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.

An isothermal phase diagram for the Fe‐Cr‐O system is presented for 1300°C which, with a minor modification, holds for 1000°C. It is observed that Fe‐Cr alloys containing over 13% Cr are in equilibrium with Cr2O3 containing a little dissolved Fe2O3 . Alloys of lower chromium content are in equilibrium with iron chromite of variable composition of the type FeCr2O4 . Evidence is presented indicating that when the spinel forms next to the metal a pronounced internal oxidation occurs. However, higher chromium alloys with Cr2O3 next to the metal show no tendency for internal oxidation. An oxygen pressure‐composition diagram for the entire system is also presented. Marker experiments made during the oxidation of 20% Cr and 30% Cr alloys suggest that Cr2O3 is formed almost entirely by cation (Cr+3) diffusion.

No abstract

No abstract

No abstract