This zinc-nickel plating is one of the most effective of the various steel parts plating coatings that exist for use in harsh, corrosive situations. Here’s why it is so effective and therefore often the preferred choice for steel parts.
Alloy Composition and Why It Matters
The coating is made up of Zinc and Nickel. The coating is made up of an alloy of Zinc and Nickel. In terms of the weight of the coating, Zinc Nickel coating is made up of 12% to 15% Nickel by weight. This is an important point as the amount of Nickel in the coating plays a big role in the coating’s durability.
How the Coating Acts as a Sacrificial Anode
As Zinc Nickel Plating is less noble than steel, when moisture comes into contact with a damaged area, the coating will corrode prior to the base metal. This ‘sacrificial’ action means that even small scratches or chips will not expose the steel to red rust. In fact, the surrounding coating will continue to protect the area.
Performance Compared to Plain Zinc
Plain zinc coatings will tend to corrode rapidly above 120 degrees C, making them unsuitable for use under car bonnets or in other high temperature applications. However, Zinc Nickel coatings retain protective properties up to around 200 degrees C and offer higher corrosion resistance than plain zinc in salt-laden or other chemically aggressive environments, due to the higher content of nickel.
Salt Spray Performance in Practice
Typical film thickness for Zinc Nickel plating is in the region of 8-12 microns. The time to white corrosion and red rust in neutral salt spray testing to ISO 9227 is over 1,000 hours for white corrosion and 500 hours + for red rust. The thickness of the coating will increase the performance of the coating. There is more on Zinc Nickel Plating at https://www.poeton.co.uk/surface-treatments/plating/zinc-nickel-plating/.
The Role of Passivation
After plating the Zinc Nickel coating has a passivation treatment applied to the surface. This could be a chromate passivation or a trivalent passivate. This additional layer gives a further chemical barrier to corrosion and, in particular, enhances the neutral salt spray test results. Early stages of corrosion are slowed down and the overall corrosion rate is improved. This is in addition to the properties of the Zinc Nickel alloy itself.
It helps you specify the correct thickness and after-treatment required for the actual application conditions of the part to be coated.
