Effect of Dilute Alloy Additions of Molybdenum, Niobium, and Vanadium on Zirconium Corrosion

Abstract

An alloy development effort was undertaken to improve the corrosion performance of zirconium-based alloys for high-burnup applications. As part of that effort, three binary alloys were studied to establish the relationship between alloy chemistry, processing, microstructure, and corrosion behavior. the binary additions included molybdenum, niobium, or vanadium at concentrations ranging from 0.1 to 2.5 weight percent. Process variables included cooling rate from the beta phase, aging temperature from 500 to 700°C, and aging times up to 1000 h. The corrosion behavior of these alloys was then studied by testing in 360°C water and 427°C steam and compared to Zircaloy-4. The effect of alloy content and processing was monitored by characterizing the microstructure of the material. In general, quenching from the beta phase formed martensite for all three alloys with essentially all alloy additions remaining in solution. Subsequent working and aging treatments resulted in precipitation and precipitate growth. These changes in the microstructure were correlated to changes in the corrosion behavior. Typically, corrosion performance initially improved with aging followed by gradual degradation with continued aging. While this trend was observed for all three alloys, the optimum aging treatment was dependent upon alloy chemistry and composition. The results showed that the corrosion behavior was dependent upon the amount of alloying element present in the alpha zirconium matrix, provided the precipitates remained less than a critical size. This critical size was alloy dependent. The vanadium alloy was observed to be more sensitive to heat treatment than the molybdenum and niobium alloys. For all three alloys, corrosion performance superior to Zircaloy-4 could be achieved by an optimum aging treatment. The details of the relationship between heat-treatment, microstructure, and corrosion response are presented and discussed.