Abstract
The influence of oil/metal interactions on the oxidation of automotive crankcase oils (monograde and multigrade) is investigated in a modified turbine oil oxidation stability test apparatus, using oil soluble organometallic salts and fresh elemental metals of zinc, nickel and aluminium to simulate the homogeneous and heterogeneous oil/metal interactions respectively. The presence of the metals in the oils exhibits a catalyst-inhibitor conversion phenomenon by acting as auto-oxidants. Although the action of the metals in the two forms appeared non-specific, zinc and nickel were found to be more active oxidation catalysts in the heterogeneous form. Furthermore, the metals were found to cause significant reductions in sludge deposit formation in the oils by interfering with the polymerization of the oxidation products, through reactions that may involve metal-directed decarboxylation of carboxylic acids. In general, the metals showed more catalytic effects in the monograde oil relative to the multigrade oil and this is related to the differences in the types of hydrocarbons in the base oils.
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