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 Fe 4 Al 13 particles (containing Si and Zn atoms) epitaxially grown on a steel substrate. The solidifying layer contained three primary phases (Mg 2 Si, Al, and MgZn 2 ) as well as the Al/Zn eutectoid and Zn/MgZn 2 /Al ternary eutectic phases. Crystallographic analysis data revealed the serial formation of the Fe 4 Al 13 , Mg 2 Si, and Al phases via heterogeneous nucleation. The primary Al particles (including 37 wt% Zn and 4 wt% Mg) included multiple Zn and MgZn 2 precipitates and were surrounded by Al/Zn eutectoid phases (containing ∼70 wt% Zn). The primary MgZn 2 phase was generated after the Mg 2 Si phase was partially transformed into the Mg 2 Zn 11 phase. A Zn/MgZn 2 /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.