Zinc Nickel coatings are available in various thicknesses and surface finishes to balance corrosion protection, appearance and functionality.
Thickness Bands and Salt Spray Performance
As for the thickness of the Zinc Nickel coating, typical thicknesses for a coated part would be in the region of 5-8 µm, 8-15 µm, or 15-25 µm thick. A coating of this thickness would typically withstand around 200 hours to white corrosion in neutral salt spray testing (NSSL), achieving 500 hours+ or more in similar tests, whilst a coating of 15-25 µm would withstand in excess of 1,000 hours prior to red rust appearance.
How Nickel Content Affects the Deposit
The alloy is plated at 12-15% nickel by weight as a plateau. This means that the zinc nickel coating is slightly darker than a standard zinc coating. This coating corrodes the steel sacrificially in a controlled manner. It gradually protects the steel from corrosion. A coating with less than 12% nickel would be more likely to behave like a pure zinc coating, whilst a coating with more than 15% nickel would start to lose its sacrificial corrosion properties.
Clear or Yellow Chromate Passivate
The coating is then sealed with a chromate passivate. The clear trivalent chromate passivate gives a silver-grey appearance and is often the preferred solution where a clean, low-visibility finish is required, such as for interior fasteners. Yellow chromate passivates give a slightly iridescent tint and provide additional barrier protection on top of the sacrificial coating.
Bright Versus Satin Finish
A bright coated surface is acceptable for use where surface reflectivity is the norm or even preferred. A dull or ‘satin’ finish will provide a surface that scatters light. This is particularly useful for visible parts on aircraft and for underbody parts for cars. Zinc Nickel Coating is covered in more detail at www.poeton.co.uk/surface-treatments/plating/zinc-nickel-plating.
Embrittlement Relief on High-Strength Steel
Parts on high strength steel (above 1,000 MPa) are baked off after plating to “relieve” the hydrogen which has penetrated into the steel during the plating process. Typically baking at 200°C for a minimum of 4 hours is specified and this must be carried out within 4 hours of plating. Note that thicker plating and harder steel can give rise to difficulties in relieving hydrogen and longer baking times may be required.
By choosing the thickness, passivate and finish at the specification stage you can avoid expensive rework later.
