The surface and bulk oxidation of zirconium

The oxidation of zirconium at high temperatures involves both the formation of a surface oxide, and absorption of oxygen into the bulk Zr.The hcp lattice of Zr is reported to absorb up to 29.8 at.% oxygen, which occupies the interstitial sites.The simultaneous surface oxidation and diffusion of oxygen into the bulk Zr were observed at temperatures of 1073, 1173 and 1243 K. Experimental results were compared to predicted results from a model developed here to evaluate values of the oxidation kinetic parameters.Samples of Zr, 0.025 cm thick, were saturated with oxygen by soaking surface-oxidized samples in an argon atmosphere at 1243 K.The initial surface oxidation properties of saturated Zr were compared to pure Zr by AES-analysis.The model of the oxidation process assumes that diffusion of 02 -through the oxide layer is rate limiting.The oxide growth at the oxide Zr interface is described by a mass balance.The principle parameter in the model is the product,D/δ., the diffusion coefficient of oxygen in ZrO2 and the oxygen concentration difference across the oxide.The model was calibrated by two completely separate methods.The first compared model predicted curves of mass vs. time to those produced experimentally.The second method compared predicted oxide thicknesses at specific times and temperatures with those observed experimentally.A value for 7delta;C.from the literature was assumed.Then D. as a function of temperature was determined for both experimental methods.From the mass gain curve fitting method the diffusion coefficient, D.=1.6X10-5EXP(-20700/RT) was found.From the oxide thickness comparison the diffusion coefficient, D.=2.0X10-2EXP(-34300/RT) was found.It is concluded that the primary oxygen diffusion path is along grain boundaries, and that D. is a function of extent of oxidation due to changing oxide crystal size during oxidation.