Electrochemical behavior of plasma electrolytic oxide coatings on rare earth element containing Mg alloys

In this work, ceramic-like oxide coatings were produced on rare earth element containing Elektron21 (E21) and WE43 Mg alloys using plasma electrolytic oxidation (PEO) process, and in addition to these alloys, AZ31B Mg alloy was used for comparison studies. Surface morphology and chemical composition of coatings were determined using scanning electron microscope (SEM) equipped with energy dispersive X-ray spectrometer (EDS). Phase analysis of coatings was carried out by X-ray diffraction (XRD). Electrochemical corrosion tests were undertaken by means of potentiodynamic polarization and electrochemical impedance spectroscopy techniques in 3.5% NaCl solution at ambient temperature. SEM results showed that the coatings have two layered microstructure with a porous top layer and a dense barrier layer. The SEM images of E21 and WE43 indicated that intermetallic compounds contributed into the coating structure during PEO processing. Electrochemical corrosion test results showed that the corrosion rates of PEO-coated substrates greatly decreased when compared to that of bare Mg alloys. However, bare rare-earth containing alloys showed increased corrosion resistance compared to bare AZ31B alloy.