Mono-nickel System – Double nickel System – Triple nickel System
Decorative chromium plating
Chromium is used to exploit its characteristics of inoxidability, brilliance, and resistance to corrosion. Chromium coatings are used for ennobling less expensive and easier to process materials, such as steel and plastic. This coating is highly reflective and has a typical bluish white colour, which is maintained over time due to some intrinsic characteristics of chromium, such as resistance to tarnishing, corrosion, abrasion, and scratch.
The term chromium plating is also used to synthetically define the traditional sequence of electroplating of copper, nickel, and chromium, or, more simply, of nickel (mono or multi-layer), and chromium. An under-layer of nickel is preferred for its characteristics of corrosion protection of the substrate, for the particular white colour which imparts to chromium, and for the corrosion protection, which, in turn, receives from the chromium layer.
Stainless steel is the only material that can be chromium-plated directly, although, in some cases, the application of a pre-layer of nickel is preferred. 
Decorative chromium coatings have very low thickness, from 0.05 to 0.5 μm; thicker deposits tend to be less brilliant and are subject to visible fractures. Decorative chromium deposits are obtained with chromic acid-based electrolytes (CrM), catalysed with sulphuric acid or mixed catalysts, and have a characteristic white colour with blue reflections. After 1975 were introduced baths based on salts such as sulphate or chromium chloride – where the chromium is in the form of trivalent cation (Cr3 +) – which produce deposits of variable colour, from pewter to white metal, similar, but not identical, to the typical colouring of hexavalent chromium deposits. For highly corrosion resistant chromium plating, double or triple nickel substrates need to be deposited in suitable proportions and with different characteristics (e.g., columnar, lamellar, and microporous), while copper can be deposited as an additional substrate.
Mono-nickel System
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| Advantages
Relatively noble nickel Disadvantages It needs polishing |
Slow but uncontrolled corrosion |
Mono-nickel system
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| Advantages
It does not need polishing Disadvantages Nickel is less noble |
Fast and uncontrolled corrosion |
Double nickel system
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| Advantages
It does not need polishing The basic material is protected |
Disadvantages
Less noble nickel in contact with chrome |
| Fast local corrosion of the glossy nickel layer with immediate visual defects | |
Triple nickel system
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Chrome
Microporous nickel
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| Advantages
It does not need polishing The basic material is protected |
Disadvantages
Less noble nickel in contact with chrome |
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| Slow and uniform corrosion of the glossy nickel layer | |
