1. The necessity of pickling and passivating stainless steel:
Austenitic stainless steel has good corrosion resistance, high temperature oxidation resistance, good low temperature performance and excellent mechanical and energy generation. Therefore, it is widely used in chemical industry, petroleum, power, nuclear engineering, aerospace, marine, medicine, light industry, textile and other sectors. Its main purpose is to prevent corrosion and rust. The corrosion resistance of stainless steel mainly relies on the surface passivation film. If the film is incomplete or defective, the stainless steel will still be corroded. In engineering, pickling and passivation are usually carried out to maximize the corrosion resistance potential of stainless steel. During the forming, assembly, welding, weld inspection (such as flaw detection, pressure test) and construction marking of stainless steel equipment and components, surface oil, rust, non-metallic dirt, low melting point metal pollutants, paint, welding, etc. Slag and spatter, etc., these substances affect the surface quality of stainless steel equipment and components, destroy the oxide film on the surface, reduce the general corrosion resistance and local corrosion resistance of steel (including pitting corrosion, crevice corrosion), and even cause Stress corrosion cracking.
Stainless steel surface cleaning, pickling and passivation not only maximize corrosion resistance, but also prevent product contamination and achieve aesthetics. According to GBl50-1998 "Steel Pressure Vessels", "the surface of containers made of stainless steel and composite steel plates with anti-corrosion requirements should be pickled and passivated." This regulation is for pressure vessels used in the petrochemical industry. Because these equipment are used in situations where they are in direct contact with corrosive media, pickling and passivation are necessary to ensure corrosion resistance. For other industrial sectors, if it is not for anti-corrosion purposes and is only based on cleanliness and aesthetic requirements, pickling and passivation are not required for stainless steel materials. However, the welds of stainless steel equipment also need to be pickled and passivated. For nuclear engineering, some chemical equipment and other applications with strict requirements, in addition to pickling and passivation, high-purity media must be used for final fine cleaning or finishing treatments such as mechanical, chemical and electrolytic polishing.
2． Principle of pickling and passivation of stainless steel
The corrosion resistance of stainless steel is mainly due to the surface being covered with an extremely thin (about 1nm) dense passivation film. This film is isolated from the corrosive medium and is stainless steel. The basic barrier of protection. Stainless steel passivation has dynamic characteristics and should not be regarded as a complete stop of corrosion, but a diffusion barrier that greatly reduces the anode reaction speed. Usually, membranes tend to be damaged in the presence of reducing agents (such as chloride ions), while membranes can be maintained or repaired in the presence of oxidants (such as air).
Stainless steel workpieces will form an oxide film when placed in the air, but the protection of this film is not perfect enough. Usually, thorough cleaning, including alkali washing and pickling, and then passivation with oxidants are required to ensure the integrity and stability of the passivation film. One of the purposes of pickling is to create favorable conditions for passivation treatment and ensure the formation of a high-quality passivation film. Because an average of 10 μm thick layer on the surface of stainless steel is corroded by pickling, the chemical activity of the acid makes the dissolution rate of defective parts higher than other parts on the surface, so pickling can make the entire surface even and balanced, and some of the original Hidden hazards that could easily cause corrosion are eliminated. But more importantly, through pickling and passivation, iron and iron oxides are dissolved preferentially than chromium and chromium oxides, and the chromium-poor layer is removed, causing chromium to be enriched on the surface of stainless steel. This chromium-rich passivation film The potential can reach +1.0V (SCE), which is close to the potential of precious metals, improving the stability of anti-corrosion. Different passivation treatments will also affect the composition and structure of the film, thereby affecting the rust resistance. For example, through electrochemical modification treatment, the passivation film can have a multi-layer structure, forming CrO3 or Cr2O3 in the barrier layer, or forming a glassy state. The oxide film enables stainless steel to exert maximum corrosion resistance.
Domestic and foreign scholars have conducted extensive research on the formation of stainless steel passivation film. Take the Beijing University of Science and Technology’s research on photoelectron spectroscopy (XPS) of 316L steel passivation film in recent years as an example to briefly describe [1]. Stainless steel passivation is the dissolution and adsorption of water molecules on the surface for some reason. Under the catalytic action of oxidants, oxides and hydroxides are formed, and undergo conversion reactions with the Cr, Ni, and Mo elements that make up stainless steel, and finally form stable The phase-forming film prevents film damage and corrosion.
The reaction process is:
Fe·H2O*≈[FeOH·O*]ad+H++e
[FeOH·O*]ad≈[FeO·O *]ad+H++e
[FeO·O*]ad+H2O≈FeOOH+O*十H++e
[FeO·O*]ad≈FeO +O*
FeOOH+Cr+H2O≈CrOOH+Fe·H20
2FeOOH≈Fe203+H20
2CrOOH≈Cr203+H20
< p>MO+3FeO+3H2O≈MOO3+3Fe·H2ONi+FeO+2H20≈NiO+Fe·H20
(where Os represents the catalyst in the passivation process, and The concentration in the passivation film remains unchanged, ad represents the adsorption intermediate)[page]
It can be seen that Fe2O3, Fe(OH)3, or γ-FeOOH, Cr203 exists on the surface of the 316L passivation film. , CrOOH or Cr(OH)3, MO exists in the form of MOO, and 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 passivating treatment methods
There are many methods for pickling and passivating stainless steel equipment and parts according to different operations. , its scope of application and characteristics are shown in Table 1.
Table 1 Comparison of stainless steel pickling and passivation methods
Method application scope, advantages and disadvantages
The dipping method is used for stainless steel that can be placed in a pickling tank or passivation tank. Parts, but not suitable for large equipment. The pickling solution can be used for a long time, with high production efficiency and low cost; large-volume equipment is filled with acid and the dipping liquid consumption is too large
The brushing method is suitable for large equipment Internal surface and local treatment requires manual operations, poor working conditions, and the acid cannot be recovered
The paste method is used at installation or maintenance sites, especially for manual welding processing, poor working conditions, and high production costs.
The spray method is used at the installation site. It uses less liquid on the inner wall of large containers, is less expensive, and is faster, but requires a spray gun and a splicing ring system.
The circulation method is used for large equipment. , such as heat exchangers and tube shells are easy to process and construct, and the acid can be reused, but piping and pumps are needed to connect the circulation system
The electrochemical method can be used for parts, and the brush method can be used for on-site treatment. The equipment surface treatment technology is more complicated and requires a DC power supply or a potentiostat
3.2 Examples of pickling and passivation treatment recipes
3.2.1 General treatment [2]
p>According to ASTMA380-1999, taking 300 series stainless steel as an example,
(1) Pickling
Reagent HNO36%~25%+HF0.5%~8 % (volume fraction);
Temperature 21~60℃; time as needed;
Or ammonium citrate 5%~10% (mass fraction);
< p>Temperature 49～71℃; time 10～60min.(2) Passivation
Agent HNO320%~50% (volume fraction);
Temperature 49~71℃; time 10~30min;
p>
Or temperature 2l～38℃; time 30～60min;
or agent HNO320%～50%+Na2Cr207H2022%～6% (mass fraction);
Temperature 49～54℃; time 15～30min;
or temperature 21～38℃; time 30～60min.
(3) Descaling and pickling
Pharmaceutical H2SO48%~11% (volume fraction);
Temperature 66~82℃; 6 inches between 5~ 45min;
And the agent HNO36%~25%+HF 0.5%~8% (volume fraction);
Temperature 21~60℃;
Or HNO315% ~ 25% + HFl% - 8% (volume fraction).
3.2.2 Treatment by paste method
(1) Use Guangzhou Petrochemical’s new urea stainless steel equipment to passivate the internal surface welds and base metal and repair the surface to polish the local passivation of the welds Take this as an example [3]
Pickling paste:
25% HNO~+4% HF+7l% condensed water (volume fraction) and BaSO, adjust to paste.
Passivation paste:
Add 30% HNO3 or 25% HNO3 + 1% (mass fraction) K2Cr207 and BaSO7 to a paste.
The coated surface should be washed for 5 to 30 minutes with condensed water until pH=7. Chemical passivation by spraying hydrogen peroxide can also be used for a single piece of equipment.
(2) Take the patent m of Shanghai Daming Iron Works as an example.
Pickling passivation paste:
HN038%～14% (used as passivating agent);
HFl0%～15% (used as corrosive agent); < /p>
Magnesium laurate 2.2% to 2.7% (as thickener)
Magnesium nitrate 60% to 70% (as filler to improve adhesion and penetration property); [page]
Sodium polyphosphate 2.3% ~ 2.8% (used as corrosion inhibitor);
Water (viscosity adjustment).
3.2.3 Electrochemical treatment
Take the Xiamen University patent [5] as an example. The treatment method is: use the stainless steel workpiece to be treated as an anode and control the constant potential. Carry out anodizing treatment, or use the stainless steel workpiece as the cathode first, control the constant potential for cathodic treatment, then use the stainless steel workpiece as the anode, control the constant potential for anodizing treatment, and continue to change its constant potential for passivation treatment. The electrolyte solution will be uniform Use HN03. After such treatment, the properties of the stainless steel passivation film are improved and the corrosion resistance is greatly improved. The pitting corrosion critical potential (Eb) is increased by about 1000mV (in 3% NaCl), and the uniform corrosion resistance is improved by three orders of magnitude (in 20% to 30% H2S04 at 45°C).
4. Application scope of pickling and passivation of stainless steel
4.1 Pickling and passivation treatment in the manufacturing process of stainless steel equipment
4.1.1 Cleaning and pickling and passivation after cutting [6]
After cutting, contaminants such as iron filings, steel powder and cooling emulsion will usually remain on the surface of stainless steel workpieces, which will cause stains and rust on the stainless steel surface, so it should be degreased and removed. oil, and then clean it with nitric acid, which not only removes iron filings and steel dust, but also passivates it.
4.1.2 Cleaning and pickling passivation before and after welding [7]
Since grease is a source of hydrogen, gas pores will be formed in welds where grease is not removed. L, and low-melting-point metal contamination (such as zinc-rich paint) will cause cracking after welding, so before welding stainless steel, the groove and the surface within 20mm on both sides must be cleaned. Oil stains can be scrubbed with acetone. Paint and rust stains should be cleaned with emery cloth or Remove with a stainless steel wire brush and then wipe clean with acetone.
No matter what welding technology is used in stainless steel equipment manufacturing, it must be cleaned after welding. All welding slag, spatter, stains and oxidation color must be removed. Cleaning methods include mechanical cleaning and chemical cleaning. Mechanical cleaning includes grinding, polishing, sandblasting, etc. Carbon steel brushes should be avoided to prevent surface rust. In order to obtain the best corrosion resistance, it can be soaked in a mixture of HNO3 and HF, or pickled passivation paste can be used. In fact, mechanical cleaning and chemical cleaning are often used in combination.
4.1.3 Cleaning of forged castings [6]
Stainless steel workpieces that have been thermally processed such as forging and casting often have a layer of oxide scale, lubricant or oxide pollution on the surface. , pollutants include graphite, molybdenum disulfide and carbon dioxide. It should be treated by shot blasting, salt bath and multiple pickling. For example, the treatment process of stainless steel turbine blades in the United States is:
Salt bath (10min) → water quenching (2.5min) → sulfuric acid washing (2min) → cold water washing (2min) → alkaline permanganate bath ( 10min) → cold water washing (2min) → sulfuric acid washing (1rain) → cold water washing (1min) → nitric acid washing (1.5min) → cold water washing (1min) → hot water washing (1min) → air drying.
4.2 Pickling and passivation treatment before new equipment is put into production
Many stainless steel equipment and pipelines in large chemical, chemical fiber, fertilizer and other equipment require pickling and passivation before they are put into production. change. Although the equipment has been pickled in the manufacturing plant to remove welding slag and oxide scale, contamination by grease, mud, sand, rust, etc. will inevitably occur during storage, transportation, and installation. In order to ensure that devices and equipment test products (especially If the quality of chemical intermediates and refined products can meet the requirements, to ensure a successful test run, pickling and passivation must be carried out. For example, the stainless steel equipment and pipelines of H2O2 production equipment must be cleaned before being put into production. Otherwise, if there are contaminants and heavy metal ions, the catalyst will be poisoned. In addition, if there are grease and free iron ions on the metal surface, it will cause the decomposition of H2O2, violently releasing a large amount of heat, causing fire or even explosion. Similarly, for oxygen pipelines, trace amounts of oil dirt and metal particles may cause sparks and serious consequences.
4.3 Pickling and passivation treatment in on-site maintenance
In the production equipment of refined terephthalic acid (PTA), polyvinyl alcohol (PVA), acrylic fiber, acetic acid, etc. Among equipment materials, austenitic stainless steel 316L, 317, and 304L are widely used. Since the materials contain harmful ions such as Cl-, Br-, SCN-, formic acid, or due to dirt and material agglomeration, pitting corrosion and gaps will occur in the equipment. Corrosion and weld corrosion. When shutting down for maintenance, the equipment or components can be fully or partially pickled and passivated to repair the passivation film to prevent the expansion of local corrosion. For example, the stainless steel pipes of the Shanghai Petrochemical PTA plant dryer were updated and overhauled, and the stainless steel heat exchangers of the acrylic fiber plant were overhauled and pickled and passivated.
4.4 Descaling and cleaning of equipment in service
Stainless steel equipment in petrochemical plants, especially heat exchangers, will deposit various dirt on the inner wall after a certain period of operation. Such as carbonate scale, sulfate scale, silicate scale, iron oxide scale, organic scale, catalyst scale, etc., which affect the heat exchange effect and cause corrosion under the scale. It is necessary to choose a suitable cleaning agent for descaling. You can use nitric acid, nitric acid + hydrofluoric acid, sulfuric acid, citric acid, EDTA, water-based cleaning agents, etc., and add an appropriate amount of corrosion inhibitor. After descaling and cleaning, dulling can be performed if necessary. chemical treatment. For example, the stainless steel heat exchangers of Shanghai Petrochemical PTA, acetic acid, acrylic and other equipment have been descaled and cleaned.
5. Precautions for pickling and passivation of stainless steel
5.1 Pre-treatment for pickling and passivation
If there is surface dirt on the stainless steel workpiece before pickling and passivation, it should be cleaned mechanically , then remove oil and grease. If the pickling solution and passivating solution cannot remove grease, the presence of grease on the surface will affect the quality of pickling and passivation. Therefore, oil removal and degreasing cannot be omitted. Alkali solution, emulsifier, organic solvent, steam, etc. can be used.
5.2 Control of Cl- in pickling liquid and rinse water
Some stainless steel pickling liquids or pickling pastes are added with hydrochloric acid, perchloric acid, ferric chloride and Sodium chloride and other chloride ion-containing corrosion media are used as the main agent or auxiliary agent to remove the surface oxide layer, and trichlorethylene and other chlorine-containing organic solvents are used for grease removal, which is not suitable from the perspective of preventing stress corrosion cracking. In addition, industrial water can be used for preliminary flushing water, but the final cleaning water requires strict control of halide content. Deionized water is usually used. For example, when using water for hydrostatic testing of petrochemical austenitic stainless steel pressure vessels, the C1- content should not exceed 25 mg/L. If this requirement cannot be met, sodium nitrate can be added to the water to make it meet the requirements. If the C1- content exceeds the standard, it will Destroying the passivation film of stainless steel is the root cause of pitting corrosion, crevice corrosion, stress corrosion cracking, etc.
5.3 Process control in pickling and passivation operations
Nitric acid solution alone is effective in removing free iron and other metal contaminants, but it is not effective in removing iron oxide scale. Corrosion products, tempering films, etc. are ineffective. Generally, HNO3+HF solution should be used. For convenience and operational safety, fluoride can be used instead of HF [2]. HNO3 solution alone does not need to add corrosion inhibitor, but when HNO3+HF pickling, Lan-826 needs to be added. Use HNO3+HF pickling. To prevent corrosion, the concentration should be maintained at a ratio of 5:1. The temperature should be lower than 49°C. If it is too high, HF will volatilize.
For the passivation solution, HNO3 should be controlled between 20% and 50%. According to electrochemical tests, the quality of the passivation film treated with an HNO3 concentration less than 20% is unstable and prone to pitting corrosion [8] , but the HNO3 concentration should not be greater than 50% to prevent over-passivation.
Using a one-step method to remove oil and pickle and passivate. Although the operation is simple and saves man-hours, the pickling and passivation liquid (paste) will contain corrosive HF, so the quality of the final protective film is not as good as many footwork.
The acid concentration, temperature and contact time are allowed to be adjusted within a certain range during the pickling process. As the use time of the pickling solution increases, attention must be paid to changes in acid concentration and metal ion concentration. Care should be taken to avoid over-pickling. The titanium ion concentration should be less than 2%, otherwise severe pitting corrosion will occur. Generally speaking, increasing the pickling temperature will speed up and improve the cleaning effect, but may also increase the risk of surface contamination or damage.
5.4 Control of pickling under sensitized conditions of stainless steel [2]
Some stainless steels are sensitized due to poor heat treatment or welding, and the use of HNO&HF pickling may produce intergranular Corrosion. Cracks caused by intergranular corrosion can concentrate halides during operation, cleaning, or subsequent processing, causing stress corrosion. These sensitized stainless steels are generally not suitable for descaling or pickling with HNO3+HF solution. If such pickling is necessary after welding, ultra-low carbon or stabilized stainless steel should be used.
5.5 Pickling of stainless steel and carbon steel assemblies
For stainless steel and carbon steel assemblies (such as stainless steel tubes, tube plates and carbon steel shells in heat exchangers) , if HNO3 or HNO3+HF is used for pickling and passivation, carbon steel will be severely corroded. At this time, appropriate corrosion inhibitors such as Lan-826 should be added. When the stainless steel and carbon steel assembly is in a sensitized state and cannot be pickled with HNO3+HF, glycolic acid (2%) + formic acid (2%) + corrosion inhibitor can be used at a temperature of 93°C for 6 hours or ammonium EDTA. Base neutral solution + corrosion inhibitor, temperature: 121°C, time: 6h, then rinse with hot water and immerse in 10mg/L ammonium hydroxide + 100mg/L hydrazine [3].
5.6 Post-treatment of pickling and passivation
After pickling and rinsing with water, the stainless steel workpiece can be treated with 10% (mass fraction) NaOH + 4% (mass fraction) Soak the alkali permanganate solution of KMnO4 in 71-82°C for 5-60 minutes to remove the pickling residue, then rinse thoroughly with water and dry. If spots or stains appear on the surface of stainless steel after pickling and passivation, they can be eliminated by scrubbing with fresh passivation solution or higher concentration of nitric acid. Stainless steel equipment or components that are finally pickled and passivated should be protected and covered or wrapped with polyethylene film to avoid contact between different metals and non-metals.
The treatment of acidic and passivation waste liquids should comply with national environmental protection discharge regulations. For example, fluoride-containing wastewater can be treated with lime milk or calcium chloride. The passivation solution should not use dichromate as much as possible. If there is chromium-containing wastewater, ferrous sulfate can be added for reduction treatment.
Pickling may cause hydrogen embrittlement of martensitic stainless steel. If necessary, heat treatment can be used to remove oxygen (heat to 200°C and keep warm for a period of time).
6． Stainless steel pickling passivation quality inspection [8]
Since chemical inspection will destroy the passivation film of the product, inspection is usually performed on a sample plate. Examples of methods are as follows:
(1) Copper sulfate titration test
Use 8gCuS04+500mLH22~3mLH2S04 solution to drop into the surface of the sample and keep it wet. If no copper appears within 6 minutes Precipitation is qualified.
(2) Potassium ferricyanide titration test
Use 2mLHCl+1mLH2S04+1gK3Fe(CN)6+97mLH20 solution to drop on the surface of the sample, and measure the number and appearance of blue spots generated The length of time is used to identify the quality of the passivation film.