By understanding the process and what is needed at each stage you will be able to give the correct specification to the plating company and avoid problems that can occur with the finish of the parts on delivery.
Cleaning and Degreasing
First the substrate is thoroughly cleaned to remove any oils, scale and contamination from previous processes. This is typically done by an alkaline soak at temperatures around 60-70°C, or by an ultrasonic clean. It is essential to remove all contamination from the substrate surface as any left behind will cause the coating to peel off.
Acid Activation
Once cleaned, parts are immersed in a diluted acid solution, (Hydrochloric or Sulphuric Acid), for a short period of time to remove any surface oxide from the base metal. This enables the Zinc/Nickel alloy to form a strong bond with the substrate. The part is then rinsed to remove any residual acid from the surface.
Electroplating in the Zinc-Nickel Bath
The parts are then submerged in an alkaline or acid chloride bath of zinc and nickel salts. The nickel content of the deposited alloy is typically in the region of 12 to 15%. The current density is controlled within a specific range of 1 to 4 A/dm² in order to achieve the required alloy ratio and to obtain a uniform deposit. There is more on Zinc Nickel Coating at www.poeton.co.uk/surface-treatments/plating/zinc-nickel-plating.
Chromate Passivation
After plating the parts are given a chromate passivate. The chromate used is a trivalent chrome, in line with the current views on hexavalent chrome under the European REACH regulations. The salt-spray resistance of a properly passivated Zinc Nickel Coating is greatly increased, typical figures are in excess of 1,000 hours before the onset of red rust, compared to a few hundred hours without a passivate.
Hydrogen Embrittlement Relief
High tensile steel parts with tensile strengths greater than approximately 1,000 Mpa are baked following the plating process. This is to relieve the parts of the risk of hydrogen induced cracking. A typical bake cycle is at 190 to 220 degree C for 8 hours or more.
Each step of the process is critical in its own right. By skipping or taking short cuts in the initial stages of the process, coating failure can so often be the result in the latter stages.
