Astro Pak offers a full selection of cleaning and surface polishing services tailored to the individual need of each client. Individually, mechanical polishing, electropolishing, and electrolytic cleaning can be standalone processes. However, combining one or more of these services together will optimally address certain requirements. When used with our chemical cleaning and passivation services, customers will experience the ultimate in corrosion and contamination protection for extended, reliable performance.
Astro Pak uses custom-designed tooling to ensure optimal contact with intricate stainless-steel surfaces, along with proprietary chemistry, for superior results. The surface roughness reduces to a level at or below the originally specified surface finish. “Ra,” or surface roughness, is confirmed using microscopic and profilometry testing to ensure absolute quality assurance. Our profilometers gauge surface roughness while Ultrasonic testers measure tank wall thickness. Recording these measurements before and after remediation allows our highly trained technicians to produce results that meet or exceed industry standards.
Astro Pak performs all three services on-site to help customers maintain or improve upon the surface finish conditions of vessel interiors or exteriors with minimal downtime. When used as part of the pre-commissioning of a vessel, our services increase system reliability by overcoming manufacturing and installation-generated issues. Our specially trained confined space entry crews are highly skilled in servicing equipment that are difficult to access. They can work 24/7 to quickly return process vessels, large or small, back into service. In addition, we also offer these services in our nationwide network of shops.
What is Mechanical Polishing?
Mechanical Polishing is a surface finishing process whereby abrasives are used to smooth and/or polish a surface. This results in uniform surface conditioning to meet or exceed ASME BPE and ASTM B912 standards. Mechanical polishing removes corrosion, pitting, scratches or etchings which compromise the passive layer of stainless steel, creating a low-chromium area that is more susceptible to corrosion. This damage can be caused by mechanical actions or chemical reactions. Mechanical Polishing also smooths welds where ferrules and valves have been replaced or added. Mechanical polishing alone is often sufficient for dry product vessels. For other applications, it is most often the first step prior to electropolishing and passivation.
What is Electropolishing?
Electropolishing is an electrochemical surface finishing process using a DC current delivered through an electrolyte solution to dissolve the surface of metal to a limited depth. The end result is a finish that meets or exceeds biopharmaceutical surface finish requirements. It can reduce Ra measurements by up to half that from mechanical polishing alone.
Additionally, electropolishing removes impurities, inclusions, and other embedded contaminants, also known as the Bielby layer, which forms during mechanical polishing. The result is a smooth, mirror-like finish where residue, contaminants and even single-cell bacteria, will have no place to lodge. Often, electropolishing is the final process in preparing a vessel for service. However, combining electropolishing with passivation achieves a smooth chromium rich surface with improved cleanability and superior resistance to corrosion. This level of finish is critical for high purity applications.
Benefits of Electropolishing
- Improve Cleanability
- Enhance Durability
- Improve Sterility
- Increase Corrosion Resistance
Microscopic View of Electropolishing
Electrolytic cleaning is a short-form version of Electropolishing using similar tooling and chemistries. Rouge and other metallic surface contaminants can be rapidly removed from austenitic stainless (300 series) with an ELC (electrolytic cleaning) process.
In fact, since these contaminants are not ‘bonded’ to the surface nearly as tightly as the base metal, their removal takes considerably less time. Rouge is released from the surface rather quickly compared to base metal, and the substrate is left in a decontaminated state which is then ready for additional polishing, passivation, or a return to production. Weld color or heat tint can also be removed using electrolytic cleaning, as this process is limited to the outermost surface of the metal and thus does not require a deep electropolishing process to remove.
Frequently Asked Questions
Electropolishing smoothes the surface by removing material to form a more glassy or shiny surface. Passivation improves the surface chemistry to reduce corrosion potential by removing iron from the top few molecular levels with no effect to surface finish.
Electropolishing is good for creating a surface that is cleaner, smoother, and more representative of the base metal than any other method currently in use. This lends itself to bio-pharmaceutical applications where vessels and equipment must be easily verified as being clean. Electropolishing also reduces surface area (vacuum applications), passivates the surface to a degree, and removes any imbedded impurities that may be present following mechanical polishing.
Electropolishing removes material while electroplating deposits material. The two processes are very similar in that both use electrolytes to carry an electrical current, but with different polarity.
An electropolished surface is only as durable as the base metal itself. Rough handling and chloride damage are the two principle types of damage we repair most often.
Most metals can be electropolished, but some of the necessary chemicals are very hazardous. With our current chemistries, we do not electropolish titanium or aluminum.
ASTM B-912 is the most common spec requested, but current BPE SF4 specs are a close second. There are many different semiconductor industry specs as well.
Electropolishing removes the peaks and cleans the valleys. It only removes surface material and does not fill in gaps.
Mechanical polishing uses abrasives to cut the surface, while electropolishing uses chemicals and electricity to dissolve the surface.
Electropolishing removes the peaks and cleans the valleys. It only removes surface material and does not fill in. Typically, to achieve BPE specs, two to four ten-thousandths of an inch of surface material is removed.
Electropolishing can reduce existing surface roughness (Ra) by up to 50%, but Ra is only part of the story. EP’ed surfaces are flatter and smoother on a microscopic level, an order of magnitude finer than Ra alone can explain.
Astro Pak’s field methods allow us to progressively polish very large vessels. Complexity is usually more of a challenge than size.
That depends on the definition of ‘better’. Electropolishing provides a smoother surface, but does not provide the enhanced chrome / iron ratio that proper passivation does.
Not with our current chemistries. Most metals can be electropolished, but some of the necessary chemicals are very hazardous.