Inhibitors for Reducing Hydrogen Ingress During Corrosion of Zirconium Alloys

Zirconium alloys, used in manufacturing nuclear core components, particularly fuel cladding (in most reactor types) and pressure tubes in CANDU® (Canada Deuterium Uranium) reactors, are usually subject to aqueous corrosion and hydrogen uptake during service. The build up of hydrogen concentration during operation can be a life-limiting factor for these components. In order to slow down the rate of hydrogen uptake and to prolong the operating life of these critical reactor components, a remedial action was targeted. A comprehensive experimental program has been carried out to identify potential inhibiting agents to be introduced into the environment of the corrosion process. The inhibiting agents were expected to intervene in the corrosion reactions to reduce the hydrogen uptake rate. Screening experiments were conducted in short-term exposures (up to 30 days) of coupons from several zirconium alloys, such as Zircaloy-2, Zircaloy-4 and Zr-2.5 weight % Nb, in high temperature (340°C) aqueous out-reactor environments simulating the CANDU heat transport coolant with various chemical compound additives. Several additives of nitrogen-containing compounds appeared to reduce hydrogen ingress significantly; up to 90 % reduction was observed. The beneficial effect of the presence of nitrogen on hydrogen uptake in these exposures might be related to its role in the production of ammonia from hydrogen liberated by the zirconium corrosion reactions. The ammonia generation mechanism has been proposed in the Ceramics literature as a means of increasing the yield of cubic zirconia during the hydrothermal oxidation of Zr in the presence of calcium nitrate. The formation of ammonia is associated with a significant reduction in the hydrogen available for pickup by the alloy. This hypothesis was confirmed in our experiments by the detection of ammonia in the post-exposure solutions. For all tested solutions with nitrogen-containing additives, ammonia was detected in the post-exposure solutions, while no ammonia was found in the post-exposure control solution or other solutions with non-nitrogen additives such as boric acid. Results from post-exposure analyses, oxide characterization, and a preliminary investigation into radiolysis implications are presented.