Metal Finishing Guide Book

2013

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CURRENT SUPPLY Direct current must be used for hull cell tests. The current supply should be capable of supplying a maximum of 12 V. Twelve volts is normally not required, but it is beneficial to have the capability. Amperage is normally 5 amps or less, but 10 amps is again beneficial to have if needed. Ripple should be less than 5%. The ammeter should read in 0.2-amp increments or less to allow for proper control. CURRENT DISTRIBUTION For the tests to be effective, the current density should have a wider range than is encountered in practice. The equation below shows a logarithmic curve as follows: Current density = I(C1 – C2 log L) where L indicates the distance along the cathode, I indicates total cell current, and C1 and C2 are constants based on the nature of the electrolyte. Hull determined these constants from a number of electrolytes and found that they did not differ from one another. As a result, the values were averaged, and a general-purpose formula was derived: For the 1,000-ml cell: Current density at any point = I(18.8 – 28.3 log L) For the 267-ml cell: Current density at any point = I(27.7 – 48.7 log L) This applies to the limits of L = 0.25 and L = 3.25 inches, where the current density is in amps/ft2, I is in amperes, and L is in inches. Deviations from these are to be expected, particularly from cyanide electrolytes, as these solutions may have fluctuations in current due to cathode polarization. PART II TESTING METHODS FOR ELECTROPLATING BATHS The jiggle cell is unique in that it is the best test equipment for mimicking actual plating tank conditions. The jiggle cell utilizes a bent cathode panel, and it can be used for most plating solutions provided that the materials of construction are adequate for the solution being tested. Most jiggle cells are made of either polypropylene or acrylic. JIGGLE CELL CATHODE The jiggle cell cathode is usually 1 inch wide × 7 inches long. The jiggle cell cathode panel is usually bent on a mandrel supplied by the equipment manufacturer. Figure 1 shows brass- and zinc-plated steel panels as purchased, after bending on a mandrel. Figure 1 depicts the various 1-inch-square areas after bending the panel. 473

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