Good corrosion protection is only achieved with a sufficiently thick layer or with an appropriate combination of layers. A thin layer of chromium on iron will offer almost no protection, so use at least the combination nickel-chromium. Another advantage is the nickel layer underneath, because the nickel (bright nickel) brings out the shine better. If you also want to improve corrosion protection in a reducing atmosphere, use the layer combination copper-nickel-chrome, as the copper does a better job here.
In general, the following applies:
Depending on the metal formed, the corrosion protection is very different. There are also big differences depending on the different types of electrolytes. Quite a few types deposit with microscopically fine pores - in these areas the protection is not present. To close the pores, higher layer thicknesses are necessary. A combination of several layers improves the protection considerably. The different layers complement each other and the corrosion protection increases exponentially, true to the motto "1+1=5".
Examples of corrosion protection
Nickel:
A pure nickel layer only has good corrosion protection from 25µm, but in the layer combination nickel-chrome or also copper-nickel-chrome, the protection is greatly improved.
Zinc:
A coating thickness of around 10µm is recommended for zinc. Zinc has a long-distance effect, which also provides cathodic protection for uncovered iron areas (e.g. pores or mechanically damaged areas).
Zinc-nickel:
Here the combination of 2 protective elements comes together. On the one hand the active zinc and the passive nickel. Both elements form a common layer with increased protection. The average layer thicknesses are between 5µm and 10µm. The layers are corrosion resistant even at temperatures of up to 180°C, which is why zinc-nickel layers are ideal for protecting components of combustion engines.
Here in the example, a chromium-plated frame, with an obviously insufficient layer thickness, or unsuitable design of the base layer:
