The effect of microbiological involvement on the topography of corroding mild steel in coastal seawater

Microbiological factors have been implicated in the accelerated corrosion of mild steel in seawater for many years. To date few successful medium-term trials have been carried out to quantify the effect of micro-organisms on the surface of steel exposed to seawater. Presented herein are the results of a two and a half year investigation in which natural seawater was taken from a tributary of Port Stephens, Australia, and split into two streams; natural (raw) seawater and sterile seawater. The waters were pumped directly into tanks in which 25 mm x 25 mm polished steel coupons were suspended. Coupons were removed on an irregular basis and examined. After about a year the coupons in natural water had developed what are commonly referred to as tubercles whereas the oxide build-up on the coupons in sterile seawater was relatively even. There was also a significant difference in size and numbers of pits. Furthermore, the cleaned surfaces of the steel when viewed using a scanning electron microscope (SEM) in all cases showed differences in oxide formation and topography between coupons recovered from natural and sterile water streams. Since the only difference between the two streams of seawater was the removal of microbiological matter, differences in topography and in pitting are most likely the result of bacterial activity.