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

In this study the different surface states that manifest in the corrosion p rocess of 1018 carbon steel in alkaline sour environment, solution prepared specifically to mimic the sour waters occurring in the catalytic oil refin ery plants of the Mexican Oil Company (PEMEX) (0.1 M (NH4)(2)S and 10 ppm N aCN at pH 9.2) were prepared and characterized. The surface states of the c arbon steel were formed by treating the surface with cyclic voltammetry at different switching potentials (Egimel+), commencing at the corrosion poten tial (E-corr = -0.890 V vs sulfate saturated electrode, SSE). The surface s tates thus obtained were characterized using electrochemical impedance spec troscopy and scanning electron microscopy techniques. It was found that for Egimel+ = -0.7 and -0.6 V vs SSE a first product of corrosion formed, char acterized by a high passivity. Moreover, it was very compact (with a thickn ess of 0.047 mum). However, at more anodic potentials (Egimel+ > -0.5 V vs SSE) a second corrosion product with non-protective properties (porous with a thickness of 0.4 mum and very active) was observed. The diffusion of ato mic hydrogen (H-0) was identified as the slowest step in the carbon steel c orrosion process in the alkaline sour media. The H-0 diffusion coefficients in the first and second products that formed at the carbon steel-sour medi um interface were of the order of 10(-15) and 10(-12) cm(2)/s respectively. (C) 2001 Elsevier Science Ltd. All rights reserved.