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The general features of the a-c impedance at a metal-electrolyte interface as a function of frequency are depicted, and recently developed measurement techniques are described. It is established that at low and very low frequencies, down to the millihertz domain, relaxation phenomena are found in those cases where the assumptions used in Stern's derivation are not met. Under such conditions polarization resistance techniques can hardly be used owing to experimental and theoretical reasons. Charge transfer resistance, when used instead of polarization resistance in a Stern's type equation, is proved to overcome these difficulties on the basis of a theoretical derivation. Correlation between charge transfer resistance and corrosion rate is illustrated by practical examples of pure iron and anodized aluminum alloys.