Natural Oxidation of Fe‒Ni Alloys: Hard X-ray Photoemission Study

Abstract

The natural oxide formation on Fe_1−xNi_x (x = 0.60, 0.40, 0.36) random bulk alloy surfaces have been studied using Hard X-ray Photoelectron Spectroscopy (HAXPES) technique at room temperature employing a photon energy of 4 keV with a total resolution of 0.8 eV in a vacuum of about 10⁻¹⁰ Torr. Intentionally, the alloy surfaces were not subjected to in-situ surface cleaning, as our motive is to study the surface oxidation. HAXPES is capable of producing both surface and bulk features of the alloys. All the core levels such as 2s, 2p, 3s, 3p of both Fe and Ni were investigated and multiple features corresponding bulk and surface were observed. Alloy surfaces were found to have predominantly Fe oxides such as FeO and Fe₂O₃ compared to Ni oxides indicating the segregation of Fe to the surface. The 3p core levels of Fe and Ni exhibit exchange splitting and spin orbit splitting respectively with lower surface oxide contribution due to the enhanced bulk sensitivity at higher kinetic energies. Fe 2s core level exhibits correlation induced multiplet features. The surface oxide increases with the Fe content in the alloy.