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.

steel 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 must choose different media according to the material of the steel, for example, pickling carbon steel with sulfuric acid and hydrochloric acid, pickling stainless steel with 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

continuous 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, and 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 one-piece and continuous.

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, therefore, 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.

Production line layout diagram of push-pull pickling unit

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

Production line layout diagram of full continuous pickling unit

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

With the improvement of people’s living standards, the requirements on the environment are getting higher and higher. New technologies and new 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.

According to reports, in the past, the surface treatment of steel coils had to be completed by pickling and oiling, which had high production costs and caused some pollution to the environment, and the treatment of waste acid had also become a big problem.

The newly-developed pickling-free treatment line uses the new EPS treatment technology, eliminating the pickling process of steel coils, eliminating the generation of waste, recycling all the media, and making the surface of the steel coils cleaner and more corrosion resistance.

The EPS patent technology is developed based on the SCS technology.

The basic principle is to use a special device for EPS processing in a closed space.

That is, the upper and lower surfaces of the sheet steel plate are sprayed with the EPS working medium, a kind of 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 a clean surface is a so-called 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 they are immersed in acid solution or sprayed with acid solution on the surface, these basic oxides undergo a series of chemical changes with acids.

Structure of iron oxide scale

Fig.3 Structure of iron oxide scale

The scales on the surface of carbon structural steel or low-alloy steel are loose, porous and cracked.

In addition, the scales are repeatedly bent, straightened, and conveyed along with the strip on the pickling unit, the pore cracks further increased and expanded.

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 iron oxide scale and metal iron as well as 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

The most widely used pickling media are hydrochloric acid and sulfuric acid, but in actual production, hydrochloric acid pickling accounts for a larger proportion.

From the characteristics of the two media, hydrochloric acid can dissolve all scales without generating pickling residues. And hydrochloric acid basically does not corrode the base metal, so after pickling, the surface of the board is smooth and silvery.

Therefore, the hydrochloric acid pickling method has the advantages of good product quality, high production capacity, less loss of metals and acids, and low cost.

In addition, the iron loss of hydrochloric acid pickling is 20% lower than that of sulfuric acid pickling.

The hydrochloric acid pickling iron loss is 0.4% to 0.5% of the acid pickling strip, and sulfuric acid is 0.6% to 0.7%.

The hydrochloric acid pickling rate is higher, which is 2 times the sulfuric acid pickling rate.

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

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