Pitting case histories

Figure 6.10 A perforated carbon steel pipe at a weld-backing ring. The gaping pit was caused by sulfate-reducing bacteria (see Case History 6.1).

After only 4 months of service, the main condenser at a large fossil utility began to perforate. Initial perforations were due to erosion-corrosion (see Case History 11.5). Small clumps of seed hairs entering the condenser after being blown into the cooling tower were caught on surfaces. The entrapped seed hairs acted as sieves, filtering out small silt and sand particles to form lumps of deposit (Fig. 6.24A and B). Immediately downstream from each deposit mound, an erosion-corrosion pit was found. 

Localized deterioration Corrosion (especially pitting and intergranular attack), erosion, cavitation, mechanical wear, and so on (see Case History 9.8). 

Steel phases have an influence on the rate of corrosion. Ferrite has a weak resistance to pitting. The presence of martensite can increase the hydrogen fragilization of steel. Intermetallic phases as Fe2Mo in high Ni content alloys can influence the corrosion resistance. The precipitate CuA12 in aluminum alloys the series 2000 is more noble than the matrix, with corrosion around the precipitate. The majority of case histories reported in the literature have involved austenitic stainless steels, aluminum alloys, and to a lesser degree, some ferritic stainless steels and nickel-based alloys.31... 

Obtain a clear, factual definition of the symptoms. A poor definition of symptoms is one of the most common troubleshooting pit-falls. In the debutanizer case history above, the following definitions were used by different people to describe the symptoms of a reboiler tube leak problem ... 

Nevertheless, some case histories have been reported, especially for piping systems and heat exchangers. In the latter, biofilms are known to cause heat transfer problems. Remarkably, corrosion rates increase, when cells inside the biofilms die, possibly due to the production of ammonia and carbon dioxide, which may result in pitting and stress corrosion cracking of copper and its alloys. 

There are large numbers of reported case histories of MIC on stainless steel in water and aqueous waste systems. They are related to different industrial applications such as freshwater storage and circulation systems in nuclear power plants [103, 113,116,142] and cooling water systems in chemical process industries [117,118]. There are basically three cases (a) crevice corrosion under unexpected deposits, (b) sensitivity of pitting and crevice corrosion to trace of H2S, and (c) crevice corrosion in natural seawater. Most of these reports are not well documented concerning the microorganisms involved in the process. However, some general features are... 

Case histories of MIC on aluminum and aluminum alloys have been mainly reported for aircraft fuel storage tanks and heat exchanger tubes using water with different salinities. In all cases, pitting corrosion occurred. 

Case histories of MIC on copper and its alloys have been reported for piping systems and heat exchangers. Whereas heat transfer problems are observed with growing biofilms, the corrosion increases after the death of the microorganisms within the biofilm. This is believed to be due to the production of ammonia and carbon dioxide upon the death of cells that may result in pitting corrosion and/or stress corrosion cracking for copper and its alloys [72,92]. 

Schmidtt CR (1986) Anomalous microbiological tuberculation and aluminum pitting corrosion — case histories. In Dexter SC (ed) Biologically induced corrosion proceedings of the international conference on biologically induced corrosion. National Association of Corrosion Engineers, Houston... 

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