How Soluble Anodes Used in Zinc Electroplating?

How Soluble Anodes Used in Zinc Electroplating? 

Anodes used for electroplating zinc are of two types - soluble and insoluble. The soluble anodes are obviously zinc to provide the metal to be plated. With insoluble anodes, the zinc is furnished to the electrolyte by leaching material containing zinc oxide.

In the electroplating of zinc with soluble anodes, the zinc ions in the electrolyte are maintained during operations by the electrolytic dissolution of the zinc of which the anodes are comprised. The plating or electrogalvanizing solution automatically maintains itself by dissolving zinc from the anodes in the exact quantities being plated out on the steel that is being coated.

All zinc anodes, but least of all high purity zinc, tend to be dissolved by a direct chemical attack when the plating bath is not in operation, resulting in loss of the metal by corrosion, and saturation of the electrolyte to such a high degree that it cannot be operated efficiently. With Special High grade zinc, because of the high anode efficiency, the electrolyte gradually tends to become supersaturated with zinc. As a result of demands from industry for improvements, research was carried out to ascertain the effects of alloying anodes with certain metals which are not harmful to the bath.

To minimize anode dissipation during idle time, and promote uniform corrosion during operation, anodes alloyed with aluminum and mercury for cyanide solutions or magnesium and calcium for acid electrolytes were introduced. These additions were found to eliminate excessive differences between anode and cathode current efficiency. Excessive anode dissolution has also been minimized by the use of auxiliary sheet steel, nickel plated sheet steel, or sheet nickel anodes along with the zinc anodes. Auxiliary anodes have likewise been found to be helpful when plating steel with zinc if recessed parts are being treated, in that they ensure a uniform deposit over an irregularly shaped surface.