This is mostly a repost of my last posting with these recipes, but I
have now tested the citric acid/battery acid recipe so I have updated
those sections.

Well, I finally dug out my electropolishing recipes and typed them in.
I have personally used the Vacuum Technology and citric acid/battery
acid recipes but not the others. Anyway, here's what I have from a
couple of old vacuum technology books and other sources.

From Fundamentals of Vacuum, author unknown, which credits Armco Steel
Corp. for the recipe
Citric acid 50-60 parts by volume
Sulfuric acid 15 parts by volume (64 parts if battery acid)
Water to make 100 parts
Temperatu 85-94 deg C (185-201 deg F)
Current density: 0.5-1 amp/sq. in.
Voltage: 6-12 V
Cathode: copper or 18-8 stainless steel

From Vacuum Technology, by Andrew Guthrie
Water 20%
Sulfuric acid 20%
Phosphoric acid 60%
Temperatu less than 80 deg C (176 deg F)
Current density: a few mintues at 5 amp/sq. in.

From the Metals Handbook, 8th. ed., Vol. 2
Water 14% by weight
Sulfuric acid 41% by weight
Phosphoric acid 45% by weight
Temperatu 170-230 deg F (77-110 deg C)
Current density: 200-350 amp/sq. ft. (1.4-2.4 amp/sq. in.)

From the Vacumetrics, Inc. Electropolisher Manual
Distilled water 22% by volume
Sulfuric acid 15% by volume
Phosphoric acid 63% by volume
Temperatu apparently room temperature or simply self-heated because
their unit does not have a heater

The sulfuric and phosphoric acids listed here are the concentrated
liquids. Concentrated sulfuric acid has approximately 18 moles/liter of
H2SO4 (specific gravity 1.84, 96.0 weight percent) while battery acid is
about 4.25 moles/liter (specific gravity 1.25, 33.3 weight percent) or
4.24 times weaker, which means it's much too weak for use in any of the
phosphoric acid recipes and really a little too weak for the citric acid
recipe. Since other metals and salts will degrade the performance,
always use distilled water in these recipes.

A tremendous amount of heat is liberated when concentrated sulfuric acid
is diluted with water. Always start by adding the distilled water to
the mixing tank, then slowly adding the sulfuric acid with stirring,
making sure the temperature does not rise over about 70-80 deg C
(160-180 deg F). Then add the phosphoric acid the same way. The
concentrated sulfuric-phosphoric acid solutions are hygroscopic. They
will absorb moisture from the air and can actually overflow the storage
tank in a matter of weeks if it is not kept well sealed and it's in a
humid room.

To mix the citric acid recipe using concentrated sulfuric acid, start
with about 20 parts of water in the tank, slowly add the sulfuric acid
with stirring, then add the citric acid, and finally add the remainder
of the water with more stirring. Note that 64 parts of battery acid and
50-60 parts of citric acid add up to 114-124 parts which don't quite fit
into 100 parts, so the battery acid really isn't quite concentrated
enough to make this exact recipe. 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
the "throwing power", or whatever the comparable term is for
electropolishing. Polishing a flat stainless steel sheet held parallel
to the single copper cathode, the front surface polished up nicely but
the back side also got about half as much polishing action as the front,
so with the normal arrangement of cathode plates on all sides and with
just a little stirring around I expect quite uniform polishing action.
There is a lot of heat put into the solution from the current (my guess
is that the voltage drop at the electrode surfaces is maybe two volts
each and I was running at 10-14 volts total so at 10 amps and 14 volts
that puts (14-2-2 volts) * 10 amps = 100 watts of heat into the liquid
so if you warm it up to begin with it will stay nice and hot while you
polish. I should be able to replenish the water lost by electrolysis
with distilled water, but from other experiments I know that if the
sulfuric acid concentration gets too low you tend to get pitting and
dulling of the surface instead of polishing so if that happens I will
use battery acid instead of distilled water to make up the volume.

Citric acid is available from www.mcmaster.com and in the canning or
spice sections of some groceries, and battery acid is usually available
at local auto parts stores. The concentrated acids are hazardous
materials and expensive to ship, and some chemical supply companies
won't sell to individuals.

The solution volume needs to be at least five times the workpiece
volume, and preferably much bigger than that (but the electrode spacing
requirements usually take care of this). Otherwise the gas given off at
the workpiece and electrode causes too much foaming and the solution
heats too much from the current passing through it. The cathode area
needs to be several times the workpiece surface area, and the workpiece
to cathode distance needs to be at least two or three workpiece
diameters. Ideally arrange multiple cathodes all around and under the
workpiece. Otherwise the polishing action will be localized on the side
facing the cathode and will be very nonuniform. Additional stirring
also helps make the polishing more uniform, especially with larger
workpieces. For small pieces just stirring it around by hand, plus the
evolved gases, is enough. A magnetic stirrer hotplate is ideal if you
have one, otherwise a lance across the bottom of the tank with several
small holes drilled into it and fed with compressed air works okay for
stirring but accelerates evaporation and makes the fumes worse. I use a
small water bath because I had one and its thermostatted, and I use a
small polyethylene bowl sitting in the water bath as my polishing tank.
It gets a little soft at 85-90 deg C but I never try to pick it up while
hot.

In the past I've used lead sheeting and stainless steel as a cathode
with the sulfuric/phosphoric acid recipe, and lead with the citric acid
recipe, with good results. With a different citric acid recipe with
much less sulfuric acid, stainless steel gave very poor results as a
cathode. I have not tried stainless steel with this citric acid recipe.
Instead, I split some 5/8" copper tubing scraps and flattened them out
to make 2" wide rectangular strips, bent into "L" shapes, and use at
least one on each side of the bowl so there is cathode surface on
opposite sides of, and below the workpiece. The copper slowly corrodes
if you store it in the solution (I'm sure it is dissolved oxygen from
air causing this) so remove the cathodes for storage.

The power supply can be as simple as a variac controlling the primary of
a transformer, with a bridge rectifer on the secondary connected to the
electrodes. That's what I use. I scrounged a variac, bought a surplus
110 V primary 13.8 V at 20 amps secondary transformer from www.mpja.com
(I don't think that exact one is still available), and a 50 V 50 A rated
bridge also from mpja. Put the rectifier on a heatsink, add a small
muffin fan to blow across the heatsink and then across the tank to carry
the fumes away from me, and clamp an ac current mulitmeter to one of the
leads from secondary to rectifier to monitor the current. Mpja also has
a current shunt and dc meter if you want direct measurement. The fuse
in the variac should protect the tranformer but the rectifer seems to
pop much faster than the fuse when I get careless and let the wires
touch or when I'm trying for too much current. The current densities
and voltage requirements are such that I doubt a simple battery charger
will work well, but feel free to experiment with your charger :-). I
use 308L stainless steel welding rod to make hangars, but pay attention
to the currents and note that the composition of stainless steel is
purty close to that of nichrome. Something like 10 amps will make 1/16"
diameter rod glow red in air, and even the 1/8" stuff will glow and
oxidize pretty quickly at 30-40 amps. The portion that is submerged
will usually be okay, cooled by the liquid, but remember that all that
current is entering the workpiece at whatever spot it is contacting the
rod so sometimes you get arcs and burn marks if the connection isn't
perfect. I try to wrap the welding rod around the workpiece when I can,
to get a solid connection and spread out the current. I've had arcs
erode the threads when I tried just sticking the wire into a small
threaded hole to polish a small piece. I have a large alligator clip on
the leads from the rectifer. The negative lead stays on the copper
cathode and I clip the positive lead onto the stainless steel hangar a
few inches above the surface of the solution to keep the clip as
uncorroded as possible, and then I just dip the workpiece into the
solution, turn on the variac and set the current by eye, watching the
bubble streams, and swish the piece around while it polishes. Pull it
every five or ten seconds for a look, and typically ten to thirty
seconds is enough. Definitely a "feel" type of operation but easy to
learn. The workpiece needs to be clean to start with. No oil, grease,
dirt, or fingerprints. Wear gloves while cleaning it with hot soapy
water, rinse well with tap water, ideally give it a quick rinse in
distilled water (if practical), shake the water off and start polishing.
I don't like to dry it first because any spots that form will interfere
with the polishing, and sometimes it helps to swish the piece around in
the solution for several seconds to kind of wash it off and to let it
heat up before turning on the current.

Be safe, and enjoy.

--
Regards,
Carl Ijames carl.ijames at verizon.net

"Carl Ijames" wrote in message
...
This is mostly a repost of my last posting with these recipes, but I
have now tested the citric acid/battery acid recipe so I have updated
those sections.

Thanks, Carl Saved for future reference.

Harold

"Carl Ijames" wrote:

This is mostly a repost of my last posting with these recipes, but I
have now tested the citric acid/battery acid recipe so I have updated
those sections.

Thanks for the info. One thing wasn't clear: are these recipes for
electropolishing stainless, or are they for some other metal(s)?

Bert

Stainless steel, specifically 300 series stuff. Haven't tried 400
series stuff, and other metals need different recipes.

--
Regards,
Carl Ijames carl.ijames at verizon.net

"Bert" wrote in message
...
"Carl Ijames" wrote:

This is mostly a repost of my last posting with these recipes, but I
have now tested the citric acid/battery acid recipe so I have updated
those sections.

Thanks for the info. One thing wasn't clear: are these recipes for
electropolishing stainless, or are they for some other metal(s)?

Bert