Plating, metal deposition

Aqueous Electrodeposition. The theory of electro deposition is well known (see Electroplating). Of the numerous metals used in electro deposition, only 10 have been reduced to large-scale commercial practice. The most commonly plated metals are chromium, nickel, copper, zinc, rhodium, silver, cadmium, tin, and gold, followed by the less frequendy plated metals iron, cesium, platinum, and palladium, and the infrequendy plated metals indium, mthenium, and rhenium. Of these, only platinum, rhodium, iddium, and rhenium are refractory. 

Plating T anks. An electroless plating line consists of a series of lead-lined (for plastics etching) or plastic-lined tanks equipped with filters and heaters, separated by rinse tanks (24). Most metal tanks, except for passivated stainless steel used for electroless nickel, cannot be used to hold electroless plating baths because the metal initiates electroless plating onto itself. Tank linings must be stripped of metal deposits using acid at periodic intervals. 

Reference 38 is a good guide to the selection of plate thickness test methods. Test methods may vary with the purity and electrochemical activity of the deposit. Metals deposited from commercial plating solutions are seldom pure. For example, zinc deposits from the three commonly used baths, ie, cyanide, chloride, and zincate, vary significantly in purity and activity (39). Standard ASTM test methods for determining plate thickness are... 

Ductility Tests. The ductihty of plated metals differs considerably from the corresponding thermally cast metals. Additionally, ductihty which is an important property if parts are to be deformed after plating, varies with the chemical composition of the plating solution, as well as the operating conditions of a given plating process. Ductihty can also be important when plated parts are stressed in use. Some metal deposits have coefficients of... 

Composite Plate—an electrodeposit that consists of two or more layers of metals deposited separately. 

Flame Plating—the deposition of a hard metal coating onto a substrate via application of molten metal at supersonic velocities. 

Peen Plating-the deposition of the coating metal (in powder form) on the substrate via a tumbling action in the presence of peening shot. 

Vacuum Deposition-also vapor deposition or gas plating the deposition of metal coatings by means of precipitation (sometimes in vacuum) of metal vapor onto a treated surface. The vapor may be produced by thermal decomposition, cathode sputtering or evaporation of the molten metal in air or an inert gas. 

Satisfactory service of an electroplated article is not achieved, however, unless adequate care is given to the choice of deposited metal, its thickness, the technique of application, and the design of the article. The choice of metal deposit is primarily determined by the basis metal, i.e. the metal from which the article is made, and the actual conditions to which the plated article will be subjected during service. In addition, however, attractive appearance and reasonable cost are also important considerations. 

At the start the cathode is invariably a metal different from that to be deposited. Frequently, the aim is to coat a base metal with a more noble one, but it may not be possible to do this in one step. When a metal is immersed in a plating bath it will corrode unless its potential is sufficiently low to suppress its ionisation. Fortunately, a low rate of corrosion is tolerable for a brief initial period. There are cases where even when a cathode is being plated at a high cathodic (nett) current density, the substrate continues to corrode rapidly because the potential (determined by the metal deposited) is too high. No satisfactory coating forms if the substrate dissolves at a high rate concurrently with electrodeposition. This problem can be overcome by one or more of the following procedures ... 

Corrosion potentials in plating baths The standing potentials of steel and copper (before application of current) are shown in Table 12.2, together with the standing potential of the plated metal and the potential below which hydrogen should, in theory, be evolved. The potential of the cathode during deposition at a typical current density is also given. 

Electroless Plating formation of a metallic coating by chemical reduction catalysed by the metal deposited. 

Electroplating is the electrolytic deposition of a thin film of metal on an object. The object to be electroplated (either metal or graphite-coated plastic) constitutes the cathode, and the electrolyte is an aqueous solution of a salt of the plating metal. Metal is deposited on the cathode by reduction of ions in the electrolyte solution. These cations are supplied either by the added salt or from oxidation of the anode, which is made of the plating metal (Fig. 12.16). 

The quantity of metal deposited during a plating process is linked stoichiometrically to the current flow through Equation, as Example illustrates. 

Actually, it is recognized that two different mechanisms may be involved in the above process. One is related to the reaction of a first deposited metal layer with chalcogen molecules diffusing through the double layer at the interface. The other is related to the precipitation of metal ions on the electrode during the reduction of sulfur. In the first case, after a monolayer of the compound has been plated, the deposition proceeds further according to the second mechanism. However, several factors affect the mechanism of the process, hence the corresponding composition and quality of the produced films. These factors are associated mainly to the com-plexation effect of the metal ions by the solvent, probable adsorption of electrolyte anions on the electrode surface, and solvent electrolysis. 

Samples of high area powders and of supported metals may be applied to the CaF2 support plate by a spraying technique, previously described In detall(ll). In Figure 1, we show a half plate design In which a supported metal deposit, produced by H2 reduction of metal Ions held on the support, occupies one half of the plate while the pure support occupies the other half. 

One must realize that once complete metal deposition has been attained, the emf across the electrodes cannot be switched off before the cathode has been taken out of the solution and rinsed with water, otherwise the metal deposit may start to redissolv e in the solution as a consequence of internal electrolysis by the counter emf. After disconnection the electrode is rinsed with acetone and dried at 100-110° C for 3-4 min. The analytical result is usually obtained from difference in weight of the dry cathode before and after electrolysis. In a few instances a copper- or silver-plated Pt cathode or even an Ag cathode is used, e.g., Zn and Bi are difficult to remove entirely from Pt, as they leave black stains and on heating form an alloy with the noble metal for this and other reasons (see below) the experimenter should consult the prescriptions in handbooks149. 

Electrolytic recovery systems work best on concentrated solutions. For optimal plating efficiency, recovery tanks should be agitated ensuring that good mass transfer occurs at the electrodes. Another important factor to consider is the anode/cathode ratio. The cathode area (plating surface area) and mass transfer rate to the cathode greatly influence the efficiency of metal deposition. 

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