Pre-transition oxidation behaviour of pre-hydrided Zircaloy-2

Oxidation tests with hydrogen present in three different states were performed: (i) Hydrogen in solid solution in the zirconium alloy, corresponding to the initial oxidation prior to precipitation of hydrides. (ii) Uniformly distributed hydrides, simulating a situation where hydrides start to precipitate and (iii) massive surface hydride, claimed to be the main cause of accelerated oxidation. The pre-hydrided samples and a reference (REF) sample were oxidised in steam in a static autoclave at 400°C and 10 MPa for 3, 10 and 25 days. The oxide formed was characterised by X-ray diffraction (XRD), transmission electron microscopy (TEM), secondary ion mass spectrometry (SIMS) and electrochemical impedance spectroscopy (EIS). Based on the results obtained, it is concluded that the oxidation of massive zirconium hydride resembles the oxidation of zirconium metal. This fact shows that the commonly observed accelerated oxidation of zirconium alloys cannot be due to an increased oxidation rate of the hydride compared to the metal,but must require a combined effect of, for example interfacial roughness and hydride precipitation. The results are discussed in relation to the mechanism of corrosion and hydriding of zirconium alloys.