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Pickling and passivation treatment: Pickling and passivation procedure: degreasing → pickling → passivation → washing → drying, etc. a) Process tests must be conducted on the workpiece before acid cleaning and passivation, in order to determine appropriate formulations and operating procedures (including safety protocols). Samples of the pickling and passivation solutions should be taken regularly for testing, so as to adjust their concentrations in a timely manner ； b) For stainless steel pressure vessels with carbon steel components, effective measures must be taken during the pickling process to prevent corrosion of the carbon steel parts ； c) Pickling and passivation should primarily be carried out by immersion; wet wiping or the use of pickling and passivation pastes is also acceptable. (1) Removal of oil stains: Stainless steel parts should have any oil stains and other foreign substances thoroughly removed before acid washing or passivation. Inspection method and requirements: Sprinkle water on the surface of the substrate; a uniform and continuous water film will form on the surface. This water film should remain intact for at least 30 seconds, with no other foreign substances or residues that could affect the quality of the part present on the surface. The cleaning method can involve using gasoline, alkaline solutions, or cleaning agents to remove oil stains, followed by rinsing with water; it is not allowed to scrub the surface of the workpiece with carbon steel brushes. (2) Acid washing: Typically, the treatment involves several steps such as loosening the oxide scale, etching, and removing the residues from the etching process. Operators must wear acid-resistant clothing, gloves, masks, and protective goggles while carrying out these operations. When pickling inside a container, a properly inspected gas mask must be worn, and the operation must be carried out under the supervision of a qualified person. The requirements for the pickling process are as follows: a) Remove the oxide scale: The solution contains 80 g/L to 120 g/L of nitric acid at a concentration of 66%; the solution temperature is room temperature, and the treatment time is less than 60 minutes ； b) Etching: The solution contains 200 g/L to 250 g/L of sulfuric acid at a concentration of 98%, and 80 g/L to 120 g/L of hydrochloric acid at a concentration of 36%; the solution temperature is 40°C to 60°C, and the process continues until the oxide scale is completely removed ； c) Removal of erosion residues: The solution contains 30 g/L to 50 g/L of nitric acid at a concentration of 66%, and 5 g/L to 15 g/L of hydrogen peroxide at a concentration of 30%; the solution temperature is room temperature, and the treatment time is 10 s to 60 s. Pickling can also be carried out using a pickling paste; the formula for this paste (for reference) is: 20 mL of hydrochloric acid (specific gravity 1.19), 100 mL of water, 30 mL of nitric acid (specific gravity 1.42), and 150 g of bentonite. Pickling paste pickling is mainly suitable for large containers or local treatment. At room temperature, apply the pickling paste evenly to the clean equipment (to a thickness of about 2 mm–3 mm), leave it there for one hour, and then gently brush it away with clean water or a stainless steel brush until a uniform white etched finish is achieved. The surface of the stainless steel after pickling must not show any visible traces of the cleaning solution, nor any patches with uneven coloring. Welds and surfaces that have been heat-treated must not exhibit any oxidized colors. After pickling, it must be rinsed thoroughly with clean water to ensure that no residual cleaning solution remains. (3) Passivation: Since the corrosion products formed on the metal surface during the corrosion process (the main component of the corrosion products of stainless steel is Cr2O3) are very dense and adhere firmly to the metal surface, they impede the corrosion process. This results in a decrease in the corrosion rate; this phenomenon is known as passivation. When treating parts, components, or finished products with a passivation solution, they must be completely immersed in the passivation solution to prevent severe corrosion of the portions above the liquid level. Stainless steel passivation can also be achieved using a passivation paste. The formula for this paste (for reference): 30 mL of nitric acid (67% concentration), 4 g of potassium dichromate, along with bentonite (100–200 mesh), which is added and mixed until a paste-like consistency is obtained. Passivation paste is mainly suitable for large containers or local treatment. Apply the passivation paste evenly to the surface of the pickled container at room temperature (about 2 mm to 3 mm thick), and check after 1 hour until a uniform passivation film has formed on the surface. (4) Washing and drying: The passivated stainless steel should be thoroughly cleaned immediately after being removed from the passivation solution; once it is at a neutral pH, any water residues should be wiped off or dried with hot air. If necessary, an additional step of dilute alkali neutralization can be added after the first wash to remove residual acid in complex cavities. The chloride content in water must not exceed 25 mg/L. (5) Recording: The operator shall record the main process parameters of pickling and passivation, and retain the recorded data. The records must be kept for at least 7 years. (6) Inspection after pickling and passivation a. Inspection of the passivation film: For stainless steel pressure vessels that require resistance to intergranular corrosion, inspection of the passivation film should be avoided on the surfaces in contact with the medium after surface treatment. The inspection of the passivation film shall be carried out in accordance with the specifications provided in the design drawings; if no such specifications are available, the blue-spot method shall be used (a solution is prepared by mixing 1 g of hematite salt with 3 mL of 65%–85% nitric acid and 100 mL of water. The filter paper is soaked in this solution and then applied to the surface to be tested, or the solution can be applied directly to that surface. If the passivation film on the surface is incomplete or contaminated with iron ions, a blue color will appear). The presence of blue spots within 30 seconds indicates that the sample fails the test. b. Color requirements: 1) The mechanically processed parts, after pickling, should retain the original color of the stainless steel ； 2) After pickling, the surface of heat-treated parts should be a dull light gray to dark gray due to differences in material composition ； 3) The surface of the parts after acid pickling and chemical passivation is a grayish color with a slight yellow tint. c. Unacceptable defects: 1) Residual oxides, mechanical impurities, dirt, etc. on the surface of the parts after pickling or passivation ； 2) Areas requiring passivation were not passivated ； 3) The parts have been corroded ； 4) The parts are not thoroughly dried, with residual moisture ； 5) There are severe color differences on the main surfaces of the same part. d. The inspection records of the passivation film must be retained for at least 7 years. e. Discharge of pickling and passivation solutions: The pickling solutions that need to be discharged must undergo neutralization; they can be released only after meeting the **wastewater discharge standards.