Electroless Nickel

Common Uses

Electroless nickel is used normally as an engineering coating to impart corrosion and wear resistance to a workpiece. Platers also commonly use the process on aluminum to provide a solderable surface and to improve lubricity and the release of molds and dies. Because of these properties, this technology is used widely in petroleum, chemicals, plastics, optics, printing, mining, aerospace, nuclear, automotive, electronics, computers, textiles, paper, and food machinery manufacturing (Fields 1982).

Common Bath Solutions

Metal finishers have used electroless nickel since the 1950s. The most common baths use nickel sulfate salts with sodium hypophosphite as the reducing agent. Platers frequently use hypophosphite in metal applications and a warm, alkaline hypophosphite solution in plastics applications. In either case, decomposition of the sodium hypophosphite during the reduction reaction results in the formation of a compound that increases deposition rates. Generally, this occurs between five and seven metal turnovers (Fields 1982).

Process Characteristics

Some advantages of this process include:

• Uniform deposition without variances in thickness
• Platable on non-conducting materials such as plastic
• Solderability
• Good wear
• Corrosion resistance

Some disadvantages of the system include:

• High chemical costs
• Embrittlement
• Poor welding characteristics of nickel phosphorous deposits
• Copper strike needed to plate certain alloys
• Slow plating rates

Another disadvantage is that the baths have a tendency to decompose spontaneously causing the entire tank to become nickel plated. When this occurs, the tank must be drained of the plating solution and filled with a nitric acid solution to dissolve the metal and repacify the tank. The nitric acid solution can be retained and used several times, but at some point it must be disposed (Davis 1992).

Bath Life Extension

Because of the frequency of bath change-out, the primary pollution prevention goal in electroless nickel baths is bath life extension. Bath life extension technology performs two functions: removal of the chemical byproducts formed during the processing of parts and the continuous addition of bath chemicals to maintain the overall chemical balance of the bath. Typical byproducts of the process are orthophosphite, sulfate, and sodium ions. Process bath chemicals and operating parameters such as nickel concentration, hypophosphite, reducing agents, complexing agents, pH, temperature, and bath stabilizers influence the effectiveness of different bath life extension methods (DoD 1996).

Recovery technologies such as ion exchange and reverse osmosis have been used to remove contaminates. Other methods include the precipitation of orthophosphite contaminants with calcium or magnesium ions, however, this method is only useful if the sulfate ion also is removed. Some treatments have extended the bath life from seven to ten times the original life (extensions such as these can reduce waste generation by 90 percent) while others have claimed increases of 50 times the original. Facilities should be aware that the concentration of inhibitors, catalysts, and exaltants will change as the lifetime of the bath is extended, requiring monitoring and additions of the chemicals (Bishop 1993).

Regeneration of Electroless Nickel Baths

Electroless nickel solutions are degraded by the buildup of orthophosphite, a breakdown product of sodium hypophosphite that platers use in the solution as a reducing agent. Studies are underway to see if electrodialysis is capable of removing the orthophosphite selectively, increasing the life of the solution vastly. Initial results for this are not promising, however, research centers such as the Toxics Use Reduction Institute continue to work with companies on making this technology feasible (Palepu).

Prolonging Bath Life with Lime

A pilot-scale study was conducted by TecKote, in Brampton, Canada, to determine if it is possible to precipitate out phosphite contamination of electroless nickel baths using lime. The test procedure was as follows:

• A plating bath was prepared using a proprietary electroless nickel solution
• Nickel salt and hypophosphite solution were mixed with deionized water
• Parts (i.e., activated steel panels) were immersed in the solution
• A 15-percent lime slurry was pumped into the plating bath at a constant rate of 20 milliliters per minute.

The plating run lasted 44 hours and yielded some promising results. The test found that the addition of lime slurry doubled the life expectancy of the plating solution, however, the process also produced a sludge that was determined to be hazardous. The study did not determine whether this process would yield cost savings for plating facilities (Richmond 1991).

Immersion (Displacement) Plating

Immersion plating is a process similar to electroless plating. In this process, the metal finish is placed on the workpiece by displacing base metal from the workpiece with another metal ion in the plating solution. The metal ions in the plating solution have a lower oxidation potential than the displaced metal. This process, like electroless plating, uses chemical reactions to apply a metal finish to the substrate. Immersion plating differs from electroless plating in that the reducing agent is the base metal of the workpiece and not a chemical additive, as is the case in electroless plating (Davis 1994).

The thickness of deposits obtained in immersion plating is limited because deposition stops when the entire surface of the base metal is coated. Higher temperatures and agitation can increase the reaction rate of the immersion process. These baths usually are inexpensive to operate and deposit well. Other benefits of immersion include its ability to deposit on difficult surfaces such as bores or holes. When working with this solution, be aware of the safety hazards associated with bases and acids (Hirsch 1993). Table 16 identifies deposit-base pairs that can use this plating technique without a cyanide solution.

Table 16. Immersion Plating Formulations (Hirsch 1993)