Editing Passivation (chemistry)

{{Short description|Physico-chemical processes of protecting a surface from a chemical reaction}}
{{for-multi|the concept in nonlinear control|Feedback passivation|the concept in spacecraft|Passivation (spacecraft)}}
{{Use dmy dates|date=November 2022}}

In [[physical chemistry]] and engineering, '''passivation''' is [[coating]] a material so that it becomes "passive", that is, less readily affected or [[corrode]]d by the environment. Passivation involves creation of an outer layer of shield material that is applied as a microcoating, created by chemical reaction with the base material, or allowed to build by spontaneous [[oxidation]] in the air. As a technique, passivation is the use of a light coat of a protective material, such as [[Metal oxide adhesion|metal oxide]], to create a shield against [[corrosion]].<ref>{{cite web |title=Passivation vs Electropolishing – What are the differences? |url=https://www.electro-glo.com/passivation-vs-electropolishing-what-are-the-differences/ |website=electro-glo.com |date=10 June 2019 |access-date=6 February 2022}}</ref> Passivation of [[silicon]] is used during fabrication of [[microelectronic]] devices.<ref>[[IUPAC]] [http://goldbook.iupac.org/P04443.html Goldbook]</ref> Undesired passivation of electrodes, called "fouling", increases the circuit resistance so it interferes with some [[Electrochemical engineering|electrochemical applications]] such as [[electrocoagulation]] for  wastewater treatment, [[Amperometry|amperometric chemical sensing]], and [[Electrosynthesis|electrochemical synthesis]].<ref name="Yang 2013">{{cite journal | last1=Yang | first1=Xiaoyun | last2=Kirsch | first2=Jeffrey | last3=Fergus | first3=Jeffrey | last4=Simonian | first4=Aleksandr | title=Modeling analysis of electrode fouling during electrolysis of phenolic compounds | journal=Electrochimica Acta | volume=94 | year=2013 | issn=0013-4686 | doi=10.1016/j.electacta.2013.01.019 | pages=259–268}}</ref>

When exposed to air, many metals naturally form a hard, relatively [[Chemically inert|inert]] surface layer, usually an [[oxide]] (termed the "native oxide layer") or a [[nitride]], that serves as a passivation layer - i.e. these metals are "self-protecting". In the case of [[silver]], the dark [[tarnish]] is a passivation layer of [[silver sulfide]] formed from reaction with environmental [[hydrogen sulfide]]. Aluminium similarly forms a stable protective oxide layer which is why it does not "rust". (In contrast, some base metals, notably [[iron]], oxidize readily to form a rough, [[Porosity|porous]] coating of [[rust]] that adheres loosely, is of higher volume than the original displaced metal, and sloughs off readily; all of which permit & promote further oxidation.) The passivation layer of oxide markedly slows further oxidation and corrosion in room-temperature air for [[aluminium]], [[beryllium]], [[chromium]], [[zinc]], [[titanium]], and silicon (a [[metalloid]]). The inert surface layer formed by reaction with air has a thickness of about 1.5&nbsp;nm for silicon, 1–10&nbsp;nm for [[beryllium]], and 1&nbsp;nm initially for [[titanium]], growing to 25&nbsp;nm after several years. Similarly, for aluminium, it grows to about 5&nbsp;nm after several years.<ref>{{cite web |url=http://www.semi1source.com/glossary/default.asp?searchterm=native+oxide |title=Semiconductor Glossary |website=semi1source.com |access-date=6 February 2022}}</ref><ref>{{harvnb|Bockris|Reddy|1977|p= 1325}}</ref><ref>{{cite book |last=Fehlner |first=Francis P |title=Low-Temperature Oxidation: The Role of Vitreous Oxides, A Wiley-Interscience Publication |publisher=John Wiley & Sons |location=New York |date=1986 |isbn=0471-87448-5}}</ref>

In the context of the [[semiconductor device fabrication]], such as silicon [[MOSFET|MOSFET transistors]] and [[solar cell]]s, '''surface passivation''' refers not only to reducing the chemical reactivity of the surface but also to eliminating the [[dangling bond]]s and other defects that form electronic [[surface state]]s, which impair performance of the devices. Surface passivation of silicon usually consists of high-temperature [[thermal oxidation]].

==Mechanisms==
[[File:Pourbaix Diagram of Iron.svg|thumb|[[Pourbaix diagram]] of iron.<ref>[http://people.bath.ac.uk/chsataj/CHEY0016%20Lecture%2015.htm University of Bath] {{webarchive|url=https://web.archive.org/web/20090303234614/http://people.bath.ac.uk/chsataj/CHEY0016%20Lecture%2015.htm |date=3 March 2009 }} & [http://www.wou.edu/las/physci/ch412/pourbaix.htm Western Oregon University]</ref>]]
There has been much interest in determining the mechanisms that govern the increase of thickness of the oxide layer over time. Some of the important factors are the volume of oxide relative to the volume of the parent metal, the mechanism of oxygen diffusion through the metal oxide to the parent metal, and the relative chemical potential of the oxide. Boundaries between micro grains, if the oxide layer is crystalline, form an important pathway for oxygen to reach the unoxidized metal below. For this reason, [[Glass|vitreous]] oxide coatings – which lack grain boundaries – can retard oxidation.<ref>Fehlner, Francis P, ref.3.</ref> The conditions necessary, but not sufficient, for passivation are recorded in [[Pourbaix diagram]]s. Some [[corrosion inhibitor]]s help the formation of a passivation layer on the surface of the metals to which they are applied. Some compounds, dissolved in solutions ([[Chromate ion|chromate]]s, [[molybdates]]) form non-reactive and low solubility films on metal surfaces.

It has been shown using [[Electrochemical scanning tunneling microscope|electrochemical scanning tunneling microscopy]] that during iron passivation, an [[Extrinsic semiconductor|n-type semiconductor]] Fe(III) oxide grows at the interface with the metal that leads to the buildup of an electronic barrier opposing electron flow and an electronic [[depletion region]] that prevents further oxidation reactions. These results indicate a mechanism of "electronic passivation".<ref>{{Cite journal |last1=Dı́ez-Pérez |first1=I. |last2=Gorostiza |first2=P. |last3=Sanz |first3=F. |date=2003 |title=Direct Evidence of the Electronic Conduction of the Passive Film on Iron by EC-STM |url=https://iopscience.iop.org/article/10.1149/1.1580823 |journal=Journal of the Electrochemical Society |language=en |volume=150 |issue=7 |pages=B348 |doi=10.1149/1.1580823|bibcode=2003JElS..150B.348D }}</ref><ref>{{Cite journal |last1=Díez-Pérez |first1=I. |last2=Sanz |first2=F. |last3=Gorostiza |first3=P. |date=2006-10-01 |title=Electronic barriers in the iron oxide film govern its passivity and redox behavior: Effect of electrode potential and solution pH |url=https://linkinghub.elsevier.com/retrieve/pii/S1388248106002803 |journal=Electrochemistry Communications |volume=8 |issue=10 |pages=1595–1602 |doi=10.1016/j.elecom.2006.07.015 |issn=1388-2481}}</ref><ref>{{Cite journal |last1=Díez-Pérez |first1=Ismael |last2=Sanz |first2=Fausto |last3=Gorostiza |first3=Pau |date=2006-06-01 |title=In situ studies of metal passive films |url=https://linkinghub.elsevier.com/retrieve/pii/S1359028607000162 |journal=Current Opinion in Solid State and Materials Science |volume=10 |issue=3 |pages=144–152 |doi=10.1016/j.cossms.2007.01.002 |bibcode=2006COSSM..10..144D |issn=1359-0286}}</ref> The electronic properties of this semiconducting oxide film also provide a mechanistic explanation of [[corrosion]] mediated by [[chloride]], which creates [[surface states]] at the oxide surface that lead to electronic breakthrough, restoration of anodic currents, and disruption of the electronic passivation mechanism ("transpassivation").<ref>{{Cite journal |last1=Díez-Pérez |first1=I. |last2=Vericat |first2=C. |last3=Gorostiza |first3=P. |last4=Sanz |first4=F. |date=2006-04-01 |title=The iron passive film breakdown in chloride media may be mediated by transient chloride-induced surface states located within the band gap |url=https://linkinghub.elsevier.com/retrieve/pii/S1388248106000464 |journal=Electrochemistry Communications |volume=8 |issue=4 |pages=627–632 |doi=10.1016/j.elecom.2006.02.003 |issn=1388-2481}}</ref>

==History==
===Discovery and etymology===
The fact that iron doesn't react with concentrated [[nitric acid]] was discovered by [[Mikhail Lomonosov]] in 1738 and rediscovered by [[James Keir]] in 1790, who also noted that such pre-immersed Fe doesn't reduce [[silver]] from [[Silver nitrate|nitrate]] anymore.<ref name=":0">{{Cite book |last=Lu |first=Xinying |url=https://books.google.com/books?id=FXitEAAAQBAJ&pg=PA2 |title=Passivation and Corrosion of Black Rebar with Mill Scale |date=2023-02-10 |publisher=Springer Nature |isbn=978-981-19-8102-9 |language=en}}</ref> In the 1830s, [[Michael Faraday]] and [[Christian Friedrich Schönbein]] studied that issue systematically and demonstrated that when a piece of [[iron]] is placed in dilute [[nitric acid]], it will dissolve and produce [[hydrogen]], but if the iron is placed in concentrated nitric acid and then returned to the dilute nitric acid, little or no reaction will take place. In 1836, Schönbein named the first state the active condition and the second the passive condition while Faraday proposed the modern explanation of the oxide film described above (Schönbein disagreed with it), which was experimentally proven by [[Ulick Richardson Evans]] only in 1927.<ref name=":0" /> Between 1955 and 1957, [[Carl Frosch]] and [[Lincoln Derrick]] discovered surface passivation of silicon wafers by silicon dioxide, using passivation to build the first silicon dioxide field effect transistors.<ref>{{Cite patent|number=US2802760A|title=Oxidation of semiconductive surfaces for controlled diffusion|gdate=1957-08-13|invent1=Lincoln|invent2=Frosch|inventor1-first=Derick|inventor2-first=Carl J.|url=https://patents.google.com/patent/US2802760A}}</ref><ref>{{Cite journal |last1=Frosch |first1=C. J. |last2=Derick |first2=L. |date=1957-09-01 |title=Surface Protection and Selective Masking during Diffusion in Silicon |url=https://iopscience.iop.org/article/10.1149/1.2428650 |journal=Journal of the Electrochemical Society |language=en |volume=104 |issue=9 |pages=547 |doi=10.1149/1.2428650 |issn=1945-7111}}</ref><ref>{{Cite journal |last1=Huff |first1=Howard |last2=Riordan |first2=Michael |date=2007-09-01 |title=Frosch and Derick: Fifty Years Later (Foreword) |url=https://iopscience.iop.org/article/10.1149/2.F02073IF |journal=The Electrochemical Society Interface |volume=16 |issue=3 |pages=29 |doi=10.1149/2.F02073IF |issn=1064-8208}}</ref>

==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>

==See also==
* [[Cold welding]]
* [[Deal–Grove model]]
* [[Pilling–Bedworth ratio]]

==References==
{{Reflist}}

==Further reading==
*{{Citation |author=ASTM |author-link=ASTM International |title=ASTM A967: Standard specification for chemical passivation treatments for stainless steel parts |edition=Rev 05e2 |date=1 March 2010 |doi=10.1520/A0967-05E02 |url=http://www.astm.org/Standards/A967.htm |postscript=. ''The most common commercial spec for passivation of stainless steel parts. Used in various industries; latest revision is active for new designs; legacy designs may still require older revisions or older standards, if the engineering has not been revisited.''}}
*{{Citation |author=SAE |author-link=SAE International |title=AMS 2700: Passivation of corrosion resistant steels. |date=8 July 2011 |edition=Rev D |url=http://standards.sae.org/ams2700d/ |postscript=. ''[[SAE International#Aerospace standards|AMS specs]] are frequently used in the aerospace industry, and are sometimes stricter than other standards. Latest revision is active for new designs; legacy designs may still require older revisions or older standards, if the engineering has not been revisited.''}}
*{{Citation |author=SAE |author-link=SAE International |title=AMS QQ-P-35: Passivation treatments for corrosion-resistant steel |date=16 February 2005 |edition=Rev A |url=http://standards.sae.org/amsqqp35a |postscript=. ''AMS-QQ-P-35 superseded U.S. federal spec QQ-P-35 on 4 April 1997. AMS-QQ-P-35 itself was canceled and superseded in February 2005 by AMS 2700.''}}
*{{Citation |author=U.S. government |title=QQ-P-35: Federal specification: Passivation treatments for corrosion-resistant steel |edition=Rev C |url=http://www.everyspec.com/FED_SPECS/Q/QQ-P-35C_NOTICE-3_21310/ |postscript=. ''U.S. federal spec QQ-P-35 was superseded by AMS-QQ-P-35 on 4 April 1997 as part of the changeover instituted by the [[United States Military Standard#Origins and evolution|Perry memo]]. Both are now outdated; they are inactive for new designs, but legacy designs may still require their use, if the engineering has not been revisited.''}}
*[[Chromate conversion coating]] (chemical film) per MIL-DTL-5541F for aluminium and aluminium alloy parts
*A standard overview on black oxide coatings is provided in MIL-HDBK-205, ''Phosphate & Black Oxide Coating of Ferrous Metals''. Many of the specifics of Black Oxide coatings may be found in MIL-DTL-13924 (formerly MIL-C-13924). This Mil-Spec document additionally identifies various classes of Black Oxide coatings, for use in a variety of purposes for protecting ferrous metals against rust.
*{{Citation | surname=Budinski | given=Kenneth G. | title=Surface Engineering for Wear Resistance | publisher=Prentice Hall | place=[[Englewood Cliffs, New Jersey]] | year=1988 | page= 48 | postscript =.}}
*{{Citation | surname=Brimi | given=Marjorie A. | title=Electrofinishing | publisher=American Elsevier Publishing Company, Inc | place=[[New York, New York]] | year=1965 | pages= 62–63 | postscript =.}}
* {{Citation |last1= Bockris |first1= John O'M. | first2= Amulya K. N. |last2= Reddy |title= Modern Electrochemistry: An Introduction to an Interdisciplinary Area |volume = 2 |publisher= Plenum Press |year= 1977 |isbn= 0-306-25002-0 |postscript =.}}
* Passivisation : Debate over Paintability http://www.coilworld.com/5-6_12/rlw3.htm {{Webarchive|url=https://web.archive.org/web/20160304043217/http://www.coilworld.com/5-6_12/rlw3.htm |date=4 March 2016 }}
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[[Category:Corrosion prevention]]
[[Category:Surface finishing]]
[[Category:German inventions]]
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