Plating Q&A: Converting a Brass Plating Bath
How do you convert a brass plating bath into a bronze plating bath?
Q. Can a brass plating bath be converted into a bronze plating bath?
A. Conceptually, converting a brass plating bath into a bronze plating bath is possible, though the conversion depends on the type of brass plating bath you are using.
|
Brass Plating Bath |
|
|
COMPONENT |
CONCENTRATION |
|
Copper cyanide |
4 oz/gal |
|
Zinc cyanide |
1.25 oz/gal |
|
Sodium cyanide |
7.5 oz/gal |
|
Sodium carbonate |
4 oz/gal |
|
OPERATING CONDITIONS |
RANGE |
|
Metallic copper |
2.8 oz/gal |
|
Metallic zinc |
0.7 oz/gal |
|
Free cyanide |
2 – 2.5 oz/gal |
|
Temperature |
75 – 100°F |
|
Current density |
3 – 5 ASF |
|
Voltage |
2 – 3 volts |
|
Anodes |
Brass 80% copper / 20% zinc |
|
Bronze Plating Bath |
|
|
Component |
Concentration |
|
Copper cyanide |
4.0 oz/gal |
|
Zinc cyanide |
0.3 oz/gal |
|
Sodium cyanide |
5.0 oz/gal |
|
Rochelle salt |
2.0 oz/gal |
|
OPERATING CONDITIONS |
RANGE |
|
Metallic copper |
2.8 oz/gal |
|
Metallic zinc |
0.17 oz/gal |
|
Free cyanide |
0.3 oz/gal |
|
Temperature |
75 – 100°F |
|
Current density |
2 – 4 ASF |
|
Anodes |
Brass 92% copper / 8% zinc |
The two plating baths are quite similar. By slightly altering the composition of the bath, you can change the color of the deposit, resulting in a brass or bronze finish.
But then is the bronze bath a true bronze? This is where you have to take into account the parts that you are plating and their use. A true bronze has different physical properties compared with that of the bronze that consists of only copper and zinc. Typically, a bronze consists of copper and tin. While the copper zinc bath will give you a bronze-like color, is not a true bronze.
My advice is to not convert your brass bath, particularly if it is functioning well. It would be better to prepare a separate bath that is a true bronze plating bath using copper and tin.
Originally published in the September 2015 issue.
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