How to pickle the surface of stainless steel

Acid cleaning and passivation process: Oil removal → Decontamination → Acid cleaning → Rinse with clean water → Passivation → Rinse with clean water → Drying. 2) Pre-treatment before acid cleaning and passivation: 2.1 Stainless steel containers or components that have been manufactured must be inspected for compliance with the requirements specified in the drawings and process documents; only after passing these inspections can they undergo acid cleaning and passivation pre-treatment. 2.2 Clean the weld seam and the slag and spatter on both sides of it; use gasoline or a cleaning agent to remove oils and other contaminants from the surface of the machined parts of the container. 2.3 When removing foreign objects from both sides of the weld, use a stainless steel wire brush, a stainless steel shovel, or a grinding wheel for removal; after cleaning, rinse thoroughly with clean water (with a chloride ion content of no more than 25 mg/l). 2.4 When the oil contamination is severe, use a 3-5% alkaline solution to remove the oil, and then rinse thoroughly with clean water. 2.5 The scale on hot-worked stainless steel parts can be removed by mechanical sandblasting; the sand must be pure silicon or alumina. 2.6 Establish safety measures for pickling and passivation, and determine the necessary tools and personal protective equipment. 3) Formulas for pickling, passivation solutions and pastes 3.1 Pickling solution formula: 20% nitric acid (specific gravity 1.42), 5% hydrofluoric acid, with the remainder being water. The above are volume percentages. 3.2 Acid cleaning paste formula: 20 milliliters of hydrochloric acid (specific gravity 1.19), 100 milliliters of water, 30 milliliters of nitric acid (specific gravity 1.42), and 150 grams of bentonite. 3.3 Passivation solution formula: 5% nitric acid (specific gravity 1.42), 4 grams of potassium dichromate, with the remainder being water. The above volume fraction of voids applies when the passivation temperature is at room temperature. 3.4 Passivation paste formula: 30 ml of nitric acid (67% concentration), 4 grams of potassium dichromate, along with bentonite (100–200 mesh), all mixed together until a paste is formed. 4) Acid washing and passivation process 4.1 Acid washing and passivation can only be carried out on containers or components that have undergone pretreatment. 4.2 Acid pickling is mainly used for the overall treatment of relatively small, unfinished components, and can be carried out by spraying. When the solution temperature is between 21–60°C, check every 10 minutes or so until a uniform white etched finish is achieved. 4.3 The pickling paste is mainly suitable for large containers or local treatment. At room temperature, apply the pickling paste evenly and thinly over the equipment (to a thickness of about 2-3 mm). After waiting for one hour, gently brush it away using clean water or a stainless steel brush until a uniform white etched finish is achieved. 4.4 Passivation solutions are mainly suitable for the overall treatment of small containers or components, and can be applied by immersion or spraying. When the solution temperature is between 48–60°C, inspections should be carried out every 20 minutes; when the temperature is between 21–47°C, inspections should be done hourly, until a uniform passivation film forms on the surface. 4.5 The passivation paste is mainly suitable for large containers or local treatment; apply it evenly to the surface of the pickled container at room temperature (about 2–3 mm thick), and check after 1 hour until a uniform passivation film has formed on the surface. 4.6 After acid washing and passivating containers or components, their surfaces must be rinsed thoroughly with clean water. Finally, an acidic litmus paper should be used to check any part of the rinsed surface to ensure that the pH value is between 6.5 and 7.5; thereafter, the surface should be dried either by wiping or with compressed air. 4.7 After acid washing and passivation, containers and components must not be subjected to impacts or scratches that could damage the passivation film during handling, lifting, and storage. 5) Precautions 5.1 When preparing the solution, water should be added to an acid-resistant container in the proper proportions, and then acid should be added slowly in the same proportions, to prevent splashing and injury caused by adding acid too quickly. 5.2 Acid washing and passivation: For the first time, testing should be carried out first on a small area of a component or on similar materials. 5.3 During the pickling process, operators must be careful and vigilant, monitoring the situation regularly to prevent excessive corrosion of the container surface. Effective measures should be taken for carbon steel components to prevent corrosion. 5.4 After pickling, the stainless steel surface shall show no obvious signs of corrosion, no patches with uneven coloring, and the welds and heat-treated surfaces shall be free of oxidized colors. 5.5 Operators must wear acid-resistant clothing, gloves, masks, and protective goggles while operating. When carrying out pickling inside a container, it is necessary to wear a gas mask that has passed inspection, and the operation must be conducted under the supervision of a qualified person. 5.6 Austenitic stainless steel components requiring corrosion resistance shall undergo heat treatment as specified in the drawings, followed by pickling and passivation. 5.7 The acid pickling and passivation formulas, as well as the operating procedures, concentration ratios, volume ratios, liquid temperature, and operation time mentioned above, can be adjusted slightly as appropriate depending on the circumstances. The pickling and passivation solution can be reused, but it is necessary to maintain the appropriate concentration and volume ratio. 5.8 After acid cleaning and passivation, blue dot testing is conducted; once the test is successful, the surface is sealed and left ready for spraying with ceramic coating.