Growth mechanism and corrosion resistance of layered double hydroxide film on magnesium alloy without external addition of magnesium and aluminum salts

This study first reports in-situ growth of layered double hydroxide (LDH) coatings on magnesium alloy surface without external adding magnesium (Mg2+) and aluminum (Al3+) salts with the aid of a chelating agent at ambient pressure and relatively low temperature (< 95 °C). The morphology, composition, and structure of the deposits on the magnesium alloy surface were analyzed using scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), etc. Combined with open-circuit potential (OCP)-time curves, the mechanism of growth and formation process of the LDH film was proposed. The process was divided into four stages, i.e., dissolution of the Mg substrate, formation of Mg(OH)2, nucleation of LDH by transformation of Mg(OH)2 phase, and growth of LDH phase. The corrosion resistance of the LDH coating before and after treatment with vanadate and tungstate was evaluated through electrochemical impedance spectroscopy (EIS), Tafel curves, and immersion measurements. Compared to bare magnesium alloy and pristine and vanadate-treated LDH, the LDH coating modified with tungstate exhibited the most effective corrosion protection for magnesium alloy AZ31, without obvious signs of corrosion after exposure to a 3.5 wt.% NaCl solution for seven days.