Oil Oxidation and Its Impact on Your Productivity

Oil oxidation is one of the key challenges facing equipment reliability. As many Mobil SHC Club members know, oxidation can occur over time due to high heat, causing sludge formation that can accelerate equipment wear. If not addressed in time, these impacts can result in equipment failure, hurting the productivity of any operation.

But, the topic of today’s tip is not the dangers of oxidation – that’s been covered fairly thoroughly on this Club and elsewhere. Today’s focus is the actual oxidation mechanism, which is a topic that is not discussed as frequently but which can provide valuable insight into how to protect against this phenomenon.

Oil oxidation mechanism

Oil oxidation is a chemical reaction between the lubricating oil and oxygen. It is best summarized in the following reactions:

RH → R∙ + H∙   (Initiation)

R∙ + O2 → ROO∙ + RH → ROOH + R∙   (Propagation)

R∙ + R’∙ → R-R’   (Branching and Termination)

The rate of oxidation is catalyzed by high temperatures, the presence of water, as well as metals such as copper. The free radical (R∙) formed during the initiation phase is highly reactive and combines with oxygen to form additional radicals in propagation. This leads to the formation of weak carboxyl acids and the development of sludge and varnish during the branching and termination phase.

Oxidation and oil formulations

The rate of oil oxidation can be controlled by the oil formulation. Choices formulators can make include the use of:

  • High quality base stocks: The higher the saturate level of the base stocks, the greater the resistance to oxidation. Base stock groups I through IV have increasing resistance to oxidation.
  • Anti-oxidation additives: Oils may contain an anti-oxidant that can help stop the oxidation chain reaction. These react with free radicals to provide a break in the oxidation cycle. There are two types of anti-oxidants used in lubricating oils:
    • Primary: comprised of aromatic amines and hindered phenolics.
    • Secondary: often phosphates and sulfur containing compounds.

The abatement of the oxidation process through the use of primary anti-oxidants is represented as follows:

R∙ + antioxidant → RH + anti-oxidant∙

Secondary anti-oxidants act by specifically decomposing peroxide radicals formed during the propagation phase.

Many lubricant manufactures will use a combination of saturated base oils and anti-oxidation additives to control oil oxidation. For example, most engine oils use quality base stocks (Groups II, III or IV) and additives like zinc dithiophosphate (secondary anti-oxidant) to resist the oxidation process. Industrial oils may use quality base stocks (Groups II through V) and additives (primary and secondary anti-oxidants), separately or together, to minimize the effects of oxidation.

As is most often the case, the choice of chemistry depends upon the resources available, the application and the operating environment.

Oxidation in used oil

In used oils, there are a number of tests that are used to determine whether or not oxidation is occurring.

Methods include:

  • Oil Viscosity (ASTM D445): A rise in the reported viscosity may suggest that oxidation is taking place.
  • Total Acid Number (ASTM D664): Measures the presence of weak carboxylic acids resulting from oxidation.
  • Total Base Number (ASTM D2896): Measures the ability of the oil to neutralize the acids formed.
  • FTIR Infrared Analysis (ASTM E2412): May identify oxidation reaction products, and contamination by organic materials in used lubricants.

All of these tests, when considered separately or together, help identify when oil oxidation is occurring, and may determine when a corrective action is needed.

Hopefully this tip provided you with helpful additional insight into how oxidation occurs and what decisions you can make to best protect against it. If you have any further questions or comments, leave a note in the section below!