THE EFFECT OF SULFATE-REDUCING BACTERIA ON THE ELECTROCHEMICAL-BEHAVIOR OF CORROSION-RESISTANT ALLOYS IN SEA-WATER

Corrosion potential (E(corr)), polarisation resistance (R(p)) and potentiodynamic polarisation measurements were conducted on UNS N04400 (Monel 400) and UNS N08904(904L) stainless steel in de-aerated sea water in the presence and absence of sulphate-reducing bacteria (SRB). These measurements as well as morphological studies were conducted after different exposure intervals of up to 26 d to the test environment. In the absence of SRB both alloys were readily passivated. The anodic polarisation behaviour of both alloys was remarkably affected by the presence of SRB. Thus exposure of alloy 400 to an SRB environment for 5 hr led to the appearance of an active dissolution peak and prolonged exposure led to a diminishing of the passive region. The anodic polarisation curves pertaining to 904L alloy showed a change in behaviour after 14 d where an active/passive transition was recorded and pitting attack below scattered bacterial colonies was detected. However, under open circuit conditions no pitting was observed. E(corr) for both alloys shifted in the active direction and R, decreased with the increase of exposure time to SRB. EDX analysis indicated sulphide-rich layers on alloy 400 which were responsible for the loss of passivity.