Steel, as a substrate for zinc nickel plating, is extremely common, primarily because of the protective layer that the zinc nickel coating provides for the Steel substrate.
Why Steel Suits Zinc Nickel Well
Zinc Nickel forms a sacrificial layer over Steel parts. The coating will corrode prior to the base metal, providing Galvanic protection to the Steel part. Zinc Nickel coating on Steel provides very effective protection to the Steel part, as the Electrochemical potential of the Steel is close to that of the Zinc Nickel coating.
Preparing Steel Before Plating
Degreasing and de-scaling of the steel part is a prerequisite to initiate the plating process. The surfaces must be free from mill scale, rust, stain and other contaminations that can hinder adhesion. High strength steel parts typically greater than 1,000 MPa require an acid clean prior to plating. This process in turn can induce hydrogen into the part and require a subsequent bake-out.
Hydrogen Embrittlement and Bake-Out
There is also the risk of hydrogen embrittlement in high strength steels. High strength steels that have been acid cleaned and then electroplated must be bake-out at temperatures of around 190°C to 220°C for a minimum period of 4 hours. This process removes the hydrogen from the steel and is covered in HSE guidance on the processes involved in electroplating.
Salt Spray Performance on Steel
The zinc nickel coating on steel typically lasts between 720 hours (or more) of white corrosion and over 1,000 hours of red rust on the steel before it starts to affect the steel underneath. This explains why this coating is often specified for automotive and off-highway fasteners, brackets and structural fixings.
Specifying Alloy Ratio and Thickness
A coating with a 12% to 16% nickel alloy content provides the best corrosion protection. In terms of thickness, 8 to 12 microns is typical for general steel items. However, fasteners require a thin coating to allow for thread tolerances and therefore 5 to 8 microns is typical. More on Zinc Nickel Coating can be found at //www.poeton.co.uk/surface-treatments/plating/zinc-nickel-plating/.
It’s worth taking the time to get the specification right for your part before it is passed on to the plater, otherwise you risk a lot of time and potential cost to put it right.
