Chromium Metal and Compounds

Is Chromium a Metal?

Yes, chromium (Cr, element 24) is a metal and is what is known as a transition metal. It is hard, silver in color, and has a blue tinge color. It is used to protect metal components such as ball bearings or the chrome on old cars.

Chromium (atomic number 24) is a brittle, corrosion-resistant metal and a critical component of stainless steel.

Chromium is an alloying agent used in steel, cast iron, and nonferrous alloys of nickel, copper, aluminum alloy, and cobalt. It is hard, brittle, corrosion-resistant, can be welded, machined, forged, and is widely used in electroplating. Chromium is not resistant to hydrochloric acid and cannot be used in its pure state because of its difficulty to work.

Chromium looks very similar to stainless steel and was very popular for chromium plating of car bumpers up to the 1950s.

Is There Naturally Occurring Chromium?

Chromium is a naturally occurring element found in gases, volcanic dust, soil,  plants, animals, and rocks. When found in the environment is found as (Cr III) trivalent chromium, and hexavalent chromium (Cr VI). Less common is metallic chromium (Cr 0) which is the result of some type of industrial process.

Human Metabolism and Health Risks

Chromium (III) is an essential nutrient and dietary element found in fat metabolism, protein, and glucose. the human body reduces chromium (VI) to chromium (III).

Chromium (VI) is more toxic than chromium (III). Chromium (VI) toxicity targets the respiratory tract and when inhaled in a high concentration of chromium trioxide can cause wheezing, coughing, and breathing difficulty in humans. Studies of workers on toxic substances have established that breathing Chromium VI results in an increased risk of lung cancer. Studies are based on the use of chromic acid mists and dissolved Cr (VI) aerosols. Chromium (VI) is a human carcinogen when inhaled.

Chromium supplementation is not recommended by the Food and Nutrition Board of the National Academy of Sciences since it “does not produce abnormalities that can be reversed with the addition of chromium.” In 2014 the European Food Safety Authority Panel on Dietetic Products Nutrition and Allergies reached the conclusion that there is “no evidence that chromium is an essential nutrient” and does not recommend chromium supplements.(1)

How is Chromium Used?

Chromium is one of the most widely used alloys. It is used as an alloying agent to harden steel, in cast iron (0.25 to 0.35 percent) and in nonferrous alloys of nickel, copper, aluminum, and cobalt. It is also used in electroplating for appearance and wear, in powder metallurgy, and to make mirrors and stainless steel.

Chromium is Brittle and Hard with a High Melting Point

Chromium compounds (referred to as chromium (III) or chromium (VI), are used for:

• Cooling tower water treatment
• Wood preservation
• Leather tanning and preservation – Almost all leather gets it tan color from using chrome, which is a toxic waste produced when making chromium.
• Dyes and pigment manufacturing
• Chrome plating – Chrome plating with chromium makes steel look like it is covered with a polished mirror.
• Drilling muds
• Textiles
• Copy machine toner

Chromium in nature colors gemstones into alexandrite, emeralds, and rubies. It is also used as a catalyst and pigment to make emerald green glass.

Pure Chromium metal in its purest form is used only 1/2 of 1% of the time. In most cases, it is used as an alloy or for anti-corrosion for wear-resistant ball bearings or silver trim on a car.

History

Chromium was first discovered in the 18th century by geologist Johann Lehmann. The metal was named “Siberian red lead.” The French Chemist Louis Vauquelin demonstrated that “Siberian Red Lead” was a unique unknown chemical element (at the time it was named crocoite).

The name chrome is based on the Greek word chroma which means color despite the fact that Chromium itself as a pale grey metal did not have a distinct color. While he thought the metal would have limited use since it could not be melted, he did believe that it could serve as a metal with a colored pigment. Today you will see colors such as:

• Chromium oxide (VI) – dark red
• Chromate – orange-red
• Sodium chromate – bright yellow – the familiar yellow color was first used for school buses, a color referred to as “chrome yellow.” Due to the lead content, this has since been changed from the use of chromium to an azo dye.
• Chrome (III) oxide (III) – Chrome green
• Chromium (II) chloride – light blue
• Chromium (III) chloride – violet

The chromium metal color changes based on the number of chromium ion’s six coordination sites have chloride instead of water.

 Capabilities

Chromium alloys can be welded, machined, and forged. Chromium is never used in its pure state.

 Limitations

Chromium is not resistant to hydrochloric acid, and cannot be used in the pure state because of its brittleness and difficulty to work with.

Properties

Chromium has a specific gravity of 7.19; a melting point of 3300°F (1816°C); Brinell hardness number of 110 to 170; is resistant to acids other than hydrochloric, and is wear, heat, and corrosion-resistant.

Key Facts

How is Chromium Produced?

Chromium metal and ferrochromium alloy are commercially produced from chromite by silicothermic or aluminothermic reactions, or by roasting and leaching processes. Chromium metal has proven of high value due to its high corrosion resistance and hardness. A major development was the discovery that steel could be made highly resistant to corrosion and discoloration by adding metallic chromium to form stainless steel.

Reference

(1) National Institutes of Health