Many other type of nickel plating solution have been advocated, mostly based on nickel sulfate, although nickel chloride and nickel sulfamate bath can be worked more quickly. All these yield matt deposits and are called dull plating baths. The electroplated coating from the Watt bath is smooth but of a matt, pearly-milk-white texture and must be mechanically polished if, as is usual, a high luster is required. This process is called coloring or color buffing, and is fairly readily accomplished by pressing each part of the surface of the article against a revolving wheel built up of soft cloth discs. The wheel is dressed with very fine line or similar powder bound in grease, the powder flowing or smearing the surface rather than abrading it.
The major modern change in the practical of nickel plating has been the use of brightening and leveling additions to the nickel plating solution of the Watt bath type. These are complex organic substance which; although present in solution only in small quantities, are absorbed on the growing electroplate and modify its metallurgical structure. Not only is the size of the crystallize grain in it reduced, but the additions have a special effect of restraining the growth at any place which tend to grow out the general level. In this way the electroplate is kept microscopically smooth and hence it is bright and lustrous are plated. It is also much harder and more wear resisting which is an important advantage in service. The advantage to electroplater is not only the cost and labor of mechanical polishing is eliminate, but also that the article can proceed direct to chromium plating without unracking or rewiring. Rinsing and drying must, however, be carefully carried out to avoid water staining.
The operation and control of nickel plating calls for much care and experience. Temperature and clarity of solution, and the pH value must be constantly checked, e.g. twice daily; analysis for the main constituents, especially nickel, is required at longer intervals. Nickel plating solutions are especially sensitive to trace a contamination by traces of copper, zinc and chromium, and by organic materials of a colloidal or glue like nature. The inorganic contaminants can be eliminated by long continued plating onto a piece of scrap sheet metal at a low current density; organic material are removed by oxidation with permanganate or by absorption on activated charcoal.
In addition to its use as undercoating to chromium, nickel plating is often used alone for protection of chemical plant and food processing equipment, or for parts, such as the electrodes of thermionic valves, which must be protected from scaling by the action of heat.Post by Aluminum Anodizing.