Abstract
The oxidation reaction of zirconium in air was studied in the temperature range 650°∼850°C. The reaction was followed up by measurements of weight change, chemical analyses of nitrogen absorbed in the oxidized specimens, and electron microscopic observations of the surface and cross-section of specimens. Above 750°C, a significant breakaway phenomenon in the oxidation rate was observed, and the rate changed from an initial cubic or parabolic law to a linear one. Analyses of nitrogen indicated that the absorbed nitrogen, concentrated in the metal phase near the metal/oxide interface, could not be stable in the post-break away stage. The results of electron microscopic observations indicated that the cracking in the inner oxide layer did have a correlation with the change in kinetics, but the cracking at the surface had no correlation to it. Electron micrographs of cross-sections also showed that the metal phase near the oxide layer had a greatly different appearance in structure in the post-breakaway stage from that in the initial stage. The mechanism of heavy break-away phenomenon in oxidation zirconium in air was discussed on the basis of the observations in electron micrographs with emphasis on the function of nitrogen.
