Early Stages of High-Temperature Oxidation Behavior and Its Phase Evolution of 9% Ni Steel

Abstract

Herein, the effects of oxygen partial pressure (5–20%) and oxidation time (0.5–6 h) on the growth behavior and phase structure of 9% Ni steel oxide layer are studied by means of high-temperature oxidation experiment, scanning electron microscopy, and X-ray diffraction. The results show that the inner and outer oxide layers of 9% Ni steel grow opposite to each other along the initial surface direction of the vertical matrix during the high-temperature oxidation process. From the perspective of the phase, the phase transition of Fe₂O₃→Fe₃O₄→FeO mainly undergoes from the outside and the inside, the results of thermodynamic calculations show that ${\Delta }_r{G}_m\left({\text{Fe}}_2{O}_3\right)$<${\Delta }_r{G}_m\left({\text{Fe}}_3{O}_4\right)$<${\Delta }_r{G}_m\left(\text{FeO}\right)$<0, and the main factor affecting the phase transition is the diffusion concentration of free O and Fe atoms. During the high-temperature oxidation process of 9% Ni steel, the diffusion of O and Fe atoms along grain boundaries promotes preferential oxidation at these boundaries. The columnar structure formed in the outer oxide layer of 9% Ni steel may be related to the uneven diffusion rate of Fe atoms at different positions at the interface between the inner and outer oxide layers.