Relationship between Stress Corrosion Cracking Susceptibility in High Temperature Water and Intergranular Corrosion Tests of Ni-base Alloy 600

Abstract

This work was carried out to obtain some information about relationship between intergranular stress corrosion cracking (IGSCC) of Ni-base Alloy 600 in aerated acidic high temperature solutions and acid intergranular corrosion (IGC). Specimens were solution treated at 900-1150°C and post heat treated at 550-750°C for 5-50 hours. The application of electrochemical potentiokinetic reactivation method (EPR) to Ni-base Alloy 600 was developed. The IGC susceptibility was sensitively detected by a potentio sweep from -400mV to +600mV (vs. SCE) in 0.25M H₂SO₄+10^-3M KSCN solutions, and after 10 minutes rest at +600mV a back scan was performed to -400mV. The most severe intergranular attack was observed when specimens were solution treated at 1100-1150°C and post heat treated at 650-700°C for 5-20hours. IGSCC susceptibility in aerated acidic high temperature solution showed good agreement with IGC sus- ceptibility detected by EPR. Disappearance of Cr-depleted zone by re-diffusion of chromium resulted in low susceptibility to IGC and IGSCC. These results show that cracking mechanism of Ni-base Alloy 600 in aerated high temperature water is dissolution of Cr-depleted zone.