The Effect of ZDP's on the Surface Chemistry of Hydrocarbon Films on Oxidized Iron Substrates

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Abstract
The thermal chemistry of lubricant molecules adsorbed on poly-crystalline iron substrates has been studied using X-ray photoelectron spectroscopy (XPS). The lubricants examined were a synthetic, poly-α-olefin base oil and a base oil containing three different zinc dialkyldithiophosphates (ZDP's); isooctyl ZDP, isopropyl ZDP, and an aryl ZDP. The major focus of the work was to determine the influence of the base oil and additives on the thermal reduction of the native iron oxide passivating film. The kinetics of iron oxide film reduction were analyzed using a sequential reaction scheme, i.e., Fe111⇌Fe11⇌Fe0, with temperature-dependent rate constants determined by fitting to time- and temperature-dependent XPS data. The authors conclude that the presence of base oil accelerates oxide film reduction and that ZDP moderates this effect to some degree.