Metalworking fluid degradation assessment by measurements of the electrical parameters at different temperatures

Metalworking fluids (MWFs) are currently used in the metal cutting industry for lubrication and to lower the temperature of both machine tools and worked pieces. Most MWFs are emulsions obtained by mixing an oil product with tap water in proper concentration (usually in the range 1% to 10%). In addition MWFs usually contain also other compounds, such as anti-microbial agents, emulsifiers and anti-corrosion products. MWFs degrade over time due to microorganism growth and contamination by processing by-products (such as tramp oil). Once worn-out the fluid must be properly disposed according to regulations using techniques such as chemical waste treatment, membrane filtration, evaporation or biological treatment. Since MWFs disposal and replacement represent a cost and have an impact on the environment, the product life-cycle must be extended as long as possible. Thus, assessment of the MWF degradation must be carried out at regular times. This work presents a technique, based on Electrical Impedance Spectroscopy, to assess MWF degradation and estimate the fluid pH value, a parameter playing an important role in the degradation process. The proposed method is based on the measurement of fluid impedance spectrum between 20 Hz and 2 MHz at four different temperatures (15, 25, 35 and 45 °C). The data are analysed by means of Principal Component Analysis (PCA) and the results show the feasibility of fluids clustering according to the contamination level with good accuracy.