Electropolishing and cleaning stainless steel and aluminum

[Evan]

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.

[Walter]

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.

[doctor demo]

Just seeing if I can bump the counter up to 43.

Steve

[Evan]

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.

[dp]

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.

[kmccubbin]

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

Kerry

[Evan]

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.

[barts]

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

[macona]

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.

[whitis]

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/6835300/description.html

Here is a listing of a large number of electropplishing recipes.

http://www.fischione.com/product_support/model_110_application_notes.asp

Caswell plating's take on why they don't offer electropolishing kits:

http://forum.caswellplating.com/metal-polishing-questions/1221-anyone-here-do-electro-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/internalops/fod/oc/600-699/655-6.htm

Perchlorite solutions are even worse.

[motorcyclemac]

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.

[Evan]

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.

[SDL]

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

[Evan]

Yes, threads should be protected. The process depends on the fact that the pointy bits are eroded faster than the flat areas. The large initial current is due to the many microscopic burrs and edges that are present from previous machining operations.

Protecting threads is easy as they can be wrapped with electrical tape for OD threads or plugged with a plastic threaded plug for ID threads. Since the process is a form of electroplating the action is mostly line of sight to the cathode and the closer it is the faster the reduction in high points. As long as the suface area to be treated is visible it will be acted on by the process.

In the case of a firearm virtually no erosion will take place in the ID of the barrel or cylinder of a revolver. Some rounding of OD corner detail is likely and engraving could be obliterated. Serial numbers should be masked.

[whitis]

I have no intention of patenting anything. Patents are a goldmine of information though.

The question wasn't whether you were going to patent it but rather whether someone else already has and whether the patent still holds. Patents can be a wealth of information ... that you can't use. Anyone here who uses the process in any commercial capacity needs to be concerned about the patents. And unfortunately, the stuff that is likely to be still covered by patents is the newer stuff that meets modern environmental standards.

Propylene glycol MSDS: "Whatever cannot be saved for recovery or recycling should be managed in an appropriate and approved waste disposal facility. Processing, use or contamination of this product may change the waste management options. State and local disposal regulations may differ from federal disposal regulations. Dispose of container and unused contents in accordance with federal, state and local requirements. "
Of course, the MSDS for water might say the same thing. It also says:
"When released into the soil, this material is expected to readily biodegrade. When released into the soil, this material is expected to leach into groundwater. When released into water, this material is expected to readily biodegrade. When released into the air, this material is expected to be readily degraded by reaction with photochemically produced hydroxyl radicals. When released into the air, this material is expected to have a half-life between 1 and 10 days. "

http://www.jtbaker.com/msds/englishhtml/p6928.htm

Another MSDS for Propylene Glycol gives instructions to treat a land or water spill like other hazardous wastes:

http://www.google.com/search?&q=propylene%20glycol%20disposal

Yet says it is not a hazardous waste.
This one says that aqueous solutions under 95% are don't have a flash point but that over 22% flamable vapors are produced when heated. And the flash point if over 95% is near the boiling point of water.

http://www.ppe.com/msds/Propylene%20Glycol.pdf

But MSDS tend not to tell you how you can really dispose of stuff.

There has been at least one case of a small airport being prevented from discharging into the sanitary sewer system. In other places, it is allowed:

http://www.cdphe.state.co.us/hm/hhw/howto/antifreeze.htm

Propylene glycol doesn't sound all that bad but disposal is still a little ambiguous even for the pure form, let alone mixtures.

I had the suspicion that glycerin may work and apparently this is the case, though most of the articles are pay to view. Nitric/sulfuric acids and glycerine, though, can make nitroglycerin. May not patented unless you are electopolishing semiconductor wafers aided by ultrasonics or using an acid other than phosphoric: http://www.faqs.org/patents/app/20080213995

http://www.freepatentsonline.com/EP0941373.html

http://www.patentstorm.us/patents/5096550/description.html

http://www.patentstorm.us/patents/7501051/claims.html

The second of these, incidentally, involves polishing carbide cutting tool inserts. The fourth one patents certain rather odd concentrations. Glycerine is a waste product of biodiesel - unfortunately in that case it is often contaminated with methanol. Apparently, adding glycerin to the sanitary sewers can substantially increase methane bio-gas production to the point that at least one treatment plant deliberately adds it. Also is compostable.

http://www.journeytoforever.org/biodiesel_glycerin.html

Phosphoric acid might be useable (after pH correction?) in composting or soil additive as phosphorous is a critical soil nutrient. At least in small quantities that can't leach into waterways (phosphates cause trouble). If some soap is used, a potassium based soap would be preferable to a sodium one but the quantities are very small. Phosphoric acid is also used to separate glycerin in biodiesel so there is some information on phosphoric acid/glycerin combinations:

http://www.journeytoforever.org/biodiesel_glycsep.html

"In the electropolishing of aluminum and for coloring by electrochemical means, glycerin used as an inhibitor prevents excessive etching and helps produce a smooth, white surface."

http://www.sdascience.org/docs/Uses_of_Glycerine.pdf

Thing is even if the ingredients are safe, when you mix acids, organics, and metals and apply electricity, who knows what other compounds you might produce?

There can be some surprise contaminants of alloys. Cobalt-60 can be a contaminant of some stainless steel (particularly from a particular plant in india, and possibly 4 others) or in irradiated metals. Otis elevator had to replace the buttons in 600 elevators. Oak Ridge discusses eliminating from electopolishing solutions:

http://www.osti.gov/bridge/servlets/purl/211395-F8y1hE/webviewable/211395.pdf

Lead is an additive in some free machining steels.

Drain disposal of laboratory Quantity Chemicals to the Sanitary Sewer (1 liter/day max):

http://www.ehs.uconn.edu/Chemical/Lab%20Drain%20Disposal.pdf

"Solutions containing any amounts of Arsenic, Barium, Cadmium, Chromium, Lead, Mercury, Selenium, Silver, Copper, Nickel, Osmium and Zinc should not be discarded into the sanitary sewer system."
An electopolishing solution would probably contain serveral.

Quantity, of course, makes a big difference. What in small quanities could be beneficial in large quantities could be bad news. Soil normally contains a variety of elements, including those metals likely to be found in an electropolishing solution.

http://www.answers.com/topic/soil-chemistry

Anyone who could come under scrutiny by the EPA or other agencies would do well to dot their i's and cross their t's.

But you might have the beginnings of a fairly green process here.

Detail proceedures for an electopolishing solution for stainless steel using glycerin, phosphoric acid, and sulfuric acid:

http://psfcwww2.psfc.mit.edu/esh/epolish.htm

http://www.metalast.com/documents/Technical_Bulletins/Processes/TB-SS.pdf

Electropolishing stainless steel micro-needles with "in a solution containing water, phosphoric acid and glycerin in a ratio of 1:3:6"

http://aiche.confex.com/aiche/2006/techprogram/P59329.HTM