Effect of steel substrate roughness on microstructure and properties of hot-dip Zn–Al–Mg coating

In this study, Zn-9Al-3Mg-0.2Si coatings with corrosion resistance were prepared on steel substrates with different roughness using the same hot-dip galvanizing parameters. The microstructure of the coating surface was analyzed using different analyzing methods. The microstructure of the coatings primarily consists of Al-rich dendrites, MgZn 2 intermetallic compound, and lamellar eutectic structures of Zn/Al/MgZn 2 or Zn/Al/Mg 2 Zn 11 . The results show that with increased surface roughness of the steel substrate, more nucleation sites were provided during solidification, leading to refined Al-rich dendrite grains with a smaller area fraction. The rough steel substrate surface increased the number of nucleation points, and their uneven distribution caused grains to grow with different orientations at various locations. Nanoindentation tests revealed that the refined grains resulted in a substantial enhancement of the coating's mechanical properties. The corrosion resistance of the material was assessed through the utilization of two analytical methods: the Tafel polarization curve and the electrochemical impedance spectroscopy. When the steel substrate roughness was 1.028 μm, the coating exhibited a dense structure and uniform grain orientation, with the lowest self-corrosion current density (1.2746 × 10 -6 A/cm 2 ). This was due to the presence of more ternary eutectic phases, less Al-rich phases and a more uniform grain orientation.