Effects of distance between anode and cathode on properties and load model of micro-arc oxidation coatings on magnesium alloys

The micro-arc oxidation coatings were prepared on AZ31B magnesium alloys in silicate-containing electrolyte at different anode-cathode distances. The thickness, microstructure, elements and phase composition of the coatings were measured by TT260 digital eddy current thickness gauge, SEM, EDS and XRD. The surface porosity of the coatings was analyzed by Image-J, and the corrosion resistance of the film was investigated by electrochemical experiments. Based on the waveform diagram, the equivalent circuit corresponding to the micro-arc oxidation system was established, and the equivalent load value was calculated to analyze the relationship between equivalent load value and corrosion resistance of the film. The results show that, with the increase of anode-cathode distances, the number of micro-pores increases while the size of micro-pores decreases, and the porosity gradually decreases from 26% to 19%. The film thickness decreases generally with the increase of the distance, but increases slightly at the distance of 200 mm. While the corrosion resistance of the film shows a tendency to become better first and then worse, that is, the corrosion resistance of the coating is the worst at the distance of 10 mm, and the corrosion resistance is the best the distance of at 200 mm. Their corrosion current density differs by three orders of magnitude. The calculated equivalent resistance R1 has a corresponding relationship with the corrosion resistance of the coatings. The greater the equivalent resistance R1, the better the corrosion resistance, which can be used to evaluate the corrosion resistance of the film layer on site. © 2020, Science Press. All right reserved.