Reoxidation of chromium with densified Cr2O3 Scales

Preoxidized chromium specimens have been high vacuum annealed at 1200° and 1300°C to produce densified Cr ₂O₃ scales. These specimens have been reoxidized at the same temperatures at 10^–6 atm O₂. The initial reoxidation is linear with time and is concluded to reflect a volume diffusion controlled transport through the densified scale. The corresponding parabolic rate constant (w² = k_pt)is given by k_p=1.4 · 10^–2 exp(–235,000/RT)(gram of O) ²/cm⁴ sec. It is tentatively concluded that outward chromium diffusion predominates in an inner layer of the Cr₂O₃ scales and inward oxygen diffusion in an outer layer. Under the experimental conditions it has not been possible to maintain growth of the Cr₂O₃ scales controlled by volume diffusion. The new oxide layer consists of fine crystallites; the oxide grows at grain boundaries within the scales. This causes sideways growth of the scale, breakdown of the originally densified layer, and an increased rate of reaction.