Classification and Brief Introduction of Pickling of Steel and Steel Pipes

Definition and Classification of Steel Pipe Pickling

• 1. Definition of Pipe Pickling: According to a certain concentration, temperature and speed, acid is chemically removed from the surface of the steel pipe, called pickling.
• 2. Pickling classification of steel pipes: According to the types of acids: sulfuric acid pickling, hydrochloric acid pickling, nitric acid pickling, hydrofluoric acid pickling. Pickling must select different media according to the material of steel, such as pickling carbon steel with sulfuric acid and hydrochloric acid, pickling stainless steel with nitric acid hydrofluoric acid and so on.

According to the shape of steel: wire pickling, forging pickling, steel plate pickling, strip pickling and so on.

According to the type of pickling equipment, it can be divided into trough pickling, semi-continuous pickling, full continuous pickling and tower pickling.

Removal of oxide scale on steel pipe surface

Removal methods of oxide scale on steel pipe surface: mechanical method, chemical method and electrochemical method

Mechanical methods: polishing, rolling, high pressure water washing, brushing, shot blasting, sand blasting, scaling, etc.

Chemical method: The iron oxide scale is removed by chemical reaction of acid, alkali and other chemical substances with iron oxide scale on steel surface.

Electrochemical method: When pickling, direct current is applied to both sides of steels immersed in acid solution through electrodes to accelerate the removal of iron oxide scale.

Main Forms of Pickling

At present, there are three main pickling methods for steel steels in the world: single steel pickling method (steel plate pickling method, Single Coil pickling method, single bundle wire pickling method), continuous pickling method and semi-continuous pickling method.

Single-piece pickling is the most original and simplest pickling method. It is characterized by pickling the whole coil of wire or loosened coil without uncoiling, and then hanging in the pool for washing. Due to the poor quality of pickling, low production efficiency, especially the great environmental pollution, it has been vigorously eliminated.

Continuous pickling is a high-yield, high-quality and fast-developing pickling method. It ensures the continuous operation of the unit by installing welding machine and looper device on the unit. However, the unit is long, the equipment is complex and the investment is high. There are two main forms of continuous pickling line: continuous horizontal pickling line and continuous tower pickling line.

Semi-continuous pickling is relative to single-piece pickling and continuous pickling. Because it does not need to install welding machines (or simple welding machines such as sewing machines) and large capacity loopers, it is necessary to shut down when changing coils. This method is most suitable for production with output of 20-900,000 t/a.

With the improvement of people's living standards and the increasing demand for the environment, new technologies and production lines for iron oxide scale removal with high efficiency, pollution-free and good quality are emerging.

The new technology of EPS treatment eliminates the pickling of steel coil, eliminates the generation of waste, recycles all the media, makes the surface of steel coil cleaner and has better corrosion resistance.

EPS patented technology is developed on the basis of SCS technology. Its basic principle is that EPS treatment is carried out in a sealed space with special equipment. That is, the upper and lower surfaces of plate and silk steel plate are sprayed by EPS working medium, i.e. mixture of steel sand and water. Under the action of certain spraying force, the oxide or iron oxide scales on the surface of plate are removed without any residual oxides. It can make the surface of steel plate smooth and clean, which is called green surface.

Principle of pickling

Pickling is the process of removing oxide scales on the surface of steel pipes by chemical methods, so it is also called chemical pickling.

Iron oxide scales (Fe203, Fe304, Fe0) formed on the surface of strip steel are alkaline oxides insoluble in water. When they are immersed in acid or sprayed with acid on the surface of strip steel, a series of chemical changes can take place between these alkaline oxides and acid.

Because the oxide scales on the surface of carbon structural steel or low alloy steel have the properties of looseness, pore and crack, and the oxide scales are straightened, stretched and rectified together with the strip steel in the pickling machine, so that these pore cracks are further increased and enlarged. Therefore, while the acid solution reacts with the oxide scales chemically, it also passes through cracks and pores. It reacts with the matrix iron of steel. That is to say, three kinds of chemical reactions between iron oxide scale and metal iron and acid solution were carried out at the beginning of acid pickling.

• (1) The oxide scale on the surface of the steel tube reacts with acid and is dissolved (dissolved).
• (2) Metallic iron reacts with acid to produce hydrogen and mechanically peels off the scale of iron oxide (mechanical peeling).
• (3) Atomic hydrogen produced reduces iron oxides to ferrous oxides which are easy to react with acids, and then is removed by reacting with acids.

Comparison of hydrochloric acid pickling and sulfuric acid pickling

The most widely used pickling media are hydrochloric acid and sulphuric acid, but the use of hydrochloric acid pickling occupies a larger proportion in actual production. In terms of the characteristics of the two media, hydrochloric acid can dissolve all iron oxide scales without producing pickling residues. And hydrochloric acid basically does not corrode the base metal, so after pickling, the surface of the plate is smooth and bright.

Therefore, the hydrochloric acid pickling method has the advantages of good product quality, high production capacity, less loss of metals and acids, low cost and so on. In addition, the iron loss of hydrochloric acid pickling is 20% lower than that of sulfuric acid pickling. The iron loss of hydrochloric acid pickling is 0.4%~0.5% of that of acid pickling strip steel and 0.6%~0.7% of that of sulfuric acid pickling strip steel. The pickling rate of hydrochloric acid is 2 times higher than that of sulfuric acid.

In recent years, due to the development of recovery and treatment technology of hydrochloric acid waste acid (that is, the development of acid regeneration technology), hydrochloric acid pickling method has been widely used.

Source: China Stainless Steel Pipes Manufacturer - Yaang Pipe Industry Co., Limited (www.yaang.com)

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