Editing Corrosion (section)

== Resistance to corrosion ==
Some metals are more intrinsically resistant to corrosion than others (for some examples, see [[galvanic series]]). There are various ways of [[corrosion inhibitor|protecting metals from corrosion]] (oxidation) including painting, [[hot-dip galvanization]], [[cathodic protection]], and combinations of these.<ref>{{cite web |url=http://www.pipingtech.com/technical/bulletins/corrosion_protection.htm |title=Methods of Protecting Against Corrosion |publisher=Piping Technology & Products |access-date=31 January 2012 |archive-date=10 February 2012 |archive-url=https://web.archive.org/web/20120210195115/http://www.pipingtech.com/technical/bulletins/corrosion_protection.htm |url-status=live }}</ref>

=== Intrinsic chemistry ===
[[File:GoldNuggetUSGOV.jpg|thumb|[[Gold nugget]]s do not naturally corrode, even on a geological time scale.]]

The materials most resistant to corrosion are those for which corrosion is [[thermodynamics|thermodynamically]] unfavorable. Any corrosion products of [[gold]] or [[platinum]] tend to decompose spontaneously into pure metal, which is why these elements can be found in metallic form on Earth and have long been valued. More common "base" metals  can only be protected by more temporary means.

Some metals have naturally slow [[chemical kinetics|reaction kinetics]], even though their corrosion is thermodynamically favorable. These include such metals as [[zinc]], [[magnesium]], and [[cadmium]]. While corrosion of these metals is continuous and ongoing, it happens at an acceptably slow rate. An extreme example is [[graphite]], which releases large amounts of energy upon [[oxidation]], but has such slow kinetics that it is effectively immune to electrochemical corrosion under normal conditions.

=== Passivation ===
{{Main|Passivation (chemistry)}}
Passivation refers to the spontaneous formation of an ultrathin film of corrosion products, known as a passive film, on the metal's surface that act as a barrier to further oxidation. The chemical composition and microstructure of a passive film are different from the underlying metal. Typical passive film thickness on aluminium, stainless steels, and alloys is within 10 nanometers. The passive film is different from oxide layers that are formed upon heating and are in the micrometer thickness range – the passive film recovers if removed or damaged whereas the oxide layer does not. Passivation in natural environments such as air, water and soil at moderate [[pH]] is seen in such materials as [[aluminium]], [[stainless steel]], [[titanium]], and [[silicon]].

Passivation is primarily determined by metallurgical and environmental factors. The effect of pH is summarized using [[Pourbaix diagram]]s, but many other factors are influential. Some conditions that inhibit passivation include high pH for aluminium and zinc, low pH or the presence of [[chloride]] ions for stainless steel, high temperature for titanium (in which case the oxide dissolves into the metal, rather than the electrolyte) and [[fluoride]] ions for silicon. On the other hand, unusual conditions may result in passivation of materials that are normally unprotected, as the alkaline environment of [[concrete]] does for [[steel]] [[rebar]]. Exposure to a liquid metal such as [[mercury (element)|mercury]] or hot [[solder]] can often circumvent passivation mechanisms.

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}}</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 |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.<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>