Civil Engineering Handbook

Operating Parameters of Low Cyanide Zinc Systems

Temperature control is as critical, if not more critical, in the low-cyanide bath as in the regular or midcyanide bath. The optimum operating temperature for most proprietary baths is 29 °C (84 °F), and the permissible range is more restricted than for the standard cyanide bath. Adequate cooling facilities are therefore mandatory and are more critical for low-cyanide than for the standard system.

Cathode Current Density. The average cathode current densities used in most low-cyanide processes are the same as in the standard cyanide bath. However, some proprietary baths do not have the extreme high-current-density capabilities of the standard cyanide bath, and burning on extremely high-current-density areas may be more of a problem with the low-cyanide bath than with the conventional baths.

Agitation. Unlike the standard cyanide bath, where agitation is usually nonexistent, air or mechanical agitation of the low-cyanide bath is common and is often quite useful in obtaining the optimum high-current-density plating range of the bath.

Filtration. Most low-cyanide baths appear to operate much more cleanly than the standard or midcyanide bath. The bath is a poor cleaner, and soils that may be removed and crystallized out of high-cyanide baths are not as readily affected by the low-cyanide bath.

Efficiency. The efficiency of the low-cyanide bath on aging is much more dependent on the particular addition agent used than the standard cyanide bath, because there is a substantial difference in various proprietary systems. In a new low-cyanide bath, current efficiency is slightly higher than that of a standard or midcyanide system. However, as the bath ages, current efficiency tends to drop, possibly because of the formation of additive breakdown products, and the efficiency of a bath after 2 or 3 months of operation may be as much as 30% below that of a higher cyanide system, especially at higher current densities. As in the standard cyanide bath, increasing the sodium hydroxide content, zinc metal content, and operating temperature increases the efficiency of the low-cyanide bath. However, increasing these variables has markedly harmful effects on the bright operating range of a low-cyanide bath that usually override the benefit of increased efficiency. The effects of bath constituents and temperature on the plating characteristics of the bright low-cyanide zinc systems are given in Table 4. Figure 4 shows the effect of sodium cyanide concentration on cathode efficiency.

Fig. 4 Effect of sodium cyanide concentration on the cathode efficiency of low-cyanide zinc solutions. • :20 g/L

(2.5 oz/gal) NaCN; «:8 g/L (1 oz/gal) NaCN; • :30 g/L (4 oz/gal) NaCN; A:15 g/L (2 oz/gal) NaCN

Bright Throwing Power and Covering Power. The bright covering power of a low-cyanide bath operated at low current density is intrinsically not as good as that of a standard or midcyanide bath. In most operations, however, the difference is negligible except on extremely deep recessed parts. The vast majority of parts that can be adequately covered in a standard cyanide bath can be similarly plated in a low-cyanide bath without any production problems, such as excessively dull recessed areas or stripping by subsequent bright dipping.

Increasing the brightener and cyanide contents, within limits, improves the bright low-current-density deposition to a visible degree. Problems with bright throwing power at extremely low current densities are often solved by raising the cyanide content to approximately 15 g/L (2 oz/gal), which in effect returns the system to the lower range of the midcyanide bath.