Impedance characterization of industrial lubricants

This paper examines the application of electrochemical impedance spectroscopy (EIS) for analysis of the electrochemical properties of industrial lubricants and focuses on establishing a relationship between lubricant chemical composition and EIS data. Presented data demonstrate a need to rely on several EIS output parameters collected over wide frequency, electrochemical dc potential, temperature and electrode geometry ranges in order to assess complex processes and decomposition pathways occurring both at the interface and in the bulk solution. Impedance characteristics for three frequency regions were studied—bulk solution-influenced high frequency response, medium frequency parameters representing adsorption of surface active lubricant additives at the electrode interface, and low frequency characteristics related to diffusion and charge transfer processes. The modeling and analysis of the EIS data as a combination of changes in bulk, adsorption, diffusion and charge transfer resistances, capacitances and constant phase element (CPE) parameters allow studying the conditions of a lubricant. The electrochemical structure of a typical industrial lubricant through the use of an equivalent circuit (EC) model is presented. The model can be used for analysis of the impedance results as a reflection of chemical composition and changes in the system.