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Standard Procedure for Acid Washing and Passivation of Stainless Steel 2007-08-30 23:03 1 Subject Matter and Scope 1.1 Subject Matter This procedure specifies the requirements, methods, and precautions for removing oil and rust from the surface of stainless steel containers (including their components), as well as for carrying out acid washing and passivation processes. 1.2 Scope of Application These procedures are applicable to the pickling and passivation treatment of chromium and nickel austenitic stainless steel vessels manufactured by our company. 2 Referenced Documents The following referenced standards and documents shall be the latest versions. When the present procedures conflict with new standards or document contents, the conflicting parts shall be governed by the relevant provisions of the latest standards and documents. \"General Regulations for the Manufacturing of Stainless Steel Pressure Vessels\" \"General Inspection Regulations\" 3 Acid washing and passivation process: Oil removal, cleaning of contaminants, rinsing with clean water, passivation, rinsing again with clean water, and drying. 4 Pre-treatment before acid washing and passivation: 4.1 For stainless steel vessels or components that have been manufactured, acid washing and passivation pre-treatment can only be carried out after checking that all specified items meet the requirements outlined in the drawings and process documents. 4.2 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. 4.3 When removing foreign objects from both sides of the weld, use a stainless steel wire brush, a stainless steel shovel, or a grinding wheel to clean them away; after cleaning, rinse thoroughly with clean water (with a chloride ion content of no more than 25 mg/l). 4.4 When the oil contamination is severe, use a 3-5% alkaline solution to remove the oil, and then rinse thoroughly with clean water. 4.5 The oxide scale on thermally processed stainless steel parts can be removed by mechanical sandblasting; the sand must be pure silicon or alumina. 4.6 Establish safety measures for pickling and passivation, and determine the necessary tools and personal protective equipment. 5 Formulas for pickling solutions, passivation solutions, and pastes 5.1 Formula for the pickling solution: 20% nitric acid (specific gravity 1.42), 5% hydrofluoric acid, with the remainder being water. The above are volume percentages. 5.2 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. 5.3 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. 5.4 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. 6 Acid Cleaning and Passivation Procedures 6.1 Acid cleaning and passivation can only be performed on containers or components that have undergone pretreatment first. 6.2 Acid pickling is mainly used for the overall treatment of smaller, 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. 6.3 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 with clean water or a stainless steel brush until a uniform white etched finish is achieved. 6.4 Passivation solutions are mainly suitable for the overall treatment of small containers or components; immersion or spraying methods can be used. When the solution temperature is between 48–60°C, inspections should be carried out every 20 minutes, while when the temperature is between 21–47°C, inspections should be done hourly, until a uniform passivation film forms on the surface. 6.5 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. 6.6 After pickling and passivating containers or components, their surfaces must be rinsed thoroughly with clean water. Finally, use acidic litmus paper to test any part of the rinsed surface to ensure that the pH value is between 6.5 and 7.5, after which the surface should be dried either by wiping or with compressed air. 6.7 After acid washing and passivation, containers and components must not be bumped, scratched, or have their passivation film damaged during handling, lifting, or storage.