The oxidation of iron around 200 °C

Phenomena associated with a critical temperature at ca . 200° C in the oxidation of mild steel sheet in air (for example, a change of slope in the curve relating reciprocal of absolute temperature with logarithm of oxygen uptake in constant time) have been investigated by gravimetric, microchemical and electron-diffraction methods. The evidence is interpreted on the basis of predominant metal-ion diffusion above and predominant oxygen-ion diffusion below 200° C, in the oxide film. In the film an outer layer of rhombohedral α -Fe 2 O 3 , overlies a layer of cubic oxide which, initially nearer γ -Fe 2 O 3 , tends towards Fe 3 O 4 —rapidly and completely above 200° C, slowly and incompletely below 200° C. Above 200° C the rate of oxidation is controlled by the diffusion of ions across the film (Wagner mechanism), the parabolic law is followed, and oxygen uptake derived from the weight increase of the specimen is coincident with the oxygen content of the film on removal from the substrate. The film, being produced outside the original metal/air interface, is relatively insensitive to the mode of preparation of the surface, and interface colours readily develop. Below 200° C, oxidation again involves ion diffusion across the film, but the rate is controlled by a boundary reaction with a consequent logarithmic relationship with time. Since oxidation now proceeds at an oxide-metal interface the optical homogeneity of the oxidized layer is very sensitive to the initial condition of the surface. Abraded surfaces carry initially a heterogeneous layer (abrasion-produced oxide in a matrix of iron) in which oxidation can proceed without contributing to the overlying (strippable) film on which the development of interference colours depends. Thickening of the film and progression of colours may be long delayed during the period of oxygen uptake within the substrate. This period may be eliminated or. reduced either by removal of the heterogeneous layer by acid etching or by its modification by vacuum annealing. Although the disturbed substrate on abraded specimens serves in this way to provide evidence for the inward diffusion of oxygen, it exerts no appreciable control over the total oxygen uptake either above or below 200° C.