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What is the corrosion of metals? Corrosion: types of corrosion, methods of protection

Corrosion lends itself to many materials, such as metal, ceramic, wood, as a result of exposure to them. As a rule, this effect is achieved due to the instability of the structure, which is affected by the thermodynamics of the environment. In the article we will understand in detail what metal corrosion is, what types it has, and also how you can protect yourself from it.

Some general information

Among the people, the word "rust" is quite popular, which refers to the process of corrosion of metal and various alloys. For polymers, people use the concept of “aging”. In fact, these words are synonymous. A striking example is the aging of rubber products that actively interact with oxygen. Some plastic products can quickly become unusable due to precipitation. How quickly the corrosion process will occur depends entirely on the conditions in which the product is placed. The humidity of the environment is especially affected. The higher its value, the faster the metal will become unusable. Experimentally, scientists have found that about 10% of products in production are simply written off due to corrosion. The types of this process are different, their classification depends on the type of environment in which the products are located, the speed and nature of the flow. Next, we consider in more detail the types of corrosion. Now every person should understand what metal corrosion is.

artificial aging

The corrosion process is not always destructive and renders certain materials unusable. Often, due to corrosion, the coating acquires additional properties, necessary for a person. That is why artificial aging has become popular. It is most often used if we are talking about aluminum and titanium. Only with the help of corrosion it is possible to achieve increased strength of materials. In order to complete the destruction process correctly, it is necessary to use heat treatment. Given that the natural aging of materials under certain conditions is a rather slow process, there is no need to clarify that when using this method, the material must have a special hardening. You also need to understand all the risks that are associated with this method. For example, although the strength of the material increases, but the ductility decreases as much as possible. With ease, now the reader will be able to answer the question of what is the corrosion of an artificial type of metal.

Heat treatment reviews

This method densifies the molecules of the material, respectively, the structure changes. Often, thermal protection is necessary to strengthen pipelines, as it allows you to protect the material from rust, as well as minimize the pressure that is exerted on the structure if it is underground. Users of this technique leave reviews in which they describe that this method protection is as effective as possible and really shows good results. Such processing is desirable to apply only in the industrial sector. Due to the fact that the chambers for firing and performing other processes necessary to obtain reliable protection are expensive, the method is not popular. Such protection of metal from corrosion is quite effective.

Classification

At the moment, there are more than 20 rust options. The article will describe only the most popular species corrosion. Conventionally, they are divided into the following groups, which will help to understand in more detail what metal corrosion is.

Chemical corrosion is the interaction with a corrosive environment. In this case, the oxidation of the metal and the reduction of the oxidizing agent occur simultaneously in one cycle. Both materials are not separated by space. Consider other types of metal corrosion.

Electrochemical corrosion is the interaction of a metal with an electrolyte. The atoms are ionized, the oxidizing agent is reduced, and these two processes occur over several cycles. Their speed is completely dependent on the potential of the electrodes.

Gas corrosion is the rusting of metal with a small amount of liquid. Moisture should not exceed 0.1%. Also, this type of corrosion can occur in a gaseous environment at high temperatures. Most often this species is found in the industry associated with the chemical industry and oil refining.

In addition to the above, there are many more types of corrosion of materials. There are biological, target, contact, local and other types of rusting.

Electrochemical corrosion and its features

In electrochemical corrosion, the destruction of the material occurs due to its contact with the electrolyte. As the last substance, there may be condensate, rainwater. It should be noted that the more salts in the liquid, the higher the electrical conductivity. Accordingly, the corrosion process will proceed quite quickly. Speaking of the most popular places that are susceptible to corrosion, it should be noted rivets in a metal structure, welded joints, as well as simply places where the material is damaged. It happens that an iron alloy during its creation is coated with special substances that have anti-corrosion properties. However, this does not prevent the rusting process, but only slows it down. Enough a prime example can be called galvanizing. Zinc has a negative potential when compared to iron. Because of this, the last material will be restored, and the zinc will be damaged. If there is an oxide film on the surface, the destruction process will become lengthy. Electrochemical corrosion has several types, but it should be noted that all of them are dangerous and, as a rule, it is impossible to stop this type of metal corrosion.

Chemical corrosion

Chemical corrosion is quite common. For example, if a person notices scale, then he must understand that it appeared as a result of metal combination, that is, interaction, with oxygen. As a rule, if the ambient temperature is high, the corrosion process will be markedly accelerated. A liquid can participate in rusting, that is, water, salt, any acid or alkali, salt solutions. If it's about chemical corrosion metals, such as copper or zinc, that their oxidation leads to a stable corrosion process of the film. The rest form iron oxide. Further, all the chemical processes that will occur will lead to the appearance of rust. It will not provide protection in any way, but, on the contrary, contributes to the occurrence of corrosion. With the help of galvanizing at the moment it is possible to protect many materials. Other means of protection against chemical corrosion of metals have also been developed.

Types of concrete corrosion

The brittleness of concrete can be caused by one of three types of corrosion. Quite often there is a change in the structure of this material. Let's take a look at why this is happening.

The most common type of corrosion should be called the destruction of cement stone. As a rule, this occurs when liquid and atmospheric precipitation constantly act on the material. Because of this, the structure of the material is destroyed. There are more below detailed examples metal corrosion:

interaction with acids. If the cement stone is constantly exposed to these materials, then a rather aggressive element is formed, which is harmful to the coating. This is calcium bicarbonate.
Crystallization of sparingly soluble substances. This is about corrosion. Due to the fact that fungi, spores and other substances enter the pores, the concrete coating begins to quickly collapse.

Corrosion: ways to protect

Manufacturers often suffer huge losses due to corrosion, so a lot of work is being done to avoid this process. Moreover, it should be noted that most often corrosion does not lend itself to the metal itself, but to huge metal structures. Manufacturers spend a lot of money on their creation. Unfortunately, it is almost impossible to provide 100% protection. However, if you properly protect the surface, that is, carry out abrasive blasting, you can delay the corrosion process for several years. They also fight with paintwork. It reliably protects the material. If the metal is underground, then it must be treated with special materials. This is the only way to achieve maximum protection metal from corrosion.

Measures to prevent aging

As mentioned above, the corrosion process cannot be stopped. But you can maximize the time during which the material will collapse. Also, in production, as a rule, they try to get rid of the factors that affect the aging process as much as possible. For example, in factories, each structure is periodically treated with solutions and polishes. It is they who save the material from negative impact on the metal from the side of mechanical, temperature and chemical conditions. In order to understand this in more detail, it is necessary to study the definition of corrosion of metals. If we talk about slowing down the effect of aging, then it should be noted that heat treatment can be used for this. Under normal operating conditions, this method will avoid the rapid destruction of the material as much as possible. Welders, so that the seams on the product do not part, use firing at a temperature of 650 degrees. This technique will reduce the intensity of aging.

Active and passive methods of struggle

Active anti-corrosion methods act by changing the structure of the electric field. To do this, you need to use direct current. The voltage must be such that the product has enhanced characteristics. A fairly popular method would be to use a “sacrificial” anode. It protects the material by its own destruction. The conditions for corrosion of metals are described above.

As for passive protection, a paintwork is used for this. It completely protects the product from the ingress of liquid, as well as oxygen. Thanks to this, the surface is maximally protected from destruction. Zinc, copper, nickel coating should be used. Even if the layer is severely destroyed, it will still protect the metal from rusting. Of course, you need to understand that passive protection methods will only be relevant if the surface does not have cracks or chips.

Reviews about the paint and varnish protection of metals

At the moment, paintwork protection is very popular. It is efficient, flexible to use, and inexpensive. However, if long-term use of a metal structure is necessary, then this method of protection will not work. More than 7-8 years paint and varnish coatings will not be able to protect the material. Accordingly, they will have to be updated. Most likely, it will be necessary to carry out restoration and replace the surface of the material. Among other disadvantages of this coating, limitations in terms of use should be noted. If it is necessary to strengthen pipes that are underground or water, then paint protection will not work. Therefore, it should be understood that if it is necessary for the structure to be used for more than 10 years, other methods of protection should be resorted to.

Galvanizing in detail

Having considered the main types of corrosion, it is also necessary to discuss the most effective methods of protection. One of these is galvanizing. It allows you to protect the material from severe damage by changing physical and chemical properties. At the moment, this method is considered economical and efficient, given that almost 40% of all mined material on Earth is spent on zinc processing. It is important to treat the material with an anti-corrosion coating.

Galvanizing is carried out for steel sheets, fasteners, appliances and huge metal structures. In general, with the help of such spraying, products of any size and shape can be protected. Zinc has no decorative purpose, although it may occasionally be added to the alloy to give it a sheen. In general, you need to understand that this metal will provide maximum protection against corrosion even in the most aggressive conditions.

Rust protection features

When working with metal, any person understands that before applying protective materials, it is necessary to prepare the surface. Often all the difficulties lie precisely in this stage. In order to create a special barrier that will allow rust to reach the metal, it is necessary to introduce the concept of a compound. Thanks to him, the kit will form protection against corrosion. In this case, electrical insulation takes place. It is usually quite difficult to protect against corrosion of ferrous metals.

Due to the specifics of the use of various means of protection, it is necessary to understand the operating conditions of the material. If the metal will be located underground, then it is necessary to use multilayer coatings that will have not only anti-corrosion properties, but also enhanced protection from mechanical damage. If we are talking about communications that actively interact with oxygen and gases, you should use a tool that minimizes the effects of water and oxygen. Respectively, increased attention on the part of the manufacturer will be given insulation against moisture, steam and low temperatures. In this case, additives and special plasticizers should be added, because the causes of metal corrosion are different and all types should be protected.

Mix "Urizol"

The Urizol mixture should be considered separately, as it is used to coat the pipeline. It is also suitable for fittings, fittings, valve assemblies and those products that are constantly in contact with oil or gases. This composition is needed in order to get rid of the influence of underground and atmospheric influences. Often this mixture is also used for the insulation of concrete materials. This substance is applied very simply, without any difficulty. In order to treat the surface, it is necessary to use a sprayer. This is the only way to avoid corrosion of metals and alloys of similar products. As soon as the components are combined, the reaction begins. This results in polyurea. After that, the mixture passes into a gel-like and non-fluid state, and after some time becomes solid. If the polymerization rate is slow, smudges will begin to form. They are harmful, because they make it difficult to increase the thickness of the coating. It should be noted that this mixture retains a sticky state for a long time. Due to this, all layers will be as uniform as possible, and intermediate thickness measurements will be equal to each other. If the polymerization process is too fast, then the adhesion of the composition will decrease. In this case, the thickness of the resulting layer for insulation will be uneven. By the way, the spray gun will quickly clog if the coating speed is too fast. Metal corrosion factors will not appear if everything is done correctly. In order to prevent such situations, it is necessary to carefully select the components and follow the manufacturing rules.

Paints and enamels

Protection of metal-plastic structures can be carried out using three methods.

Coatings have already been described. They are simple, have a variety of colors, and with the help of them you can easily process huge surfaces. Since the process of metal corrosion is quite fast, then you should immediately think about coating with materials.

The second type is plastic coatings. As a rule, they are made of nylon, PVC. This coating will provide maximum protection against water, acids and alkalis.

The third type is rubber coating. Often it is used to protect tanks and other structures from the inside.

Phosphating and chromating

The metal surface must be properly prepared for the protection process. Which methods will be used depends entirely on the type of surface. For example, ferrous metals are protected by phosphating. Non-ferrous metals can be processed by both methods. In general, if we talk about chemical preparation, it is necessary to clarify that it takes place in several stages. To begin with, the surface is degreased. Then it is washed with water. Next, a conversion layer is applied. After that, it is washed again with two types of water: drinking and demineralized, respectively. The next thing to do is passivation. Chemical treatment should be carried out by spraying, immersion, steam jet and water jet methods. The first two methods must be applied using special units that will fully prepare the surface for work. Which method to choose, it is necessary to decide depending on the size, configuration of the product, and so on. In order to better understand this issue, one should know the equations for the reactions of corrosion of metals.

Conclusion

The article described what corrosion is and what types it has. Now any person after reading this article will be able to understand how to protect any material from aging. By and large, this is quite easy to do, knowing all the necessary instructions. The main thing is to understand all the characteristics of the environment in which the material is used. If the products are located in a place where constant vibrations occur, as well as there are strong loads, then cracks will occur in the paintwork. Because of this, moisture will begin to get on the metal, respectively, the corrosion process begins immediately. In such cases, it is better to additionally use rubber sealants and gaskets, then the coating will last a little longer.

In addition, it must be said that the design, with premature deformation, will quickly deteriorate and age. Accordingly, this can lead to completely unforeseen circumstances. This will bring material damage and may result in the death of a person. Accordingly, special attention should be paid to corrosion protection.

The phrase "corrosion of metal" contains much more than the name of a popular rock band. Corrosion irrevocably destroys the metal, turning it into dust: of all the iron produced in the world, 10% will completely collapse in the same year. The situation with Russian metal looks something like this - all the metal smelted per year in every sixth blast furnace in our country becomes rusty dust before the end of the year.

The expression "costs a pretty penny" in relation to metal corrosion is more than true - the annual damage caused by corrosion is at least 4% of the annual income of any developed country, and in Russia the amount of damage is calculated in ten figures. So what causes the corrosion processes of metals and how to deal with them?

What is metal corrosion

The destruction of metals as a result of electrochemical (dissolution in moisture-containing air or aquatic environment- electrolyte) or chemical (formation of metal compounds with chemical agents of high aggression) interaction with the external environment. The corrosion process in metals can develop only in some areas of the surface (local corrosion), cover the entire surface (uniform corrosion), or destroy the metal along the grain boundaries (intergranular corrosion).

The metal under the influence of oxygen and water becomes a loose light brown powder, better known as rust (Fe 2 O 3 ·H 2 O).

Chemical corrosion

This process occurs in media that are not conductors of electric current (dry gases, organic liquids - petroleum products, alcohols, etc.), and the intensity of corrosion increases with increasing temperature - as a result, an oxide film is formed on the surface of metals.

Absolutely all metals, both ferrous and non-ferrous, are subject to chemical corrosion. Active non-ferrous metals (for example, aluminum) under the influence of corrosion are covered with an oxide film that prevents deep oxidation and protects the metal. And such a low active metal as copper, under the influence of air moisture, acquires a greenish coating - patina. Moreover, the oxide film protects the metal from corrosion not in all cases - only if the crystal-chemical structure of the resulting film is consistent with the structure of the metal, otherwise the film will not help in any way.

Alloys are subject to a different type of corrosion: some alloy elements do not oxidize, but are reduced (for example, in a combination of high temperature and pressure in steels, carbides are reduced by hydrogen), while the alloys completely lose the necessary characteristics.

Electrochemical corrosion

The process of electrochemical corrosion does not require the obligatory immersion of the metal in the electrolyte - a thin electrolytic film on its surface is sufficient (electrolytic solutions often impregnate the medium, surrounding metal(concrete, soil, etc.)). The most common cause of electrochemical corrosion is the widespread use of household and technical salts (sodium and potassium chlorides) to remove ice and snow on roads in winter - cars and underground utilities are especially affected (according to statistics, annual losses in the United States from the use of salts in winter are 2.5 billion dollars).

The following happens: metals (alloys) lose some of their atoms (they pass into the electrolytic solution in the form of ions), electrons replacing the lost atoms charge the metal with a negative charge, while the electrolyte has a positive charge. A galvanic couple is formed: the metal is destroyed, gradually all its particles become part of the solution. Electrochemical corrosion can be caused by stray currents arising from leakage from electrical circuit parts of the current into aqueous solutions or into the soil and from there - into a metal structure. In those places where stray currents exit the metal structures back into the water or into the soil, the destruction of metals occurs. Especially often, stray currents occur in places where ground electric vehicles are moving (for example, trams and electric railway locomotives). In just a year, stray currents with a power of 1A are able to dissolve iron - 9.1 kg, zinc - 10.7 kg, lead - 33.4 kg.

Other causes of metal corrosion

Radiation, waste products of microorganisms and bacteria contribute to the development of corrosive processes. Corrosion caused by marine microorganisms causes damage to the bottoms of marine vessels, and corrosion processes caused by bacteria even have their own name - biocorrosion.

The combined effect of mechanical stresses and the external environment greatly accelerates the corrosion of metals - their thermal stability decreases, surface oxide films are damaged, and electrochemical corrosion is activated in those places where inhomogeneities and cracks appear.

Measures to protect metals from corrosion

An inevitable consequence of technological progress is the pollution of our living environment, a process that accelerates the corrosion of metals, since the external environment shows more and more aggression towards them. There are no ways to completely eliminate the corrosion destruction of metals; all that can be done is to slow down this process as much as possible.

To minimize the destruction of metals, you can do the following: reduce the aggression of the environment surrounding the metal product; increase the resistance of the metal to corrosion; eliminate the interaction between the metal and substances from the external environment that exhibit aggression.

For thousands of years, mankind has tried many ways to protect metal products from chemical corrosion, some of them are still used today: coating with grease or oil, other metals that are less corrosive (the most ancient method, which is already more than 2 thousand years old - tinning (tin coating)).

Anti-corrosion protection with non-metallic coatings

Non-metallic coatings - paints (alkyd, oil and enamel), varnishes (synthetic, bituminous and tar) and polymers form a protective film on the surface of metals, excluding (with its integrity) contact with the external environment and moisture.

The use of paints and varnishes is advantageous in that these protective coatings can be applied directly at the assembly and construction site. Methods for applying paints and varnishes are simple and amenable to mechanization, damaged coatings can be restored "on the spot" - during operation, these materials have a relatively low cost and their consumption per unit area is small. However, their effectiveness depends on the observance of several conditions: compliance with the climatic conditions in which the metal structure will be operated; the need to use exclusively high-quality paints and varnishes; strict adherence to the technology of application to metal surfaces. Paints and varnishes are best applied in several layers - their quantity will provide the best protection against atmospheric effect onto a metal surface.

Polymers such as epoxy resins and polystyrene, polyvinyl chloride and polyethylene can act as protective coatings against corrosion. In construction work, reinforced concrete embedded parts are covered with coatings from a mixture of cement and perchlorovinyl, cement and polystyrene.

Protection of iron against corrosion by coatings from other metals

There are two types of metal inhibitor coatings - protective (zinc, aluminum and cadmium coatings) and corrosion resistant (silver, copper, nickel, chromium and lead coatings). Inhibitors are applied chemically: the first group of metals has a high electronegativity with respect to iron, the second - a large electropositivity. The most widespread in our everyday life are metal coatings of iron with tin (tinplate, it is used to produce cans) and zinc (galvanized iron - roofing), obtained by pulling sheet iron through a melt of one of these metals.

Cast iron and steel fittings, as well as water pipes are often galvanized - this operation significantly increases their resistance to corrosion, but only in cold water (when hot water is wired, galvanized pipes wear out faster than non-galvanized ones). Despite the effectiveness of galvanizing, it does not provide perfect protection - the zinc coating often contains cracks, which require preliminary nickel plating of metal surfaces (nickel plating) to eliminate them. Zinc coatings do not allow the application of paints and varnishes on them - there is no stable coating.

The best solution for corrosion protection- aluminum coating. This metal has a lower specific gravity, which means it is consumed less, aluminized surfaces can be painted and the paint layer will be stable. In addition, the aluminum coating, compared to galvanized coating, is more resistant to aggressive environments. Aluminizing is not very common due to the difficulty of applying this coating to a metal sheet - aluminum in the molten state exhibits high aggression towards other metals (for this reason, aluminum melt cannot be kept in a steel bath). Perhaps this problem will be completely solved in the very near future - the original way of performing aluminization was found by Russian scientists. The essence of the development is not to immerse the steel sheet in the aluminum melt, but to raise the liquid aluminum to the steel sheet.

Improving corrosion resistance by adding alloying additives to steel alloys

The introduction of chromium, titanium, manganese, nickel and copper into the steel alloy makes it possible to obtain alloyed steel with high anti-corrosion properties. The high proportion of chromium imparts special resistance to the steel alloy, due to which an oxide film of high density is formed on the surface of the structures. The introduction of copper (from 0.2% to 0.5%) into the composition of low-alloy and carbon steels makes it possible to increase their corrosion resistance by 1.5-2 times. Alloying additives are introduced into the composition of steel in compliance with the Tammann rule: high corrosion resistance is achieved when there is one atom of the alloying metal for eight iron atoms.

Measures to counter electrochemical corrosion

To reduce it, it is necessary to reduce the corrosive activity of the medium by introducing non-metallic inhibitors and to reduce the number of components capable of starting an electrochemical reaction. In this way, there will be a decrease in the acidity of soils and aqueous solutions in contact with metals. To reduce the corrosion of iron (its alloys), as well as brass, copper, lead and zinc, carbon dioxide and oxygen must be removed from aqueous solutions. In the electric power industry, chlorides are being removed from the water, which can affect localized corrosion. Liming the soil can reduce its acidity.

Stray current protection

It is possible to reduce the electrical corrosion of underground utilities and buried metal structures subject to several rules:

the section of the structure that serves as a source of stray current must be connected with a metal conductor to the rail tramway;
heating network routes should be located at the maximum distance from the railroads on which electric transport moves, to minimize the number of their intersections;
the use of electrically insulating pipe supports to increase the transient resistance between the soil and pipelines;
at the inputs to objects (potential sources of stray currents), it is necessary to install insulating flanges;
on flanged fittings and stuffing box compensators, install conductive longitudinal jumpers - to increase the longitudinal electrical conductivity on the protected section of pipelines;
in order to equalize the potentials of pipelines located in parallel, it is necessary to install transverse electrical jumpers in adjacent sections.

Protection metal objects equipped with insulation, as well as small steel structures, is performed using a protector that acts as an anode. The material for the protector is one of the active metals (zinc, magnesium, aluminum and their alloys) - it takes on most of the electrochemical corrosion, collapsing and preserving the main structure. One magnesium anode, for example, provides protection for 8 km of pipeline.

Abdyuzhanov Rustam, especially for rmnt.ru

What is metal corrosion

Destruction of metals as a result of electrochemical (dissolution in a moisture-containing air or water environment - electrolyte) or chemical (formation of metal compounds with highly aggressive chemical agents) interaction with the external environment. The corrosion process in metals can develop only in some areas of the surface (local corrosion), cover the entire surface (uniform corrosion), or destroy the metal along the grain boundaries (intergranular corrosion).

The metal, under the influence of oxygen and water, becomes a loose light brown powder, better known as rust (Fe2O3 H2O).

Chemical corrosion

Absolutely all metals, both ferrous and non-ferrous, are subject to chemical corrosion. Active non-ferrous metals (for example, aluminum) under the influence of corrosion are covered with an oxide film that prevents deep oxidation and protects the metal. And such a little active metal as copper, under the influence of air moisture, acquires a greenish coating - patina. Moreover, the oxide film protects the metal from corrosion not in all cases - only if the crystal-chemical structure of the formed film is consistent with the structure of the metal, otherwise the film will not help in any way.

Electrochemical corrosion

The process of electrochemical corrosion does not require the obligatory immersion of the metal in the electrolyte - a thin electrolytic film on its surface is sufficient (electrolytic solutions often impregnate the environment surrounding the metal (concrete, soil, etc.)). The most common cause of electrochemical corrosion is the widespread use of household and technical salts (sodium and potassium chlorides) to remove ice and snow on roads in winter - cars and underground utilities are especially affected (according to statistics, annual losses in the United States from the use of salts in winter are 2.5 billion dollars).

The following happens: metals (alloys) lose some of their atoms (they pass into the electrolytic solution in the form of ions), electrons replacing the lost atoms charge the metal with a negative charge, while the electrolyte has a positive charge. A galvanic couple is formed: the metal is destroyed, gradually all its particles become part of the solution. Electrochemical corrosion can be caused by stray currents that occur when part of the current leaks from an electrical circuit into aqueous solutions or into the soil and from there into a metal structure. In those places where stray currents exit the metal structures back into the water or into the soil, the destruction of metals occurs. Especially often, stray currents occur in places where ground electric vehicles are moving (for example, trams and electric railway locomotives). In just a year, stray currents with a power of 1A are able to dissolve iron - 9.1 kg, zinc - 10.7 kg, lead - 33.4 kg.

Other causes of metal corrosion

Radiation, waste products of microorganisms and bacteria contribute to the development of corrosive processes. Corrosion caused by marine microorganisms causes damage to the bottoms of marine vessels, and corrosive processes caused by bacteria even have their own name - biocorrosion.

The combination of exposure to mechanical stresses and the environment greatly accelerates the corrosion of metals - their thermal stability decreases, surface oxide films are damaged, and electrochemical corrosion is activated in those places where inhomogeneities and cracks appear.

Measures to protect metals from corrosion

An inevitable consequence of technological progress is the pollution of our environment, a process that accelerates the corrosion of metals as the external environment becomes increasingly aggressive towards them. There are no ways to completely eliminate the corrosion destruction of metals; all that can be done is to slow down this process as much as possible.

For thousands of years, mankind has tried many ways to protect metal products from chemical corrosion, some of them are still used today: coating with grease or oil, other metals that corrode to a lesser extent (the oldest method, which is more than 2 thousand years old, is tinning (coating tin)).

Anti-corrosion protection with non-metallic coatings

Non-metallic coatings - paints (alkyd, oil and enamels), varnishes (synthetic, bituminous and tar) and polymers form a protective film on the surface of metals, excluding (with its integrity) contact with the external environment and moisture.

Polymers - epoxy resins and polystyrene, polyvinyl chloride and polyethylene can act as protective coatings against corrosion. In construction work, reinforced concrete embedded parts are covered with coatings from a mixture of cement and perchlorovinyl, cement and polystyrene.

Protection of iron against corrosion by coatings from other metals

There are two types of metal inhibitor coatings - protective (zinc, aluminum and cadmium coatings) and corrosion resistant (silver, copper, nickel, chromium and lead coatings). Inhibitors are applied chemically: the first group of metals has a high electronegativity with respect to iron, the second - a large electropositivity. The most widespread in our everyday life is metal coatings of iron with tin (tinplate, tin cans are made from it) and zinc (galvanized iron - roofing), obtained by pulling sheet iron through a melt of one of these metals.

Cast iron and steel fittings, as well as water pipes are often galvanized - this operation significantly increases their resistance to corrosion, but only in cold water (when hot water is connected, galvanized pipes wear out faster than non-galvanized ones). Despite the effectiveness of zinc plating, it does not provide perfect protection - zinc coating often contains cracks, which require preliminary nickel plating of metal surfaces (nickel plating) to eliminate them. Zinc coatings do not allow applying paints and varnishes on them - there is no stable coating.

The best solution for corrosion protection is aluminum coating. This metal has a lower specific gravity, which means it is consumed less, aluminized surfaces can be painted and the paint layer will be stable. In addition, the aluminum coating, compared to galvanized coating, is more resistant to aggressive environments. Aluminizing is not very common due to the difficulty of applying this coating to a metal sheet - aluminum in the molten state exhibits high aggression towards other metals (for this reason, aluminum melt cannot be kept in a steel bath). Perhaps this problem will be completely solved in the very near future - the original way of performing aluminization was found by Russian scientists. The essence of the development is not to immerse the steel sheet in the aluminum melt, but to raise the liquid aluminum to the steel sheet.

Improving corrosion resistance by adding alloying additives to steel alloys

Measures to counter electrochemical corrosion

Stray current protection

It is possible to reduce the electrical corrosion of underground utilities and buried metal structures subject to several rules:

the section of the structure that serves as a source of stray current must be connected with a metal conductor to the tram rail;

heating network routes should be located at the maximum distance from the railroads on which electric transport moves, to minimize the number of their intersections;

the use of electrically insulating pipe supports to increase the transient resistance between the soil and pipelines;

at the inputs to objects (potential sources of stray currents), it is necessary to install insulating flanges;

on flanged fittings and stuffing box compensators, install conductive longitudinal jumpers - to increase the longitudinal electrical conductivity on the protected section of pipelines;

in order to equalize the potentials of pipelines located in parallel, it is necessary to install transverse electrical jumpers in adjacent sections.

The protection of metal objects provided with insulation, as well as small steel structures, is carried out using a protector that acts as an anode. The material for the protector is one of the active metals (zinc, magnesium, aluminum and their alloys) - it takes on most of the electrochemical corrosion, collapsing and preserving the main structure. One magnesium anode, for example, provides protection for 8 km of pipeline.

Corrosion of metals, as you know, brings a lot of trouble. Is it not for you, dear car owners, to explain what she threatens: give her free rein, so only tires will remain from the car. Therefore, the sooner the fight against this disaster begins, the longer the car body will live.

To be successful in the fight against corrosion, it is necessary to find out what kind of "beast" it is and understand the reasons for its occurrence.

Today you will know

Is there any hope?

The damage done to mankind by corrosion is colossal. According to various sources, corrosion "eats" from 10 to 25% of the world's iron production. Turning into a brown powder, it irretrievably dissipates throughout white light, as a result of which not only we, but also our descendants are left without this most valuable structural material.

But the trouble is not only that metal is lost as such, no - bridges, cars, roofs, architectural monuments are destroyed. Corrosion spares nothing.

The Eiffel Tower, the symbol of Paris, is terminally ill. Made of ordinary steel, it inevitably rusts and collapses. The tower has to be painted every 7 years, which is why its mass increases by 60-70 tons each time.

Unfortunately, it is impossible to completely prevent the corrosion of metals. Well, except to completely isolate the metal from the environment, for example, place it in a vacuum. 🙂 But what is the use of such "canned" parts? The metal must "work". Therefore, the only way to protect against corrosion is to find ways to slow it down.

AT time immemorial for this, fat, oils were used, and later they began to cover iron with other metals. First of all, low-melting tin. In the writings of the ancient Greek historian Herodotus (5th century BC) and the Roman scientist Pliny the Elder, there are already references to the use of tin to protect iron from corrosion.

An interesting incident occurred in 1965 at the International Symposium on Corrosion Control. An Indian scientist spoke about a society for the fight against corrosion, which has existed for about 1600 years, and of which he is a member. So, one and a half thousand years ago, this society took part in the construction of temples of the Sun on the coast near Konarak. And despite the fact that these temples were flooded by the sea for some time, the iron beams are perfectly preserved. So even in those distant times, people knew a lot about the fight against corrosion. So, not everything is so hopeless.

What is corrosion?

The word "corrosion" comes from the Latin "corrodo" - to gnaw. There are also references to the late Latin "corrosio - corrosive". But anyway:

Corrosion is the process of metal destruction as a result of chemical and electrochemical interaction with the environment.

Although corrosion is most commonly associated with metals, it also affects concrete, stone, ceramics, wood, and plastics. In relation to polymeric materials, however, the term degradation or aging is more often used.

Corrosion and rust are not the same

In the definition of corrosion in the paragraph above, the word “process” is not in vain highlighted. The fact is, corrosion is often identified with the term "rust". However, these are not synonyms. Corrosion is precisely a process, while rust is one of the results of this process.

It is also worth noting that rust is a corrosion product exclusively of iron and its alloys (such as steel or cast iron). Therefore, when we say “steel rusts”, we mean that the iron in its composition rusts.

If rust only applies to iron, then other metals don't rust? They don't rust, but that doesn't mean they don't corrode. They just have different corrosion products.

For example, copper, corroding, is covered with a beautiful greenish coating (patina). Silver tarnishes in air - this is a deposit of sulfide on its surface, whose thin film gives the metal a characteristic pinkish color.

Patina is a corrosion product of copper and its alloys.

The mechanism of the course of corrosion processes

The variety of conditions and environments in which corrosion processes occur is very wide, so it is difficult to give a single and comprehensive classification of the occurring corrosion cases. But despite this, all corrosion processes have not only a common result - the destruction of the metal, but also a single chemical entity - oxidation.

Simplified, oxidation can be called the process of electron exchange of substances. When one substance is oxidized (donates electrons), the other, on the contrary, is reduced (receives electrons).

For example, in a reaction...

… a zinc atom loses two electrons (is oxidized), and a chlorine molecule adds them (is reduced).

Particles that donate electrons and are oxidized are called reducing agents, and particles that accept electrons and are reduced are called oxidizers. These two processes (oxidation and reduction) are interrelated and always occur simultaneously.

Such reactions, which are called redox reactions in chemistry, underlie any corrosion process.

Naturally, the tendency to oxidation in different metals is not the same. To understand which ones have more and which ones have less, remember school course chemistry. There was such a thing as an electrochemical series of voltages (activity) of metals, in which all metals are arranged from left to right in order of increasing “nobility”.

So, the metals located in the row to the left are more prone to donating electrons (and hence to oxidation) than the metals to the right. For example, iron (Fe) is more susceptible to oxidation than the more noble copper (Cu). Some metals (for example, gold) can donate electrons only under certain extreme conditions.

We will return to the activity series a little later, but now let's talk about the main types of corrosion.

Types of corrosion

As already mentioned, there are many criteria for the classification of corrosion processes. So, corrosion is distinguished by the type of distribution (solid, local), by the type of corrosive medium (gas, atmospheric, liquid, soil), by the nature of mechanical effects (corrosion cracking, Fretting phenomenon, cavitation corrosion) and so on.

But the main way to classify corrosion, which makes it possible to most fully explain all the subtleties of this insidious process, is classification according to the mechanism of flow.

According to this criterion, two types of corrosion are distinguished:

chemical
electrochemical

Chemical corrosion

Chemical corrosion differs from electrochemical corrosion in that it occurs in media that do not conduct electricity. Therefore, with such corrosion, the destruction of the metal is not accompanied by the appearance of an electric current in the system. This is the usual redox interaction of the metal with the environment.

The most typical example of chemical corrosion is gas corrosion. Gas corrosion is also called high-temperature corrosion, since it usually proceeds at elevated temperatures, when the possibility of moisture condensation on the metal surface is completely excluded. This type of corrosion can include, for example, corrosion of elements of electric heaters or nozzles of rocket engines.

The rate of chemical corrosion depends on temperature - as it rises, corrosion accelerates. Because of this, for example, during the production of rolled metal, fiery splashes scatter in all directions from the hot mass. It is scale particles that are chipped off the surface of the metal.

Scale is a typical product of chemical corrosion, an oxide resulting from the interaction of hot metal with atmospheric oxygen.

In addition to oxygen, other gases can have strong aggressive properties towards metals. These gases include sulfur dioxide, fluorine, chlorine, hydrogen sulfide. For example, aluminum and its alloys, as well as steels with a high chromium content (stainless steels), are stable in an atmosphere that contains oxygen as the main aggressive agent. But the picture changes dramatically if chlorine is present in the atmosphere.

In the documentation for some anti-corrosion preparations, chemical corrosion is sometimes called "dry", and electrochemical - "wet". However, chemical corrosion can also occur in liquids. Only in contrast to electrochemical corrosion, these liquids are non-electrolytes (i.e., non-conductive, such as alcohol, benzene, gasoline, kerosene).

An example of such corrosion is the corrosion of iron parts of a car engine. Sulfur present in gasoline as an impurity interacts with the surface of the part, forming iron sulfide. Iron sulfide is very brittle and easily peels off, leaving a fresh surface for further interaction with sulfur. And so, layer by layer, the detail is gradually destroyed.

Electrochemical corrosion

If chemical corrosion is nothing more than a simple oxidation of a metal, then electrochemical corrosion is destruction due to galvanic processes.

Unlike chemical corrosion, electrochemical corrosion proceeds in media with good electrical conductivity and is accompanied by the appearance of a current. To "start" electrochemical corrosion, two conditions are necessary: galvanic couple and electrolyte.

Moisture on the metal surface (condensate, rainwater, etc.) acts as an electrolyte. What is a galvanic couple? To understand this, let's go back to the activity series of metals.

We look. On the left are the more active metals, on the right are the less active ones.

If two metals with different activity come into contact, they form a galvanic pair, and in the presence of an electrolyte, a flow of electrons occurs between them, flowing from the anode to the cathode sections. In this case, the more active metal, which is the anode of the galvanic couple, begins to corrode, while the less active metal does not corrode.

Diagram of a galvanic cell

For clarity, let's look at a few simple examples.

Let's say a steel bolt is secured with a copper nut. What will corrode, iron or copper? Let's look at the activity row. Iron is more active (to the left), which means that it will be destroyed at the junction.

Steel bolt - copper nut (steel corrodes)

What if the nut is aluminum? Let's look at the activity row again. Here the picture changes: already aluminum (Al), as a more active metal, will lose electrons and break down.

Thus, the contact of a more active "left" metal with a less active "right" metal enhances the corrosion of the first.

As an example of electrochemical corrosion, one can cite the cases of destruction and flooding of ships, the iron skin of which was fastened with copper rivets. Also noteworthy is the incident that occurred in December 1967 with the Norwegian ore carrier Anatina, en route from Cyprus to Osaka. AT pacific ocean a typhoon hit the ship and the holds were filled with salt water, resulting in a large galvanic couple: copper concentrate + steel hull of the ship. After some time, the steel hull of the ship began to soften and it soon gave a distress signal. Fortunately, the crew was rescued by a German ship that came to the rescue, and Anatina herself somehow made it to the port.

Tin and zinc. "Dangerous" and "safe coatings

Let's take another example. Let's say the body panel is covered with tin. Tin is a very corrosion-resistant metal, in addition, it creates a passive protective layer, protecting iron from interaction with the external environment. So the iron under the tin layer is safe and sound? Yes, but only until the tin layer gets damaged.

And if this happens, a galvanic couple immediately appears between tin and iron, and iron, which is a more active metal, will begin to corrode under the influence of galvanic current.

By the way, there are still legends about the supposedly “eternal” tinned bodies of the “Victory” among the people. The roots of this legend are as follows: when repairing emergency vehicles, the craftsmen used blowtorches for heating. And suddenly, for no apparent reason, tin begins to flow from under the flame of the burner! Hence the rumor that the body of the "Victory" was completely tinned.

In fact, everything is much more prosaic. The stamp equipment of those years was imperfect, so the surfaces of the parts turned out to be uneven. In addition, the then steels were not suitable for deep drawing, and the formation of wrinkles during stamping became business as usual. A welded but not yet painted body had to be prepared for a long time. The bulges were smoothed out with emery wheels, and the dents were filled with tin solder, especially a lot of which was near the windshield frame. Only and everything.

Well, you already know whether a tinned body is “eternal”: it is eternal until the first good hit with a sharp stone. And there are more than enough of them on our roads.

But with zinc, the picture is quite different. Here, in fact, we beat electrochemical corrosion with its own weapon. The protective metal (zinc) is to the left of iron in the voltage series. This means that in case of damage, it will not be steel that will be destroyed, but zinc. And only after all the zinc has corroded, the iron will begin to break down. But, fortunately, it corrodes very, very slowly, keeping the steel for many years.

a) Corrosion of tinned steel: when the coating is damaged, the steel is destroyed. b) Corrosion of galvanized steel: when the coating is damaged, the zinc is destroyed, protecting the steel from corrosion.

Coatings made from more active metals are called " safe", and from the less active ones -" dangerous". Safe coatings, in particular galvanizing, have long been successfully used as a way to protect car bodies from corrosion.

Why Zinc? After all, in addition to zinc, in the series of activity relative to iron, several more elements are more active. Here's the catch: the farther two metals are from each other in the activity series, the faster the destruction of the more active (less noble). And this, accordingly, reduces the durability of anti-corrosion protection. So for car bodies, where, in addition to good metal protection, it is important to achieve a long service life of this protection, galvanizing is the best fit. Moreover, zinc is available and inexpensive.

By the way, what will happen if you cover the body, for example, with gold? First, it will be oh so expensive! 🙂 But even if gold would become the cheapest metal, this cannot be done, since it will do our “piece of iron” a disservice.

After all, gold is very far from iron in the activity series (furthest), and at the slightest scratch, iron will soon turn into a pile of rust covered with a golden film.

The car body is exposed to both chemical and electrochemical corrosion. But the main role is still assigned to electrochemical processes.

After all, it’s a sin to hide, galvanic couples in a car body and a small truck: these are welds, and contacts of dissimilar metals, and foreign inclusions in sheet metal. The only thing missing is an electrolyte to “turn on” these galvanic cells.

And the electrolyte is also easy to find - at least the moisture contained in the atmosphere.

In addition, under real operating conditions, both types of corrosion are enhanced by many other factors. Let's talk about the main ones in more detail.

Factors Affecting Car Body Corrosion

Metal: chemical composition and structure

Of course, if car bodies were made of commercially pure iron, their corrosion resistance would be impeccable. Unfortunately, or perhaps fortunately, this is not possible. Firstly, such iron is too expensive for a car, and secondly (more importantly) it is not strong enough.

However, let's not talk about high ideals, but let's get back to what we have. Take, for example, steel grade 08KP, widely used in Russia for stamping body parts. When examined under a microscope, this steel is as follows: fine grains of pure iron mixed with grains of iron carbide and other inclusions.

As you may have guessed, such a structure gives rise to many microvoltaic cells, and as soon as an electrolyte appears in the system, corrosion will slowly begin its destructive activity.

Interestingly, the corrosion process of iron is accelerated by sulfur-containing impurities. Usually it gets into iron from coal during blast-furnace smelting from ores. By the way, in the distant past, not stone, but charcoal, which practically did not contain sulfur, was used for this purpose.

Including for this reason, some metal objects of antiquity during their centuries-old history practically did not suffer from corrosion. Take a look, for example, at this iron pillar, which is located in the courtyard of the Qutub Minar in Delhi.

It has been standing for 1600 (!) years, and at least something. Along with the low humidity in Delhi, one of the reasons for such a striking corrosion resistance Indian iron is, just the same, low content in sulfur metal.

So, in reasoning in the manner of “before, the metal was cleaner and the body did not rust for a long time,” there is still some truth, and a lot of it.

By the way, why don't stainless steels rust then? But because chromium and nickel, used as alloying components of these steels, stand next to iron in the electrochemical series of voltages. In addition, upon contact with an aggressive environment, they form a strong oxide film on the surface, which protects the steel from further corrosion.

Chrome nickel steel is the most typical stainless steel, but there are other grades besides it. stainless steels. For example, light stainless alloys may include aluminum or titanium. If you have been to the All-Russian Exhibition Center, you must have seen the obelisk "To the Conquerors of Space" in front of the entrance. It is lined with titanium alloy plates and there is not a single speck of rust on its shiny surface.

Factory body technology

The thickness of sheet steel, from which the body parts of a modern car are made, is usually less than 1 mm. And in some places of the body, this thickness is even less.

A feature of the process of stamping body panels, and indeed, any plastic deformation of the metal, is the occurrence of unwanted residual stresses during deformation. These stresses are negligible if the punching equipment is not worn and the strain rates are set correctly.

Otherwise, a kind of “time bomb” is laid in the body panel: the arrangement of atoms in crystal grains changes, so the metal in a state of mechanical stress corrodes more intensively than in a normal state. And, characteristically, the destruction of the metal occurs precisely in the deformed areas (bends, holes), which play the role of the anode.

In addition, when welding and assembling the body at the factory, a lot of cracks, overlaps and cavities are formed in it, in which dirt and moisture accumulate. Not to mention welds, forming with the base metal all the same galvanic pairs.

Influence of the environment during operation

The environment in which metal structures are operated, including cars, is becoming more and more aggressive every year. In recent decades, the content of sulfur dioxide, nitrogen oxides and carbon has increased in the atmosphere. This means that cars are no longer washed with water, but with acid rain.

Since we are talking about acid rain, let's return once again to the electrochemical series of voltages. The observant reader will notice that it also includes hydrogen. Reasonable question: why? But why: its position shows which metals displace hydrogen from acid solutions, and which do not. For example, iron is located to the left of hydrogen, which means it displaces it from acid solutions, while copper, which is to the right, is no longer capable of such a feat.

It follows that acid rain is dangerous for iron, but not for pure copper. But this cannot be said about bronze and other copper-based alloys: they contain aluminum, tin and other metals that are in the row to the left of hydrogen.

It has been noticed and proved that in the conditions of a big city, bodies live less. In this regard, the data of the Swedish Institute of Corrosion (SHIK) are indicative, which found that:

in countryside Sweden, the rate of destruction of steel is 8 microns per year, zinc - 0.8 microns per year;
for the city, these figures are 30 and 5 microns per year, respectively.

The climatic conditions in which the car is operated are also important. So, in a marine climate, corrosion is activated approximately twice.

Humidity and temperature

How great is the effect of moisture on corrosion, we can understand the example of the previously mentioned iron column in Delhi (remember the dryness of the air as one of the reasons for its corrosion resistance).

Rumor has it that a foreigner decided to reveal the secret of this stainless iron and somehow broke off a small piece from the column. What was his surprise when, on the ship on the way from India, this piece became covered with rust. It turns out that in the humid sea air, stainless Indian iron turned out to be not so stainless after all. In addition, a similar column from Konarak, located near the sea, was hit very hard by corrosion.

The corrosion rate at relative humidity up to 65% is relatively low, but when the humidity rises above the specified value, corrosion accelerates sharply, since at such humidity a layer of moisture forms on the metal surface. And the longer the surface remains wet, the faster corrosion spreads.

That is why the main centers of corrosion are always found in the hidden cavities of the body: they dry much more slowly than open parts. As a result, stagnant zones form in them, a real paradise for corrosion.

By the way, the use of chemical reagents to combat ice corrosion is also on hand. Mixed with melted snow and ice, anti-icing salts form a very strong electrolyte, capable of penetrating anywhere, including hidden cavities.

With regard to temperature, we already know that increasing it activates corrosion. For this reason, there will always be more traces of corrosion near the exhaust system.

Air access

Interesting all-??? thing this corrosion. As interesting as it is insidious. For example, do not be surprised that a shiny steel cable, seemingly completely untouched by corrosion, may turn out to be rusted inside. This is due to the uneven access of air: in those places where it is difficult, the threat of corrosion is greater. In corrosion theory, this phenomenon is called differential aeration.

The principle of differential aeration: uneven access of air to different areas metal surface leads to the formation of a galvanic cell. In this case, the area intensively supplied with oxygen remains unharmed, and the area poorly supplied with oxygen corrodes.

A striking example: a drop of water that has fallen on the surface of a metal. The area under the drop and therefore less supplied with oxygen plays the role of an anode. The metal in this area is oxidized, and the role of the cathode is played by the edges of the drop, which are more accessible to the influence of oxygen. As a result, iron hydroxide, a product of the interaction of iron, oxygen, and moisture, begins to precipitate at the edges of the drop.

By the way, iron hydroxide (Fe 2 O 3 nH 2 O) is what we call rust. A rust surface, unlike the patina on a copper surface or an aluminum oxide film, does not protect the iron from further corrosion. Initially, rust has a gel structure, but then it gradually crystallizes.

Crystallization begins inside the rust layer, while outer shell The gel, which is very friable and brittle when dry, flakes off and the next layer of iron is exposed. And so on until all the iron is destroyed or the system runs out of oxygen and water.

Returning to the principle of differential aeration, one can imagine how many opportunities exist for the development of corrosion in hidden, poorly ventilated areas of the body.

Rust ... everything!

As they say, statistics know everything. Earlier, we mentioned such a well-known center for the fight against corrosion as the Swedish Corrosion Institute (SHIK) - one of the most authoritative organizations in this field.

Once every few years, scientists of the institute conduct an interesting study: they take the bodies of well-worked cars, cut out the “fragments” most beloved by corrosion from them (sections of thresholds, wheel arches, door edges, etc.) and evaluate the degree of their corrosion damage.

It is important to note that among the studied bodies there are both protected (galvanized and / or anticorrosive) and bodies without any additional anticorrosion protection (simply painted parts).

So, SHIK claims that best protection car body is just a combination of "zinc plus anticorrosive". But all other options, including “just galvanizing” or “just anticorrosive”, according to scientists, are bad.

Galvanization is not a panacea

Proponents of the refusal of additional anti-corrosion treatment often refer to factory galvanization: with it, they say, no corrosion threatens the car. But, as Swedish scientists have shown, this is not entirely true.

Indeed, zinc can serve as an independent protection, but only on smooth and smooth surfaces, moreover, not subject to mechanical attacks. And on the edges, edges, joints, as well as places regularly exposed to "shelling" with sand and stones, galvanizing gives in to corrosion.

In addition, not all cars have fully galvanized bodies. Most often, only a few panels are coated with zinc.

Well, we must not forget that zinc, although it protects steel, is inevitably consumed in the process of protection. Therefore, the thickness of the zinc "shield" will gradually decrease over time.

So the legends about the longevity of galvanized bodies are true only in cases where zinc becomes part of the overall barrier, in addition to regular additional anti-corrosion treatment of the body.

It's time to finish, but the topic of corrosion is far from exhausted. We will continue to talk about the fight against it in the following articles under the heading "Anti-corrosion protection".

Ph.D. V.B. Kosachev, A.P. Gulidov, NPK "Vector", Moscow

The article provides information on the corrosion of metals, which can be useful for a wide range of engineering and technical workers associated by the nature of their activity with the implementation of practical measures to protect the equipment of heat supply organizations from corrosion.

Corrosion and its social significance

Any corrosion process leads to changes in the properties of structural materials. The result of the process is a "corrosion effect" that worsens functional characteristics metal equipment, environment and technical systems, regarded as a "damage effect" or "corrosion damage".

Obviously, the economic losses associated with the corrosion of metals are determined not so much by the cost of the corroded metal, but by the cost of repair work, losses due to the temporary cessation of the functioning of engineering systems, the cost of preventing accidents, in some cases absolutely unacceptable from the point of view of environmental safety. Estimates of the costs associated with corrosion (according to foreign sources) lead to the conclusion that the total annual cost of combating the consequences of corrosion is 1.5-2% of the gross national product. Some of these costs are unavoidable; it would be unrealistic to completely eliminate all corrosion damage. However, corrosion losses can be significantly reduced by best use in practice of the accumulated knowledge about corrosion processes and methods of protection against corrosion, which anti-corrosion services have at the moment.

Corrosion processes

The concept of "corrosion of metals" includes a large group chemical processes leading to the destruction of the metal. These processes differ sharply from each other in external manifestations, in the conditions and environments in which they occur, as well as in the properties of the reacting metals and the resulting reaction products. However, there is every reason to combine them, because despite the sharp differences, all these processes have not only a common result - the destruction of the metal, but also a single chemical essence - the oxidation of the metal.

The cause of corrosion is the thermodynamic instability of metals, as a result of which most of them are found in nature in an oxidized state (oxides, sulfides, silicates, aluminates, sulfates, etc.). Thus, corrosion can be defined as a spontaneous process that occurs when a metal interacts with the environment, accompanied by a decrease in the Gibbs free energy and destruction of the metal. Corrosion occurs at the interface between two phases "metal - environment", i.e. it is a heterogeneous multi-stage process and consists of at least three main stages that are repeated many times:

1 supply of reacting substances (including a corrosive agent) to the interface;

2 the actual reaction of the interaction of metal with a corrosive environment, the result of which is the transition of a certain amount of metal into an oxidized form with the formation of corrosion products, and a corrosive agent into a reduced form;

3 removal of corrosion products from the reaction zone.

Mechanisms of corrosion processes

According to the mechanism of the metal oxidation process, chemical and electrochemical corrosion are distinguished.

Chemical corrosion . This type of corrosion includes such processes of metal oxidation and reduction of a corrosive agent, in which the transfer of metal electrons is carried out directly to the atoms or ions of the oxidizing agent (corrosive agent), which is most often atmospheric oxygen.

2Me + O 2 --> 2MeO (1)

In the practice of heat supply, the most common and practically important type of chemical corrosion is gas corrosion - corrosion of metals in dry gases (air, fuel combustion products) at high temperatures. The main factors affecting the rate of gas corrosion are:

3 nature of the metal (alloy);

4 composition of the gaseous medium;

5 mechanical properties of the resulting corrosion products (oxide films);

6 temperature.

So, for iron, the main component of carbon steels used for the manufacture of screens of the furnace space and the convective part of hot water boilers, the dependence of the gas corrosion rate on temperature is close to exponential, Fig. 1. Temperature affects the composition of oxide films formed on steel and the laws of their growth, Table. 1. Their mechanical properties and, accordingly, depend on the composition of oxide films. protective properties, since a dense continuous oxide film can protect the metal from further oxidation. The partial pressure of oxygen also affects the rate of gas corrosion. When a number of metals are oxidized at a constant and sufficiently high temperature with an increase in the partial pressure of oxygen (Po 2), the oxidation rate first increases sharply, and then, when a certain critical value (P o 2) is reached, it sharply decreases and remains quite low over a wide pressure range, Figure 2. The heating regime has a great influence on the rate of oxidation of metals. Temperature fluctuations (variable heating and cooling), even in small intervals, cause the destruction of oxide films due to the occurrence of large internal stresses, as a result of which the rate of metal oxidation increases sharply.

To protect against gas corrosion, heat-resistant alloying of steels is used, protective (reducing) atmospheres are created, thermal diffusion (based on aluminum, silicon and chromium) and sprayed (based on oxides of aluminum, magnesium, zirconium) protective coatings are used.

electrochemical corrosion. This type of corrosion is the most common and includes those cases when the processes of metal oxidation and reduction of the oxidizing component proceed separately in a liquid electrolyte medium, i.e. in a medium that conducts electricity. Such media can be: natural water, aqueous solutions of salts, acids, alkalis, as well as air, soil and heat-insulating structures containing electrolyte (moisture) in a certain amount. Thus, the process of electrochemical corrosion is a combination of two coupled reactions occurring:

anodic (oxidation) Me → Me z+ + ze - (2),

and cathodic (recovery) D + ze - → (Dze -) (3),

where D is a depolarizer (oxidizing agent) that attaches metal electrons to itself. The following can act as a depolarizer: oxygen dissolved in the electrolyte, hydrogen ions (H +) and some metals. General scheme electrochemical corrosion process of the metal is shown in Figure 3, and a particular case of iron rusting is described by the reaction:

2Fe + 2H 2 O + O 2 → 2Fe 2+ + 4 OH - (4).

The appearance of cathode-anode galvanic cells on carbon steels (the main structural material of pipelines) in contact with electrolytes occurs mainly due to the differentiation of the steel surface into areas with different electrode potentials (the theory of local corrosion elements). The reasons for differentiation can be different:

7 heterogeneity of the metal structure (in carbon steels there are phases - ferrite and cementite, structural components - pearlite, cementite and ferrite, which have different electrode potentials);

8 presence of oxide films, impurities, non-metallic inclusions, etc. on the steel surface;

9 uneven distribution of the oxidizer at the "metal-electrolyte" interface, for example, different humidity and aeration in different parts of the metal surface;

10 uneven temperature distribution;

11 dissimilar metal contact.

Summary data for N.D. Tomashov about galvanic corrosion vapors (Table 2), the formation of which is possible on existing pipelines of heating networks in the presence of moisture or its traces, allow us to state that all cases of rusting of pipelines and metal structures of heating networks occur as a result of electrochemical corrosion.

The main types of electrochemical corrosion

and the nature of corrosion damage to the metal

Depending on the conditions of the process of electrochemical corrosion (type of corrosive medium), atmospheric, soil, microbiological and liquid (acid, alkali, salt, marine and freshwater) corrosion are distinguished. Depending on the operating conditions, any of the above types of corrosion can occur when such operational factors as friction, cavitation, stresses in the metal, and external sources of direct and alternating current are applied.

Table 3 presents possible types electrochemical corrosion of pipelines and capacitive equipment of heat supply enterprises, as well as unfavorable operational factors that contribute to an increase in the rate of corrosion processes. Figures 5-9 show the most typical corrosion damage to structural carbon steels caused by various types electrochemical corrosion.

Methods of protection against electrochemical corrosion

Protection against electrochemical corrosion is a set of measures aimed at preventing and inhibiting corrosion processes, maintaining and maintaining the operability of equipment and structures during the required period of operation.

Methods for protecting metal structures from corrosion are based on targeted action, leading to a complete or partial decrease in the activity of factors contributing to the development of corrosion processes. Corrosion protection methods can be conditionally divided into methods of influencing the metal and methods of influencing the environment, as well as combined methods. The classification of methods is shown in Figure 10.

Among the methods of influencing metal, in the practice of protecting equipment and pipelines of heat supply organizations, protective and insulating coatings of permanent action (polymer, glass enamel, metal zinc and aluminum) are most widely used. Impact on a corrosive environment (water) is used to protect capacitive equipment and pipelines from internal corrosion by its inhibition and deaeration.

It is possible to significantly reduce the rate of corrosion processes in pipelines by applying electrochemical protection. With this type of protection, the electrochemical potential of the pipeline is shifted to the required (protective) potential range (polarization of the structure) by connecting it to an external current source - a cathodic protection station or a protector.

It should be noted that the protection option for a particular object should be selected based on an analysis of its operating conditions. At the same time, the requirements for indicators characterizing required quality object work, technological features application of the chosen method (methods) of protection and the economic effect achieved at the same time.

The complication of the operating conditions of equipment and, first of all, heat pipelines, the appearance of specific air and water pollution require constant improvement of corrosion protection methods. Based on the analysis of generalized information on corrosion damage to various equipment of heat supply enterprises, it can be concluded that the main directions in improving corrosion protection methods in heat supply are: the introduction of anti-corrosion and waterproofing coatings for the outer surfaces of pipelines with improved consumer properties; application for hot water supply of pipes with glass-enamel and polymeric internal coatings; the use of combined protection options with the sharing of installations electrochemical protection and protective coatings.

Table 1

Table 3

No. p \ p	Type of electrochemical corrosion	Pipeline laying method (type of equipment)	Additional corrosion factors
1.	atmospheric corrosion	External surfaces of pipelines of ground and channel laying (at the level of flooding and siltation of the channel, not reaching the insulating structures). Surfaces of various metal structures and equipment not in contact with water and soil.	Internal stresses in the metal of the pipeline and metal structures, shock-mechanical impact of a drop from the ceiling. Characteristic corrosion damages: uniform corrosion, in places of a drop corrosion by spots is possible.
2.	Underground corrosion	External surfaces of pipelines of channelless laying (in case of violation of the integrity of the insulation), channel laying (periodic flooding and silting of the channel, accompanied by moistening of the thermal insulation).	Internal stresses in the metal, corrosion by external direct and alternating current, drop impact. Characteristic corrosion damage: uneven corrosion, corrosion by spots, when exposed to stray currents, through damage to the pipeline wall is possible.
3.	underwater corrosion	External surfaces of pipelines of channel laying. (Permanent flooding of the channel in the absence of thermal insulation on the pipeline). Internal surfaces of pipelines and chemical water treatment equipment (deaerators, filters, etc.)	Internal stresses in metal, corrosion by external direct and alternating current. If the pipeline is not completely submerged, corrosion along the waterline is possible. Characteristic corrosion damage: uneven corrosion, when exposed to stray currents, through damage to the pipeline wall, ulcerative lesions in the waterline area are possible. On pipelines of hot water supply, the process of microbiological corrosion by iron bacteria is possible. Characteristic corrosion damage: pitting corrosion (for internal surfaces of pipelines), pitting corrosion, uneven corrosion.