24th International Colloquium Tribology - January 2024 197 Copper Wire Resistance Corrosion Test for Assessing Potential Fluids as E-Thermal Fluids in BEVs Immersion Cooling Applications Bernardo Tormos 1* , Vicente Bermúdez 1 , Jorge Alvis-Sanchez 1 , Leonardo Farfan-Cabrera 2 1 Universitat Politècnica de València - CMT - Clean Mobility & Thermofluids, Valencia, Spain 2 Tecnológico de Monterrey - Escuela de Ingeniería y Ciencias, Puebla, Mexico. * Corresponding author: E-mail (betormos@mot.upv.es) 1. Introduction The electrification of the transport sector worldwide is paving the way for Battery Electric Vehicles (BEVs) to become a prominent part of the modern automotive landscape. To enhance BEVs’ efficiency, safety, and lifespan, immersion cooling is emerging as a promising solution to manage the thermal loads of high-capacity batteries (enabling fast charging), ensuring safe and optimal performance [1] . For such cooling systems, selecting an appropriate dielectric fluid is critical to prevent adverse effects on the immersed components, particularly copper components, commonly found in electric applications. While assessing various aspects of potential E-Thermal fluids, material compatibility holds significant importance, primarily owing to the anticipated extended lifespan of these fluids (fill for life). Consequently, conducting compatibility assessment tests becomes imperative, and copper being the primary conductor of electricity in this application, both the fluid and the copper materials must not negatively affect each other. The Copper Wire Resistance Corrosion Test (CWRCT) method has emerged as an important tool for assessing the compatibility of fluids with copper materials [2] . This study presents a CWRCT designed to assess different fluids as potential E-thermal fluids for BEV: a base stock Polyalphaolefin (PAO), a base stock API GIII, a synthetic base stock Diester, an electrical insulating oil (TRANSF), and a fully formulated dielectric coolant (AMPC). The experiment was conducted in collaboration with the Tecnológico de Monterrey University (TEC), where the PAO, the insulating oil, and the fully formulated oil were tested at their facilities in Puebla, Mexico, and the Diester, the API GIII, and another batch of PAO were also tested at CMT - Clean Mobility & Thermofluids research center at Universitat Politècnica de València (UPV), Spain. 2. Materials and methods The experimental setup consisted of placing 500 mL of each test fluid in a 1 L beaker. A 1 m length of 64 microns diameter (42 American Wire Gauge [AWG] caliber) copper wire was submerged in each fluid, while another wire was placed above the fluid to evaluate both oil and vapor phases. The test was conducted at a working temperature of 130°C (± 2ºC) for 336 hours. A DDM (Digital Data Multimeter) was employed to measure the resistance of the copper wires with a 1mA direct current. Figure 1. CWRCT in-house TEC setup The change (increase) in the resistance is an indicator of the corrosiveness of the fluid to the copper due to removal of material, since the resistance of a wire is provided by the formula: (1) Where R is the resistance of the wire, ρ is the resistivity of copper, L is the length of the wire, and d is the diameter of the wire. If the fluid is corrosive to the copper, the wire’s cross-sectional area will decrease, increasing the overall resistance. 3. Results and discussion Qualitative (SEM) and quantitative (resistance measurements and ICP measurements) aspects are considered in the results. 3.1 Resistance measurements The measurements obtained at Tecnológico de Monterrey showed that the wire in the PAO vapour phase failed after 140 hours, while the rest increased their resistance by less than 1,5%. Figure 2. Resistance measurements at Tecnológico de Monterrey (TEC). 198 24th International Colloquium Tribology - January 2024 Copper Wire Resistance Corrosion Test for Assessing Potential Fluids as E-Thermal Fluids in BEVs Immersion Cooling Applications Oscillations observed in the resistance measurements appear as a consequence of the high fluctuations in ambient temperature between day and night; taking into account that heating plates were used to control temperature; therefore, there is a higher difference in resistance between the vapor and oil liquid phases. Measurements obtained at Universitat Politècnica de Valencia (Figure 3), where a thermal bath was used in the setup, show more stable conditions (no oscillations) and smaller differences between vapor and oil phases. Figure 3. Resistance measurements. CMT With the exception of the wires exposed to Diester and PAO (TEC) in the vapor phase, most wires did not exhibit a substantial increase in resistance over the course of the experiment. When wire failure occurred, the presence of green droplets around the wire was observed (Figure 4). This phenomenon is often indicative of the generation of Copper (II) hydroxide (Cu(OH)2) as a corrosion byproduct, typically due to the presence of moisture. Figure 4. Green droplets were found in the PAO (TEC) and Diester wires in vapor phase. Table 1. SEM Element analysis FLUID Cu C O Si S PAO OIL TEC 65,45 27,69 6,71 - 0,15 PAO VAP TEC 30,65 43,19 25,84 0,18 0,07 AMPC OIL 67,49 25,95 6,38 0,08 - AMPC VAP 55,62 36,38 7,61 - - TRANSF OIL 61,96 26,54 10,76 0,13 0,55 TRANS VAP 66,98 28,84 4,18 - - PAO OIL CMT 72,91 22,83 4,09 0,1 - PAO VAP CMT 65,56 30,86 3,49 0,09 - DIEST OIL 73,32 21,7 4,74 - 0,05 DIEST VAP 34,86 44,68 20,16 0,06 0,17 G_III_OIL 72,66 23,07 4,27 - - G_III_VAP 60,13 33,39 6,4 - 0,07 3.2 SEM Portions of the wires were observed under a Scanning Electron Microscope (SEM) to analyze the effects of corrosion and quantify the elements found on the surface. Table 1 shows the material analysis on the surface of the wire and the percentage of elements found. The wires most affected by corrosion were the vapor phase of the PAO (TEC) and the vapor phase of the Diester; both showed a significant decrease in copper found on the surface and high amounts of oxygen. Figure 5 also shows the images of these two wires at 800x magnification and 20kV of power at the SEM. Figure 5. SEM images: a) PAO_VAP_TEC (Left) and b) DIEST_VAP (Right) 4. Conclusions Overall, resistance measurements, green droplets, SEM images, and the surface analysis of the failed copper wires are complementary evidence of corrosion. Ambient conditions might play an important role in the process of corrosion since the PAO only failed in one location. The methods of corrosion might differ, given the exponential failure of the PAO_VAP_TEC and the linear increase in resistance of DIEST_VAP. Further research must be conducted to determine viability of fluids as a E-thermal fluids. Other factors such as thermal and electrical performance, costs, biodegradability, and sustainability have yet to be considered, and they could greatly influence its suitability. 5. Acknowledgements This research was partly funded by the project CIAI- CO/ 2021/ 013 from GVA, Generalitat Valenciana. The authors also want to acknowledge “Programa de Apoyo para la Investigación y Desarrollo” (PAID-01-22) for financing the PhD. Studies of J. Alvis-Sanchez at Universitat Politècnica de València. References [1] Pambudi, N.A.; Sarifudin, A.; Firdaus, R.A.; Ulfa, D.K.; Gandidi, I.M.; Romadhon, R. The immersion cooling technology: Current and future development in energy saving. Alexandria Engineering Journal 2022, 61, 9509-9527. [2] Hunt, Gregory J., Michael P. Gahagan, and Mitchell A. Peplow. “Wire resistance method for measuring the corrosion of copper by lubricating fluids. Lubrication Science 29.4 (2017): 279-290.