Corrosion inhibitor additives are the family of surface-active chemistries that protect metal surfaces — steel, copper, brass, bronze, aluminium — from chemical attack inside a lubricated system. The family divides by target metal: sulfonate and amine inhibitors protect ferrous surfaces from rust and acid, while thiadiazoles and benzotriazoles passivate copper and its alloys against sulfur and acid attack. A complete additive package usually carries both — because a lubricant that protects steel but corrodes copper, or vice versa, is only half a solution. Understanding the family, and the tests that prove it, is essential for any formulation that touches mixed-metal systems.
The Corrosion Inhibitor Family by Target Metal
Protecting Steel and Iron: The Rust Side
- Metal sulfonates (T701/T702) — the workhorse; adsorbed hydrophobic film that blocks water and oxygen.
- Amine and amide inhibitors — ashless, multifunctional, used in industrial oils and metalworking fluids.
- Phosphate esters — combine anti-wear and anti-corrosion in one molecule.
- Overbased sulfonates — add acid neutralisation to film protection.
Validated by: ASTM D665 (rust), humidity cabinet (ASTM D1748), salt spray (ASTM B117).
Protecting Copper and Yellow Metals: The Passivation Side
Copper, brass and bronze are attacked by the very chemistries that protect steel — especially sulfur from EP additives and acids from oxidation. The specialist inhibitors:
- Thiadiazole derivatives (T561) — five-membered ring compounds with sulfur and nitrogen. They chemisorb onto copper surfaces, forming a protective film, and also offer mild anti-wear/EP activity. The default choice in gear oils and engine oils where sulfur chemistry is present.
- Benzotriazole (T706) and derivatives (T705 series) — nitrogen heterocycles that form a copper-benzotriazolate film; the classic metal deactivator for turbine, hydraulic and gear oils.
- Tolyltriazole and other azoles — alternatives with slightly different solubility and film properties.
Validated by: ASTM D130 (copper strip) at the application temperature, plus D4048 for greases.
Protecting Aluminium and Other Alloys
- Amine phosphates and specific carboxylates protect aluminium surfaces in hydraulic and metalworking applications.
- Zinc and magnesium alloys — niche chemistries for specific systems (e.g. some transmission and coolant applications).
The Interaction Problem: EP Chemistry vs Copper
The central conflict in corrosion inhibitor additive selection: the sulfur that gives gear oils their extreme-pressure performance is the same sulfur that corrodes copper. Sulfurized olefins react with steel to form protective films at high contact temperature — but they also react with copper at much lower temperatures, producing black copper sulfide.
The solution is the copper passivator: thiadiazole or benzotriazole chemistry that occupies the copper surface first, blocking the sulfur reaction while allowing the EP chemistry to do its steel-protection job. This is why:
- Every sulfur-bearing gear oil carries a copper passivator.
- The passivator dose is set by the D130 result, not by intuition.
- The balance is validated at the operating temperature — a pass at 100 °C can fail at 121 °C.
Reading the Tests: ASTM D130 and Beyond
- ASTM D130 — copper strip corrosion: a polished copper strip is immersed in the oil for 3 hours at a set temperature (100 °C standard, 121 °C severe service). The strip is rated 1a (slight tarnish) to 4c (severe corrosion). Gear oil specs typically require 1a–2a.
- ASTM D4048 — the copper strip test for greases.
- ASTM D665 — rust test for steel protection.
- ASTM D1748 / D3603 — humidity and water-exposure rust tests.
A professional supplier provides D130 data at the temperature of your application, and D665 for the steel side, with the passivator dose clearly stated.
Building a Complete Corrosion Protection System
A robust package is layered by mechanism:
- Film formers — sulfonates for steel, azoles/thiadiazoles for copper; the primary defence.
- Acid neutralisers — overbased detergents and amine borates; remove the acid that attacks both metal types.
- Oxidation control — antioxidants slow the acid generation that drives corrosion.
- Metal deactivators — stop the catalytic effect of copper ions, which accelerate oxidation and re-corrode.
The four layers work as a system: slow the acid, neutralise what forms, block it at the surface, and protect the metals that would otherwise catalyse more damage.
Related Reading
FAQ
Q: What is the difference between a corrosion inhibitor and a metal deactivator? A: Corrosion inhibitors form protective films or neutralise corrosive species; metal deactivators specifically passivate reactive metal surfaces (especially copper) and suppress their catalytic effect on oxidation. In practice the terms overlap, and azole/thiadiazole products do both.
Q: What is T561? A: A thiadiazole derivative used as a copper corrosion inhibitor and metal deactivator, with mild anti-wear activity. It is the standard choice for gear oils and engine oils carrying sulfur EP chemistry.
Q: Why does gear oil corrode copper without a passivator? A: The sulfurized olefin EP additive that protects steel also attacks copper, forming copper sulfide. A thiadiazole or benzotriazole passivator blocks the copper surface first.
Q: How is copper corrosion tested? A: ASTM D130 — a copper strip in oil for 3 hours at temperature, rated 1a to 4c. Gear oil specs typically require 1a–2a. Test at your application temperature, not just 100 °C.
Q: Can one corrosion inhibitor protect both steel and copper? A: Not reliably. Steel protection (sulfonates, amines) and copper protection (azoles, thiadiazoles) are different chemistries for different mechanisms — complete packages carry both.
Conclusion
Corrosion inhibitor additives are a matched set, not a single product: sulfonates and amines for steel, thiadiazoles and benzotriazoles for copper, antioxidants to slow the acid source, and overbased chemistry to neutralise what forms. Building the right combination for your metallurgy — and proving it with D665, D130 and humidity data — is the professional standard. Minglan Chemical supplies the full corrosion inhibitor range — T701/T702 sulfonates, T705/T706 benzotriazole derivatives, T561 thiadiazoles and complete packages — with complete test documentation. Contact us to protect your system's metals.