Fasteners with threads, recesses and tight tolerances are difficult to plate well.
Why Fasteners Are a Plating Challenge
Because of the many recesses in a thread (recesses down to 0.3 mm deep) even a small amount of current variation in the plating current can cause an extremely uneven amount of deposit thickness in the thread. The difference can cause a nut not to run freely on a bolt.
Alloy Composition and Corrosion Resistance
Standard zinc or zinc-iron coatings have a limited corrosion resistance in comparison to Zinc Nickel Plating of the same thickness. Zinc Nickel Plating typically contains 12% to 15% Nickel by weight. When subjected to salt-spray testing, threaded fasteners coated with Zinc Nickel Plating can last 1,000 hours or more before red rust appears.
Hydrogen Embrittlement Relief
Fasteners made from high tensile steel above grade 10.9 can suffer from hydrogen absorption during the electroplating process. To alleviate hydrogen embrittlement relief requirements for fasteners include a bake at 190 – 220°C within 4 hours of plating to allow the material to relieve any absorbed hydrogen prior to failure by brittle fracture.
Thickness and Thread Fit
8 to 12 μm is a typical thickness for fasteners, but then they have to be gauged after coating to check that the thread has not been interfered with. Fasteners specified to close-tolerance thread classes (e.g. ISO General Purpose fine) need to have the coating thickness confirmed with the supplier before ordering.
Which Sectors Use It
Automotive, Aerospace/Defence use Zinc-Nickel Plating on Fasteners., under bonnet and require long-term corrosion protection and dimensional stability. There is more on Zinc Nickel Plating at www.swmf.co.uk/surface-coatings/zinc-nickel-alloy-plating/.
If you are specifying a fastener coating for a demanding environment then consider zinc-nickel coating.
