Unexpected corrosion of stainless steel in low chloride waters – microbial aspects

Abstract Stainless steels EN 1.4301 and 1.4401/1.4404 are normally considered corrosion resistant in low chloride natural waters like drinking water. However, a number of corrosion failures have been observed in e.g. fire extinguisher systems and drinking water installations, where stagnant conditions or periods of low water consumption have occurred prior to the failure. Typically the corrosion attacks appear within 2-3 years in weld nuggets, heat affected zones or in crevices like e.g. press fitting pipe connections. The failure mode is pitting and crevice corrosion leading to leaks and rust stains on the outside of the installation. Corrosion may occur in water qualities with rather low chloride contents and fairly low conductivity, which would usually not be considered especially corrosive towards stainless steel. One key parameter is the ennoblement documented on stainless steel in drinking water qualities, due to the formation of a biofilm. In itself, this is not enough to initiate pitting in these water qualities, but combined with a geometrically or metallurgically vulnerable area, corrosion may accelerate. The mechanism is linked to the naturally occurring microbial activity, where the localisation and growth of specific bacteria depend on the environment. Inside a crevice the oxygen content will decrease and anaerobic, stagnant conditions will form leading to growth of e.g. sulphate-reducing bacteria, whereas the heat tint on a heat affected zone with its high content of iron facilitates the growth of iron oxidising bacteria. A number of failure cases from Danish and Finnish stainless steel installations are discussed with the objective to identify key parameters, suggest possible mechanisms and discuss whether prediction is possible. The paper includes a short literature review, practical experience with corrosion in connections in stainless steel installations - either welded connections or press fittings - and suggested mechanisms for the microbiologically influenced corrosion of stainless steel in low chloride water. This cooperation was facilitated by COST D33 “Nanoscale electrochemical and bio-processes at solid-aqueous interfaces of industrial materials”.