Comparison of 1 pickling 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
2 Example of pickling passivation treatment formula
2. 1 General handling
(1) pickling
HNO 3 6% ~ 25%+HF 0.5% ~ 8% (volume fraction);
The temperature is 265438 0℃ ~ 60℃; Time required;
Or 5%- 10% (mass fraction) of ammonium citrate;
The temperature is 49℃ ~ 765438 0℃; Time 10 minutes to 60 minutes.
(2) Passivation
20% ~ 50% (volume fraction) of HNO3;
The temperature is 49 ~ 765438 0℃; Time 10 minutes to 30 minutes;
Or the temperature is 2l-38 DEG C; The time is 30min ~ 60min；;
Or HNO3 20% ~ 50%+Na2Cr207H2O22% ~ 6% (mass fraction);
The temperature is 49 DEG C to 54 DEG C; Time 15 minutes to 30 minutes;
Or the temperature is 21℃ ~ 38℃; The time is 30 min ~ 60 min.
(3) descaling and pickling
H2SO4 8% ~ 1 1% (volume fraction);
The temperature is 66 DEG C to 82 DEG C; 6 inches between 5 min and 45 min;
And HNO 3 6% ~ 25%+HF 0.5% ~ 8% (volume fraction);
The temperature is 265438 0℃ ~ 60℃;
Or HNO3 15% ~ 25%+HF L% ~ 8% (volume fraction).
2.2 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.
Pickling paste:
25% HNO 3+4% HF+7L% condensed water (volume fraction) and BaSO4, and mixed into paste.
Passivating paste:
Mix 30% HNO3 or 25% HNO3+ 1% (mass fraction) K2Cr2O7 and BaSO4 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:
HNO 38% ~ 14% (as passivator); Hf 10% ~ 15% (corrosive);
2.2% ~ 2.7% magnesium stearate (as thickener)
60% ~ 70% of magnesium nitrate (used as filler to improve adhesion and air permeability);
2.3% ~ 2.8% of sodium polyphosphate (as corrosion inhibitor);
Water (viscosity adjustment).
2.3 electrochemical treatment
The stainless steel workpiece to be treated is used as the anode to control the constant potential of anodic oxidation, or the stainless steel workpiece is used as the cathode to control the constant potential of cathodization first, and then the stainless steel workpiece is used as the anode to control the constant potential of anodic oxidation and continue to change the constant potential of passivation. Nitric acid is used as electrolyte solution. 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, 45℃).
3. Scope of application of stainless steel pickling passivation
3. 1 pickling passivation in the manufacturing process of stainless steel equipment
3. 1. 1 Cleaning, pickling and passivation after cutting.
After the stainless steel workpiece is cut, dirt such as iron filings, steel powder and cooling emulsion will usually remain on the surface, 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.
3. 1.2 Cleaning and pickling passivation before and after welding
Because grease is the source of hydrogen, pores 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, mechanical cleaning is usually combined with chemical cleaning.
3. 1.3 Cleaning of forged castings
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).
3.2 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.
3.3 Pickling passivation treatment in field maintenance
Equipment materials for PTA, PVA, acrylic fiber, acetic acid, etc.
Austenitic stainless steels 3 16L, 3 17 and 304L are widely used in materials. Because all materials contain harmful ions such as Cl-, Br-, SCN- and formic acid, or because of dirt and material caking, equipment will suffer from pitting corrosion, crevice corrosion and weld corrosion. 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.
4. Matters needing attention in stainless steel pickling passivation
4. 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.
4.2 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.
4.3 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 should be used in general, and fluoride can be used instead of HF 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%. The electrochemical test shows that the quality of passivation film treated with nitric acid concentration less than 20% is unstable and pitting corrosion is easy to occur, but the nitric acid concentration should not be greater than 50%, so over-passivation should be prevented.
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.
4.4 Pickling Control of Stainless Steel under Sensitization Condition
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.
4.5 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.
4.6 Post-treatment of pickling and passivation
After pickling and water washing, stainless steel workpiece can be soaked in NaOH+4% solution containing 10% (mass fraction) and permanganate solution containing 4% (mass fraction) for 5 ~ 60 min at 7 1 ~ 82℃ to remove pickling residue, and then washed thoroughly with water and dried. 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 lead to hydrogen embrittlement of martensitic stainless steel. If necessary, deoxidation can be carried out by heat treatment (heating to 200℃)
Warm for a while).
5. stainless steel pickling passivation Quality Inspection
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 8g of copper sulfate+500ml of H2O+2 ~ 3ml of sulfuric acid 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
2 ml HCl+1ml H2SO4+1gk3fe (cn) 6+97 ml H2O solution was dropped on the surface of the template, and the quality of the passive film was identified by the number of blue spots produced and the length of their appearance.
A multifunctional stainless steel pickling agent consists of a component A and a component B, wherein the component A contains 60-80% sulfuric acid and 20-40% nitric acid, and the component B contains 70-90% hydrochloric acid and 10-30% LK corrosion inhibitor, and the weight ratio of the component A to the component B is 90-98% and 2-/kloc-. It is the pretreatment or intermediate treatment of electroplating, enamel, rolling and other processes.
(1) The method of removing scale and rust on steel surface with acidic solution is called pickling. Iron oxides (Fe3O4, Fe2O3, FeO, etc. ) such as scale and rust react with acid solution to generate salt, which is dissolved in acid solution and removed. Acids used for pickling include sulfuric acid, hydrochloric acid, phosphoric acid, nitric acid, chromic acid, hydrofluoric acid and mixed acids. The most commonly used are sulfuric acid and hydrochloric acid. The pickling process mainly includes immersion pickling, spray pickling and pickling for rust removal. Generally, dipping pickling method is adopted, and spraying method can be used in mass production. Steel parts are usually pickled in 10% ~ 20% (volume) sulfuric acid solution at 40℃. When the iron content in the solution exceeds 80g/L and the ferrous sulfate exceeds 2 15g/L, the pickling solution should be replaced. Steel is pickled with 20% ~ 80% (by volume) hydrochloric acid solution at room temperature, which is not easy to cause over-corrosion and hydrogen embrittlement. Because acids have a great corrosive effect on metals, it is necessary to add corrosion inhibitors. After cleaning, the metal surface becomes silvery white, and at the same time, the surface is passivated, which improves the corrosion resistance of stainless steel.
(2) In order to eliminate the adsorption on the surface of diatomite carrier and reduce the tailing of chromatographic peaks, the carrier should be treated with acid washing or alkali washing before use. Pickling is to soak the carrier in 6mol/L hydrochloric acid for 2 hours or heat it in concentrated hydrochloric acid for 30 minutes, filter it, wash it with water to neutrality and dry it. Acid cleaning can remove impurities such as iron, aluminum, calcium and magnesium on the surface, but it can't remove silanol groups. Pickling carrier is suitable for analyzing acidic samples.
The purpose of pickling is to remove scale. Both hot working stainless steel and welding heat affected zone will produce scale, which will affect corrosion resistance. Commonly used are pickling and acid paste pickling, and pickling includes dipping and brushing.
Pickling solution formula (volume fraction): nitric acid (density 1.42g/cm3)20%, hydrofluoric acid 5%, and the rest is water. The pickling temperature is room temperature.
Washing solution formula (volume fraction): 50% hydrochloric acid and 50% water.
Acid solution formula: hydrochloric acid (density 1. 19g/cm3)20mL, water 100mL, nitric acid (density 1.42g/
Cm3)30mL, bentonite 150g.
Immersion washing method is suitable for smaller equipment and parts. Soak the equipment and components in pickling solution for 25 ~ 45 minutes, and then rinse them with clean water after taking them out. Scrubbing is suitable for large equipment. Brush it to white with a brush dipped in pickling solution, and then rinse it with clear water. Acid paste pickling is also suitable for large equipment. The method is to coat the prepared acid paste on the weld surface and its heat affected zone of the weldment, stay for a few minutes, and then rinse it with clear water.
Passivation is carried out after pickling. The formula of passivation solution is: 5mL nitric acid, potassium dichromate 1g and 95mL water. The treatment temperature is room temperature and the treatment time is 65438±0h h. The treatment method is to wipe the passivation solution on the weld surface, then rinse it with cold water, then scrub it carefully with cloth, and finally rinse it with hot water and blow dry it.
After passivation, the appearance of stainless steel is all silvery white, which has high corrosion resistance.