ENHANCED OXIDATION OF NICKEL IN ATOMIC OXYGEN

The oxidation of nickel in atomic oxygen has been investigated at the conditions close to those of a low Earth orbit (at an atomic oxygen in cident flux of 10(16)-6 X 10(16) atoms cm(-2) s(-1)) and in the temper ature range 773-1373 K. The rate constant of nickel oxidation in atomi c oxygen at low pressures was found to be higher by orders of magnitud e than that in diatomic oxygen having the same pressure even at these rather high temperatures. As regards the dependence of oxidation rate on oxygen pressure, while in diatomic oxygen the parabolic rate consta nt of nickel oxidation is proportional to P-O2(0.35), in atomic oxygen it is proportional to P-O(0.72) (the power is twice that in O-2) Also , the activation energy of oxidation decreases from 182 kJ mol(-1) for molecular oxygen to 76 kJ mol(-1) for atomic oxygen. A model is propo sed explaining these observations, which is based on the assumption th at atomic oxygen in the gas phase and the oxygen chemisorbed on the Ni O surface are close to equilibrium