Total Acid Number (TAN)
Total Acid Number (TAN) is a measure of acid concentration present in a lubricant. The acid concentration of a lubricant depends on the presence of additives, acidic contamination, and oxidation by-products. The depletion of an additive package may cause an initial decrease in the TAN of fresh oil.
The TAN value alone cannot be used to predict the corrosive nature of an oil, as the test only measures the total quantity of acidic species present, rather than the specific chemical identities. Two samples might have the same TAN value, but one may contain high levels of corrosive acids while the other contains relatively benign organic acids. Negative effects attributed to elevated TAN values include increased viscosity, corrosion risk, and the formation of gums and resins.
In crude oil, naphthenic acids are primarily responsible for corrosion problems—a phenomenon known as Naphthenic Acid Corrosion (NAC). TAN testing is vital across industries to assess oxidation levels and component failure risks.
Typical TAN Ranges across Oil Types
| Application Type | Typical TAN Range (mg KOH/g) |
|---|---|
| Compressor & Turbine Oils | 0.01 – 0.25 |
| Gearbox & Lube Oils | 0.1 – 10.0 |
| Lubricant Additive Concentrates | 20 – 200 |
Measurement Methods: Potentiometric titration, color-indicator titration, and spectroscopic methods (such as FTIR) are commonly used to measure TAN.
Importance of TAN
- Serves as a crucial guide for quality control in fresh lubricating oil formulations and tracks degradation during service.
- Essential for preventive maintenance to protect machinery from severe internal corrosion and premature component failure.
- Quantifies both weak organic acids and strong inorganic acids present in the oil sample.
Read also Complete Guide to Lubricants