Chromium

The hardest pure metal on the Mohs scale at about 8.5, harder than hardened steel, which is why hard chrome plating is the standard wear surface on hydraulic cylinder rods. It is what makes stainless steel stainless: above roughly 10.5% chromium the steel grows a self-repairing chromium oxide film a few nanometres thick, and scratching it simply grows it back. Bulk chromium is too brittle to use structurally, so it is nearly always an alloying element or a coating.

Properties

Density

7,190 kg/m³ — 7.19 g/cm³

Melting point

1,907 °C

Tensile strength

250–300 MPa

Young's modulus

279 GPa

Thermal conductivity

93.9 W/(m·K)

Hardness

8.5 Mohs

Thermal expansion

4.9 µm/(m·K)

Floats in water

No

Magnetic

No

What it is used for

• The corrosion-resisting element in all stainless steel
• Hard chrome plating on hydraulic rods and moulds
• Decorative plating on trim and fittings
• Refractory bricks and green pigments

How it compares

Does it rust?

Chromium does not rust — rust is iron oxide specifically — but it does corrode, forming its own oxide or salts over time.

Is it magnetic?

Chromium is not ferromagnetic. Most metals are not.