Bright dipping of brass, what chemically happens?

[Capntom]

We have brass parts that get Bright dipped. The line is composed of H2SO4, sodium dichromate and water. What is actually happening to the brass that reveals the cleaner luster? I have heard it "cleans and seals". Can someone explain in a few sentences?

 

[CoryPad]

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.

...

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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