Editing Anodizing (section)

==Widely used specifications==
The most widely used anodizing specification in the US is a [[United States Military Standard|U.S. military spec]], MIL-A-8625, which defines three types of aluminium anodizing. Type I is chromic acid anodizing, Type II is sulphuric acid anodizing, and Type III is sulphuric acid hard anodizing. Other anodizing specifications include more MIL-SPECs (e.g., MIL-A-63576), aerospace industry specs by organizations such as [[SAE International|SAE]], [[ASTM International|ASTM]], and [[International Organization for Standardization|ISO]] (e.g., AMS 2469, AMS 2470, AMS 2471, AMS 2472, AMS 2482, ASTM B580, ASTM D3933, ISO 10074, and BS 5599), and corporation-specific specs (such as those of Boeing, Lockheed Martin, Airbus and other large contractors). AMS 2468 is obsolete. None of these specifications define a detailed process or chemistry, but rather a set of tests and quality assurance measures which the anodized product must meet. BS 1615 guides the selection of alloys for anodizing. For British defense work, a detailed chromic and sulfuric anodizing processes are described by DEF STAN 03-24/3 and DEF STAN 03-25/3 respectively.<ref>[ftp://avalon.iks-jena.de/mitarb/lutz/standards/dstan/03/024/00000300.pdf |DEF STAN 03-24/3]{{dead link|date=May 2025|bot=medic}}{{cbignore|bot=medic}}</ref><ref>[ftp://avalon.iks-jena.de/mitarb/lutz/standards/dstan/03/025/00000300.pdf |DEF STAN 03-25/3]{{dead link|date=May 2025|bot=medic}}{{cbignore|bot=medic}}</ref>

===Chromic acid (Type I)===
The oldest anodizing process uses [[chromic acid]]. It is widely known as the Bengough-Stuart process but, due to the safety regulations regarding air quality control, is not preferred by vendors when the additive material associated with type II doesn't break tolerances. In North America, it is known as Type I because it is so designated by the MIL-A-8625 standard, but it is also covered by AMS 2470 and MIL-A-8625 Type IB. In the UK it is normally specified as Def Stan 03/24 and used in areas that are prone to come into contact with propellants etc. There are also Boeing and Airbus standards. Chromic acid produces thinner, 0.5 μm to 18 μm (0.00002" to 0.0007")<ref name="MIL">US Military Specification MIL-A-8625, [http://assist.daps.dla.mil/quicksearch/ ASSIST database] {{webarchive|url=https://web.archive.org/web/20071006205659/http://assist.daps.dla.mil/quicksearch/ |date=2007-10-06 }}</ref> more opaque films that are softer, ductile, and to a degree self-healing. They are harder to dye and may be applied as a pretreatment before painting. The method of film formation is different from using sulfuric acid in that the voltage is ramped up through the process cycle.

===Sulfuric acid (Type II & III)===
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[[Sulfuric acid]] is the most widely used solution to produce an anodized coating. Coatings of moderate thickness 1.8 μm to 25 μm (0.00007" to 0.001")<ref name="MIL" /> are known as Type II in North America, as named by MIL-A-8625, while coatings thicker than 25 μm (0.001") are known as Type III, hard-coat, hard anodizing, or engineered anodizing. Very thin coatings similar to those produced by chromic anodizing are known as Type IIB. Thick coatings require more process control,<ref name="Edwards"/> and are produced in a refrigerated tank near the freezing point of water with higher voltages than the thinner coatings. Hard anodizing can be made between 13 and 150 μm (0.0005" to 0.006") thick. Anodizing thickness increases wear resistance, corrosion resistance, ability to retain lubricants and [[PTFE]] coatings, and electrical and thermal insulation. Sealing Type III will improve corrosion resistance at the cost of reducing abrasion resistance. Sealing will reduce this greatly. Standards for thin (Soft/Standard) sulfuric anodizing are given by MIL-A-8625 Types II and IIB, AMS 2471 (undyed), and AMS 2472 (dyed), BS EN ISO 12373/1 (decorative), BS 3987 (Architectural). Standards for thick sulphuric anodizing are given by MIL-A-8625 Type III, AMS 2469, BS ISO 10074, BS EN 2536 and the obsolete AMS 2468 and DEF STAN 03-26/1.

===Organic acid ===
Anodizing can produce yellowish integral colors without dyes if it is carried out in weak acids with high voltages, high current densities, and strong refrigeration.<ref name="Edwards"/> Shades of color are restricted to a range which includes pale yellow, gold, deep bronze, brown, grey, and black. Some advanced variations can produce a white coating with 80% reflectivity. The shade of color produced is sensitive to variations in the metallurgy of the underlying alloy and cannot be reproduced consistently.<ref name="sheasby ch8" />

Anodizing in some organic acids, for example [[malic acid]], can enter a 'runaway' situation, in which the current drives the acid to attack the aluminium far more aggressively than normal, resulting in huge pits and scarring. Also, if the current or voltage are driven too high, 'burning' can set in; in this case, the supplies act as if nearly shorted and large, uneven and amorphous black regions develop.

Integral color anodizing is generally done with organic acids, but the same effect has been produced in laboratories with very dilute sulfuric acid. Integral color anodizing was originally performed with [[oxalic acid]], but [[sulfonated]] [[aromatic compounds]] containing oxygen, particularly [[sulfosalicylic acid]], have been more common since the 1960s.<ref name="sheasby ch8" /> Thicknesses of up to 50 μm can be achieved. Organic acid anodizing is called Type IC by MIL-A-8625.

===Phosphoric acid ===
Anodizing can be carried out in phosphoric acid, usually as a surface preparation for adhesives. This is described in standard ASTM D3933.

===Borate and tartrate baths===
Anodizing can also be performed in [[borate]] or [[tartrate]] baths in which aluminium oxide is insoluble. In these processes, the coating growth stops when the part is fully covered, and the thickness is linearly related to the voltage applied.<ref name="Edwards"/> These coatings are free of pores, relative to the sulfuric and chromic acid processes.<ref name="Edwards"/> This type of coating is widely used to make electrolytic capacitors because the thin aluminium films (typically less than 0.5 μm) would risk being pierced by acidic processes.<ref name="sheasby ch7"/>

===Plasma electrolytic oxidation===
[[Plasma electrolytic oxidation]] is a similar process, but where higher [[voltages]] are applied. This causes sparks to occur and results in more crystalline/ceramic type coatings.