Effects of NOx on liquid phase oxidation and inhibition at elevated temperatures

Nitrogen oxides (NO_x =NO₂ +NO) play a role in degradation of lubricating oils in internal combustion engines. Interactions of NO_x with fuel derived species have been shown to lead to varnish and deposit formation. Mechanisms of interactions of NO_x with additive and base oil components have not, however, been investigated in detail. The present work describes investigations of the effects of NO₂ on oxidation processes and the fate of NO_x in a model lubricant system. NO₂ acts as a free radical initiator which accelerates the initial rate of uninhibited hydrocarbon oxidation and increases the rate of antioxidant consumption in inhibited systems. NO_x consumption increases after the end of the inhibition period. In addition, NO₂ and NO react with oxidation intermediates or products in a complex series of reactions which can include direct reaction with antioxidants, formation of nitro-oxidation products, conversion of NO₂ to NO and regeneration of NO₂ from NO. The significance of results obtained in this study with respect to mechanisms of oil degradation is discussed.