Chromium (Cr)
Chromium is the essential element that makes stainless steel 'stainless' — a minimum of 10.5% Cr creates the self-healing passive oxide layer that provides corrosion resistance. Beyond stainless steel, chromium is critical for hard chrome plating, superalloys, and refractory applications. It is the hardest pure metal element.
Properties
- Density
- 7,190 kg/m³
- Melting Point
- 1,907 °C
- Boiling Point
- 2,671 °C
- Thermal Conductivity
- 93.9 W/(m·K)
- Electrical Resistivity
- 1.27 × 10⁻⁷ Ω·m
- Tensile Strength
- 280–370 MPa
- Yield Strength
- 140–210 MPa
- Hardness
- 1,060 HV (pure Cr)
- Elongation
- 0–2%
- Crystal Structure
- BCC
Common Grades
- Ferrochrome HC (60–70% Cr): High-carbon ferrochrome. Primary feedstock for stainless steel production.
- Ferrochrome LC (<0.1% C): Low-carbon ferrochrome for low-carbon stainless grades (304L, 316L).
- Cr 99%+ (Electrolytic): High-purity chromium metal. Superalloy production, sputtering targets.
- Chrome Oxide (Cr₂O₃): Green chromium oxide. Pigments, refractory bricks, polishing compounds.
Applications
- Stainless Steel: Essential alloying element (10.5–30% Cr) for all stainless steel grades
- Chrome Plating: Hard chrome for hydraulic cylinders, decorative chrome for automotive trim
- Superalloys: Oxidation resistance in nickel and cobalt superalloys for jet engines
- Refractories: Chrome-magnesite bricks for steelmaking furnaces, cement kilns
FAQ
How does chromium make steel stainless?
When steel contains at least 10.5% chromium, the chromium reacts with oxygen to form a thin, invisible, self-healing chromium oxide (Cr₂O₃) layer on the surface. This passive layer is only 1–5 nm thick but extremely tenacious — if scratched, it reforms within seconds in the presence of oxygen.
What is the difference between hard chrome and decorative chrome plating?
Hard chrome (industrial chrome) is applied at 20–500 μm thickness directly onto steel for wear resistance and low friction (hydraulic rods, molds). Decorative chrome is much thinner (0.25–0.5 μm) applied over nickel plating for appearance (automotive trim, bathroom fittings). Different processes and chemistries.
Is hexavalent chromium (Cr⁶⁺) dangerous?
Yes — hexavalent chromium compounds (used in chrome plating and some pigments) are classified as carcinogenic (IARC Group 1). Strict workplace exposure limits apply (5 μg/m³ in EU). Trivalent chromium (Cr³⁺), used in passivation and some plating, is much less toxic and is replacing Cr⁶⁺ in many applications under REACH regulation.