Case Study: How Chromium Protects Steel

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passivation, corrosion

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Stainless steel was discovered by accident in 1913 by Harry Brearly, a British metallurgist. At the time, Brearly was experimenting with different steel alloys in an attempt to improve gun barrels. After months of research, he noticed all of his barrels had corroded except for the one composed of 14% chromium. Brearly had made one of the most important discoveries of the 20th century, stainless steel. Stainless steel is known and used for its ability to resist corrosion. Ordinary iron-carbon steels corrode easily in air. Oxygen combines with iron to produce a red oxide known as rust. Although chromium corrodes in a similar manner, its oxide, Cr2O3 forms a hard surface layer preventing oxygen from reacting with iron. Simply, chromium acts as a passive layer because its electric potential (-.56V) is more negative in relation to that of iron (-.44V). The photograph compares the two materials: iron-carbon steel versus the stainless steel with the Cr2O3. Brearly's invention is found in many everyday objects. For example, kitchen sinks are made of the most common type of stainless steel, 18:8 -- 18% chromium and 8% nickel. More corrosion resistant stainless steels such as those used in building construction contain molybdenum as well as chromium.

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Author:
Dave Goodman
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