Oxidation of Fe—Cr Binary Alloys

ISSN： 0007-0599

Source： British Corrosion Journal, Vol.3, Iss.2, 1968-03, pp. : 61-67

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Abstract

The reaction kinetics between high purity oxygen and six binary Fe–Cr-alloys, containing 2–40% Cr, at temperatures between 650° and 950° have been studied for periods of up to 50 hours' duration. A precise method of surface preparation was employed to secure reproducible results. Oxidised samples were investigated by metallography, electron-probe microanalysis and X-ray diffraction.In all cases the rate of oxidation decreased progressively with the duration of exposure although some short periods of increased oxidation were observed on some alloys at the lower temperatures. The scale thickness after 40 hours' oxidation varied from 0·3 μm at 650° to 14 μm at 950° for Cr-rich oxides and to 600 μm for Fe-rich oxides. The 20% Cr-alloy was generally the most resistant to oxidation. The 8% Cralloy was about as resistant as the others at 650°–750° but oxidised more rapidly at higher temperatures. For alloys containing more than 16% Cr, the activation energies for oxidation were almost constant over the temperature range, at about 47kcal/mole, but a value of 60kcal/mole was observed for the 16% Cr-alloy.Differences in oxidation rate between one alloy and another probably reflected the effects of varying amounts of Fe in substitutional solid solution in the ‘Cr₂O₃’ scale perhaps involving changes in the defect concentration. Both the kinetic results and the structural studies are consistent with lattice diffusion mechanisms, but further work is required to obtain a complete understanding of the complex blistered scales.