Construction and anticorrosive mechanism of external cation/internal anion-selective bipolar magnesium alloy MAO/WPU coatings

Magnesium alloy micro-arc oxidation (MAO) film/waterborne polyurethane (WPU) composite coating has been widely applied in the automotive, electronic communication, and aerospace fields because of high corrosion resistance, environmental friendliness, and low cost. Although the high corrosion resistance of magnesium alloy MAO/WPU coatings mainly depends on their compactness, the ion selectivity of the coating determines the migration rate of corrosive medium and thus significantly affects corrosion behavior of magnesium alloy beneath the coating. In view of this, a novel MAO/WPU composite bipolar coating with inner anion-selective and outer cation-selective was constructed on the magnesium alloy for enhancing its anti-corrosion performance. Through regulating the ion selectivity of the MAO film and the WPU coating respectively, two types of external cation/internal anion-selective and external cation/internal cation-selective coatings were successfully prepared. Corrosion resistance of the former is significantly superior to the latter through electrochemical impedance spectra measurement in the salt-water immersion accelerated test. To further elucidate the influence of ion selectivity on the corrosion resistance, the I-V curve characteristics are analyzed and ion-transporting behavior of the bipolar coatings is revealed, whose outer-cation/inner-anion structure resembles that of a semiconductor p-n junction, exhibiting unidirectional cation conduction and anion blocking. The EDS elemental analysis results confirm that coating failure is mainly driven by localized Cl⁻ enrichment. This work offers a novel insight into improving the corrosion resistance of anti-corrosion coating by governing its ion selectivity.