Microbial Influenced Corrosion of passive alloys in natural or industrial waters and test methodology.

International audienceAerobic bacteria settled on the surface of passive alloys like stainless steels, nickel alloys , titanium alloys, freely exposed to natural waters (seawater, river water,) act in synergistic way as promoters of crevice corrosion onset, being the cathodic reaction(s) accelerated by the aerobic biofilm. Sulphate reducing bacteria (SRBs), active under shielded areas, in mature biofilm or under anaerobic conditions, decrease the passivation of these passive materials leading to an increase of the anodic reaction by the anaerobic biofilm. In laboratory, the effect of aerobic bacteria can be simulated by sterile seawater in which an enzyme (oxidase) is added. Based on this mechanism, a test for crevice corrosion and pitting behaviour of stainless steels has been successfully implemented by 20 laboratories in aerobic conditions. In anaerobic conditions, the effects of BSR on the crevice corrosion potentials of stainless steels can be simulated by adding Na$_2$S to sterile de-aerated seawater in such a way that the same total sulphide concentrations and pH are obtained. Corrosion methods are deduced from these results to test the behaviour of stainless steels and more generally of passive alloys under mixed conditions