Electropolishing and Passivation

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briphi

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I've only made a few knives for fun, using CPM S35vn, and even when I sand them to 1000 grit on my grinder I can still see small grind lines.

I was just reading about electropolishing, and was thinking I could electropolish them instead of having to buy a buffer. Has anyone electropolished before?

To be clear, I'm not talking about passivation. More like reverse electro-plating. Seems like it should work, but I can't find much info about it here. Anyone doing it?
 
Despite the similar sounding name, it is not the same thing as polishing on a buffer. It will leave a bright micro-pitted surface like fine sandblasting that has been cleaned in muriatic acid.
Also disregarding the dangerous chemicals and industrial equipment that would be needed, it does not make the surface smoother. It removes oxidation and surface colors a well as any contamination from other things that were impacted into the surface in machining and finishing. If there are scratches there before the electro-polishing, they will be there afterward.
 
Electropolishing can be done with citric acid, and does not seem to require 'dangerous' chemicals. Even still, I have lots of experience with the safe handling sulfuric acid. The only other thing required is a DC current source and low voltage.

And wikipedia leads me to believe that it does indeed make the surface of the metal smoother:

"To achieve electropolishing of a rough surface, the protruding parts of a surface profile must dissolve faster than the recesses. This process, referred to as anodic leveling, is achieved by a mass transport limited dissolution reaction. Anodic dissolution under electropolishing conditions deburrs metal objects due to increased current density on corners and burrs."

It should remove upto a thousandth off of all metal surfaces, smoothing out any scratches or imperfections in that layer.
 
I would stick to the buffer method. The electro polisher will be time consuming and make the tool marks bigger than they were before it removes them.
 
It will level the surface to some degree, but only from protrusions not indentations (like a scratch.) It is relying on the current flowing differently off of small whiskers and burrs. It will also cause the tip of the blade to erode faster than the body.

The level of finishing with 1000+ grit paper and compound is much finer than the 1-2 thousandths finishing level cited there. In other words, if you want to remove burrs or surface undulations it will, but also sacrificing the sharpness or any corners or edges on your piece.

I advocate using a vibratory system with corn cob media and polishing compounds. This leaves a uniform polished surface with almost zero elbow grease and won't kill your sharp lines. (Provided good prep to 800-1000 grit prior to polish) From there it is simple to buff up or use micronic paper to get a scratch free mirror shine. Keep in mind in use a high polish will show even the slightest scratch....

Trust me, we all wish there was a magic pot to dump our blades in at 220 grit and pull them out high polished with nice sharp details... Unfortunately automated processes can't descriminate the flats from the sharps like we can with manual processes.

-Eric
 
You may be right, it may not take enough off in a reasonable amount of time to eliminate the grind lines. I may just buy a buffer anyway.

It does look like even a mirror polished surface can have it's corrosion abilities enhanced by electropolish while still keeping the mirror finish. Check out this pdf: http://www.euro-inox.org/pdf/map/Electropolishing_EN.pdf page 9 has some electron microscope pictures of stainless steel surface finishes after having been ground, buffed, and electropolished. You can actually see the crystal structure of the steel in the electropolished part.

I will probably experiment with electropolish just to see what it does. If its useless for knife making, I can always use the parts for anodizing or plating anyway.
 
When we electroplate steel outside corners get thicker plating as plating rate and therefore thickness is greater at those corners.The reverse should be true when electropolishing .
 
So I have done some more research, and it does appear that electropolishing should be exactly what I need. I really don't understand why more people don't try it.

I found one website for instance, where a guy was experimenting in his shop and electropolished a brushed metal stainless steel outlet cover. It removed the brushed scratches and brought it to a dull mirror finish. See here: http://bbs.homeshopmachinist.net/th...ing-and-cleaning-stainless-steel-and-aluminum

Further, I found a book online and free, called "Universal Metal Finishing Guidebook". It can be found here: http://metalfinishing.epubxp.com/title/12238
Electropolishing information begins on page 85. On page 145 it says "Burrs, belt lines, scratches, scales, and other imperfections anywhere on the surface that is immersed and charged will be polished and refined." To me that indicates removing scratch lines is common practice.

I've placed an order for phosphoric and sulfuric acids to prepare my electrolyte bath, as well as some nitric acid which is used as a post process bath to remove the sulfates and phosphates that stubbornly won't wash off with water after the polish. All of the acids were quite easy to get at duda diesel. Meanwhile I'm going to search for a power supply with enough amps to get the right current density.

I'll report back once I make more progress, in case anyone is interested.
 
In case anyone is following along, I have done the following calculations:

For the electropolishing bath, I have choosen to use a solution of sulfuric acid 15%, phosphoric acid 63% as described on page 90 of the aforementioned universal Metal Finishing Guidebook. This requires a current density on the metal of 50 A/ft^2.

Assuming an average knife if 8" long, 1" wide, and 1/4" thick, we get 20.5 inch^2. Converting to feet yields 0.142 ft^2. 50 A/ft^2 / 0.142 ft^2 = 7.1 Amps. So you would need a power supply capable of delivering at least 7.1 Amps of power for the electrolpolishing formula.

On page 86, the book states that the ideal range for electropolishing steel is 9-13V. After much searching and research I have decided to buy the VOLTEQ Power Supply HY1520EX 15V 20A Over Voltage Over Current Protection. Apparently the "EX" line is important for electroplating applications as the over voltage/over current protection is required. They caution not to get the regular "E" line, as I guess you might burn it out. This power supply ran me $140.

The book also recommends no more than 5 amps per gallon of solution, otherwise it will over heat, and even as is it may require cooling. Because of our 7.1 amp requirement, we need at least 1.5 gallons of solution. This means that buying about 1 gallon of phosphoric and 950ml of sulfuric should do the trick (these are common sizes/amounts).

Now I just need to go about buying a tank suitable for electropolish. The book recommends something made out of SS 316L, since the operating temperature of the solution will be between 80 and 175 F, it may start to melt or weaken plastics. Just have to look for something thats about 2 gallons and proportionally tall and narrow.
 
Would be interested to see before and after pics as well. The process of electropolishing removes surface material. It removes the peaks faster than the valleys to smooth out the overall surface finish. Good luck! If you need help don't hesitate to reach out - [Removed]
 
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Very interesting, I to would like to see some before and after pictures.

I think you will have trouble finding a go stainless container in 316L it's usually pretty expensive.

maybe Electropolish.com would consider giving you knife a free "bath" and you can decide from there if its would continuing with?

I do have a few questions though:
-Does this at all change the grain size or the set up of the grain?
-Would it work on carbon steel?
-Would it take away harder metal slower than soft?
Thanks-I'm looking forward to this.
 
Will not change grain size .
Will work on carbon steel and other metals , copper etc.
Electropolishing is used for electron microscope proceedures and it will etch different components differently --martensite vs ferrite vs carbide .
 
I used to work at company that, among other things, electro polished Auminum in preparation for anodization. Here is what I recall: A concentrated mixture of nitric and sulphuric acid heated to about 180 degrees F to which mist reducers were added. A highly agitated bath utilizing compressed air added at the bottom of the tank. A very high capacity exhaust system. Titanium racks to hold the parts. Titanium steam coils to heat the bath. A pit under the tanks made of acid resisting bricks and pitch mortar. Rubber aprons, gloves, sleeves, boots, goggles, an eye wash fountain, and a safety shower. A large solid state rectifier.
The parts achieved a mirror finish after being cold forged in a drop hammer.
 
Generally, when a poster that has a long dissertation on what he is going to try, doesn't come back to tell us how successful he was- it means he wasn't.
 
His only posts were the seven he made in this thread. He last posted nearly a year ago. I would not expect an update.


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