The sulfuric acid is used to remove oxides that form during processing. The chromate treatment is used in a solution to remove stains.
Here is a good summary from the ASM Handbook, Volume 5, Surface Engineering:
Pickling in solutions containing 4 to 15 vol% sulfuric acid or 40 to 90 vol% hydrochloric acid is used for the removal of oxides formed on the surface of copper-base materials during mill processing and fabricating operations. The sulfuric acid solution is used to remove black copper oxide scale on brass extrusions, forgings, and machined parts; oxide on copper tubing, forgings, and machined parts; and light annealing scale or tarnish.
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By far the most common defect encountered in pickling brass is the presence of red stain on the metal. Caused by cuprous oxide, this defect may be removed by pickling in a solution of 10% sulfuric acid containing 98 g/L (13 oz/gal) of ferric sulfate at temperatures up to 60 °C (140 °F), or in a solution of 12% sulfuric acid containing 45 g/L (6 oz/gal) of sodium dichromate at room temperature.
Regards,
Cory
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