It is as important to get the thickness specified correctly as it is to select the correct coating to apply to a part. If you specify the coating incorrectly then the part may be rejected or fail in service.
Match Thickness to Salt-Spray Performance
The thickness of a Zinc Nickel Plating deposit typically is specified within a band of 5 to 25 microns. The higher end of the specified thickness will provide a longer corrosion life, in terms of hours to white corrosion in a salt-spray test. For example, an 8 micron deposit with passivation would achieve approximately 240 to 500 hours to white corrosion, whereas 12 to 15 micron thickness would achieve approximately 720 hours or more to white corrosion.
Account for Geometry and Substrate
Coatings are prone to uneven build up on parts of complex geometry. The coating build up on the inside of through holes, blind holes and recessed areas can be 30% to 50% less than the coating build up on the external surfaces of the part. The coating build up on steel substrates tends to be more even than on cast components with porosity that can affect the coating adhesion and local thickness.
Include Post-Plate Treatments in Your Stack-Up
As mentioned above, post plate treatments such as passivation and a topcoat sealant are commonly used. 3 micron for passivation and 1-3 micron for a topcoat sealant. The dimensional change due to the plated coating and these processes should be taken into account when calculating the total change in the part and ensuring that it is still within tolerance. com/en-GB/standards/ and should be referenced when writing the drawing notes for zinc nickel plated parts. For more on this, take a look at www.poeton.co.uk/surface-treatments/plating/zinc-nickel-plating/.
Write the Drawing Note Clearly
Note on drawing: The minimum and maximum thickness of the Zn-Ni plate, the method of measurement (X-ray fluorescence or magnetic gauge) and the surface on which the measurement is taken (significant surface) must be clearly stated on the drawing. Typical note: Zn-Ni plate 8 micron min, 15 micron max, measured on significant surface per BS EN ISO 2177.
Plan Tolerance Relief on Close-Fit Parts
025 mm) prior to plating. 024 mm. This should be taken into account when designing the part prior to machining.
Specifying zinc nickel plating thickness correctly can save hours in redesign for costly re-make.