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What environmental components cause chemical corrosion. Metal corrosion and its types

The phrase “metal corrosion” contains much more than the name of a popular rock band. Corrosion irreversibly destroys metal, turning it into dust: of all the iron produced in the world, 10% will be completely destroyed in the same year. The situation with Russian metal looks something like this: all the metal smelted in a 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 amounts to ten figures. So what causes corrosion processes in metals and how to deal with them?

What is metal corrosion

Destruction of metals as a result of electrochemical (dissolution in a moisture-containing air or aqueous medium - 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 grain boundaries (intercrystalline corrosion).

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 environments 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 susceptible to chemical corrosion. Active non-ferrous metals (for example, aluminum) under the influence of corrosion are covered with an oxide film, which 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 does not protect the metal from corrosion 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 another type of corrosion: some elements of the alloys are not oxidized, but are reduced (for example, in a combination of high temperature and pressure in steels, carbides are reduced by hydrogen), and the alloys completely lose the necessary characteristics.

Electrochemical corrosion

The process of electrochemical corrosion does not necessarily require immersing the metal in an electrolyte - a thin electrolytic film on its surface is sufficient (often electrolytic solutions permeate the environment surrounding the metal (concrete, soil, etc.)). The most common cause of electrochemical corrosion is the widespread use of household and industrial salts (sodium and potassium chlorides) to remove ice and snow on roads in winter period- cars and underground communications(according to statistics, annual losses in the USA from the use of salts in winter amount to 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. Electro chemical corrosion can cause 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 metal structures back into water or soil, metal destruction occurs. Stray currents occur especially often in places where ground electric transport moves (for example, trams and electric railway locomotives). In just one year, stray currents with a force of 1A are capable of dissolving 9.1 kg of iron, 10.7 kg of zinc, and 33.4 kg of lead.

Other causes of metal corrosion

The development of corrosion processes is facilitated by radiation and waste products of microorganisms and bacteria. Corrosion caused by marine microorganisms damages bottoms sea vessels, and corrosion processes caused by bacteria even have their own name - biocorrosion.

The combination of the effects of mechanical stress and external environment accelerates the corrosion of metals many times over - their thermal stability decreases, surface oxide films are damaged, and in those places where inhomogeneities and cracks appear, electrochemical corrosion is activated.

Measures to protect metals from corrosion

Inevitable consequences technical progress is the pollution of our environment - a process that accelerates the corrosion of metals, since the external environment is increasingly aggressive towards them. There are no ways to completely eliminate the corrosive 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 metal product; increase metal resistance to corrosion; eliminate interaction between the metal and substances from the external environment that exhibit aggression.

Over thousands of years, mankind has tried many ways to protect metal products from chemical corrosion, some of them are still used today: coating with fat or oil, other metals that corrode to a lesser extent (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 enamels), varnishes (synthetic, bitumen and tar) and polymers form a protective film on the surface of metals, excluding (while intact) contact with the external environment and moisture.

The advantage of using paints and varnishes is that these protective coatings can be applied directly on the installation and construction site. The 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 compliance with 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. It is best to apply paints and varnishes in several layers - their quantity will provide better protection against weathering on the metal surface.

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 made from a mixture of cement and perchlorovinyl, cement and polystyrene.

Protection of iron from corrosion by coatings of 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 greater electronegativity with respect to iron, the second has greater electropositivity. The most widespread in our everyday life are metal coatings of iron with tin (tinplate, 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 supplied, galvanized pipes wear out faster than non-galvanized ones). Despite the effectiveness of galvanizing, it does not provide ideal protection - the zinc coating often contains cracks, the elimination of which requires preliminary nickel plating of metal surfaces (nickel plating). Zinc coatings do not allow paint and varnish materials to be applied to them - there is no stable coating.

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

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

Measures to counteract electrochemical corrosion

To reduce it, it is necessary to reduce the corrosive activity of the environment by introducing non-metallic inhibitors and reducing the number of components that can start an electrochemical reaction. This method will reduce the acidity of soils and aqueous solutions in contact with metals. To reduce corrosion of iron (its alloys), as well as brass, copper, lead and zinc, it is necessary to remove carbon dioxide and oxygen from aqueous solutions. The electrical power industry removes chlorides from water that can affect localized corrosion. By liming the soil you can reduce its acidity.

Stray current protection

It is possible to reduce electrical corrosion of underground communications and buried metal structures by following several rules:

the section of the structure serving 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 rail roads along which electric vehicles travel, minimizing the number of their intersections;
the use of electrically insulating pipe supports to increase the transition resistance between the soil and pipelines;
at inputs to objects (potential sources of stray currents), it is necessary to install insulating flanges;
install current-conducting longitudinal jumpers on flange fittings and gland expansion joints to increase 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 areas.

Protection of metal objects equipped with insulation, as well as small steel structures, is carried out using a protector that functions 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, breaking down and preserving the main structure. One magnesium anode, for example, protects 8 km of pipeline.

Rustam Abdyuzhanov, specially for rmnt.ru

physical-chemical or chemical interaction between a metal (alloy) and the environment, leading to deterioration of the functional properties of the metal (alloy), environment or technical system incorporating them.

The word corrosion comes from the Latin “corrodo” “to gnaw” (Late Latin “corrosio” means “corrosion”).

Corrosion is caused by a chemical reaction between metal and substances environment flowing at the interface between metal and medium. Most often, this is the oxidation of the metal, for example, by atmospheric oxygen or acids contained in solutions with which the metal is in contact. Metals located in the voltage series (activity series) to the left of hydrogen, including iron, are especially susceptible to this.

As a result of corrosion, iron rusts. This process is very complex and includes several stages. It can be described by the summary equation:

Fe + 6 H 2 O (moisture) + 3 O 2 (air) = 4 Fe (OH ) 3

Iron hydroxide(

III ) is very unstable, quickly loses water and turns into iron oxide ( III ). This compound does not protect the iron surface from further oxidation. As a result, the iron object can be completely destroyed.

Many metals, including quite active ones (for example, aluminum), when corroded, become covered with a dense, well-bonded oxide film, which does not allow oxidizing agents to penetrate into deeper layers and therefore protects the metal from corrosion. When this film is removed, the metal begins to interact with moisture and oxygen in the air.

Aluminum under normal conditions is resistant to air and water, even boiling water, but if mercury is applied to the surface of aluminum, the resulting amalgam destroys the oxide film pushes it from the surface, and the metal quickly turns into white flakes of aluminum metahydroxide:

4Al + 2H 2 O + 3O 2 = 4AlO(OH)Amalgamated aluminum reacts with water to release hydrogen: Al + 4 H 2 O = 2 AlO (OH) + 3 H 2

Some rather inactive metals are also susceptible to corrosion. In humid air, the surface of copper becomes covered with a greenish coating (patina) as a result of the formation of a mixture of basic salts.

Sometimes when metals corrode, it is not oxidation that occurs, but the reduction of some elements contained in the alloys. For example, at high pressures and temperatures, carbides contained in steels are reduced by hydrogen.

The destruction of metals in the presence of hydrogen was discovered in the mid-nineteenth century. The French engineer Sainte-Claire Deville studied the causes of unexpected ruptures of gun barrels. During their chemical analysis, he found hydrogen in the metal. Deville decided that hydrogen saturation was the reason for the sudden drop in steel strength.

Hydrogen caused a lot of trouble to the designers of equipment for one of the most important industrial chemical processes - ammonia synthesis. The first devices for this synthesis lasted only tens of hours, and then shattered into small parts. Only adding titanium, vanadium or molybdenum to steel helped solve this problem.

Corrosion of metals can also include their dissolution in liquid molten metals (sodium, lead, bismuth), which are used, in particular, as coolants in nuclear reactors.

In terms of stoichiometry, the reactions that describe the corrosion of metals are quite simple, but in terms of their mechanism they belong to complex heterogeneous processes. The corrosion mechanism is determined primarily by the type of aggressive environment.

When a metal material comes into contact with a chemically active gas, a film of reaction products appears on its surface. It prevents further contact between metal and gas. If counter diffusion of reacting substances occurs through this film, then the reaction continues. The process is facilitated at high temperatures. During corrosion, the product film continuously thickens and the metal is destroyed. Metallurgy and other industries that use high temperatures suffer heavy losses from gas corrosion.

Corrosion is most common in electrolyte environments. In some technological processes metals come into contact with molten electrolytes. However, most often corrosion occurs in electrolyte solutions. The metal does not have to be completely immersed in the liquid. Electrolyte solutions can be present in the form of a thin film on the surface of the metal. They often permeate the environment surrounding the metal (soil, concrete, etc.).

During the construction of the metro bridge and the Leninskie Gory station in Moscow, they added large number sodium chloride to prevent freezing of concrete that has not yet set. The station was built in as soon as possible(in just 15 months) and opened on January 12, 1959. However, the presence of sodium chloride in the concrete caused the destruction of the steel reinforcement. 60% of reinforced concrete structures were subject to corrosion, so the station was closed for reconstruction , lasting almost 10 years. Only on January 14, 2002, the metro bridge and the station, called Vorobyovy Gory, were re-opened.

Using salts (usually sodium or calcium chloride) to remove snow and ice from roads and sidewalks also causes metals to break down faster. Suffer greatly vehicles and underground communications. It is estimated that in the United States alone, the use of salts to combat snowfall and ice leads to losses of about $2 billion per year due to engine corrosion and $0.5 billion in additional repairs of roads, underground highways and bridges.

In electrolyte environments, corrosion is caused not only by the action of oxygen, water or acids on metals, but also by electrochemical processes. Already at the beginning of the 19th century. Electrochemical corrosion was studied by English scientists Humphry Davy and Michael Faraday. The first theory of electrochemical corrosion was put forward in 1830 by the Swiss scientist De la Rive. It explained the occurrence of corrosion at the point of contact of two different metals.

Electrochemical corrosion leads to the rapid destruction of more active metals, which in various mechanisms and devices come into contact with less active metals located to the right in the electrochemical voltage series. Use of copper or brass parts in iron or aluminum structures, which operate in seawater, significantly increases corrosion. There are known cases of destruction and sinking of ships whose iron plating was fastened with copper rivets.

Separately, aluminum and titanium are resistant to seawater, but if they come into contact in one product, for example, in a housing for underwater photographic equipment, the aluminum is very quickly destroyed and the housing leaks.

Electrochemical processes can also occur in a homogeneous metal. They are activated if there are differences in the composition of the metal grain in the bulk and at the boundary, inhomogeneous mechanical stress, microimpurities, etc. Many of our compatriots, including Vladimir Aleksandrovich Kistyakovsky (1865–1952) and Alexander Naumovich Frumkin (1895–1976), took part in the development of the general theory of electrochemical corrosion of metallic materials.

One of the reasons for the occurrence of electrochemical corrosion is stray currents, which appear due to the leakage of part of the current from electrical circuits into the soil or aqueous solutions, where they fall on metal structures. Where the current exits these structures, the dissolution of the metal begins again into the soil or water. Such zones of destruction of metals under the influence of stray currents are especially often observed in areas of ground electric transport (tram lines, railway transport on electric traction). These currents can reach several amperes, which leads to large corrosion damage. For example, the passage of a current of 1 A for one year will cause the dissolution of 9.1 kg of iron, 10.7 kg of zinc, 33.4 kg of lead.

Corrosion can also occur under the influence of radiation, as well as waste products of bacteria and other organisms. The development of bacteria on the surface of metal structures is associated with the phenomenon of biocorrosion. The fouling of the underwater part of ships with small marine organisms also affects corrosion processes.

When the metal is simultaneously exposed to the external environment and mechanical stress, all corrosion processes are activated, since this reduces the thermal stability of the metal, destroys oxide films on the metal surface, and intensifies electrochemical processes in places where cracks and inhomogeneities appear.

Corrosion leads to huge irreversible losses of metals; about 10% of the produced iron is completely destroyed every year. According to the Institute of Physical Chemistry of the Russian Academy of Sciences, every sixth blast furnace in Russia works in vain all smelted metal turns into rust. The destruction of metal structures, agricultural and transport vehicles, and industrial equipment causes downtime, accidents, and deterioration in product quality. Taking into account possible corrosion leads to increased metal costs in the manufacture of high-pressure apparatus, steam boilers, metal containers for toxic and radioactive substances, etc. This increases overall corrosion losses. Considerable amounts of money have to be spent on anti-corrosion protection. The ratio of direct losses, indirect losses and costs for corrosion protection is estimated as (34):1:1. In industrialized countries, damage from corrosion reaches 4% of national income. In our country it amounts to billions of rubles a year.

Corrosion problems are constantly getting worse due to the continuous growth in metal production and the tightening of their operating conditions. The environment in which metal structures are used is becoming more and more aggressive, including due to its pollution. Metal products used in technology operate under conditions of increasingly high temperatures and pressures, powerful flows of gases and liquids. Therefore, the issues of protecting metal materials from corrosion are becoming increasingly relevant. It is impossible to completely prevent metal corrosion, so the only way to combat it is to find ways to slow it down.

The problem of protecting metals from corrosion arose almost at the very beginning of their use. People tried to protect metals from atmospheric influences with the help of fat, oils, and later by coating with other metals and, above all, low-melting tin (tinning). In the works of the ancient Greek historian Herodotus (5th century BC) and the ancient Roman scientist Pliny the Elder (1st century BC) there are already references to the use of tin to protect iron from rusting. Currently, the fight against corrosion is carried out in several directions at once: they are trying to change the environment in which a metal product operates, influence the corrosion resistance of the material itself, and prevent contact between the metal and aggressive substances of the external environment.

Corrosion can be completely prevented only in an inert environment, for example, in an argon atmosphere, but in the vast majority of cases it is impossible to actually create such an environment during the operation of structures and mechanisms. In practice, to reduce the corrosive activity of a medium, they try to remove the most reactive components from it, for example, they reduce the acidity of aqueous solutions and soils with which metals may come into contact. One of the methods of combating corrosion of iron and its alloys, copper, brass, zinc, and lead is the removal of oxygen and carbon dioxide from aqueous solutions. In the energy sector and some branches of technology, water is also freed from chlorides, which stimulate local corrosion. To reduce soil acidity, liming is carried out.

The aggressiveness of the atmosphere strongly depends on humidity. For any metal there is a certain critical relative humidity, below which it is not subject to atmospheric corrosion. For iron, copper, nickel, zinc it is 50-70%. Sometimes, to preserve items of historical value, their temperature is artificially maintained above the dew point. In closed spaces (for example, in packaging boxes), humidity is reduced using silica gel or other adsorbents. The aggressiveness of the industrial atmosphere is determined mainly by fuel combustion products ( cm. ENVIRONMENTAL POLLUTION). The prevention of acid rain and the elimination of harmful gas emissions help reduce losses from corrosion.

The destruction of metals in aqueous environments can be slowed down using corrosion inhibitors, which are added in small quantities (usually less than 1%) to aqueous solutions. They promote passivation of the metal surface, that is, the formation of a thin and dense film of oxides or other poorly soluble compounds, which prevents the destruction of the main substance. For this purpose, some sodium salts (carbonate, silicate, borate) and other compounds are used. If razor blades are immersed in a solution of potassium chromate, they will last much longer. Organic inhibitors are often used, which are more effective than inorganic ones.

One method of corrosion protection is based on the development of new materials that have higher corrosion resistance. The search for substitutes for corrosive metals is ongoing. Plastics, ceramics, glass, rubber, asbestos and concrete are more resistant to environmental influences, but in many other properties they are inferior to metals, which still serve as the main structural materials.

Noble metals are practically resistant to corrosion, but for wide application They are too expensive, so they are used only in the most critical parts, such as non-corrosive electrical contacts. Nickel, aluminum, copper, titanium and alloys based on them have high corrosion resistance. Their production is growing quite quickly, but even now the most accessible and widely used metal remains quickly rusting iron. Alloying is often used to impart corrosion resistance to iron-based alloys. This is how stainless steel is obtained, which, in addition to iron, contains chromium and nickel. The most common stainless steel in our time, grade 188 (18% chromium and 8% nickel), appeared in 1923. It is completely resistant to moisture and oxygen. First tons stainless steel in our country were smelted in 1924 in Zlatoust. Nowadays, many grades of such steels have been developed, which, in addition to chromium and nickel, contain manganese, molybdenum, tungsten and other chemical elements. Surface alloying of inexpensive iron alloys with zinc, aluminum, and chromium is often used.

To resist atmospheric corrosion, thin coatings of other metals that are more resistant to moisture and atmospheric oxygen are applied to steel products. Chromium and nickel platings are often used. Because chrome platings often contain cracks, they are usually applied over less decorative nickel platings. To protect tin cans from corrosion in organic acids contained in food products, a significant amount of tin is consumed. For a long time, cadmium was used to coat kitchen utensils, but it is now known that this metal is hazardous to health and cadmium coatings are used only in technology.

To slow down corrosion, varnishes and paints, mineral oils and lubricants are applied to the metal surface. Underground structures are covered with a thick layer of bitumen or polyethylene. Internal surfaces steel pipes and tanks are protected with cheap cement coatings.

To make the paintwork more reliable, the metal surface is thoroughly cleaned of dirt and corrosion products and subjected to special treatment. For steel products, so-called rust converters containing orthophosphoric acid (H 3 PO 4) and its salts are used. They dissolve residual oxides and form a dense and durable film of phosphates, which can protect the surface of the product for some time. Then the metal is coated with a primer layer, which should adhere well to the surface and have protective properties (usually red lead or zinc chromate is used). Only after this can varnish or paint be applied.

One of the most effective methods The fight against corrosion is electrochemical protection. To protect drilling platforms, welded metal bases, and underground pipelines, they are connected as a cathode to an external current source. Auxiliary inert electrodes are used as an anode.

Another version of such protection is used for relatively small steel structures or additionally insulated metal objects (for example, pipelines). In this case, a protector is used - an anode made of a relatively active metal (usually magnesium, zinc, aluminum and their alloys), which gradually collapses, protecting the main object. With the help of one magnesium anode, up to 8 km of pipeline is protected. Tread protection is widespread; for example, in the USA, about 11.5 thousand tons of aluminum are spent annually on the production of protectors.

Protection of one metal by another, more active metal located in the voltage series to the left is effective without imposing a potential difference. The more active metal (for example, zinc on the surface of iron) protects the less active metal from destruction.

Electrochemical methods of combating corrosion also include protection against destruction of structures by stray currents. One of the ways to eliminate such corrosion is to connect a metal conductor to the section of the structure from which the stray current flows with the rail along which the tram or electric train moves.

Elena Savinkina

LITERATURE Fremantle M. Chemistry in action. In 2 parts. M., Mir, 1991
Stepin B.D., Alikberova L.Yu. Chemistry book for home reading. M., Chemistry, 1994

Metal corrosion or alloy occurs, as a rule, at the phase interface, i.e. at the contact boundary solid with gas or liquid.

Corrosion processes are divided into the following types: according to the mechanism of interaction of the metal with the environment; by type of corrosive environment; by type of corrosion damage to the surface; by volume of destroyed metal; by the nature of additional influences to which the metal is exposed simultaneously with the action of a corrosive environment.

According to the mechanism of interaction of the metal with the environment, chemical and electrochemical corrosion are distinguished.

Corrosion occurring under the influence of the vital activity of microorganisms is classified as biological corrosion, and corrosion occurring under the influence of radioactive radiation is referred to as radiation corrosion.

Based on the type of corrosive environment involved in the corrosion destruction of a metal or alloy, they distinguish between corrosion in non-electrolyte liquids, corrosion in solutions and melts of electrolytes, gas, atmospheric, underground (soil) corrosion, stray current corrosion, etc.

Depending on the nature of the change in the surface of a metal or alloy or the degree of change in their physical and mechanical properties, during the corrosion process, regardless of the properties of the environment, corrosion damage can be of several types.

1. If corrosion covers the entire surface of the metal, then this type of destruction is called - complete corrosion. Continuous corrosion refers to the destruction of metals and alloys under the influence of acids, alkalis, and the atmosphere. Continuous corrosion can be uniform, that is, the destruction of the metal occurs at the same rate over the entire surface, and uneven, when the corrosion rate in individual areas of the surface is not the same. An example of uniform corrosion is corrosion in the interaction of copper with nitric acid, iron with hydrochloric acid, zinc with sulfuric acid, aluminum with alkali solutions. In these cases, corrosion products do not remain on the metal surface. Iron pipes also corrode in the same way. outdoors. This is easy to see if you remove the layer of rust; underneath, a rough metal surface is found, evenly distributed throughout the pipe.

2. Alloys of some metals are susceptible to - selective corrosion, when one of the elements or one of the structures of the alloy is destroyed, while the rest remain practically unchanged. When brass comes into contact with sulfuric acid, component-selective corrosion occurs - corrosion of zinc, and the alloy is enriched in copper. Such destruction is easy to notice, since reddening of the surface of the product occurs due to an increase in the concentration of copper in the alloy. With structural-selective corrosion, predominantly destruction of any one structure of the alloy occurs; for example, when steel comes into contact with acids, ferrite is destroyed, but iron carbide remains unchanged. Cast iron is especially susceptible to this type of corrosion.

3. For local corrosion On the surface of the metal, lesions are found in the form of individual spots, ulcers, and dots. Depending on the nature of the lesions, local corrosion occurs in the form of spots, that is, lesions that are not very deep into the thickness of the metal; ulcers - lesions deeply deepened into the thickness of the metal; points, sometimes barely visible to the eye, but penetrating deeply into the metal. Corrosion in the form of pits and spots is very dangerous for such structures where it is important to maintain conditions of tightness and impermeability (tanks, apparatus, pipelines used in the chemical industry).

4. Subsurface corrosion starts from the metal surface in cases where protective coating(films, oxides, etc.) destroyed in certain areas. In this case, destruction occurs predominantly under the coating, and corrosion products are concentrated inside the metal. Subsurface corrosion often causes swelling and delamination of the metal. It can only be determined under a microscope.

5. Crevice corrosion- destruction of metal under gaskets, in gaps, threaded fasteners, riveted joints, etc. It often develops in the area of the structure located in the gap (crack).

6. Intergranular corrosion- destruction of the metal along the boundaries of crystallites (grains) with loss of its mechanical strength; the appearance of the metal does not change, but it is easily destroyed into individual crystals under mechanical influence. This is explained by the formation of loose, low-strength corrosion products between the grains of the metal or alloy. Chromium and chromium-nickel steels, nickel and aluminum alloys are susceptible to this type of corrosion. To avoid intergranular corrosion, in recent years stainless steels with a reduced carbon content have been widely used or carbide-forming agents - titanium, tantalum, niobium (in 5-8 times the carbon content) are introduced into their composition.

When a metal or alloy is simultaneously exposed to highly aggressive environments and mechanical tensile stresses, corrosion cracking, or transgranular corrosion, is possible. In this case, destruction occurs not only along the boundaries of the crystallites, but the metal crystallite itself is divided into parts. This is a very dangerous type of corrosion, especially for structures bearing mechanical loads (bridges, axles, cables, springs, autoclaves, steam boilers, internal combustion engines, water and steam turbines etc.).

Corrosion cracking depends on the design of the equipment, the nature of the aggressive environment, the structure and structure of the metal or alloy, temperature, etc. For example, corrosion cracking of carbon steels very often occurs in alkaline environments at high temperatures; stainless steels - in solutions of chlorides, copper sulfate, orthophosphoric acid; aluminum and magnesium alloys - under the influence of sea water; titanium and its alloys - under the influence of concentrated nitric acid and solutions of iodine in methanol.

It should be noted that depending on the nature of the metal or alloy and the properties of the aggressive environment, there is a critical stress above which corrosion cracking is often observed.

Based on the nature of the additional influences to which the metal is exposed, simultaneously with exposure to an aggressive environment, one can distinguish stress corrosion, friction corrosion and cavitation corrosion.

7. Stress corrosion- corrosion under simultaneous exposure to a corrosive environment and permanent or temporary stresses. The simultaneous impact of cyclic tensile stresses and a corrosive environment causes corrosion fatigue, i.e., premature destruction of the metal occurs. This process can be represented as follows: first, local corrosion appears on the surface of the product in the form of ulcers, which begin to act as a stress concentrator; the maximum stress value will be at the bottom of the ulcers, which has a more negative potential than the walls, as a result of which the metal will be destroyed deep, and the ulcer will turn into a crack. Propeller shafts are susceptible to this type of corrosion. Car springs, ropes, cooled rolls of rolling mills, etc.

8. Friction corrosion- metal destruction caused by simultaneous exposure to a corrosive environment and friction. When two surfaces move oscillatingly relative to each other under conditions of exposure to a corrosive environment, abrasion corrosion, or fretting corrosion, occurs. It is possible to eliminate corrosion due to friction or vibration the right choice structural material, reducing the coefficient of friction, using coatings, etc.

9. Gas corrosion- this is the chemical corrosion of metals in a gaseous environment with a minimum moisture content (usually no more than 0.1%) or at high temperatures. This type of corrosion occurs frequently in the chemical and petrochemical industries. For example, in the production of sulfuric acid at the stage of sulfur dioxide oxidation, in the synthesis of ammonia, in the production of nitric acid and hydrogen chloride, in the processes of synthesis of organic alcohols, oil cracking, etc.

10. Atmospheric corrosion is the corrosion of metals in an atmosphere of air or any moist gas.

11. Underground corrosion- This is the corrosion of metals in soils and soils.

12. Contact corrosion is a type of corrosion caused by the contact of metals having different stationary potentials in a given electrolyte.

CORROSION OF METALS– physical-chemical or chemical interaction between a metal (alloy) and the environment, leading to deterioration of the functional properties of the metal (alloy), environment or technical system that includes them.

The word corrosion comes from the Latin “corrodo” - “to gnaw” (Late Latin “corrosio” means “corrosion”).

Corrosion is caused by a chemical reaction between the metal and environmental substances that occurs at the interface between the metal and the environment. Most often, this is the oxidation of the metal, for example, by atmospheric oxygen or acids contained in solutions with which the metal is in contact. Metals located in the voltage series (activity series) to the left of hydrogen, including iron, are especially susceptible to this.

As a result of corrosion, iron rusts. This process is very complex and includes several stages. It can be described by the summary equation:

4Fe + 6H 2 O (moisture) + 3O 2 (air) = 4Fe(OH) 3

Iron(III) hydroxide is very unstable, quickly loses water and turns into iron(III) oxide. This compound does not protect the iron surface from further oxidation. As a result, the iron object can be completely destroyed.

Aluminum under normal conditions is resistant to air and water, even boiling water, but if mercury is applied to the surface of aluminum, the resulting amalgam destroys the oxide film - pushes it from the surface, and the metal quickly turns into white flakes of aluminum metahydroxide:

4Al + 2H 2 O + 3O 2 = 4AlO(OH)

Amalgamated aluminum reacts with water to release hydrogen:

2Al + 4H 2 O = 2AlO(OH) + 3H 2

Corrosion of metals can also include their dissolution in liquid molten metals (sodium, lead, bismuth), which are used, in particular, as coolants in nuclear reactors.

Corrosion is most common in electrolyte environments. In some technological processes, metals come into contact with molten electrolytes. However, most often corrosion occurs in electrolyte solutions. The metal does not have to be completely immersed in the liquid. Electrolyte solutions can be present in the form of a thin film on the surface of the metal. They often permeate the environment surrounding the metal (soil, concrete, etc.).

During the construction of the metro bridge and the Leninskie Gory station in Moscow, large amounts of sodium chloride were added to the concrete to prevent freezing of the concrete that had not yet set. The station was built in the shortest possible time (in just 15 months) and opened on January 12, 1959. However, the presence of sodium chloride in the concrete caused the destruction of the steel reinforcement. 60% of reinforced concrete structures were subject to corrosion, so the station was closed for reconstruction , lasting almost 10 years. Only on January 14, 2002, the metro bridge and the station, called Vorobyovy Gory, were re-opened.

Using salts (usually sodium or calcium chloride) to remove snow and ice from roads and sidewalks also causes metals to break down faster. Vehicles and underground communications are severely affected. It is estimated that in the United States alone, the use of salts to combat snowfall and ice leads to losses of about $2 billion per year due to engine corrosion and $0.5 billion in additional repairs of roads, underground highways and bridges.

Electrochemical corrosion leads to the rapid destruction of more active metals, which in various mechanisms and devices come into contact with less active metals located to the right in the electrochemical voltage series. The use of copper or brass parts in iron or aluminum structures that operate in seawater significantly increases corrosion. There are known cases of destruction and sinking of ships whose iron plating was fastened with copper rivets.

Electrochemical processes can also occur in a homogeneous metal. They are activated if there are differences in the composition of the metal grain in the bulk and at the boundary, inhomogeneous mechanical stress, microimpurities, etc. Many of our compatriots participated in the development of the general theory of electrochemical corrosion of metallic materials, including Vladimir Aleksandrovich Kistyakovsky (1865–1952) and Alexander Naumovich Frumkin (1895–1976).

One of the reasons for the occurrence of electrochemical corrosion is stray currents, which appear due to the leakage of part of the current from electrical circuits into the soil or aqueous solutions, where they fall on metal structures. Where the current exits these structures, the dissolution of the metal begins again into the soil or water. Such zones of destruction of metals under the influence of stray currents are especially often observed in areas of ground electric transport (tram lines, electric railway transport). These currents can reach several amperes, which leads to large corrosion damage. For example, the passage of a current of 1 A for one year will cause the dissolution of 9.1 kg of iron, 10.7 kg of zinc, 33.4 kg of lead.

Corrosion leads to huge irreversible losses of metals; about 10% of the produced iron is completely destroyed every year. According to the Institute of Physical Chemistry of the Russian Academy of Sciences, every sixth blast furnace in Russia works in vain - all the smelted metal turns into rust. The destruction of metal structures, agricultural and transport vehicles, and industrial equipment causes downtime, accidents, and deterioration in product quality. Taking into account possible corrosion leads to increased metal costs in the manufacture of high-pressure apparatus, steam boilers, metal containers for toxic and radioactive substances, etc. This increases overall corrosion losses. Considerable amounts of money have to be spent on anti-corrosion protection. The ratio of direct losses, indirect losses and costs for corrosion protection is estimated as (3–4):1:1. In industrialized countries, damage from corrosion reaches 4% of national income. In our country it amounts to billions of rubles a year.

The problem of protecting metals from corrosion arose almost at the very beginning of their use. People tried to protect metals from atmospheric influences with the help of fat, oils, and later by coating with other metals and, above all, low-melting tin (tinning). In the works of the ancient Greek historian Herodotus (5th century BC) and the ancient Roman scientist Pliny the Elder (1st century BC) there are already references to the use of tin to protect iron from rusting. Currently, the fight against corrosion is carried out in several directions at once - they are trying to change the environment in which a metal product operates, influence the corrosion resistance of the material itself, and prevent contact between the metal and aggressive substances of the external environment.

The aggressiveness of the atmosphere strongly depends on humidity. For any metal there is a certain critical relative humidity, below which it is not subject to atmospheric corrosion. For iron, copper, nickel, zinc it is 50–70%. Sometimes, to preserve items of historical value, their temperature is artificially maintained above the dew point. In closed spaces (for example, in packaging boxes), humidity is reduced using silica gel or other adsorbents. The aggressiveness of the industrial atmosphere is determined mainly by fuel combustion products ( cm. ENVIRONMENTAL POLLUTION). The prevention of acid rain and the elimination of harmful gas emissions help reduce losses from corrosion.

Noble metals are practically resistant to corrosion, but they are too expensive for widespread use, so they are used only in the most critical parts, for example, for the manufacture of non-corrosive electrical contacts. Nickel, aluminum, copper, titanium and alloys based on them have high corrosion resistance. Their production is growing quite quickly, but even now the most accessible and widely used metal remains quickly rusting iron. Alloying is often used to impart corrosion resistance to iron-based alloys. This is how stainless steel is obtained, which, in addition to iron, contains chromium and nickel. The most common stainless steel in our time, grade 18–8 (18% chromium and 8% nickel), appeared in 1923. It is quite resistant to moisture and oxygen. The first tons of stainless steel in our country were smelted in 1924 in Zlatoust. Nowadays, many grades of such steels have been developed, which, in addition to chromium and nickel, contain manganese, molybdenum, tungsten and other chemical elements. Surface alloying of inexpensive iron alloys with zinc, aluminum, and chromium is often used.

To resist atmospheric corrosion, thin coatings of other metals that are more resistant to moisture and atmospheric oxygen are applied to steel products. Chromium and nickel platings are often used. Because chrome platings often contain cracks, they are usually applied over less decorative nickel platings. Protecting tin cans from corrosion by organic acids found in food products requires a significant amount of tin. For a long time, cadmium was used to coat kitchen utensils, but it is now known that this metal is hazardous to health and cadmium coatings are used only in technology.

To slow down corrosion, varnishes and paints, mineral oils and lubricants are applied to the metal surface. Underground structures are covered with a thick layer of bitumen or polyethylene. The internal surfaces of steel pipes and tanks are protected with cheap cement coatings.

One of the most effective methods of combating corrosion is electrochemical protection. To protect drilling platforms, welded metal bases, and underground pipelines, they are connected as a cathode to an external current source. Auxiliary inert electrodes are used as an anode.

Elena Savinkina

Chemical and physicochemical reactions that occur during the interaction of the environment with metals and alloys, in most cases lead to their spontaneous destruction. The process of self-destruction has its own term - “corrosion”. The result of corrosion is a significant deterioration in the properties of the metal, as a result of which products made from it quickly fail. Every metal has properties that allow it to resist destruction. Corrosion resistance, or, as it is also called, chemical resistance of a material, is one of the main criteria by which metals and alloys are selected for the manufacture of certain products.

Depending on the intensity and duration of the corrosion process, the metal can be subjected to either partial or complete destruction. The interaction of a corrosive environment and metal leads to the formation of phenomena such as scale, oxide film and rust on the metal surface. These phenomena differ from each other not only in appearance, but also in the degree of adhesion to the surface of metals. For example, during the oxidation of a metal such as aluminum, its surface is covered with a film of oxides, which is characterized by high strength. Thanks to this film, destructive processes are stopped and do not penetrate inside. If we talk about rust, then the result of its influence is the formation of a loose layer. The corrosion process in this case very quickly penetrates the internal structure of the metal, which contributes to its rapid destruction.

Indicators by which the classification of corrosion processes is carried out:

type of corrosive environment;
conditions and mechanism of occurrence;
nature of corrosion damage;
type of additional effects on metal.

According to the mechanism of the corrosion process, both chemical and electrochemical corrosion of metals and alloys are distinguished.

Chemical corrosion- this is the interaction of metals with a corrosive environment, during which a simultaneous oxidation of the metal and restoration of the oxidizing component of the environment are observed. Products interacting with each other are not separated spatially.

Electrochemical corrosion- this is the interaction of metals with a corrosive environment, which is an electrolyte solution. The process of ionization of metal atoms, as well as the process of reduction of the oxidizing component of a given corrosive environment, occur in different acts. The electrode potential of the electrolyte solution has a significant impact on the rate of these processes.

Depending on the type of aggressive environment, there are several types of corrosion.

Atmospheric corrosion represents the self-destruction of metals in an air atmosphere or in a gas atmosphere characterized by high humidity.

Gas corrosion is the corrosion of metals that occurs in a gaseous environment in which the moisture content is minimal. The absence of moisture in the gas environment does not the only condition, promoting self-destruction of the metal. Corrosion is also possible at high temperatures. This type of corrosion is most common in the petrochemical and chemical industries.

Radiation corrosion represents the self-destruction of a metal under the influence of radioactive radiation of varying degrees of intensity.

Underground corrosion is corrosion that occurs in soils and various soils.

Contact corrosion represents a type of corrosion, the formation of which is facilitated by the contact of several metals that differ from each other by stationary potentials in a specific electrolyte.

Biocorrosion is the corrosion of metals that occurs under the influence of various microorganisms and their vital activity.

Corrosion by current (external and stray)- another type of metal corrosion. If a metal is exposed to current from external source, then this is corrosion by external current. If the effect is carried out through stray current, then this is stray current corrosion.

Corrosive cavitation is a process of self-destruction of metals, the occurrence of which is promoted by both impact and corrosive effects of the external environment.

Stress Corrosion is metal corrosion caused by the interaction of a corrosive environment and mechanical stresses. This type corrosion poses a significant danger to metal structures that are subject to severe mechanical stress.

Fretting corrosion- a type of metal corrosion that is caused by a combination of vibration and exposure to a corrosive environment. To minimize the likelihood of corrosion due to friction and vibration, it is necessary to carefully approach the choice of structural material. It is also necessary to use special coatings and, if possible, reduce the coefficient of friction.

Based on the nature of the destruction, corrosion is divided into continuous and selective.

Complete corrosion completely covers the metal surface. If the rate of destruction over the entire surface is the same, then this is uniform corrosion. If the destruction of metal in its various areas occurs with at different speeds, then the corrosion is called uneven.

Selective corrosion implies the destruction of one of the alloy components or one structural component.

Local corrosion, which appears in the form of separately scattered spots on the surface of the metal, represents depressions of different thicknesses. The lesions may be shells or points.

Subsurface corrosion forms directly on the surface of the metal, after which it actively penetrates deeper. This type of corrosion is accompanied by delamination of metal products.

Intergranular corrosion manifests itself in the destruction of metal along grain boundaries. By appearance metal is quite difficult to determine. However, the strength and ductility of the metal change very quickly. Products made from it become fragile. This type of corrosion is most dangerous for chromium and chromium-nickel steels, as well as for aluminum and nickel alloys.

Crevice corrosion is formed in those areas of metals and alloys that are located in threaded fasteners, various gaps and under all kinds of gaskets.

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