Preparation of superhydrophobic silica film on Mg–Nd–Zn–Zr magnesium alloy with enhanced corrosion resistance by combining micro-arc oxidation and sol–gel method

To improve the corrosion resistance, a superhydrophobic silica film on Mg–3.0Nd–0.2Zn–0.4Zr (wt.%, NZ30K) magnesium alloy was successfully prepared by the combination of micro-arc oxidation (MAO) and sol–gel method using tetraethoxysilane (TEOS) and methyltriethoxysilane (MTES) as precursors. The MAO layer with highly porous surface was formed as an intermediate layer to ensure the excellent adhesion of superhydrophobic silica film to the substrate. The parameters of the sol–gel process such as molar ratios of TEOS/C2H5OH and MTES/TEOS had significant effects on the surface morphologies and water-repellent properties of the silica films. A superhydrophobic silica film with static contact angle of 151° was obtained as the molar ratios of TEOS/C2H5OH and MTES/TEOS were 1/30 and 1/2 respectively. It is shown that the superhydrophobic silica film can significantly improve the corrosion resistance of NZ30K magnesium alloy by nearly three orders of magnitude by analysis of the potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) tests in 3.5 wt.% NaCl solution. 168 h immersion test further demonstrates that the superhydrophobic silica film can effectively provide long-term corrosion protection for NZ30K magnesium alloy. In addition, the superior corrosion protection performance of the superhydrophobic silica film compared with that of the pure silica film without the addition of MTES can be attributed to the good coverage of hydrophobic methyl groups on its surface, which could effectively decrease the direct contact surface to corrosive medium.