Microstructural evolution of pre-oxidized Cr-coated Zry-4 during annealing in argon

Abstract

The annealing behavior of the pre-oxidized Cr-coated Zry-4 at 1200 °C in argon is systematically investigated. A Cr2O3 scale with a thickness of ∼9 μm formed on the sample surface after pre-oxidation of the Cr-coated Zry-4 in steam at 1200 °C for 30 min. During annealing in inert atmosphere, the thickness of the Cr2O3 scale decreases with the increase of the annealing time because of the reaction between the Cr2O3 scale and the outward diffused Zr at the Cr2O3/Cr interface, the decomposition of Cr2O3, and the inward diffusion of O into the Zircaloy substrate along the ZrO2 precipitates at the Cr grain boundaries. The Cr2O3 scale has completely transformed into Cr after annealing for one hour. Pores and Cr grains are observed inside the Cr2O3 scale after annealing due to the decomposition of Cr2O3. The ZrO2 precipitates at Cr grain boundaries gradually grow outward during the annealing and finally reach the outer surface of the Cr coating. The results in this paper provide new insights into the thickness decrease mechanism and the decomposition behavior of the Cr2O3 scale.