Mill scale, often shortened to just scale, is the flaky surface of hot rolled

steel Steel is an alloy made up of iron with added carbon to improve its strength and fracture resistance compared to other forms of iron. Many other elements may be present or added. Stainless steels that are corrosion- and oxidation-resistan ...

, consisting of the mixed

iron oxide Iron oxides are chemical compounds composed of iron and oxygen. Several iron oxides are recognized. All are black magnetic solids. Often they are non-stoichiometric. Oxyhydroxides are a related class of compounds, perhaps the best known of wh ...

iron(II) oxide Iron(II) oxide or ferrous oxide is the inorganic compound with the formula FeO. Its mineral form is known as wüstite. One of several iron oxides, it is a black-colored powder that is sometimes confused with rust, the latter of which consists of ...

(FeO),

iron(III) oxide Iron(III) oxide or ferric oxide is the inorganic compound with the formula Fe2O3. It is one of the three main oxides of iron, the other two being iron(II) oxide (FeO), which is rare; and iron(II,III) oxide (Fe3O4), which also occurs naturally a ...

(), and

iron(II,III) oxide Iron(II,III) oxide is the chemical compound with formula Fe3O4. It occurs in nature as the mineral magnetite. It is one of a number of iron oxides, the others being iron(II) oxide (FeO), which is rare, and iron(III) oxide (Fe2O3) which also occu ...

(, magnetite). Mill scale is formed on the outer surfaces of plates, sheets or profiles when they are being produced by rolling red hot iron or steel billets in

rolling mill In metalworking, rolling is a metal forming process in which metal stock is passed through one or more pairs of rolls to reduce the thickness, to make the thickness uniform, and/or to impart a desired mechanical property. The concept is simil ...

s. Mill scale is bluish-black in color. It is usually less than thick, and initially adheres to the steel surface and protects it from atmospheric corrosion provided no break occurs in this coating. Because it is electrochemically cathodic to steel, any break in the mill scale coating will cause accelerated corrosion of steel exposed at the break. Mill scale is thus a boon for a while until its coating breaks due to handling of the steel product or due to any other mechanical cause. Mill scale becomes a nuisance when the steel is to be processed. Any paint applied over it is wasted, since it will come off with the scale as moisture-laden air gets under it. Thus mill scale can be removed from steel surfaces by flame cleaning,

pickling Pickling is the process of preserving or extending the shelf life of food by either anaerobic fermentation in brine or immersion in vinegar. The pickling procedure typically affects the food's texture and flavor. The resulting food is cal ...

, or

abrasive blasting Sandblasting, sometimes known as abrasive blasting, is the operation of forcibly propelling a stream of abrasive material against a surface under high pressure to smooth a rough surface, roughen a smooth surface, shape a surface or remove su ...

, which are all tedious operations that consume energy. This is why shipbuilders and

steel fixer A steel fixer (UK, ironworkers or "rod busters" in the United States) is a tradesman who positions and secures steel reinforcing bars, also known as rebar, and steel mesh used in reinforced concrete on construction projects. The work involves ...

s used to leave steel and

rebar Rebar (short for reinforcing bar), known when massed as reinforcing steel or reinforcement steel, is a steel bar used as a tension device in reinforced concrete and reinforced masonry structures to strengthen and aid the concrete under tension. ...

delivered freshly rolled from mills out in the open to allow it to 'weather' until most of the scale fell off due to atmospheric action. Nowadays, most

steel mill A steel mill or steelworks is an industrial plant for the manufacture of steel. It may be an integrated steel works carrying out all steps of steelmaking from smelting iron ore to rolled product, but may also be a plant where steel semi-fini ...

s can supply their product with mill scale removed and steel coated with shop primers over which

welding Welding is a fabrication process that joins materials, usually metals or thermoplastics, by using high heat to melt the parts together and allowing them to cool, causing fusion. Welding is distinct from lower temperature techniques such as b ...

or painting can be done safely. Mill scale generated in rolling mills will be collected and sent to a

sinter plant Sinter plants agglomerate iron ore fines (dust) with other fine materials at high temperature, to create a product that can be used in a blast furnace. The final product, a sinter, is a small, irregular nodule of iron mixed with small amounts o ...

for

recycling Recycling is the process of converting waste materials into new materials and objects. The Energy recycling, recovery of energy from waste materials is often included in this concept. The recyclability of a material depends on its ability t ...

In art

Mill scale is sought after by select

abstract expressionist Abstract expressionism is a post–World War II art movement in American painting, developed in New York City in the 1940s. It was the first specifically American movement to achieve international influence and put New York at the center of th ...

artists as its effect on steel can cause unpredicted and seemingly random abstract organic visual effects. Although the majority of mill scale is removed during its passage through scale breaker rolls during manufacturing, smaller structurally inconsequential residue can be visible. Leveraging this processing vestige by accelerating its corrosive effects through the metallurgical use of

phosphoric acid Phosphoric acid (orthophosphoric acid, monophosphoric acid or phosphoric(V) acid) is a colorless, odorless phosphorus-containing solid, and inorganic compound with the chemical formula . It is commonly encountered as an 85% aqueous solution, w ...

or in conjunction with selenium dioxide can create a high contrast visual substrate onto which other compositional elements can be added.

In refractory production

Mill scale can be used as a raw material in granular refractory. When this refractory is cast and preheated, these scales provide escape routes for the evaporating water vapor, thus preventing cracks and resulting in a strong, monolithic structure.

In reduced iron powder production

Rolling Mill scale is complex oxide contains around 70% iron with traces of nonferrous metals and alkaline compounds. The reduced iron powder may be obtained through converting mill scale into a single highest oxide i.e. hematite (Fe₂O₃) and its reduction with hydrogen. Shahid and Choi reported the reverse co-precipitation method for the synthesis of

Magnetite Magnetite is a mineral and one of the main iron ores, with the chemical formula Fe2+Fe3+2O4. It is one of the oxides of iron, and is ferrimagnetic; it is attracted to a magnet and can be magnetized to become a permanent magnet itself. With ...

from mill scale and used for multiple environmental applications such as nutrient recovery, ballasted coagulation in

Activated sludge The activated sludge process is a type of biological wastewater treatment process for treating sewage or industrial wastewaters using aeration and a biological floc composed of bacteria and protozoa. It uses air (or oxygen) and microorganism ...

process, and heavy metal remediation in aqueous environment.

References

{{DEFAULTSORT:Mill Scale Metallurgy Oxides Steelmaking Metalworking terminology

In art

In refractory production

In reduced iron powder production

See also

References