Alkaline and Acid Plating Solutions Used for Zinc Plating Process

Jul 19, 2021

Alkaline and Acid Plating Solutions Used for Zinc Plating Process

These are two general plating baths types: one utilizes an acid electrolyte and the other an alkaline electrolyte. The former is known as sulfate, sulfate-chloride, chloride, chloride-acetate, and fluoborate baths, depending upon their chief constituents. The alkaline electrolytes are mostly cyanide baths, although zincate and pyrophosphate baths also have their advocates.

The choice between the types used is based upon a number of factors. For instance, in considering which bath to use, the higher plating rate and low operating costs of acid baths have to be balanced against the superior throwing power of the alkaline cyanide baths. Thus, the acid baths, although having poor throwing power, are used largely for plating strip and wire because of their high plating rate and lower operating costs. Alkaline cyanide baths, with their better throwing power, are used in plating objects of irregular shape.

Acid baths
Of the acid baths, the sulfate solution is the most widely used. Reasonably priced zinc sulfate is the principal salt in this bath and because it is highly soluble in water, a solution can be highly concentrated, with the advantage that the general ratio of zinc to other type ions makes possible an increased rate of zinc deposition. Because of high anode efficiency and the tendency of anodes to corrode, additions of sulfuric acid must be made periodically to maintain acidity. Also, sodium acetate or aluminum chloride are added to the bath because their buffering action helps regulate solution pH.

The sulfate bath is the most commonly used electrolyte for zinc electrodeposition. As mentioned earlier, it is used extensively for coating wire and strip. Deposits from this bath are whiter but coarser-grained than those deposited from cyanide baths.

Chloride baths made from solutions of zinc, sodium, and aluminum chlorides yield very good deposits. The higher conductivities of these baths allow them to be run at high current densities, without requiring agitation or other precautions to prevent overheating the electrolytes.

Alkaline baths
The alkaline baths are generally used for batch zinc electroplating and are capable of providing a variety of deposits. While the cyanide baths have formed the basis for most of the alkaline electrolytes, comparatively recently baths employing ammonium salts have been developed. The relative advantages of alkaline baths such as superior throwing power and disadvantages with respect to plating rates and operating costs have already been discussed.

Alkaline cyanide baths are made up largely of zinc cyanide in solutions of sodium cyanide and sodium hydroxide. They are found to be the most easily controlled of all the zinc plating baths. The cyanide electrolytes are employed with comparatively low current densities and soluble anodes.