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Pickling passivation of stainless steel

Table of Contents

1. The need for stainless steel pickling passivation

Austenitic stainless steel has good corrosion resistance, high temperature oxidation resistance, better low temperature performance and excellent mechanical and processing properties. Therefore, it is widely used in chemical, petroleum, power, nuclear engineering, aerospace, marine, pharmaceutical, light industry, textile and other sectors. Its main purpose is to prevent corrosion and rust. Stainless steel corrosion resistance mainly relies on the surface passivation film, if the film is incomplete or defective, stainless steel will still be corroded. Engineering is usually carried out pickling passivation treatment, so that the corrosion resistance potential of stainless steel to a greater extent. In stainless steel equipment and components in forming, assembly, welding, weld inspection (such as flaw detection, pressure resistance test) and construction markings and other processes bring surface oil, rust, non-metallic dirt, low melting point metal contaminants, paint, welding slag and spatter, etc., these substances affect the surface quality of stainless steel equipment and components, damage its surface oxide film, reducing the steel’s resistance to overall corrosion performance and resistance to local corrosion performance ( including pitting corrosion, crevice corrosion), and can even lead to stress corrosion rupture.

Stainless steel surface cleaning, pickling and passivation, in addition to maximizing corrosion resistance, there is a role in preventing product contamination and access to aesthetics. In GBl50-1998 “Steel Pressure Vessels” stipulates that “the surface of stainless steel and composite steel plate manufactured by the vessel with anti-corrosion requirements should be pickled and passivated”. This provision is for the use of petrochemical pressure vessels, because these devices are used in direct contact with corrosive media, from the assurance of corrosion resistance and corrosion resistance, pickling passivation is necessary. For other industrial sectors, such as not for anti-corrosion purposes, based only on the cleanliness and aesthetic requirements, and the use of stainless steel materials are not pickling passivation. But the welds of stainless steel equipment also need to be pickling passivation. For nuclear engineering, some chemical devices and other use of strict requirements, in addition to pickling passivation, but also the use of high-purity media for the final fine cleaning or mechanical, chemical and electrolytic polishing and other finishing treatment.

Effective way to prevent rust in stainless steel

2. Stainless steel pickling passivation principle

Stainless steel corrosion resistance is mainly due to the surface covered with a very thin (about 1nm) dense passivation film, this film 1nm corrosive media isolation, is the basic barrier to stainless steel protection. Stainless steel passivation has dynamic characteristics, should not be seen as a complete stop of corrosion, but the formation of a barrier layer of diffusion, so that the anodic reaction rate is greatly reduced. Usually in the presence of reducing agents (such as chloride ions) tend to destroy the film, while in the presence of oxidizing agents (such as air) can maintain or repair the film.

Extremely thin, dense passivation film

Stainless steel workpiece placed in the air will form an oxide film, but the protection of this film is not perfect. Usually a thorough cleaning, including alkaline and pickling, followed by passivation with an oxidizing agent is 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 to ensure the formation of a high-quality passivation film. Because the surface of stainless steel through pickling so that an average of 10μm thick layer of the surface is corroded away, the chemical activity of the acid makes the defective parts of the dissolution rate than other parts of the surface, so pickling can make the entire surface tends to be uniformly balanced, some of the original susceptibility to corrosion is removed from the hidden danger. But more importantly, through pickling passivation, so that iron and iron oxides than chromium and chromium oxides preferentially dissolved, removing the chromium-poor layer, resulting in chromium enrichment in the stainless steel surface, the potential of this chromium-rich passivation film up to +1.0V (SCE), close to the potential of precious metals, to improve the stability of corrosion resistance. Different passivation treatment will also affect the composition and structure of the film, thus affecting the stainless steel, such as through electrochemical modification process, the passivation film can be made to have a multilayer structure, the formation of CrO or CrO in the barrier layer, or the formation of glassy oxide film, so that stainless steel can play the maximum corrosion resistance.

Scholars at home and abroad have conducted a lot of research on the generation of stainless steel passivation film. In recent years, Beijing Science University of 316L steel passivation film photoelectron spectroscopy (xps) research as an example for a brief description. Stainless steel passivation is the surface layer for some reason dissolved with the adsorption of water molecules, under the catalytic effect of oxidizing agents, the formation of oxides and hydroxides, and the composition of stainless steel cr, Ni, Mo elements conversion reaction, the final formation of a stable phase-forming film, preventing the destruction of the film and corrosion.

3. Stainless steel pickling passivation methods and processes

3.1 Pickling passivation treatment method comparison

Stainless steel equipment and parts pickling passivation treatment according to the operation of different methods, its scope of application and characteristics are shown in Table 1.

Table 1 stainless steel pickling passivation method comparison

Serial number Method Scope of application Advantages and disadvantages
1 Impregnation method For parts that can be put into the pickling tank or passivation tank, but not for large equipment pickling solution can be used for a longer period of time High production efficiency and low cost; large volume equipment filled with acid impregnation consumes too much liquid
2 Painting method Suitable for large equipment internal surface and local treatment material operation Poor labor conditions and unrecoverable acid
3 Paste method For installation or maintenance sites, especially for manual handling of welding sections Poor labor conditions and high production costs
4 Spraying method For installation sites, large vessel interiors Low fluid consumption, low cost, high speed, but need to configure the gun and cutter ring system
5 Circular method For large equipment, such as heat exchangers, shell and tube Easy to handle construction, the acid can be reused, but it needs to be connected to the circulation system with pipes and pumps.
6 Electrochemical method Can be used for both parts and surface treatment of field equipment by electric brush method More complex technology, need DC power supply or constant potential meter

4. Application scope of stainless steel pickling passivation

4.1 Pickling passivation treatment in the manufacturing process of stainless steel equipment

4.1.1 Cleaning and pickling passivation after cutting processing

Stainless steel workpiece by cutting processing on the surface usually residual iron chips, steel and cooling emulsion and other dirt, will make the stainless steel surface stains and rust, so should be degreased and degreased, and then cleaned with nitric acid, not only to remove the iron chips steel, but also passivation.

Good corrosion resistance of stainless steel

4.1.2 Cleaning and pickling passivation before and after welding

As grease is a source of hydrogen, in the weld without removing grease will form pores, and low melting point metal pollution (such as zinc-rich paint) will cause cracking after welding, so stainless steel must be cleaned before welding the bevel and both sides of the surface within 20mm, oil can be scrubbed with acetone, paint rust should first be removed with a sand cloth or stainless steel wire brush, and then wipe clean with acetone.

Stainless steel equipment manufacturing regardless of the welding technology, after welding are to be cleaned, all welding slag, spatter, stains and oxidation color are to be removed, removal methods include mechanical and chemical cleaning. Mechanical cleaning with grinding, polishing and sandblasting shot, etc., should avoid the use of carbon steel brushes to prevent surface rust. In order to obtain the best corrosion resistance, it can be immersed in a mixture of HNO3 and HF, or the use of pickling passivation paste. In fact, mechanical cleaning is commonly used in combination with chemical cleaning.

Pickling passivation of stainless steel

4.1.3 Cleaning of forged and cast parts

After hot processing such as forging and casting stainless steel workpiece, the surface often has a layer of oxide, lubricant or oxide pollution, pollutants including graphite, molybdenum disulfide and carbon dioxide, etc.. Should be treated by shot blasting, salt bath treatment and multi-pass pickling treatment. Such as the United States stainless steel turbine blade treatment process for

Salt bath (10min) → water quenching (2.5min) → sulfuric acid wash (2min) → cold water wash (2min) → alkaline permanganate bath (10min) → cold water wash (2min) → sulfuric acid wash (1rain) → cold water wash (1min) → nitric acid wash (1.5min) → cold water wash (1min) → hot water wash (1min) → air drying.

4.2 Pickling passivation treatment before the commissioning of new devices

Many large chemical, chemical fiber, fertilizer and other devices of stainless steel equipment and pipelines in production before the start of the requirements for pickling passivation. Although the equipment has been pickled in the manufacturing plant, in addition to welding slag and oxide skin, but in storage, transportation, installation process and inevitably caused by grease, mud and sand, rust and other pollution, in order to ensure that the device and equipment test products (especially chemical intermediates and refined products) quality can meet the requirements to ensure a successful commissioning, must be pickled passivation. Such as H2O2 production device stainless steel equipment and piping, must be cleaned before production, otherwise if there is dirt heavy metal ions will make the catalyst poisoning. In addition, such as metal surfaces with grease and free iron ions will cause the decomposition of H2O2, violent release of large amounts of heat, causing a fire, or even an explosion. Similarly for oxygen pipeline, the presence of trace oil and metal particles may also produce sparks and serious consequences.

4.3 Acid pickling and passivation treatment in field maintenance

In the refined terephthalic acid (PTA), polyvinyl alcohol (PVA), acrylic, acetic acid and other production equipment materials, a large number of austenitic stainless steel 316L, 317, 304L, because the material contains Cl-, Br-, SCN-, formic acid and other harmful ions, or due to dirt, material agglomeration, will produce pitting, crevice corrosion and weld corrosion on the equipment. In the parking maintenance can be a comprehensive or local pickling passivation treatment of equipment or components to repair its passivation film to prevent the expansion of local corrosion. Such as Shanghai Petrochemical PTA device dryer stainless steel pipe update overhaul and acrylic device stainless steel heat exchanger overhaul have been acid pickling passivation.

4.4 In-service equipment descaling cleaning

Petrochemical plant stainless steel equipment, especially heat exchangers, after a certain period of operation, the inner wall will deposit a variety of dirt, such as carbonate scale, sulfate scale, silicate scale, iron oxide scale, organic scale, catalyst scale, etc., affecting the heat transfer effect, and will cause corrosion under the scale. Need to choose the right cleaning agent for descaling, can use nitric acid, nitric acid + hydrofluoric acid, sulfuric acid, citric acid, EDTA, water-based cleaning agent, etc., and add the right amount of corrosion inhibitor. After descaling and cleaning, if necessary, then passivation. Chemical treatment. Such as Shanghai Petrochemical PTA, acetic acid, acrylic and other devices of stainless steel heat exchanger have been descaled and cleaned.

5. Stainless steel pickling passivation precautions

5.1 Pre-treatment of pickling passivation

Stainless steel workpiece pickling passivation before the surface dirt, etc., should be cleaned by mechanical cleaning, and then degreasing degreasing. If the pickling solution and passivation solution can not remove the grease, the presence of grease on the surface will affect the quality of pickling passivation, for this reason oil removal and degreasing can not be omitted, you can use alkali, emulsifier, organic solvents and steam, etc.

5.2 pickling solution and rinse water Cl- control

Some stainless steel pickling solution or pickling paste using the addition of hydrochloric acid, perchloric acid, ferric chloride and sodium chloride and other aggressive media containing chloride ions as the main agent or additives to remove the surface oxide layer, in addition to grease with trichloroethylene and other organic solvents containing chlorine, from the prevention of stress corrosion rupture is not very suitable. In addition, the initial rinse water can be used for industrial water, but the final cleaning water requires strict control of halide content. Usually use deionized water. Such as petrochemical austenitic stainless steel pressure vessel for water pressure test water, control C1-content does not exceed 25mg / L, such as the inability to meet this requirement, the water can be added to sodium nitrate treatment, so that it meets the requirements, C1-content exceeds the standard, will destroy the passivation film of stainless steel, is the root cause of pitting, crevice corrosion, stress corrosion rupture, etc..

5.3 Pickling passivation operation in the process control

Nitric acid solution alone for the removal of free iron and other metal dirt is effective, but the removal of iron oxide, thick corrosion products, tempering film, etc. is not effective, should generally use HNO3 + HF solution, for convenience and operational safety, available fluoride instead of HF. HNO3 solution alone can not add corrosion inhibitor, but HNO3 + HF pickling, you need to add Lan-826. use HNO3 + HF pickling, in order to prevent corrosion, the concentration should be kept at a ratio of 5:1. The temperature should be lower than 49℃, if it is too high, HF will volatilize.

For passivation solution, HNO3 should be controlled between 20% and 50%. According to electrochemical test, the quality of passivation film treated with HNO3 concentration less than 20% is unstable and easy to produce pitting, but the concentration of HNO3 should not be greater than 50% to prevent over-passivation.

The one-step process of degreasing pickling passivation, although easy to operate and save man-hours, but the pickling passivation solution (paste) will have aggressive HF, so its final protective film quality is not as good as the multi-step method.

The acid concentration, temperature and contact time can be adjusted within a certain range during the pickling process. With the growth of pickling solution usage time, the change of acid concentration and metal ion concentration must be paid attention to. Care should be taken to avoid over pickling and titanium ion concentration should be less than 2%, otherwise it will lead to serious pitting. In general, improve the pickling temperature will accelerate and improve the cleaning effect, but may also increase the risk of surface contamination or damage.

5.4 Stainless steel sensitization conditions of pickling control

Some stainless steel due to poor heat treatment or welding caused by sensitization, using HNO3 + HF pickling may produce intergranular corrosion, caused by intergranular corrosion cracks in operation, or cleaning, or subsequent processing, can concentrate halide, and cause stress corrosion. These sensitized stainless steel is generally not suitable for descaling or pickling with HNO3 + HF solution. After welding, such as this pickling must be carried out, should be used ultra-low carbon or stabilized stainless steel.

5.5 Stainless steel and carbon steel combination of pickling

Stainless steel and carbon steel combination parts (such as heat exchangers in stainless steel tubes, plates and carbon steel shell), pickling passivation if the use of HNO3 or HNO3 + HF will seriously corrode carbon steel, when the appropriate corrosion inhibitor should be added such as Lan-826. when stainless steel and carbon steel combination parts in the sensitized state, can not be used HNO3 + HF pickling, can be used hydroxyacetic acid (2%) + formic acid (2%) + corrosion inhibitor, temperature 93 ℃, time 6h or EDTA ammonium-based neutral solution + corrosion inhibitor, temperature: 121 ℃, time: 6h, followed by hot water rinse and immersion in 10mg / L ammonium hydroxide + 100mg / L hydrazine.

5.6 Post-treatment of pickling passivation

Stainless steel workpiece by pickling and water rinsing, available with 10% (mass fraction) NaOH + 4% (mass fraction) KMnO4 alkali generated permanganate solution in 71 ~ 82 ℃ soaked 5 ~ 60min to remove the pickling residue, and then rinsed thoroughly with water and dried. Stainless steel surface pickling passivation after the appearance of spots or stains, available fresh passivation solution or higher concentration of nitric acid scrubbing and elimination. Final pickling and passivation of stainless steel equipment or parts should pay attention to the protection, available polyethylene film cover or wrap, to avoid contact with non-metallic foreign metal.

The treatment of acidic and passivation waste liquid should comply with national environmental protection emission regulations. Such as fluorine wastewater can be treated with lime milk or calcium chloride. Passivation solution as far as possible without dichromate, such as chromium-containing wastewater, can add ferrous sulfate reduction treatment.

Pickling may cause hydrogen embrittlement of martensitic stainless steel, such as the need for heat treatment to oxygen (heated to 200 ℃ holding time).

6. Stainless steel pickling passivation quality inspection

As the chemical test will destroy the passivation film of the product, usually in the sample for inspection. Examples of methods are as follows.

(1) copper sulfate titration test

With 8gCuS04 + 500mLH20 + 2 ~ 3mLH2S04 solution drop into the surface of the sample plate, maintain the wet state, such as 6min no copper precipitation for qualified.

(2) titration test of potassium pertechnetate

Using 2mLHCl+1mLH2S04+1gK3Fe(CN)6+97mLH20 solution drop on the surface of the sample plate, through the number of blue spots generated and the length of time to identify the quality of passivation film.

Conclusion

This article mainly explains the stainless steel pickling passivation treatment, the use: a comprehensive pickling passivation of stainless steel, remove all kinds of oil, rust, oxide skin, weld spots and other dirt, the surface becomes uniform silver-white after treatment, greatly improving the corrosion resistance of stainless steel, applicable to various types of stainless steel parts, plates and their equipment. Features are simple operation, easy to use, economical and practical, while adding high-efficiency corrosion inhibitor, fog inhibitor, to prevent the metal from over-corrosion and hydrogen embrittlement phenomenon, inhibit the generation of acid fog. It is especially suitable for small and complex workpieces, which are not suitable for paste application, and is better than similar products in the market.

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