Interaction of titanium with atomic and molecular oxygen

Abstract

Oxidation of α-titanium has been studied under conditions simulating low Earth orbits i.e. in atomic oxygen at an incident flux of 1016–1017 particles cm–2 s–1. The temperature range was 873–1123 K, in which α-modification of Ti exists. The kinetics of oxidation in atomic oxygen and in low-pressure (0.01–0.1 Pa) diatomic oxygen have been investigated in the initial stages of the process (i.e. before oxide scale formation) in relation to oxygen flux and temperature. The flux of oxygen dissolving in the metal across and metal/gas interface was determined by electrical resistance measurements. Linear rate law was observed at the initial stages of oxidation. For the dependence of reaction rate on diatomic oxygen pressure (or incident flux), the order with respect to pressure was found to be below unity (0.65 at 973 K and 0.82 at 1073 K). A high value of activation energy was obtained, 183 kJ mol–1. A kinetic explanation of these observations has been proposed. Strong acceleration of oxidation in dissociated oxygen was found only at temperatures below 973 K. The reaction rate increase four-fold with a degree of oxygen dissociation of 28.7% at 923 K and two-fold at 973 K. This increase in oxidation rate was confirmed by gravimetry and measurements of the titanium lattice parameter.