Correlation between the surface chemistry of annealed IF steels and the growth of a galvanneal coating

Abstract

An attempt is made to establish possible relationships between the quantity of oxides and hydroxides of the alloying elements on the outer interstitial-free steel surface after the annealing process and the characteristics of the galvanneal coating formed. XPS measurements reflect a clear influence of the alloying element contents and the water vapour content or dew point (DP) of the atmosphere on the oxide and hydroxide coverage of the steel surface. In general, much less Fe–Zn intermetallic formation is seen on the Ti steel substrate than on the Ti–Nb steel. The reason for this may be related with the higher oxide/(Fe + Mn) atomic ratio on the external surface of the Ti steel substrate. With the Ti–Nb–P steel, an absence of Fe was observed in the coatings obtained on the surface of this steel annealed in atmospheres with a DP of −45 or −10 °C. A clear direct relationship has been found between the inhibition of growth of Fe–Zn intermetallic compounds and the fraction of the steel substrate surface covered by manganese oxides as a result of the annealing process. For the Ti–Nb–P steel the increase in the DP of the galvannealing atmosphere to 10 °C resulted in the incorporation of a significant Fe content in the coating. XPS analysis suggests a change in the surface film, and the formation of a phosphate enriched layer, instead of manganese oxide. A lower fraction of oxide coverage on the annealed steel substrate resulted in less of an impediment to the diffusion of iron atoms from the steel substrate to the zinc coating in the galvannealing process