What Is Electropolishing?

By: Robert L. Davis
Electropolishing is the electrolytic removal of metal in a highly ionic solution by means of an electrical potential and current. A less technical description of the process would be "reverse plating". Electropolishing is normally used 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 enhance the material properties of a work piece in addition to changing its physical 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 is electropolished. 

What Materials Can Be Electropolished?

Almost any metal can be electropolished. The metal can be ferrous or non-ferrous. A typical listing of metals and alloys that can be electropolished are as follows: 
Aluminum-non Silicon & wrought 
Beryllium-Copper Alloys 
Nickel Silver 
Stainless Steels 
Hastelloy Alloys 

Why Electropolish? 

Electropolishing 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 surface finish. At first glance the leveling effect is important because it can, in many instances, improve the visual appearance of the part. Its real usefulness, however, is in the manner in which it smoothes the part. 

This smoothing effect results in leveling of the grain boundaries of the metal. In fact, the effect can be so pronounced at times, that visual identification of the boundaries can no longer be made. With the grain boundaries smoothed 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 normally desirable to have all surfaces in the metal stress relieved to the point where the bulk properties of the material are characteristics throughout the part. There is an exception in the case of materials that require the additional fatigue strength. These material may have had their surface purposely work hardened. This work hardening induces compressive stresses into the surface of the material. Be aware that electropolishing can easily remove this work hardened layer. It should also be noted that a uniform bead blasting of the material after electropolishing will normally restore the fatigue strength, in addition to letting the part retain the benefits of the electropolishing. 

The important thing to remember about electropolishing is that it is versatile. You can improve almost anything you can put in the tank. The better the part to begin with, the better the result. 

This removal of discontinuities in the edges of the grain boundaries will also remove sites for chemicals, dirt and microorganisms to be trapped. A third benefit of the leveling is the reduction of the total surface area of the grain boundaries exposed to the process. This significantly reduces the amount of material from the grain boundary areas exposed to chemical attack. In the areas of high vacuum work, the smoothing effect is useful because it effectively reduces the total process surface area of the work piece. This reduces the gas load on the system allowing one to reach higher vacuums more easily. 

Electropolishing also has a quality control and inspection aspect to its nature. Since the process is carried out in the presence of aggressive chemicals, when a defective part comes through the process line, the chemicals in the electropolishing solution tend to attack or uncover the defect in the part. This is a dramatic but effective way of double checking the quality of the material being processed. 

It should further be pointed out that with certain kinds of steels and stainless steels the surfaces of the part become passivated when they are electropolished. This passivation is useful in many processes related to the bio-genetic, pharmaceutical and semiconductor companies. 

Practical Limitations: 

While it is possible to electropolish almost all metal, there are several metals and alloys that do not lend themselves to standard industrial processing. Some of the factors that determine suitability are related to the process chemistry, while others are related to the processing of the raw metals. Cast metals are, in general, very porous and quite difficult to electropolish as well as alloys that contain quantities of carbon, sulfur or silicon. Most electropolishing solutions dissolve silver, limiting the process general use to the alloys of silver. 

The best way to find out what can be done is to ask questions and investigate. 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 told, it is standard practice to try a sample of any new or unfamiliar part before any major processing is started. Electropolishing is a very effective quality control and inspection technique. If something is wrong with the metallurgy of a part, electropolishing points it out by etching away the defective material. This etching is non-reversible and is almost never repairable. 

One last thing to remember is that most electropolishiers do not provide their services on all the metals listed above. Like any other business, the electropolisher 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 metals 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 electropolishing solutions can polish a very broad range of materials. 

If you are interested in having a part or material electropolished, investigate and ask questions. There is a lot of information you can get over the phone, and most electropolishers will run a test sample or two for free. 

Electropolishing Of Stainless Steel: A Detailed Look At The Process

Electropolishing can be difficult to explain because it has the potential to perform several material and chemical processing steps simultaneously. Also, if we tried to explain every detail for every metal that can be polished, the result would be an entire book. Since this is intended to be an overview, electropolishing will be explained from the view point of only one metal. The metal of choice because of its dynamic range and our experience with the material shall be stainless steel. Stainless steels come in a wide variety of grades and compositions from an equally wide variety of manufacturing processes. We feel that if one can understand the subtleties of electropolishing stainless steel, then they should have enough understanding of the process to tackle most of the problems associated with the other metals. 

We will begin our discussion of the process by introducing you to the terms used by the industry. The principle definitions we will consider are: 

Work Piece (Anode): The metal piece, part or component that is being electropolished. In this explanation, the work piece is made out of stainless steel. This piece of metal is connected to the positive side of the electrical rectifier and functions as a sacrificial anode in the process. 

Electropolishing Bath: A mixture of phosphoric and sulfuric acids into which the work piece is placed for processing. This bath is the ionic solution which wets the work piece and carries the metal ions from anode to cathode. 

Cathode: The material that is connected to the negative end of the rectifier that 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 densities to the surface of the work piece. 

Anode Film(Drag Out): The thick viscous film of electropolishing solution that forms on the surface of the work piece during the electropolishing process. 

How Does Electropolishing Work:

Electropolishing will occur when metal can dissolve anodically through a highly polarizing 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 ions that creates a desirable and practical electropolishing procedure. 

Lets 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 edges 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 (cathode). Next to the processing tank is a rinse tank filled with deionized water. Next to the rinse tank is a final rinse area where parts can be hosed down with fresh deionized water.

In electropolishing, you connect the work piece to the positive side of the 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 (actual length depends upon the part) you turn off the power and you take the part out of the tank. The part will be covered by a thick film or "drag out". The electrical connection is removed from the part and it is taken to the 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 all remaining traces of the anode film. 

What Happens Inside The Tank: 

To 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 equilibrium point and maintains this point by precipitating out additional metals to the system as sludge. 

The electropolishing effect occurs because as the current is applied, the electropolishing film at the surface of the metal changes its characteristics. As the current is applied to the work piece, the electropolishing solution becomes thicker and takes on the characteristics of an insulator or resistor. It is important to note that the greater the film thickness the higher the resistance or insulation properties of the film. This film must be assumed to have a nominal thickness that is independent of the microstructure of the metal. 

This means that the metal closest to the surface of the work piece has a very thick covering of anode film solution and is, for the most part electrically cut off from the cathode. The further you get from the work piece the thinner the anode film and the more charge received by the metal from the cathode. With the film thickness independent of the microstructure, surface irregularities 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. This makes them dissolve faster than the lower peaks. The medium peaks receive a lower current than the higher peaks so they dissolve more slowly. It is this effect of differential dissolution rates at the work surface that creates the leveling effect in electropolishing. 

As the polishing process takes place, hydrogen is given off at the cathodes and oxygen is given off at the outer edge of the anode film. If one observes 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. 

A Simplified View Of The Theory:

We feel that information on a process without an intuitive feel for how its controlling mechanisms operate leaves one in the world of the theorists rather than in the world of industrial applications. In an effort to bring reader beyond the theory, we have put together several short descriptions for the mechanisms that we feel control electropolishing. We believe that these mechanisms work simultaneously in the electropolishing process. All compliment each other and interact to various degrees. A change in any single mechanism can affect the results of the process. They all make sense when viewed together and, with the exception of the "deep cone effect", they are all interdependent on each other. 
First Mechanism; Chemical Saturation Effect: Or "Why Does It Really Work"
Chemistry has known for a long time that when a solution has reached its saturation 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 is our contention that electropolishing solution saturates, but does not supersaturate 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 process proceeds, metal ions are given off of the metal surface and go into solution. This rapid increase of heavy metal ions into solution is what we believe creates the anode film. As the anode film becomes saturated with metal ions, the electropolishing process slows down or stops in response to the increase in the anode film Metal Ion Saturation Level.
Second Mechanisms; Lightning Rod Effect Or "What Caused Those Funny Marks"
Ben 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 peaks that protrude into the anode film will have a significantly greater charge concentration on them as compared to the valleys of the work surface. Since the electropolishing process by nature, is a metal removal process by ionic solution, then the points in the metal with the greatest ionic charges will have a greater electromotive potential for ionization into solution and thus will be removed faster than the points in the metal with less electromotive potential (i.e. the valleys of the material surface) 

It is important to note here that this effect is one of the most important control mechanisms in electropolishing. This mechanism is fundamental at the surface level of the part. It is also very important on a macro level. Charge will collect on macro-surfaces just as well as on micro-surfaces. If you have a discontinuity, you will see charge concentration on the work piece (i.e. a hole, notch, bend, angle, slot, ect.) at this discontinuity. If there is a concentration of charge, the electropolishing process should go faster at that point. A faster reaction creates more oxygen relative to the rest of the work piece. This differential in oxygen production disrupts the anode film. A disruption in the continuity of the anode film creates a mark on the part.

Third Mechanisms; The Viscosity Effect:
We feel that as the electropolishing solution approaches its saturation point, its viscosity greatly increases. This creates a stagnant layer of Saturated Electropolishing Solution or anode film on the surface of the part. Like all viscous material it does not want to move unless it is forced to move.
Fourth Mechanisms; The Osmosis Effect:
By natural osmosis, the metal ions in solution at the edge of the "Anode Film" (furthest away from the work surface) will naturally migrate into the main 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 layer of the anode film becoming more active than the inner layer. This creates a 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 piece. 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.
Fifth Mechanisms; Gas Mixing/Pump Effect:
As the electropolishing process takes place, oxygen is formed as a natural part of the process. The oxygen is generated at the outermost edge of the anode film (i.e. not at the work piece surface). Since this oxygen is generated as a gas, it will form bubbles and rise to the surface. This bubbling works as a pump and moves the main body of the electropolishing solution along the surface of the anode film. This movement causes the electropolishing solution to mix with the "Anode Film Solution" at the surface of the anode 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 active, the high spots of the work surface will be dissolved while the lower regions of the work piece are inaccessible. 
Sixth Mechanism; The Parabolic Mirror Or Deep Cone Effect:
The Mechanisms shown above operate on principles associated with ionic solutions. There is, however, a sixth mechanism that should theoretically occur that complements the process, but is not directly associated with ionic solutions. To explain this effect, remember that if you have a smooth reflective parabolic dish, all light that falls onto the dish is reflected upward to a central focal point. (It is important to note that all light is reflected outward 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 still 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 the cone is steep, much of the light will not reflect to a central point beyond the rim of the cone, but instead be reflected to a point inside the cone. 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 note 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. 

All this leads to the fact that if the electric charge that is associated with the electropolishing electrical current is a form of electromagnetic radiation, 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 light. 

Now let us imagine that the surface of the work piece before polishing is similar to a side view of the rocky mountains. If this is true, then the imperfections of the surface could be said to resemble Deep Cones or Parabolas. Since the electropolishing takes place by using a DC current process, the charge from the cathode should be transmitted to the anode in the fashion that light is transmitted. Assuming that this charge is the electromotive force that causes the electropolishing process to take place, the electropolishing effect would occur at places where the electromagnetic radiation from the 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 assumption. 

If the energy is reflected back into the wall of the cone at a different place (usually higher in the cone because of the cone angle), the new point that 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 photoelectric effect, then a ion will be knocked loose from the surface of the work piece every time sufficient electromagnetic energy is delivered to the surface 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 angle, the more likely that a reflected beam of radiation will be reflected and successful in striking the wall of the cone in another place. 

This 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 anodes in an electroplating operation. (This again is speculation, on my part but it all ties into the basic phenomenon). 

Final Result:
In 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 lower spots are changed very little. This creates a smoothing effect to the metal surface. It also means that by nature of the process, the total amount of dimensional change required to obtain the polish effect is very small. (Dimensional reduction of the work piece is on the order of 2.5 ten thousandths of an inch / 0.00025 in.) 

The Passivation Effect: 

Almost any stainless steel that you can buy has been rolled, machined and/or manipulated 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 re-manufacture will have an appreciable amount of steel worked into its surface. 

Note: In the industry this surface impregnated steel is referred to as "free iron". 

This free iron corrodes with no real difficulty. The corrosion process of iron is a very aggressive reaction. This reaction will in almost all cases start corrosion in the stainless steel. Once started, the corrosion of the stainless will continue to take place without the presence of free iron. 

The corrosion information that I have seen on electropolishing uses a salt spray test as its basis for comparing corrosion rates. This does not mean to say that other tests have not been used. It's just that the information that I have seen used this test. 

The corrosion of free iron in salt water creates both chemical and electrical 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 known 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 grain structure will be very much like Swiss cheese. Stainless steel after exposure to chlorine tends to become brittle and loses all its strength. Electropolishing 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, electropolishing removes material from the surface of the metal selectively. For example, electropolishing does not readily remove the carbon from the metal because 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 surface of the metal as the electropolishing process takes place. 

Note: 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 in the industry as "smut". It is usually removed from the metal surface before it is used in service. Smutting is not a common problem associated with electropolishing. 

If the carbon present is not a problem, as you electropolish a part you start enriching the surface with chromium and nickel. At some point, a chemical reaction takes place during processing of the part. The chromium reacts and forms chromium oxide. Further, if the surface is very rich in chromium, 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 oxidizing to form a chromium oxide passivation layer. This mechanism is referred to in the industry as "passivation." 

The term "passivation" is used widely in the stainless steel processing industry, especially in the areas of food and pharmaceuticals. Many people refer to this oxide surface as a chrome-nickel oxide and imply that both metals join to form a protective coating. However, when you look into the chemistry you will find that chromium oxides are always noted for their corrosion resistance and nickel oxides are not. We feel that the nickel either sits on the surface as an elemental metal or that it combines into some compound 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 stainless steel. The major difference between it and stainless steel is the ratio of materials. The correct ratios for 316L stainless steel is approximately 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 parts iron, 16 parts chromium, and 0.1 parts carbon. 

After seeing these and knowing that electropolishing enriches the surface of stainless with chromium, nickel and carbon, we now have developed our own theory as to how this mechanism works. 

We feel that as electropolishing solution removes the iron ions from the surface of the part, it leaves much of the chromium, nickel and carbon behind on the surface of the metal. We think that the nickel, chromium, iron and carbon combine to form Nichrome. If the reaction goes to completion, there will be excess chromium left over. As it turns out, there will be almost as much free chromium left over as there is Nichrome. This leaves the free chromium to react with the oxygen of the electropolishing process to form 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 "smutting" problem with the electropolishing processing. 

The 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. The nitric acid then oxidizes the chromium rich surface and the part becomes passive. Nitric acid does not attack the chromium oxide so when the entire surface of the stainless steel becomes passive, all significant chemical reactions on the stainless steel stops; a very nice self regulating reaction. 

In order to passivate or do a good passivation, you must have a clean active part. In some cases, you may have to chemically strip the existing passive layer from the part before you can re-passivate it. 

Electropolishing does not require any stripping of the existing passivation layer. The electromotive 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 existed. 

It in significant to note that passivation by electropolishing and chemical passivation are typically considered equivalent techniques to produce the identical results. This does not say anything about their respective surface finishes, but merely that both surfaces will be equally passive. 

The big distinction to notice is the time factor. It takes from two to eight hours to chemically passivate a part. It takes anywhere from thirty seconds to eight minutes to passivate the same part by electropolishing it. 

One last notation. Electropolishing is an excellent technique for metal cleaning in preparation for welding. The electropolishing process removes virtually all of the surface contaminates in the metal. It also stress relieves the surface of the metal. It reduces the hydrogen present in the parent material and enriches the welding surface with chromium and nickel. Finally, it forms a passivation layer over the parent metal so that it does not oxidize. 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 work easier, cleaner and more uniform. 

Processing Problems; Why Didn't It Work?

The results from a failed electropolishing job can be anywhere from frustrating to heartbreaking. We have seen parts that just don't shine right. We have also seen parts that have been partially or totally dissolved by the process. Like anything else, the source of the problem can be anywhere from operator error to the material defects in the parts. This section is directed primarily at the known pitfalls of the process that can be avoided by proper planning and practical process testing. 
Uneven Finish:
An uneven finish is usually caused by an uneven mass transfer through the Anodic film during the process. Any disruption of the film tends to leave a visual mark on the work piece. 

The most common appearance failure of a part is due to bubble tracks. This effect, which was described earlier, is usually handled by careful electrical connection and setup of the part. With a little thought and communication with the customer, the part can be oriented so that the cosmetic side of the part generates bubbles that have a free path to the surface. This does not always eliminate the problem. Many times the marks are just transferred 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 lower rate. This reduces the pumping energy of the bubbles and thus the disruption 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 the bubbles and prevents them from tracking up the work piece surface in a regular and predictable manner. 

Another frequent cosmetic failure of electropolishing is what we refer to as zebra stripes or leopard spots. Oils and glues must be completely removed from the metal surface before a part can be electropolished. If left on, these 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. 

It is also wise to remember that dirt and oils are trapped within the metal's surface as it is rolled in the mills. It is not uncommon to have the electropolishing process remove the outer layer of metal to uncover small oil pockets. When this happens, the part comes out with small patches or leopard spots of area where the finish is dull. 

Much of the appearance grade stainless steel commercially produced is shipped 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 areas with a dull finish. The characteristic pattern from this contamination is a zebra stripe.

Porosity In The Part:
Porosity of a part after electropolishing is of particular concern because it can be mechanically destructive to the part. There are several causes that can create this effect, and most of these are related to material defects. 

Porosity 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 rolling 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 between the blades. If you electropolish metal which has lots of trapped surface 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 electropolishing 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 the bottom of the anode film and consequently not removed. They will show up as a porous finish on the part. 

Another common Porosity problem is caused by improperly heat-treating certain types of stainless steels. As you may know, chromium is added to make the stainless steel tough. In the metal making process the chromium joins with the carbon to form chromium carbides. These particles distribute themselves throughout the metal crystalline structure to provide strength to the part. If the steel is subsequently heated these chromium carbides can move within the structure of the metal. If the metal is not properly quenched, the chromium carbides tend to migrate to the grain boundaries of the steel. Chromium carbide is attacked by electropolishing. If the material is uniformly distributed within the grain boundaries of the metal, electropolishing will literally dissolve all the chromium in these boundaries. Problems related to heat treating normally show up as severely etched or dissolved parts. 

It should be noted here that the electropolisher did not create the damage described here. Carbide precipitation in the grain boundaries is normally considered to be a rejectable defect in materials because the parts will have lost all their strength. The part was already defective when he received it. Electropolishing merely performed a quality control inspection of the heat treatment. 

Cast metals by nature of their manufacturing process at times can contain large pores in their structure. If such a part is electropolished, it will behave much like the part with dirt entrapment. 

A frequent trap in the pharmaceutical and dairy industry is to send out material that has been mechanically polished. In a mechanically polished process, one smears the surface layers of the metal over with elbow grease and rubbing compound. This smearing is reported to seal off the surface of the metal like folding over a piece of aluminum wrap over a piece of food at home. 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. Occasionally, someone will electropolish an article that has been manufactured like this 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 mechanical polishing, it will also be a little porous.

Dull Finishes:
Many of the problems associated with dull finishes are related partly to geometry 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. These problems are generally beyond your control so you will have to ask questions and inquire about these kinds of problems. 

As 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 speculation for this has to do with the deep cone effect. The more curved the surface the more effective conservation of energy. This creates a higher brighter polish.

It Didn't Polish Like The Last Batch:
In electropolishing, both material composition and initial finish have a tremendous effect on final finish. If you make two parts from the same piece of stock and machined them to two different surface finishes, their final finishes after electropolishing will be different. If you take two parts of different alloys and machine them identically, you should also expect to have two different finishes after electropolishing. The more consistent the parts are before polishing, the more repeatable the process.
Quality Control:
Electropolishers usually love to talk about their quality and hate to talk about their quality control. The electropolisher has the problem of having no incoming quality control. The materials he polishes are brought to him by his customers. The materials of construction and surface finishes have been selected by the customer for his needs and purposes, not the electropolisher's. 

Quality control usually means that you have standards for materials that are brought into the shop. You then turn these materials into goods that meet a defined set of product standards. It is a generally accepted rule that if the raw materials are not up to specification, the final product will not be worth while. This is why incoming quality control is usually a description of packaging for final shipment. 

As a result, the typical quality assurance manual will state how the parts are handled. This is usually a detailed procedure describing material handling and process procedures. It will describe the cleaning of the part before and after processing. There is usually a description of packaging for final shipment. There is a practical difficulty in the writing of quality control specification, for electropolishing. If every shipment is a special case, how do you determine in advance how much material should be removed? How do you determine the process temperature or rectifier settings? 

Electropolishing is as much of an art as a science. It requires insight, intuition and an artist's touch. It is amazing what a good electropolisher can do. He can usually take several pieces of reasonably similar material and adjust the process so that all the parts come out acceptable. They may not all look identical, but they will usually be acceptable. When one is working with 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 shine. Take off an extra 0.00025" and remove some unacceptable scratches. There is a lot one can do to a part whose manufacturing tolerance is +/-.001", when you are electropolishing. It is very easy for a good supplier to turn out high quality parts. It is very hard to quantify how he does it.

Common Questions About The Process:

Does Electropolishing Clean Parts?
Electropolishing tries to remove the surface layer of the work piece. If it does so, then everything that was on the surface of the piece will be removed. If this is considered cleaning, then electropolishing will indeed clean parts. In general, it is best to remove all impurities and surface residue before polishing. 

Surface impurities can interfere with the process and make the final surface uneven. This uneven appearance is unacceptable to many customers. Care should be 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.

Does Electropolishing Sterilize Parts?
The electropolishing process also does not sterilize parts. It does, however, make the surface of the material so that it can be sterilized. Electropolishing removes most of the surface pits and crevices from equipment surfaces, eliminating traps and adhesion sites. Many consider this cleaner surface with dramatically fewer traps and pits to be able to become more sterile than surfaces which have not been electropolished.
How Does Electropolishing Help Me Achieve A Better Vacuum?
Electropolishing helps achieve a better vacuum by reducing the effective surface area inside a vacuum system. It also creates a passivation layer over the surface. This passivation layer acts an a sealer over the metal surface. This sealer helps prevent outgassing of gasses diffused into the metal.
Can You "Crack" Or Break An Electropolished Surface?
Bending 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 passivation layer and grain boundaries of the material will be disrupted in the areas affected by the distortion. This will destroy the integrity of the passivation layer in the affected area. 

Parts should always, if possible, be bent and machined prior to electropolishing. This allows the electropolishing process to smooth out the grain boundaries 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 bent at various angles. The tubing is then cut open to determine how much damage and distortion was done in the bending process. The results are usually related to particle counts given off by the damaged areas. Structural effects to the tubing are not usually a factor. 

Does Electropolishing Remove The Silver From The Nuts Of Fittings?
The electropolishing solution does an excellent job of removing silver from almost anything. As a result all silver surfaces must be carefully masked of before polishing. If you are not familiar with the polisher's masking 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 processing. This is a difficult problem for anyone just starting out, and the solutions to the problem are considered proprietary.
What Will Any Final Surface Finish (RA.) Be After Polishing And Will You Guarantee That Finish?
In 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, the less reliable the estimate. 

Most electropolishers will not guarantee surface finish because of the process sensitivity to metallurgy, machining and surface impurities. If a part has any of the above problems, the final surface finish may not be smooth or even across the part's surface. The fault is in the part, not the process. It is also beyond the control of the electropolisher. 

In specification work, one is typically paid only for work done to specification. 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 did a good job. That there is no government specification covering electropolishing because of these limitations. This does not mean the process is not viable; nor does it mean that it does not work. It is just very difficult to define.