Base oil oxidation detection using novel chemical sensors and impedance spectroscopy measurements

Abstract

Lubricating oils are employed in machinery to minimise friction, remove heat and wear particles, protect against corrosion, etc. Mineral base oils are derived from natural crude oil through a distillation process and are divided into five main groups depending on volume of sulphur, saturate and viscosity index, according to the American Petroleum Institute (API). As lubricating base oil oxidises, acidic by-products are formed and oil viscosity increases due to progressive polycondensation of higher molecular weight products. In this work, thick film (TF) potentiometric sensors based on ion-selective electrodes (ISE) have been developed for oil acidity measurement to enable online oil condition monitoring. A TF ruthenium oxide (RuO2) working electrode and three types of TF reference electrodes were fabricated and tested in oxidised base oil samples prepared at the Shell Houston laboratories. The TF sensors were evaluated by comparison with acid number (AN) measurement and also compared with oil conductivity measurements using impedance spectroscopy. The results show that the conductivity, viscosity and AN of the oil samples increase with the level of oil oxidisation. The output of the TF sensors detected the acidity increase of the oxidised oil samples at both 50 and 80°C and displayed a linear relationship with oil AN.