Black (Gun-metal grey) Nickel Plating
This type of nickel plating is predominantly decorative, as it does not possess strong protective qualities, mainly because of the thin deposits that are obtained with it and due to its nature. The deposits, in fact, are not made of pure nickel, but of a mixture of nickel sulphide and other substances including zinc, thiocyanate, etc.. Consequently, adhesion is quite poor.
The best results, as for glossy appearance and colouring, are obtained on cadmium and zinc; but as regards resistance to corrosion it is better to operate on an electroplating of nickel.
In the end, however, black nickel-plated objects still need to be oiled or painted with transparent varnish to protect the deposit and give it a translucent effect, unless the black nickel plating is carried out in Nickel-Tin alloy baths.
There are plenty of baths that can be used for black nickel plating. Most are based on zinc sulphate (ZnSO) and sodium thiocyanate (NaCNS).
Zinc causes the well-known black streaks that radiate vertically on objects already in normal nickel plating baths, where it is present as impurity.
Such baths take some practice, especially with regard to temperature, pH, time of immersion, arrangement of objects, and use of anodes. These can be made of nickel, or insoluble (stainless steel, carbon), or mixed nickel and insoluble.
Usually, baths are depleted more rapidly in zinc and thiocyanate than in nickel. Hence, the use of insoluble anodes. The tanks are coated with rubber or polyvinyl chloride.