Marine immersion corrosion observations for archaeological and other data, including from shipwrecks, wrought iron anchors, and cast iron bridge piers are used to estimate corrosion losses in natural unpolluted coastal and near-coastal seawaters for exposures up to about 600 y. Empirical functions for the development of corrosion loss with time were then developed, standardized to 20°C mean seawater temperature. The resulting slightly nonlinear corrosion loss trend, represented by a modified form of the power law, is consistent with phase 4 of the previously proposed bi-modal corrosion model. These observations support the notion that the rate of long-term corrosion is controlled by outward diffusion, through the rust layers, of gaseous hydrogen generated by the cathodic hydrogen evolution reaction under predominantly anaerobic corrosion conditions. The power-law trend also provides good extrapolation from shorter-term data. For practical purposes for exposures <100 y in seawaters with a mean temperature of around 20°C, a linear model, with a longer-term corrosion rate 0.06 mm/y at 20°C, is sufficiently accurate.

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