Many metals (aluminum, copper, iron, steel) are attacked by air to create undesirable surface oxidation. While oxidation can be beneficial and protect the surface (note the green color of the Statue of Liberty) often this oxidation needs to be removed. While abrasive blasting is highly recommended for this process, large quantities of small parts can be difficult to blast one at a time.
Vibratory or barrel tumbling the parts in an abrasive or polishing media can quickly and effectively remove oxidation and prep a surface for further processing. In combination with the appropriate chemical compounds choosing the right type of media is important. Using an abrasive media will typically produce a clean, matte surface finish. Polishing media will enhance the compound cleaning action to leave a bright surface finish.
A mild acid cleaner designed for tumbling is an ideal choice for gentle oxidation removal. While more aggressive acids (mineral acids like muriatic) will also remove oxidation, care should be taken not to attack the underlying metal. In a pinch, a quick additive to use would be something like vinegar (acetic acid), citric acid or oxalic acid products (consumer bar cleaners).