Mechanism studies have shown that the primary factor controlling the hydrogen absorption of zirconium alloys in gaseous atmospheres containing hydrogen is the rate at which oxidant (H2O) is transferred to the metal surface from the environment. The oxidation rate of Zircaloy-2 is shown to be pressure independent from 1500 psi to below 1 mm water vapor pressure. When an oxidant is present remarkable inhibition of gas phase hydriding at 400 C occurs even up to 1000 psi hydrogen pressure. However, at sufficiently low oxidant concentrations in the presence of an inert carrier gas, the oxidation rate can be reduced, due to local depletion of the oxidant. Once the oxidation rate is diffussion limited, catastrophic hydriding can occur at low hydrogen pressure (7 mm Hg), and relatively low temperatures (150 C), in a few days' time, with pre-existing autoclave films offering only transitory protection.
The electrical resistance of ZrO2 corrosion films decreased in the order of 106 ohms when the atmosphere was shifted from oxidizing to nonoxidizing, probably due to an increase in the oxygen anion vacancy concentration. This suggests the initiation of gas phase hydriding may be related to a higher hydrogen diffusion rate made possible by the higher concentration of oxygen vacancies in the film.
