Pickling and passivation of stainless steel surfaces

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1. Necessity of acid washing and passivation for stainless steel: Austenitic stainless steel possesses good corrosion resistance, resistance to high-temperature oxidation, satisfactory performance at low temperatures, as well as excellent mechanical and processing 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 aesthetics. According to GBl50-1998 \"Steel Pressure Vessels\", the surfaces of vessels manufactured from stainless steel and composite steel plates that require corrosion protection shall be pickled and passivated. This regulation 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 units, and other applications with stringent requirements, in addition to pickling and passivation, high-purity media are used for final thorough cleaning, or mechanical, chemical, and electrolytic polishing treatments are employed for further refinement. 2. Principle of acid washing and passivation of stainless steel: The corrosion resistance of stainless steel is primarily due to a very thin (about 1 nm) layer of dense passivation film on its surface; this film isolates the surface from corrosive agents and serves as the fundamental barrier for protecting 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. Generally, it tends to damage the membrane in the presence of reducing agents (such as chloride ions), while it can maintain or repair the membrane in the presence of oxidizing agents (such as air). When stainless steel parts are exposed to air, an oxide film forms on them, but this film does not provide sufficient protection. Typically, a thorough cleaning is required first, including alkali and acid cleaning, followed by passivation with an oxidizing agent, 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. Since pickling removes an average layer of 10μm thick from the surface of stainless steel through corrosion, 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 acid 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 exhibit maximum corrosion resistance. Scholars at home and abroad have conducted extensive research on the formation of stainless steel passivation films. A brief overview is given using recent research by Beijing University of Science and Technology on the photoelectron spectroscopy (XPS) of the passivation film on 316L steel as an example. Stainless steel passivation involves the dissolution of the surface layer due to certain factors and the adsorption of water molecules; under the catalysis of oxidants, oxides and hydroxides are formed, which undergo reaction transformations with the Cr, Ni, and Mo elements that make up the stainless steel. This process results in the formation of a stable protective film that prevents the destruction of the film and the occurrence of corrosion. Its reaction mechanism is as follows: Fe•H2O + O* ≈ ad + H++e; ad ≈ ad + H++e; ad + H2O ≈ FeOOH + O* + H++e; ad ≈ FeO + O*; FeOOH + Cr + H2O ≈ CrOOH + Fe•H2O; 2FeOOH ≈ Fe2O3 + H2O; 2CrOOH ≈ Cr2O3 + H2O; MO + 3FeO + 3H2O ≈ MOO3 + 3Fe•H2O; Ni + FeO + 2H2O ≈ NiO + Fe•H2O. (Here, Os represents the catalyst in the passivation process, and its concentration remains constant during passivation; ad denotes the adsorbed intermediate.) ) It can be seen that on the outermost layer of the 316L passivation film, there are Fe2O3, Fe(OH)3, or γ-FeOOH; Cr2O3, CrOOH, or Cr(OH)3; and MO in the form of MOO. The main components of the passivation film are CrO3, FeO, and NiO. 3. Methods and processes for pickling and passivating stainless steel 3.1 Comparison of pickling and passivation methods There are various methods for pickling and passivating stainless steel equipment and components, depending on the procedures used; their applicable ranges and characteristics are shown in Table 1. 3.2 Examples of pickling and passivation formulations 3.2.1 General treatment According to ASTMA380—1999, taking 300-series stainless steel as an example: (1) Pickling – Solution: 6%–25% HNO3 + 0.5%–8% HF (by volume) ； Temperature 21～60℃ ； Time as needed ； or 5%–10% (by mass) ammonium citrate as a chemical agent ； Temperature 49–71℃ ； Time: 10–60 min. (2) Passivation solution: HNO3 at 20％～50％ (volume fraction) ； Temperature 49–71℃ ； Time: 10–30 min ； or temperature 21～38℃ ； Time: 30–60 min ； Or chemical solution: 20%–50% HNO3 + 22%–6% Na2Cr2O7·2H2O (by mass) ； Temperature 49–54℃ ； Time: 15–30 min ； or temperature 21～38℃ ； Time: 30–60 minutes. (3) Descaling pickling solution: H2SO4 8％～11％ (volume fraction) ； Temperature 66～82℃ ； 5–45 min in 6 inches ； And chemical solution: HNO3 6％～25％ + HF 0.5％～8％ (by volume) ； Temperature 21～60℃ ； Or 15％–25％ HNO3 + 1％–8％ HFl (by volume). 3.2.2 Treatment by paste method (1) Taking the local passivation of the welds and base metal on the inner surface of new urea production equipment made of stainless steel at Guangzhou Petrochemical, as well as the passivation of surfaces that have been ground after repair, as an example: The pickling paste is prepared by mixing 25% HNO3 + 4% HF + 71% condensed water (by volume) with BaSO4 to form a paste. Passivation paste: 30% HNO3 or 25% HNO3 + 1% (by mass) K2Cr2O7 mixed with BaSO4 to form a paste. Coat the surface for 5–30 minutes, then rinse with condensate water until the pH reaches 7; chemical passivation using hydrogen peroxide spraying can also be applied to individual devices. (2) Taking the patent m of Shanghai Daming Iron Factory as an example. Pickling passivation paste: HN03 at 8%–14% (as a passivator) ； HFl 0%–15% (as a corrosive agent) ； Magnesium stearate: 2.2%–2.7% (as a thickening agent); Magnesium nitrate: 60%–70% (as a filler to improve adhesion and permeability) ； Sodium polyphosphate 2.3%–2.8% (as a corrosion inhibitor) ； Water (to adjust viscosity). 3.2.3 Electrochemical treatment: Taking a patent from Xiamen University as an example, the treatment method involves using the stainless steel workpiece to be treated as the anode, and carrying out anodization by controlling the potential at a constant level; or alternatively, the stainless steel workpiece is first used as the cathode for cathodization at a constant potential, after which it is again used as the anode for anodization at a constant potential, with the potential being adjusted further to achieve passivation. HNO3 is used as the electrolyte solution in all cases. After such treatment, the properties of the stainless steel passivation film are improved, and its corrosion resistance is **enhanced. The pitting critical potential (Eb) increases by about 1000 mV (in 3% NaCl), and the resistance to uniform corrosion improves by three orders of magnitude (in 20%–30% H2SO4 at 45°C). 4. Application scope of stainless steel pickling and passivation 4.1 Pickling and passivation during the manufacturing of stainless steel equipment 4.1.1 Cleaning, pickling, and passivation after machining After machining, stainless steel workpieces often have residues such as iron shavings, steel particles, and cooling emulsions on their surfaces, which can cause stains and rust on the surface of the stainless steel. Therefore, it is necessary to carry out degreasing, followed by cleaning with nitric acid, in order to remove the iron shavings and steel particles while also achieving passivation. 4.1.2 Cleaning, pickling, and passivation before and after welding: Since grease is a source of hydrogen, gas bubbles can form in welds where grease has not been removed. Moreover, contamination by low-melting-point metals (such as zinc-rich paint) can cause cracking after welding; therefore, it is necessary to clean the groove area as well as the surfaces within 20 mm on either side before welding stainless steel. Grease can be removed by wiping with acetone, while paint and rust should first be removed using sandpaper or a stainless steel brush, followed by cleaning with acetone. Regardless of the welding technique used in the manufacturing of stainless steel equipment, it is necessary to clean the material after welding; all slag, spatter, stains, and oxidation residues must be removed. The methods for removal include mechanical cleaning and chemical cleaning. Mechanical cleaning includes grinding, polishing, and sandblasting/shot blasting. The use of carbon steel brushes should be avoided to prevent rusting on the surface. To achieve the best corrosion resistance, it can be immersed in a mixture of HNO3 and HF, or acid pickling passivation paste can be used. In practice, mechanical cleaning and chemical cleaning are often used in combination for cerium-4. 4.1.3 Cleaning of forged and cast parts: Stainless steel components that have undergone heat treatment such as forging or casting often have an oxide layer, lubricants, or other contaminants on their surface; these contaminants include graphite, molybdenum disulfide, carbon dioxide, and others. It should be treated by shot blasting, salt bath treatment, and multiple acid washing processes. The treatment process for stainless steel turbine blades in the United States is as follows: salt bath (10 min) → water quenching (2.5 min) → sulfuric acid washing (2 min) → cold water washing (2 min) → alkaline permanganate bath (10 min) → cold water washing (2 min) → sulfuric acid washing (1 min) → cold water washing (1 min) → nitric acid washing (1.5 min) → cold water washing (1 min) → hot water washing (1 min) → air drying. 4.2 Acid washing and passivation treatment before the commissioning of new units: Stainless steel equipment and pipelines in many large-scale chemical, textile, fertilizer, and other types of plants require acid washing and passivation before they are put into operation. Although the equipment has been pickled in the manufacturing plant to remove slag and scale, contamination by grease, sediment, rust, etc. is inevitable during storage, transportation, and installation. To ensure that the quality of the devices and equipment as well as the products produced during testing (especially chemical intermediates and refined products) meets the required standards and to guarantee a successful first test run, pickling and passivation are necessary. For stainless steel equipment and pipelines in H2O2 production plants, cleaning must be carried out before operation; otherwise, contaminants and heavy metal ions can poison the catalyst. Furthermore, substances such as grease and free iron ions on metal surfaces can cause the decomposition of H2O2, releasing large amounts of heat, which can lead to fires or even explosions. Similarly, for oxygen pipelines, trace amounts of oil and metal particles can also generate sparks, leading to serious consequences. 4.3 Acid washing and passivation during on-site maintenance In the equipment and materials used in production facilities for purified terephthalic acid (PTA), polyvinyl alcohol (PVA), acrylic fibers, acetic acid, etc., austenitic stainless steels such as 316L, 317, and 304L are widely employed. Due to the presence of harmful ions such as Cl-, Br-, SCN-, and formic acid in these materials, or as a result of dirt and material aggregation, pitting, crevice corrosion, and weld corrosion can occur in the equipment. During shutdown for maintenance, the equipment or components can be subjected to full or partial pickling and passivation treatments to repair their passivation layer and prevent the spread of local corrosion. Acid washing and passivation have been carried out for tasks such as the renewal and maintenance of stainless steel pipes in the PTA unit at Shanghai Petrochemical, as well as the maintenance of stainless steel heat exchangers in the acrylic fiber unit. 4.4 Descaling and cleaning of operating equipment: In petrochemical plants, stainless steel equipment, especially heat exchangers, accumulate various types of deposits on their inner surfaces over time as a result of operation. These deposits include carbonate scale, sulfate scale, silicate scale, iron oxide scale, organic deposits, and catalyst residues. Such deposits impair the heat exchange efficiency and can also lead to corrosion beneath the scale. An appropriate cleaning agent must be selected for descaling; options include nitric acid, nitric acid + hydrofluoric acid, sulfuric acid, citric acid, EDTA, water-based cleaners, etc., with an appropriate amount of corrosion inhibitor added. After descaling and cleaning, passivation can be performed if necessary. Chemical treatment. The stainless steel heat exchangers in facilities such as Shanghai Petrochemical’s PTA, acetic acid, and acrylic fiber plants have all been cleaned to remove scale. 5. Precautions for acid washing and passivation of stainless steel 5.1 Pretreatment before acid washing and passivation: If there are surface contaminants on the stainless steel parts prior to acid washing and passivation, they should be removed through mechanical cleaning, followed by oil and grease removal. If the pickling solution and passivation solution cannot remove grease, the presence of grease on the surface will affect the quality of pickling and passivation; therefore, degreasing is essential, and this can be achieved using alkalis, emulsifiers, organic solvents, and steam. 5.2 Control of Cl- in pickling solutions and rinsing water: Some stainless steel pickling solutions or pastes use chloride-containing corrosive agents such as hydrochloric acid, perchloric acid, ferric chloride, and sodium chloride as main components or additives to remove the surface oxide layer. Chlorine-containing organic solvents such as trichloroethylene are used to remove grease, but these are not very suitable for preventing stress corrosion cracking. Furthermore, industrial water can be used for the initial rinsing water, but strict control is required over the halide content in the water used for the final cleaning. Deionized water is usually used. For water used in hydraulic testing of petrochemical austenitic stainless steel pressure vessels, the C1- content should be kept below 25 mg/L. If this requirement cannot be met, sodium nitrate can be added to the water to achieve the desired level. An excessive C1- content can destroy the passivation layer of stainless steel, leading to pitting, crevice corrosion, stress corrosion cracking, and other issues. 5.3 Process control in pickling and passivation operations: Nitric acid solution is effective for removing free iron and other metallic contaminants, but it is ineffective against removing iron oxide scale, thick corrosion products, and tempering films. Generally, a HNO3+HF solution should be used; for convenience and operational safety, fluorides can be used as a substitute for HF. A pure HNO3 solution can be used without a corrosion inhibitor, but when using HNO3+HF for pickling, Lan-826 must be added. Use HNO3+HF for pickling; to prevent corrosion, the concentration should be maintained at a 5:1 ratio. The temperature should be below 49°C; if it is too high, HF will evaporate. For the passivation solution, the concentration of HNO3 should be maintained between 20% and 50%. According to electrochemical tests, a passivation film formed at a HNO3 concentration lower than 20% has unstable quality and is prone to pitting; however, the HNO3 concentration should not exceed 50% either, in order to avoid over-passivation. Although treating with the one-step method for degreasing, pickling, and passivation is simple and saves time, the pickling and passivation solution (paste) contains corrosive HF; as a result, the quality of the resulting protective film is inferior to that obtained using the multi-step method. During the pickling process, the concentration of the acid, temperature, and contact time can be adjusted within certain limits. As the acid cleaning solution is used for an extended period of time, it is necessary to pay attention to changes in acid concentration and metal ion concentration. Care should be taken to avoid over-acid cleaning; the titanium ion concentration should be below 2%, otherwise severe pitting will occur. Generally, increasing the pickling temperature accelerates and improves the cleaning effect, but it may also increase the risk of surface contamination or damage. 5. Control of pickling under sensitized conditions in stainless steel: Certain stainless steels become sensitized due to improper heat treatment or welding; pickling with HNO3 & HF may cause intergranular corrosion. Cracks resulting from this intergranular corrosion can concentrate halides during operation, cleaning, or subsequent processing, thereby leading to stress corrosion. These sensitized stainless steels are generally not suitable for descaling or pickling using an HNO3+HF solution. If such pickling is necessary after welding, ultra-low carbon or stabilized stainless steel should be used. 5. Acid pickling of stainless steel and carbon steel assemblies: When acid pickling and passivation are carried out on assemblies made of stainless steel and carbon steel (such as stainless steel tubes, tube sheets, and carbon steel shells in heat exchangers), the use of HNO3 or HNO3+HF can cause severe corrosion of the carbon steel; in such cases, an appropriate corrosion inhibitor such as Lan-826 should be added. When stainless steel and carbon steel assemblies are in a sensitized state and cannot be pickled using HNO3+HF, hydroxyacetic acid (2%) + formic acid (2%) + a corrosion inhibitor at a temperature of 93°C for 6 hours can be used, or an ammonium EDTA neutral solution + a corrosion inhibitor at a temperature of 121°C for 6 hours. After that, the components should be rinsed with hot water and immersed in a solution containing 10 mg/L of ammonium hydroxide + 100 mg/L of hydrazine. 5.6 Post-treatment of acid washing and passivation: After acid washing and water rinsing, the stainless steel parts can be immersed in an alkaline permanganate solution containing 10% (by mass) NaOH + 4% (by mass) KMnO4 at 71–82°C for 5–60 minutes to remove residues from the acid washing process. Subsequently, they should be thoroughly rinsed with water and dried. If speckles or stains appear on the surface of stainless steel after pickling and passivation, they can be removed by scrubbing with fresh passivation solution or nitric acid of higher concentration. The stainless steel equipment or components that have been acid-washed and passivated should be properly protected; they can be covered or wrapped with polyethylene film to prevent contact between different metals and non-metals. The treatment of acidic and passivation waste liquids must comply with **environmental discharge regulations**. For fluoride-containing wastewater, lime milk or calcium chloride can be used for treatment. Try to avoid using chromates in the passivation solution; if chromium-containing wastewater is present, ferrous sulfate can be added for reduction treatment. Pickling can cause hydrogen embrittlement in martensitic stainless steel; if necessary, deoxidization can be achieved through heat treatment (heating to 200°C and holding for a period of time). 6. Quality inspection of stainless steel pickling and passivation: Since chemical testing can damage the product’s passivation layer, inspections are usually carried out on samples. Examples of the methods are as follows: (1) Copper sulfate titration test: 8 g of CuSO4 + 500 mL of H2O + 2–3 mL of H2SO4 solution is applied to the surface of the sample and kept moist; if no copper precipitation occurs within 6 minutes, the result is considered satisfactory. (2) Potassium ferricyanide titration test: 2 mL of HCl + 1 mL of H2SO4 + 1 g of K3Fe(CN)6 + 97 mL of H2O solution is applied to the surface of the sample, and the quality of the passivation film is assessed based on the number of blue spots that appear and the time it takes for them to appear.