Understanding the Formation Mechanism of Al-Rich Interfacial Layer during Galvanizing of Cu Pre-coated High-Strength Steel Sheet

This work aims at understanding the formation of a beneficial thin continuous aluminum (Al)-rich interfacial layer with uniform distribution of fine equiaxed Fe-Al crystals formed at the substrate/coating interface during the initial stages of interfacial reactions as well as hindrance of Fe dissolution in Zn layer as soon as copper (Cu) pre-coated high-strength steel substrate is immersed into the molten zinc (Zn) bath containing 0.2 wt.% Al. Pre-coated Cu does not allow the immediate interaction and interdiffusion of Fe and Zn because of the positive heat of mixing between Fe and Cu. As the Cu dissolves in liquid Zn, counterdiffusion of Al and Zn toward the steel interface allows Fe to react with Al and Zn forming the thin Fe-Al
inhibition layer with finer equiaxed grains. Moreover, dissolved Cu in the Zn layer would also prevent Fe diffusion in Zn in the time span during dipping, which is beneficial. Atom probe tomography (APT) analysis at the substrate/coating interface location reveals the presence of 56.22 ± 1.36 at.% Al, 35.01 ± 1.31 at.% Fe, 6.33 ± 0.66 at.% Zn, and 0.032 ± 0.01 at.% Cu, similar to the intermetallic compound of Fe2Al(5-x)Znx, as well Cu in the Zn layer supporting the above reasoning.