Corrosion Pit Growth on Steel for Welded Structures under Repeated Stress in Seawater

Abstract

The integrity of offshore structures such as wind turbines should be managed against corrosion fatigue. In this study, steel for welded structures, SM490A, was used for corrosion fatigue tests under welding residual stress and for constant displacement corrosion tests under four-point bending in synthetic seawater. Corrosion fatigue life of the specimens with compressive residual stress was longer than that with tensile residual stress. Effects of welding residual stress and constant surface stress on corrosion pit growth were not recognized by scattered pit size distribution, but a pit growth model was formulated in the following equations, where d_p,m is the mean corrosion pit depth (μm), A and B are fitting parameters, Δσ is the nominal stress range (MPa), and t is the time (month). d_p,m = A t ^B,　A = 0.0953Δσ + 3.26,　B = －0.0023Δσ + 0.892 The reason why corrosion fatigue characteristics are affected by residual stress is considered to be that residual stress affects the critical stress intensity factor range for corrosion fatigue crack initiation, the time required for corrosion fatigue crack initiation from a corrosion pit, and the time required for corrosion fatigue crack propagation.