Nickel Anode Materials

Most nickel plating processes are operated with soluble nickel anode materials. Nickel from the anode is converted into ions that enter the plating solution to replace those discharged at the cathode. In addition, the anode distributes current to the parts being plated and influences metal distribution.

The simplest way to satisfy anode requirements is to suspend nickel bars from hooks placed on an anode bar so that the nickel, not the hook, is immersed in the plating solution. Nickel anode materials are encased in cloth anode bags to prevent insoluble anode residues from entering the solution and causing roughness at the cathode. The use of bars or electrolytic nickel strip is still practiced but has been supplanted in most regions of the world by the use of titanium anode baskets. The baskets used in nickel plating are generally made of titanium mesh strengthened by solid strips of titanium at tops, bottoms, and edges. The baskets are encased in cloth anode bags, suspended on the anode bar by hooks that are an integral part of the baskets, and loaded with small pieces of nickel. The mesh facilitates the free flow of plating solution. Baskets that incorporate hoppers at the tops facilitate basket loading and help prevent pieces of nickel from falling into the tank.

Titanium anode baskets were quickly accepted because of their many advantages. The basket anode is large and unchanging, ensuring a uniform anode area giving constant current distribution and consistent thickness for repeat batches of the same work. Anode maintenance involves topping-up the load to keep the baskets filled. Conforming baskets can be made in virtually any size and shape. The anode-to-cathode distance can be made constant, thereby contributing to good current distribution. Lowest-cost, primary forms of nickel can be used to fill the baskets. Baskets can be semiautomatically or automatically filled with nickel, and that practice is growing in progressive plating shops. One limitation is that titanium cannot be used in concentrated fluoborate solutions or those containing fluoride ions; small amounts of fluoride in solution activate titanium, causing it to corrode.

The available forms of nickel for titanium baskets include high-purity electrolytic nickel squares about 25 x 25 mm, pure electrolytic nickel in button-like shapes about 22 mm in diameter, and sulfur-activated, electrolytic nickel button-shape pieces about 25 mm in diameter. Other popular forms of nickel for plating with baskets are made in spherical shapes by a gas-refining process; the spherical forms are also available in sulfur-free and sulfur-containing grades.

The sulfur-activated forms dissolve relatively uniformly at high current densities and at 100% anode efficiency even in the absence of chloride ions, whereas sulfur-free forms dissolve nonuniformly and require the presence of chloride ions in solution to dissolve efficiently. The need for chloride ions is due to the tendency for pure nickel to become passive in nickel sulfate solutions. Although the tendency for passivity persists even in the presence of chlorides, the chloride ion attacks the passive oxide film that forms when current flows through the anode, and nickel can be dissolved through pits on the surface. The sulfur-containing materials do not form oxide films, and they dissolve at low anode potentials. The small amount of sulfur in the nickel lowers the surface resistance to current flow, the practical effect being to reduce power costs. The unique advantage of the spherical forms of nickel is product flowability, which facilitates automatic basket loading and filling of conforming, semicylindrical, and other complicated basket shapes.

The anode affects the quality of nickel primarily through its effect on current distribution and thickness uniformity. Most anode materials available today are made to strict specifications of purity and are unlikely to introduce significant amounts of impurities into the solution.

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