What Makes Zinc Nickel Coating Last in Salt Spray

Zinc nickel coating – what makes it last in salt spray?

Zinc nickel alloy coatings consistently outlast plain zinc coatings in salt spray tests but the reason is not just the presence of nickel in the coating. There are several important decisions that are made during the plating and finishing process that affect the life of a coating in salt spray testing.

The Nickel Content Window

A coating with a certain alloy ratio (around 12-15% nickel) forms a special alloy phase called gamma phase, which is by far more corrosion resistant than other phases. The deposit composition is a critical parameter and must be specified, verified during plating and also be within the limits of the finisher’s equipment.

How the Coating Sacrifices Itself

The Zinc nickel coating is anodic to the steel substrate. This means that it will corrode before the steel but will protect the steel from corrosion when the coating is scratched or chipped. However, the alloy corrodes more slowly than plain zinc and so extends the period of the sacrificial coating.

The Role of Passivate Treatment

The passivate treatment is applied after plating to greatly increase the corrosion performance of zinc nickel plated parts in salt spray tests. A simple specification of a “zinc nickel coating” will not necessarily get the best life in salt spray for parts made of steel, it is essential that the plating process and finishing process is specified in full.

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Coating Thickness and Hours to Red Rust

Thickness of deposit is another aspect, generally speaking thicker is better as there is more material to ‘eat’ away before exposing the underlying steel. Typically an 8 micron thick deposit would yield less hours to red rust than a 15 micron thick deposit on an identical substrate with identical passivation.

Hydrogen Embrittlement Relief

Parts made from high-strength steel (above 1000 MPa tensile strength) can become brittle as a result of taking on hydrogen during the plating process. A relief bake is typically carried out within a few hours of plating at a temperature of around 190 to 200 degrees C to release the hydrogen from the part.

When specifying a coating it is not sufficient to simply specify a name, all of the above points must be clearly specified.