Comparative investigation of Cu ion and adipic acid addition on electrochemical and microstructure characteristics of vanadium conversion coating on AZ31 Mg alloy

In this study V-based conversion coating with and without modification was developed to enhance the anti-corrosion resistance of AZ31 Mg alloy. Copper (Cu) and adipic acid (AA) were used as inorganic and organic additives and their roles on electrochemical and microstructural properties of conversion coating were investigated and followed with thorough discussion. Field emission scanning electron microscope (FE-SEM), X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) were utilized in order to characterize surface of the coatings. In addition, the electrochemical behavior was evaluated by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). FE-SEM and AFM images showed that the surface of vanadium conversion coating (VCC) contained several micro-cracks and the addition of Cu produced more compact and finer surface. XPS results indicated the precipitation of V2O5, VO(OH)3, V(OH)5, VO2 and Mg(OH)2 on the surface of three different conversion coatings. According to the electrochemical results, the highest corrosion resistance was obtained for Cu modified conversion coating (polarization resistance (Rp) = 14,350 Ω·cm2) that displayed significant improvement in corrosion resistance of bare substrate (polarization resistance (Rp) = 253 Ω·cm2).