Corrosion susceptibility of mooring chain steel under cyclic stress

Abstract

Corrosion-fatigue is a severe form of degradation that results from the joint effect of cyclic tensile stresses and of a corrosive environment and is particularly relevant in mooring chains of offshore structures. The work deals with the corrosion-fatigue of a high-strength low-carbon steel alloy used in mooring chains. While many studies are concerned with the nucleation of cracks starting from corrosion pits, the work deals rather with the opposite effect, i.e., how the cyclic stress may influence the nucleation and growth of pits. A range of applied cyclic stresses were applied both to cylindrical specimens and to trunk conical-shaped specimens, the corrosion degree being estimated by quantitative image analysis of the corroded surface, using the fraction of corroded area as an index. The cyclic tensile stress enhanced the nucleation/growth of pits on the surface. The corroded area in cylindrical specimens increases exponentially with the stress when the Smith-Watson-Topper (SWT) model is considered. The trunk conical specimen geometry reveals the effect of stress for a single exposure time, an advantage compared to the cylindrical geometry, in which the effect of exposure time cannot be isolated from that of the applied stress. The extent of surface corrosion increases with the local mean stress, except for a narrow region near the crack, likely due to local cathodic protection induced by the anode inside the crack. Correlations are proposed for the stress limits corresponding to a certain reduction in corrosion-fatigue life, and for the surface corrosion damage as a function of the local SWT parameter.