The initial situation
- Previous standard: zinc coating (applied by electroplating, thickness 8 - 12 µm) with chromate layer (0.3 µm)
- This surface finish has proven to be optimal for mechanically stressed components
- Good corrosion protection, up to 100 hours until occurrence of white rust (salt spray test according to DIN 50021)
- The decisive factor for good corrosion protection is the ″self-healing effect″ of the chromate layer in case of damage: the Cr(VI)-ions ″flow″ to the damaged area and close it up. This effect can only be achieved with Cr(VI)
The problem (zinc)
- The basic is the same zinc layer as used in yellow chromizing.
- Instead of the chromate layer, a passivation and additional sealing layer is used.
- Should the sealing layer be damaged, corrosion problems are the consequence.
- In this case, there is no ″self-healing effect″ as there is with a Cr(VI) surface.
The solution: plus
- A zinc-nickel basic layer is many times more corrosion resistant compared to zinc.
- A passivation with subsequent sealing provides a higher gloss finish and improves frictional properties.
- Zinc-nickel does not develop pronounced white rust like zinc, but merely a light grey haze.