Use of DC Electrochemical Methods for Evaluation of Paint Films on Steel, Aluminum, and Zinc^☆

Degradation of paints that were applied on steel, aluminium, and zinc and exposed to air-saturated seawater has been studied by using direct current (DC) electrochemical methods and by visual examination of long-term exposed specimens. The DC measurements were performed on surfaces with a continuous paint film, and the long-term specimens were polarized by a defect in the paint film. The DC measurements on coated aluminium, steel, and zinc have been found to give information about the mechanism of corrosion. The results illustrate the importance of the properties of the metal substrate and of the paint film. Coated steel galvanically coupled to bare steel was found to have a high potential and, hence, to act as a cathode vs bare steel. For coated aluminium, however, initiation of corrosion at the metal/coating interface was found to give a much lower potential. The behavior of coated zinc was found to be similar to that of coated steel. The potential was high compared to bare zinc, which means that coated zinc acts as cathode. The results from the long-term exposure test were found to be in accordance with the results from the short-term tests for all three substrates with respect to the mechanism of corrosion.