CHEMISTRY OF THE EXTREME-PRESSURE LUBRICANT ADDITIVE LEAD NAPHTHENATE ON STEEL SURFACES

The adsorption and chemical reactivity of the extreme-pressure (EP) oil additive lead naphthenate (Pbnp) on steel surfaces are examined with X-ray photoelectron spectroscopy. In addition, the chemical compositions of AISI 440C and 52100 steel surfaces are studied as a function of sample cleaning treatment, including solvent cleaning, and treatments with acidic and basic solutions. At room temperature, Pbnp is shown to physisorb on the iron oxide overlayer present on both steel surfaces following solvent cleaning. A chemisorbed form of Pbnp is characterized by Pb 4f peaks chemically shifted by 0.3-0.4 eV to lower binding energy and significantly lower intensity of the C 1s feature associated with the Pbnp carboxylate group. This form of Pbnp is observed on acid- and base-pretreated surfaces that lose their iron oxide overlayers. The chemisorbed Pbnp surface species is also observed when the steel surfaces are scratched while being immersed in the Pbnp solution. When the Pbnp treated steel surfaces are heated to simulate the EP conditions for which Pbnp is used, most of the Pbnp on the oxide-covered surfaces desorbs. In addition, metallic Pb is readily formed on the scratched surfaces and on surfaces with significant amounts of metallic Fe. These results point to two possible modes of boundary protection: chemisorbed Pbnp under mild wear conditions and a layer of metallic Pb under severe wear conditions.