Galvanized steel surfaces can be protected against corrosion in natural waters by a superficial coating of zinc diphosphate, obtained by the partial oxidation of zinc in a solution containing diphosphate ions. Impedance measurements were used to study the improvement in corrosion protection resulting from this layer and to develop a quality control method. The mechanism of both anodic and cathodic processes remains the same in the presence of the coating as in its absence, so that the relationshipR tIcorr=constant is valid (R t is charge transfer resistance;I corr is intensity of the corrosion current). Thereby, a protection factor, defined as the ratio of the corrosion currents in the absence and in the presence of the coating, can be estimated from the corresponding charge transfer resistances determined experimentally from impedance diagrams. Impedance data obtained on several test-pieces are in good agreement with the coating morphology, as shown by direct scanning electron microscope (SEM) observations, leading to a simple criterion for quality control.