Recovery of Motor #'s

[Kurt S]

Rich Fields (CRG) pulled this together:
The standard for metal etching is covered by ASTM (www.astm.org) E340, Standard Test Method for Macroetching Metals and Alloys. I strongly encourage anyone considering attempting this to obtain a copy of this either from the website (for a fee) or from your local library in Volume 3.01 of the ASTM Book of Standards.

The procedure involves use of acid, and there are obvious safety issues associated with this. Use safety glasses and acid-resistant gloves, read the safety precautions, if diluting - add acid to solvent rather than solvent to acid, be careful of fumes, do this in a ventilated area, etc.

The theory, for those interested, is that the cold-work of the stamping process changes the metal microstructure locally around the imprint, sufficiently so that there is a resulting difference in resistance (increase or decrease) to acid attack in the portion that has been cold-worked. If the original stamp is removed, the material underneath retains this change in microstructure, which is not visible to the naked eye (until being revealed through etching).

There is a limit to how much material can be removed during the decking process before etching will not reveal the original stamp. Obviously the deeper the current surface is below the original surface the less distinct any characters will be.

For illustration, here are the approximate steps for cast iron:

1) Prepare or purchase etching solution. There are numerous types of etching solutions. The "general purpose" solution in the standard for iron and steel is a 50/50 mix of concentrated hydrochloric acid and water. An easily obtainable close approximation of this is swimming pool muriatic acid, available from swimming pool and home improvement stores, usually already in a diluted form. (Note: a 160F-180F solution is suggested in the test method but I strongly recommend avoiding use of a heated acid solution in the garage. A room-temperature solution will work - but will take a little longer.)

2) Remove all grease, dirt, and paint from area to be etched.

3) Mask off portions of the casting that you don't want etched. You don't want the etching solution washing down your cylinder bores!

4) Clean the pad with solvent and/or cleaning solution. "Any grease, oil, or other residue will produce uneven attack."

5) Saturate a cotton ball held in tongs with the etching solution, uniformly swabbing the area of interest. Wait for the "structure to be developed" (characters to appear). Since the estimate development time is 15-30 minutes you may have to repeat the swabbing periodically to keep the surface uniformly wet with etchant. When the characters become visible or it becomes obvious that they won't, you can stop.

6) Rinse the casting with hot water to remove the etchant.

7) If necessary, "desmut" the surface by vigorous scrubbing with a vegetable fiber brush.

8) Blow the casting dry with compressed air.

[Edited by Kurt S (02-19-2002 at 11:18 AM).]