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

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 Fe Zn layer composed of δ(FeZn 10 ) and Γ(Fe 11 Zn 40 ) 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 Al 2 O 3 and Fe Al phases; a Mg-rich layer interwoven with Mg Al phases, Mg Zn 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 Fe Zn layer formed first on the substrate. Subsequently, increased Al atoms combined with Fe atoms within the Fe Zn phase to form an Fe Al layer, thereby inhibiting Fe Zn layer growth. Finally, active Mg atoms form a Mg-rich layer on the surface of the Fe Al layer. Mg-rich layer combines with Al atoms to form a Mg Al phase and induces lattice distortion, thereby inhibiting Fe Al layer growth. Throughout the process, the removal of HCl exposes the Fe Zn 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/cm 2 ) markedly lower than the ZA diffusion coating (5.499 × 10 −5 A/cm 2 ). While the ZAM diffusion coating exhibited a weight gain of 1.182 ± 0.075 mg/cm 2 after 120 h of oxidation at 500 °C, which was slightly higher than the ZA diffusion coating but considerably lower than Q235 steel.