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

Abstract

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 are discussed. The results of this investigation suggest a new model for the antiwear activity of ZDP. ZDP prevents surface oxide reduction, maintaining an easily sheared surface layer, and thus preventing metal–metal contact and wear.

Phosphate Esters, Thiophosphate Esters and Metal Thiophosphates as Lubricant Additives

“…A number of observations have been reported, including the necessity of phosphate being present in the oil to form a wear resistant surface, and the observation that once the phosphate is removed from the solution, the sulfur causes corrosion within the wear scar [36]. Other techniques have shown that ZDDP prevents the reduction of surface oxides by the lubricant basestock, leaving an easily sheared surface layer that prevents direct contact between the surfaces [37]. It is commonly thought that ZDDP forms a series of pads on the surface of the bearing.…”

Additive properties of saturated very long chain fatty acids in castor and jojoba oils

“…On the other hand, ZDTP has been known to be a good antioxidant additive in mineral oil base-stocks; similar to the natural biodegradable base-stocks observed in Table 3. Two studies [17], [18] reported its antioxidant mechanism to be through the decomposition of peroxides and terminating chain reactions involving peroxy-radicals formed during hydrocarbon oxidation.…”

The Tribology and Chemistry of Phosphorus‐Containing Lubricant Additives

“…Studies have demonstrated that phosphate must be present in the oil for a wear-resistant surface and once the phosphate is depleted, sulfur causes corrosion inside the wear scar [35]. ZDDP has also been shown to stop the reduction of surface metal oxides the lubricant, forming an easily sheared layer that prevents direct contact between the two surfaces [36]. A commonly proposed mechanism for the reduction of wear involves the formation of pads that cover most of the surface of the bearing.…”