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

s iron(II) oxide (FeO), iron(III) oxide (), and iron(II,III) oxide (, 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 sim ...

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 food preservation, preserving or extending the shelf life of food by either Anaerobic organism, anaerobic fermentation (food), fermentation in brine or immersion in vinegar. The pickling procedure typically affects th ...

, 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 fo ...

s used to leave steel and rebar 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-fi ...

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 br ...

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

or in conjunction with selenium dioxide can create a high contrast visual

substrate Substrate may refer to: Physical layers *Substrate (biology), the natural environment in which an organism lives, or the surface or medium on which an organism grows or is attached ** Substrate (locomotion), the surface over which an organism lo ...

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 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