A coating on a fastener can fail early in its life, lead to seized threads, rust staining etc. Even disassembling a part of a structure that has been taken apart for repairs can be expensive. So what does Zinc Nickel Plating actually do for fasteners?
Sacrificial Protection at the Thread Root
The coating is acting as a sacrificial anode protecting the substrate beneath from corrosion. This is particularly important at thread roots, and under-head contact faces. These are the areas where stress concentrations, and surface damage, have created areas of bare metal. The surrounding Zinc Nickel coating is protecting this area from corrosion.
Far Longer Salt Spray Performance
Salt spray tests on plain zinc coatings show a failure to red rust at around 200 hours. Zinc nickel coatings containing 12 to 15 % nickel, typically last 1,000 hours or more before any indication of base metal corrosion appears. This gives a long time between inspections and eventual replacement, especially for products that are left outside and subjected to road or marine corrosion.
Threads Still Assemble After Plating
Fasteners that are plating thickness of 8 to 15 microns and have been coated using good process control, are able to be assembled without re-tapping of the threads, even when mass produced.
More on Zinc Nickel Plating can be found at www.poeton.co.uk/surface-treatments/plating/zinc-nickel-plating/.
Reduced Bimetallic Corrosion
Bimetallic corrosion is a phenomenon where corrosion is increased between two dissimilar metals. This can occur between a steel fastener and an aluminium or magnesium casting. The potential difference between the two metals can drive the corrosion and in severe cases can cause rapid failure. Zinc nickel has a potential closer to aluminium than do steel fasteners. As a result, the galvanic driving force is decreased and the corrosion rate is lowered.
Stability at Elevated Temperatures
These plain zinc coatings then start to deteriorate at temperatures of around 120°C and above. However, a Zinc Nickel coating will continue to provide adequate protection to the underlying steel up to temperatures of approximately 200°C. Therefore, this coating is particularly suitable for fasteners that are exposed to elevated temperatures.
Specifying the right coating at the outset saves considerably more time than replacing corroded fasteners later.
