Effect of the carbon dioxide pressure on the electrochemical behavior of 3Cr low alloyed steel at high temperature

Electrochemical corrosion behavior of 3Cr steel in CO ₂- containing solution at a high temperature was investigated by various electrochemical measurements and analysis as well as thermodynamic calculations of ionic concentrations and equilibrium electrode potentials. A conceptual model was developed to illustrate the electrochemical corrosion mechanism of 3Cr steel in the CO ₂-containing sodium chloride solution. Comparing the corrosion potentials of 3Cr steel in the test solution under different CO ₂ pressures with the conceptual model, it is found that anodic reactions of the 3Cr steel contain a direct dissolution of Fe, and the formation of corrosion scales, FeCO ₃ and Cr(OH) ₃, by Fe+HCO3-=FeCO ₃+H ⁺+2e and Cr + 3OH ^- = Cr(OH) ₃. With the CO ₂ pressure increasing, the corrosion potential has a positive shift. It indicates that the CO ₂ pressure has a greater effect on the cathodic reaction than that of anodic reaction. And the corrosion current has positive linear relationship with the increase of CO ₂ pressure. It is attributed to the concentration increasing of the reactants of the cathodic reaction. According to analysis of the electrochemical impedance spectroscopy, the scale forming reactions dominate the corrosion process when the CO ₂ pressure is lower than 0.6 MPa and the dissolution of Fe, followed by the consecutive mechanism with adsorbed intermediate products, takes up the dominant part in the anodic process when the CO ₂ pressure exceeds 0.6 MPa.