Stainless Steel Pipe De-greasing, Cleaning and Passivation

I. De-greasing of Stainless Steel Pipes

Before cleaning and passivating stainless steel pipes, they must undergo de-greasing to remove residual oils from the inner walls due to processing. Typically, industrial alcohol or dichlorodifluoromethane (CCL2F2) is used as de-greasing agents in pharmaceutical water systems.

Most stainless steel pipes used in pharmaceutical water systems are small-diameter pipes. De-greasing can be achieved through immersion by placing the pipe into a tank filled with de-greasing agent for approximately 10 minutes. During this time, the pipe should be turned 3-4 times. After removal, it should be blown dry using oil-free compressed air or nitrogen. The criteria for successful de-greasing include:

  1. Wiping the surface with a clean white filter paper; absence of oil stains indicates success.
  2. Blowing the de-greased part with oil-free steam; if condensate can suspend a particle of camphor less than 1mm in diameter, the process is deemed successful.
  3. Using ultraviolet light (wavelength 320-380nm) to check the surface; absence of oil fluorescence signifies completion.
  4. Sampling the solvent after de-greasing; if the fat content does not exceed 350mg/L, the process is considered successful.
  5. For solvent-degreased pipes, all remaining solvents must be thoroughly purged until no odor remains. For pipes degreased using an alkaline solution, rinse thoroughly with oil-free water until neutral pH is reached. Chloride content in water used for rinsing stainless steel pipes should not exceed 25ppb.

II. Cleaning of Stainless Steel Pipes

Both interior and exterior surfaces of welded parts need to be cleaned and pickled to restore their resistance to microbial corrosion. Subsequently, thorough rinsing with hot water is required. When installing gas pipelines, sterile-filtered dry compressed air should be used throughout the installation to ensure cleanliness. Installation and cleaning procedures must be established and approved by users. Guidelines for cleaning steam pipelines will be discussed later. Below is a reference cleaning procedure:

  1. Manual Cleaning Method: Use a clean cloth to drag along the inner surface of the pipe or blow it clean with compressed air until there’s no dirt left.
  2. Chemical Cleaning Method: This method is typically applied post-welding installation. The basic steps involve:
    • A chemical solution containing 15% nitric acid (HNO3), 1% hydrofluoric acid (HF), and 83% water (H2O).
    • Heating the solution to 49-60°C, pumping it into the pipe system, and soaking for 15 minutes. Then drain and rinse with deionized water until neutral pH is reached.

III. Passivation of Stainless Steel Pipes

During welding, heating and cooling alter the microstructure and surface condition of austenitic stainless steel, making it more susceptible to corrosion compared to non-welded areas. Without proper passivation, materials like 304L and 306L can develop rust within about a year when used in injection water systems. Passivation forms a protective layer on the surface that enhances its resistance to oxidation and corrosion at room temperature.

For passivation methods:

  1. Example Method One: Circulate a solution of 10% nitric acid and 0.5% chromic acid at 60°C for one hour, followed by thorough draining, rinsing, and reinstallation of cleaned components.
  2. Example Method Two: Flush with a mixture of 80% purified water and 20% nitric acid (65%-68% concentration) at ambient temperature under pressure of 0.2-0.3MPa for two hours.
  3. Example Method Three: Rinse with a solution of 94% purified water and 6% sodium nitrite under static conditions for half an hour, then maintain for two hours.
  4. Example Method Four: Circulate an 8% nitric acid solution prepared with distilled water at 49-52°C for one hour.
  5. Example Method Five: Clean with 10% trisodium phosphate at 70°C for half an hour and then with 2% nitric acid for another thirty minutes before final rinsing.

These methods ensure thorough cleaning and passivation, enhancing the longevity and performance of stainless steel piping systems in pharmaceutical applications.