The mechanism of low-temperature oxidation of zirconium

The kinetics of oxide-film growth on zirconium was modelled on the basis of coupled currents of cations and electrons (by both tunnelling and thermionic emission) through a homogeneous surface-charge field. Good agreement is obtained with experimental growth curves for T = 304-573 K and pO2 = 1.3 x 10-6-1.3 x 10-4 Pa. The growth rate is limited by the field-assisted cation migration in the developing oxide film. Up to oxide-film thicknesses of about 2-3 nm, a constant kinetic potential is sustained during growth by the relatively fast forward and reverse electron currents. Beyond this critical thickness for T > 473 K, the net electron flux by tunnelling drops to zero and electron transport by thermionic emission becomes rate-determining, while cation transport remains rate-limiting. Changes of the energy barrier for cation motion and of the metal-oxide and oxide-oxygen work functions are related to the transformation of the amorphous, non-stoichiometric oxide film into crystalline ZrO2