Zinc nickel plating changes more than the appearance of fasteners. The way in which they can be torqued is also affected and in some ways can catch the unwary engineer out if he or she uses tables of recommended torque values for uncoated fasteners.
Coating Thickness and Effective Diameter
Typical deposits of Zinc Nickel have a surface finish of around 8 – 15 microns per surface. The coating adds to the effective diameter of threads thereby increasing the “tightness” of fit. This needs to be taken into account
when revising torque values for use with plated fasteners on close tolerance threaded components and even 10 microns can be significant.
How Lubricity Changes Friction
A very important property of Zinc Nickel Plating is that it is very smooth and lubricious. This results in a lower friction coefficient than a bare steel or zinc plated fastener. The effect of this is that the same amount of torque will produce a greater tensile force than would be calculated using the torque values for uncoated or zinc plated fasteners.
Finding the Right Torque Values If you are looking for Zinc Nickel Plating, see https://www.poeton.co.uk/surface-treatments/plating/zinc-nickel-plating/.
Do not refer to torque values listed for uncoated fasteners or fasteners that have been zinc plated. Obtain the friction coefficient of the coating system from your plating supplier and use a torque-tension formula to recalculate the required torque values for your specific application. For further information on the above torque considerations with zinc nickel coated fasteners,
Hydrogen Embrittlement and Re-Torquing
The hydrogen embrittlement risk for highly stressed fasteners (approx > 1,000 MPa tensile strength) during the plating process can be relieved by bake-out. However, re-torquing the fasteners too soon after plating can result in delayed fracture. For information on the risks of hydrogen embrittlement in plated fasteners see Guidance on hydrogen embrittlement risks in plated fasteners.
Check Whether Existing Torque Tables Apply
A zinc nickel plated bolt is a dry (non-lubricated) fastener and therefore should not have torque values for a dry fastener referenced in the technical data. The torque values for a coated fastener in a lubricated joint must be specified on the engineer’s drawing and explained to the assembly staff before use.
Getting the torque correct for coated fasteners is simple provided you recognise the changes the coating has made to the friction and fit of the fastener.
