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 similar to 1 mu m 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 similar to 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 mu m 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.