Inhibitors and conductivity
All of us once were warned that water and electricity do not make a good combination. In the light of the enlarging importance of electrification of vehicle fleets, a specific physical property of an engine coolant, electrical conductivity, increasingly gets into the spotlight. Electrical conductivity, or in other words the capability of a fluid to conduct electricity, is dependant of the amount of electrolytes that dissolve into the polar solvent where they form both anions and cations. These charged ions are able to effectively carry an electrical current when a potential difference is applied. One could envisage the ions as very small particles bringing a charge from place A to B within a liquid.
Electrical conductivity of an engine coolant
An engine coolant consists of both a suitable solvent to dissolve electrolytes, water, and a significant amount of electrolytes or salts, being the corrosion inhibitors and other ionic additives. In Organic Additive Technology, organic acids are typically neutralized with sodium (NaOH) or potassium hydroxide (KOH) and are thus present in the water-glycol mixture as carboxylate salts being ionic species. Inorganic corrosion inhibitors, such as silicates, phosphates, etc… are ions as well and can as well contribute to the total electrical conductivity of an engine coolant.
In some cases, electrical conductivity is applied to estimate the quality of an engine coolant, just as electrical conductivity is used to assess quality of wastewater. Indeed, a higher electrical conductivity relies to having more inhibitors in a coolant. However, this relation is far from linear, especially when considering higher amounts of electrolytes in the solution (Figure 1) as is the case in a coolant. Moreover, each type of ion (and there are many different ones in a coolant) contributes differently to the total electrical conductivity depending its charge and size. Small ions such as H+ or OH- have a higher mobility in a fluid, while larger ions typically experience more ‘drag’. Also one needs to account for the amount of water molecules that ‘stick’ to an ion to determine its effective size in water. Therefore, electrical conductivity should not be considered as a measure for coolant quality.
Figure 1: Non-linear behaviour of NaCl solution at high NaCl concentration in water
More concrete: when two coolants with an exact same inhibitor content and thus very similar performance, are neutralized either with NaOH or with KOH; then the coolant with KOH has approx. 35% higher electrical conductivity (Figure 2). Also the glycol used as freezing point depressant plays a role in the electrical conductivity; the same inhibitor combination is typically 30% less conductive in a 50v% solution of propylene glycol in water than in a 50v% ethylene glycol solution. In water only, the conductivity is approx. three times higher than in the ethylene glycol solution. These differences are explained by the other mobility or drag of ions in the different solutions. Also temperature affects electrical conductivity since ionic mobility in enhanced at higher temperatures.
Figure 2: Comparison of electrical conductivity of coolant with the same inhibitor type and concentration. Variation of counter-ion and base fluid.
Electrical conductivity in e-Mobility
Given the ionic nature of most additives in an engine coolant, conductivities are typically well above 1000 μS/cm, where 5000 μS/cm is not an exception in water-glycol mixtures (Figure 2). Applying such liquids on electrical components (battery, electric motor, power electronics,…) requires these to be physically separated for the component to avoid short circuits or performance loss through eddy (stray) currents. In electrical vehicles this is ensured with bottom cooling plates, heat sinks or coolant jackets where a standard engine coolant flows through to ensure cooling (or heating) of the part. For safety reasons, one may consider to reduce the conductivity to lower values. However, as described above, this is not without effect on either the heat transfer (increasing glycol:water ratio) or corrosion protection (reduction of inhibitors). In this perspective, we are developing e-coolants with reduced conductivity whilst maintaining a performance level accepted by the automotive industry.
Finally, in a fuel cell vehicle, the fuel cell requires cooling directly on bipolar plates, which also collects and conducts the electricity generated in the fuel cell. Hence, the electrical conductivity of the coolant needs to be and stay as low as possible. To ensure operation and safety of the system, an ion exchange device is installed in the fuel cell cooling system to remove all ions that generate over time due to corrosion, oxidation or impurities. On the coolant side, completely new, non-ionic additive technologies are under development at Arteco to ensure a Fuel cell coolant with long lasting performance.