'RUST (O.E.' rust, a word which appears in many Teutonic languages, cf. Du. roest, Ger. rost); in origin it is allied with "ruddy" and "red," the reddish-brown powdery substance which forms on the surface of iron or steel exposed to atmospheric corrosion. Formerly the process was regarded as oxidation pure and simple, and, although it was known that iron did not rust in dry air, yet no attempt was made to explain why water was necessary to the action. F. Crace-Calvert in 1871 showed that the carbon dioxide of the atmosphere was a factor; and in 1888 Crum Brown published the theory - termed the "carbonic acid theory" - that water and carbon dioxide react with iron to form ferrous carbonate and hydrogen, the ferrous carbonate being subsequently oxidized by moist oxygen to ferric hydrate and regenerating carbon dioxide, which again reacts with more iron. This theory was controverted by Wyndham Dunstan, who attempted to prove that carbon dioxide was not necessary to rusting; and in place of the acid theory, he set up a scheme which involved the production of hydrogen peroxide. G. T. Moody has since shown that when all traces of carbon dioxide are removed (which is a matter of great experimental difficulty) iron may be left in contact with oxygen and water for long periods without rust appearing, but on' the admission of carbon dioxide specks are rapidly formed. It also appears that rust changes in composition on exposure to the atmosphere, both the ferrous oxide and carbonate being in part oxidized to ferric oxide. Acids, other than carbonic, may promote rusting; this is particularly the case with ironwork exposed to the acids - sulphurous, nitric, &c. - contained in smoke. It is probable that the action depends upon the presence of iron, oxygen and water, and some acid which makes the water an electrolyte.
Steel differs in many ways from iron in respect of atmospheric corrosion; the heterogeneous nature of steel gives occasion to a selective rusting, ferrite is much more readily attacked than the cementite and pearlite; moreover, the introduction of other elements may retard rusting; this is particularly the case with the nickel-steels.
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