Effect of Fuel-to-Air Ratio on Oxidation and Interfacial Structure in Galvanizing of a Dual-Phase Steel

Automotive-grade high-strength steels are galvanized for improved corrosion resistance. However, selective oxidation of alloying elements during annealing heat-treatment may influence the subsequent zinc (Zn) coating quality. The formation of internal and external oxides depends on the alloy composition, especially the Si/Mn ratio, and the oxygen potential of the annealing atmosphere. In this work, a dual-phase (DP) steel was intercritically annealed with varied fuel-to-air ratios in a direct-fired furnace and then galvanized in a Zn bath with 0.2 wt% Al. The type of internal and external oxides and the interfacial structures between the steel substrate, the Al-Fe-Zn inhibition layer, and the Zn coating were examined by using site-specific focused ion beam (FIB) and transmission electron microscopy (TEM). It was found that for the high fuel-to-air ratio sample, the external oxide is a Mn-Si-O complex oxide and the internal oxide presents an inhomogeneous structure; for the medium fuel-to-air ratio sample, the external oxides are both MnO and Mn-Si-O oxides and the internal oxide presents a Si-O core surrounded by a Mn-Si-O shell; for the low fuel-to-air ratio sample, both the external and internal oxides are Mn-Si-O. The inhibition layer in all the samples appears to be continuous. Lift-off of surface oxides was observed in the high and medium fuel-to-air ratio samples. Possible mechanisms are discussed.