Electroplating, development

Apphcations of ultrasound to electrochemistry have also seen substantial recent progress. Beneficial effects of ultrasound on electroplating and on organic synthetic apphcations of organic electrochemistry (71) have been known for quite some time. More recent studies have focused on the underlying physical theory of enhanced mass transport near electrode surfaces (72,73). Another important appHcation for sonoelectrochemistry has been developed by J. Reisse and co-workers for the electroreductive synthesis of submicrometer powders of transition metals (74). 

Table 1 Hsts many of acetamide s important physical properties. Acetamide, CH2CONH2, dissolves easily ia water, exhibiting amphoteric behavior. It is slow to hydroly2e unless an acid or base is present. The autodissociation constant is about 3.2 x 10 at 94°C. It combines with acids, eg, HBr, HCl, HNO, to form soHd complexes. The chemistry of metal salts ia acetamide melts has been researched with a view to developing electroplating methods. The hterature of acetamide melts and complexes, their electrochemistry and spectroscopy, has been critically reviewed (9).

The depressed prices of most metals in world markets in the 1980s and early 1990s have slowed the development of new metal extraction processes, although the search for improved extractants continues. There is a growing interest in the use of extraction for recovery of metals from effluent streams, for example the wastes from pickling plants and electroplating (qv) plants (276). Recovery of metals from Hquid effluent has been reviewed (277), and an AM-MAR concept for metal waste recovery has recentiy been reported (278). Possible appHcations exist in this area for Hquid membrane extraction (88) as weU as conventional extraction. Other schemes proposed for effluent treatment are a wetted fiber extraction process (279) and the use of two-phase aqueous extraction (280). 

Electroplating. Many developments in electroplating (qv) have been driven by increased coating functionaHty and economy, others by environmental and legislative compliance. This field has expanded rapidly. Only some of the developments are discussed herein. 

Nickel salts and soaps have been used in electrosensitive copy paper for image development. Nickel bis-(3,5 di-Z fZ-butylsaHcylate) [68569-24-4] has been studied in pressure-sensitive color developer sheets (201). It has also been used for color stabili2ation of color copy paper (see Electroplating). 

Lesser amounts of sodium thiocyanate are used in color toning photographic paper, as a stabilizer in rapid film development, and as a sensitizing agent in color negative-film emulsions. It is also used as a brightener in copper electroplating. 

Around 1800, the attack of chromite [53293-42-8] ore by lime and alkaU carbonate oxidation was developed as an economic process for the production of chromate compounds, which were primarily used for the manufacture of pigments (qv). Other commercially developed uses were the development of mordant dyeing using chromates in 1820, chrome tanning in 1828 (2), and chromium plating in 1926 (3) (see Dyes and dye intermediates Electroplating Leather). In 1824, the first chromyl compounds were synthesized followed by the discovery of chromous compounds 20 years later. Organochromium compounds were produced in 1919, and chromium carbonyl was made in 1927 (1,2). 

In 1979, a viable theory to explain the mechanism of chromium electroplating from chromic acid baths was developed (176). An initial layer of polychromates, mainly HCr3 0 Q, is formed contiguous to the outer boundary of the cathode s Helmholtz double layer. Electrons move across the Helmholtz layer by quantum mechanical tunneling to the end groups of the polychromate oriented in the direction of the double layer. Cr(VI) is reduced to Cr(III) in one-electron steps and a colloidal film of chromic dichromate is produced. Chromous dichromate is formed in the film by the same tunneling mechanism, and the Cr(II) forms a complex with sulfate. Bright chromium deposits are obtained from this complex. 

Current density can be increased without impairing the quaUty of the copper by polishing the cathode surface by brief periodic current reversals (PCR). Reversed current electrolysis, first developed for electroplating, was tested in 1952 for copper refining. Although good results were obtained, no suitable electrical equipment for current reversal was available. The thyristor-controUed siUcon rectifier, introduced in the 1960s, provided a means for... 

Potassium cyanide [151 -50-8] KCN, a white crystalline, deUquescent soHd, was initially used as a flux, andlater for electroplating, which is the single greatest use in the 1990s. The demand for potassium cyanide was met by the ferrocyanide process until the latter part of the nineteenth century, when the extraordinary demands of the gold mining industry for alkah cyanide resulted in the development of direct synthesis processes. When cheaper sodium cyanide became available, potassium cyanide was displaced in many uses. With the decline in the use of alkah cyanides for plating the demand for potassium cyanide continues to decline. The total world production in 1990 was estimated at about 4500 t, down from 7300 t in 1976. 

SemiadditiveMethod. The semiadditive method was developed to reduce copper waste. Thin 5.0 lm (4.5 mg/cm ) copper foil laminates are used, or the whole surface may be plated with a thin layer of electroless copper. Hole forming, catalysis, and electroless copper plating are done as for subtractive circuitry. A strippable reverse—resist coating is then appHed. Copper is electroplated to 35 p.m or more, followed by tin or tin—lead plating to serve as an etch resist. The resist is removed, and the whole board is etched. The original thin copper layer is quickly removed to leave the desired circuit. This method wastes less than 10% of the copper. 

Cadmium. In 1989, U.S. consumption of cadmium for coatings was 1474 t (7), compared to 1552 t in 1970, 2089 t in 1979, and 1230 t in 1985. Cadmium plating amounts to about 15% of total cadmium production (see Cadmiumand cadmium alloys). Of the cadmium being plated in 1989, 30% was for automotive parts, over 22% for electronics, and 18% for industrial fasteners. Because of cadmium s high and weU-pubHcized toxicity and very tight waste restrictions, there are considerable efforts to develop alternative materials, and the quantities of cadmium used in electroplating ate expected to decrease. The price of cadmium anodes in early 1993 was about l/kg. 

Special containers have been developed for anesthetic ether to prevent deterioration before use. Their effectiveness as stabHizers usuaHy depends on the presence of a lower oxide of a metal having more than one oxidation state. Thus the sides and the bottoms of tin-plate containers are electroplated with copper, which contains a smaH amount of cuprous oxide. Staimous oxide is also used in the linings for tin containers. Instead of using special containers, iron wire or certain other metals and aHoys or organic compounds have been added to ether to stabHize it. 

Although the basic principles of electroplating remain unchanged, the extent of development and variety of application have widened substantially. In this section some notable developments will be cited. 

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