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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DanD
 
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Default 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   Report Post  
Ron Moore
 
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Default 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   Report Post  
R. O'Brian
 
Posts: n/a
Default 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



  #4   Report Post  
DanD
 
Posts: n/a
Default 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   Report Post  
Grant Erwin
 
Posts: n/a
Default 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   Report Post  
DanD
 
Posts: n/a
Default 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   Report Post  
Carl Ijames
 
Posts: n/a
Default 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   Report Post  
TheAndroid
 
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Default 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   Report Post  
DanD
 
Posts: n/a
Default 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]


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Unknown
 
Posts: n/a
Default 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.





  #11   Report Post  
Carl Ijames
 
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Default 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


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Junior Member
 
Posts: 2
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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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