Influence of the prior oxide film on the growth and structure of oxides formed on Fe26Cr alloys at 600°C

Abstract

A Fe26 Cr alloy has been oxidized at 600°C in 5 × 10 −3, 5 × 10 −2 and 5 × 10 −1 torr oxygen to examine the influence of the prior oxide film on the growth and structure of oxides formed at high temperature. Different prior oxides were produced either by electropolishing or by annealing the electropolished specimen in vacuum at 600°C. Auger electron spectroscopy (AES) showed the Cr content of the prior oxide film to be increased from ∼ 50 to ∼ 95% during annealing, and electron diffraction indicated a change in oxide structure from amorphous to crystalline. At 5 × 10 −3 torr, electropolished Fe26 Cr oxidizes faster than the vacuum annealed specimens because the amorphous prior oxide gives rise to a finer-grained cubic oxide with more grain boundary easy diffusion paths for cation transport. From AES and electron back-scattering Fe 57 Mössbauer spectroscopy it is concluded that this cubic oxide is a duplex layer of inner γ-Cr 2O 3 and outer Fe 3O 4. The oxidation rate slows markedly when nucleated α-Fe 2O 3 covers the cubic oxide. With increased oxidation time Fe 3O 4 converts to α-Fe 2O 3 and the γ-Cr 2O 2 to α-Cr 2O 3. Annealed Fe26 Cr oxidizes slower primarily because of a lower cation transport through a coarser-grained cubic oxide rather than because of a higher Cr content in the prior oxide. α-Fe 2O 3 nucleates at an earlier stage in the oxidation and essentially stifles the reaction. The extent of Cr incorporation into any of the Fe oxides produced in 5 × 10 −3 torr oxygen is small (⋞ 5%). Increasing the oxygen pressure from 5 × 10 −3 to 5 × 10 −2 and 5 × 10 −1 torr has little effect on the mechanism of oxidation of vacuum annealed Fe26 Cr, except that the overall extent of oxidation is less because of earlier α-Fe 2O 3 formation and, after a few hours of oxidation, up to ∼ 20% Cr is incorporated into the α-Fe 2O 3 lattice. On electropolished Fe26 Cr at 5 × 10 −2 and 5 × 10 −1 torr oxygen nodules of α-Cr 2O 3 form and continue to grow both at grain boundaries and within the grains. Possible mechanisms for this nodule formation, which is exclusive to electropolished specimens oxidized at the higher pressures, are considered.