Cyanide Zinc Baths

Bright cyanide zinc baths may be divided into four broad classifications based on their cyanide content: regular cyanide zinc baths, midcyanide or half-strength cyanide baths, low-cyanide baths, and microcyanide zinc baths. Table 1 gives the general composition and operating conditions for these systems.

Table 1 Composition and operating conditions of cyanide zinc baths

Constituent

Standard cyanide bath(a)

Mid or half-strength cyanide bath(b)

Optimum

Range

Optimum

Range

g/L

oz/gal

g/L

oz/gal

g/L

oz/gal

g/L

oz/gal

Preparation

Sodium cyanide

42

5.6

30-41

4.0-5.5

20

2.7

15-28

2.0-3.7

Sodium hydroxide

79

10.5

68-105

9.0-14.0

75

10.0

60-90

8.0-12.0

Sodium carbonate

15

2.0

15-60

2.0-8.0

15

2.0

15-60

2.0-8.0

Sodium polysulfide

2

0.3

2-3

0.3-0.4

2

0.3

2-3

0.3-0.4

Brightener

(g)

(g)

1-4

0.1-0.5

(g)

(g)

1-4

0.1-0.5

Analysis

Zinc metal

34

4.5

30-48

4.0-6.4

17

2.3

15-19

2.0-2.5

Total sodium cyanide

93

12.4

75-113

10.0-15.1

45

6.0

38-57

5.0-7.6

Sodium hydroxide

79

10.5

68-105

9.0-14.0

75

10.0

60-90

8.0-12.0

Ratio: NaCN to Zn

2.75

0.37

2.0-3.0

0.3-0.4

2.6

0.3

2.0-3.0

0.2-0.4

Constituent

Low-cyanide bath(c)

Microcyanide bath(d)

Optimum

Range

Optimum

Range

g/L

oz/gal

g/L

oz/gal

g/L

oz/gal

g/L

oz/gal

Preparation

Zinc cyanide

94(b)

1.3(e)

7.5-14(b)

1.0-1.9

(f)

(f)

(f)

(f)

Sodium cyanide

7.5

1.0

6.0-15.0

0.8-2.0

1.0

0.1

0.75-1.0

0.4-0.13

Sodium hydroxide

65

8.7

52-75

6.9-10.0

75

10.0

60-75

8-10

Sodium carbonate

15

2.0

15-60

2.0-8.0

Sodium polysulfide

Brightener

(g)

(g)

1-4

0.1-0.5

(g)

(g)

1-5

0.1-0.7

Analysis

Zinc metal

7.5

1.0

0.8-1.5

7.5

1.0

6.0-11.3

0.8-1.5

Total sodium cyanide

7.5

1.0

6.0-15.0

0.8-2.0

1.0

0.1

0.75-1.0

0.1-0.13

Sodium hydroxide

75

10

60-75

8.0-10.0

75

10.0

60-75

8-10

Ratio: NaCN to Zn

1.0

0.1

1.0

0.1

Note: Cathode current density: limiting 0.002 to 25 A/dm2 (0.02 to 250 A/ft2); average barrel 0.6 A/dm2 (6 A/ft2); average rack 2.0 to 5 A/dm2 (20 to 50 ft2). Bath voltage: 3 to 6 V, rack; 12 to 25 V, barrel.

(a) Operating temperature: 29 °C (84 °F) optimum; range of 21 to 40 °C (69 to 105 °F).

(b) Operating temperature: 29 °C (84 °F) optimum; range of 21 to 40 °C (69 to 105 °F).

(c) Operating temperature: 27 °C (79 °F) optimum; range of 21 to 35 °C (69 to 94 °F).

(d) Operating temperature: 27 °C (79 °F) optimum; range of 21 to 35 °C (69 to 94 °F).

(f) Dissolve zinc anodes in solution until desired concentration of zinc metal is obtained.

(g) As specified

Cyanide baths are prepared from zinc cyanide (or zinc oxide sodium cyanide), and sodium hydroxide, or from proprietary concentrates. Sodium polysulfide or tetrasulfide, commonly marketed as zinc purifier, is normally required in standard, midcyanide, and occasionally low-cyanide baths, to precipitate heavy metals such as lead and cadmium that may enter the baths as an anode impurity or through drag-in.

Standard cyanide zinc baths have a number of advantages. They have been the mainstay of the bright zinc plating industry since the early 1940s. A vast amount of information regarding standard cyanide bath technology is available, including information on the technology of operation, bath treatments, and troubleshooting.

The standard cyanide bath provides excellent throwing and covering power. The ability of the bath to cover at very low current densities is greater than that of any other zinc plating system. This capability depends on the bath composition, temperature, base metal, and proprietary additives used, but it is generally superior to that of the acid chloride systems. This advantage may be critical in plating complex shapes. This bath also tolerates marginal preplate cleaning better than the other systems.

Cyanide zinc formulas are highly flexible, and a wide variety of bath compositions can be prepared to meet diverse plating requirements. Zinc cyanide systems are highly alkaline and pose no corrosive problems to equipment. Steel tanks and anode baskets can be used for the bath, substantially reducing initial plant investment.

The cyanide system also has a number of disadvantages, including toxicity. With the possible exception of silver or cadmium cyanide baths, the standard cyanide zinc bath containing 90 g/L (12 oz/gal) of total sodium cyanide is potentially the most toxic bath used in the plating industry. The health hazard posed by the high cyanide content and the cost for treating cyanide wastes have been the primary reasons for the development of the lower-cyanide baths and the switch to alkaline noncyanide and acid baths. Although the technology for waste treatment of cyanide baths is well developed, the cost for the initial treatment plant may be as much as or more than for the basic plating installation.

Another disadvantage is the relatively poor bath conductivity. The conductivity of the cyanide bath is substantially inferior to that of the acid bath, so substantial power savings may be had by using the latter.

The plating efficiency of the cyanide system varies greatly, depending on such factors as bath temperature, cyanide content, and current density. In barrel installations at current densities up to 2.5 A/dm2 (25 A/ft2), the efficiency can range within 75 to 90%. In rack installations, the efficiency rapidly drops below 50% at current densities above 6 A/dm2 (60 A/ft2).

Although the depth of brilliance obtained from the cyanide zinc bath has increased steadily since 1950, none of the additives shows any degree of the intrinsic leveling found in the acid chloride baths. The ultimate in depth of color and level deposits reached in the newer acid baths cannot be duplicated in the cyanide bath.

Midcyanide Zinc Baths. In an effort to reduce cyanide waste as well as treatment and operating costs, most cyanide zinc baths are currently at the so-called midcyanide, half-strength, or dilute cyanide bath concentration indicated in Table 1. Plating characteristics of midcyanide baths and regular cyanide baths are practically identical. The only drawback of the midcyanide bath, compared with the standard bath, is a somewhat lower tolerance to impurities and poor preplate cleaning. This drawback is seldom encountered in practice in the well-run plant. Greater ease of rinsing, substantially less dragout, and savings in bath preparation, maintenance, and effluent disposal costs are responsible for the prominence of this type of bath.

Low-cyanide zinc baths are generally defined as those baths operating at approximately 6 to 12 g/L (0.68 to 1.36 oz/gal) sodium cyanide and zinc metal. They are substantially different in plating characteristics from the midcyanide and standard cyanide baths. The plating additives normally used in regular and midstrength cyanide baths do not function well with low metal and cyanide contents. Special low-cyanide brighteners have been developed for these baths.

Low-cyanide zinc baths are more sensitive to extremes of operating temperatures than either the regular or midcyanide bath. The efficiency of the bath may be similar to that of a regular cyanide bath initially, but it tends to drop off more rapidly (especially at higher current densities) as the bath ages. Bright throwing power and covering power are slightly inferior to those of a standard midcyanide bath. However, most work that can be plated in the higher cyanide electrolytes can be plated in the low-cyanide bath. Despite the fact that low-cyanide baths have significantly lower metal and cyanide contents, they are less sensitive to impurity content than the standard or midcyanide bath. Heavy metal impurities are much less soluble at lower cyanide contents. The deposit from a low-cyanide bath is usually brighter than that from a regular or midcyanide system, especially at higher current densities. These baths are used extensively for rack plating of wire goods. Unlike the other cyanide systems, low-cyanide baths are quite sensitive to sulfide treatments to reduce impurities. Regular sulfide additions may reduce the plating brightness and precipitate zinc.

Microcyanide zinc baths are essentially a retrogression from the alkaline noncyanide zinc process discussed in the following section. In the early history of alkaline baths it was often difficult to operate within its somewhat limited parameters; many platers used a minimal amount of cyanide in these baths, 1.0 g/L (0.13 oz/gal), for example. This acted essentially as an additive, increasing the overall bright range of the baths. However, it negated the purpose of the alkaline noncyanide bath, which is to totally eliminate cyanide.

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