Dependence of corrosion potential and corrosion rate of a low-alloy steel upon depth of aqueous solution

The Kelvin vibrating-capacitor technique has been used to measure the corrosion potential and the oxygen-consumption method has been applied to measure the corrosion rate of a low-alloy steel under very thin layers of electrolyte in air. The layer thickness dependencies of the corrosion rate and of corrosion potential are due to kinetic control by oxygen diffusion, and the latter dependence permits the Tafel slope of the anodic dissolution reaction to be evaluated. The anomalously large value of the Tafel parameter may be understood in terms of partial passivation of the metal surface by the intermediate reaction product, FeOH. This signifies a distinct difference from corrosion in the same system under bulk solutions.