The influence of concretion on the long-term corrosion rate of steel shipwrecks in the Belgian North Sea

Some three hundred mainly steel shipwrecks from both World Wars lie buried at shallow depths along the Belgian North Sea coast. They were examined recently to estimate corrosion rates over periods in excess of 100 years. The rate was estimated by comparing the measured in-situ steel plate thicknesses with archived ship information. The estimates show distinctly lower long-term corrosion rate compared to that predicted by the Melchers (Modeling of marine immersion corrosion for mild and low alloy steels – part 1: phenomenological model. Corrosion. 2003;59(4):319–334) corrosion model, when parameterised for local North Sea conditions. Concretion after 50 years has a multi-layer structure for which SEM-EDS and XRD measurements show the innermost layer, close to the metal surface, consisting of akaganeite, and the outer layer mostly of calcium carbonates, silicates, and some siderite. In between there is a considerable layer of hard magnetite. The latter is proposed as the reason for the low long-term corrosion rate (0.016 mm y–1) in an environment with calcium carbonate available in abundance.