Stainless steel pickling and passivation

Our company also encounters such problems when carrying out acid cleaning and passivation; we try to identify a dozen or so points on the surface of the heads after acid cleaning, but there are no specific regulations or standards available.

Stainless steel is resistant to acids by nature; pickling is merely a method for degreasing, and rinsing it with clean water is sufficient. Steel and aluminum products need to be passivated after pickling. (For reference)

A formula of 20% nitric acid + 15% hydrofluoric acid + 65% water. There are many formulas for acid pickling and passivation solutions as well as blue spot test reagents, and these should be analyzed, screened, and compared in advance. The pickling and passivation treatment involves the use of the pickling – passivation – blue spot test ； There are also methods that combine acid washing, passivation, and inspection using the blue spot test. The latter eliminates one processing step. Below are two integrated formulas for pickling and passivation, provided for your selection: Formula 1: 20% nitric acid, 10% hydrofluoric acid, 70% water. Instructions: First, prepare the solution according to the formula and soak the material at room temperature for 15–30 minutes; after that, rinse it thoroughly with clean water. Recipe 2: 10%–15% nitric acid, with the remainder being water. Note: First, prepare the solution according to the recipe and soak it at room temperature for 1–1.5 hours; after that, rinse it thoroughly with clean water. The acid washing and passivation of stainless steel can be carried out by immersing it in an acid washing and passivation solution; if the use of a liquid is not convenient, it is acceptable to apply an acid washing and passivation paste. After surface treatment, the area should be rinsed thoroughly with water, and phenolphthalein test paper should be used to check the degree of cleaning. Blue dot test: Prepare a solution by adding 1 gram of potassium ferrocyanide K3 to 3 milliliters of (65%~85%) nitric acid HNO3 and 100 milliliters of water (it is advisable to prepare it just before use). Then, after soaking filter paper in the solution, it is attached to the surface to be tested, or the solution is applied or dropped directly onto that surface; blue spots should appear within 30 seconds, and the presence of such blue spots indicates that the sample is unqualified. It should be noted that this test must be carried out after the surface treated with acid washing and passivation has basically dried. After the test, the test liquid should also be rinsed clean. The basic principle of the blue spot test is as follows: if the surface passivation layer is imperfect or there is iron ion contamination, free iron ions will be present, and then the following reaction occurs: Fe2+ + K = KFe↓ (deep blue) + 2K

It should be to randomly select a few points; it’s not practical to conduct a blue-dot method test on the entire area.

There are standards; what exactly are they? Could you tell me the standard numbers?

Standard Procedure for Acid Washing and Passivation of Stainless Steel 1 Subject Matter and Scope 1.1 Subject Matter This procedure specifies the requirements, methods, and precautions for removing oil and rust from the surfaces of stainless steel containers (including their components), as well as for carrying out acid washing and passivation. 1.2 Scope of Application These procedures apply 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 should 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 Rules for the Manufacturing of Stainless Steel Pressure Vessels\" \"General Inspection Rules\" 3 Acid washing and passivation process: Oil removal, cleaning of contaminants, rinsing with clean water, passivation, rinsing with clean water, drying. 4 Pre-treatment before acid washing and passivation: 4.1 After the stainless steel vessels or components have been manufactured, it is necessary to check that they meet the requirements specified in the drawings and process documents for the relevant items before proceeding with acid washing and passivation pre-treatment. 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 scale on heat-treated 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, passivation solutions and pastes 5.1 Pickling solution formula: 20% nitric acid (specific gravity 1.42), 5% hydrofluoric acid, and the remainder is water. The above are volume percentages. 5.2 Pickling paste formula: 20 ml of hydrochloric acid (specific gravity 1.19), 100 ml of water, 30 ml 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 is obtained at a passivation temperature of room temperature. 5.4 Blunting paste formula: 30 ml of nitric acid (67% concentration), 4 grams of potassium dichromate, along with bentonite (100–200 mesh), 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 preprocessing. 6.2 Pickling solution is mainly used for the overall treatment of relatively small, unfinished components, and can be applied 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 Pickling paste pickling 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. 6.4 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. 6.5 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 2–3 mm thick), and check after 1 hour until a uniform passivation film has formed on the surface. 6.6 Containers or components that have been pickled and passivated must have their surfaces rinsed thoroughly with clean water. Finally, an acidic litmus paper should be used to test 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. 6.7 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. 7 Precautions 7.1 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 that could cause injury due to too rapid addition of acid. 7.2 For pickling and passivation treatment, the first operation should be tested on a small area of a component or on similar materials first. 7.3 During the pickling process, operators must be careful and observe closely to prevent excessive corrosion on the surface of the containers. Effective measures should be taken for carbon steel components to prevent corrosion. 7.4 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. 7.5 Operators must wear acid-resistant clothing, gloves, masks, and protective goggles while operating. When pickling inside a container, it is necessary to wear a gas mask that has passed inspection, and the operation must be carried out under the supervision of a qualified person. 7.6 Austenitic stainless steel components requiring corrosion resistance shall undergo heat treatment as specified in the drawings, followed by acid washing and passivation. 7.7 The aforementioned acid pickling and passivation formulas, as well as the operating procedures, concentration ratios, volume ratios, liquid temperature, and operation time, 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. 24. What is passivation? Answer: The phenomenon in which the chemical reactivity of an active metal or alloy **decreases, resulting in it assuming a noble metal state, is called passivation. If the corrosion products formed on a metal as a result of the action of a medium have a dense structure, forming a thin film (often invisible) that covers the metal’s surface tightly, this changes the surface condition of the metal. As a result, the metal’s electrode potential shifts toward positive values, leading to a corrosion-resistant passive state. For example, when Fe transforms to Fe2+, the standard electrode potential is –0.44 V; after passivation, this value jumps to +0.5–1 V, resulting in the corrosion-resistant properties typical of noble metals. This thin film is known as a passivation film. The blue dot method is merely an intuitive testing approach without any specific testing criteria. As long as the reagent is applied, blue will appear if the surface passivation film is imperfect or there is iron ion contamination. At this point, it indicates that the passivation film is unsatisfactory, and re-passivation is required. Conversely, the absence of blue dots indicates compliance. However, the reagents must be prepared in a certain ratio. One of the reagents has the following formula: 1 gram of hematite salt + 3 milliliters of 65–85% HNO3 + 100 milliliters of water. After configuration, soak filter paper in the solution and attach it to the surface to be tested, or apply the solution directly to the surface.

Is the standard for floor 8 the enterprise standard? Well, the method we use is similar to it: 30 milliliters of nitric acid (67% concentration) and 4 grams of potassium dichromate are used, along with bentonite (100–200 mesh), and all these ingredients are mixed together until a paste is formed. This method works perfectly well.

Does stainless steel need to be pickled and then passivated? Otherwise, it will corrode more quickly, right? Moreover, for steel and aluminum products, they should be pickled but not passivated; otherwise, potassium dichromate might have an impact, right? Steel should be treated to be black or blue (including phosphating), while aluminum products should have their surfaces treated by cathodic oxidation.

The method for pickling and passivating stainless steel involves using 10% concentrated nitric acid mixed with 30% concentrated hydrochloric acid; nitric acid is added first followed by hydrochloric acid, and then the mixture is poured into water. The ratio of water to acid should be 1:1, and after adjusting this ratio, pickling and passivation can take place. It is also possible to add the adjusted acid and water to bentonite with a particle size of 100–200 mesh and stir until a paste-like consistency is achieved; this method is also fully effective.