Citrate gel conversion coating on AZ31 magnesium alloys

A citrate gel conversion system was developed in this study. The citrate gel, which was obtained by adding ethanol into sodium citrate aqueous solution, was dissolved in glycerin to form the conversion solution. After the consecutive steps of roll coating and heating, a citrate gel conversion coating was made on AZ31 magnesium alloys. Microstructure characterization showed that the citrate gel coating was thin and free of cracks resulting from dehydration. As evaluated by electrochemical analyses, this coating apparently reduced the corrosion current density and enhanced the polarization resistance. With the addition of copper sulfate, copper-containing clusters were formed on top of the coating due to the replacement reaction between magnesium substrate and cupric ions, and resulted in a further increase in both the corrosion potential and polarization resistance.