What is steel pickling
Table of Contents
Steel pickling means to make the strip steel have a clean and active surface to ensure the surface quality of the steel produced. In order to obtain such a surface, the steel is placed in a pickling tank to remove the iron oxide from the surface (except when annealed in a protective or reducing atmosphere).
The basic principle of steel pickling
Iron oxides (FeO, Fe3O4, Fe2O3) on the steel surface are oxides that do not dissolve in water, and when they are immersed in the acid, these oxides undergo a series of chemical reactions with the acid, respectively.
Because the iron oxide on the surface of carbon structural steel or low-alloy steel has a loose, porous and cracked structure, coupled with the iron oxide in pickling along with the steel through straightening, pulling and bending of the transmission repeatedly, so that these pores cracks further increase and expand. Therefore, while the acid solution reacts chemically with the iron oxide, it can also react chemically with the iron of the steel matrix by penetrating through cracks and pores. In other words, the chemical reaction between all 3 oxides and metallic iron and acid solution is carried out simultaneously at the beginning of pickling, so the process of pickling includes the following 3 effects.
Various iron oxides on the steel surface react chemically with the acid to produce iron salts dissolved in water and dissolved in the acid solution. If pickling with hydrochloric acid or sulfuric acid, the generation of ortho-iron and ferrous chloride or sulfate can be dissolved in the acid solution, thus removing the iron oxide from the surface of the strip. This effect is generally called the dissolution effect.
Mechanical stripping effect
Steel surface iron oxide in addition to various oxides of iron, but also part of the metallic iron, and iron oxide and porous, then the acid solution can be through the pores and cracks in the iron oxide and iron oxide or matrix iron action, and accordingly produce a large amount of hydrogen. The expansion pressure generated by this hydrogen gas can then strip the iron oxide from the steel surface. This effect of peeling off the iron oxide by the expansion pressure of the hydrogen gas generated in the reaction is generally called mechanical peeling.
When metallic iron interacts with acid, hydrogen atoms are first produced. Some of the hydrogen atoms combine with each other to become hydrogen molecules, which contribute to the stripping of the iron oxide. The other part of the hydrogen atoms rely on their chemical activity and strong reducing ability to reduce the high-valent iron oxides and high-valent iron salts to low-valent iron oxides and low-valent iron salts that are easily soluble in acid solutions.
Classification of pickling
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 steel, for example, pickling carbon steel with sulfuric acid and hydrochloric acid, pickling stainless steel with a mixture of nitric acid and hydrofluoric acid, etc.
According to the shape of steel: wire pickling, forging pickling, steel plate pickling, strip pickling, etc. According to the type of pickling equipment: tank pickling, semi-continuous pickling, full continuous pickling, tower pickling. Steel surface treatment is a prerequisite for the quality of the entire coating and plays a significant role in the overall corrosion protection. The content of steel surface treatment is to remove oil, moisture, dirt, oxide, rust, old coating and other non-steel substances on the steel surface. Acid washing is a convenient, efficient and scientific method to remove impurities.
At present, there are three main methods of steel pickling used in the world: the single-piece steel pickling method (steel plate pickling method, single-roll pickling method, single bundle of wire pickling method), continuous pickling method and semi-continuous pickling method.
The single-piece pickling method is one of the most primitive and simple pickling methods, which is characterized by the fact that no uncoiling is required, and the whole roll of wire rod or loosely rolled strip steel coil is pickled and then lifted and placed into a pool for rinsing. Due to the poor quality of pickling, low production efficiency, especially the pollution of the environment is very large, is now vigorously eliminated. Continuous pickling is a high production and high quality, faster development of the pickling method, it is through the set on the unit set welding machine and live set device, so that the process section in the change of coil without stopping, to ensure continuous operation of the unit, but the unit is long, complex equipment, higher investment. There are two main forms of continuous pickling line, which are continuous horizontal pickling unit and continuous tower type pickling unit.
Semi-continuous pickling is relative to single-piece and continuous type, as it does not need to set up welding machine (or set up simple welding machine such as stitching machine, etc.) and large capacity live set, therefore, it has to stop when changing rolls, and this method is most suitable for the production of 200,000 to 900,000 t/a. Steel component blanks are generally pickled before surface plating and spraying to remove surface oxides, thus producing pickling waste and pickling wastewater. Among them, the pickling wastewater contains high acid concentration and can be recovered as recycled acid. While a large amount of rinse water, i.e. pickling wastewater, contains low acid content and is very uneconomical to be recycled, so it is discharged as wastewater. The main hazards of acid-containing wastewater are corrosion of hydraulic structures such as sewerage pipes and reinforced concrete; hindering biological reproduction in biological treatment of wastewater; acidic wastewater can poison fish, kill crops and affect the growth of aquatic crops. Acidic wastewater infiltrates into the soil and causes soil calcification over time, destroying the loose state of the soil, thus affecting the growth of crops. Human and animal consumption of acidic water can cause gastrointestinal inflammation and even burns.
The necessity of steel pickling
The hot rolled strip steel coil produced by the hot rolling mill is rolled and coiled, and the iron oxide skin generated on the surface of the strip under the corresponding conditions can cover the surface of the strip very firmly and cover up the defects on the surface of the strip.
From the point of view of rolling, if these strips with iron oxide are sent directly to the cold rolling mill to roll will bring many problems: First, rolling under the conditions of massive downward pressure will press the iron oxide into the strip matrix, affecting the surface quality and processing performance of the cold rolled sheet, and even cause scrap; Second, the iron oxide is broken into the emulsion system for cooling and lubricating the rolls will damage the circulating equipment and shorten the emulsion’s Third, the damage to the surface finish and processing accuracy are very high and expensive cold rolls. Therefore, it is necessary to remove the iron oxide from the surface of the strip before cold rolling to ensure the surface quality of the cold rolled strip produced.
From the point of view of producing hot-dip galvanized steel, direct hot-dip galvanizing with hot-rolled strip is also necessary to make the strip have a clean and active surface before galvanizing, as is the case with cold-rolled steel. In order to obtain such a surface, the hot rolled strip needs to be treated to remove iron oxide from the surface, usually by pickling.
There are generally two types of iron oxide on steel pipes: one is the oxide – rust – generated at room temperature, in a humid environment, and with SO2, CO2 dissolved in moisture to act as a catalyst. This rust layer is reddish-brown in color and is composed of trivalent and trivalent iron compounds with oxygen and crystalline water; the other is the oxide iron generated at high temperature, commonly known as iron scale, which is dark black or dark brown. It is generated in two cases: one is the rolling iron generated during rolling or welding, and the other is the iron oxide generated during heat treatment due to the partial pressure of oxygen greater than the equilibrium oxygen partial pressure of iron (except when annealed in a protective or reducing atmosphere).
In the production of hot-dip galvanized steel tubes, electro-galvanized steel tubes, coated steel tubes or plastic-clad steel tubes, this layer of iron must be removed, otherwise it will have a serious impact on product quality, and even fail to produce qualified products. Before galvanizing, even if a very thin layer of iron is present, it is not possible to plate a complete and continuous zinc layer. Failure to remove surface oxides during coating or compounding can cause blistering and reduce the corrosion resistance of the coated layer.
Other most common pickling of steel profiles is that of hot rolled angles (e.g. for power transmission towers) which must also be pickled prior to galvanizing.
For wire, whether hot-rolled wire or semi-finished steel wire after heat treatment, the surface has a hard and brittle layer of iron oxide. On the one hand, these iron oxide affects the future processing process, so that the lubrication coating can not be firmly combined with the steel matrix; on the other hand, when drawing, because the iron oxide is very hard and brittle, which will scratch the surface of the mold and steel wire; then the iron oxide between the mold and steel wire, not only will increase the friction between the steel wire and the mold, increasing the friction, in serious will cause broken wire; sometimes the iron oxide may be pressed into the steel wire matrix, resulting in the plating of the “fin skin” and other defects. Therefore, it is necessary to remove these iron oxide layer before the wire is drawn.
The same for stainless steel and special steel hot rolled material, in the cold rolling or cold drawing process before processing, are to be pickled. Steel parts, metal products, in the surface treatment such as electroplating, phosphating, coating are also required before the pickling process.
Pickleability of steel
(1) Iron oxide on the surface of hot-rolled steel
Hot-rolled steel can be pickled and the formation of iron oxide as many factors, such as the adhesion strength of iron oxide, steel composition, the type and degree of mechanical deformation, the structure and thickness of iron oxide, surface contamination (such as pollution caused by grease), surface defects, the type and composition of the pickling agent and the working conditions when pickling. The following is an analysis of the structure and acid resistance of iron oxide skins only.
In the iron oxide skin, the fulvic is only present on the surface near the steel plate, while the Fe3O4 and Fe2O3 in the outer layer of the iron skin are more difficult to dissolve in the acid solution. However, due to the existence of cracks and pores in the iron skin layer (especially through the broken scale or after pulling correction), the acid solution will be able to reach the metal surface and the fulvic layer through these cracks and pores; the dissolution of metallic iron and fulvic reduces the adhesion between the iron skin and the metal, and under the action of hydrogen gas generated during the reaction between the acid and metallic iron, the iron oxide will fall off from the substrate. At the same time, the insoluble Fe3O4 and Fe2O3, are also reduced to the easily soluble FeO, thus making the iron oxide peel off from the steel surface.
Another important factor affecting the pickling property is the denseness of the iron skin. Fourdrinier has a natural maximum porosity, while Fe2O3 layer and Fe3O4 layer is dense, they will block all the other pores in the iron skin within the oxide layer; thus preventing the acid penetration; strip steel in the cooling process, although some cracks will be formed, but also can not ensure that the acid penetration into the depth of the iron oxide. Especially for the hot-rolled strip produced by modern mills, the thickness of the iron is stable and the density is relatively high, therefore, in order to improve the acid washability of the iron oxide, it is still necessary to use scale-breaking equipment to increase the cracks.
When pickling the strip, it is found that the iron oxide on the surface of the strip tail (head of the pickled coil) is easier to wash off. This is because the strip tail rolling temperature is generally 30 to 50 ℃ lower than the middle and head, and in the rolled by the water falling from the winder to strengthen the cooling, so the process of iron oxide formation in the strip tail ends earlier, iron oxide is thinner, and FeO is too late to convert.
The most difficult to pickle is the strip head (pickling strip coil tail) of iron oxide. This is because the formation process of iron oxide on the head of the strip is slower than the end of the tail, and the iron oxide layer thickened. In addition, the slow cooling of iron oxide prompted the decomposition of FeO into Fe3O4 or Fe2O3, is also the cause of pickling difficult to wash.
In the strip pickling will also find that the edge of the strip will appear on the edge of the black edge not washed off, this is because in the middle edge of the strip length, the iron oxide cooling more slowly, and the surrounding oxygen to the strip surface pathway is more open, so that the iron oxide here in the Fe203 layer increased significantly.
(2) Rust layer generated by longer exposure to the atmosphere
As mentioned earlier, in the rust layer generated on the steel surface, not only is the hydroxy iron oxide relatively loose and easily dissolved, but the Fe in the rust layer will be further oxidized.
The presence of FeSO4-7H2O, FeSO4-H2O and Fe2(SO4)3 has been confirmed in the rust layer of steel in industrial areas.
The main components of iron oxide are the three oxides of iron (FeO, Fe3O4, Fe2O3). They are water-insoluble amphoteric oxides, which can undergo a complex decomposition reaction with strong bases and acids under appropriate conditions to produce salts that can be dissolved in water.
Through the complex decomposition reaction, the iron oxide on the surface of steel can be removed from the steel surface. However, in industry, in addition to the surface treatment of stainless steel using strong alkali to remove the surface of iron, chromium and silicon oxides, are used to remove iron oxide from the steel surface by using aqueous solutions of strong acids for chemical treatment of steel.
In the industrial production of pickling chemical treatment, there are steel as an electrode, pass the current to improve the quality of pickling and pickling speed, the so-called electrolytic pickling method.
The main methods used for pickling steel are sulfuric acid pickling and hydrochloric acid pickling.
Disadvantages of steel pickling process
Pickling of steel is widely used in the removal of iron oxide from the surface of cold-rolled sheet blanks, i.e., hot-rolled sheets, such as hot-rolled sections needing phosphating or plating before surface treatment processing to remove iron oxide; welded pipes in galvanizing or other hot dip plating, electroplating processing before surface pretreatment; annealed steel, such as pipes, profiles, wire, etc., before cold-drawing processing; steel processing parts for electroplating, electro-brush plating before rust removal treatment. Electric brush plating before the rust removal treatment and stainless steel and special steel production process of similar treatment. Pickling is still an indispensable process in steel production and steel surface treatment. However, the use of steel pickling treatment process also brings some problems as follows.
(1) Large consumption of steel materials and acid
In the process of pickling, some mainly through the chemical reaction between acid and iron, dissolving the corresponding metal oxide dissolution to remove the iron. Some, such as the use of sulfuric acid for steel pickling, but also with the help of acid and steel chemical reaction generated by the hydrogen bubble stripping effect to remove iron oxide. Therefore, in the pickling process, a large amount of acid consumption is inevitable. Although the use of pickling corrosion inhibitor in pickling can reduce the metal consumption of steel, but there is still a considerable amount of metal iron loss.
(2) Possible reduction of physical properties of steel
During the pickling process, a chemical reaction occurs between metallic iron and acid and hydrogen is produced. Since the chemical level of hydrogen in the pickling solution is higher than the chemical level of hydrogen in the steel being pickled, the generated hydrogen will penetrate into the steel and accumulate to cause hydrogen embrittlement, which will affect the mechanical properties of the steel or the subsequent processing.
(3) Will bring a series of environmental pollution problems
The use of pickling process for steel or parts for surface treatment, due to the variety of steel, product specifications and the scale of production vary, resulting in the production equipment and production environment has a great difference. Such as the sealing of pickling tank, corrosion of production equipment, ventilation and exhaust of pickling workshop, acid mist discharge and treatment, as well as sewage treatment and discharge, the treatment methods and treatment levels vary greatly. This will bring many environmental protection problems, which need to be dealt with and solved one by one.
(4) Disposal of waste acid and iron salts
The pickling of iron and steel consumes a large amount of iron metal and generates a large amount of waste acid with the corresponding acid and iron oxide or iron salt solution. In order to recycle and utilize these waste acids and iron salts, a large investment is needed to build the corresponding recycling and processing equipment. Especially for some small and small batches of iron and steel parts for pickling treatment, it is often difficult to centralize the pickling waste solution for treatment, and once it happens to be discharged directly, it will cause serious pollution to the environment.
In the pickling process, a large amount of acid mist is generated, which not only damages the health of workers, but also corrodes the plant and equipment, while the volatilization of a large amount of acid mist causes acid loss and increases the cost of pickling. Pickling wastewater also contains a large number of heavy metal ions, and the pollution of water bodies caused by its exceedance, the biological toxicity, and ultimately the harm to human health are self-evident.
Source: China Stainless Steel Pipes Manufacturer – Yaang Pipe Industry (www.epowermetals.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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