Steel Pickling: The Essential Guide

Recently, the environmental protection department has strictly controlled the pickling production enterprises, and some equipment and processing plants that do not meet environmental protection requirements have been ordered to be closed down or demolished for rectification.

Therefore, many people have become concerned about the problems related to pickling.

I. Definition and Classification of Steel Pickling

1. Definition of pickling

Acids are chemically removed from iron oxide scales at a certain concentration, temperature and speed, which is called pickling.

2. Pickling classification

According to the type of acid:

• Sulfuric acid pickling
• Hydrochloric acid pickling
• Nitric acid pickling
• Hydrofluoric acid pickling

Pickling requires the selection of different media depending on the type of steel. For instance, carbon steel is pickled using sulfuric acid and hydrochloric acid, while stainless steel is pickled using a mixed acid of nitric acid and hydrofluoric acid.

According to the shape of the steel:

• Wire pickling
• Forging pickling
• Steel plate pickling
• Strip steel pickling

According to the type of pickling equipment:

• Tank pickling
• Semi-continuous pickling
• Full continuous pickling
• Tower pickling

Ⅱ. Removal Method of Iron Oxide Scale

Removal method of iron oxide scale including:  Mechanical, chemical and electrochemical methods.

Mechanical method:

• Polishing
• Barrel grinding
• High-pressure water washing
• Brushing
• Shotblasting
• Sandblasting
• Scale breaking

Chemical method:

Chemical substances such as acids and alkalis are used to chemically react with the scale on the steel surface to remove the scale.

Electrochemical method:

During pickling, direct current is applied to both sides of the steel immersed in the acid solution through the electrode to accelerate the removal of scale.

Ⅲ. The Main Form of Steel Pickling

At present, there are three main methods for pickling steel in the world:

• Single-piece steel pickling (steel pickling, single coil pickling, single bundle wire pickling)
• Continuous pickling
• Semi-continuous pickling

Single-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 coil of wire or loose coiled steel coil is pickled and then hoisted in the pool for washing.

Due to the poor quality of pickling, the low efficiency of production, especially the environmental pollution, it is being vigorously eliminated.

Continuous pickling is a kind of high-yield, high-quality and fast-developing pickling method.

It is equipped with a welding machine and a looper device on 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 pickling is relative to single-piece and continuous pickling.

Since it does not need to set up 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 coil.

This method is most suitable for the production of 200,000 to 900,000 t/a.

Fig.1 Production line layout diagram of push-pull pickling unit

Fig.2 Production line layout diagram of full continuous pickling unit

As people’s living standards improve, the demand for a cleaner environment is increasing. To meet this need, new technologies and production lines with high efficiency, no pollution, and good quality for removing iron oxides are constantly emerging.

On July 20, 2013, with the roll-free pickling coil being rolled off the production line of Taiyuan Iron and Steel Group’s hot rolling mill, China’s first steel coil surface pickling-free treatment line was officially put into production.

In the past, the surface treatment of steel coils had to be completed by pickling and oiling, which had high production costs, caused environmental pollution, and resulted in waste disposal problems.

The newly-developed pickling-free treatment line uses the new EPS treatment technology, which eliminates the pickling process of steel coils, eliminates the generation of waste, recycles all the media, and makes the surface of the steel coils cleaner and more corrosion-resistant.

The EPS patent technology is developed based on SCS technology. The basic principle is to use a special device for EPS processing in a closed space. The upper and lower surfaces of the steel plate are sprayed with the EPS working medium, a mixture made of steel grit 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 sheet can be made smooth and clean, which is called a “green” surface.

Ⅳ. The Principle of Steel Pickling

Pickling is the process of chemically removing the scale on the metal surface, so it is also called chemical pickling. The scales (Fe203, Fe304, Fe0) formed on the surface of the strip are all alkaline oxides that are insoluble in water. When the metal is immersed in an acid solution or sprayed with an acid solution on the surface, these basic oxides undergo a series of chemical changes with the acids.

Fig.3 Structure of iron oxide scale

The scale on the surface of carbon structural steel or low-alloy steel is loose, porous, and cracked. In addition, the scale is repeatedly bent, straightened, and conveyed along with the strip on the pickling unit, which further increases and expands the pore cracks.

Therefore, while the acid solution chemically reacts with the iron oxide scale, it also reacts with the base iron of the steel through cracks and pores. That is to say, at the beginning of pickling, there are three kinds of chemical reactions between the iron oxide scale, metal iron, and acid solution.

• The iron oxide scale is dissolved by chemical reaction with acid (dissolution).
• Metal iron reacts with acid to generate hydrogen, and mechanically peels off the scale (mechanical peeling effect).
• Generated atomic hydrogen reduces iron oxides to ferrous oxides that are prone to acid action, and then removed by the action of acid (reduction).

Ⅴ. Hydrochloric Acid Pickling vs. Sulfuric Acid Pickling

Hydrochloric acid is the most widely used pickling medium, with a larger proportion than sulfuric acid in actual production.

Hydrochloric acid can dissolve all scales without generating pickling residues and basically does not corrode the base metal, resulting in a smooth and silvery surface after pickling.

This method has advantages such as good product quality, high production capacity, less loss of metals and acids, and low cost.

The iron loss of hydrochloric acid pickling is 20% lower than that of sulfuric acid pickling, with an iron loss rate of 0.4% to 0.5% compared to sulfuric acid’s 0.6% to 0.7%.

Moreover, 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 hydrochloric acid waste acid.

VI. Iron and Steel Pickling Rust Removal Formulation

1. Iron, Cast Steel Oxide Rust Removal Solution

• Industrial Sulfuric Acid (Relative Density 1.84): 75~100g/L
• Table Salt: 200~500g/L
• KC Corrosion Inhibitor: 3~5g/L
• Industrial Hydrochloric Acid (Relative Density 1.18): 110~150g/L
• Pickling Temperature: 20~60°C
• Pickling Time: 5~10min

This ambient 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, chromium steel, and chromium nickel steel, and also acts as a dust inhibitor.

To prevent acid mist, add a 10% mist suppressant.

This rust removal solution works quickly and, at room temperature, rust removal takes no longer than 10 minutes. Thanks to the dust and ash inhibitor, the metal surface is clean and displays a grey-white surface after pickling.

The Refinery, dealing with 200m, Φ200mm~300mm steel pipes lined with rubber, first carried out internal pipe rust removal using this rust remover, which proved to be very effective. All the rubber linings passed inspection.

2. Carbon Steel Water Cooler Rust Removal Solution

Before applying a corrosion-resistant coating to a carbon steel water cooler, it needs to be pickled to remove rust. The washing solution composition is as follows (mass fraction):

• Industrial Hydrochloric Acid: 48.6~64.6%
• Industrial Water: 51.4~35.4% (30% content)
• Hexamethylenetetramine: 0.3% of the acid solution

Pickling Process: Room temperature pickling for 30~60 minutes, then rinse with water until neutral. Finally, carry out phosphating or passivation. This formula is widely used domestically for rust removal in water-cooled heat exchangers, with good coating results.

3. Large Iron and Steel Oxide Rust Removal Solution

• Industrial Hydrochloric Acid (30%): 350g/L
• Aniline: 0.3g/L
• Hexamethylenetetramine: 0.8g/L
• Water: Residue
• 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 a 10% solution of sodium nitrite at a temperature of 30~40°C, passivate for 30 minutes to prevent rusting.

4. High-Alloy Steel Rust Removal Solution

• Industrial Hydrochloric Acid (30%): 12~28g/L
• Rohdine: 1~2g/L
• Nitric Acid (Relative Density 1.33~1.38): 110~120g/L
• Water: Residue

Rust Removal Process: At a temperature of 40~50°C, remove rust for 15~16 minutes, then rinse with water. This formula is mainly suitable for rust removal 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 pickle for 2~3 minutes to remove rust. This formula is mainly suitable for the removal of 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: Residue

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 industry, “two-in-one” oil removal and rust removal, “three-in-one” oil removal, rust removal, phosphating or passivation multi-functional rust removers have emerged.

Rust Removal Process: Room temperature pickling rust removal for 2~10 minutes, then rinse with water until neutral, and finally carry out rust prevention treatment.

Its formula is shown in the following table:

Table 1 Rust Removal Solution Formula

This rust prevention solution has strong rust prevention capability, but the rust prevention time is relatively short, so it is suitable for rust prevention between processes.

The “three-in-one” degreasing, rust removal, and passivation (or phosphating) solution is suitable for metal equipment treatment before painting, thereby 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 spots.

“Three-in-one” Degreasing, Rust Removal, and Passivation Composition:

• Oxalic Acid: 150g/L
• Emulsifier: 10g/L
• Thiourea: 10g/L
• Water: Residue

The working conditions are rust removal temperature 85°C, rust removal time 2~2 minutes.

“Three-in-one” Degreasing, Rust Removal, and Phosphating Solution Composition:

• Phosphoric Acid: 50~300g/L
• M-P Organosilicon Acid Solution: 0.1g/L
• Thiourea: 3~5g/L
• Water: Residue

The working conditions are a temperature of 85°C and a time of 2~3 minutes.

The composition of the “four-in-one” degreasing, rust removal, phosphating, and passivation solution is shown in the following table.

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 transparent zinc dihydrogen phosphate. Dilute with distilled water to 2/3 of the total volume, then add calculated amounts of zinc nitrate, magnesium chloride, phosphoric acid, tartaric acid, and potassium dichromate. After stirring until dissolved, add the solution of ammonium molybdate that has been dissolved in a small container in two batches, and stir evenly. Finally, add 601 cleaning agent and dilute to the total volume.

The 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 the solvent and is used for oil removal. The phosphating process of the “four-in-one” 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 17~25 points, the total acidity is 170~220 points, and the ratio of free acidity to total acidity is (1:7)~(1:10).

Use 0.1mol/L NaOH standard solution to titrate 10mL of phosphating solution. When phenolphthalein is used as an indicator, the milliliters of NaOH consumed is the “point” number 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 serious, OP emulsifier can be added to enhance the decontamination ability. Steel parts with blue oxide scales 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 enhances protective capabilities.

Next, I will introduce two more “four-in-one” degreasing, rust removal, phosphating, and passivation solution formulas suitable for removing oxide scales.

The process of the first formula is more complex than other formulas. It is divided into two steps. 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 the oil and rust are removed, you should first rinse with water. After the water is neutral, phosphating and passivation treatments can be carried out.

Table 1 Formula for removing oil and rust (oxide scale)

Note: The amount of sulfuric acid can be increased if the oxide scale is thick.

Table 2 Phosphating and passivation solution formula

Note: The ratio of free acidity to total acidity: (1:12) ~ (1:18)

Phosphating process parameters: Use a 1:1 dilution of the phosphating solution with water, treatment time is 15~30 minutes, treatment temperature is 10~60°C, total acidity is 200~250 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 phosphating film thickness is 5~8μm; the adhesion is grade I; the impact resistance is 500N·cm; the copper sulfate drop test >150S; the sodium chloride solution soaking test >8h; indoor rust prevention (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 an amount of 2~3kg/m², and the total rust removal time is 60 minutes. After rust removal, rinse with water.

Finally, wipe dry with a mixture of acetic acid and ammonia solution to aid in rust prevention. The formula for the acid pickling paste is as follows (please provide the table):

Table 3 Rust Prevention Acid Pickling Paste Formula