Editing Passivation (chemistry) (section)

==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>