Decorating electrolytic plating

Electroplating. The second-largest appHcation for nickel chemicals is as electrolytes ia nickel electroplating (qv). In ordinary plating systems, nickel present ia the electrolyte never forms on the finished workpiece the latter results from dissolution and transfer from nickel anodes. Decorative nickel plating is used for automobile bumpers and trim, appHances, wire products, flatware, jewelry, and many other consumer items. A comprehensive review of nickel electroplating has been compiled (164). 

The process consists of pre-etching, etching, etch neutralization, catalyst appHcation, catalyst activation, and plating. Most commercial appHcations, except REl/EMl shielding, use the initial copper or nickel deposit as a base for subsequent electrolytic plating of electrolytic copper, nickel, or chromium. The exact types and thicknesses of metal used are determined by part usage, eg, automotive exterior, decorative, plumbing, and others (24). 

Electrolytic plating of metals is performed for both protective and decorative reasons. The arrangement and chemistry of the plating bath must be such that a coat of even thickness, non-porous, adherent metal is produced. The "throwing power is a measure of the ability of the metal ions to reach remote sections of the article to be plated. Accurate control of current, potential, plating bath chemistry, and temperature is essential in order to obtain high-quality results. 

Silver is used as a coating material for decoration purposes furthermore its chemical and electrical properties are utilized, respectively, in various equipment and environments in the process industry and in electric contacts and other small components in electrical and electronic equipment. Silver coatings are commonly applied by electrolytic plating (see p. 288). A coating of copper or nickel is often used under the silver top coat. Silver may be attacked by sulphides in industrial atmospheres. To prevent attack, sometimes a thin layer of rhodium is used on top of the silver coating alternatively, a passivating chemical treatment is carried out. 

Electroless, electrolytic plating, and vacuum metallizing are processes used to deposit metal surfaces on plastics materials. However, metal surfaces can also be provided by adhesives or hot-stamp methods. Some finished plastic parts must have shiny metallic surfaces. Besides providing a decorative finish, metal coatings may provide an electrical conducting surface, a wear- and corrosion-resistant surface, or added heat deflection. 

G. A. Krulik, Electrolytic Plating, Plastics Finishing and Decoration, D. Satas, ed.. Van Nostrand Reinhold Co., New York, 1986. 

Figure M.3 Layers of construction (a) electroless plating, (b) electrolytic plating, and (c) vacuum metallizing. (Ref Muccio, E.A., Finishing and Decorating Plastic Part , Plastic Part Technology, ASM International, Materials Park, OH, 1991)...

There are presently very few electroplating processes which utilize a divided cell, primarily due to the increased cell voltage (and, hence, power costs), constructional complexity and increased maintenance requirements. One example is the "Envirochrome process for decorative chromium plating which has been trial-manufactured during the last Bve years. This utilizes a Cr(iii) electrolyte which avoids problems associate with the use and disposal of Cr(vi) solutions. 

The Electrolytic Corrosion Test. Also developed for use on nickel—chromium and copper—nickel—chromium decorative automobile parts is the electrolytic corrosion (EC) test (44). Plated specimens or parts are made anodic in a corrosive electrolyte under controlled conditions for 2 min, and then tested for penetration to the substrate. 

The main use of the chromium metal so produced is in the production of non-ferrous alloys, the use of pure chromium being limited because of its low ductility at ordinary temperatures. Alternatively, the Cr203 can be dissolved in sulphuric acid to give the electrolyte used to produce the ubiquitous chromium-plating which is at once both protective and decorative. 

Chromium(lll) sulfate is used as the electrolyte for obtaining pure chromium metal. It is used for chrome plating of other metals for protective and decorative purposes. Other important applications of this compound are as a mordant in the textile industry in tanning leather to dissolve gelatin to impart green color to paints, varnishes, inks, and ceramic glazes and as a catalyst. 

Nickel plating is also extensively used for decorative and corrosion resistance applications. In most cases, it is electrolytically deposited. However, some applications can require high purity films or good conformal coverage that can be better performed by the CVD process. 

Metal-plated articles are common in our society. Jewelry and tableware are often plated with silver. Gold is plated onto jewelry and electrical contacts. Copper is plated onto many objects for decorative purposes (Figure 21-5). Automobiles formerly had steel bumpers plated with thin films of chromium. A chrome bumper required approximately 3 seconds of electroplating to produce a smooth, shiny surface only 0.0002 mm thick. When the metal atoms are deposited too rapidly, they are not able to form extended lattices. Rapid plating of metal results in rough, grainy, black surfaces. Slower plating produces smooth surfaces. Tin cans are steel cans plated electrolytically with tin these are sometimes replaced by cans plated in a fraction of a second with an extremely thin chromium film. 

On the other hand, plastics have served well in the decorative segment of the wheel. Wheel covers, also known as hubcaps, are decorative parts attached to the steel wheel that covers the lug nuts. Generally, painted acrylonitrile-butadiene-styrene (ABS) and thermoplastic polyolefin (TPO) are used most often in making hubcaps. Chrome-plated emulsion ABS also is used in this application for customers who prefer the appearance of chrome. Chrome plating is a finishing treatment utilizing the electrolytic deposition of chromium. Many other plastics such as mass ABS and TPO have been attempted in this application with limited success. 

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