Pickling is the process of cleaning and processing a metal workpiece. Chemical, acidic, alkaline, electrochemical - there are many ways to perform this technological operation. Where is metal etching used, why is it used in industry, what are the processing methods using this technology, all these issues are discussed in detail in the article below.

What is etching

This is a technology for removing the top layer from the surface of a metal part. The technology is used to clean workpieces from scale, rust, oxides and remove the top layer of metal. Using this method, the top layer is removed to search for internal defects and study the macrostructure of the material.

Using etching, they clean the part and increase surface adhesion. This is done for the subsequent connection of the metal surface with another workpiece, before applying paint, enamel, galvanic coating and other protective coatings.

The method allows you not only to quickly clean the part, but also to create the desired pattern on the metal surface. Using this method, the finest channels and complex images are cut out on a metal surface. You can clean large parts or rolled products. The processing depth is adjustable with an accuracy of several microns, which makes it possible to produce complex parts with small grooves and other complex elements.

Application of etching in industry

1. For cleaning parts made of carbon, low-alloy and high-alloy steel, titanium and aluminum from oxide film.
2. To improve adhesion before applying galvanic and other types of protective coatings.
3. To prepare steel surfaces for hot-dip galvanizing.
4. To conduct macroanalysis to detect the formation of intergranular corrosion in stainless steels.
5. This technology is used to process small metal parts, such as watch gears.
6. Copper processing is used to make semiconductor chips and printed circuit boards in electronics. This method applies a conductive pattern to the microcircuit.
7. For quick cleaning of hot rolled metal products, heat-treated parts, from oxides.
8. In the aircraft industry, this technology is used to reduce the thickness of aluminum sheets to reduce the weight of the aircraft.
9. In the manufacture of metal inscriptions and drawings. Etching produces relief images drawn by removing a layer of metal according to a specific stencil.

Types of etching

The main types of metal processing used in industry:

• electrolytic - there are cathode and anodic;
• chemical;
• plasma.

Electrolytic etching

Electrolytic or galvanic metal processing is used to quickly clean parts, apply engravings and produce grooves. Metal parts are immersed in an acid or salt electrolyte. The part becomes a cathode - a negative electrode or an anode - a positive electrode. Therefore, two types of electrolytic etching are classified - cathodic and anodic.

1. Cathodic etching. The method is used to remove scale from the surface of carbon steel products after hot rolling or oil quenching. In cathodic etching, the material for the anode is lead, and the electrolyte is a solution of hydrochloric, sulfuric acid or an alkali metal salt. During the electrolysis process, hydrogen gas is actively released at the cathode, which interacts with iron and removes scale. During the cathode method, the metal surface is actively saturated with hydrogen, which increases the fragility of the workpiece. Therefore, the cathode method is not used for thin-walled products.
2. Anodic electrochemical cleaning. This is the most common method in mechanical engineering. The process involves mechanically tearing off the oxide film on the anode with oxygen and mixing metal molecules with the electrolyte. An electrolyte is a solution of acids or salts of the metal being processed. Lead, copper and other metals are used as the cathode. During anodic treatment, the surface of the product becomes clean, with slight roughness, and the metal dissolves in the electrolyte. With this method, there is a risk of reducing the thickness of the workpiece and over-etching.

Chemical etching

The chemical treatment method is used to clean the surface of a part from oxide film, scale and rust for workpieces made of the following materials:

• ferrous metals;
• stainless and heat-resistant steels;
• titanium and its alloys;
• aluminum

For etching, sulfuric, hydrochloric or nitric acid is used. The workpiece is immersed in an acid or alkaline solution, molten salt and kept for the required time interval. The required cleaning time can range from 1 to 120 minutes.

The cleaning process occurs due to the release of hydrogen when the acid interacts with the metal. Acid molecules penetrate through pores and cracks under the oxide film. There they interact with the metal surface and hydrogen is released. The released gas tears off the oxide film and cleans the part.

Simultaneously with the oxides, the metal being treated dissolves in the acid. To prevent this process, corrosion inhibitors are used.

Plasma etching

With the ion-plasma method, cleaning and removal of the surface layer occurs by bombarding the part with ions of inert gases that do not react chemically with the molecules of the material being processed. Allows you to make high-precision notches and grooves with an accuracy of up to 10 nm. The technology is used in microelectronics.

The plasma-chemical method involves excitation of plasma in a chemically active medium, which causes the formation of ions and radicals. Active particles falling on a metal surface cause a chemical reaction. In this case, light compounds are formed, which are removed from the surrounding air by vacuum pumps.

The method is based on chemical reactions that occur when using reactive gases, such as oxygen, which are highly reactive. These gases actively interact in the gas discharge plasma. Unlike plasma treatment in inert gases, with this cleaning method, the active gas reacts only with certain molecules.

The disadvantage of this method is the lateral expansion of the grooves.

Etchants

Pickling of carbon steels is carried out in an 8-20% solution of sulfuric or 10-20% hydrochloric acid. With the obligatory addition of corrosion inhibitors (KS, ChM, UNIKOL) to eliminate the fragility of the material and reduce the possibility of over-etching.

Products made of stainless or heat-resistant steel are processed using a solution consisting of: 12% hydrochloric, 12% sulfuric, 1% nitric acid. If required, processing is done in several stages. The first is that scale is loosened in 20% hydrochloric acid. The second stage is immersion in a 20-40% nitric acid solution to completely remove surface contaminants.

The thick layer of scale that forms on stainless steel is removed during its production by 75-85% molten caustic soda with 20-25% sodium nitrate. After which, complete removal of oxides is carried out in 15-20% nitric acid.

Processing of aluminum and alloys based on it involves removing the refractory oxide film from the surface of the workpiece. For this purpose, alkaline or acidic solutions are used. Usually 10-20% alkali is used, at a temperature of 50-80 ºС, the etching procedure takes less than 2 minutes. The addition of sodium chloride and sodium fluoride to the alkali makes this process more uniform.

The purification of titanium and its alloys, carried out after heat treatment, is carried out in several stages. At the first stage, scale is loosened in concentrated caustic soda. Then the scale is removed in a solution of sulfuric, nitric or hydrofluoric acid. To remove the remaining pickling sludge, use hydrochloric or nitric acid with the addition of a small amount of hydrofluoric acid.

When processing copper and its alloys, etchants from hydrogen peroxide, chromic acid and the following salts are used:

• copper chloride;
• ferric chloride;
• ammonium persulfate.

This information material describes in detail the pickling process used in metallurgical plants. The method allows you to quickly clean the metal surface from oxides, scale, rust and other contaminants. Thanks to etching, it is possible to apply various designs to metal, create complex microcircuits and make microscopic channels of the desired shape.

Stainless steel often requires surface treatment to achieve the desired aesthetic or performance properties. Treatment with shot blasting and sandblasting devices is limited due to the high probability of hardening. Modern production uses etching of stainless steel, after preliminary thermal or mechanical treatment. The complexity of this process, compared to conventional black, low-alloy steels, is explained by the presence of a chromium oxide film that acts as a protective barrier. It is this that forms hard scale that does not interact well with reagents. Technological influences may cause color changes on the surface. These include welding, soldering, and other operations involving high temperatures. Iridescent tarnish can be removed by etching. For different chemical compositions of stainless steel, individual pickling methods and compositions have been developed, taking into account the influence of the steel elements, to achieve maximum results.

Predominant methods of etching stainless steel steels are alkaline and acidic, which can be intensified by electrolysis or occur without it.

Acid etching

Maximum effect pickling stainless steel with acids is achieved by sequential interaction of the stainless steel surface in baths with two types of acids - sulfuric and nitric. The sequence of stages is as follows

1. Degreasing, removal of large snags, scale
2. Pickling in a sulfuric acid bath (concentration 10-12%) or sulfuric acid bath (8% sulfuric acid, 4% hydrochloric acid). In this case, corrosion of scale and roughness on the surface occurs. The ideal temperature for the process is between 60 and 80 degrees Celsius. Monitoring this parameter is important for process control. The duration of treatment depends on the steel grade, the presence of a controlled ratio, and the concentration of acids. If the bath is depleted, pitting corrosion may occur. For example, steel with 18% Cr, 8% Ni requires 23 to 45 minutes of pickling in a sulfuric acid bath. Reducing the processing time by half can be achieved if this operation is carried out in a controlled atmosphere.
3. Rinsing with plenty of running water
4. Immersion of the workpiece in a bath filled with a solution of nitric acid and hydrofluoric acid (10 - 20, 1-2 weight percent, respectively). At a bath temperature of 60–70 degrees, the treatment time is 7–15 minutes.
5. Repeated rinsing with large volumes of water

The presented method is basic and has many variations. Etching in one nitrate bath with an admixture of hydrofluoric acid increases the etching time to 30 minutes. Sodium fluoride can act as a substitute for hydrofluoric acid. Increasing the concentration of hydrofluoric acid to 10% allows the process to be carried out at low temperatures, avoiding preliminary immersion in sulfuric acid.

Reducing the etching time in sulfuric acid can be achieved by adding no more than 5% sodium chloride. This move gives the desired effect in 15 minutes, but at the same temperature, about 80 degrees Celsius.

Be careful: if it is necessary to carry out the procedure in a room with insufficient aspiration, replace the components of the second stage of etching. Acids produce harmful fumes when etching. A solution of ferrous sulfate (7%) and hydrofluoric acid (2%) is proposed as a replacement.

To correctly select the acid etching method, you need to know and take into account the state of the oxide film on the surface of stainless steel. Appearance can tell you about the composition of the film. The green color of the scale indicates a high content of chromium oxides. Accordingly, the action of acidic environments will be difficult and will require more time.

Electrolytic etching

One option common in modern factories is electrolytic etching. A workpiece or part placed in an acid bath is connected to a positive or negative terminal. When current passes through the surface of stainless steel, oxygen is released. The gaseous phase has a mechanical effect on the oxide film. This helps speed up the processing process and the quality of the resulting surface.

Etching with ready-made pastes

Modern industry offers a variety of etching products on the market. pastes for stainless steel . Their main purpose is local processing of welds, the consequences of changes in the uniformity of surface coloring under the influence of temperature. The principle of working with such pastes is simple and can be used even in small workshops.

• Apply the paste in a thick layer up to 2 cm, using a brush
• Exposure 60-90 minutes
• Water jet rinsing

The use of pastes is advisable for processing welding seams of stainless steel grades. The treated seam is able to resist corrosion even in the damp conditions of a car wash.

Alkaline etching

Treating the surface of stainless steel with molten caustic soda is called alkaline etching. It should be noted that during this process the oxide film is destroyed, while the chemicals do not react with the metal. An increase in temperature promotes corrosion of the oxide film, improving the quality of the treated surface. Rapid cooling in liquid also helps to improve the treated surface.

It is almost impossible to achieve 100% results with this type of processing. Residual films from chromium oxides, nickel and iron oxides are possible on the metal. Among the recommendations for final finishing of such defects is a short-term treatment in a nitrate bath.

Alkaline etching methods

The following methods are distinguished:

• Aging in soda. The sodium nitrate content should range from 20-40%, heated to a temperature of 460-500 degrees Celsius. Etching in such an environment lasts for 15 minutes. Some austenitic grades of stainless steel are prohibited from being heated above 450 degrees. This can lead to intergranular corrosion. This is followed by a rinsing step in a large amount of water, followed by a 5-minute immersion in a sulfuric acid bath and up to 10 minutes in a nitrate bath.
• Known in England since the first half of the 19th century, the etching method is combined with passing an electric current through the part being etched. At a current density of 11 A/m2, 15 seconds is sufficient. This reaction rate is associated with the electrolysis process. The release of sodium and hydrogen at the cathode contributes to the reduction of oxides. The reduced metal is deposited on the surface. This type of etching allows you to obtain degreased metal, characterized by purity and uniformity. This method uses soda. Variations are possible with the composition and addition of calcium chloride. This method is used for etching flat, rod blanks, and drawn products.
• Treatment with sodium hydrides is based on reduction by exposing the metal to sodium and hydrogen. The presence of sodium hydride is achieved by the interaction of hydrogen and sodium, which is in a molten state. A cylinder without a bottom plane is placed in molten caustic soda. The top plane has a hole. Sodium is poured into this hole, it reacts on the surface of the bath. A stream of hydrogen is passed through a spot of sodium on caustic soda. A hydride is formed and diffuses throughout the bath. Achieving the required concentration of 1-2% sodium hydride occurs within controlled threshold values. In the absence of an air separation product, dissociated ammonia is used. The parts are heated in such a bath to 400 degrees Celsius. Stainless steels show good pickling results with this technique and duration of 4-17 minutes. After etching, it is recommended to thoroughly rinse the parts. If necessary, carry out additional treatment in a nitrate bath. Given the high cost of this method, its obvious advantage is the fact that the metal does not interact with the etchant. Metal losses are minimal. Lower process temperatures reduce coolant costs and reduce operational safety.

There are certain rules that must be followed for any of the presented methods. Among them, priority is the treatment of the metal surface before etching, removal of the oxide film, and degreasing. The etching process is no less important.

Bath materials

Choosing the right material for making etching baths is a difficult task for chemists and materials scientists.

• ceramic coated
• glass covered brick
• wood, lead-coated concrete
• rubber derivatives
• Certain grades of stainless steel for acid baths.

The content of nitrogenous acid with impurities of hydrofluoric or hydrochloric acid allows the use of the same materials. The only exceptions are lead as a coating, ceramics with a high silicon content, due to their interaction. It is quite possible to use steel in alkali baths, monitoring the progress and intensity of electrolysis in close proximity to the material. Under certain conditions and acid content, its temperature, and nature, it is possible to use stainless steel grades for pickling tanks. Such, for example, as 8Х18Н8М or 10Х20Н25М4.

From the information provided in this review, we can conclude that the processing mode, the chemical composition of the bath, the need for additional mechanical processing, and the use of electrolysis should be determined based on specific initial conditions (steel grade, state of the oxide film, technological capabilities) and regulated in the context of the expected final result .

As you know, metal is quite difficult to process at home without special skills and tools, especially if it is such a hard metal as steel. However, you can call on chemistry to help: there is such a chemical process - electrolysis. It flows on the electrodes when an electric current passes through electrolyte solutions. Those. If you take a metal workpiece as an electrode and use ordinary salt water as an electrolyte, then when a current passes through it, the metal will begin to be etched, in other words, atoms from the surface of the metal will begin to “fly away.” Thus, metal processing does not always require any special skills and tools, because electricity can do everything for us.

In this article we will look at how to etch an inscription or design on a metal plate. For this you will need:

• Plastic or glass container.
• Salt.
• Metal plate.
• Power supply 5 - 12 volts.
• Connecting wires.

Electrochemical engraving on metal

Making a board without etching

"Documentation" - Technical information by application electronic components, features of construction of various radio engineering And electronic circuits, as well as documentation on the features of working with engineering software and regulatory documents (GOST).

The method of manufacturing a printed circuit board by cutting has long been known and has many supporters, since it does not require computer equipment and a home chemical laboratory. You only need a simple cutter, which can be easily made from a piece of hacksaw blade.

To develop this method, I want to offer a more advanced tool and, of course, a different cutting technique. I make the cutter on a sharpener from an old MZ tap (or M3.5, M4). The cutter drawing is shown in Fig. 1. On it, thin solid lines show the contours of the tap before it is processed, and thin dashed lines show the invisible bottom of the remaining (upper in the picture) groove.

After turning, the cutter must be inserted into the handle, for which an old felt-tip pen or fountain pen will do. The cutter shank is heated in the flame of a gas stove to a temperature at which the plastic of the future handle melts, and, held with pliers, inserted into a previously prepared hole. The cutting edge is sharpened and finished using fine-grained sandpaper.

The design is applied to the foil of the board blank in any way, even with a pencil. Using the described tool is completely different from working with a traditional cutter and is more reminiscent of the techniques of a linocut maker using a graver. A groove in the foil to the base material of the board is cut by moving away from you, and not towards you, turning the handle left and right around the longitudinal axis.

The decisive factor in the work is the choice of the optimal cutting angle a (Fig. 2). If the angle is too large, the cutter penetrates into the material, begins to require more and more force and stops completely, and if it is too small, it “jumps out” of the material. At the optimal angle - it is determined experimentally - the edge cuts off a narrow strip of foil, slightly gripping the base layer.

Each cut is made in one pass, and not several, like with a conventional cutter, so even with little skill, the speed of making the board is very high. I spend about half an hour on a board measuring 100x60 mm.

Publication date: 06.12.2007

Readers' opinions

• Alfred / 06/18/2012 - 05:31
  This is the ideal awensr. Everyone should read this
• EvgEniy / 05/22/2012 - 12:10
  I had never thought about this method before... Thanks for the interesting idea! Now I want to make an electronically controlled machine that will do this itself (a la plotter)
• 000 / 24.12.2011 - 00:48
  Well, you can, of course, disable a simple board, but if something is more serious, alas, this method will look like masochism
• Zvezdochet / 04/13/2011 - 14:11
  To be honest, this is the first time I’ve heard of this method. And I can’t imagine a cutting technique with any other tool, but what is described in the article is an ingenious and simple method!

The starting metal for cold rolling is rolled stock obtained in hot rolling mills, called rolled stock. A mandatory operation in the production technology of cold-rolled products is metal for rolling. Since the surface of hot-rolled metal is covered with a layer of scale, it becomes necessary to remove it to obtain a high-quality metal surface.
The removal efficiency depends on its physicochemical composition, its thickness and structure, as well as on the etching conditions. Optimal pickling conditions are created when the scale

contains the maximum amount of wustite (iron oxide - FeO), and hematite (Fe2O3) is absent. This is due to the fact that wustite is highly soluble in acids, while hematite is an insoluble compound. Such conditions for the formation of scale are typical for low temperatures at the end of rolling. Reducing the temperature at which the strips are wound into a roll does not affect the thickness of the scale layer, but reduces the risk of hematite appearing on the edges and ends of the strip.

There are acid and mechanical. On pickling lines, scale is removed by combining both methods of removing it: first, the strip passes through a descaling machine and a skin-passing cage, where the scale is cracked and mechanically removed, and then the scale remaining on the strip is dissolved in acid solutions (chemical method).
based on the interaction of scale with acids. In this case, the scale undergoes chemical transformations and is separated from the base metal. In addition, scale removal also occurs as a result of the release of hydrogen gas, which accumulates under the scale and tears it away from the metal.

Known 2 etching method: immersing the metal in the bath and supplying the solution in the form of jets under pressure.

The most common acids used for pickling steel are sulfuric and. When using sulfuric acid, not only the dissolution of scale oxides occurs, but also pure iron, which leads to an increase in metal waste and an increase in acid consumption. Therefore, sulfuric acid etching is usually carried out in the presence of inhibitors - substances that slow down the process of dissolution of pure metal without reducing the rate of scale etching. The disadvantages of sulfuric acid etching also include: contamination of the etching solution with sludge, uneven removal of scale, lack of regeneration of spent etching solutions and low demand for the regeneration by-product - iron sulfate.

Etching in hydrochloric acid occurs in the outer and inner layers of scale. Hydrochloric acid dissolves not only wustite quite well, but also higher iron oxides. At the same time, the scale does not fall off to form sludge at the bottom of the bath or strip, but almost completely goes into solution. It is believed that metal losses during hydrochloric acid etching are ~25% less than during etching in sulfuric acid due to a decrease in the dissolution of pure iron. When etching in hydrochloric acid, the intensity of dissolution of scale increases, and over-etching is less common. Etching in hydrochloric acid results in a cleaner surface than etching in sulfuric acid. The great advantage of hydrochloric acid is the possibility of complete regeneration of spent hydrochloric acid etching solutions. Pickling, as a rule, is carried out in a hot solution, then the strip is wrung out by pairs of squeezing rollers, washed, dried, and the edge is trimmed. The strip processed in this way is transmitted to .