Effect Of β-quenching And Annealing Treatments On The Corrosion Resistance Of Zircaloy-4

Zirconium alloys have been commonly used as the nuclear fuel cladding for a lowthermal neutron absorption cross-section and good corrosion resistance. Theimprovement in corrosion resistance can deep burn-up, extend refueling periodin-reactor. In this paper, Zircaloy-4specimens were treated by β-quenching withdifferent cooling rates and annealing treatments （480―600°C/2―200h）, andcorroded in three dirrerent water chemistries （360°C/18.6MPa/deionized water,0.01M LiOH and0.04M LiOH） by autoclave tests. The microstructure ofspecimens and oxide films were observed by TEM and SEM respectively. The Feand Cr concentrations in α-Zr matrix was examined by APT and WDS. The effect ofβ-quenching and annealing treatments on the corrosion resistance of Zircaloy-4specimens is discussed. The main results and conclusions are listed as follows:1. The effect of β-quenching rate and annealing parameters on the microstructureof Zircaloy-4specimens is as follows:（1） β-quenching rate has a great effecton the width of lath grains. Most of lath grains are small and there are a fewresidual β-Zr in specimens500°C/s-β-WQ. But the lath grains of specimens100°C/s-β-WQ are much bigger, and the annealing treatments （480―600°C/2―200h） have little effect on the width of lath grains.（2）Few second phaseparticals （SPPs） are found in specimens500°C/s-β-WQ. While in specimens100°C/s-β-WQ, fine SPPs in lath grains and coarse SPPs at the grainboundaries are found. Even after annealed at480―600°C for2―200h, thegrowth of SPPs is not obvious.（3） Almost all Fe and Cr dissolve in α-Zr forspecimens500°C/s-β-WQ. Fe and Cr supersaturate in α-Zr for specimens100°C/s-β-WQ. And the concetrations Fe and Cr in α-Zr decrease withincreasing annealing temperature and time after annealing treatment.2. The effect law of heat treatments on the corrosion resistance of Zircaloy-4isdifferent.（1） The corrosion resistance is similar for all specimens corroded in deionized water.（2） When corroded in lithiated water with0.01M LiOH, thespecimens with fast β-quenching rate show the worst corrosion resistance,while the other specimens show close corrosion resistance.（3） When corrodedin lithiated with0.04M LiOH, the heat treatments have a great effect on thecorrosion resistance. The specimens with fast β-quenching rate show the worstcorrosion resistance, and the specimens with low β-quenching rate exhibit thebest corrosion resistance. But when the specimens with low β-quenching ratewere then annealed at480―600°C for2―200h, the corrosion resistancedecreases with increasing annealing temperature. These results show that theeffect degree of microstructure on the corrosion resistance ofZircaloy-4increases with increasing LiOH concetration.3. The specimens with fast β-quenching rate, in which there is β-Zr and thehighest Fe and Cr concentrations in α-Zr, show the worst corrosion resistanceno matter in which kind of water chemistry. This indicates the exsitence ofβ-Zr and exorbitant Fe and Cr concentrations in α-Zr are harmful to thecorrosion resistance, and the harmful effect increases with increasing LiOHconcentration.4. Specimens100°C/s-β-WQ+540°C/200h,100°C/s-β-WQ+600°C/2h and100°C/s-β-WQ show the best corrosion resistance in deionized water, lithiatedwater with0.01M LiOH and0.04M LiOH respectively. This indicates withincreasing LiOH concentration, much higher Fe and Cr concentrations in α-Zrare needed to weaken the harmful effect of LiOH to get excellent corrosionresistance.5. Specimens corroded in different concentrations of LiOH have an obviousdifference in the morphology of oxide films.（1） For the specimens corrodedin deionized water, the outer surface of oxide films is even and consist ofequiaxedgrains with nanometer size.（2） For the specimens corroded inlithiated water, the ZrO₂columnar whiskers with hundreds of nonmeter inlength stretch out of the oxide film surface, coarsen and are formed into a bigcolumnar grains with increasing corrosion time.（3） According to the fracture surface morphology of oxide films, the proportion of equiaxed grainsincreases with increasing LiOH concentration. This indicates LiOHaccelerates the microstructural evolution of oxide films to decrease thecorrosion resistance.