The Effect of Niobium Additions on the Corrosion Behavior of Zircaloy-4

Abstract

Alloying additions of 0.5 and 1.0 wt% niobium, respectively, have been added to Zircaloy-4 in an attempt to improve its high-temperature corrosion resistance. Ingots of these modified alloys were fabricated to a 0.76-mm-thick sheet via a processing sequence compatible with commercial tubing production and were given one of four different final anneals. Subsequent testing indicated that the niobium additions had little or no effect on corrosion resistance in 360°C water. In 427°C steam, however, the 0.5%-niobium addition provided increased resistance to spalling, while the 1.0%-niobium addition decreased both cumulative weight gains and post-transition corrosion rates. The weight gains exhibited by the 0.5%-niobium alloy were relatively insensitive to final heat treatment, whereas the 1.0%-niobium alloy suffered a degradation in properties as the extent of the final anneal increased. These trends in corrosion performance were subsequently correlated with the second-phase particle size distributions present in the alloys, the best performance being obtained when the mean particle diameter was <400 to 500 Å. It was concluded that both niobium additions improved the corrosion performance of Zircaloy-4 at elevated temperatures, but that the best performance was obtained at the 1.0-wt%-niobium level.