Austenitic stainless steel has good corrosion resistance, high temperature oxidation resistance, good low temperature performance and excellent mechanical and R energy generation. Therefore, it is widely used in chemical industry, petroleum, electric power, nuclear engineering, aerospace, ocean, medicine, light industry, textile and other departments. Its main purpose is to prevent corrosion and rust. The corrosion resistance of stainless steel mainly depends on the surface passivation film. If the film is incomplete or defective, stainless steel will still be corroded. In engineering, pickling and passivation are usually carried out to make the corrosion resistance of stainless steel play a greater role. In the process of forming, assembling, welding, weld inspection (such as flaw detection and pressure test) and construction marking of stainless steel equipment and components, surface oil stain, rust, nonmetal dirt, low melting point metal pollutants, paint, welding slag and splash will be brought, which will affect the surface quality of stainless steel equipment and components, destroy the oxide film on their surfaces, and reduce the overall corrosion resistance and local corrosion resistance (including pitting corrosion and crevice corrosion) of steel.
Cleaning, pickling and passivation of stainless steel surface can not only maximize corrosion resistance, but also prevent product pollution and obtain beautiful appearance. GBl50- 1998 "Steel Pressure Vessels" stipulates that "the surfaces of vessels made of stainless steel and composite steel plates with anti-corrosion requirements should be passivated by pickling". This regulation is aimed at pressure vessels used in petrochemical industry, because these equipment are used in direct contact with corrosive media, and it is necessary to put forward pickling passivation from the perspective of ensuring corrosion resistance. For other industrial sectors, stainless steel materials do not need to be passivated by pickling, if it is not for the purpose of anticorrosion, but only for the requirements of cleanliness and beauty. However, the weld of stainless steel equipment needs pickling passivation. For nuclear engineering, some chemical plants and other demanding applications, in addition to pickling passivation, high-purity medium should be used for final fine cleaning or mechanical, chemical and electrolytic polishing.
2. stainless steel pickling passivation principle
The corrosion resistance of stainless steel is mainly due to its surface covered with a very thin (about 1nm) dense passivation film, which is the basic barrier to protect stainless steel. Passivation of stainless steel has dynamic characteristics, which should not be regarded as a complete stop of corrosion, but a diffusion barrier, which greatly reduces the anode reaction speed. Generally speaking, the membrane is easy to be destroyed in the presence of reducing agent (such as chloride ion), while the membrane can be maintained or repaired in the presence of oxidizing agent (such as air).
When stainless steel workpiece is placed in air, it will form an oxide film, but the protection of this film is not perfect. Generally, before passivation with oxidant, thorough cleaning, including alkaline cleaning and acid cleaning, is needed 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 high-quality passivation film. Because the surface of stainless steel with an average thickness of 10μm is corroded by pickling, the chemical activity of the acid solution makes the dissolution rate of the defective part higher than that of other parts on the surface, so pickling can make the whole surface tend to be uniform and balanced, and some hidden dangers that were easy to cause corrosion have been eliminated. But more importantly, through pickling passivation, iron and iron oxides are dissolved before chromium and chromium oxides, and the chromium-poor layer is removed, resulting in the enrichment of chromium on the stainless steel surface. The potential of this chromium-rich passivation film can reach+1.0V (SCE), which is close to that of precious metals, and the corrosion resistance stability is improved. Different passivation treatments will also affect the composition and structure of the film, thus affecting the antirust performance. For example, through electrochemical modification, the passivation film can have a multilayer structure, CrO3 or Cr2O3 can be formed in the barrier layer, or a glassy oxide film can be formed, so that stainless steel can exert its maximum corrosion resistance.
Scholars at home and abroad have done a lot of research on the formation of stainless steel passivation film. Taking the photoelectron spectroscopy study of passivation film of 3 16L steel by University of Science and Technology Beijing in recent years as an example, this paper briefly describes [1]. Passivation of stainless steel means that the surface layer is dissolved for some reason and absorbed by water molecules. Under the catalysis of oxidant, oxides and hydroxides are formed, which react with cr, Ni and Mo elements that make up stainless steel, and finally form a stable phase-forming film, thus preventing the damage and corrosion of the film. The reaction process is as follows:
Fe H2O+O *≈[FeOH O *]ad+H ++ e
[FeOH O*]ad≈[FeO O*]ad+H++e
[feo o *] ad+H2O ≈ feooh+o * ten H++e
[FeO O*]ad≈FeO+O*
Fe ooh+Cr+H2O≈CrOOH+Fe H2O
2 Fe ooh≈fe2o 3+H2O
2 rooh≈cr2o 3+H2O
MO+3FeO+3H2O≈MOO3+3Fe H2O
ni+FeO+2h2o≈NiO+Fe H2O
(Among them, Os represents the catalyst in the passivation process, the concentration in the passivation ion remains unchanged, and ad represents the adsorption intermediate. ) [Page]
It can be seen that there are Fe2O3, Fe(OH)3 or γ -FeOOH, cr2o 3, CrOOH or Cr(OH)3 and MO on the surface of 3 16L passivation film, and the main components of the passivation film are CrO3, FeO and NiO.
3. stainless steel pickling passivation's methods and technologies.
3. Comparison of1pickling passivation treatment methods
There are many methods for pickling passivation of stainless steel equipment and parts according to different operations. See table 1 for its application scope and characteristics.
Table 1 stainless steel pickling passivation method comparison
Advantages and disadvantages of the scope of application of the method
Immersion method is used for parts that can be put into pickling tank or passivation tank, and is not suitable for large equipment. The pickling solution can be used for a long time, with high production efficiency and low cost. Large-capacity equipment filled with acid consumes too much liquid.
Brush coating method is suitable for the internal surface and local treatment of large equipment, and the working conditions are harsh, so the acid solution cannot be recovered.
Sticking method is used in installation or maintenance site, especially in manual operation of welding site treatment, with poor working conditions and high production cost.
Spraying method is used in the installation site, which has the advantages of less liquid consumption, low cost and high speed, but it needs to be equipped with spray gun and ring string system.
Circulation method is convenient for large equipment, such as heat exchanger and shell-and-tube treatment, and acid solution can be reused, so it is necessary to connect pipes and pumps in the circulation system.
Electrochemical method can be used not only for parts, but also for surface treatment of field equipment by brush coating. The technology is complex and requires DC power supply or potentiostat.
3.2 Example of pickling passivation treatment formula
3.2. 1 General handling [2]
According to ASTM A 380- 1999, only 300 series stainless steel is taken as an example.
(1) pickling
HNO 36% ~ 25%+HF 0.5% ~ 8% (volume fraction);
Temperature 21~ 60℃; Time required;
Or 5%- 10% (mass fraction) of ammonium citrate;
The temperature is 49 ~ 765438 0℃; Time 10 ~ 60 minutes.
(2) Passivation
HNO 320% ~ 50% (volume fraction);
The temperature is 49 ~ 765438 0℃; Time 10 ~ 30 minutes;
Or the temperature is 2l-38 DEG C; The time is 30-60 min;
Or HNO 320% ~ 50%+Na2Cr207H2022% ~ 6% (mass fraction);
The temperature is 49-54 DEG C; Time 15 ~ 30 minutes;
Or the temperature is 21~ 38℃; The time is 30 ~ 60 minutes.
(3) descaling and pickling
H2SO4 48% ~ 1 1% (volume fraction);
The temperature is 66-82 DEG C; 6 inches for 5 to 45 minutes;
And HNO 36% ~ 25%+HF 0.5% ~ 8% (volume fraction);
Temperature 21~ 60℃;
Or HNO315% ~ 25%+hfl% ~ 8% (volume fraction).
Paste treatment
(1) Take the passivation of internal surface weld and base metal and local passivation of polished weld on maintenance surface of newly-built urea stainless steel equipment in Guangzhou Petrochemical Company as an example [3].
Pickling paste:
25% HNO+4% HF+7L% condensed water (volume fraction) and BaSO, and adjust to paste.
Passivating paste:
Mix 30% HNO3 or 25% HNO3+ 1% (mass fraction) K2Cr207 and BaSO7 into paste.
Coat the surface for 5 ~ 30 minutes, rinse it with condensed water to pH=7, or spray hydrogen peroxide on a single device for chemical passivation.
(2) Take the patent M of Shanghai Daming Iron Works as an example.
Pickling passivation paste:
HN 038% ~ 14% (as passivator);
0% ~ 15% HFL (as corrosive agent);
2.2% ~ 2.7% magnesium stearate (as thickener)
60% ~ 70% of magnesium nitrate (used as filler to improve adhesion and air permeability); [Page]
2.3% ~ 2.8% of sodium polyphosphate (as corrosion inhibitor);
Water (viscosity adjustment).
electrochemical treatment
Take Xiamen University patent [5] as an example, its treatment method is: take the stainless steel workpiece to be treated as anode and control the constant potential for anodic oxidation, or take the stainless steel workpiece as cathode and control the constant potential for anodic oxidation first, then take the stainless steel workpiece as anode and control the constant potential for anodic oxidation, and continue to change its constant potential for passivation. The electrolyte is HN03. After this treatment, the properties of stainless steel passivation film are improved and the corrosion resistance is greatly improved. The pitting critical potential (Eb) is increased by about 1000mV (in 3% NaCl solution), and the uniform corrosion resistance is improved by three orders of magnitude (in 20% ~ 30% H2SO4 solution, at 45℃).
4. Scope of application of stainless steel pickling passivation
4. 1 pickling passivation in the manufacturing process of stainless steel equipment
4. 1. 1 Cleaning and pickling passivation after cutting [6]
After the stainless steel workpiece is cut, the surface usually remains dirt such as iron filings, steel powder and cooling emulsion, which will cause stains and rust on the stainless steel surface. Therefore, it is necessary to degrease and degrease, and then clean with nitric acid, which not only removes iron filings and steel powder, but also passivates it.
4. 1.2 Cleaning and pickling passivation before and after welding [7]
Because grease is the source of hydrogen, gas will be formed in the weld if grease is not removed, and low melting point metal pollution (such as zinc-rich paint) will cause post-weld cracking. Therefore, before welding stainless steel, the groove and the surface within 20mm on both sides must be cleaned, the oil stain can be scrubbed with acetone, and the paint rust can be removed with emery cloth or stainless steel wire brush first, and then wiped with acetone.
No matter what welding process is used in the manufacture of stainless steel equipment, it should be cleaned after welding to remove all welding slag, spatter, stains and oxidation color. Cleaning methods include mechanical cleaning and chemical cleaning. Mechanical cleaning includes grinding, polishing and sandblasting. Carbon steel brushes should be avoided to prevent the surface from rusting. In order to obtain the best corrosion resistance, it can be soaked in the mixed solution of HNO3 and HF, or it can be used with pickling passivation paste. In fact, 4 californium 1 mechanical cleaning is often combined with chemical cleaning.
4. 1.3 Cleaning of forged castings [6]
After forging, casting and other hot working, the surface of stainless steel workpiece is often polluted by a layer of oxide scale, lubricant or oxide, and the pollutants are graphite, molybdenum disulfide and carbon dioxide. Shot peening, salt bath treatment and multi-pass pickling treatment should be adopted. For example, the treatment of stainless steel turbine blades in the United States is as follows:
Salt bath (1 0min) → water quenching (2.5min) → sulfuric acid washing (2min) → cold water washing (2min) → alkaline permanganate bath (10min) → cold water washing (2min) → sulfuric acid washing (1rain).
4.2 Pickling and passivation treatment before the new unit is put into production
Many large-scale chemical, chemical fiber, chemical fertilizer and other stainless steel equipment and pipelines need pickling passivation before they are put into production. Although the equipment has been pickled before leaving the factory to remove welding slag and scale, it will inevitably cause pollution such as grease, mud and rust during storage, transportation and installation. In order to ensure the quality of commissioning products (especially chemical intermediates and refined products) of devices and equipment meet the requirements and ensure the success of one commissioning, pickling passivation must be carried out. For example, the stainless steel equipment and pipelines of H2O2 production plant must be cleaned before production, otherwise the heavy metal ions in the dirt will poison the catalyst. In addition, if there are grease and free iron ions on the metal surface, it will cause H2O2 to decompose and release a lot of heat violently, causing fire and even explosion. Similarly, for the oxygen pipeline, the existence of trace oil and metal particles may also produce sparks, causing serious consequences.
4.3 Pickling passivation treatment in field maintenance
Austenitic stainless steels 3 16L, 3 17 and 304L are widely used for refining equipment materials such as terephthalic acid (PTA), polyvinyl alcohol (PVA), acrylic fiber and acetic acid. Because all materials contain harmful ions such as Cl-, Br-, SCN- and formic acid, or because dirt and materials are caked, spots will be caused on the equipment. When stopping for maintenance, all or part of equipment or parts can be passivated by pickling, and the passivation film can be repaired to prevent local corrosion from spreading. For example, the stainless steel tube of the dryer in PTA plant of Shanghai Petrochemical Company and the stainless steel heat exchanger in acrylic fiber plant have been passivated by pickling.
4.4 descaling and cleaning of in-use equipment
Stainless steel equipment in petrochemical plants, especially heat exchangers, will deposit various scales on the inner wall after a certain period of operation, such as carbonate scale, sulfate scale, silicate scale, iron oxide scale, organic scale and catalyst scale. , will affect the heat transfer effect, resulting in corrosion under the scale. Appropriate cleaning agents should be selected for descaling, such as nitric acid, nitric acid+hydrofluoric acid, sulfuric acid, citric acid, EDTA and water-based cleaning agents. , and add an appropriate amount of corrosion inhibitor. After descaling and cleaning, it can be passivated if necessary. Chemical treatment, for example, stainless steel heat exchangers in PTA, acetic acid and acrylic fiber plants of Shanghai Petrochemical Company have been descaled and cleaned.
5. Precautions for stainless steel pickling passivation
5. 1 Pretreatment of pickling passivation
If there is dirt on the surface of stainless steel workpiece before pickling passivation, it should be cleaned mechanically first, and then degreased. If pickling solution and passivation solution can't remove grease, the presence of surface grease will affect the quality of pickling passivation. Therefore, degreasing cannot be omitted, and alkali solution, emulsifier, organic solvent and steam can be used.
5.2 Control of Cl- in Pickling Solution and Cleaning Water
For some stainless steel pickling solutions or pickling pastes, it is not suitable to prevent stress corrosion cracking by using chlorine-containing corrosive media such as hydrochloric acid, perchloric acid, ferric chloride and sodium chloride as main agents or additives to remove the surface oxide layer and chlorine-containing organic solvents such as trichloroethylene to remove grease. In addition, industrial water can be used as initial washing water, but the halide content of final washing water is strictly controlled. Deionized water is usually used. For example, the content of C 1- in the water for hydrostatic test of austenitic stainless steel pressure vessels in petrochemical industry should be controlled within 25mg/L. If this requirement cannot be met, sodium nitrate can be added to the water to make it meet the requirements. If the content of C 1- exceeds the standard, the passivation film of stainless steel will be destroyed, which is the root of pitting corrosion, crevice corrosion and stress corrosion cracking.
5.3 Process control in pickling passivation operation
Nitric acid solution alone is effective for removing free iron and other metal pollutants, but it is ineffective for removing mill scale, thick corrosion products, tempered films, etc. HNO3+HF solution is generally used, and fluoride can be used instead of HF[2] for convenience and safety. No corrosion inhibitor can be added to nitric acid solution alone, but Lan-826 needs to be added to nitric acid+hydrofluoric acid pickling. Use nitric acid+hydrofluoric acid for pickling. To prevent corrosion, the concentration should be kept at 5: 1. The temperature should be lower than 49℃. If it is too high, HF will volatilize.
For passivation solution, nitric acid should be controlled between 20% and 50%. According to electrochemical test, the quality of passivation film treated with nitric acid concentration less than 20% is unstable and pitting corrosion is easy to occur [8], but the nitric acid concentration should not be greater than 50%, so it is necessary to prevent over-passivation.
Using one-step method to treat degreasing pickling passivation is simple and saves working hours, but there will be corrosive HF in pickling passivation solution (paste), so the quality of the final protective film is not as good as that of multi-step method.
During pickling, it is allowed to adjust the acid concentration, temperature and contact time within a certain range. With the increase of pickling time, we must pay attention to the changes of acid concentration and metal ion concentration to avoid excessive pickling. The concentration of titanium ions should be less than 2%, otherwise it will lead to serious pitting corrosion. Generally speaking, increasing the pickling temperature will accelerate and improve the cleaning effect, but it may also increase the risk of surface pollution or damage.
5.4 Pickling control under the condition of stainless steel sensitization [2]
Some stainless steels are sensitized due to heat treatment or poor welding, HNO &；; HF pickling may cause intergranular corrosion, and cracks caused by intergranular corrosion will concentrate halides during operation, cleaning or subsequent processing, leading to stress corrosion. These sensitized stainless steels are generally not suitable for rust removal or pickling with HNO3+HF solution. If pickling is necessary after welding, ultra-low carbon or stabilized stainless steel should be used.
5.5 Pickling of stainless steel and carbon steel components
For stainless steel and carbon steel components (such as stainless steel tubes, tubesheets and carbon steel shells in heat exchangers), if HNO3 or HNO3+HF is used for pickling passivation, carbon steel will be seriously corroded. At this time, appropriate corrosion inhibitor such as Lan-826 should be added. When the assembly of stainless steel and carbon steel can't be washed with HNO3+HF in sensitized state, glycolic acid (2%)+ formic acid (2%)+ corrosion inhibitor can be used at 93℃ for 6h or ammonium EDTA-based neutral solution+corrosion inhibitor at 12 1℃ for 6h, then it can be washed with hot water and immersed in 65438.
5.6 Post-treatment of pickling and passivation
After pickling and water washing, stainless steel workpiece can be soaked in 1 crude permanganate solution containing 10% (mass fraction) NaOH+4% and 4% (mass fraction) KMnO4 at 7 1 ~ 82℃ for 5 ~ 60 min to remove pickling residue, and then thoroughly cleaned with water. Spots or stains appear on the back surface of stainless steel pickling passivation, which can be removed by scrubbing with fresh passivation solution or high concentration nitric acid. Stainless steel equipment or components that are finally passivated by pickling shall be protected and covered or wrapped with polyethylene film to avoid contact between dissimilar metals and nonmetals.
The treatment of acidic and passivation waste liquid shall conform to the national environmental protection discharge regulations. For example, fluorine-containing wastewater can be treated with lime milk or calcium chloride. Passivation solution does not need 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, and oxygen can be removed by heat treatment (heating to 200℃ for a period of time) if necessary.
6. stainless steel pickling passivation Quality Inspection [8]
Because chemical inspection will destroy the passive film of the product, it is usually carried out on the sample plate. Examples of methods are as follows:
(1) copper sulfate titration test
Drop 8 gcus04+500ml H2O+2 ~ 3ml H2SO4 solution on the surface of the sample plate and keep it moist. If copper does not precipitate within 6 minutes, it is qualified.
(2) Potassium ferricyanide titration test
Drop 2ml HCl+1ml H2SO4+1gk3fe (cn) 6+97ml H2O solution on the surface of the template, and judge the quality of the passive film by the number of blue spots produced and the length of them.