Metal corrosion - main types of corrosion. According to the type of corrosive environment. Protection of iron from corrosion by coatings of other metals

Metal corrosion or alloy occurs, as a rule, at the phase boundary, that is, at the boundary of contact of a solid with a gas or liquid.

Corrosion processes are divided into the following types: according to the mechanism of interaction of the metal with the environment; by type of corrosive environment; by type of corrosion damage to the surface; by volume of destroyed metal; by the nature of additional influences to which the metal is exposed simultaneously with the action of a corrosive environment.

According to the mechanism of interaction of the metal with the environment, chemical and electrochemical corrosion are distinguished.

Corrosion that occurs under the influence of the vital activity of microorganisms is classified as biological corrosion, while corrosion that occurs under the influence of radioactive radiation- to radiation corrosion.

Based on the type of corrosive environment involved in the corrosion destruction of a metal or alloy, they distinguish between corrosion in non-electrolyte liquids, corrosion in solutions and melts of electrolytes, gas, atmospheric, underground (soil) corrosion, stray current corrosion, etc.

By the nature of the change in the surface of a metal or alloy or by the degree of change in their physical and mechanical properties during the corrosion process, regardless of the properties or environment corrosion damage There are several types.

1. If corrosion covers the entire surface of the metal, then this type of destruction is called - complete corrosion. Continuous corrosion refers to the destruction of metals and alloys under the influence of acids, alkalis, and the atmosphere. Continuous corrosion can be uniform, that is, the destruction of the metal occurs at the same rate over the entire surface, and uneven, when the corrosion rate in individual areas of the surface is not the same. An example of uniform corrosion is corrosion in the interaction of copper with nitric acid, iron with hydrochloric acid, zinc with sulfuric acid, aluminum with alkali solutions. In these cases, corrosion products do not remain on the metal surface. Iron pipes also corrode in the same way. outdoors. This is easy to see if you remove the layer of rust; underneath it is revealed rough surface metal, evenly distributed throughout the pipe.

2. Alloys of some metals are susceptible to - selective corrosion, when one of the elements or one of the structures of the alloy is destroyed, while the rest remain practically unchanged. When brass comes into contact with sulfuric acid, component-selective corrosion occurs - corrosion of zinc, and the alloy is enriched in copper. Such destruction is easy to notice, since reddening of the surface of the product occurs due to an increase in the concentration of copper in the alloy. With structural-selective corrosion, predominantly destruction of any one structure of the alloy occurs; for example, when steel comes into contact with acids, ferrite is destroyed, but iron carbide remains unchanged. Cast iron is especially susceptible to this type of corrosion.

3. For local corrosion Lesions in the form of individual spots, ulcers, and dots are found on the surface of the metal. Depending on the nature of the lesions, local corrosion occurs in the form of spots, that is, lesions that are not very deep into the thickness of the metal; ulcers - lesions deeply deepened into the thickness of the metal; points, sometimes barely visible to the eye, but penetrating deeply into the metal. Corrosion in the form of pits and spots is very dangerous for such structures where it is important to maintain conditions of tightness and impermeability (tanks, apparatus, pipelines used in the chemical industry).

4. Subsurface corrosion begins from the metal surface in cases where the protective coating (films, oxides, etc.) is destroyed in certain areas. In this case, destruction occurs mainly under the coating, and corrosion products are concentrated inside the metal. Subsurface corrosion often causes swelling and delamination of the metal. It can only be determined under a microscope.

5. Crevice corrosion- destruction of metal under gaskets, in gaps, threaded fasteners, riveted joints, etc. It often develops in the area of the structure located in the gap (crack).

6. Intergranular corrosion- destruction of the metal along the boundaries of crystallites (grains) with loss of its mechanical strength, appearance The metal does not change, but it is easily destroyed into individual crystals under mechanical stress. This is explained by the formation of loose, low-strength corrosion products between the grains of the metal or alloy. Chromium and chromium-nickel steels, nickel and aluminum alloys are susceptible to this type of corrosion. To avoid intergranular corrosion, recent years Stainless steels are widely used reduced content carbon or carbide formers are introduced into their composition - titanium, tantalum, niobium (in 5-8 times the amount of carbon content).

When a metal or alloy is simultaneously exposed to highly aggressive environments and mechanical tensile stresses, corrosion cracking, or transgranular corrosion, is possible. In this case, destruction occurs not only along the boundaries of the crystallites, but the metal crystallite itself is divided into parts. This is very dangerous look corrosion, especially for structures bearing mechanical loads (bridges, axles, cables, springs, autoclaves, steam boilers, internal combustion engines, water and steam turbines etc.).

Corrosion cracking depends on the design of the equipment, the nature of the aggressive environment, the structure and structure of the metal or alloy, temperature, etc. For example, corrosion cracking of carbon steels very often occurs in alkaline environments at high temperatures; stainless steels - in chloride solutions, copper sulfate, phosphoric acid; aluminum and magnesium alloys - under the influence of sea water; titanium and its alloys - under the influence of concentrated nitric acid and solutions of iodine in methanol.

It should be noted that depending on the nature of the metal or alloy and the properties of the aggressive environment, there is a critical stress above which corrosion cracking is often observed.

Based on the nature of the additional influences to which the metal is exposed, simultaneously with exposure to an aggressive environment, one can distinguish stress corrosion, friction corrosion and cavitation corrosion.

7. Stress corrosion- corrosion under simultaneous exposure to a corrosive environment and permanent or temporary stress. The simultaneous impact of cyclic tensile stresses and a corrosive environment causes corrosion fatigue, i.e., premature destruction of the metal occurs. This process can be imagined as follows: first, local corrosion appears on the surface of the product in the form of ulcers, which begin to act as a stress concentrator, maximum value tension will be at the bottom of the ulcers, which has a more negative potential than the walls, as a result of which the destruction of the metal will go deeper, and the ulcer will turn into a crack. Propeller shafts are susceptible to this type of corrosion. Car springs, ropes, cooled rolls of rolling mills, etc.

8. Friction corrosion- metal destruction caused by the simultaneous influence of a corrosive environment and friction. When two surfaces move oscillatingly relative to each other under conditions of exposure to a corrosive environment, abrasion corrosion, or fretting corrosion, occurs. It is possible to eliminate corrosion due to friction or vibration the right choice structural material, reducing the coefficient of friction, using coatings, etc.

9. Gas corrosion is the chemical corrosion of metals gas environment at minimum moisture content (usually no more than 0.1%) or at high temperatures. In chemical and petrochemical industry This type of corrosion is common. For example, in the production of sulfuric acid at the stage of sulfur dioxide oxidation, in the synthesis of ammonia, in the production of nitric acid and hydrogen chloride, in the processes of synthesis of organic alcohols, oil cracking, etc.

10. Atmospheric corrosion is the corrosion of metals in an atmosphere of air or any moist gas.

11. Underground corrosion- This is the corrosion of metals in soils and soils.

12. Contact corrosion is a type of corrosion caused by the contact of metals having different stationary potentials in a given electrolyte.

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

There are 3 features that characterize corrosion:

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

Types of metal corrosion

The most common are the following types of metal corrosion:

Uniform – covers the entire surface evenly
Uneven
Electoral
Local stains – individual areas of the surface are corroded
Ulcerative (or pitting)
Spot
Intercrystalline - spreads along the boundaries of a metal crystal
Cracking
Subsurface

Main types of corrosion

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

Chemical corrosion of metals

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

Chemical corrosion of metals can be gas or liquid.

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

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

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

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

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

where V ok is the volume of the formed oxide

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

M ok – molar mass the resulting oxide

ρ Me – metal density

n – number of metal atoms

A Me is the atomic mass of the metal

ρ ok - density of the formed oxide

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

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

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

Below are the values α for some metal oxides

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

Electrochemical corrosion of metals

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

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

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

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

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

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

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

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

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

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

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

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

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

Electrochemical corrosion may leak into different environments. Depending on the nature of the environment, the following types of electrochemical corrosion are distinguished:

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

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

A: Fe – 2e – = Fe 2+

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

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

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

Methods of protection against metal corrosion

The main method of protection against metal corrosion is creation of protective coatings– metallic, non-metallic or chemical.

Metal coatings.

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

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

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

Non-metallic coatings.

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

Chemical coatings.

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

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

phosphating – obtaining a protective film of phosphates (Fe 3 (PO 4) 2, Mn 3 (PO 4) 2);

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

blueing – the metal surface interacts with organic substances;

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

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

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

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

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

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

Categories,

Metal corrosion is a widespread cause of deterioration of various metal parts. Metal corrosion (or rusting) is the destruction of metal under the influence of physical and chemical factors. Factors that cause corrosion include natural precipitation, water, temperature, air, various alkalis and acids, etc.

1

Metal corrosion is becoming a serious problem in construction, at home and in production. Most often, designers provide for the protection of metal surfaces from rust, but sometimes rusting occurs on unprotected surfaces and on specially treated parts.

Metal alloys form the basis of human life; they surround him almost everywhere: at home, at work, and during leisure. People don’t always notice metal things and parts, but they constantly accompany them. Various alloys and pure metals are the most produced substances on our planet. Modern industry produces various alloys 20 times more (by weight) than all other materials. Even though metals are considered to be some of the strongest substances on Earth, they can break down and lose their properties through rusting processes. Under the influence of water, air and other factors, a process of oxidation of metals occurs, which is called corrosion. Despite the fact that not only metal, but also rocks can corrode, processes associated specifically with metals will be discussed below. It is worth paying attention to the fact that some alloys or metals are more susceptible to corrosion than others. This is due to the speed of the oxidation process.

Metal oxidation process

The most common substance in alloys is iron. Corrosion of iron is described by the following chemical equation: 3O 2 +2H 2 O+4Fe=2Fe 2 O 3. H 2 O. The resulting iron oxide is that red rust that spoils objects. But let's look at the types of corrosion:

Hydrogen corrosion. On metal surfaces practically never occurs (although theoretically possible). In this regard, it will not be described.
Oxygen corrosion. Similar to hydrogen.
Chemical. The reaction occurs due to the interaction of the metal with some factor (for example, air 3O 2 +4Fe = 2Fe 2 O 3) and occurs without the formation of electrochemical processes. So, after exposure to oxygen, an oxide film appears on the surface. On some metals, such a film is quite strong and not only protects the element from destructive processes, but also increases its strength (for example, aluminum or zinc). On some metals, such a film peels off (destroys) very quickly, for example, sodium or potassium. And most metals deteriorate quite slowly (iron, cast iron, etc.). This is how, for example, corrosion occurs in cast iron. More often, rusting occurs when the alloy comes into contact with sulfur, oxygen, or chlorine. Due to chemical corrosion, nozzles, fittings, etc. rust.
Electrochemical corrosion of iron. This type Rust occurs in environments that conduct electricity (conductors). The destruction time of different materials during electrochemical reactions is different. Electrochemical reactions are observed in cases of contact between metals that are located at a distance in a series of tensions. For example, a product made of steel has copper soldering/fastenings. When water hits the connections, the copper parts will be the cathodes and the steel will be the anode (each point has its own electrical potential). The speed of such processes depends on the amount and composition of the electrolyte. For reactions to occur, the presence of 2 different metals and an electrically conductive medium is required. In this case, the destruction of alloys is directly proportional to the current strength. The higher the current, the faster reaction, the faster the reaction, the faster the destruction. In some cases, alloy impurities serve as cathodes.

Electrochemical corrosion of iron

It is also worth noting the subtypes that occur during rusting (we will not describe it, we will only list it): underground, atmospheric, gas, with different types immersion, solid, contact, friction-induced, etc. All subspecies can be classified as chemical or electrochemical rusting.

2

Corrosion of reinforcement and welded structures often occurs during construction. Corrosion often occurs due to non-compliance with the rules for storing the material or failure to perform work on processing the rods. Corrosion of reinforcement is quite dangerous, since reinforcement is laid to strengthen structures, and as a result of the destruction of the rods, a collapse is possible. Corrosion of welds is no less dangerous than corrosion of reinforcement. This will also significantly weaken the seam and may lead to tearing. There are many examples where rust on power structures leads to the collapse of premises.

Other common cases of rusting in everyday life are damage to household tools (knives, cutlery, tools), damage to metal structures, damage to vehicles (both land, air and water), etc.

Perhaps the most common rusty things are keys, knives and tools. All these items are subject to rust due to the fact that friction removes the protective coating, which exposes the base.

The base is subject to destruction processes due to contact with aggressive environments (especially knives and tools).

Destruction due to contact with aggressive media

By the way, the destruction of things that are often used in everyday life can be observed almost everywhere and regularly, at the same time, some metal objects or structures can remain rusty for decades and will perform their functions properly. For example, a hacksaw, which was often used to saw logs and left for a month in a barn, will quickly rust and may break during work, and a pole with road sign It can stand for ten or even more years rusty and not collapse.

Therefore, all metal items should be protected from corrosion. There are several methods of protection, but they are all chemical. The choice of such protection depends on the type of surface and the destructive factor acting on it.

To do this, the surface is thoroughly cleaned of dirt and dust in order to eliminate the possibility of the protective coating not getting on the surface. It is then degreased (for some types of alloy or metal and for some protective coatings this is necessary), after which a protective layer is applied. Most often, protection is provided by paints and varnishes. Depending on the metal and factors, different varnishes, paints and primers are used.

Another option is to apply a thin protective layer of another material. This method is usually practiced in production (for example, galvanizing). As a result, the consumer practically does not need to do anything after purchasing the item.

Applying a thin protective layer

Another option is to create special alloys that do not oxidize (for example, stainless steel), but they do not guarantee 100% protection; moreover, some things made from such materials oxidize.

Important parameters of protective layers are thickness, service life and rate of destruction under active adverse influences. When applying a protective coating, it is extremely important to accurately fit into the permissible layer thickness. Typically, manufacturers of paints and varnishes indicate it on the packaging. So, if the layer is larger than the maximum allowable, this will cause excessive consumption of varnish (paint), and the layer may be destroyed under strong mechanical stress, more thin layer may wear off and shorten the protection period of the base.

A correctly selected protective material and correctly applied to the surface guarantees 80% that the part will not be subject to corrosion.

3

Many people in everyday life do not think about how to protect their things from rye. And they get a problem in the form of a damaged item. How to properly solve this problem?

Removing rust from a part

In order to restore a thing or part from rust, the first step is to remove all the red plaque to a clean surface. It can be removed with sandpaper, files, or strong reagents (acids or alkalis), but drinks like Coca-Cola have earned particular fame for this. To do this, the item is completely immersed in a container with a miracle liquid and left for some time (from several hours to several days - the time depends on the item and the damaged area).

Red spots on steel products

According to the UN, each country loses from 0.5 to 7-8% of its gross national product per year due to corrosion. The paradox is that less developed countries lose less than developed ones. And 30% of all steel products produced on the planet are used to replace rusted ones. Therefore, it is highly recommended that you take this issue seriously.

Types of corrosion

Chemical corrosion >>> Electrochemical corrosion >>> Gas corrosion >>> Atmospheric corrosion >>> Underground corrosion >>> Biocorrosion >>> Contact corrosion >>> Radiation corrosion >>> Corrosion cavitation >>> Fretting corrosion >>> Intergranular corrosion >>> Crevice corrosion >>>

Corrosion processes are classified according to the mechanism of interaction of metals with the external environment; by type of corrosive environment and process conditions; by the nature of corrosion damage; according to the types of additional influences to which the metal is exposed simultaneously with the action of a corrosive environment.

According to the mechanism of the process, they distinguish chemical and electrochemical corrosion of metals.

Chemical corrosion is a process of interaction of a metal with a corrosive environment, in which the oxidation of the metal and the reduction of the oxidizing component of the environment occur simultaneously in one act. The interaction products are not spatially separated. Electrochemical corrosion- this is the process of interaction of a metal with a corrosive environment (electrolyte solution), in which the ionization of metal atoms and the reduction of the oxidizing component of the corrosive environment do not occur in one act and their rates depend on the electrode potential.

Depending on the type of corrosive environment and conditions of occurrence, several types of corrosion are distinguished. Gas corrosion- this is the chemical corrosion of metals in a gaseous environment with a minimum moisture content (usually no more than 0.1%) or at high temperatures. This type of corrosion occurs frequently in the chemical and petrochemical industries. For example, in the production of sulfuric acid at the stage of sulfur dioxide oxidation, in the synthesis of ammonia, in the production of nitric acid and hydrogen chloride, in the processes of synthesis of organic alcohols, oil cracking, etc.

Atmospheric corrosion is the corrosion of metals in an atmosphere of air or any moist gas.

Underground corrosion- This is the corrosion of metals in soils and soils.

Biocorrosion- This is corrosion that occurs under the influence of the vital activity of microorganisms.

Contact corrosion is a type of corrosion caused by the contact of metals having different stationary potentials in a given electrolyte.

Radiation corrosion- This is corrosion caused by the action of radioactive radiation.

Corrosion by external current and corrosion by stray current. In the first case, this is metal corrosion that occurs under the influence of current from an external source. In the second case - under the influence of stray current.

Stress Corrosion- corrosion caused by simultaneous exposure to a corrosive environment and mechanical stress. If these are tensile stresses, then cracking of the metal may occur. This is a very dangerous type of corrosion, especially for structures experiencing mechanical loads (axles, springs, autoclaves, steam boilers, turbines, etc.). If metal products are subjected to cyclic tensile stress, corrosion fatigue can occur. The fatigue limit of the metal decreases. Car springs, ropes, and rolling mill rolls are susceptible to this type of corrosion.

Corrosive cavitation- metal destruction caused by simultaneous corrosion and impact effects external environment.

Fretting corrosion is corrosion caused by both vibration and exposure to a corrosive environment. Corrosion due to friction or vibration can be eliminated by the correct choice of structural material, reducing the coefficient of friction, using coatings, etc.

Corrosion is called solid , if it covers the entire surface of the metal. Continuous corrosion can be uniform if the process occurs at the same speed over the entire surface of the metal, and uneven when the process speed is not the same on different parts of the surface. Uniform corrosion is observed, for example, when iron pipes corrode in air. At selective corrosion one structural component or one component of the alloy is destroyed. Examples include graphitization of cast iron or dezincification of brass.

Local (localized) corrosion covers individual areas of the metal surface. Local corrosion can be expressed in the form of individual spots, not very deep into the thickness of the metal; ulcers - destruction that looks like a shell, deeply deepened into the thickness of the metal, or points (pittings) penetrating deeply into the metal. The first type is observed, for example, during corrosion of brass in sea water. Pit corrosion observed in steels in soil, and pitting - in austenitic chromium-nickel steel in sea water.

Subsurface corrosion begins on the surface, but then spreads deep into the metal. Corrosion products end up concentrated in metal cavities. This type of corrosion causes swelling and delamination of metal products.

Intergranular corrosion characterized by metal destruction along grain boundaries. It is especially dangerous because the appearance of the metal does not change, but it quickly loses strength and ductility and is easily destroyed. This is due to the formation of loose, low-strength corrosion products between the grains. Chromium and chromium-nickel steels, nickel and aluminum alloys are especially susceptible to this type of destruction.

Crevice corrosion causes destruction of metal under gaskets, in gaps, threaded fasteners, etc.

DEFINITION

When in contact with the environment, many metals, as well as metal-based alloys, can be subject to destruction due to chemical interaction (ORR with substances in environment). This process is called corrosion.

A distinction is made between corrosion in gases (gas corrosion), which occurs at high temperatures in the absence of moisture on metal surfaces, and electrochemical corrosion (corrosion in electrolyte solutions, as well as corrosion in a humid atmosphere). As a result of gas corrosion, oxide, sulfide, etc. are formed on the surface of metals. films. Furnace fittings, parts of internal combustion engines, etc. are subject to this type of corrosion.

As a result of electrochemical corrosion, metal oxidation can lead to both the formation of insoluble products and the transition of the metal into solution in the form of ions. This type of corrosion affects pipelines located in the ground, underwater parts of ships, etc.

Any electrolyte solution - aqueous solution, and water contains oxygen and hydrogen, capable of reduction:

O 2 + 4H + +4e = 2H 2 O (1)

2H + +2e=H 2 (2)

These elements are oxidizing agents that cause electrochemical corrosion.

When writing the processes occurring during electrochemical corrosion, it is important to take into account standard electrode potentials (EP). Thus, in a neutral environment, the EC of process 1 is equal to 0.8B, therefore, metals whose EC is less than 0.8B (metals located in the activity series from its beginning to silver) are subject to oxidation by oxygen.

The EP of process 2 is -0.41V, which means that only those metals whose potential is lower than -0.41V (metals located in the activity series from its beginning to cadmium) are subject to oxidation with hydrogen.

The rate of corrosion is greatly influenced by impurities that a particular metal may contain. Thus, if a metal contains non-metallic impurities, and their EC is higher than the EC of the metal, then the corrosion rate increases significantly.

Types of corrosion

There are several types of corrosion: atmospheric (corrosion in humid air at normal conditions), corrosion in the soil, corrosion due to uneven aeration (access of oxygen to different parts metal product in solution is not the same), contact corrosion (contact of 2 metals with different EP in an environment where moisture is present).

When corrosion occurs on the electrodes (anode and cathode), electrochemical reactions, which can be written by the corresponding equations. Yes, in an acidic environment electrochemical corrosion proceeds with hydrogen depolarization, i.e. Hydrogen is released at the cathode (1). In a neutral environment, electrochemical corrosion occurs with oxygen depolarization—water is reduced at the cathode (2).

K (cathode) (+): 2H + +2e=H 2 - reduction (1)

A (anode) (-): Me – ne →Me n + – oxidation

K (cathode) (+): O 2 + 2H 2 O + 4e → 4OH - - reduction (2)

In the case of atmospheric corrosion, the following electrochemical reactions occur on the electrodes (and at the cathode, depending on the environment, various processes can occur):

A (anode) (-): Me→Me n + +ne

K (cathode) (+): O 2 + 2H 2 O + 4e → 4OH - (in alkaline and neutral environments)

K (cathode) (+): O 2 + 4H + + 4e → 2H 2 O (in acidic medium)

Corrosion protection

Used for corrosion protection following methods: use of chemically resistant alloys; protection of the surface of metals with coatings, which most often use metals that are coated in air with oxide films that are resistant to the effects of the external environment; treatment of corrosive environments; electrochemical methods ( cathodic protection, protector method).

Examples of problem solving

EXAMPLE 1

EXAMPLE 2

Exercise

The part consists of an alloy of iron and nickel. Which metal will corrode faster? Write down the equations of the anodic and cathodic processes during atmospheric corrosion. The values of standard electrode potentials are E(Fe 2+ /Fe) = - 0.444V, E(Ni 2+ /Ni) = -0.250V.

Solution

First of all, active metals (those with the most negative values standard electrode potentials), in this case it is iron.