What are the surface treatment requirements for medical grade degreased stainless steel seamless

The core requirements for the surface treatment of medical de-greased seamless stainless steel tubes are: clean without residues, smooth surface, stable passivation, and bio-safety. They must also meet the requirements of medical contact safety, sterilization tolerance, and usage stability. The specific requirements are as follows: 

Core requirements for degreasing treatment:

Thoroughly remove organic substances such as oils, oil stains, and cutting fluids adhering to the product during production and processing.

After degreasing, there should be no visible oil stains or oil spots on the surface. Under ultraviolet light or fluorescent detection, no fluorescent reaction should occur.

The residual amount of organic substances should comply with medical standards (typically ≤ 5mg/m²), and no residual cleaning agents (such as acid-base cleaning solutions) should be present.

The commonly used degreasing process is "alkaline degreasing + acid pickling + rinsing", and the final rinsing water should be neutral (pH 6.5 - 7.5). 

2. Surface roughness requirements

For ordinary medical scenarios (such as infusion tubes, external devices): Inner and outer surface roughness Ra ≤ 0.8 μm.

For implantable devices (such as orthopedic screws, cardiovascular stents): Surface roughness Ra ≤ 0.2 μm (requires electrolytic polishing EP treatment).

The surface should be free of burrs, scratches, depressions, and oxide films. The depth of scratches shall not exceed 10% of the wall thickness, and there should be no sharp edges.

The inner wall should be smooth and uniform, avoiding steps, wrinkles, and preventing drug residue or bacterial growth. 

3. Requirements for passivation treatment

After degreasing, a passivation treatment must be carried out to form a uniform and dense Cr₂O₃ passivation film (thickness 5-10nm).

The passivation film must be tested by the blue dot test: after adding the blue dot reagent, no blue dots should appear within 30 seconds, indicating that the passivation film is intact.

The passivation process should be environmentally friendly and non-toxic. Commonly used methods are nitric acid passivation or citric acid passivation. Avoid using passivation agents containing chromium (VI).

After passivation, thorough rinsing is required, with residual passivation solution ≤ the limit value to prevent chemical reactions with body fluids and drugs. 

4. Containment of Pollutants and Residues

Solid pollutants such as metal debris, dust, fibers, etc. must comply with ISO 14644-1 Class 7 or higher standards. Particle pollutants should meet the standard of ≤ medical limit for heavy metal ions (such as lead, cadmium, mercury), and the amount of nickel ions released should be ≤ 0.5 μg/cm²/week (in accordance with ISO 10993-15).

There should be no visible rust, pitting, or color spots. The surface color should be uniform and consistent, and there should be no local oxidation or color change. 

5. Sterilization Compatibility Requirements

After surface treatment, it must be able to withstand common medical sterilization methods, including high-temperature and high-pressure steam (134℃, 0.2MPa), ethylene oxide (EO), and gamma-ray sterilization.

After repeated sterilization (≥100 times), there should be no peeling, detachment, or cracking on the surface, and the passivation film remains intact. The corrosion resistance does not diminish.

After sterilization, there should be no odor or release of harmful substances, and it should not affect the stability of the drug or the tissue compatibility with the human body. 

6. Compliance and Testing Requirements

The surface treatment must comply with ASTM A967 (standard for stainless steel passivation) and GB/T 25150 (specification for surface treatment of medical stainless steel).

A surface roughness test report, a degreasing cleanliness test report, and a passivation film integrity test report must be provided.

For medical devices in direct contact with the human body, the surface treatment must pass the relevant biological compatibility tests (such as cytotoxicity, sensitization, etc.) as per ISO 10993.