Effect of Coating Current Frequency on Corrosion Behavior and Microstructure of PEO Coated AZ31 Magnesium Alloy

In recent years the Plasma electrolytic oxidation (PEO) coatings on Mg, Ti and Al substrates are widely investigated. One of the important parameters which affects these coatings properties is the electrical current which used in the process. However, in this study the effect of frequencies over 1000Hz on corrosion behavior and microstructure of PEO coated samples of Mg AZ31 was investigated. The samples have been coated in a basic bath with 10g/l Na3PO4 and 2g/l KOH under DC pulsed unipolar current with 12% duty cycle in 1000, 2000, 3000Hz frequency. Microscopy investigations of surface and cross sections of samples showed no significant difference between three samples. Also, the X-Ray Diffraction studies showed Mg and MgO as dominant phases of all coatings and there were no phosphate containing phases in the coatings. The electrochemical tests including OCP, Potentiodynamic polarization and EIS showed superior corrosion resistance for 2000Hz sample and it concluded that this sample may be the optimum specimen.