Zinc alloy electroplating is an electrogalvanization process for corrosion protection of metal surfaces and increasing their wear resistance.
Contents
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 (CrVI) 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 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, 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former Zn/Ni (94%/6%) which was a blend between pure zinc and the crystallographic gamma phase of Zn/Ni (86%/14%), was withdrawn from the European specifications.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 high 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 CrVI 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.