Fundamental understanding of microstructure and Cr on supercritical CO2 corrosion resistance of high-strength low-alloy steels

In order to understand and improve the corrosion resistance of high-strength low-alloy (HSLA) steels under supercritical CO₂ (s-CO₂) environment, the effects of microstructure and chromium content of experimental steels on the corrosion of pipeline steels transporting s-CO₂ were investigated by rotating cage method and electrochemical techniques. The complex microstructure of HSLA steels is identified and quantified and it is related to s-CO₂ corrosion rate. It was shown that the corrosion resistance of the experimental steels in s-CO₂ environment was significantly reduced with the increase in coarse granular bainite (GB) and martensite-austenite (MA) in the microstructure. Electrochemical tests showed that the corrosion mechanisms of the steels were the same, but the electrochemical impedance of the higher-Cr test steel was lower in both the bare and the rusted samples, which was due to its internal coarse GB and rough MA constituents. However, the impedance of 0.6Cr steel is reduced due to its GB and coarse MA components. Kelvin probe force microscopy characterization tests confirm that there is a large potential difference between the coarse MA components and the substrate, which reduces the local corrosion resistance of the material. However, MA tends to be a companion microstructure of coarse GB, which plays an important role in promoting the occurrence of localized corrosion on the surface of experimental steels in s-CO₂ environ-ments, and tends to lead to a serious deterioration of the corrosion performance of HSLA steels in s-CO₂ environments.