Corrosion Inhibition and the Zero Charge Potential of Metals

Abstract

THE electrode potential of a mild steel specimen immersed in sodium chloride solutions has been measured in conditions of very rapid aeration produced by bubbling air vigorously over the specimen. Corrosion is rapid, but the potential does not fall to the values observed in conditions of moderate aeration. It rises to a maximum value that can be maintained for an appreciable time even though corrosion continues at a rapid rate. (This maximum potential must be related to the maximum rate of cathodic depolarization as limited by the rate of diffusion of oxygen across the boundary layer on the surface of the metal.) The relationships between potential, chloride concentration and surface preparation of the steel are specific and reproducible and will be described elsewhere. At lower rates of aeration, lower (less-positive) steady potentials can similarly be maintained. This method of potential stabilization was used to test the theory put forward by De¹ for inhibition in neutral and acid solution. De suggested that inhibitive anions are adsorbed preferentially on metal/solution interfaces that are positive to the zero charge potential (potential of electrocapillary maximum) of the metal, that is, he suggested, on cathodic surfaces in neutral solution. Evidence for such preferential adsorption has been found by Hoar and Holliday², by Antropov et al. ³ and others for inhibition in acid solutions; but so far as I am aware no direct evidence exists for its validity in neutral solutions.