XPS study of the initial growth of oxide films on Inconel 600 alloy

Abstract

The gas phase oxidation of Inconel 600 alloy has been studied under very mild oxidation conditions. Depth-composition profiles of thin oxide films formed on the alloy have been studied using X-ray photoelectron spectroscopy (XPS) and ion bombardment. The oxide and metal phases within the surface film are reproducibly differentiated using the XPS chemical shift. The film composition and structure were studied as a function of oxidation time, temperature, oxygen concentration and surface topography. Oxidation at 100°C results in formation of a duplex NiO-Cr 2O 3 layer. At 280°C a duplex layer also forms consisting of a NiFe 2O 4 spinel, near the surface, and Cr 2O 3 near the metal interface. Iron diffusion is found to be strongly affected by the method of preparation; chromium diffusion is not. Oxygen concentration affects the relative concentration of nickel oxide produced during oxidation. The chromium oxide phase, however, is unaffected by oxygen concentration at this temperature, suggesting that growth of this protective layer is severely limited by the low outward diffusion rate of chromium ion. The initial oxide film is more enriched in chromium at 500°C than at 300°C, and some evidence of internal oxidation is seen. Oxidation of the nickel phase occurs after a limiting thickness of Cr 2O 3 is established. At 700°C, a surface oxide phase of CrO 2 is established initially. The optimum conditions for formation of a pure chromium oxide layer on Inconel 600 are high temperature, annealed alloy surfaces and a low oxidant concentration.