Effect of Mg on Microstructure and Growth Kinetics of Zn-22.3Al-1.1Si Coating

Abstract

The effects of Mg on the microstructure and growth kinetics of the hot-dip Zn-22.3Al-1.1Si-x Mg (x = 0, 0.2, 0.4, and 0.6) coatings were investigated in detail. Scanning electron microscopy with energy dispersive spectroscopy and X-ray diffraction studies revealed the presence of η-Zn, α-Al, Zn/Al eutectoid, (Si), Al/Si eutectic, and Zn/Al/Mg₂Zn11 ternary eutectic on the top surface of these Mg-containing coatings. Especially, a small amount of MgZn₂ phase appears in top surface of Zn-22.3Al-1.1Si-0.6Mg coating. Five phases are found in the alloy layers, i e, Fe₂Al₅, FeAl₃, τ_5C, τ_5H, and τ₁. The addition of 0.2% Mg can delay the emergence of FeAl₃ phase. When the Mg content is more than 0.2%, the outer layers of coating change from τ_5C to τ_5H phase. The growth of the inhibition layer is diffusion controlled for various Mg content baths. Mg improves the corrosion resistance of the Mg-containing coatings, and the Zn-22.3Al-1.1Si-0.6Mg coating possesses the highest protective properties.

Keywords

Mg / microstructure / growth kinetics / inhibition layer / diffusion