Editing Passivation (chemistry) (section)

==Specific materials==

===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.

===Carbon===
In [[carbon quantum dot]] (CQD) technology, CQDs are small carbon [[nanoparticles]] (less than [[Orders of magnitude (length)#10 nanometres|10 nm]] in size) with some form of surface passivation.<ref name="Wang & Hu 2014">{{cite journal |doi=10.1039/C4TC00988F |title=Carbon quantum dots: Synthesis, properties and applications |journal=Journal of Materials Chemistry C |volume=2 |issue=34 |pages=6921–39 |year=2014 |last1=Wang |first1=Youfu |last2=Hu |first2=Aiguo |doi-access=free }}</ref><ref>{{cite journal |doi=10.1021/acsami.5b00448 |pmid=25845394 |title=Carbon Quantum Dots and Applications in Photocatalytic Energy Conversion |journal=ACS Applied Materials & Interfaces |volume=7 |issue=16 |pages=8363–76 |year=2015 |last1=Fernando |first1=K. A. Shiral |last2=Sahu |first2=Sushant |last3=Liu |first3=Yamin |last4=Lewis |first4=William K. |last5=Guliants |first5=Elena A. |last6=Jafariyan |first6=Amirhossein |last7=Wang |first7=Ping |last8=Bunker |first8=Christopher E. |last9=Sun |first9=Ya-Ping }}</ref><ref>{{cite journal |doi=10.1038/nbt994 |pmid=15258594 |title=In vivo cancer targeting and imaging with semiconductor quantum dots |journal=Nature Biotechnology |volume=22 |issue=8 |pages=969–76 |year=2004 |last1=Gao |first1=Xiaohu |last2=Cui |first2=Yuanyuan |last3=Levenson |first3=Richard M |last4=Chung |first4=Leland W K |last5=Nie |first5=Shuming |s2cid=41561027 }}</ref>

===Ferrous materials===
[[File:Tempering standards used in blacksmithing.JPG|thumb|[[Tempering (metallurgy)|Tempering]] colors are produced when steel is heated and a thin film of iron oxide forms on the surface. The color indicates the temperature the steel reached, which made this one of the earliest practical uses of thin-film interference.]]
[[Ferrous]] materials, including steel, may be somewhat protected by promoting oxidation ("rust") and then converting the oxidation to a metalophosphate by using [[phosphoric acid]] and add further protection by surface coating. As the uncoated surface is water-soluble, a preferred method is to form [[manganese]] or zinc compounds by a process commonly known as [[parkerizing]] or [[phosphate conversion]]. Older, less effective but chemically similar electrochemical conversion coatings included [[black oxide|black oxidizing]], historically known as [[bluing (steel)|bluing]] or [[browning (steel)|browning]]. Ordinary [[steel]] forms a passivating layer in [[alkali]] environments, as [[reinforcing bar]] does in [[concrete]].

====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.

===Titanium===
[[File:Anodized titanium table.jpg|thumb|right|Relation between voltage and color for anodized titanium.]]
The surface of [[titanium]] and of titanium-rich alloys oxidizes immediately upon exposure to air to form a thin passivation layer of [[titanium oxide]], mostly [[titanium dioxide]].<ref name="Chen 2001">{{cite journal | last1=Chen | first1=George Zheng | last2=Fray | first2=Derek J. | last3=Farthing | first3=Tom W. | title=Cathodic deoxygenation of the alpha case on titanium and alloys in molten calcium chloride | journal=Metallurgical and Materials Transactions B | volume=32 | issue=6 | year=2001 | issn=1073-5615 | doi=10.1007/s11663-001-0093-8 | pages=1041–1052| bibcode=2001MMTB...32.1041C | s2cid=95616531 }}</ref> This layer makes it resistant to further corrosion, aside from gradual growth of the oxide layer, thickening to ~25&nbsp;nm after several years in air. This protective layer makes it suitable for use even in corrosive environments such as sea water. Titanium can be anodized to produce a thicker passivation layer. As with many other metals, this layer causes [[thin-film interference]] which makes the metal surface appear colored, with the thickness of the passivation layer directly affecting the color produced.

===Nickel===
[[Nickel]] can be used for handling elemental [[fluorine]], owing to the formation of a passivation layer of [[Nickel(II) fluoride|nickel fluoride]]. This fact is useful in [[water treatment]] and [[sewage treatment]] applications.

===Silicon===
In the area of [[microelectronics]] and [[photovoltaic]] [[solar cell]]s, surface passivation is usually implemented by [[thermal oxidation]] at about 1000&nbsp;°C to form a coating of [[silicon dioxide]]. Surface passivation is critical to [[solar cell efficiency]].<ref>{{cite book |last1=Black |first1=Lachlan E. |title=New Perspectives on Surface Passivation: Understanding the Si-Al2O3 Interface |date=2016 |publisher=Springer |isbn=9783319325217 |url=https://core.ac.uk/download/pdf/156698511.pdf}}</ref> The effect of passivation on the efficiency of solar cells ranges from 3–7%. The surface resistivity is high, >&nbsp;100 Ωcm.<ref>{{cite journal |doi=10.1002/1099-159X(200009/10)8:5<473::AID-PIP337>3.0.CO;2-D|title=Surface passivation of crystalline silicon solar cells: A review|year=2000|last1=Aberle|first1=Armin G.|journal=Progress in Photovoltaics: Research and Applications|volume=8|issue=5|pages=473–487}}</ref>

=== Perovskite ===
The easiest and most widely studied method to improve [[perovskite solar cell]]s is passivation. These defects usually lead to deep energy level defects in solar cells due to the presence of hanging bonds on the surface of perovskite films.<ref>{{Cite journal |last=Stranks |first=Samuel |date=2020 |title=Performance-limiting nanoscale trap clusters at grain junctions in halide perovskites |url=https://doi.org/10.1038/s41586-020-2184-1 |journal=Nature |volume=580 |issue=7803 |pages=360–366|doi=10.1038/s41586-020-2184-1 |pmid=32296189 |bibcode=2020Natur.580..360D |s2cid=215775389 }}</ref><ref>{{Cite journal |last=Jinsong |first=Huang |date=2020 |title=Resolving spatial and energetic distributions of trap states in metal halide perovskite solar cells |url=https://www.science.org/doi/10.1126/science.aba0893 |journal=Science |volume=367 |issue=6484 |pages=1352–1358|doi=10.1126/science.aba0893 |pmid=32193323 |arxiv=2008.06789 |bibcode=2020Sci...367.1352N |s2cid=213193915 }}</ref> Usually, small molecules or polymers are doped to interact with the hanging bonds and thus reduce the defect states. This process is similar to Tetris, i.e., we always want the layer to be full. A small molecule with the function of passivation is some kind of square that can be inserted where there is an empty space and then a complete layer is obtained. These molecules will generally have lone electron pairs or pi-electrons, so they can bind to the defective states on the surface of the cell film and thus achieve passivation of the material. Therefore, molecules such as [[Carbonyl group|carbonyl]],<ref>{{Cite journal |last=Nazeeruddin |first=Mohammad Khaja |date=2020 |title=Gradient band structure: high performance perovskite solar cells using poly(bisphenol A anhydride-co-1,3-phenylenediamine) |url=https://pubs.rsc.org/en/content/articlelanding/2020/ta/d0ta05496h#! |journal=Journal of Materials Chemistry A |volume=8 |issue=17113}}</ref> nitrogen-containing molecules,<ref>{{Cite journal |last=Yang |first=Yang |date=2019 |title=Constructive molecular configurations forsurface-defect passivation of perovskite photovoltaics |url=https://www.science.org/doi/10.1126/science.aay9698 |journal=Science |volume=366 |issue=6472 |pages=1509–1513|doi=10.1126/science.aay9698 |pmid=31857483 |bibcode=2019Sci...366.1509W |osti=1574274 |s2cid=209424432 |hdl=11424/244343 |hdl-access=free }}</ref> and sulfur-containing molecules<ref>{{Cite journal |last=Snaith |first=Henry J. |date=2014 |title=Enhanced Photoluminescence and Solar Cell Performance via Lewis Base Passivation of Organic-Inorganic Lead Halide Perovskites |url=https://pubs.acs.org/doi/10.1021/nn5036476 |journal=ACS Nano |volume=8 |issue=10 |pages=9815–9821|doi=10.1021/nn5036476 |pmid=25171692 }}</ref> are considered, and recently it has been shown that π electrons can also play a role.<ref>{{Cite journal |last=Zhou |first=Zhongmin |date=2021 |title=Reducing Defects Density and Enhancing Hole Extraction for Efficient Perovskite Solar Cells Enabled by π-Pb2+ Interactions |url=https://doi.org/10.1002/anie.202102096 |journal= Angewandte Chemie International Edition|volume=60 |issue=32|pages=17356–17361 |doi=10.1002/anie.202102096 |pmid=34081389 |s2cid=235321221 }}</ref>

In addition, passivation not only improves the photoelectric conversion efficiency of perovskite cells, but also contributes to the improvement of device stability. For example, adding a passivation layer of a few nanometers thickness can effectively achieve passivation with the effect of stopping water vapor intrusion.<ref>{{Cite journal |last=Fang |first=Junfeng |date=2018 |title=In-situ cross-linking strategy for efficient and operationally stable methylammoniun lead iodide solar cells |journal=Nature Communications |volume=9 |issue=1 |page=3806 |doi=10.1038/s41467-018-06204-2 |pmid=30228277 |pmc=6143610 |bibcode=2018NatCo...9.3806L }}</ref>