Cathodic polarization of variously corroded iron electrodes in acid perchlorate solutions

Abstract

The steady-state cathodic reaction taking place on iron electrodes in deaerated, acid perchlorate solutions free of iron salts in the hydrogen evolution reaction. However, reduction of corrosion products formed during preceding anodic polarization will also occur in various potential regions. Cathodic polarization diagrams recorded with rotating disc electrodes ( rde) show three reaction regions. In region I, starting below the corrosion potential, the reaction is influenced by pH of the solution and by potential. This is followed, at lower potentials, by region II, where the reaction is limited by diffusion of hydrogen ions to the electrode. The limiting cd is determined by pH of the solution and by the speed of rotation of the rde. At still lower potentials, in region III, there is a further increase of cd due to discharge of water molecules. This reaction is influenced by potential, but not by pH or rde rotation rate. Experiments lasting a few minutes demonstrate that the behaviours in the regions I and II depend on the type of preceding anodic polarization; the alternative effects will be cathodic peak in region I, attributed to the reduction of a previously formed oxidic iron compound approaching the composition of magnetite, or an additional contributon to the cd due to reduction of a different iron compound and in excess of the quantity accounted for the hydrogen evolution. A detailed, rather novel and powerful type of analysis of the reaction recorded in region I indicates a low and constant coverage by adsorbed hydrogen atoms giving rise to a parallel combination of a Volmer—Tafel mechanism (the minor part) and a Volmer—Heyrovsky mechanism (the major part) for the hydrogen evolution reaction.