Economical N1 base alloys with modified chemical composition range of UNS N08535, were investigated in mild sour environment. The mild sour environment considered was 25% NaCl solution containing 0.2MPa H2S and 3.0MPa CO2. Corrosion resistance of 25Cr-3Mo alloy was improved by Cu addition not less than 1.48mass% and Ni content not less than 31.9mass%. Based on test results, an economical alloy which consisted of 25Cr-32Ni-3Mo- 1.5Cu has been developed. The critical temperature for SCC resistance of the economical alloy in a mild sour environment was 50°F(28°C) bigher than that of conventionrd alloys containing 3mass% Mo in hostile environments with high H2S partial pressure and CH3COOH. The surface film formed on the economical Ni base alloy was analyzed by secondary ion mass spectrometer(SIMS) afler 48 and 720h exposures in a mild sour environment. The surface film, consisting of an outer Cr-O layer, a middle Ni-S layer, and an inner Cr-O layer, was still growing at 48h, and tier 720h the film became a single Cr-O layer with sulfide on the film surface and with Cu enrichment in the film in contact with the mother metal Furthermore, The role of alloying elements such as Fe, Cr, Ni, Mo and Cu in SCC resistance was discussed from the
viewpoint of stability of corrosion product based on the volubility of oxide and sulfide calculated by thermodynamic method.
Keywords: Hydrogen sulfide, Oil country tubular goods, Stress corrosion cracking, Ni base alloy, SolubiIity, Corrosion product