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What is stainless steel pickling and passivation

What is stainless steel pickling and passivation?

In order to improve the appearance and corrosion resistance of stainless steel, the processed stainless steel must be pickled and passivated. Remove the oxide scale generated after welding and high-temperature processing to make it silver and bright, and form a layer of oxide film with chromium as the main material on the treated surface, so as to avoid secondary oxygen corrosion and achieve the purpose of passivation, so as to improve the surface anti-corrosion quality of stainless steel products and prolong their service life.

0a08163631484a35a97a29c6bab37d61 - What is stainless steel pickling and passivation

Pickling can remove the oxide layer on the workpiece surface. Passivation is to form a new chromium rich protective layer on the workpiece surface. It is often easier to rust without treatment after pickling, and the anti-oxidation ability can be greatly improved after passivation.
The anti-corrosion function of stainless steel is mainly due to the fact that the surface is covered with a very thin (about 1nm) dense passive film, which can isolate corrosive substances and is the basic barrier for stainless steel protection. The passivation of stainless steel has dynamic characteristics, which should not be regarded as the complete cessation of corrosion, but the formation of diffusion barrier layer, which greatly reduces the anode reaction speed. Generally, it tends to destroy the film in the presence of reducing agent (such as chloride ion), but can maintain or repair the film in the presence of oxidant (such as air).
Although stainless steel workpiece placed in air will form oxide film, the protection of this film is not perfect. Generally, thorough cleaning, including alkali washing and pickling, and then passivation with oxidant 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 high-quality passivation, because the average surface area of stainless steel is 10 μ The surface of a layer with a thickness of M is corroded, and the chemical activity of the acid solution makes the dissolution rate of the defective part higher than that of other parts on the surface. Therefore, pickling can make the whole surface tend to be uniform and balanced, and some hidden dangers that are easy to cause corrosion are removed.

Through acid pickling and passivation, the oxides of iron and iron are preferentially dissolved than those of chromium and chromium, and the chromium poor layer is removed, resulting in the enrichment of chromium on the surface of stainless steel. The potential of this chromium rich passivation film can reach + 1.0V (SCE), which is close to the potential of precious metals and improves the stability of corrosion resistance. 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, form CrO3 or Cr2O3 in the barrier layer, or form a glassy oxide film, so that stainless steel can give full play to the maximum corrosion resistance.

Pickling can remove the oxide layer on the workpiece surface. Passivation is to form a new chromium rich protective layer on the workpiece surface. It is often easier to rust without treatment after pickling, and the anti-oxidation ability can be greatly improved after passivation.

Principle of pickling and passivation

The corrosion resistance of stainless steel is mainly due to the addition of a high content of Cr element in the steel (such as 316L w (Cr) = 16.00 – 18.00). Cr element is easy to oxidize and can quickly form a dense Cr2O3 oxide film on the surface of the steel, which improves the electrode potential and corrosion resistance of the steel in the oxidizing medium, The corrosion resistance of stainless steel mainly depends on the very thin (about 1nm) dense passive film covered on the surface. This passive film is isolated from the corrosive medium and is the basic barrier for stainless steel protection. If the passive film is incomplete or defective, the stainless steel will still be corroded.
The surface oil stain, scratch, rust, impurities, low melting point metal pollutants, paint, welding slag, spatter, etc. caused by the lifting, assembly, welding, weld inspection (such as dye flaw detection and pressure test) and processing of tank plates, equipment and accessories of stainless steel chemical ship affect the surface quality of stainless steel and damage its surface passive film, It reduces the surface corrosion resistance and is easy to interact with the corrosive medium in the chemicals to be transported later, causing pitting corrosion, intergranular corrosion and even stress corrosion cracking.
Stainless steel chemical ships usually use seawater to wash tanks in operation. Seawater is rich in chloride ions, which has a great corrosive effect on the passivation film. Under bad working conditions, pickling passivation is indispensable.
The passive film of stainless steel has dynamic characteristics, which should not be regarded as the complete cessation of corrosion, but the formation of a diffused protective layer. Generally, it tends to destroy the passive film in the presence of reducing agent (such as chloride ion), and it can protect and repair the passive film in the presence of oxidant (such as air).
Stainless steel placed in the air will form an oxide film, but the protection of this film is not perfect. The average thickness of the stainless steel surface is 10 mm through pickling μ The surface of one layer of M is corroded, and the chemical activity of the acid solution makes the dissolution rate of the defective part higher than that of other parts on the surface. Therefore, pickling can make the whole surface tend to be uniform and balanced. More importantly, through pickling and passivation, iron and iron oxides are dissolved prior to chromium and chromium oxides, and the chromium poor layer is removed to enrich chromium on the surface of stainless steel, Under the action of oxidant passivation, a complete and stable passivation film is produced on the surface. The potential of this chromium rich passivation film can reach + 1.0V (SCE), which is close to the potential of precious metals, and the stability of corrosion resistance is improved.

Passivation method of pickling

According to different operation modes, stainless steel pickling and passivation treatment mainly includes six methods: immersion method, paste method, brushing method, spray method, circulation method and electrochemical method. Among them, immersion method, paste method and spray method are more suitable for pickling and passivation of stainless steel chemical ships and equipment.

Immersion method

Stainless steel pipeline, elbow and small parts are most suitable for this method, and the treatment effect is the best. Because the treated parts can be fully immersed in pickling and passivation solution, the surface reaction is complete and the passivation film is dense and uniform. Chuandong shipyard has achieved good results in batch pickling of “Ninghua 417” stainless steel pipeline by using the pickling workshop of our factory. This method is suitable for continuous batch operation, but it needs to continuously supplement new solution with the decrease of solution reaction concentration. Its disadvantage is that it is not suitable for high-capacity equipment and pipelines with long and wide shape due to the limitation of acid tank shape and capacity; If it is not used for a long time, the effect will be reduced due to solution volatilization and other reasons, and special site, acid tank and heating equipment are required.

Paste method

At present, stainless steel pickling passivation paste has been widely used in China and a series of products are supplied. The main components are nitric acid, hydrofluoric acid, corrosion inhibitor and viscous agent in a certain proportion. The ship standard stainless steel pickling passivation paste CB/T3595-94 has specific inspection rules and manual operation, which is suitable for on-site construction. It is suitable for stainless steel chemical ship weld treatment, welding discoloration, deck top The coating and passivation of corners, dead corners, back of escalator and large area in tank are applicable.
The advantages of paste method are that it does not need special equipment and site, heating equipment, flexible on-site operation, acid pickling and passivation are completed at one time, and strong independence; The passivation paste has a long shelf life. Each coating treatment uses a new passivation paste for one-time use. The reaction stops after one layer of passivation on the surface, which is not easy to be over corroded and is not limited by the subsequent flushing time. Passivation can be strengthened in weak links such as welds. The disadvantages are poor operating environment, high labor intensity, high cost and poor treatment effect on the inner wall of stainless steel pipeline, which needs to be combined with other methods.

Spray method

It is suitable for pickling and passivation of single products with fixed site and closed environment or equipment with simple internal structure, such as spray pickling process on plate production line and pickling of inner wall of liquid cargo tank for stainless steel chemical ship. Chuandong shipyard prepares the liquid cargo tank of “Yingchun” ship of Sinochem 3450t stainless steel chemical ship under construction for pickling and passivation by spraying method. The utility model has the advantages of fast continuous operation speed, simple operation mode, little corrosion impact on workers, and the pipeline can be pickled again in the liquid transfer process. The solution utilization rate is high. This method has many restrictions, such as:

  • 1. All scaffolds in the tank shall be removed, the tank and piping system shall be cleaned, and there shall be no residue, impurity, etc. in the pipeline.
  • 2. All deep well pumps, valves, tank washers and other liquid cargo systems of the whole ship shall be commissioned for standby.
  • 3. If the pickling solution stays in the tank for too long, it will react and cause excessive corrosion of stainless steel. Therefore, it must be operated continuously. The shipyard must cooperate highly and prepare a large amount of cleaning water at any time. In case of power failure, water cut-off and shutdown, serious consequences will be caused.
  • 4. Waste acid and waste water shall be discharged in large containers or replaced by a tank temporarily, but neutralized and discharged as soon as possible to avoid corrosion of the tank.
  • 5. With the increase of reaction time and solution impurities, the effective components of acid washing solution gradually decrease. The solution concentration must be detected at any time and new solution must be supplemented in time.
  • 6. The residence time of spraying liquid on the deck, four walls, dead corners and the back of escalator is short, and the pickling effect is slightly worse than that of the bottom plate.

Process flow of pickling and passivation

  • Pretreatment (surface degreasing and cleaning) → water cleaning → acid pickling → passivation → water cleaning → inspection → drying

Pretreatment before pickling and passivation

  • (1) Pickling and passivation pretreatment can be carried out for stainless steel containers or parts after manufacturing according to the requirements of drawings and process documents and after the specified items are inspected as qualified. Note: entrusted pickling and passivation business is not considered.
  • (2) The weld and welding slag and spatter on both sides shall be cleaned; Gasoline or cleaning agent shall be used to remove oil stains and other dirt on the surface of the machined parts of the container. When removing foreign matters on both sides of the weld, remove them with stainless steel wire brush, stainless steel shovel or grinding wheel. After removal, wash them with clean water (the chloride ion content in the water does not exceed 25mg / L). When the oil stain is serious, remove the oil stain with 3-5% alkali solution and rinse it with purified water.
  • (3) The oxide scale of stainless steel hot working parts can be removed by mechanical sand blasting. The sand must be pure silicon or alumina.
  • (4) Formulate safety measures for pickling and passivation, and determine necessary appliances and labor protection articles.

Formulation of pickling and passivation solution and paste

  • A. Pickling solution formula: nitric acid (specific gravity 1.42) 20%, hydrofluoric acid 5%, and the rest is water. The above is volume percentage.
  • B. Pickling paste formula: 20ml hydrochloric acid (specific gravity 1.19), 100ml water, 30ml nitric acid (specific gravity 1.42), 150g bentonite.
  • C. Passivation solution formula: nitric acid (specific gravity 1.42) 5%, potassium dichromate 4G, and the rest is water. The passivation temperature is room temperature.
  • D. Passivation paste formula: 30ml nitric acid (concentration 67%), 4G potassium dichromate, add bentonite (100-200 mesh) and stir until paste.

Operation of pickling and passivation

  • (1) Only pretreated containers or parts can be pickled and passivated.
  • (2) Pickling solution pickling is mainly used for the overall treatment of small unprocessed parts, which can be sprayed and brushed. When the solution temperature is 21-60 ℃, check it every 10 minutes or so until it shows a uniform white acid etching finish.
  • (3) Pickling paste pickling is mainly suitable for large containers or local treatment. Evenly clean the outside of the pickling paste on the equipment (about 2-3mm thick) at room temperature. After staying for one hour, gently brush it with clean water or stainless steel wire brush until it presents a uniform white acid etching finish.
  • (4) Passivation solution is mainly applicable to the overall treatment of small containers or components. Immersion or spray brushing can be used. When the solution temperature is 48-60 ℃, check it every 20 minutes, and when the solution is 21-47 ℃, check it every hour until a uniform passivation film is formed on the surface.
  • (5) The passivation paste is mainly suitable for large containers or local treatment. Apply the passivation paste evenly on the pickled container surface (about 2-3mm) at room temperature and check it after 1 hour until a uniform passivation film is formed on the surface.
  • (6) The surface of pickling passivation container or parts must be washed with clean water. Finally, test any part of the washing surface with acid litmus test paper to make the pH value between 6.5-7.5, and then wipe dry or blow dry with compressed air.
  • (7) Containers and parts shall be handled after pickling and passivation. It is forbidden to bump and scratch the passivation film during hoisting and storage.

Process control of pickling and passivation of stainless steel

1. Pretreatment of pickling and passivation
Before pickling and passivation of stainless steel workpiece, if there is surface dirt, it shall be mechanically cleaned and then degreased. If the acid washing solution and passivation 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 and steam can be used.
2. Control of Cl- in pickling solution and flushing water
Some stainless steel pickling solutions or pickling pastes use chloride ion containing corrosive media such as hydrochloric acid, perchloric acid, ferric chloride and sodium chloride as the main agent or auxiliary agent to remove the surface oxide layer, and trichloroethylene and other chlorinated organic solvents for grease removal, which is not suitable to prevent stress corrosion cracking.
In addition, industrial water can be used for preliminary flushing, but the halide content of final cleaning water shall be strictly controlled. Deionized water is usually used. For example, the C1 – content of water for hydrostatic test of petrochemical austenitic stainless steel pressure vessel shall not exceed 25mg / L. if this requirement cannot be met, sodium nitrate can be added to the water to meet the requirements. If the C1 – content exceeds the standard, the passive film of stainless steel will be damaged, which is the root cause of pitting corrosion, crevice corrosion, stress corrosion cracking, etc.
3. Process control in pickling and passivation operation
Nitric acid solution alone is effective for removing free iron and other metal dirt, but it is ineffective for removing iron oxide scale, thick corrosion products, tempering film, etc. HNO3 + HF solution shall be generally used. Fluoride can be used to replace HF for convenience and operation safety.
No corrosion inhibitor can be added to HNO3 solution alone, but Lan-826 needs to be added when HNO3 + HF pickling. HNO3 + HF pickling shall be used. In order to prevent corrosion, the concentration shall be maintained at the ratio of 5:1. The temperature should be lower than 49 ℃. If it is too high, HF will volatilize. For the passivation solution, HNO3 should be controlled between 20% – 50%. According to the electrochemical test, the quality of the passivation film treated with HNO3 concentration less than 20% is unstable and prone to pitting corrosion, but the HNO3 concentration should not be greater than 50% to prevent over passivation. Although the one-step process is easy to operate and saves man hours, there will be corrosive HF in the pickling passivation solution (paste), so the quality of the final protective film is not as good as that of the multi-step process. During pickling, it is allowed to adjust the acid concentration, temperature and contact time within a certain range. With the increase of service time of acid washing solution, attention must be paid to the changes of acid concentration and metal ion concentration, and over pickling should be avoided. The titanium ion concentration should be less than 2%, otherwise serious pitting corrosion will be caused. 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. Control of pickling under stainless steel sensitization
Some stainless steels are sensitized due to poor heat treatment or welding. Pickling with HNO3 + HF may produce intergranular corrosion. Cracks caused by intergranular corrosion can concentrate halides during operation, cleaning or subsequent processing, resulting in 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 shall be used.
5. Pickling of stainless steel and carbon steel assembly
For stainless steel and carbon steel assemblies (such as stainless steel tubes, tubesheets and carbon steel shells in heat exchangers), pickling and passivation. If HNO3 or HNO3 + HF is used, carbon steel will be seriously corroded. At this time, appropriate corrosion inhibitor such as Lan-826 shall be added. When the stainless steel and carbon steel assembly cannot be pickled with HNO3 + HF in the sensitized state, glycolic acid (2%) + formic acid (2%) + corrosion inhibitor can be used, the temperature is 93 ℃, the time is 6h, or EDTA ammonium based neutral solution + corrosion inhibitor, the temperature is 121 ℃, the time is 6h, and then washed with hot water and immersed in 10mg / L ammonium hydroxide + 100mg / L hydrazine.
6. Post treatment of pickling and passivation
After pickling and water washing, stainless steel workpieces can be soaked in alkaline 1 permanganate solution containing 10% (mass fraction) NaOH + 4% (mass fraction) KMnO4 at 71-82 ℃ for 5-60min to remove pickling residues, and then thoroughly washed with water and dried. After pickling and passivation, spots or stains on the surface of stainless steel can be eliminated by scrubbing with fresh passivation solution or high concentration nitric acid. Stainless steel equipment or parts that are finally pickled and passivated shall be protected and covered or wrapped with polyethylene film to avoid contact between dissimilar metals and nonmetals.
The treatment of acid and passivation waste liquid shall comply with the national environmental protection discharge regulations. For example, fluoride containing wastewater can be treated with lime milk or calcium chloride. Dichromate shall not be used for passivation solution as far 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, it can be deoxidized by heat treatment (heated to 200 ℃ for a period of time).
7. Quality inspection for pickling and passivation of stainless steel
Since chemical inspection will damage the passive film of the product, it is usually inspected on the sample plate. Examples of methods are as follows:
(1) Copper sulfate titration test
Drop 8g CUSO4 + 500ml H20 + 2-3ml H2SO4 solution into the sample surface and keep it wet. It is qualified if there is no copper precipitation within 6min.
(2) High iron titration test
Drop 2ml HCl + 1ml H2SO4 + 1g K3Fe (CN) 6 + 97ml H20 solution on the surface of the template. The quality of the passive film is identified by the number of blue spots and the length of time.
8. Application examples
(1) Passivation of long string parts
When passivating long series parts, the upper and lower ends of the plated parts enter and exit the passivation solution successively. At the same time, when the workpiece swings in the solution, the swing amplitude of the lower end of the plated part is much larger than that of the upper end; On the other hand, when staying in the air after passivation, the solution flows down from the upper end, the surface of the lower plated part is attached with more solution than the upper plated part, and the lower end has more chemical reaction time with the solution. These make the color of the lower passivation film darker than that of the upper passivation film. In order to reduce the color difference, it is recommended that such parts enter and exit the passivation solution horizontally (the lower end is hooked with a hook), and prevent the anode used from being too long.
(2) Passivation of long parts
When passivating long parts, if the passivation tank cannot accommodate them, temporary measures can be taken. Use bricks or wooden strips to process a frame that can accommodate the plated parts. The frame is lined with plastic cloth and can be used after injecting passivation solution. This method is convenient and can avoid quality problems such as uneven film layer or joint marks.
(3) Passivation of flat parts
During passivation of general plane parts, the contact between the edge part and the passivation solution will be more intense than that in the middle part when swinging in the passivation tank, resulting in uneven color of the passivation film at this part. This problem can be solved by stirring with compressed air, and the effect is very good. In order to improve the uniformity of the passivation film, attention should also be paid to the uniformity of current distribution in the galvanizing process. If necessary, the edge of the plated part should be shielded to prevent the coating roughness at this part due to excessive current and affect the color of the passivation film.  
(4) Surface passivation
After passivation of smooth surface parts, because the surface is smooth, the passivation solution is difficult to adsorb on its surface and quickly loses, so the residence time in the solution and air during passivation should be appropriately prolonged, otherwise the passivation film of the workpiece appears light.  
(5) Passivation of easy to catch water parts
The solution shall be avoided to avoid excessive loss, consumption and pollution of the passivation solution, and the quality of the passivation film at this part can also be guaranteed.
(6) Passivation of small parts
The whole string of bound workpieces can be passivated in the plastic basket to avoid falling out of the group due to shaking and falling into the groove during passivation.

Quality inspection of pickling and passivation

The quality inspection of stainless steel pickling and passivation effect can be carried out with reference to the ship standard stainless steel pickling and passivation paste CB / t3595-94 and the international standard inspection of surface passivation film formed by stainless steel electrolytic polishing, polishing and pickling passivation ISO15730. Generally, it can be divided into appearance inspection and chemical test. The chemical test mainly includes man-made seawater hanging piece corrosion test copper sulfate titration test, potassium ferricyanide titration test (blue dot test), etc. the inspection of pickling and passivation effect of “Ninghua 417” wheel is carried out in the following three ways:
Appearance inspection: the pickling and passivation surface of stainless steel shall be uniform silver white, smooth and beautiful, without obvious corrosion traces, and the weld and heat affected zone shall be free of oxidation color and uneven spots.
Residual liquid test: use phenolphthalein test paper to check the flushing degree of residual liquid on the surface of stainless steel. If the pH value is neutral, it is qualified.
Blue spot test: the basic principle of the blue spot test method is that if the surface passivation film is incomplete or contaminated with iron ions, there will be free iron ions. When potassium ferricyanide solution meets iron ions, it will react to form blue precipitation. The reaction formula is as follows:

  • K++Fe+2+[Fe(CN)6]-3→KFe[Fe(CN)6]↓

Dissolve 10g potassium ferricyanide in 50ml distilled water with a 100ml beaker, add 30ml concentrated nitric acid after dissolution, then transfer it to a 1000ml volumetric flask, dilute it with distilled water to the scale, that is, the blue dot test solution, and the storage period is one week. Test method: drop the blue dot test solution on the test paper attached to the stainless steel surface, and it is unqualified if the blue dot is displayed on the test paper after 30 seconds. It should be noted that The passivation film of stainless steel will be damaged by chemical test. Generally, the test plate and the product can be pickled and passivated at the same time, and then the above test can be carried out on the test plate.

What is the difference between stainless steel passivation and stainless steel pickling passivation?

Stainless steel passivation belongs to the natural color passivation of the workpiece. Without changing the original color and size of the workpiece, a layer of dense passivation protective film is formed. Stainless steel passivation is generally applicable to SUS301, SUS304, SUS316, SUS316L and SUS321, not sus303 and SUS201, and can effectively improve the anti rust ability of the workpiece by 3-20 times, The high salt spray performance of stainless steel passivation of Kemeng company is greatly improved, far ahead of similar products in the same industry.
The pickling and passivation of stainless steel belongs to silver white passivation effect. The passivation vision is similar to the matte of stainless steel electrolytic polishing. It is visually close to the color of aluminum, white but without gloss. Stainless steel pickling and passivation is generally applicable to the removal of rust, weld spots, oil stain, black and yellow oxide scale produced by sus300 series austenitic stainless steel (sus303, 304, 316, 316L, etc.) during forming, assembly and welding. At the same time, stainless steel workpieces are fully passivated to greatly improve the corrosion resistance of stainless steel workpieces. Truly achieve the effect of acid pickling and passivation.
Both have the effect of passivation and rust prevention, but compared with the rust prevention time, stainless steel passivation is better than stainless steel pickling passivation. Therefore, different technological processes can be selected according to different customer requirements, different duration of high salt fog and different materials.
20210914220238 80438 - What is stainless steel pickling and passivation

This is the passivation effect (primary color passivation).

20210914220247 89574 - What is stainless steel pickling and passivation

This is the pickling passivation effect (the surface is white but has no gloss).

Source: China Stainless Steel Pipes Manufacturer – Yaang Pipe Industry (www.epowermetals.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

If you want to have more information about the article or you want to share your opinion with us, contact us at sales@epowermetals.com

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