Effect of AC interference on the stress corrosion cracking susceptibility of X65 steel under cathodic protection

Abstract The influence of alternating current (AC) on the stress corrosion cracking (SCC) susceptibility of X65 steel under cathodic protection (CP) was studied by slow strain rate testing (SSRT) and electrochemical techniques. The X65 steel exhibits an increase in SCC susceptibility and brittle morphology with increasing AC current density and CP potential. The effect of AC interference on the SCC susceptibility is more pronounced at −0.77 VSCE than −1.12 VSCE. The imposed AC signals at −0.77 VSCE decrease elongation, percentage reduction in area and produce a ductile to brittle transition in the fracture morphology, mainly controlled by an anodic dissolution mechanism. At −1.12 VSCE, AC interference enhances the kinetics of the cathodic reactions producing higher hydrogen concentration on the steel surface, that leads to hydrogen assisted cracking. Pitting corrosion was observed under CP potential and AC interference. The pitting severity in terms of size and depth, increases with increasing AC current density, especially at −1.12 VSCE. The maximum pit depth of 90 µm was found at the CP potential of −1.12 VSCE and 90 A/m2 AC (3 V rms). Microcracking and crack evolution from the pits were also observed.