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Glyceregia: What's the Glycerol for?

  • 1.  Glyceregia: What's the Glycerol for?

    Pittsburgh Chpt Admin
    Posted 01-21-2022 10:02
    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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  • 2.  RE: Glyceregia: What's the Glycerol for?

    Posted 01-22-2022 00:51
    Stephen,
    I probably only used this etchant a handful of times in a career, as I dealt quite rarely with superalloys and super stainlesses.  But here is my understanding.  In general the carrier is chosen to regulate the activity / dissociation of the active ingredients.  For instance, alcoholic etchants will vary in their effect if they are mixed in isopropyl < methanol < ethanol as I recall.  You have to choose between the strength you want and the drawbacks, which are expense and difficulty of handling (ethanol is usually taxed/regulated, methanol is more toxic), what the industrial experience has been, stability, and so on.

    The glycerol I believe acts to allow slower and therefore more controllable etching than aqua regia with the same acids.  However glycerol is a hydrocarbon and glyceregia is known for having a pretty short shelf life, and the references refer to it needing to be used quickly and even undergoing violent reaction which is temperature dependent.

    I hope this is of value.
    Paul Tibbals



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  • 3.  RE: Glyceregia: What's the Glycerol for?

    FASM
    Posted 01-23-2022 13:49
    Paul - I believe you're on the right track. The related "mixed acids" etch uses glacial acetic acid to dilute the aqua regia. I suspect that it (like glycerol) also helps to bind water (these etches do not tolerate water, and even a humid day can make them unpredictable). 

    --
    John Grubb



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  • 4.  RE: Glyceregia: What's the Glycerol for?

    Posted 01-22-2022 00:55

    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
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  • 5.  RE: Glyceregia: What's the Glycerol for?

    Pittsburgh Chpt Admin
    Posted 01-24-2022 09:05
    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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  • 6.  RE: Glyceregia: What's the Glycerol for?

    Posted 01-24-2022 09:41
    Is there a particular brand of toilet bowl cleaner? I'm just curious what the main active ingredient would be.

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    Ken Kirby
    Snap-on, Inc.
    Kenosha WI
    (262) 748-3836
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  • 7.  RE: Glyceregia: What's the Glycerol for?

    Posted 01-24-2022 13:06
    I think he's just looking for an alternate source of the HCl, though as there are other ingredients in toilet bowl cleaners generally you are now in the realm of research!  Actually you can find somewhat less pure HCl than lab grade sold with the pool cleaning / concrete etching supplies as Muriatic acid.

    As long as we're discussing hiding etchants away, I can attest to having used methanol rather than ethanol in the general purpose picral and nital mixtures.  But don't store nital, particularly the methanol version, and even more particularly in the higher 10% mix, in a tightly capped bottle.  The nitric acts to slowly oxidize the alcohol apparently and it builds up gas pressure.  When kept in plastic bottles you will see bulging, and in a glass container will eventually burst it.  Of course that will be contained in your secondary containment where your etchants are stored safely apart, right?

    There are lots of wetting agents out there that are in etchant recipes.  I believe we used a photographic process addition to one of ours.  These were in the old recipe lists that are in metallographic texts.

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    Paul Tibbals
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  • 8.  RE: Glyceregia: What's the Glycerol for?

    Pittsburgh Chpt Admin
    Posted 01-24-2022 14:17
    I am afraid I cannot share the brand of toiler bowl cleaner; that's top secret intellectual property! Ha ha!

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    Stephen Rooney
    R&D Metallurgist
    Ellwood Materials Technologies
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  • 9.  RE: Glyceregia: What's the Glycerol for?

    Posted 01-24-2022 10:29
    It acts as a wetting agent on the polished surface. I used combination of Hydrochloric acid, Nitric acid and Hydrofluoric acid. It was made in very small amount and was discarded immediately after use, the reagent was used in the past to etch Ni, Nb, and some R&D super alloys which we developed for Injection molding barrels.

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    Sanjay Kulkarni
    Materials Engineer
    MSSC
    Troy, MI
    248-840-1056
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  • 10.  RE: Glyceregia: What's the Glycerol for?

    W Michigan Chapter Adm
    Posted 01-25-2022 08:37
    You say you used glycerol with a HCl/Nitric/HF etch? Do you mind sharing the proportions? We use Keller's and Tucker's etch (both are HCl/Nitric/HF) on wrought aluminum at my lab and I am always interested in ways to improve our efforts. Thanks!

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    David Betz
    Sr. Laboratory Engineer
    Hydro Aluminum Metals, USA
    dbetzasm@gmail.com
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  • 11.  RE: Glyceregia: What's the Glycerol for?

    Posted 01-26-2022 10:06
    Hello David,
    I used the above combination in equal parts for Super Alloys like Inconel 600, and other patented super alloys. For Aluminum, I used Sodium Hydroxide + Water  (90:10 mix) to keep it simple. I used Keller's reagent in the beginning and did not observe any difference in microstructure, mainly I was interested in grainsize.
    One thing  I wanted to try was, to use Alconox solution in ultrasonic bath. Immerse polished mount in bath and  and keep cleaning for 10-15 minutes. I think it will bring out the grain structure. I know it does good job on Magnesium alloys, I used that all the time for TTMP fine grained alloys.
    Regards,

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    Sanjay Kulkarni
    Materials Engineer
    MSSC
    Troy, MI
    248-840-1056
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  • 12.  RE: Glyceregia: What's the Glycerol for?

    W Michigan Chapter Adm
    Posted 01-27-2022 10:44
    Sanjay,

    1-10% NaOH is not a bad macroetch, but for wrought aluminum I find it more typically used to identify seam welds and detect inflow. It isn't as "safe," but Tucker's etch does a much better job at developing grains:  640 ml HCl (38%); 320 ml HNO3 (70%); 40 ml HF (49%).  Of course, neither are a good etch for microscopy.
    (Tuckers on Left, NaOH on Right)

    When I need to use a microscope, I have used Keller's on occasion, but without much success (although I don't have a lot of experience with it). Typically, labs (including ours) seem to like electrolytic methods (Barker's) and polarized light. For aluminum grains, I prefer Keller's followed by color tint with Weck's, when I can. Can be tricky to get to turn out well, but worth it.


    I am intrigued by the Alconox solution. Might have to try at some point. Does the bath need to be heated?

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    David Betz
    Sr. Laboratory Engineer
    Hydro Aluminum Metals, USA
    dbetzasm@gmail.com
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  • 13.  RE: Glyceregia: What's the Glycerol for?

    Posted 01-27-2022 15:39
    Ultrasonic Cleaning Method for Etching: I don't remember. It has been many years since I used that method. You can try both ways. I had it written down in company manual, but I don't have it with me. I am working with steel alloys after spending many years in non ferrous alloys :)
    Regards,

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    Sanjay Kulkarni
    Materials Engineer
    MSSC
    Troy, MI
    248-840-1056
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  • 14.  RE: Glyceregia: What's the Glycerol for?

    Posted 01-27-2022 16:28
    I used the Glyceregia few times during the last year for etching austenitic stainless steels and nickel alloys. I had also the same question in my mind about the role of the glycerol. We tried different mixtures of the hydrochloric acid, the nitric acid and the glycerol. From these trials, it became clear that the HCl is playing the major role to obtain a smooth and uniform etch (Figures 1). If the level of HNO3 is higher than that of the HCl, the microstructure looks pixelled, and etch becomes less smooth or and less uniform (Figure 2). I believe that the role of glycerol is a corrosion inhibitor, it is used to relax the reaction with the metal surface, giving more control on timing of the process. The higher the level of the glycerol the longer the time to achieve a proper etch. It took more than 30 minutes to partially etch SS 304 when glycerol was 50% of the mixture. When we replaced the glycerol with some H2O2 to shorten the etching time, the etch took only few seconds and was uncontrollable, but at the same time different details of the microstructure were revealed such as twin boundaries, and some etch pits also appeared (Figure 3 and Figure 4). The Glyceregia etchant, in my experience, does not spread well on the surface, it lacks wettability with the steel surface, compared with Nital for example.  I hope this helps. 


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    Waleed Khalifa
    Principal and CEO
    Arabic Consultancy Center for Engineering Materials, Inspection
    Maadi, Cairo
    Egypt
    01098163293
    accmiw@ymail.com
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