Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 463 of 707

commonly used. ANODE The anodes are typically 2.25 × 2.25 inches and are fitted close to the cell wall. In addition, the anodes are nor- mally flat. If the solution is subject to high anode polarization, then the anode should be corrugated, ridged, or made from gauze to increase the effective anode area. Figure 1: Hull cell and temperature controller. Figure 2: Brass- and zinc-coated steel panels. The anode material should be care- fully chosen. Impurities in the anode composition can affect the outcome of the tests. Therefore, the same anode material should always be used for a series of tests. It is common practice to use filter paper or a small anode bag to trap any anode fines or sludge that may interfere with the test. However, it is not sound practice to use filter paper because it could impede proper solution replenish- ment in the anode film. If quantita- tive evaluation of the solution con- ditions and the anode is required, then the anode area will usually need to be increased to provide the proper anode-to-cathode ratio. CATHODE The cathode is used to judge the condition of the plating solution. The ideal con- dition would entail having the test panel made from the same composition as the work being plated, however, this is easier said than done. Generally, polished brass panels and zinc-coated steel panels are used. The condition and composition of the cathode surface have a great effect on the outcome of the test. Therefore, the cathode material and preparation must be standardized. Steel panels are zinc coated to prevent rusting during storage; usually the zinc is stripped in a 50%/volume solution of hydrochloric acid, rinsed, scratched with 400-grit wet/dry sandpaper, wiped with a clean paper towel to remove the smut, then rinsed and placed in the electrolyte for the plating test. Operators must be careful not to touch or fingerprint the area that will undergo testing. Scratching the bottom of the test panel provides the operator with an idea of the leveling ability of the plating solution. Brass panels are generally coated with a peelable plastic. It is common practice to remove the coating; scratch the bottom edge with 400-grit sandpaper; soak or direct-current clean the panel in a suitable brass cleaner; rinse; place in 5% by vol- ume sulfuric acid; rinse; and then place in the electrolyte for testing. 462

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