Pickling and Passivation of Steel Pipes

What Is Pickling & Passivation

Pickling and passivation are two forms of chemical metal finishing that provide protective properties to metal especially against rust. Or in other terms, pickling and passivation are the name of the two processes where the metal is submerged in a bathing liquid that removes imperfections and rust from the surface of steel pipe.

The corrosion resistance of the stainless steel pipe - Pickling and Passivation of Steel Pipes

What Is The Difference Between Pickling & Passivation
On the surface there does not seem to be a big difference between the two processes. The differences come down to the intensity of the treatments.

  • Pickling uses acids that take off the surface of the metal to treat impurities as a sublevel basis.
  • Pickling leaves a greater change to the steel pipe.
  • Passivation uses either nitric acid or citric acid that are not recognized as being as aggressive as the acids used in pickling.
  • Passivation does not typically go below the surface of the metal and does not change the properties of the metal. The oxide layer left by passivation is only .0000001 inch thick. To put that in perspective it is 1/100,000 as thick as a human hair.

What Are The Advantages Of Pickling & Passivation
Both types of metal finishing have great advantages for steel pipes.

  • Removes impurities that are left from the metal being manufactured.
  • Protects the metal from pollutants getting into the steel pipe and causing rusting and damage in the future.
  • Leaves the surface of the steel pipe smooth and with no imperfections.
  • Increases the durability of the steel pipes.

Why the steel pipe to pickling

Pickling carbon steel pipe is a very caustic process. A pickling reservoir is essentially a pure acid bath that will strip everything off the steel. Any foreign matter, rust, grease, oil, everything. The pickling process is so harmful that it is not allowed in any Metro Detroit plant and pickling facilities are strictly regulated in areas where it is allowed. It is so caustic that any building housing the pickling reservoir will be literally destroyed within 30 years. Environmentally safe, our pickled carbon steel pipe or stainless steel comes directly from the mill.rolled stainless steel – from the mill
Part of the pickling process is to coat the steel in a light oil after stripping it bare. This has to be done immediately or it will start to rust. Pickling is very prevalent in steel used for hydraulics. No foreign materials including bits of rust or steel flakes can be present as they would not be filtered through a hydraulic pump for example.
For stainless steel it is referred to as Pickle and Passivation. Pickling is done first, where again any foreign particles are removed and the stainless steel surface is metallically cleaned (purified).  Passivated where a dense passive layer can form only on a pure surface from the chromium oxide. This ensures corrosion resistance to other elements in stainless steel.

Steel pipe pickling procedure
So called pickling is using hydrofluoric acid and nitric acid solution to wash steel surface oxide generated after heat-treated. Used in the solution composition and ratio values: HF (3-8%), HNO3 (10-15%), H2O (the remaining amount) when processing solution temperature at 40-60 °C.
Steel pipe pickling procedure:
Clean up debris in oil spill → Loaded baskets → Chemical degreasing → Hot water cleaning  → Cold water Cleaning  →  Chemical rust → high pressure hair water purge →  The flow of water to wash  → Hot water cleaning → Blow-dry → test
The pickling process should pay attention to the following points:
The first point is the acceptance of incoming listed, specifications, quantity and straightening cut quality unqualified acceptance of pause the pickling and timely notice of the quality control department or workshop.
The second point is ready to work. The first is to add acid in accordance with the ratio value of the shop floor control, added acid should be evenly distributed in the pickling tank, rather than adding in one part of the acid tank; then control the size of the amount of steam, in order to control the temperature of the acid steels generally control the temperature of the acid to slightly steaming prevail (about 60 degrees), and should not control the temperature is too high or feeling of boiling acid; remaining is to organize the process of site steel pipe card, to distinguish between the different frame number, Because of the presence of different kinds of steel pickling time difference.
The third point of the the pickling process control, and also the most important step. Different products steel pipes and finished steel pickling and degreasing. Which, lifting products steel pipes can be directly used nylon rope to the pickling tank, hanging the fashionable steel pipe should tail high head low, so that the acid can successfully enter the pipe bore; finished steel pipe hanging into the acid bath pickling, at appropriate intervals time lifting steel pipe back into the acid tank, to be the oxidation of the surface of the steel pipe inside and outside Picchi this off to remove the steel pipe, gradually branched substantially flush with high-pressure water jets to wash pipe bore, then steel pipe into the hanging folders in the acid soak appropriate time steel pipes hanging out the acid tank should be soaked for 10 minutes in the sink and then gradually branched rinse the inside and outside surfaces. To oil pipe should go sump off of the surface oil and lubrication lime soak in the sink for 10 minutes and then rinse the inner hole-by-branch flush; rolling the finished products to the oil pipe in the sink to soak for a period of time sure the steel pipe has cooled completely, the steel pipe into the acid bath for half a minute, and then into the sink and flush-by-branch.
The fourth point is the pickling process after the inspection work. Finished pickling steel pipe test focused primarily oxide cleanliness of the inner and outer surfaces of the steel pipe, especially in the cleanliness of the inner bore, the best use yarn DON by formula inspection, the sampling amount should be not less than 10%. Followed by acid blot testing the steel surface and steel pipe color inspection failure should be re-pickling; away oil the surface of the steel pipe should be no oil spill, particular attention should be no residual acid outflow pipe ends.
Pickling Technics for Carbon steel pipe
Carbon steel pipe pickling method currently are used in the construction and management of two trough pickling pickling inner loop. 
(1) trough pickling. The installed piping removed after decomposition into pickling bath soak treatment after passing their secondary installation. This method is suitable for larger diameter tube short, straight, easy to disassemble, piping and a small amount of on-site piping system hydraulics hydraulic piping construction of small amounts of occasions, such as pumping stations, valve stations inside, can be used trough pickling method. 
(2) pipe loop pickling. In the installed hydraulic components in the hydraulic lines disconnected or removed with a hose, receivership, rinse cover joints, constitute rinse loop. Acid pump acid into the circulating loop pickling. The pickling method is more advanced construction technology in recent years, with the pickling speed, good effect, the process is simple, easy to operate, reducing pollution on the human body and the environment, reduce labor intensity and shorten the pipeline installation period, resolve long lines and complex piping pickling difficult problem, and to avoid secondary pollution problems trough pickling easy assembly occurs, has been widely used in the construction of large hydraulic system piping. 
Pickling formula and process piping will cause unreasonable tube wall can not be completely ex oxide, wall over corrosion, corrosion of the pipe wall and the tube again residual chemical reactions sediment and other phenomena.
Pickling and Passivation of Stainless Steel
1. Pickling passivation pretreatment
Stainless steel pickling passivation before any surface dirt, etc., should be mechanically cleaned, degreasing and then degreasing. If the pickling solution and the passivation solution can not remove the grease, the grease on the surface will affect the quality of the pickling passivation. Therefore, degreasing and degreasing can not be omitted. Lye, emulsifier, organic solvent and steam can be used. 
2. Pickling and flushing water Cl- control
Some stainless steel pickling or pickling by adding hydrochloric acid, perchloric acid, ferric chloride and sodium chloride and other chloride-containing corrosive media as the main agent or additives to remove the surface oxide layer, with the exception of grease trichlorethylene such as chlorine-containing organic solvents, from the prevention of stress corrosion cracking is not appropriate. In addition, industrial water may be used for the initial rinse water, but the halide content is strictly controlled for the final rinse water. Usually deionized water. Such as petrochemical austenitic stainless steel pressure vessel water pressure test control C1- content of not more than 25mg / L, such as can not meet this requirement, can be added to the water treatment of sodium nitrate to meet the requirements, C1-content exceeded, will be Destroying stainless steel passivation film is the root cause of pitting, crevice corrosion, stress corrosion cracking and the like. 
3. Pickling passivation operation of the process control
Nitric acid solution alone is used to remove free iron and other metal contaminants is effective, but to remove the scale, thick corrosion products, tempering film, etc., generally should be used HNO3 + HF solution, for the convenience and safety of operation, fluoride substitute for HF. HNO3 alone solution can be added without corrosion inhibitor, but need to add Lan-826 when HNO3 + HF pickling. Use HNO3 + HF pickling, to prevent corrosion, the concentration should maintain a 5: 1 ratio. The temperature should be below 49 ℃. If it is too high, HF will volatilize. 
4. Stainless steel pickling control sensitization conditions
Some stainless steels are sensitized by poor heat treatment or welding, pickling with HNO & HF may cause intergranular corrosion, and cracks caused by intergranular corrosion may condense halides during operation, during cleaning, or during subsequent processing, resulting in Stress corrosion. These sensitized stainless steel generally should not use HNO3 + HF solution descaling or pickling. After welding such as the need for such pickling, ultra-low carbon or stabilized stainless steel should be used.
5. Stainless steel and carbon steel assembly pickling
For stainless steel and carbon steel assemblies (eg, stainless steel tubes in heat exchangers, tube sheets, and carbon steel housings), pickling passivates severe corrosion of carbon steels with HNO3 or HNO3 + HF and proper corrosion inhibition agents such as Lan-826. When the combination of stainless steel and carbon steel in the sensitized state, can not be washed with HNO3 + HF, glycolic acid (2%) + formic acid (2%) + corrosion inhibitor, temperature 93 ℃, time 6h or ammonium EDTA base neutral solution + corrosion inhibitor, temperature: 121 °C, time: 6h, then rinsed with hot water and immersed in 10 mg / L ammonium hydroxide + 100 mg / L hydrazine.
6. Pickling passivation of the post-processing
Stainless steel parts after pickling and water rinse, available 10% (mass fraction) NaOH + 4% (mass fraction) KMnO4 alkaline permanganate solution soaked in 71 ~ 82 ℃ 5 ~ 60min to remove acid wash the residue, rinse thoroughly with water and allow to dry. After the stainless steel surface by pickling passivated spot or stain, available fresh passivation solution or higher concentration of nitric acid to eliminate. The final pickling passivation stainless steel equipment or components should pay attention to protection, polyethylene film can be covered or bandaged, to avoid contact with different metals and non-metallic.

Source: China Steel Pipes Manufacturer – Yaang Pipe Industry (www.metallicsteel.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@metallicsteel.com

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References:

  • www.yaang.com
  • https://www.steeljrv.com/stainless-steel-pipe-pickling.html

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