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Electropolishing and cleaning stainless steel and aluminum

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  • SDL
    replied
    Originally posted by Evan
    It will change dimensionally but I highly doubt that it is enough to worry about. I wouldn't expect to see a change of more than a couple of tenths at the most.
    We use electro polishing a lot on stainless at work via subcontract polishers like these http://www.anopol.co.uk/electropolishing.htm to produce these UV Systems http://www.hanovia.com/products/pmt.aspx.

    The process is capable of halving the surface finish but when doing complex pipe shapes electrodes are required to get to the inside surfaces.

    Threads can be a problem as they can reduce in size dramatically if small compared to the main mass, i.e. an M5 stud on a big tank.

    If doing welded fabrications they are normally pickled in a acid tank overnight before going into the e.pol tanks

    Steve Larner

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  • Evan
    replied
    I have no intention of patenting anything. Patents are a goldmine of information though. In the case of the patents I found on this subject the writer tried to obscure the chemicals used by referring to them by the least often used chemical name for the product. As example the propylene glycol was referred to as a propylene oxide derived polyol. The dish soap was called out as a fatty acid ester from lauryl sulphate.

    I have used propylene glycol because it is non toxic and is not regulated so far as disposal is concerned. Neither is phosphoric acid and both ingredients are available in food grade quality. The only issue at all is the small amount of dissolved metals from the levelling process. Most of that should be plated out of solution on the cathode. The primary dissolved metal will be iron and that is also not regulated as many water supplies contain significant amounts of iron. That leaves very small amounts of nickel and chromium. Only the chromium is of interest and then only if it is present in hexavalent form.

    The reactions should produce trivalent chromium which is an essential trace element for plants and humans. The solution can be poured out on the ground without harm.

    Propylene Glycol antifreeze is readily available at any RV dealer for use in protecting the potable water supply lines.

    Mac,

    It will change dimensionally but I highly doubt that it is enough to worry about. I wouldn't expect to see a change of more than a couple of tenths at the most.
    Last edited by Evan; 11-29-2009, 02:06 AM.

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  • motorcyclemac
    replied
    Ooooh... I think you may have just shown how to save me a LOT of time with stainless steel revolvers. I might just have to try this out. It would clean and polish all the hard to get parts.

    Evan...have you measured the material? Does it change it dimensionally?

    Cheers
    Mac.

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  • whitis
    replied
    Keep it up, this is interesting.

    You mentioned some patents but didn't give patent numbers or whether they are expired and whether your version would be covered.

    Increasing the water may be a bad way to reduce the current (better done with current regulation or lower voltage) as it may lead to pitting instead of polishing.

    One thorny issue is solution maintenance/disposal. Ideally, you want to be able to measure the solution so you can maintain the concentrations, use your successive rinse water baths to replace evaporation for each previos stage (i.e. shooting for zero waste water emissions), precipitate out the metal and separate the components or render them harmless when the solution gets old. Unfortunately, all the metal probably does not plate out on the cathode. But it seems you have a safer solution to use than most.

    Here is a non-expired patent using salt and antifreeze:
    http://www.patentstorm.us/patents/68...scription.html

    Here is a listing of a large number of electropplishing recipes.
    http://www.fischione.com/product_sup...tion_notes.asp

    Caswell plating's take on why they don't offer electropolishing kits:
    http://forum.caswellplating.com/meta...polishing.html

    Hydrogen gas produced can lead to an explosion hazard. Usually it blows away but if you are polishing the inside of a tank...
    http://www.hse.gov.uk/foi/internalop...-699/655-6.htm
    Perchlorite solutions are even worse.

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  • macona
    replied
    Interesting. I have a little electropolishing setup from Tektronix and it does have a heater to heat the solution. There are probably a lot of different recipes out there. Everything I have heard is the stuff is pretty nasty.

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  • barts
    replied
    Great post Evan - thanks!

    Keep in mind that most antifreeze in the States is ethylene glycol rather than propylene glycol. The former is toxic to felines and attractive to them as well, so kitties beware! The latter is generally marketed as "green" antifreeze.

    - Bart

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  • Evan
    replied
    I will be running some more experiments with this. I want to see what effect is has on other alloys of steel. I also want to do some more tests on aluminum. I will keep it updated as I have results to report.

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  • kmccubbin
    replied
    Add me to the interested list! I've been waiting impatiently for the update.

    Kerry

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  • dp
    replied
    Just so you know, I'm very interested and have dropped your post into my local folder. I just didn't have anything to add - the original post was complete, shovel ready, etc.

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  • Evan
    replied
    I am really surprised so few people seem interested in this. I recall quite a few comments on this subject in the past.

    Anyway, I did some more testing to day.

    Number one. I think it would be a good idea to greatly reduce the dish soap content, perhaps to less than a gram per litre. It produces a lot of very tough and very dense fine foam that poses a problem overflowing the container.

    2: You do not need a heater. The main problem is keeping the solution cool, not hot. When the part is first dunked it will pull very heavy current unless some sort of current limiting is used. I think a panel or board with a set of headlights might be a good idea.

    Another way to reduce the current draw at the expense of longer treatment times is to reduce the acid percentage in favour of more water.

    I ran a part today that was a pretty good test. It's a brushed stainless steel electrical faceplate. I was curious if the process would be able to remove the fine scratches of the brushed finish. It can and leaves a nice finish of it's own. It isn't mirror bright but it is a very fine satin almost a mirror finish.

    I intentionally left out one ingredient from the bath formula and that may be required to achieve a true mirror finish. That is 1/2 part of sulphuric acid which I didn't want to use inside the shop. This time of year it is too cold to do these experiments outside so I went with a shop safe formula. Even so this is a very acceptable process for producing an even and good looking finish on stainless steel.



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  • doctor demo
    replied
    Just seeing if I can bump the counter up to 43.

    Steve

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  • Walter
    replied
    Thanks Evan!

    This is particularly interesting to me as we work with alot of stainless and this may well prove to be something we can make use of. We already have a dedicated setup for de-rusting and could easily convert to do this in. At the very least it's something I'm gonna have to play with.

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  • Electropolishing and cleaning stainless steel and aluminum

    I did some experiments this afternoon electropolishing stainless steel. It turns out that it is stupid simple to get a reasonable result. I took information from some patents and deciphered the very clever ploy of using the chemical formulae for dish soap and antifreeze and discovered the ingredients of the bath.

    1 part by volume of 80% phosphoric acid
    1 part by volume of propylene glycol (auto antifreeze)
    1 part water
    1/4 part ethanol or isopropanol
    5 grams liquid dish soap per litre of water

    Use a stainless steel container as the cathode (negative electrode)

    Line the container with one or two layers of plastic window screen to prevent short circuits.

    This bath does not contain acids that are likely to pose a problem to health. Phosphoric acid is non toxic and is not a particularly dangerous chemical. You do not want to get it in your eyes so wear appropriate eye protection. It does not cause a high intensity exothermic reaction if water is added to it but it is still a good idea follow the rule of adding acid to water, not water to acid.

    For these initial tests I used 2 12 volt batteries to supply the power.

    I don't have a lot of pictures of the actual treatment in progress because the process does release some fumes. The fumes are very benign, smelling very mild and mostly like dish washing soap. I ran the test in my garage shop as fumes of phosphoric acid will not cause rust. Phosphoric acid is used for rust conversion on iron. But, I didn't trust having the camera in the vicinity of the fumes for long.

    My test was cut short because I accidentally shorted a piece to the pan causing a pinhole leak. Hence my suggestion to line the pan with some plastic window screen.

    The process draws a lot of current, anywhere from 25 to 150 amps per square foot. Car batteries are the best bet. I will be doing some more experiments and I am going to try using just 12 volts to see what happens. The solution needs to be hot but I discovered that all you need to heat it is to process a part and the current flow will quickly heat up the solution.

    Correct temperature is 150 to 220 F. Nothing seems to very critical about this so feel free to change the formula for the bath or use a different power supply. The part is always positive.

    Here is the setup:



    Some results:None of these were in the bath more than a few minutes. I had a time limit because of company staying here returning from a funeral.

    The results are obvious even after just 30 seconds.

    The last of these images is aluminum and it seems that this bath may work just as well with aluminum as SS. It certainly cleans it in a hurry.






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