The zinc nickel plating process for fasteners has many variables to consider for a successful coating process.
Allow for Coating Thickness on Threads
Deposits of zinc nickel plating typically are in the order of 8-15 microns. On a threaded article the coating will cover the thread flanks and roots. Thus it is essential that the thread tolerance class be specified before plating, in order to allow for the required coating thickness. On external threads a tolerance class of 6g is usually satisfactory to allow for the coating without causing subsequent assembly problems.
Salt Spray Performance to Expect
A properly applied zinc nickel coating to 12 to 15 microns will typically outperform a zinc plating in a salt spray test. Zinc nickel coatings are specified by automotive OEM’s and off-highway equipment manufacturers for the coating of fasteners to be used for chassis and powertrain components which are subjected to normal road use and may from time to time be subjected to road salt and wet conditions.
Hydrogen Embrittlement and Stress-Relief Baking
High-strength fasteners are susceptible to hydrogen embrittlement after being electroplated. By baking the part at 190-210°C within four hours of plating, the hydrogen in the steel is driven out. The baking process must be agreed with the plating supplier for high-strength fasteners.
Torque-Tension Behaviour
Zinc Nickel coatings have a mild lubricating effect, which must be taken into account when specifying torque-tension relationships for bolts. This effect, which is similar to that of a light oil, results in reduced friction under the bolt head and between the threaded shank of the bolt and the receiving part’s threads. Therefore torque values for coated fasteners must be increased to achieve the same tension as for unlubricated fasteners.
Zinc Nickel Coating is covered in more detail at poeton.co.uk/surface-treatments/plating/zinc-nickel-plating/.
When to Choose It Over Plain Zinc or Hot-Dip Galvanising
When choosing between a plain zinc coating, or a hot-dip galvanised coating, plain zinc offers some degree of corrosion protection however this is often short of the useful life afforded by a zinc nickel coating. Hot-dip galvanising produces a thick coating, typically in the order of 45-85 microns. A major issue with such a coating is that it often fills threaded holes completely. Parts that have been hot-dip galvanised must subsequently be re-tapped in order to be of any use.
To avoid later problems in assembly it is essential to define thread tolerance and requirements with respect to hydrogen embrittlement in advance by the engineer who has specified coating for threaded parts.
