Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 188 of 707

parts directly into the chromium tank. Since the parts are not held by a rack, poor or no contact with cathode leads is common. Contact must depend upon grav- ity, while being dampened by the solution, to hold the part against the cathode contact points. To increase the likelihood of good contact and satisfactory plating, platers use low current density, low temperatures, and specially designed barrels and trays. Cathode contact points in barrels are constructed from wire mesh, solid steel lin- ers, button contact points less than an inch apart, or steel bars that tumble the parts as they move. Large barrels with small loads also help to increase the fre- quency of contact and reduce temperature build up inside the confined space of the barrel. In general, the highest practical current density possible, without burn- ing, should be used. Screw and spiral design plating equipment are also used. The parts are put into the barrel at one end and transferred through the barrel for plating inside the threads of the screw. This permits a continuous flow of plated parts. Vibratory agitation and centrifugal force barrels are also available. In all cases, the barrels must be constructed so that the hydrogen gas generated during plating can escape from the barrel rather than being trapped and possibly exploding. Tray plat- ing requires that parts be layered onto a metallic screen and vibrated or tumbled during plating. Trivalent chromium solutions have recently been tried for barrel and tray plating of chromium. Since current interruptions do not hurt the deposit in triva- lent chromium processes, and burning is not a problem, this technology will prob- ably become more popular in the future. HCA-50® Catalyzed Chromium High Efficiency • High Value Chromium plating demands increased efficiency and increased performance at the lowest possible operating cost. Call us and we'll show you how to meet these objectives. • Chrome Catalyst • Fume Control • Chromic Acid • Lab Support CHEMICAL LLC 866-461-8882 • www.metalfinishing/advertisers 187

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