Editing Passivation (chemistry) (section)

===Aluminium===
[[Aluminium]] naturally forms a thin surface layer of [[aluminium oxide]] on contact with [[oxygen]] in the atmosphere through a process called [[oxidation]], which creates a physical barrier to corrosion or further oxidation in many environments. Some [[aluminium alloy]]s, however, do not form the oxide layer well, and thus are not protected against corrosion. There are methods to enhance the formation of the oxide layer for certain alloys. For example, prior to storing [[hydrogen peroxide]] in an aluminium container, the container can be passivated by rinsing it with a dilute solution of [[nitric acid]] and peroxide alternating with [[deionized water]]. The nitric acid and peroxide mixture [[oxidize]]s and dissolves any impurities on the inner surface of the container, and the deionized water rinses away the acid and oxidized impurities.<ref>[http://www.aluminiumanodisers.co.uk/aluminium_passivation.html Aluminum Passivation]</ref>

Generally, there are two main ways to passivate aluminium alloys (not counting [[plating]], [[painting]], and other barrier coatings): [[chromate conversion coating]] and [[anodizing]]. [[Alclad]]ing, which metallurgically bonds thin layers of pure aluminium or alloy to different base aluminium alloy, is not strictly passivation of the ''base'' alloy. However, the aluminium layer clad on is designed to spontaneously develop the oxide layer and thus protect the base alloy.

Chromate conversion coating converts the surface aluminium to an aluminium chromate coating in the range of {{convert|0.00001|-|0.00004|in|nm|sigfig=2}} in thickness. Aluminium chromate conversion coatings are amorphous in structure with a gel-like composition hydrated with water.<ref>[http://www.cybershieldinc.com/chemical-conversion-coating-on-aluminum/ Chemical Conversion Coating on Aluminum]</ref> Chromate conversion is a common way of passivating not only aluminium, but also [[zinc]], [[cadmium]], [[copper]], [[silver]], [[magnesium]], and [[tin]] alloys.

Anodizing is an electrolytic process that forms a thicker oxide layer. The anodic coating consists of hydrated aluminium oxide and is considered resistant to corrosion and abrasion.<ref>Aluminum Anodizing Process [http://www.superiormetals.us/aluminum-anodizing-process.htm] {{Webarchive|url=https://web.archive.org/web/20190320194200/http://www.superiormetals.us/aluminum-anodizing-process.htm|date=20 March 2019}}</ref> This finish is more robust than the other processes and also provides [[electrical insulation]], which the other two processes may not.