PNNL

Abstract

Coatings of metal specimens is known to have an impact on hydrogen gettering (hydrogen ab-sorption). The coating can have one or multiple functions, such as enhancing gettering, preventing gettering and/or preventing oxidation of the metal substrate. It is known that contaminants and surface texture can impact hydrogen gettering/absorption performance, but has not previously been thoroughly explored. This study evaluates the role of different post-plating heat treatments of nickel plated zircaloy-4 getters (NPGs) and the role of the heat treatments on gettering rates and texture. The nickel-plating is applied to prevent oxidation of Zircaloy-4 surface and also enhances gettering. Typically, the nickel-plating must be heat treated before desirable gettering can occur. Our NPG getter with known satisfying performance has been pre-heat treated in air followed by activation heat treatment in vacuum at higher temperature. Here we were interested in finding out if both heat treatment steps are necessary to obtain desirable gettering performance or if one step can be omitted. XPS analysis showed that if the nickel surface is not heat treated before bonding the nickel to the zirconium, there will be carbon contaminants on the surface which significantly re-duces gettering. We studied texture of Zircaloy-4 using SEM/EBSD to compare NPGs with both heat treatment steps with NPGs that had no post-plating heat treatment to learn if the degree of cold-work can be impacted by the heat treatment steps. We did not observe any differences in texture between them. We measured gettering rates of both pretreated and activated NPGs and NPGs that had been activated without first pre-heat treated. We found that the NPGs without the first post-heating step had up to 7 times slower gettering rate and obtained higher plateau pressures due to the contaminated surface. Thus, the pre-heat treatment in air before activation is necessary to avoid slower gettering rates and higher plateau pressures.