Glossary

Chromium oxide refers primarily to chromium(III) oxide, with the chemical formula Cr₂O₃. It’s a dark green, extremely stable compound that forms when chromium reacts with oxygen. This oxide is one of the most important and naturally occurring compounds of chromium—it’s the same protective film that makes stainless steel “stainless.”

Formation and Structure

When chromium in an alloy (like stainless steel or chromoly) is exposed to air, it reacts with oxygen to form a thin, dense, self-healing layer of Cr₂O₃ on the metal’s surface. This layer is only a few atoms thick, but it’s chemically inert and adheres tightly, preventing oxygen and moisture from reaching the metal beneath.

This process is known as passivation, and it’s why stainless steels and chromium-plated parts resist rust and corrosion far better than carbon steel.

Properties

- Chemical formula: Cr₂O₃

- Color: Deep green (also used as a pigment known as chromium green or chrome oxide green)

- Melting point: ~2,435°C (4,415°F)

- Hardness: Very high—used as an abrasive and in polishing compounds

- Chemical stability: Resistant to acids, bases, and oxidation

Industrial and Engineering Uses

Corrosion Protection:
The protective oxide film on chromium-bearing metals shields fasteners, tools, and machinery from rust.

Pigments and Coatings:
Used in paints, ceramics, and glass as a durable green pigment.

Polishing and Abrasives:
Finely ground chromium oxide is used in metal polishing compounds (“green rouge”).

Refractory Applications:
Its heat resistance makes it ideal for furnace linings and ceramics.

In Fasteners and Alloys

In fastener materials like stainless steel, chromium-molybdenum (chromoly) steel, and plated bolts, chromium oxide is what prevents corrosion. If the surface is scratched, the oxide layer re-forms automatically, sealing the exposed metal—making it a key reason why chromium-alloyed steels have such long service lives.