Kinetics and Mechanism of High Temperature Internal Oxidation of Ni-14wt%W Alloy at 1000°C

Abstract

Ni-14wt%W model-alloy has been used to study the kinetics and the mechanism of high temperature internal oxidation in presence of an external oxide scale. Oxidations have been done in air at 1000°C for different oxidation times and have allowed the evaluation of the oxidation kinetic from mass gain and from oxide thickness measurements. The oxidation kinetic which follows a parabolic oxidation rate indicates a diffusive behaviour of species during the oxide formation. Cross-section observations by Scanning Electron Microscopy (SEM) and analysis by Grazing Incidence X-Rays Diffraction (GIXRD) show a multi-layered structure of the oxide. Starting from the surface towards the bulk alloy, the scale is layered as follows : (i) a pure nickel oxide (NiO) in the outer part (ii) a porous NiO matrix containing NiWO4 second phase and (iii) an internal oxidation zone of tungsten, first forming WO3 sub-micrometer oxides that progressively transform into NiWO4 oxide precipitates in the alloy.