The effect of seawater sterilisation on the corrosion of mild steel

Initiation of pitting of mild steel in seawater usually is attributed to anode-cathode reactions set up by the small manganese sulphide (MnS) inclusions present in most steels. On the other hand there is now considerable evidence that microbiological factors are involved in the early corrosion of mild steel in natural seawater. Presented herein are the results of a two year investigation of whether microbiological factors are likely to have a significant role in the pitting of mild steel exposed to seawater. In the experiment, natural seawater was pumped from a Port Stephens tributary and split into two streams. One stream flowed slowly through a tank in which 25 mm x 25 mm polished steel coupons were suspended and was then released back to the estuary. The second stream was passed through a series of filters and sterilized using ultra-violet (UV) lamps prior to entering a similar but sealed tank, also fitted with UV sterilization, where identical coupons were suspended, before being returned to the estuary. Coupons were removed at various time intervals and examined both visually and using a scanning electron microscope (SEM). In all cases the metal surfaces show a considerable difference in topography between those recovered from natural and those from sterilised seawater. Since the only significant difference between the exposure conditions was the removal of living microbiological material it is concluded that microbiological factors are responsible, directly or indirectly, for the more severe pitting observed on the coupons exposed to natural seawater.