Search our quarter-million Q&As

Home of the finishing HOTLINE since 1989

White Stain/Contamination of Chemical Polished Al Extruded and Machined Parts

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

The white contaminant was verified to be Aluminium Phosphate. Fallout is about 6-10%. Chemical Polish bath used @ 115°C:
Phosphoric acid: 70% (vol)
Nitric acid: 10 % (vol)
acetic acid ⇦ on eBay or Amazon [affil link] : 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 volume for CP process given the following raw chemical?

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

Thanks

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.

Sorry! Finishing.com is temporarily Read-Only.
Ted Mooney is retiring but I have several offers to take it over.
We're working hard to make sure we find it the best new home.