Acids in the oil

Year of publication: 1999


Base oils used for the production of lubricating oils contain neither acids nor alkaline components. The pH value of these base oils, to which no active ingredients have yet been added, is pH = 7. The base oils are therefore completely neutral. In the production of lubricants, additives are added to the base oils, which are usually a little "acidic". Especially additives to improve wear or corrosion protection are based on slightly acidic compounds.
Even engine oils whose additives have a primarily alkaline effect already have some acid-reacting components as fresh oil. These production-related acid components are harmless.

The formation of acids in oil

However, negative effects occur when the proportion of acids in the oil increases. There are several reasons for this:
The acid load of engine oils increases in the course of their use due to combustion products.
"Blow-by" gases enter the oil circuit. Sulfur, which stems from the fuel, adds further acidity to the oil. They all have to be neutralized and rendered harmless by the engine oils. This means that the BN of an oil in internal combustion engines, also known as the alkaline reserve, decreases during operation.

Although no acidic combustion products contaminate the oil in hydraulic oils and industrial lubricants, such lubricants also become acidic through oxidation. To neutralize used oils, more caustic potash solution is consumed in the laboratory if the oil has been aged by reaction with air-oxygen over long periods of use or by high temperature. During oil aging, the typical hydrocarbon chains become "acidic" due to the accumulation of oxygen.

Wear protection additives, often referred to as EP additives, protect the surface from localized welding and from increased wear by forming metal salts on the surface that are relatively easy to remove. The reaction products resulting from the wear mechanism also have an acidic effect.

Negative impacts of acids

Acids in the oil increase the corrosion effect. Non-ferrous metals in particular, such as copper, are then attacked by chemical wear and dissolved in the oil. Iron also corrodes faster in acidic oil in the form of rust. This also promotes mechanical abrasive wear. The corrosion particles removed can in turn cause further wear in a kind of chain reaction. An increased proportion of acids in the oil promotes oxidation and oil aging, among other things. An oxidized oil usually becomes "thicker". The increase in viscosity can go so far that the oil is no longer adequately delivered to the lubrication points. Important additives are consumed to neutralize the acids, which are then no longer available in sufficient quantities to provide wear protection. This reduces the performance of the oil; oil change intervals become shorter.


  • OELCHECK determines the BN (base number) for engine oils and the AN (neutralization number) for all other oils using analysis kit no. 2.
    These two values offer a complementary statement for an oil change extension for the respective oil type.
  • The BN describes the absorption capacity of the oil for the acidic components from the combustion gases which result from the combustion process.
  • The AN indicates the degree of acidification.
  • An engine oil is worse the lower the BN has become compared to the fresh oil.
  • A hydraulic oil or an industrial lubricant is worse the higher the AN or NZ or neutralization number has become compared to the fresh oil.