Effect of worn metals on the oxidation of lubricating oils

Worn metals (copper, iron, Ni-Cr and aluminium) are found in used motor oils as a result of the friction of movable engine parts. Their effect on the deterioration rate of these oils was studied. Although iron and Ni-Cr were found to exist at lower concentrations than copper, their effect on the deterioration in the detergency power of the oils was stronger. This effect can be attributed to the high reactivity of their ions, which form complex compounds with the polar groups of the detergent additives tested. Copper was found to increase the oils' oxidation rate substantially and this can be attributed to its being a heterogeneous oxidizing catalyst, which resulted in the high oil deterioration. However, because copper has a low propensity to form complex compounds, its effect on the detergency power was relatively weak. When combinations of these metals were present the same characteristics were exhibited.