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Black nickel plating results in a dense black nickel finish onto any metal.

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The Nickel plating process produces a ductile, semi-bright nickel deposit.

It is generally used as a fast plating process for Engineering applications, e.g. pumps, gears, pistons and rollers etc. Electroless Nickel can produce uniform coatings over complex shapes or where there are tight tolerances to work with. The hardness of Electroless Nickel as plated is approximately 500 VPN but can be increased to values as high as 900-1000 VPN by heat treatment.

Deposits of up to 80 microns can be achieved. Electroless Nickel is also available in a Black Finish.

Bright nickel plating baths use combinations of organic agents to achieve bright nickel deposits. There are two classes of these organic additives. The first class is the aromatic sulphonic acids, sulphonamides and sulphonamides that contain the functional group =C-SO-2. Saccharin is a widely used example of this type of brightener. Nickel deposits plated using these additives are mirror bright initially; however as the nickel builds, brightness diminishes. This first class of brighteners incorporates sulphur into the bright nickel, reducing corrosion resistance.

Brighteners in the second class, also called levellers, have inorganic metal ions and organic compounds. These may include butynediol, coumarin, ethylene cyanohydrin and formaldehyde. These are used as levelling agents because they increase surface smoothness as the nickel deposit thickness increases.

Semi-Bright Nickel. At first, coumarin was used to obtain a high-levelling, ductile, semi-bright and sulphur-free nickel deposit from a Watts nickel bath. However, coumarin-free solutions are now available. A semi-bright nickel finish is semi-lustrous, as the name implies. However, it was specifically developed for its ease of polishing and buffing. Or, if subsequently bright nickel plated, buffing can be eliminated. Brightness and smoothness are dependent on operating conditions (see Table I).

The reason semi-bright nickel finishes are so easily buffed and/or polished is that the structure of the deposit is columnar, whereas the structure of a bright nickel finish is plate-like (lamellar). However, the structure can be changed with additives, a change in pH, current density or even an increase in solution agitation. This is not a problem unless it affects properties such as internal stress.

Internal stress can be compressive or tensile. Compressive stress is where the deposit expands to relieve the stress. Tensile stress is where the deposit contracts. Highly compressed deposits can result in blisters, warping or cause the deposit to separate from the substrate. Deposits with high tensile stress can also cause warping in addition to cracking and reduction in fatigue strength.

Watts baths and high-chloride type baths can produce high tensile stress. During bright-nickel plating, stress-reducing additives are used, but these codeposit sulphur materials that affect the physical and/or engineering properties of the deposit. Saccharin is often used as a stress reducing agent. Nickel sulphamate baths can deposit pure low-stressed finishes without using additives.

Other Types of Nickel. To obtain other types of finishes such as satin nickel, organic additives are used and deposition conditions are altered. Deposits from a Watts bath are usually 7-10 mm thick, with the appearance dependent on the temperature and/or pH. At higher temperatures and a pH of 4.5-5.0, nickel deposits are matte. At 122F and a pH of 2.5-3.5, deposits are bright.

Black nickel plating is lustrous and has a black or dark grey colour. Plating is done with little or no agitation. Occasionally it is necessary to remove hydrogen gas (bubbles) from the part's surface using wetting agents. The pH of the bath ranges from 5-6, and the temperature varies from ambient to 140F. Current density remains at approximately 0.5 A/dm2.

The coatings average 2 mm thick and corrosion resistance is limited, therefore they are usually lacquered or coated with oil or grease. If the black nickel must have good corrosion resistance, an undercoating such as bright or dull nickel, zinc or cadmium is necessary.