Thermal stability of coolant
Since the start of the use of cooling liquids in combustion engines stability against heat is one of the major challenges. Stability of the cooling liquid is in many cases a question of the simultaneous action of excessive heat and oxygen. The base fluid, i.e. the combination of water and organic freeze depressants, such as monoethylene glycol (MEG) and monopropylene glycol (MPG) and combinations of the aforementioned substances, are prone to heat induced oxidation in the presence of air. The corrosion inhibitor package, consisting of organic acids (OAT) and mineral (inorganic) corrosion inhibitors are in most cases less prone to heat induced oxidation, but might suffer from heat induced alteration of their physical properties (insolubility, agglomeration, precipitation) or chemical nature (oligomerization, polymerization). Both effects, the thermal-oxidative stress on the base fluids and the heat induced alteration of the physical and chemical properties of the inhibitor package negatively affect the performance of cooling liquids (hereinafter “coolants”).
Thermal–oxidative stress on the base fluid
In the thermo-oxidative degradation of MEG, MPG and potentially glycerol/water mixture the reaction with oxygen (from air dissolved in the base fluid and always present in the expansion tank of a cooling system) is the first step of degradation. Although this primary step in degradation of the base fluid also takes place at ambient temperature, the average temperature of a coolant in operation, i.e. 90°C, accelerates the oxidation reaction dramatically. As a result of heat induced oxidation several acidic products are produced in a cascade, mainly gylcolate, oxalate and formate in case of MEG. As one could expect, the formation of acids drives the pH value down into the acidic rim below pH 7, the more and the faster the lower the reserve alkalinity of the coolant formulation is.
Fig.1: Thermo-oxidative degradation of Mono-Ethylene-Glycol (MEG, here “EG”); L. An, R. Chen, Journal of Power Sources, Vol. 329, 2016, p.481-504
Low pH values affect the efficiency of corrosion inhibitors and the corrosion resistance of many metallurgies. For instance, cast iron parts, present in engines and cooling systems lose their passivation layer in an acidic environment (pH <7). Mono Propylene Glycol (MPG) and glycerol (Gly) as freeze depressants in coolant formulations suffer from similar degradation patterns as MEG. Due to their different chemical structures the resulting acid cocktail is more complex: Formic, Oxalic, Acetic, Glycolic, Glyceric, Lactic and Pyruvic acid are formed, beside many non-acidic oxidation products. The resistance of the coolant against thermo-oxidative stress is mainly determined by its capacity to absorb acid formed during oxidation. Mainly strong buffer compounds such as borate, phosphate and amines contribute to the acid compensation capacity (expressed in terms of reserve alkalinity), but also the Organic Acid Corrosion inhibitors (OATs) can contribute substantially. Other additives that contribute to thermal stability of a coolant formulation, are compounds with metal deactivation and antioxidant properties. These compounds either reduce the catalytic effect of corrosion metals or interrupt the oxidation chain reaction of gylcols as shown in Fig. 1. Most of those compounds are nitrogen, sulphur or phosphorous based compounds.
Thermal stress on the corrosion inhibitor package
The corrosion inhibitor package consists of 2 basic chemistries: Organic Acid Corrosion Inhibitors (OATs) and Mineral (or Inorganic) Corrosion Inhibitors. Whereas the Organic Acid Corrosion inhibitors, are very stable under thermal stress and have retention rates of higher than 90% after long term heat exposure, many mineral inhibitors undergo physical and chemical alteration under the same conditions. As a result, the retention of mineral corrosion inhibitors after long term heat stress is in general substantially lower than the one of organic corrosion inhibitors. Unfortunately, in laboratory experiments and in the field, the effect of excessive heat on mineral corrosion inhibitors such as, Silicate, Phosphate, Borate or Molybdate, cannot be easily separated from effects that other parameters than heat can have, for example pH, surface effects, fluid dynamic effects, presence of corrosion metal cations, brazing residues or other impurities. Therefore, low retention rates of mineral inhibitors and the potential appearance of gels, precipitates or deposits, originating from mineral corrosion inhibitors have to be considered as the result of multiple factors (as mentioned above) and not of the action of heat alone.
Fig.2: Retention rates of major mineral corrosion inhibitors, combined action of heat stress and other parameters present in vehicle tests
Many mineral corrosion inhibitors undergo under excessive heat condensation reactions. Especially silicates tend to produce complex 3-dimensional structures that are no longer soluble in the coolant. Phosphate, borate and molybdate undergo similar reactions, may co-precipitate with silicates and potentially accelerate gel and precipitate formation under heat stress.
Conclusion
The thermal stability of coolants is subject to thermal-oxidative stress to the base fluids and to thermal stress towards the corrosion inhibitor package. Whereas the base fluid see its physical and chemical properties changed mainly by oxidation of glycols, thermal stress on the inhibitor package tends to deactivate mineral corrosion inhibitors through condensation and polymerization reactions. In contrast, OAT corrosion inhibitors are much less prone to oxidation, decomposition or chemical alteration due to thermal stress.