Designing for the chemical conversion coating on AZ91 magnesium alloy by equalising interface potential difference

The formation of intermetallic compounds leads to heterogeneous chemical compositions and galvanic corrosion in alloy materials, which are widely recognised as primary contributors to non-uniform conversion coatings and poor corrosion resistance. This study implements a strategy aimed at regulating surface chemical composition and equalising interfacial potential differences to develop an Al-Mg-V chemical conversion coating on AZ91 magnesium alloy. The surface characteristics and corrosion resistance of the coating are investigated. The results demonstrate that the coating integrates the discretely distributed intermetallic compounds (Mg ₁₇Al ₁₂) into a continuous and stable cathode phase. The framework structure composed of Mg and Al effectively equalises the interfacial potential differences, reducing the potential difference from 300 mV to 40 mV. The coating enhances the polarization resistance of the AZ91 substrate by approximately 400 times and reduces the self-corrosion current density by three orders of magnitude. After a 186-h neutral salt spray test, no corrosion is observed. The corrosion inhibiting structure formed by V hydroxides/oxides fill the pores and acts as a passivator on the surface, and imparts self-repair capabilities to the coating. The design concept of this chemical conversion coating is expected to be applied to other alloy materials.