STUDIES OF THE REACTION OF ATOMIC AND MOLECULAR-OXYGEN WITH NIOBIUM AND NIOBIUM (1AT-PERCENT ZIRCONIUM) (0.7AT-PERCENT CARBON) (PWC-11) ALLOY AT LOW-PRESSURES

The interaction of atomic and diatomic oxygen with niobium and niobium -(1 at.% zirconium)-(0.7 at.% carbon) alloy has been studied between 7 73 and 1373 K at an oxygen pressure in the range 9 X 10(-3)-7 X 10(-2) Pa. At the initial stage of oxidation, when the dissolution of oxygen in niobium or alloy takes place, both the diatomic and atomic oxygen absorption probabilities are independent of the incident flux, apparen tly the rate of reaction is determined by the oxygen chemisorption on the metal surface. Atomic and molecular oxygen absorption probability at rather high temperatures are also almost independent of temperature , whereas at lower temperatures molecular oxygen absorption probabilit y demonstrates a strong temperature dependence. Oversaturation of the metal surface with dissolved oxygen is observed if oxidation occurs in dissociated oxygen at a temperature below 973 K. At the oxide film gr owth stage, oxidation occurs according to a parabolic law, consequentl y the reaction rate is determined by diffusion through the oxide film. Nevertheless dissociation of oxygen increases the reaction rate.