Effect of electric field on oxide layer structure at zirconium oxidation in H2O, CO2 and H2O/CO2 supercritical fluids

Abstract

Oxidation of zirconium plates in supercritical fluids H2O, CO2 and H2O/CO2 was studied at 823 K and 24–39 MPa. An experimental cell was a flat capacitor, where the zirconium plate acted as the anode and the copper plate of the same size was the cathode. The interelectrode gap was 2 mm. The experiments were carried out in the absence of electric field and in the electric field of 300 kV m−1. It has been found that in all cases the oxidized layer consists of two parts: the outer loose layer and the inner dense layer forming an interface with Zr. At oxidation in a medium containing H2O, there is non-uniformity in distribution of oxygen near the boundary region of the loose and dense layers. When Zr is oxidized in the H2O/CO2 mixture, zirconium carbonate is found on the surface. In the electric field, an average specific rate of an increase in the sample mass decreases linearly with an increase in the fraction of CO2 in the H2O/CO2 mixture, and without the electric field it increases. Apparently, the reason for this is formation and subsequent decomposition of zirconium carbonate, leading to additional destruction of the oxidized layer and accelerated oxidation. When Zr is oxidized in CO2 in the presence of the electric field, a 12-fold increase in the rate of zirconium oxidation is registered, and this is associated with an increase in the surface leakage current due to formation of amorphous carbon.