Workpiece electroplating

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). 

Electrochemical machining is a process based on the same principles used in electroplating except that the workpiece is the anode and the tool is the cathode. Electrolyte is pumped between the electrodes and a potential is applied, resulting in rapid removal of metal. 

In electropolishing, the metal workpiece is made the anode rather than the cathode. Instead of deposition onto the surface of the workpiece, some of the metal dissolves, leaving a bright, polished surface. High points dissolve at a faster rate than recessed areas. Electropolishing is performed to improve adhesion of subsequent electroplates, to deburr and Finish parts, and for decorative purposes (Schaer 1971). 

Metal powder can also be coated onto the workpiece. The 3M Company has developed a cold welding technique in which the workpiece, the metal powder, water, glass shot and additives are tumbled together in a barrel. Coatings are limited to ductile metals such as Cd, An, Sn, Pb, In, Ag, Cu, brass, and tin/lead solder the method is generally suitable only for small parts, and it doesn t produce a fine surfaced, cosmetic coating. Costs are comparable to those for electroplating with afterbake (Kirk-Othmer 1981). 

There are several possible sources of impurities in the electrolytes and reasons for their potential accumulation during use. Key amongst the sources, are the unavoidable side-reactions. Others include the widespread practice in electroplating processes of using the more convenient open systems that allow easier handling of workpieces. Consequently the absorption of atmospheric gases and particles might introduce impurities. 

The overall concept for recovery, regeneration and reuse in electroplating is shown in Figure 11.17. It includes the recovery stage, in which the workpieces are rinsed for further cleaning and the diluted electrolyte received. The diluted solution... 

Workpieces to be plated may be put through a variety of pretreating processes, including surface cleaning, surface modification, and rinsing. A schematic flowsheet of a typical electroplating plant, including surface treatment and waste treatment, is depicted in Fig. 2. 

Electroforming is to produce or reproduce a metal workpiece by electrodeposition in a plating bath over a base form (mold) or mandrel, which is subsequently removed. In some cases, the mandrel or mold may remain within the finished metal workpiece. A mandrel is a form used as a cathode in electroplating. 

A deposit may be removed rapidly and selectively from the substrate by immersion in a suitable electrolyte, particularly in the reclamation of damaged or faulty electroplated workpieces. The technique may be carried out under open-circuit conditions or metal dissolution may be accelerated by anodic stimulation. The electrolyte may be chosen such that the substrate is immune or passive under the process conditions. In some circumstances, the rate of formation of this layer is enhanced by carrying out the coating electrochemically with the surfaces as the anode. Commonly, however, the layer is sprayed on and dried. 

A wide spectrum of process operations is seen in electroplating. Both jig and barrel plating may be carried out in a line plating operation (where the workpiece is moved automatically in a predetermined fashion from one tank to another, including precleaning and postplating operations) or a more flexible (and labour-intensive) vat plating may involve manual movement of the workpiece between the tanks. 

Steel articles are plated by immersing them in a solution of nickel sulfate or nickel chloride at 70°C and at a pH of 3.5. The workpiece is immersed for five minutes, washed with a solution of sodium carbonate, and heated to 750°C. Metals, such as cobalt, palladium and aluminum are added to catalyze the reaction. A coating thickness of 0.03 mm is produced. The chemically deposited coatings have a higher resistance than electroplated coatings. 

Electrochemical decontamination is the reverse procedure of electroplating. The upper oxide-layers containing the contaminants can be removed from the metal surface using a direct electric current between the workpiece and a cathode in an electrolyte. The method can be applied by immersing the workpiece to be decontaminated in an electrolyte tank as an anode. 

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