Passivating Brass Shell Cases after Pickling/Polishing

Question:

I am running a mass finishing process that requires an aqueous passivation of the brass as a final process to help prevent corrosion. Can you recommend a chemical additive we can use with our rinse water? J.R.

Answer:

Any solution containing one of the compounds in the azole family is recommended for tarnish protection of copper and brass alloys. Two widely used compounds in this family are benzotriazole and tolyltriazole. Both can be purchased individually as a solid, but I would not recommend this since they can be difficult to get into solution and are used at such low concentration (parts per million level). Cobratec is a trademark name of PMC Specialties Group that has supplied both inhibitors for many years both in solid form and as pre-dissolved liquid mixtures. Although not as widely documented as benzotriazole or tolyltriazole, I have also had good experience with citric acid as an anti-tarnish for copper parts in something that is close to a mass-finishing operation (a barrel cleaning line). About a one-half ounce of solid citric acid dissolved in a gallon of water gives good anti-tarnish protection for extended storage periods. I have also been told that the parts tumble and separate better after this operation, although I’m not sure why.

Both of the above inhibitors are good at providing medium-term corrosion protection (anti-tarnish) with indoor storage conditions. Short of a lacquer or some other type of coating, there is nothing that will give you long term corrosion protection in a severely corrosive environment.

Whatever you do, avoid some steel rust inhibitors that contain amines. Many steel rust inhibitors that offer moderate indoor storage protection contain triethanolamine. If this is applied to brass, and then stored in a humid environment, it is possible that some of the amine could turn to ammonia which will result in stress corrosion cracking of you brass parts.