Revealing the influence of Cr and Al additions on the corrosion of carbon steel in a supercritical CO2–SO2 environment

Abstract

The formation mechanism of corrosion product films of X65 and 5Cr2Al steels in CO₂-saturated aqueous phase environments containing 100 ppm and 300 ppm SO₂ impurities was investigated. Under both conditions, the corrosion rate of X65 was approximately ten times that of 5Cr2Al, with the main reaction being the formation of FeCO₃. In the CO₂-100 ppm SO₂ system, the inner layer of 5Cr2Al formed Cr(OH)₃ and Al(OH)₃ layers, while the outer layer formed FeCO₃. When the SO₂ concentration increased to 300 ppm, Cr and Al reacted synergistically with SO₂, forming a double-layer structure with an inner layer composed of Cr(OH)₃ and Al(OH)₃ and an outer layer composed of dense elemental sulfur. This film layer structure exhibited excellent corrosion resistance. In situ electrochemical measurement results in supercritical CO₂ environments with 300 ppm SO₂ further reveal that corrosion is aggressive for X65. While a semi-passivation phenomenon occurred for 5Cr2Al steel. This study indicates that a dense S elemental film can form only when Cr and Al are present and the SO₂ concentration reaches 300 ppm. 5Cr2Al is a suitable material choice for the injection tube in a mixed CO₂–SO₂ environment.