Modelling immersion corrosion of structural steels in natural fresh and brackish waters

The multi-phase mean-value model previously proposed for modelling the marine immersion corrosion of low carbon and low alloy structural steels is examined for application to fresh and brackish waters. Use is made of field data for brackish and fresh waters available in the literature, supplemented with data or estimates for water temperature, pH, hardness and nutrient levels. It is shown that the data exhibits consistency with the model and that it is a function of average water temperature. Corrosion in brackish and fresh waters corrosion depends on water hardness, pH and nutrient levels, with higher pH levels and lower water hardness associated with higher aerobic levels of corrosion but these are not significant for anaerobic corrosion. In the anaerobic phases 3 and 4 of the model, the available data and associated trends are interpreted as showing that elevated levels of nutrients produce higher rates of corrosion. Conversely, these phases showed very low rates of corrosion for fresh waters with very low nutrient levels. Consistent with basic corrosion theory and with laboratory observations, salinity by itself is not a clear distinguishing characteristic. The model provides a new approach to interpreting the available data for corrosion in fresh and brackish waters. It permits plausible explanations for previous apparently inconsistent observations for corrosion in brackish waters. Finally, it reinforces the need for full and detailed reporting of corrosion testing programs, including details of precise timing, location, orientation and environmental conditions including means and variations in water temperatures, DO, salinity, pH, water hardness, carbonates and various nutrients.