Synergetic enhancement of corrosion resistance in ZM2 magnesium alloy via a hybrid cold spraying/micro-arc oxidation duplex coating

Abstract

In this study, a duplex coating consisting of an initial cold-sprayed (CS) Al underlayer and a subsequent micro-arc oxidation (MAO) top-layer was developed on a ZM2 cast Mg alloy substrate to enhance its corrosion resistance. The role of the CS Al underlayer in regulating the microstructural evolution, phase transformation, and corrosion behavior was systematically investigated. Surface polishing of the CS (CS(P)) layer reduces roughness and homogenizes the electric field during the following MAO treatment, thereby suppressing localized micro-discharges and promoting the formation of a denser and more uniform ceramic layer. The analyses reveal that the MAO layer consists γ-Al₂O₃ nanocrystals embedded in an amorphous matrix. This kind of structure effectively minimizes the pore formation and improves the coating integrity of the MAO layer. Electrochemical measurements demonstrate that a significant enhancement in corrosion resistance has been achieved. The corrosion current density decreases by 3 orders of magnitude, while the polarization resistance increases to 2.38 × 10⁵ Ω·cm², approximately 280 times higher than that of the bare ZM2 substrate. During 14-day long-term immersion, the CS(P)+MAO duplex coating exhibits exceptional stability, with a negligible cumulative hydrogen evolution (0.65 mL·cm⁻²) and a superior corrosion rate (0.124 mm·y⁻¹). Surface characterizations of the CS(P)+MAO duplex coating reveal no evident macroscopic pitting or microcracks. The excellent corrosion resistance of the CS(P)+MAO bilayer coating arises from the synergistic protection provided by a dense microstructure, the physical barrier of the CS Al underlayer, and the chemical stability and high interfacial resistance of the γ-Al₂O₃-rich ceramic phase in the MAO top-layer.