The reason steel fasteners corrode is that steel is one of the most reactive materials in any assembly. The surface of the fastener needs to be a coating that the environment attacks instead of the steel.
A Sacrificial Barrier That Takes the Hit
Zinc nickel plating creates a more reactive surface than steel, which then becomes the target for any environmental attack. The coating corrodes before the steel beneath it, and continues to corrode as long as there is coating left.
Why Nickel Content Matters
Pure zinc is a very aggressive material and will sacrifice itself very quickly. However, by adding a percentage of nickel to the coating, typically around 12-15% by weight, the consumption rate of the coating can be increased dramatically. This alloyed coating is more noble than pure zinc, but is still very reactive and will corrode in preference to steel. As a result, the coating will last several times longer than a pure zinc coating before the steel beneath it starts to corrode.
Coverage at the Difficult Spots
Crevice corrosion typically is found at the thread roots or under the heads of fasteners. As moisture can penetrate into these areas and the oxygen can be lacking, the attack can be severe. In electroplating zinc-nickel, these recessed areas are filled up equally well as the surfaces of the fasteners. Thus, the most sensitive parts of a fastener are also protected.
Passivation Adds a Second Line of Defence
Most processes will then apply a chromate or trivalent passivation to the surface to further extend the life of the zinc-nickel plated surface by sealing the surface to slow down the early stages of zinc dissolution. A topcoat sealer is then typically applied to provide further extended protection. There is more on Zinc Nickel Plating at www.swmf.co.uk/surface-coatings/zinc-nickel-alloy-plating/.
What Salt Spray Hours Tell You
Salt spray corrosion test results are measured and expressed in hours to the first red rust (e.g. 720 hours) and qualify for various end use applications including automotive underbody fasteners as well as other marine and outdoor structural applications which require longer life (e.g. 1,000 hours or more).
A fastener that meets these thresholds will last longer than the assembly it is fastening.
