Definition
A metal deactivator (MDI) is an additive that passivates catalytically active metal surfaces to prevent them from accelerating oxidation of the lubricant. Copper and copper alloys are particularly problematic catalysts — even trace dissolved copper ions can initiate and propagate radical chain oxidation at concentrations as low as 1 ppm.
The primary metal deactivators used in lubricants are:
- Benzotriazole (BTA) — forms a stable Cu-BTA chelate complex on copper surfaces; most widely used
- Tolyltriazole (TTA, methyl-BTA) — slightly more oil-soluble than BTA; similar effectiveness
- Dimercaptothiadiazole derivatives (DMTD) — effective for copper and ferrous metals in gear oil applications
Metal deactivators are used at low concentrations (0.01–0.1%) and are synergistic with antioxidants. They serve dual functions as both corrosion inhibitors (passive film on surface) and metal deactivators (chelation of dissolved metal ions).