High quality surfaces of magnesium alloy AZ31 by adjusting appropriate electropolishing parameters

• Electropolishing in a phosphoric acid-ethanol electrolyte in the transient and passive region enables the smoothest magnesium surfaces. • With increasing water content in the electrolyte, a white phosphate-containing surface film forms on the magnesium surface. • By reducing the water content in the electrolyte, the suitable potential range can be extended to the active and transpassive region. • The ideal potential range can be easily recognized during electropolishing, as the formation of gas bubbles on the surface is reduced to a minimum. Magnesium alloy AZ31 is a light material with a good mechanical stability and is used in various engineering applications. Although its tendency to localized corrosion is a limiting factor in its use. Electropolishing is a widely used process for improving the surface roughness and corrosion behavior of metals. However, there is a lack of knowledge about the electropolishing of magnesium and its alloys. In this study, an optimal electropolishing process for AZ31 was developed to improve the surface properties by varying the electrolyte concentration and the applied potential. The electrolyte composition was a mixture of phosphoric acid, ethanol and deionized water. The applied potentials were selected based on measured current density potential curves. Thereby, electropolishing was performed up to an electric charge of 18 As. The experimental results indicate that the electropolishing process should be carried out at a low current density to avoid bubble evolution and surface defects. Therefore, the concentration of the electropolishing electrolyte should have an appropriate low conductivity, and the applied potential should be in the transient or passive region of the polarization curve recorded prior to electropolishing. It could be shown that an optimized electropolishing process improved the surface of AZ31 by providing a bright and mirror-like surface and a lower roughness compared to a mechanically ground surface.