The importance of oxidation stability

Why is oxidation stability so important and what makes the difference in coolants on that topic?

For a better understanding of the impact of oxidation on a coolant, some general information is shared on the chemical processes.

What is oxidation?

Oxidation is a reaction of a molecule, atom or ion with loss of electrons resulting in an increase of the oxidation state, which refers to the degree of oxidation of an atom in a molecule. It is the charge of an atom if all bonds it formed were ionic bonds. Oxidation always goes together with a reduction reaction, where a molecule, atom or ion has a gain of electrons resulting in a reduced oxidation state.

An example of such an oxidation reaction is the oxidation of metal atom by oxygen gas. For example, the oxidation of aluminium by oxygen is given below, where aluminium forms aluminium oxide. The reaction illustrates that the oxidation reaction has changed the aluminium metal.

Another example of an oxidation reaction related to coolants is the oxidation of the base fluid such as molecule ethylene glycol.

Why is oxidation stability so important for coolants?

The example and definition above show that oxidation changes the structure of a molecule. When the base fluid is oxidized under the influence of oxygen present in the cooling system it can form acidic components such as glycolic acid, formic acid and oxalic acid. These small molecule acids can more easily interact with the metal surface through the carboxylate group and accelerate metal corrosion. The formation of these acids can also result in a reduction of the pH. Where a typical coolant pH is between 8 and 9, strong oxidation of the base fluid can reduce pH below 7, decreasing coolant stability and increasing corrosion rates of certain metallurgies such as cast iron.

Next to oxidation of the base fluid, certain additive molecules are more sensitive to oxidation  degradation wherein the changing molecules may have some negative characteristics. The changes can range from coolant colour changes, to a loss of the corrosion inhibition function and to the formation of new components with decreased solubility resulting in deposit formation and finally reduced heat transfer properties.

In conclusion oxidation of the coolant has a negative effect on the coolant function.

How can coolant make the difference?

Oxidation in a cooling system is especially caused by oxygen and heat further catalysed by presence of metals. Unfortunately, these three parameters causing oxidation are always present in a cooling system. To cope with this oxidation environment it is important that the coolant is sufficiently robust. An increased buffer capacity (e.g. by selection of a buffer) can reduce the negative effects of oxidation on pH reduction.

Because some additives are more sensitive to oxidation, the choice of more oxidation resistant additives can increase the coolant stability reducing the negative effects on the coolant function. It is demonstrated as well in lab test as in the field that OAT’s (Organic acid corrosion inhibitors, technology) are very resistant to oxidation, making it a preferred corrosion inhibitor to provide long lasting protection under high temperature oxidation conditions.  The use of anti-oxidants, components that inhibit the oxidation reaction, may also be considered.