Microstructural evolution and oxidation behavior of hot-dip 55 wt.% Al–Zn–Si coated steels

Abstract

The effects of temperature and heating rate on the oxidation kinetics of hot-dip 55wt.%Al–Zn–Si coated steels were investigated experimentally and theoretically. The microstructure evolution was examined by scanning electron microscope (SEM) and X-ray diffraction (XRD). The Fe2Al5Zn0.4 phases formed at the initial stage of oxidation gradually transformed into FeAl with the prolongation of time. The irregularity protuberances appeared on the surface of specimens at 550 °C were formed owing to the rapid growth of Γ phase. The main oxide, ZnO, was generated at 650 and 750 °C, but rarely observed at 550 °C. The weight gain at different temperatures was determined by thermo-gravimetric (TG) balance. The calculated results from Real Physical Picture (RPP) model suggested that the first stage of isothermal oxidation was controlled by chemical reaction and the second stage was controlled by diffusion at 650∼750 °C. The activation energies were 180.4 kJ/mol for isothermal oxidation and 171.2 kJ/mol for non-isothermal oxidation, respectively. All the calculated and predicted results agreed well with the experimental data, which confirmed the validity of RPP model.