Problems with Zinc Anodes
What causes zinc plating bath anodes to polarize?
Q. We use non-cyanide alkaline zinc plating baths. I find that the anodes polarize when the current is on. What is causing this problem? –M.J.P.
A. The two most likely causes of polarization of anodes are either a high current density or a low sodium hydroxide concentration.
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 can be removed to cause 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 60 or 75 percent of the anode area consisting of plain steel.
A 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.