Why Does Oil Degrade?

When exposed to normal conditions in a plant, any oil will start to pick up moisture as soon as it comes in contact with the air. Oil is very hygroscopic — meaning it picks up moisture easily. Moisture levels in oil will normally increase to a maximum of about 300ppm and then stabilize. These levels are normal and will cause no damage.

If an oil is used in a total loss application — such as a drip-feed — pick-up of moisture and dirt is not a factor, as the oil is used once then discarded. (Drip-feed “waste oil” is an ideal candidate for our purification service).


Most oils are used in enclosed reservoirs, however, where they re-circulate through the operating system. As the oil re-circulates, it lubricates, cools and flushes away debris from normal wear particles. This debris, depending on the size of the particles, may settle in the oil reservoir, be removed by the filters, or simply be re-circulated, if it is too small to be removed.

These re-circulated particles cause damage through abrasive wear each time they circulate through the operating system. Research conducted over the past few years demonstrates that particles as small as 5 microns, when present in large quantities, will cause severe pump and valve wear by acting as a lapping compound.


A lot of dirt is introduced into the oil reservoir through the air. Very few oil reservoirs have efficient air filters. Dirt particles light enough to float in the air are drawn into the reservoir every time the oil level in the tank goes down. When this happens, some of the dust particles settle out into the oil.

Oil reservoirs with levels that change frequently can pump many times their volume of air every hour. This exposes the oil in the tank to potentially very large amounts of dirt. This is how new oil delivered from most oil companies becomes contaminated with dirt.


The additives present in most oils contain chemicals that work to extend the life of the oil. These consist mainly of antioxidants, rust preventives, anti-foam agents, de-emulsifiers, etc.

Water, heat and oxygen will, individually or combined, cause damage to both the base oil and the additive system. Water will react with many oil additives and hydrolyze them, fracturing the additive into two or more chemical fragments. These reaction products may or may not be oil-soluble — and they may act as catalysts for the further decomposition of the oil or additives.

Oxygen will react with the additives and the oil to form oxidation by-products. These by-products will generally be acidic in nature at first, and may act as catalysts for further oxidation.


Antioxidants protect against oxidation only if they are present in sufficient quantities. When an antioxidant falls below a certain level it will become less effective. As it is further reduced, it will become completely ineffective.

Antioxidants are used up slowly under normal conditions. They protect by being consumed as they react with and neutralize oxidation products. Both heat and moisture will accelerate consumption of an antioxidant.

As the additive quantity falls below critical level (which is different for each oil), the rate of oxidation will increase. Oxidation byproducts will catalyze more oxidation — meaning that the more byproducts are present, the faster oxidation will occur.

Eventually, the level of oxidation will reach a point where the lubricant must be removed and replaced with new oil. Purification is not recommended at this stage.