitesinin de etkili olduðu bu araþtýrmacýlar tarafýndan ifade edilmiþtir. Jung ve arkadaþlarý33 katodik depolarizasyonun SRB aktivitesinin fonksiyonu olduðu ileri sürmüþlerdir. Desulfovibrio desulfuricans (KCTC 2360) aktif geliþme fazýndayken, sülfat indirgenmesi ve hidrojenin uzaklaþmasýyla kuvvetli bir þekilde katodik depolarizasyon gerçekleþmiþtir. Kültür yaþlandýkça, sülfat tükenmiþ, metabolik ürünler besi ortamýnda birikmiþ ve katodik depolarizasyon durmuþtur. Rainha ve Fonseca34 SRB kültür yaþýnýn korozyon hýzýna etkisini incelemiþlerdir. Çalýþmada 1 günlük Desulfovibrio desulfuricans kültürü ile elde edilen korozyon akým yoðunluklarý, 5 günlükle elde edilenin yaklaþýk 2 katý kadardýr. Araþtýrmacýlar yaþlý kültürde bulunan yüksek miktarda sülfürün, demir sülfür pasif filmi oluþturacak kadar çok olduðunu ileri sürmüþlerdir. Bu araþtýrýcýlar ayrýca yumuþak çeliðin bakteri kültür ortamýnda elde edilen korozyon potansiyellerinin steril ortamdakine göre daha negatif yöne kaydýðýný belirtmiþlerdir. SRB'nin geliþme aþamalarýnýn deniz suyu ortamýndaki karbon çeliðinin korozyonunu etkilediði Kuang ve arkadaþlarý35 tarafýndan da elektrokimyasal olarak belirlenmiþtir. Potansiyodinamik polari-

KOROZYON, 15 (1-2), 2007 5

zasyon ölçümlerine göre, karbon çeliðinin korozyon hýzý bakterinin eksponansiyel büyüme fazýnda artmýþ, gerileme ve ölme fazýnda ise duraðan kalmýþtýr. Elektrokimyasal empedans spektroskopisi (EIS) ölçümlerinden hesaplanan polarizasyon direnci (Rp) bakterinin eksponansiyel büyüme fazýnda azalmýþ, gerileme ve ölme fazýnda ise duraðan kalmýþtýr. Araþtýrmacýlar, çeliðin korozyonda hýzýnýn, aktif SRB sayýsýndan çok metabolizma ürünlerinin ortamda birikmesinin etkili olduðunu ileri sürmüþlerdir. Ýlhan-Sungur ve arkadaþlarý36 soðutma kulesi suyundan izole ettikleri Desulfovibrio sp.' nin galvanize çelik üzerinde neden olduðu korozyonu incelemiþler, hücre sayýsý ile kütle kaybý yöntemiyle hesaplanan korozyon hýzýnýn orantýlý olmadýðýný göstermiþlerdir. Karbonhidrat analizleri ise biyofilmde bulunan toplam karbonhidrat miktarýnýn kütle kaybýyla ters orantýlý olduðunu göstermiþtir. Sülfat indirgeyen bakterilerin neden olduðu korozyonun hýzýný besi ortamý özellikleri de etkilemektedir. D. desulfuricans'ýn çelik üzerinde neden olduðu korozyona iki farklý besi ortamýnýn (Laktat/sülfat ve laktat/nitrat) etkisi Fonseca ve arkadaþlarý(8) tarafýndan incelenmiþtir. Laktat/sülfat ortamýna SRB inokülasyonundan sonra Ekor daha katodik deðerlere kaymýþtýr. Bakteri inoküle edilmiþ ortamda elde edilen Ikor deðerleri, steril ortama oranla daha yüksektir. Demirin anlýk çözünme hýzýnýn sülfür çökelmesinden daha hýzlý gerçekleþtiði ileri sürülmüþtür. D. desulfuricans'ýn iki besi ortamýnýndaki çelik üzerinde korozif etkisini Feio ve arkadaþlarý37 da incelemiþlerdir. Kütle kaybý ölçümleri laktat/nitrat ortamýnda geliþen D. desulfuricans'ýn çeliðin korozyonunu hýzlandýrmadýðýný, ancak laktat/sülfat ortamýnda geliþen SRB'nin çeliðin korozyon hýzýný oldukça arttýrdýðýný göstermiþtir. Laktat/sülfat ortamýnda bakterinin metabolik aktivitesinin 7 günden sonra durduðunu, ortamda laktat ve sülfat olsa bile üretilen H2S'in toksik etkisi nedeniyle hücre geliþiminin sürdürülemediði bildirilmiþtir. Laktat/nitrat ortamýnda ise nitrat indirgenmesi sonucu üretilen amonyaðýn bakteri üzerinde toksik etkisi olmadýðý, geliþimin ortamdaki laktat ve nitrat konsantrasyonlarýyla sýnýrlandýðý bulunmuþtur. Besi ortamýndaki demir konsantrasyonunun korozyon hýzýný etkilediði Çetin ve arkadaþlarý38 tarafýndan gösterilmiþtir. Steril besi ortamýnda demir konsantrasyonu arttýkça korozyon potansiyelleri (Ekor) daha anodik deðerlere kayarken, korozyon akým yoðunluklarý (Ikor) artmýþtýr. 10 mg/L FeSO4 .7H2O içeren ortama Desulfotomaculum sp.'nin

6

KOROZYON, 15 (1-2), 2007

inokülasyonundan 5 gün sonra en yüksek Ikor deðeri elde edilirken, 100 mg/L FeSO4 .7H2O içeren ortamda bakteri inokülasyonundan 1 gün sonra en yüksek Ikor deðerine ulaþýlmýþtýr. Artan demir konsantrasyonlarýnýn bakterinin geliþimini arttýrdýðý belirlenmiþtir. Çelik kuponlarýn bakteri bulunan besi ortamýnda 1 ay bekletilmesinden sonra korozyon ürünleri üzerinde yapýlan EDAX analizleri S ve P piklerinde de artýþ olduðunu göstermiþtir (Þekil 4). Ayrýca Lee ve arkadaþlarý39 ile Lee ve Characklis40'e göre düþük Fe+2 konsantrasyonlarýnda geçici ve yapýþkan demir sülfür filmi oluþmakta ve bu durum Ikor deðerlerini de düþürmektedir. Buna karþýlýk demirce zengin ortamlarda biyofilm altýnda koruyucu bir tabaka olarak bulunan demir sülfür filmi oluþmamakta ve korozyon akým yoðunluklarý artmaktadýr.

Þekil 4. Desulfotomaculum sp. içeren besiortamýnda 1 ay bekletilmiþ çelik kuponun a) SEM mikrografý, b) EDAX spektrumu38. Figure 4. a) SEM micrographs and b) EDS spectrum of steel coupon after immersion for 1 months in the culture medium with Desulfotomaculum sp.38.

Sheng ve arkadaþlarý25, bakterinin ürettiði hidrojen sülfürün katodik indirgenmesi ile korozyon hýzýnýn arttýðýný ileri sürmüþlerdir. Katodik reaksiyon aþaðýdaki gibidir. H S + e- → SH- + ½ H 2

2

Ayrýca hidrojen sülfür ve oksidasyon ürünleri de çeliði korozyona uðratabilir. Demir sülfürlerin oluþumu da dolaylý olarak metali depolarize edebilir. Anodik reaksiyonlar FeS oluþumuyla hýzlanmaktadýr. Fe + S-2 → FeS + 2eKDT' nin zayýf bir yaný anodik reaksiyonun uyarýlmasýnda sülfürlerin rolü hakkýnda bilgi vermemesidir. Ancak Wang ve Liang41 laktatlý deniz suyu ortamýnda 10 CrMoAl çeliðinin korozyonuna SRB etkisini incelemiþlerdir. SRB'li ortamda 7 ve 14 günlük inkübasyon sürelerinden sonra elde edilen potansiyodinamik polarizasyon eðrilerine göre korozyon potansiyeli zamanla -614 mV' dan -718 mV'a ve -768 mV'a kaymýþtýr. Ayný potansiyel uygulandýðýnda anodik akým yoðunluðunun SRB'li ortamda

steril ortamdakinden daha yüksek olduðu görülmüþtür. SRB'nin bu tip çelikte anodik depolarizasyon iþlemini hýzlandýrdýðý bildirilmiþtir. Bu araþtýrýcýlara göre demir sülfürle birleþerek FeS oluþturmakta ve anodik çözünmeyi hýzlandýrmaktadýr. Laktat eklenmiþ deniz suyu ortamýnda ortama bakteri eklenmesi korozyon hýzýný 0.017 mm/a'dan 0.019 mm/a'ya çýkartýrken, C vitamini ve ferro amonyum sülfat içeren ortamda SRB eklenmesi korozyon hýzýný 0.046 mm/a'dan 0.073 mm/a'ya çýkarmýþtýr. Korozyon ürünlerinin EDAX analizinde ise SRB'li ortamda steril ortama göre kükürt içeriðinin oldukça arttýðý görülmüþ ve bu durum korozyon ürünlerinden FeS oluþumuna baðlanmýþtýr. KDT'de dikkate alýnmayan bir nokta da demir yüzeyinde oksitler, sülfürler, hidroksitler ve hatta biyofilmler gibi birikintilerin olmasýdýr. Metal-çözelti ara yüzeyinde biyolojik ve inorganik iþlemler ayný anda ancak farklý yönlerde gerçekleþir. Korozyon ürünü tabakalarý ile biyofilm arasýnda aktif bir etkileþim vardýr. Korozyon ve korozyon ürünlerinin birikimi metal yüzeyinden çözeltiye doðru, biyofilm oluþumu ise çözeltiden metal yüzeyine doðru oluþur. Korozyon ürünleri pasif filmin metal yüzeyine yapýþmasýný arttýrarak korumayý arttýrabilir veya biyofilm parçalandýðýnda koruyucu filmlerin ayrýlmasý korozyonu arttýrabilir11. Jung ve arkadaþlarý33 çalýþmalarýnda bakteriyel sülfat indirgenmesi sonucu oluþan sülfürlerin anodu baþlangýçta uyardýðýný ve bunun sonucu olarak açýk devre potansiyellerinin daha negatif deðerlere kaydýðýný göstermiþlerdir. Bu araþtýrýcýlar zamanla demir sülfürün elektrot yüzeyinde koruyucu bir film oluþturarak potansiyeli daha pozitif deðerlere kaydýrdýðýný ileri sürmüþlerdir. Paslanmaz çelik ve karbon çeliðinin korozyonu üzerinde jeotermal sahalardan izole edilen mezofilik (Desulfotomaculum sp.) ve termofilik SRB'nin etkisini Alfaro-Cuevas-Villanueva ve arkadaþlarý42 incelemiþlerdir. Mikrobiyel aktivitenin tüm korozyon iþlemini etkilediði ve özellikle çukurcuk korozyonu ile yerel korozyona yol açtýðý belirlenmiþtir. Anodik polarizasyon eðrileri metal yüzeyinde korozyon ürünleri filmi ve biyofilm oluþumu nedeniyle önce pasifleþme ve daha sonra aktifleþmenin gerçekleþtiðini göstermiþtir. Biyojenik sülfürlerin korozif etkisi ortamda bulunan diðer korozif iyonlarla (klorürler) ve metal yüzeyindeki biyofilmde bulunan mikrobiyal birlikteliklerin etkisiyle arttýrýlabilir (Þekil 5). Antony ve arkadaþlarý43 yaptýklarý çalýþmada NaCl içeren besi ortamýnda 2205 dupleks paslanmaz çeliðinin korozyonuna Desulfovibrio desulfuri-

Þekil 5. Karbon çeliðinin anoksik ortamda biyokorozyonu4. Figure 5. Biocorrosion of carbon steel in anoxic medium4.

cans etkisini incelemiþlerdir. Steril ortamda Ekor deðerinin 40 gün boyunca fazla deðiþmediði, bakterili ortamda ise negatif yöne kaydýðý ve -0.53 V (SCE) deðerinde sabit kaldýðý gösterilmiþtir. SRB'li ortamda 40 gün süreyle býrakýlan çelik kuponlar SEM ile incelendiðinde yüzeylerinde biyofilmin oluþtuðu, biyofilmin altýnda çiziklerin, küçük çukurlarýn ve siyah renkli bir yüzey filminin bulunduðu görülmüþtür. Kuponlarýn 14 gün SRB'li ortamda bekletildikten ve anodik olarak polarize edildikten sonra alýnan SEM görüntüleri, çeliðin çukur korozyonu yanýnda çatlak korozyonuna da uðradýðýný ve tane sýnýrlarýnda yarýlmalar olduðunu göstermiþtir. Düþük Cr ve Mo içeriðine sahip bölgelerden taneler ayrýlmýþtýr. Araþtýrmacýlar polarize olmuþ ya da olmamýþ kuponlarda biyofilm altýndaki pasif filmde demir sülfür ve diðer metal sülfürlerin bulunduðu ve pasif filmin sülfürlenmesinin katodik reaksiyonu depolarize ettiðini ileri sürmüþtür. SRB'nin ürettiði veya hidrojen sülfürün inorganik fosfor bileþikleriyle reaksiyonundan oluþan uçucu fosfor bileþikleri de korozyonda etkilidir44. Çelik yüzeyinde oluþmuþ koruyucu demir sülfür tabakasý parçalandýðýnda uçucu ve oldukça aktif fosfor bileþikleri metal üzeride korozyona yol açmaktadýr. 2. MÝKROBÝYEL KOROZYONUN ENGELLENMESÝ VE KONTROLÜ Mikroorganizmalarýn aktiviteleri sonucunda metal donanýmlarda çukur korozyonu oluþmasý, demir sülfür gibi korozyon ürünleri veya biyofilmlerin týkanýklýk yaratmasý ve güvenlik riskleri oluþmasý kaçýnýlmazdýr. Yýpranan donanýmýn deðiþimi ve onarým sýrasýnda sistemin çalýþamamasý nedeniyle yapýlan

KOROZYON, 15 (1-2), 2007 7

harcamalar yüksek olduðundan endüstiyel sistemlerde mikrobiyel aktivitenin kontrolü ve engellenmesi gereklidir. Günümüzde son derece önemli bir konu olan ancak uygulamada yeteri kadar üzerinde durulmayan biyokorozyonun kontrolü, endüstriyel yatýrým ve üretim maliyetlerini etkileyen en önemli faktörler arasýndadýr. Biyokorozyon ciddi ekonomik kayýplara yol açmaktadýr. Amerika petrol ve gaz endüstrilerinde mikrobiyel korozyondan kaynaklanan yýllýk zararýn 1-2 milyar dolar civarýnda olduðu tahmin edilmektedir 26,45. Endüstride mikrobiyolojik korozyonu engellemek için, yaygýn korozyon kontrol yöntemleri (kaplamalar, katodik koruma, korozyon inhibitörleri vs.) yanýnda biyositler de kullanýlmaktadýr46. Mikroorganizmalarý öldüren ya da geliþmelerini engelleyen biyositler, tek bir kimyasaldan ya da farklý kimyasallardan oluþabilir. Endüstriyel bir biyosit, geniþ mikrobiyel spektrumda etkili olmalý, üretimi ucuz olmalýdýr. Kendisi fazla toksik ve korozif olmamalý ve ayrýca inhibe edici etkisini diðer bileþiklerin bulunduðu ortamlarda ve iþletme koþullarýnda uzun süre koruyabilmelidir47. Klor, klor dioksit, ozon, brom gibi inorganik maddeler veya izotiazolonlar, kuaterner amonyum bileþikleri, aldehitler (gluteraldehit, akrolein, formaldehit) gibi organik maddeler biyosit olarak kullanýlmaktadýr48,49. Biyositler oksitleyici veya oksitleyici olmayan toksik maddeler olabilir. Etkin inhibisyon için, bu iki grup biyositin veya ayný grupta bir kaç biyositin kombinasyonlarý da kullanýlmaktadýr. Bunun nedeni bakterinin zaman içinde kullanýlan biyositlere karþý direnç geliþtirmesidir50-52. Oksitleyici (klorür, bromür, hipoklorür, peroksit vd.) ve oksitleyici olmayan (gluteraldehit, formaldehit, kuaterner amonyum bileþikleri) biyositler bir arada kullanýlarak korozyon kontrolü arttýrýlýp maliyeti de düþürülebilir. Kuaterner amonyum bileþikleri ve poliaminler gibi bazý organik biyositler, hem biyosit hem de korozyon inhibitörü olarak görev yaparlar. Bunlar metal yüzeyine yapýþarak koruyucu bir film oluþtururlar ve pek çok mikroorganizma için toksiktirler. Diðer biyositler korozyonu sadece bakteri geliþimini engelleyerek durdururlar53. Ortam þartlarý, mikroorganizma tipi, biyofilm oluþumu gibi faktörler kullanýlan biyositlerin kullanýmýný etkilenmektedir. Ortamda çelik kupon yüzeyi bulunmasý SRB'nin ürettiði EPS'nin nükleik asit, protein ve polisakkarit içeriðini etkilemektedir. Çevre koþullarý mikroorganizmanýn fizyolojisinde moleküler seviyede deðiþikliklere yol açmaktadýr54. Gaylarde ve Johnston55 inokülüm bileþenlerindeki farklýlýklarýn, mikroorganizmanýn geliþtirilme þartlarýnýn,

8

KOROZYON, 15 (1-2), 2007

ortamda oksijen veya metal kupon bulunmasýnýn ve farklý ortam pH'ýnýn bakterilerde biyosit duyarlýlýðýna etkisi olduðunu göstermiþtir. Desulfovibrio vulgaris, D. desulfuricans ve anaerobik Vibrio anguillarum bakterilerine Myacide AS (25-100 mg/L), Vantocil IB (50-200 mg/L) ve gluteraldehitin (50200 mg/L) etkisinin incelendiði çalýþmada, ayný biyosit konsantrasyonunda saf kültür yerine karýþýk SRB kültürü kullanýldýðýnda SRB'leri öldürmek için gerekli temas süresinin arttýðý görülmektedir. Ortamda oksijen bulunduðunda bakterilerin biyositlere karþý daha duyarlý olduðu bildirilmiþtir V. anguillarum içeren kültürde SRB türünü öldürmek için gerekli temas süresinin arttýðý gözlenmiþtir. Bu bakterinin biyositi adsoblayarak ya da modifiye ederek etkisiz hale getirdiði düþünülmektedir. Bakterilerin biyosit testinden önce demir plakalý ortamda geliþtirilmiþ olmasý gluteraldehite karþý direnci arttýrmýþtýr. Demirin membran geçirgenliðini etkileyerek veya membran yapýsýný saðlamlaþtýrarak, ya da oluþan demir sülfürlerin biyositlerin baðlanma bölgelerini kapatmasýyla biyositlere karþý direnç saðladýðý düþünülmektedir. Bakterilerin ortam pH'ý 7.2 olduðunda genellikle biyositlere karþý daha dirençli olduðu ifade edilmiþtir. Salgýlanan polimerler ve bakteri tabakasýndan oluþan biyofilmin bakterileri biyositlere karþý korumasý nedeniyle, ortamda metal kupon bulunduðunda genellikle daha yüksek konsantrasyonlarda biyosit kullanýlmasý gerektiði bildirilmiþtir. Çetin ve arkadaþlarý56 tarafýndan gerçekleþtirilen çalýþmada da ortamda çelik kupon bulunduðunda SRB'nin geliþimin engellemek için uygulanan formaldehit veya gluteraldehit konsantrasyonunun arttýrýlmasý gerektiði belirlenmiþtir. Kupon yüzeyinde oluþan biyofilm tabakasýnýn kalýnlýðý nedeniyle biyositin difüzyonu engellenmekte ve iç tabakada yer alan hücreler korunmaktadýr. Fang ve arkadaþlarý10, SRB içeren biyofilmlerin toksik metallerin ve kimyasallarýn (Cr, Cd, Zn, Al, Pb, gluteraldehit, fenol) etkisiyle kümeleþtiði ve EPS üretiminin arttýðýný gözlemiþlerdir. Kümeleþme ile kimyasalla temas eden biyofilmin yüzey alaný azaltýlarak bakterinin kimyasala karþý daha dirençli olmasý saðlanmýþtýr. Artan EPS tabakasý da kimyasallara karþý doðal bir difüzyon bariyeri oluþturmuþ ve kümeleþme oluþumuna katkýda bulunmuþtur. Kümeleþme olan alanlarýn katot olarak davrandýðý, diðer yüzeylerin deniz suyundaki klor ve sülfatýn etkisine açýk olduðu için anot olarak davrandýðý ileri sürülmüþtür. Ayrýca EPS tabakasýnýn asidik yapýda olmasý ve demiri baðlama özelliði nedeniyle korozyonun hýzlandýðý görülmüþtür. Gardner ve Stewart57 petrol sahasýndan izole

edilen karýþýk SRB kültürünü sürekli sistemde geliþtirmiþler ve biyofilm aktivitesine biyosit etkisini sülfür üretiminin ölçümüyle belirlemiþlerdir. Planktonik (yüzen) SRB ile yapýlan kontrol deneylerinde 10 mg S 1-1 sülfür konsantrasyonuna ulaþmak için yaklaþýk 47 saat gerekliyken, ortama 50 mg 1-1 gluteraldehit eklendiðinde 143 saat gerekmektedir. Biyofilm deneylerinde ise kontrolde yaklaþýk 10 mg S 1-1 sülfür konsantrasyonuna 1.7 saatte ulaþýlýrken, 500 mg 1-1 gluteraldehit eklendiðinde bu süre yaklaþýk 62 saate uzamýþtýr. Planktonik hücrelerin sülfür üretimini durdurmak için gereken dozlar biyofilmler için yeterli olmamýþtýr. Bu çalýþmada gluteraldehitin biyofilmin kalýnlýðýný deðiþtirmediði ve biyofilme nüfuz etmesine raðmen bakterilerin biyofilm etkisiyle biyosite karþý daha dirençli olduðu gösterilmiþtir. Biyositlerin biyofilme difüzlenmesini kolaylaþtýrmak için biyodispersanlarýn kullanýmý ile ilgili çalýþmalar da yapýlmýþtýr. Wiatr ve Fedyniak58 tarafýndan yapýlan çalýþmada Desulfovibrio desulfuricans, Desulfotomaculum nigrificans ve Clostridium sporogenes türlerine metronidazol ve biyodispersan karýþýmýnýn etkisi incelenmiþtir. Deneylerde kullanýlan bakteri karýþýmý bu türlerin saf kültürleri ile endüstriyel biyofilmlerden elde edilen aerobik bakteri ve mantarlardan oluþmaktadýr. Biyosit 50 mg/L'lik miktarda uygulandýðýnda yaklaþýk 4 günde SRB'ler ölmektedir. Ancak anaerobik reaktörde bu konsantrasyon uygulandýðýnda, biyositin kalýn biyofilm tabakasýna nüfuz edememesi nedeniyle biyosit etkisiz kalmýþtýr. Daha yüksek deriþimlere çýkýlmasý maliyet açýsýndan uygun bulunmamýþtýr. Bu yüzden biyosit biyodispersan ile beraber uygulanmýþtýr. Metronidazol ve biyodispersan karýþýmýnýn sürekli besleme ile reaktöre verilmesiyle 32 saat sonunda SRB'ler % 99 oranýnda azalmýþtýr. Fabrika ortamýnda soðutma suyu sisteminde uygulanan 100-120 mg/L'lik biyosit biyodispersan karýþýmý SRB sayýsýný 3 hafta içinde 3×106 cfu(*)/g'dan 300 cfu/g'a düþürmüþtür. Oksitleyici ajanlardan klorür yaygýn olarak korozyonun engellenmesinde kullanýlmaktadýr. Ancak biyofilmlere diffüzlenmekte yetersiz kaldýðý için karýþýk kültürlerden oluþan biyofilmlerde etkinliðini azaltmaktadýr. Ayrýca SRB'ler klorüre direnç geliþtirdiði için daha yüksek konsantrasyonlara çýkýlmasý veya biyosit deðiþikliði gerekli olmaktadýr59. Franklin ve arkadaþlarý60 Pseudomonas, Bacillus, Erwinia, Acinetobacter ve SRB türlerinden oluþan bakteri kültürü ile korozyon çalýþmalarý yapmýþlardýr. Biyosit olarak klor ve klor-bromun etkisi bakteriyel aktivitenin belirlenmesi ile ölçülmüþtür. Biyo-

filmlere 16 mg/L'lik biyosit uygulandýðýnda bakteri sayýlarý ve aktiviteleri düþmüþtür, daha sonra ortama verilen 2 mg/L'lik biyosit ile biyositin etkisi sürdürülmüþtür. Yüksek konsantrasyonda biyosit uygulandýðýnda biyositin kendisinin korozif etkisinden dolayý hem steril hem de inoküle edilmiþ sistemlerde korozyon hýzýnýn arttýðý elektrokimyasal empedans spektroskopi ölçümleri ile belirlenmiþtir. Sürekli ve kesikli olarak klorlanan deniz suyu ortamýnda paslanmaz çeliðin korozyonu ve bakteri aktivitesi Gundersen ve arkadaþlarý(61) tarafýndan incelenmiþtir. Bakterileri öldürmek için sürekli olarak 0.1 mg/L'lik konsantrasyonda klor uygulanmasý yeterli olmaktadýr. Kesikli uygulamada günlük olarak 30 dakika süreyle uygulanan 1 mg/L klor yeterli bulunmuþtur. Bu çalýþmada üç tip paslanmaz çeliðin klorsuz deniz suyu ortamýnda gümüþ/ gümüþ klorür referans elektroda karþý okunan açýk devre potansiyellerinin bakteri aktivitesinin etkisinde zamana baðlý olarak arttýðý gösterilmiþtir. Sürekli biyosit uygulamasýnda farklý çeliklerin biyosite farklý tepki verdikleri ve düþük biyosit deriþimlerinde potansiyelin 40 gün süreyle yavaþça artmakta olduðu gözlenmiþtir. Bu durumun klorun okside edici özelliðinden kaynaklandýðý ve çukurcuk korozyonu tehlikesini devam ettiði bildirilmiþtir. Bakteri aktivitesi kesikli klorlanan ortamda az da olsa devam ettiði ve çeliklerin açýk devre potansiyelinin klorsuz deniz suyundakinden daha düþük olduðu bulunmuþtur. Araþtýrmaya göre metal üzerinde 0.2 mg/L den daha düþük klor uygulanmasý korozyona neden olmamaktadýr. Hardy62 kuaterner amonyum bileþiklerinin 50 mg/L altýndaki konsantrasyonlarda uygulandýðýnda Desulfovibrio vulgaris'e karþý etkisiz kaldýðýný gluteraldehitin ise 50 mg/L konsantrasyonda bakteri aktivitesini kontrole göre % 80 den fazla azalttýðýný radyorespirometri yöntemiyle belirlemiþtir. Desulfovibrio vulgaris kültür ortamýna gluteraldehit artan deriþimlerde eklendiðinde sülfür üretimini tamamen durdurmak için geçen zaman azalmaktadýr. Çalýþmada biyositlerin aktivitesini ortamda bulunan ditiyonat ve sülfür gibi indirgeyici ajanlarýn durdurduðu gözlenmiþtir. Besi ortamý olarak laktat eklenmiþ ve tuz oraný % 42 olan formasyon suyu ile % 19 olan kuzey deniz suyu kullanýldýðýnda gluteraldehitin yüksek tuzlu ortamda daha etkin olarak sülfür üretimini engellediði görülmüþtür. Ýzotiyazolon biyosit olan Kathon FP'nin etkinliðinin incelendiði bir çalýþmada 50 ve 100 mg/L'lik biyosit deriþimleri Hormoconis resinae mantarýndan oluþan biyofilm üzerinde etkiliyken, H. resinae ve SRB'den oluþan karýþýk biyofilmde daha yüksek

* Cfu (colony forming unit): koloni oluþturucu birim, besiyerinde geliþip koloni oluþturabilen mikroorganizma sayýsý

KOROZYON, 15 (1-2), 2007 9

deriþimde biyosit kullanýldýðýnda H.resinae'nýn etkilendiði görülmüþtür. Bu deriþimde sesil SRBlerin de % 99 oranýnda azaldýðý gösterilmiþtir 63. Bazý durumlarda mikrobiyel aktiviteyi durdurmak korozyonu engellememektedir. Gayosso ve arkadaþlarý64 tarafýndan yapýlan çalýþmada gaz boru hatlarýndan izole edilen ve Desulfovibrio vietnamensis'i de içeren bakteriyel kültür kullanýlarak, polarizasyon direnci, elektrokimyasal empedans spektroskopisi teknikleriyle bu kültürün korozyon hýzý, mekanizmasý ve hýza biyositin etkisi incelenmiþtir. Gluteraldehitin (200 mg/L) hem planktonik hem de sesil bakterilerin geliþimini engellediði, öldürücü dozda biyosit eklendiðinde korozyon hýzýnýn yavaþladýðý, ancak biyofilm etkisinin sürdüðü ve ara fazda biriken korozif metabolitler ve demir sülfür gibi galvanik etkisi olan ve çeliðe katodik etkisi olan korozyon ürünlerinin bir miktar korozyona yol açtýðý görülmüþtür. Keresztes ve arkadaþlarý65 Desulfovibrio desulfuricans'ýn paslanmaz çelik ve pirincin üzerinde neden olduðu korozyon hýzýna N-hidroksi-metil-amino asitlerin etkisini araþtýrmýþlardýr. N-hidroksi-metil-glisin'in (GLY) 100 mg/L'lik N-hidroksi-metil-fenilalanin (PHE) 500 mg/L'lik deriþim kullanýldýðýnda besiyerinin ancak 20 gün sonra siyahlaþtýðý bildirilmiþtir. Polarizasyon direnci ölçümlerine göre SRB'li ortamda çeliðin korozyon hýzýnýn kontrole göre oldukça arttýðý gözlenmiþ, 250 mg/L PHE kullanýldýðýnda SRB geliþiminin 3-4 gün sonra baþladýðý ve bundan sonra korozyon hýzýný arttýrdýðý tespit edilmiþtir. Ayný konsantrasyonda uygulanan GLY' nin geliþimi tamamen durdurduðu ve korozyon hýzýnýn steril besi ortamýyla ayný olduðu bildirilmiþtir. Metal olarak pirinç kullanýldýðýnda korozyon hýzýnýn daha düþük olduðu gözlenmiþtir. Biyositler etkisini; DNA, RNA ile reaksiyona girerek, DNA-protein çapraz bað oluþumunu arttýrarak, hücre membranýnda ve sitoplazmadaki proteinlerin -OH, -NH2, -COOH, -SH gibi gruplarýyla etkileþime girerek, hücre zarýna zarar verip yarý geçirgen özelliklerini bozarak, hücresel madde alýþveriþini engelleyerek ve hatta hücre içeriðinin dýþarý akmasýna neden olarak göstermektedir66. Çoðu biyosit potansiyel karsinojendir. Endüstride mikroorganizmalarýn kontrolü için kullanýlan biyositler sonuçta akarsulara veya arýtým sistemi sularýna karýþmaktadýrlar67,68. Biyosit kullanýmýyla ekolojik ve çevresel problemlere yol açýlmamasý ve çevrenin korunmasý için endüstride kullanýmlarýna yasal sýnýrlamalar getirilmiþtir. Bu bakýmdan yeni yöntemlerin geliþtirilmesi veya varolanlarýn dikkatli seçilmesini gerektirmekte-

10

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dir. Biyosit kullanýmýna alternatif ve çevreyle dost olan iki yeni yaklaþým mikrobiyel populasyonun yapýsýný sýcaklýk, besin konsantrasyonu ve akýþý gibi faktörlerin deðiþtirilmesiyle etkilemeye dayanmaktadýr. Birinci yaklaþým ortama nitratlý bileþikler ekleyerek sülfat indirgemesini engellenmesidir11. Petrol sahalarýnda kuyulara nitrat eklenmesi ortamdaki parçalanabilir petrol bileþenleri için SRB ile rekabet eden nitrat indirgeyen ve sülfür okside eden bakterilerin geliþimini uyarmaktadýr. Ayrýca nitrat indirgenmesinin ara ürünleri ve nitrit sülfür üretimini engellemektedir69,70. Biyosit kullanýlmadan MIC'in engellenmesi için diðer bir yaklaþým da biyofilmlerin kullanýlmasýdýr. Çeþitli çeliklerle yapýlan çalýþmalarda biyofilmdeki bazý bakterilerin varlýðý ve aktiviteleriyle korozyon inhibisyonunun gerçekleþtiði gösterilmiþtir71. Biyofilm korozyon ürünleri ile metal arasýnda bir difüzyon engeli oluþturarak metal çözünmesini engelleyebilir ya da biyofilmdeki aerobik mikroorganizmalar oksijeni tüketerek metal yüzeyinde oksijen deriþiminin azalmasýna yol açabilirler. Mikroorganizmalarýn ürettiði siderofor gibi metabolik ürünler de korozyon inhibitörü olarak görev yapabilirler. Ayrýca bazý mikroorganizmalar ürettikleri antibiyotiklerle korozyona yol açan bakterilerin geliþimini durdurabilirler. Mansfeld72 tarafýndan yapýlan bir çalýþmada çeþitli bakterilerin farklý metallerin ve alaþýmlarýn korozif ortamlardaki korozyon hýzýný azalttýðý ifade edilmiþtir. Bazý Shewanella suþlarý yapay deniz suyunda alüminyum 2024, çelik ve pirinçte çukur korozyonu oluþumunu engellemektedir. Shewanella ana ve S. algae bulunan ortamlarda korozyon potansiyeli (Ekor) daha negatif deðerlere kaymaktadýr. Buna karþýlýk, Bacillus türleriyle yapýlan korozyon inhibisyonu çalýþmalarýnda Ekor'un bakterili ortamda daha pozitif deðerlere kaydýðý gözlenmiþtir. Shewanella suþlarýnýn metal yüzeyinde oksijen deriþimini düþürerek ve anaerobik bir ortam oluþturarak, Bacillus türlerinin ise üretilen inhibitör maddeler yoluyla korozyon inhibisyonu gerçekleþtirdiði ileri sürülmüþtür. KAYNAKÇA 1. 2. 3. 4. 5. 6. 7. 8. 9.

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42. R. Alfaro-Cuevas-Villanueva, R. Cortes-Martinez, J.J. García-Díaz, R. Galvan-Martinez, R. Torres-Sanchez, Mater. Corros. 57 (2006) 543. 43. P.J. Antony, S. Chongdar, P. Kumar, R. Raman. Electrochim. Acta. 52 (2007) 3985. 44. W.P. Iverson, G.J. Olson, Proc. Conf. Microbial Corrosion, the National Physical Laboratory and the Metals Society, NPL Teddington, London. (1983) 46 p. 45. D. Bermont-Bouis, M. Janvier, P.A.D. Grimont, I. Dupont, T. Vallaeys, J. Appl. Microbiol. 102 (2007) 161. 46. R. Zuo, Appl. Microbiol. Biotechnol. 76 (2007) 1245. 47. H.A.Videla, Int. Biodeter. Biodegr. 49 (2002) 259. 48. C.W.S. Cheung, I.B. Beech, S.A. Campbell, J. Satherley, D.J. Schiffrin, Int. Biodeter. Biodegr. 33 (1994) 299. 49. C.J. Hurst, Manual of environmental microbiology. ASM press, USA. (2002) 1138 p. 50. A.D. Russell, J. Appl. Microbiol. Symposium Supplement 92 (2002) 1S. 51. M. Heinzel, Int. Biodeter. Biodegr. 41 (1998) 225. 52. J.S. Chapman, Int. Biodeter. Biodegr. 51 (2003) 271. 53. J.B. Davis, Petroleum Microbiology. Elsevier Publishing Company, NewYork. (1967) 603 p. 54. V. Zinkevich, I. Bogdarina, H. Kang, M.A.W. Hill, R. Tapper, I.B. Beech, Int. Biodeter. Biodegr. 37 (1996) 163. 55. C.C. Gaylarde, J.M. Johnston, Proc. Conf. Microbial Corrosion, the National Physical Laboratory and the Metal Society, NPL Teddington, London. (1983) 91 p. 56. D. Çetin, S. Bilgiç, G. Dönmez, ISIJ Int. 47 (2007) 1023. 57. L.R. Gardner, P.S. Stewart, J. Ind. Microbiol. Biot. 29 (2002) 354. 58. C.L. Wiatr, O.X. Fedyniak, J.Ind. Microbiol. 7 (1991) 7. 59. S.G. Choudhary, Hydrocarb. Processing. 77 (1998) 91. 60. M.J. Franklin, D.E. Nivens, A.A. Vass, M.W. Mittelman, R.F. Jack, N.J.E. Dowling, D.C. White, Corrosion. 47( 1991) 128. 61. R. Gundersen, B. Johansen, P.O. Gartland, L. Fiksdal, I. Vintermyr, R. Tunold, G. Hagen, Corrosion. 47(1991) 800. 62. J.A. Hardy, Proc. Conf. Microbial Corrosion, the National Physical Laboratory and the Metal Society, NPL Teddington, London (1983) 98 p. 63. P.S. Guiamet, C.C. Gaylarde, W. J. Microbiol. Biot. 12 (1996) 395. 64. M.J.H. Gayosso, G.Z. Olivares, N.R Ordaz, R.G. Esquivel, Mater. Corros. 56 (2005) 624. 65. Zs. Keresztes, J. Telegdi, J. Beczner, E. Kálmán, Electrochim. Acta, 43 (1998) 77. 66. S. Kailasam, K.R. Rogers, Chemosphere. 66 (2007) 165. 67. L.L. Sano, A.M. Krueger, P.F. Landrum, Aquat. Toxicol. 71 (2005) 283. 68. H.W. Leung, Ecotox. Environ. Safe. 49 (2001) 26. 69. C. Hubert, M. Nemati, G. Jenneman, G. Voordouw, Appl. Microbiol. Biotechnol. 68 (2005) 272. 70. I. Davidova, M.S. Hicks, P.M. Fedorak, J.M. Suflita, J. Ind. Microbiol. Biot. 27 (2001) 80. 71. B. Little, J. Lee, R. Ray Biofouling, 23 (2007) 87. 72. F. Mansfeld, Electrochim. Acta, 52 (2007) 7670.

YAZAR Demet Çetin, Ankara Üniversitesi, Fen Fakültesi, Biyoloji Böl. Beþevler/Ankara E-mail: [email protected]

KOROZYON, 15 (1-2), 2007 11

THE USE OF SEMIEMPIRICAL CALCULATIONS IN CORROSION INHIBITOR STUDIES ABSTRACT Semiempirical methods are particularly significant in the study of electrochemistry and provide researchers with a relatively quick way of studying the structure and behavior of corrosion inhibitors. In this review article, the semiempirical quantum chemical studies on the efficiencies of various corrosion inhibitors that have been carried out so far have been introduced and their results have been summarized. KOROZYON ÝNHÝBÝTÖRÜ ÇALIÞMALARINDA YARI DENEYSEL HESAPLAMALARIN KULLANIMI Yarý deneysel yöntemler özellikle elektrokimya çalýþmalarýnda önemlidirler ve araþtýrmacýlara korozyon inhibitörlerinin yapýlarýný ve davranýþlarýný incelemede çok kolaylýk saðlamaktadýrlar. Bu derlemede çeþitli korozyon inhibitörleri üzerine þu ana kadar yapýlan yarý-ampirik kuantum kimyasal çalýþmalar ortaya konmuþ ve bu çalýþmalarýn sonuçlarý özetlenmiþtir.

1. INTRODUCTION Quantum chemical methods have already proven to be very useful in determining molecular structure as well as in elucidating electronic structure and reactivity1. Thus, it has become the common practice to carry out quantum mechanical calculations in corrosion inhibition studies. The concept of assessing the efficiency of a corrosion inhibitor with the help of computational chemistry is to search

for compounds with desired properties using chemical intuition and experience into a mathematically quantified and computerized form. Once a correlation between structure and activity or property is found, any number of compounds, including those not yet synthesized, can be readily screened on the computer2. To study molecules and molecular structures, scientific models have to be considered. The word "model" has a special meaning in science. It does not mean sitting down immediately at a personal computer and drawing on the screen, although modellers may spend some of their time on that activity. It means having a set of mathematical equations which are capable of representing accurately the chemical phenomenon under study3,4. The study of corrosion processes and their inhibition by organic inhibitors is a very active field of research5, many researchers report that the inhibition effect mainly depends on some physicochemical and electronic properties of the organic compound molecule which related to its functional groups, steric effects, electronic density of donor atoms, and orbital character

* Corrosponding outhor, E-mail: [email protected]

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KOROZYON, 15 (1-2), 2007

Gökhan GECE

of donating electrons, and so on6,7. The inhibiting mechanism is generally explained by the formation of a physical and/or chemical adsorption film on the metal surface8,9. It is well known that the organic compounds act as inhibitors rich in heteroatoms such as sulphur, nitrogen and oxygen10,11. These compounds and their derivatives are excellent corrosion inhibitors in a wide range of media and are selected essentially from empirical knowledge based on their macroscopical physicochemical properties. Recently, theoretical prediction of the efficiency of corrosion inhibitors has become very popular in parallel with the progress in computational hardware and the development of efficient algoritms which assisted the routine development of molecular quantum mechanical calculations12. Due to the enormous complexity of this sort of study (i.e. atoms from the metallic surface, inhibitor molecules, solvent molecules), the theoretical study of the corrosion inhibition processes cannot be achieved in a rigorous way from the viewpoint of quantum chemistry.

2. QUANTUM CHEMICAL DESCRIPTORS Quantum-chemical methods and molecular modeling techniques enable the definition of a large number of molecular quantities characterizing the reactivity, shape and binding properties of a complete molecule as well as of molecular fragments and substituents. The use of theoretical descriptors presents two main advantages: firstly, the compounds and their various fragments and substituents can be directly characterized on the basis of their molecular structure only; and secondly, the proposed mechanism of action can be directly accounted for in terms of the chemical reactivity of the compounds under study13. Quantum-chemically derived descriptors are fundamentally different from experimentally measured quantities, although there is some natural overlap. Unlike experimental measurements there is no statistical error in quantum-chemical calculations. There is inherent error however, associated with the assumptions required to facilitate the calculations. In most cases the direction but not the magnitude of the error is known14. In using quantum chemistry-based descriptors with a series of related compounds, the computational error is considered to be approximately constant throughout the series. The prominent quantum chemical descriptors can be subdivided as follows: 2.1. Atomic Charges All chemical interactions are either electrostatic (polar) or orbital (covalent). Electrical charges in the molecule are obviously the driving force of electrostatic interactions. The local electron densities or charges are important in many chemical reactions and physico-chemical properties of compounds. Thus, charge-based descriptors have been widely employed as chemical reactivity indices or as measures of weak intermolecular interactions. Many quantum-chemical descriptors are derived from the partial charge distribution in a molecule or from the electron densities on particular atoms. Most modern semiempirical methods use Mulliken population analysis15 for the calculation of the charge distribution in a molecule. In fact, this definition of atomic charge is arbitrary and other definitions are available, although none of them corresponds to any directly experimentally measurable quantity16. Moreover, semiempirical methods are mostly parametrized to reproduce heats of formation, ionization potentials, and/or geometric characteristics of the molecules. Therefore the cal-

culated atomic charges may be less reliable. For these reasons the values of atomic charges calculated by different semiempirical methods are in sometimes poor agreement with each other. However, these numerical quantities are easy to obtain and they give at least a qualitative picture of the charge distribution in a molecule17. Atomic partial charges have been used as static chemical reactivity indices18. The calculated σ- and π-electron densities on a particular atom also characterize the possible orientation of the chemical interactions and thus, are often considered to be directional reactivity indices. In contrast, overall electron densities and net charges on atoms are considered as nondirectional reactivity indices19. The latter are obtained by subtracting the number of valence electrons belonging to the atom according to the classical valence concepts from the total electron density on the atom. Such calculated net atomic charges are suitable for characterizing interactions according to classical point-charge electrostatic model20. Other common charge-based descriptors are the most positive and the most negative net atomic charges and the average absolute atomic charge. Atomic charges are also used for the description of the molecular polarity of molecules. 2.2. Molecular Orbital Energies Energies of the HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) are very popular quantum chemical descriptors. These orbitals play a major role in governing many chemical reactions and determining electronic band gaps in solids. According to the frontier molecular orbital theory of chemical reactivity, the formation of a transition state is due to an interaction between the frontier orbitals (HOMO and LUMO) of reacting species21. Thus, the treatment of the frontier molecular orbitals separately from the other orbitals is based on the general principles governing the nature of chemical reactions. The energy of the HOMO is directly related to the ionization potential and chracterizes the susceptibility of the molecule toward attack by electrophiles. The energy of the LUMO is directly related to the electron affinity and characterizes the susceptibility of the molecule toward attack by nucleophiles. The concept of hard and soft nucleophiles and electrophiles has been also directly related to the relative energy of the HOMO/LUMO orbitals. Hard nucleophiles have a low-energy HOMO; soft nucleophiles have a high-energy HOMO; hard electrophiles have a high-energy LUMO; and soft

KOROZYON, 15 (1-2), 2007 13

electrophiles have a low-energy LUMO22. The HOMO-LUMO gap, i.e. the difference in energy between the HOMO and LUMO, is an important stability index23. A large HOMO-LUMO gap implies high stability for the molecule in the sense of its lower reactivity in chemical reactions24. The concept of "activation hardness" has been also defined on the basis of the HOMO-LUMO energy gap25. The qualitative definition of hardness is closely related to the polarizability, since a decrease of the energy gap usually leads to easier polarization of the molecule25. 2.3. Dipole Moment (µ) The polarity of a molecule is well known to be important for various physicochemical properties and many descriptors have been proposed to quantify the polarity effects. The most obvious and most often used quantity to describe the polarity is the dipole moment of the molecule26. The total dipole moment, however, reflects only the global polarity of a molecule. Local polarities can be represented by local dipole moments, but these are conceptually difficult to define. First approximations of these quantities can be obtained by considering the atomic charges in the localized regions of the molecule. 2.4. Energy The total energy calculated by quantum chemical methods is also a good descriptor. The total energy of a system is composed of the internal, potential and kinetic energy. Kohenberg and Kohn27 proved that the total energy of a system including that of the many body effects of electrons (exchange and correlation) in the presence of static external potential (for example, the atomic nuclei) is a unique functional of the charge density. The minimum value of the total energy functional is the ground state energy of the system. The electronic charge density which yields this minimum is then the exact single particle ground state energy. 3. SEMIEMPIRICAL METHODS In principle, any observable property of an atomic or molecular system can be obtained from the Schrödinger equation. Over the past decades the semiempirical molecular orbital methods have been used widely in computational studies. Semiempirical approaches neglect many smaller integrals to speed up the calculations. In order to compensate for the errors caused by these approximations, empirical parameters are introduced into the rema-

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KOROZYON, 15 (1-2), 2007

ining integrals and calibrated against reliable experimental or theoretical reference data. This strategy can only be successful if the semiempirical model retains the essential physics to describe the properties of interest. Provided that this is the case, the parameterization can account for all other effects in an average sense, and it is then a matter of validation to establish the numerical accuracy of a given approach. In current practice, semiempirical methods serve as efficient computational tools which can yield fast quantitative estimates for a number of properties. This may be particularly useful for correlating large sets of experimental and theoretical data, for establishing trends in classes of related molecules, and for scanning a computational problem before proceeding with higher-level treatments28. There remains the need to improve semiempirical methods with regard to their accuracy and range of applicability, without compromising their computational efficiency. In addition, there is the need to develop new algorithms in order to exploit modern computer architectures and to extend semiempirical calculations to ever larger molecules. Over the years, a large number of methods with different acronyms have been published, including MNDO29, AM130, PM331. Different semiempirical methods differ in the details of the approximations (e.g. the core-core repulsion functions) and in particular in the values of the parameters. The semiempirical methods can be optimized for different purposes. The MNDO, AM1 and PM3 methods were designed to reproduce heats of formation and structures of a large number of organic molecules. Other semi-empirical methods are specifically optimized for spectroscopy ( e.g. INDO/S or CNDO/S). MNDO (Modified Neglect of Differential Overlap) is based on the NDDO (Neglect of Diatomic Differential Overlap) approximation and in turn NDDO an improvement version of INDO (Intermediate Neglect of Differential Overlap) method. INDO itself is an improvement on the CNDO (Complete Neglect of Differential Overlap) approximation. There are several such semi-empirical LCAO MO methods, developed for specific purposes. AM1 (Austin Model 1), is a semiempirical method based on the Neglect of Differential Diatomic Overlap integral approximation. Specifically, it is a generalization of the modified Neglect of Diatomic Differential Overlap approximation. AM1 was developed by Michael Dewar and co-workers and published in 198530. AM1 is an attempt to improve the MNDO model by reducing the repulsion of atoms

at close separation distances. The atomic core-atomic core terms in the MNDO equations were modified through the addition of off-center attractive and repulsive Gaussian functions. The complexity of the parameterization problem increased in AM1 as the number of parameters per atom increased from 7 in MNDO to 13-16 per atom in AM1. PM3 (Parameterized Model number 3), is another semiempirical method based on the Neglect of Differential Diatomic Overlap integral approximation. The PM3 method uses the same formalism and equations as the AM1 method. The only differences are: a) PM3 uses two Gaussian functions for the core repulsion function, instead of the variable number used by AM1 (which uses between one and four Gaussians per element); b) the numerical values of the parameters are different. The other differences lie in the philosophy and methodology used during the parameterization: whereas AM1 takes some of the parameter values from spectroscopical measurements, PM3 treats them as optimizable values. The method was developed by J. J. P. Stewart and first published in 198931. 4. CORROSION INHIBITORS STUDIED BY SEMIEMPIRICAL METHODS The inhibition of corrosion in acid solutions can be affected by the addition of a variety of organic molecules. Compounds containing nitrogen, oxygen and sulphur have shown vast applications as corrosion inhibitors. The influence of some heterocyclic compounds, i.e. some oxadiazole derivatives, on the corrosion of mild steel in acid solutions, has been investigated by Lagrenée et al.32, and Bentiss et al.33,34. Beside using experimental methods as such as mass loss measurements, polarisation curves and AC impedance methods, they used AM1 semiempirical method to obtain the electronic properties of those compounds. They calculated EHOMO, ELUMO, ∆E (ELUMO-EHOMO) and dipole moment (µ) and found a highly significant multiple correlation coefficient between experimental and theoretical data. Semiempirical calculations for the efficiency of some imidazole derivatives as acidic corrosion inhibitors for zinc and iron have been performed by Bereket et al.35,36 using AM1, PM3, MNDO and MINDO/3 methods. Charges on nitrogen atoms, total energy, ionization potential, EHOMO, ELUMO, ∆E (ELUMO-EHOMO) and dipole moment (µ) have been calculated and correlated with experimental results. A satisfactory agreement was found between theoretical and experimental data. Similar studies on the-

se compounds have been carried out by Öðretir et al.37,38 in order to search inhibition mechanism of corrosion via metal-ligand interaction using semiempirical methods. According to their conclusion, semiempirical calculations can be used to elucidate the mechanism of inhibition. Popova et al.39 investigated the effect of molecular structure of some different azole derivatives as inhibitors on corrosion of mild steel in acidic medium by using AM1 quantum chemical method. Some triazole derivatives (triazole (TA), 3-amino1,2,4-triazole (ATA) and 3,5- diamino-1,2,4-triazole (DTA)) (Fig.1) on copper corrosion in 0.5M HCl have been studied by El Issami et al.40 using AM1, MNDO and PM3 methods. According to the results of their electrochemical and gravimetric measurements (Table 1), the efficiency of the triazolic compounds follows the sequence: TA < ATA < DTA. They aimed to show the formation of the copper-ATA or copper-DTA complexes and to find a correlation between the highest occupied molecular orbital energy (EHOMO) and inhibition efficiencies. Therefore, they calculated quantum chemical indices such as EHOMO, ELUMO and energy gap (∆E) and ∆Hƒ (energy of formation) by using AM1, MNDO and PM3 methods (Table 2). The plot of the inhibition efficiency of triazoles against EHOMO (Figure 2) is linear with a slope close to unity and regression coefficient R=0.964. By comparing their experimental and theoretical results, they concluded that high values of EHOMO indicate a tendency of the molecule to donate electrons to appropriate acceptor molecules with low energy of empty atomic orbitals and the energy of the lowest unoccupied molecular orbitals indicates the ability of the molecule to accept electrons. Also, the less negative HOMO and the smaller energy gap are reflected in stronger chemisorption bond and greater inhibitor efficiency Zhang et al.41 also studied some triazole derivatives using Parker, Parr and Pople (PPP) method. They concluded that EHOMO, ELUMO and π-electron density were consistent with the inhibition efficiencies of the compounds. Similar results were obtained by the study of Qafsaoui et al.42 using HartreeFock approximation. A more detailed and comparative study of triazole, oxadizale and thiadiazole derivatives on the corrosion inhibition of steel has been carried out by El Ashry et al.43 using AM1, PM3, MINDO/3 and MNDO semiempirical methods. They correlated quantum chemical descrip-

KOROZYON, 15 (1-2), 2007 15

Figure 1. Molecular structures of TA, ATA and DTA Þekil 1. TA, ATA ve DTA'nýn molekül yapýlarý

Table 1. Inhibition efficiencies obtained using Icorr, Rp and mass loss data Çizelge 1. Ikor, Rp ve kütle kaybý verilerinden elde edilen inhibisyon verimliliði

Table 2. Calculated theoretical parameters for TA, ATA and DTA. Çizelge 2. TA, ATA ve DTA için hesaplanmýþ teorik parametreler.

Figure 2. Correlation between HOMO energy and IE(%) of triazolic compounds Þekil 2. Triazol bileþiklerinin HOMO enerjisi ile %IE arasýndaki kolerasyon

tors such as total negative charge on the molecule, EHOMO, ELUMO, dipole moment (µ) and molecular volume to corrosion inhibition efficiency. A satisfactory agreement with experimental data was reported43. Zhang et al.44 studied the adsorption behaviour of some thiazole derivatives at Fe surface through the molecular dynamics simulation(1) and the quantum chemical calculations. Thereto, some pyrazole45-47 and pyridine48-51 derivatives have been investigated as corrosion inhibitors in acidic media by means of semiempirical quantum chemical methods. Due to the presence of -C=N- group, electronegative nitrogen, sulphur and/or oxygen atoms in the molecule, Schiff bases should be good corrosion inhibitors. Recently some Schiff bases as effective corrosion inhibitors for steel52,53, aluminum54,55 and copper56,57 in acidic media have been investigated by using semiempirical molecular orbital methods. El Ashry et al.58 correlated the structural characteristics of hydrazides and Schiff bases (Fig. 3) with their corrosion inhibition efficiency at different inhibitor concentrations in aqueous acid solutions and investigated the relation between the inhibition efficiency and quantum chemical calculation parameters, EHOMO, ELUMO, dipole moment, total negative charge on molecules, and linear solvation energy. These researchers used a non-linear regression analysis to correlate quantum chemical parameters (EHOMO, ELUMO, µ, TE), LSER (Vi,π*) and inhibitor concentrations (Ci) with the experimental inhibition efficiencies obtained by mass loss methods for compounds 1-18. Thus, a composite index of more than one quantum parameter, which might affect the inhibition efficiency of molecules was correlated with the experimental corrosion inhibition efficiencies (Figure 4). According to their results, the inhibition efficiency of the Schiff bases increases with increasing EHOMO and decreasing ELUMO; reverse results were obtained for the hydrazides. A highly significant multiple correlation coefficient (r >0.96) was obtained between experimental and calculated efficiencies. Some amides and derivatives e.g. urea, thiourea, thioacetamide and thiosemicarbazide have been found to be good inhibitors for mild steel in acid solutions59-64. Several quantum chemical studies59-64 have been carried out on these compounds

1 Molecular dynamics simulations generate information at the microscopic level, including atomic positions and velocities. The conversion of this microscopic information to macroscopic observables such as pressure, energy, heat capacities, etc., requires statistical mechanics.

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KOROZYON, 15 (1-2), 2007

used extensively in oil field corrosion. A lot of work on imidazoline inhibitors has been studied experimentally65,66. However, its theoretical study67,68 is fewer relatively. Wang et al.68 measured the averaged percentage protection experimentally (using mass loss and polarization curve methods) and obtained molecular geometry and charge density by MNDO calculation. Their theoretical prediction has been Table 3. Quantum chemical parameters of BPMU and MPMU64. Çizelge 3. BPMU ve MPMU' nun kuantum kimyasal parametreleri64. Inhibitor

Figure 3. Molecular structures of hydrazides (1-4) and Schiff bases (518) Þekil 9. Hidrazürlerin (1-4) ile Schiff bazlarýnýn (5-18) molekül yapýlarý

by using semiempirical methods. Zhang et al.64 (Table 3), Fang and Li62 found a strong relationship between inhibition effect and the ∆E (ELUMO-EHOMO) (Table 4), whereas Kutsán et al.63 found a relationship between dipole moment (µ) and corrosion parameters. As shown in Table 3, BPMU has the smaller HOMO-LUMO gap compared with MPMU and thus is the better inhibitor. As can be clearly seen from Table 4, the inhibition efficiency decreases as the EHOMO level dec-

Figure 4. Plot of average calculated efficiencies versus experimental efficiencies of compounds 1-1858. Þekil 4. Bileþiklerin (1-18) ortalama hesaplanan etkinliklerinin deneysel etkinliklerine karþý grafiði 58.

reases. Urea molecule has the lowest EHOMO value, also has the worst inhibition efficiency. An amount of 1.422 eV energy difference between the EHOMO values of urea and that of TSC which has the highest EHOMO corresponding to 55% change in inhibition efficiency. TU and TA have very close EHOMO values, compared with their very close experimental efficiencies. Imidazoline based corrosion inhibitors are well known to have high corrosion inhibitor and are

EHOMO (eV)

ELUMO (eV)


E (eV)

IE (%)

Bis-piperidiniummethyl-urea (BPMU)

-9.449

-2.673

6.776

93.7

Mono-piperidiniummethyl-urea (MOMU)

-9.067

-1.923

7.144

79.8

Table 4. Quantum chemical parameters of urea, thiourea, thioacetamide and thiosemicarbazide 62. Çizelge 4. Urea, thiourea, thioacetamide ve thiosemicarbazide' nin kuantum kimyasal parametreleri62. Inhibitor

EHOMO (eV)

ELUMO (eV)


E (eV)

IE (%)

Urea (U)

-9.945

1.432

11.377

21.98

Thiourea (TU)

-8.596

0.706

9.302

70.11

Thioacetamide (TA)

-8.576

0.755

9.331

70.13

Thiosemicarbazide (TSC)

-8.523

0.458

8.981

76.95

verified by experimental results well. The environmental requirements that are currently imposed on the development of cleaner chemical inhibitors represent a strong motivation for the study of inhibition by natural tannins. Although anticorrosive action of natural tannins has been known for a long time, only in the past decade tannins have been systematically investigated as metal corrosion inhibitors both experimentally and theoretically in various media69-71. Martinez et al.69,70 calculated the molecular properties of chestnut and mimosa tannins most relevant to their action as corrosion inhibitors. These were, namely, the geometrical structure of the molecule, the dipole moment (µ), HOMO and LUMO energies, the HOMO-LUMO energy gap. According to the results of their work, the analysis of the action of metal corrosion inhibitors by quantum chemical values may, to a great extent, eliminate the empirical approach to the research work in this sphere. And it may also facilitate a rational selection and design of new inhibitors. As an alkaloid, berberine could be readily abstracted from natural coptis72, and the hydrochloric berberine has also been commonly used as a nontoxic antibiotic for years in China73. Since little known about the inhibition behaviour of berberine for metallic materials in acidic media, Li et al.74 used experimental and quantum chemical methods to discuss the correlation of inhibition effect and molecular structure of berberine. The authors

KOROZYON, 15 (1-2), 2007

17

predicted that the adsorption of berberine on the mild steel surface in sulfuric acid may be achieved by the interaction between iron atoms and cyclic molecular π orbital so they calculated HOMO and LUMO energies. The density distribution of HOMO/LUMO indicated that there are several feasible absorbed sites in one berberine molecular being in favor of the strong adsorption and high inhibition efficiency. Attempts to connect corrosion inhibition with structural properties of prospective molecules by using semiempirical quantum chemical approach have been based and developed along with the theories of reactivity and various compounds; i.e. zinc di-alkyl-di-thiophosphates75,76, potassium ethyl xanthate77, phthalocyanines78, polymethylene amines79, benzyl triphenyl phosphonium bromide80, hydrazine carbodithioic acid derivatives81, aliphatic amines82, pyrimidine derivatives83, piperazine derivatives84, amino acids and hydroxy carboxylic acids85, phenyl-N,N-dimorpholinemthanes86, aniline trimers87, triblock copolymers88, quarternary ammonium salts89, organophosphorus compounds90 and para-chlorobenzene nitriles91 have been investigated. Parameters of electron structure have been extensively used for the correlation with corrosion inhibition effect of these organic compounds. Although there is no general way to predict the potential of a compound to be good corrosion inhibitor or to find a universal type of correlation, Babic-Samardzija et al.92 made attempt to correlate some molecular parameters of N-heterocyclic amines (piperidine (pip), 2-methyl piperidine (2mp), 3methyl piperidine (3mp), 2,6-dimethyl piperidine (26dp), 3,5-dimethyl piperidine (35dp)) with their corrosion inhibition efficiency. For this purpose, they used PM3, AM1 and MNDO semiempirical methods. The effect of the molecular structure of these compounds on their inhibiting properties has been considered in terms of their electronic and chemical structure. First, the effect on electron density, i.e. charge on the nitrogen atom and on the whole heterocyclic ring. Second, the effect on structural changes in terms of bond distances and angles. Total energy (Etot) has been obtained after geometric optimization with respect to all the nuclear coordinates. Table 5 shows the effect of the -CH3 methyl group in various locations on some electronic and structural characteristics of piperidine. The negative charge on the nitrogen atom (-QN) of the piperidine is slightly higher than on its derivatives. The

18

KOROZYON, 15 (1-2), 2007

sum of the net charge of the six atoms from the cyclic ring (-Qring) was calculated for the nonprotonated piperidine and its methyl derivatives and showedthat their inhibitor efficiency was related to this effect, as shown in Figure 5. According to the results of their computational study, the relationship between some molecular parameters and the inhibiting properties of amino compounds show that a definite dependence exists. Nevertheless, a number of neglected parameters that could be involved in such correlations, Table 5. Total energy, charge on the nitrogen and the sum of the net charge of six atoms from the cyclic ring, bond distance and corresponding angle for the piperidine and its methyl derivatives obtained from PM3 method 92. Çizelge 5. PM3 metodundan bulunan toplam enerji, azot üzerindeki elektrik yükü, ve çevrimsel halkadan altý atomun üzerindeki net yükler toplamý, bað aralýðý, ve piperidine ve metil türevlerine ait açýlar 92.

such as surface and solution characteristics, give at least a simplified explanation, though the correlation is not so simple and straightforward as might be expected. But it is clear that the inhibition properties of these N-heterocyclic amines could be related to the charge on nitrogen atom and sum of the net charge of the six atoms from the cyclic ring. Babic-Samardzija and Hackerman93 also used molecular modeling to gain some insight into structural and electronic effects of polypyrazolylborates (Fig. 6) in relation to their inhibiting efficiencies and adsorption behaviour. They used Tafel measurements, linear polarization resistance and electrochemical impedance spectroscopy to investigate two polypyrazolylborates as corrosion inhibitors for iron in acidic media. According to the findings of their electrochemical measurements, both polypyrazolylborates were fair-to-good inhibitors against acidic iron corrosion. Some parameters of interest (Table 6), were obtained by using the MNDO method. It is obvious

Figure 6. The structure of dihydrobis(1-pyrazolyl)borate (Bp) and hydrotris(1-pyrazolyl)borate (Tp). Þekil 6. Dihidrobis(1-pirazolil)borat (Bp) ile hidro(1-pirazolil)borat(Tp)'nin yapýsý.

Figure 5. Relation between inhibitor efficiency and sum of the net charge the heterocyclic ring (-Qring) of piperidine and its methyl derivatives 92. Þekil 5. Piperidinin heterohalkasýnýn (-Qhalka) ve metil türevlerinin toplam net yükü ile inhibitör etkinliði arasýndaki iliþki92.

that Bp had a lower binding energy and heat of formation as well as lower total energy than did Tp. The energy gap (∆) between the EHOMO and ELUMO shows that the higher inhibition effect could be related to the lower energy difference, i.e. to the Bp molecules that more easily could be excited and which more readily could undergo a charge transfer interaction with the metal surface. The high negative charge density of Bp and Tp has been determined alongside the nitrogen bonds (Fig. 7). For complexing, nitrogen atoms as electron donors are available for coordination to the iron. That interaction is more likely to occur with the Pyr-B Pyr structure of Bp (Fig. 7(a)). The steric hindrance of the three pyrazole rings of Tp (Fig.7(b)) and their orientation towards iron surface will tend to lower its sorption. This could explain the enhanced inhibition of Bp compared to Tp in both acids. Up to now, several studies on corrosion inhibitors concerning the semiempirical calculations have been reviewed. As prescribed, the use of semiempirical methods has been a subject of intense interest in corrosion inhibitor studies especially in recent years. Consequently, it can be said that in such studies two different approaches have been used. In the first approach, the empirical method, each functional group in an inhibitor molecule is assumed to contribute a unique, independent and additive increment of corrosion protection; these increments are determined from the corrosion rates

by correlation of the molecular fragments with inhibitor performance. In the second approach, the semiempirical method, quantum chemical properties are correlated with inhibitor performance; determining the descriptor parameters is a most important aspect of this approach94. Table 6. Calculated quantum chemical parameters by MNDO method for the Bp and Tp. Çizelge 6. Bp ve Tp için MNDO metodu ile hesaplanmýþ kuantum kimyasal parametreler. Compound

EHOMO(eV) ELUMO (eV) E (eV)

µ (D)

-Etot (kJ/mol)

-Ebind (kJ/mol)

Bp

1.5778

5.2416

3.6638

2.759

171.677

8.231

Tp

0.9389

4.7530

3.8142

4.962

251.381

11.616

Figure 7. Charge density distributions of the Bp and Tp molecules obtained after molecular modeling (MNDO) 93. Þekil 7. Bp ve Tp moleküllerinin moleküler modellemeden (MNDO) sonra elde edilen yük yoðunluk daðýlýmlarý ) 93.

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N. 11 (2000) 1075. 87. Jr. L. T. Sein, Y. Wei, S. A. Jansen, Comput. Theor. Polym. Sci. 11 (2001) 83. 88. A. Yurt, V. Bütün, B. Duran, Mater. Chem. Phys. (In Press). 89. G. Bereket, M. Gülec, A. Yurt, Anti-Corr. Meth. Mater. 53 (2006) 52. 90. P. Mutombo, N. Hackerman, Anti-Corr. Meth. Mater. 45 (1998) 413. 91. X. Xuejun, C. Shuman, P. Ling, G. Xunjie, P. Keru, Anti-Corr. Meth. Mater. 51 (2004) 101. 92. K. Babic-Samardzija, K. F. Khaled, N. Hackerman, Anti-Corr. Meth. Mater. 52 (2005) 11.

93. K. Babic-Samardzija, N. Hackerman, Anti-Corr. Meth. Mater. 53 (2006) 19. 94. N. Khalil, Electrochim. Acta 48 (2003) 2635.

AUTHOR Gökhan Gece, Ankara University, Faculty of Scieences, Chemistry Dept., Ankara/Turkey • E-mail: [email protected]

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KLORÜRLÜ ORTAMLARDA BETONARME ÇELÝÐÝNÝN ELEKTROKÝMYASAL DAVRANIÞLARINA ASETAT ÝYONUNUN ETKÝLERÝ

ÖZET Bu çalýþmada %3.5 NaCl içeren ortamdaki betonarme çeliðinin korozyonuna oda sýcaklýðýnda asetat iyonunun etkisi araþtýrýlmýþtýr. Çalýþmalarda elektrokimyasal üç elektrot tekniði uygulanmýþtýr. Elde edilen deneysel sonuçlar da, asetat iyonunun betonarme çeliðinin korozyon hýzýný azaltacak yönde etki ettiði saptanmýþtýr. Ayrýca, deðiþik bekleme süreleri için korozyon potansiyeli ile polarizasyon direnci arasýnda doðrusal bir iliþkinin olmadýðý belirlenmiþtir. THE EFFECT OF ACETATE ION ON ELECTROCHEMICAL BEHAVIOUR OF REINFORCING STEEL IN CHLORIDE SOLUTIONS In this work ,the effect of acetate ion on the corrosion of reinforcing steel in chloride solutions at room temperature were investigated through using the three electrode technique in electrochemical studies. The results obtained indicate, that the acetate ion reduces the corrosion rate of reinforcing steel. In addition, it was observed, that there is no direct correlation between corrosion potential and polarization resistance for various waiting times.

1. GÝRÝÞ Korozyon, metallerin içinde bulunduklarý ortam ile kimyasal veya elektrokimyasal reaksiyonlara girerek metalik özelliklerini kaybetmeleri olayýdýr. Metallerin büyük bir kýsmý su ve atmosfer etkisine dayanýklý olmayýp normal koþullar altýnda bile korozyona uðrayabilir. Bütün metaller doðada mineral olarak bulunduklarý hale dönüþmek eðilimin* Ýletiþimle görevli yazar: E-mail: [email protected]

22

KOROZYON, 15 (1-2), 2007

dedir. Bu nedenle korozyon olaylarý enerji açýða çýkararak kendiliðinden yürür. Bazý soy metaller hariç teknolojik öneme sahip bütün metal ve alaþýmlar korozyona uðrayabilir1,2. Betonarme yapýlarda donatý korozyonunun gözlenen en belirgin göstergesi beton örgünün çatlamasýdýr. Bu yapýlarda olasý hasarlarýn teþhis ve kontrolü için, gözle görülen semptomlar yardýmlar ile korozyonun kalitatif olarak belirlenmesine ve korozyonla ilgili bazý parametrelerin tespit edilmesine ihtiyaç vardýr. Betonarme yapýlaþmalarda kendisine oldukça fazla kullaným alaný bulmuþ olan demir ve demirli malzemelerin korozyonuna yönelik birçok çalýþma vardýr. Son yýllarda, ekonomik ve sosyal problemler nedeni ile önem kazanan donatý korozyonu ile ilgili araþtýrmalarýn sayýsý artmakta, donatý korozyonu deney yöntemleri geliþtirilmektedir. Gerek bu deneylerin gerekse korozyon hasarý sonrasý tespit ve onarýmýn zahmetli ve pahalý bir iþ olduðu belirlenmiþtir. Hasarýn geciktirilmesi ve en aza indirilmesi için projenin hazýrlanmasýndan itibaren donatýnýn korozyonu ve neden olacaðý hasarlar dikkate alýnmalýdýr. Bu amaçla deneysel verilerin elde edilmesindeki kolaylýk ve sonuçlarýn güvenilirliði

Caner MENEKÞE Güray KILINÇÇEKER*

açýsýndan elektrokimyasal yöntemlerle korozyon hýzýnýn belirlenmesi oldukça kullanýþlýdýr. Son zamanlarda, betonarme yapýlardaki çeliðin korozyonunu önlemek amacýyla kullanýlabilecek organik maddeler belirlenmeye çalýþýlmýþtýr3. Beton en kuru koþullarda bile bir elektrolit ortamý kabul edilebilir. Beton boþluklarý içinde daima bir miktar su bulunur. Bu su, çimento klinker bileþiklerinin hidratasyonu sýrasýnda açýða çýkan kalsiyum hidroksit ve az miktarda alkali hidroksitleri nedeni ile alkali özelliktedir. Beton boþluk suyunun pH deðeri doygun halde yaklaþýk olarak 12,5 civarýndadýr. Beton yüksek pH derecesinde bir elektrolit olduðundan beton içinde yürüyen korozyon olayýnda anod ve katod reaksiyonlarý aþaðýdaki gibidir4-6.

Katodik reaksiyonun yürümesi için katot yüzeyine oksijen difüzyonu zorunludur. Atmosferden beton içindeki çelik yüzeylerine oksijen taþýnýmý iki yolla mümkün olabilir: 1. Eðer beton kuru halde

ise, yani beton boþluklarý su ile dolu deðilse, atmosferde beton iç bölgelerine oksijen taþýným hýzý oldukça yüksektir. 2. Aksi halde oksijenin su içinde çözünmüþ olarak metal yüzeyine kadar yayýnmýþ olmasý gerekecektir. Beton içine oksijen yayýnma hýzý; beton kalitesine, özellikle beton permeabilitesine baðlýdýr. Beton permeabilitesi baþta su / çimento oraný olmak üzere betonun hazýrlanmasý ve kürü sýrasýnda bir çok faktöre baðlýdýr. Beton hazýrlanmasý sýrasýnda kullanýlmýþ olan su / çimento oraný ne kadar küçük tutulabilirse, beton boþluk yüzdesi ve permeabilitesi de o kadar yüksek olacaktýr. Çimento havada veya su altýnda su ile etkileþerek katýlaþabilen ve katýlaþtýktan sonra suya dayanýklý, temelde kireç, kil, silis ve demir oksitten oluþan hidrolik baðlayýcý bir maddedir. Çimentonun ilkel maddeleri kil ve kalkerdir. Bu maddeler belirli oranlarda karýþtýrýldýktan sonra yüksek sýcaklýkta piþirilir. Yüksek sýcaklýkta ilkel maddeler önce ayrýlýr7. Kalker'in ayrýþmasý sonucunda SiO2, Al2O3 ve Fe2O3 oluþur: CaCO3 › CaO + CO2 (900°C) Kil içinde bulunan en önemli madde 2SiO2.Al2O3.2H20 formülüne sahip kaolinittir. • 2SiO2.Al2O3.2H20 → 2SiO2 + Al2O3 + 2H20 Sýcaklýk artýþý ile kaolinik • 700 - 900°C'de CaO.Al2O3, CaO.SiO2, ve az miktarda CaO.Fe2O3 • 900 - 950°C'de 5CaO.3Al2O3 • 940 - 1200°C'de 2CaO.SiO2 • 1200 - 1300°C'de 3CaO.Al2O3, 4CaO.Fe2O3, ve • 1200 - 1450°C'de 3CaO.SiO2 yapýlarýna dönüþür. Bu karmaþýk ürünlere klinker adý verilir. Çimentonun içindeki oksitlerin her biri kýsaltýlmýþ olarak bir sembolle gösterilir: CaO = C, Al2O3 = A, SiO2 = S, Fe2O3 = F, H 20 = H 2. MATERYAL VE METOD Betonarme çeliðinin elektrokimyasal davranýþlarý, pH'sý 8 olan %3,5 NaCl, 0,1 M asetat ve %3,5 NaCl + 0,1M asetat içeren ortamlarda; oda sýcaklýðýnda (298 K) incelenmiþtir. Deneylerde kullanýlan kimyasal maddeler analitik saflýktadýr. Çalýþma elektrodu olarak, 10 mm çapýnda 5 cm uzunluðunda teknik saflýkta nervürlü demir çubuklar, TSE'ye uygun portland çimentosunun kullanýldýðý beton harcýna gömülerek çalýþma elektrodu hazýrlanmýþtýr. Karþý elektrot olarak alaný 1,0 cm2 olan pilatin

Çizelge 1. Portland çimentosu içindeki fazlar. Table 1. Phases present in portland cement.

Bileiin Adı

Oksitleri

Tri kalsiyum silikat

3CaO.SiO2

C3S

Di kalsiyum silikat

2CaO.SiO2

C2S

Tri kalsiyum alüminat

3CaO.Al2O3

C3A

Tetra kalsiyum alümina ferri 4CaO. Al2O3.Fe2O3

Kısa Gösterilii

C4AF

levha, ve karþýlaþtýrma elektrodu olarak da standart kalomel elektrot (SCE) kullanýlmýþtýr. Çalýþmalarda, akým-potansiyel eðrileri potansiyokinetik olarak üç elektrot tekniði ile elde edilmiþtir (CHI 604 Elektrokimyasal Analiz Cihazý). Deney süresince çalýþma elektrodu %3,5 NaCl çözeltisi içerisinde bekletilerek çözelti deriþimi sabit kalmasý için destile su ilavesi yapýlmýþtýr. Çalýþma elektrotlarý, kalomel elektroda karþý ölçülen denge potansiyelinden itibaren 0,004 V.s-1 hýzla, katodik yönden anodik yöne doðru polarize edilerek akým-potansiyel eðrileri çizilmiþtir (-2.0 V - + 1.8 V aralýðýnda). Ayrýca, polarizasyon direnci elektrokimyasal impedans spektroskopisiyle 100 kHz - 0,01 Hz aralýðýnda 0,01 mV'luk genlikler uygulayarak belirlenmiþtir. 3. DENEYSEL SONUÇLAR VE TARTIÞMA 3.1. Karma Suyu Karýþýmýnda Sadece %3.5 NaCl Ýçeren Betonarme Çeliðinin Elektrokimyasal Davranýþý Betonarme çeliðinin sabit sýcaklýkta (298 K), karma suyunda sadece % 3,5 NaCl içeren sulu ortamda elde edilen Tafel eðrileri, Nyquist ve Bode diyagramlarý sýrasýyla Þekil 1, 2 ve 3' de verilmektedir. Bekleme sürelerinden sonra belirlenen açýk devre potansiyeli; 1. günde -0,535 V, 2. günde 0,578 V, 7. günde -0,536 V, 28. günde -0,557 V, 60. günde -0,846 V ve 90. günde -0,669 V olarak ölçülmüþtür. Þekil 1' de verilen akým-potansiyel eðrilerinden elde edilen anodik (+0,800 V) ve katodik (1,700 V) potansiyellere karþý gelen akým yoðunluklarý sýrasýyla 2,2440.10-2 A/cm2 ve 4,3410.10-2 A/cm2 dir.. Betonarme çeliðin'in; %3,5NaCl içeren ortamda elde edilen Tafel eðrilerinde, katodik tepkime O2 iyonlarýnýn indirgenmesidir. Anodik tepkimeyi ise; düþük potansiyellerde Fe+2 iyonlarý, yüksek potansiyellerde de Fe+3 iyonlarýnýn çözünme-çökmesinin oluþturduðu görülmektedir. Þekil 2'de verilen Nyquist ve Bode diyagramlarýnda klorürlü ortam koþullarýnda betonarme çeliði'nin yüzeyinde 1. günden 7. güne kadar yaklaþýk 1000 Ω' luk direncin oluþtuðu, bu direncin 7. gün-

KOROZYON, 15 (1-2), 2007 23

log i (A.cm-2)

den sonra hýzla düþtüðü ve 90. Gün sonunda yaklaþýk 1/10 oranýnda azalarak 100 Ω seviyelerine düþtüðü görülmektedir.

E (V/Ag,AgCl)

Þekil 1. Betonarme Çeliði'nin; % 3,5 NaCl içeren ortamlardaki akým yoðunluðu-potansiyel eðrileri. Figure 1. Current density-potential curves of Reinforcing Steel in solutions of 3.5 % NaCl.

a

  
*

1. Gün 2. Gün 7. Gün 28. Gün 60. Gün 90. Gün

ði'nin akým yoðunluðu- potansiyel eðrisi, Nyquist ve Bode diyagramý Þekil 3 ve 4'te verilmiþtir. 0.1M asetat içeren ortamda elde edilen akým-potansiyel eðrisinden (Þekil 3), betonarme çeliði'nin korozyon potansiyeli; 1. günde -0,531 V, 2. günde -0,522 V, 7. günde -0,493 V, 28. günde -0,514 V, 60. günde -0,711 V ve 90. günde -0,589 V olarak ölçülmüþtür. Anodik (+0,800) ve katodik (-1,700) yöndeki korozyon akým yoðunluklarýnda dikkate deðer bir azalma meydana gelmiþtir (0,1M asetat iyonu için anodik ve katodik akým yoðunluklarý sýrasýyla 1,3670.10-2 ve 2,4110.10-2 A/cm2 dir). Asetat iyonunun betonarme çeliðinin yüzeyindeki çözünmüþ oksijen deriþimini düþürerek katodik tepkime hýzýnýn azalmasýna neden olduðu anlaþýlmaktadýr8-9. Temas suyunda % 3.5 NaCl bulunan ortamlara daldýrýlan, karma suyu 0.1 M asetat içeren betonarme elektrotlarýn 90. günün sonunda elde edilen Nyquist ve Bode diyagramlarý Þekil 4.a ve 4.b'de verilmektedir. Þekil 4.a ve 4.b'ye bakýldýðýnda, 1. ve 2. günden itibaren betonarme çeliðinin üzerinde polarizasyon direncinin yüksek olduðu ve 7. günden 60. güne kadar yavaþ bir þekilde 60. günden sonra ise hýzlý bir þekilde azaldýðý görülmektedir. Dolayýsýyla bu diyagramlardan, betonarme çeliði için yüzeyinde 1. ve 2. günde kararlý bir pasif taba-

1. Gün 7. Gün 28. Gün 90. Gün

log i (A.cm-2)

%3.5NaCl

 
*

0.1M Asetat

b

E (V/Ag,AgCl)  
*

1. Gün 7. Gün 28. Gün 90. Gün

c

Þekil 2. Betonarme çeliði'nin; sadece % 3,5 NaCl içeren ortamlardaki Nyquist (a) ve Bode (b-c) diyagramlarý. Figure 2. Nyquist (a) and Bode (b-c) diagrams of reinforcing steel in solutions of 3.5 % NaCl.

3.2 Karma Suyu Karýþýmýnda Asetat Ýyonu Ýçeren Betonarme Çeliðinin Elektrokimyasal Davranýþý 0.1M asetat içeren ortamdaki betonarme çeli-

24

KOROZYON, 15 (1-2), 2007

Þekil 3. Betonarme çeliði'nin; 0,1M asetat iyonu içeren ortamlardaki akým yoðunluðu-potansiyel eðrileri. Figure 3. Current density-potential curves of reinforcing steel in solutions of 0,1M acetate ion.

kanýn oluþtuðu ve sonra bu pasif tabakanýn 60. güne kadar yavaþ 60. günden sonra ise hýzlý bir þekilde kararlýlýðýný yitirmeye baþladýðý 90. günün sonunda ise polarizasyon direncinin ilk gündekinden yaklaþýk 1/3 oranýnda azalarak polarizasyon direncinin en alt seviyeye indiði anlaþýlmaktadýr. 3.3 Karma Suyu Karýþýmýnda %3.5 NaCl + Asetat Ýyonu Ýçeren Betonarme Çeliðinin Elektrokimyasal Davranýþý Korozyon hýzý belirleme yöntemleri sonucunda elde edilen Tafel eðrileri, Nyquist ve Bode diyag-

ramlarý Þekil 5 ve 6'da verilmektedir. Akým-potansiyel eðrisinden (Þekil 5) korozyon potansiyelleri dikkate alýndýðýnda, ortama asetat iyonu ilave edilmesiyle korozyon potansiyellerinin daha pozitif deðerlere kaydýðý anlaþýlmaktadýr (1. günde -0,507 V, 2. günde -0,523 V, 7. günde -0,534 V, 28. günde -

daldýrýlan ve karma suyunda %3.5 NaCl ile 0.1 M asetat içeren betonarme elektrotlarýn Elektrokimyasal Ýmpedans Spektroskopisi yöntemi sonucu elde edilen Nyquist ve Bode diyagramlarýnda Þekil 5 ve 6'ya bakýldýðýnda, 1., 2., 7., 28., 60. ve 90. gün koþullarýndaki direnç deðerlerinin sýrasýyla; 1082 Ω, 1211 Ω, 1164 Ω, 832 Ω, 644 Ω ve 107 Ω, olduðu görülmektedir. Dolayýsýyla bu diyagramlardan, betonarme çeliði için yüzeyinde 1., 2. ve 7. günde kararlý bir pasif tabakanýn oluþtuðu ve sonra bu pasif

Þekil 5. Betonarme çeliði'nin; %3,5 NaCl + 0,1 M asetat iyonu içeren ortamlardaki akým yoðunluðu potansiyel eðrileri. Figure 5. Current density-potential curves of reinforcing steel in solutions of %3.5 NaCl + 0.1 M acetate ion.

Þekil 4. Betonarme çeliðinin; sadece 0,1 M asetat iyonu içeren ortamlardaki Nyquist (a) ve Bode (b-c) diyagramlarý. Figure 4. Nyquist (a) and Bode (b-c) diagrams of reinforcing steel in solutions of 0.1 M acetate ion.

0,546 V, 60. günde -0,655 V ve 90. günde -0,653 V). Asetat iyonunun betonarme çeliðinin yüzeyine adsorbe olmasý sonucu korozyon tepkimesinin katodik yöndeki akým yoðunluklarýnda dikkate deðer bir azalmanýn meydana geldiði görülmektedir (0.1M asetat iyonu içeren çözeltideki katodik akým yoðunluðu 1,8630.10-2 A/cm2 dir). Bununla birlikte, anodik polarizasyon davranýþlarý, sadece klorür içeren ortamlarda elde edilenlere kýsmen benzemektedir. Ortamda bulunan asetat iyonunun, hem yüzeyi kapatma etkisi yaparak, hem de korozif bileþenleri uzaklaþtýrarak (çözünmüþ oksijeni azaltarak) akým yoðunluklarýnýn azalmasýna neden olduðu görülmektedir. Bu davranýþýn nedenin yüzeyde oluþan oksit tabakasýnýn zamanla poröz yapý kazanmasý ve su difüzyonunun artmasý gösterilebilir10,11. Temas suyunda % 3.5 NaCl bulunan ortamlara

tabakanýn 60. güne kadar yavaþ 60. günden sonra ise hýzlý bir þekilde kararlýlýðýný yitirmeye baþladýðý anlaþýlmaktadýr. Temas suyunda %3,5 NaCl, karma sularýnda ise; %3,5 NaCl, 0,1M asetat iyonu ve %3,5 NaCl+0,1M asetat iyonu içeren ortamlarda elde edilen akým-potansiyel eðrilerinden elde edilen anodik (+0,800 V) ve katodik (-1,700 V) akým yoðunluklarý ile, kurulan denge anýndaki potansiyel deðerleri, açýk devre potansiyelinden itibaren ±7 mV potansiyel aralýðýnda elde edilen akým-potansiyel eðrilerinden belirlenen polarizasyon dirençleri ile birlikte Çizelge 2'de verilmektedir. Nyquist diyagramlarýndan bulunan polarizasyon dirençleri; NaCl için 1. günden 28. güne kadar sabit gittiði 28. günden sonra hýzlý bir þekilde azaldýðý, asetatlý ortamlarda ise 1. ve 2. günden itibaren betonarme çeliðinin üzerinde polarizasyon direncinin yüksek olduðu ve 7. günden 60. güne kadar yavaþ bir þekilde 60. günden sonra hýzlý bir þekilde azaldýðý ve NaCl + asetatlý ortamlarda ise, 7. güne kadar polarizasyon direncinin arttýðý, 7. günden 60. güne kadar azaldýðý ve 90. gün sonunda kararlýlýðýný kaybettiði görülmektedir. Akým-potansiyel eðrisinden ise akým yoðunluðu en fazla olan %3,5 NaCl içeren, en az akým yoðunluðuna sahip olanýn ise 0,1 M asetat iyonu içeren ortamlar olduðu anlaþýlmaktadýr.

KOROZYON, 15 (1-2), 2007 25

Þekil 6. Betonarme Çeliði'nin; sadece %3,5 NaCl + 0,1 M Asetat iyonu içeren ortamlardaki Nyquist (a) ve Bode (b-c) diyagramlarý. Figure 6. Nyquist(a) and Bode (b-c) diagrams of Reinforcing Steel in solutions of 3.5 % NaCl + 0.1 M Acetate ion.

Çizelge 2. Karma suyu içeriðine göre farlý bileþimlerde olan betonarme elektrotlardan potansiyokinetik yöntemlerle elde edilen elektrokimyasal nicelikler. Table 2. Electrochemical data obtained through potentiokinetic techniques from concrete specimens of different compositions.

4. SONUÇLAR Betonarme çeliðinin; Cl-, CH3COO- ve Cl- + CH3COO- içeren ortamlardaki korozyonunu konu alan bu araþtýrmadan elde edilen sonuçlar aþaðýdaki þekilde özetlenebilir: 1. Asetat iyonlarýnýn bu koþullarda betonarme çeliðinin yüzeyinde sýnýrlý seviyede bir inhibisyona neden olduðu akým-potansiyel eðrileri ve Nyquist diyagramlarýndan anlaþýlmaktadýr12,13. 2. Asetat iyonu içeren klorürlü ortamlarda, betonarme çeliðinin katodik akým yoðunluðu ve korozyon potansiyeline karþý gelen anodik akým yoðunluðu dikkate alýndýðýnda, korozyon hýzýnda önemli miktarda azalma görülmektedir. Ayrýca, betonarme çeliðinin yüzeyinde meydana gelen polarizasyon direnci 1. günden 90. güne kadar oldukça büyüktür. 3. Klorürlü ortamlara Asetat iyonu ilavesi ile hazýrlanan betonarme elektrodun polarizasyon direncinin, sadece klorür iyonu ile hazýrlanmýþ olan ortamlara oranla daha yüksek olduðu saptanmýþtýr. Akým yoðunluðu-potansiyel eðrilerinden, polarizasyon direncindeki artýþla orantýlý olarak, korozyon potansiyeli dolayýnda korozyon hýzýnda belirgin bir azalmanýn meydana geldiði anlaþýlmaktadýr. TEÞEKKÜR

Yazarlar, bu çalýþmayý destekleyen Çukurova Üniversitesi Araþtýrma Projeleri Birimi'ne teþekkür ederler. (Proje No: FEF2006YL46). KAYNAKÇA 1. 2. 3. 4. 5. 6. 7. 8.

9. 10. 11. 12. 13. 14.

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KOROZYON, 15 (1-2), 2007

S. Üneri, (1980), Korozyon 1, Korozyon Mühendisliði, SEGEM Yayýnlarý, Ankara. M. Erbil, (1980), Korozyon Ýnhibitörleri ve Ýnhibitör Etkinliklerinin Saptanmasý, Ankara Üniversitesi Fen Fakültesi Fizikokimya Kürsüsü, Ankara. H. Kahyaoðlu, (Ekim 1998 ), Doktora Tezi Çukurova Üniversitesi , Fen Bilimleri Enstitüsü, Kimya Anabilim Dalý. 4. Batis ,E.Rakanta, (2005), Corrosion of Steel Reinforcement Due to Atmospheric Pollution Cement and Concrete Research 27, 269-275. A.B. Yýlmaz, Ý. Dehri ve M. Erbil, ( 2003 ), Karma Suyu pH Deðiþiminin Beton BasýnçDayanýmýnave Betonarme Demirinin Korozyon Potansiyeline Etkisi, IV. Elektrokimya Günleri 146-149. T. Kikuchi, K. Aramaki, ( 2000 ), The Inhibition Effects of Anion and Cation Inhibitors on Corrosion of Iron in an Anhydrous Acetonitrile Solution of FeCl3 Corrosion Science 42, 817- 829. 7. O. Kayali . B. Zhu, ( 2005 ), Corrosion Performance of MediumStrength and Silica Fume High Strength Reinforced Concrete in a Chloride Solution Cement & Concrete Composites 27, 117-124. 8. G. Kýlýnççeker, B.Yazýcý, A.B. Yýlmaz, M. Erbil, British Corrosion Journal, (2002), Vol. 37(1), p. 1-8. 9. Jinagyuan Hou, D.D.L. Chung, ( 2000 ), Effect of Admixtures in

Concrete on the Corrosion Resistance of Steel Reinforced Concrete Corrosion Science 42, 1489 -1507. 15. 10. S. Masadeh , Corrosion and Rehabilitation of Steel Reinforced Concrete Structure Exposed to Dead Sea Water 2006. X. International Corrosion Symposium- Adana. 198-206. 16. 11. A.A. Gürten, K. Kayakýrýlmaz, M. Erbil, The Effect of Thiosemicarbazide on Corrosion Resistance of Steel Reinforcement in Concrete 2007. Construction and Building Materials 21. 669-676. 17. 12. G. Qiao, , J. Ou, 2007, Corrosion Monitoring of Reinforcing Steel in Cement Mortar by EIS and ENA. Electrochimica Acta 52. 8008-

8019. 18. 13. G. Kýlýnççeker, B. Yazýcý, M. Erbil, H. Galip, "The effect of phosphate ions (PO3-4) on the corrosion of iron in sulphate solutions," Turk. J. Chem., 1999, 23, 41-50.

YAZARLAR Caner Menekþe, Çukurova Üniversitesi, Fen-Edebiyat Fakültesi, Kimya Böl., Adana. Güray Kýlýnççeker, Çukurova Üniversitesi, Fen-Edebiyat Fakültesi, Kimya Böl., Adana.

KOROZYON, 15 (1-2), 2007 27

EFFECT OF WELDING PARAMETERS ON THE SUSCEPTIBILITY TO HYDROGEN CRACKING IN LINE PIPE STEELS IN SOUR ENVIRONMENTS ABSTRACT This paper is concerned about the hydrogen induced cracking (HIC) behavior of welded steels that are used in petroleum lines under sour petroleum environments. In order to investigate the effect to HIC of welding parameters applied in pipe production, welds were done with different line energies. Two different API X-65 steels were used in welding operations. The specimens taken from welded zones were tested in a set-up described in the NACE standard TM 02 84. The specimens were examined under the microscope and the crack lengths were measured with the aid of a computer program. The results obtained were discussed in view of metallurgical and welding parameters aspects. And, it was concluded that the metallurgical parameters of steels used in pipe production were more important than welding parameters regarding their effect on HIC. It was shown that the composition and microstructure of steels were the major factors that control the susceptibility to HIC. EKÞÝ PETROL ORTAMLARINDA KULLANILAN ÇELÝKLERÝN HÝDROJENLE ÇATLAMA DUYARLILIÐINA KAYNAK PARAMETRELERÝNÝN ETKÝSÝ Bu makalede petrol hatlarýnda kullanýlan kaynaklý çeliklerin ekþi petrol ortamlarýnda rastlanan hidrojen tetikli çatlama (HIC) davranýþlarý tartýþýlmaktadýr. Borularýn üretimi esnasýnda kullanýlan kaynak parametrelerinin hidrojen tetikli çatlamaya etkilerini incelemek amacýyla farklý ýsý girdilerinde kaynaklar yapýldý. Ýki farklý API X-65 çeliði ile üretilen kaynaklý parçalardan alýnan örnekler ilgili NACE standardýnda (TM 02 84) önerilen deney düzenegini kullanarak test edildi. Örnekler metalografik olarak incelendi ve çat-

lak boylarý bir bilgisayar programý yardýmý ile ölçüldü. Elde edilen sonuçlar metalurjik ve kaynak parametreleri açýsýndan tartýþýlarak deðerlendirildi. Bu araþtýrmadan elde edilen sonucu, boru üretiminde kullanýlan çeliklerin metalurjik parametrelerinin hidrojen tetikli çatlama davranýþlarýna etkisinin kaynak parametrelerinden daha önemli olduðu þeklinde genellemek mümkündür. Çeliklerin kimyasal bileþimi ve iç yapýlarýnýn hidrojen tetikli çatlama üzerinde doðrudan etkili olduðu gösterildi.

1. INTRODUCTION Line pipe steels used in sour environments (containing H2S) are prone to hydrogen induced cracking (HIC), also known as stepwise cracking (SWC), depending on metallurgical and environmental factors. The metallurgical factors consist of alloying elements, microstructure, segregation, and the amount and shape of non-metallic inclusions. Some of the environmental factors that can cause HIC are the partial pressures of hydrogen sulfide (H2S) and carbon dioxide (CO2), temperature, pH of the medium, moisture, and the presence of aggressive ions. Many failures of sour gas line pipes have occurred around the world as a result of HIC as documented by a large number of publications 1,2,3,4.Considerable effort has been devoted seeking

* Corrosponding outhor, E-mail: [email protected]

28

KOROZYON, 15 (1-2), 2007

Özgür YAVAÞ* Mustafa DORUK

a through understanding of the mechanism of HIC so that a laboratory test method can be developed to identify and quantify material susceptibility to HIC, which would allow for production of steels with greater resistance to HIC. NACE International has developed a standard test, TM-02-84, to evaluate the performance of the line pipe5. The method describes procedures for evaluating the resistance of welded line pipe steels to stepwise cracking induced by hydrogen absorption aqueous sulfide corrosion. Although the relative H2S susceptibility can be related to the steel microstructure and sour gas environment6,7,8,9, there have been published few data about relation between H2S susceptibility and the production parameters of the line pipes. The aim of this study is to identify the welding parameters of the line pipe that yield the highest resistance to sour environments. NACE International laboratory test method was applied to evaluate the susceptibility to cracking of welding and heat affected zone (HAZ). Based on the generated data, it was tried to identify the welding parameters and the metallurgical variables that are expected to have a pronounced effect on

the resistance of welded structures to HIC in sour environments. 2. EXPERIMENTAL PROCEDURES 2.1. Materials The testing materials were high strength low alloy line pipe steels. The grade of steels are complies with API 5L X-65. Totally three different steel sheets delivered by two different manufacturers were used for the experiments. They are listed in Table 1 including also their thicknesses. In the rest of the paper they will be referred as to 'Steel 1', 'Steel 2' and 'Steel 3'. The chemical composition of the steels is given in Table 2. The carbon equivalent values in the table are calculated from the following formula;

Table 4. Melt off rates of welding wire Çizelge 4. Kaynak çubuklarýnýn ergime hýzý

CE = %C + %Mn/6 + %Ni/15 + %Cr/5 + %Mo/4 + %Cu/13.

weld speed, voltage and current. Line energies were calculated from the following formula by 90 percent efficiency;

Table 1. Materials tested. Çizelge 1. Test edilen malzemeler.

Line Energy (kJ / cm) = 0,9 x 60 x Volt (V) x Current (A) / Weld speed (cm / min)

The mechanical properties of steels can be seen in Table 3 compared to the minimum required values according to API 5L for X-65 grade. Table 2. Chemical analysis of steels Çizelge 2. Çeliklerin kimyasal bileþimleri

Table 3. Mechanical properties of steels Çizelge 3. Çeliklerin dayanç özellikleri

2.2. Preparation of Welding Specimens A portable submerged arc welding machine was used to do the welds with different line energies. Potential difference and current together with welding speed of the machine can be controlled to obtain a high quality weld. Welding wire and flux used in welding procedure were fed manually. The speed of welding wire was automatically adjusted under each level as indicated in Table 4. 2.3. Implementation of Tests Nine different line energies were chosen, to cover a sufficiently wide range of combination of

The same line energy values were applied for three different steel groups. It is noted that the weld speed, voltage, and current values were different for the same line energies because they depend on the wall thickness of sheets. . Lincoln P223 weld flux and 3.2 mm diameter S2MoTiB weld wire were used for all welding procedure. While the line energies were changed for each test specimen, all other parameters (the quality of weld, weld flux, weld wire, weld wire diameter and free wire length) were kept constant. In line of this approach, altogether 27 specimens were prepared for further examinations that are summarized in Tables 5, 6 and 7. Test specimens were finished by using machining. The dimensions of the test piece were 350 x 1500 mm with a longitudinal nick, 7.0 mm deep and 60° flank angle. In all welding operations with different line energies, these nicks with the same geometry and dimensions were filled with weld. A view of these specimens is shown in Figure 1. 2.4. The Hydrogen Cracking Sensitivity Test To simulate the sour environment, the HIC test apparatus according to NACE TM 0284 was used. Figure 2 shows a schematic representation of the

Figure 1. Design of welding specimens (All dimensions in mm). Þekil 1. Kaynaklý örneklerin tasarýmý (Bütün ölçüler mm)

KOROZYON, 15 (1-2), 2007 29

Table 5. Weld parameters for 'Steel 1'. Çizelge 5. 'Çelik 1' için kaynak parametreleri.

Figure 2. Schematic presentation of typical test system5. Þekil 2. Tipik test sisteminin þematik gösterimi5.

Table 6. Weld parameters for 'Steel 2'. Çizelge 6. 'Çelik 2' için kaynak parametreleri.

Table 7. Weld parameters for 'Steel 3'. Çizelge 7. 'Çelik 3' için kaynak parametreleri.

test system. For each test, two coupons were taken from the material to be tested and one coupon was cut containing a weld. The shape of the coupons has been standardized as a rectangle 20 x 100 mm. The top and bottom faces were lightly machined until they are flat. The cut coupons were ground on a wet endless belt, and finish ground on dry 320 grit silicon carbide papers. The specimens were then degreased in acetone and handled with clean gloves. Synthetic sea water was chosen for the test solution which was prepared in accordance with the requirements of ASTM D1141-52. The initial pH of the solution was 8.2. Test solution was deaerated in a closed container by bubbling nitrogen through it at a rate of 100 ml/minute for one hour. The test

30

KOROZYON, 15 (1-2), 2007

specimens were then placed horizontally in the solution with their wide faces vertical and their narrow faces horizontal. The lower face was raised from the cell bottom on bars of glass. The specimens were placed in the solution quickly in order to prevent oxygen pickup. The solution was then saturated by bubbling H2S of 99.5 vol.% purity at the rate of 200 mI/minute for one hour through an open ended tube with a 5.0 mm internal diameter. After one hour the rate of purging decreased to 5.0 ml/minute to maintain a small positive pressure of H2S in the test cell by the use of an outlet trap to prevent oxygen contamination from the air for 96 hours. The pH of the solution at the end of the test was 4.2. At the same time, the H2S concentration in the solution was determined by idiometric titration and the result was 2500 ppm. After purging, the specimen was removed from the solution, washed in running water, wire brushed to remove loose deposits, washed in acetone and dried in petroleum ether and cold air. Test specimens were sectioned transversely at three points. The intention of the sectioning procedures is to examine for cracks on a plane transverse to the rolling direction. 2.5. Other Tests For hardness levels of the welds, three measurements were done according to ASTM A370 standard. The points of measurement are shown in Figure 3. Impact toughness values of the welds were determined at 0°C by using a impact test machine. For each weld, three different tests were performed. For metallographic examination the sections obtained from test specimens were mounted in epoxy resin and polished stepwise. Etching was done by 3.0 % Nital solution. Cracking was examined by eye, and microscopically at magnifications of 100X. For each crack observed, the length of stepwise propagation is determined by using a computer program. 3. RESULTS AND DISCUSSION 3.1. HIC Test Results

Table 10. HIC test results for 'Steel 3'. Çizelge 10. 'Çelik 3' için HIC test sonuçlarý.

Figure 3. Hardness measurement points. Þekil 3. Sertlik ölçümü yapýlan noktalar.

The HIC test results are tabulated in Tables 8, 9 and 10. Although for all three steels the same line energies were applied, only the specimens taken from the Steel 1 were cracked under the majority of test conditions (Table 8). Therefore, the discussion will be focused mainly on results obtained with Steel 1. In order to reveal the possible correlations between the hardness, carbon equivalent of steel and the crack length, a group analysis was applied through arranging the data for parameters 1 to 9 (Table 11). In addition, the variation of carbon equivalent together with the hardness of HAZ as a function of line energies is given in Figure 4. And, the Figure 5 shows the correlation between the crack length and the hardness of HAZ. The information which can be inferred from the present data can be summarized as follows: - The specimens with longer crack length have also higher hardness in HAZ and larger carbon equivalents. This is particularly the case for parameters '1', '5', and '7'. - The variations of crack length and HAZ hardness with line energy included are also in support of this conclusion. Table 8. HIC test results for 'Steel 1'. Çizelge 8. 'Çelik 1' için HIC test sonuçlarý.

Table 9. HIC test results for 'Steel 2'. Çizelge 9. 'Çelik 2' için HIC test sonuçlarý.

- However, it appears to be not possible to define threshold values for hardness and carbon equivalent that would be large enough to induce hydrogen cracking. 3.2. Effect of Metallurgical Variables Steels delivered by two different steel producers show different cracking behavior under wet H2S environment. First group specimens of Steel 2 showed resistance to cracking while the other one (Steel 1) welded with some line energies cracked. The carbon equivalent values calculated from the chemical compositions of steels are given in Table 2. As seen, the cracked steel has higher carbon equivalent compared with other two. The conclusion which can be drawn at this stage is that a strict control of chemical composition of steel is of primary significance as far as the resistance against the HIC is concerned. Evidently, the grain size of steel is another important factor which can directly be correlated with susceptibility to cracking. As indicated in Figure 6, the Steel 2 which resisted the cracking under all conditions tested has finer grains compared to Steel 1. 3.3. Effect of Hardness In order to shed more light on factors that are responsible for HIC, the variation of average hardness as a function of weld parameters 1 t0 9 of all steels for three regions (material, HAZ and weld) are plotted in Figures 7, 8 and 9. For all parameters the hardness values of Steel 1 are higher than that of the other two. At this stage of discussion it would be worthwhile to compare data included in Table 8 and Figures 7 to 9. Even though there is not a one-to-one correlation exist, it can generally be claimed that the susceptibility to HIC increases with increasing hardness. 3.4. Impact Toughness Test Results. Impact toughness test results are tabulated in Table 12. The values included in the table are ave-

KOROZYON, 15 (1-2), 2007 31

rage of three measurements . This data are in acceptable agreement with the HIC test results (compare with data given in Tables 8 and 11). Table 11. HIC test results for 'Steel 1'. Çizelge 11. 'Çelik 1' için HIC test sonuçlarý.

and hardness value is determined and shown in the Figure 4. Carbon equivalent of steel is the major parameter which affects the chemical composition of the weld and HAZ. The high value of carbon equivalent of welding wire can induce HIC. Cooling rates of HAZ after welding is an additional factor which controls the increase in hardness. On the other hand, the cooling rate depends on the line energy, thickness and the temperature of wel-

Steel 1 (100X) grain size = 8

Steel 2 (100X) grain size = 8

Figure 6. Comparison of micro structures of Steel 1 and Steel 2. Þekil 6. Çelik 1 ve Çelik 2' nin mikro yapýlarýnýn karþýlaþtýrýlmasý.

220

Steel 1

200

Steel 2 Steel 3

Figure 4. Variation of hardness and carbon equivalents as a function of welding parameters (Steel 1). Þekil 4. Sertlik ve karbon eþdeðerlerinin kaynak parametrelerine gore deðiþimi (Çelik 1).

180 1

10 19

2

3

4

5

6

7

8

9

11 20

12 21

13 22

14 23

15 24

16 25

17 26

18 27

Figure 7. Variation of hardness of steels as a function of welding parameters Þekil 7. Çelik sertliklerinin.kaynak parametrelerine gore deðiþimi

220

Steel 1

200

Steel 2 Steel 3

180 1 10 19

Figure 5. Variation of hardness and crack length as a function of welding parameters (Steel 1). Þekil 5. Sertlik ve çatlak boyunun kaynak parametrelerine gore deðiþimi (Çelik 1).

3.5. Effect of Welding Parameters There is no direct relation between welding parameters and HIC susceptibility of Steel 1, as indicated by comparison of the line energy values with crack lengths in Table 8. However, the welding parameters that increase the hardness of the HAZ may induce susceptibility to HIC. Beside the carbon equivalents, the cooling rate of HAZ after welding has to be taken into consideration as an additional factor which would control the hardness. The relationship between the carbon equivalent

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KOROZYON, 15 (1-2), 2007

2 11 20

3 12 21

4 13 22

5 14 23

6 15 24

7 16 25

8 17 26

9 18 27

Figure 8. Variation of hardness of HAZ as a function of welding parameters. Þekil 8. HAZ bölgesi sertliklerinin kaynak parametrelerine gore deðiþimi

270

250

Steel 1 Steel 2 230

Steel 3

210 1

2

3

4

5

6

7

8

9

10 19

11 20

12 21

13 22

14 23

15 24

16 25

17 26

18 27

Figure 9. Variation of hardness of weldments as a function of welding parameters. Þekil 9. Kaynak malzemesi sertliklerinin kaynak parametrelerine gore deðiþimi

ded plates. In order to prevent fast cooling, preheating of parts to be welded would be recommended. In addition, the higher line energies may also cause reduction in cooling rate. Like the thick section parts, larger welding area may also be useful to have slow cooling. The welding area can be enlarged by reducing the welding speed and increasing the current.

meters that cause slow cooling rates may be preferred to prevent HIC. REFERENCES 1. M. Elboujdarni, V.S. Sastri, and R.W. Revie "Field Measurement of Hydrogen in Sour Gas Pipelines", corrosion Vol. 50, No.8, August Table 12. Relation of welding parameters and crack length (Steel 1). Çizelge 12. Kaynak parametreleri ile çatlak uzunluðu arasýndaki iliþki (Çelik 1).

Table 12. Impact toughness test results. Çizelge 12. Çentik darbe deneyi sonuçlarý.

4. CONCLUSIONS This study has led to the following conclusions; 1. Metallurgical parameters are more effective than welding parameters in determining HIC susceptibility of steel in wet H2S environments. 2. Carbon equivalent and average grain size values are the predominant metallurgical parameters that affect the HIC resistance. Fine grained steels with low carbon equivalent are more resistant against cracking in wet H2S environments. 3. There is a direct relation between carbon equivalent and HAZ hardness values. However, there is not a threshold hardness value below which the cracking could be eliminated. 4. After welding, slow cooling may produce softer microstructures. Therefore, the welding para-

1994, p.636 - 640 2. T. Hara, H. Asahi and H. Ogawa, "Conditions of Hydrogen Induced Corrosion Occurrence" of X-65 Grade Line Pipe Steels in Sour Environment", Corrosion Vol.60, No 12, December 2004, p.1113 - 1121 3. G.J. Biefer "The Stepwise Cracking of Line Pipe Steels in Sour Environments" Materials Performance, Vol.21, No.6, June 1982, p.19 - 34 4. T.P. Greenveld, R.R. Fissler, "Hydrogen Induced Damage in Sour Gas Gathering Lines" Proc. Nace Western Regron Conf. 1979, Calgay 5. NACE Standard TN-02-84, "Test Method Evaluation of Pipeline Steels for Resistance to Stepwise Cracking" (Houston, TX, Nace, 1984) 6. H.Y. Liou, R.I. Shieh, F.I Wei and S.C. Wang "Roles of Microalloying Elements in HIC Resistant Property of High Strength Low Alley Steels", Corrosion- Vol. 49, No.5, May 1993, p.389 - 398 7. L. Albarran, A. Aguilar, L. Martinez and H.F. Lopez "Corrosion and Cracking Behavior in an API X-80 Steel Exposed to Sour Gas Environments", Corrosion Vol.58, No.9, September 2002, p.783 - 792 8. R.D McCright "Effects of Environmental Species and Metallurgical Structure on the Hydrogen Entry into Steel" pp 306-325 the Ohio State University Corrosion Center, Columbus, Ohio 9. M. Kimura, N. Totsuka, T. Kursu, I. Hane, Y. Nakai "Effect of Environmental Factors on Hydrogen Permeation in Line Pipe Steel" Corrosion 85, pp 237, May 1985

AUTHORS Özgür Yavaþ, NOKSEL Çelik Boru Sanayi A:Þ:, Hendek/Adapazarý, Turkey. Mustafa Doruk, Middle East Technical University, Metallurgical and Materials Eng.Dept., Ankara/Turkey.

KOROZYON, 15 (1-2), 2007 33

MATERIALS CHALLENGES IN REFINING OF HEAVY CRUDE OIL* ABSTRACT It is known that the global energy demand is increasing and this is putting pressure on the oil producing countries to increase their production capacities. The increase in production capacity quite often is achieved through producing large quantities of heavy crude oil. Although heavy oil is usually blended with lighter crude, but the deteriorating quality of the feedstock is pressing the refineries due to increased risk of corrosion, equipment failures and downtime of process units. The most damaging impurities in crude oil are inorganic salts, organic chlorides, organic acids, and sulfur compounds. To make matters worse, many of the compounds are unstable during refining operations and they break into smaller components or combine with other constituents, concentrating corrodants in certain units, such as the breakdown of sulfur compounds and organic chlorides. The paper presents the current situation of heavy crude oil, background of the problems encountered during refining and mitigation methods in addition to a failure investigation of 321 stainless steel tubes used in a charge heater handling heavy crude oil. The investigation shows an un-usual form of sulfidation, which was a result of the combined effects of the nature of the crude and the operating conditions. AÐIR HAM PETROLUN RAFÝNASYONUNDA MALZEME SORUNLARI Bilindiði gibi, küresel enerji gereksinimi hýzla artmakta ve bu olgu petrol ürten ülkelerin üretim kapasitesini giderek artýrma zorunlluluðunu beraberinde getirmektedir. Üretim kapasitesindeki artýþ, çoðu kez, artan miktarlarda ham petrol rafinasyonu ile gerçekleþir.Aðýr ham pet-

rolün daha hafif olanla karýþtýrýlarak iþlenmesine karþýn, yan ürünün halen zarar verici nitelikte olmasý korozyon riskini artýrmaktadýr. Bu techizat hasarlarý yanýnda üretime ara verme zorunluluðunu doðurmaktadýr. Ham petrolün içerdiði zararlý unsurlar inorganic tuzlar, organic klorürler, organic asitler ve kükürt bileþikleridir. Bu bileþiklerden pek coðunun rafinasyon iþlemi koþullarýnda kararlý olmamalarý ve parçalanarak diðer bazýlarý ile bileþik oluþturmalarý sistemin bazý birimlerinde korozyona yol açan unsurlarýn yoðunlaþmasýna neden olur. Bunun baþlýca örnekleri kükürt bileþikleri ve organic klorürlerin parçalanmasýdýr. Bu yazýda, aðýr ham petrol rafinasyonunun günümüzdeki durumu özetlenmekte ve karþýlaþýlan sorunlarla önlenmeleri yönünde alýnabilecek tedbirler hakkýnda bilgi verilmektedir. Ayrýca, aðýr ham petrolün rafinasyon birimlerinde kullanýlan AISI 321 paslanmaz çeliðinden mamul ýsýtýcý borularda gözlemlenen bir hasarýn analizi verilmektedir. Bu araþtýrme söz konusu hasarýn olðan dýþý bir sürfürlenmeden kaynaklandýðýný göstermiþtir. Bu durum ham petrolün özellikleri ile iþletme koþullarýnýn ortak sonucu olarak yorumlanmýþtýr.

1. STATUS OF HEAVY CRUDE OIL The increasing world energy demand has pushed the oil producing countries to start exploiting heavy oil reservoirs, which had been neglected or little used and to increase the oil exploration activities. Currently, some heavyweight producers such as Saudi Arabia, Venezuela and Iran produce large quantities of heavy (American Petroleum Institute API Gravity <

H.M. Shalaby

20) sour crude with high sulfur content. Others such as Nigeria, the United Arab Emirates, Angola and Libya pump a higher quality, light sweet crude, with low sulfur content. Most of the world refineries, are equipped with alloys capable of handling sweet light crude, which is most suitable for refining into petrol, gas oil and heating oil. On the other hand, refining of heavy crude or a blend containing heavy crude is difficult and is associated with operational problems. The problems arise from the increased risk of corrosion, equipment failures, and downtime of process units. These effects are caused primarily by the high sulfur and salt contents of these crudes, including organic chlorides1,2. To make matters worse, many of the compounds are unstable during refining operations and they break into smaller components or combine with other constituents, concentrating corrodants in certain units, such as the breakdown of sulfur compounds and organic chlorides. On the other hand, refineries are increasingly faced with competing requirements of increasing their margins while maintaining high levels of reliability and on stream availability of the-

* Paper presented in the10th International Corrosion Symposium held on 1-4 November 2006 in Adana/Turkey

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KOROZYON, 15 (1-2), 2007

ir plants. With crude costs accounting for around 80% of a refinery's expenditures, cheaper crudes can have a very positive impact on refinery margins if proceed successfully. Traditionally low quality crudes "opportunity crudes or disadvantage crudes" attract a discount in the crude oil market due to the perceived processing problems. If not properly solved, these problems can quickly erode any margin gains obtained from buying a cheaper feedstock. But for those refiners that know how to handle these problems, these crudes can represent a significant improvement to their bottom line. Recent trends in the crude oil supply market indicate that within the next few years processing heavier crudes will be less a matter of choice than of necessity. The global conventional crude oil slate will become markedly different from that of today, as supplies of conventional light, sweet crude decrease, and they are replaced by crudes lower in gravity and higher in acidity, sulfur and other impurities. Heavy crude oil production is predicted to grow by some 2 to 4 million barrels per day (BPD) over the next decade3. These current events are facing the oil industry in general with many decisions and technological challenges not only regarding the methodologies of producing heavy oil, transportation and refining of heavy oil, but also evaluating the value and optimum utilization of this produced oil, including crude oil segregation, up-grading and blending approaches. 2. MAIN PARAMETERS AFFECTING CRUDE CORROSIVITY 2.1. Inorganic Salts Crude oil is a mixture of many different compounds, generally combinations of carbon and hydrogen, all with their own unique physical properties. Crude oil as such is not corrosive. However, it contains corrosive impurities, such as inorganic salts. Inorganic salts are present in the brine produced with the crude oil or picked up as a contaminant. The bulk of the salts are sodium chloride (NaCl), magnesium chloride (MgCl2) and calcium chloride (CaCl2), reflecting the composition of seawater. However, the total salt content by weight can vary from less than 3 pounds per thousand barrels (PTB) of crude oil to 300 PTB or more in heavy crude oil1. When the crude oil is preheated, most of the MgCl2 and CaCl2 begin to hydrolyze at about 120OC and form hydrogen chloride (HCl) vapor. At

370OC, approximately 95% of the MgCl2 and 15% of the CaCl2 have hydrolyzed. The chemical reaction for MgCl2 is: MgCl2 + H2O → 2HCl + MgO

(1)

A similar reaction occurs for the CaCl2. The NaCl, being more temperature stable, does not hydrolyze to any appreciable extent1. The HCl vapor thus formed is not corrosive at temperatures above the water dew point. For this reason, there is no corrosive acid attack in the preheat system where no liquid water is present. However, in the pre-flash and atmospheric columns of crude units, the HCl is carried up the columns with the hydrocarbon where being highly water soluble it dissolves in the condensing water to form hydrochloric acid. This highly corrosive acid can create severe corrosion problems in the top of column, the overhead line, the overhead exchanger and condensers. The source of the condensing water can be the crude oil, stripping steam, or carryover from the desalter1,4. The resulting corrosion reaction with steel is: Fe + 2HCl → FeCl2 + H2

(2)

The presence of H2S keeps the reaction going as follows: FeCl2 + H2S → 2HCl + FeS

(3)

The formation of additional HCl thus perpetuates the cycle. Ammonia is often added to neutralize HCl. Other sources of ammonia include carry-over from desalter wash water and streams imported from hydrotreating units. However, above the water dew point, HCl can react with NH3 to form solid ammonium chloride (NH4Cl). The temperature at which NH4Cl forms is dependent upon the partial pressures of HCl and NH3. NH4Cl is hygroscopic so may absorb moisture even though water is not condensing. Wet NH4Cl is highly corrosive to many materials. NH4Cl deposition can occur in the tops of the columns as well as in overhead and reflux piping and overhead condensers1,4. To minimize the effects of inorganic salts, the refiner often washes the crude oil with water and uses a desalting vessel to remove the added water and most of the inorganic contaminants from the

KOROZYON, 15 (1-2), 2007 35

crude prior to distillation in the crude unit1,5. 2.2. Organic Chlorides Organic chlorides (often called phantom chlorides) constitute a contaminant in crude oil. Organic chlorides are also known as "undesaltable chlorides" or "rogue halogens." The sources of these chlorides vary considerably and are always different for each case. However, they often result from the carry-over of chlorinated solvents, which are used in the oilfields. They can also be picked up by the crude during transportation in contaminated tanks or lines. Organic chlorides are not removed in the desalters, thus their presence does not become evident until it is too late. Organic chlorides can decompose in the heaters, forming HCl, causing erratic pH control and accelerated corrosion in crude unit overhead system as well as downstream units1,2. 2.3. Organic Acids Many crude oils contain organic acids, but seldom do they constitute a serious corrosion problem. However, a few crudes contain sufficient quantities of organic acid, generally naphthenic acids, to cause severe problems in units operating above 230oC. Naphthenic acid corrosion (NAC) often occurs in the same places as high temperature sulfur attack such as heater tube outlets, transfer lines, column flash zones, and pumps1. The term naphthenic acid, as commonly used in the petroleum industry, refers collectively to all of the organic acids present in the crude oil. The name is derived from the early discovery of monobasic carboxylic acids in petroleum. These acids are based on a saturated single-ring structure6. Later, more extensive laboratory studies showed an astonishing variety of organic acids to be present in crude oil. These include fatty acids as low in molecular weight as formic and acetic as well as saturated and unsaturated acids based on single and multiple five and six-membered rings. The general chemical formula of naphthenic acids is R(CH2)nCOOH, where R is one or more cyclopentane ring and n is typically greater than 12. The amounts of the naphthenic acids present in crude oils vary from one crude to another around the world. Since the naphthenic acids are organic acids, variations in molecular weight, boiling point, and ring structure can influence both their fraction characteristics and chemical reactivity7. The total acid content in crude oils is expressed as the total acid number (TAN), which is measured

36

KOROZYON, 15 (1-2), 2007

in units of milligrams of potassium hydroxide required to neutralize a gram of oil. The common values of TAN range from 0.1 to 3.5, but severe situations have been found for some hydrocarbon fractions having TAN values in excess of 108. It is not unusual, however, for crudes from a given oilfield to change acid number over a period of years6. The corrosion reaction processes of naphthenic acids are described typically by; Fe + 2RCOOH = Fe(RCOO)2 + H2 Fe(RCOO)2 + H2S = FeS + 2RCOOH

(4) (5)

The iron naphthenates are soluble in oil and the surface is relatively film free. In the presence of H2S, a sulfide film is formed which can offer some protection depending on the acid concentration7. A major controversy in the prediction and control of NAC is that at low temperatures, certain sulfur compounds may reduce the severity of NAC9. For many years, conventional wisdom held that NAC generally did not occur in crudes with a TAN less than 0.3 or 0.5 mg per gram6,10. However, there has been some discussion that crude with TAN of less than 0.5 could still cause significant corrosion problems depending on the specific acids found in that crude and the sulfur content of the crude11. Since the naphthenic acids are a mixture in any particular crude and vary considerably from crude to crude, two different crudes with the same TAN will not necessarily have the same corrosivity12. In discussing acid content and corrosivity, Craig13 has reported data showing that typically only about 5% of the naphthenic acids present in crude are corrosive. Thus, where NAC is found, corrosion rates are affected by the activity of the particular naphthenic acids present, as well as by their concentration. NAC occurs primarily in crude units and vacuum units. NAC in fluid catalytic cracking units and in units that handle cracked products is negligible since these acids decompose between 400OC and 480OC. Catalysts also decompose the acids in hydrodesulfurizer units. Problems with NAC can occur in the feed preheat sections when the oil starts to vaporize, and in the lower and side stream equipment of naphthenic acid-containing streams11. NAC typically has a localized pattern, particularly at areas of high velocity and, in some cases, where condensation of concentrated acid vapors can occur in crude distillation units14,15. The attack also is described as lacking corrosion products.

Damage is in the form of unexpected high corrosion rates on alloys that would normally be expected to resist sulfidic corrosion (particularly steels with more than 9% Cr). In some cases, even very highly alloyed materials (i.e., 12% Cr, type 316 stainless steel (SS) and type 317 SS, and in some severe cases even 6% Mo SS's) have been found to exhibit sensitivity to corrosion under these conditions16. 2.4. Sulfur Compounds Most crude oils vary greatly in both the amount of sulfur and the type of sulfide species present16. Because sulfur is the primary corrodant, the corrosion that these species cause is referred to as sulfidic corrosion. Sulfidic corrosion usually is a general mass loss or wastage of the exposure surface, with the formation of a sulfide corrosion scale. Sulfidic corrosion is another type of high temperature attack. Its temperature range overlaps NAC, starting at about 260OC and increasing from there17. The total sulfur content usually does not accurately predict the level of the sulfur-caused corrosion. Not all of the sulfur is in a form that is potentially reactive with the metal5. The total sulfur content includes species such as thiophenes, mercaptans, and sulfides, which show very different corrosivity. The proportion of each of these species varies with the boiling point of the fraction: mercaptans are predominant in the light fractions while thiophenes concentrate in the residue. The more important factor may be the capability of the sulfur compounds to form H2S, which usually is a more reactive compound18. H2S can occur naturally in crude oils or can be formed during the refining process by thermal decomposition of sulfur compounds. As shown in Figure 1, H2S evolution behavior (and most likely corrosiveness) does not accurately relate to the sulfur content of the crude18. Despite this limitation, the sulfur content of crude oil or hydrocarbon fractions has been used to indicate the potential for sulfidic corrosion in refinery equipment. Sulfur at a level of 0.2% and above is known to be corrosive to carbon and low alloy steels at temperature 232OC to 455OC. At high temperature conditions, the presence of naphthenic acids was found to increase the severity of sulfidic corrosion8. Presumably, the presence of these organic acids disrupt the sulfide film thereby promoting sulfidic corrosion on alloys that would normally be expected to resist this form of attack (i.e., 12% Cr and higher alloys). When sulfur is the only contaminant, McCo-

Figure 1. H2S released from crudes as a function of temperature 18 Þekil 1. Ham petrolden salgýlanan H2S' in sýcaklýða baðlý olarak deðiþimi18

nomy curves, with other factors, are used to predict the relative corrosivity of crude oils and their various fractions. The current form of these curves is shown in Figure 2a, while Figure 2b shows the relevant correction factors19. These curves reveal some of the basic information needed to understand and potentially mitigate sulfidic corrosion in process applications. Important elements include: 1. The increased severity of corrosion with sulfur concentration and service temperature between 250OC and 400OC, and 2. The benefit of increased Cr content in steels to reduce the corrosion rates. The McConomy curves are useful in estimating the corrosion rate that will be expected based solely on sulfur content. However, for high TAN fractions, laboratory and plant data19 show that corrosion rates of carbon steel and 5% Cr were similar. At high sulfur and low TAN, the calculated rates were higher than the actual rates. Conversely, at low sulfur and high TAN, the calculated rates were much lower than actual ones. Also the calculated rates for 9% Cr and 317 SS were very low compared to experimental data. Thus, corrosion data from McConomy curve should be carefully evaluated based on TAN levels 21.

KOROZYON, 15 (1-2), 2007 37

(a) (a)

(b)

on survey information. There are a series of curves indicating the corrosion rates for various steels (0 through 18% Cr) vs. H2S content and temperatures. Figure 3 shows curves for 5 and 9% Cr steel in naphtha diluents22. 2.5. Temperature In addition to the concentration of corrosive acids, the temperature of the fluid has a great influence on the corrosion rate. There appears to be little doubt that NAC occurs over the temperature range of about 220-400OC 6,12,23,24. Typically, no corrosion damage is found at temperature above 400OC. Most likely, this is because of the decomposition of naphthenic acids and/or the formation of coke on the hot metal surface. At temperatures lower than 220OC, the corrosion rate is not high enough to cause problems. NAC is of most concern in areas of crude distillation units that combine velocity with temperatures. Unlike sulfidic corrosion, which increases in severity with increasing temperature, NAC can vary in severity with temperature, depending on the specific naphthenic acids present in the crude oil5. 2.6. Velocity (a)

(b) (b)

Figure 2. (a) Modified McConomy curve; (b) correction factors.19 Þekil 2. (a) Deðiþtirilmiþ McConomy eðrisi, (b) Düzeltme faktörleri.19

The McConomy curves do not directly take into account the influence of velocity, which can be significant in some cases, or the effect of hydrogen in the process environment. Sulfidic corrosion rates in hydrogen-containing environments generally are higher than in hydrogen-free process environments5. The Couper-Gorman diagrams have been developed to account for hydrogen effects, based

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KOROZYON, 15 (1-2), 2007

Figure 3. Couper-Gorman curves for: (a) 5 %; and (b) 9 % Cr steel in naphtha22. Þekil 3. Couper-Gorman eðrileri (a) % 5 ve (b) % 9 Cr içeren çeliklerin nafta içindeki tavýrlarý22.

Both sulfidic and naphthenic acid corrosion can be accelerated by velocity of the flowing process environment or by local turbulence. The wall shear stress produced by the flowing media contributes an added mechanical means to remove the normally protective sulfide films. The wall shear stress is proportional to velocity but also takes into account the physical properties of the flowing media25. In general, the higher the acid and/or sulfur content or the lower the degree of vaporization, the greater the sensitivity to velocity. However, an added complexity is that changes in vaporization can influence the partitioning of the acid between the liquid and vapor phases. In fact, in some cases, it appears possible to obtain very high corrosion rates even at very low levels of naphthenic acid content (i.e., TAN ≈ 0.3) and low sulfur content when combined with high temperature and high velocity5. Blanco et al.26 presented corrosion data that had been collected from transfer lines handling crude oils at about 360OC. The data indicated that the corrosion rate of plain carbon steel is considerably higher than that of 5Cr-0.5Mo containing steel when the crude oil flows at high velocities. 3. MITIGATION METHODS 3.1. Blending Blending is the most common and preferred method of reducing the severity of high-temperature corrosion in crude oil refining systems. It is accomplished by diluting a high TAN feedstock with a low TAN crude or fraction. This technique reduces the overall TAN value to a level that corresponds to an acceptably low rate of corrosive attack16. Blending could also be carried out with high sulfur crudes to decrease corrosion of the equipment by forming a protective iron sulfide film21. Problems can be encountered, however, when making blending decisions regarding new sources of crude containing significant levels of naphthenic acid or in applying experience obtained from one plant or unit to another. 3.2. Corrosion Inhibitors Injection of corrosion inhibitors may provide adequate and economic protection if it is closely monitored and used for specific fractions that are known to be particularly severe, or that fluctuate with feedstock quality. Experience with inhibitors used to mitigate high temperature crude oil corrosion is limited but growing rapidly. The impact of inhibitors on product economics and on downstream processes needs to be assessed5. Traditional

filming amine inhibitors are ineffective in this application27. This ineffectiveness stems from inadequate thermal stability and the absence of a sulfide scale, which many filming amines require to be effective. Inhibitors, which have been found successful, can be divided into two broad categories, i.e. phosphorus containing and no-phosphorus. The phosphorus-containing formulations are generally more effective than the non-phosphorus, but bring with them the concern about poisoning downstream catalysts12. If properly selected and applied, corrosion inhibitors have been shown to reduce corrosion rates by 80 to 90 % from the non-inhibited rates28. This level of protection may be adequate, depending on the non-inhibited corrosion rates, to ensure operation without corrosion failures over a typical run length of 4 years. However, because of the high cost of the chemicals, many refineries choose to apply the corrosion inhibitors as a temporary solution to a critical situation, for example to extend the life of corroded piping or furnace tubes until the next schedule shutdown of the unit, when more permanent solutions such as replacement or metallurgical upgrades can be implemented. The main limitations of corrosion inhibitors is that, when injected with the crude feed, they cannot protect the column trays in contact with a condensing vapour phase, and also (at low dosage levels) the high velocity areas in the vacuum transfer line, areas which are susceptible to severe naphthenic corrosion. The cost of the inhibitors can be as high as 5 to 10 US$ cents per bbl of treated fluid. This represents an annual cost of about US$ 3 to 6 million for a refinery processing 200,000 BPD if the full crude stream is treated. 3.3. Materials of Construction The selection of materials of construction has a significant impact on the operability, economics, and reliability of refining units. For this reason, materials selection should be a cooperative effort between the materials engineer and plant operations and maintenance personnel. Reliability can often be equated to predictable materials performance under a wide range of exposure conditions. Ideally, a material should provide some type of warning before it fails; materials that fracture spontaneously and without bulging as a result of brittle fracture or stress corrosion cracking (SCC) should be avoided. Uniform corrosion of equipment can be readily detected by various inspection techniques. In contrast, isolated pitting is potentially much

KOROZYON, 15 (1-2), 2007 39

more serious because leakage can occur at highly localized areas that are difficult to detect. The effect of environment on the mechanical properties of a material can also be significant. Certain exposure conditions can convert a normally ductile material into a very brittle material that may fail without warning. A material must not only be suitable for normal process conditions but must also be able to handle transient conditions encountered during start-up, shutdown, emergencies, or extended standby. It is often during these time periods that equipment suffers serious deterioration or that failure occurs29. Austenitic stainless steels have excellent corrosion resistance, but are subject to SCC by chlorides. If sensitized, they are also subject to polythionic acids30,31. As the nickel content is increased above 30%, austenitic alloys become, for all practical purposes, immune to chloride SCC. Nickel also forms the basis for many high-temperature alloys, but nickel alloys can be attacked and embrittled by sulfur-bearing gases at elevated temperatures32. The general consensus is that chromium alloying below around 18 % is insufficient to ensure protection against naphthenic acid corrosion. However, the formation of protective and stable sulfide scales seems to be favored by an increase in the chromium content of an alloy. In cases where TAN values are low and sulfur is the primary corrodant, 5 to 12% Cr steels are usually used. For severe conditions of sulfidic attack created by temperature and/or sulfur content, a minimum of 9% Cr is typically preferred. Higher alloys, such as type 316SS (with nominally 18% Cr and 2% Mo min.) or type 317 SS (18% Cr with 3% Mo min.) are commonly used when TAN is above 0.5 and in the atmospheric column when TAN is above 1.55. 4. FAILURE INVESTIGATION OF CHARGE HEATER TUBES A leak occurred in the charge heater of a refinery plant handing Kuwait's heavy crude oil (Ratawi-Burgan), which has API of 17-24 and total sulfur content of 3.8-4.4. The crude is the charge (tube side) in the radiant and convection sections. The temperature at the inlet is 130OC, while it is 345OC at the outlet. Visual inspection revealed that some of the 321 SS convection tubes at the inlet had black oil/coke deposits. No leak was detected upon pressurizing the heater at 405 psig. However, when one of the tubes was removed and was lightly grounded externally at the black colored

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KOROZYON, 15 (1-2), 2007

area, a circumferential crack was observed visually (Figure 4a). The crack was not associated with bulging of the tube. The presence of this crack raised the possibility that the remaining heater tubes in operation may also have cracks. The provided tube was 13.83 cm in outside di-

(a)

(b)

Figure 4. Photographs showing: (a) circumferential single crack; and (b) deposited black layer on internal surface. Þekil 4. (a) Çevresel tek çatlak ve (b) Ýç yüzey üzerinde oluþmuþ siyah katmaný gösterir fotograflar

ameter. The internal tube surface was covered with a non-uniform black deposited layer (Figure 4b). The minimum thickness of the deposited layer was ≈ 1.86 mm, while the maximum thickness was ≈ 4.61 mm. The maximum deposit thickness was found at the failure location. The minimum tube thickness was ≈ 5.92 mm, while the maximum thickness was ≈ 6.96 mm. Thus, the tube suffered reduction in thickness of ≈ 1.04 mm maximum. The black deposited layer consisted of carbon (coke), iron sulfide, calcite, sodium chloride, hematite and magnetite (see Figure 5). Figure 6 shows optical micrographs taken for a cross-section cut from the tube at the crack location. It can be seen from the figure that only one large size crack existed in the tube with a few short branches. The internal tube surface was irregular in shape and was covered in some areas with a scale layer (Figure 6a). In addition, grain separation and subsurface small grain boundary cracking or precipitations were seen at the internal surface. The external tube surface was free of any signs of corrosion damage. The crack and its few short branches were relatively thick and filled with corrosion product scale. The tips of the branches of the crack were blunted (Figure 6b). The short branches were mostly transgranular in nature. Some grain separation can be seen at the crack walls near the internal surface. The linear marks around the crack near the internal surface appeared like crack branches emanating from the crack. SEM examinations of the polished cross-section cut at the failure location clearly revealed that the grain boundary separation, the linear marks, and the crack and its branches were a result of pre-

(a) 450X

(b) 450X

Figure 5. XRD pattern of black deposited layer found on the internal tube surface. Þekil 5. Boru iç yüzeyinde çökelen siyah katmanýn X-ýþýnlarý kýrýným diyagaramý.

(c) 550X 550X (c)

(a) (a) 200X 200X

(b) (b)200X 200X

Figure 6. Optical micrographs of cross-section of failed tube, showing: (a) nature of the crack at internal surface and linear cracks emanating from the walls of the main crack; and (b) transgranular nature of short crack branches near external surface (10% oxalic acid etch). Þekil 6. Hasarlý borunun dýþ kesitinden alýnan metalografik görüntüler: (a) Ýç yüzeydeki çatlak ve ana çatlak duvarýndan baþlayan parallel çatlaklar. (b) Dýþ yüzeye yakýn, taneler içi kýsa çatlak oluþumlarý (% 10 oksalik asit çözeltisinde daðlanmýþ).

cipitation of a corrosion product compound (see Figure 7). The precipitation appeared similar to those resulting from high temperature reactions, such as oxidation or sulfidation. EDS analyses of the precipitates inside the crack (or inside the branches) in bulk of the material revealed that the precipitates are rich in Fe and S (Figure 8a). When the corrosion products inside the crack close to the internal surface were analyzed, the precipitates were found rich in Ca (Figure 8b). The investigation clearly indicated that cracking of the tube was due to high temperature sulfidation. This result was rather surprising and unexpected as sulfidation usually causes general metal loss and occurs at temperatures > 260OC, which is higher than the operational inlet temperature in the convection section of the furnace, which is 130OC or 266OF. The only possible explanation for the occurrence of high temperature sulfidation is the formation of a hot spot along the length of the tube. This view is supported by the fact that the internal tube surface was found covered with a non-uniform

Figure 7. SEM micrographs of polished cross-section of tube, showing: (a) scale precipitated inside the grain boundaries near internal surface; (b) scale build-up and penetration at the internal tube surface; and (c) scale precipitated inside the crack in mid-section. Þekil 7. Borunun dik kesitinden alýnan SEM görüntüleri: (a) Ýç yüzeye yakýn bölgelerde, tane sýnýrlarýnda çökelen kabuki, (b) Kabuk oluþumu ve boru iç yüzeyine penetrasyonu, (c) Orta kesitte, çatlak içinde çökelen kabuk.

(a) (a)

(b) (b) Figure 8. EDS analyses made for the scale inside the crack in: (a) bulk of material; and (b) near internal surface. Þekil 8. Çatlak içinde çökelen kabuðun EDS analizi: (a) malzeme içinde ve (b) iç yüzey yakýnýnda.

black deposited layer. The maximum deposit thickness was seen near the failure location. In addition, the layer was very dense and hard that a chisel was used to remove sufficient quantity for XRD. This nature of the deposited layer could be attribu-

KOROZYON, 15 (1-2), 2007 41

ted to its composition, which consisted of calcite in addition to coke, iron sulfide, sodium chloride, hematite, and magnetite (see Figure 5). According to ASM Handbook33, the presence of internal scale or deposits causes an increase in tube metal temperature because the scale or deposits have a lower thermal conductivity than the steel tube. Such temperature increase can lead to creep and accelerated oxidation in addition to other failure mechanisms. It appears that in the present case, the increase in metal temperature did not lead to creep, but caused high temperature sulfidation. Thus, it seems that the tube was exposed to temperatures greater than 260OC, but less than 540OC, which is the starting temperature for creep. Refinery furnace tubes are usually fabricated from several grades of steels for elevated temperature applications, mainly ferritic, i.e. carbon steels, C-Mo steels, and Cr-Mo steels. The materials are selected for their stress rupture or creep rupture properties combined with corrosion resistance34. The tubes are usually designed for limited service time of 10 years35. SSs are sometimes used as furnace tubes for highly corrosive feeds. SSs are preferred over high nickel alloys because nickel is prone to form low melting nickel-nickel sulfide eutectic. However, sulfur-bearing gases under reducing conditions greatly accelerate the attack of SSs with high nickel contents. Over the service time, the tubes are subjected to several degradation phenomena that may cause operational problems. Damage to the inside surface of the tube can result from the aggressive action of the feed. The principal corrosive impurity in crude oils is sulfur and sulfur compounds36. In the present case, high temperature sulfide corrosion caused by these species resulted in localized sulfide scale formation and formation of subsurface corrosion products (Figures 6 and 7). Iron sulfide scale precipitated along the grain boundaries and within the grains. The localized nature of the attack led to the un-usual appearance of the crack in the circumferential direction instead of the usual general metal loss, scaling and thinning of the tube. The crack, thus, appeared to have initiated intergranularly at the internal surface and converted to transgranular or stress-rupture cracking near the external surface. Ratawi-Burgan crude contains high salt content. This might explain the presence of calcite as part of the scale. This might also explain the occurrence of the failure in the convection section whe-

42

KOROZYON, 15 (1-2), 2007

re the operating temperature is suitable for the precipitation of calcite. It can be safely assumed that the nature of the crude was the catalytic factor for the occurrence of high temperature sulfidation and its localized nature. 5. CONCLUSIONS The paper presented the current situation of heavy crude oil and background of the problems that might be encountered during refinery operations and the measures to be taken to overcome the problems. The paper also included a failure investigation of 321 SS tubes employed in a charge heater handling heavy crude. The investigation presented an example of an un-usual form of corrosion encountered in heavy crude. This example pointed to the materials challenges that might be encountered during refining of heavy crudes. REFERENCES 1. A. Bagdasarian, J. Feather, B. Hull, R. Stephenson and R. Strong, Crude unit corrosion and corrosion control, CORROSION/96, Paper No. 615, Houston, Texas, NACE International, 1996. 2. M.V. Veazey, Phantom chlorides create real problems for refiners, Materials Performance, Vol. 41, No. 5, p. 16, 2002. 3. S.D. Kapusta, A.C. Ooms, F.G.A. van den Berg, J.W. Buijs, and A.J. Smith, Processing opportunity crudes: a new strategy for crude selection, paper presented at Petrotech 2001, New Delhi, India, Paper R075, 2001. 4. D. Clarida, J. Johnston, M. McConnell and R. Strong, Corrosion and fouling experiences in crude units using low base-strength neutralizers, Materials Performance, Vol. 36, No. 7, p. 46, 1997. 5. R.D. Kane and M.S. Cayard, Understanding critical factors that influence refinery crude corrosiveness, Materials Performance, p. 48, July 1999. 6. R.L. Piehl, Naphthenic acid corrosion in crude distillation units, Materials Performance, Vol. 27, No. 1, p. 37, 1988. 7. E. Slavcheva, B. Shone and A. Turnbull, Review of naphthenic acid corrosion in oil refining, British Corrosion Journal, Vol. 34, No. 2, p. 125, 1999. 8. R.D. Kane and M.S. Cayard, A comprehensive study on naphthenic acid corrosion, CORROSION/2002, Paper No. 02555, Houston, Texas, NACE International, 2002. 9. Heller, Corrosion of refinery equipment by naphthenic acid, NACE 8B-163 Publication Report of NACE Tech. Comm. T8, Materials Protection, September 1963. 10. L. Kaley, et al., Corrosion in the oil refining industry, CORROSION/96, Houston, Texas, NACE International, 1996. 11. M.J. Nugent and J.D. Dobis, Experience with naphthenic acid corrosion in low TAN crudes, CORROSION/98, Paper No. 577, Houston, Texas, NACE International, 1998. 12. M.J. Zetlmeisl, Naphthenic acid corrosion and its control, CORROSION/96, Paper No. 218, Houston, Texas, NACE International, 1996. 13. H.L Craig Jr., Naphthenic acid corrosion in the refinery, CORROSION/95, Paper No. 333, Houston, Texas, NACE International, 1995. 14. W.A. Derungs, Naphthenic acid corrosion - An old enemy of the petroleum industry, Corrosion, Vol. 12, No. 12, p. 41, 1956. 15. J. Gutzeit, Naphthenic acid corrosion in oil refineries, Materials Performance, Vol. 16, No. 10, p. 24, 1977. 16. S. Tebbal and R.D. Kane, Review of critical factors affecting crude corrosivity, CORROSION/96, Paper No. 607, Houston, Texas, NACE International, 1996. 17. American Petroleum Institute, High temperature crude oil corrosivity studies, API 943, September 1974. 18. R.L. Piehl, Correlation of corrosion in a crude distillation unit with

chemistry of crudes, Corrosion, Vol. 16, p. 139, 1960. 19. J. Gutzeit, High temperature sulfidic corrosion of steels, Process Industries Corrosion-Theory and Practice, ed. B.J. Moniz and W.J. Pollock, Houston, TX, NACE, p. 367, 1986. 20. S. Tebbal, R.D. Kane and K. Yamada, Assessment of the corrosivity of crude fractions from varying feedstock, CORROSION/97, Houston, Texas, NACE International, 1997. 21. R.D. Kane and M.S. Cayard, Assess crude oil corrosivity. Hydrocarbon Processing Journal, p. 97, October 1998. 22. A.S. Couper and J.W. Gorman, Computer correlations to estimate high temperature hydrogen sulfide corrosion in refinery streams, Materials Protection, p. 31, January 1971. 23. H. Craig, Conference, CORROSION/96, Paper No. 603, Houston, Texas, NACE International, 1996. 24. E. Babaian-Kibala, H.L. Craig, Jr, G.L. Rusk, K.V. Blachard, T.J. Rose, B.L. Uehlein, R.C. Quinter and M.L. Summer, Materials Performance, Vol. 32, No. 4, p. 50, 1993. 25. R.D. Kane, R.D. et al., Assessment of flow induced corrosivity in petroleum environments, Proceedings Eurocorr 98, EFC, Ultrecht, The Netherlands, September 1998. 26. F. Blanco, B. Hopkinson and I. Ramirez, Proc. ARPEL Meeting, Mexico City, Mexico, August 1957. 27. G.L. Scattergood, R.C. Strong and W.A. Lindley, Naphthenic acid corrosion, an update of control methods, CORROSION/87, Paper No. 197, Houston, Texas, NACE International, 1987. 28. S.D. Kapusta, A. Ooms, A. Smith, F. van den Berg, and W. Fort, Safe processing of acid crudes, CORROSION/2004, Paper No. 04637, Houston, Texas, NACE International, 2004.

29. J. Gutzeit, R.D. Merrick and L.R.Scharfstein, Corrosion in petroleum refining and petrochemical operations, ASM Handbook, Corrosion, Vol. 13, ASM International, 1987. 30. A. J. Brophy, Stress corrosion cracking of austenitic stainless steels in refinery environments, Materials Performance, Vol. 13, No. 5, p. 9, 1974. 31. A.S. Couper and H.F. McConomy, Stress corrosion cracking of austenitic stainless steels in refineries, Proc. API, Vol. 46 (III), p. 321, 1966. 32. J.F. Mason, Jr., The selection of materials for some petroleum refinery applications, Corrosion, Vol. 12, No. 5, p. 199t, 1956. 33. ASM Handbook, Failure analysis and prevention, Vol. 11, ASM International, 1986. 34. API 530, third edition, API, Washington D.C., 1988. 35. R.A. White and E. F. Ehmke, Materials Selection for Refineries and Associated Facilities, NACE, Houston, Texas, p. 131, 1991. 36. J. Gutzeit, High temperature sulfidic corrosion of steels, in Process Industry Corrosion - The Theory and Practice, NACE 1986, Delia Cuellar, Page 43

AUTHOR H.M. Shalaby, Kuwait Institute for Scientific Research Petroleum Research and Studies Center P.O. Box 24885, Safat-13109, Kuwait E-mail: [email protected] Fax: (965) 398-0445

KOROZYON, 15 (1-2), 2007 43

INDUSTRIAL APPLICATIONS OF ANODIC PROTECTION IN THE CZECH REPUBLIC** ABSTRACT Industrial installations of anodic protection have been used successfully in the Czech Republic for more than 25 years. Within this time, the anodic protection has been applied for six different corrosion systems in more than 70 objects. The consequence of anodic corrosion protection, considering the most frequent industrial applications (in concentrated sulphuric acid and sodium hydroxide), is improvement of the protective effect of a passive layer formed in the course of anodic polarization compared to a passive layer formed spontaneously. ÇEK CUMHURÝYETÝ'NDE ANODÝK KORUMANIN SANAYÝDE UYGULAMALARI Anodik korumanýn sanayideki baþarýlý uygulamalarý Çek Cumhuriyeti'nde 25 yýlý aþkýn bir süreden beri devam etmektedir. Bu sure içinde, anodic koruma altý farklý korozyon sistemine ve yirmi beþden fazla tesiste uygulanmýþtýr. Deriþik sülfürik asit ve sodium hidroksit baþta olmak üzere, çeþitli ortamlarda korozyonu önlemek amacý ile uygulanan anodic korumanýn ana katkýsý metal yüzeyinde oluþan pasif filmin koruyucu etkisini güçlendirmektir.

1. INTRODUCTION Anodic protection (AP) is characterized as an intentional passivation of metal by current passage when a protected object is an anode (Figure 1) or as change of a metal potential within the passive range by current passage1-3. Anodic protection constitutes a primary corrosion

measure taken usually in strongly aggressive industrial solutions. Considering the fact that concentrated solutions usually with extreme pH values are concerned, and that the protected surfaces are exposed to enforced or natural environmental convection, in case of AP, changes of the electrolyte composition in the surface layer (due to hydrolysis and migration) cannot be counted on, though the current densities related to the area of the polarized surface are comparable for both anodic and cathodic protection. The use of anodic protection is related to passivable metals having a sufficient width of a stable passive range with low corrosion rate (usually 0.1 mm.a-1) in case of which non-uniform forms of corrosion do not occur at the protection potential. The corrosion environment is required to have a sufficient specific conductivity ( 1 S m-1), minimum environmental erosion incidence and stable chemical composition (the environment must not be decomposed at the protection potential). The time to activation, after the polarisation current is switched off, must be also sufficiently long (min. 15 to 30 minutes).

P. Novák* M.Mokrá

2. CZECH INDUSTRIAL APPLICATIONS OF ANODIC PROTECTION Industrial installations of the anodic corrosion protection have been in operation in the Czech Republic for 25 years already. Since 1979, the anodic protection has been applied for tens of appliances, at the moment there are 52 installations in operation. A three - electrode potentiostatic system is used to all anodically protected equipments. The AP system electric device consists of an automatically regulated main supply of direct current with, in most cases, by a backup supply, a gauge and power cabling, a gauge and controlling panel and a light and tone signalisation of the AP system status. For new installations, the main and the backup AP system regulation circuits are identical and interchangeable by mere setting-up protection parameters, and enables computerised collection of data and an overall computer control over the AP system operation. The installed output per one AP unit does not exceed 4.5 kW. The energetic needs of the AP system are negligible, the operational requirements for current are below 10 amps, when the

* Corrosponding author ** Paper presented in the10th International Corrosion Symposium held on 1-4 November 2006 in Adana/Turkey

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2.1. Stainless Steel in Concentrated Sulphuric Acid The passivity of an austenitic stainless steels CrNi18/10 (Mo,Ti) in hot concentrated sulphuric acid (93 - 98 %) enables using AP of cooling loops in sulphuric acid plants. This is the most frequent industrial application of AP worldwide. Currently, there are more than 50 objects anodically protected in the Czech Republic, out of which the sulphuric acid tube coolers are the most frequent2,3 ( Table 1). The main reason of using AP in this case is heavy erosion corrosion, which was detected in equipment made of stainless steel if left without anodic protection. The stability of the stainless steel passive state in 93 - 98 % acid strongly depends on weak contamination of the acid by iron ions. Effective anodic protection of a CrNi 18/10Ti steels in flowing hot (1000C) 98.5 % sulphuric acid is possible only in the passive range from +0.85 V (SHE) within optimum protection potential approx. +1.0 V (SHE). To achieve effective anodic protection the mini-

Table 1. Anodically protected coolers of sulphuric acid in Czech industry (July 2006). Çizelge 1. Çek sanayisinde anodic korumalý sülfürik asit soðutucularý (Temmuz/2006).

only) (Figure 3) merely in pure acid, while in acid with increasing iron content the shape is of a type II (two free corrosion potentials), exceeding 7 ppm Fe type III (free corrosion potential in passivity only). Although the installation of anodically protected coolers in sulphuric acid with the concentration of 93 - 99 % is known worldwide, the installations of 75-78 % sulphuric acid AP coolers in plants pro10

1 -2

supply voltage is lower than 10 V. The price of the regulation device does not depend on the size of the protected object. Concerning accuracy and reliability, the greatest demands are posed upon the reference electrodes that must prove to be stable enough under operation, and to have a minimum service life of one year. The cathodes' service life is given by their corrosion resistance at cathodic polarisation in the environment; however, the minimum required is a period of one year. The number of reference electrodes and cathodes depends on the size, design and geometry of the protected equipment.

|j| (Am )

Figure1. Prevailing explanation of anodic protection principle. Þekil 1. Anodik koruma sisteminin günümüzde geçerli sayýlan açýklama-

mum concentration of iron ions in flowing sulphuric acid at 100oC must be 7 ppm (solubility of ferric sulphate). The stability of the passive sulphate layer during acid flow is conditioned by origin of trivalent iron compounds whose solubility in concentrated acid is about two orders lower than that of bivalent iron compounds. Spontaneous passivation results in the formation of Fe2+ sulphate passive layer while anodic polarization above +0.85 V (SHE) leads to the formation of Fe3+ sulphates. This is the reason why cooling systems with anodic protection do not need any significant limitations of acid flow rate and why erosion corrosion does not occur in them, in contrast to systems without anodic protection An example of experimentally potentiodynamic curves of austenitic stainless steel in hot concentrated acid is depicted at Figure 2. The curve has a shape of type I (free corrosion potential in activity

0,1

acid with more than 7 ppm Fe

pure acid (dashed)

0,01 -0,5

0

0,5

1

1,5

2

E (V/SHE)

Figure 2. Potentiodynamic curves for stainless steel (CrNi-18/10) in 98.5 % H2SO4 at 1000C (1 mV/s, 1000 rpm). Þekil 2. Bir paslanmaz çeliðin (CrNi-18/10) % 98.5 deriþimli ve 1000C sýcaklýktaki sülfürik asit ortamýnda elde edilen potansiyodinamik polarizasyon eðrileri (Tarama hýzý= 1.0 mV/s,1000 Dev/dak).

KOROZYON, 15 (1-2), 2007 45

cessing calcining gases from ferrosins production are rather unusual due to the threshold concentration and temperature. The potentiodynamic curve of stainless steel 'CrNi-18/10' in 78 % sulphuric acid at 900C has a shape of type I (free corrosion potential in activity only) (Figure 3) and stainless steel 'CrNiMo-18/10' has a shape of type II (two free corrosion potentials). Considering sulphuric acid coolers, cathodes made of stainless steel 'CrNiMo-18/10+Ti' are used and placed, in most cases, across the tube bank. Mercurosulphate electrodes (MSE) filled with concentrated acid are used as reference electrodes. The protection potential is adjusted in range +0.3 up to +0.4 V (MSE), and the protection current is up to 5 amps. Stainless steel 'CrNi-18/10 + Ti' was also used for the anodically protected vessel (60m3) for storing pure sulphuric acid (94 to 99 %) at temperatures up to 400C. 2.2. Carbon Steel in Concentrated Sulphuric Acid Similar to the stainless steel coolers in hot sulphuric acid (94 - 98 %), the AP of carbon steel storage tanks of concentrated sulphuric acid (92 - 97 % up to 500C (Table 2) is a case of transition of spontaneously formed salt layer (Fe2+) to significantly less soluble layer containing Fe3+. The potentiodynamic curve has a shape of type III (Figure 3) (free corrosion potential in passivity only). The protection potential has been set up to the value of +0.65 V(MSE), protection currents considerably depend on the volume of the vessel's content, and range within tenths and units of amps. Reference electrodes and cathode material are identical as in case of acid coolers. In addition to corrosion protection, the AP of sulphuric acid storage tanks made of carbon steel ensures high purity of the stored acid. The anodic protection keeps the iron content in the stored acid on the level of units ppm, while in case of without AP there are tens of ppm.

Table 2. Anodically protected sulphuric acid tanks made from carbon steel in Czech industry (July 2006). Çizelge 2. Çek sanayisinde karbon çeliðinden mamul anodic korumalý sülfürik asit tanklarý (Temmuz/2006). Medium 92 – 97 % H2SO4

Temp.[oC] max. 50

Material CSN 11373

Equipment storage tank

max. 40 dispatch tank

46

KOROZYON, 15 (1-2), 2007

m3 Pcs. 600 4 1500 1 80 1 180 2 100 2 90 1

year 1997 2000 2000 1993 2002 1997

2.3. Carbon Steel in Concentrated Sodium Hydroxide In case of anodically protected carbon steel storage tanks of sodium hydroxide, change of potential in the passive area leads to values at which corrosion cracking is eliminated. The potentiodynamic curve of carbon steel in concentrated sodium hydroxide has a shape of type III (Figure 3) (free corrosion potential in passivity only). There are anodically protected three storage tanks in Czech industry (carbon steel CSN11416, 80 m3) of 43 % sodium hydroxide (max. temp. 400C). Electrode system consists of Hg/HgO reference electrodes and a cathode in a form of a nickel wire. The protection potential is adjusted on the value -0.45 V (Hg/HgO), the protection current depend, to a great extent, on the hydroxide temperature and range within 3 to 15 amps. 2.4. Carbon Steel in Liquid UAM Fertilizer Application of AP to ammonium nitrate storage vessels were the most ample group of AP installations in 1985-89 when various farms operated 22 anodically protected liquid fertiliser storage vessels (600m3) made of carbon steel. The fertiliser consists of 42 % of ammonium nitrate, 32 % of urea and 26 % of water. The cathodes were made of stainless steel. These installations are not operated anymore due to the limited use of the UAM fertiliser, due to other method of carbon steel corrosion protection (for a passivation is sufficient content of 0.2 % NH3 to have pH value of the fertilizer higher than 7.5) and low expertise of the farms' staff. 2.5. Stainless Steel in Chemical (Electroless) Nickel Plating Bath The vessels for chemical nickel deposition are another group of anodically protected industrial installations in the Czech Republic. Instead of corrosion resistance, the AP prevents in this case undesirable deposition of nickel on the stainless steel tank walls. The AP system used for chemical nickel bathes is based on an automatically controlled supply of direct current that is connected with a nickel reference electrode and stainless steel cathodes. The protection potential is +0.95 V (Ni), the polarisation current depends on the tank's size and the operation conditions, and ranges between 0.5 and 2.5 A. 3. DISCUSSION In the most widespread technical applications, the anodic protection thus does not lead to passi-

vation of the protected surface, but merely to the change of potential in passivity to values at which a passive layer shows better corrosion resistance than the one formed spontaneously. Anodic protection is applied particularly to systems with qualitative course of E-j curves of the types II, or III (Fi-

Type I

gure 3). It improves safeness of the anodic protection systems, as the imminent danger of activation is not threatening, as it is in case of type I applications, and thus the problems with passivation by current from external source are eliminated.

Type II Figure 5. Three types oftypes potential - current (j)(E) cumulative curves Figure 3. Three of(E) potential – current (Σ -dashed) for passivable metal in electrolyte with increasing oxidi(j) cumulative curves (!- -dashed) for which passivable zing power (A - anodic and C cathodic curve) represent systems for anodic protection. metal suitable in electrolyte with increasing oxidizing

power (A -oksitleme anodickapasitesine and C - cathodic curve) which Þekil 5. Artan sahip ortamlarda pasifleþebilir metallerin anodik korunmasýna uygun üç ayrý represent systems suitable forsistemleri anodictanýmlayan protection. potansiyel(E)-akým (j) diyagramlarý (A-anodik ve C-katodik eðriler. Toplam eðriler çizgiile gösterilmiþ). !ekil 3 noktalý Artan oksitleme kapasitesine sahip

.

ortamlarda pasifle!ebilir metallerin anodik korunmasına uygun sistemleri tanımlayan üç ayrı potansiyel(E)-akım (j) diyagramları (Aanodik ve C-katodik e" riler. Toplam e" riler noktalı çizgiile gösterilmi!).

Type III 4. CONCLUSIONS The consequence of anodic corrosion protection, considering the most frequent industrial applications (in concentrated sulphuric acid and sodium hydroxide), is improvement of the protective effect of a passive layer formed in the course of anodic polarization compared to a passive layer formed spontaneously. It improves safeness of the anodic protection systems, as the imminent danger of activation is not threatening, and thus the problems with passivation by current from external source are eliminated. Reliable operation of an anodically protected equipment is conditioned by professional skills and expertise of a plant's operating and maintenance staff that should follow, when regularly checking the anodic protection

system, the operational instructions for each installation. No substantial failure has been reported on at the anodically protected equipments. Installations of anodic protection have been used for more than 25 years in the Czech industry successfully. REFERENCES 1. O.L.Jr.Riggs, C.E.Locke, Anodic protection, Theory and practice in prevention of corrosion, 284 pp., Plenum Press New York and London, 1981 (in English). 2. V.S.Kuzub, Anodic protection of metals against corrosion, 182 pp., Khimiya, Moscow, 1983 (in Russian). 3. P.Novák, Anodic protection against corrosion, 239 pp., SNTL Prague, 1987 (in Czech).

AUTHORS P.Novák, Department of Metals and Corrosion Engineering, ICT Prague, Technická 5, CZ-166 28 Prague, Czech Republic M.Mokrá , BG SYS HT, Ltd., Holubova 389, CZ-530 03 Pardubice, Czech Republic

KOROZYON, 15 (1-2), 2007 47

ASELSAN’DA KOROZYON EĞİTİMİ KURSU (12-14 Kasım 2007, Ankara)

Aselsan tesislerinde yapılan bu etkinlikte, • Korozyonun temel ilkeleri • Korozyonun önlenmesinde temel yaklaşımlar • Atmosferik korozyon • Deniz ortamında korozyon • Yüzey kaplamaları ile korozyonu önleme • Korozyon türleri • Korozyona dirençli metal ve alaşımlar konuları Prof. Dr. Semra Bilgiç, Prof. Dr. A. Fuat Çakır, Prof. Dr. Mehmet Erbil ve Prof. Dr. Mustafa Doruk tarafından verilmiş olup bu eğitime Ar-Ge Bölümünden 36 makine mühendisi katılmıştır.