Letter 8082

White stain/contamination of chemical polished Al extruded and machined parts 

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Problem: White contamination/stain on chemical polished Al parts (6061).

The white contam. was verified to be Aluminium Phosphate. Fallout is about 6-10%.

Chemical Polish bath used @ 115ƒC:
Phosphoric acid: 70% (vol)
Nitric acid: 10 % (vol)
Acetic acid: 20 % (vol)

What could be the cause & remedy? Is there any simple selective titration method to determine the respective acid (nitric acid, phosphoric, acetic acid content) on the CP bath mixture?

What is the recommended bath composition by vol for CP process given the following raw chemical?

Nitric acid strength: 65%
Phosphoric acid strength: 70%
Acetic acid strength: 70%

Is acetic acid commonly used in CP process? If yes, what is the purpose?

Thanks

Tan Thong Guan
BJ Industries - Singapore

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Tan,

Try reducing your temperature slightly. Chemical polishing of Al is generally carried out at ~80 degrees C. It may be your chemical solution is too volatile at this increased temperature. Also, it could be as simple as inadequate rinsing. The rinse post polishing should be heated also. Fast changes in temperature are known to cause increased precipitation in chemical processes. Could explain the salt on the surface.

John Martyn
- Ireland

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