Alloying Reactions in Hot Dip Galvanizing and Galvannealing Processes

The iron-zinc alloying reactions which take place during hot dip galvanizing and galvannealing processes were investigated, with special reference to the formation and growth behavior of iron-zinc intermetallic compounds. The SEM observation and X-ray diffraction analysis of the intermetallic compounds which had formed on laboratory galvanized Ti stabilized interstitial-free steel and on low carbon aluminum killed steel sheets showed that there were two types of elementary iron-zinc alloying reactions; one was the formation of ζ and δ, crystals and another was so-called outburst reaction.The ζ crystal nucleated on the Fe-Al intermetallic compound which had been formed on the steel surface, and grew into the melt as a single phase crystal. Simultaneously, the outburst structure was generated as a result of the direct reaction between iron and molten zinc, and grew as a multiple phase structure (ζ, δ1 and Γ phases). The ζ phase was generated at the isothermal galvannealing temperature of less than 773 K, and it was explained in terms of the iron-zinc equilibrium phase diagram (peritectic temperature of ζ phase). The effects of solute element in steel on the thickness of Γ phase were explained by its suppression effect toward the outburst reaction.