The coating of a fastener is almost as important as the material of the fastener itself, as it can be subjected to some of the toughest conditions on a component’s assembly.
Which Fasteners Suit This Process
Bolts, nuts, screws and spring clips all can be plated using the Zinc Nickel Plating process. Fasteners of a small diameter can be barrel plated in high volume, while larger items of a complex shape are better suited to being rack plated to ensure even coverage.
Alloy Composition and Hydrogen Embrittlement
The 12-15% nickel content of the Zinc Nickel Plating process produces a less acidic bath than conventional zinc processes, thus reducing the steel’s absorption of hydrogen. For high-tensile strength fasteners, delayed brittle fracture under load (embrittlement) due to hydrogen absorption can occur.
Controlling Thickness on Threaded Parts
The typical thickness of a coating is 8-12 micrometers and the plater will control this to meet the customer’s thread gauges to ensure nuts run freely. Thread gauges will be checked prior to despatch.
Baking and Passivation
High tensile strength fasteners above grade 10.9 will require baking of the plated item at a temperature of around 190-220°C for a period of approximately 4 hours. Following baking the plated item will require a final trivalent chromate passivation layer to be applied to provide increased corrosion protection, adding a further 200-400 hours to the salt spray test.
Where It Gets Specified
Zinc nickel plating is commonly specified for fasteners to be used in automotive underbody fixings, aerospace parts such as brackets and fittings and other offshore equipment parts. For Zinc Nickel Plating, see swmf.co.uk/surface-coatings/zinc-nickel-alloy-plating/.
Specifying the correct coating from the outset can help to prevent subsequent rework and failures in service.
