It is not subject to the rusting process. Anti-corrosion

Ministry of Education of the Russian Federation

Pacific State Economic University

ABSTRACT

Discipline:Chemistry

Topic: Metal corrosion

Completed:

Student of group 69

Krivitskaya Evgenia

Nakhodka

Corrosion of non-metallic materials

As operating conditions become more severe (increasing temperature, mechanical stress, environmental aggressiveness, etc.), non-metallic materials are also exposed to the action of the environment. In connection with this, the term “corrosion” began to be used in relation to these materials, for example, “corrosion of concrete and reinforced concrete”, “corrosion of plastics and rubber”. This refers to their destruction and loss of operational properties as a result of chemical or physico-chemical interaction with the environment. But it should be taken into account that the mechanisms and kinetics of processes for non-metals and metals will be different.

Metal corrosion

The formation of galvanic couples is usefully used to create batteries and accumulators. On the other hand, the formation of such a pair leads to an unfavorable process, the victim of which becomes whole line metals, - corrosion. Corrosion is understood as electrochemical or chemical destruction occurring on a surface. metal material. Most often, during corrosion, the metal is oxidized to form metal ions, which, upon further transformations, produce various corrosion products. Corrosion can be caused by either a chemical or an electrochemical process. Accordingly, a distinction is made between chemical and electrochemical corrosion of metals.

Chemical corrosion

Chemical corrosion is the interaction of the metal surface with (corrosion- active) environment, not accompanied by the occurrence of electrochemical processes at the phase boundary. In this case, the interactions of metal oxidation and reduction of the oxidizing component of the corrosive environment occur in one act. For example, the formation of scale when iron-based materials react at high temperatures with oxygen:

4Fe + 3O 2 → 2Fe 2 O 3

At electrochemical corrosion ionization of metal atoms and reduction of the oxidizing component of a corrosive environment do not occur in one act and their rates depend on the electrode potential of the metal (for example, rusting of steel in sea ​​water).

Electrochemical corrosion

The destruction of metal under the influence of galvanic cells arising in a corrosive environment is called electrochemical corrosion. Not to be confused with electrochemical corrosion is the corrosion of a homogeneous material, such as the rusting of iron or the like. Electrochemical corrosion (the most common form of corrosion) always requires the presence of an electrolyte (condensate, rainwater, etc.) with which the electrodes are in contact - either various elements material structure, or two different contacting materials with different redox potentials. If ions of salts, acids, or the like are dissolved in water, its electrical conductivity increases, and the rate of the process increases.

Corrosive element

When two metals with different redox potentials come into contact and are immersed in an electrolyte solution, such as rainwater with dissolved carbon dioxide CO 2 , a galvanic cell is formed, the so-called corrosion cell. It is nothing more than a closed galvanic cell. In it, a slow dissolution of a metallic material with a lower redox potential occurs; the second electrode in a pair, as a rule, does not corrode. This type of corrosion is especially characteristic of metals with high negative potentials. So, a very small amount of impurities on the surface of a metal with a large redox potential is already sufficient for the appearance of a corrosive element. Particularly at risk are places where metals with different potentials come into contact, such as welds or rivets.

If the dissolving electrode is corrosion-resistant, the corrosion process slows down. This is the basis, for example, for the protection of iron products from corrosion by tinning or galvanizing - tin or zinc has a more negative potential than iron, therefore, in such a pair, iron is reduced, and tin or zinc must corrode. However, due to the formation of an oxide film on the surface of tin or zinc, the corrosion process is greatly slowed down.

Hydrogen and oxygen corrosion

If reduction of H 3 O + ions or H 2 O water molecules occurs, they speak of hydrogen corrosion or corrosion with hydrogen depolarization. Ion reduction occurs according to the following scheme:

2H 3 O + + 2e − → 2H 2 O + H 2

2H 2 O + 2e − → 2OH − + H 2

If hydrogen is not evolved, which often happens in neutral or strongly alkaline environment, oxygen is reduced and here we speak of oxygen corrosion or corrosion with oxygen depolarization:

O 2 + 2H 2 O + 4e - → 4OH -

A corrosive element can form not only when two different metals come into contact. A corrosive element is also formed in the case of one metal, if, for example, the surface structure is heterogeneous.

Anti-corrosion

Corrosion results in billions of dollars in losses every year, and solving this problem is an important task. The main damage caused by corrosion is not the loss of metal as such, but the enormous cost of products destroyed by corrosion. That is why the annual losses from it in industry developed countries so great. True losses from it cannot be determined by assessing only direct losses, which include the cost of a collapsed structure, the cost of replacing equipment, and the cost of measures to protect against corrosion. Even greater damage comes from indirect losses. These include equipment downtime when replacing corroded parts and assemblies, product leakage, and disruption of technological processes.

Ideal corrosion protection of 80% is ensured proper preparation surface, and only 20% by the quality of the paints and varnishes used and the method of their application. . The most productive and effective method surface preparation before further protection of the substrate is abrasive blasting .

Typically, there are three areas of corrosion protection methods:

1. Structural

2. Active

3. Passive

To prevent corrosion, they are used as structural materials. stainless steels , Corten steels , non-ferrous metals .

As protection against corrosion, application of any coatings, which prevents the formation of a corrosive element (passive method).

Oxygen corrosion of galvanized iron

Oxygen corrosion of tin-coated iron

Paint coating, polymer coating and enameling must, first of all, prevent the access of oxygen and moisture. Coating, for example, of steel with other metals such as zinc, tin, chromium, and nickel is often also used. The zinc coating protects the steel even when the coating is partially destroyed. Zinc has a more negative potential and corrodes first. Zn 2+ ions are toxic. In the manufacture of cans, tin coated with a layer of tin is used. Unlike galvanized sheet, when the tin layer is destroyed, the iron begins to corrode, and more intensely, since tin has a more positive potential. Another way to protect metal from corrosion is to use a protective electrode with a high negative potential, for example, made of zinc or magnesium. For this purpose, a corrosion element is specially created. The protected metal acts as a cathode, and this type of protection is called cathodic protection. The dissolving electrode is called, accordingly, a sacrificial protection anode. This method is used to protect sea vessels, bridges, boiler plants, and underground pipes from corrosion. To protect the ship's hull, zinc plates are attached to the outer side of the hull.

If we compare the potentials of zinc and magnesium with iron, they have more negative potentials. But nevertheless, they corrode more slowly due to the formation of a protective oxide film on the surface, which protects the metal from further corrosion. The formation of such a film is called metal passivation. In aluminum it is enhanced by anodic oxidation (anodizing). When a small amount of chromium is added to steel, an oxide film forms on the surface of the metal. The chromium content in stainless steel is more than 12 percent.

Cold galvanizing system

The cold galvanizing system is designed to enhance the anti-corrosion properties of a complex multi-layer coating. The system provides complete cathodic (or galvanic) protection of iron surfaces against corrosion in various aggressive environments

The cold galvanizing system is available in one, two or three packages and includes:

binder - compositions on chlorinated rubber, ethyl silicate, polystyrene, epoxy, urethane, alkyd (modified) basis are known;

Anticorrosive filler - zinc powder ("zinc dust"), with a content of more than 95% of metallic zinc, having a particle size of less than 10 microns and a minimum degree of oxidation .;

hardener (in two- and three-pack systems)

One pack cold galvanizing systems are supplied ready to use and require only thorough mixing of the composition prior to application. Two- and three-pack systems can be supplied in multiple packages and require additional preparation steps before application (mixing binder, filler, hardener).

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

1

Corrosion refers to the destruction of materials caused by physical and chemical or purely chemical influence 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 at 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 at different rates, which depend on the type of corrosive environment, the structure of the 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.
• 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 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.

Basic material for studying the topic:

§ 13, page 81.

Gabrielyan, O. S.

Chemistry. 9th grade: Bustard, 2013.

Additional material on the topic “Corrosion of metals”

Corrosion, rusting, rust is the spontaneous destruction of metals as a result of chemical or physico-chemical interaction with the environment. In general, this is the destruction of any material, be it metal or ceramics, wood or polymer. The cause of corrosion is the thermodynamic instability of structural materials to the effects of substances in the environment in contact with them. Example - oxygen corrosion of iron in water:

Iron hydroxide Fe(OH) 3 is what is called rust.

In everyday life, the term “rusting” is more often used for iron (steel) alloys. Less well known are cases of corrosion of polymers. In relation to them, there is the concept of “aging”, similar to the term “corrosion” for metals. For example, the aging of rubber due to interaction with atmospheric oxygen or the destruction of some plastics under the influence of precipitation, as well as biological corrosion. The rate of corrosion, like any chemical reaction, is very dependent on temperature. An increase in temperature of 100 degrees can increase the corrosion rate by several orders of magnitude.

Classification of types of corrosion

Corrosion processes are characterized by a wide distribution and diversity of conditions and environments in which they occur. Therefore, there is not yet a unified and comprehensive classification of the occurrence of corrosion cases.

Depending on the type of aggressive environment in which the destruction process occurs, corrosion can be of the following types:

gas corrosion;

atmospheric corrosion;

corrosion in non-electrolytes;

corrosion in electrolytes;

underground corrosion;

biocorrosion;

corrosion due to stray currents.

According to the conditions of the corrosion process, the following types are distinguished:

contact corrosion;

crevice corrosion;

corrosion at incomplete immersion;

corrosion at full immersion;

corrosion during variable immersion;

friction corrosion;

intergranular corrosion;

Stress corrosion.

By nature of destruction:

continuous corrosion covering the entire surface:

• uniform;

  uneven;

  selective;

local (local) corrosion covering individual areas:

• ulcerative;

  point;

  end-to-end;

  intergranular (delamination in deformed workpieces and knife-edge in welded joints).

The main classification is made according to the mechanism of the process. There are two types:

chemical corrosion;

electrochemical corrosion.

Corrosion of non-metallic materials

As operating conditions become more severe (increasing temperature, mechanical stress, environmental aggressiveness, etc.), non-metallic materials are also exposed to the action of the environment. In connection with this, the term “corrosion” began to be used in relation to these materials, for example, “corrosion of concrete and reinforced concrete”, “corrosion of plastics and rubber”. This refers to their destruction and loss of operational properties as a result of chemical or physico-chemical interaction with the environment. But it should be taken into account that the mechanisms and kinetics of processes for non-metals and metals will be different.

Metal corrosion

Rust is the most common type of corrosion.

Corrosion of metal.

Corrosion of metals is the destruction of metals due to their chemical or electrochemical interaction with a corrosive environment. For the corrosion process, the term “corrosion process” should be used, and for the result of the process, “corrosion destruction”. The formation of galvanic couples is usefully used to create batteries and accumulators. On the other hand, the formation of such a pair leads to an unfavorable process, the victim of which is a number of metals - corrosion. Corrosion refers to the electrochemical or chemical destruction of a metal material that occurs on the surface. Most often, during corrosion, the metal is oxidized to form metal ions, which, upon further transformations, produce various corrosion products. Corrosion can be caused by either a chemical or an electrochemical process. Accordingly, a distinction is made between chemical and electrochemical corrosion of metals.

Types of corrosion

There are 4 main types of corrosion: electrochemical corrosion, hydrogen, oxygen corrosion and chemical.

Electrochemical corrosion

The destruction of metal under the influence of galvanic cells arising in a corrosive environment is called electrochemical corrosion. Corrosion of a homogeneous material, for example, rusting of iron, etc., should not be confused with electrochemical corrosion. Electrochemical corrosion (the most common form of corrosion) always requires the presence of an electrolyte (Condensate, rainwater, etc.) with which the electrodes are in contact - either different elements of the material structure, or two different contacting materials with different redox potentials. If ions of salts, acids, or the like are dissolved in water, its electrical conductivity increases and the speed of the process increases.

Corrosive element

When two metals with different redox potentials come into contact and are immersed in an electrolyte solution, such as rainwater with dissolved carbon dioxide CO 2 , a galvanic cell is formed, the so-called corrosion cell. It is nothing more than a closed galvanic cell. In it, a slow dissolution of a metallic material with a lower redox potential occurs; the second electrode in a pair, as a rule, does not corrode. This type of corrosion is especially characteristic of metals with high negative potentials. So, a very small amount of impurities on the surface of a metal with a large redox potential is already sufficient for the appearance of a corrosive element. Particularly at risk are places where metals with different potentials come into contact, such as welds or rivets.

If the dissolving electrode is corrosion-resistant, the corrosion process slows down. This is the basis, for example, for protecting iron products from corrosion by galvanizing - zinc has a more negative potential than iron, therefore, in such a pair, iron is reduced, and zinc must corrode. However, due to the formation of an oxide film on the surface of zinc, the corrosion process slows down greatly.

An example of large-scale electrochemical corrosion is the incident that occurred in December 1967 with the Norwegian ore carrier Anatina. Anatina), traveling from Cyprus to Osaka. A typhoon that struck in the Pacific Ocean led to the entry of salt water into the holds and the formation of a large galvanic couple: a copper concentrate with the steel hull of the ship, which soon softened, and the ship sent a distress signal. The crew was rescued by a German ship that arrived in time, and the Anatina itself barely made it to the port.

Hydrogen and oxygen corrosion

If reduction of H 3 O + ions or H 2 O water molecules occurs, they speak of hydrogen corrosion or corrosion with hydrogen depolarization. Ion reduction occurs according to the following scheme:

2H 3 O + + 2e − → 2H 2 O + H 2

2H 2 O + 2e − → 2OH − + H 2

If hydrogen is not released, which often happens in a neutral or strongly alkaline environment, oxygen is reduced and we speak of oxygen corrosion or corrosion with oxygen depolarization:

O 2 + 2H 2 O + 4e − → 4OH −

A corrosive element can form not only when two different metals come into contact. A corrosive element is also formed in the case of one metal, if, for example, the surface structure is heterogeneous.

Chemical corrosion

Towel warmer electrocorrosion

Chemical corrosion is the interaction of a metal surface with a corrosive environment, not accompanied by the occurrence of electrochemical processes at the phase boundary. In this case, the interactions of metal oxidation and the reduction of the oxidizing component of the corrosive environment occur in one act. For example, the formation of scale when iron-based materials react at high temperatures with oxygen:

During electrochemical corrosion, the ionization of metal atoms and the reduction of the oxidizing component of the corrosive environment do not occur in one act and their rates depend on the electrode potential of the metal (for example, rusting of steel in sea water).

Types of corrosion

Gas corrosion

Atmospheric corrosion

Corrosion by partial immersion

Waterline corrosion

Corrosion at full immersion

Corrosion under variable immersion

Underground corrosion

Biocorrosion

Corrosion by external current

Stray current corrosion

Contact corrosion

Friction corrosion

Fretting corrosion

Complete corrosion

Uniform corrosion

Uneven corrosion

Local corrosion

Subsurface corrosion

Pitting corrosion

Corrosion spots

Through corrosion

Layer corrosion

Filiform corrosion

Structural corrosion

Intergranular corrosion

Selective corrosion

Graphitization of cast iron

Dezincification

Crevice corrosion

Knife corrosion

Corrosion ulcer

Corrosion cracking

stress corrosion

Corrosion fatigue

Corrosion fatigue limit

Corrosion brittleness

Anti-corrosion

Corrosion results in billions of dollars in losses every year, and solving this problem is an important task. The main damage caused by corrosion is not the loss of metal as such, but the enormous cost of products destroyed by corrosion. That is why the annual losses from it in industrialized countries are so great. True losses from it cannot be determined by assessing only direct losses, which include the cost of a collapsed structure, the cost of replacing equipment, and the cost of measures to protect against corrosion. Even greater damage comes from indirect losses. These include equipment downtime when replacing corroded parts and assemblies, product leakage, and disruption of technological processes.

Ideal corrosion protection is 80% ensured by proper surface preparation, and only 20% by the quality of the paints used and the way they are applied. The most productive and effective method of surface preparation before further protection of the substrate is abrasive blast cleaning.

Typically, there are three areas of corrosion protection methods:

Structural

Active

Passive

To prevent corrosion, stainless steels, Corten steels, and non-ferrous metals are used as structural materials. When designing a structure, they try to isolate as much as possible from the ingress of a corrosive environment, using adhesives, sealants, rubber gaskets.

Active methods of combating corrosion are aimed at changing the structure of the electrical double layer. A constant electric field is applied using a direct current source, the voltage is selected in order to increase the electrode potential of the protected metal. Another method is to use a sacrificial anode, a more active material that will break down, protecting the item being protected.

As protection against corrosion, the application of a coating that prevents the formation of a corrosive element (passive method) can be used.

Oxygen corrosion of galvanized iron

Oxygen corrosion of tin-coated iron

Paint coating, polymer coating and enameling must, first of all, prevent the access of oxygen and moisture. Coating, for example, of steel with other metals such as zinc, tin, chromium, and nickel is often also used. The zinc coating protects the steel even when the coating is partially destroyed. Zinc has a more negative potential and corrodes first. Zn 2+ ions are toxic. In the manufacture of cans, tin coated with a layer of tin is used. Unlike galvanized sheet, when the tin layer is destroyed, the iron begins to corrode, and more intensely, since tin has a more positive potential. Another way to protect metal from corrosion is to use a protective electrode with a high negative potential, for example, made of zinc or magnesium. For this purpose, a corrosion element is specially created. The protected metal acts as a cathode, and this type of protection is called cathodic protection. The soluble electrode is called, respectively, the anode of the sacrificial protection. This method is used to protect sea vessels, bridges, boiler plants, and underground pipes from corrosion. To protect the ship's hull, zinc plates are attached to the outer side of the hull.

If we compare the potentials of zinc and magnesium with iron, they have more negative potentials. But nevertheless, they corrode more slowly due to the formation of a protective oxide film on the surface, which protects the metal from further corrosion. The formation of such a film is called metal passivation. In aluminum it is enhanced by anodic oxidation (anodizing). When a small amount of chromium is added to steel, an oxide film forms on the surface of the metal. The chromium content in stainless steel is more than 12 percent.

Thermal spraying

To combat corrosion, thermal spray methods are also used. Using thermal spraying, a layer of another metal/alloy is created on the surface of a metal, which is more resistant to corrosion (insulating) or, conversely, less resistant (protective). This layer allows you to stop corrosion of the protected metal. The essence of the method is as follows: particles of a metal mixture, for example zinc, are applied to the surface of the product with a gas jet at high speed, resulting in the formation of protective layer thickness from tens to hundreds of microns. Thermal spraying is also used to extend the life of worn-out equipment components: from restoring steering racks in car repair shops to oil production companies.

Thermal diffusion zinc coating

To operate metal products in aggressive environments, more durable anti-corrosion protection of the surface of metal products is necessary. Thermal diffusion zinc coating is anodic in relation to ferrous metals and electrochemically protects steel from corrosion. It has strong adhesion (adhesion) to the base metal due to the mutual diffusion of iron and zinc in the surface intermetallic phases, so peeling and chipping of coatings does not occur during impacts, mechanical loads and deformations of the processed products.

Diffusion galvanizing, carried out from the vapor or gas phase at high temperatures (375-850 °C), or using rarefaction (vacuum) - at temperatures from 250 °C, is used to coat fasteners, pipes, fittings and other structures. Significantly increases the resistance of steel and cast iron products in environments containing hydrogen sulfide (including against hydrogen sulfide corrosion cracking), industrial atmosphere, sea water, etc. The thickness of the diffusion layer depends on temperature, time, galvanizing method and can be 0.01-1. 5 mm. The modern process of diffusion galvanizing allows the formation of a coating on the threaded surfaces of fasteners, without complicating their subsequent screwing. Microhardness of the coating layer Hμ = 4000 - 5000 MPa. Diffusion zinc coating also significantly increases the heat resistance of steel and cast iron products, at temperatures up to 700 °C. It is possible to obtain alloyed diffusion zinc coatings, which are used to improve their performance characteristics.

Cadmium plating

Coating steel parts with cadmium is done using methods similar to galvanizing, but provides stronger protection, especially in sea water. It is used much less frequently due to the significant toxicity of cadmium and its high cost.

Chrome plating

Corrosion impairs the performance of pipelines.

The economic losses from metal corrosion are enormous. In the United States, according to the latest NACE data [corrosion damage and costs to combat it amounted to 3.1% of GDP ($276 billion). In Germany, this damage amounted to 2.8% of GDP. According to experts from various countries, these losses in industrialized countries range from 2 to 4% of the gross national product. At the same time, metal losses, including a mass of failed metal structures, products, and equipment, amount to 10 to 20% of annual steel production.

Collapse of the Silver Bridge.

Rust is one of the most common causes of bridge failures. Since rust has a much larger volume than the original mass of iron, its build-up can lead to uneven fit of structural parts to each other. This caused the destruction of the Mianus River Bridge in 1983 when the bearings of the lifting mechanism rusted internally. Three drivers died when they fell into the river. Investigations showed that the road drain was blocked and not cleaned, and wastewater penetrated the bridge supports. On December 15, 1967, the Silver Bridge connecting Point Pleasant, West Virginia, and Kanauga, Ohio, unexpectedly collapsed into the Ohio River. At the time of the collapse, 37 cars were moving on the bridge, and 31 of them fell along with the bridge. Forty-six people were killed and nine were seriously injured. In addition to the loss of life and injuries, the main transportation route between West Virginia and Ohio was destroyed. The cause of the collapse was corrosion

The Kinzu Bridge in Pennsylvania was destroyed in a 2003 tornado primarily because the central main bolts had rusted, significantly reducing its stability.

Homework

Alloys

Attention!!!

To get a “3” grade, it is enough to complete only the first part of the work; to get a “4” grade, you need to complete the entire part of the “3” grade of work without errors, and also without errors, the entire part of the “4” grade of work. To receive a grade of “5” you must complete all the work without errors!!!

Rating "3"

1. Which metal as a simple substance is more susceptible to corrosion?

1) 1s 2 2s 2 2p 6 3s

2) 1s 2 2s 2 2p 6 3s 2 3p 6 3d 5 4s 1

3) 1s 2 2s 2 2p 6 3s 2 3p 6 3d 10 4s 2

4) 1s 2 2s 2 2p 6 3s 2 3p 1

2. Chemical corrosion is caused by

1) water and oxygen

2) oxides of carbon and sulfur

3) salt solutions

4) all of the above factors.

3. When Ni and Fe come into contact in an acid solution

1) iron will dissolve

2) iron will be restored

3) nickel will dissolve

4) oxygen will be released

Rating "4"

4. Methods of corrosion protection, in which substances that reduce the aggressiveness of the environment are introduced into the working environment, are called

5. A method of corrosion protection in which an iron sheet is coated with a layer of tin.

6. Corrodes most actively

1) chemically pure iron

2) iron coated with a layer of tin

3) technical hardware

4) iron-titanium alloy

Rating "5"

7. Alloying element that imparts corrosion resistance to steel

8. The mass of copper released on a plate placed in a solution of copper (II) chloride if zinc weighing 13 g reacted

8. The mass of copper released on an iron plate placed in a solution of copper (II) sulfate, if iron weighing 11.2 g reacted.

CORROSION OF METALS
spontaneous physical and chemical destruction and transformation of useful metal into useless chemical compounds. Most environmental components, whether liquids or gases, contribute to the corrosion of metals; Constant natural influences cause rusting of steel structures, damage to car bodies, the formation of pitting (etching pits) on chrome coatings, etc. In these examples, the surface of the metal is visibly destroyed, but the concept of corrosion includes cases of internal destructive action, for example at the interface between metal crystals. This so-called structural (intercrystalline) corrosion occurs outwardly imperceptibly, but can lead to breakdowns and even accidents. Often, unexpected damage to metal parts is related to stress, particularly that associated with metal corrosion fatigue. Corrosion is not always destructive. For example, the green patina often seen on bronze sculptures is copper oxide, which effectively protects the metal underneath the oxide film from further atmospheric corrosion. This explains the excellent condition of many ancient bronze and copper coins. Corrosion control is carried out using protective methods developed on the basis of well-known scientific principles, but it remains one of the most serious and difficult problems modern technology. OK. 20% of the total amount of metals is lost annually due to corrosion, and huge amounts of money are spent on corrosion protection.
Electrochemical nature of corrosion. M. Faraday (1830-1840) established a connection between chemical reactions and electric current, which was the basis of the electrochemical theory of corrosion. However, a detailed understanding of corrosion processes came only at the beginning of the 20th century. Electrochemistry as a science arose in the 18th century. thanks to the invention of A. Volt (1799) of the first galvanic element (voltaic column), with the help of which a continuous current was obtained by converting chemical energy into electrical energy. A galvanic cell consists of one electrochemical cell in which two various metal(electrodes) are partially immersed in an aqueous solution (electrolyte) capable of conducting electricity. The electrodes outside the electrolyte are connected by an electrical conductor (metal wire). One electrode (the "anode") dissolves (corrodes) in the electrolyte, producing metal ions that go into solution, while hydrogen ions accumulate on the other electrode (the "cathode"). The flow of positive ions in the electrolyte is compensated by the passage of electron current (electrical current) from the anode to the cathode in an external circuit.

Metal ions, passing into solution, react with the components of the solution, producing corrosion products. These products are often soluble and do not prevent further corrosion of the metal anode. Thus, if two adjacent areas, for example on the surface of steel, differ even slightly from each other in composition or structure, then in a suitable (for example, humid) environment a corrosion cell will form at this location. One area is anode to the other, and it is this area that will corrode. Thus, all small local inhomogeneities of the metal form anodic-cathode microcells, for this reason the metal surface contains numerous areas potentially susceptible to corrosion. If the steel is lowered into plain water or almost any water-containing liquid, then a suitable electrolyte is already ready. Even in a moderately humid atmosphere, moisture condensation will settle on the metal surface, leading to the formation of an electrochemical cell. As already noted, an electrochemical cell consists of electrodes immersed in an electrolyte (i.e., two half-cells). The potential (electromotive force, EMF) of an electrochemical cell is equal to the potential difference between the electrodes of both half-cells. Electrode potentials are measured relative to a hydrogen reference electrode. The measured electrode potentials of metals are reduced to a series of voltages, in which noble metals (gold, platinum, silver, etc.) are at the right end of the series and have a positive potential value. Ordinary base metals (magnesium, aluminum, etc.) have strongly negative potentials and are located closer to the beginning of the row to the left of hydrogen. The position of the metal in the stress series indicates its resistance to corrosion, which increases from the beginning of the series to its end, i.e. from left to right.
See also ELECTROCHEMISTRY; ELECTROLYTES.
Polarization. The movement of positive (hydrogen) ions in the electrolyte towards the cathode, followed by a discharge, leads to the formation of molecular hydrogen at the cathode, which changes the potential of this electrode: a reverse sign (stationary) potential is established, which reduces the overall cell voltage. The current in the cell drops very quickly to extremely small values; in this case the cell is said to be "polarized". This condition suggests a reduction or even cessation of corrosion. However, the interaction of oxygen dissolved in the electrolyte with hydrogen can negate this effect, which is why oxygen is called a “depolarizer.” The polarization effect sometimes manifests itself as a decrease in the corrosion rate in stagnant waters due to lack of oxygen, although such cases are unusual since the effects of convection in a liquid medium are usually sufficient to supply dissolved oxygen to the cathode surface. Uneven distribution of the depolarizer (usually oxygen) over the metal surface can also cause corrosion, since this creates an oxygen concentration cell in which corrosion occurs in the same way as in any electrochemical cell.
Passivity and other anode effects. The term passivation was originally used to refer to the corrosion resistance of iron immersed in a concentrated solution of nitric acid. However, this is a more general phenomenon, since under certain conditions many metals are in a passive state. The phenomenon of passivity was explained in 1836 by Faraday, who showed that it was caused by an extremely thin oxide film formed as a result chemical reactions on the metal surface. Such a film can be restored (change chemically), and the metal again becomes active upon contact with a metal that has a more negative potential, for example, iron in the vicinity of zinc. In this case, a galvanic couple is formed in which the passive metal is the cathode. Hydrogen released at the cathode restores its protective oxide film. Oxide films on aluminum protect it from corrosion, and therefore anodized aluminum, resulting from the anodic oxidation process, is used both in decorative purposes, and in everyday life. In a broad chemical sense, all anodic processes occurring on a metal are oxidative, but the term “anodic oxidation” implies the targeted formation of a significant amount of solid oxide. A film of a certain thickness is formed on aluminum, which is the anode in a cell whose electrolyte is sulfur or phosphoric acid. Many patents describe various modifications of this process. The initially anodized surface has a porous structure and can be painted in any desired color. The introduction of potassium dichromate into the electrolyte gives a bright orange-yellow tint, while potassium hexacyanoferrate(II), lead permanganate and cobalt sulfide color the films blue, red-brown and black, respectively. In many cases, water-soluble organic dyes are used, and this gives a metallic luster to the painted surface. The resulting layer must be fixed, for which it is enough to treat the surface with boiling water, although boiling solutions of nickel or cobalt acetates are also used.
Structural (intercrystalline) corrosion. Various alloys, in particular aluminum, increase their hardness and strength with aging; the process is accelerated by subjecting the alloy to heat treatment. In this case, submicroscopic particles are formed, which are located along the boundary layers of microcrystals (in the intercrystalline space) of the alloy. Under certain conditions, the region immediately adjacent to the boundary becomes an anode with respect to the interior of the crystal, and in a corrosive environment, the boundaries between crystallites will be preferentially subject to corrosion, with corrosion cracks penetrating deeply into the metal structure. This "structural corrosion" seriously affects mechanical properties. This can be prevented either by properly selected heat treatment regimes or by protecting the metal with a corrosion-resistant coating. Cladding is the cold coating of one metal with another: a high-strength alloy is rolled between thin strips of pure aluminum and compacted. The metal included in such a composition becomes corrosion-resistant, while the coating itself has little effect on the mechanical properties.
See also METAL COATINGS.
Corrosion prevention. During electrochemical corrosion, the resulting products often dissolve (go into solution) and do not prevent further destruction of the metal; in some cases you can add to the solution chemical compound(inhibitor), which reacts with primary corrosion products to form insoluble compounds with protective properties that are deposited on the anode or cathode. For example, iron corrodes easily in a dilute solution of common salt (NaCl), but adding zinc sulfate to the solution produces slightly soluble zinc hydroxide at the cathode, and adding sodium phosphate produces insoluble iron phosphate at the anode (examples of cathodic and anodic inhibitors, respectively). Such protection methods can only be used in cases where the structure is completely or partially immersed in a liquid corrosive environment. Cathodic protection is often used to reduce the rate of corrosion. In this method, an electrical voltage is applied to the system such that the entire structure to be protected is the cathode. This is accomplished by connecting the structure to one pole of a rectifier or DC generator, while an external chemically inert anode, such as graphite, is connected to the other pole. For example, in the case of pipeline corrosion protection, an insoluble anode is buried in the ground near them. In some cases, additional protective anodes are used for these purposes, for example, suspended inside water storage containers, with the water in the container acting as an electrolyte. Other methods of cathodic protection allow sufficient current to flow from some other source through a structure that becomes the cathode entirely and contains possible local anodes and cathodes at the same potential. To do this, a metal with a more negative potential is connected to the protected metal, which plays the role of a sacrificial anode in the galvanic couple formed and is destroyed first. Zinc sacrificial anodes have been used since 1825, when the famous English chemist H. Davy proposed using them to protect copper plating on wooden ship hulls. Anodes based on magnesium alloys are widely used to protect the hulls of modern ships from corrosion in seawater. Sacrificial anodes are more often used compared to anodes connected to external current sources, since they do not require energy consumption. Surface painting is also used to protect against corrosion, especially if the structure is not completely immersed in liquid. Metal coatings can be applied by metal spraying or electroplating (e.g. chrome plating, galvanizing, nickel plating).
Types of specific corrosion. Stress corrosion is the destruction of metal under the combined action of static load and corrosion. The main mechanism is the initial formation of corrosion pitting and cracks followed by structural failure caused by stress concentrations in these cracks. The details of the corrosion mechanism are complex and not always understood; they may be associated with residual stresses. Pure metals, as well as brass, are not prone to corrosion under stress. In the case of alloys, cracks appear in the intercrystalline space, which is the anode in relation to the internal regions of the grains; this increases the likelihood of corrosion along the intercrystalline boundaries and facilitates the subsequent process of cracking along them. Corrosion fatigue is also a consequence of the combined action of mechanical stress and corrosion. However, cyclic loads are more dangerous than static ones. Fatigue cracking often occurs in the absence of corrosion, but the destructive effect of corrosion cracks, which create stress concentrations, is obvious. It is likely that all so-called fatigue mechanisms involve corrosion, since surface corrosion cannot be completely eliminated. Corrosion due to liquid metals is a special form of corrosion that does not involve an electrochemical mechanism. Liquid metals are of great importance in cooling systems, in particular nuclear reactors. Liquid potassium and sodium and their alloys, as well as liquid lead, bismuth and lead-bismuth alloys are used as coolants. Most structural metals and alloys, when in contact with such a liquid medium, are subject to destruction to one degree or another, and the corrosion mechanism may be different in each case. First, the material of the container or pipes in a heat transfer system may dissolve to a small extent in the liquid metal, and since solubility generally varies with temperature, the dissolved metal may precipitate out of solution in the cooled portion of the system, thereby clogging passages and valves. Secondly, intercrystalline penetration of liquid metal is possible if there is a selective reaction with alloying additives of the structural material. Here, as in the case of electrochemical intergranular corrosion, the mechanical properties deteriorate without visible manifestations and without changing the mass of the structure; however, such cases of destructive impact are rare. Third, liquid and solid metals can react to form a surface alloy, which in some cases serves as a diffusion barrier to further attack. Erosion corrosion (impact, cavitation corrosion) refers to the mechanical impact of liquid metal flowing in a turbulent mode. In extreme cases, this leads to cavitation and erosive failure of the structure.
See also CAVITATION. The corrosive effects of radiation are being intensively studied in connection with the development of nuclear energy, but there is little information on this issue in the open press. The commonly used term "radiation damage" refers to all changes in the mechanical, physical or chemical nature of solid materials that are caused by exposure to the following types of radiation: ionizing radiation (X-rays or g), light charged particles (electrons), heavy charged particles (a-particles) and heavy uncharged particles (neutrons). It is known that the bombardment of a metal by heavy particles of high energy leads to disturbances at the atomic level, which, under appropriate circumstances, can be the site of electrochemical reactions. However, a more important change occurs not in the metal itself, but in its environment. Such indirect effects arise as a result of the action of ionizing radiation (for example, g-rays), which does not change the properties of the metal, but aqueous solutions causes the formation of highly reactive free radicals and hydrogen peroxide, and such compounds contribute to an increase in the rate of corrosion. In addition, a corrosion inhibitor such as sodium dichromate will be reduced and lose its effectiveness. Under the influence of ionizing radiation, oxide films also become ionized and lose their corrosion-protective properties. All of the above features are highly dependent on the specific conditions associated with corrosion.
Oxidation of metals. Most metals react with atmospheric oxygen to form stable metal oxides. The rate at which oxidation occurs is highly dependent on temperature, and at normal temperatures only thin film oxide (on copper, for example, this is noticeable by the darkening of the surface). At higher temperatures, the oxidation process proceeds faster. Noble metals are an exception to this rule, since they have a low affinity for oxygen. It is assumed that gold does not oxidize at all when heated in air or in oxygen, and the weak oxidation of platinum at temperatures up to 450 ° C stops when heated to higher temperatures. Conventional structural metals oxidize to form four types of oxide compounds: volatile, dense, protective or non-porous. A small number of refractory metals, such as tungsten and molybdenum, become brittle at high temperatures and form volatile oxides, so a protective oxide layer does not form and at high temperatures the metals must be protected by an inert atmosphere (noble gases). Ultralight metals tend to form oxides that are too dense, which are porous and do not protect the metals from further oxidation. For this reason, magnesium oxidizes very easily. Protective oxide layers form on many metals, but they are usually only moderately protective. An oxide film on aluminum, for example, completely covers the metal, but cracks develop under compressive stress, apparently due to changes in temperature and humidity. The protective effect of oxide layers is limited to relatively low temperatures. Many "heavy metals" (eg copper, iron, nickel) form non-porous oxides which, although they do not crack, do not always protect the base metal. Theoretically, these oxides are of great interest and are being actively studied. They contain less than a stoichiometric amount of metal; missing metal atoms form holes in the oxide lattice. As a result, atoms can diffuse through the lattice, and the thickness of the oxide layer constantly increases.
Application of alloys. Since all known structural metals are prone to oxidation, structural elements that are at high temperatures in an oxidizing environment should be made from alloys that contain a metal that is resistant to the action of the oxidizer as an alloying element. These requirements are met by chromium, a fairly cheap metal (used in the form of ferrochrome), which is present in almost all high-temperature alloys that meet the requirements of oxidation resistance. Therefore, all stainless steels alloyed with chromium have good oxidation resistance and are widely used in household and industry. Nichrome alloy, which is widely used as wire for spirals of electric furnaces, contains 80% nickel and 20% chromium and is completely resistant to oxidation at temperatures up to 1000 ° C. Mechanical properties are no less important than oxidation resistance, and it often turns out that that certain alloy elements (such as chromium) impart both high-temperature strength and oxidation resistance to the alloy, so that the problem of high-temperature oxidation did not become a serious problem until fuel oil containing vanadium was used (in gas turbine engines) or sodium. These contaminants, together with sulfur in the fuel, produce combustion products that are extremely corrosive. Attempts to solve this problem have culminated in the development of additives that, when burned, form harmless volatile compounds with vanadium and sodium. Fretting corrosion does not include electrochemical corrosion or direct oxidation in the gas phase, but is mainly a mechanical effect. This is damage to the joints metal surfaces as a result of abrasion at their small multiple relative displacements; observed in the form of scratches, ulcers, shells; is accompanied by jamming and reduces resistance to corrosion fatigue, as the resulting scratches serve as starting points for the development of corrosion fatigue. Typical examples are damage in the grooves of the turbine blades due to vibration, abrasion of the compressor impellers, wear of the gear teeth, threaded connections etc. At small repeated displacements, the protective oxide films are destroyed, rubbed into powder, and the corrosion rate increases. Fretting corrosion of steel is easily identified by the presence of red-brown oxide particles. The fight against fretting corrosion is carried out by improving designs, using protective coatings, elastomeric gaskets, lubricants.
see also
Great Soviet Encyclopedia

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Metal corrosion- Corrosion: physicochemical interaction between the metal and the environment, as a result of which the properties of the metal change and often deterioration of the functional characteristics of the metal, the environment or the technical system that includes them occurs...

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

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, 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 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 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

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.

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

• Anode - the 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 is 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, they 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 various parts material.
• Marine corrosion– occurs in 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 the operation of electric railways, tram lines and other units.

Methods of protection against metal corrosion

The main method of protecting metal from corrosion is creation of 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 may 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 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.

Tread protection – one of the types of 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.