Thanks for the helpful replies! I can certainly confirm that Glyceregia has a short shelf-life (about 20 min before it starts performing unpredictably). It also has a syrupy quality that is unlike other etchants, so observing safe lab practices of cleaning off samples thoroughly before placing them under a microscope or touching non-lab bench surfaces is slightly more challenging. For all that, it works well.
I have heard something of the line of reasoning that Paul describes in relation to Picric and its alcohol based variant, Picral, for etching of prior austenite grain boundaries. Some people also swear by a methanol based version vs. an ethanol based version and explain that everything etches much more clearly in methanol. They will go so far as to squirrel some away, despite lab safety policies prohibiting its use.
We have found the formulations that work for our purposes, but even speaking with metallographers in different departments I experience the "conclusion" that David is talking about. We get into some good-natured arguments about our various approaches. Fortunately, the proof is in the pudding and we give credence to whoever is able to produce the best micrograph.
As an aside, I did learn that you can substitute toilet bowl cleaner for the HCl in Glyceregia if you happen to be in a pinch. Due to my own negligence, we did not have enough HCL on hand and it seems to be more difficult these days to get chemicals shipped promptly. It gives an interesting pattern effect, likely due to the settling of whatever other chemicals happen to be in the cleaner, and its always fun to see how different ingredients in the same etchant react, being wise about what one decides to mix. That's probably the weirdest ingredient substitution I've done, the other being the classic Dawn dish soap as a wetting agent.
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Stephen Rooney
R&D Metallurgist
Ellwood Materials Technologies
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Original Message:
Sent: 01-22-2022 00:54
From: David Sapiro
Subject: Glyceregia: What's the Glycerol for?
As far as I know, no one really understands the effects of [most] etchants on a fundamental level, as with other forms of localized corrosion in general. Bryan Webler had a student working on this sort of thing while I was at CMU, but I'm not sure if he had any conclusions. If I had to guess, the glycerol is used as a surfactant/wetting agent that disrupts the layer of water molecules against the metal. This may affect the amount of acid in contact with the metal or the metal dissolution rate into the solution.
Even less helpful: my experience in corrosion research is that everyone's conclusion is "I don't understand it, but everyone else is clearly wrong", so you may get some conflicting answers.
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David Sapiro
Senior Structural Materials Engineer
Seattle WA
dosapiro@gmail.com
Original Message:
Sent: 01-21-2022 10:02
From: Stephen Rooney
Subject: Glyceregia: What's the Glycerol for?
Hello!
I used a glyceregia etch for the first time a couple of weeks ago and was impressed by the results when used in a stainless steel. This makes me wonder what exactly the glycerol does when added to the etchant. Does anyone have any insight here?
Thank you!
Stephen
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Stephen Rooney
R&D Metallurgist
Ellwood Materials Technologies
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