effects of sulfides. Hardened steel, or welds not stress relieved in medium 

 carbon steel, may crack due to stress corrosion. Hydrogen sulfide presence 

 can lead to corrosion of the vapor side of copper alloy heat exchangers. 

 Small amounts of ammonia may also be present in polluted seawater, causing 

 aggressive attack and stress corrosion cracking of copper- zinc alloys. The 

 copper-nickel alloys are preferred when ammonia pollution is expected and 

 the 90-10 copper nickel alloy (UNS No. C70600) has demonstrated satisfactory 

 performance in many applications where sulfide pollution has been present. 



f . Effects of Marine Organisms . Biofouling and biological fouling are 

 common terms that refer to the settlement and growth of living organisms on 

 materials exposed to the marine environment. Some metals, such as titanium 

 and the nickel-chromium-high molybdenum alloys, are completely corrosion 

 resistant under fouling. Copper base alloys exhibit varying degrees of 

 resistance to biofouling. Other materials such as aluminum, carbon steel, 

 and stainless steel both foul and suffer increased corrosion due to bio- 

 fouling. On structures such as wharves and breakwaters, biofouling may not 

 be of as much importance. However, biofouling causes increased wave action 

 loadings on such structures. Increased flow blockage and decreased heat 

 transfer efficiency are other problems encountered as result of marine 

 biofouling on metal structures in marine service. 



Biofouling resistance is highest for copper and the 90-10 copper nickel 

 alloy. Brass and bronze have good resistance but 70-30 copper nickel alloy, 

 aluminum bronze, zinc (galvanizing), and Monel alloy 400 have only fair 

 biofouling resistance. The high resistance to biofouling of many of the 

 copper-base materials have been attributed by some researchers to the in- 

 hospitable nature of the green cupric hydroxychloride corrosion product that 

 forms on these materials. This film is itself loosely attached so that any 

 marine organisms that do attach to this film are soon removed. Monel, 

 carbon steel, aluminum, and stainless steel exhibit poor corrosion resistance 

 under biofouling. Carbon steel suffers general corrosion, whereas Monel, 

 aluminum, and stainless steel exhibit pitting and crevice corrosion. 

 Crevice corrosion is caused by differential oxygen cells produced when 

 oxygen is prevented from reaching the metal surface under barnacles. 



Corrosion rates on carbon steel may be reduced a little when biofouling 

 is present due to reduced velocity of water at the metal surface; however, 

 corrosion rates remain relatively high. Biofouling will periodically slough 

 off when the corrosion product breaks off. The high general corrosion rate 

 of carbon steel in seawater is attributed to marine organisms known as 

 anaerobic bacteria. Principal groups are the sulphate-reducing and the 

 iron-consuming bacteria. The sulphate-reducing bacteria require oxygen, 

 which is derived from the reduction of compounds such as sulphates, sulphites , 

 thiosulphates, or organic substances rather than dissolved oxygen. These 

 bacteria liberate hydrogen sulphite which attacks iron severely, removing 

 hydrogen from the cathodic areas of the steel with the formation of iron 

 sulphide. The iron-consuming bacteria do not actually consume iron as food 

 but do require iron in solution for growth. 



Biofouling has been controlled by using copper base alloys, antifouling 

 coatings, mechanical cleaning of the surface, or environmental controls. 

 Environmental control measures include increased flow velocity, elevated 



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