Effect of flexure on rusting of ship's steel plating

Data collected from ageing ships indicate that the rate of corrosion of ship's plating may increase with long periods of exposure when combined with plate flexure. The reasons for this have not been clarified. Herein, the possibility is investigated that the observed effect is due to the loss of protective rusts under high strains in the steel plating, as might be caused by high sea-states and during loading and unloading. Recently, relatively short-term exposure studies of the tensile strain-induced fracture of rusts formed on structural steels during natural marine exposure show an increase in corrosion rate of some 10–15%, but only when the strains are near and beyond the elastic limit of the steel. This has implications for ship management. An interpretation is given for the corrosion loss curves reported in the literature and it is shown that, qualitatively at least, they are consistent with earlier observations and models for the corrosion of steel in marine environments. It is proposed that the validity of the conclusions be tested using long-term exposures under corrosion and flexure conditions in actual ships.