THB MOST common method of hydrogenation of zirconium for experimental purposes is annealing in gaseous hydrogen. Like autoclave treatment, gaseous hydrogenation is a relatively slow process, particularly if the material is not carefully cleaned or oxidizes simultaneously.
Electrolytic hydrogenation in molten salts containing hydrogen was found to be a rapid process for stainless steel and this method was therefore tried also on zirconium.
Two different molten salt electrolytes were used: 3 parts KOH plus 2 parts NaOH at temperatures above 220ยฐC and NaOH above 320ยฐC. l-mm thick sheet specimens of Zircaloy-2 were cathodes, the iron crucible being the anode. Hydrogen evolution started at a cell voltage of 2-2 V, when the cd exceeded 2 A/dmB. Only about O-1 % of the hydrogen evolved was absorbed by the material. The hydrogen content varied linearly with the cd. To obtain an amount corresponding to 250 ppm in 5 hr at 4OO"C, 15 A/dm2 was required. A slight difference in the rate of absorption was noticed between Zircaloy from different manufacturers. No particular surface preparation was needed.
At the lowest temperatures indicated for the respective electrolytes, a surface layer of hydride was formed, while the hydrogen content of the interior remained essentially unchanged during the relatively short periods of time involved. To distribute the hydrogen uniformly in the specimen, annealing at a higher temperature was required. At 400ยฐC, 5 hr was required to obtain an even distribution of 500 ppm in a 0.7-mm thick tube specimen.
Observation in a microscope of specimens cooled in the diffusion furnace at the same rate as during autoclave treatment showed that the hydride structure was normal. There was no difference in structure at varying rates of hydrogenation.