Study on the growth mechanism and properties of Zn-Al-Mg alloy thermal diffusion coatings

Abstract

Zn-Al-Mg powder co-diffusion was performed on Q235 steel using a mechanical energy-assisted diffusion process. The resulting ZAM alloy diffusion coating comprised an inner FeZn layer composed of δ(FeZn₁₀) and Γ(Fe₁₁Zn₄₀) phases, while the outer layer exhibited a multiphase structure distributed discontinuously in island-like or dot-like patterns on the δ-phase surface. The multiphase structure exhibits layered characteristics extending outward from the δ phase: an Al-rich layer composed of Al₂O₃ and FeAl phases; a Mg-rich layer interwoven with MgAl phases, MgZn phases, and MgO; and an amorphous ZnO layer. The growth mechanism is investigated by this study, revealing that during the primary co-diffusion phase, a 20 μm FeZn layer formed first on the substrate. Subsequently, increased Al atoms combined with Fe atoms within the FeZn phase to form an FeAl layer, thereby inhibiting FeZn layer growth. Finally, active Mg atoms form a Mg-rich layer on the surface of the FeAl layer. Mg-rich layer combines with Al atoms to form a MgAl phase and induces lattice distortion, thereby inhibiting FeAl layer growth. Throughout the process, the removal of HCl exposes the FeZn layer and restores its normal growth. Eventually, under this dynamic mechanism, a complex, multi-phase outer structure is formed. The self-corrosion current (J_corr) of the ZAM diffusion coating (1.673 × 10⁻⁵ A/cm²) markedly lower than the ZA diffusion coating (5.499 × 10⁻⁵ A/cm²). While the ZAM diffusion coating exhibited a weight gain of 1.182 ± 0.075 mg/cm² after 120 h of oxidation at 500 °C, which was slightly higher than the ZA diffusion coating but considerably lower than Q235 steel.