Q. What causes the buildup of carbonates in an alkaline non-cyanide zinc plating bath? How do we prevent this from happening? P.P.
A. A lot of platers are surprised to discover that the alkaline zinc plating bath is susceptible to carbonate buildup. Most of us assume that this only occurs in alkaline cyanide baths. Surprise—the alkaline non-cyanide bath is also susceptible to this problem. There are two ways this happens:
Our atmosphere contains carbon dioxide, which combines with water to create a form of carbonic acid, H2CO3 (the same acid in soda pop). The carbonic acid reacts with the alkaline solution to form carbonates. Air agitation or vigorous mixing will make the problem worse.
A smaller but important factor is the formation of oxygen at the anode. The oxygen can react with organic materials that may be present in the plating bath as additives. This can be a more serious problem if insoluble anodes are used in the bath.
Alkaline zinc plating baths have a tendency to “grow” (that is, zinc concentration increases) if zinc anodes are used, so insoluble anodes are often used instead.
Another factor is current density. If the anodic current density is too high, more oxygen will be generated at the anode and the rate of carbonate formation will increase.
Carbonate concentration must be monitored on a regular basis. When the concentration gets too high, the carbonate must be removed either by freezing out or by using careful addition of materials that will form insoluble carbonates that can be filter out of the bath.
If you want to learn more about zinc plating, try to locate a copy of the book, Zinc Plating by Herb Geduld. This book is also out of print but used copies do become available fairly often on www.amazon.com.