About the blog: What Things Are Made Of


The United States relies on imports for dozens of commodities in everyday use. Often enough, that reliance is 100%. In this book I aim to provide awareness of the hidden geology and mineralogy behind common things, and to develop an appreciation for the global resource distribution that underpins our society. While concerns about oil import reliance are in the news every day, our needs for other minerals are comparable and are typically unknown even to technologically aware Americans.

Sunday, February 7, 2010

Consider the fork

Perhaps your family heirloom cutlery is sterling silver (that is, 92.5% by weight silver and 7.5% copper), but most likely your “silverware” consists of stainless steel.

Stainless steel has been around for only about 100 years, although chromium’s corrosion resistance in iron alloys was known by the 1820s. In the 1910s metallurgists invented ways of reducing chromium’s carbon content to make commercial stainless steel possible. Chromium makes steel “stainless,” not literally true, but such alloys are far less likely to rust.

Chromium finds its greatest use in stainless steel alloys, but dinnerware represents a tiny portion of end use. Structural elements in cars, aircraft, and appliances, together with building construction, take huge volumes of stainless steel. Most stainless steel alloys contain 10% or more chromium, with iron proportions at 70% to 90%.

While iron and chromium dominate the chemistry of stainless steel, your knife or fork probably contains other metals that make the alloy more machinable, resistant to abrasion, or simply more shiny. Nickel, manganese, molybdenum, titanium, and aluminum all contribute desirable properties to stainless steel. Dinnerware alloys commonly contain nickel (10%) and chromium (18%) combined with iron (72%). Nickel makes the alloy stronger, and chromium increases hardness along with its corrosion resistance.

The U.S. has no primary chromite (iron-magnesium chromate, (Fe,Mg) Cr2O4, chromium’s principal ore) production from mines, but our import dependency is only 39% thanks to a remarkable 61% of supply generated by recycling. Imports come mainly from South Africa and Kazakhstan, which together with India produce three-quarters of the world’s chromium—one of few elements whose production isn’t dominated by China.

Chromium metal hit record prices in 2008, averaging $11,078 per ton. U.S chromium consumption is declining with the recession, but in 2008 we used more than 400,000 tons worth more than $1 billion. With prices like that, it’s no surprise that one chromite deposit about 10 miles south of Coos Bay, Oregon, is being evaluated and permitting is in the works. Additional chromite reserves exist in the 2.7-billion-year-old Stillwater Complex of south-central Montana.

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