The synthesis of electrode polarisation curves

Abstract

By summing the currents for the anodic and cathodic reactions which can occur on an electrode as a function of potential, it is possible to model the typical log ( i) vs potential polarisation curves for an electrode. The effects of the solution chemistry in the electrolyte, such as pH, ambient oxygen pressure and ionic concentrations, can be incorporated via the changes in the appropriate Nernst equation. The electrode reactions considered are the oxidation of the electrode metal, the deposition from the dissolved metal ions, the oxidation of hydroxyl ions or of water to give oxygen gas, the reduction of hydrogen ions or of water to form hydrogen gas and the reduction of dissolved oxygen gas. The oxidation of the metal is itself considered in terms of the dissolution to a hydrated soluble cationic species, the formation of a passivating species on the anode and the breakdown of the passive film by pitting, by the formation of a soluble higher oxidation species of the metal or by the breakdown of water. Single electrode reactions are discussed for (1) the effects of pH and dissolved oxygen concentration on the corrosion of an iron electrode, (2) the pitting corrosion of aluminium and (3) the electrodeposition of zinc. The model allows multi-electrode systems to be examined and the area of each electrode to be varied. The present work is restricted to two-electrode systems such as (1) the corrosion of the iron/zinc couple as a function of pH and electrode area and (2) a differential aeration cell. In this case, the variables are the areas of the electrodes, the differing oxygen concentrations and the changes in the ferrous ion concentration as the process proceeds. The effects of introducing an ionic resistance, through for example a paint film, are studied and it is shown that there is a critical resistance for each system, of the order of 10 5 to 10 6 Ω, below which there is no effect.