View source for Chemistry:Mill scale

{{more references|date=December 2010}}
[[File:Eisenhammerschlag Fe3 O4.jpg|thumb|Mill scale on an anvil]]
'''Mill scale''', often shortened to just '''scale''', is the flaky surface of [[Engineering:Rolling (metalworking)|hot rolled]] steel, consisting of the mixed [[Chemistry:Iron oxide|iron oxide]]s [[Chemistry:Iron(II) oxide|iron(II) oxide]] ({{chem2|FeO}}, [[Chemistry:Wüstite|wüstite]]), [[Chemistry:Iron(III) oxide|iron(III) oxide]] ({{chem2|Fe2O3}}, [[Chemistry:Hematite|hematite]]), and [[Chemistry:Iron(II,III) oxide|iron(II,III) oxide]] ({{chem2|Fe3O4}}, [[Chemistry:Magnetite|magnetite]]). 

Mill scale is formed on the outer surfaces of plates, sheets or profiles when they are produced by passing red hot iron or steel billets through rolling mills.<ref>{{Cite journal |last=Cunha |first=Adriano Ferreira da |last2=Mol |first2=Marcos Paulo Gomes |last3=Martin's |first3=Máximo Eleotério |last4=Assis |first4=Paulo Santos |date=March 2006 |title=Caracterização, beneficiamento e reciclagem de carepas geradas em processos siderúrgicos |journal=Rem: Revista Escola de Minas |volume=59 |issue=1 |pages=111–116 |doi=10.1590/S0370-44672006000100014 |issn=0370-4467 |doi-access=free}}</ref> Mill scale is bluish-black in color. It is usually less than {{convert|0.1|mm|abbr=on}} 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 [[Engineering:Flame cleaning|flame cleaning]], [[Engineering:Pickling (metal)|pickling]], or [[Engineering:Abrasive blasting|abrasive blasting]], which are all tedious operations that consume energy. This is why shipbuilders and steel fixers 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 [[Engineering:Steel mill|steel mill]]s can supply their product with mill scale removed and steel coated with shop [[Engineering:Primer (paint)|primers]] over which [[Chemistry:Welding|welding]] or painting can be done safely.

Mill scale generated in rolling mills will be collected and sent to a [[Engineering:Sinter plant|sinter plant]] for [[Physics:Recycling|recycling]].

==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. {{Citation needed|date=October 2018}} Although the majority of mill scale is removed from steel 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 [[Chemistry:Phosphoric acid|phosphoric acid]] or in conjunction with [[Chemistry:Selenium dioxide|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 [[Chemistry:Refractory metals|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==
Mill scale is a complex oxide that contains around 70% iron with traces of nonferrous metals and alkaline compounds. Reduced iron powder may be obtained by conversion of mill scale into a single highest oxide i.e. hematite ({{chem2|Fe2O3}}) followed by reduction with hydrogen. Shahid and Choi reported the reverse co-precipitation method for the synthesis of magnetite ({{chem2|Fe3O4}}) from mill scale and used for multiple environmental applications such as nutrient recovery,<ref>{{cite journal |last1=Shahid |first1=Muhammad Kashif |last2=Kim |first2=Yunjung |last3=Choi |first3=Young-Gyun |title=Magnetite synthesis using iron oxide waste and its application for phosphate adsorption with column and batch reactors |journal=Chemical Engineering Research and Design |date=August 2019 |volume=148 |page=169-179 |doi=10.1016/j.cherd.2019.06.001 |url=https://doi.org/10.1016/j.cherd.2019.06.001}}</ref> ballasted coagulation in [[Chemistry:Activated sludge|activated sludge]] process, and heavy metal remediation in an aqueous environment.<ref>{{cite journal |last1=Shahid |first1=Muhammad Kashif |last2=Choi |first2=Younggyun |title=Characterization and application of magnetite Particles, synthesized by reverse coprecipitation method in open air from mill scale |journal=Journal of Magnetism and Magnetic Materials |date=1 February 2020 |volume=495 |page=165823 |doi=10.1016/j.jmmm.2019.165823 |url=https://doi.org/10.1016/j.jmmm.2019.165823}}</ref>

==See also==
* [[Chemistry:Dross|Dross]]
* [[Engineering:Firescale|Firescale]]
* Hammer paint 
* [[Chemistry:Slag|Slag]]
* Slag (welding)

==References==
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{{DEFAULTSORT:Mill Scale}}

[[Category:Metallurgy]]
[[Category:Oxides]]
[[Category:Steelmaking]]
[[Category:Metalworking terminology]]

{{Sourceattribution|Mill scale}}