Microbiologically influenced corrosion of stainless steel weldments

Stainless steels (SS) of the basic 18 Cr -8 Ni compositions are commonly used in nuclear power plants especially in high purity environments. There are several reports of MIC attack on SS when untreated or potable water is used for hydrotesting. Investigations' on the behaviour of SS components in seawater-cooled plants by earlier workers have demonstrated that biofilm formed on SS can increase the corrosion potential considerably leading to pitting of more anodic areas like welds and heat effected zones. In the present study, an attempt was made to understand the role of microorganisms in the corrosion of SS weldments in a fresh water reservoir. This reservoir water is used in the service water system of the Fast Breeder Test Reactor at Kalpakkam. The monitoring of open circuit potential (OCP) of AISI Type 304 SS with time in the raw reservoir water showed a steady positive shift of OCP to about 200 mV in 30 days. This positive shift of OCP was not observed in the sterile water. Rise in OCP was also observed on specimens with pure algal biofilms, but not with bacterial biofilms in the dark conditions and in the pure culture bacterial biofilms. Biofilm characterisation studies in the fresh water reservoir showed that photosynthetic algal-dominated biofilms is developing in this environment. To understand the effect of this algal-influenced ennoblement on the SS-welds, AISI Type 304 SS frames with welded regions were also exposed in the reservoir. Within one and a half year, weld started corroding and after five years heavily corroded SS frames were withdrawn and total microbiological and metallurgical analysis were done. Results are discussed in the light of available literature.