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  • 1.  Electroetching Probe Selection

    Posted 10-12-2020 16:59
    Hello All,
    I am trying to figure out the reasoning behind the selection of electrode materials used in electrolytic etching. I know that stainless steel and platinum are the most common materials used as the electrodes, but I am wondering why. I assume it has something to do with electronegativity to ensure the electrodes are more "noble" than the sample to be etched, but maybe it has more to do with corrosion resistance?
    Basically my goal with the question is to ensure I am using an appropriate material for electroetching nickel based alloys while keeping a reasonable price point for the electrode materials. I am also interested in understanding the reasons for electrode material selection.
    Thanks!

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    Alex
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  • 2.  RE: Electroetching Probe Selection

    Posted 10-16-2020 08:49
    ​@George Vander Voort - would you be able to assist in this inquiry??

    Regards- Carrie​

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    Carrie Hawk
    ASM International
    Community Engagement Specialist

    440-338-5497
    carrieh@asminternational.org
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  • 3.  RE: Electroetching Probe Selection

    Posted 10-16-2020 11:42
    Alex:
    Your message shows that you are aware of why noble metals like platinum and stainless steels (austenitic and ferritic) are used as the anode when doing an electrolytic etch.  Noble metals have the advantage of creating an anode that works well and also resists corrosive attack if not cleaned off after use.  I have used these metals all my career in metallography.  When I worked for Bethlehem Steel in the Bethlehem Plant and at Homer Research Labs, I did not do much electrolytic etching.  But, when I joined Carpenter Technology Corporations research lab, I started using electrolytic etching due to the alloys being made.  When I worked at Buehler Ltd I also used electrolytic etching for certain metals and alloys.  The etch results were always very good and uniform.

    George Vander Voort

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    George Vander Voort FASM
    Principle
    Vander Voort Consulting L.L.C.
    Wadsworth IL
    (847) 623-7648
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  • 4.  RE: Electroetching Probe Selection

    Posted 10-16-2020 13:27
    Edited by Alex Woodman 10-16-2020 13:42
    George,
    Thank you very much for your input, it is helpful. If I may ask a bit more, I would like to know if there are any particular advantages to using a platinum anode, aside from greater nobility. Basically, is there any effective difference between an etch made using platinum vs. 316 aside from their price?
    Thank you again,

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    Alex Woodman
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  • 5.  RE: Electroetching Probe Selection

    Posted 10-16-2020 17:27
      |   view attached
    Dear Alex:

    I have never had a problem using a SS anode.  I often electrolytic etch austenitic stainless steels using aqueous 60% nitric acid at no more than 1.5 V dc for 1 to 2 minutes. I have attached a PPP showing etch results.

    Best wishes,

     

    George F. Vander Voort

    (1) 847-623-7648

    Consultant-Struers Inc.

    Vander Voort Consulting LLC

    http://www.georgevandervoort.com/" style="margin: 0">www.georgevandervoort.com 





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  • 6.  RE: Electroetching Probe Selection

    Posted 10-19-2020 03:01
    Relating to the electrode composition question, at my lab the practice was to use platinized titanium in some situations and stainless in others.  My understanding of the general process is that the EMF of the electrolytic process is adding to the driving force of the corrosion process.  The composition of the etchant influences the half-cell reaction occurring at the sample surface, and experimentation has resulted in the use of the various etchants to give the desired effect, whether general corrosion, accelerated attack at the grain boundaries, staining effects, and so on.

    The EMF generally will act to be protective at the cathodic end, though depending on pH, ions present, prior passivation, applied voltage, and surface condition, corrosion might be possible at that side.  The effects within the solution from changes in the half-cell reaction at the cathode end might vary with the electrode composition though I have not ever experimented or researched this.

    So, I'd recommend doing some tests.  We did not do large amounts of electrolytic etching and I do not ever remember replacing any of the electrodes we used, either stainless or otherwise.  Handle it like a corrosion test - measure the weight of the electrodes to the microgram before and after known amount of current flow.  Also make sure your microstructural results are identical by using replicate samples and doing image analysis.



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