It is shown that the inhibiting action of sodium chromate on localized corrosion of aluminum exposed to a sodium chloride solution is mainly the result of polarization of the local anodes. Sodium chromate would be an even more effective inhibitor of corrosion in this case if it did not depolarize the local cathodes. In order to obtain dependable information on the mechanism of inhibitor action, the influence of the inhibitor on the behavior of the local cells must be studied. Polarization curves obtained by the use of currents from an external source are of value in studying the mechanism of inhibitor action only if information on local cell behavior can be derived from them. Two indirect methods of estimating the anodic and cathodic polarization curves of the local cells are described. These methods make it possible to obtain the local cell polarization curves without the necessity of locating or isolating the local anodes and cathodes. The practical application of these methods is limited by the assumption that the areas of the local anodes and cathodes are not altered by the application of external current. One of the two methods has additional limitations. The relationship of "reversible" potentials of metals as calculated from free energy data and as measured in aqueous solutions is discussed. It is indicated that polarization plays an important role in the measurement of potentials commonly accepted as reversible. Agreement between potentials calculated from free energy data and measured "reversible" potentials is dependent upon local cell action of a specific nature. Departure from this specific local cell action accounts for the fact that the measured value of the apparent single potential does not, in most cases, correspond to the value calculated from free energy data.
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Mears et al. (1950) studied this question.