Preparation of Superhydrophobic Anti-Corrosion Coatings on Magnesium Alloy Surfaces via Phosphating-Hydrothermal Process

In order to enhance the corrosion resistance of AZ91D magnesium alloy in chloride media, a superhydrophobic composite coating was developed through a process involving zinc-based phosphating, hydrothermal treatment, and stearic acid modification. Initially, a zinc-based phosphating film was prepared in a phosphating solution comprising Na 3 PO 4 12H 2 O, ZnSO 4 7H 2 O, NaNO 2 , and NaF. Subsequently, micro-nanostructures were grown in an alkaline hydrothermal system composed of ZnSO4 7H2O and sodium citrate. The formation of a low-surface-energy layer was achieved through the process of stearic acid impregnation. SEM, XRD, XPS, and EDS analyses revealed a multilevel rough surface morphology composed of radial petal-like and plate-like structures, primarily consisting of ZnO, Mg(OH) 2 , and zinc/magnesium stearate. Wetting tests demonstrate that the contact angle of the coating firstly increases and subsequently decreases in accordance with rising ZnSO 4 concentrations. At a concentration of 0.04 mol/L, the contact angle reaches 160.6°, with a rolling angle of approximately 7°, thereby achieving a typical superhydrophobic state. Electrochemical impedance spectroscopy indicates that the composite coating enhances charge transfer resistance by approximately seven orders of magnitude compared to the bare AZ91D sample, with low-frequency impedance |Z| reaching 109 Ω cm 2 . Following a 168-h immersion in a 3.5 wt.% NaCl solution, the mass loss of the coated sample was found to be negligible, the solution pH remained essentially unchanged, and the surface Cl content after corrosion was only about 0.1%, a result that significantly outperforms that of the substrate. The system has been demonstrated to provide highly efficient and stable corrosion protection for magnesium alloys through the synergistic action of Mg(OH) 2 sealing and the hydrophobic stearate layer.