Metal Finishing Guide Book


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Impurity Maximum Conc. (ppm) Iron Copper Zinc Lead Chromium 50 40 50 2 10 (hexavalent) Aluminum Organic impurities 60 solution related Purification Treatment High pH and electrolysis High pH and electrolysis High pH and electrolysis Electrolysis High pH. It may be necessary to precede this with a potassium permanganate-lead carbonate treatment followed by lead removal. High pH Activated carbon; activated carbon plus electrolysis Table VI. Maximum Concentration of Impurities and Purification Treatments Purification of Solutions Nickel-plating baths freshly prepared from technical salts contain organic and inorganic impurities that must be removed before the bath is operated. Older baths gradually become contaminated from drag-over from preceding treatments, from components that are allowed to fall off the rack and allowed to remain in the tank, from corrosion products from auxiliary equipment, from tools dropped into the tank, and from other sources. It is more effective to keep impurities out of the plating bath than to deal with rejects and production interruptions resulting from the use of impure solutions. The maximum concentrations of impurities normally permissible in nickel plating solutions and recommended treatments for their removal are shown in Table VII. The electrolytic treatment referred to in the table, known as "dummying," involves placing a large corrugated cathode in the solution and plating at low current densities, 2 and 5 A/ft2. Copper, lead, and certain sulfurbearing organic addition agents are best removed at 2 A/ft2, whereas iron and zinc are more effectively removed at 5 A/ft2. A corrugated cathode is preferred because it gives a wider current density range. At 2 A/ft2, impurities should be removed after the solution has been operated for 2 A-hr/gal; at 5 A/ft2, 5 A-hr/ gal should be sufficient. The high pH treatment requires transferring the nickel solution to an auxiliary treatment tank. Sufficient nickel carbonate is added to bring the pH above 5.2. Approximately 0.5 to 1.0 ml/L of 30% hydrogen peroxide is added. The bath is agitated and kept warm for 2 hr. The pH is adjusted to the optimum level after the bath is filtered back into the main plating tank. The solution may then be electrolyzed at low current density until deposit quality is acceptable. When organic impurities are to be removed, activated carbon is added prior to the high pH treatment described above. Approximately 0.13 to 0.4 oz/gal (1 to 3 g/L) of activated carbon is commonly added to the solution in the auxiliary treatment tank. The nickel carbonate and hydrogen peroxide are then added. The solution is then filtered. Electrolytic purification is often desirable at this point. After a new bath has been prepared, the high pH treatment, treatment with activated carbon, and electrolysis at low current densities are performed sequentially until the quality of the deposit as determined by the tests discussed in the next section is acceptable. 303

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