Zinc Nickel a Corrosion Protection Coating
Sacrificial, anodic coatings, specifically zinc deposits, have provided corrosion protection to components and parts across many industries with an increasing demand in the automotive and industrial sectors. Today, typical sacrificial applications for fasteners, fuel systems, braking systems and drive assemblies require an increasing demand for higher corrosion performance from anodic coatings. Electrochemical alloys can be designed to provide the highest corrosion potentials to meet these increased demands. Today zinc-nickel alloy systems deposited from alkaline and acid chemistry types can provide excellent performance in many applications. However, comparing and contrasting these deposits and the systems that provide the zinc-nickel alloys will provide a good knowledge basis for selecting the best system for the required application.
The Increasing Popularity of Zinc-Nickel Deposits
In the last decade, electroplated zinc-nickel deposits have become increasingly requested. The main reason for this is the ability of zinc-nickel to offer excellent corrosion protection when applied over ferrous substrates. Galvanic corrosion happens in an aqueous environment, such as a 5% neutral salt solution, when two metals with different electrode potentials come in contact with one another. When this happens the sacrificial metal, which has a more negative potential, loses electrons and oxidizes to become an aqueous cation. The greater the difference in potential between the two metals, the faster this dissolution happens. Reactive metals such as zinc and magnesium have a more negative potential compared to cast iron and steel, while noble metals like silver and platinum have a less negative potential.
Zinc-nickel is sacrificial to cast iron and steel, though not quite as sacrificial as pure zinc. In this way, a zinc-nickel layer over steel would corrode less quickly than a pure zinc layer of comparable thickness. It is also seen that cadmium has a very close sacrificial potential to steel, and would provide superior corrosion resistance in a salt water environment. However, due to the well-known toxicity of cadmium and its compounds, zinc-nickel deposits have been desired by industry as a more environmentally safe alternative to cadmium.
Of particular interest in the zinc-nickel alloy system is the gamma phase zinc-nickel (γ Zn-Ni). The reason for this is that it has been widely accepted that γ Zn-Ni provides better corrosion resistance and ductility than other phases when applied over cast iron or steel. The binary phase diagram in Fig. 33 depicts how the majority of this γ phase is found when the nickel content ranges between 14 – 25 wt% in a temperature range of 0 – 700°C. In the electroplating industry, a deposit containing between 12 – 16 wt% nickel is usually what is specified and desired, although specification ranges may be opened to 10 – 18 wt%.
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If you enjoyed reading this article you may also like to read our previous post on what is zinc plating?