The marine corrosion of structural steels in brackish and fresh waters

High-quality models are required to describe the corrosion of structural steels in marine, brackish and fresh waters and these models should, preferably, be based on a proper understanding of the corrosion processes involved. The models should then be used to interpret field data, rather than attempting to derive models empirically from field data. On this basis a multi-phase corrosion–time model has been proposed previously for mild and low alloy steels subject to immersion in 'at-sea' conditions. This model is reviewed and extended to brackish and fresh waters in this paper. The scientific principles show that salinity is not the critical issue for the corrosiveness of water. Instead, the critical parameters are the temperature, pH and calcium carbonate solubility when corrosion is governed by oxidation. For later corrosion, governed by anaerobic bacterial activity, the available nutrient level and rate of supply of nutrients control the observed rate of corrosion. The data available at the present time for estimating all the parameters of the new model are somewhat preliminary and new corrosion monitoring programmes will be required to augment the information and to fully characterize the corrosivity of coastal and harbour waters. However, indicative trends for the parameters, based on currently available data, are given and permit the construction of corrosion loss curves for practical applications.