Coupons of nitrided and non-nitrided 9Cr and 13Cr were exposed to a simulated downhole environment (250 F, 200 psia CO2, 1 psia H2; aqueous phase of 5 percent NaCl). Coupons were fixtured in a static autoclave such that galvanic effects between sets of coupons could be determined. One group of 16 coupons was exposed to the vapor phase of the autoclave and 1 group of sixteen coupons was exposed to the liquid phase.
Vapor phase corrosion was found to be minimal from the standpoint of weight loss. Minor indications of shallow pitting were observed for 9Cr and 13Cr materials, however, the corrosion was insensitive to nitriding type.
In the liquid phase, general corrosion was pronounced for nitrided 13Cr and both nitrided and non-nitrided 9Cr. The results indicate that nitriding is not beneficial to (has no effect on) materials that undergo general corrosion. For materials that otherwise would be
passive, nitriding depletes chromium to the point that general corrosion becomes possible.
No galvanic effects were evident from the data. No differences were noted for OPQ nitrided as opposed to ion nitrided materials.