Citrate gel conversion coating on AZ31 magnesium alloys

• A non-aqueous citrate gel conversion coating system on AZ31 Mg plates is developed.
• Citrate gel conversion coating on AZ31 Mg plate is nearly free of cracks.
• Citrate gel conversion coating exhibits an enhanced corrosion resistance.
• Citrate gel conversion coating formed with cupric ions shows obvious passivation.

A citrate gel conversion coating on AZ31 magnesium alloy is developed. The conversion solution is composed of glycerin and the citrate gel, which is isolated by adding ethanol into sodium citrate aqueous solution. Microstructure characterization shows that the coating is about 200 nm thick and free of cracks. This coating markedly reduces the corrosion current density and enhances the polarization resistance in potentiodynamic polarization and elevates the total impedance in electrochemical impedance measurement. Moreover, the addition of copper sulfate results in the formation of copper-rich clusters on the coating and an increase in the corrosion potential.