Crevice corrosion behaviors of corrosion-resistant alloys (CRAs) with various Ni, Cr, and Mo contents were investigated in conditions simulating the sour gas environment encountered in oil and gas production. Crevice corrosion occurred more easily in a 0.1 MPa H2S environment than in a 1.0 MPa H2S environment. Ni, Cr, and Mo all improved crevice corrosion resistance in electrochemical and immersion tests. The improving effect of Ni and Cr on crevice corrosion resistance reached saturation at 20 percent of their contents. The effect of Mo was remarkable. Alloys containing more than 6% Mo exhibited excellent crevice corrosion resistance, which could not be achieved by the increment in Ni and Cr contents.
The onset of the crevice corrosion on CRAs in H2S–Cl environment was investigated by electrochemically studying the pH drop in the crevice solution and the depassivation pH (pHd). These are considered to determine the extent of crevice corrosion resistance in comparison to that in O2–Cl− environment. The effect of Ni on crevice corrosion differed in H2S–Cl− and O2–Cl− environments. It was proven that Ni accelerates the pH drop and raises the pHd inside crevices by reducing the H2S concentration through Ni–S compound formation in H2S–Cl− environments.
It has been shown that the crevice corrosion frequency from the immersion test in 0.1 MPa H2S was better correlated with the pHd in the deaerated solution rather than the pHd in the H2S containing solution. The crevice corrosion resistance under 0.1 and 1 MPa H2S is discussed in relation to the pHd dependent on the H2S concentration in the crevice.
