Electrochemical behavior of carbon steel in alkaline sour environments measured by electrochemical impedance spectroscopy

We evaluated the corrosion process of 1018 carbon steel in an alkaline sour environment, using a very specific solution to simulate the conditions of the sour waters occurring in the catalytic plants of the PEMEX (Mexican Oil Company) oil refinery, 0.1 M (NH₄)₂S and 10 ppm NaCN at pH 9.2. Using polarization curves and electrochemical impedance spectroscopy, we characterized the carbon steel surfaces following diverse immersion times in the alkaline sour environment. The corrosion process in these media involves a series of steps occurring simultaneously. At initial immersion times, the corrosion rate is rapid and a protective film of maximum thickness forms. After 50 h of immersion, growth and dissolution of the film reach equilibrium. As the protective film is formed from corrosion products, different diffusing processes occur through it. Fe²⁺ ions and atomic hydrogen (H°) diffuse through the film. The diffusion of atomic hydrogen is the limiting step of the corrosion process. An accelerated assay of the corrosion process shows that diffusion of atomic hydrogen causes the rupture (blister) of the passive film formed in these media, increasing H° diffusion up to a critical time where the corrosion process increases.