Polishing Technology and Quality Requirements for Stainless Steel Reactors

Polishing Technology Fundamentals

• Creating a dense passivation layer for corrosion resistance
• Achieving low surface roughness to minimize media adhesion
• Ensuring easy cleaning and sterility compliance

Mechanical Polishing (Internal & External Surfaces)

1. Equipment: Rotary polishing machines with variable-speed drive
2. Tools: Flexible polishing discs, progressive grit polishing compounds
3. Process Steps:
   • Coarse grinding (80-120 grit) to remove welding marks
   • Medium grinding (240-320 grit) for surface leveling
   • Fine polishing (400-600 grit) to achieve smoothness
   • Final buffing with diamond paste for mirror finish

• Used for straight pipes and internal channels
• Specialized tools like flexible shafts and brush polishers
• Achieves roughness (Ra) ≤ 0.8μm for general applications

Electrolytic Polishing (Electrochemical Polishing)

• Anodic dissolution combined with passivation layer formation
• Low-voltage (6-12V), high-current (5-20A/dm²) electrolysis in acidic solutions
• Chemical reaction:
  Metal + Electrolyte → Metal Ion + Passivation Film

1. Uniform polishing of complex geometries (internal tubes, corners)
2. Micro-topography refinement:
   • Convex areas dissolve faster due to higher current density (tip effect)
   • Achieves Ra 0.2-0.4μm (mirror finish)
3. Enhanced passivation:
   • Increases chromium oxide (Cr₂O₃) content in the surface layer
   • Improves corrosion resistance by 30-50% compared to mechanical polishing

• Mixture of phosphoric acid, sulfuric acid, and glycerol
• Temperature controlled at 50-80°C for optimal film formation

Quality Control Standards

1. Surface Roughness (Ra):
   • Pharmaceutical grade: ≤ 0.4μm (internal surfaces)
   • Food grade: ≤ 0.8μm (external surfaces)
2. Passivation Layer Testing:
   • Blue dot test (ASTM A967) to verify chromium content
   • Salt spray test (ASTM B117) for corrosion resistance (≥ 500 hours)
3. Visual Inspection:
   • No visible scratches, pits, or discoloration under 10x magnification
   • Uniform gloss without polishing marks

Industry-Specific Requirements

• Pharmaceutical Applications:
  ▪ Inner surfaces must be electrolytically polished to prevent drug residue
  ▪ Sanitary clamp connections polished to match tube finish
• Food Industry:
  ▪ External surfaces treated with mechanical polishing for easy cleaning
  ▪ Welds ground and polished to eliminate crevices

Process Flow at Suote

1. Pre-treatment: Degreasing and acid pickling
2. Mechanical grinding: 3-stage progressive grit process
3. Electrolytic polishing (for critical surfaces)
4. Post-treatment: Deionized water rinsing and passivation
5. QC: Surface roughness measurement and visual inspection

Conclusion