What is Mechanical Polishing?
Mechanical polishing is a finishing process whereby abrasives are used to smooth and/or polish a surface. Mechanical polishing removes corrosion, pitting, scratches or etchings all of 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 damage 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 typically reduces Ra by a further 50% of the resultant Ra from the final mechanical polishing step.
Additionally, electropolishing removes impurities, inclusions, and other embedded contaminants. 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
What is Electrolytic Cleaning?
When Astro Pak electropolishes, we process for a specified amount of time at a specified current density. Meeting this time + current setpoint ensures we have matched the parameters our process is qualified to, therefore meeting our roughness and passive layer composition acceptance criteria.
When Astro Pak electrolytic cleans (also referred to as “electrochemical cleaning” or “electrocleaning”), we are using the same tooling as electropolishing, but the process is performed at lower power and is not required to run for a specified amount of time. Electrolytic cleaning is processed until a visual criteria is met, while electropolishing requires a specific amount of process time regardless of whether the surface appears clean before the time is met.
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 that is then ready for additional polishing, passivation, or a return to production.
Weld color, or heat tint, can also be removed using electrolytic cleaning, also referred to as electrolytic or electrochemical pickling, as this process is limited to the outermost surface of the metal and thus does not require a deep electropolishing process to remove.
Conclusion
Understanding the differences between mechanical polishing, electropolishing, and electrolytic cleaning is key to selecting the right process for optimal surface enhancement. Combining these techniques with chemical cleaning and passivation provides a robust solution for superior corrosion resistance and contamination protection, resulting in a uniform conditioned surface that meets or exceeds ASME BPE and ASTM B912 standards. This approach not only ensures the best possible surface conditions but also extends the lifespan and reliability of your equipment.
Tailored to meet your specific needs, from pre-commissioning to maintenance, and available for on-site service to minimize downtime, the choice of method hinges on application requirements and the desired finish quality. Selecting the right process is essential for achieving the highest standards of purity and performance.
