PITTING CORROSION BY BACTERIA ON CARBON-STEEL, DETERMINED BY THE SCANNING VIBRATING ELECTRODE TECHNIQUE

The scanning vibrating electrode technique (SVET) has provided a non-destructive, on-line method for locating the presence and position of anodic electrochemical activity on the surface of carbon steel coupons. Using the SVET, changes in current densities over the steel with time were mapped in the presence and in the absence of bacteria. In a sterile liquid medium, the maps showed highly localized anodic current densities, which subsequently became inactive. Analysis of current maps and the open circuit potential (OCP) showed the potential transients were due to pit initiation and repassivation processes. When in the same bulk fluid, an aerobic bacterium, isolated from a corrosion tubercle, was grown, similar trends of pit initiation and repassivation were observed for several hours. However, after extended exposure to bacteria, local anodic activity did not repassivate. The corrosion then propagated and spread, until a large area of the sample was anodic. Recovery of the spent growth medium after growth of the bacteria, when used as the bulk solution, did not induce this irreversible pitting. These results indicated that the growth of bacteria altered surface conditions and prevented the initiated anodic sites from repassivating. The on-line non-destructive localized measurements possible by SVET can provide insight into the mechanisms of microbial influenced corrosion (MIC).