Zinc is a relatively low-cost metal and as a result standard zinc plating is usually much cheaper than any zinc alloy plating. Zinc is a great coating for steel substrates because zinc acts as an anode to steel and sacrifices itself to protect the substrate from corrosion. Typically, a steel substrate will not begin to corrode until almost all of the sacrificial zinc coating has gone.
Zinc-nickel coatings will normally last many times as long in a corrosive environment compared to standard zinc. For many applications zinc-nickel alloy plating is superior to standard zinc plating. Due to the complexity of zinc-nickel plating, it has only become cost effective in recent years as technology has improved. Zinc-nickel is now seen by many as an improvement on zinc plating and as an alternative to toxic cadmium plating.
Standards for zinc-nickel plating typically require unpassivated parts coated with 10µm of zinc-nickel to withstand 500 hours or more of neutral salt spray testing before red corrosion. For comparison zinc plated and passivated parts of a similar thickness are expected to achieve only 120 hours - considerably less.
Zinc-nickel has good unpassivated corrosion resistance, although it can be improved even further with passivates and top coats. By comparison zinc plating is very reliant on the passivation and topcoating for protection. In this way, zinc nickel may perform much better than zinc for higher temperature applications (passivates lose effectiveness at elevated temperature).
Zinc-nickel may be preferable in applications where not all parts will be the same base metal because it reduces the galvanic potential with other commonly used materials, especially aluminium without being excessively sacrificial itself. It is common to use zinc-nickel plating on components made of materials such as carbon steel or stainless steel, when they’ll be assembled with parts made of aluminium.
Zinc-nickel has now been in widespread use for many years in the automotive industry and elsewhere. It has been found to perform reliably in typical uses. However, zinc plating has been in use for over a century in various forms, so it is very well understood by designers, surface treatment providers and buyers alike. It’s extensively standardised and many guidelines for its effective use exist. Where there is less concern about higher corrosion resistance or galvanic potential, the simplicity and predictability of zinc can be an advantage.
Excluding passivation and supplementary treatments, zinc’s performance is primarily a function of thickness, which can be easily and cheaply evaluated by the receiver’s inspector - using equipment like a magnetic thickness meter or by measuring pre/post-plating dimensions with a micrometer or similar. Zinc-nickel’s performance is a function of thickness and the zinc-nickel ratio (composition), which may vary across the part. This cannot be non-destructively inspected with simple methods and is typically inspected using X-ray fluorescence (XRF), requiring expensive equipment and trained operatives. All reputable zinc-nickel electroplaters should have inspection equipment that can measure this and use it for batch or periodic inspection, but nonetheless some providers do not. Customers must trust that this is being measured accurately and reliably or rely on sampled confirmation by a third party or periodic auditing. This may add complexity and risk.
Both zinc and zinc-nickel plating are at risk of causing hydrogen embrittlement when applied to high tensile strength steels and de-embrittlement is required.
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