Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 194 of 707

Metallic effects 100 150 200 250 300 350 400 450 500 50 0 0 5 10 15 impurites Chart 1, Impurities (g/L) versus Milliohms 250 g/l to >300 g/l. Further increase in chromic acid does not seem to give additional benefits. Other impurities, such as chlorides, phosphates, nitrates and organics, will not only have an impact on the solution but will also affect the deposit characteris- tic. The goal should be to maintain a bath free of such impurities. FIXTURES FOR FUNCTIONAL CHROME PLATING A fixture is essentially a rack and is used to hold the work and to transfer current to the work or workload. This area is probably an art for functional chrome plat- ing and is the heart of getting the deposit to the areas needed. The work of fix- turing is beyond the scope of this article but an excellent book, Hard Chrome Plating by Robert Guffie, provides a great deal of information on this subject. The requirement for many chrome plating jobs is to deposit chrome onto a spe- cific area. This requires masking areas of the work piece(s) so that plating does not occur on unwanted areas. To accomplish this, a wide range of products from waxes to paints and lacquers are used. Care should be given in selecting the materials used for masking, as the wrong material can contaminate the chemistry. Shields and thieves are also employed to prevent deposits on certain areas or min- imize the deposit and improve distribution of the deposit into other areas. Again, this is a highly specialized area of plating and is beyond the scope of this article. POWER SUPPLY As with other plating operations, a source of power delivering a direct current (DC) is required. With hard chrome or any hexavalent process, this source of DC power must be relatively pure with respect to ripple or AC component of DC. Most modern-day DC power sources for plating are provided by converting AC to DC by a rectifier. The rectifier suppliers all have special means to filter this AC com- ponent that will deliver a direct current with no more than 5% ripple. Problems from high ripple can range from poor adhesion and cohesion to 193 20 25 (milliohms)

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