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#REDIRECT [[Electrogalvanization]]
'''Zinc alloy electroplating''' is an [[electrogalvanization]] process for [[corrosion]] protection of metal surfaces and increasing their wear resistance.


{{R from merge}}
==History==
Modern development started during the 1980s with the first alkaline Zn/Fe (99.5%/0.5%) deposits and Zn/Ni (94%/6%) deposits. Recently,{{when|date=July 2014}} the reinforcement of the corrosion specifications of major European car makers and the [[End of Life Vehicles Directive]] (banishing the use of hexavalent chromium (Cr<sup>VI</sup>) conversion coating) required greater use of alkaline Zn/Ni containing between 12 and 15% Ni (Zn/Ni 86/14).<ref name="ref-1">[https://docs.google.com/viewer?a=v&q=cache:87OBrYAgFqQJ:www.legris.com/jahia/webdav/site/legris/shared/BP/Documentation/directives_europeenes_FR.pdf+norme+ELV+chrome+6&hl=en&gl=fr&pid=bl&srcid=ADGEEShSfDYjhdI2SITSFrmNEQUrF4jHSsXkLGJV25rKp2mtXiOq6baWdhNEgDT3H8YMs-W_rS8fMNoVrfk9B7cRzN86MAgD5kEmqZcgYuxK0wr9mzFVFtFIKKPcCOj5JqiMGAiERhyb&sig=AHIEtbQPXwhLbUJOv_JDD5R1gPps5jsnTA European Directives] (in French)</ref>
Only Zn/Ni (86%/14%) is an alloy while lower content of iron, cobalt and nickel leads to co-deposits. Zn/Ni (12–15%) in acidic and alkaline electrolytes is plated as the gamma crystalline phase of the Zn-Ni binary [[phase diagram]].

==Processes==
The corrosion protection is primarily due to the anodic potential dissolution of zinc versus iron. Zinc acts as a sacrificial anode for protecting iron (steel). While steel is close to -400 mV, depending on alloy composition, [[electroplating|electroplated]] zinc is much more anodic with -980 mV. [[Steel]] is preserved from corrosion by cathodic protection. Alloying zinc with cobalt or nickel at levels less than 1% has minimal effect on the potential; but both alloys improve the capacity of the zinc layer to develop a chromate film by conversion coating. This further enhances corrosion protection.

On the other hand, Zn/Ni between 12% and 15% Ni (Zn/Ni 86/14) has a potential around -680 mV, which is closer to cadmium -640 mV. During corrosion, the attack of zinc is preferred and the dezincification leads to a consistent increase of the potential towards steel. Thanks to this mechanism of corrosion, this alloy offers much greater protection than other alloys.

For cost reasons, the existing market is divided between alkaline Zn/Fe (99.5%/0.5%) and alkaline Zn/Ni (86%/14%). The use of former alkaline and acidic Zn/Co (99.5%/0.5%) is disappearing from the specifications because Fe gives similar results with less environmental concern. The former Zn/Ni (94%/6%) which was a blend between pure zinc and the crystallographic gamma phase of Zn/Ni (86%/14%), was withdrawn from the European specifications. A specific advantage of alkaline Zn/Ni (86%/14%) involves the lack of [[hydrogen embrittlement]] by plating. It was proved{{by whom|date=December 2011}} that the first nucleation on steel starts with pure nickel, and that this layer is plated 2&nbsp;nm thick prior to the Zn-Ni.<ref name="ref-2">
{{cite journal
| last = Duprat
| first = J.J.
| last2 = Kelly
| first2 =Mike
| author3= (Coventya)
| title = Dedicated processes for electroplating on fasteners
| journal = Fastener Technology International
| volume =
| issue =
| pages = 56–60
| date = August 2010
| url=https://docs.google.com/viewer?a=v&q=cache:Y43oVXFP1zoJ:www.nasf.org/staticcontent/Duprat%2520Paper.pdf+El+Hajjami,+Besan%C3%A7on+University,+2007&hl=en&gl=fr&pid=bl&srcid=ADGEESiVywdKzKQfP4eYg0A0hOikhglM-frWsAMvQyNzk6yGvgOTZiBlrA6MzLCPRedxzonu7ZvWzwom0S_Rfl6PycuEE_yTlEfuw8Dvv8N6eqKDP917lqVkPHah9IIxUxQmteBcrIOU&sig=AHIEtbSj1QqkGTDWmI3wLt-m3cmiEEQa7A
}}</ref> This initial layer prevents [[hydrogen]] from penetrating deep into the [[steel]] substrate, thus avoiding the serious problems associated with hydrogen embrittlement. The value of this process and the initiation mechanism is quite useful for high strength steel, [[tool steel]]s and other substrates susceptible to hydrogen embrittlement.
A new acidic Zn/Ni (86%/14%) has been developed which produces a brighter deposit but offers less metal distribution than the alkaline system, and without the aforementioned nickel underlayer, does not offer the same performance in terms of hydrogen embrittlement.
Additionally, all the zinc alloys receive the new Cr<sup>VI</sup> free conversion coating films which are frequently followed by a top-coat to enhance corrosion protection, [[wear resistance]] and to control the [[coefficient of friction]].

===Bath compositions===
*Composition of electrolyte for plating alkaline zinc-iron at 0,5% in Fe:

{| class="wikitable" style="text-align:center; width:40%;"
|+ Electrolyte
|-
! scope=col | Parameters
! scope=col | Composition in g/L
|-
|[[Zinc]]
|6–20
|-
|[[Iron]]
|0.05–0.4
|-
|Caustic soda
|120
|}

*Composition of electrolyte for plating acidic zinc-cobalt at 0,5% in Co:

{| class="wikitable" style="text-align:center; width:40%;"
|+ Electrolyte
|-
! scope=col | Parameters
! scope=col | Composition in g/L
|-
|Zinc
|25–40
|-
|[[Cobalt]]
|2–5
|-
|Total [[chloride]]
|130–180
|-
|[[Potassium chloride]]
|200–250
|-
|[[Boric acid]]
|25
|}

*Composition of electrolyte for plating alkaline zinc-nickel 4-8% in Ni:

{| class="wikitable" style="text-align:center; width:40%;"
|+ Electrolyte
|-
! scope=col | Parameters
! scope=col | Composition in g/L
|-
|Zinc
|7.5–10
|-
|[[Nickel]]
|1.8–2
|-
|Caustic soda
|100–120
|}

*Composition of electrolyte for plating alkaline zinc-nickel at 12–15% in Ni:

{| class="wikitable" style="text-align:center; width:40%;"
|+ Electrolyte
|-
! scope=col | Parameters
! scope=col | Composition in g/L
|-
|Zinc
|7–12
|-
|Nickel
|1–2.5
|-
|Caustic soda
|120
|}

*Composition of electrolyte for plating acidic zinc-nickel at 12–15% in Ni:

{| class="wikitable" style="text-align:center; width:40%;"
|+ Electrolyte
|-
! scope=col | Parameters
! scope=col | Composition in g/L
|-
|Zinc
|30–40
|-
|Nickel
|25–35
|-
|Total chloride
|150–230
|-
|Boric acid
|25
|}

==References==
{{reflist}}

==External links==
* [https://docs.google.com/viewer?a=v&q=cache:oOTy7GlkHw0J:https://dspace.lboro.ac.uk/dspace-jspui/bitstream/2134/3202/3/Hull_and_his_cell_Gabe.pdf+Hull_and_his_cell_Gabe.pdf&hl=en&gl=uk&pid=bl&srcid=ADGEESilUVhKkP7cAhTuBe8BZ9bY1lGuGf-dX_biuRdvxYb_WtyFF4kftqbbkK9XlajNNJy-CAJQSt09nLwz_zIhoyGmocewIQcfanIM8BQahbaAwzMNqhrCMnER7t6U4OLHVc1ly72P&sig=AHIEtbTZDka6XtMUQee1l1_AIV1o84u8iQ The Hull Cell]
* {{cite journal
| last = Thiery
| first = L.
| last2 =Raulin
| first2 = F.
| title = Advances in trivalent passivates on zinc and zinc alloy
| journal = Galvanotechnik
| volume = 98
| issue = 4
| pages = 862–869
| year = 2007
| language =
| url = https://stneasy.fiz-karlsruhe.de/html/english/login1.html}}

* {{cite journal
|last=El Hajjami
|first=A
| author2 =Gigandet, M.P.; De Petris-Wery, M.; Catonné, J.C.; Duprat, J.J.; Thiery, L.; Pommier, N.; Raulin, F.; Starck, B.; Remy, P.;
| title = Characterization of thin Zn-Ni alloy coatings electrodeposited on low carbon steel
| journal = Applied Surface Science
| volume = 254
| issue = 2
| pages = 480–489
| year = 2007
| doi=10.1016/j.apsusc.2007.06.016
}}

* {{cite journal
|last=Pommier
|first=N. (Coventya)
| author2 = Thiery, L. (Coventya); Gigandet, M.P.; Tachez, M.
| title = Electrochemical study of the degradation of an organomineral coating: polarization resistance and electrochemical impedance spectroscopy measurements
| journal = Ann. Chim. Sci. Mater.
| volume = 23
| issue = 1–2
| pages = 397–400
| year = 1998
| language =
| jstor =
| issn =
| doi = 10.1016/S0151-9107(98)80101-3
}}

* {{cite web
|title=Modern Electroplating, 5th Edition
|publisher=Wiley
|url=http://media.wiley.com/product_data/excerpt/46/04711682/0471168246.pdf}}

* {{cite book
| last = Geduld
| first = H.
| title = Zinc Plating
| publisher = Finishing Publications
| year = 1998
| location =
| pages =
| language =
| isbn = 978-0904477108
}}

* {{cite journal
|last=Wojczykowski
|first=K.
|title=New Developments in Corrosion Testing: Theory, Methods and Standards
|journal=Surfin Proceedings
|year=2010
|series=Session 7
| url=https://docs.google.com/viewer?a=v&q=cache:0a21RLgTfG8J:www.nasf.org/staticcontent/WojczykowskiNewDevelopmentsPres.pdf+WojczykowskiNewDevelopmentsPres.pdf&hl=en&gl=uk&pid=bl&srcid=ADGEEShohbfVtddVkxnEVdvJy_lv1M7Vz9Zaq9BpoPfQry7SHAc5A07N2mg6QbGgU6xkV62iivIG-qpADYCeYCTfNrRuyslmVD817XJi_is0FQvXDnr1vc10BwqF33_YUH7us1HxBazE&sig=AHIEtbQHOW4YHPTZaZYjO_tUJ8L8MQAwJw&pli=1
}}

* {{cite journal
|last=Jimenez
|first=A.
|title=Membrane Technology for electroplating processes
|journal=Surfin Proceedings
|year=2010
|series=Session 4
|url=http://www.nasf.org/staticcontent/JimenezMembraneTechnologyPap.pdf
}}


[[Category:Zinc alloys]]
[[Category:Zinc alloys]]

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