^ Electropolished Parts: Smooth & Glossy – Image Courtesy of ozguroral at ShutterStock.com
Old Process, New Applications
Are you tempted to believe that your Sheaffer pen is made from silver? It is so glossy and so smooth. The same goes for surgical instruments and those shiny pipes they use these days for gas exploration. And those shiny containers in the food industry? What makes them so lustrous?
Chances are, their makers have electro-polished them to provide such a brightly catchy and reflective look. Also known as electrolytic polishing or electrochemical polishing, electropolishing is a metal finishing process that chemically removes metal ion by ion in a controlled manner.
Such removal brings out the cleaner and smoother underlying layer that is free from thermal and mechanical stresses as also from ingrained contaminants. Furthermore, this layer is more passive i.e. corrosion resistant.
Utilized in the 1930s for upgrading the aesthetics of fountain pens and cookware, the process has now found application in multiple areas such as the medical, semiconductor, food, and pharmaceutical sectors that require components with remarkably clean and smooth surfaces.
For the same reasons of smoothness and hygiene, electropolishing finds application in making components that handle extremely pure gases and liquids. Electro-polished surfaces are also more compatible for physical vapor deposition (PVD).
Stainless Steel (SS) is widely used for commercial and consumer applications on account of its great strength and corrosion resistance. Such vast use makes it the most electro-polished metal.
The Electropolishing Process
If you observe an average metal surface under a microscope, it appears as a series of hills (micro-projections) and valleys (micro-depressions). The undulations are not immediately apparent to the naked eye. Nevertheless, they do exist.
Although mechanical finishing processes such as polishing, grinding, blasting, buffing, and sanding smoothen the surface, their action is limited. And they leave the surface smeared because by their very nature they are macro-level processes.
Electropolishing however is a micro-level process and as such complements the macro-level, mechanical processes. It does not leave behind any smears and provides a smooth and streamlined surface that is almost devoid of micro-projections and micro-depressions.
Again, electropolishing is an inherently impact-free process. It does not impact the workpiece – chemically, thermally, or mechanically. Therefore you can conveniently electro-polish very small and ultra-fragile metal parts.
As opposed to electroplating that deposits a metal film on another metal, electropolishing removes metal. Both processes employ electrolysis i.e. the passage of direct electric current through a salt solution for decomposing the solution and, sometimes, the electrode(s) into ions and directing their movement.
Electrolytic cells contain one positively and one negatively charged electrode immersed in a salt solution. The positively charged electrode is the anode while the negative electrode is the cathode.
In electropolishing, operators connect the workpiece to the anode. An inert metal such as lead or SS normally makes the cathode. Operators then dip both electrodes inside a temperature-controlled, acidic electrolyte.
When you pass direct electric current through this setup, the material of the workpiece steadily dissolves in the electrolytic solution and moves towards the cathode. This removes material from the surface of the anode.
Another thing you will witness when you pass current is the evolution of large amounts of oxygen at the anode. The oxygen forms a dense gaseous layer that facilitates the process of electropolishing. Hydrogen evolves at the cathode.
But for the surface of the workpiece to be made smooth, electropolishing must remove the micro-projections faster than it removes the micro-depressions. Such selective material removal is called anodic leveling.
Two major phenomena make anodic leveling possible:
- the tendency of the electric current to flow more from the micro-projections and less from the micro-depressions
- the viscous boundary layer formed at the workpiece surface
In fluid mechanics, boundary layer is the thin layer of almost-stationary fluid (liquid or gas) that immediately surrounds an object moving through the same fluid. Effects of viscosity are very potent in this layer. The atmosphere, for example, is the boundary layer surrounding the earth.
Then again, the electrolytic bath forms a film over the metal surface once you immerse the workpiece into it. This film is thickest over the micro-depressions and thinnest over the micro-projections.
Now, electrical resistance is minimum over the thinnest portions of the film. This means, current density is maximum over micro-projections. Metal removal rates are therefore maximum over these projections.
Metal removal rate is directly proportional to the composition and efficiency of the electrolyte, amount of current passed through the electrolyte, and the duration of passage of current.
Operators optimize these factors to obtain controllable metal removal of 0.0001 to 0.0025 inches. The beauty of electropolishing lies in its versatility, scalability, and customizability. This also makes the success of the process dependent on the skill of the operator-technician.
Electropolishing normally lowers surface roughness by half. If the roughness of a surface, for example, is 50 micro-inches (µ-in), electropolishing can reduce it to 25 µ-in.
For every subsequent dose of electropolishing, however, the amount of decrease in roughness decreases. This is because the surface becomes increasingly fine.
If, for example, the initial dose of electropolishing has lowered the roughness from 50 µ-in to 25 µ-in, the next dose may lower the surface roughness to 20 µ-in only.
Roughness Arithmetic Average (Ra) is the measure of surface roughness. This is the average height of the microscopic surface irregularities (micro-projections and micro-depressions) from the mean line measured within the sampling length.
Electropolishing works equally well on small hollow needles and large vessels and pipes. The normal duration for electropolishing is 1 to 20 minutes although operators sometimes use longer durations.
Technicians commonly employ a current density of 0.1 A/m2 (ampere per square meter). In electromagnetism, current density is the quantity of current flowing through per unit cross sectional area of a conducting material.
Other, typical process parameters include:
- For stainless steel (SS) of the 200 and 300 series: 150-1600F for 5-10 minutes at a direct electric voltage of 3-15V
- For SS of the 400 series: 1750F for 5 minutes at a direct electric voltage of 5-6V
- For carbon steels: 140-1600F for 5-10 minutes at a direct electric voltage of 3-6V
- For brass and copper: 70-1500F for 1-5 minutes at a direct electric voltage of 1-2V that may be scaled up to 15V
- For aluminum and its alloys: 100-1600F for 1-5 minutes at a direct electric voltage of 1-10V
Safety concerns include:
- highly corrosive and acidic electrolytic solutions
- buildup of oxygen and hydrogen
- large currents used to electro-polish large parts
Trapped gases can turn explosive in the presence of a spark. Stir the solution lightly and allow them to escape. For protection against large currents, use electrical equipment such as cables, rectifiers, and bus bars of the required capacity and quality.
Ensure that you train your operators and technicians. Also ensure that they use personal protective equipment (PPE) when handling the corrosive and acidic electropolishing solutions. For the same reason, use equipment that resists corrosion and acids.
Always ascertain that you dispose the rinse water in an environment-friendly manner as required by applicable laws. This disposal is more of a problem when you electro-polish large components on-site where equipment for handling such water may not be available.
Before electropolishing, you have to mechanically polish surfaces that are blasted with grit, sand, or glass-beads and those that are polished with coarse abrasives. You also have to clean all electro-polished parts to remove the electrolytic solution.
Electropolishing of high-quality welds is not an issue. Average and below-average welds present a challenge as electropolishing these welds brings out the concealed defects that you have to deal with.
Defects include inclusions, voids, alloy separation, phase modifications, and carbide precipitation. As cumbersome as it is, addressing these defects is a much better option than waiting for the joint to fail later. Why be sorry later when you can be safe now?
Merits, Applications, & Process Standards
Electropolishing boosts a metal’s and particularly stainless steel’s:
- Passivatione. corrosion resistance. Electropolishing removes iron and nickel from the surface layer of SS at a faster rate than it does chromium
A chromium-rich surface has a much better corrosion resistance. Please note that we add chromium to iron while making SS precisely because of chromium’s superior corrosion resistance
- Micro-Hygiene: electropolishing removes the microscopic scratches, smears, debris, and abrasives from the metal surface that mechanical polishing cannot
As a result, the surface is free from impurities. These impurities can facilitate corrosion by scratching the metal surface and/or providing access to the causatives of corrosion
- Fatigue Resistance: electropolishing removes the mechanically and/or thermally stressed metal surface layer thereby upgrading the metal’s fatigue resistance. This further hikes the corrosion resistance of the metal
- Aesthetics, Polish, and Reflectivity: the featureless and stress-free surface looks better, feels better and maintains the clean look for ultra-long durations
You can conveniently apply electropolishing to complex shaped and delicate work-pieces with very tight tolerances because the process does not induce mechanical, thermal, and chemical stresses. Also, electropolishing does not disturb the component’s original dimensions.
Smoother and cleaner surfaces are easier to sterilize and do not allow the accumulation or clogging of microscopic contaminants. Electropolishing therefore finds application in:
- sectors that require the highest standards of hygiene such as the medical, surgical, pharmaceutical, and food industries
- SS drums of washing machines
- thin metal samples for transmission electron microscopy, for an electro-polished surface does not deform the surface layers of the sample
- ultra high vacuum (UHV) components
- pipes for exploration and extraction of subterranean gases
Sharp, small burrs that you cannot remove using mechanical polishing are ideal targets for removal by electropolishing. Particularly, if these burrs dot the insides of minute holes. For removing large burrs, however, you have to use mechanical polishing.
- ASME BPE Standards for Electropolishing Bioprocessing Equipment
- ASTM B 912-02 (2008): Passivation of Stainless Steels Using Electropolishing
- SEMI F 19: Electropolishing Specifications for Semiconductor Applications
- ASTM E1558: Standard Guide for Electrolytic Polishing of Metallographic Specimens
A process with multiple merits, electropolishing is perhaps the ultimate finishing process. As useful as it is, the process is not a complete alternative to mechanical finishing processes, but more of a complementary procedure.
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