Q. Our company brass plates zinc parts. Most of the time we obtain a yellow brass finish, which is acceptable for subsequent finishing steps. Occasionally, we get a pinkish-red brass that is not acceptable and must be rejected. What are some possible causes for this color change? S.B.
A. The most likely cause of the color variation is a change in the amount of copper and zinc in your brass plating bath. The first thing that comes to mind is that you copper content in the plating bath is too high. The table below gives you some idea of color of the plate versus the amount of copper in the plating bath:
| Approximate Copper Content | Brass Plate Color |
|---|---|
| 86-88% | Copper pink |
| 80-82% | Light pink |
| 67-72% | Lemon yellow |
| 62-66% | Straw yellow |
| 57-60% | Golden |
| 42-44% | Dull light pink |
| 30% or less | Silvery white |
*The one surprise in this table is that, at this copper concentration, the brass deposit may appear to have a pinkish color.
If the brass deposit has too much copper content, here are some suggested fixes:
- Add 20 ml of ammonia per gal of bath.
- Lower the plating bath temperature by 2-4°F.
- Increase the pH by adding sodium hydroxide (caustic soda).
- Increase the current density by a small amount.
- Increase the free cyanide content.
If the deposit has too much zinc content, here are some suggested fixes:
- Lower the current density by a small amount.
- Lower the pH with sodium bicarbonate.
- Raise the bath temperature.
- Add copper cyanide to the plating bath.
Always make sure the metal ratio in solution of copper/zinc is no greater than 8:1 and no less than 2:1. If your plating solution has too high of a copper content, you can adjust the ratio of your copper/zinc anodes by adding some auxiliary zinc anodes to the bath.
