CHROMIUM

Name: Chromium

Symbol: Cr

Group: 6

Period: 4

Block: d

Category: Transition Metals

Atomic Number: 24

Atomic Mass: 51.9961 u

Electrons per Shell: 2, 8, 13, 1

Electronegativity: 1.66

Density: 7.19 g/cm³

Melting Point: 1907 ºC

Boiling Point: 2671 ºC

Thermal Conductivity: 93.9 W/(m·K)

Electrical Conductivity: 7.9 × 10⁶ S/m

Magnetic Order: Antiferromagnetic

Ordinary State: Solid

Oxidation States: -2, -1, 0, +1, +2, +3, +4 +5, +6

Mohs hardness: 8.5

Vickers Hardness: 1060 MPa

Brinell hardness: 687 MPa

Most stable isotopes: Cr-50 (4.35%), Cr-52 (83.79%), Cr-53 (9.50%), Cr-54 (2.36%)

Discoverer: Louis Nicolas Vauquelin (1797)

Chromium compounds have been known for at least two millennia; for instance, the Terracotta Army of the Qin Dynasty was coated with red chromium oxide to inhibit wear, though this process is not equivalent to modern chroming. The red-colored mineral crocoite (PbCrO4), discovered in 1761 in the Berezovks mines, first drew significant attention due to its striking hue. In 1797, Louis Nicolas Vauquelin successfully isolated the metallic element, naming it Chromium from the Greek word chroma (meaning color) because its salts exhibit such a wide spectrum of colors. The vivid red color of the gemstone ruby is, in fact, due to trace amounts of chromium present in its structure. Although the metal was isolated relatively easily, obtaining high-purity chromium proved challenging until Hans Goldschmidt refined it in 1890 using a thermite-like process. This groundwork paved the way for future metallurgical milestones, notably Léon Guillet's development of ferritic stainless steel in 1911. 

Chromium is a distinctive mirror-white metal and is the hardest of all pure metals on the Mohs scale (8.5), though it is inherently brittle and possesses low ductility. It adopts a body-centered cubic crystal structure and is chemically stable due to a crucial natural defense mechanism: it spontaneously forms an extremely thin, passive layer of chromium(III) oxide (Cr2O3) on its surface. This layer provides exceptional corrosion resistance, shielding the bulk metal against oxidizing acids and most alkalis, preventing the tarnishing seen in other metals. This protective layer is the key reason chromium is essential for manufacturing stainless steel, even in non-nickel-containing grades, and why the pure metal is used in corrosion-resistant static parts like laboratory crucibles and specialized piping. However, this protective layer is vulnerable to penetration by pure halogens such as fluorine and chlorine.

Pure chromium is highly resistant to corrosion thanks to its Cr₂O₃ oxide layer, which protects it against oxidizing acids and alkalis.

It does not tarnish like other metals, but it is vulnerable to halogens like fluorine and chlorine, which perforate its protective layer.

Chromium is primarily used as an alloy in:

• High-speed steels (up to 4%)

• Stainless steels (minimum 10.5%)

• It improves hardness and corrosion resistance. In alloys such as Stellite (chrome-cobalt), it provides toughness.

It is also used in:

• Chrome plating to protect metals

• Superalloys such as Nichrome

• Jewelry, for its shine (e.g., 316L steel)

Chromium-iron alloys (such as Cr-Fe 95:5) are used in applications such as:

• High temperature

• Corrosive environments

For example, in pipes for corrosive liquids. Their ductility improves with 5% iron, but their high cost and manufacturing difficulty limit their use. They are produced using powder metallurgy in inert atmosphere furnaces.

It is crucial to distinguish between:

• Stainless steel: contains bulk chromium

• Chrome-plated steel: has only a surface layer of chromium

Chrome plating protects through electrolysis, but once the layer is damaged, the underlying steel corrodes.