1912

Stainless Steel

Pierre Berthier (1782–1861), Elwood Haynes (1857–1925), Harry Brearley (1871–1948)

Most simple iron alloys begin to rust quickly on exposure to water and oxygen. The process causing this is the oxidation of the metal to iron oxide, and it continues because the oxide is brittle and flaky, constantly exposing the metal underneath to further oxidation. It also reacts with the iron atoms next to it, converting them to iron oxide and spreading the reaction farther into the metal.

But if you add enough chromium to the mix (more than 10 percent), you can produce steel that remains shiny under most ordinary conditions. One reason this works, oddly, is that chromium is even easier to oxidize than iron is. You might guess that some sort of chromium rust would lead to another pile of flakes and powder, but the chromium oxide that forms is hard, dense, and unreactive, creating a protective layer, a molecular skin, on the surface of the metal, which keeps oxygen from getting to any of the metal atoms below.

French metallurgist Pierre Berthier first recognized this property in 1821, but the metalworkers of the time could only make brittle objects with the resulting steel. In the early 1900s, though, metallurgists in France, Germany, England, and the U.S. found several routes to stainless alloys, with the most useful and easily prepared ones discovered in 1912 by the metallurgists Elwood Haynes and Harry Brearley, who were American and English respectively. This started off a large, new industry and any number of patent fights, but by the 1920s stainless steel was becoming a familiar part of life in the industrialized world, used in a range of products, from cutlery and scalpels to aircraft engines and automobile trim.

The protective-layer principle (known as passivation) is also used in metal alloys for handling fluorine gas, which otherwise tends to react with everything. And it keeps some other reactive metals tamed, most notably aluminum. See the entry on thermite for what happens when aluminum finally does react with oxygen in the air, and be glad that—thanks to an invisible layer of aluminum oxide— this has no chance of happening to your roll of aluminum foil.

The U.S. Steel building in Pittsburgh is seen in this bullet-riddled reflection. Stainless steel and other alloys that are essential to modern civilization have created huge industrial firms (and fortunes).