Definition
Oxidation stability describes a lubricant’s resistance to chemical degradation by oxygen at elevated temperatures. Oxidation is the primary aging mechanism for lubricating oils and leads to viscosity increase, acid formation, sludge, and varnish deposits.
The oxidation process is a free-radical chain reaction:
1. Initiation: Formation of alkyl radicals (R•) from base oil molecules
2. Propagation: R• + O₂ → ROO• → ROOH → further radicals
3. Termination: Radical scavengers (antioxidants) interrupt the chain
Key test methods:
- ASTM D2272 (RBOT) — rotating bomb oxidation test; measures oxidation induction time under oxygen pressure
- ASTM D943 (TOST) — turbine oil oxidation stability test; measures time to acid number rise
- CEC L-085 (TEOST) — thermo-oxidation engine oil simulation test for deposit tendency