Improving the Corrosion Resistance of Magnesium Alloy AZ31 by a Duplex Anodized and Sol-Gel Coating

Magnesium and its alloys have found application in several areas viz., aerospace, electronics industry, and automobile, owing to their specific strength-to-weight ratio, thermal conductivity, electromagnetic shielding, etc. However, poor corrosion resistance limits their applicability for widespread use. Anodization and micro arc oxidation (MAO) are commonly used to enhance the corrosion protection of magnesium alloys. The anodized or MAO layers are porous, and hence, organic sealants are conventionally used for sealing the pores. There are not many reports on the use of organic-inorganic hybrid coatings as sealants on anodized layers of magnesium alloys. In this study, the hybrid sol-gel silica-based coatings were investigated as sealants on anodized magnesium alloy AZ31 substrates. A thick (∼20 µm) and porous oxide layer was deposited by anodization using sodium silicate as electrolyte. Ambient curable hybrid silica coatings with and without corrosion inhibitors like cerium oxide and 8-hydroxyquinoline were deposited by dip coating method. The anodized layer and the duplex coating were analyzed using X-ray diffraction and field emission scanning electron microscopy with energy-dispersive X-ray analysis (EDAX) attachment. Electrochemical impedance spectroscopy and potentiodynamic polarization studies were used to assess the corrosion protection performance of these coatings in 0.61M NaCl solution. The porous anodized magnesium oxide layer improved the binding strength of the adjacent sol-gel layer; the sol-gel layer physically sealed the porous anodized coating and offered higher corrosion protection. The duplex coating prevented the diffusion of aggressive ions onto the magnesium alloy AZ31 and enhanced the corrosion resistance significantly.