Chemical corrosion is a process consisting in the destruction of a metal when interacting with an aggressive external environment. The chemical variety of corrosion processes is not related to the impact electric current. With this type of corrosion, an oxidative reaction occurs, where the material being destroyed is at the same time a reducing agent of the elements of the environment.

The classification of a variety of an aggressive environment includes two types of metal destruction:

• chemical corrosion in non-electrolyte liquids;
• chemical gas corrosion.

Gas corrosion

The most common type of chemical corrosion - gas - is a corrosive process that occurs in gases at elevated temperatures. This problem is typical for many types of work. technological equipment and details (fittings of furnaces, engines, turbines, etc.). In addition, ultra-high temperatures are used in the processing of metals under high pressure (heating before rolling, stamping, forging, thermal processes, etc.).

Features of the state of metals at elevated temperatures are determined by their two properties - heat resistance and heat resistance. Heat resistance is the degree of stability of the mechanical properties of a metal at ultrahigh temperatures. Under the stability of mechanical properties is meant the retention of strength for a long time and resistance to creep. Heat resistance is the resistance of a metal to the corrosive activity of gases at elevated temperatures.

Development speed gas corrosion driven by a number of indicators, including:

• atmospheric temperature;
• components included in the metal or alloy;
• parameters of the environment where gases are located;
• duration of contact with the gaseous medium;
• properties of corrosive products.

The corrosion process is more influenced by the properties and parameters of the oxide film that appears on the metal surface. Oxide formation can be chronologically divided into two stages:

• adsorption of oxygen molecules on a metal surface interacting with the atmosphere;
• the contact of a metal surface with a gas, resulting in a chemical compound.

The first stage is characterized by the appearance of an ionic bond, as a result of the interaction of oxygen and surface atoms, when the oxygen atom takes away a pair of electrons from the metal. The resulting bond is distinguished by exceptional strength - it is greater than the bond of oxygen with the metal in the oxide.

The explanation for this connection lies in the action of the atomic field on oxygen. As soon as the metal surface is filled with an oxidizing agent (and this happens very quickly), at low temperatures, due to the van der Waals force, the adsorption of oxidizing molecules begins. The result of the reaction is the appearance of the thinnest monomolecular film, which becomes thicker over time, which complicates the access of oxygen.

At the second stage, a chemical reaction occurs, during which the oxidizing element of the medium takes valence electrons from the metal. Chemical corrosion is the end result of the reaction.

Characteristics of the oxide film

The classification of oxide films includes three types:

• thin (invisible without special devices);
• medium (temper colors);
• thick (visible to the naked eye).

Appeared oxide film has protective capabilities - it slows down or even completely inhibits the development of chemical corrosion. Also, the presence of an oxide film increases the heat resistance of the metal.

However, a truly effective film must meet a number of characteristics:

• be non-porous;
• have a solid structure;
• have good adhesive properties;
• differ in chemical inertness in relation to the atmosphere;
• be hard and wear resistant.

One of the above conditions - a solid structure is particularly important. The continuity condition is the excess of the volume of oxide film molecules over the volume of metal atoms. Continuity is the ability of oxide to cover the entire metal surface with a continuous layer. If this condition is not met, the film cannot be considered protective. However, there are exceptions to this rule: for some metals, for example, for magnesium and elements of the alkaline earth group (excluding beryllium), continuity is not a critical indicator.

Several techniques are used to determine the thickness of the oxide film. The protective qualities of the film can be determined at the time of its formation. To do this, the rate of oxidation of the metal, and the parameters of the change in rate over time, are studied.

For an already formed oxide, another method is used, consisting in the study of the thickness and protective characteristics of the film. To do this, a reagent is applied to the surface. Next, experts fix the time it takes for the penetration of the reagent, and based on the data obtained, they draw a conclusion about the film thickness.

Note! Even the finally formed oxide film continues to interact with the oxidizing environment and the metal.

Corrosion development rate

The rate at which chemical corrosion develops depends on temperature regime. At high temperatures, oxidative processes develop more rapidly. Moreover, the decrease in the role of the thermodynamic factor of the reaction does not affect the process.

Of considerable importance is cooling and variable heating. Due to thermal stresses, cracks appear in the oxide film. Through the gaps, the oxidizing element enters the surface. As a result, a new layer of the oxide film is formed, and the former one peels off.

The components of the gaseous medium also play an important role. This factor is individual for different types of metals and is consistent with temperature fluctuations. For example, copper quickly corrodes if it comes into contact with oxygen, but is resistant to this process in a sulfur oxide environment. For nickel, on the contrary, sulfur oxide is destructive, and stability is observed in oxygen, carbon dioxide and aquatic environment. But chromium is resistant to all of the listed media.

Note! If the oxide dissociation pressure level exceeds the pressure of the oxidizing element, the oxidizing process stops and the metal becomes thermodynamically stable.

The alloy components also affect the rate of the oxidative reaction. For example, manganese, sulfur, nickel, and phosphorus do nothing to oxidize iron. But aluminum, silicon and chromium make the process slower. Cobalt, copper, beryllium and titanium slow down the oxidation of iron even more. Additions of vanadium, tungsten and molybdenum will help to make the process more intensive, which is explained by the fusibility and volatility of these metals. The slowest oxidation reactions proceed with the austenitic structure, since it is most adapted to high temperatures.

Another factor on which the corrosion rate depends is the characteristics of the treated surface. A smooth surface oxidizes more slowly, while an uneven surface oxidizes faster.

Corrosion in non-electrolyte liquids

Non-conductive liquid media (i.e. non-electrolyte liquids) include such organic substances as:

• benzene;
• chloroform;
• alcohols;
• carbon tetrachloride;
• phenol;
• oil;
• petrol;
• kerosene, etc.

In addition, a small amount of inorganic liquids, such as liquid bromine and molten sulfur, are considered non-electrolyte liquids.

At the same time, it should be noted that organic solvents themselves do not react with metals, however, in the presence of a small amount of impurities, an intense interaction process occurs.

The sulfur-containing elements in the oil increase the corrosion rate. Also, corrosive processes are enhanced by high temperatures and the presence of oxygen in the liquid. Moisture intensifies the development of corrosion in accordance with the electromechanical principle.

Another factor rapid development corrosion - liquid bromine. At normal temperatures, it is especially destructive to high-carbon steels, aluminum and titanium. The effect of bromine on iron and nickel is less significant. Lead, silver, tantalum and platinum show the greatest resistance to liquid bromine.

Molten sulfur reacts aggressively with almost all metals, primarily lead, tin and copper. Sulfur affects carbon steels and titanium less and almost completely destroys aluminum.

Protective measures for metal structures located in non-conductive liquid media are carried out by adding metals that are resistant to a particular environment (for example, steels with a high chromium content). Also, special protective coatings are used (for example, in an environment where there is a lot of sulfur, aluminum coatings are used).

Corrosion protection methods

Corrosion control methods include:

The choice of a specific material depends on the potential efficiency (including technological and financial) of its use.

Modern principles of metal protection are based on the following methods:

1. Improving the chemical resistance of materials. Chemically resistant materials (high-polymer plastics, glass, ceramics) have successfully proven themselves.
2. Isolation of the material from the aggressive environment.
3. Reducing the aggressiveness of the technological environment. Examples of such actions include the neutralization and removal of acidity in corrosive environments, as well as the use of various inhibitors.
4. Electrochemical protection (imposition of external current).

The above methods are divided into two groups:

1. Chemical resistance enhancement and insulation are applied before the steel structure is put into service.
2. Reducing the aggressiveness of the environment and electrochemical protection are used already in the process of using a metal product. The use of these two techniques makes it possible to introduce new methods of protection, as a result of which protection is provided by changing operating conditions.

One of the most commonly used methods of metal protection - galvanic anti-corrosion coating - is not economically viable for large surface areas. The reason is the high cost of the preparatory process.

The leading place among the methods of protection is the coating of metals with paints and varnishes. The popularity of this method of combating corrosion is due to a combination of several factors:

• high protective properties (hydrophobicity, repulsion of liquids, low gas permeability and vapor permeability);
• manufacturability;
• ample opportunities for decorative solutions;
• maintainability;
• economic justification.

At the same time, the use of widely available materials is not without drawbacks:

• incomplete wetting of the metal surface;
• impaired adhesion of the coating to the base metal, which leads to the accumulation of electrolyte under the anti-corrosion coating and, thus, contributes to corrosion;
• porosity, leading to increased moisture permeability.

And yet, the painted surface protects the metal from corrosion processes even with fragmentary damage to the film, while imperfect galvanic coatings can even accelerate corrosion.

Organosilicate coatings

Chemical corrosion practically does not apply to organosilicate materials. The reasons for this lie in the increased chemical stability of such compositions, their resistance to light, hydrophobic properties and low water absorption. Also, organosilicates are resistant to low temperatures, have good adhesive properties and wear resistance.

The problems of metal destruction due to the effects of corrosion do not disappear, despite the development of technologies to combat them. The reason is the constant increase in the production of metals and the increasingly difficult operating conditions for products made from them. It is impossible to finally solve the problem at this stage, so the efforts of scientists are focused on finding ways to slow down corrosion processes.

Materials made of metals under the chemical or electrochemical influence of the environment are subject to destruction, which is called corrosion. Corrosion of metals 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- this is a spontaneous process that occurs due to the instability of the thermodynamic system metal - components of the environment.
• Corrosion- This 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 spots - corrode certain areas of the surface
5. Ulcerative (or pitting)
6. dotted
7. Intercrystalline - propagates along the boundaries of the 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 the occurrence of such chemical reactions in which, after the destruction of the metal bond, the metal atoms and the atoms that make up the oxidizing agents form. Electric current between individual sections of the metal surface in this case does not occur. This type of corrosion is inherent in media that are not capable of conducting electric current - these are gases, liquid non-electrolytes.

Chemical corrosion of metals is gas and liquid.

Gas corrosion of metals - this is the result of the action of aggressive gas or vapor media 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 the complete destruction of the metal (if the metal is active), and in other cases, a protective film can form on its surface (for example, aluminum, chromium, zirconium).

Liquid corrosion of metals - can occur in such non-electrolytes 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 character.

For chemical corrosion the rate of destruction of the metal is also proportional to the rate at 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 by the value Pilling-Bedwords factor: (α = V ok / V Me) in relation to the volume of the formed oxide or any other compound to the volume of the metal consumed for the formation of this oxide

α \u003d V ok / V Me \u003d M ok ρ Me / (n A Me ρ ok),

where V ok is the volume of the formed oxide

V Me is the volume of metal consumed for the formation of oxide

M ok - molar mass of the resulting oxide

ρ Me - metal density

n is the number of metal atoms

a-me- atomic mass metal

ρ ok is the 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 the metal from corrosion. They are formed during the oxidation of alkali and alkaline earth metals (excluding beryllium) with oxygen.

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

For values α > 2.5 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	SnO 2	1,33	W	WO3	3,35
Ni	NiO	1,52

Electrochemical corrosion of metals

Electrochemical corrosion of metals- this is the process of destruction of metals in a different environment, which is accompanied by the appearance of an electric current inside the system.

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

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

The very process of removing electrons from the cathode sections is called depolarization, and the substances that contribute to the removal are called depolarizers.

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

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

2H + +2e - \u003d H 2 hydrogen ion discharge

2H 3 O + + 2e - \u003d H 2 + 2H 2 O

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

O 2 + 4H + + 4e - \u003d H 2 O dissolved oxygen recovery

O 2 + 2H 2 O + 4e - \u003d 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 \u003d -0.4 V). Their corrosion is possible even in neutral aqueous media, in which there is no oxygen or other oxidizing agents.
2. Intermediate activity metals (thermodynamic instability) - located between cadmium and hydrogen (E 0 \u003d 0.0 V). In neutral environments, in the absence of oxygen, they do not corrode, but corrode in acidic environments.
3. Inactive metals (intermediate thermodynamic stability) - are between hydrogen and rhodium (E 0 \u003d +0.8 V). They are resistant to corrosion in neutral and acidic environments where oxygen or other oxidizing agents are absent.
4. noble metals (high thermodynamic stability) - gold, platinum, iridium, palladium. They can corrode only in acidic environments in the presence of strong oxidizing agents.

Electrochemical corrosion can take place in various environments. Depending on the nature of the medium, 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 the environment of any moist gas. This is the most common type of corrosion.

For example, when iron interacts with environmental components, some of its sections serve as an anode, where iron is oxidized, while others serve as a cathode, where oxygen is reduced:

A: Fe - 2e - \u003d Fe 2+

K: O 2 + 4H + + 4e - \u003d 2H 2 O

The cathode is the surface where there is more oxygen inflow.

• soil corrosion- depending on the composition of the soil, as well as its aeration, corrosion can proceed more or less intensively. acidic soils the most aggressive, and the sandy ones the least.
• Aeration corrosion- occurs with uneven access of air to various parts of the material.
• marine corrosion- flows in sea water, due to the presence of dissolved salts, gases and organic substances in it .
• Biocorrosion- occurs as a result of the vital 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 action of stray currents in underground structures, as a result of the work of electrical railways, tram lines and other units.

Metal Corrosion Protection Methods

The main way to protect against metal corrosion is creation of protective coatings- metallic, non-metallic or chemical.

Metallic coatings.

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

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

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

Not metal coatings.

Such coatings may be inorganic ( cement mortar, vitreous mass) and organic (high molecular weight 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 - obtaining a protective film of phosphates (Fe 3 (PO 4) 2, Mn 3 (PO 4) 2);

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

blueing – metal surface interacts with organic substances;

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

Change in the composition of technical metal It also improves the corrosion resistance of the metal. In this case, such compounds are introduced into the metal that increase its corrosion resistance.

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

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

Protective protection - one of the types of electrochemical protection - is as follows.

Plates of a more active metal, which is 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.

Categories ,

Corrosion is the process of spontaneous destruction of the surface of materials due to interaction with the environment. Its cause is thermodynamic instability. chemical elements to certain substances. Formally, polymers, wood, ceramics, rubber are subject to corrosion, but the term “aging” is more often used for them. The most serious damage is caused by the rusting of metals, for the protection of which high-tech countermeasures are being developed. But we will talk about this later. Scientists distinguish between chemical and electrochemical corrosion of metals.

Chemical corrosion

It usually occurs when a metal structure is exposed to dry gases, liquids or solutions that do not conduct electric current. The essence of this type of corrosion is the direct interaction of the metal with an aggressive environment. Elements chemically corrode during heat treatment or as a result of long-term operation at sufficiently high temperatures. This applies to gas turbine blades, fittings for melting furnaces, parts of internal combustion engines, and so on. As a result, certain compounds are formed on the surface: oxides, nitrides, sulfides.

It is a consequence of the contact of a metal with a liquid medium capable of conducting an electric current. Due to oxidation, the material undergoes structural changes leading to the formation of rust (an insoluble product), or metal particles pass into a solution of ions.

Electrochemical corrosion: examples

It is divided into:

• Atmospheric, which occurs when there is a liquid film on the metal surface, in which gases contained in the atmosphere (for example, O 2, CO 2, SO 2) are able to dissolve to form electrolyte systems.
• Liquid, which flows in a conductive liquid medium.
• Groundwater, which flows under the influence of groundwater.

Causes

Since usually any metal that is used for industrial needs is not ideally pure and contains inclusions of a different nature, the electrochemical corrosion of metals occurs due to the formation of a large number of short-circuited local galvanic cells on the iron surface.

Their appearance can be associated not only with the presence of various (especially metallic) impurities (contact corrosion), but also with surface heterogeneity, crystal lattice defects, mechanical damage etc.

Interaction mechanism

The process of electrochemical corrosion depends on chemical composition materials and features of the external environment. If the so-called technical metal is covered with a wet film, then in each of the said galvanic microelements, which are formed on the surface, two independent reactions take place. The more active component of the corrosion pair donates electrons (for example, zinc in a Zn-Fe pair) and passes into the liquid medium as hydrated ions (that is, it corrodes) according to the following reaction (anodic process):

M + nH 2 O \u003d M z + * nH 2 O + ze.

This part of the surface is the negative pole of the local microelement, where the metal dissolves electrochemically.

On the less active surface area, which is the positive pole of the microelement (iron in a Zn-Fe pair), electrons are bound due to the reduction reaction (cathode process) according to the scheme:

Thus, the presence of oxidizing agents in the water film, which are able to bind electrons, makes it possible to continue the anodic process. Accordingly, electrochemical corrosion can develop only if both anodic and cathodic processes occur simultaneously. Due to inhibition of one of them, the rate of oxidation decreases.

polarization process

Both of the above processes cause polarization of the respective poles (electrodes) of the microelement. What are the features here? Usually, the electrochemical corrosion of metals is more significantly slowed down by cathode polarization. Therefore, it will increase under the influence of factors that prevent this reaction and are accompanied by the so-called depolarization of the positive electrode.

In many corrosion processes, cathodic depolarization is carried out by the discharge of hydrogen ions or by the reduction of water molecules and corresponds to the formulas:

• In an acidic environment: 2H + + 2e \u003d H 2.
• In alkaline: 2H 2 O + 2e \u003d H 2 + 2OH -.

Potential range

The potential that corresponds to these processes, depending on the nature of the aggressive medium, can vary from -0.83 to 0 V. For a neutral aqueous solution at temperatures close to the standard, it is approximately -0.41 V. Therefore, hydrogen ions, contained in water and in neutral aqueous systems, can only oxidize metals with a potential less than -0.41 V (located in the voltage series up to cadmium). Considering that some of the elements are protected by an oxide film, the number of metals subject to oxidation in neutral media by hydrogen ions is insignificant.

If the wet film contains dissolved air oxygen, then it is capable, depending on the nature of the medium, of binding electrons by the effect of oxygen depolarization. In this case, the scheme of electrochemical corrosion is as follows:

• O 2 + 4e + 2H 2 O \u003d 4OH - or
• O 2 + 4e + 4H + = 2H 2 O.

The potentials of these electrode reactions at temperatures close to standard vary from 0.4 V ( alkaline environment) up to 1.23 V (acidic). In neutral media, the potential of the oxygen reduction process under these conditions corresponds to a value of 0.8 V. This means that dissolved oxygen is able to oxidize metals with a potential of less than 0.8 V (located in a series of voltages up to silver).

The most important oxidizers

Types of electrochemical corrosion are characterized by oxidizing elements, the most important of which are hydrogen ions and oxygen. At the same time, a film containing dissolved oxygen is much more corrosive than moisture, where there is no oxygen, and which is capable of oxidizing metals exclusively with hydrogen ions, since in the latter case the number of types of materials capable of corroding is much less.

For example, carbon impurities are present in steel and cast iron mainly in the form of iron carbide Fe 3 C. In this case, the mechanism of electrochemical corrosion with hydrogen depolarization for these metals is as follows:

• (-) Fe - 2e + nH 2 O = Fe 2+ nH 2 O (rust may form);
• (+) 2H + + 2e \u003d H 2 (in an acidified environment);
• (+) 2H 2 O + 2e \u003d H 2 + 2OH - (in a neutral and alkaline medium).

The corrosion mechanism of iron, which contains copper impurities, in the case of oxygen depolarization of the cathode is described by the equations:

• (-) Fe - 2e + nH 2 O = Fe 2+ nH 2 O;
• (+) 0.5O 2 + H 2 O + 2e \u003d 2OH - (in an acidified environment);
• (+) 0.5O 2 + 2H + + 2e \u003d H 2 O (in a neutral and alkaline medium).

Electrochemical corrosion proceeds at different rates. This indicator depends on:

• potential difference between the poles of a galvanic microelement;
• the composition and properties of the electrolyte environment (pH, the presence of corrosion inhibitors and stimulants);
• concentration (feed rate) of the oxidizing agent;
• temperature.

Protection methods

Electrochemical protection of metals against corrosion is achieved in the following ways:

• Creation of anticorrosive alloys (alloying).
• Increasing the purity of the individual metal.
• Applying various protective coatings to the surface.

These coatings, in turn, are:

• Non-metallic (paints, varnishes, lubricants, enamels).
• Metallic (anodic and cathodic coatings).
• Formed by special surface treatment (passivation of iron in concentrated sulfuric or nitric acids; iron, nickel, cobalt, magnesium in alkali solutions; formation of an oxide film, for example, on aluminum).

Metallic protective coating

The most interesting and promising is the electrochemical protection against corrosion by another type of metal. According to the nature of the protective effect, metallized coatings are divided into anodic and cathodic. Let's dwell on this point in more detail.

An anode coating is a coating formed by a more active (less noble) metal than the one that is being protected. That is, protection is carried out by an element that is in a series of voltages up to the base material (for example, coating iron with zinc or cadmium). With local destruction of the protective layer, the less noble metal coating will corrode. In the zone of scratches and cracks, a local galvanic cell is formed, the cathode in which is the protected metal, and the anode is the coating, which is oxidized. The integrity of such a protective film does not matter. However, the thicker it is, the slower electrochemical corrosion will develop, and the beneficial effect will last longer.

A cathodic coating is a coating with a metal with a high potential, which, in a series of voltages, is after the protected material (for example, spraying low-alloy steels with copper, tin, nickel, silver). The coating must be continuous, since if it is damaged, local galvanic cells are formed, in which the base metal will be the anode, and protective layer- cathode.

How to protect metal from oxidation

Electrochemical corrosion protection is divided into two types: sacrificial and cathodic. Protective coating is similar to anode coating. A large plate of a more active alloy is attached to the material to be protected. A galvanic cell is formed, in which the base metal serves as a cathode, and the protector serves as an anode (it corrodes). Usually, zinc, aluminum or magnesium-based alloys are used for this type of protection. The protector gradually dissolves, so it must be replaced periodically.

Lots of trouble in public utilities and in the industry as a whole delivers electrochemical corrosion of pipelines. In the fight against it, the method of cathodic polarization is most suitable. For this metal structure, which is protected from destructive oxidation processes, is connected to the negative pole of some external direct current source (it then becomes a cathode, while the rate of hydrogen evolution increases, and the corrosion rate decreases), and a low-value metal is attached to the positive pole.

Electrochemical protection methods are effective in a conductive environment (sea water is a prime example). Therefore, protectors are often used to protect the underwater parts of marine vessels.

Processing of aggressive environment

This method is effective when the electrochemical corrosion of iron occurs in a small volume of conductive liquid. In this case, there are two ways to deal with destructive processes:

• Removal of oxygen from the liquid (deaeration) as a result of purging with an inert gas.
• The introduction of inhibitors into the environment - the so-called corrosion inhibitors. For example, if the surface is destroyed as a result of oxidation with oxygen, organic substances are added, the molecules of which contain certain amino acids (imino-, thio- and other groups). They are well adsorbed on the metal surface and significantly reduce the rate of electrochemical reactions leading to destruction of the surface contact layer.

Output

Of course, chemical and electrochemical corrosion brings significant damage both in industry and in everyday life. If the metal did not corrode, the service life of many items, parts, assemblies, mechanisms would increase significantly. Now scientists are actively developing alternative materials that can replace metal, which are not inferior in terms of performance characteristics However, it is probably impossible to completely abandon its use in the short term. In this case, advanced methods of protecting metal surfaces from corrosion come to the fore.

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

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

What is metal corrosion

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

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

Chemical corrosion

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

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

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

Electrochemical corrosion

The process of electrochemical corrosion does not require mandatory immersion of the metal in the electrolyte - a thin electrolytic film on its surface is sufficient (electrolytic solutions often impregnate the environment surrounding the metal (concrete, soil, etc.)). The most common cause of electrochemical corrosion is the widespread use of household and technical salts (sodium and potassium chlorides) to remove ice and snow on roads in winter period- Cars and underground utilities are especially affected (according to statistics, annual losses in the United States from the use of salts in the winter period 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 the metal structures back into the water or into the soil, the destruction of metals occurs. Especially often, stray currents occur in places where ground electric vehicles are moving (for example, trams and electric railway locomotives). In just a year, stray currents with a power of 1A are able to dissolve iron - 9.1 kg, zinc - 10.7 kg, lead - 33.4 kg.

Other causes of metal corrosion

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

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

Measures to protect metals from corrosion

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

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

Mankind has tried many methods of protection for thousands of years. metal products from chemical corrosion, some of them are used to this day: coating with grease or oil, other metals that are less corrosive (the oldest method, which is more than 2 thousand years old, is tinning (tin coating)).

Anti-corrosion protection with non-metallic coatings

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

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

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

Protection of iron against corrosion by coatings from other metals

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

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

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

Improving corrosion resistance by adding alloying additives to steel alloys

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

Measures to counter electrochemical corrosion

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

Stray current protection

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

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

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

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Chemical corrosion

External factors of gas corrosion. Gas corrosion is a special case of chemical corrosion and is possible only under conditions that exclude the occurrence of electrochemical processes. A characteristic feature of gas corrosion is the absence of moisture on the metal surface. Therefore, in most cases we are talking about corrosion at elevated temperatures at which water is in the gas phase. However, based on the definition, one can imagine gas corrosion at room temperature, but under conditions of a high degree of dryness, natural or artificially created. So, when drying with silica gel to the dew point - 30 °С the moisture content of the air will be 0.333 g/m3. At + 20 °С it corresponds to the humidity of the air only 2 % . In such conditions

viah flow of electrochemical corrosion is virtually eliminated. In industry, cases of gas corrosion are quite common - from the destruction of parts, heating furnaces to metal corrosion during its heat treatment.

The rate of gas corrosion is affected by a number of factors, primarily such as the temperature and composition of the gaseous medium.

An increase in temperature markedly increases the rate of corrosion. In the first approximation, this relationship can be described

on the well-known physical chemistry Arrhenius equation

log K = A -

where To- speed reaction; AND and AT- constants; T- absolute temperature (°C).

It follows from the equation that the logarithm of the corrosion rate is linearly related to the reciprocal of the absolute temperature. This dependence in some cases (for example, for copper in the temperature range 700 - 900 °С) is fully confirmed, but more often it is more complex, which is associated with the influence of secondary reactions, the nature and properties of corrosion products, etc.

In a clean air environment, corrosion is reduced to the interaction of metal with oxygen. Iron already at temperature 300 °С covered in air with scale, i.e., an oxide film visible to the naked eye. The composition of the scale includes magnetite F 3 O 4 and hematite Fe2O3. With increasing temperature, up to 575 °С, the corrosion rate remains approximately constant, but starting from 575 0 C, increases sharply. This fact is associated with the appearance of wustite (iron oxide FeO).

Two groups of reactions proceed in parallel on the surface of carbon steel during corrosion: oxidation of iron to oxides with the formation of scale and decarburization reactions with the participation of iron carbide (cementite) according to the following equation:

Fe 3 C + O 2 → 3Fe + CO 2.

Thus, the surface layer of the metal is depleted in cementite. With prolonged heating, the depth of the decarburized layer can be several millimeters. This significantly affects the properties of the metal, and above all, its hardness and strength. Decarburization is also observed in the presence of carbon dioxide, water vapor or other oxidizing agents and proceeds according to similar reactions:

Fe 3 C + CO 2 → 3Fe + 2CO,

Fe 3 C + H 2 O → 3Fe + CO + H 2.

An increase in gas pressure, other things being equal, also greatly accelerates gas corrosion.

Hydrogen has a specific effect on the corrosion resistance of steel, causing at elevated temperature and pressure the so-called hydrogen embrittlement, i.e., a sharp decrease in strength. Hydrogen brittleness is explained not only by the decarburization of steel due to the reduction of cementite with hydrogen, but also by such phenomena as the molization of atomic hydrogen in the steel crystal lattice and the formation of water and methane vapor along the grain boundaries of the metal. Each of the processes leads to the generation of gas, which creates enormous pressure in the closed volume of the metal. This, in turn, causes the appearance of numerous microcracks that reduce the strength of the metal.

Gas corrosion is highly susceptible and many non-ferrous

metals, although each of them refers to certain gases in different ways. This can be illustrated by the data in Table. 2, in which the corrosion rate of metals for clarity is given in relative units, while the corrosion rate of iron in oxygen is taken as 100.

Tab. 9 convincingly demonstrates the influence of the nature of the metal on the corrosion rate. For example, if the corrosion of tungsten slows down in the transition from oxygen to water vapor by about 20 times, and copper - in 3,5 times, then the corrosion rate of iron, on the contrary, increases.

Table 9

Gas corrosion of a number of metals in certain environments

(temperature 800 °С, duration 24 hours)

oxide films. A significant influence on the rate of gas corrosion is exerted by the resulting corrosion products, their physicochemical and mechanical properties. In most cases, corrosion occurs in an oxidizing environment; in this case, an oxide film is formed on the metal surface as a corrosion product. However, a thin oxide film on the metal usually appears already at room temperature. The properties of the resulting oxide film have a decisive influence on the further course of the corrosion process. In the case of a sharp deceleration of the process up to the complete cessation of corrosion, one speaks of the onset of passivity of the metal surface.

Thermodynamics of gas corrosion. The thermodynamic possibility of the process of gas corrosion with the formation of an oxide film is determined by the magnitude of the change in the free energy of the system. There is a convenient form for determining the thermodynamic possibility of corrosion due to metal oxidation, which is reduced to comparing the elasticity of dissociation of the resulting oxidation reaction product with the partial pressure of oxygen in the gas phase.

Indeed, if the partial pressure of oxygen P O and elasticity of oxide dissociation P MeO in the metal oxidation reaction mMe + nO 2 Me m O 2 n are equal, then the reaction will be in equilibrium. If P O> P MeO, then the reaction proceeds from left to right in the direction of oxide formation. If P O< P MeO, then the oxide spontaneously dissociates into oxygen and metal. Therefore, a comparison of the elasticity of dissociation of a given oxide at a given temperature, for example, with the partial pressure of oxygen in air ( P O 0.2 at at atmospheric pressure) allows you to find the boundary by thermodynes

cal probability of the process of metal oxidation in air. So, judging by the data in Table. 10, silver already at 400 °K unable to oxidize. For copper, this boundary lies in the region 2000 °K.

Table 10

Elasticity of dissociation of silver and copper oxides

temperature dependent

Process	300 0 K	400 0 K	500 0 K	800 0 K	1200 0 K	1600 0 K
Ag 2 O 2 Ag + O 2	8,4 ∙ 10 -5	6,9 ∙ 10 -1		-	-	-
Cu 2 O 2 Cu + O 2	-	-	0,56 ∙ 10 - 30	3,7 ∙ 10 - 16	2 ∙ 10 - 8	1,8 ∙ 10 - 4

Properties of oxide films. Depending on the formation conditions, oxide films can have a thickness from monomolecular to several millimeters. There are thin, medium and thick films. Thin films have thicknesses ranging from a few angstroms to 400 Å. They are invisible and can be detected and measured by the so-called optical method of reflecting polarized light.

Medium films have a thickness 400 - 5000 Å and are visible to the naked eye due to the appearance of tint colors (a phenomenon of light interference known from physics). Their thickness can be measured by various methods, among which the most accessible are gravimetric (weight) and electrometric (cathode reduction method).

Film thickness above 5000 Å(i.e. thicker 0.5 microns) are determined by the gravimetric method or by the cathodic reduction method, as well as using a microscope, micrometer or other similar measuring instruments. They are usually easily visible to the naked eye.

It should be noted that when studying the phase composition and

The structures of oxide films are widely used by electron microscopic, electron graphic and X-ray methods of investigation.

In table. 11 gives examples of oxide films on iron. Attention is drawn to the clear dependence of the film thickness on the conditions of its formation, as well as the thickness range itself - from 15 Å before 0.6mm.

It would be erroneous to assume that the thicker the oxide film, the more reliably it protects the metal from corrosion. In reality, the situation is rather the opposite, namely, thin films have the best protective properties. However, the film thickness, strictly speaking, is not yet a criterion for the protective ability.

In order for the oxide film to have protective properties, it must first of all be continuous, non-porous. The condition for the continuity of the oxide film was formulated by Pilling and Bedworth: if the volume of metal oxide is less than the volume of the metal from which the film was formed, then the film

formed discontinuous; if the volume of metal oxide is greater than the volume of metal, then the film can be non-porous, compact.

This can be explained by the following inequalities:

< 1 the film cannot be continuous; at > 1 the film may be continuous.

In its turn

V Me = and V Me O = ,

where AND- atomic weight of the metal (i.e., a gram is considered - an atom of the metal); - metal density; M- molecular weight

metal oxides; n- the number of metal atoms in the oxide molecule; D is the density of the oxide.

Table 11

The thickness of the oxide film on iron

depending on conditions

The continuity condition is necessary and essential, but not the only one for characterizing protective

properties of the oxide film. When too large values V Me O / V Me The film experiences such high internal stresses that it collapses, losing its continuity. For example, in relation VWO / VW = 3.35 tungsten oxide film has very weak protective properties.

The film must have good adhesion to the metal, must be sufficiently strong and elastic. The coefficients of thermal expansion of the film and metal should be close enough. Finally, the film must be chemically resistant when exposed to a corrosive environment.

An important condition is the need for orientation

ionic correspondence of the resulting film to the metal. The essence of orientational correspondence is reduced to the requirement of maximum similarity between the crystal lattices of the metal and the resulting oxide with a minimum displacement of atoms. Most often, in the presence of an oxide crystal structure close to the metal structure, the protective properties of such a film are better than oxide that is not oriented with respect to the metal.

Laws of growth of oxide films. If a non-continuous oxide film is formed as a result of corrosion, oxygen gets free access to the metal surface. In this case, the corrosion rate should be a constant value:

where y- the thickness of the oxide film. After integration, we obtain the equation

y = k + A,

expressing the linear dependence of the film thickness on time. Constant AND indicates the presence of some oxide film by the time the oxidation starts ( y = a at = 0 ). As follows from the equation, the film growth rate in this case does not depend on its thickness. Corrosion can proceed at a constant rate up to the complete transformation of the metal into oxide, as occurs during the oxidation of magnesium in an oxygen environment.

However, often the actual rate of oxidation, while remaining constant, turns out to be lower than the theoretical rate of the chemical reaction of metal oxidation. This discrepancy is explained by the presence of the thinnest, up to several monomolecular layers, continuous film of pseudomorphic oxide at the metal-metal oxide interface. Pseudomorphic oxide has a high degree orientation correspondence to the metal and is, thus, a kind of crystallographic continuation of the lattice of the oxidized metal, differing at the same time in parameters from the metal oxide lattice. Being non-porous, it makes it difficult for oxygen to penetrate the metal surface.

Thus, even in the case of the formation of a thick and loose oxide film on the metal, the corrosion rate will ultimately be limited not by the rate of the oxidation reaction, but by the rate of oxygen diffusion through the compact pseudomorphic oxide.

If an oxide with sufficiently good protective properties is formed during corrosion, then the corrosion rate will depend on the ratio of the rates of mutual diffusion through the film of oxygen atoms to the metal surface and metal atoms to the oxide-gas interface. It can be shown that in this case, as the film thickness increases, the corrosion rate will slow down according to the equation

After integrating and combining the constants, we obtain a parabolic dependence of the oxide film thickness on the corrosion duration:

y 2 \u003d k + A.

This dependence is observed during the oxidation of copper, nickel, and tungsten. Given a parabolic corrosion versus time curve, one can determine the corrosion rate at any point on the curve. It will be expressed as the tangent of the slope of the tangent passing through the given point, since

tg = .

Finally, under certain conditions, the deceleration of the rate of metal oxidation with an increase in the thickness of the oxide film occurs more intensely than required by the parabolic law. In these cases, the oxidation rate is related to the film thickness by the exponential dependence

After integration, we arrive at the logarithmic equation

y \u003d ln (k).

The logarithmic film growth law is experimentally confirmed during the oxidation of aluminum and zinc in air in the temperature range 20 - 255 °C, copper - up to 100 °С, iron up 385 °С.

It is important to emphasize that the patterns of film growth on metal can vary depending on the conditions. Thus, the oxidation of iron at a temperature below 385 °С obeys a logarithmic law, in the region above this temperature and up to 1000 °С- parabolic, and at oxygen pressure below 1 mmHg Art. and temperature 700 - 950 °С- linear.

Film destruction. During the growth of the oxide film, significant internal stresses arise in it. Therefore, if the resulting film is not strong enough or has little adhesion to the metal, or is too inelastic, or for other reasons mentioned above (for example, the difference in the thermal expansion coefficients of the metal and the film), it collapses. The nature of the destruction is related to the cause that caused it. If the film strength is high and the adhesion to the metal is not good enough, bubbles form. Large bubbles usually lead to ruptures (Fig. 68, a), and protective

a B C D E

Figure 68. Types of destruction of oxide films.

a- bubble with rupture; b- microbubbles in the oxide layer (vacuum porosity); in- flaking; G- cracking at

shift; d- cracking at the corners and ribs.

film properties are sharply reduced. In other cases, small bubbles form in the oxide layer (Fig. 68, b), and then the protective properties of the film may even increase, since such a “vacuum porosity” prevents the diffusion of reacting atoms or ions and thus inhibits the corrosion process. Peeling of the oxide may be observed (Fig. 68, in), as well as cracking on the surface (Fig. 68, G) or on the corners and edges (Fig. 68, d).

Methods of protection against gas corrosion. The main method of protection against gas corrosion is reduced to the use of alloyed alloys with the so-called heat resistance. To reduce the rate of iron oxidation at 900 °С twice is enough to enter 3,5 % aluminum, and four times - about 5,5 % . The concentration of the alloying component may be negligible. So, molten magnesium oxidizes so vigorously in air that it is capable of spontaneous combustion. However, with the introduction of only 0,001 % beryllium, the rate of oxidation of magnesium is sharply reduced.

The action of alloying elements is explained by the formation on the surface of the metal protective films. They are either formed from the alloying component only or consist of mixed oxides of the alloying component and the parent metal. Spinel-type oxides have the best protective properties. The spinel structure of the oxide is characterized by a high degree of compactness of ions in the lattice and the virtual absence of vacant sites; this is what determines their high thermodynamic stability. An example of spinels are the oxides FeO ∙ Cr 2 O 3 on the surface of chromium steel or NiO ∙ Cr 2 O 3 on the surface of chromium-nickel steel.

The second method of combating gas corrosion is the use of a protective atmosphere. Depending on the nature of the metal, the gas medium should not contain oxidizing agents (for steel) or, conversely, reducing agents (for copper). In some cases, inert gases are used - nitrogen, argon. In practice, this method is found only in special cases: during heat treatment and welding. So, annealing of steel is carried out in an atmosphere containing a mixture of nitrogen, hydrogen and carbon monoxide. Welding of aluminum-magnesium and titanium parts proceeds successfully in an argon atmosphere.

The third method of reducing the rate of gas corrosion is to protect the metal surface with special heat-resistant coatings.

roofs. In some cases, the surface, for example, of a steel part, is thermally coated with an iron-aluminum or iron-chromium alloy. Both alloys have high protective properties, and the process itself is called

by aluminizing and thermochromizing, respectively. In other cases, the surface is protected with a layer of cermet - a mixture of metal and oxides. Ceramic-metal coatings (cermets) are interesting in that they combine the refractoriness, hardness and heat resistance of ceramics with the ductility and conductivity of metal. Refractory oxides are used as a non-metallic component Al2O3, MgO and compounds - such as carbides and nitrides. The metal components are the metals of the iron group, as well as chromium, tungsten, molybdenum.