Zinc Nickel Plating Thickness Limits
Thickness of the coating also matters a lot. As it turns out, too little of a good thing is not better than nothing at all.
The Standard Thickness Band
Typically the coating thickness for zinc-nickel plating is specified to be within the range of 5µm up to 25µm, depending upon the requirements for corrosion protection against the service conditions of the component against dimensional change.
Thin Deposits for Tight Tolerances
The 5 to 8 µm thickness range results in minimal dimensional growth making it suitable for applications where threaded parts or very tight bore diameters are concerned. Low corrosion protection is the major trade-off but is usually acceptable for internal usage or very lightly exposed items.
Thicker Deposits for Harsh Environments
Parts that will be used in under-bonnet applications or at sea can be plated with a coating of 15 to 25 µm thickness. These types of coatings contain a nickel content of 12 to 15% and can achieve in excess of 1,000 hours in neutral salt spray testing, see Zinc Nickel Plating Archives for examples of members work.
Hydrogen Embrittlement and Baking
Higher thickness coatings are susceptible to Hydrogen Embrittlement (HE) of high-strength materials, e.g. steel above 1,000MPa tensile strength. Baking of such parts at 190°C to 220°C within a few hours of plating can remove hydrogen that has penetrated into the material during plating.
What the Standards Specify
ASTM B841 is the appropriate specification for electrodeposited zinc-nickel coatings. When specifying coating thickness for critical safety components, reference this document for acceptable alloy composition and coating thickness.
Specifying zinc nickel plating to suitable thickness for application and component dimensions, rather than selecting the average of the normal thickness range, ensures fit for purpose specification.