Zinc alloy electroplating

Zinc alloy electroplating is an electrogalvanization process for corrosion protection of metal surfaces and increasing their wear resistance.

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, 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^VI) conversion coating) required greater use of alkaline Zn/Ni containing between 12 and 15% Ni (Zn/Ni 86/14). 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 a 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 acts as a 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. 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former Zn/Ni (94%/6%) which was a blend between pure zinc and the crystallographic gamma phase of Zn/Ni (86%/14%), was withdrawn from the European specifications.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 that the first nucleation on steel starts with pure nickel, and that this layer is plated 2 nm thick prior to the Zn-Ni. 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 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^VI 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:

Electrolyte

Parameters	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:

Electrolyte

Parameters	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:

Electrolyte

Parameters	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:

Electrolyte

Parameters	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:

Electrolyte

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

External links

• The Hull Cell

• Thiery, L.; Raulin, F. (2007). "Advances in trivalent passivates on zinc and zinc alloy". Galvanotechnik. 98 (4): 862–869.

• El Hajjami, A (2007). "Characterization of thin Zn-Ni alloy coatings electrodeposited on low carbon steel". Applied Surface Sciences. 254: 480–489. doi:10.1016/j.apsusc.2007.06.016. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)CS1 maint: extra punctuation (link)

• Pommier, N. (Coventya) (1998). "Electrochemical study of the degradation of an organomineral coating: polarization resistance and electrochemical impedance spectroscopy measurements". Ann. Chim. Sci. Mat. 23: 397–400. doi:10.1016/S0151-9107(98)80101-3. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)

• "Modern Electroplating, 5th Edition," (PDF). Wiley.

• Geduld, H. (1998). Zinc Plating. Finishing Publications.

• Wojczykowski, K. (2010). "New Developments in Corrosion Testing: Theory, Methods and Standards". Surfin proceedings. Session 7. Grand Rapids, MI.

• Jimenez, A. (2010). "Membrane Technology for electroplating processes" (PDF). Surfin proceedings. Session 4. Grand Rapids, MI.