Power, throwing

The selection of a particular deposition process depends on the material to be deposited and its availabiUty rate of deposition limitations imposed by the substrate, eg, maximum deposition temperature adhesion of deposit to substrate throwing power apparatus required cost and ecological considerations. Criteria for CVD, electro deposition, and thermal spraying are given in Table 2 (13). 

The simplest electroplating baths consist of a solution of a soluble metal salt. Electrons ate suppHed to the conductive metal surface, where electron transfer to and reduction of the dissolved metal ions occur. Such simple electroplating baths ate rarely satisfactory, and additives ate requited to control conductivity, pH, crystal stmcture, throwing power, and other conditions. 

Examples of plating solutions having good throwing power include cyanide plating baths such as copper, zinc, cadmium, silver, and gold, and noncyanide alkaline zinc baths. Examples of poorer throwing power baths are acid baths such as copper, nickel, zinc, and hexavalent chromium. 

Free KCN = surplus KCN over that required to complex the CuCN the titratable KCN. NaCN can be substituted on an equimolar basis with the CN content. Current density range is less and throwing power is less but NaCN costs less. 

The most common plafing bath uses fluoride to complex the fin. A typical solution contains 45 g/L staimous chloride, 300 g/L nickel chloride hexahydrate, and 55 g/L ammonium bifluofide. It is operated at pH 2.0—2.5 usiag ammonium hydroxide temperature is 65—75°C and current about 200 A/m. The bath has excellent throwing power. Air agitation is avoided. The deposit is bright without additives. Anodes are cast nickel, and the fin is replenished by additions of staimous chloride. AHoy anodes of 72% fin have been used to a much lesser extent. Tia-nickel deposits are covered by ASTM (136) and ISO (137) specifications. One other bath based on pyrophosphate has appeared ia the Hterature, but does not seem to be ia commercial use. 

Some electroplating solutions produce more uniform deposits than others, and the ability of a solution to deposit uniform coatings is measured by its throwing power (see Section 12.1). 

On this scale, zero represents the case when M = P, and electrolyte resistance is the main factor. Throwing power can be worse, down to a limit T = - 100% when A/ = oo, i.e. no deposit at all on the far cathode. Conversely, when M < P, T is positive. Were M to reach 1-0 despite the difference in position, 7" = -F 100%. At one time +100% was regarded as an unrealisable limit, but conditions have been found for which T = -f150% in a Haring-Blum cell. 

While the sulphate bath has a high plating speed, its throwing power is poor, and this limits its application to articles of simple shapes. 

Deposit uniformity The uniformity of a deposit is an important factor in its overall corrosion resistance and is a function of geometrical factors and the throwing power of the plating solution. A distinction is made here between macro-throwing power, which refers to distribution over relatively large-scale profiles, and micro-throwing power, which relates to smaller irregularities... 

The copper cyanide bath has excellent macro-throwing power and is chosen whenever irregular-shaped parts are to be plated. The sulphate bath is not inferior when parts with very narrow recesses, i.e. with width of opening less than 6 mm, are to be plated, although its macro-throwing power is... 

Concern over the health hazards of the hexavalent chromium solutions used to form the top coat of conventional nickel plus chromium coatings have encouraged research into trivalent chromium plating solutions. A process with better throwing power and improved covering power than those of hexavalent chromium has been described by Smart etal". A process for depositing a chromium-iron, or chromium-nickel-iron alloy, has been outlined by Law. ... 

---