Effect of long periods of corrosion on the fatigue lifetime of offshore mooring chain steel

R4-grade mooring chain steel specimens were subjected to alternating phases of saltwater corrosion and air fatigue to replicate seasonal load variations on chains in service. No significant difference in fatigue lifetimes was registered between precorroded specimens subjected to continuous fatigue and those periodically interrupted by phases of accelerated corrosion. Moreover, eight months of natural corrosion of specimens with fatigue cracks did not have a large effect on their remaining fatigue lifetimes. Retardation of fatigue crack growth by crack tip blunting from corrosion is deemed unlikely based on observations from the current work and the literature. Corrosion accelerated by anodic polarization is demonstrated to be unsuitable for replicating natural corrosion of fatigue cracks.