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August 1, 2017

Nickel Electroplating Voltage

The low voltage direct current is supplied to the tank from the normal mains alternating current supply via a step down transformer and a static rectifier. It is now usual current supply can be adjusted by variation of the transformer ratio, and the serves to adjust the magnitude of the current. A voltage range of 3 to 7 V is sufficient for this. It is unusual to have a switch in the low voltage circuit, as it is highly desirable that always an ammeter in circuit. The current to be applied must be calculated from the surface area of the article to be plated. At 35oC with moderate agitation, 15 to 30 A/ft2 of area can be used, whereas at higher temperature and with more violent agitation of the solution, current densities of up to 100 A/ft2 are possible. If these current densities are seriously exceeded the electroplate is dark colored and powdery; this is called burning. The rate of growth of thickness of the deposit is strictly proportional to the current density, and at 20 A/ft2 almost exactly 0.001 in (25 µm) per hour. It is thus possible to calculate the time for which the articles must remain in the plating tank to attain any desired thickness of deposit. During this time the current must not be interrupted for any appreciable time, or the plating is liable to be laminated and to peel off. It is, obviously, often difficult to compute the exact surface area of complex articles; in such cases the operator applies the voltage known from experience to produce the desired current density, but this method of current adjustment is not too reliable.

The thickness of current plating to applied must be determined in the light of the type of basis metal and, particularly, with regard to the protection from corrosion which is desired. Although a thin deposit of nickel, e.g. 0.0001 in thick, is visually continuous, it is found that it contains numerous, invisible tiny pores or holes. As the thickness of the plate is increased the number of these decrease until at about 0.001 in (25 µm) thickness; the number in negligible. This thickness (0.001 in) is considered to be minimum reliable thickness if good protection to steel against the weather outdoors is required. Of course if the purpose of the electroplating is mostly decorative, and on a metal not easily corroded, a lesser thickness will suffice. It must, however, be borne in mind that on an article of intricate shape, the current density at any point may differ from the average current density, because of the shape of the electrical field. It will be appreciably less in recesses and appreciably greater on projecting areas. The thickness of plating will accordingly be much less than the reliable minimum in recesses, even enough the average thickness is adequate. Some electroplating solutions compensate for this to some extent by a property called throwing power. In any case some reduction of thickness must be allowed for in the subsequent polishing operation.