The surface chemistry of zinc dialkyldithiophosphate, an antiwear additive, on oxidized iron and steel foils.

The chemisorption and thermal chemistry of zinc dialkyldithiophosphate (ZDP) were studied on iron oxide and AISI 52100 steel substrates using X ray photoelectron spectroscopy (XPS). Solutions of alkyl and aryl ZDP's in a synthetic hydrocarbon oil (poly alpha olefin (PAO)) were used. Surface oxide layers on both iron and steel foils were reduced to metallic iron when heated with a film of PAO on the surface. The addition of ZDP to the films prevented reduction. XPS revealed that a surface sulfate was reduced to sulfide upon heating. Several possible mechanisms for the surface antioxidant activity of ZDP. ZDP prevents surface oxide reduction, maintaining an easily sheared surface layer, preventing metal metal contact and wear.