Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 233 of 707

Impurity Iron Copper Zinc Lead Chromium Maximum Conc. (ppm) 50 40 50 2 10 (hexavalent) 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. Aluminum Organic impurities 60 solution related High pH Activated carbon; activated carbon plus electrolysis Table VI. Maximum Concentration of Impurities and Purification Treatments 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 agi- tated 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 elec- trolyzed at low current density until deposit quality is acceptable. When organic impurities are to be removed, activated carbon is added pri- or 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 aux- iliary 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 per- formed sequentially until the quality of the deposit as determined by the tests discussed in the next section is acceptable. CONTROLLING THE PROPERTIES OF NICKEL DEPOSITS Methods that measure thickness, adhesion, and corrosion resistance of nickel coat- ings are available as means of quality control. Properties such as porosity, ductility, tensile strength, internal stress, hardness, and wear resistance are important to con- trol the quality of electroplated articles. Some of these properties may be measured by the following methods. Thickness Micrometer readings are often used to determine the thickness of a coating at a particular point when the deposit thickness exceeds 125 ┬Ám (0.005 in.). Other methods for determining the thickness of electrodeposited coatings can be found in ASTM standards. ASTM standard B 487 describes a method based on metallographic examination of cross-sections of the plated object. Alternate tests involve magnetic (ASTM B 530) and coulometric (ASTM B 504) measurements of thickness. 232

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