Evolution of near-surface concentration profiles of Cr during annealing of Fe–15Cr polycrystalline alloy

Abstract

For the development of surface modification of Fe–Cr alloys, the surface chemistry of Fe–15 at.% Cr alloy specimen concerning the initial stage of oxidation and the effects of hydrogen during annealing and following oxidation was investigated. Samples were exposed to a heat-treatment atmosphere; annealing, oxidation and annealing followed by oxidation in air and N 2–H 2 gas mixtures at 800 °C. After the heat treatments, scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) were employed to characterize the morphology, elemental depth profiles and the chemical states of the elements. In the initial stage of oxidation, the chemistry of the oxide layer on the surface transformed as following sequence: Fe-rich oxide → duplex oxide layer (outer Fe-rich and inner Cr-rich) → Cr-rich mono-phase layer (M 2O 3). In the transformation, the reaction rate for the formation of Cr-rich oxide layer was controlled by the diffusion of cations. The annealing in H 2-containing atmospheres increased the Cr content at the surface. The increase of hydrogen content in the atmosphere further increased the Cr to Fe ratio in near-surface, and the thickness of the layer affected by the heat treatment. The selective oxidation of chromium occurred as internal Cr 2O 3 formation, as a function of the Cr content, rather than the oxygen partial pressure. Hydrogen facilitated the diffusion of chromium probably by cleaning of fast diffusion paths. The addition of hydrogen in the annealing atmosphere prior to the oxidation was beneficial to obtain a Cr-rich oxide layer on the surface.