HIGH-TEMPERATURE OXIDATION BEHAVIOR OF AMORPHOUS ZR-NI ALLOYS IN AIR

Oxidation behavior of amorphous Zr-33 Ni-67 and Zr65Ni35 alloy ribbons of ca. 20 mu m thickness was studied in air at 800 K by DSC, TGA, XRD and SEM/EDX. It was found that the preferential oxidation of Zr takes place on both Zr-Ni alloys. Zirconium in the alloy was almost complet ely oxidized with a high oxidation rate before the oxidation of Ni sta rted because of the small alloy thickness. The oxidation process of th e pre-crystallized Zr33Ni67 alloy was essentially the same as for the amorphous counter part. The oxidation rate followed the parabolic rate law after the completion of preferential oxidation of zirconium, and the parabolic rate constant was ca. 5.4 x 10(-3) g(2) . m(-4). s(-1). The oxidation of the amorphous Zr65Ni35 alloy had two parabolic stages within the experimental time span up to 18000 s; the parabolic rate c onstant was 2.2 x 10(-3)g(2) . m(-4). s(-1) in the first stage and 1.0 2 x 10(-2) g(2) . m(-4). s(-1) in the second stage which began by the cracking of the surface film Oxidation products were ZrO2 and NiO, met allic nickel remained on both alloys, and Zr2Ni and ZrNi, on the Zr65N i35 alloy within the experimental time span. Tetragonal and monoclinic ZrO2 were produced. Tetragonal ZrO2 was major on the Ni-rich alloy, a nd the monoclinic ZrO2 on the Zr-rich alloy. Porous cocoon- or worm-li ke NiO developed on the surface by diffusion of nickel through pinhole s and/or cracks of the Zr-enriched protective surface him. Most of the oxidized nickel was, however, embedded inside.