Operating Parameters of Standard Cyanide and Midcyanide Zinc Solutions

Anodes. Almost every physical form of zinc anode material has been used in cyanide zinc plating, the type and prevalence varying from country to country. In the United States, cast zinc balls approximately 50 mm (2 in.) in diameter, contained in spiral steel wire cages, are by far the most common anode material. A practical variation of this is the so-called flat top anode, with a flat surface to distinguish it from cadmium ball anodes. The use of ball anodes provides maximum anode area, ease of maintenance, and practically complete dissolution of the zinc anodes with no anode scrap formation.

One of the most economical forms of anode material is the large cast zinc slabs that form the prime material for subsequent ball or elliptical anode casting. Although these have the disadvantage of bulky handling and the need for specially fabricated anode baskets, their lower initial cost makes their use an important economic factor in the larger zinc plating shop.

Three grades of zinc for anodes are conventionally used for cyanide zinc plating: prime western, intermediate, and special high-grade zinc. The zinc contents of these are approximately 98.5%, 99.5%, and 99.99%, respectively. The usual impurities in zinc anodes are all heavy metals, which cause deposition problems unless continuously treated. Nearly troublefree results can consistently be obtained through the use of special high-grade zinc. A typical composition of special high-grade zinc is:

Constituent

Amount, %

Zinc

99.9930

Lead

0.0031

Cadmium

0.0017

Iron

0.0010

Copper

Trace

Control of Zinc Metal Content. Zinc anodes dissolve chemically as well as electrochemically in cyanide baths, so effective anode efficiency will be above 100%. This causes a buildup in zinc metal content, because cathode efficiencies are usually substantially less than 100%. A number of procedures have been developed to control this tendency.

In a conventional new zinc cyanide installation, approximately ten spiral anode ball containers should be used for every meter of anode rod. These should be filled initially, and after 1 or 2 weeks of operation they should be adjusted to compensate for anode corrosion and dragout losses so that the metal content remains as constant as possible. During shutdown periods in excess of 48 h, most cyanide zinc platers remove anodes from the bath. In large automatic installations, this may be done by using a submerged steel anode bar sitting in yokes that can be easily lifted by hoist mechanisms.

One of the prime causes of zinc metal buildup is the very active galvanic cell between the zinc anodes and the steel anode containers. This is evidenced by intense gassing in the area of anodes in a tank not in operation. Zinc buildup from this source can be eliminated by plating the anode containers with zinc before shutdown, which eliminates the galvanic couple.

Temperature. Probably no operating variable is as important and as often overlooked in the operation of cyanide zinc baths as operating temperature. Cyanide zinc solutions have been reported operating between the rather wide limits of 12 to 55 °C (54 to 130 °F), with the vast majority of baths operating between 23 to 32 °C (73 to 90 °F). The exact operating temperature for a given installation depends on the type of work processed, the finish desired, and the engineering characteristics of the plating system. Bath temperature has an effect on a great many variables in the cyanide zinc systems, so the optimum temperature is generally a compromise. Increasing the bath temperature:

• Increases cathode efficiency

• Increases bath conductivity

• Increases anode corrosion

• Produces duller deposits over a broad range of current densities

• Reduces covering power

• Reduces throwing power

• Increases breakdown of cyanide and addition agents

Lowering the bath temperature has the opposite effects. Thus, if a plater is primarily concerned with plating of pipe or conduit where deposit brilliance is not of great importance and covering and throwing power are not critical, operating the bath at the highest practical temperature to give optimum conductivity and plating efficiency would be preferred. For general bright plating of fabricated stampings, a lower bath temperature should be used, permitting the required excellent covering and throwing power and bright deposits.

The effects of higher bath temperature can be compensated to a substantial extent by increasing the total-cyanide-to-zinc ratio of the solution. The exact optimum ratio varies slightly for a given proprietary system, as shown in Table 3.

Table 3 Effect of bath temperature on total-cyanide-to-zinc ratio

Temperature

Total-NaCN-

Total-NaCN-

to-Zn

ratio

to-Zn ratio

°C

°F

(standard cyanide bath)

(midcyanide bath)

22

72

2.6

2.2

26

79

2.7

2.3

30

86

2.8

2.4

34

93

2.9

2.5

38

100

3.0

2.6

42

108

3.2

2.7

46

115

3.3

3.0

Cathode Current Densities. Bright cyanide zinc solutions operate at wide-ranging cathode current densities varying from extremely low, less than 0.002 A/dm2 (0.02 A/ft2), to above 25 A/dm2 (250 A/ft2) without burning (i.e., the formation of dark, coarse electrodeposits). Current density limits depend on bath composition, temperature, cathode film movement, and addition agents used.

Average current densities vary but are approximately 0.6 A/dm2 (6 A/ft2) in barrel plating and 2 to 5 A/dm2 (20 to 50 A/ft2) in still or rack plating. Barrel zinc plating is a complex phenomenon in which a large mass of parts is constantly tumbled in the plating cylinder at varying distances from the cathode contact surfaces. At any given time, a part may have an infinitesimally low current density or it may even be deplating, and in another instant, near the outer surface of the tumbling mass, current density may approach 20.0 A/dm2 (200 A/ft2). In general, the bulk of deposition takes place in the lower current density range of 0.2 to 1 A/dm2 (2 to 10 A/ft2).

Average cathode current densities are generally easier to maintain in rack and still line operations and range from approximately 2 to 5 A/dm2 (20 to 50 A/ft2). However, the actual current density of any particular area of a given part will vary greatly, depending on part configuration, anode-to-cathode distance, bath shape, and other factors affecting the primary and secondary current distribution characteristics. In most cases, with proper attention to racking and work shape, current density variations can be kept within practical limits on fabricated parts so that if a minimum average thickness of 4 pm (0.15 mil) is required on a specific part, variations from approximately 2.5 to 8 pm (0.09 to 0.3 mil) occur at various areas on the part.

Cathode current efficiencies in barrel cyanide zinc plating vary between 75 and 93%, depending on temperature, formulation, and barrel current densities. In rack or still plating, however, there is quite a wide variation in current efficiencies when higher current densities are used, especially above 3 A/dm2 (30 A/ft2). The effects of zinc metal content, sodium hydroxide content, and the cyanide-to-zinc ratio on cathode current efficiency are shown in Fig. 3. As can be seen from the graphs, the current efficiency in the most commonly used baths drops dramatically from approximately 90% at 2.5 A/dm2 (25 A/ft2) to 50% at 5 A/dm2 (50 A/ft2). An improvement in current efficiency can be obtained by using a high-strength bath; however, this is offset by the relatively poor throwing power of the solution, higher brightener consumption, higher operating costs, and maintenance difficulties. The lower standard bath concentration, which gives practically identical results, is used for practically all plating installations except a selected few rack tanks that plate conduit or large flat surfaces with no critical recessed areas.

Fig. 3 Effects of bath composition variables and cathode current density on cathode efficiency in cyanide zinc plating. (a) Effect of NaCN/Zn ratio. 60 g/L (8 oz/gal) Zn (CN); 17.5 to 43.7 g/L (2.33 to 5.82 oz/gal) NaCN; 75.2 g/L (10 oz/gal) NaOH; 2.0-to-1 to 2.75-to-1 ratios of NaCN to zinc. Temperature: 30 °C (86 °F). (b) Effect of zinc metal content. 60.1, 75.2, and 90.2 g/L (8, 10, and 12 oz/gal) Zn (CN); 43.7, 54.6, and 65.5 g/L (5.82, 7.27, and 8.72 oz/gal) NaCN; 75.2 g/L (10 oz/gal) NaOH; 2.75-to-1 ratio of NaCN to zinc. Temperature: 30 °C (86 °F). (c) Effect of NaOH content. 60.1 g/L (8 oz/gal) Zn(CN); 43.6 g/L (5.8 oz/gal) NaCN; 150.4 and 75.2 g/L (20 and 10 oz/gal) NaOH; 2.75-to-1 ratio of NaCN to zinc. Temperature: 30 °C (86 °F)

Fig. 3 Effects of bath composition variables and cathode current density on cathode efficiency in cyanide zinc plating. (a) Effect of NaCN/Zn ratio. 60 g/L (8 oz/gal) Zn (CN); 17.5 to 43.7 g/L (2.33 to 5.82 oz/gal) NaCN; 75.2 g/L (10 oz/gal) NaOH; 2.0-to-1 to 2.75-to-1 ratios of NaCN to zinc. Temperature: 30 °C (86 °F). (b) Effect of zinc metal content. 60.1, 75.2, and 90.2 g/L (8, 10, and 12 oz/gal) Zn (CN); 43.7, 54.6, and 65.5 g/L (5.82, 7.27, and 8.72 oz/gal) NaCN; 75.2 g/L (10 oz/gal) NaOH; 2.75-to-1 ratio of NaCN to zinc. Temperature: 30 °C (86 °F). (c) Effect of NaOH content. 60.1 g/L (8 oz/gal) Zn(CN); 43.6 g/L (5.8 oz/gal) NaCN; 150.4 and 75.2 g/L (20 and 10 oz/gal) NaOH; 2.75-to-1 ratio of NaCN to zinc. Temperature: 30 °C (86 °F)

Sodium carbonate is present in every cyanide and alkaline zinc solution. It enters the bath as an impurity from the makeup salts (sodium hydroxide and sodium cyanide may contain anywhere from 0.5 to 2% sodium carbonate) or as a deliberate addition to the initial bath (15 to 30 g/L, or 2.0 to 4 oz/gal).

The harmful effects of sodium carbonate in cyanide zinc plating are not as critical as in cyanide cadmium plating. Sodium carbonate does not begin to affect normal bath operation until it builds to above 75 to 105 g/L (10 to 14 oz/gal). Depending on overall bath composition and the type of work being done, a carbonate content in this range results in a slight decrease in current efficiency, especially at higher current densities, decreased bath conductivity, grainier deposits, and roughness, which becomes visible when the carbonate crystallizes out of cold solutions.

The carbonate content of zinc baths builds up by decomposition of sodium cyanide and absorption of carbon dioxide from the air reacting with the sodium hydroxide in the bath. Carbonates are best removed by one of the common cooling or refrigeration methods rather than by chemical methods, which are simple in theory but extremely cumbersome in practice. When an operating cyanide zinc bath has reached the point that excessive carbonates present a problem, it undoubtedly is contaminated with a great many other dragged-in impurities, and dilution is often a much quicker, although expensive, method of treatment. Alkaline noncyanide baths do not suffer from the effects of carbonate buildup.

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