What environmental components cause corrosion? Types of corrosion - how metal rusts

All types of corrosion appear for one reason or another. The key one is considered to be instability from the point of view of thermodynamics of materials to compounds that exist in working environments where metal products operate.

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Corrosion refers to the destruction of materials caused by physico-chemical or purely chemical influences of the environment. First of all, corrosion is divided by type into electrochemical and chemical, and by nature into local and continuous.

Local corrosion can be knife-like, intercrystalline, through (through corrosion is known to car owners who do not monitor the condition of the body of their vehicle), pitting, subsurface, filamentous, ulcerative. It also exhibits brittleness, cracking, and staining. Continuous oxidation can be selective, uneven and uniform.

The following types of corrosion are distinguished:

• biological – caused by the activity of microorganisms;
• atmospheric – destruction of materials under the influence of air;
• liquid – oxidation of metals in non-electrolytes and electrolytes;
• contact – formed during the interaction of metals with different values ​​of stationary potentials in an electrolytic environment;
• gas – becomes possible with elevated temperatures in gas atmospheres;
• white - often found in everyday life (on objects made of galvanized steel, on heating radiators);
• structural – relates to the heterogeneity of materials;
• crevice - occurs exclusively in cracks and gaps present in metal products;
• soil – observed in soils and soils;
• fretting corrosion – is formed when two surfaces move (oscillating) in relation to each other;
• external current – ​​destruction of a structure caused by the influence of electric current coming from any external source;
• stray currents.

In addition, there is the so-called corrosion erosion - rusting of metals during friction, stress corrosion caused by mechanical stress and the influence of an aggressive environment, cavitation (corrosion process plus impact contact of the structure with the external atmosphere). We have listed the main types of corrosion, some of which we will discuss in more detail below.

2

A similar phenomenon is usually recorded when there is close interaction (tight contact) of plastic or rubber with metal or two metals. In this case, the destruction of materials occurs at the point of their contact due to the friction that occurs in this area, caused by the influence of a corrosive environment. In this case, the structure is usually subject to a relatively high load.

Most often, fretting corrosion affects moving contacting steel or metal shafts, bearing elements, various bolted, splined, rivet and keyed joints, ropes and cables (that is, those products that perceive certain oscillatory, vibration and rotational stresses).

In essence, fretting corrosion is formed due to the influence of an active corrosive environment in combination with wear of a mechanical nature.

The mechanism of this process is as follows:

• Corrosion products (oxide film) appear on the surface of contacting materials under the influence of a corrosive environment;
• this film is destroyed by friction and remains between the contacting materials.

Over time, the process of destruction of the oxide film becomes more and more intense, which usually causes the formation of contact destruction of metals. Fretting corrosion occurs with at different speeds, which depends on the type of corrosive environment, the structure of materials and the loads acting on them, and the temperature of the environment. If a white film appears on the contacting surfaces (the process of metal discoloration is observed), we are most often talking about the fretting process.

The negative consequences of fretting corrosion for metal structures can be mitigated in the following ways:

• Use of viscous lubricating compounds. This technique works if the products are not subject to excessive loads. Before applying the lubricant, the surface of the metals is saturated with phosphates (slightly soluble) of manganese, zinc or ordinary iron. This method protection against fretting corrosion is considered temporary. It remains effective as long as due to sliding protective composition is not completely removed. Lubricants, by the way, are not used to protect structures made of.
• Competent choice of materials for the manufacture of the structure. Fretting corrosion occurs extremely rarely if the object is made of hard and soft metals. For example, it is recommended to coat steel surfaces with silver, cadmium, tin, and lead.
• The use of additional coatings with special properties, gaskets, cobalt alloys, materials with a low coefficient of friction.

Sometimes fretting corrosion is prevented by creating surfaces in contact with each other with a minimum amount of slip. But this technique is used very rarely, due to the objective complexity of its implementation.

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This type of corrosion destruction of materials is understood as corrosion to which structures and structures operating in the surface atmospheric part are exposed. Atmospheric corrosion can be wet, damp or dry. The last of these proceeds according to a chemical scheme, the first two - according to an electrochemical scheme.

Atmospheric corrosion of the wet type becomes possible when there is a film of moisture on the metals that is small in thickness (no more than one micrometer). Condensation of wet droplets occurs on it. The condensation process can proceed according to the adsorption, chemical or capillary scheme.

Atmospheric corrosion of the dry type occurs without the presence of a wet film on the surface of metals. In the first stages, the destruction of the material occurs quite quickly, but then the rate of rusting slows down significantly. Dry atmospheric corrosion can occur much more actively if the structures are exposed to any gas compounds present in the atmosphere (sulfur dioxide and other gases).

Atmospheric corrosion wet type formed at 100% air humidity. It affects any objects that are used in water or are constantly exposed to moisture (for example, doused with water).

Atmospheric corrosion causes serious damage to metal structures, so various techniques are being created to combat it:

• Reducing air humidity (relative). Relatively simple and yet very effective method, which consists of dehumidifying the air and heating the rooms where metal structures are used. Atmospheric corrosion with this technique is greatly slowed down.
• Coating surfaces with non-metallic (varnishes, paints, pastes, lubricants) and metallic (nickel and zinc) compounds.
• Alloying of metals. Atmospheric corrosion becomes less violent in cases where phosphorus, titanium, chromium, copper, aluminum, and nickel are added to the metal in small quantities. They stop the anodic process or transfer steel surfaces to a passive state.
• Use of inhibitors - volatile or contact. Volatile compounds include dicyclohexylamine, benzoates, carbonates, and monoethanolamine. And the most famous contact type inhibitor is sodium nitrite.

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Gas corrosion is observed, as a rule, at elevated temperatures in an atmosphere of dry vapors and gases. Enterprises in the chemical, oil and gas and metallurgical industries suffer the most from it, as it affects containers where processing is carried out chemical compounds and substances, engines of special machines, chemical plants and units, gas turbines, equipment for heat treatment and melting of steel and metals.

Gas corrosion occurs during oxidation:

• carbon dioxide (carbon dioxide corrosion);
• hydrogen sulfide (hydrogen sulfide corrosion);
• hydrogen, chlorine, various halogens, methane.

Gas corrosion is most often caused by exposure to oxygen. The destruction of metals during this process proceeds according to the following scheme:

• ionization of the metal surface (electrons and cations appear that saturate the oxide film);
• diffusion (to the gas phase) of electrons and cations;
• weakening of interatomic bonds in the oxygen molecule caused by adsorption (physical) of oxygen on the metal surface;
• adsorption chemical type, leading to the creation of a dense film of oxides.

After this, oxygen ions penetrate deep into the film, where they come into contact with metal cations. Gas corrosion, caused by the influence of other chemical compounds, follows a similar principle.

The phenomenon of hydrogen corrosion of steel is noted in technological equipment, which operates in hydrogen atmospheres at high (from 300 MPa) pressures and temperatures above +200 °C. This corrosion is formed due to the contact of carbides included in steel alloys with hydrogen. Visually, it is poorly visible (the surface of the structure has no obvious damage), but at the same time the strength indicators of steel products are significantly reduced.

There is also the concept of hydrogen depolarization corrosion. This process can occur at a certain value of partial pressure in the medium with which the electrolyte is in contact. Typically, the phenomenon of corrosion with hydrogen depolarization is observed in two cases:

• with low activity in an electrolytic solution of metal ions;
• at increased activity hydrogen ions in the electrolyte.

Carbon dioxide corrosion affects oil equipment and pipelines that operate in environments containing carbon dioxide. These days, this type of corrosion failure is prevented by operating with low levels of alloying. Optimal results, as practice has shown, are observed when using alloys with a chromium content of 8 to 13 percent.

Corrosion is the destruction of metal, ceramic, wood and other materials as a result of chemical or physical-chemical interaction. As for the reasons for the occurrence of such an undesirable effect, they are different. In most cases, this is structural instability to thermodynamic influences environment. Let's take a closer look at what corrosion is. Types of corrosion also need to be considered, and it wouldn’t hurt to talk about protection against it.

Some general information

We are used to hearing the term “rusting”, which is used in the case of corrosion of metal and alloys. There is also such a thing as “aging,” which is characteristic of polymers. Essentially, it's the same thing. A striking example is the aging of rubber products due to active interaction with oxygen. In addition, some plastic elements are destroyed by exposure. The rate of corrosion directly depends on the conditions in which the object is located. Thus, rust on a metal product will spread faster the higher the temperature. Humidity also affects: the higher it is, the faster it becomes unsuitable for further use. It has been experimentally established that approximately 10 percent of metal products are irretrievably written off, and corrosion is to blame. Types of corrosion are different and are classified depending on the type of environment, the nature of the course, etc. Let's look at them in more detail.

Classification

Currently, there are more than two dozen rusting options. We will present only the most basic types of corrosion. Conventionally, they can be divided into the following groups:

• Chemical corrosion is a process of interaction with a corrosive environment, in which the reduction of the oxidizing agent occurs in one act. The metal and the oxidizing agent are not separated spatially.
• Electrochemical corrosion is the process of interaction of a metal with the ionization of atoms and the reduction of the oxidizing agent in different acts, but the rate largely depends on the electrode potential.
• Gas corrosion - chemical rusting of metal with a minimum moisture content (no more than 0.1 percent) and/or high temperatures ah in a gaseous environment. Most often this type is found in the chemical and oil refining industries.

In addition, there are still a huge number of rusting processes. All of them are corrosion. Types of corrosion, in addition to those described above, include biological, radioactive, atmospheric, contact, local, targeted rusting, etc.

Electrochemical corrosion and its features

With this type of destruction, the process occurs when the metal comes into contact with the electrolyte. The latter can be condensate or rainwater. The more salts and acids a liquid contains, the higher the electrical conductivity, and therefore the speed of the process. As for the places of metal structures most susceptible to corrosion, these are rivets, welded joints, and places of mechanical damage. If the structural properties of the iron alloy make it resistant to rust, the process slows down somewhat, but still continues. A striking example is galvanizing. The fact is that zinc has a more negative potential than iron. For this simple reason, the iron alloy is restored, but the zinc alloy is corroded. However, the presence of an oxide film on the surface greatly slows down the destruction process. Of course, all types of electrochemical corrosion are extremely dangerous and sometimes it is even impossible to fight them.

Chemical corrosion

This change in metal is quite common. A striking example is the appearance of scale as a result of the interaction of metal products with oxygen. High temperature in this case acts as an accelerator of the process, and liquids such as water, salts, acids, alkalis and salt solutions can participate in it. If we talk about materials such as copper or zinc, their oxidation leads to the formation of a film that is resistant to further corrosion. Steel products form iron oxides. Further developments lead to the appearance of rust, which does not provide any protection against further destruction, but, on the contrary, contributes to it. Currently, all types of chemical corrosion are eliminated using galvanization. Other means of protection may also be used.

Types of concrete corrosion

Changes in the structure and increase in the fragility of concrete under the influence of the environment can be of three types:

• Destruction of parts of cement stone is one of the most common types of corrosion. It occurs when a concrete product is systematically exposed to atmospheric precipitation and other liquids. As a result, calcium oxide hydrate is washed out and the structure is disrupted.
• Interaction with acids. If cement stone comes into contact with acids, calcium bicarbonate is formed - an aggressive chemical element for a concrete product.
• Crystallization of sparingly soluble substances. In essence, this means biocorrosion. The bottom line is that microorganisms (spores, fungi) enter the pores and develop there, resulting in destruction.

Corrosion: types, methods of protection

Billions of dollars in annual losses have led people to fight against these harmful effects. We can say with confidence that all types of corrosion lead to the loss not of the metal itself, but of valuable metal structures, the construction of which costs a lot of money. It is difficult to say whether it is possible to provide 100% protection. However, with proper surface preparation, which consists of abrasive blasting, it is possible to achieve good results. The paint coating reliably protects against electrochemical corrosion when applied correctly. And special surface treatment will reliably protect against metal destruction underground.

Active and passive methods of control

The essence of active methods is to change the structure of the double electric field. For this, a direct current source is used. The voltage must be selected in such a way that the product to be protected increases. Another extremely popular method is the “sacrificial” anode. It breaks down, protecting the base material.

Passive protection involves the use of paint and varnish. The main task is to completely prevent moisture and oxygen from entering the protected surface. As noted above, it makes sense to use zinc, copper or nickel plating. Even a partially destroyed layer will protect the metal from rusting. Of course, these types of protection against metal corrosion are effective only when the surface does not have visible defects in the form of cracks, chips, and the like.

Galvanizing in detail

We have already looked at the main types of corrosion, and now I would like to talk about best methods protection. One of these is galvanizing. Its essence lies in the fact that zinc or its alloy is applied to the surface being treated, which gives the surface some physical and chemical properties. It is worth noting that this method is considered one of the most economical and efficient, and this despite the fact that approximately 40 percent of the world's production of this element is spent on zinc metallization. Steel sheets, fasteners, as well as instruments and other metal structures can be galvanized. The interesting thing is that using metallization or spraying you can protect a product of any size and shape. Zinc has no decorative purpose, although with the help of some special additives it becomes possible to obtain shiny surfaces. In principle, this metal is capable of providing maximum protection in aggressive environments.

Conclusion

So we told you about what corrosion is. Types of corrosion were also considered. Now you know how to protect the surface from premature rusting. By and large, this is extremely simple to do, but where and how the product is used is of considerable importance. If it is constantly subjected to dynamic and vibration loads, then there is a high probability of cracks in the paintwork, through which moisture will enter the metal, as a result of which it will gradually deteriorate. However, the use of various rubber gaskets and sealants in areas where metal products interact can slightly extend the life of the coating.

Well, that's all on this topic. Remember that premature failure of a structure due to corrosion can lead to unforeseen consequences. At an enterprise, large material damage and human casualties are possible as a result of rusting of the supporting metal structure.

Corrosion of metals can manifest itself in various forms, the main ones being:

1. General corrosion, also known as uniform corrosion. General corrosion is the most common type of destruction of metals and is caused by chemical or electrochemical reactions. General corrosion results in deterioration of the entire metal surface, but is considered one of the safest forms of corrosion because it is predictable and controllable.

2. Local (localized) corrosion. Unlike general corrosion, this type of corrosion is focused on one area of ​​the metal structure.

Localized corrosion is classified into three types:

2.1 Pitting: corrosion in the form of a small hole or cavity in metal. It usually occurs as a result of depassivation of a small area of ​​the surface. The affected area becomes the anode and some of the remaining metal becomes the cathode, resulting in localized galvanic reactions. This form of corrosion can often be difficult to detect due to the fact that the affected area is usually relatively small and may be hidden beneath the surface.

2.2 Crevice: Like pitting, crevice corrosion is localized in certain place. This type of corrosion is often associated with a stagnant micro-zone of aggressive media, such as under gaskets, washers and clamps. An acidic environment, or lack of oxygen in narrow crevices, can lead to this type of corrosion.

2.3 Filament Corrosion: Occurs under painted or metallized surfaces when water or a humid environment disturbs the coating. Filiform corrosion begins with small defects in the coating and spreads, causing structural damage.

3. Galvanic corrosion begins when two different metals are placed together in a corrosive electrolyte environment. A galvanic couple is formed between two metals, one of the metals is the anode, and the other is the cathode. In this case, metal ions move from the anodized material to the cathode metal.

In the presence of an electrochemical effect, the anodic site is destroyed much more severely than the cathode. Without a flow of charged particles, both metals corrode equally. For galvanic corrosion to exist, three conditions must be present: electrochemically dissimilar metals, direct contact of these metals, and exposure to an electrolyte.

4. The destruction of metal from environmental influences can be the result of a combination of environmental conditions affecting the material, or from one of the factors. Chemical exposure, temperature and conditions associated with mechanical stress (especially tensile forces) can lead to the following types of corrosion: corrosion fatigue cracking, stress corrosion cracking, hydrogen cracking, liquid metal embrittlement in contact with liquid metal.

5. Erosion-corrosion wear occurs when exposed to aggressive particles and environmental flow, cavitation, as a result of which the protective oxide layer on the metal surface is constantly removed, and the base metal corrodes.

6. Intergranular corrosion is chemical or electrochemical destruction at the grain boundaries of a metal. This phenomenon often occurs due to impurities in the metal, which are usually concentrated at the grain boundaries.

7. Selective leaching (or alloy failure) is the corrosion of one of the elements in the alloy. The most common type is zinc leaching from brass. Corrosion results in porous copper.

8. Frictional corrosion occurs as a result of wear and/or vibration on an uneven, rough surface. As a result, depressions and grooves appear on the surface. Frictional corrosion often occurs in rotating machine parts, in bolt assemblies and bearings, and on surfaces subject to vibration during transportation.

9. High temperature corrosion most often occurs in gas turbines, diesel engines and other machines containing vanadium or sulfates, which can form compounds with a low melting point when burned. These compounds are very corrosive to metal alloys, including stainless steels.

High temperature corrosion can also occur at high temperatures as a result of oxidation, sulfidation and carbonization of the metal.

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 chemical reaction metal with environmental substances flowing at the boundary of 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.

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:

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

Some rather inactive metals are also susceptible to corrosion. In humid air the surface of copper is 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 it was hydrogen saturation that was the reason for the sudden drop in the strength of steel.

Hydrogen has caused a lot of trouble for 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 a large number of 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 degrade 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 between 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. 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.

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 very quickly breaks down 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. In developing general theory Many of our compatriots participated in electrochemical corrosion of metal 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 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. Fouling of the underwater part of ships with small particles marine organisms also affects corrosion processes.

When the metal is simultaneously exposed to the external environment and mechanical stresses, 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 irregularities 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 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 devices high pressure, 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 industrial developed countries Corrosion damage 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 increase 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 increasingly high temperatures and pressures, powerful streams 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 some 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). Reducing losses from corrosion helps to prevent acid rain and elimination of harmful gas emissions.

Destruction of metals in aquatic 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 of the methods 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 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. Many grades of steel have now been developed, which, in addition to chromium and nickel, contain manganese, molybdenum, tungsten and others 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 for covering kitchen utensils used cadmium, 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 fit well on 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 anti-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

Types of corrosion

Chemical corrosion >>> Electrochemical corrosion >>> Gas corrosion >>> Atmospheric corrosion >>> Underground corrosion >>> Biocorrosion >>> Contact corrosion >>> Radiation corrosion >>> Corrosion cavitation >>> Fretting corrosion >>> Intergranular corrosion >>> Crevice corrosion >>>

Corrosion processes are classified according to the mechanism of interaction of metals with the external environment; by type of corrosive environment and process conditions; by the nature of corrosion damage; according to the types of additional influences to which the metal is exposed simultaneously with the action of a corrosive environment.

According to the mechanism of the process, they distinguish chemical and electrochemical corrosion of metals.

Chemical corrosion is a process of interaction of a metal with a corrosive environment, in which the oxidation of the metal and the reduction of the oxidizing component of the environment occur simultaneously in one act. The interaction products are not spatially separated. Electrochemical corrosion- this is the process of interaction of a metal with a corrosive environment (electrolyte solution), in which the ionization of metal atoms and the reduction of the oxidizing component of the corrosive environment do not occur in one act and their rates depend on the electrode potential.

Depending on the type of corrosive environment and conditions of occurrence, several types of corrosion are distinguished. Gas corrosion- this is 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.

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

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

Biocorrosion- This is corrosion that occurs under the influence of the vital activity of microorganisms.

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

Radiation corrosion- This is corrosion caused by the action of radioactive radiation.

Corrosion by external current and corrosion by stray current. In the first case, this is metal corrosion that occurs under the influence of current from an external source. In the second case - under the influence of stray current.

Stress Corrosion- corrosion caused by simultaneous exposure to a corrosive environment and mechanical stress. If these are tensile stresses, then cracking of the metal may occur. This is a very dangerous type of corrosion, especially for structures experiencing mechanical loads (axles, springs, autoclaves, steam boilers, turbines, etc.). If metal products are subjected to cyclic tensile stress, corrosion fatigue can occur. The fatigue limit of the metal decreases. Car springs, ropes, and rolling mill rolls are susceptible to this type of corrosion.

Corrosive cavitation- metal destruction caused by simultaneous corrosive and impact effects of the external environment.

Fretting corrosion is corrosion caused by both vibration and exposure to a corrosive environment. Corrosion due to friction or vibration can be eliminated by the correct choice of structural material, reducing the coefficient of friction, using coatings, etc.

Corrosion is called solid , if it covers the entire surface of the metal. Continuous corrosion can be uniform if the process occurs at the same speed over the entire surface of the metal, and uneven when the process speed is not the same on different parts of the surface. Uniform corrosion is observed, for example, when iron pipes corrode in air. At selective corrosion one structural component or one component of the alloy is destroyed. Examples include graphitization of cast iron or dezincification of brass.

Local (localized) corrosion covers individual areas of the metal surface. Local corrosion can be expressed in the form of individual spots, not very deep into the thickness of the metal; ulcers - destruction that looks like a shell, deeply deepened into the thickness of the metal, or points (pittings) penetrating deeply into the metal. The first type is observed, for example, during corrosion of brass in sea water. Pitting corrosion was observed in steel in soil, and pitting corrosion was observed in austenitic chromium-nickel steel in sea water.

Subsurface corrosion begins on the surface, but then spreads deep into the metal. Corrosion products end up concentrated in metal cavities. This type of corrosion causes swelling and delamination of metal products.

Intergranular corrosion characterized by metal destruction along grain boundaries. It is especially dangerous because the appearance of the metal does not change, but it quickly loses strength and ductility and is easily destroyed. This is due to the formation of loose, low-strength corrosion products between the grains. Chromium and chromium-nickel steels, nickel and aluminum alloys are especially susceptible to this type of destruction.

Crevice corrosion causes destruction of metal under gaskets, in gaps, threaded fasteners, etc.