The oxidation of iron at 175 to 350° C

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Abstract
The oxidation of iron, previously freed from oxide by hydrogen treatment, has been studied at 175 to 350° C; five methods (gravimetric, electrometric, film -transfer followed by chemical or microscopic examination, X -rays and electron diffraction) have been used to identify and estimate the different oxides. If specimens are exposed to air at room temperature without subsequent hydrogen treatment, complications are introduced, magnetite being formed where otherwise it would be absent. On hydrogen-reduced iron powder, ferrous oxide, magnetite and α-ferric oxide appear when oxygen is admitted, but the oxidation is not isothermal, since a glow appears on admission of oxygen even when the initial temperature is only 40° C. On hydrogen-reduced Swedish sheet, α-ferric oxide alone appears at 175 and 225° C, while at 300, 325 and 350° C, a duplex film of magnetite (not γ-ferric oxide) overlaid with α-ferric oxide is formed; at 250° C, the film consists of a single layer (α-ferric oxide) for 8 h, but then magnetite appears below it at certain places (distinguishable by the more advanced interference colours) and spreads laterally, the rate of magnetite formation increasing rapidly with time. Most of the films studied display interference tints in the usual sequence, when they are on the metal; after transfer to glass, they show different colours, which in the case of the duplex films are themselves different according as they are viewed from the magnetite side or the α-ferric oxide side. This can easily be explained. Some experiments on pure iron give results similar to those on Swedish iron. The growth law on sheet is parabolic ( W 2 = Kt + K ' ) at 325 and 350° C but logarithmic ( W = K ' In ( Kt + K ´)) at 175, 225, 250, 275 and 300° C.

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