ELECTROCHEMICAL STUDIES ON CORROSION UNDER A WATER FILM

The corrosion mechanism and corrosion rate were studied as a function of the water film thickness using Kelvin probe (non-contacting reference electrode) and impedance methods. Cathodic polarization curves were measured at different film thicknesses on copper, platinum, iron and gold electrodes. The diffusion-limiting current of oxygen increased with decreasing thickness to around 20-30 mu m, and then decreased with decreasing thickness. The corrosion rate of copper as a function of the water film thickness was measured by the impedance method. It was confirmed that the rate is constant for a thick film, increases with decreasing thickness to give a maximum and then decreases, as reported by Tomashov. The water film thickness was measured by the solution resistance of the water film. Using the impedance method, the corrosion rates of steels were monitored during wet and dry cycles. The importance of monitoring the corrosion rate, during evaluation or conventional corrosion tests, is illustrated by the results.