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Metalworking (rec.crafts.metalworking) Discuss various aspects of working with metal, such as machining, welding, metal joining, screwing, casting, hardening/tempering, blacksmithing/forging, spinning and hammer work, sheet metal work. |
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Electropolishing
I've been experimenting with electropolishing stainless steel with
limited success. I've tried various electrolytes - hydrochloric acid (not good), citric acid (better, but still etches my part), and Phosphoric acid (more aggressive than citric, but still does not polish well). I have tried various current densities, but still get the same results - light frosting (sort of like a fine bead blast finish), but no bright polished finish.I'm using anodic polarity with the part to be polished connected to the power supply (+) and the cathode (also stainless) connected to the (-) terminal. I have looked at the finishes under a microscope and can see the reason for the lack-luster finish. I have also read several postings, etc. where various blends of sulfuric and phosphoric acids and am wondering if this is my problem. Does anyone have any experience with electrolytes used to electropolish? Any hints would be greatly appreciated. BTW, the stainless I'm working with is 303. Thanks. Dan |
#2
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Electropolishing
I did a search under lycos for "electro polishing chemicals" and got a lot
of returns. Finishing.com seems to be a bit of a clearing house for finishing products. Others are chemical suppliers. Reinventing the wheel?? Might be easier to just source the correct stuff. Respectfully, Ron Moore "DanD" wrote in message ... I've been experimenting with electropolishing stainless steel with limited success. I've tried various electrolytes - hydrochloric acid (not good), citric acid (better, but still etches my part), and Phosphoric acid (more aggressive than citric, but still does not polish well). I have tried various current densities, but still get the same results - light frosting (sort of like a fine bead blast finish), but no bright polished finish.I'm using anodic polarity with the part to be polished connected to the power supply (+) and the cathode (also stainless) connected to the (-) terminal. I have looked at the finishes under a microscope and can see the reason for the lack-luster finish. I have also read several postings, etc. where various blends of sulfuric and phosphoric acids and am wondering if this is my problem. Does anyone have any experience with electrolytes used to electropolish? Any hints would be greatly appreciated. BTW, the stainless I'm working with is 303. Thanks. Dan |
#3
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Electropolishing
The Metals Handbook, Vol 2,"Heat Treating, Cleaning and Finishing", 8th
edition gives a couple of recipes: #1 sulphuric acid 41% phosphoric acid 45% water 14% operating temp: 170-230F current density: 200-350 A/sq.ft. #2 phosphoric acid 56% chromic acid 12 % water 32% operating temp: 80-175F current density 100-1000 A/sq.ft. All percentages by weight. You might want to get a copy of the book. It contains articles on electropolishing and every other finishing method you could want. Copies are available on the used book sites Randy "DanD" wrote in message ... I've been experimenting with electropolishing stainless steel with limited success. I've tried various electrolytes - hydrochloric acid (not good), citric acid (better, but still etches my part), and Phosphoric acid (more aggressive than citric, but still does not polish well). I have tried various current densities, but still get the same results - light frosting (sort of like a fine bead blast finish), but no bright polished finish.I'm using anodic polarity with the part to be polished connected to the power supply (+) and the cathode (also stainless) connected to the (-) terminal. I have looked at the finishes under a microscope and can see the reason for the lack-luster finish. I have also read several postings, etc. where various blends of sulfuric and phosphoric acids and am wondering if this is my problem. Does anyone have any experience with electrolytes used to electropolish? Any hints would be greatly appreciated. BTW, the stainless I'm working with is 303. Thanks. Dan |
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Electropolishing
"R. O'Brian" wrote:
The Metals Handbook, Vol 2,"Heat Treating, Cleaning and Finishing", 8th edition gives a couple of recipes: #1 sulphuric acid 41% phosphoric acid 45% water 14% operating temp: 170-230F current density: 200-350 A/sq.ft. #2 phosphoric acid 56% chromic acid 12 % water 32% operating temp: 80-175F current density 100-1000 A/sq.ft. All percentages by weight. You might want to get a copy of the book. It contains articles on electropolishing and every other finishing method you could want. Copies are available on the used book sites Randy Thanks Ron and Randy for the helpful info. I think I will try changing my electrolyte to the H2SO4 - H3PO4 mix at elevated temps as suggested by the Metals Handbook. Hopefully it will do the trick. Will report back on my results. Kind regards, Dan |
#5
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Electropolishing
Where do you buy H3P04 in technical grade? - GWE
DanD wrote: "R. O'Brian" wrote: The Metals Handbook, Vol 2,"Heat Treating, Cleaning and Finishing", 8th edition gives a couple of recipes: #1 sulphuric acid 41% phosphoric acid 45% water 14% operating temp: 170-230F current density: 200-350 A/sq.ft. #2 phosphoric acid 56% chromic acid 12 % water 32% operating temp: 80-175F current density 100-1000 A/sq.ft. All percentages by weight. You might want to get a copy of the book. It contains articles on electropolishing and every other finishing method you could want. Copies are available on the used book sites Randy Thanks Ron and Randy for the helpful info. I think I will try changing my electrolyte to the H2SO4 - H3PO4 mix at elevated temps as suggested by the Metals Handbook. Hopefully it will do the trick. Will report back on my results. Kind regards, Dan |
#6
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Electropolishing
www.chemsavers.com or search Ebay for Phosphoric Acid. Chemsavers has
an Ebay Store where I bought 500 mL of reagent H3PO4 for $25 + $7.50 shipping. Grant Erwin wrote: Where do you buy H3P04 in technical grade? - GWE DanD wrote: "R. O'Brian" wrote: The Metals Handbook, Vol 2,"Heat Treating, Cleaning and Finishing", 8th edition gives a couple of recipes: #1 sulphuric acid 41% phosphoric acid 45% water 14% operating temp: 170-230F current density: 200-350 A/sq.ft. #2 phosphoric acid 56% chromic acid 12 % water 32% operating temp: 80-175F current density 100-1000 A/sq.ft. All percentages by weight. You might want to get a copy of the book. It contains articles on electropolishing and every other finishing method you could want. Copies are available on the used book sites Randy Thanks Ron and Randy for the helpful info. I think I will try changing my electrolyte to the H2SO4 - H3PO4 mix at elevated temps as suggested by the Metals Handbook. Hopefully it will do the trick. Will report back on my results. Kind regards, Dan |
#7
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Electropolishing
I've been experimenting with electropolishing stainless steel with
limited success. I've tried various electrolytes - hydrochloric acid (not good), citric acid (better, but still etches my part), and Phosphoric acid (more aggressive than citric, but still does not polish well). I have tried various current densities, but still get the same results - light frosting (sort of like a fine bead blast finish), but no bright polished finish.I'm using anodic polarity with the part to be polished connected to the power supply (+) and the cathode (also stainless) connected to the (-) terminal. I have looked at the finishes under a microscope and can see the reason for the lack-luster finish. I have also read several postings, etc. where various blends of sulfuric and phosphoric acids and am wondering if this is my problem. Does anyone have any experience with electrolytes used to electropolish? Any hints would be greatly appreciated. BTW, the stainless I'm working with is 303. I've done some 304 with phosphoric/sulfuric acid solutions, and with citric/sulfuric acid solutions, and a little 316, but not any 303. Go to google groups and search for "electropolish ijames" and you'll find a long posting I made here giving recipes and conditions. If the solution conductivity or ionic strength isn't right it is really easy to "electrodull", which is what my coworkers call your frosted, dull finish :-). The cathode composition is also important; I get good results most consistently with copper. Here is an excerpt: [quote] However, I have used 50 parts of citric acid and 50 parts battery acid with good results. This solution is supersaturated with citric acid at room temperature so all will not dissove without heating, and some some citric acid will precipitate out each time you cool the solution but it seems to redissolve okay with heating the next time the solution is used. I started with about 1/2 cup of each acid in a small bowl, and then gave the bowl about a minute and a half in a microwave oven which made it too hot to touch but got all the citric acid dissolved. I then electropolished some small pieces, between one and four square inches, and got nice polishing action between about two and 10 amps/sq. in. with about 3 sq. in. of copper sheet as cathode, which was a little on the small side. Below that it really didn't seem to damage the surface, but the polishing was so slow it wasn't worth doing - I got noticeable brightening after about 4-6 minutes at 2 amp/sq. in. and a nicely polished surface after about a minute or two at 5-8 amps/sq. in. Ten amps/sq. in. was okay but the foaming was pretty bad in my litte test bowl. I was also impressed with [endquote] -- Regards, Carl Ijames carl.ijames at verizon.net |
#8
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Electropolishing
I put some info in the dropbox two or three years ago about my
electropolishing experiences. It was pretty easy. I used the electrolyte formula I found in the original process patent. |
#9
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Electropolishing
Thanks Carl. Despite my efforts, I'm still doing electro-dulling! Your
comment about using copper for the cathode might be what I'm missing. I've been using SS for the cathode. I think my lack of success is due to a combination of things. Wrong cathode material, electrolyte out of balance, and low electrolyte temperature. I have however played with various current densities. One other thing I have learned is that by using 308 stainless wire (commonly used in MIG welding) as wire hangars, there must be enough of an iron content to dirty up the electrolyte with a reddish-brown emission. Back to the lab... more later. Thansk to all, Dan "Carl Ijames" wrote: I've been experimenting with electropolishing stainless steel with limited success. I've tried various electrolytes - hydrochloric acid (not good), citric acid (better, but still etches my part), and Phosphoric acid (more aggressive than citric, but still does not polish well). I have tried various current densities, but still get the same results - light frosting (sort of like a fine bead blast finish), but no bright polished finish.I'm using anodic polarity with the part to be polished connected to the power supply (+) and the cathode (also stainless) connected to the (-) terminal. I have looked at the finishes under a microscope and can see the reason for the lack-luster finish. I have also read several postings, etc. where various blends of sulfuric and phosphoric acids and am wondering if this is my problem. Does anyone have any experience with electrolytes used to electropolish? Any hints would be greatly appreciated. BTW, the stainless I'm working with is 303. I've done some 304 with phosphoric/sulfuric acid solutions, and with citric/sulfuric acid solutions, and a little 316, but not any 303. Go to google groups and search for "electropolish ijames" and you'll find a long posting I made here giving recipes and conditions. If the solution conductivity or ionic strength isn't right it is really easy to "electrodull", which is what my coworkers call your frosted, dull finish :-). The cathode composition is also important; I get good results most consistently with copper. Here is an excerpt: [quote] However, I have used 50 parts of citric acid and 50 parts battery acid with good results. This solution is supersaturated with citric acid at room temperature so all will not dissove without heating, and some some citric acid will precipitate out each time you cool the solution but it seems to redissolve okay with heating the next time the solution is used. I started with about 1/2 cup of each acid in a small bowl, and then gave the bowl about a minute and a half in a microwave oven which made it too hot to touch but got all the citric acid dissolved. I then electropolished some small pieces, between one and four square inches, and got nice polishing action between about two and 10 amps/sq. in. with about 3 sq. in. of copper sheet as cathode, which was a little on the small side. Below that it really didn't seem to damage the surface, but the polishing was so slow it wasn't worth doing - I got noticeable brightening after about 4-6 minutes at 2 amp/sq. in. and a nicely polished surface after about a minute or two at 5-8 amps/sq. in. Ten amps/sq. in. was okay but the foaming was pretty bad in my litte test bowl. I was also impressed with [endquote] |
#10
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Electropolishing
On Mon, 31 Oct 2005 16:47:01 GMT, DanD wrote:
,;Thanks Carl. Despite my efforts, I'm still doing electro-dulling! Your ,;comment about using copper for the cathode might be what I'm missing. ,;I've been using SS for the cathode. I think my lack of success is due ,;to a combination of things. Wrong cathode material, electrolyte out of ,;balance, and low electrolyte temperature. I have however played with ,;various current densities. One other thing I have learned is that by ,;using 308 stainless wire (commonly used in MIG welding) as wire ,;hangars, there must be enough of an iron content to dirty up the ,;electrolyte with a reddish-brown emission. ,; I got into this thread late but there probably is enough iron in any stainless to dirty up the electrolyte. 300 stainless steels are the easiest to electropolish. The cathode material shouldn't have much influence. You need to run these at a hellish high current density. I used a mixture of perchloric acid, acetic acid, and acetic anhydride. you won't be able to get perchloric acid and if you could I wouldn't recommend it. Too easy to get into serious trouble if you don't know what you are doing. The chances of you knowing enough about perchloric acid to keep out of trouble are very low. Get copies of the good books on metal finishing. There should be enough information there to get you started. The Metal Finishing Guidebook, ASM International's "Metals Handbook-- Volume 5: Surface Engineering, and (especially) The Electroplating Engineering Handbook. Also I believe there is a "Metal Finishing Handbook". It has been a long time so that title may not be correct. I would suggest using your 308 stainless wire. Get a notebook and record composition of the electrolyte, temperature of bath, voltage, and current density. You can change the current density at constant voltage by changing the length of wire in the bath. Note how the wire looks with time. I would suggest H2SO4 + H3PO4 for the bath. You will need to work at a high temperature. How high? That is why you setup the above conditions for variables. Change one thing at a time and record the results with comments. When you are polishing wire you can run at a high current density without heating up the bath too much from the IR heating. Here are a couple references: ******************************* Electropolishing of 316L Stainless Steel for Anti-corrosion Passivation Authors: Hocheng H.; Kao P.S.; Chen Y.F. Source: Journal of Materials Engineering and Performance, Volume 10, Number 4, August 2001, pp. 414-418(5) Publisher: ASM International full text options Abstract: 316L stainless steel is deemed an indispensable material in the semiconductor industry. In many instances, the surface of the production equipment needs to be treated for low-corrosion passivation, good finish, weldability, and cleanliness. The process characteristics of electropolishing meet these requirements well. The current study investigates the effects of the major processing parameters on the anti-corrosion performance and the surface roughness. The electrolyte with 10% water content and a ratio between H2SO4 and H3PO4 of 4 and 6 has been proven to be successful, showing no corrosion pitting points on the specimen surface. The electrolyte temperature of 85 ± 10 °C and the electrical current density of 0.5 to 1.0 A/cm2 are found to be optimal. The processing time beyond 3 to 5 min produces no further improvement. The addition of 10% glycerin provides a very fine surface (maximum roughness of 0.05 m), while the anti-corrosion performance is deteriorated. The results obtained are useful for the manufacture of the semiconductor equipment. and **************** Source: Transactions of the Institute of Metal Finishing, Volume 83, Number 1, February 2005, pp. 51-53(3) ******************** If you are going to dilute concentrated sulfuric acid use ice cubes to make up the amount of water you want. Pour the acid onto the ice cubes. The solution will be hot but it won't splatter. Wear safety glasses even if you don't think it will splatter. Wear safety glasses when running your electrolysis. You will get a lot of gas evolution and that can spread solution around where you don't want it. Google with "electropolishing stainless solutions" will get you a lot of reading material. Don't forget that you are working with some dangerous chemicals. Be careful, wear appropriate safety equipment, and keep a voluminous supply of water where you can find it without using your eyes. You might need that skill. |
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Electropolishing
Thanks Carl. Despite my efforts, I'm still doing electro-dulling! Your
comment about using copper for the cathode might be what I'm missing. I've been using SS for the cathode. I think my lack of success is due to a combination of things. Wrong cathode material, electrolyte out of balance, and low electrolyte temperature. I have however played with various current densities. One other thing I have learned is that by using 308 stainless wire (commonly used in MIG welding) as wire hangars, there must be enough of an iron content to dirty up the electrolyte with a reddish-brown emission. You're welcome. It's so rare I have anything to contribute here, and I've learned so much over the years. I don't understand it, but I've seen test workpieces (usually scraps of 1/4" OD 304 tubing, about 1-3" submerged, I scotchbrite to dullness and then polish to test the setup each time I turn everything on before going on to the workpiece) go from great to ugly as I change nothing but the cathode from different pieces of 304 stainless steel sheet to copper to lead sheet. The lead and copper "always" work (famous last words :-)), but apparently identical pieces of 304ss work or don't work at random. I wound up splitting two 1' long pieces of 5/8" copper tubing from the scrap bin with some small snips to get two sheets 2"x1', and bent each into an L that stuck up out of the solution and went ~1/3 of the way across the bottom of my work tank. Connected them together with a piece of 8 ga wire and I've been using them for 3 years now. My work tank is a plastic 1/2 gallon ice cream container - nice rectangular shape and big enough for the 1-2" sized stuff I usually do. I used to use concentrated H2SO4-H3PO4 but that stuff is nastier if you get it on you and is hygroscopic (it sucks water out of the air and will gradually overflow the tank). I've been happy with the citric acid-H2SO4 solution once I worked out the best ratio (equal parts battery acid and citric acid powder by volume, heat to dissolve and use warm-to-hot, I usually run at 90 deg C but I have a nice thermostatted water bath that I keep at that temp for other stuff). Also it's easier to get battery acid and citric acid (auto parts store and www.mcmaster.com) than it is to get concentrated H2SO4 and H3PO4. I pretty much always use 308 wire for hangars and I've never seen the solution go reddish brown. It starts out clear and slowly turns green from the chromium (plus bluish from the nickel). Oh, watch out for the hangar wire - you _will_ burn yourself a few times. 304SS is actually pretty close to nichrome, composition-wise, and pushing 5-25 amps down a 1/16" diameter piece of 308 will get it to glow red hot pretty often. It's the times it doesn't glow that get you, it's so easy to forget how hot it gets. ----- Regards, Carl Ijames carl.ijames at verizon.net |
#12
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SS to a mirrorfinish adjusted plasma polishing. Technology based on safe salts. Examples of work technology - Plasma Polishing- Pvdcoaters
Video polishing process - www.youtube.com/c/Pvdcoaters |
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