Oxidation, The Oil Killer
by Jonathan Sowers, CLS,
Marketing Manager, Technical Services
with Andrew DePompei, Chemist

Nearly 150 years ago, August Wilhelm von Hofmann, a German scientist working in England at the time, recognized that under atmospheric conditions oxygen caused the deterioration of rubber. This is the first recorded observation of the chemical reaction we term oxidation.1 Normally, the bonds holding atoms together will not split in a way that leaves a molecule with an odd, unpaired electron. However, in the presence of a catalyst (heat, light, etc.), weak bonds will split, and free radicals are formed. Free radicals are very unstable and react quickly with other compounds, trying to capture the needed electron to gain stability. Generally, free radicals attack the nearest stable molecule, essentially "stealing" its electron. As a result of this reaction, in certain cases the attacked molecule itself becomes a free radical, beginning a chain reaction. Once initiated, the process cascades, finally resulting in the complete degradation of the original substance.

Thus, oxidation may be defined as a chemical reaction of a substance with oxygen, resulting in the loss of electrons from the atoms of the original substance. The corrosion of metals is a form of oxidation; for example, rust on iron is iron oxide. With most lubricants, we are concerned with the oxidation reaction occurring with the hydrocarbon molecules in the lubricant’s base oil. One purpose of lubricant formulation for a defined application is to control the deterioration in a planned manner over an established period of time.2 Oxidation still occurs, but the reactions are minimal unless stimulated by catalytic forces and the introduction of numerous free radicals. Sources of free radicals in oil include oxygen and nitro-oxides (nitrogen dioxide, nitric and nitrous oxide). Catalytic forces that accelerate the oxidation include contaminants, such as dirt and wear metals, and ultraviolet (UV) light. However, the most influential catalyst is temperature.

Oxidation is temperature-dependant and, as a chemical reaction, is subject to the Arrhenious equation, which states that for every 10°C temperature increase the rate of oxidation will double or triple (and that increase is compounded). For example, a reaction rate of unity at 300°C will multiply by between 2 and 3 at 310°C, multiply by between 4 and 9 at 320°C, multiply by between 8 and 27 at 330°C, and so on.3

Results of the oxidation include chemical and physical changes in the oil. Chemical byproducts of oxidation are organic acids, usually carboxylic acids, which promotes corrosion and the creation of even more free radicals and hyperoxides in the oil. Mineral oil lubricants are made up of chains of hydrocarbon molecules. Physically, the oxidation reaction at the molecular level breaks the chain and forms two smaller molecules. This change in the physical structure impacts the film strength of the oil and degrades the load-carrying properties. This in turn results in more friction, more wear, and increased temperatures–all of which contribute to even higher rates of oxidation. The change in the hydrocarbon’s molecular structure also causes the new molecules to behave differently with respect to viscosity properties, which increase.

Degradation of the oil due to oxidation results in increased viscosity, sludge, and sediment, which leads to varnish deposits, base oil breakdown, additive depletion, loss of foam properties, higher temperatures, acids, corrosion, and wear.

There are three stages to the oxidation process:

1. Initiation—hydrocarbon oxidation begins with a loss of electrons, forming “free radicals” and hyperoxides. This reaction breaks the hydrocarbon molecule chain.
2. Propagation—the free radicals formed in the initiation stage react with stable molecules in the oil resulting in a degraded oil molecule and a new free radical to continue the chain reaction.
3. Termination—The cycle stops the oxidation mechanism with the formation of oxidation byproducts such as carboxylic acids and long chain hydrocarbons.4

Antioxidant additives enhance the effectiveness of the termination step, reducing oxidation throughout the oil. They break the cycle by converting free radicals to stable molecules and by stopping the propagation. There are two groups of oxidation inhibitor additives, based on how they inhibit oxidation. 1) “Primary Antioxidants” break the cycle. These are hindered phenolic and aromatic amines. 2) “Secondary Antioxidants” include sulfur compounds and ZDP (zinc dithiophosphates), which act as peroxide decomposers.5

Oil analysis offers a way to monitor the oxidation of the oil through a variety of tests. Laboratory tests that monitor the rate and state of oxidation in oil include:

• Viscosity
• Total acid number
• FTIR – oxidation
• Appearance (darkening)
• Odor (rotten eggs, burnt)
• Varnish Potential (MPC & Gravimetrics, Ultracentrifuge, RULER, ISO PC, Blotter)
• RPVOT

The appropriate tests for the lubricant and application can generate alerts when the oil needs to be changed before serious damage results. These tests also frequently identify the cause of excessive oxidation so preventive measures may be taken to reduce or eliminate the problem.

References:
1. Wikipedia http://en.wikipedia.org/wiki/August_Wilhelm_von_Hofmann
2. The Handbook of Lubrication and Tribology, Vol. I, Section 29, “The Degradation of Lubricants in Service Use,” 29.3, 2006
3. Ditto, 29.6
4. Practicing Oil Analysis, “The Lubricant Nemesis – Oxidation,” Dr. David Wooten, March 2006
5. STLE Advanced Lubrication, “Lubricants and Fuel Additives,” Chevron Oronite, 1994

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