Comparison of the characteristics of corrosion scales covering 3Cr steel and X60 steel in CO₂-H₂S coexistence environment

The characteristics of corrosion scales on 3Cr and X60 steel were investigated by using weight-loss measurement and electrochemical test. The results manifested that corrosion scales of X60 steel was composed of a S-rich layer, while that of 3Cr steel was composed of an outer Cr-rich layer and an inner S-rich layer. Cr(OH)₃ and FeS deposited competitively on the surface of 3Cr steel. The deposition of FeS, which inhibited the precipitation of Cr(OH)₃, preferentially accumulated closely to the surface to form the inner layer, while Cr(OH)₃ deposited away from the surface to form the outer layer. The corrosion degree of X60 and 3Cr steel was perceived as serious corrosion under the test conditions. In the selection of tubing and casing for sulphur-bearing gas wells, 3Cr steel should be further evaluated according to specific working conditions.