Degradation mechanism of electrodeposited coatings in alkaline solution

Abstract

Electrodeposited organic coatings are used as primers or as a single-layer coating for corrosion protection of domestic appliances. In this application, it is important to avoid any possible coating degradation that could be caused by detergent solution which may accidentally come in contact with the coating. These kinds of coatings are often tested by immersion of the coated metal sheet with a cross scribe in a detergent solution (pH 11, temperature 74 °C) for about 10 days, measuring the extent of delamination away from the scribe at the end of the test. In these tests, a high dispersion of data is noted: specimens that have been prepared to be equivalent show a range of delamination from almost zero to very high levels, unacceptable for the application. The aim of this work is to define the reasons for the anomalous and premature failure of some specimens. The experimental work was carried out using mainly electrochemical techniques and surface analysis. A mechanism of delaminative degradation has been proposed, based on the chemical conversion layer (zinc phosphate) of the steel substrate, the porosity of the organic coating (dependent on the deposition conditions) and the particular environment (high temperature and pH). The final results reveal the cause of the problem and allow a definition of a testing approach, based on electrochemical impedance measurements, capable of checking the coating quality during production. This test is conducted in well controlled conditions and with a simplified data analysis procedure.