On employing a sectioning technique with Cr51 as the tracer, the diffusion of Cr ions in nitrogen‐equilibrated Cr2O3 was determined from 1045° to 1550°C. Volume diffusion coefficients in hot‐pressed compacts of three different impurity levels conform to the expression D=0.137expfalse(−61,100/RTfalse)cm2sec−1 . Additions of 2 normalw/o CeO2 and 1 normalw/normalo Y2O3 do not cause a significant change. Use of lower density sintered compacts provides erroneous results. Conductivity measurements were performed in nitrogen, argon, and air over a wide temperature range; no dependence on oxygen partial pressure was observed. Calculated cation transference numbers vary from 10−3 to 10−6. Postulating a simplified model for the defect structure of Cr2O3 , the activation enthalpy for hole motion is 8.4 kcal mole−1; the enthalpy of defect formation is 91 kcal mole−1; the activation enthalpy for cation motion is about 31 kcal mole−1.

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Oxygen‐18 exchange between gaseous oxygen, held at a pressure of 125 mm Hg in a Pt10Rh chamber, and spheres of α‐Cr2O3 containing three or less grains was determined from 1100° to 1450°C. Isotope equilibrium on crystal surfaces appears to be quickly established, and the rate‐determining factor is self‐diffusion conforming to the relation D = 15.9 exp(‐101,000/ RT) cm2sec−1. Changing sphere diameters caused no detectable variation in diffusion coefficients. Anions are the much slower diffusing species in this oxide.

By conducting continuous weight-gain measurements from 500 to 1300 C on a variety of Fe-Al, Fe-Cr-Al, Ni-Al and Co-Al alloys in oxygen at 100 torr and examining the reaction products which form, reasons for the enexpected ineffectiveness of preferential aluminum oxidation have been explored. With Fe-4.4 percent Al and Fe-12 percent Al, there are two different regions of parabolic growth- these are associated with the appearance of γ-A12O3 below 900 C and α-Al2O3 above. The latter is normally more protective but possesses a greater tendency for cracking and spalling; adding 24.6 percent chromium promotes its formation at lower temperatures. With Ni-5.0 percent Al, two displaced regions are also found; here, nickel penetrates alumina to provide intermediate NiO · Al2O3 and an outer layer of NiO. With Co-4 percent Al, somewhat less cobalt penetrates γ-Al2O3 to provide CoQ · Al2O3 and an outer layer of Co3O4. When both CoO and α-Al2O3 are stable phases, the latter grows mainly by internal oxidation and offers little protection.

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