Replacing Chromic Acid

Question:

We epoxy powder coat a product line that is about 50% brass, 25% bronze and 25% nickel-plated brass. Our pretreatment consists of an alkaline cleaner followed by an 0.2 oz/gal chromic acid solution. Could you recommend a replacement for the chromic acid? D. R.

Answer:

Since you do not indicate what the function of the chromic acid solution is for, I will have to make some assumptions. Since your brass and bronze parts may become discolored (oxidized) during the alkaline cleaning process, you are probably including the chromic acid solution for tarnish removal. Assuming this is the case, you have a couple different options.

First would be a direct replacement of the solution. I have found that a citric acid step at the end of a copper or brass cleaning line can do a good job in removing the tarnish that can develop from upstream processing of parts. It can work with as little as a quarter to one ounce per gallon of solution. It will also be helpful to heat the tank slightly (not necessary above 120°F).

The second way to approach the problem may be to eliminate the tarnish from the upstream processing entirely. If you see the oxidation occurring in the cleaning tank, it could be possible to use a slightly less alkaline cleaner. If it were less aggressive, it may not produce as much oxide. Additionally, the formula of the cleaner could be modified with the addition of base metal inhibitors that would minimize or eliminate the formation of the oxide. Some potential ingredients that could be included are silicates and triazoles. Additionally, if you are using compressed air in an attempt to agitate the bath, I would recommend against it. It will maintain the bath with a higher level of dissolved oxygen, resulting in more oxidation of your parts (as well as some detrimental effects on the cleaner itself).

One or both of these solutions should allow you to eliminate the use of the chromic acid from your process helping minimize the amount of hazardous waste you generate and minimize exposure of your employees to a very hazardous chemical.