Recently, the environmental protection department has strictly controlled pickling production enterprises, and some processing equipment and factories that do not meet environmental protection requirements have been ordered to be closed or demolished for rectification.
Therefore, many people were worried about the problems related to pickling.
I. Definition and Classification of Steel Pickling
1. Definition of pickling
Acids are chemically removed from iron oxide scale at a certain concentration, temperature and speed, which is called pickling.
2. Pickling classification
According to the type of acid:
- Sulfuric acid pickling
- Pickling with hydrochloric acid
- Nitric acid pickling
- Hydrofluoric acid pickling
Pickling requires the selection of different media depending on the type of steel. For example, carbon steel is pickled with sulfuric acid and hydrochloric acid, while stainless steel is pickled with a mixture of nitric acid and hydrofluoric acid.
According to the shape of the steel:
- Wire stripping
- Pickling Forging
- Steel sheet pickling
- Pickling steel strips
According to the type of blasting equipment:
- Tank stripping
- Semi-continuous pickling
- Complete continuous blasting
- Tower stripping
Ⅱ. Iron oxide scale removal method
Iron oxide scale removal method, including: Mechanical, chemical and electrochemical methods.
Mechanical method:
- Polishing
- Barrel grinding
- High pressure water washing
- Brushing
- Blasting
- Sand blasting
- Scale break
Chemical method:
Chemical substances such as acids and alkalis are used to chemically react with the scale on the surface of the steel to remove it.
Electrochemical method:
During pickling, direct current is applied to both sides of the steel immersed in the acid solution through the electrode to accelerate scale removal.
Ⅲ. The main way of pickling steel
Currently, there are three main steel pickling methods in the world:
- One-piece steel pickling (steel pickling, single coil pickling, single bundle wire pickling)
- Continuous pickling
- Semi-continuous pickling
One-piece steel pickling is the most primitive and simple pickling method.
Its characteristic is that it does not need to open the coil.
The entire wire coil or loose steel coil is pickled and then hoisted into the pool for washing.
Due to poor pickling quality, low production efficiency, mainly environmental pollution, it is being vigorously phased out.
Continuous pickling is a kind of high-yield, high-quality and fast-developing pickling method.
It is equipped with welding machine and looping device in the unit, so that the process section does not stop when the coil is changed to ensure the continuous operation of the unit.
But the unit is long, the equipment is complicated and the investment is high.
There are two main types of continuous pickling lines, which are continuous horizontal pickling units and continuous tower pickling units.
Semi-continuous blasting is relative to continuous and single-piece blasting.
Since it is not necessary to assemble a welding machine (or a simple welding machine such as a sewing machine) and a large capacity looper, it is necessary to stop the machine when changing the bobbin.
This method is best suited for production of 200,000 to 900,000 t/a.
Fig.1 Layout diagram of push-pull pickling unit production line
Fig.2 Production line layout diagram of complete continuous pickling unit
As people's living standards improve, the demand for a cleaner environment increases. To meet this need, new technologies and production lines with high efficiency, pollution-free and good quality for removing iron oxides are constantly emerging.
On July 20, 2013, with the rollless pickling coil being withdrawn from the hot rolling mill production line of Taiyuan Iron and Steel Group, China's first steel coil surface non-pickling treatment line was officially put into operation production.
In the past, the surface treatment of steel coils had to be completed by pickling and lubrication, which entailed high production costs, caused environmental pollution and resulted in waste disposal problems.
The newly developed non-pickling treatment line uses new EPS treatment technology, which eliminates the pickling process of steel coils, eliminates waste generation, recycles all media, and makes the surface of steel coils cleaner and stronger to corrosion. .
EPS patent technology is developed based on SCS technology. The basic principle is to use a special device for processing EPS in a closed space. The upper and lower surfaces of the steel plate are sprayed with EPS working medium, a mixture made of steel shot and water, and the oxide or scale on the surface of the steel plate is removed under a certain spraying force without any oxide remaining.
Therefore, the surface of the steel plate can be smooth and clean, which is called a “green” surface.
Ⅳ. The Principle of Steel Pickling
Pickling is the process of chemically removing scale from the metal surface, which is why it is also called chemical pickling. The scales (Fe203, Fe304, Fe0) formed on the surface of the strip are all water-insoluble alkaline oxides. When metal is immersed in an acidic solution or sprayed with an acidic solution on the surface, these basic oxides undergo a series of chemical changes with the acids.
Fig.3 Iron oxide scale structure
The scale on the surface of carbon structural steel or low alloy steel is loose, porous and cracked. Furthermore, the scale is repeatedly bent, straightened and transported together with the strip in the pickling unit, which further enlarges and expands the pore cracks.
Therefore, although the acid solution reacts chemically with the iron oxide scale, it also reacts with the base iron in the steel through cracks and pores. In other words, at the beginning of pickling, three types of chemical reactions occur between the iron oxide scale, the metallic iron and the acid solution.
- Iron oxide scale is dissolved by chemical reaction with acid (dissolution).
- The metallic iron reacts with the acid to generate hydrogen and mechanically peels off the scale (mechanical peeling effect).
- The generated atomic hydrogen reduces iron oxides to ferrous oxides which are prone to acid action and then removed by acid action (reduction).
Ⅴ. Hydrochloric acid pickling vs. Sulfuric acid pickling
Hydrochloric acid is the most commonly used pickling medium, with a greater proportion than sulfuric acid in actual production.
Hydrochloric acid can dissolve all scale without generating pickling residue, and basically does not corrode the base metal, resulting in a smooth, silvery surface after pickling.
This method has advantages such as good product quality, high production capacity, lower loss of metals and acids and low cost.
The iron loss of hydrochloric acid pickling is 20% less than that of sulfuric acid pickling, with an iron loss rate of 0.4% to 0.5% compared to 0.6% to 0.7 % sulfuric acid.
Furthermore, hydrochloric acid pickling is 2 times faster than sulfuric acid pickling.
In recent years, hydrochloric acid pickling has been widely used due to the development of acid regeneration technology that recovers and treats residual acid from hydrochloric acid.
SAW. Formulation for removing rust when pickling iron and steel
1. Iron Oxide and Cast Steel Rust Removal Solution
- Industrial sulfuric acid (relative density 1.84): 75~100g/L
- Table salt: 200~500g/L
- Corrosion inhibitor KC: 3~5g/L
- Industrial hydrochloric acid (relative density 1.18): 110~150g/L
- Pickling temperature: 20~60°C
- Pickling time: 5~10min
This room temperature rust remover quickly cleans rust and oxidation from iron surfaces and also has degreasing properties. Its composition and operating conditions are as follows:
- Industrial sulfuric acid (relative density 1.84): 150~200g/L
- Hexamethylenetetramine: 3g/L
- Triethanolamine: 2g/L
- Industrial hydrochloric acid (30% content): 200~300g/L
- Table salt: 200~300g/L
- Rust removal temperature: 15 ~ 25°C
- Sodium Dodecyl Sulfate: 10g/L
- Rust removal time: 2~5min
Note: Table salt controls the corrosive action of H2SO4 on carbon steel, chrome steel and chrome nickel steel and also acts as a dust inhibitor.
To prevent acid fog, add a 10% fog suppressant.
This rust removal solution works quickly, and at room temperature, rust removal takes no more than 10 minutes. Thanks to the dust and ash inhibitor, the metal surface is clean and presents a grayish-white surface after pickling.
The Refinery, which deals with 200m, Φ200mm~300mm rubber-coated steel pipes, first carried out internal pipe rust removal using this rust remover, which proved to be very effective. All rubber liners have passed inspection.
2. Rust removal solution for carbon steel water cooler
Before applying a corrosion-resistant coating to a carbon steel water cooler, it needs to be pickled to remove rust. The composition of the washing solution is as follows (mass fraction):
- Industrial Hydrochloric Acid: 48.6~64.6%
- Industrial Water: 51.4~35.4% (30% content)
- Hexamethylenetetramine: 0.3% acid solution
Pickling process: Pickling at room temperature for 30 ~ 60 minutes, then rinse with water until neutral. Finally, perform phosphating or passivation. This formula is widely used internally for rust removal in water-cooled heat exchangers, with good coating results.
3. Great solution for iron oxide and steel rust removal
- Industrial Hydrochloric Acid (30%): 350g/L
- Aniline: 0.3g/L
- Hexamethylenetetramine: 0.8g/L
- Water: Waste
- Acetic acid: 0.8g/L
Rust removal process: At a temperature of 30~50°C, remove rust for 1 hour, then rinse with water until the pH value is 7. Finally, using 10% sodium nitrite solution at a temperature 30~40°C, passivate for 30 minutes to prevent rust.
4. High Alloy Steel Rust Removal Solution
- Industrial hydrochloric acid (30%): 12~28g/L
- Rohdina: 1~2g/L
- Nitric acid (relative density 1.33~1.38): 110~120g/L
- Water: Waste
Rust removal process: At a temperature of 40~50°C, remove rust for 15~16 minutes and then rinse with water. This formula is mainly suitable for removing rust from high alloy steels.
5. Light rust removal solution (mass fraction)
- Chromic anhydride: 15
- Water: 76.5
- Phosphoric Acid: 8.5
Pickling process: Heat the rust removal solution to 85~95°C and marinate for 2~3 minutes to remove the rust. This formula is primarily suitable for removing light rust from precision parts, bearings and the like.
6. Rust Remover for Precision Instruments and Parts
- Industrial sulfuric acid (relative density 1.84): 15g/L
- Chromic Anhydride: 150g/L
- Water: Waste
Rust removal solution operating conditions: Rust removal temperature 80 ~ 90 ° C, rust removal time, 10 ~ 20 minutes.
7. Iron and steel degreasing, rust removal, phosphating and passivation cleaning solution
With the development of the industry, multifunctional “two-in-one” oil and rust removal, “three-in-one” oil removal, rust removal, phosphating or passivation removers have emerged.
Rust removal process: Pickling rust removal at room temperature for 2 to 10 minutes, then rinse with water until neutral, and finally perform rust prevention treatment.
Its formula is shown in the following table:
Table 1 Rust Removal Solution Formula
| Raw material name | Content 1% | Forgive the name | Happy/% |
| disodium hydrogen phosphate Sodium nitrite Sodium bicarbonate |
3.5 6.2 1 |
Glycerol Water |
1.6 87.7 |
This rust prevention solution has strong rust prevention ability, but the rust prevention time is relatively short, so it is suitable for inter-process rust prevention.
The “three-in-one” solution of degreasing, rust removal and passivation (or phosphating) is suitable for treating metal equipment before painting, thus achieving the purpose of degreasing, rust removal and passivation (or phosphating). However, it is not suitable when there is a lot of scale and heavy rust stains.
“Three in one” composition for degreasing, rust removal and passivation:
- Oxalic Acid: 150g/L
- Emulsifier: 10g/L
- Thiourea: 10g/L
- Water: Waste
Working conditions are rust removal temperature of 85°C, rust removal time of 2 to 2 minutes.
Composition of the “three in one” degreasing, rust removal and phosphating solution:
- Phosphoric Acid: 50~300g/L
- MP organosilicon acid solution: 0.1g/L
- Thiourea: 3~5g/L
- Water: Waste
Working conditions are a temperature of 85°C and a time of 2 to 3 minutes.
The composition of the “four in one” degreasing, rust removal, phosphating and passivation solution is shown in the following table.
| “Four in One” Formula/(g/L) | Rust removal temperature/°C | Rust removal time/min | ||||
| Material name | 1# | two# | 1# | two# | 1# | two# |
| Phosphoric acid (80% content) | 110~180 | 110 | 50~60 | 55~65 | 25 | 5~15 |
| Zinc oxide | 30~50 | 25 | ||||
| Zinc Nitrate | 150~170 | 150 | ||||
| Magnesium chloride | 15~30 | 3 | ||||
| Potassium Tartrate | 0.2~0.4 | 5 | ||||
| Ammonium molybdate | 0.8~1.2 | 1 | ||||
| Sodium dodecyl sulfate | 20~40 | 30 | ||||
| Manganese Phosphate | / | 10 | ||||
| Potassium dichromate | / | 0.2~0.3 | ||||
| Water | Residual | Residual | ||||
Take formula 2# as an example, pour a certain amount of zinc oxide into a container and make it into a paste with distilled water. While stirring, add phosphoric acid until it dissolves into clear zinc dihydrogen phosphate. Dilute with distilled water to 2/3 of the total volume and then add calculated amounts of zinc nitrate, magnesium chloride, phosphoric acid, tartaric acid and potassium dichromate. After stirring until dissolved, add the ammonium molybdate solution that was dissolved into a small container in two batches and stir evenly. Finally, add cleaning agent 601 and dilute to full volume.
601 cleaning agent is an anionic surfactant, which has good permeability and wettability, is easily soluble in water, acid resistant, heat resistant and does not react with metal ions, so it is very stable in solvent and is used for removing oil. The “four in one” phosphating process is the same as the general phosphating principle. Oil removal and rust removal occur at the same time, and phosphoric acid has a soaking effect on iron, forming a dense phosphating film on the surface of the steel. Ammonium molybdate and potassium dichromate act as passivators.
The free acidity of the “four in one” treatment solution is 17 to 25 points, the total acidity is 170 to 220 points, and the ratio of free acidity to total acidity is (1:7) ~ (1:10).
Use standard 0.1mol/L NaOH solution to titrate 10mL of phosphating solution. When phenolphthalein is used as an indicator, the milliliters of NaOH consumed are the “point” of total acidity. When methyl orange is used as an indicator, the milliliters of NaOH consumed is the “point” number of free acidity.
Steel parts with gold and rainbow colors can be processed directly. If the oil pollution is severe, OP emulsifier can be added to increase the decontamination ability. Steel parts with blue oxide scale cannot be treated with this method because the oxide scale is too thick. The amount of scale that this method can dissolve is 7~10g/m².
On the surface of steel parts treated with “four in one”, a thin film of insoluble phosphates of metals such as zinc, iron and magnesium is formed. This film has protective properties and can adsorb paint, which improves the adhesion of the paint film and increases the protection ability.
Next, I will present two more “four-in-one” formulas of degreasing, rust removal, phosphating and passivation solutions suitable for removing oxide scale.
The process of the first formula is more complex than other formulas. It is divided into two stages. The first step is to remove oil and rust (oxide scale) (see table 1 for the formula); The second step is phosphating and passivation treatment (see Table 2 for the formula). It should be noted that after removing the oil and rust, it must first be rinsed with water. After water neutrality, phosphating and passivation treatments can be carried out.
Table 1 Formula for removing oil and rust (oxide scale)
| Oil and rust removal formula | Working Conditions for Oil and Rust Removal Solution | ||
| Ingredient | Happy | Rust removal temperature /°C |
Rust removal time /min |
| Industrial Sulfuric Acid (Relative Density 1.84) | 60~65mL/L | 75~85 | 5~20 |
| Thiourea | 5~7g/L | ||
| Sodium Dodecylbenzene Sulfonate | 20~50mL/L | ||
| Water | Residual | ||
Note: The amount of sulfuric acid can be increased if the oxide scale is thick.
Table 2 Phosphating and passivation solution formula
| Phosphating and Passivation Formulation | Phosphating and Passivation Working Conditions | ||
| Ingredients | Concentration/(g/L) | Phosphating Temperature /°C |
Phosphating time /min |
| Phosphoric acid | 58 | 65~75 | |
| Zinc oxide | 15 | ||
| Zinc Nitrate | 200 | ||
| Chromium Dihydrogen Phosphate (calculated as Potassium Dichromate) | 0.3~0.4 | ||
| Titanium Sulfate Oxide (not required for welding) | 0.1~0.3 | ||
| Tartaric acid | 5 | ||
| Sodium dodecyl sulfate | 15mL | ||
| OP emulsifier | 15mL | ||
| Water | Residual | ||
Note: The ratio of free acidity to total acidity: (1:12) ~ (1:18)
Phosphating process parameters: Use 1:1 dilution of phosphating solution with water, treatment time is 15 to 30 minutes, treatment temperature is 10 to 60°C, total acidity is 200 to 250 points.
Drying time: natural drying for 24 hours or drying at 120°C for 30~60 minutes. The appearance of the phosphating film is dark gray, the film is densely crystallized, continuous and uniform; the thickness of the phosphating film is 5~8μm; adhesion is grade I; impact resistance is 500N·cm; the copper sulfate drop test >150S; the immersion test in sodium chloride solution >8h; rust prevention indoors (T20°C RH86%) >60 days.
8. Acid Pickling Paste
Acid pickling paste can be used to remove rust. The recommended application thickness is 1~2mm, with a quantity of 2~3kg/m², and the total rust removal time is 60 minutes. After removing the rust, rinse with water.
Finally, dry with a mixture of acetic acid and ammonia solution to help prevent rust. The formula of acid pickling paste is as follows (provide table):
Table 3 Acid Pickling Paste Formula for Rust Prevention
| Per quantity of 300g /g |
Per quantity of 300g /g |
||
| Industrial Hydrochloric Acid (30% concentration) | 4.3 | 1.2 | 1% |
| Phosphoric Acid (relative density 17) | 18.6 | 0.2 | / |
| Industrial Sulfuric Acid (relative density 1.84) | 40.3 | 5.5 | 5.33% |
| Hexamethylenetetramine | 0.8 | 0.1 | 0.10% |
| Bentonite (120#) or Diatomaceous Earth, Yellow Ocher | 200 | 200 | 7.60% |
| oxalic acid | / | / | 0.07% |
| Asbestos wool | / | / | 6.50% |
| Water | 36 | 93 | Residual |
