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AISI 316L and Surface Treatments for Pharma Use - CNC Manufacturing

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Preparing Components in AISI 316L and Selection of Surface Treatments for Pharma Use - CNC Manufacturing: A Technical Approach

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15 ottobre 2024
The preparation of pharmaceutical-grade components from AISI 316L stainless steel involves multiple stages, including precise CNC machining and specific surface treatments to meet the regulatory and functional requirements of the sector. The selection of correct tolerances and surface finishes is vital to ensuring components are suitable for sterile environments and high-precision applications, such as volumetric pumps or manifolds. This article delves into the technical aspects of CNC machining for AISI 316LVM, along with surface treatments like electropolishing and passivation, based on standards such as ASME BPE 2024 [1], ASTM F138 [2], and ISO 10993 [3].
1. AISI 316L in Pharmaceutical Applications
1.1 Chemical Composition and Key Properties
AISI 316L stainless steel is particularly suited for Pharma applications due to its low carbon content (<0.03%), which minimizes carbide precipitation and reduces the risk of intergranular corrosion during repeated sterilization cycles [1]. The addition of molybdenum (2-3%) enhances resistance to pitting and crevice corrosion, particularly in chloride-rich environments, such as those found in cleanroom environments or saline-based Pharma processes [4]. In any case, the process must be thoroughly validated and inspected. When the environment is not suitable for AISI 316LVM, ceramic-based components (@Neoceram) and PEEK (polyether ether ketone) components can be used as alternatives.
1.2 Mechanical Properties and Use Case Selection
The choice of AISI 316L over other materials is based on its balance between mechanical strength and corrosion resistance. Typical tensile strength values are around 485 MPa, with elongation at break often exceeding 40% [5]. These properties make AISI 316L suitable for components that require a combination of toughness and flexibility, such as manifolds and fluid distribution components. However, for high-precision equipment like volumetric pumps, which have tolerance requirements in the micrometer range, the challenge lies in achieving not only the necessary dimensions but also the required surface finishes [6].
2. CNC Machining of AISI 316L for Pharma Use
2.1 Tolerances Based on Component Application
The CNC machining process for AISI 316L varies significantly based on the final component's functional requirements. For instance:
  • Volumetric pumps: Require tolerances as tight as some microns to ensure there are no gaps that could cause leakage or contamination during fluid transfer [6]. These components also demand highly controlled surface roughness to meet sterility [1].
  • Manifolds and simpler fluid distribution components: Can typically maintain standard tolerances of 0.1 mm, which are sufficient for non-critical connections or fluid management systems. However, the surface finish remains crucial, particularly in preventing microbial contamination [7].
2.2 Surface Roughness and Standards: ASME BPE for Stainless Steel in Pharma
Surface roughness (Ra) plays a significant role in ensuring the cleanliness and sterility of pharmaceutical equipment. According to the ASME BPE-2024 (Bioprocessing Equipment) Standard [1], surface roughness directly impacts the ability of surfaces to resist bacterial growth and biofilm formation, particularly in high-purity environments such as cleanrooms and sterile filling areas. ASME BPE specifies different roughness levels depending on the application.
Mechanical polish (MP), widely used in standard pharmaceutical processes, generally results in roughness values of:
  • Ra ≤ 0.50 μm (20 μin) for non-critical areas [1].
  • Ra ≤ 0.38 μm (15 μin) for surfaces that come into direct contact with the product [1].
For applications requiring ultra-high purity, such as biotechnology and parenteral drug production, electropolishing is essential. This process further reduces the roughness to:
  • Ra ≤ 0.38 μm (15 μin) as a standard [15].
  • In extreme cases, surfaces can reach Ra ≤ 0.25 μm (10 μin) [15], which is critical for environments where even the smallest surface imperfections could harbor contaminants.
The smoother the surface, the easier it is to clean, which directly influences CIP (Clean-In-Place) efficiency [17], reducing cleaning times and improving process turnaround. Additionally, smoother surfaces minimize the risk of contamination during sterilization, which is a core requirement for pharmaceutical processes governed by FDA regulations [16]. Obviously this process, controlled and verified costs so to reduce the processes steps and manual operations we are working on custom manufacturing units that are able to process also little batches without a big amount of reprogramming time.
3. Surface Treatments for AISI 316L Components in Pharma
3.1 Electropolishing
Electropolishing is a crucial step in surface preparation for pharmaceutical applications. The process smooths microscopic peaks and valleys on the metal surface, improving its corrosion resistance and making it easier to sterilize [9]. Standard electropolishing conditions for AISI 316L involve a current density of 0.1-0.3 A/cm² in a sulfuric and phosphoric acid bath, which removes up to 50 µm of material and reduces surface roughness to Ra ≤ 0.4 µm [10].
3.2 Passivation: Enhancing Corrosion Resistance
Post-electropolishing, the passivation process is essential to remove any remaining free iron from the surface, thereby enhancing the natural chromium oxide layer that protects against corrosion [11]. Passivation for AISI 316L typically involves immersion in a nitric acid solution (20-25% concentration) at temperatures of 50-60°C for 30-45 minutes, following ASTM A967 guidelines [12]. The passivated surface meets the strict requirements for corrosion resistance in pharmaceutical processes, including compliance with ASME BPE 2024 and GMP standards [1,13].
3.3 Alternative Surface Treatments
In addition to electropolishing and passivation, coatings like PTFE or PVD are used in applications where low friction or enhanced wear resistance is required. For example, in dynamic systems such as valves or mixers, PTFE coatings can significantly reduce the risk of surface damage and contamination [14].
4. Regulatory Requirements and Surface Finishing Standards
4.1 Compliance with ASME BPE-2024 and GMP Standards
Both surface treatments and machining processes for AISI 316L components must comply with the ASME BPE-2024 standards, which define acceptable surface finishes, tolerances, and material traceability [1]. The standard emphasizes maintaining Ra values below 0.5 µm for product contact surfaces, and passivation is mandated for components that undergo repeated sterilization cycles [13]. Additionally, the FDA's GMP requirements specify that all treatments must be fully documented and traceable, ensuring quality control throughout the manufacturing process [8].
Conclusion
AISI 316L stainless steel remains the material of choice for many pharmaceutical components, thanks to its excellent combination of corrosion resistance and mechanical properties. However, achieving the necessary precision and surface finish requires careful selection of machining techniques and surface treatments such as electropolishing and passivation. Compliance with standards like ASME BPE 2024 ensures that the components will meet the rigorous requirements of the pharmaceutical industry.
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Bibliography
  1. ASME BPE 2024. Bioprocessing Equipment Standard, 2024.
  2. ASTM F138. Standard Specification for Stainless Steel Bar and Wire for Surgical Implants (UNS S31673), 2013.
  3. ISO 10993-1. Biological evaluation of medical devices – Part 1: Evaluation and testing within a risk management process, 2018.
  4. W. Revie and H. H. Uhlig. Corrosion and Corrosion Control, 4th Edition, Wiley, 2008.
  5. ASTM A276. Standard Specification for Stainless Steel Bars and Shapes, 2021.
  6. S. Kalpakjian and S. Schmid. Manufacturing Processes for Engineering Materials, 6th Edition, Pearson, 2017.
  7. FDA. Pharmaceutical GMPs for the 21st Century – A Risk-Based Approach, 2004.
  8. R. K. Gupta et al. Surface Finish and Contamination Control in Bioprocess Equipment, BioPharm International, 2020.
  9. J. R. Davis. Surface Engineering for Corrosion and Wear Resistance, ASM International, 2001.
  10. ASTM A380. Standard Practice for Cleaning, Descaling, and Passivation of Stainless Steel Parts, Equipment, and Systems, 2013.
  11. ASTM A967. Standard Specification for Chemical Passivation Treatments for Stainless Steel Parts, 2017.
  12. M. Ohring. Materials Science of Thin Films, 2nd Edition, Academic Press, 2002.
  13. B. Crook. The Microbiological Quality of Pharmaceuticals: Sterility, Bioburden and Data Integrity, CRC Press, 2019.
  14. T. S. Sudarshan. Surface Modification Technologies XV, Technomic Publishing, 2002.
  15. Herring, D. "Electropolishing for Corrosion Resistance." Heat Treating Progress, 2015.
  16. FDA. "Sterility Testing and Surface Requirements." U.S. Food and Drug Administration, 2020.
  17. Brindley, D. "Passivation of Stainless Steel in Pharmaceutical Applications." Journal of Bioprocess Engineering, 2017.