OXIDE SCALE must be completely removed from hot-worked or hot-rolled steel before subsequent processing is initiated, in order to prevent wear on dies and rolls and avoid surface defects in the final product. This oxide scale originates during the hot working or hot rolling of steel, when the surface of the metal reacts with oxygen in the air to form oxides of iron, or mill scale. The scale actually consists of three iron oxides with different proportions of iron and oxygen. Hematite, Fe2O3, which contains 30.1% oxygen, is the outermost oxide in the scale layer, whereas wustite, FeO, with 22.3% oxygen, is the innermost oxide. Magnetite, Fe3O4, contains 27.6% oxygen; when all oxides are present, the middle layer in the scale is magnetic. At temperatures above 566 °C (1050 °F), wustite is the predominant oxide, but during cooling below 566 °C (1050 °F), a portion of it is transformed to iron and magnetite (4FeO = Fe3O4 + Fe). In cases of rapid cooling, which can occur with rod and bar, substantial amounts of wustite are retained in the cooled product. When cooling after hot rolling is relatively slow, as it is with coiled strip, magnetite is the main oxide constituent of the scale in the cooled product.

Pickling is the most common of several processes used to remove the scale from steel surfaces. The term pickling refers to the chemical removal of scale by immersion in an aqueous acid solution. The process originated in the late 1700s, when sheets of steel were descaled by immersion in vats of vinegar. Wide variations are possible in the type, strength, and temperature of the acid solutions used, depending on time constraints (batch vs. continuous operations), as well as the thickness, composition, and physical nature (cracks) of the scale. Pickling is applicable for many types of forgings and castings, for merchant bar, blooms, billets, sheet, strip, wire, and for tubing.

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