In metallurgy, stainless steel, also known as inox steel or inox from French "''inoxydable''", is a steel alloy with a minimum of 10.5%〔(【引用サイトリンク】 url = http://www.worldstainless.org/Files/issf/non-image-files/PDF/TheStainlessSteelFamily.pdf )〕 chromium content by mass.
Stainless steel does not readily corrode, rust or stain with water as ordinary steel does. However, it is not fully stain-proof in low-oxygen, high-salinity, or poor air-circulation environments.〔(【引用サイトリンク】url = http://www.stainless-steel-blog.com/2013/09/06/why-is-stainless-steel-stainless/ )〕 There are different grades and surface finishes of stainless steel to suit the environment the alloy must endure. Stainless steel is used where both the properties of steel and corrosion resistance are required.
Stainless steel differs from carbon steel by the amount of chromium present. Unprotected carbon steel rusts readily when exposed to air and moisture. This iron oxide film (the rust) is active and accelerates corrosion by forming more iron oxide; and, because of the greater volume of the iron oxide, this tends to flake and fall away. Stainless steels contain sufficient chromium to form a passive film of chromium oxide, which prevents further surface corrosion by blocking oxygen diffusion to the steel surface and blocks corrosion from spreading into the metal's internal structure.〔Jianhai Qiu. ("Stainless Steels and Alloys: Why They Resist Corrosion and How They Fail" ). Corrosionclinic.com. Retrieved on 29 June 2012.〕 Due to the similar size of the steel and oxide ions, they bond very strongly and remain attached to the surface.
Passivation occurs only if the proportion of chromium is high enough and oxygen is present.
The corrosion resistance of iron-chromium alloys was first recognized in 1821 by French metallurgist Pierre Berthier, who noted their resistance against attack by some acids and suggested their use in cutlery. Metallurgists of the 19th century were unable to produce the combination of low carbon and high chromium found in most modern stainless steels, and the high-chromium alloys they could produce were too brittle to be practical.
In the late 1890s Hans Goldschmidt of Germany developed an aluminothermic (thermite) process for producing carbon-free chromium. Between 1904 and 1911 several researchers, particularly Leon Guillet of France, prepared alloys that would today be considered stainless steel.
Friedrich Krupp Germaniawerft built the 366-ton sailing yacht ''Germania'' featuring a chrome-nickel steel hull in Germany in 1908.〔(【引用サイトリンク】title=A Proposal to Establish the Shipwreck Half Moon as a State Underwater Archaeological Preserve )〕 In 1911, Philip Monnartz reported on the relationship between chromium content and corrosion resistance. On 17 October 1912, Krupp engineers Benno Strauss and Eduard Maurer patented austenitic stainless steel as Nirosta.〔(【引用サイトリンク】title=ThyssenKrupp Nirosta: History )〕〔(German Patent Office: Deutsches Reichspatent Nr. 304126, erteilt am 18. Oktober 1912 )〕〔(German Patent Office: Deutsches Reichspatent Nr. 304159, erteilt am 21. Dezember 1912 )〕
Similar developments were taking place contemporaneously in the United States, where Christian Dantsizen and Frederick Becket were industrializing ferritic stainless steel. In 1912, Elwood Haynes applied for a US patent on a martensitic stainless steel alloy, which was not granted until 1919.〔Carlisle, Rodney P. (2004) (Scientific American Inventions and Discoveries ), p. 380, John Wiley and Sons, ISBN 0-471-24410-4〕
Also in 1912, Harry Brearley of the Brown-Firth research laboratory in Sheffield, England, while seeking a corrosion-resistant alloy for gun barrels, discovered and subsequently industrialized a martensitic stainless steel alloy. The discovery was announced two years later in a January 1915 newspaper article in ''The New York Times''.〔 The metal was later marketed under the "Staybrite" brand by Firth Vickers in England and was used for the new entrance canopy for the Savoy Hotel in London in 1929.〔Howse, Geoffrey (2011) ''A Photographic History of Sheffield Steel'', History Press, ISBN 0752459856〕 Brearley applied for a US patent during 1915 only to find that Haynes had already registered a patent. Brearley and Haynes pooled their funding and with a group of investors formed the American Stainless Steel Corporation, with headquarters in Pittsburgh, Pennsylvania.
In the beginning stainless steel was sold in the US under different brand names like "Allegheny metal" and "Nirosta steel". Even within the metallurgy industry the eventual name remained unsettled; in 1921 one trade journal was calling it "unstainable steel". In 1929, before the Great Depression hit, over 25,000 tons of stainless steel were manufactured and sold in the US.
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