Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 224 of 707

and/or shields before thickness can be controlled within a specified range. Fig. 1. Current distribution is not uniform over a shaped article. Areas remote from the anode receive a smaller share of the available current than areas near the anode. DECORATIVE NICKEL PLATING The development of bright and semib- right nickel-plating solutions, multilay- er nickel coatings, and microporous and microcracked chromium have resulted in great improvements in the appear- ance and corrosion performance of dec- orative nickel coatings. Modern deco- rative nickel-chromium coatings are brilliant, highly leveled, and long-last- ing. Decorative Processes The solutions used for decorative plating contain organic addition agents that modify the growth of the nickel deposit to produce fully bright, semibright, and satinlike surfaces. The basic constituents— nickel sulfate, nickel chloride, and boric acid—serve the same purposes as they do in the Watts solution (Table III). Nickel sulfate is the principal source of nickel ions; nickel chloride improves anode dissolution and increases solution conductivity; boric acid helps to produce smoother, more ductile deposits. Anionic antipitting or wetting agents are required to reduce the pitting due to the clinging of hydrogen bubbles to the prod- ucts being plated. Nonfoaming wetting agents that lower surface tension are available for air-agitated solutions. The composition and operating conditions given in Table III for the Watts solu- tion are typical of many decorative nickel-plating solutions, but wide variations in the concentrations of nickel sulfate and nickel chloride are possible. Since most dec- orative nickel-plating processes are proprietary, composition and operating condi- tions should be controlled within the limits recommended by the suppliers. Bright Nickel Solutions Bright nickel solutions contain at least two types of organic addition agents, which complement each other and yield fully bright nickel deposits. One type produces deposits that are mirror-bright initially, but are unable to maintain the mirrorlike appearance of the deposit as its thickness is increased. This class includes compounds like benzene disulfonic acid, and benzene trisulfonic acid, benzene sulfonamide and sulfonimides such as saccharin. The presence of the sul- fon group and an unsaturated bond adjacent to the sulfon are critical character- istics. Adsorption of the addition agent occurs by virtue of the unsaturated bond, onto growth sites, points or edges of crystals, and at dislocations. The organic com- pound is reduced electrochemically at the cathode, and this is accompanied by the reduction and incorporation of sulfur (as the sulfide) in the deposit. Fully bright nickel deposits typically contain 0.06% to 0.12% sulfur. These reactions control the structure and growth of the nickel as it is deposited. The second type may be termed leveling agents because they make the surface smoother as the thickness of the deposit is increased. They are sulfur-free, bath- soluble organic compounds containing unsaturated groups and generally intro- duce small amounts of carbonaceous material into the deposit. Typical examples 223

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