Grain boundary effect on bacterial attachment

Abstract

AISI Type 304 L stainless steel is a widely used material in industries due to its strength and resistance to corrosion. However, corrosion on SS is reported largely at weld or its adjacent areas. Bacteria were observed to colonize preferentially near welds and the reason for this is described as the surface roughness. In the present study, the influence of yet another important metal surface condition ie. substratum microstructure on the bacterial adhesion has been evaluated. 304 L SS weld samples were prepared and machined to separate weld metal, Heat Affected Zone (HAZ) and base metal portions. These coupons were molded in resin so that only the surfaces polished to 3 micrometer finish were exposed to the experimental medium with bacteria, Pseudomonas sp., isolated from a corrosive environment in Japan. The coupons were exposed for varying duration. The area of bacterial attachment showed significant difference with time of exposure and the type of coupons. Generally, the weld metal samples showed more attachment while the base metal the least. The area of attachment was inversely proportional to the average grain size of the three samples. As the bacteria started colonizing, the attachment largely occurred on the grain boundaries of base metal (after 8 h, 84.62 percent and 15.38 percent of total number of bacteria attached in the Field Of View (FOV) at grain boundary and matrix, respectively) and on the austenite-ferrite interface in the weld metal (after 8 h, 88.33 percent and 11.77 percent of total number of bacteria attached in the FOV at boundary and matrix, respectively). Weld area had more grains and hence more grain boundary/unit area than base metal and hence resulted in more bacterial attachment. The SEM observation showed this increased attachment of Pseudomonas sp. resulted in the initiation of MIC on weld coupons by 16 days. Therefore, the results provide data to support the fact that substratum microstructure influences bacterial attachment which, in turn leads to corrosion.