MICROBIALLY SUSTAINED PITTING CORROSION OF 304-STAINLESS-STEEL IN ANAEROBIC SEAWATER

A system has been developed in which a small anode and large cathode can be induced in a concentric stainless steel electrode. A current of approximately 11 mu A cm(-2) was applied for 72 h while bacteria were allowed to colonize the electrode in a closed system, in artificial seawater under anaerobic conditions. Once the applied current was removed, the resultant galvanic current was monitored and a flow of nutrients resumed to an open system. Only a co-culture of SRB and Vibrio sp. maintained a current of approximately 3 mu A cm(-2) for >200 h. No current was maintained with pure axenic cultures or in sterile controls. Bacterial counts revealed that the resultant current was dependent on the number and type of bacteria on both the anode and cathode. It would appear that SRB are necessary on the cathode, leading to high charge transfer resistance above 100 k Omega cm(2), while a mixed consortium is necessary on the anode giving low charge transfer resistance below 1 k Omega cm(2). These results would appear to give further evidence for the previously proposed cathodic depolarization theory as a mechanism for MIC and for another anodic reaction involving a mixed consortium.