Black chromium plating

1. Characteristics and properties

High current densities and the parameters of the bath cause the deposition of superimposed chromium oxide layers, with a chromium content of 60-70% and a hardness of 250-350 HV 0.1, which although not as scratch and wear resistant as bright chromium coatings, is among the black galvanic layers the hardest and as corrosion resistant as a micro-cracked chromium layer. The uniformly matt layers, which are heat-resistant up to approx. 600°C, are usually 2-5µm thick and are first coated with bright or matt nickel or copper-plated, depending on customer requirements. After black chromium plating on frames, the coatings – which initially look dull and sooty – are usually after-treated by dipping or rubbing with oil-containing emulsions.

The black chrome layer as a metallic element contains no prohibited substances, but since the structure of the layer is very amorphous and black chrome electrolytes contain CrVI, electrolyte residues accumulate in the layer which cannot be removed even by thorough rinsing.  For this reason, black chrome coatings are not RoHS compliant.

2. Benefits and areas of application

Black chrome plating is mostly used for decorative reasons, e.g. for antennas, windscreen wipers, microscopes, surgical instruments, etc.

3. Basic material

Basically, steels, copper and copper alloys can be directly black chromium plated. Zinc die-cast workpieces require additional undercopper plating, nickel alloys require additional activation with nickel strike, aluminium requires a specifically coordinated pre-treatment.

4. Required delivery condition

The goods provided must be largely metallically bright. Residual oils or residual impurities must be removable with conventional aqueous alkaline degreasers. Oils containing silicone, alkali/alkaline earth and high-pressure additives must be avoided at all costs.

5. Pre-treatment

Black chrome layers are standardised according to DIN EN 12540 and are provided with a semi-gloss or matt nickel layer as required. The chrome layer is strongly influenced by the texture of the base material or the intermediate layer.