Corrosion Inhibitors

Corrosion inhibitors are additives that slow the electrochemical corrosion of metal parts and equipment in water-based fluids. In industrial manufacturing they split by the metal they protect: azole film-formers such as benzotriazole guard copper, brass, and other yellow metals, while anodic inhibitors such as sodium nitrite and sodium molybdate passivate ferrous surfaces. A mixed-metal system — a cast-iron machine wetted with brass fittings — usually needs both a yellow-metal azole and a ferrous inhibitor, because each protects metals the other does not.

Benzotriazole (BTA) is the standard yellow-metal inhibitor. It forms a thin, insoluble complex on copper and brass that blocks the corrosion reaction, and it stays effective at low dose, often a fraction of a percent of the fluid. Specify BTA wherever a metalworking fluid, coolant, or cleaner contacts copper alloys. It does little for bare steel, so it complements rather than replaces a ferrous inhibitor.

For ferrous protection the choice is between sodium nitrite and sodium molybdate, and that is the real trade-off on this page. Sodium nitrite is a strong, low-cost anodic inhibitor, but it is not used in fluids containing secondary amines, because the two can react to form N-nitrosamines; many modern water-soluble metalworking fluids use sodium molybdate or amine-carboxylate chemistry instead. Molybdate works at higher dose and cost but avoids that reaction. Confirm regulatory status for your application and jurisdiction.

• Yellow-metal (copper, brass, bronze) protection in metalworking fluids and coolants
• Ferrous corrosion inhibition in water-soluble cutting and grinding fluids
• Closed-loop and recirculating system corrosion control
• Interim and in-process rust protection on machined ferrous parts
• Mixed-metal system protection where steel and copper alloys share one fluid

• Benzotriazole — PubChem
• Sodium molybdate — PubChem
• Sodium nitrite — PubChem