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Carld
07-20-2009, 04:32 PM
I read in a thread that aluminum sulfate or sulfide will dissolve a steel tap. Which is it, aluminum sulfate or aluminum sulfide?

I have searched this site and PM and could not find the recent thread in which someone said the dissolved a tap out of an aluminum plate.

This time I will write it in my MH book.

Evan
07-20-2009, 05:09 PM
That will take a bit longer than forever. Use battery acid instead. The thread you are probably thinkg of was about anodizing. An andodizing bath will remove steel from aluminum in a hurry. In fact, if you mix up half and half battery acid with water and line a plastic container with aluminum foil then you can do the same. Hook up the part to the positive side of a battery charger or even a car battery and the negative to the aluminum foil. Put the part in the bath with some sort of spacers so it doesn't touch the foil. Do this out side, you don't want even a trace of acid fumes in the air where your machines are. They will turn to rust overnight. Be sure to wear safety glasses when using acid and always add acid to water, never water to acid.

If you are doing a plate you could likely simplify this a lot. Use modeling clay and build a dam around the hole with the tap and plug the bottom of the hole if there is one. Place a few lumps of clay near the hole and put an aluminum plate on them with a wire attached. Hook up the positive to the work piece and connect the negative to the small piece of aluminum on the clay lumps over the hole. Pour in just enough acid/water solution to fill the dam so it touches the cathode. Check in about half an hour to an hour depending on the size of the tap.

When you are finished neutralize the acid with baking soda and either pour on the ground or down the drain. It's harmless.

Carld
07-20-2009, 07:44 PM
Thanks Evan.

JCHannum
07-20-2009, 08:03 PM
The actual chemical in question is plain alum, aluminum sulfate. It is slow, but it does work. It works best when heated.

The problem with either acid or alum is in getting them to flow into the hole, the tap will pretty well plug the hole, making it difficult to get either chemical into contact with the tap to dissolve it.

BobWarfield
07-20-2009, 08:04 PM
http://www.homemodelenginemachinist.com/index.php?topic=270.0

Alum is correct.

BW

DR
07-20-2009, 08:16 PM
Many tooling houses sell kits of acid and clay to do this. Not sure what the acid is, nitric maybe?

I bought one to take a tap out of an expensive 7075 part. The tap was the forming type, with a TIN coating. There was some doubt whether it would work well in that case.

Following the instructions, there were a few bubbles at first, then nothing. After the better part of a day there was no noticeable effect on the tap. And, I had refreshed the acid several times.

This was for the birds, you could grow very old waiting for the tap to disappear.

Just about ready to toss the whole thing, I whipped out my industrial grade heat gun. Blasting the part way beyond the comfortable-to-touch heat range the acid starting really doing some business. The tap was almost totally gone in about 20 minutes.

Duffy
07-20-2009, 08:38 PM
Many tooling houses sell kits of acid and clay to do this. Not sure what the acid is, nitric maybe?

I bought one to take a tap out of an expensive 7075 part. The tap was the forming type, with a TIN coating. There was some doubt whether it would work well in that case.

Following the instructions, there were a few bubbles at first, then nothing. After the better part of a day there was no noticeable effect on the tap. And, I had refreshed the acid several times.

This was for the birds, you could grow very old waiting for the tap to disappear.

Just about ready to toss the whole thing, I whipped out my industrial grade heat gun. Blasting the part way beyond the comfortable-to-touch heat range the acid starting really doing some business. The tap was almost totally gone in about 20 minutes.
As a matter of interest, the RATE of the acid attack will approximately DOUBLE for each 10* centigrade, (18* F,) increase in solution temperature. So it is a lot quicker warm than cold. Duffy

Evan
07-20-2009, 10:15 PM
Mix in one drop of liquid dish soap and there will be no problem getting the acid to flow into the hole.