Effects of cooling rate on the microstructure and properties of hot-dipped Zn–Al–Mg coatings

Appropriate hot-dip coating process parameters are very important for enhancing the cracking resistance and corrosion resistance of Zn–Al–Mg coatings. In this study, five different Zn–6Al–3Mg coatings were prepared by controlling the cooling rate: ZAM1 (0.1 °C/s), ZAM2 (1 °C/s), ZAM3 (5 °C/s), ZAM4 (30 °C/s), and ZAM5 (400 °C/s). The microstructure of each coating mainly included an Al-rich phase, Zn/Al/MgZn2 or Zn/Al/MgZn11 ternary eutectic, and primary MgZn2 phase. A smaller Zn phase appeared in the ZAM3 and ZAM5 coatings, and there was no primary MgZn2 phase in ZAM5. The surface crack morphology of each coating after the tensile and bending tests was observed using scanning electron microscopy. The cracking resistance of the ZAM5 coating was the highest, followed by those of ZAM1, ZAM2, ZAM3, and ZAM4. The cracking resistance of the coating was correlated to the nucleation quantity of the primary MgZn2 phase, because the primary MgZn2 phase is an intermetallic compound that is difficult to plastically deform. The ZAM2 coating exhibited the best corrosion resistance, as determined by neutral salt spray and electrochemical tests. This result was attributed to its higher ternary eutectic area fraction and lower Al-rich phase area fraction.