Chromium

Chromium — Material Page

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Chromium (Cr, atomic number 24) is a lustrous, hard, silvery-blue transition metal in Group 6 of the periodic table, with one of the highest melting points (1,907 °C) and hardness values (Mohs 8.5) of any non-refractory metal. It adopts a body-centered cubic (BCC) structure and has an anomalous electron configuration ( 3d⁵ 4s¹) where a 3d electron is promoted to half-fill the d shell — reflecting the exceptional stability of the half-filled d⁵ configuration. This electronic structure gives chromium a rich and colorful coordination chemistry spanning oxidation states from –2 to +6, with +3 (trivalent, Cr³⁺) being the most thermodynamically stable and +6 (hexavalent, Cr⁶⁺) being a powerful oxidizer of significant industrial and toxicological importance. Chromium is antiferromagnetic below 311 K (the Néel temperature), one of only a few elements to exhibit antiferromagnetism — its spin density wave ground state has been the subject of fundamental condensed matter physics research. Its density of 7.19 g/cm³, elastic modulus of 279 GPa, and low coefficient of thermal expansion (4.9 µm/m·°C) make it a valuable alloying addition for improving stiffness and thermal stability in structural applications.

Chromium's defining industrial property is its ability to form a thin, self-healing, adherent Cr₂O₃ passive film in oxidizing environments — the foundation of stainless steel's corrosion resistance and the basis of virtually all industrial chromium applications. When added to iron at ≥10.5 wt%, chromium produces stainless steel by enabling spontaneous passivation in air and aqueous environments: the Cr₂O₃ film (2–3 nm thick) reforms within milliseconds if scratched, providing continuous protection against rust, pitting, and crevice corrosion. Stainless steel consumes approximately 85% of global chromium production (~32 million tonnes ferrochromium/year), used in everything from surgical instruments and food processing equipment to architectural cladding and chemical plant vessels. The passive film's stability extends to high temperatures — Cr₂O₃ is stable to ~2,270 °C — enabling chromium additions to nickel superalloys (Inconel, Hastelloy) to provide oxidation resistance in gas turbine hot sections operating above 1,000 °C.

Beyond corrosion protection, chromium occupies key niches in hard coatings, functional thin films, optical applications, and advanced ceramics. Hard chrome plating (electrodeposited Cr, 50–250 µm) achieves hardness of 800–1,000 HV and a friction coefficient against steel of ~0.17 — the standard wear-resistant coating for hydraulic cylinder rods, landing gear, and precision tooling, though increasingly challenged by REACH restrictions on hexavalent chromium plating baths. Physical vapor deposition (PVD) chromium films serve as adhesion layers under gold and other noble metals in semiconductor and MEMS devices. Chromium-doped laser gain media — Cr:Al₂O₃ (ruby, the first laser), Cr:YAG, Cr:LiSAF, Cr:forsterite — cover a uniquely broad tunable range from 650 nm to 1.3 µm. Chromium carbide (Cr₃C₂) and Cr₂O₃-based thermal spray coatings provide wear resistance in oil and gas valve seats and pump sleeves at temperatures where other coatings fail. In pigments, Cr₂O₃ is the definitive permanent green pigment and chrome yellow (PbCrO₄) — though increasingly restricted — has historically been one of the most lightfast yellows available.

General Properties

Property	Value	Notes
Atomic Number	24	Group 6 (chromium group), Period 4; transition metal; anomalous electron configuration 3d⁵ 4s¹ rather than expected 3d⁴ 4s² — d⁵ half-filling stabilization
Atomic Mass	51.996 u	Four stable isotopes; ⁵²Cr dominates at 83.79%; ⁵³Cr (I = 3/2) is NMR-active and used in geochemical isotope ratio studies
Density (20 °C)	7.19 g/cm³	Similar to iron (7.87 g/cm³); slightly less dense, contributing a modest mass reduction when alloyed; BCC structure with a = 2.885 Å
Melting Point	1,907 °C (2,180 K)	Among the highest of the Period 4 transition metals; only exceeded by W, Re, Os, Ir, Mo in practical use; enables use as an alloying element in refractory and high-temperature structural applications
Boiling Point	2,671 °C (2,944 K)	Significant vapor pressure above ~1,700 °C — relevant for evaporation source design and PVD thin film deposition of Cr adhesion layers
Thermal Conductivity	93.9 W/m·K	Moderate thermal conductivity; lower than Cu or Al but higher than stainless steel (~16 W/m·K); relevant for heat dissipation in Cr-coated tool and die applications
Coefficient of Thermal Expansion	4.9 µm/m·°C	Very low CTE among transition metals — about ¼ that of aluminum and ½ that of steel; contributes to dimensional stability of Cr-containing alloys across temperature cycles
Electrical Resistivity	125 nΩ·m (20 °C)	Higher than most transition metals due to spin disorder scattering from the antiferromagnetic spin density wave; resistivity anomaly near the Néel temperature (311 K)
Crystal Structure	Body-Centered Cubic (BCC)	BCC structure stable from RT to melting point; no allotropic transformations; antiferromagnetic below 311 K (Néel temperature) with an incommensurate spin density wave along ⟨100⟩ — a canonical model system in condensed matter physics

Mechanical Properties

Property	Value	Notes
Hardness	Mohs 8.5; ~1,060 HV	Hardest of all pure transition metals accessible at room temperature (excluding the ultra-refractory metals W, Re, Os); harder than steel (~200 HV) and titanium (~200 HV); the hardness of electroplated hard chrome (~800–1,000 HV) approaches that of pure Cr
Elastic (Young's) Modulus	279 GPa	Higher than iron (200 GPa) and most engineering metals; chromium additions to steel and nickel superalloys increase modulus and stiffness — important for turbine blade creep resistance
Poisson's Ratio	0.21	Lower than most metals (typical ~0.3); reflects the strongly directional covalent character of Cr–Cr bonding in the BCC lattice
Ductile-to-Brittle Transition	~–20 °C to +200 °C (depends on purity)	Pure chromium is brittle at room temperature due to interstitial impurities (N, O, C) that pin dislocations; high-purity Cr (>99.9%) is ductile above ~100 °C; the brittleness of commercial Cr limits its direct structural use but does not affect its role as an alloying element

Thermal & Environmental Properties

Property	Value	Notes
Corrosion Resistance	Excellent (self-passivating)	Forms a 2–3 nm Cr₂O₃ passive film in air and aqueous environments that reforms within milliseconds if damaged; ≥10.5 wt% Cr in iron enables stainless steel passivation; passive film stable in dilute acids but attacked by reducing acids (HCl, H₂SO₄) and strongly oxidizing conditions (concentrated HNO₃ can passivate at high concentration but break down at intermediate concentration)
High-Temperature Oxidation	Cr₂O₃ protective to ~1,000 °C; volatile CrO₃ above ~1,000 °C in O₂	Cr₂O₃ provides excellent oxidation protection to ~1,000 °C; above this temperature, volatile CrO₃(g) forms in high pO₂ environments, causing catastrophic "chromium evaporation" in SOFC interconnects and high-temperature turbine components — a major materials engineering challenge
Oxidation States	–2, 0, +1, +2, +3, +4, +5, +6	+3 (Cr³⁺) is the most stable state — forms octahedral complexes with characteristic green/violet color; +6 (Cr⁶⁺, chromate/dichromate) is a powerful oxidizer and IARC Group 1 carcinogen — use heavily regulated under REACH and RoHS; +2 (Cr²⁺) is a strong reducing agent in aqueous solution
Toxicology (Cr⁶⁺)	IARC Group 1 carcinogen (Cr⁶⁺ compounds)	Hexavalent chromium compounds (CrO₃, K₂Cr₂O₇, chromate pigments) are carcinogenic by inhalation and dermal contact; OSHA PEL 5 µg/m³ (Cr⁶⁺); Cr⁶⁺ electroplating restricted under REACH Annex XIV; Cr³⁺ and Cr⁰ (metal) are not classified as carcinogens and have very low acute toxicity

Chemical Properties

Property	Value / Behavior	Notes
Surface Oxide	Cr₂O₃ (eskolaite; corundum structure)	Cr₂O₃ is a wide-bandgap semiconductor (Eg ~3.4 eV), an excellent refractory (mp 2,435 °C), and a stable green pigment (Pigment Green 17); the passive film is amorphous in the first 1–2 nm and crystalline at greater depth; Cr₂O₃ is also the active component in chromia-based catalysts
Acid Resistance	Resistant to dilute HNO₃, H₂SO₄ (via passivation); dissolved by HCl	The passive film dissolves in reducing acids (HCl, dilute H₂SO₄) exposing the metal to attack; resistant to dilute HNO₃ and acetic acid; concentrated HNO₃ passivates; nitric-hydrofluoric acid mixtures used for dissolution in analytical chemistry
Magnetic Properties	Antiferromagnetic below 311 K (Néel temp)	Cr is the canonical itinerant antiferromagnet with an incommensurate spin density wave (SDW) along ⟨100⟩; the SDW nesting wavevector connects parallel flat sections of the Fermi surface; extensively studied as a model for itinerant magnetism and SDW physics; Cr/Fe and Cr/Mn thin-film superlattices show giant magnetoresistance (GMR) and exchange coupling
Coordination Chemistry	Rich color chemistry across oxidation states	Cr³⁺ octahedral complexes give ruby (Cr³⁺ in Al₂O₃, red), emerald (Cr³⁺ in Be₃Al₂Si₆O₁₈, green), and chrome alum KCr(SO₄)₂·12H₂O (violet) their characteristic colors; Cr⁶⁺ chromates are yellow, dichromates orange; the element name derives from Greek chroma (color), coined by Vauquelin in 1798

Identifier	Value
Symbol	Cr
Atomic Number	24
CAS Number	7440-47-3
UN Number	UN3089 (powder)
EINECS Number	231-157-5

Isotope	Type	Notes
⁵⁰Cr	Stable	4.345% natural abundance; I = 0; used as an enriched spike isotope for chromium isotope dilution mass spectrometry (IDMS) in environmental and geological samples; ⁵⁰Cr is also a double electron capture candidate but no decay has been observed
⁵²Cr	Stable	83.789% natural abundance; most abundant chromium isotope; I = 0; used as the reference isotope in Cr isotope ratio measurements; daughter of ⁵²Mn (t½ = 5.6 days) — the ⁵²Mn/⁵²Cr system records early solar system nucleosynthesis events in meteorites
⁵³Cr	Stable	9.501% natural abundance; I = 3/2, NMR-active; ⁵³Cr NMR used to probe chromium coordination chemistry; radiogenic daughter of ⁵³Mn (t½ = 3.7 Myr) — the ⁵³Mn–⁵³Cr chronometer dates early solar system differentiation events and aqueous alteration in meteorite parent bodies to within ±1 Myr of solar system formation
⁵⁴Cr	Stable	2.365% natural abundance; I = 0; enriched ⁵⁴Cr shows nucleosynthetic anomalies in meteorites (Δ⁵⁴Cr) that distinguish carbonaceous from non-carbonaceous chondrites — used as a proxy for solar system material mixing and planetary building block provenance
⁵¹Cr	Radioactive	t½ = 27.7 days (electron capture); 320 keV gamma emitter; produced by neutron activation of ⁵⁰Cr; used as a radiotracer for red blood cell labeling in erythrocyte survival studies and gastrointestinal protein loss measurements; also used in industrial tracers and reactor activation analysis

Scientific & Research Applications

Use Case	Form Typically Used	Description
Chromium-Doped Laser Gain Media	Cr:Al₂O₃ (ruby), Cr:YAG, Cr:LiSAF crystals	Chromium-doped crystals provide the broadest tunable gain range of any solid-state laser ion. Ruby (Cr³⁺:Al₂O₃) was the gain medium of the first laser (Maiman, 1960), emitting at 694 nm. Cr:YAG and Cr:forsterite cover 1.3–1.6 µm for telecom and OCT applications. Cr:LiSAF and Cr:LiCAF are directly diode-pumped, tunable from 800–1,000 nm, enabling compact ultrafast sources for spectroscopy and biomedical imaging.
PVD Adhesion & Barrier Layers	Cr sputtering targets, Cr evaporation sources	Chromium thin films (5–50 nm) deposited by sputtering or evaporation are the standard adhesion interlayer between SiO₂ or polymer substrates and noble metal films (Au, Pt, Ag) in semiconductor devices, MEMS, sensors, and microfluidic chips. Cr provides strong adhesion via oxide bonding to SiO₂ and prevents delamination of overlying metallization under thermal cycling or mechanical stress.
Spintronics & Magnetism Research	Cr single crystals, Cr/Fe and Cr/Mn multilayer films	Chromium's antiferromagnetic spin density wave (SDW) ground state makes it a canonical system for studying itinerant magnetism and quantum criticality. Cr/Fe multilayer superlattices were among the first systems in which giant magnetoresistance (GMR) was discovered (Grünberg and Fert, Nobel Prize 2007) — the GMR effect used in all modern hard disk drive read heads originates from oscillatory exchange coupling through Cr spacer layers.
Nuclear Reactor Cladding Research	Cr-coated Zr alloy cladding, FeCrAl alloy tube	Chromium coatings (5–15 µm PVD or cold spray) on zirconium alloy fuel cladding are the leading accident-tolerant fuel (ATF) concept for light water reactors — Cr provides oxidation protection and reduces hydrogen generation rate in loss-of-coolant accident scenarios by several orders of magnitude compared to uncoated Zircaloy. FeCrAl alloys (Fe-Cr-Al with ≥5% Cr) are also under development as ATF cladding.
Chromium Isotope Geochemistry	Enriched ⁵³Cr, ⁵⁴Cr spike solutions; high-purity Cr standard	The ⁵³Mn–⁵³Cr extinct radionuclide system dates early solar system differentiation events in meteorite parent bodies; ⁵⁴Cr nucleosynthetic anomalies distinguish carbonaceous from non-carbonaceous solar system material. In Earth science, the Cr³⁺/Cr⁶⁺ isotope fractionation (δ⁵³Cr) is a proxy for the emergence of oxidative weathering and early atmospheric oxygenation in Precambrian rock records.
Catalysis Research	Cr₂O₃ powder, CrOₓ/SiO₂ Phillips catalyst	The Phillips catalyst (CrOₓ supported on silica, activated at 600–900 °C in O₂) is responsible for approximately one-third of all polyethylene produced globally — it polymerizes ethylene at low pressure without a cocatalyst via a surface Cr²⁺/Cr³⁺ redox cycle that remains mechanistically debated. Chromia-alumina catalysts are used for dehydrogenation of propane to propylene and alkane isomerization in petroleum refining.

Industrial & Commercial Applications

Sector	Form / Alloy Used	Description
Stainless Steel Production	Ferrochromium (FeCr, 50–70% Cr); Cr metal	Chromium is the defining alloying element in stainless steel — ≥10.5 wt% Cr enables spontaneous passivation in air and aqueous environments. Austenitic grades (304: 18% Cr, 8% Ni; 316: 17% Cr, 10% Ni, 2.3% Mo), ferritic grades (430: 17% Cr), and martensitic grades (420: 13% Cr) together consume ~85% of world chromium production. Global stainless steel output exceeds 55 million tonnes/year.
Nickel Superalloys & High-Temperature Alloys	Cr metal addition to Ni-base superalloy melts	Chromium additions of 10–20 wt% to nickel superalloys (Inconel 718: 17–21% Cr; Hastelloy X: 20–23% Cr; René 80: 14% Cr) provide oxidation and hot corrosion (Type I: Na₂SO₄-induced; Type II: low-temperature sulfidation) resistance in gas turbine hot sections. The protective α-Cr₂O₃ scale must be maintained without spalling across thermal cycles from idle to full power.
Hard Chrome Plating	Electrodeposited Cr from CrO₃/H₂SO₄ bath (Cr⁶⁺) or trivalent Cr bath	Hard chrome electroplating deposits 50–250 µm of chromium with hardness 800–1,000 HV and a wear rate 20–50× lower than hardened steel, providing the standard wear and corrosion protection for hydraulic cylinder rods, aerospace landing gear, printing rolls, and precision measuring tools. Cr⁶⁺ plating baths are restricted under REACH Annex XIV (authorization required in EU from 2024); trivalent Cr (Cr³⁺) processes are the approved alternative.
Chromium Carbide & Oxide Thermal Spray Coatings	Cr₃C₂–NiCr powder, Cr₂O₃ powder (HVOF/plasma spray)	Chromium carbide-nickel chromium (Cr₃C₂–25NiCr) HVOF coatings provide wear and corrosion resistance at elevated temperatures (up to 900 °C) — the standard coating for paper mill rolls, pump impellers, and oil and gas valve components where hard chrome plating is inadequate at temperature. Cr₂O₃ plasma spray coatings (hardness ~1,800 HV) protect against abrasive wear in textile machinery guides and pump liners.
Pigments & Colorants	Cr₂O₃ (Pigment Green 17), chrome oxide green, chromate pigments	Chromium(III) oxide (Cr₂O₃) is the definitive permanent green pigment for camouflage coatings, artists' colors, and ceramic glazes — outstanding lightfastness, heat stability to 1,500 °C, and chemical resistance. Viridian (Cr₂O₃·2H₂O, hydrated) provides a more transparent, cooler green for fine arts. Chrome yellow (PbCrO₄) and molybdate orange are highly regulated due to Pb and Cr⁶⁺ content but remain used in industrial marking paints and traffic signs where no substitute matches their opacity and durability.
Refractory & Foundry Applications	Chromite sand (FeCr₂O₄), chrome-magnesia refractories	Chromite sand (natural FeCr₂O₄) is the standard mold and core sand for steel and high-alloy casting — its very low thermal expansion, high thermal conductivity, and resistance to metal penetration produce superior casting surface finish compared to silica sand. Chrome-magnesia refractories (MgO–Cr₂O₃) line electric arc furnace roofs and steel ladle walls, providing resistance to slag attack and thermal shock up to 1,700 °C.

Purity	Main Use
99%	General metallurgical applications and stainless steel production — suitable as a master alloy addition to steel and nickel-base alloy melts where trace impurities (Fe, Si) are tolerated or removed in subsequent processing
99.7%	High-performance alloys and surface coatings — used in precision alloy formulations for superalloy and specialty steel production where controlled impurity levels ensure predictable corrosion resistance and mechanical properties
99.9%	Thin-film research and catalysis studies — the standard purity for PVD evaporation sources, Cr:Al₂O₃ and Cr:YAG laser crystal growth, and Phillips catalyst precursor synthesis where sub-1,000 ppm metallic impurities are required
99.95%	Semiconductor and vacuum applications — used in Cr adhesion layer evaporation sources for MEMS and semiconductor device fabrication, and in spintronics thin-film growth where metallic impurities would introduce magnetic scattering and alter GMR behavior
99.99%	Sputtering targets and microelectronics manufacturing — the highest purity for magnetron sputtering targets used in semiconductor interconnect adhesion layers, optical coatings, and fundamental antiferromagnetism and SDW research where sub-100 ppm total impurity content is required

Synonym / Alternative Name	Context
Cr	Chemical symbol
Chromium metal	Standard commercial and regulatory designation for the elemental form; used in REACH, RoHS, and transport regulations
Elemental chromium	General scientific term distinguishing the pure metal from chromium compounds (Cr₂O₃, CrO₃, Cr₃C₂, etc.); important in toxicology where Cr⁰ and Cr³⁺ are non-carcinogenic while Cr⁶⁺ compounds are IARC Group 1
Chrome	Common commercial and colloquial term used for chromium metal and chromium-plated finishes; the name derives from Greek chroma (color), coined by Nicolas Louis Vauquelin who discovered the element in 1798 from the mineral crocoite (PbCrO₄)
Cromo	Spanish and Italian language equivalent
Chrom	German language equivalent

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