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Chloride in acid copper bath?

The acid copper plating recipes I have seen all state a small quantity of chloride ion is required, typically 40 to 70ppm. Yet I can't find it's exact purpose and get different answers. I have read that it improves throwing power, I have also read it works in conjunction with brightener and another is, from letter 4836, the formation of a black film on the anode. I have been successfully using my plating tank for about 1 year now containing only acid, copper and rain water. Matte finish is desirable. Anodes are from old 1/2" diameter solid copper electrical cables and appear light gray pink. I do notice some precipitation of anode when in idle state for long periods. Does the chloride ion cause a black film which prevents anode corrosion thus keeping copper content from rising ?

Adam Seychell
- Melbourne, Australia
2001