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A freely corroding metal may be represented as an electrode under the simultaneous influence of at least two electrochemical reactions, one anodic and one cathodic. If the corroding medium is mildly acidic, aerated water there are two cathodic reactions of significance, reduction of water and reduction of oxygen, while additional anodic reactions must be considered if the metal has multiple oxidation states or contains alloying ingredients or if its potential is sufficiently positive to oxidize water. The separate behavior of these reactions may be delineated by polarizing the electrode and measuring the current that passes as a function of the potential. To a good approximation these reactions may be regarded as independent. In this paper the current carried by a single electrode reaction at a given potential is expressed by a system of equations that incorporate rate control by electron transfer, diffusion, and passivation, and correct for ohmic loss in the electrolyte; the total current passed by the electrode and measurable in the external circuit is expressed as a simple algebraic sum of these values. Features of illustrative polarization curves (potential versus log current) are explained in the context of this model, and the technique is applied to two experimental systems, one entailing seven reactions with twenty-four parameters which were fit to the model via a regressive procedure.
electrode, model, polarization, multi-reaction
University of Connecticut, Storrs, CT
Materials Science Laboratory, GTE Labs. Inc., Waltham, MA