Plating Baths

Most cadmium plating is done in cyanide baths, which generally are made by dissolving cadmium oxide in a sodium cyanide solution. Sodium cyanide provides conductivity and makes the corrosion of the cadmium anodes possible.

Cyanide Baths. Compositions and operating conditions of four cyanide baths are given in Tables 1(a) and 1(b). Note that for each of these baths a ratio of total sodium cyanide to cadmium metal is indicated; maintenance of the recommended ratio is important to the operating characteristics of the bath.

Table 1(a) Compositions of cadmium plating cyanide solutions

Solution No.

Ratio of total sodium cyanide to cadmium metal

Composition(a)

Cadmium oxide

Cadmium metal

Sodium cyanide

Sodium hydroxide(b)

Sodium carbonate(c)

g/L

oz/gal

g/L

oz/gal

g/L

oz/gal

g/L

oz/gal

g/L

oz/gal

1

4:1

23

3

19.8

2.62

78.6

10.4

14.4

1.90

30-75

4-10

2

7:1

23

3

19.8

2.62

139

18.4

14.4

1.90

30-45

4-6

3

5:1

26

3.5

23.1

3.06

116

15.3

16.6

2.19

30-60

4-8

(a) Metal-organic agents are added to cyanide solutions to produce fine-grain deposits. The addition of excessive quantities of these agents should be avoided, because this will cause deposits to be of inferior quality and to have poor resistance to corrosion. The addition of these agents to solutions used for plating cast iron is not recommended.

(b) Sodium hydroxide produced by the cadmium oxide used. In barrel plating, 7.5 g/L (1 oz/gal) is added for conductivity.

(c) Sodium carbonate produced by decomposition of sodium cyanide and absorption of carbon dioxide, and by poor anode efficiency. Excess sodium carbonate causes anode polarization, rough coatings, and lower efficiency. Excess sodium carbonate may be reduced by freezing, or by treatment with calcium sulfate.

Table 1(b) Operating conditions of cadmium plating cyanide solutions

Solutions No.

Current density(a)

Operating temperature

Remarks

Range

Average

A/m2

A/ft2

A/m2

A/ft2

°C

°F

1

55650

5-60

270

25

2732

8090

For use in still tanks. Good efficiency, fair throwing power. Also used in bright barrel plating

2

110860

1080

270

25

2732

8090

For use in still tanks and automatic plating. High throwing power, uniform deposits, fair efficiency. Not for use in barrel plating

3

55970

5-90

380

35

2429

7585

Primarily for use in still tanks, but can be used in automatic plating and barrel plating. High efficiency and good throwing power

4

55-

5-

540

50

27-

80-

Used for plating cast iron. High speed and high efficiency1®

(a) For uniform deposits from cyanide solutions, the use of a current density of at least 215 A/m2 (20 A/ft2) is recommended. Agitation and cooling of solution are required at high current densities.

(a) For uniform deposits from cyanide solutions, the use of a current density of at least 215 A/m2 (20 A/ft2) is recommended. Agitation and cooling of solution are required at high current densities.

(b) Agitation and cooling are required when current density is high (above 215 A/m , or 20 A/ft2).

For still tank or automatic plating of steel, selection of a bath on the basis of cyanide-to-metal ratio depends on the type of work being plated and the results desired:

• For parts with no recesses and when protection of the basis metal is the sole requirement, Solution 1 in Table 1(a) (ratio, 4 to 1) is recommended.

• For plating parts with deep recesses and when a bright, uniform finish is required, Solution 2 in Table 1(a) (ratio, 7 to 1) is recommended.

• For all-purpose bright plating of various shapes, Solution 3 in Table 1(a) (ratio, 5 to 1) is recommended.

• For high-speed, high-efficiency plating, Solution 4 in Table 1(a) (ratio, 4.5 to 1) is recommended.

Although the use of brighteners produces maximum improvement in uniformity and throwing power (that is, the ability of an electroplating solution to deposit metal uniformly on an irregularly shaped cathode) in Solution 3 in Tables 1(a) and 1(b), brighteners also improve these properties in Solutions 1 and 2.

Normally, the sodium hydroxide content of cyanide baths is not critical. Usual limits are 7.5 to 26 g/L (1.0 to 3.5 oz/gal); the preferred concentration for best results is 15 ± 4 g/L (2 ± 0.5 oz/gal). Sodium hydroxide contributes to conductivity and, in excess, affects the current-density range for obtaining bright plate. Analytical procedures useful in the maintenance of cyanide baths are outlined in the section "Chemical Analysis of Cyanide Cadmium Plating Baths" in this article.

In recent years, the need for pollution control of cyanide solutions has led to the development of noncyanide cadmium electroplating baths, shown in Table 2. Noncyanide baths generate little hydrogen embrittlement and are used to electroplate hardened, high-strength steels. Both the sulfate and the fluoborate baths have been used for some time as a substitute for cyanide baths, and working data are available. The fluoborate bath is characterized by high cathode efficiency, good stability, and relatively little production of hydrogen embrittlement (see the section "Selective Plating" in this article). The major disadvantage of the fluoborate bath is its poor throwing power. It is widely used in barrel plating operations. If this bath is used for still plating at high current density, air agitation is desirable. Wire and strip geometries can readily be plated in a fluoborate bath. Practically all of the other acid-type baths shown in Table 2 are supplied to electroplaters as proprietary baths. Because each proprietary bath has its own peculiarities, it is advisable to obtain all proper operating information from the supplier to obtain the desired results.

Table 2 Concentration of commercial noncyanide cadmium plating baths

Bath

Proprietary(a)

Fluoborate(b)

Acid sulfate(c)

g/L

oz/gal

g/L

oz/gal

Ammonium chloride

11-23

1.5-3.0

Ammonium fluoborate

60

8

Ammonium sulfate

75-115

10-15

Boric acid

27

3.6

Cadmium

4-11

0.5-1.5

95

12.6

Cadmium fluoborate

244

32.2

Cadmium oxide

7.6-11 g/L (1.0-1.5 oz/gal)

(a) Proprietary requires a current density of 22 to 160 A/m2 (2 to 15 A/ft2) and an operating temperature of 16 to 38 °C (61 to 100 °F).

(b) Fluoborate requires a current density of 325 to 650 A/m2 (30 to 60 A/ft2) and an operating temperature of 21 to 38 °C (70 to 100 °F).

(c) Acid sulfate requires a current density of 110 to 660 A/m2 (10 to 61 A/ft2) and an operating temperature of 16 to 32 °C (61 to 90 °F).

Brighteners. The most widely used, and probably the safest, brightening agents for cyanide baths are organics such as:

• Some sulfonic acids

These materials form complexes with the electrolyte in cyanide baths and influence the orientation and growth of electrodeposited crystals, resulting in the formation of fine longitudinal crystals, and hence a bright deposit. Care should be taken not to add the brighteners in too large an amount. Too much brightener can result in dullness, pitting, blistering, and general poor quality and appearance. It is difficult to remove the excess brightener. Many organic brighteners are available as proprietary materials. When these are used, manufacturers' recommendations regarding amounts and other conditions of use should be followed.

Another method of brightening consists of the use of trace quantities of metallic nickel, cobalt, molybdenum, and selenium. The concentration of these elements in the bath is much more critical than the concentration of the organic brighteners. Poor bright dipping qualities or poor ductility and corrosion resistance of the coating may result from an excess of these metals. Certain proprietary brighteners contain both metallic and organic compounds. Brighteners for the noncyanide baths are also proprietary products.

Rough or pitted deposits should not be encountered in a well-balanced, carefully operated bath. However, if the concentration of metal is too low or the ratio of metal to cyanide varies from recommended values, roughness may result. Other factors that may contribute are contamination by dust, dirt, oil, metallic particles, or soap. Excessive concentrations of sodium carbonate and too high a temperature or current density also promote surface roughness.

Pitted deposits usually are the result of metallic impurities or an excessive amount of decomposed organic addition agents. The interfering metals are antimony, lead, silver, arsenic, tin and thallium. Pitting may also result from the presence of nitrates.

Correction of roughness or pitting may require a complete solution clean-up, including removal of excess sodium carbonate, purification with zinc dust, treatment with activated carbon, and filtration.

Formation and Elimination of Carbonate. Sodium carbonate forms in the cyanide bath as a result of the decomposition of sodium cyanide and the reaction of sodium cyanide with carbon dioxide from the air. The preferred method of agitation, if used, is mechanical because air agitation accelerates the buildup of carbonates.The buildup also results from failure to keep ball anode racks full or from the use of a large area of insoluble steel anodes.

Maximum concentrations of sodium carbonate that can be present in the bath without adverse effect on operating efficiency and deposit characteristics depend on the metal content of the bath. For example, carbonate can be present in concentrations up to 60 g/L (8 oz/gal) if the metal content is 19 g/L (2.5 oz/gal), and up to 30 g/L (4 oz/gal) if metal content is 30 g/L (4oz/gal), without deleterious effects. Exceeding these concentrations results in anode polarization, depletion of the metal content of the bath, and poor, irregular, and dull deposits.

To remove carbonates, the preferred method is to freeze them out by reducing the temperature to 1 to 3 °C (35 to 40 °F) in an outside treatment tank. This lowers the solubility of the carbonates, and the resulting precipitate is allowed to settle. The next step is to pump or filter the clear solution back to the plating tank, readjust the solution based on analysis, and properly dispose of the settled precipitate and solution. It is also possible to remove carbonates by treating the solution with calcium sulfate or calcium cyanide. The equipment supplier should be consulted about which procedure should be applied. Continuous purification equipment that maintains a preset level of carbonate is now available in state-of-the-art equipment.

Purification and Filtration. Whenever it is convenient, continuous filtration is advisable. If a solution is contaminated by impurities such as copper, tin, lead, or other metals, the following treatment is recommended.

Transfer the solution to an auxiliary tank of the same size as the plating tank; stir in 0.7 to 1 kg (1.5 to 2 lb) of purified zinc dust per 400 L (100 gal). Continue to stir for about 1 h, then allow to settle for no more than 6 h. Filter through a well-packed filter. If the solution contains excess organic impurities, such as decomposed brighteners, it should be treated with activated carbon and filtered. Pumps and filter parts should be made of iron or steel for alkaline cyanide baths. The solution attacks brass or bronze, and heavy copper contamination results.

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