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Pyrophoric iron powders are passivated in 19 steps by N2/O2mixtures. During each step the oxygen uptake is determined by weighing and a sample for magnetic measurements is taken. The magnetic saturation as a function of the oxygen content exhibits a characteristic shape with a break between two straight lines. The initial steep slope can be explained by a 1:1 reaction of oxygen and iron in the surface layer, assuming this layer to be nonmagnetic and applying linear superposition of the magnetic moments. After the break higher iron oxides might be expected.
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Schnitker et al. (1980) studied this question.
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