ESCA study of the oxidation of a nickel–molybdenum alloy

Abstract

AbstractThe surface composition of Ni–6at% Mo (110) alloy was analysed by ESCA after surface preparation treatments and after oxidation at 25°C and 500°C (p = 10−4 Torr). Ion etching causes a marked enrichment of molybdenum on the surface. After annealing at 500°, the concentration of segregated molybdenum is ˜25%. At 800°C there is no segregation of the metal elements, but sulphur segregates to the surface. Oxidation at 25°C gives a thin oxide layer (5–6 Å) with a composition which reflects the composition of the initial surface, so that the oxidation of a surface enriched with molybdenum gives a molybdenum‐rich oxide. Oxidation at 500°C gives a molybdenum‐enriched oxide. The Mo concentration of the oxide is 12%, i.e. twice that in the bulk. The lateral growth of oxide islands seems to be already complete after a very short oxidation time (2 min) and then a parabolic growth of the oxide layer is observed. The chemical shifts of Ni2+ (2p3/2) and Mo6+ (3d5/2) in the oxide formed on the alloy are respectively 2.1 eV and 4.2 eV, while they are 1.6 eV for NiO and 4.8 eV for MoO3. Adsorbed sulphur has a marked influence on the oxidation. The oxide growth (also parabolic) is faster in the presence of sulphur at 500°C. The oxide film grows on top of the sulphur layer.