Improved corrosion resistance of ZrO2/MgO coating for magnesium alloys by manipulating the pore structure

Magnesium (Mg) alloys are popular among structural materials due to their high specific strength and good machinability. However, poor corrosion resistance limits its application in harsh environments, which requires ceramic coating to overcome the shortage of corrosion sensitivity. In this work, ZrO2/MgO ceramic coating was in situ synthesized by plasma electrolytic oxide (PEO) technology, coatings with different pore structures were fabricated by adjusting the pH of the electrolyte, and X-ray computed tomography (XCT) is used to quantitatively investigate the pore structure in ceramic coating. As the pH of the electrolyte increases from 3 to 12, the Zr(OH)4 colloidal particles from electropositive to electronegative charges. Electropositive Zr(OH)4 moves to the anode by diffusion, while electronegative Zr(OH)4 participates in the growth of the coating by electromigration. Due to the electromigration mode at pH = 12, the ZrO2 deposited around the pores caused a change in the pore structure from interconnected to isolated, which resulted in a reduction of the overall porosity to 8.06%. Isolated pores effectively prevent the penetration of corrosive media, resulting in the corrosion current density (icorr) being nearly 46 times lower than the coating with pH = 3. This study presents a novel strategy of adjusting pore structure to enhance the corrosion resistance of ZrO2 ceramic coating, which may have implications for various applications.