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.