Microbially influenced corrosion on stainless steel in waste water treatment plants: Part 2

Abstract

Microbially influenced corrosion (MIC)on stainless steel in freshwater is commonly related to the development of an active biofilm on the surface. A known effect of the microbial activity that causes the formation of a biofilm on stainless steel surfaces is the ennoblement of the stainless steel, as shown in the field test presented in part 1 of this study. Ennoblement may increase the risk of local corrosion if the potential rises above the breakdown potential of the passive layer for the steel grade. An aim of this investigation was to examine the reasons for corrosion when ennoblement occurs on stainless steels and to provide guidance on the use of particular grades of stainless steel in the final stages of waste water treatment systems operating under aerobic conditions. In this paper (part 2), the corrosion resistance of stainless steels in distilled waters containing various amounts of chlorides, nitrates, and sulphates has been evaluated according to a factorial design. A risk window was then designed for predicting MIC on stainless steel in aerobic waste water treatment plants based on these results. Another aim of the investigation, carried out in a waste water treatment plant, was to test coupons with residual weld oxides on the surface and to compare the corrosion resistance with pickled welded coupons. All coupons were exposed in the final stage of the plant where no ennoblement of the open circuit potential occurred in the beginning. Nevertheless, after several weeks, high potentials were recorded following heavy rainfall. After field test, visual inspection revealed corrosion on AISI 304 stainless steel.