Rapid Formation of a Superhydrophobic Surface on a Magnesium Alloy Coated with a Cerium Oxide Film by a Simple Immersion Process at Room Temperature and Its Chemical Stability

We have developed a facile, simple, time-saving method of creating a superhydrophobic surface on a magnesium alloy by a simple immersion process at room temperature. First, a crystalline CeO2 film was vertically formed on the magnesium alloy by immersion in a cerium nitrate aqueous solution for 20 min. The density of the crystals vertically with respect to the magnesium alloy increased with increasing immersion time. Next, the film were covered with fluoroalkylsilane (FAS: CF3(CF2)7CH2CH2Si(OCH3)3) molecules within 30 min by immersion in a toluene solution containing FAS and tetrakis(trimethylsiloxy)titanium (TTST: (CH3)3SiO)4Ti). TTST was used as a catalyst to promote the hydrolysis and/or polymerization of FAS molecules. The FAS-coated CeO2 film had a static contact angle of more than 150°, that is, a superhydrophobic property. The shortest processing time for the fabrication of the superhydrophobic surface was 40 min. The contact angle hysteresis decreased with an increase in the immersion time in the cerium nitrate aqueous solution. The chemical stability of the superhydrophobic surface on magnesium alloy AZ31 was investigated. The average static water contact angles of the superhydrophobic surfaces after immersion in the solutions at pH 4, 7, and 10 for 24 h were found to be 139.7 ± 2, 140.0 ± 2, and 145.7 ± 2°, respectively. In addition, the chemical stability of the superhydrophobic surface in the solutions at pH ranging from 1 to 14 was also examined. The superhydrophobic surfaces had static contact angles of more than 142° in the solutions at pH ranging from 1 to 14, showing that our superhydrophobic surface had a high chemical stability. Moreover, the corrosion resistance of the superhydrophobic surface on the magnesium alloy was investigated using electrochemical measurements.