The progression of corrosion loss and maximum pit depth, once initiated, is of much interest for predicting the future or remaining long-term life for steel infrastructure systems such as nuclear waste disposal storages and offshore pipelines. These often operate under adverse environmental exposures, including seawater or aggressive ground waters. Failure can have substantial economic and environmental consequences. The model widely used for the prediction of long-term corrosion is the strictly concave power law function. However, it is inconsistent with many longer-term empirical field observations for corrosion loss and for maximum pit depth. These often exhibit a monotonic bi-modal trend. Earlier it was proposed that this may be due in part to microbiological influences. Herein an experiment is described in which mild steel coupons were exposed for 2.5 years to natural seawater and also, separately, to sterilised seawater. It was found that coupons in both streams exhibited the bi-modal behaviour. It is proposed that the bi-modal phenomenon largely is the result of a change in the nature of the corrosion reactions as the rust layers deposited on the exterior surface of the steel become increasingly less permeable, particularly to oxygen and that eventually hydrogen evolution and diffusion becomes the rate controlling mechanism.