Abstract: Stainless steel pipelines in LNG projects will inevitably be damaged during transportation, hoisting or prefabrication welding. In order to repair and protect stainless steel pipelines in time, it is necessary to pickle and passivate the welding positions and damaged areas of stainless steel pipelines. The process and treatment requirements of pickling and passivation of stainless steel pipes in LNG projects are explained in this article.
 
Pickling and passivation adopt chemical methods to clean, pickle and passivate the outer and inner surfaces of stainless steel and other damaged parts, that is, pickling is used to remove all kinds of oil, rust, oxide scale, welding spots and other dirt on the welding spots and damaged areas of stainless steel; then, passivation is used for protection. The inner and outer surfaces of the stainless steel pipeline are uniform silver-white after pickling and passivation treatment, and a uniform and dense passivation film can be formed, which can greatly improve the corrosion resistance of the stainless steel pipeline.
 
During the fabrication of stainless steel pipes, the welding process can damage the outer and inner surfaces of the pipe near the welding joint. Stainless steel pipelines may damage the exterior and interior surfaces of the pipeline due to transportation, hoisting, or other factors. Pickling can remove the oxide film and rust in this part, and then passivation is taken to protect the pipeline.
 
1. Requirements for pickling and passivation
Use pickling passivation paste or pickling solution to clean surfaces near welding joints and other damaged areas of stainless steel pipelines, including the inner and outer surfaces of the pipeline.
 
2. The processes of pickling and passivation
Preparations → safety measures → pickling passivation tests → formal pickling passivation → cleaning and drying acceptance → inspection
 
3. Preparations before pickling and passivation
3.1 Preparations before pickling and passivation
(1) Complete the welding, pressure release and non-destructive testing of pickling and passivation pipelines.
(2) According to the process requirements, the pipeline to be pickled and passivated must be checked and qualified.
(3) Prepare the cleaning package and cleaning tracking table for the pipeline.
 
3.2 Cleaning welding spots and damaged areas of the pipeline before pickling and passivation
Clean the welding seam one by one according to the direction of the pipeline's cleaning package, and make a record; there should be no residual contaminants such as paint, oil, welding slags, heat treatment formations, impurities, debris, metal residues and abrasive debris after cleaning according to the following methods. Document the record. The pipeline should be cleaned as follows:
 
(1) Use a stainless steel wire brush, a stainless steel shovel or alumina grinding wheel to clean the welding slag, spatter and other foreign objects in the areas on both sides of the welding.
(2) Use clean fresh water to remove grease, oil, mud, sand and all other contaminants before prefabrication or final cleaning preparation. The pH of the cleaning fresh water should be between 6 and 8. For stainless steel pipes, the chloride ion content should not exceed the minimum value required by the project.
(3) For places with serious oil contamination, use an alkali solution with a concentration between 3% and 5% to clean, and rinse with clean water.
 
3.3 Safety measures
(1) All personnel involved in pickling and passivation must be aware of its danger, and operators of pickling and passivation must be trained; they can only operate if they are qualified after training.
(2) Prepare sufficient water and medicines for an emergency.
(3) Prepare the necessary labor protection articles, including acid-resistant clothing, gloves, masks, and protective goggles.
(4) Special recycling boxes: The residues from pickling and passivation should not be discarded casually. Special containers for collecting residues should be provided on site. After neutralization, they can be put at designated places.
(5) Set isolation belts, signal lights and warning signs in the operation area.
(6) There must be a safe passage running through the entire construction area.
(7) The operator must be careful and observe in time to prevent excessive corrosion on the surface of the pipeline. Effective measures should be taken for carbon steel parts to prevent corrosion.
(8) For the process of pickling and passivation, special attention and supervision are required. Safety passages that meet the standards need to run through the entire area, and testing, cleaning and all operations must be carried out in accordance with the announced health and safety work plan. Raise special vigilance when using hazardous chemicals.
 
3.4 Pickling passivation tests
Before the formal pickling and passivation, the pickling passivation test should be carried out for stainless steel pipelines with small diameters or pipes made of similar materials to observe the pickling passivation effect.