Editing Passivation (chemistry) (section)

====Stainless steel====
[[File:Pre and post passivation.png|thumb|The fitting on the left has not been passivated, the fitting on the right has been passivated.]]
[[Stainless steel]]s are corrosion-resistant, but they are not completely impervious to rusting. One common mode of corrosion in corrosion-resistant steels is when small spots on the surface begin to rust because [[Grain boundary|grain boundaries]] or embedded bits of foreign matter (such as grinding [[swarf]]) allow water molecules to oxidize some of the iron in those spots despite the alloying [[chromium]]. This is called [[rouging]]. Some grades of stainless steel are especially resistant to rouging; parts made from them may therefore forgo any passivation step, depending on engineering decisions.<ref>{{cite web|title=Stainless Steel Passivation|url=http://www.arrowcryogenics.com/stainless-steel-passivation.htm|publisher=Arrow Cryogenics|access-date=28 February 2014|archive-url=https://web.archive.org/web/20140304073548/http://www.arrowcryogenics.com/stainless-steel-passivation.htm|archive-date=4 March 2014|url-status=dead}}</ref>

Common among all of the different specifications and types are the following steps: Prior to passivation, the object must be cleaned of any contaminants and generally must undergo a validating test to prove that the surface is 'clean.' The object is then placed in an acidic passivating bath that meets the temperature and chemical requirements of the method and type specified between customer and vendor. While nitric acid is commonly used as a passivating acid for stainless steel, citric acid is gaining in popularity as it is far less dangerous to handle, less toxic, and biodegradable, making disposal less of a challenge. Passivating temperatures can range from ambient to {{convert|60|°C|°F|abbr=on}}, while minimum passivation times are usually 20 to 30 minutes. After passivation, the parts are neutralized using a bath of aqueous [[sodium hydroxide]], then rinsed with clean water and dried. The passive surface is validated using humidity, elevated temperature, a rusting agent (salt spray), or some combination of the three.<ref>{{Cite web |url=http://www.cartech.com/techarticles.aspx?id=1566 |title=Carpenter Technical Articles – HOW TO PASSIVATE STAINLESS STEEL PARTS |access-date=8 May 2013 |archive-date=22 October 2013 |archive-url=https://web.archive.org/web/20131022223046/http://www.cartech.com/techarticles.aspx?id=1566 |url-status=dead }}</ref> The passivation process removes exogenous iron,<ref name=delstar>{{Cite web|url=http://www.delstar.com/stainless-steel-passivation|title = Stainless Steel Passivation Services – A967 & A380 &#124; Delstar Metal Finishing, Inc}}</ref> creates/restores a passive oxide layer that prevents further oxidation ([[rust]]), and cleans the parts of dirt, scale, or other welding-generated compounds (e.g. oxides).<ref name=delstar /><ref>{{cite web |url=http://www.euro-inox.org/pdf/map/Passivating_Pickling_EN.pdf |title=Pickling and Passivating Stainless Steel |work=Euro Inox |access-date=2013-01-01 |url-status=dead |archive-url=https://web.archive.org/web/20120912054431/http://www.euro-inox.org/pdf/map/Passivating_Pickling_EN.pdf |archive-date=12 September 2012 |df=dmy-all }}</ref>

Passivation processes are generally controlled by industry standards, the most prevalent among them today being ASTM A 967 and AMS 2700. These industry standards generally list several passivation processes that can be used, with the choice of specific method left to the customer and vendor. The "method" is either a [[nitric acid]]-based passivating bath, or a [[citric acid]]-based bath, these acids remove surface iron and rust, while sparing the chromium. The various 'types' listed under each method refer to differences in acid bath temperature and concentration. [[Sodium dichromate]] is often required as an additive to oxidise the chromium in certain 'types' of nitric-based acid baths, however this chemical is highly toxic. With citric acid, simply rinsing and drying the part and allowing the air to oxidise it, or in some cases the application of other chemicals, is used to perform the passivation of the surface.

It is not uncommon for some aerospace manufacturers to have additional guidelines and regulations when passivating their products that exceed the national standard. Often, these requirements will be cascaded down using [[Nadcap]] or some other accreditation system. Various testing methods are available to determine the passivation (or passive state) of stainless steel. The most common methods for validating the passivity of a part is some combination of high humidity and heat for a period of time, intended to induce rusting. Electro-chemical testers can also be utilized to commercially verify passivation.