When metals interact with substances environment compounds are formed on their surface that have completely different properties than the metals themselves. IN ordinary life we often repeat the words “rust››, “rusting”, seeing a brown-yellow coating on products made of iron and its alloys.
Rusting is a special case of corrosion.
Corrosion is the process of spontaneous destruction of metals under the influence of the external environment.
However, almost all metals are subject to destruction, as a result of which many of their properties deteriorate (or are completely lost): strength, ductility, shine decrease, electrical conductivity decreases, and friction between moving machine parts increases, the dimensions of parts change, etc.
In its own way chemical nature corrosion is an oxidation-reduction process. Depending on the environment in which it occurs, two types of corrosion are distinguished.

Types of corrosion

1.Chemical corrosion occurs in a non-conducting electricity environment.
This type of corrosion occurs when metals interact with dry gases or non-electrolyte liquids (gasoline, kerosene, etc.). Parts and components of engines, gas turbines, and rocket launchers are subject to such destruction. Chemical corrosion is often observed during metal processing. high temperatures.

3 Fe + 2O 2 = Fe 3 O 4
4 Al + 3O 2 = 2Al 2 O 3

Most metals are oxidized by atmospheric oxygen, forming oxide films on the surface. If this film is strong, dense, and well bonded to the metal, then it protects the metal from further destruction. Such protective films appear in Zn, AI, Cr, Ni, Sn, Pb, Nb, Ta, etc. In iron, it is loose, porous, easily separated from the surface and therefore is not able to protect the metal from further destruction.

II. Electrochemical corrosion occurs in a conductive environment (in the electrolyte) with the appearance of an electric current inside the system. The underwater parts of ships are subject to electrochemical corrosion, steam boilers, underground pipelines, metal structures located in humid air. As a rule, metals and alloys are heterogeneous and contain inclusions of various impurities. When they come into contact with electrolytes, some areas of the surface begin to act as an anode (donate electrons), while others act as a cathode (receive electrons).

To protect iron from corrosion, all kinds of coatings are used: paint, a layer of metal (tin, zinc). In this case, paint and tin protect against corrosion until protective layer intact The appearance of cracks and scratches in it allows moisture and air to penetrate to the surface of the iron, and the corrosion process resumes, and in the case of a tin coating it even accelerates, since tin serves as a cathode in the electrochemical process.
Galvanized iron behaves differently. Since zinc acts as an anode, it protective function remains even if the zinc coating is damaged. Cathodic protection Widely used to reduce corrosion of underground and subsea pipelines and steel supports of high-voltage transmissions, oil platforms and piers.

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 to a specific location. This type of corrosion is often associated with a micro-stagnation zone aggressive environment eg 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. Electrochemical corrosion begins when two various metal are 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.

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.

1

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.

3

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 of the wet type is formed at one hundred percent 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.

4

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 tanks where chemical compounds and substances are processed, 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 of a 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;
• with increased activity of 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– spontaneous oxidation of metals, harmful to industrial practice (reducing the durability of products). This word comes from Latin corrodere- corrode. The environment in which a metal corrodes (corrodes) is called corrosive or aggressive. In this case, corrosion products are formed: chemical compounds containing metal in oxidized form. In cases where metal oxidation is necessary to carry out any technological process, the term “corrosion” should not be used. For example, we cannot talk about corrosion of a soluble anode in a galvanic bath, since the anode must oxidize, sending its ions into the solution, for the desired process to occur. It is also impossible to talk about corrosion of aluminum during the aluminothermic process. But the physical and chemical essence of the changes occurring with the metal in all such cases is the same: the metal is oxidized. Consequently, the term “corrosion” has not so much a scientific as an engineering meaning. It would be more correct to use the term "oxidation" regardless of whether it is harmful or beneficial to our practice. In the standardization system (GOST 5272-68), metal corrosion is defined as the destruction of metals due to their chemical and electrochemical interaction with a corrosive environment. In the ISO (international standardization) system, this concept is somewhat broader: a physical and chemical interaction between a metal and the environment, as a result of which the properties of the metal change, and the functional characteristics of the metal, the environment or the technical system that includes them often deteriorate.

Objects affected by corrosion– metals, alloys (solid solutions), metal coatings, metal structures of machines, equipment and structures. The corrosion process is represented as a corrosion system consisting of metal and a corrosive environment. A corrosive environment contains one or more substances that react with the metal. It can be liquid or gaseous. A gaseous medium that oxidizes a metal is called oxidizing gas environment. A change in any part of a corrosion system caused by corrosion is called corrosive effect. Corrosive effect, worsening functional characteristics metal, coating, medium or including them technical systems, are regarded as damage effect or how corrosive damage(according to the ISO system). As a result of corrosion, new substances are formed, including oxides and salts of the corroding metal, these are - corrosion products. Visible products of atmospheric corrosion, consisting mainly of hydrated iron oxides, are called rust, gas corrosion products – scale. The amount of metal converted into corrosion products over a certain time is referred to as corrosion losses. Corrosion losses per unit of metal surface per unit of time characterize corrosion rate. Damage effect associated with losses mechanical strength metal is defined by the term - corrosion damage, its depth per unit time is called corrosion penetration rate. The most important concept is corrosion resistance. It characterizes the ability of a metal to resist the corrosive effects of the environment. Corrosion resistance is determined qualitatively and quantitatively - by the rate of corrosion under given conditions, by a group or resistance score on an accepted scale, using optical instruments. Metals with high corrosion resistance are called corrosion resistant. Factors influencing the rate, type, distribution of corrosion and related to the nature of the metal (composition, structure, internal stresses, surface condition) are called internal corrosion factors. Factors influencing the same corrosion parameters, but related to the composition of the corrosive medium and process conditions (temperature, humidity, medium exchange, pressure, etc.) are called external factors corrosion. In some cases, it is advisable to divide corrosion factors in accordance with Table 4.

Table 4

Corrosion factors

2. Classification of metal corrosion processes

It is customary to classify corrosion according to the mechanism, conditions of the process and the nature of destruction. According to the mechanism of occurrence, corrosion processes, according to GOST 5272-68, are divided into two types: electrochemical And chemical. Electrochemical corrosion includes the process of interaction of a metal with a corrosive environment, in which the ionization of metal atoms and the reduction of oxidizing agents of the environment occur in more than one act and depend on the electronic potential (the presence of conductors of the second type). Let's consider several types of electrochemical corrosion:

1) atmospheric– characterizes the process under humid conditions air environment. This is the most common type of corrosion, since most structures are operated in atmospheric conditions. It can be divided as follows: into outdoors, with the possibility of precipitation getting on the surface of the vehicle, or with protection from it in conditions of limited air access and in a confined air space;

2) underground– destruction of metal in soils and soils. A type of this corrosion is electrochemical corrosion under influence stray currents. The latter arise in the ground near sources of electric current (electricity transmission systems, electrified transport routes);

3) liquid corrosion, or corrosion in electrolytes. Its special case is underwater corrosion– destruction metal structures immersed in water. According to the operating conditions of metal structures, this type is divided into corrosion during complete and partial immersion; in case of partial immersion, the process of corrosion along the waterline is considered. Aquatic environments may differ in corrosive activity depending on the nature of the substances dissolved in them (sea, river water, acidic and alkaline solutions of the chemical industry, etc.). With underwater corrosion, corrosion processes of equipment are possible in non-aqueous liquid media, which are divided into non-electrically conductive and electrically conductive. Such environments are specific to chemical, petrochemical and other industries. Chemical corrosion refers to a process in which the oxidation of the metal and the reduction of the environment represent a single act (the absence of conductors of the second type). Chemical corrosion– this is the destruction of metals in oxidizing environments at high temperatures. There are two types: gas(i.e. oxidation of the metal when heated) and corrosion in non-electrolytes:

A) characteristic feature Gas corrosion is the absence of moisture on the metal surface. The rate of gas corrosion is influenced primarily by the temperature and composition of the gaseous medium. In industry, cases of this corrosion are often encountered: from the destruction of parts of heating furnaces to corrosion of metal during heat treatment.

b) corrosion of metals in non-electrolytes, regardless of their nature, comes down to a chemical reaction between the metal and the substance. Organic liquids are used as non-electrolytes.

A special group should include types of corrosion under conditions of mechanical stress (mechanical corrosion). This group includes: actual stress corrosion, characterized by the destruction of metal under simultaneous exposure to a corrosive environment and constant or variable mechanical stresses; corrosion cracking– with simultaneous exposure to a corrosive environment and external or internal mechanical tensile stresses with the formation of transgranular cracks.

Distinguish independent species corrosion:

1) friction corrosion– metal destruction caused by the simultaneous influence of a corrosive environment and friction;

2) fretting corrosion– destruction during oscillatory movement of two surfaces relative to each other under exposure to a corrosive environment;

3) corrosion cavitation– destruction due to impact of the environment;

4) corrosion erosion– under abrasive influence of the environment;

5) contact corrosion– destruction of one of two metals that are in contact and have different potentials in a given electrolyte.

A distinction must be made between corrosion and erosion. Erosion o latin word erodere(destroy) - gradual mechanical destruction of metal, for example, during abrasion of rubbing parts of mechanisms.

An independent type of corrosion - biocorrosion– this is the destruction of metal, in which a biofactor acts as a significant factor. Bioagents– microorganisms (fungi, bacteria) that initiate or stimulate the corrosion process.

According to the nature of destruction, corrosion is divided into continuous (or general) and local (local). Continuous corrosion covers the entire surface of the metal, and it can be uniform or uneven. Local corrosion occurs with the destruction of individual areas of the metal surface. The types of this corrosion are: pitting, spot corrosion and through corrosion.

Subsurface corrosion begins at the surface, but develops primarily below it in such a way that the corrosion products are concentrated inside the metal. Its variety is layer-by-layer corrosion, propagating predominantly in the direction of plastic deformation of the metal.

Structural corrosion is associated with the structural heterogeneity of the metal. Its variety is intergranular– destruction of the metal along the boundaries of crystallites (grains) of the metal; intragranular– destruction of metal along crystallite grains. It is observed during corrosion cracking that occurs under the influence of external mechanical loads or internal stresses.

Knife corrosion– localized destruction of metal in the fusion zone of welded joints in liquid environments with high corrosive activity.

Crevice corrosion– intensification of the process of metal destruction in the gaps between two metals.

Selective corrosion– destruction of one structural component or one component of the metal in highly active environments. There are a number of varieties: graphitization of cast iron (dissolution of ferritic or pearlite components) and dezincification (dissolution of the zinc component) of brass.

3. Types of corrosion damage

Corrosion, depending on the nature of the metal, the aggressive environment and other conditions, leads to various types of destruction. Figure 13 shows sections through a corroded metal sample, showing possible changes in surface topography as a result of corrosion.

Rice. 11. Schematic illustration various types corrosion: a – uniform corrosion; b – spot corrosion; c, d – corrosion by ulcers; d – pitting (pitting); e – subsurface corrosion; НН – initial metal surface; CC – surface relief changed due to corrosion.

Sometimes corrosion occurs at a uniform rate over the entire surface; in this case, the surface becomes only slightly rougher than the original (a). Different corrosion rates are often observed in individual areas: spots (b), ulcers (c, d). If the ulcers have a small cross-section, but a relatively large depth (d), then they speak of pitting corrosion. In some conditions, a small ulcer extends deeper and wider below the surface (e). Uneven corrosion is much more dangerous than uniform corrosion. Uneven corrosion, with a relatively small amount of oxidized metal, causes a large reduction in the cross-section in selected places. Pitting or pitting corrosion can lead to the formation of through holes, e.g. sheet material, with little metal loss.

The above classification is, of course, conditional. Numerous forms of failure are possible, falling between the characteristic types shown in this figure.

Some alloys are subject to a peculiar type of corrosion that occurs only along the boundaries of crystallites, which are separated from each other thin layer corrosion products (intercrystalline corrosion). Here the loss of metal is very small, but the alloy loses strength. This is very dangerous look corrosion that cannot be detected during external inspection of the product.

4. Corrosion protection methods

To weaken the corrosion process, it is necessary to influence either the metal itself or the corrosive environment. The main areas for combating corrosion are:

1) alloying the metal, or replacing it with another, more corrosion-resistant one;

2) protective coatings (metallic and non-metallic) of organic or inorganic origin;

3) electrochemical protection; there are cathodic, anodic and sacrificial protection as a variant of cathodic protection.

For example, for atmospheric corrosion, coatings of organic and inorganic origin are used; Electrochemical protection is effective against underground corrosion;

4) introduction of inhibitors (substances that slow down the reaction rate).

Metal materials under chemical or electrochemical influence of the environment are subject to destruction, which is called corrosion. Metal corrosion is caused, as a result of which metals pass into an oxidized form and lose their properties, which renders metallic materials unusable.

There are 3 features that characterize corrosion:

• Corrosion- From a chemical point of view, this is a redox process.
• Corrosion is a spontaneous process that occurs due to the instability of the thermodynamic system metal - environmental components.
• Corrosion is a process that develops mainly on the surface of the metal. However, it is possible that corrosion can penetrate deep into the metal.

Types of metal corrosion

The most common are the following types of metal corrosion:

1. Uniform – covers the entire surface evenly
2. Uneven
3. Electoral
4. Local stains – individual areas of the surface are corroded
5. Ulcerative (or pitting)
6. Spot
7. Intercrystalline - spreads along the boundaries of a metal crystal
8. Cracking
9. Subsurface

Main types of corrosion

From the point of view of the mechanism of the corrosion process, two main types of corrosion can be distinguished: chemical and electrochemical.

Chemical corrosion of metals

Chemical corrosion of metals - this is the result of such chemical reactions, in which after destruction metal connection, metal atoms and atoms that are part of the oxidizing agents form. In this case, no electric current occurs between individual sections of the metal surface. This type of corrosion is inherent in media that are not capable of conducting electric current - these are gases and liquid non-electrolytes.

Chemical corrosion of metals can be gas or liquid.

Gas corrosion of metals – this is the result of the action of aggressive gas or steam environments on the metal at high temperatures, in the absence of moisture condensation on the metal surface. These are, for example, oxygen, sulfur dioxide, hydrogen sulfide, water vapor, halogens. Such corrosion in some cases can lead to complete destruction metal (if the metal is active), and in other cases a protective film may form on its surface (for example, aluminum, chromium, zirconium).

Liquid corrosion of metals – can occur in non-electrolytes such as oil, lubricating oils, kerosene, etc. This type of corrosion, in the presence of even a small amount of moisture, can easily acquire an electrochemical nature.

For chemical corrosion the rate of metal destruction is proportional to the speed with which the oxidizing agent penetrates the metal oxide film covering its surface. Metal oxide films may or may not exhibit protective properties, which is determined by continuity.

Continuity such a film is estimated to be Pilling-Badwords factor: (α = V ok /V Me) in relation to the volume of the formed oxide or any other compound to the volume of metal spent on the formation of this oxide

α = V ok /V Ме = М ok ·ρ Ме /(n·A Me ·ρ ok),

where V ok is the volume of the formed oxide

V Me is the volume of metal consumed to form the oxide

M ok – molar mass of the formed oxide

ρ Me – metal density

n – number of metal atoms

A Me - atomic mass metal

ρ ok - density of the formed oxide

Oxide films, which α < 1 , are not continuous and through them oxygen easily penetrates to the surface of the metal. Such films do not protect metal from corrosion. They are formed by the oxidation of alkali and alkaline earth metals (except beryllium) with oxygen.

Oxide films, which 1 < α < 2,5 are solid and are able to protect the metal from corrosion.

With values α > 2.5 the continuity condition is no longer met, as a result of which such films do not protect the metal from destruction.

Below are the values α for some metal oxides

metal	oxide	α	metal	oxide	α
K	K2O	0,45	Zn	ZnO	1,55
Na	Na2O	0,55	Ag	Ag2O	1,58
Li	Li2O	0,59	Zr	ZrO2	1.60
Ca	CaO	0,63	Ni	NiO	1,65
Sr	SrO	0,66	Be	BeO	1,67
Ba	BaO	0,73	Cu	Cu2O	1,67
Mg	MgO	0,79	Cu	CuO	1,74
Pb	PbO	1,15	Ti	Ti2O3	1,76
Cd	CdO	1,21	Cr	Cr2O3	2,07
Al	Al2O2	1,28	Fe	Fe2O3	2,14
Sn	SnO2	1,33	W	WO 3	3,35
Ni	NiO	1,52

Electrochemical corrosion of metals

Electrochemical corrosion of metals is the process of destruction of metals in various environments, which is accompanied by the appearance of an electric current within the system.

With this type of corrosion, an atom is removed from the crystal lattice as a result of two coupled processes:

• Anode – metal in the form of ions goes into solution.
• cathodic – electrons formed during the anodic process are bound by a depolarizer (the substance is an oxidizing agent).

The process of removing electrons from the cathode sites is called depolarization, and the substances that promote removal are called depolarizers.

The most widespread corrosion of metals with hydrogen and oxygen depolarization.

Hydrogen depolarization carried out at the cathode during electrochemical corrosion in an acidic environment

2H + +2e - = H 2 hydrogen ion discharge

2H 3 O + +2e - = H 2 + 2H 2 O

Oxygen depolarization carried out at the cathode during electrochemical corrosion in a neutral environment

O 2 + 4H + +4e - = H 2 O dissolved oxygen reduction

O 2 + 2H 2 O + 4e - = 4OH -

All metals, in their relation to electrochemical corrosion, can be divided into 4 groups, which are determined by their values:

1. Active metals (high thermodynamic instability) - these are all metals that are in the range of alkali metals - cadmium (E 0 = -0.4 V). Their corrosion is possible even in neutral aqueous environments in which there is no oxygen or other oxidizing agents.
2. Intermediate activity metals (thermodynamic instability) - located between cadmium and hydrogen (E 0 = 0.0 V). IN neutral environments, in the absence of oxygen, do not corrode, but are subject to corrosion in acidic environments.
3. Low-active metals (intermediate thermodynamic stability) - are between hydrogen and rhodium (E 0 = +0.8 V). They are resistant to corrosion in neutral and acidic environments in which there is no oxygen or other oxidizing agents.
4. Noble metals (high thermodynamic stability) – gold, platinum, iridium, palladium. They can be subject to corrosion only in acidic environments in the presence of strong oxidizing agents.

Electrochemical corrosion can occur in various environments. Depending on the nature of the environment, the following types of electrochemical corrosion are distinguished:

• Corrosion in electrolyte solutions- in solutions of acids, bases, salts, in natural water.
• Atmospheric corrosion– in atmospheric conditions and in any humid gas environment. This is the most common type of corrosion.

For example, when iron interacts with environmental components, some of its sections serve as the anode, where iron oxidation occurs, and others serve as the cathode, where oxygen reduction occurs:

A: Fe – 2e – = Fe 2+

K: O 2 + 4H + + 4e - = 2H 2 O

The cathode is the surface where the oxygen flow is greater.

• Soil corrosion– depending on the composition of the soil, as well as its aeration, corrosion can occur more or less intensely. Acidic soils the most aggressive, and the sandy ones the least.
• Aeration corrosion— occurs when there is uneven access of air to different parts of the material.
• Marine corrosion- flows into sea ​​water, due to the presence of dissolved salts, gases and organic substances in it .
• Biocorrosion– occurs as a result of the activity of bacteria and other organisms that produce gases such as CO 2, H 2 S, etc., which contribute to metal corrosion.
• Electrocorrosion– occurs under the influence of stray currents in underground structures, as a result of electrical work railways, tram lines and other units.

Methods of protection against metal corrosion

The main method of protecting metal from corrosion is Creation protective coatings – metallic, non-metallic or chemical.

Metal coatings.

Metal coating is applied to the metal that needs to be protected from corrosion with a layer of another metal that is resistant to corrosion under the same conditions. If the metal coating is made of metal with more negative potential ( more active ) than the protected one, it is called anodic coating. If the metal coating is made of metal with more positive potential(less active) than the protected one, then it is called cathode coating.

For example, when applying a layer of zinc to iron, if the integrity of the coating is compromised, the zinc acts as an anode and will be destroyed, while the iron is protected until all the zinc is used up. The zinc coating is in this case anodic.

Cathode the coating to protect the iron may, for example, be copper or nickel. If the integrity of such a coating is violated, the protected metal is destroyed.

Non-metallic coatings.

Such coatings can be inorganic ( cement mortar, glassy mass) and organic (high molecular compounds, varnishes, paints, bitumen).

Chemical coatings.

In this case, the protected metal is subjected to chemical treatment in order to form a corrosion-resistant film of its compound on the surface. These include:

oxidation – obtaining stable oxide films (Al 2 O 3, ZnO, etc.);

phosphating – receiving protective film phosphates (Fe 3 (PO 4) 2, Mn 3 (PO 4) 2);

nitriding – the surface of the metal (steel) is saturated with nitrogen;

blueing – the metal surface interacts with organic substances;

cementation – obtaining on the surface of the metal its connection with carbon.

Changing the composition of technical metal also helps to increase the metal's resistance to corrosion. In this case, compounds are introduced into the metal that increase its corrosion resistance.

Changes in the composition of the corrosive environment(introduction of corrosion inhibitors or removal of impurities from the environment) is also a means of protecting metal from corrosion.

Electrochemical protection is based on connecting the protected structure to the cathode of an external source direct current, causing it to become a cathode. The anode is scrap metal, which, when destroyed, protects the structure from corrosion.

Tread protection - one of the types electrochemical protection– is as follows.

Plates of a more active metal, called protector. The protector - a metal with a more negative potential - is the anode, and the protected structure is the cathode. The connection of the protector and the protected structure with a current conductor leads to the destruction of the protector.

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