Editing Anodizing (section)

==Aluminium==
[[File:Colored aluminium key blanks.jpg|thumb|upright|Colored anodized aluminium key blanks]]
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Aluminium alloys are anodized to increase corrosion resistance and to allow [[dyeing]] (coloring), improved [[lubrication]], or improved [[adhesion]]. However, anodizing does not increase the strength of the aluminium object. The anodic layer is [[insulator (electricity)|insulative]].<ref>{{harvnb|Davis|1993|p=376}}.</ref>

When exposed to air at room temperature, or any other gas containing oxygen, pure aluminium [[passivation (chemistry)|self-passivates]] by forming a surface layer of [[amorphous]] [[aluminum oxide|aluminium oxide]] 2 to 3 [[nanometer|nm]] thick,<ref>{{harvnb|Sheasby|Pinner|2001|p=5}}.</ref> which provides very effective protection against corrosion. Aluminium alloys typically form a thicker oxide layer, 5–15&nbsp;nm thick, but tend to be more susceptible to corrosion. Aluminium alloy parts are anodized to greatly increase the thickness of this layer for corrosion resistance. The corrosion resistance of aluminium alloys is significantly decreased by certain alloying elements or impurities: [[copper]], [[iron]], and [[silicon]],<ref>{{harvnb|Sheasby|Pinner|2001|p=9}}.</ref> so [[Aluminium alloy|2000-, 4000-, 6000 and 7000-series Al alloys]] tend to be most susceptible.

Although anodizing produces a very regular and uniform coating, microscopic fissures in the coating can lead to corrosion. Further, the coating is susceptible to chemical dissolution in the presence of high- and low-[[pH]] chemistry, which results in stripping the coating and corrosion of the substrate. To combat this, various techniques have been developed either to reduce the number of fissures, to insert more chemically stable compounds into the oxide, or both. For instance, sulphuric-anodized articles are normally sealed, either through hydro-thermal sealing or precipitating sealing, to reduce porosity and interstitial pathways that allow corrosive ion exchange between the surface and the substrate. Precipitating seals enhance chemical stability but are less effective in eliminating ionic exchange pathways. Most recently, new techniques to partially convert the amorphous oxide coating into more stable micro-crystalline compounds have been developed that have shown significant improvement based on shorter bond lengths.

Some aluminium aircraft parts, architectural materials, and consumer products are anodized. Anodized aluminium can be found on [[MP3 player]]s, [[smartphones]], [[multi-tool]]s, [[flashlight]]s, [[cookware]], [[camera]]s, [[sporting goods]], [[firearms]], [[window frame]]s, [[roof]]s, in electrolytic capacitors, and on many other products both for corrosion resistance and the ability to retain dye. Although anodizing only has moderate wear resistance, the deeper pores can better retain a lubricating film than a smooth surface would.

Anodized coatings have a much lower thermal conductivity and coefficient of linear expansion than aluminium. As a result, the coating will crack from [[thermal stress]] if exposed to temperatures above 80&nbsp;°C (353 K). The coating can crack, but it will not peel.<ref name="Edwards">{{Cite book
  | last = Edwards
  | first = Joseph
  | title = Coating and Surface Treatment Systems for Metals
  | publisher = Finishing Publications Ltd. and ASM International
  | year = 1997
  | pages = 34–38
  | isbn = 978-0-904477-16-0 }}</ref> The melting point of aluminium oxide is 2050&nbsp;°C (2323K), much higher than pure aluminium's 658&nbsp;°C (931K).<ref name="Edwards"/> This and the insulativity of aluminium oxide can make welding more difficult.

In typical commercial aluminium anodizing processes, the aluminium oxide is grown down into the surface and out from the surface by equal amounts.<ref>{{cite book|last=Kutz|first=Myer|title=Handbook of Environmental Degradation of Materials|url=https://archive.org/details/handbookenvironm00kutz_735|url-access=limited|publisher=William Andrew|location=Norwich, NY |isbn=978-0-8155-1749-8 |page=[https://archive.org/details/handbookenvironm00kutz_735/page/n355 353]| chapter=Protective coatings for aluminum alloys|date=2005-06-02}}</ref> Therefore, anodizing will increase the part dimensions on each surface by half the oxide thickness. For example, a coating that is 2 [[μm]] thick will increase the part dimensions by 1 μm per surface. If the part is anodized on all sides, then all linear dimensions will increase by the oxide thickness. Anodized aluminium surfaces are harder than aluminium but have low to moderate wear resistance, although this can be improved with thickness and sealing.

===Process===
====Desmut====
A desmut solution can be applied to the surface of aluminium to remove contaminants. Nitric acid is typically used to remove smut (residue), but is being replaced because of environmental concerns.<ref>{{cite journal |doi=10.4028/www.scientific.net/MSF.569.309|title=Development of Free Nitric acid, Non-P Desmut Solution for Surface Treatment Aluminium Alloys|year=2008|last1=Son|first1=Seong Ho|last2=Kwon|first2=Dae Chol|last3=Jeong|first3=Do Won|journal=Materials Science Forum|volume=569|pages=309–312|s2cid=95989141}}</ref><ref>{{cite journal |title=Smut and Desmutting |date=February 1, 2001 |author=Larry Chesterfield |journal=Products Finishing |url=https://www.pfonline.com/articles/smut-and-desmutting |accessdate=September 10, 2021}}</ref><ref>{{cite book | last = Brace | first = Arthur | title = The technology of anodizing aluminum | publisher = Technicopy Limited | location = Stonehouse | year = 1979 | isbn = 0905228081 }}</ref><ref>{{cite book | last = Wernick | first = S | title = The surface treatment and finishing of aluminium and its alloys | publisher = ASM International Finishing | location = Ohio Teddington | year = 1987 | isbn = 0904477096 }}</ref>

====Electrolysis====
The anodized aluminium layer is created by passing a [[direct current]] through an electrolytic solution, with the aluminium object serving as the anode (the positive electrode in an electrolytic cell). The current releases [[hydrogen]] at the [[cathode]] (the negative electrode) and [[oxygen]] at the surface of the aluminium anode, creating a build-up of aluminium oxide. [[Alternating current]] and pulsed current is also possible but rarely used. The voltage required by various solutions may range from 1 to 300 V DC, although most fall in the range of 15 to 21 V. Higher voltages are typically required for thicker coatings formed in sulfuric and organic acid. The anodizing current varies with the area of aluminium being anodized and typically ranges from 30 to 300 [[ampere|A]]/[[meter|m]]<sup>2</sup>.

Aluminium anodizing (eloxal or '''El'''ectrolytic '''Ox'''idation of '''Al'''uminium)<ref>{{Cite web|title=Anodizing - WELCO Welding & Coating Solutions - Bruck i.d. Opf.|url=https://www.welco.eu/en/services/anodizing|access-date=2021-04-12|website=www.welco.eu}}</ref> is usually performed in an [[acid]]ic solution, typically sulphuric acid or chromic acid, which slowly [[Dissolution (chemistry)|dissolve]]s the aluminium oxide. The acid action is balanced with the oxidation rate to form a coating with nanopores, 10–150&nbsp;nm in diameter.<ref name="Edwards"/> These pores are what allow the electrolyte solution and current to reach the aluminium [[Substrate (materials science)|substrate]] and continue growing the coating to greater thickness beyond what is produced by auto-passivation.<ref name="sheasby ch6">{{harvnb|Sheasby|Pinner|2001|pp=327–425}}.</ref> These pores allow for the dye to be absorbed, however, this must be followed by sealing or the dye will not stay. Dye is typically followed up by a clean nickel acetate seal. Because the dye is only superficial, the underlying oxide may continue to provide corrosion protection even if minor wear and scratches break through the dyed layer.{{citation needed|date=November 2015}}

Conditions such as electrolyte concentration, acidity, solution temperature, and current must be controlled to allow the formation of a consistent oxide layer. Harder, thicker films tend to be produced by more concentrated solutions at lower temperatures with higher voltages and currents. The film thickness can range from under 0.5 [[micrometre|micrometer]]s for bright decorative work up to 150 micrometers for architectural applications.

===Dual-finishing===
{{unreferenced section|date=February 2024}}
Anodizing can be performed in combination with [[chromate conversion coating]]. Each process provides corrosion resistance, with anodizing offering a significant advantage when it comes to ruggedness or physical wear resistance. The reason for combining the processes can vary, however, the significant difference between anodizing and chromate conversion coating is the electrical conductivity of the films produced. Although both stable compounds, chromate conversion coating has a greatly increased electrical conductivity. Applications where this may be useful are varied, however the issue of grounding components as part of a larger system is an obvious one.

The dual finishing process uses the best each process has to offer, anodizing with its hard wear resistance and chromate conversion coating with its electrical conductivity.

The process steps can typically involve chromate conversion coating the entire component, followed by a masking of the surface in areas where the chromate coating must remain intact. Beyond that, the chromate coating is then dissolved in unmasked areas. The component can then be anodized, with anodizing taking to the unmasked areas. The exact process will vary dependent on service provider, component geometry and required outcome. It helps to protect aluminium article.