Microarc Electrodeposition of Ceramic Films on Double Electrodes of AZ91D Magnesium Alloy by Symmetrical AC Pulse Method

Abstract

Using the two direction symmetrical alternating current pulse method, ceramic films were successfully electrodeposited simultaneously on anodic and cathodic AZ91D magnesium specimens by microarc oxidation. This process was named double electrode microarc electrodeposition (DEME). The thickness, morphology and composition of the DEME films were studied using SEM and X-ray diffraction patterns. The rare earth element Ce was detected in both the anodic and cathodic films when CeF₄·H₂O was added to the film forming an electrolyte. The improvement in corrosion resistance of AZ91D after DEME treatment was shown by comparing potentiodynamic scanning polarisation curves and the electrochemical impedance spectroscopy of the specimens with and without DEME treatment. By immersing the samples with and without DEME treatment in 3.5%NaCl solution, it was found that the corrosion rate of specimens treated using the DEME process was much lower than that of untreated specimens. In order to reduce their porosity, samples with DEME films were further painted with three different kinds of coatings: sol-gel, polysilicone and epoxy lacquer. Samples coated with DEME film and epoxy coatings could sustain a 336 h neutral salt spray test.