The Role of Nickel in Low Alloy Steels Exposed to H₂S Containing Environments. Part II: Effect of the Electrochemical Potential and Stress Level on Trench Formation

In the first part of this paper, trenches were reported for Ni-containing steels tested using the slow strain rate test (SSRT) method in H₂S-containing environments at the open-circuit potential (OCP). Trenches are deep elongated pits, and their appearance is more similar to small blunt cracks. These features can develop a sharp sulfide stress crack at their bottom under certain conditions, which are still not fully understood. In this second and final part, the effect of the electrochemical potential and the stress level in trench nucleation and growth was investigated for the same set of Ni-containing steels with up to 5 wt% Ni. The anodic nature of the trench formation mechanism was verified, and under an anodic polarization a critical stress value for trench formation, σ_trench, was estimated from SSRTs and finite element modeling. Applying a cathodic potential suppressed trench formation, but not cracking, because cracks nucleated and propagated by hydrogen stress cracking. The resulting environmental and stress-level dependencies for Ni steels confirmed that trenches could be considered a form of environmental-assisted cracking. It is concluded that the main role of trenches is to provide favorable spots for hydrogen stress crack nucleation at OCP, but their presence is neither necessary nor sufficient for cracking occurrence.