Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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(use analysis as for total sodium cyanide in cyanide zinc solutions) pH Temperature of operation 10.2-10.4 75-95° F (25-35° C) Zinc oxide may be used instead of zinc cyanide, but it must be lead free and preferably French (USP) process. Since the zinc oxide generates hydroxide, the car- bonate used should be all bicarbonate to keep the pH down. This solution is a general purpose solution. For barrel plating, the concen- tration should be higher for added efficiency. The concentration should be two- thirds the listed concentration for still tank plating and one-third for flash plating. The weaker solutions give more uniform plating thickness and better uni- formity color in deep recesses. The efficiency of plating is governed by three factors. The first is copper con- tent with the efficiency increasing with copper content. The second is cyanide con- tent with the efficiency decreasing as the cyanide to zinc ratio rises above 3:1. The higher cyanide decreases the throw in deep recesses. The normal ratio of cyanide to zinc is three up to five for yellow brass. Efficiency increases with temperature especially with dilute solutions. Ammonia controls the plating alloy. Adding ammonia increases the zinc content in the plating alloy. With no ammonia, a red-colored brass is produced. A large excess of ammonia can produce white, banded deposits. An increase in temperature eliminates this. Ammonia also brightens the deposit. Carbonate in large amounts is an impurity, but about 4 oz/gal is essential to stabilize the plating. Carbonate in low pH solutions exists as an equilibrium of carbonate and bicarbonate known as sesquicarbonate. The pH may vary from 9.8 to 10.8 without adverse effect. To raise the pH, add sodium hydroxide. Lower pH solutions may release cyanide in ventilation air so the pH should normally be kept above 10.3. Addition agents generally do not improve the plating from a solution in proper balance, but may be useful in extending the plating range. Wetting or foam- ing agents may be useful in suppressing fumes, but must be chosen carefully since many wetting agents used in cleaners and other solutions may cause organic con- tamination and brown plate. Under normal conditions it is not necessary to add ammonia to the solution if the temperature of operation is 90° F or less. If the solution is not operated for more than a few days, an addition may be necessary. Air agitation also strips ammonia from the solution. A second type of solution uses the same basic formula as the first, but at the higher concentrations and at temperatures of 140° F and higher. Since ammonia is fugitive at these temperatures, addition agents to replace the ammonia are nec- essary. Ethylene diamine, propylene diamine, and monoethanolamine or mixtures thereof and similar amine compounds are recommended at concentrations of 0.25-1% by volume. These materials are stable at higher temperatures and require only small additions to maintain color. A third, entirely different, type of brass plating solution was developed by DuPont in 1938. It involves the use of sodium hydroxide to complex the zinc and higher concentrations of copper. It gives high plating efficiency. A typical for- mulation is as follows: Copper cyanide 10 oz/gal (75 g/L) 173

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