Characterization of the Adsorption Mechanism of Manganese Phosphate Conversion Coating Derived Tribofilms

Casing connections in the oil and gas industry are typically coated with zinc and/or manganese phosphate for corrosion protection during storage. The presence of phosphate coatings is also known to give beneficial tribological performance. The coating allows the system to run without problems long after it is worn off. This is because of two mechanisms. Glaze layer formation on the coated surface and, as will be shown, tribofilm formation on the uncoated counter-surface. An investigation into the mechanism behind this tribofilm formation is presented in this paper. The aim is to develop lubricants that exploit these mechanisms. A pin-on-disc set-up was used to investigate the interaction of a manganese phosphated disc and bare counter surface. Six base oils with different polarity and viscosity were used. The resulting tribofilms were analysed using optical microscopy, scanning electron microscopy, X-ray diffractometry, X-ray photoelectron spectroscopy, focused ion beam, and atomic force microscopy. The tribofilm is robust, amorphous, and only forms in the presence of a lubricant under sliding conditions and adsorbs on substrates with a large variation in chemical composition. It is concluded that the tribofilm consists of physisorbed manganese phosphate and formation is shear stress activated.