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)(2)S and 10 ppm NaCN at pH 9.2. Using po larization curves and electrochemical impedance spectroscopy, we characteri zed the carbon steel surfaces following diverse immersion times in the alka line sour environment. The corrosion process in these media involves a seri es of steps occurring simultaneously. At initial immersion times, the corro sion 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 degrees) diff use through the film. The diffusion of atomic hydrogen is the limiting step of the corrosion process. An accelerated assay of the corrosion process sh ows that diffusion of atomic hydrogen causes the rupture (blister) of the p assive film formed in these media, increasing H degrees diffusion up to a c ritical time where the corrosion process increases. (C) 2000 Elsevier Scien ce Ltd. All rights reserved.