by Chris Woodford. Last updated: September 18, 2017.
Nothing sparkles quite like chrome! If you have to get a vintage Pontiac looking great for the streets of Havana, you'll know one of the secrets is to buff up the chrome "trim"—the fenders, headlamps, door handles, and radiator grille—so they really gleam in the sun. Car makers love relatively inexpensive chromium-plated parts because they shine like silver, making vehicles that sell for thousands look like a million dollars!
It's ironic that we think of chromium as shiny, silvery, and essentially colorless; Louis Vauquelin (1763–1829), the French chemist who discovered it in 1797, chose the name from chromos, the Greek word for color, noting the many brightly colored compounds that chromium formed. Let's find out more about this intriguing element!
Photo: It's the chromium-plating that makes old cars like this Austin Cambridge look really special. Unfortunately, keeping chrome looking this good is really hard work!
What is chromium?
Photo: Photo of a chromium-coated aerospace part courtesy of NASA Glenn Research Center (NASA-GRC).
If it shines like silver but it's hard like steel, it's probably chromium: a transition metal element from group 6 of the periodic table. From its shiny nature, you might think it's rare and valuable, but chromium is relatively common: roughly the 21st most widespread element in Earth's rocky crust.
Where and how is chromium produced?
Chromium reacts readily with oxygen so we never find it in Earth's crust in its pure metal state. Most commercial chromium comes from chromium iron ore, also called chromite (FeCr2O4), mined in such diverse countries as Albania, Brazil, Cuba, Finland, India, Kazakhstan, Russia, South Africa, Turkey, the Ukraine, and Zimbabwe; of these, South Africa, Kazakhstan, and Turkey produce approximately three quarters of the world's chromite. One US company mines the ore in Oregon.
Chart: Worldwide chromium mining figures for 2016. Between them, Kazakhstan and South Africa have roughly 95 percent of the world's chromium reserves though, as you can see here, they currently mine a smaller proportion of the chromium we actually use. Source: US Geological Survey: Minerals Commodity Summaries: Chromium: January 2017.
According to the US Geological Survey (USGS), major exporters of chromium to the United States include South Africa, Kazakhstan, and Russia. Russia produces most of its chromium electrolytically (by electrolysis), the United States uses both electrolytic and aluminothermic production (heating with aluminum in smelters), while the other countries use mostly aluminothermic methods. In 2016, the world produced around 30 million tons of chromite ore, of which the vast majority (over 95 percent) ended up in metal production. The United States currently uses about 5 percent of the total chromite ore produced worldwide. As of 2017, total world reserves of chromite ore are estimated at over 12 billion tons—enough to supply world needs for centuries. About 95 percent of this is concentrated in just two countries: South Africa and Kazakhstan.
Physical and chemical properties
Photo: Chrome cleanup: Chromium isn't always shiny and nice to look at. This toxic lagoon of chromium sludge at Portsmouth, Ohio was caused by nuclear power production. The chromium was used to keep the cooling water system free of corrosion but, as a byproduct, made a sludge that was stored in lagoons like this. More environmentally friendly cooling systems are now used and the lagoon has since been cleaned up. Photo courtesy of US Department of Energy.
Physically speaking, chromium is pretty dull: it's a typical metal: hard, tough, and lustrous, and highly resistant to both heat and chemical action. It slowly reacts with oxygen in air (which is why you have to keep on polishing those fenders) and with acids, but it doesn't react with seawater or moist air—and that's why car fenders don't rust like iron and steel car parts and bodywork. It's paramagnetic (weakly attracted to magnets) and its crystals take one of two forms: body-centered cubic or hexagonal close-packed.
Chemically, it's much more colorful—quite literally: although pure chromium is silvery-white, it forms all kinds of colorful compounds, including chromates and dichromates. Many gems (including chromium garnets, emeralds, rubies, serpentines, and some sapphires) get their colors from the chromium compounds they contain. Many pigments (the color chemicals in paints) also contain chromium compounds, including chrome yellow (lead chromate), red, orange, and green. Potassium dichromate is a light-sensitive chemical used in old-fashioned photography processes before digital cameras came along.
What is chromium used for?
Photo: Lots of chromium ends up in stainless steel, an alloy used for making such things as cutlery and surgical equipment. Stainless steel typically contains 10–30% chromium.
Most chromium is used to make hard, rustproof metallic alloys, including stainless steel (with a high chromium content, typically used in rustproof cutlery and surgical equipment) and other chromium-steel alloys (containing less chromium) used for such things as armor plating and oil pipes. In electroplated form, chromium is still used to make automobile "trim" and the shiny coating used in bathroom and door "furniture" (such as faucets and door handles), though inexpensive plastic finishes are now widely used instead.
Photo: These vitamin tablets contain "trace" amounts of chromium: around 25 μg (micrograms). To put that in context, they also contain 14 mg (milligrams) of iron—560 times more. Even though the chrome content really is absolutely minute, it's still vital for our health. Found in such foods as grain, potatoes, and fruit, it's used in the body's production of insulin.
Chromium's heat-resistant properties make it useful in refractory bricks (those used to line furnaces and kilns), while its hardness is useful in an alloy called stellite (a tough material made from cobalt, tungsten, and chromium) used for making very hard cutting tools. Chromium compounds such as chrome alum (chromium potassium sulfate) and chromic acid (chromium (VI) oxide) are used in tanning leather and dyeing clothes, while other compounds of chromium are important industrial catalysts.
Minute (trace) quantities of chromium are essential for a healthy human diet, but some chromium compounds—notably hexavalent chromium, or Cr (VI)—are extremely toxic and carcinogenic (cancer-causing). In the United States, as of 2009, the OSHA (Occupational Safety and Health Administration) limits a worker's exposure of hexavalent chromium to 5 micrograms per cubic meter of air (average exposure over an 8-hour period).