Alkaline Noncyanide Baths

Alkaline noncyanide baths are a logical development in the effort to produce a relatively nontoxic, cyanide-free zinc electrolyte. Approximately 15 to 20% of zinc plated at present is deposited from these baths. Bath composition and operating parameters of these electrolytes are given in Table 2. The operating characteristics of an alkaline noncyanide system depend to a great extent on the proprietary additives and brightening agents used in the bath, because the zinc deposit may actually contain 0.3 to 0.5 wt % C, which originates from these additives. This is ten times as much carbon as is found in deposits from the cyanide system.

Table 2 Composition and operating characteristics of alkaline noncyanide zinc baths

Constituent

Optimum(a)

Range(b)

g/L

oz/gal

g/L

oz/gal

Preparation

Zinc oxide

9.4

1.3

7.5-21

1-2.8

Sodium hydroxide

65

8.6

65-9G

8.6-12

Proprietary additive

(c)

(c)

3-5

G.4-G.7

Analysis

Zinc metal

7.5

1.G

6^-17^

G.8-2.3

Sodium hydroxide

75.0

10.0

75-112

10.0-14.9

(a) Operating conditions: temperature, 27 °C (81 °F) optimum; cathode current density, 0.6 A/dm2 (6 A/ft2); bath voltages, 3 to 6 rack.

(b) Operating conditions: temperature, 21 to 35 °C (69 to 94 °F) range; cathode current density, 2.0 to 4.0 A/dm2 (20 to 40 A/ft2); bath voltages, 12 to 18 barrel.

(c) As specified

Alkaline noncyanide baths are inexpensive to prepare and maintain, and they produce bright deposits and cyanide-free effluents. An alkaline noncyanide zinc bath with a zinc metal content of 7.5 to 12 g/L (1.0 to 1.6 oz/gal) used at 3 A/dm2 (30 A/ft2) produces an acceptably bright deposit at efficiencies of approximately 80%, as shown in Fig. 1. However, if the metal content is allowed to drop 2 g/L (0.26 oz/gal), efficiency drops to below 60% at this current density. Raising the metal content much above 17 g/L (2.3 oz/gal) produces dull gray deposits, lower-current-density plating areas, and poor distribution; however, additives have been developed to address this problem. Increasing sodium hydroxide concentration increases efficiency, as shown in Fig. 2. However, excessively high concentrations will cause metal buildup on sharp-cornered edges. Alkaline noncyanide zinc is a practical plating bath having hundreds of thousands of gallons in use in large captive plating installations.

Fig. 1 Cathode current efficiency of alkaline noncyanide zinc baths as related to zinc metal contents. NaOH, 80 g/L (11 oz/gal); Na2COs, 15 g/L (2 oz/gal)

Fig. 2 Effect of zinc and sodium hydroxide concentration on the cathode efficiency of noncyanide zinc solutions. Temperature: 26 °C (77 °F). • : 7.5 g/L (1 oz/gal) Zn, 75 g/L (10 oz/gal) NaOH; •: 7.5 g/L (1.0 oz/gal) Zn, 150 g/L (20 oz/gal) NaOH; • : 11 g/L (1.5 oz/gal) Zn, 110 g/L (15 oz/gal) NaOH; A: 15 g/L (2.0 oz/gal) Zn, 150 g/L (20 oz/gal) NaOH; • : 11 g/L (1.5 oz/gal) Zn, 150 g/L (20 oz/gal) NaOH.

Fig. 2 Effect of zinc and sodium hydroxide concentration on the cathode efficiency of noncyanide zinc solutions. Temperature: 26 °C (77 °F). • : 7.5 g/L (1 oz/gal) Zn, 75 g/L (10 oz/gal) NaOH; •: 7.5 g/L (1.0 oz/gal) Zn, 150 g/L (20 oz/gal) NaOH; • : 11 g/L (1.5 oz/gal) Zn, 110 g/L (15 oz/gal) NaOH; A: 15 g/L (2.0 oz/gal) Zn, 150 g/L (20 oz/gal) NaOH; • : 11 g/L (1.5 oz/gal) Zn, 150 g/L (20 oz/gal) NaOH.

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