Volatilization and Transport Mechanisms During Cr Oxidation at 300 °C Studied In Situ by ToF-SIMS

Abstract

The oxidation of chromium at 300 °C was investigated in situ by ToF-SIMS for three different oxygen pressures ($P_{{{\text{O}}_{2} }} = 2.0 \times 10^{ - 7}$, 6.0 × 10⁻⁷ and 2.0 × 10⁻⁶ mbar). Sequential exposure to the ¹⁸O isotopic tracer was performed to reveal the governing transport mechanism in the oxide film. The evolution of the oxide thickness was monitored. Volatilization of Cr₂O₃ was evidenced. A model was used to describe the kinetics resulting from the measurements. Both the parabolic and volatilization constants showed a dependence on oxygen partial pressure like $P_{{{\text{O}}_{2} }}^{ - 1/n}$, with n = 1.9 ± 0.1, indicating a defect structure mainly consisting of oxygen vacancies. The re-oxidation in ¹⁸O₂ shows a growth of the oxide layer at the metal/oxide interface, demonstrating an oxidation process governed by anionic transport via oxygen vacancies. The diffusion coefficient of oxygen in the oxide was determined by fitting the ToF-SIMS depth profiles. It is 2.0 × 10⁻¹⁸ cm² s⁻¹.