Controlling the Corrosion of Copper Alloys in Engine Oil Formulations: Antiwear, Friction Modifier, Dispersant Synergy

The next generation of engine oil under development has been formulated to maintain beneficial oil lubrication properties at increased engine operating temperatures, increased drain-oil intervals, and with the recirculation of exhaust gas back through the engine (EGR). These conditions result in the formation of degradation products from decomposed fuel, additives, and base oil. Decomposition products containing reactive sulfur can result in the corrosion of copper alloys. Sulfur-containing compounds currently used in these formulations can include zinc dithiophosphates (ZDP), molydithiophosphates, molydithiocarbamates, and molybdic acid/amine complexes, along with sulfur containing detergents and antioxidants. Interactions among these components and others in the formulation often determine the propensity of these formulations for corrosion.

This paper will discuss the results of corrosion bench tests used to screen oil formulations for copper corrosion. Copper test coupon weight loss and surface appearance were determined along with the ppm concentration of copper found in the end of test oil. Test coupons were analyzed with X-ray photoelectron spectroscopy (XPS) to determine the resulting surface chemistry and to understand the mechanism for corrosion. Results of this study were used to develop oils which limit the corrosion of copper.