Susceptibility of Stainless Steel Weldments to Microbiologically Influenced Corrosion
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SUSCEPTIBILITY OF STAINLESS STEEL WELDMENTS TO MICROBIOLOGICALLY INFLUENCED CORROSION
SUSAN WATKINS BORENSTEIN Pacific Gas and Electric Co., PO Box 77000 F1634, San Francisco, CA 94106
ABSTRACT Microbiologically influenced corrosion (MIC) is the term used for the phenomenon where corrosion is initiated or accelerated by microorganisms. Biofilms of bacteria form on metal surfaces when exposed to natural waters. The activity of these biofilms and how they attach themselves to metal surfaces directly influence corrosion mechanisms. This paper describes the mechanisms for MIC and the factors which influence the susceptibility of austenitic stainless steel weidments to MIC. The metallurgical, microbiological and electrochemical factors that influence MIC are discussed. Case histories of MIC-related failures and field test results of austenitic stainless steel weldments in various welded conditions are presented.
INTRODUCTION For the last several years there has been renewed interest in MIC, although corrosion initiated or accelerated by microorganisms has been recognized since the 30's. 1 For example, power generation facilities have had many problems with MIC failures requiring costly repair 2-6 Analyses of costly pitting failures at nuclear power plants, chemical process plants, and pulp and paper mills have indicated that stainless steels are susceptible to MIC. 7 tn particular, attack in the form of pitting at or adjacent to weldments in austenitic stainless steels often occurs. Although pitting is very localized, pitting failures can be devastating and lead to expensive repairs. Many MIC failures in industry result in pitting or general corrosion to metals. In many cases the engineers who perform the failure analyses do not consider MIC until it is too late to obtain scrapings, water samples and corrosion deposit samples for analysis. The following paper summarizes how biofilms are formed, mechanisms for MIC, how corrosion is associated with weldments, and case studies of MIC. In addition to discussing case histories, this paper reviews how welding, heat treatment and other metallurgical and process variables relate to corrosion resistance.
MECHANISMS FOR MIC One common example of MIC failures results when untreated or poorly treated water remains in stainless steel components or piping systems after hydrostatic testing. Hydrostatic testing, also known as hydrotesting, consists of filling the system with water and checking for leaks and structural integrity under pressure. Although stagnant water conditions, per se, are not likely to produce direct corrosive attack because of the inherent corrosion resistance of the materials, they are ideal for MIC. It is very important to remember that natural water and even potable water contains bacteria. The concept of "good water" is often treated very lightly. Costly failures have resulted from the failure to keep the water properly treated. In general, water will pick up significant amounts of contaminants and organisms if stored more than three days in equipment or sto
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