Guest Videos: Chromium, A Critical Mineral


Most people think of one or more of the following when they hear the word “chromium”:

  • Shiny, shiny chrome
  • The open-source platform underlying Google’s browser
  • Health supplements
  • The “stainless” part of stainless steel
  • Chemistry class

These are all common enough (plus the software has nothing to do with the physical element). So why does the United States call chromium a strategic mineral?

What is chromium?

In nature, this is a dull gray metal; its ore–chromite–is not spectacular, either.

However, the element takes its name from the the Greek word chroma, meaning “color,” because all of its compounds look fabulous.


chromium quartet

Purple chrome alum crystal (Ra’ike via Wikimedia, CC BY-SA 3.0); ruby (Tim Evanson, CC BY-SA 2.0); green test tube solution (Leiem via Wikimedia, CC BY-SA 4.0); and orange Chromax supplement (Sbharris via Wikimedia, CC BY-SA 3.0)


Chromium has always been popular with artists and painters. Mixed with lead, it makes red paint. Renoir varied the proportions a bit and got just the right shade of orange for his work.

With a few more tweaks, chrome also gives us school-bus yellow.

Chromium shows so many colors because it’s a transition metal. The group’s chemistry is complex but it underlies the rainbow of transition-metal compounds.

This video shows various transition metal colors nicely. Here’s a link, if you also want to get into the science details more fully.



However, chromium is not considered vital to the US economy and national security because it makes school buses highly visible.

Chromium uses

The main use of chromium is to make stainless steel and other alloys.

When exposed to air, chromium metal forms a thin but very dense layer of molecules that oxygen can’t penetrate. Treat steel with chromium (usually in the form of ferrochrome) and, besides becoming a little stronger, the steel won’t rust–it is now “stainless.”

Of course, the actual production of stainless steel isn’t that simple.



Since chromium is hard and has a high melting point, it can also be used in superalloys, protecting everything from jet engines and land-based energy turbine blades to military-grade specialty metals.

Chromium sources

Per Fortier and others (see source list below):

U.S. chromite reserves are small, with no mining, resulting in chromium-bearing materials being produced from imported chromite ores and ferrochromium. Globally, South Africa has the largest chromite reserves and is the leading source of chromium-bearing imports. Limited substitutes exist for chromium in alloy applications; however, recycling is extensive, accounting for about 40 percent of consumption.

There is no shortage of chromium anywhere in the world, but the lack of substitutes and US dependence on imports are why decision-makers are looking more closely at its supply line.

Without chromium, jet engine and energy plant turbines would be more expensive and probably less reliant. As for stainless steel and chrome, it’s hard to imagine a world without them.



Here is the magic process that made those wheels and other parts so shiny:




Featured image: Atoma, via Wikimedia, CC BY 2.5.



Sources:

Bowman, R. 2000. Superalloys: A Primer and History, 9th International Symposium on Superalloys. http://www.tms.org/meetings/specialty/superalloys2000/superalloyshistory.html Last accessed July 9, 2018.

Fortier, S. M.; Nassar, N. T.; Lederer, G. W.; Brainard, J.; and others. 2018. Draft critical mineral list–summary of methodology and background information–U. S. Geological Survey technical input document in response to Secretarial Order No. 3359. Open-File Report 2018-1021. (PDF).

Schulte, R. F., and Bryden, B. N. 2018. Chromium in March 2018. US Geological Survey Mineral Industry Surveys. https://minerals.usgs.gov/minerals/pubs/commodity/chromium/mis-201803-chrom.pdf

Zientek, M. L.; Loferski, P. J.; Parks, H. L.; Schulte, r. F.; and Seal, R. R. II. 2017. Chapter N: Platinum Group Elements, in Critical Mineral Resources of the United States—Economic and Environmental Geology and Prospects for Future Supply: U.S. Geological Survey Professional Paper 1802, Schulz, K. J.; DeYoung, Jr., J. H.; Seal, II, R. R.; and Bradley, D. C., eds. https://pubs.er.usgs.gov/publication/pp1802

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