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"Magnesium electropolishing and annealing"
Current question:July 6, 2021
Q. Hi there,
I am electropolishing 0.2mm diameter magnesium wire to improve its surface properties as part of my final year university project.
I currently use aluminium cathodes, 5 parts ethanol to 3 parts 85% phosphoric acid electrolyte to polish at 2.5V. It polishes very effectively but I have no idea how to tell if the solution has aged too much.
I have read many incredibly helpful threads on this website that explain how to tell when the electrolyte needs replaced/filtered/decanted but they mostly refer to sulfuric/phosphoric acid baths for steel. As the wire is so fine and I don't have access to an SEM or an AFM, I have no way to tell quantitatively or qualitatively using the wire, when the electrolyte has aged too much or is working less effectively.
Would anyone be able to assist by suggesting ways I could measure if my electrolyte has aged too much for this particular setup? Or just any advice regarding how to preserve the solution to keep optimum polishing conditions as long as possible or even just best practices.
- Glasgow, Scotland
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I am studying the growth of fluoride and oxide films on magnesium and am having a few problems with surface preparation. I am using pure (99.9%) Mg foil and start the processing by cleaning and degreasing.
Two problems 1) Electropolishing seems to always leave the surface pits or undulations. I have tried several different recipes, and have had the best luck with ethanol saturated with sodium thiocyanide, electropolished at constant voltage (35 V) and done at low temp (0 °C). This is still not satisfactory, even by eye (I want a smooth mirror finish). I have good luck electropolish Al with a galvanostatic method, but cannot reproduce with Mg. Suggestions? 2) I would like to quantify the effects of annealing on the grain boundaries. I understand that one can make the boundaries visible via chemical etch but have had no luck finding specifics. Currently I am annealing under nitrogen/hydrogen at 350 ° C for around 1/2 hour.
Any Help is much appreciated!
Thank you,Kevin Hobbs
- Norman, Oklahoma
A. Hello again,
I suppose I am going to partially answer my own question here but I should report that I have improved on the electropolish situation by including a sulfuric acid pickle in my preclean. I am now able to get a mirror finish by eye, although some imperfections are still visible under the optical microscope. Suggestions are still welcome.
Thanks again,Kevin Hobbs [returning]
- Norman, Oklahoma
I have problems in EBSD (electron-backscattered diffraction) analyses of as-cast (partially wrought) Mg alloys. The problem is that the surface is microscopically bumpy after chemical polishing. Mechanical polishing, finished with the OP-S solution, also doesn't work. It seems that it has to be electropolished. I am wondering if anybody has electropolished as-cast Mg alloy. If so, I'd appreciate the polishing conditions including the solution, voltage, and temperature. Thanks a lot.Faramarz Zarandi
McGill University - Montreal, QC, Canada
July 30, 2010
A. Dear Mr. F. Zarandi,
I had the same problem as you mentioned to find out the solution, time and voltage for electropolishing of twin-roll cast Mg alloys. I found the solution of 15% Nitric Acid in Ethanol, 12 V for 10-15 seconds works properly. Bear in mind that the temperature of this electropolishing stage must be as close as possible to -30 °C. This is very important issue: otherwise, due to the heat release during mixing of Acid with Ethanol, explosion might be possible. The 30% Nitric Acid in ethanol might be a good solution for Mg electropolishing but I haven't tried it before.
- London, United Kingdom
January 24, 2012
Q. Hello Sir,
I am doing electropolishing of Mg alloys myself. May I ask you what temperature you use? I am using -45 to -55 °C myself, with 90V using a Strueres Tenupul system.
MSc. Fady Refaat Elsayed
PhD Student, Graduate School of Pure and Applied Science, University of Tsukuba
Research Assistant, Nano-Structure Analysis Group
Magnetic Materials Center
- Tsukuba, Japan
June 20, 2017
I am working on Mg alloy system. I am using orthophosphoric acid and ethanol (3:5 ratio) as electrolyte and SS cathode. I did at 3 V for 30 sec and 3 mins at 1.5V. I am not getting a surface for EBSD.
- Nagpur, Maharashtra, India
Ed. note: Our FAQ on "Finishing of Magnesium" includes an electropolishing formulation which readers may wish to review.