Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 192 of 707

ity to oxidize Cr (III) to Cr (VI). Proper filming of the anodes is very important. New anodes should be dum- mied at plating current for several hours to develop the proper lead peroxide film. If the bath sits idle for several days it should be dummied again for 2–3 hours to reactivate the anodes. If the bath is to be idle for long periods of time it would be best to remove the anodes altogether. Some chrome platers will use rippled anodes, believing that such will give more surface area. As the anode works, however, the anode forms lead chromate in the groves of the ripple and the effective anode area is reduced. A smooth round anode is preferred to flat or rippled anodes. In addition to lead or lead alloy anodes there are specialized anodes that uti- lize platinized titanium as anodes. These are relatively expensive but allow for very precise chrome deposits. There are applications where the work is centered into a circular platinized titanium anode array and parts are plated to very close tol- erances. In this case the cost is greatly justified as it eliminates post grinding oper- ation and the need for excess chrome for grind stock. In the printing industry, for example, many companies utilize platinized titanium as gravure rolls are plated with chrome. Some very precise plating is done with platinized-titanium for I.D. plating of small ports on engines or other types of aircraft components. Some spe- cialized equipment use platinized titanium for plating shock absorber rods. While it is not generally recommended, shops have successfully used steel wire to plate into I.D. ports or small diameter holes. These, however, eventually lead to rapid bath contamination. OPERATING CONDITIONS Typical operating conditions for functional or hard chrome are given in the fol- lowing Table 1. Ordinary chemistry Temperature °F/°C Cathode Current Density Solution Agitation Anode-to-Cathode Area Anode Material Table 1, Operating Conditions As operating temperatures rise, the efficiency of the bath decreases. However, this can be offset by the ability to plate at higher current densities to further gain a faster plating rate. Table 2 shows the efficiency differences between two tem- perature variables and the resulting efficiency. This effect is even greater with the mixed catalyst chemistries. A benefit of operating at a higher temperature is this 191 120°F to 140°F 49°C to 60°C 0.25 asi to 2.5 asi 4 asd to 38 asd Mild Air (blower) or mixer 1:1 to 3:1 Lead-7%Tin or Lead 6% Antimony Mixed fluoride chemistry 130°F to140°F 54°C to 60°C 1asi to 4asi 15 asd to 62 asd Mild Air (blower) or mixer 1:1 to 3:1 Lead-7%Tin Mixed non-fluoride chemistry 130°F to 150°F 54°C to 66°C 1asi to 6 asi 15 asd to 95 asd Mild Air (blower) or mixer 1:1 to 3:1 Lead-7%Tin or Lead 6% Antimony

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