Fasteners are subject to immense stress and consequently suffer from damage to threads, blind holes and other recessed areas. In harsh environments, these small defects can cause moisture to penetrate resulting in rapid onset of corrosion.
The Alloy Ratio That Does the Work
The alloy ratio is critical to the coating’s performance and typically consists of 12% to 15% nickel with the balance of 85% to 88% zinc. This ratio of nickel to zinc is critical in achieving the sacrificial coating’s corrosion protection. In normal service the zinc will corrode prior to the steel substrate however the presence of nickel in the coating slows down this process dramatically compared to a plain zinc coating.
Salt Spray Performance in Real Numbers
Standard thickness zinc plated fasteners can be expected to last around 200 hours in salt spray test before the start of red rust. In marine, automotive or outdoor industrial use this extended life can be critical.
What Passivation Adds
Passivation is a critical final processing stage, in which a trivalent chromate solution is used to passivate the surface of the plate, sealing any micro-pores and providing protection against white corrosion products forming, as well as conferring heat resistance. Items such as under-bonnet fasteners or those that are close to exhaust manifolds are typical applications where heat resistance can be a major issue. For more on this, take a look at https://www.poeton.co.uk/surface-treatments/plating/zinc-nickel-plating/.
Why Fastener Geometry Suits This Process
The zinc nickel plating process is an electrochemical process that is carried out in a bath. This type of process is particularly suited to fasteners with complex profiles. The plating process follows all threads and enters all recesses without bridging over them. It gives an extremely even and consistent coverage on such parts.
Hydrogen Embrittlement Considerations
9 and above, are prone to hydrogen embrittlement. The coating process does, on application, absorb hydrogen, which is to be ‘baked out’ of the coating by the subsequent oven drying. Zinc nickel coating has a lower risk of hydrogen embrittlement than the acid zinc process.
Choosing the most suitable coating for fasteners is largely a matter of fact and therefore primarily the following factors influence the choice of coating.