Localized Corrosion Resistance of Precipitation Hardened Ni Based Alloys at the Recommended Limits of H2S Concentration and Temperature

Abstract The application limits of Ni based alloys in sour service conditions are largely defined in terms of the H2S partial pressure and temperature. In the present study the objective is to evaluate the susceptibility to pitting and stress corrosion cracking (SCC) for precipitation hardened alloy 725 (UNS* N07725) at conditions near and beyond the application limits as indicated in the ISO+ 15156-part 3 standard. The testing was performed using electrochemical techniques to study the pitting likelihood and by means of the slow strain rate method to assess the susceptibility to SCC. The experiments are carried out in a 1% (w/w) NaCl brine at pH 4, the variables considered are the temperature levels of 150°C and 230°C and H2S concentrations at each temperature from 14,000 mg/l up to 18,000 mg/l. The results indicate that alloy 725 exhibits pitting 150°C at H2S concentrations higher than 16,000 mg/l, and at 230°C at H2S content higher than 14,000 mg/l. The results of SSRT tests indicate that there is significant loss of ductility and mechanical strength in the alloy 725 which indicates that the susceptibility to environmentally assisted cracking increases with the H2S content and temperature the SSRT results correlate with the average pit current density, however the analysis of the fracture surfaces does not show clear evidence of electrochemical activated crack propagation associated with SCC susceptibility.