The Effect of Microbiological Involvement On The Topography of Corroding Mild Steel in Coastal Seawater

Microbiological factors have been attributed to accelerated corrosion of mild steel in seawater for some the past half century or so but 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 year investigation where raw water was taken from a Port Stephens tributary and split into two streams; raw water and sterile water. The waters were pumped directly into tanks in which 10 mm x 10 mm polished steel coupons were suspended.

Coupons were removed on an irregular basis and examined. After about a year the raw water coupons had developed what are commonly referred to as tubercles whereas the oxide build-up on the sterile water coupons was relatively even.  Of particular interest was the surface of the steel when viewed using a scanning electron microscope (SEM).  The images presented in all cases show a difference in oxide formation and topography between coupons recovered from raw and sterile water streams where the only difference was the removal of microbiological matter. Hence the pitting observed on the raw water coupons can only be attributed to bacterial activity.