General Information about Ferrous Scale?
The flaky surface often found on hot rolled metals and iron oxides is known as scale or mill scale. The thin layer of scale quickly forms on the surface of plates and sheets as they pass through the steel re-rolling billets and rolling red hot irons on their way to the rolling mill.
The bluish-black flaky scale is a mixture of iron oxides. In general, the one millimetre thick scale clinging to the steel surface offers protection against atmospheric corrosion. However, the metal is only protected for as long as the mill scale seal is unbroken.
The Composition of Mill Scale
Characteristically, the blue-gray scale is made up of a number of valuable minerals. The natural materials found in mill scale include, magnetite and an ultra thin film of Fe203, otherwise known as Hematite.
The magnetite is made up of residual black Fe0 and numerous fine metal grains, which make the coating feel rough to the touch. Scale or mill scale is found on many hot-rolled steel products. However, the scale is rarely found on descaled metal(for example, galvanized metal). The scale is also rarely found on metals processed in a protective area.
Invariably, an inner layer of wüstite Fe0 forms between the magnetite and the steel at hot rolling temperatures.
When hot-rolled strip or hot-rolled coils are cooled below 1040 °F, the majority of the wüstite is converted to Fe304 and Fe. Fe304 is predominant in the cooled metal.
The mill scale is 85% Fe0 when the steel is red hot. The remaining fifteen percent of the layer of scale is made up of Fe203 and Fe 304.
The steel processing industry regard mill scale as a pest. This is simply because scaled metal cannot be coated. The fine layer of scale moistens the metal and creates an air trap. The slightest crack in the scale will allow the steel to corrode quickly.
However, the fine layer of scale does protect the steel, hence mill scale has certain benefits. However, various processes such as pickling, flame cleaning and abrasive blasting cause the scale to crack.
The ship-building industry prefers to source steel straight from the mills. The new rolled metal will be given time to weather and atmospheric conditions will eventually remove the scale. The majority of modern steel mills now supply mill scale free metal. This type of steel is ideal because it is ready to weld.
Rolling mills gather the flaky scale and transport it to a sinter plant, where the ore is recycled. In general, current scrap metal and iron ore prices determine the scrap price paid. Quality scale fetches more money. Low grade mill scale such as that used by counterweight manufacturers and the cement industry has a lower scrap value. In fact, a low grade ore will undoubtedly be worth far less than quality scrap metal.
China is one of the leading mineral ore importers. The vast country also imports huge amounts of mill scale. However, all scale is subject to stringent checks, to ensure its meets the strict standards. In excess of one million tons of mill scale was shipped to China in the last twelve months, much of the scale was used in ferro alloy production, however, the scale is also widely used to make welding flux.
A Typical Analysis of Good Quality Mill Scale for Industrial Use: