Comparison in characterization of composite and sol-gel coating on AZ31 magnesium alloy

An Al2O3 protective coating on magnesium alloy AZ31 was prepared by a repeated direct sol-gel process annealing at 300 °C and a composite coating was also deposited using Al2O3 particles dispersed sol followed by phosphating treatment and annealing at 300 °C. The morphologies, structures and critical adhesive loads as well as corrosion properties of the coatings were comparatively investigated by scanning electron microscopy (SEM), X-ray diffractometry (XRD), nanoscratch test and electrochemical measurement. The results show that the composite coating has a more uniform, crack-free layer and improved adhesion to the substrate as compared with that of the repeated direct sol-gel coating owing to its lower heat strain. The main phases in both coatings consist of γ-Al2O3 and δ-Al2O3 derived from annealed alumina sol, and the composite coating has an anticorrosion performance which is superior to that of the repeated direct sol-gel coating.