Effects of heat treatments on near surface elemental profiles of Fe–15Cr polycrystalline alloy

Abstract

For the development of optimised surface modification of Fe–Cr alloys, the surface chemistry of an Fe–15 at.-%Cr alloy specimen during the initial stage of oxidation was investigated as well as the effects of hydrogen during annealing and following oxidation. Samples were exposed to various heat treatment atmospheres at 800°C: annealing in N2–H2 gas mixtures, oxidation in air and annealing then oxidation. After the heat treatments, scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) were employed to characterise 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 in the following sequence: Fe rich oxide → Duplex oxide layer (outer Fe rich and inner Cr rich) → Cr rich mono-phase layer (M2O3). In the transformation, the reaction rate for the formation of the Cr rich oxide layer was controlled by the diffusion of cations. Annealing in H2-containing atmospheres increased the Cr content at the surface. An increase in hydrogen content in the atmosphere further increased both the Cr to Fe ratio in the near surface region, and the thickness of the layer affected by the heat treatment. Selective oxidation of chromium occurred as internal Cr2O3 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 to the annealing atmosphere before the oxidation was beneficial to obtain a Cr rich oxide layer on the surface.