Electrochemical characterization of the different surface states formed in the corrosion of carbon steel in alkaline sour medium

Abstract In this study the different surface states that manifest in the corrosion process of 1018 carbon steel in alkaline sour environment, solution prepared specifically to mimic the sour waters occurring in the catalytic oil refinery plants of the Mexican Oil Company (PEMEX) (0.1 M (NH 4 ) 2 S and 10 ppm NaCN at pH 9.2) were prepared and characterized. The surface states of the carbon steel were formed by treating the surface with cyclic voltammetry at different switching potentials ( E λ + ), commencing at the corrosion potential ( E corr =−0.890 V vs sulfate saturated electrode, SSE). The surface states thus obtained were characterized using electrochemical impedance spectroscopy and scanning electron microscopy techniques. It was found that for E λ + =−0.7 and −0.6 V vs SSE a first product of corrosion formed, characterized by a high passivity. Moreover, it was very compact (with a thickness of 0.047 μm). However, at more anodic potentials ( E λ + >−0.5 V vs SSE) a second corrosion product with non-protective properties (porous with a thickness of 0.4 μm and very active) was observed. The diffusion of atomic hydrogen (H 0 ) was identified as the slowest step in the carbon steel corrosion process in the alkaline sour media. The H 0 diffusion coefficients in the first and second products that formed at the carbon steel–sour medium interface were of the order of 10 −15 and 10 −12 cm 2 /s respectively.