Chemical Reactivity of Triphenyl Phosphorothionate (TPPT) with Iron: An ATR/FT-IR and XPS Investigation

The surface reactivity of triphenyl phosphorothionate (TPPT) on air-oxidized iron surfaces has been studied by in situ attenuated total reflection Fourier transform infrared spectroscopy (ATR/FT-IR) and ex situ X-ray photoelectron spectroscopy (XPS). The ATR/FT-IR spectra indicate that a reaction took place at 423 K on the iron-coated germanium ATR crystal with the formation of pyrophosphates, organophosphates, and sulfates. The oxidation of the base oil (poly-a-olefin, PAO) during thermal testing, leading to the production of oxygenated compounds that can adsorb onto the iron surface and react with it, resulted in the presence of carbonates and carboxylates in the film. As the heating time increased, the corrosion of the iron coating by the aggressive species formed during the thermooxidative aging of PAO provided the cations necessary for the formation of orthophosphate compounds. The subsequent ex situ XPS analysis, besides substantiating the formation of short-chain polyphosphates, organophosphates, and sulfates on the air-oxidized iron surface, indicates the presence of oxygenated organic compounds in the outermost part of the reaction layer. Modeling the ATR/FT-IR system allowed the correlation of the changes observed in the experimental ATR/FT-IR spectra with the reflectivity alterations caused by the formation of reaction products on the iron-coated germanium ATR crystal. On the basis of these findings, a mechanism for the reaction of TPPT at the oil/iron oxide interface has been proposed.