Influence of Seawater Nutrient Content on the Early Immersion Corrosion of Mild Steel—Part 1: Empirical Observations

A previous study of the immersion corrosion of mild and low-alloy steels in natural coastal seawater found a high degree of consistency between the corrosion losses observed at different testing sites when allowance was made for average seawater temperature and sites with obvious pollution and velocity effects were excluded. However, some sites showed early corrosion rates considerably below the trend. Carefully conducted laboratory experiments using real seawater also show low trends. It is proposed that the more severe early corrosion of steels in natural coastal immersion conditions is caused by elevated nutrient levels in seawater. Not only does this increase the rate of growth of biofilms and of marine fouling as normally assumed but it also raises the rate of metabolism of bacteria. It is contended that the latter is the principal cause of increased early corrosion at higher nutrient levels. A detailed analysis of an extensive range of field data provides support for this contention. It was found that low rates of early corrosion usually are associated with pristine waters that have much lower nutrient contents compared to typical coastal waters. The implications carry over to the various parameters describing the previously proposed corrosion loss model. It is found that elevated nutrient levels increase the early corrosion rate but reduce the maximum corrosion that occurs under aerobic conditions, and reduce, slightly, the time to transition to anaerobic conditions.