UNS N04400 and UNS N05500 (in this paper: Alloy 400 and Alloy K-500, respectively) are Ni-Cu alloys that are included in NACE Standard MR0175. Alloy K-500 is the precipitation hardening version of Alloy 400. Both alloys are used in sour service in oil and gas production, e.g., as small diameter tubing, fittings, in small valves, and as some components in downhole packers, subsurface safety valves, etc. The corrosion of Alloy 400 and Alloy K-500 were studied in laboratory experiments by exposing them to simulated sour, aqueous production environments with cover gases containing up to 200 psi of H2S and up to 400 psi of CO2 (1400 and 2760 kPa) while at temperatures as high as 350°F (177°C). Tests were also conducted in dry H2S gas. In general, the corrosion rate of each alloy increases with temperature, H2S partial pressure, temperature cycling, occurrence of alternate wetting and drying, and mechanical straining. Alloy 400, unlike Alloy K-500, was found to be limited only by general corrosion, as it was not found to be susceptible to stress corrosion cracking or localized corrosion in these simulated sour service environments. The alloy condition, i.e., solution annealed (Alloy 400 and Alloy K-500) or solution annealed and aged (Alloy Ko500), also affected the corrosion rate. The corrosion product produced on Alloys 400 and K-500 during exposure to sour environments is not adherent when dry; spalling and/or blistering of the product occurs. Therefore, alternating
wetting and drying and/or dynamic straining could produce higher corrosion rates than observed in these laboratory tests. The adherence of the corrosion product is significantly better on Alloy K-500; no blistering or spalling. The corrosion product on the Alloy 400 was determined to be primarily Ni3S2 plus other nickel sulfides. Explanations for these variations and effects are offered in this paper.