To help lubricating oils last longer and perform more effectively, users need to understand the impact of oxidation, and deploy the right tools to keep it under control.
Modern lubricating oils are precision-engineered fluids, designed to deliver high performance and a long life in demanding conditions. All oils are vulnerable to one common element, however. As oil circulates through machines, its organic molecules react with oxygen from the air, forming new, more complex compounds. Over time, these oil oxidation reactions will degrade the oil until it can no longer fulfil its role.
Oil oxidation has a number of undesirable effects. It increases the viscosity of the oil, which can stop lubricant films performing as expected. The products of oxidation are acidic and can corrode critical components. Oxidation products in the oil can also accumulate on surfaces, creating a sticky “varnish” that increases friction in moving parts, or builds deposits that block pipes and jam valves.
What causes oil oxidation? The simple answer is “exposure to air”, but the rate at which oxidation occurs is driven by a number of other factors. High temperatures can significantly increase the oxidation rate, for example. And impurities in the oil such as copper particles or ferrous compounds can act as catalysts, driving the rate higher still. Unfortunately for users, hot, contaminated environments are extremely common in industrial applications.
To maximize the working life of their products, lubricant manufacturers incorporate antioxidants into their formulations. Commonly used additives include aromatic amines and phenolics. Antioxidants can stop oxidation in its tracks, binding to the free radicals formed when oil begins to oxidate and preventing the process from propagating.
While antioxidants can significantly slow the degradation of the oil in a machine, they don’t last forever. Eventually, all the antioxidant material will be consumed, leaving oil exposed to attack by the oxygen in the air.
Oil oxidation tests
To keep their machines healthy, users want to understand the degree of oxidation that has already occurred in their oil, and its remaining useful life. Answering those questions isn’t straightforward.
There are some characteristics that can indicate heavily oxidated oil, including an increase in viscosity, an unpleasant odour or significant discolouration. But, by the time those effects are obvious during routine maintenance, the oil may already be severely degraded, potentially damaging the machine.
For an earlier warning, owners need to use more sophisticated analytical techniques. The most commonly used oil oxidation tests include:
Fourier Transform Infrared (FTIR) spectrometry. A sample of oil is exposed to multiple wavelengths of infrared light. Different molecules within the oil absorb the light at different rates, so analysing the wavelengths that pass through the sample gives an indication of its composition. Comparing the infrared “fingerprints” of used and new oil can reveal the presence of unwanted materials within the used sample, including oxidation products as well as water, soot and other contaminants.
Total Acid Number (TAN). The organic by-products of oil oxidation are acidic, so measuring the acidic constituents of sample using potentiometric or colorimetric techniques can indicate the presence of oxidation. This approach requires repeated measurements over time to identify trends in the acid content. Many additives used to reduce oxidation are themselves acidic, so the TAN of the oil in a machine may initially fall, as additives are consumed, before rising again as oxidation progresses.
Rotating Pressure Vessel Oxidation Test (RPVOT). This test calculates the remaining oxidation capacity of the oil. A sample is placed in a sealed container and pressurized with pure oxygen. The vessel is then rotated at high speed in a heated bath to encourage oxidation. As the oxygen inside reacts with the oil, the pressure in the container falls. The rate of this pressure drop is then compared with the equivalent rate for new oil. As long as they know the time since the last oil change, this measurement allows equipment owners to calculate the remaining useful life (RUL) of the oil in a machine.
Remaining Useful Life Evaluation Routine (RULER). Another way of calculating RUL, this technique forces oxidation by applying a variable voltage to a sample of oil mixed with an electrolyte solution. The testing machine measures the peak current in the sample, which is lower when fewer antioxidants are present. Once again, test results are compared with those of new oil to estimate the fraction of antioxidants remaining in the sample.
Reducing oil oxidation
The battle against oil oxidation begins with the selection of a high-quality oil containing a suitable blend of antioxidant additives. Equipment owners can give their oil the best chance of a long life by following good operating and lubricant management practices. These include storing oil in suitable air-tight containers, minimizing exposure to high temperatures, and efficient filtration to remove particles, water and other contaminants.
Regular monitoring of the oil using one or more of the methods described above will indicate its condition, allowing owners to intervene before oxidation becomes too advanced. Where testing indicates that the antioxidant additives in the oil are close to being consumed, it is possible to “re-additivate” the oil, by adding manufacturer-recommended additive materials, or conducting a partial oil change to introduce additive-rich oil into the system.
These approaches have their limits, however. As oil circulates, it will gradually accumulate oxidation products and contaminant particles, many of which are too small to be trapped by conventional filters. Even if it is topped-up with fresh antioxidant additives, old oil gradually becomes more susceptible to oxidation. Until recently, that meant that owners had no choice but to periodically dispose of their oil and start again with new.