Humid and/or polluted air attacks all metals, including nickel, albeit at a much slower rate than e.g. steel. The chromium layer counteracts these attacks while preserving the gloss of the surface and increasing its resistance to mechanical stresses.
Nickel is anodic relative to chromium. This means that corrosion – albeit slowly – starts in the nickel layer. The chromium coating cannot be made completely dense. This can be counteracted in various ways by making the chromium layer microcracked, about 250 cracks/cm in each direction, or microporous, about 10,000 micropores/cm2. This increases the exposed nickel surface relative to the chrome surface, which reduces the rate of attack. Due to the fact that chromium is inherently nonductile and hard, which often causes cracks in the chromium layer, this alone does not provide good corrosion protection and must be combined with an underlying nickel layer. To preserve the shine of polished stainless steel, however, it is very suitable.
Unlike hard chromium plating, there are currently bright chromium plating processes based on trivalent chromium that are not affected by the REACH ban.