INTRODUCTION
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.
Careful observations of the corrosion of steel surfaces in natural seawater have shown that pitting initiates almost immediately after first immersion and that these pits grow quickly in depth to about 100 microns1. Usually this is attributed to anode-cathode reactions set up by the small manganese sulfide (MnS) inclusions present in most steels2. It also has been proposed that the region immediately surrounding the MnS inclusion rather then the inclusion itself may be responsible for the observed pitting3. Although there is much research still on-going regarding the mechanisms involved in pitting corrosion, the possibility that microbiological activity can have some part in the pitting process for mild steels does not appear to have been given much attention, although the present authors have argued that the changes of the mild steel corroded surfaces with increased exposure time are likely to be the result of microbiological influences4. They observed that topography of the surface changes considerably with time and includes pit growth both in depth and in area, with subsequent pit coalescence and eventually the initiation of newer pitting.
It is now well-established that upon first immersion of a steel surface in natural seawater it is soon covered by a thin biofilm that harbours micro-organisms5 that appear to have an interaction with the corrosion process6. Other evidence points to microorganisms being involved in later corrosion, particularly when anaerobic niches form within the corrosion products, which then provide an appropriate local environment for sulfate reducing bacteria (SRB)7.
These observations are included in the mathematical models for uniform corrosion developed by the authors8 and also have been used in models for maximum pit depth9.
