Localized corrosion and repassivation behavior of Ni600 and SS304 under conditions relevant to SCC

Abstract

In this study, the corrosion kinetics and repassivation behavior of stainless steel 304 (SS304) and nickel alloy 600 (Alloy 600) were investigated and compared under pit and crack-like environments. The corrosion kinetics of nickel Alloy 270 (pure nickel) was also assessed for reference. To evaluate the kinetics of metal dissolution under aggressive conditions, downward potential scans were performed on one-dimensional (1D) pit samples. These scans were used to determine the dissolution current density across a range of temperatures at a fixed metal cation concentration at the metal surface corresponding to the saturation level at 20 °C (C_{sat_20}). Repassivation behavior was evaluated using constant potential and constant temperature tests. In the first approach, downward potential scans at 25 °C were employed to identify the potential and pit depth at which repassivation occurs for each alloy. In the second method, downward temperature scans under constant applied potential were used to assess the corresponding temperature and pit depth at repassivation. Under charge-transfer-controlled conditions, pure nickel exhibits the slowest dissolution kinetics, followed by Alloy 600, with SS304 exhibiting the fastest rate. However, SS304 demonstrates a greater tendency to repassivate compared to Alloy 600. The critical concentration for repassivation was 60 % of the saturation concentration for SS304 and 15 % for Alloy 600. Density Functional Theory simulations provided mechanistic insights into these observations. A mechanistic explanation of the effects of electrochemistry on stress corrosion cracking resistance of Ni- and Fe-based alloys was proposed.