INTERACTION OF TITANIUM WITH ATOMIC AND MOLECULAR-OXYGEN

Oxidation of alpha-titanium has been studied under conditions simulati ng low Earth orbits i.e. in atomic oxygen at an incident flux of 10(16 )-10(17) particles cm(-2) s(-1). The temperature range was 873-1123 K, in which alpha-modification of Ti exists. The kinetics of oxidation i n 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 o xide 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 ord er 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 p roposed. Strong acceleration of oxidation in dissociated oxygen was fo und 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-fo ld at 973 K. This increase in oxidation rate was confirmed by gravimet ry and measurements of the titanium lattice parameter.