Polarized Zinc Anodes

Question:

I am using a non-cyanide alkaline zinc plating bath. I find that the anodes polarize when the current is on. What causes this? S.D.

Answer:

The two most likely causes of polarization of anodes are: high current density and/or a low concentration of sodium hydroxide. The rule of thumb for this type of plating bath is that the sodium hydroxide concentration should be approximately 10 times that of the zinc metal concentration. For example, if your bath has a concentration of 2 ounces per gallon of zinc then the concentration of your sodium hydroxide should be approximately 20 ounces per gallon.

In this type of zinc plating bath the anode efficiency is much higher than that of the cathode. If you are trying to maintain a one to one anode-cathode ratio, the zinc concentration in the plating bath will increase. To alleviate this problem some of the zinc anodes are removed which causes the current density on the remaining anodes to increase. The one to one ratio can be maintained by replacing some of the zinc anodes with steel anodes. It is not unusual to find as much as 75% of the anode area to consist of plain steel.