Effects of Alloying and Irradlation on the Oxidation Behaviour of Zirconium Alloys

Abstract

Observationson the oxidation behaviour of zirconium-niobium alloys in pressurised water at 300° indicate a sensitivity to oxygen content of the water and niobium content of the alloy which can be reconciled with the concept of an oxide film of duplex type, the inner portion being of the normal hypostoicheiometric n-type but changing unnder the relatively oxidising conditions in the outer layers to a hyperstoicheiometric p-type oxide contaning Interstitial oxygen ions by virtue of the presence of some Nbv ions.There are also preliminary indications of a similar type of sensitivity of corrosion rate to oxygen content of the water wIth a 0·75 wt.-% zIrcomum-ehromium alloy. Experiments carried out with Zircaloy-2 at 290° in water dosed with chromium trioxide and with a 1500 lb/in2 overpressure of oxygen to produce chromate ions in solution have shown an increase in corrosion rate suggesting that doping of the zirconia film with higher valent chromium ions is occurring during the test.There are analogies between the out-of-pile corrosion behaviour of zirconium-niobium alloys under oxidisng conditions and the behaviour of Zircaloy-2 under oxidising conditions in-pile. It is suggested that, owmg to different trapping efficiencies in the longer term there is an imbalance between the concentrations of oxygen vacancies and interstitials under irradiation, and that further, there will be an interaction with the external oxygen pressure such that the concentration of oxygen interstitials will increase at the expense of the vacancy concentration as the oxygen pressure increases. The converse appears to be the case when the 2·5 wt.-% niobium-zironium alloy is subjected to high fast neutron flux under oxidising conditions; a lower rate of oXld~~I~nISobserved compared with the out-of-pIle value under comparable conditions, and the lack of sensitivity to oxygen potential and the appearance of the oxide suggests predominant oxygen transport via vacancies, i.e n-type behaviour.