Sulfamate Baths

Sulfamate baths consist essentially of lead sulfamate with sufficient sulfamic acid to obtain a pH of about 1.5. Sulfamic acid is stable and nonhygroscopic, and is considered a strong acid. Compositions and operating conditions of two typical sulfamate baths are given in Table 3.

Table 3 Compositions and operating conditions of lead sulfamate baths pH, 1.5; temperature, 24-49 °C (75-120 °F); cathode current density, 0.5-4 A/dm2 (5-40 A/ft2); anode current density, 0.5-4 A/dm2 (540 A/ft2); anode/cathode ratio, 1:1; anode composition, pure lead

Table 3 Compositions and operating conditions of lead sulfamate baths pH, 1.5; temperature, 24-49 °C (75-120 °F); cathode current density, 0.5-4 A/dm2 (5-40 A/ft2); anode current density, 0.5-4 A/dm2 (540 A/ft2); anode/cathode ratio, 1:1; anode composition, pure lead

Bath

Lead

Animal glue

Peptone equivalent

Free sulfamic acid

g/L

oz/gal

g/L

oz/gal

g/L

oz/gal

g/L

oz/gal

1

140

18.75

5.6

0.75

150

20.1

Because the acid and the salt used in the solutions in Table 3 are highly soluble in water, sulfamate baths can be prepared either by adding constituents singly or as formulated salts to water. Solutions are usually formulated to concentrations that allow bath operation over a wide range of current densities. Lead concentration can vary from 112 to 165 g/L (15 to 22 oz/gal), while the pH is held at about 1.5. As in other lead plating solutions, additive agents (peptone gelatin or other colloids, alkyl or alkyl aryl polyethylene glycols) are required to produce smooth, fine-grained deposits.

Spongy deposits are obtained if the lead concentration is too low, the current density is too high, or the concentration of additive agent is too low. At low pH or high temperature, sulfamate ions hydrolyze to ammonium bisulfate to form insoluble lead sulfate. Ordinarily, this hydrolysis presents no problem, provided the bath is correctly operated.

Maintenance and Control. Sulfamate baths do not require much attention other than maintenance of the correct proportion of additive agents to produce the desired deposit quality. Additive agent content is evaluated by the use of the Hull cell. The pH is easily adjusted with sulfamic acid or ammonia and can be measured with a glass electrode. Lead concentration can be determined with sufficient accuracy by hydrometer readings or an occasional gravimetric analysis.

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