They cause chemical corrosion of the metal. Anti-aging measures

Corrosion- This is the destruction of solids caused by chemical and electrochemical processes that develop on the surface of the body during its interaction with the external environment. Corrosion of metals is especially damaging. The most common and most familiar form of corrosion to all of us is iron rusting. The term "corrosion" applies to metals, concrete, some plastics and other materials. Corrosion is the physical and chemical interaction of a metal with a medium, leading to the destruction of the metal.

It is difficult to take into account the higher indirect losses from downtime and reduced productivity of equipment subjected to corrosion, from disruption of the normal course of technological processes, from accidents caused by a decrease in the strength of metal structures, etc. An accurate assessment of damage from corrosion of iron and steel is, of course, impossible. However, based on some available data on the average annual replacement volume of corrugated metal roofs, wires, pipelines, steel trolleys and other corrosive iron and steel objects, it can be concluded that due to inadequate protection, annual replacement costs can on average reach 2 percent of the total volume of steel used.

About corrosion of metals

The concepts of "corrosion" and "rust" should not be confused. If corrosion is a process, then rust is one of its results. This word applies only to iron, which is part of steel and cast iron. In what follows, the term "corrosion" will mean the corrosion of metals. According to the international standard ISO 8044, corrosion is understood as a physicochemical or chemical interaction between a metal (alloy) and a medium, leading to a deterioration in the functional properties of the metal (alloy), the medium or the technical system that includes them. Rust is a layer of partially hydrated iron oxides that forms on the surface of iron and some of its alloys as a result of corrosion.

In addition to corrosion, metal (in particular, building) structures are exposed to erosion - destruction of the material surface under the influence of mechanical stress. Erosion is provoked by rains, winds, sand dust and other natural factors.
Ideal protection against corrosion by 80% is ensured by the correct preparation of the surface for painting and only 20% by the quality of the used paints and varnishes and the method of their application (ISO).

Corrosion process

Corrosion of metals is the spontaneous destruction of metals due to chemical or electrochemical interaction with the environment.

The environment in which the metal corrodes (corrodes) is called corrosive or corrosive environment. In the case of metals, speaking about their corrosion, they mean the undesirable process of interaction of the metal with the environment.

Corrosion process stages:

• supply of a corrosive medium to the metal surface;
• interaction of the environment with the metal;
• full or partial removal of products from the metal surface.

Classification of corrosion processes

By the nature of destruction, the following types of corrosion are distinguished:

Chemical corrosion Is a process in which the oxidation of the metal and the reduction of the oxidizing component of the medium occur in one act.
Chemical corrosion is possible in any corrosive environment, but most often it occurs in cases where the corrosive environment is not an electrolyte (gas corrosion, corrosion in non-electrically conductive organic liquids).

Electrochemical corrosion- this is the destruction of metals due to their electrochemical interaction with an electrolytically conducting medium, in which the ionization of metal atoms and the reduction of the oxidizing component of the medium occur in more than one act and their rates depend on the value of the electrode potential of the metal. This type of corrosion is the most common. During electrochemical corrosion, the chemical transformation of a substance is accompanied by the release of electrical energy in the form of direct current.

Biochemical corrosion- in the case when the corrosion of metal in seawater is enhanced by the surface fouling by marine organisms.
Electrocorrosion- increased corrosion under the action of anodic polarization caused by an external electric field (for example, during welding afloat, in the presence of stray currents in the water area).

By type of corrosive environment

Some corrosive environments and the destruction they cause are so characteristic that the corrosive processes occurring in them are classified by the name of these environments.
As a rule, metal products and structures are exposed to many types of corrosion - in these cases, they speak of the action of the so-called mixed corrosion.

Gas corrosion- corrosion in a gaseous environment at high temperatures.

Atmospheric corrosion- metal corrosion under atmospheric conditions at humidity sufficient for the formation of an electrolyte film on the metal surface (especially in the presence of corrosive gases or aerosols of acids, salts, etc.). A feature of atmospheric corrosion is the strong dependence of its speed and mechanism on the thickness of the moisture layer on the metal surface or the degree of moisture of the formed corrosion products.

Liquid corrosion- corrosion in liquid media.

Underground corrosion- corrosion of metal in soils and soils. A characteristic feature of underground corrosion is a large difference in the rate of oxygen delivery to the surface of underground structures in different soils (tens of thousands of times).

By the nature of destruction, corrosion is distinguished

Solid- Covers the entire metal surface
Local- Covers specific areas of corrosion
Uniform- Flows at approximately the same speed over the entire surface
Spot (pitting)- In the form of individual points with a diameter of up to 2 mm
Ulcerative- In the form of ulcers with a diameter of 2 to 50 mm
Spots- In the form of spots with a diameter of more than 50 mm and a depth of 2 mm
Subsurface- Causes metal delamination and swelling of layers
Subfilm- Leaks under a protective metal coating
Intercrystalline- In the form of destruction of grain boundaries
Selective (selective)- In the form of dissolution of individual alloy components
Crevice- Develops in crevices and narrow gaps

Corrosion of metals can manifest itself in various forms, the main of which are:

1. General corrosion, also known as uniform corrosion. General corrosion is the most common type of metal degradation and is caused by chemical or electrochemical reactions. General corrosion degrades the entire metal surface, but is considered one of the safest forms of corrosion because it is predictable and manageable.

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

Localized corrosion is classified into three types:

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

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

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

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

In the presence of an electrochemical effect, the anode section is destroyed much more strongly than the cathodic one. Without a flow of charged particles, both metals corrode in the same way. For galvanic corrosion to exist, three conditions are necessary: ​​electrochemically dissimilar metals, direct contact of these metals, and the effect of an electrolyte.

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

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

6. Intergranular corrosion is a chemical or electrochemical destruction at the boundaries of metal grains. This phenomenon is often due to impurities in the metal, which tend to concentrate at the grain boundaries.

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

8. Frictional corrosion occurs as a result of wear and / or vibration on uneven, rough surfaces. As a result, depressions and grooves appear on the surface. Frictional corrosion is common when machine parts rotate, in bolted assemblies and bearings, and surfaces subject to vibration during transport.

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

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

Ministry of Education of the Russian Federation

Pacific State University of Economics

ESSAY

Discipline: Chemistry

Topic: Corrosion of metals

Completed:

Group 69 student

Krivitskaya Evgeniya

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Corrosion of non-metallic materials

As the operating conditions become more severe (temperature rise, mechanical stress, aggressiveness of the environment, etc.), non-metallic materials are exposed to the action of the environment. In this connection, the term "corrosion" began to be applied in relation 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 physicochemical interaction with the environment. But it should be borne in mind that the mechanisms and kinetics of processes for non-metals and metals will be different.

Corrosion of metals

The formation of galvanic couples is used to advantage in the creation of batteries and accumulators. On the other hand, the formation of such a pair leads to an unfavorable process, the victim of which becomes a number of metals - corrosion. Corrosion is understood to mean the electrochemical or chemical destruction of a metallic material occurring on the surface. Most often, during corrosion, the metal is oxidized with the formation of metal ions, which, during further transformations, give various corrosion products. Corrosion can be caused by both chemical and electrochemical processes. Accordingly, a distinction is made between chemical and electrochemical corrosion of metals.

Chemical corrosion

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

4Fe + 3O 2 → 2Fe 2 O 3

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

Electrochemical corrosion

The destruction of metal under the influence of galvanic cells arising in a corrosive environment is called electrochemical corrosion. Should not be confused with electrochemical corrosion, corrosion of a homogeneous material such as iron rust 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 come 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, for example 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. It slowly dissolves a metal material with a lower redox potential; the second electrode in a pair is generally non-corrosive. This type of corrosion is especially common in metals with high negative potentials. Thus, a very small amount of impurity 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 metal contact points with different potentials, such as welds or rivets.

If the dissolving electrode is corrosion-resistant, the corrosion process is slowed down. This is the basis, for example, of 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 the reduction of H 3 O + ions or H 2 O water molecules occurs, 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 does not evolve, which often occurs in a neutral or strongly alkaline environment, oxygen is reduced, and here they speak of oxygen corrosion or corrosion with oxygen depolarization:

O 2 + 2H 2 O + 4e - → 4OH -

A corrosive element can not only form 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 not uniform.

Corrosion control

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

Ideal protection against corrosion is 80% ensured by correct surface preparation, and only 20% by the quality of the used paints and varnishes and the method of their application. ... The most productive and effective method of 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, they are used as construction materials stainless steels , corten steels , non-ferrous metals .

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

Galvanized Iron Oxygen Corrosion

Oxygen corrosion of tin-plated iron

Paint, resin and enamel coatings must, above all, prevent the access of oxygen and moisture. Plating is often also used, 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 is the first to corrode. Zn 2+ ions are toxic. In the manufacture of cans, tin covered with a layer of tin is used. Unlike galvanized sheet metal, 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 high negative potential, for example, made of zinc or magnesium. For this, a corrosive element is specially created. The protected metal acts as a cathode, and this type of protection is called cathodic protection. The dissolvable electrode, respectively, is called the anode of protective 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 enhanced by anodic oxidation (anodizing). When a small amount of chromium is added to steel, an oxide film forms on the metal surface. The chromium content of stainless steel is over 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 can be one-, two- or three-pack and includes:

Binder - known compositions on chlorinated rubber, ethyl silicate, polystyrene, epoxy, urethane, alkyd (modified) base;

· Anticorrosive filler - zinc powder ("zinc dust"), containing more than 95% metallic zinc, having a particle size of less than 10 microns and a minimum oxidation state;

Hardener (in two- and three-pack systems)

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

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

1

Corrosion means the destruction of materials caused by the physicochemical or purely chemical influence of the 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, filamentary, ulcerative. It also appears brittle, cracked and stained. Continuous oxidation can be selective, uneven, and uniform.

The following types of corrosion are distinguished:

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

2

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

Most often, fretting corrosion affects moving contacting steel or metal shafts, bearing elements, a variety of bolted, spline, riveted and keyed joints, ropes and cables (that is, those products that perceive certain vibrational, vibrational and rotational stresses).

In fact, fretting corrosion occurs 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 becomes the cause of the formation of contact destruction of metals. Fretting corrosion occurs at different rates, which depends on the type of corrosive environment, the structure of materials and the loads acting on them, and the temperature of the environment. If a white film appears on the contacting surfaces (the process of metal discoloration is observed), it is most often the fretting process.

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

• The use of lubricating viscous compositions. 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 of protection against fretting corrosion is considered temporary. It remains effective until the protective compound is completely removed due to slipping. Lubricants, by the way, are not used to protect structures from.
• Competent choice of materials for the manufacture of structures. Fretting corrosion is extremely rare when the object is made of hard and soft metals. For example, it is recommended to cover steel surfaces with silver, cadmium, tin, lead.
• The use of additional coatings with special properties, gaskets, cobalt alloys, materials with a low coefficient of friction.

Fretting corrosion is sometimes prevented by creating contact surfaces with minimal slip. But this technique is used very rarely, due to the objective complexity of its implementation.

3

This type of corrosive destruction of materials is understood as the corrosion to which structures and structures operating in the near-ground atmospheric part are exposed. Atmospheric corrosion is wet, wet and dry. The last of the indicated proceeds according to the chemical scheme, the first two - according to the electrochemical scheme.

Atmospheric corrosion of a wet type becomes possible when there is a thin film of moisture on the metals (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.

Dry-type atmospheric corrosion occurs without a wet film on the metal surface. In the first stages, the destruction of the material proceeds rather quickly, but then the rusting rate slows down significantly. Dry atmospheric corrosion can also proceed much more actively if structures are affected by any gaseous compounds present in the atmosphere (sulfur and other gases).

Wet atmospheric corrosion occurs when the air humidity is 100%. Any objects that are operated in water or are constantly exposed to moisture (for example, doused with water) are susceptible to it.

Atmospheric corrosion causes serious damage to metal structures, therefore, various methods are created to combat it:

• Decrease in humidity (relative) air. A relatively simple and at the same time very effective method, which consists in dehumidifying the air and heating the rooms where the 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 metals. Atmospheric corrosion becomes less violent when phosphorus, titanium, chromium, copper, aluminum, nickel are added to the metal in small quantities. They suspend the anodic process or transfer the steel surfaces to a passive state.
• Use of inhibitors - volatile or contact. Volatiles include dicyclohexylamine, benzoates, carbonates, monoethanolamine. And the most famous contact-type inhibitor is sodium nitrite.

4

Gas corrosion is usually noted at elevated temperatures in an atmosphere of dry vapors and gases. Enterprises of the chemical, oil and gas and metallurgical industries suffer the most from it, as it affects containers 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 commonly caused by exposure to oxygen. The destruction of metals with it goes according to the following scheme:

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

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 follows a similar principle.

The phenomenon of hydrogen corrosion of steel is noted in technological equipment that operates in hydrogen atmospheres at high (from 300 MPa) pressures and temperatures above +200 ° C. Such 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 the strength characteristics of steel products are significantly reduced.

There is also the concept of hydrogen depolarization corrosion. This process can occur at a certain value of the 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:

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

Carbon dioxide corrosion affects petroleum equipment and pipelines that operate in environments containing carbon dioxide. Today, this type of corrosion attack is prevented by low alloying operation. Practice has shown optimal results when using alloys with chromium inclusions from 8 to 13 percent.

CORROSION OF METALS
spontaneous physical and chemical destruction and transformation of a useful metal into useless chemical compounds. Most environmental components, be they liquids or gases, corrode metals; constant natural influences cause rusting of steel structures, damage to car bodies, the formation of pits (etching pits) on chrome-plated 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 (intergranular) corrosion occurs externally imperceptibly, but can lead to accidents and even accidents. Often, unexpected damage to metal parts is associated with stresses, in particular those associated with metal corrosion fatigue. Corrosion is not always destructive. For example, the green patina often seen on bronze sculptures is a copper oxide that 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 by methods of protection developed on the basis of well-known scientific principles, but it remains one of the most serious and complex tasks of 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 (volt column), with the help of which a continuous current was obtained by converting chemical energy into electrical energy. A galvanic cell consists of a single electrochemical cell in which two different metals (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, which go into solution, while hydrogen ions accumulate on the other electrode ("cathode"). The flow of positive ions in the electrolyte is compensated by passing an electron current (electric 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 will not interfere with further corrosion of the metal anode. So, 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 forms 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 immersed in ordinary 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 against a 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, followed by a discharge, leads to the formation of molecular hydrogen at the cathode, which changes the potential of this electrode: an opposite (stationary) potential is established, which reduces the total cell voltage. The cell current drops very quickly to extremely low 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, therefore oxygen is called a "depolarizer". The polarization effect sometimes manifests itself in a decrease in the corrosion rate in stagnant waters due to a lack of oxygen, although such cases are not typical, since the effects of convection in a liquid medium are usually sufficient to supply dissolved oxygen to the cathode surface. The 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 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 is caused by an extremely thin oxide film formed as a result of chemical reactions on the surface of a metal. Such a film can be reduced (changed chemically), and the metal becomes active again upon contact with a metal with a more negative potential, for example, iron in the vicinity of zinc. In this case, a galvanic pair is formed, in which the passive metal is the cathode. The hydrogen released at the cathode restores its protective oxide film. Oxide films on aluminum protect it from corrosion, and therefore anodized aluminum, obtained as a result of the anodic oxidation process, is used both for 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 the cell, the electrolyte of which is sulfuric or phosphoric acid. Many patents describe various modifications of this process. The initially anodized surface is porous 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 in 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 sufficient 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 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 inner part of the crystal, and in a corrosive environment, the boundaries between crystallites will be predominantly subject to corrosion, with corrosion cracks deeply embedded in the metal structure. This "structural corrosion" seriously affects the mechanical properties. It can be prevented either by properly selected heat treatment regimes or by protecting the metal with a corrosion-impervious coating. Cladding - 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.
Prevention of corrosion. During electrochemical corrosion, the resulting products often dissolve (go into solution) and do not prevent further destruction of the metal; in some cases, a chemical compound (inhibitor) can be added to the solution, which reacts with the 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 common salt (NaCl), however, when zinc sulfate is added to the solution, poorly 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 anode 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 corrosion rate. In this method, an electric voltage is applied to the system in such a way that the entire structure to be protected is a 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, the insoluble anode is buried in the ground near them. In some cases, additional protective anodes are used for these purposes, for example, suspended inside containers for storing water, and the water in the container acts as an electrolyte. Other methods of cathodic protection provide sufficient current flowing from some other source through the structure, which completely becomes the cathode and contains possible local anodes and cathodes at the same potential. For this, a metal with a more negative potential is connected to the metal to be protected, which in the formed galvanic pair plays the role of a protective anode and is destroyed first. Zinc protective anodes have been used since 1825, when the famous English chemist H. Davy proposed using them to protect the copper sheathing of wooden ship hulls. Anodes based on magnesium alloys are widely used to protect the hulls of modern ships from corrosion in seawater. Protective anodes are more commonly used in comparison with anodes connected to external power 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. Metallic coatings can be applied by metal spraying or by electroplating (eg chrome plating, zinc plating, nickel plating).
Types of specific corrosion. Stress corrosion is the destruction of a metal under the combined action of static loading 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 clear, they can be associated with 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 inner portions of the grains; this increases the likelihood of corrosion attack along the intergranular boundaries and facilitates the subsequent process of cracking along them. Corrosion fatigue is also the result of the combined effect 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 concentration points is obvious. Probably all so-called fatigue mechanisms involve corrosion, since surface corrosion cannot be completely ruled out. 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 in 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, 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 slightly in the liquid metal, and since solubility usually changes with temperature, the dissolved metal may precipitate out of solution in the cooled portion of the system, clogging channels and valves. Secondly, intercrystalline penetration of liquid metal is possible if there is a selective reaction with alloying additions 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 interact with the formation of a surface alloy, which in some cases serves as a diffusion barrier with respect to further action. Erosion corrosion (shock, cavitation corrosion) refers to the mechanical effect of liquid metal flowing in a turbulent mode. In extreme cases, this leads to cavitation and erosive destruction of the structure.
See also CAVITATION. The corrosive effects of radiation are being intensively studied in connection with the development of nuclear power, 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 radiation of the following types: 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 with high-energy heavy particles leads to disturbances at the atomic level, which, under appropriate circumstances, can be the places of occurrence of electrochemical reactions. However, the more important change does not take place 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 regenerate and lose its effectiveness. Under the influence 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 strongly depends on temperature, and at normal temperatures only a thin oxide film forms on the metal surface (on copper, for example, this is noticeable by the darkening of the surface). At higher temperatures, the oxidation process is 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 weak oxidation of platinum at temperatures up to 450 ° C stops when heated to higher temperatures. Ordinary structural metals are oxidized 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, therefore no protective oxide layer is formed and at high temperatures the metals must be protected by an inert atmosphere (inert gases). Ultra-light 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 in many metals, but they usually have moderate protective properties. 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 to relatively low temperatures. Many "heavy metals" (eg, copper, iron, nickel) form non-porous oxides that, while not cracking, do not always protect the base metal. Theoretically, these oxides are of great interest and are being actively studied. They contain less stoichiometric amounts of metal; the 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 a metal that is resistant to the action of an oxidizing agent 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 chromium alloyed stainless steels have good oxidation stability and are widely used in household and industrial applications. The nichrome alloy, which is widely used as wire for spirals of electric furnaces, contains 80% nickel and 20% chromium and is quite resistant to oxidation at temperatures up to 1000 ° C. Mechanical properties are not less important than oxidation resistance, and it often 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 pose serious difficulties until they began to use (in gas turbine engines) fuel oil containing vanadium as fuel 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 galvanic corrosion or direct oxidation in the gas phase, but is primarily a mechanical effect. This is damage to articulated metal surfaces as a result of abrasion at their small multiple relative displacements; observed in the form of scratches, ulcers, shells; is accompanied by seizure and reduces the resistance to corrosion fatigue, because the resulting scratches serve as starting points for the development of corrosion fatigue. Typical examples are damage in the grooves of the turbine blades during vibration, abrasion of compressor impellers, wear of gear teeth, threaded connections, etc. At small multiple displacements, the protective oxide films are destroyed, abrade 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, and lubricants.
see also
Great Soviet Encyclopedia

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