4. Pickling and passivation of stainless steel pipelines
4.1 Requirements for pickling passivation
By pickling, the oxide scale formed by high-temperature operations such as heat treatment and welding can be removed.
(1) The pickling passivation of the outer surfaces near the welding seam can be passivated with pickling paste or pickling solution. The operation time is 30 minutes or according to the technical requirements of the pickling paste manufacturer.
(2) After pickling, there should not be obvious corrosion, black flow marks and iron oxide red marks on the surface of the pipe. There shall be no uniform streaks. The surface's metallic luster shall be consistent. There should not be oxidized colors on welding seams and hot-worked surfaces.
 
4.2 Pickling passivation processes
(1) Use pickling paste on the surface near the welding seam.
(2) Clean the debris on the surface first, and then apply the pickling passivation paste with a thickness of 1 to 2mm on the surface.
(3) Clean the surface according to the thickness of the surface oxide scale and the temperature.
(4) The passivation paste can be cleaned with a stainless steel brush, and can be cleaned with a high-pressure hydraulic giant until it is cleaned.
(5) The waste liquid in the pickling and passivation process is stored in the special recycle box.
 
In addition, in the pickling and purification process, carefully observe to avoid excessive corrosion of pipelines, and effective isolation measures should be taken for carbon steel pipelines to prevent corrosion.
 
5. Cleaning and drying after pickling and passivation
5.1 Requirements for media used for cleaning and drying
(1) The requirements for fresh water used for cleaning: the pH value should be 6 to 7 and the chloride ion content should meet the minimum requirements of the project.
(2) The requirements for gas used for drying: dry, oil-free compressed air or inert gas
 
5.2 Cleaning and drying processes
(1) After pickling and passivation, the internal area of ​​the pipeline should be rinsed repeatedly with fresh water immediately, and its pH value should be tested with pH test paper until the pH value is neutral.
(2) The waste liquid in the cleaning process should be stored in the designated special recycling box to prevent environmental pollution.
(3) Rinse with hot water several times after passivation, and the pH value should be neutral. All the cleaned parts should be dried with dry, oil-free compressed air or inert gas.
 
6. Inspection of pickling and passivation
6.1 Visual inspection
Visual inspection should be carried out under good lighting conditions, including general lighting and auxiliary lighting. Inspect the inner and outer surfaces of the pipeline at least 100 feet of lighting, preferably 250 feet of lighting.
 
After pickling and passivation, the welding spot and damaged areas on the inner and outer surfaces of the pipeline shall be free from paint, oil, grease, welding flux, spatter, dust, metal, abrasive particles and chips, and shall have a uniform color. There should not be oxide skin formed by high-temperature operations such as heat treatment and welding.
 
6.2 Wipe tests
Use a damp, clean, lint-free white cotton cloth or filter paper but not saturated to check the cleanliness of accessible surfaces. If there are stains on the cotton cloth after wiping the surface of the pipeline, it means that there are residual pollutants on the pipe wall.
 
7. Conclusion
(1) In order to ensure the safety of pickling and passivation, the participants of pickling and passivation must be aware of the dangers. Conduct strict training for operators, and take safety isolation measures in accordance with regulations.
(2) All operators should be equipped with a full set of labor protection.
(3) To prevent accidents in the pickling passivation process, sufficient rinse water and medicines for an emergency should be prepared.
(4) In order to ensure the quality of pickling and passivation of stainless steel pipelines, before formal pickling and passivation, all impurities on the surface of stainless steel pipelines should be removed, and a pickling passivation test should be carried out on the surface of the same or similar materials.
(5) The pH value of the fresh water used for cleaning before and after pickling and passivation is between 6 and 7, and the chloride ion content meets the minimum requirements of the project.
(6) It is necessary to ensure the passivation effect of pickling, avoid excessive corrosion of stainless steel pipelines, and isolate carbon steel pipelines.
(7) The waste liquid should be discharged to the designated box for recycling so as not to pollute the environment.