Crack growth behavior of 725 in seawater under cathodic polarization

Hydrogen embrittlement behavior of four heats of alloy 725 (H1, H2, H3, and H4) were characterized by measuring the crack growth rate response over a range of cathodic potentials. Significant grain boundary precipitation in two heats (H1 and H3) may have contributed to higher susceptibility, as evidenced by higher values of crack growth rate. The crack growth rate increased with increasing twin fraction. The crack morphology exhibited evidence of intergranular as well as slip band cracking. The crack growth rate response of the two heats that were less susceptible (H2 and H4) were evaluated in more detail over a range of applied potentials and stress intensity factors. Evidence of nano void formation at the intersection of dislocation slip bands, and grain boundaries suggests that hydrogen stabilized vacancies may play a role in the crack growth mechanism. Crack growth rate exhibited a strong response with applied potential varying by as much as 100 times in the potential range of −1050mV SCE to −850mV SCE. Over the range of K values evaluated, the crack growth rate increased by a factor of 3to 30 with applied K depending on the applied potential. The crack growth rate was related to the water adsorption on fresh metal surfaces of alloy 725 in the anticipated crack tip conditions. A crack tip strain rate based approach was applied to model the measured crack growth response.