ALKALINE ZINC PLATING
Can you help us figure out why our alkaline zinc plating bath makes the anodes polarize when the current is on?
Q. We are using an alkaline zinc plating bath. We seem to have one significant problem with this bath: the anodes polarize when the current is on. This does not happen all the time but we are unable to figure out what is going on.
Can you help us? M.E.
A. There are two possible reasons for polarization of zinc anodes in an alkaline zinc plating bath. The most obvious is that your anode current density is too high. In the alkaline zinc plating bath, anode efficiency tends to be much higher than cathode efficiency. If you’re operating a tank with a one-to-one cathode-to-anode ratio, the zinc metal content will climb. Typically, some of the zinc anodes are removed from the plating bath to minimize the increase of the zinc content in the solution. As a result the current density on the remaining zinc anodes increases, and this can produce polarization.
To prevent this from happening you must replace some of the zinc anodes with steel anodes. In a typical alkaline zinc plating bath it’s not uncommon to see 75–85% of the anode area made up of plain steel.
The second likely cause of anode polarization is improper sodium hydroxide? concentration.? This can be easily remedied by chemical testing.
One additional comment: To prevent buildup of zinc in the plating solution during down periods, the zinc anodes should be removed from the plating tank during idle times.
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