Plating Tanks

Cylindrical or bell-shaped tanks have been used for electroless nickel plating, although rectangular tanks have been found to be the most convenient to build and operate. Rectangular tanks have been constructed from various materials in many different sizes. A common electroless nickel plating system is shown in Fig. 20.

Fig. 20 Twin tank system for electroless nickel plating. Tanks are used alternately. While one tank is being used to plate, the second is being passivated. Cylindrical tank is used to store 30% nitric acid for passivation.

Physical Dimensions. The following factors should be considered when selecting the size of an electroless nickel plating tank:

• Number of parts to be plated each day

• Plating thickness required

• Plating rate of the solution (most conventional electroless nickel solutions deposit between 12 and 25 t-1 m/h, or 0.5 and 1 mil/h)

• Type of rack, barrel, or basket used to support parts

• Number of production hours available each day to process parts

• Nominal recommended work load of 1.2 dm2/L (0.5 ft2/gal) of working solution

The size of the part or the size of the supporting rack, barrel, or basket usually defines the minimum size tank that can be used. The minimum dimension of the tank should be at least 15 cm (6 in.) greater than the maximum dimension of the part or its support to allow proper agitation and the flow of fresh solution to all surfaces. The size of the tank may have to be increased, however, to accommodate the volume of the parts required or to provide a more suitable work area to solution volume ratio.

Construction Materials. The following factors should be considered when selecting construction materials for a plating tank:

• Operating temperature of the electroless nickel plating solution usually 85 to 95 °C (185 to 205 °F)

• Tendency of tank material to become sensitized to the deposition of electroless nickel

• Cost of tank material, including both initial construction cost and its life in a production environment

With continued exposure to heated electroless nickel solutions, almost any surface eventually becomes sensitized or receptive to deposition of the coating. The more inert or passive the material selected, the less likely that plate out can occur. All material in contact with the plating solution must be repassivated periodically with 30 vol% nitric acid to minimize deposition on its surface.

The most widely used materials for tank construction have been polypropylene, stainless steel, and steel or aluminum with a 635 pm (25 mil) thick polyvinyl chloride bag liner. Contamination from bleedout of oils or other plasticizers can have harmful effects on the plating solution. Leaching linings prior to use is recommended. However, the contaminants continue to migrate to the surface and enter the solution (Ref 48). Although all of these materials have been used successfully, a 6 to 12 mm (0.25 to 0.5 in.) thick polypropylene liner installed in a steel or fiberglass support tank, has proven to be the most troublefree material and has gained the widest acceptance. Polypropylene is relatively inexpesive and is very resistant to plate out. The smooth surface of polypropylene also reduces the possibility of deposit nucleation.

When constructing a polypropylene tank, only stress relieved, unfilled virgin material should be used. Welds should be made under an inert gas shield, such as nitrogen, to prevent oxidation of the polypropylene and incomplete fusion. All welds should be spark tested at 20,000 V before use to ensure integrity.

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