Oxidation and corrosion behavior of commercial 5 wt% Al-Zn coated steel during austenitization heat treatment

Abstract

A commercial 5 wt% Al-Zn coated steel was employed to detail the microstructural evolution, oxidation, and the related corrosion behavior changes during the austenitization heat treatment. During the early stages of austenitization, Fe2Al5Znx phase forms between the coating and the steel substrate. With continued austenitization, the inner part of the coating transforms to the Γ-Fe3Zn10 phase and the outer part of the coating to the FeAl phase, accompanying with the formation of a thin surface oxide layer mainly composed of Al2O3 with minor ZnO. Subsequently, the coating transforms to the Γ-Fe3Zn10 phase and α-Fe(Zn), and finally to the single α-Fe(Zn) phase. The Al in the coating is distributed mainly on the surface of the alloy layer and suppresses the oxidation of the alloy layer. The austenitization treatment enriches the coating with Fe, which, in turn, ennobles the corrosion potential and reduces the corrosion current density of the coating. Conversely, the potential difference between the coating and the steel substrate is reduced after austenitization.