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Plating Q&A: Converting a Brass Plating Bath

How do you convert a brass plating bath into a bronze plating bath?
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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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