Microstructural characterization of corrosion resistant Zn–15Al–6Mg–0.4Si galvanized coating

A complex microstructure of the Zn–15Al–6Mg–0.4Si coating layer with excellent hardness and corrosion resistance was investigated by performing thermodynamic calculations, X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The inhibition layer with a thickness of ∼1 μm consisted of submicrometer-sized monoclinic Fe₄Al₁₃ particles (containing Si and Zn atoms) epitaxially grown on a steel substrate. The solidifying layer contained three primary phases (Mg₂Si, Al, and MgZn₂) as well as the Al/Zn eutectoid and Zn/MgZn₂/Al ternary eutectic phases. Crystallographic analysis data revealed the serial formation of the Fe₄Al₁₃, Mg₂Si, and Al phases via heterogeneous nucleation. The primary Al particles (including 37 wt% Zn and 4 wt% Mg) included multiple Zn and MgZn₂ precipitates and were surrounded by Al/Zn eutectoid phases (containing ∼70 wt% Zn). The primary MgZn₂ phase was generated after the Mg₂Si phase was partially transformed into the Mg₂Zn₁₁ phase. A Zn/MgZn₂/Al ternary eutectic microstructure with a spacing of 1.0 μm was formed during the final solidification stage. The Zn–15Al–6Mg–0.4Si coating exhibited high hardness and low corrosion rate in a 3.5 wt% NaCl solution as compared with those of the coatings with lower Al and Mg contents.