Sulfamate bath

Nickel. Nickel plating continues to be very important. Many plating baths have been formulated, but most of the nickel plating is done in either Watts baths or sulfamate baths. Watts baths contain sulfate and chloride nickel salts along with boric acid, and were first proposed in 1916 (111). Nickel was first plated from sulfamate in 1938 (112) and patented in 1943 (108). The process was brought to market in 1950 (113). Typical bath compositions and conditions are shown in Table 14. 

Both Watts and sulfamate baths are used for engineering appHcation. The principal difference in the deposits is in the much lower internal stress obtained, without additives, from the sulfamate solution. Tensile stress can be reduced through zero to a high compressive stress with the addition of proprietary sulfur-bearing organic chemicals which may also contain saccharin or the sodium salt of naphthalene-1,3,6-trisulfonic acid. These materials can be very effective in small amounts, and difficult to remove if overadded, eg, about 100 mg/L of saccharin reduced stress of a Watts bath from 240 MPa (34,800 psi) tensile to about 10 MPa (1450 psi) compressive. Internal stress value vary with many factors (22,71) and numbers should only be compared when derived under the same conditions. 

Nickel sulfamate is more soluble than the sulfate salt, and baths can be operated using higher nickel concentrations and higher currents. Sulfamate baths have been found to have superior microthrowing power, the abiUty to deposit in small cracks or crevices. Using one nickel salt, only a hydrometer and pH paper are needed to control the bath. A small amount of chloride salt was added as a proprietary. Highly purified nickel sulfamate concentrates are commercially available that can be used to make up new plating baths without further purification. 

Plating solutions used in nickel electroforming are primarily the Watts bath and the nickel sulfamate bath. Watts baths exhibit higher stress and require additives for stress control, which may affect other properties. Sulfamate baths produce much lower stress and are preferred where purer nickel or nickel—cobalt deposits ate needed. ASTM specifications are available that describe the mandrels and plating solutions (116,162). 

Fig. 8-32. Analysis of chloride, bromide, and sulfate in a nickel sulfamate bath. - Separator column 2 IonPac AS4A eluent 0.0017 mol/L NaHC03 + 0.0018 mol/L Na2C03 flow rate 1.5mL/min detection suppressed conductivity injection 50 pL sample (1 1000 diluted).

The main component in nickel sulfamate baths — sulfamate - can be determined in a single rim, in addition to the decomposition product sulfate and other bath constituents such as chloride and bromide (see Fig. 8-32). To achieve a sufficient separation between the sulfamate ions and chloride, both of which elute near the void volume, two identical anion exchangers were used in series, even though this increases the total analysis time to about 20 minutes. 

Nickel anodes are preferred in tip machines because the pH and the metal content remain stable. The pH will decrease rapidly when insoluble anodes are used. The pH should be maintained at 1.5 or higher with additions of nickel carbonate. Stress values are higher than in sulfamate baths, with values of about 20 kpsi. 

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