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Which acid to use to neutralize and brighten alkaline etched cast Aluminium?

July 23, 2008

Hi,

I needed to dip clean cast Aluminium engine parts so I etched them in a solution of 10% Potassium Hydroxide. This worked fine.
I then put them into a 10% Phosphoric Acid solution to neutralize and brighten them. However, the castings came out black. A pressure water wash took off some of this black film but still left them looking dark gray.

Interestingly, the near pure Aluminium wire I used to suspend them in the solutions came out clean and bright, as I expected the parts to also.

I guess there's something in the cast alloy that's reacting differently and producing the black film.

What acid solution should I use? Do I need to use Nitric Acid instead or a combination instead?

I anyone can shed some light, that'd be great.

Cheers,
Patrick.

Patrick Bustard
Manufacturer - Willaimstown, Victoria, Australia

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July 23, 2008

Hi, Patrick. You have analysed the problem correctly. The castings contain other alloying materials which do not dissolve away in either potassium hydroxide or phosphoric acid. But proprietary mixed-acid desmutters will be more effective than any straight acid or home-brew because that black smut could be a combination of things like both copper and silicon. See if a local distributor can offer you a desmut solution. Removing silicon requires HF or some other fluoride chemistry; that's dangerous stuff and a proprietary mixed acid will reduce the danger a bit.

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

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July 24, 2008

Your wire is an extruded part, so will etch quite uniformly whereas the casting will etch to a much rougher surface. Most aluminum wire is nearly pure aluminum so will work like Ted said.
I would use a weaker hydroxide solution so that you can control the etch a bit better. Possibly as low a 5% for a fresh solution and gradually increasing the OH to maintain the etch rate.

some of the truck stops may sell a liquid brightener for aluminum. It contains fluoride, so handle with great care! It is affordable if you only need small lots of it whereas you need to buy large amounts if you go the metal finishing chemical vendor route.

James Watts
- Navarre, Florida

Mixture of Nitric acid and HF Is good for cast aluminium

Suresh Reddy
- Bangalore, India
July 27, 2008