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, 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 stains, rust, nonmetallic dirt, low melting point metal pollutants, paint, welding slag and spatter are brought, which 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.
surface cleaning, pickling and passivation of stainless steel can not only improve corrosion resistance to the greatest extent, but also prevent product pollution and obtain beautiful appearance. In GBl5-1998 "Steel Pressure Vessels", it is stipulated 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 the situation of 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, if it is not for anti-corrosion purposes, it is only based on the requirements of cleanliness and beauty, and stainless steel materials do not need pickling passivation. However, the welding seam of stainless steel equipment needs pickling passivation. For nuclear engineering, some chemical plants and other applications with strict requirements, in addition to pickling passivation, high-purity media 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 the fact that the surface is covered with a very thin (about 1nm) dense passivation film, which is a basic barrier for stainless steel protection because of its 1n corrosion medium isolation. Passivation of stainless steel has dynamic characteristics, which should not be regarded as a complete stop of corrosion, but a diffusion barrier layer, which greatly reduces the anode reaction speed. Generally, the membrane tends 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).
the stainless steel workpiece will form an oxide film when it is placed in the air, but the protection of this film is not perfect. Generally, thorough cleaning, including alkali washing and acid washing, is required before passivation with oxidant, so as 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 1μm is corroded by pickling, the chemical activity of acid solution makes the dissolution rate of defective parts 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 removed. But more importantly, through pickling passivation, iron and iron oxides are dissolved prior to chromium and chromium oxides, and the chromium-poor layer is removed, resulting in chromium enrichment on the surface of stainless steel. The potential of this chromium-rich passivation film can reach+1.V (SCE), which is close to the potential 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 rust resistance. For example, through electrochemical modification, the passivation film can have a multi-layer 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 (xps) study of passivation film of 316L steel by Beijing University of Science and Technology 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 oxidants, 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, which prevents the destruction and corrosion of the film. The reaction process is:
Fe H2+O * ≈ [Feoh O *] AD+H++E
[Feoh O *] AD ≈ [Feo O *] AD+H++E
[Feo O *] AD+H2O ≈ FeOO.
feooh+Cr+H2O ≈ croooh+Fe H2
2feooh ≈ Fe23+H2
2crooh ≈ Cr23+H2
Mo+3feo+3H2O ≈ MoO3+3fe H2O
Ni+FeO. ) [page]
It can be seen that Fe2O3, Fe(OH)3, or γ -FeOOH, Cr23, CrOOH or Cr(OH)3 and MO exist in the form of MOO in the surface layer of the 316L passivation film, and the main components of the passivation film are CrO3, FeO and NiO.
3. stainless steel pickling passivation's methods and processes
3.1 Comparison of pickling passivation treatment methods
There are many methods for pickling passivation treatment of stainless steel equipment and parts according to the operation, and their application scope and characteristics are shown in Table 1.
Table 1 Comparison of stainless steel pickling passivation method
Advantages and disadvantages of application scope of the method
Immersion method is used for parts that can be put into pickling tank or passivation tank, but it is 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 full of acid solution, which consumes too much liquid.
Brushing method is suitable for the internal surface and local treatment of large-scale equipment, and the working conditions are poor, and the acid solution cannot be recovered.
Paste method is used for the installation or maintenance site, especially for the manual operation of welding part, which has poor working conditions and high production cost.
Spray method is used for the installation site, and the inner wall of large containers has low liquid consumption, low cost and high speed. However, it needs to be equipped with spray gun and ring-inserting system.
The circulation method is convenient for large-scale equipment, such as heat exchanger and shell treatment, and the acid solution can be recycled.
The circulation system needs to be connected with pipes and pumps. Electrochemical method can be used for both parts and brush method to treat the surface of field equipment, which is complicated. Need DC power supply or potentiostat
3.2 Examples of pickling passivation treatment formula
3.2.1 General treatment [2]
According to ASTMA38—1999, only 3 series stainless steel is taken as an example,
(1) pickling agent HNO is 36% ~ 25%+HF is .5% ~.
the temperature is 21 ~ 6℃; Time as needed;
or 5% ~ 1% (mass fraction) of ammonium citrate;
the temperature is 49 ~ 71℃; The time is 1 ~ 6 min.
(2) passivating
HNO 32% ~ 5% (volume fraction);
the temperature is 49 ~ 71℃; Time is 1 ~ 3 min;
or the temperature is 2l ~ 38℃; Time is 3 ~ 6 min;
or HNO 32% ~ 5%+na2cr27h222% ~ 6% (mass fraction);
the temperature is 49 ~ 54℃; Time is 15 ~ 3 min;
or the temperature is 21 ~ 38℃; The time is 3 ~ 6 min.
(3) descaling and pickling
H2SO4 48% ~ 11% (volume fraction);
the temperature is 66 ~ 82℃; 5 ~ 45 min between 6 inches;
and HNO 36% ~ 25%+HF .5% ~ 8% (volume fraction);
the temperature is 21 ~ 6℃;
or HNO 315% ~ 25%+HFL%-8% (volume fraction).
3.2.2 Paste treatment
(1) Take the passivation of internal surface weld and parent metal of new urea stainless steel equipment in Guangzhou Petrochemical Company and the local passivation of polished weld on maintenance surface as an example [3]
Pickling paste:
25% HNO ~+4% HF+7L% condensed water (volume fraction) and BaSO, and adjust to paste.
passivation paste:
3% HNO3 or 25% HNO3+1% (mass fraction) K2Cr27 and BaSO7 are mixed to paste.
coat the surface for 5 ~ 3 min, rinse it with condensed water until pH=7, or spray hydrogen peroxide for chemical passivation of single equipment.
(2) Take the patent M of Shanghai Daming Iron Works as an example.
pickling passivation paste:
HN 38% ~ 14% (as passivator);
% ~ 15% of hfl (as corrosive agent);
2.2% ~ 2.7% of magnesium stearate (as thickener)
6% ~ 7% of magnesium nitrate (as filler to improve adhesion and permeability); [page]
2.3% ~ 2.8% of sodium polyphosphate (as corrosion inhibitor);
water (adjusting viscosity).
3.2.3 electrochemical treatment
Take Xiamen University patent [5] as an example, the treatment method is: take the stainless steel workpiece to be treated as an anode and control the constant potential for anodizing, or take the stainless steel workpiece as a cathode and control the constant potential for cathodic treatment, then take the stainless steel workpiece as an anode and control the constant potential for anodizing, and continue to change its constant potential for passivation. Electrolyte solutions are all HN3. After this treatment, the properties of stainless steel passivation film are improved and the corrosion resistance is greatly improved. The critical potential (Eb) of pitting corrosion is increased by about 1mV (in 3% NaCl), and the uniform corrosion resistance is improved by three orders of magnitude (in 2% ~ 3% H2SO4 at 45℃).
4. Application scope of stainless steel pickling passivation
4.1 Acid pickling and passivation treatment in the manufacturing process of stainless steel equipment
4.1.1 Cleaning and acid pickling and passivation after cutting [6]
Scrap iron, steel powder and cooling emulsion usually remain on the surface of stainless steel workpiece after cutting, which will cause stain and rust on the surface of stainless steel, so degreasing and oil removal should be carried out and then cleaning with nitric acid, which not only removes the dirt.
4.1.2 Cleaning and pickling passivation before and after welding [7]
Because grease is the source of hydrogen, gas will form in the weld without grease removal, and low melting point metal pollution (such as zinc-rich paint) will cause cracking after welding, so the groove and the surface within 2mm on both sides of stainless steel must be cleaned before welding, and the oil stain can be scrubbed with acetone. Paint rust should be washed with emery cloth or stainless steel first.
No matter what welding technology is adopted in the manufacture of stainless steel equipment, it should be cleaned after welding, and all welding slag, spatter, stain and oxidation color should be removed. The cleaning methods include mechanical cleaning and chemical cleaning. Mechanical cleaning includes grinding, polishing and sand blasting, etc. Carbon steel brushes should be avoided to prevent surface rust. In order to obtain the best corrosion resistance, it can be soaked in the mixed solution of HNO3 _ 3 and HF, or pickling passivation paste can be used. In fact, it is often used in combination with chemical cleaning.
4.1.3 Cleaning of forged castings [6]
Stainless steel workpieces after hot working such as forging and casting often have a layer of oxide scale, lubricant or oxide pollution on their surfaces, and the pollutants include graphite, molybdenum disulfide and carbon dioxide. Shot peening, salt bath treatment and multi-pass pickling treatment should be adopted. For example, the treatment process of stainless steel turbine blades in the United States is:
salt bath (1min)→ water quenching (2.5 min) → sulfuric acid washing (2min)→ cold water washing (2min)→ alkaline permanganate bath (1min)→ cold water washing (2min)→ sulfuric acid washing (1rain)→ cold water washing (1min)→ nitric acid washing.
4.2 pickling and passivation treatment before the new plant is put into production
Many stainless steel equipment and pipelines in large-scale chemical, chemical fiber, chemical fertilizer and other plants require pickling and passivation before they are put into production. Although the equipment has been acid-washed in 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 the equipment and equipment to meet the requirements and ensure the success of one commissioning, acid-washing passivation must be carried out. For example, stainless steel equipment and pipelines of H2O2 production plant must be cleaned before production, otherwise 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 the decomposition of H2O2, release a lot of heat violently, cause fire and even explosion. Similarly, for the oxygen pipeline, the existence of trace oil pollution and metal particles may also produce sparks and have serious consequences.
4.3 pickling passivation treatment in field maintenance
In refining the equipment materials of terephthalic acid (PTA), polyvinyl alcohol (PVA), acrylic fiber and acetic acid, a large number of austenitic stainless steels 316L, 317 and 34L are used, because the materials all contain harmful ions such as Cl-, Br-, SCN- and formic acid, or because of dirt and material agglomeration. When stopping for maintenance, the equipment or parts can be fully or partially 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 of PTA plant in Shanghai Petrochemical Company and the stainless steel heat exchanger of acrylic fiber plant have been passivated by pickling.
4.4 descaling and cleaning of in-service equipment
After a certain period of operation, stainless steel equipment in petrochemical plants, especially heat exchangers, will deposit various scales on the inner wall, such as carbonate scale, sulfate scale, silicate scale, iron oxide scale, organic scale and catalyst scale, which will affect the heat exchange effect and cause corrosion under the scale. It is necessary to choose a suitable cleaning agent for descaling, such as nitric acid, nitric acid+hydrofluoric acid, sulfuric acid, citric acid, EDTA, water-based cleaning agent, etc., and add a proper amount of corrosion inhibitor. After descaling and cleaning, it can be blunted 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 and passivation
If there is surface dirt on the stainless steel workpiece before pickling and passivation, it should be cleaned mechanically and then degreased. If the pickling solution and passivation solution can't remove grease, the presence of grease on the surface will affect the quality of pickling passivation. Therefore, degreasing can't be omitted, and alkali solution, emulsifier, organic solvent and steam can be used.
5.2 control of Cl- in pickling solution and washing water
in some stainless steel pickling solutions or pickling pastes, corrosive media containing chloride ions such as hydrochloric acid, perchloric acid, ferric chloride and sodium chloride are used as main agents or additives to remove the surface oxide layer, and chlorine-containing organic solvents such as trichloroethylene are used to remove grease, which is not suitable for preventing stress corrosion cracking. In addition, industrial water can be used for the initial washing water, but the halide content of the final washing water is strictly controlled. Deionized water is usually used. Such as petrochemical austenitic stainless steel pressure vessels.