Hydrothermal Preparation of Calcium Aluminum Corrosion-Resistant Coatings on AZ91D Magnesium Alloy

Magnesium alloys are widely used in all kinds of fields because of their excellent mechanical properties, but their application has been prevented by poor corrosion resistance. In this paper, Mg(OH)2-Ca(OH)2/Al(OH)3/Al2O3 composite coatings with long-term corrosion resistance were fabricated on the surface of Mg alloys using the hydrothermal method. Among them, the calcium hydroxide/calcium nitrate–alumina coating successfully filled the cracks in the magnesium hydroxide coating. Meanwhile, we explored the influences of different heating times and temperatures on the coating and analyzed its composition. After immersing the coating in a 3.5% NaCl solution for 168 h, only a small portion of the surface dissolved. Electrochemical test results indicated that the corrosion potential and corrosion current density of the coating increased by three orders of magnitude, significantly improving corrosion resistance in comparison to bare samples. Adhesion tests showed that the coating exhibited good bonding performance to the substrate. This method features a simple, pollution-free preparation process and does not require complex instrumentation, thereby enhancing the longevity of the magnesium alloy.