Corrosion Potential and Corrosion Rate of Low-Alloy Steel under Thin Layer of Solution

Corrosion potential and corrosion rate of a low-alloy steel covered with a thin solution layer have been examined mainly in air using the Kelvin probe apparatus and a pressure sensor. Corrosion potential, Ecorr(V vs SHE), and corrosion rate, icorr (A·m−2), increase with decreasing thickness of the solution layer, h, in the thickness range between 200×10−6 m and 10×10−6 m, as follows:(This article is not displayable. Please see full text pdf.) \ oindentThese observed changes of Ecorr and icorr are attributed to the varied oxygen reduction rate with h which corresponds to the thickness of the oxygen diffusion layer. When h becomes lower than 10×10−6 m, the corrosion product formed on the steel surface suppresses steel dissolution; so that icorr decreases and Ecorr stays at a certain value. At h of 0.1×10−6 m, the Ecorr jump-up was observed. This might be because the solution is no longer in the form of a thin layer but in the form of fine droplets; if this is the case then Ecorr reflects a mixed potential of corrosion potential under solution droplet and work function of the water adsorbed steel surface. Ecorr measurements in nitrogen supports the above understanding of the results obtained in air.