No abstract

KINETICS OF OXIDATIONTable III. Comparison of parabolic rate constants for Sandvik and Batch B tubing at 1153 o and 1504~ 1111 Data set Parabolic Parabolic rate constant" rate constant* (Oxide) (Alpha) 8~/2, Difference* 9 ~/2, Difference * * cm~/sec x lO s (%) cm~/see x 10 8

The kinetics of oxidation of Zircaloy-4 in steam were examined under isothermal conditions from 900 ~ to 1500~C. Rate curves describing the growth of the reaction product layers, oxide-and oxygen-stabilized alpha, were obtained at 50~ intervals under carefully defined and documented experimental, conditions. The early stages of oxidation for temperatures above 1000~ were shown to be virtually ideally parabolic; below 1000~ the growth of the oxide layer was nonparabolic. Statistical evaluations of the data in terms of the rate constants and their Arrhenius parameters established the high degree of self-consistency of these measurements. The features of the experimental apparatus and techniques that were necessary to insure accurate measurements are discussed.

Thin rectangular specimens of tantalum were oxidized on one side at 500°C and 1 atm oxygen pressure. From curvature measurements, the bending stresses arising as a consequence of the oxidation process were determined and correlated with known oxidation rate behavior and oxide morphology. The simplest stress model, that of a uniform film on a substrate, was shown to be an inadequate description of the stress source, thus reflecting the complexity of the tantalum‐oxygen reaction. A model relating the stress gradient to the oxygen concentration gradient was tested and shown to be reasonably consistent with the experimental data.

The influence of anodic films up to 3000Aå thick on the subsequent high‐ temperature oxidation characteristics of tantalum was studied. At a given temperature, the anodic films decreased the gross oxygen consumption rate to an extent dependent on their thickness and also delayed the onset of the “breakaway” increase in the oxidation rate common to this metal. The result that very thin anodic films pass comparatively large quantities of oxygen to the metal while postponing the appearance of the crystalline oxide phases permitted some oxygen solution effects during the early stages of oxidation to be observed.