'Impregnation' of Defect Lattices in Iron Oxide Scales


IN the course of an X-ray examination on scales formed on steels, a structure phenomenon was observed which is believed to be of general interest. Magnesia is known to possess a considerably smaller lattice-parameter than ferrous oxide, with which it is isomorphous. If, therefore, wustite (the FeO solid solution) dissolves magnesium oxide, its parameter would be expected to decrease, as the smaller Mg++ ion replaces Fe++. To test this, a FeO-MgO solid solution was allowed to form, by coating Armco iron with pure magnesia, and oxidizing in air at 1,000° C. ; the scale was then examined by powder-photographs. The result was that the ferrous oxide had split into two phases, both of the same structure (rocksalt type), and, surprisingly, not of a decreased but an increased lattice parameter, compared with the oxide formed on non-coated iron under otherwise identical conditions. The accompanying photographs illustrate the effect. The measured dimensions were as follows:

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GOLDSCHMIDT, H. 'Impregnation' of Defect Lattices in Iron Oxide Scales. Nature 157, 478–479 (1946). https://doi.org/10.1038/157478b0

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