Fasteners are very often an after-thought and are one of the first components to be susceptible to corrosion. Selecting an appropriate coating to protect fasteners can be critical to extend the life of an assembly.
The Alloy Composition That Matters
Typical zinc plated material is 99% Zinc with the balance of metals such as copper and lead. Zinc Nickel Plating is a specially formulated alloy that is composed of approximately 12% to 15% Nickel with the balance of 85% to 88% Zinc. This specific alloy ratio produces a unique gamma-phase microstructure, which significantly increases the coating’s corrosion resistance over that of pure zinc.
How Sacrificial Protection Works
The coating will corrode before the steel fastener, as it is more reactive than steel but less than the coating itself. In practice, even small scratches or exposure of coating edges will not compromise the corrosion protection of the steel fastener.
Salt Spray Performance
Salt Spray testing on typical zinc plated items usually shows red rust to appear between 96 hours and 200 hours. In contrast, tests carried out on zinc nickel plated samples have shown red rust to appear between 500 hours and 1000 hours or more to red rust to appear under ISO 9227 (Salt Spray testing) conditions. This makes zinc nickel an ideal coating for fasteners to be used in automotive underbody, offshore and marine environments.
Hydrogen Embrittlement on High-Strength Fasteners
Any electroplating process for metal parts has the potential to absorb hydrogen. Such parts made from high tensile steel (grade 10.9 and above) can be prone to cracking under load. To prevent this, the baked plate process is often used where parts are plated with zinc-nickel. This process involves heating the plated parts to around 190 to 200°C for several hours to drive off the hydrogen. You should always check with your supplier that they use this process for high tensile steel fixings that have been zinc-nickel plated.
Use in Bimetallic Assemblies with Aluminium
5 volts from the aluminium, which will, in the presence of corrosion media, quickly create severe galvanic corrosion. Zinc nickel plating reduces this risk very significantly as its electrochemical potential is very close to that of aluminium and so there is very little potential difference between the two materials. More on Zinc Nickel Plating can be found at https://www.poeton.co.uk/surface-treatments/plating/zinc-nickel-plating/.
When specifying a coating it is crucial that you also specify the post-treatment, in order to get the best from the coating and the overall life of the assembly.