Effect of Alloying Element on Corrosion Resistance of Ni Base Alloy in Sulfur Containing Sour Environments

Abstract The effect of the alloying elements Cr, Ni, Mo, W and Cu on stress corrosion cracking(SCC) and localized corrosion of Ni base alloy was investigated in sulfur-containing sour environments at elevated temperatures. Cr, Ni, Mo, W and Cu were found to be effective elements in improving the corrosion resistance. In particular, the SCC resistance in the environment improved due to increasing the Ni content in and the addition of Cu to alloy, and the localized corrosion resistance corresponding to the decrease of the corrosion rate in the environment improved due to increasing the Mo and Cr content in the alloy. An economical corrosion resistant alloy to these types of corrosion was 22Cr-52Ni-11Mo-0.7W-0.8Cu alloy. The SCC resistance in a sour environment acidified by using acetic acid was also investigated by SSRT and the result showed that the higher the Mo content in the alloy, the higher the critical temperature for SCC occurrence. Furthermore, a mechanism of corrosion resistance in the sulfur-containing sour environments was studied by analyzing the results of surface film and calculating the results of the solubility of oxides and sulfides. The calculation results showed that the solubility of MoO2 and MoS2 is very small at low pH, and the lower the pH, the smaller the solubility of MoS2 and MoO2. It is thought from these results that the formation of Mo oxide and/or sulfide occurred on the low pH portion in the inner layer of the film underneath the sulfur deposit and this improved the corrosion resistance to both SCC and localized corrosion.