Influence of welding variables on microbiologically influenced corrosion of austenitic stainless steel weldments

Austenitic stainless steels have good corrosion resistance and are widely used for severe corrosion environments. But under certain conditions they undergo localized corrosion such as pitting corrosion. Microbiologically influenced corrosion (MIC) is the term used for the phenomenon where corrosion is initiated or accelerated by microorganisms. In the past few years, analyses of costly pitting failures have indicated that austenitic stainless steels are susceptible to MIC. In particular, for reasons not completely understood, corrosive attack in austenitic stainless steel usually takes the form of pitting at, or adjacent to, welds. It may be possible to avoid costly pitting failures if the weldment could be made less susceptible to MIC by proper selection of filler metal, welding process varibles, or heat treatment. The objective of this research was to determine how MIC responds to welding and heat treatment variables for two types of austenitic stainless steel. A series of field and laboratory experiments were conducted to investigate the influence of welding variables on MIC of austenitic stainless steel weldments. Pipe specimens of both Type 304L and 316L austenitic stainless steel were gas tungsten arc welded (GTAW) for field testing. Filler metals were selected to produce three percentages of ferrite content (low, medium, and high). Weldments of each ferrite content were evaluated in both as-deposited and solution-annealed conditions. Field testing involved direct exposure of the welded specimens for appoximately one year to a small trickle of water from a well known to produce MIC. Mounds indicating biomass formation appeared on the welds after two weeks' exposure. Laboratory studies involved measurements of corrosion potentials as a function of time and pitting potentials in microbial solutions. Field studies indicated no MIC pitting of solution annealed weldments. Approximately equal numbers of low, medium, and high ferrite content welds in the as-welded condition had MIC pitting corrosion. Laboratory studies indicated solution annealed weldments are less susceptible to pitting corrosion. Laboratory studies for the three as-deposited weld metal samples (corresponding to low, medium, and high ferrite numbers) in the sterile control and the bacterial solutions always showed pitting development. The most likely reason for this reduced susceptibility in solution annealed samples is less segregation or microsegregation of elements important to corrosion resistance.