Influence of Cr content on corrosion performance of middle Cr alloy steel in CO2 environment

The corrosion behavior of three medium-Cr alloys in CO2 aqueous solution environment was studied by high temperature autoclave accelerated test and electrochemical means (including micro-potential test). In this environment, the corrosion rate of the 5Cr sample was 1.5023 mm/a, 7Cr was 0.2471 mm/a, and 9Cr was 0.0540 mm/a. The CO2 corrosion product film consisted of two layers. The inner film was amorphous FeCO3 and Cr(OH)3, and the outer layer was granular FeCO3 and CaCO3. The dense inner film was rich in Cr, which provided effective protection for the tested steel in the CO2 corrosive environment. Increasing the Cr element content helped to increase the self-corrosion potential of the steel, reduced the corrosion current, and helped to improve the corrosion resistance of the steel. On the other hand, the increase of Cr content inhibited the transformation of pearlite, the pearlite content decreased, the proportion of acicular ferrite increased, and the microstructure was more uniform and single. Meanwhile the uniform microstructure reduced the potential difference between different regions, and the corrosion resistance of steel was improved.