Robert L. Davis
is the electrolytic removal of metal in a highly ionic solution by
of an electrical potential and current. A less technical description of
the process would be "reverse plating". Electropolishing is normally
to remove a very thin layer of material on the surface of a metal part
or component. The process is of interest because of its ability to
the material properties of a work piece in addition to changing its
dimensions. The amount of change to the metal is highly dependent upon
the metal itself and how it has been processed up to the point where it
Materials Can Be Electropolished?
any metal can be electropolished. The metal can be ferrous or
A typical listing of metals and alloys that can be electropolished are
Silicon & wrought
is used for leveling the surface of most metals. It requires a minimum
of labor and if the conditions are right, can provide a spectacular
finish. At first glance the leveling effect is important because it
in many instances, improve the visual appearance of the part. Its real
usefulness, however, is in the manner in which it smoothes the
smoothing effect results in leveling of the grain boundaries of the
In fact, the effect can be so pronounced at times, that visual
of the boundaries can no longer be made. With the grain boundaries
edge to edge, the usual sites for stress cracking have been removed. In
most instances, this will enhance a part's strength up to the value of
its bulk material properties. It should be noted here that it is
desirable to have all surfaces in the metal stress relieved to the
where the bulk properties of the material are characteristics
the part. There is an exception in the case of materials that require
additional fatigue strength. These material may have had their surface
purposely work hardened. This work hardening induces compressive
into the surface of the material. Be aware that electropolishing can
remove this work hardened layer. It should also be noted that a uniform
bead blasting of the material after electropolishing will normally
the fatigue strength, in addition to letting the part retain the
of the electropolishing.
important thing to remember about electropolishing is that it is
You can improve almost anything you can put in the tank. The better the
part to begin with, the better the result.
removal of discontinuities in the edges of the grain boundaries will
remove sites for chemicals, dirt and microorganisms to be trapped. A
benefit of the leveling is the reduction of the total surface area of
grain boundaries exposed to the process. This significantly reduces the
amount of material from the grain boundary areas exposed to chemical
In the areas of high vacuum work, the smoothing effect is useful
it effectively reduces the total process surface area of the work
This reduces the gas load on the system allowing one to reach higher
also has a quality control and inspection aspect to its nature. Since
process is carried out in the presence of aggressive chemicals, when a
defective part comes through the process line, the chemicals in the
solution tend to attack or uncover the defect in the part. This is a
but effective way of double checking the quality of the material being
should further be pointed out that with certain kinds of steels and
steels the surfaces of the part become passivated when they are
This passivation is useful in many processes related to the
pharmaceutical and semiconductor companies.
it is possible to electropolish almost all metal, there are several
and alloys that do not lend themselves to standard industrial
Some of the factors that determine suitability are related to the
chemistry, while others are related to the processing of the raw
Cast metals are, in general, very porous and quite difficult to
as well as alloys that contain quantities of carbon, sulfur or silicon.
Most electropolishing solutions dissolve silver, limiting the process
use to the alloys of silver.
best way to find out what can be done is to ask questions and
Sometimes the answer to your inquiry is known. Many times the answer is
found through testing the material or parts. Although you may not be
it is standard practice to try a sample of any new or unfamiliar part
any major processing is started. Electropolishing is a very effective
control and inspection technique. If something is wrong with the
of a part, electropolishing points it out by etching away the defective
material. This etching is non-reversible and is almost never
last thing to remember is that most electropolishiers do not provide
services on all the metals listed above. Like any other business, the
is out to perform a useful service to the business community and make a
profit for providing this service. In order to do this, he sets himself
up to polish the metals that are common to his customer base. There is
very little economic incentive to keep up tanks and solutions for the
and alloys he sees infrequently. Please do not feel cheated or angry if
he turns down your business. On the other side of this argument, some
solutions can polish a very broad range of materials.
you are interested in having a part or material electropolished,
and ask questions. There is a lot of information you can get over the
and most electropolishers will run a test sample or two for free.
Of Stainless Steel: A Detailed Look At The Process
can be difficult to explain because it has the potential to perform
material and chemical processing steps simultaneously. Also, if we
to explain every detail for every metal that can be polished, the
would be an entire book. Since this is intended to be an overview,
will be explained from the view point of only one metal. The metal of
because of its dynamic range and our experience with the material shall
be stainless steel. Stainless steels come in a wide variety of grades
compositions from an equally wide variety of manufacturing processes.
feel that if one can understand the subtleties of electropolishing
steel, then they should have enough understanding of the process to
most of the problems associated with the other metals.
will begin our discussion of the process by introducing you to the
used by the industry. The principle definitions we will consider
Piece (Anode): The metal piece, part or component that is
electropolished. In this explanation, the work piece is made out of
steel. This piece of metal is connected to the positive side of the
rectifier and functions as a sacrificial anode in the process.
Bath: A mixture of phosphoric and sulfuric acids into which the
piece is placed for processing. This bath is the ionic solution which
the work piece and carries the metal ions from anode to cathode.
The material that is connected to the negative end of the rectifier
accepts the metal ions from the work piece. The cathode is usually made
out of metal and is shaped in such a way as to provide even current
to the surface of the work piece.
Film(Drag Out): The thick viscous film of electropolishing
that forms on the surface of the work piece during the electropolishing
Does Electropolishing Work:
will occur when metal can dissolve anodically through a highly
film. Current theory also states that the highly polarizing film can be
a solid, fluid and/or a gas. It is the steady state removal of metal
that creates a desirable and practical electropolishing procedure.
start with a description of the physical process. First we have a tank.
This tank is filled with a phosphoric and sulfuric acid solution. The
of the tank are lined with metal plates that function as the cathodes.
Electrical connections are provided so that the leads from a rectifier
can be connected to the work piece (anode) and the metal side plates
Next to the processing tank is a rinse tank filled with deionized
Next to the rinse tank is a final rinse area where parts can be hosed
with fresh deionized water.
electropolishing, you connect the work piece to the positive side of
rectifier and the cathode to the negative side. You adjust your current
level, voltage level and tank temperature to the most desirable setting
for the work piece. The work piece is then lowered into the processing
tank and the power is turned on. After a suitable amount of time
length depends upon the part) you turn off the power and you take the
out of the tank. The part will be covered by a thick film or "drag
The electrical connection is removed from the part and it is taken to
drag out tank where the anode film is rinsed off by immersion. The part
is then taken to the rinse station and given a final rinse to remove
remaining traces of the anode film.
Happens Inside The Tank:
begin with, the tank is a mixture of acids that are saturated with the
metal salts of stainless steel. The solution is almost always at
point and maintains this point by precipitating out additional metals
the system as sludge.
electropolishing effect occurs because as the current is applied, the
film at the surface of the metal changes its characteristics. As the
is applied to the work piece, the electropolishing solution becomes
and takes on the characteristics of an insulator or resistor. It is
to note that the greater the film thickness the higher the resistance
insulation properties of the film. This film must be assumed to have a
nominal thickness that is independent of the microstructure of the
means that the metal closest to the surface of the work piece has a
thick covering of anode film solution and is, for the most part
cut off from the cathode. The further you get from the work piece the
the anode film and the more charge received by the metal from the
With the film thickness independent of the microstructure, surface
protrude through the anode film in proportion to their height from the
work surface. The highest ones have the least insulation from the anode
film and receive a proportionally greater current from the cathode.
makes them dissolve faster than the lower peaks. The medium peaks
a lower current than the higher peaks so they dissolve more slowly. It
is this effect of differential dissolution rates at the work surface
creates the leveling effect in electropolishing.
the polishing process takes place, hydrogen is given off at the
and oxygen is given off at the outer edge of the anode film. If one
the generation of oxygen, it moves up the outer layer of the anode film
(tank side, not the work piece side). As this occurs, the viscous anode
film can be seen to move downward.
Simplified View Of The Theory:
feel that information on a process without an intuitive feel for how
controlling mechanisms operate leaves one in the world of the theorists
rather than in the world of industrial applications. In an effort to
reader beyond the theory, we have put together several short
for the mechanisms that we feel control electropolishing. We believe
these mechanisms work simultaneously in the electropolishing process.
compliment each other and interact to various degrees. A change in any
single mechanism can affect the results of the process. They all make
when viewed together and, with the exception of the "deep cone effect",
they are all interdependent on each other.
Mechanism; Chemical Saturation Effect: Or "Why Does It Really Work"
has known for a long time that when a solution has reached its
point, that unless something special is done to the solution (to get it
to become supersaturated) it will take no more ions into solution. It
our contention that electropolishing solution saturates, but does not
at the beginning of the process, then when you turn on the current the
entire surface of the metal will have some metal removal. As the
proceeds, metal ions are given off of the metal surface and go into
This rapid increase of heavy metal ions into solution is what we
creates the anode film. As the anode film becomes saturated with metal
ions, the electropolishing process slows down or stops in response to
increase in the anode film Metal Ion Saturation Level.
Mechanisms; Lightning Rod Effect Or "What Caused Those Funny Marks"
Franklin proved that a piece of material that is charged tends to have
very pronounced concentration of charge at edges and irregular or sharp
points. If you take this down to the surface level then each of the
that protrude into the anode film will have a significantly greater
concentration on them as compared to the valleys of the work surface.
the electropolishing process by nature, is a metal removal process by
solution, then the points in the metal with the greatest ionic charges
will have a greater electromotive potential for ionization into
and thus will be removed faster than the points in the metal with less
electromotive potential (i.e. the valleys of the material surface)
Mechanisms; The Viscosity Effect:
is important to note here that this effect is one of the most important
control mechanisms in electropolishing. This mechanism is fundamental
the surface level of the part. It is also very important on a macro
Charge will collect on macro-surfaces just as well as on
If you have a discontinuity, you will see charge concentration on the
piece (i.e. a hole, notch, bend, angle, slot, ect.) at this
If there is a concentration of charge, the electropolishing process
go faster at that point. A faster reaction creates more oxygen relative
to the rest of the work piece. This differential in oxygen production
the anode film. A disruption in the continuity of the anode film
a mark on the part.
feel that as the electropolishing solution approaches its saturation
its viscosity greatly increases. This creates a stagnant layer of
Electropolishing Solution or anode film on the surface of the part.
all viscous material it does not want to move unless it is forced to
Mechanisms; The Osmosis Effect:
natural osmosis, the metal ions in solution at the edge of the "Anode
(furthest away from the work surface) will naturally migrate into the
body of the electropolishing solution. This loss of ions into the main
body of the solution reduces the ion saturation of the electropolishing
solution at the surface of the anode film. This results in the outer
of the anode film becoming more active than the inner layer. This
situation where metal can still be removed in the regions of the anode
film farthest away from the surface of the work piece. This results in
the removal of any significant high spots in the surface of the work
This assumes they are still within the active region of the anode film.
Concurrently, the metal at the surface of the work piece is effectively
cut off from activity by the saturated, viscosity stagnant anode film.
Mechanisms; Gas Mixing/Pump Effect:
the electropolishing process takes place, oxygen is formed as a natural
part of the process. The oxygen is generated at the outermost edge of
anode film (i.e. not at the work piece surface). Since this oxygen is
as a gas, it will form bubbles and rise to the surface. This bubbling
as a pump and moves the main body of the electropolishing solution
the surface of the anode film. This movement causes the
solution to mix with the "Anode Film Solution" at the surface of the
film. The mixing allows fresh electropolishing solution that is not at
saturation to mix with the anode film and reduce its outer layer below
the saturation point. Again, with the outer layer of the anode film
the high spots of the work surface will be dissolved while the lower
of the work piece are inaccessible.
Mechanism; The Parabolic Mirror Or Deep Cone Effect:
Mechanisms shown above operate on principles associated with ionic
There is, however, a sixth mechanism that should theoretically occur
complements the process, but is not directly associated with ionic
To explain this effect, remember that if you have a smooth reflective
dish, all light that falls onto the dish is reflected upward to a
focal point. (It is important to note that all light is reflected
and, none is focused inward to the center bottom of the dish. Now, for
the sake of argument, let us not use a parabolic dish, but instead use
a cone (like that of a sharp pointed ice cream cone). This cone will
be shiny and smooth like the dish. If you shine light at the cone, the
light will be reflected upwards again just like that of the dish. If
cone is steep, much of the light will not reflect to a central point
the rim of the cone, but instead be reflected to a point inside the
As light travels through this focal point, it will continue on, and in
many cases hit another portion of the cone wall. It is important to
that no light but that which directly falls into the bottom of the cone
gets to the bottom. Any light that is the slightest bit off target gets
reflected back up the cone.
this leads to the fact that if the electric charge that is associated
the electropolishing electrical current is a form of electromagnetic
then it should behave like a charge in a vacuum. If it does, then it's
"charge" should be projected into the tank and travel like a beam of
let us imagine that the surface of the work piece before polishing is
to a side view of the rocky mountains. If this is true, then the
of the surface could be said to resemble Deep Cones or Parabolas. Since
the electropolishing takes place by using a DC current process, the
from the cathode should be transmitted to the anode in the fashion that
light is transmitted. Assuming that this charge is the electromotive
that causes the electropolishing process to take place, the
effect would occur at places where the electromagnetic radiation from
cathode is not fully absorbed by the first ion of metal it strikes. The
remaining energy should be reflected back. Since we consider stainless
a pretty good reflector of light and energy, this may not be a bad
the energy is reflected back into the wall of the cone at a different
(usually higher in the cone because of the cone angle), the new point
it strikes may also absorb some of the energy and also be affected. As
you can see, if the electropolishing effect is anything like the
effect, then a ion will be knocked loose from the surface of the work
every time sufficient electromagnetic energy is delivered to the
of the work piece. Please note that only the very lowest portion of the
work piece receives electromagnetic radiation from the cathode if this
radiation falls on it directly. Also note, that the steeper the cone
the more likely that a reflected beam of radiation will be reflected
successful in striking the wall of the cone in another place.
cone effect has a preference for taking down the high spots in the work
piece over leveling the work piece at the bottom of the pits or cones.
It should also be pointed out that an electropolishing process operates
at current levels higher than those required to just dissolve the
in an electroplating operation. (This again is speculation, on my part
but it all ties into the basic phenomenon).
electropolishing, there is a very significant preference to the removal
of any high spots on the metal surface. This means that the dimensions
of the high spots are changed drastically while the dimensions of the
spots are changed very little. This creates a smoothing effect to the
surface. It also means that by nature of the process, the total amount
of dimensional change required to obtain the polish effect is very
(Dimensional reduction of the work piece is on the order of 2.5 ten
of an inch / 0.00025 in.)
any stainless steel that you can buy has been rolled, machined and/or
with carbon or tempered steel implements. This means that, in general,
all of the stainless steel that you will ever buy off the shelf or
will have an appreciable amount of steel worked into its surface.
In the industry this surface impregnated steel is referred to as "free
free iron corrodes with no real difficulty. The corrosion process of
is a very aggressive reaction. This reaction will in almost all cases
corrosion in the stainless steel. Once started, the corrosion of the
will continue to take place without the presence of free iron.
corrosion information that I have seen on electropolishing uses a salt
spray test as its basis for comparing corrosion rates. This does not
to say that other tests have not been used. It's just that the
that I have seen used this test.
corrosion of free iron in salt water creates both chemical and
chemical reactions. We believe that this reaction attacks the stainless
steel at its grain boundaries and the corrosion propagates through the
grain structure ungluing the grain structure. There is another well
chemical reaction that takes place when stainless steel is subjected to
the effects of chlorine. In this reaction, the chlorine leaches out of
the carbon severely degrades the structure of the atomic packing of the
metal. The result is that after removal of the carbon, the molecular
structure will be very much like Swiss cheese. Stainless steel after
to chlorine tends to become brittle and loses all its strength.
of stainless steel has two significant benefits besides the leveling of
the work piece surface. First, the electropolishing process will remove
all free iron from the surface of the work piece. This has the obvious
effect of eliminating the free iron corrosion up front. Secondly,
removes material from the surface of the metal selectively. For
electropolishing does not readily remove the carbon from the metal
carbon is very electrochemically neutral. Further, the process does not
readily remove chromium or nickel. The chromium, nickel and carbon, for
all practical purposes, becomes uncovered and remains sitting on the
of the metal as the electropolishing process takes place.
If the carbon that is exposed is present in any significant quantities,
it can be seen on the surface of the metal. This layer is referred to
the industry as "smut". It is usually removed from the metal surface
it is used in service. Smutting is not a common problem associated with
the carbon present is not a problem, as you electropolish a part you
enriching the surface with chromium and nickel. At some point, a
reaction takes place during processing of the part. The chromium reacts
and forms chromium oxide. Further, if the surface is very rich in
the chromium oxide will form what you can think of as a layer over the
metal surface. This is referred to as a chromium enriched surface
to form a chromium oxide passivation layer. This mechanism is referred
to in the industry as "passivation."
term "passivation" is used widely in the stainless steel processing
especially in the areas of food and pharmaceuticals. Many people refer
to this oxide surface as a chrome-nickel oxide and imply that both
join to form a protective coating. However, when you look into the
you will find that chromium oxides are always noted for their corrosion
resistance and nickel oxides are not. We feel that the nickel either
on the surface as an elemental metal or that it combines into some
that is corrosion resistant. It is worth noting that Nichrome is listed
for its corrosion resistance, particularly to sea water. If you look at
the chemical composition for Nichrome, you will find something that is
very interesting. Nichrome is made up of all the constituents of
steel. The major difference between it and stainless steel is the ratio
of materials. The correct ratios for 316L stainless steel is
74 parts iron, 16 parts chromium, 10 parts nickel and about 0.03 parts
carbon. For Nichrome the correct percentages are 60 parts nickel, 24
iron, 16 parts chromium, and 0.1 parts carbon.
seeing these and knowing that electropolishing enriches the surface of
stainless with chromium, nickel and carbon, we now have developed our
theory as to how this mechanism works.
feel that as electropolishing solution removes the iron ions from the
of the part, it leaves much of the chromium, nickel and carbon behind
the surface of the metal. We think that the nickel, chromium, iron and
carbon combine to form Nichrome. If the reaction goes to completion,
will be excess chromium left over. As it turns out, there will be
as much free chromium left over as there is Nichrome. This leaves the
chromium to react with the oxygen of the electropolishing process to
the corrosion resistant chromium oxides. In this reaction at least some
of the carbon that comes to the metal surface is used in the formation
of Nichrome. This might very well explain why you don't get a
problem with the electropolishing processing.
next step in passivation is that the nitric acid attacks the stainless
steel and free iron of the work piece. It continues to eat away at the
surface of the part till the surface becomes enriched with chromium.
nitric acid then oxidizes the chromium rich surface and the part
passive. Nitric acid does not attack the chromium oxide so when the
surface of the stainless steel becomes passive, all significant
reactions on the stainless steel stops; a very nice self regulating
order to passivate or do a good passivation, you must have a clean
part. In some cases, you may have to chemically strip the existing
layer from the part before you can re-passivate it.
does not require any stripping of the existing passivation layer. The
potential has more than enough energy to remove the outer layer of the
part (It typically removes about 2.5 ten thousandths of an inch/0.00025
in.) This will remove any passivation layer that may have previously
in significant to note that passivation by electropolishing and
passivation are typically considered equivalent techniques to produce
identical results. This does not say anything about their respective
finishes, but merely that both surfaces will be equally passive.
big distinction to notice is the time factor. It takes from two to
hours to chemically passivate a part. It takes anywhere from thirty
to eight minutes to passivate the same part by electropolishing
last notation. Electropolishing is an excellent technique for metal
in preparation for welding. The electropolishing process removes
all of the surface contaminates in the metal. It also stress relieves
surface of the metal. It reduces the hydrogen present in the parent
and enriches the welding surface with chromium and nickel. Finally, it
forms a passivation layer over the parent metal so that it does not
The reduction of contaminates allows the work piece to heat more evenly
and reduces the amount of slag produced in the welds. This makes the
easier, cleaner and more uniform.
Problems; Why Didn't It Work?
results from a failed electropolishing job can be anywhere from
to heartbreaking. We have seen parts that just don't shine right. We
also seen parts that have been partially or totally dissolved by the
Like anything else, the source of the problem can be anywhere from
error to the material defects in the parts. This section is directed
at the known pitfalls of the process that can be avoided by proper
and practical process testing.
uneven finish is usually caused by an uneven mass transfer through the
Anodic film during the process. Any disruption of the film tends to
a visual mark on the work piece.
In The Part:
most common appearance failure of a part is due to bubble tracks. This
effect, which was described earlier, is usually handled by careful
connection and setup of the part. With a little thought and
with the customer, the part can be oriented so that the cosmetic side
the part generates bubbles that have a free path to the surface. This
not always eliminate the problem. Many times the marks are just
to the back side of the part. Another method of reducing the effect is
to lower the rectifier settings so that the reaction takes place at a
rate. This reduces the pumping energy of the bubbles and thus the
of the anode film. The most effective way to reduce this problem is to
agitate the part or the tank slightly. The agitation gently disrupts
bubbles and prevents them from tracking up the work piece surface in a
regular and predictable manner.
frequent cosmetic failure of electropolishing is what we refer to as
stripes or leopard spots. Oils and glues must be completely removed
the metal surface before a part can be electropolished. If left on,
contaminants will prevent the anode layer from wetting out the surface
of the metal which inhabits metal removal. The uneven removal rate from
the surface is vary apparent.
is also wise to remember that dirt and oils are trapped within the
surface as it is rolled in the mills. It is not uncommon to have the
process remove the outer layer of metal to uncover small oil pockets.
this happens, the part comes out with small patches or leopard spots of
area where the finish is dull.
of the appearance grade stainless steel commercially produced is
with a plastic coating that protects the finish. The glue that holds on
this coating tends to get down into the pores of the metal. If this is
not diligently removed from the metal surface, it will also create
with a dull finish. The characteristic pattern from this contamination
is a zebra stripe.
of a part after electropolishing is of particular concern because it
be mechanically destructive to the part. There are several causes that
can create this effect, and most of these are related to material
can be caused by the entrapment of dirt in the surface of the metal as
it is rolled in the mill. Think of fertilizing your lawn and then
over it with a steam roller. After the process, the grass would be very
flat. It would also have all the fertilizer you put on it trapped
the blades. If you electropolish metal which has lots of trapped
dirt from the mill or mechanical finishing, there is a good chance that
you will open the surface of the metal over the dirt. Since
solution is a combination of acids, it is normal except that the pocket
will be cleaned out. If the pockets are of any depth, they will be at
bottom of the anode film and consequently not removed. They will show
as a porous finish on the part.
common Porosity problem is caused by improperly heat-treating certain
of stainless steels. As you may know, chromium is added to make the
steel tough. In the metal making process the chromium joins with the
to form chromium carbides. These particles distribute themselves
the metal crystalline structure to provide strength to the part. If the
steel is subsequently heated these chromium carbides can move within
structure of the metal. If the metal is not properly quenched, the
carbides tend to migrate to the grain boundaries of the steel. Chromium
carbide is attacked by electropolishing. If the material is uniformly
within the grain boundaries of the metal, electropolishing will
dissolve all the chromium in these boundaries. Problems related to heat
treating normally show up as severely etched or dissolved parts.
should be noted here that the electropolisher did not create the damage
described here. Carbide precipitation in the grain boundaries is
considered to be a rejectable defect in materials because the parts
have lost all their strength. The part was already defective when he
it. Electropolishing merely performed a quality control inspection of
metals by nature of their manufacturing process at times can contain
pores in their structure. If such a part is electropolished, it will
much like the part with dirt entrapment.
frequent trap in the pharmaceutical and dairy industry is to send out
that has been mechanically polished. In a mechanically polished
one smears the surface layers of the metal over with elbow grease and
compound. This smearing is reported to seal off the surface of the
like folding over a piece of aluminum wrap over a piece of food at
What is underneath the smeared surface is trapped and sealed. It should
not bother us again. It does not really matter what our true opinion of
this philosophy; what does matter is that it is a factual practice.
someone will electropolish an article that has been manufactured like
and get a big surprise. The electropolish will remove the top layer of
material and release all the entrapped rubbing compound from the part.
Needless to say, the part will not be as shiny as when you started. It
will not only lose the very fine surface finish created by the
polishing, it will also be a little porous.
of the problems associated with dull finishes are related partly to
and partly to the polishing technique. Improper technique is related to
the improper placement of cathodes around the part. Improper control of
process temperature, voltage or current can also affect the finish.
problems are generally beyond your control so you will have to ask
and inquire about these kinds of problems.
Didn't Polish Like The Last Batch:
a general rule of thumb, large flat surfaces tend to electropolish to a
refractive satin finish. Small parts or parts with significant radii of
curvature tend to electropolish to a reflective mirror finish. The
for this has to do with the deep cone effect. The more curved the
the more effective conservation of energy. This creates a higher
electropolishing, both material composition and initial finish have a
effect on final finish. If you make two parts from the same piece of
and machined them to two different surface finishes, their final
after electropolishing will be different. If you take two parts of
alloys and machine them identically, you should also expect to have two
different finishes after electropolishing. The more consistent the
are before polishing, the more repeatable the process.
usually love to talk about their quality and hate to talk about their
control. The electropolisher has the problem of having no incoming
control. The materials he polishes are brought to him by his customers.
The materials of construction and surface finishes have been selected
the customer for his needs and purposes, not the
control usually means that you have standards for materials that are
into the shop. You then turn these materials into goods that meet a
set of product standards. It is a generally accepted rule that if the
materials are not up to specification, the final product will not be
while. This is why incoming quality control is usually a description of
packaging for final shipment.
a result, the typical quality assurance manual will state how the parts
are handled. This is usually a detailed procedure describing material
and process procedures. It will describe the cleaning of the part
and after processing. There is usually a description of packaging for
shipment. There is a practical difficulty in the writing of quality
specification, for electropolishing. If every shipment is a special
how do you determine in advance how much material should be removed?
do you determine the process temperature or rectifier settings?
is as much of an art as a science. It requires insight, intuition and
artist's touch. It is amazing what a good electropolisher can do. He
usually take several pieces of reasonably similar material and adjust
process so that all the parts come out acceptable. They may not all
identical, but they will usually be acceptable. When one is working
dimensions on the order of 0.00025", there is usually some latitude in
how you process a part. Increase the reaction rate and improve the
Take off an extra 0.00025" and remove some unacceptable scratches.
is a lot one can do to a part whose manufacturing tolerance is
when you are electropolishing. It is very easy for a good supplier to
out high quality parts. It is very hard to quantify how he does it.
Questions About The Process:
Electropolishing Clean Parts?
tries to remove the surface layer of the work piece. If it does so,
everything that was on the surface of the piece will be removed. If
is considered cleaning, then electropolishing will indeed clean parts.
In general, it is best to remove all impurities and surface residue
Electropolishing Sterilize Parts?
impurities can interfere with the process and make the final surface
This uneven appearance is unacceptable to many customers. Care should
taken before using electropolishing for cleaning. Most polishers do not
want impurities in their processing tank for fear that they may affect
the bath. it may be necessary to inquire about cleaning before you send
a part out.
electropolishing process also does not sterilize parts. It does,
make the surface of the material so that it can be sterilized.
removes most of the surface pits and crevices from equipment surfaces,
eliminating traps and adhesion sites. Many consider this cleaner
with dramatically fewer traps and pits to be able to become more
than surfaces which have not been electropolished.
Does Electropolishing Help Me Achieve A Better Vacuum?
helps achieve a better vacuum by reducing the effective surface area
a vacuum system. It also creates a passivation layer over the surface.
This passivation layer acts an a sealer over the metal surface. This
helps prevent outgassing of gasses diffused into the metal.
You "Crack" Or Break An Electropolished Surface?
or distortion of any metal, polished or not, will distort its structure
and surface grain boundaries. In any metal working, care must be taken
to not destroy the integrity of the metal through excessive distortion.
If a piece of material was electropolished before distortion, the
layer and grain boundaries of the material will be disrupted in the
affected by the distortion. This will destroy the integrity of the
layer in the affected area.
Electropolishing Remove The Silver From The Nuts Of Fittings?
should always, if possible, be bent and machined prior to
This allows the electropolishing process to smooth out the grain
and passivate all the surfaces of the part. This includes the surfaces
that were destroyed by any machining, burnishing or bending of the part
prior to electrpolishing. "Cracking" is a term used in the analysis of
polished and passivated tubing. These tests are designed to compare the
effects of chempolishing and electropolishing. In Cleanliness studies,
straight lengths of tubing are electropolished and chempolished then
at various angles. The tubing is then cut open to determine how much
and distortion was done in the bending process. The results are usually
related to particle counts given off by the damaged areas. Structural
to the tubing are not usually a factor.
electropolishing solution does an excellent job of removing silver from
almost anything. As a result all silver surfaces must be carefully
of before polishing. If you are not familiar with the polisher's
technique, it would be wise to inquire. Most nuts have a rear silvered
surface at the tube end of the nut that should be addressed in his
This is a difficult problem for anyone just starting out, and the
to the problem are considered proprietary.
Will Any Final Surface Finish (RA.) Be After Polishing And Will You
general, electropolishing will improve the surface finish of a part by
two fold. This means that if a part starts out with a surface finish of
32 RMS, its expected final finish would be 16 RMS. This is a good rule
of thumb to use thinking about the technique. The coarser the finish,
less reliable the estimate.
electropolishers will not guarantee surface finish because of the
sensitivity to metallurgy, machining and surface impurities. If a part
has any of the above problems, the final surface finish may not be
or even across the part's surface. The fault is in the part, not the
It is also beyond the control of the electropolisher.
specification work, one is typically paid only for work done to
All work that is not done to specification goes unpaid. This leaves the
electropolishers in an unacceptable position when he is given the parts
by the customer. He cannot control the material and yet he must perform
work to the limitations of the material produced by others. If the work
does not meet specification, it goes unpaid despite the fact that he
a good job. That there is no government specification covering
because of these limitations. This does not mean the process is not
nor does it mean that it does not work. It is just very difficult to