Effect of annealing atmosphere on the galvanizing behavior of a dual-phase steel

The influence of annealing atmosphere oxidation potential (i.e. pH2O/pH2 ratio) on the surface condition of an industrial dual-phase steel alloyed with 2 wt.% Mn and 0.3 wt.% Mo has been studied by galvanizing simulation, with particular attention towards alloying element segregation, the occurrence of selective oxidation of the surfaces immediately prior to hot-dip galvanizing and their effect on reactive wetting and coating adherence. The surface chemistry and morphology prior to dipping have been analyzed in detail, along with their evolution during annealing, and the coating quality and chemistry were characterized and interpreted in relation to the surface condition prior to dipping. It was found that Mn segregates to the surface and gives rise to significant selective oxidation for all the experimental atmospheres, with a transition from external to internal oxidation occurring when the oxidation potential of the atmosphere increased. However, in spite of the presence of Mn oxides at the steel surface prior to dipping, samples annealed under all atmospheres exhibited good coating adherence and a well developed inhibition layer. This is attributed to the aluminothermic reduction of the Mn oxides by the Al present in the Zn bath followed by reactive wetting between the substrate and the Zn bath.