What are pickling and passivation?

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Pickling and passivation is a chemical reaction used to remove rust from the surface of steel. It involves a thorough acid washing and passivation of stainless steel to remove various types of oils, rust, oxide scales, weld spatter, and other contaminants; after treatment, the surface becomes a uniform silver-white color. This process **improves the corrosion resistance of stainless steel and is suitable for various types of stainless steel parts, sheets, and related equipment. It is simple to operate, easy to use, and cost-effective. It also contains efficient corrosion inhibitors and anti-fogging agents to prevent excessive corrosion and hydrogen embrittlement in metals, as well as to suppress the formation of acid fog. It is particularly suitable for small and complex workpieces, in situations where paste application is not feasible; it outperforms similar products on the market. Depending on the type of stainless steel and the severity of the oxide scale, it can be used in its pure form or diluted with water at a ratio of 1:1 to 1:4 ； Ferritic, martensitic, and austenitic stainless steels with low nickel content (such as 420, 430, 200, 201, 202, 300, 301, etc.) are used after dilution, while austenitic stainless steels with higher nickel content (such as 304, 321, 316, 316L, etc.) are soaked in their original form ； It is generally used at room temperature or after being heated to 50–60 degrees. Soak it for 3–20 minutes or longer (the exact time and temperature should be determined by the user based on their own testing), until the surface dirt is completely removed and a uniform silver-white color appears, with a uniform and dense passivation film formed. After treatment, take it out and rinse it thoroughly with clean water; it is advisable to rinse it further with alkaline water or lime water to neutralize it. The necessity of acid washing and passivation for stainless steel: Stainless steel possesses excellent corrosion resistance, resistance to high-temperature oxidation, good performance at low temperatures, as well as superior mechanical and electrical properties. Therefore, it is widely used in industries such as chemicals, petroleum, power, nuclear engineering, aerospace, marine, pharmaceuticals, light industry, and textiles. Its main purpose is to prevent corrosion and rust. The corrosion resistance of stainless steel relies primarily on its surface passivation film; if this film is incomplete or defective, the stainless steel will still be susceptible to corrosion. In engineering, pickling and passivation are typically carried out to maximize the corrosion resistance of stainless steel. During the processes of forming, assembling, welding, weld inspection (such as flaw detection and pressure testing), and marking stainless steel equipment and components, surface contaminants such as oil, rust, non-metallic dirt, low-melting-point metal pollutants, paint, slag, and spatter can accumulate. These substances affect the surface quality of the stainless steel equipment and components, damage their oxide layer, reduce the steel’s resistance to general corrosion as well as to local corrosion (including pitting and crevice corrosion), and may even lead to stress corrosion cracking. Cleaning, pickling, and passivation of stainless steel surfaces not only maximize corrosion resistance but also prevent product contamination and enhance appearance. According to GBl50-1998 \"Steel Pressure Vessels\", the surfaces of vessels made of stainless steel or composite steel plates that require corrosion protection must be pickled and passivated. This requirement applies to pressure vessels used in the petrochemical industry, as these devices are used in environments where they come into direct contact with corrosive substances; therefore, to ensure corrosion resistance, acid washing and passivation are deemed necessary. For other industrial sectors, where stainless steel is used not for corrosion prevention but solely for cleaning and aesthetic reasons, acid washing and passivation are not required. However, the welds of stainless steel equipment still need to be pickled and passivated. For nuclear engineering, certain chemical processing equipment, and other applications with stringent requirements, in addition to acid washing and passivation, a final fine cleaning using high-purity media or finishing treatments such as mechanical, chemical, and electrolytic polishing must be carried out. The principle of pickling and passivation of stainless steel: The corrosion resistance of stainless steel is mainly due to a very thin (about 1 nm) and dense passive film that covers its surface. This film acts as a barrier, isolating the metal from corrosive media; it is the fundamental protective layer for stainless steel. Stainless steel passivation has dynamic characteristics; it should not be viewed as a complete cessation of corrosion, but rather as the formation of a diffusing barrier that **reduces** the rate of anodic reactions. Typically, in the presence of reducing agents (such as chloride ions), it tends to destroy the film; whereas in the presence of oxidizing agents (such as air), it can maintain or repair the film. When stainless steel parts are exposed to air, an oxide film forms on them, but the protective properties of this film are not sufficient. Typically, a thorough cleaning process, including alkali washing and acid washing, must be carried out followed by passivation with an oxidant, in order to ensure the integrity and stability of the passivation film. One of the purposes of pickling is to create favorable conditions for passivation, ensuring the formation of a high-quality passivation film. Because pickling removes an average layer of 10 μm thick from the surface of stainless steel, the chemical activity of the acid causes the dissolution rate to be higher in areas with defects compared to other parts of the surface; thus, pickling helps to achieve a uniform balance across the entire surface, eliminating potential areas that are prone to corrosion. But more importantly, through pickling and passivation, iron and its oxides dissolve preferentially over chromium and its oxides, removing the chromium-poor layer and resulting in an accumulation of chromium on the surface of the stainless steel. The potential of this chromium-rich passivation film can reach +1.0V (SCE), which is close to the potential of precious metals, thereby enhancing the corrosion resistance. Different passivation treatments also affect the composition and structure of the film, thereby influencing its corrosion resistance. For example, through electrochemical modification, the passivation film can be given a multi-layer structure; CrO3 or Cr2O3 can be formed in the barrier layer, or a glassy oxide film can be created, enabling stainless steel to achieve maximum corrosion resistance. I. Methods for acid washing and passivating stainless steel: The immersion method is suitable for components that can be placed in acid washing or passivation tanks; it is not appropriate for large equipment. The acid solution can be used for an extended period of time, resulting in high production efficiency and low costs ； Filling large-volume equipment with acid results in excessive liquid consumption. The coating method is suitable for the inner surfaces of large-scale equipment and for partial treatment in situations where working conditions are poor and acid cannot be recovered. The paste method is used in installation or maintenance sites, especially for handling welding areas, where manual operations are required, working conditions are poor, and production costs are high. The spraying method is used at the installation site; it requires less liquid, is less costly, and works faster for coating the inner walls of large containers, but it necessitates the use of spray guns and a circulation system. The circulation method is used for large-scale equipment such as heat exchangers and shell-and-tube units; it is convenient for construction, the acid can be reused, and piping and pumps are required to connect the circulation system. The electrochemical method can be used for both components and for surface treatment of on-site equipment using brushes; it is a relatively complex technique that requires a DC power supply or a potentiostat. II. Acid washing and passivation process: Oil removal and dirt cleaning → Washing with clean water → Passivation → Rinsing with clean water → Drying. III. Pre-treatment before acid washing and passivation: 3.1 Stainless steel containers or components that have been manufactured must undergo inspections to ensure they meet the requirements specified in the drawings and process documents; only after passing these inspections can they be subjected to acid washing and passivation pre-treatment. 3.2 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. 3.3 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; after cleaning, rinse thoroughly with clean water (with a chloride ion content of no more than 25 mg/l). 3.4 When the oil contamination is severe, use a 3-5% alkaline solution to remove the oil, and then rinse thoroughly with clean water. 3.5 The oxide scale on heat-treated stainless steel parts can be removed by mechanical sandblasting; the sand must be pure silicon or alumina. 3.6 Formulate safety measures for pickling and passivation, and determine the necessary tools and personal protective equipment. IV. Formulas for pickling, passivation solutions and pastes 4.1 Pickling solution formula: 20% nitric acid (specific gravity 1.42), 5% hydrofluoric acid, and the remainder is water. The above are volume percentages. 4.2 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. 4.3 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. 4.4 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. V. Acid washing and passivation procedures 5.1 Acid washing and passivation can only be carried out on containers or components that have undergone pretreatment. 5.2 Pickling with pickling solution is mainly used for the overall treatment of smaller, unprocessed parts; it can be applied by spraying or brushing. When the solution temperature is between 21–60°C, check every 10 minutes or so until a uniform white etched finish is achieved. 5.3 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. 5.4 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. 5.5 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. 5.6 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 level is between 6.5 and 7.5; thereafter, the surface should be dried either by wiping or with compressed air. 5.7 During handling, lifting, and storage of containers and components after acid pickling and passivation, it is prohibited to damage or scratch the passivation film. VI. Precautions 6.1 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. 6.2 For pickling and passivation treatment, a test should be conducted first on a local surface of a small component or on a similar material. 6.3 During the pickling process, operators must be careful and attentive, monitoring the situation regularly to prevent excessive corrosion of the container surface. Effective measures should be taken to prevent carbon steel components from corroding. 6.4 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. 6.5 Operators must wear acid-resistant clothing, gloves, masks, and protective goggles while operating. When pickling inside a container, one must wear a properly inspected gas mask and carry out the process under the supervision of a designated person. 6.6 Austenitic stainless steel components requiring corrosion resistance shall undergo heat treatment as specified in the drawings, followed by pickling and passivation. 6.7 The aforementioned pickling and passivation formulations, as well as the operating conditions such as concentration, volume ratio, liquid temperature, and processing time, may be subject to minor and appropriate adjustments as needed. The pickling and passivation solution can be reused, but it is necessary to maintain the appropriate concentration and volume ratio. VII. Inspection Methods The quality inspection of surfaces that have undergone pickling and passivation can generally be considered from the following three aspects: 1. Visual inspection – The surface after pickling and passivation should appear as a uniform silver-white color. There should be no obvious signs of corrosion; the surfaces of the welds and heat-affected zones must be free from any oxidation color or uneven discoloration. 2. Inspection of residual liquid: Use phenolphthalein test paper to check the degree to which the residual liquid on the surface has been washed away; a neutral pH value indicates compliance. 3. Blue dot test (inspection of passivation film density): To assess the quality of the pickling and passivation treatment, the blue dot test can also be used as needed. This test involves preparing a solution by mixing 1 gram of potassium ferrocyanide K3 with 3 milliliters of (65%–85%) nitric acid HNO3 and 100 milliliters of water (it is advisable to prepare this solution 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 on the surface are observed within 30 seconds; the presence of such blue spots indicates failure. It should be noted that this test must be carried out after the surface that has been pickled and passivated is basically dry, and the testing solution should also be rinsed clean after the test. The basic principle of the blue spot test is as follows: if the surface passivation film is imperfect or contaminated with iron ions, free iron ions will be present, and the following reaction will occur: Fe2+ + K3PO4 → KFe(CN)6↓ (deep blue) + 2K+. The blue spot test is generally used in the manufacturing of stainless steel chemical containers; it is a very stringent testing method, and strict adherence to the prescribed procedures is necessary in order to pass the test. 4. Testing of carbon steel particles on the passivated surface: Dissolve 8 grams of CuSO4 in 500 milliliters of distilled water, add 2–3 milliliters of H2SO4, then drop the prepared solution onto the passivated surface and keep it moist; if no copper precipitation occurs within 6 minutes, it is considered acceptable. Stainless steel tanks are pickled and passivated using a pickling and passivation paste, which results in a dense and intact oxide film. This paste does not contain sulfides or oxides, and the chloride ions in it are strictly controlled. The pickling and passivation paste for stainless steel tanks is safe for enhancing the resistance of welded joints to crystallization and corrosion, thereby improving the corrosion resistance of the stainless steel tanks. It can also be applied to a range of stainless steel storage tanks, such as stainless steel filter tanks, stainless steel mixing tanks, stainless steel water tanks, etc. Suggestions: 1. Stainless steel should be stored in a dedicated area; wooden planks or rubber mats must be used as a floor covering, and it is strictly prohibited to store it together with carbon steel ； 2. All types of processing equipment, such as rolling machines and sheet cutting machines, must be cleaned; rollers and corners should be coated with varnish to ensure that stainless steel does not come into direct contact with carbon steel ； 3. The tooling fixtures are made of stainless steel or lined with stainless steel ； 4. The wire brush used for pickling should be made of stainless steel.