Not affected by the rusting process. Corrosion control

Ministry of Education of the Russian Federation

Pacific State Economic University

ESSAY

Discipline:Chemistry

Subject: Corrosion of metals

Completed:

Group 69 student

Krivitskaya Evgeniya

Nakhodka

Corrosion of non-metallic materials

As the operating conditions become more severe (increase in temperature, mechanical stress, aggressiveness of the environment, etc.), non-metallic materials are also exposed to the action of the environment. In this connection, the term "corrosion" began to be applied to these materials, for example, "corrosion of concrete and reinforced concrete", "corrosion of plastics and rubbers". 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 nonmetals and metals will be different.

Corrosion of metals

The formation of galvanic pairs 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 whole line metals, - corrosion. Corrosion is understood as electrochemical or chemical destruction occurring on the surface. metal material. Most often, during corrosion, the metal is oxidized with the formation of metal ions, which, upon further transformations, give various corrosion products. Corrosion can be caused by both chemical and electrochemical processes. Accordingly, there are chemical and electrochemical corrosion of metals.

Chemical corrosion

Chemical corrosion - the interaction of the metal surface with (corrosion active) medium that is 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 medium proceed in one act. For example, the formation of scale when iron-based materials are exposed to oxygen at high temperature:

4Fe + 3O 2 → 2Fe 2 O 3

At electrochemical corrosion the ionization of metal atoms and the reduction of the oxidizing component of the corrosive medium 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. Thus, a very small amount of impurities on the surface of a metal with a high 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 have 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 there is a reduction of H 3 O + ions or H 2 O water molecules, they speak of hydrogen corrosion or corrosion with hydrogen depolarization. The recovery of ions 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 strong 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 a single metal, if, for example, the surface structure is inhomogeneous.

Corrosion control

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 the industrial developed countries so great. True losses from it cannot be determined by evaluating only direct losses, which include the cost of a collapsed structure, the cost of replacing equipment, and the costs of measures to protect against corrosion. Even more damage is indirect losses. These are downtime of equipment when replacing corroded parts and assemblies, leakage of products, disruption of technological processes.

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

There are usually three areas of corrosion protection methods:

1. Structural

2. Active

3. Passive

To prevent corrosion as structural materials used stainless steels , corten steels , non-ferrous metals .

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

Oxygen corrosion of galvanized iron

Oxygen corrosion of tin-plated iron

Paint coating, polymer coating and enameling should, above all, prevent the access of oxygen and moisture. Often a coating is also applied, for example steel with other metals such as zinc, tin, chromium, nickel. 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, iron begins to corrode, moreover, intensively, since tin has a more positive potential. Another possibility to protect the metal from corrosion is to use a protective electrode with a large negative potential, for example, made of zinc or magnesium. For this, 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 sacrificial protection. This method is used to protect against corrosion of ships, bridges, boiler plants, pipes located underground. 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 strengthened 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 content of chromium 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 packs and includes:

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

· anti-corrosion 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, by nature - into local and continuous.

Local corrosion is knife, intergranular, through (through corrosion is known to car owners who do not monitor the condition of the body of their vehicle), pitting, subsurface, filiform, pitting. It is also manifested by brittleness, cracking and staining. Continuous oxidation can be selective, uneven and uniform.

There are the following types of corrosion:

• biological - due to the activity of microorganisms;
• atmospheric - the destruction of materials under the influence of air;
• liquid - oxidation of metals in non-electrolytes and electrolytes;
• contact - is formed during the interaction in an electrolytic environment of metals with different values ​​​​of stationary potentials;
• gas - becomes possible at elevated temperatures in gaseous atmospheres;
• white - often found in everyday life (on objects made of galvanized steel, on radiators);
• structural - is related to the heterogeneity of materials;
• crevice - occurs exclusively in the cracks and gaps present in metal products;
• soil - noted in soils and soils;
• fretting corrosion - is formed when two surfaces move (oscillatory) in relation to each other;
• external current - the destruction of the structure, caused by the impact of electric current coming from any external source;
• wandering 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 shock contact of the structure with the external atmosphere). We have given the main types of corrosion, some of which will be discussed in more detail below.

2

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

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

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

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;
• the specified 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 proceeds at different rates, which depend on the type of corrosive medium, the structure of materials and the loads acting on them, and the temperature of the medium. If a white film appears on the contacting surfaces (the process of discoloration of the metal is observed), we are most often talking about the fretting process.

The negative consequences for metal structures caused by fretting corrosion can be leveled in the following ways:

• The use of lubricating viscous compounds. This technique works if the products are not subjected to excessive loads. Before applying the lubricant, the metal surface 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 compound is not completely removed. Lubricants, by the way, are not used to protect structures from.
• Competent choice of materials for the manufacture of construction. Fretting corrosion is extremely rare if the object is made of hard and soft metals. For example, steel surfaces are recommended to be coated with silver, cadmium, tin, lead.
• Use of additional coatings with special properties, gaskets, cobalt alloys, low friction materials.

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 the corrosion that structures and structures operating in the surface atmospheric part are exposed to. Atmospheric corrosion is wet, wet and dry. The last of these flows according to the chemical scheme, the first two - according to the electrochemical one.

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

Dry-type atmospheric corrosion occurs without the presence of a wet film on the surface of metals. At the first stages, the destruction of the material is quite fast, but then the rusting rate slows down significantly. Dry atmospheric corrosion can proceed much more actively if the structures are affected by any gas compounds present in the atmosphere (sulphurous and other gases).

Atmospheric wet-type corrosion occurs at 100% humidity. Any objects that are operated in water or are constantly exposed to moisture (for example, doused with water) are subject to it.

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

• Reducing the humidity (relative) of the air. Relatively simple yet very effective method, which consists in dehumidifying the air and heating the premises where metal structures are operated. Atmospheric corrosion with this technique is greatly slowed down.
• Coating of surfaces with non-metallic (varnishes, paints, pastes, lubricant compositions) and metallic (nickel and zinc) compositions.
• Alloying of metals. Atmospheric corrosion becomes less violent when phosphorus, titanium, chromium, copper, aluminum, and nickel are added to the metal in small quantities. They stop the anode process or transfer the steel surfaces to a passive state.
• The use of inhibitors - volatile or contact. The volatiles include dicyclohexylamine, benzoates, carbonates, monoethanolamine. And the best known contact type inhibitor is sodium nitrite.

4

Gas corrosion observed, as a rule, at elevated temperatures in an atmosphere of dry vapors and gases. The enterprises of 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 installations 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.

Most often, gas corrosion is caused by exposure to oxygen. The destruction of metals during it 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 (physical) adsorption on the metal surface of oxygen;
• adsorption of the chemical type, leading to the formation of a dense film of oxides.

After that, 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 proceeds according to 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 over +200 °C. Such corrosion is formed due to the contact of carbides included in steel alloys with hydrogen. Visually, it is poorly noticeable (the surface of the structure has no obvious damage), but at the same time, the strength characteristics of steel products are significantly reduced.

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

• at low activity in the 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. Today, this type of corrosion failure is prevented by operating with a low level of alloying. Optimum results, as practice has shown, are noted when using alloys with chromium inclusions from 8 to 13 percent.

The main material for studying the topic:

§ 13, p. 81.

Gabrielyan, O. S.

Chemistry. Grade 9: 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 the general case, this is the destruction of any material, whether it be metal or ceramics, wood or polymer. The cause of corrosion is the thermodynamic instability of structural materials to the effects of substances in contact with them. An example is oxygen corrosion of iron in water:

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

In everyday life, for iron alloys (steels), the term "rust" is more often used. Cases of corrosion of polymers are less known. 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 atmospheric precipitation, as well as biological corrosion. The rate of corrosion, like any chemical reaction, is highly dependent on temperature. An increase in temperature by 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 a variety of conditions and environments in which they occur. Therefore, there is no single and comprehensive classification of the occurring cases of corrosion yet.

According to the type of aggressive media in which the destruction process takes place, 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 the nature of the destruction:

continuous corrosion covering the entire surface:

• uniform;

  uneven;

  electoral;

local (local) corrosion, covering individual areas:

• ulcerative;

  point;

  through;

  intergranular (separating in deformed blanks and knife 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 the operating conditions become more severe (increase in temperature, mechanical stress, aggressiveness of the environment, etc.), non-metallic materials are also exposed to the action of the environment. In this connection, the term "corrosion" began to be applied to these materials, for example, "corrosion of concrete and reinforced concrete", "corrosion of plastics and rubbers". 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 nonmetals and metals will be different.

Corrosion of metals

Rust is the most common form 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 “corrosive process” should be used, and for the result of the process, “corrosive destruction”. The formation of galvanic pairs 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 is understood as the electrochemical or chemical destruction of a metallic material that occurs on the surface. Most often, during corrosion, the metal is oxidized with the formation of metal ions, which, upon further transformations, give various corrosion products. Corrosion can be caused by both chemical and electrochemical processes. Accordingly, there are chemical and electrochemical corrosion of metals.

Types of corrosion

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

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 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 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 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. Thus, a very small amount of impurities on the surface of a metal with a high 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 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 zinc surface, the corrosion process is greatly slowed down.

An example of large-scale electrochemical corrosion is the incident that occurred in December 1967 with the Norwegian ore carrier Anatina (Eng. Anatina), en route from Cyprus to Osaka. A typhoon that had flown in the Pacific Ocean led to salt water entering the holds and the formation of a large galvanic couple: copper concentrate with the steel hull of the ship, which soon softened, and the ship gave a distress signal. The crew was rescued by a German ship that came to the rescue, and Anatina herself barely made it to the port.

Hydrogen and oxygen corrosion

If there is a reduction of H 3 O + ions or H 2 O water molecules, they speak of hydrogen corrosion or corrosion with hydrogen depolarization. The recovery of ions 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 occurs in a neutral or strongly alkaline environment, oxygen reduction occurs and is referred to as oxygen corrosion or oxygen depolarization corrosion:

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 a single metal, if, for example, the surface structure is inhomogeneous.

Chemical corrosion

Towel warmer electrocorrosion

Chemical corrosion is the interaction of a metal surface with a corrosive medium, which is 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 medium proceed in one act. For example, the formation of scale when iron-based materials are exposed to oxygen at high temperature:

During electrochemical corrosion, the ionization of metal atoms and the reduction of the oxidizing component of the corrosive medium 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

Corrosion at the waterline

Corrosion at full immersion

Corrosion under variable immersion

underground corrosion

Biocorrosion

Corrosion by external current

Stray current corrosion

contact corrosion

Friction corrosion

Fretting corrosion

continuous corrosion

uniform corrosion

Uneven corrosion

localized corrosion

Subsurface corrosion

Pitting

stain corrosion

through corrosion

Layered corrosion

Filiform corrosion

Structural corrosion

Intergranular corrosion

Selective corrosion

Cast iron graphitization

Dezincification

crevice corrosion

Knife corrosion

Corrosion ulcer

stress corrosion cracking

stress corrosion

Corrosion fatigue

Corrosion fatigue limit

Corrosion brittleness

Corrosion control

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 evaluating only direct losses, which include the cost of a collapsed structure, the cost of replacing equipment, and the costs of measures to protect against corrosion. Even more damage is indirect losses. These are downtime of equipment when replacing corroded parts and assemblies, leakage of products, 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.

There are usually 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 constant 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-plated iron

Paint coating, polymer coating and enameling should, above all, prevent the access of oxygen and moisture. Often a coating is also applied, for example steel with other metals such as zinc, tin, chromium, nickel. 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, iron begins to corrode, moreover, intensively, since tin has a more positive potential. Another possibility to protect the metal from corrosion is to use a protective electrode with a large negative potential, for example, made of zinc or magnesium. For this, 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 ships, bridges, boiler plants, pipes located underground 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 strengthened 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 content of chromium in stainless steel is more than 12 percent.

Thermal spraying

Thermal spraying methods are also used to combat corrosion. With the help of thermal spraying, a layer of another metal / alloy is created on the metal surface, which has a higher resistance to corrosion (insulating) or vice versa less resistant (tread). This layer allows you to stop the corrosion of the protected metal. The essence of the method is as follows: with a gas jet, particles of a metal mixture, such as zinc, are applied to the surface of the product 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: from steering rack restoration in a car service to oil companies

Thermal diffusion zinc coating

For the operation of metal products in aggressive environments, a more stable anti-corrosion protection of the surface of metal products is required. 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 there is no peeling and chipping of the coatings under impact, mechanical stress and deformation of the processed products.

Diffusion galvanizing, carried out from a vapor or gas phase at high temperatures (375-850 °C), or using a vacuum (vacuum) - at a temperature of 250 °C, is used to coat fasteners, pipes, fittings, and other structures. Significantly increases the resistance of steel, 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 makes it possible to form a coating on the threaded surfaces of fasteners, without complicating their subsequent make-up. 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 used to improve their service characteristics.

Cadmium plating

Coating steel parts with cadmium is done in a similar way to galvanizing, but gives 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 the cost of combating it amounted to 3.1% of GDP (276 billion dollars). 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 the mass of failed metal structures, products, equipment, range from 10 to 20% of the 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 bridge over the Mianus River in 1983, when the hoist bearings corroded inside. Three drivers died in a fall into the river. Studies have shown that the road drain was blocked and not cleaned, but wastewater penetrated the bridge abutments. On December 15, 1967, the Silver Bridge connecting Point Pleasant, West Virginia, and Kanauga, Ohio, suddenly collapsed into the Ohio River. At the time of the collapse, 37 cars were moving along the bridge, and 31 of them fell along with the bridge. Forty-six people died and nine were seriously injured. In addition to loss of life and injuries, the main transportation route between West Virginia and Ohio was destroyed. The collapse was caused by corrosion.

The Kinzoo Bridge in Pennsylvania was destroyed in 2003 by a tornado primarily because the central main bolts corroded, greatly reducing its stability.

Homework

Alloys

Attention!!!

To get a mark of "3" it is enough to complete only the first part of the work, to get a mark of "4", it is necessary to complete without errors the entire part of the work for a "3" and also without errors the entire part of the work for a mark of "4". To get a score of "5" you must complete all the work without errors !!!

Grade "3"

1. Which of the metals 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. Cause chemical corrosion

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. Corrosion protection methods, in which substances are introduced into the working environment that reduce the aggressiveness of the environment, are called

5. Corrosion protection method in which the iron sheet is coated with a layer of tin

6. Most actively corrodes

1) chemically pure iron

2) iron coated with a layer of tin

3) technical iron

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 entered into the reaction

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

CORROSION OF METALS
spontaneous physical and chemical destruction and transformation of a 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 pittings (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 proceeds imperceptibly from the outside, but can lead to accidents and even accidents. Often, unexpected damage to metal parts is associated with stresses, in particular, those associated with corrosion fatigue of the metal. Corrosion is not always destructive. For example, the green patina often seen on bronze sculptures is copper oxide, which effectively protects the metal beneath the oxide film from further atmospheric corrosion. This explains the excellent condition of many ancient bronze and copper coins. Corrosion control is carried out by protection methods developed on the basis of well-known scientific principles, but it remains one of the most serious and complex tasks. 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. Volta (1799) of the first galvanic cell (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 ("anode") dissolves (corrodes) in the electrolyte, forming metal ions that go into solution, while hydrogen ions accumulate on the other electrode ("cathode"). The flow of positive ions in the electrolyte is compensated by the passage of an electron current (electrical current) from the anode to the cathode in an external circuit.

Metal ions, passing into the solution, react with the components of the solution, giving corrosion products. These products are often soluble and do not prevent further corrosion of the metal anode. So, if two adjacent areas, for example, on the surface of steel, even slightly differ from each other in composition or structure, then in a suitable (for example, humid) environment, a corrosion cell is formed at this place. One area is the anode to the other, and it is this area that will corrode. Thus, all small local inhomogeneities of the metal form anode-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 condensate will settle on the metal surface, leading to the appearance 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. The electrode potentials are measured relative to the hydrogen reference electrode. The measured electrode potentials of metals are summarized in 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 series of stresses 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 with subsequent discharge leads to the formation of molecular hydrogen on the cathode, which changes the potential of this electrode: the opposite (stationary) potential is set, which reduces the total 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, so oxygen is called a "depolarizer". The effect of polarization sometimes manifests itself as a reduction in the rate of corrosion in stagnant water due to lack of oxygen, although such cases are not typical, since the effects of convection in the liquid medium are usually sufficient to supply dissolved oxygen to the cathode surface. An uneven distribution of the depolarizer (usually oxygen) over the metal surface can also cause corrosion, since this forms an oxygen concentration cell, in which corrosion occurs in the same way as in any electrochemical cell.
Passivity and other anode effects. The term "passivity" (passivation) was originally used in relation 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 passivity phenomenon was explained in 1836 by Faraday, who showed that it was caused by an extremely thin oxide film formed as a result of chemical reactions on the metal surface. Such a film can be restored (changed chemically), and the metal becomes active again upon contact with a metal having 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. The hydrogen released on 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, as well as in everyday life. In a broad chemical sense, all anodic processes occurring on the 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 the cell, the electrolyte of which is sulfuric 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 hue, 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 imparts a metallic sheen 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 (intergranular) corrosion. Various alloys, in particular aluminum, increase their hardness and strength during 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 intergranular space) of the alloy. Under certain conditions, the region immediately adjacent to the boundary becomes an anode with respect to the inner part of the crystal, and in a corrosive environment, the boundaries between crystallites will be predominantly subject to corrosion, with corrosion cracks penetrating deeply into the metal structure. This "structural corrosion" seriously affects mechanical properties. It can be prevented either with the help of properly selected heat treatment modes, or by protecting the metal with a corrosion-impervious coating. Cladding is a 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 (pass into solution) and do not prevent further destruction of the metal; in some cases, the solution can be added chemical compound(inhibitor), which reacts with primary corrosion products to form insoluble and protective compounds that are deposited on the anode or cathode. For example, iron easily corrodes in a dilute solution of ordinary salt (NaCl), however, when zinc sulfate is added to the solution, sparingly soluble zinc hydroxide is formed at the cathode, and when sodium phosphate is added, insoluble iron phosphate is formed at the anode (examples of cathodic and anodic inhibitors, respectively). Such protection methods can only be used when the structure is wholly or partially immersed in a liquid corrosive medium. Cathodic protection is often used to reduce the rate of corrosion. In this method, an electrical voltage is applied to the system in such a way that the entire structure to be protected is the cathode. This is done 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 corrosion protection of pipelines, an insoluble anode is buried in the ground near them. In some cases, additional protective anodes are used for this purpose, for example, suspended inside water storage tanks, the water in the tank acting as an electrolyte. Other methods of cathodic protection provide sufficient current to flow from some other source through the structure, which becomes the entire cathode 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 in the formed galvanic pair plays the role of a sacrificial anode and is destroyed first. Zinc protector anodes have been used since 1825, when the famous English chemist H. Davy suggested using them to protect the copper plating of wooden ship hulls. Anodes based on magnesium alloys are widely used to protect the hulls of modern ships from corrosion in sea water. Protector anodes are more commonly used than anodes connected to external current sources, since they do not require energy. Surface painting is also used to protect against corrosion, especially if the structure is not completely immersed in liquid. Metallic coatings can be applied by metal spraying or by electroplating (eg chromium plating, zinc plating, nickel plating).
Types of specific corrosion. Stress corrosion is the destruction of metal under the influence of the combined action of static load and corrosion. The main mechanism is the initial formation of corrosion pits 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, and may be related to residual stresses. Pure metals, as well as brass, are not prone to stress corrosion. In the case of alloys, cracks appear in the intergranular space, which is the anode in relation to the internal regions of the grains; this increases the likelihood of corrosion action along the intergranular 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 that create stress concentrations is obvious. It is likely that all so-called fatigue mechanisms involve corrosion, since surface corrosion cannot be completely excluded. Liquid metal corrosion 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 in contact with such a liquid medium undergo destruction to one degree or another, and the corrosion mechanism in each case may be different. First, the material of the container or pipes in the heat transfer system may dissolve to a small extent in the liquid metal, and since the solubility usually changes with temperature, the dissolved metal may precipitate out of solution in the cooled part of the system, thereby clogging the channels and valves. Secondly, intergranular penetration of liquid metal is possible if its selective reaction with alloying additives of the structural material exists. 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. Thirdly, liquid and solid metals can interact with the formation of a surface alloy, which in some cases serves as a diffusion barrier with respect to further exposure. Erosive corrosion (impact, cavitation corrosion) refers to the mechanical action of liquid metal flowing in a turbulent regime. In extreme cases, this leads to cavitation and erosional destruction 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 common term "radiation damage" refers to all changes in the mechanical, physical or chemical nature of solid materials that are due to exposure to radiation of the following types: ionizing radiation (X-ray or g), light charged particles (electrons), heavy charged particles (a-particles) and heavy uncharged particles (neutrons). It is known that the bombardment of metal by high-energy heavy particles leads to the occurrence of disturbances at the atomic level, which, under appropriate circumstances, can be the sites 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 in aqueous solutions causes the formation of highly reactive free radicals and hydrogen peroxide, and such compounds contribute to an increase in the corrosion rate. In addition, a corrosion inhibitor such as sodium dichromate will recover and lose its effectiveness. Under the action of ionizing radiation, oxide films are also 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 temperature, 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, as 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. Ordinary structural metals, on the other hand, 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 is not formed and at high temperatures the metals should be protected by an inert atmosphere (inert gases). Ultralight metals form, as a rule, too dense oxides, 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 usually have a moderate protective ability. An oxide film on aluminum, for example, completely covers the metal, but cracks develop under compressive stresses, apparently due to changes in temperature and humidity. The protective effect of oxide layers is limited by 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 is constantly increasing.
The use 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 an oxidizing-resistant metal as an alloying element. Chromium meets these requirements - a fairly cheap metal (used in the form of ferrochromium), which is present in almost all high-temperature alloys that meet the requirements for oxidation resistance. Therefore, all stainless steels alloyed with chromium have good oxidation resistance and are widely used in household and industry. An alloy of nichrome, which is commonly used as wire for electric furnace coils, contains 80% nickel and 20% chromium and is quite resistant to oxidation at temperatures up to 1000 ° C. Mechanical properties are no less important than oxidation resistance, and often it turns out that that certain alloy elements (such as chromium) give the alloy both high-temperature strength and oxidation resistance, so that the problem of high-temperature oxidation did not introduce serious problems until the use (in gas turbine engines) of fuel oil containing vanadium or sodium. These contaminants, together with the 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 articular damage. 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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Corrosion of metals- Corrosion: physical and chemical interaction between a metal and a medium, as a result of which the properties of the metal change and often there is a deterioration in the functional characteristics of the metal, the medium or the technical system that includes them ...

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

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 petroleum, 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

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.

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.
• 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 during 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 when there is an uneven supply of air to the various parts 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 electric 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.

non-metallic 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 - 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 – 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.