Zinc alloy electroplating

Zinc alloys have been developed for meeting the most challenging specifications in terms of corrosion protection, temperature and wear resistance.

History

The modern development started during the 80’s with the first alkaline Zn/Fe (99,5%/0,5%) deposits and Zn/Ni (94%/6%) deposits. Recently, the reinforcement of the corrosion specifications of the major European Car Makers and the Directive ELV that banished the use of hexavalent Chromium (CrVI) Conversion Coating required greater use of alkaline Zn/Ni between 12 and 15% of Ni (Zn/Ni 86/14).^[1] 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 Nickel in acidic and alkaline electrolytes is plated as the gamma crystalline phase of the binary diagram Zn-Ni.

Processes

The corrosion protection is primarily due to the anodic potential dissolution of zinc versus iron. Zinc is acting as sacrificial anode for protecting iron (steel). While steel is close to -400 mV, depending on alloy composition, electroplated zinc is much more anodic with -980 mV. Steel is preserved from corrosion by cathodic protection. When alloying zinc with cobalt or nickel at levels less than 1% has minimal affect on the potential but both alloys improve the capacity of the zinc layer to develop a chromate film by conversion coating. This enhances corrosion protection further. On the opposite Zn/Ni between 12% and 15% of Ni (Zn/Ni 86/14) has a potential around -680 mV 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 all other alloys.

For cost reasons the existing market is dividing 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former Zn/Ni (94%/6%) that was a blend between pure zinc and the crystallographic gamma phase of Zn/Ni (86%/14%), was withdrawn from the European specs. A specific advantage of alkaline Zn/Ni (86%/14%) involves the lack of hydrogen embrittlement by plating. It was proved that the first nucleation on steel starts with pure nickel and that this layer is plated 2 nm thick prior to the Zn-Ni.^[2] 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 hard strength steel, tool steels and other substrates susceptible to hydrogen embrittlement.

A new acidic Zn/Ni (86%/14%) has been developed that produces 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^VI free conversion coating films that are frequently followed by a top-coat to enhance corrosion protection, Wear resistance and to control the coefficient of friction.

Bath compositions

Alkaline Zinc-Iron :

Titre du tableau
Parameters	Composition in g/L
Zinc	6-20
Iron	0.05-0.4
Caustic soda	120

Acidic Zinc-cobalt:

Titre du tableau
Parameters	Composition in g/L
Zinc	25-40
Cobalt	2-5
Total Chloride	130-180
Potassium Chloride	200-250
Boric Acid	25

Alkaline Zinc-nickel 4-8%:

Titre du tableau
Parameters	Composition in g/L
Zinc	7.5-10
Nickel	1.8-2
Caustic Soda	100-120

Alkaline Zinc-nickel 12-15%

Titre du tableau
Parameters	Composition in g/L
Zinc	7-12
Nickel	1-2.5
Caustic Soda	120

Acidic Zinc-nickel 12-15%

Titre du tableau
Parameters	Composition in g/L
Zinc	30-40
Nickel	25-35
Total Chloride	150-230
Boric Acid	25

Business Fields

Initiated by the Automotive Industry, Zinc Alloys are applied for all applications where the cosmetic requirements require more than 6 years without a change in appearance and 12 years for avoiding functional corrosion. Alkaline Zn/Ni 86/14 has a microhardness of 450 HV15 and can replace hard steel components for various equipment manufacturers. Besides Automotive, Electrical, House Building, Aerospace, Fastener Industries all find benefits from Zinc Alloys.

Références

JJ. Duprat(Coventya), Mike Kelly(Coventya), «Dedicated processes for electroplating on fasteners», Fasteners Technology International, August 2010, p56-60,

www.nasf.org/staticcontent/Duprat%20Paper.pdf - Pages similaires

L. Thiery, F. Raulin : « Advances in trivalent passivates on zinc and zinc alloy », Galvanotechnik 98(4) (2007) 862-869,

http://stneasy.fiz-karlsruhe.de

El Hajjami, M.P. Gigandet, M. De Petris-Wery, J.C. Catonné, J.J. Duprat, L. Thiery, N. Pommier, F. Raulin, B. Starck, P. Remy : « Characterization of thin Zn-Ni alloy coatings electrodeposited on low carbon steel », Applied Surface Sciences, 254, (2007) 480-489,

http://dx.doi.org/10.1016/j.apsusc.2007.06.016

N. Pommier, L. Thiery (Coventya), M.P. Gigandet, M. Tachez : « Electrochemical study of the degradation of an organomineral coating: polarization resistance and electrochemical impedance spectroscopy measurements » , Ann. Chim. Sci. Mat, 1998, 23, 397-400,

http://dx.doi.org/10.1016/j.apsusc.2007.06.016