What is metal corrosion?

Chemical corrosion: electrochemical rust, as a physical phenomenon and the protection of metals from it, what reaction products cause it and its speed

Metals are most afraid of corrosion. It can render the most durable structures unusable. Huge bridges, power lines, kilometer-long pipelines become helpless in the face of the destructive process.

To prevent damage, the metal is protected. But it is important to understand that there are several types of oxidation. Electrochemical corrosion, chemical or gas - they are all similar in consequences, but effective methods of protection against each type will have their own individual measures, depending on many factors.

The essence, causes and signs of chemical corrosion

Electrochemical corrosion most often affects metals. This is due to the fact that they, as a rule, are thermodynamically unstable in the environment that surrounds them, and the latter is more often an ordinary electrolyte, that is, a conductor. Therefore, due to the metal, the corrosive environment tends to be restored by transferring particles through an electric current.

For chemical corrosion to occur, not much is needed: metal and an appropriate corrosive environment. The reasons for the high probability of oxidation are the unstable thermodynamic state of the metal and its desire to move to a more stable state. This can be expressed verbally as follows:

Initial metal + oxidizing environment = reaction = result in the form of rust.

The main sign of chemical corrosion - the oxidizing environment - is not an electrolyte as in electrochemical corrosion, that is, redox processes have a purely chemical overtone. There are two types of chemical corrosion:

• one that flows in a gaseous environment when the surrounding temperature is very high;
• The second type of chemical corrosion is associated with liquids.

For ideal protection against corrosion, it is necessary to carry out combined protection measures, since in reality the metal is affected simultaneously by both electrochemical and chemical oxidation.

Differences between chemical corrosion and electrochemical corrosion

The process of electrochemical corrosion is understood as the redox reaction of a corrosive environment due to the electrons of the metal, which are taken away from it under the influence of the electrical potential that arises in the electrolyte, which is this oxidizing environment. Ionization is accompanied by the destruction of that part that is in direct contact with the environment, and rust, visible to the naked eye, is nothing more than a restored corrosive environment.

If we talk about how chemical corrosion differs from electrochemical corrosion, there are several fundamental points:

1. The essence of electrochemical corrosion is the process occurring in the electrolyte, and this is the main thing.
2. In electrochemical oxidation, an electric current is necessarily present, which is not the case in chemical oxidation.
3. Electrochemical corrosion is not characterized by a one-time transition of particles from the metal to the oxidizing component, but is determined by the magnitude of the potential. That is, the higher the potential, the greater the speed of particle movement and the faster the recovery process of the corrosive environment. In a chemical process, the destruction of the substance is accompanied by the simultaneous restoration of the corrosive environment.

The following types of electrochemical corrosion are distinguished:

1. Intercrystalline. An electrochemical phenomenon when aluminum, nickel, and other elements experience grain destruction along its boundary, and this occurs selectively. As a result, the structure loses its strength and its plasticity properties deteriorate. The danger is that this electrochemical process may not be visually noticeable.
2. Pitting. It manifests itself as damage to pinpoint areas on elements such as copper, its alloys and others. The size of individual areas where corrosion occurs usually does not exceed 1.5 millimeters. Electrochemical pitting is of surface type, as well as open and closed.
3. Slit. A dangerous type of electrochemical corrosion, accompanied by rapid, enhanced destruction of areas where there are microcracks, gaps or cracks. Corrosion can occur in any environmental condition.

Protective protection against electrochemical corrosion

As mentioned above, the higher the potential of the metal, the faster electrochemical corrosion occurs. This means that by reducing the electrical potential, it is possible to reach a point when the oxidation process becomes impossible.

The tread or, in other words, electrochemical protection of metal structures, gas pipelines, and hulls of sea vessels is based on this principle. A metal that behaves more actively than the main one being protected is taken and connected to the last current-carrying conductor.

It turns out that this anode already reacts with the corrosive environment, and the base metal remains unharmed.

Anodic protection must be periodically renewed, because the “sacrificial metal” is also very much destroyed during operation, especially if it is not quite correctly selected for work in a particular environment.

The main cases when the use of tread protection will be justified:

1. There are insufficient funds to carry out more expensive protective measures.
2. If you need to provide protection to small metal structures.
3. When pipelines have additional surface insulation.

Protection against electrochemical corrosion, such as sacrificial protection, is used mainly for different grades of steel. Here it would be appropriate to use protectors based on cadmium, aluminum, magnesium, chromium, zinc, but not a pure element is used, but alloys.

Zinc protectors

Zinc protectors, in addition to the zinc base, contain:

• up to 0.15% cadmium with the minimum possible percentage of 0.025%;
• up to 0.5% aluminum;
• impurities of iron, lead and copper, the total amount not exceeding 0.005%.

The most effective use of zinc has been found in sea water, where such protectors successfully protect, for example, gas or oil pipelines. Another advantage of zinc is that it can be used with explosive substances. When the anode dissolves, there is no release of pollutants that can harm the environment.

The use of zinc protectors in reservoirs where the water has a fresh composition or in the soil underground is accompanied by the rapid formation of compounds such as oxides and hydroxides on the surface of the protector, which lead to inhibition of the electrochemical process of oxidation of the anode, and the protection of the base metal from electrochemical corrosion actually ceases.

Magnesium protectors

It is not advisable to use magnesium protectors in their pure form, which is explained by the rapid rusting of this metal. Therefore, corrosion protectors based on magnesium, in addition to it, have:

• maximum 5%, minimum 2% zinc;
• maximum 7%, minimum 5% aluminum;
• small content of copper, nickel and lead, no more than tenths of a percent.

Magnesium protectors are good when the environment in which they are used has a pH of no higher than 10.5, which corresponds to ordinary soils, slightly saline water bodies, or simply fresh water.

Any pipelines and metal structures located in the conditions described above are suitable for protection.

The use of magnesium in aggressive saline solutions is accompanied by the rapid formation of a poorly soluble film on its surface.

In some cases, magnesium protectors affect the metal in such a way that it becomes more brittle and cracks can form in the body of the structure. Before using magnesium to protect against galvanic corrosion of a specific steel grade, additional research must be conducted to avoid negative consequences.

Aluminum protectors

The intended purpose of aluminum protectors is to act as protection against electrochemical corrosion in environments with running water with a salty composition, for example, in coastal sea waters. The aluminum tread alloy contains:

• impurities of indium, cadmium, silicon no more than 0.02%;
• zinc – up to 8%;
• magnesium – up to 5%.

Thanks to these additional metals, there is no possibility of a hard, dissolution-retarding film forming on the tread. It is acceptable to use an aluminum protector in environments suitable for a magnesium protector.

Methods for protecting metals from chemical corrosion

Since chemical corrosion is in no way related to the potential of the metal, and protectors or cathodes will not prevent it, methods of protection against it have their own specifics. The fight against destruction of this nature can take place in three main areas:

1. Method of constructive problem solving. It involves the use of alloys that have very high corrosion resistance, or the use of bimetallic compositions, where the main structural metal is coated with a thin layer of a stable compound (for example, galvanizing).
2. A method of improving the environment where the product will be used by changing the pH and neutralizing the aggressive components of a corrosive environment.
3. A passive protection method best known for blocking access to the surface of a structure in a corrosive environment with a film of inactive and non-reactive material. This is a coating of various types of paint and varnish components to protect against corrosion.

Source: https://xn----8sbna6aihebzq3cl.xn--p1ai/drugoe/metody-zashhity-ot-himicheskoj-korrozii.html

Rust on metal: harm, types of corrosion

We, sellers of rolled metal, are faced with this obsession - rust, like no one else. And we know for sure the harm caused by corrosion. In this article we will say a few words about this problem, its manifestations, its scale.

Damage, damage

Everyone has seen those orange-brown or yellowish rust spots on metal parts. The economic damage from metal corrosion is enormous. In the USA and Germany, the estimated damage from corrosion and the costs of combating it amount to approximately 3% of GDP. At the same time, metal losses, including due to the failure of structures, products, and equipment, amount to up to 20% of the total steel production per year. For Russia, exact data on losses from corrosion have not been calculated.

It is known for certain that it was rusted metal structures that caused the collapse of several bridges in the United States, including with numerous casualties. Environmental damage is also extremely unpleasant: leakage of gas and oil due to the destruction of pipelines leads to environmental pollution.

Types of corrosion and its causes

Before talking about rust on iron, let's briefly look at its other types.

Not only metals, but also non-metallic products are susceptible to corrosion. In this case, corrosion is also called “aging”. Plastics, rubbers and other substances are subject to aging. For concrete and reinforced concrete there is a term "fatigue". They are destroyed or their performance characteristics deteriorate due to chemical and physical exposure to the environment.

Metal alloys - copper, aluminum, zinc - also corrode: during their corrosion, an oxide film is formed on the surface of the product, tightly adhering to the surface, which significantly slows down the further destruction of the metal (and the patina on copper also gives it a special charm). Precious metals are such not only because of their beauty, valued by jewelers, but also because of their resistance to corrosion.

Gold and silver are still used to coat particularly sensitive electronic contacts, and platinum is used in the space industry.

The metal can corrode in some areas of the surface (local corrosion), cover the entire surface (uniform corrosion), or destroy the metal along the grain boundaries (intercrystalline corrosion). Corrosion accelerates noticeably with increasing temperature.

Types of Rust

Iron is more susceptible to corrosion. From a chemical point of view, rust is an oxidative process (like combustion). Elements resulting from oxidation in an oxygen environment are called Oxides. There are 4 main types.

1. Yellow rust - chemical formula FeO(OH)H2O (ferrous oxide). Occurs in a humid, oxygen-poor environment. Often found underwater. In nature, it exists in the form of the mineral wustite, while being a monoxide (it contains 1 oxygen atom).

2. Brown rust - Fe2O3 (double iron oxide): grows without water and is rare.

3. Black rust - Fe3O4 (tetravalent iron oxide). Formed with low oxygen content and without water, it is therefore stable and spreads very slowly. This oxide is ferromagnetic (under certain conditions it is magnetized in the absence of an external magnetic field), therefore it is potentially applicable for the creation of superconductors.

4. Red rust - chemical formula Fe2O3•H2O (ferric oxide). Occurs under the influence of oxygen and water, the most common type, the process proceeds evenly and affects the entire surface.

Unlike all of the above types of oxidation, which are not so dangerous for iron, this one forms iron hydroxide in its thickness, which, when it begins to peel off, opens up more and more layers of metal for destruction. The reaction can continue until the structure is completely destroyed.

 It is used in iron smelting and as a dye in the food industry. It occurs naturally in nature under the name hematid.

Several types of rusting can occur simultaneously without particularly interfering with each other.

Chemical and electrochemical corrosion

Iron rusts if it contains additives and impurities (such as carbon) and comes into contact with water and oxygen. If salt (sodium and potassium chloride) is dissolved in water, the reaction becomes electrochemical and the rusting process accelerates.

The massive use of these salts both in household chemicals and for combating ice and snow makes electrochemical corrosion a very common and dangerous phenomenon: losses in the United States from the use of salts in winter amount to $2.5 billion.

When exposed to water and oxygen simultaneously, iron hydroxide is formed, which, unlike oxide, peels off from the metal and does not protect it in any way. The reaction continues either until the iron is completely destroyed or until the system runs out of water or oxygen.

Electrochemical corrosion can be caused by stray currents that occur when part of the current leaks from an electrical circuit into aqueous solutions or into the soil and from there into a metal structure.

In those places where stray currents exit metal structures back into water or soil, metal destruction occurs. Stray currents occur especially often in places where ground electric transport moves (for example, trams and electric railway locomotives).

In just one year, stray currents with a force of 1A are capable of dissolving 9.1 kg of iron, 10.7 kg of zinc, and 33.4 kg of lead.

In the second part of the article we will tell you how you can protect your metal structures from this scourge or defeat it if it is already attacking.

Source: https://www.1metallobaza.ru/blog/kak-my-stradaem-ot-rzhavchiny

Which metal is not susceptible to corrosion?

The definition of metal corrosion should be understood as the process of its destruction as a result of the negative impact of environmental factors - oxygen, water, carbon oxides and sulfur oxides in the air, water-salt solutions (for example, sea water).

In general, today there are two types of corrosion:

• chemical (metal destruction occurs under the influence of non-electrolytes (oxidizing agents) even at normal temperatures. For example, oxygen, chlorine, other types of gas);
• electrochemical (destruction of metal occurs as a result of exposure to electrolyte solutions, for example, humid air).

Technical iron is the most susceptible to corrosion. At the same time, there are metals and alloys that are resistant to such negative effects.

Ways to combat corrosion

Today, there are several ways to combat corrosion. The main list includes the protective coating of a metal surface with paints or a layer of another metal (silver, chromium, tin, zinc), as well as the addition of metal impurities to create alloys. The choice between them determines the purpose and scope of application. For example, in the food industry, metals coated with paints and varnishes to combat corrosion cannot be used.

Corrosion-resistant metals and alloys

So, it was said above that technical hardware is most vulnerable to the negative influences of the environment. It is important to note that chemically pure iron practically does not rust (as evidenced by many examples of well-preserved historical iron artifacts).

The question arises: why shouldn't modern society use pure iron? The fact is that without impurities, iron loses a number of its properties, and therefore is not suitable for use in a number of areas.

In their pure form, aluminum, copper, tin, zinc, and brass are not subject to corrosion.

Sometimes they are used to coat metal in order to prevent its direct contact with the environment. Thus, metal products are chrome-plated, nickel-plated, galvanized, etc. For example, coating iron with tin (tinning) produces tinplate, which is often used in the production of canning containers.

Light stainless alloys traditionally contain impurities of the same corrosion-resistant metals - aluminum, titanium, cupronickel, brass, bronze.

Areas of application of corrosion-resistant alloys

The resulting alloys with an admixture of titanium are actively used in the aerospace industry and recreational sectors, nickel silver in medicine, and brass in the chemical industry and mechanical engineering.

In general, by combining various metals in different ratios, you can obtain a huge number of corrosion-resistant alloys suitable for use in a number of areas of modern human activity.

We, sellers of rolled metal, are faced with this obsession - rust, like no one else. And we know for sure the harm caused by corrosion. In this article we will say a few words about this problem, its manifestations, its scale.

General information about metal corrosion

Corrosion is the destruction of solids caused by chemical and electrochemical processes developing on the surface of the body during its interaction with the external environment. Corrosion of metals causes particular damage.

The most common and most familiar type of corrosion to all of us is the rusting of iron. The term "corrosion" applies to metals, concrete, some plastics and other materials.

Corrosion is the physical and chemical interaction of a metal with its environment, leading to the destruction of the metal.

It is difficult to take into account higher indirect losses from downtime and decreased productivity of equipment subject 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, of course, is impossible.

However, based on some available data on the average annual replacement of corrugated metal roofs, wires, pipelines, steel trucks and other iron and steel objects subject to corrosion, it can be concluded that due to improper protection, annual replacement costs can average up to 2 percent of the total volume of steel used.

About metal corrosion

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 corrosion of metals.

According to the international standard ISO 8044, corrosion is understood as a physicochemical or chemical interaction between a metal (alloy) and the environment, leading to deterioration in the functional properties of the metal (alloy), environment or 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 subject to erosion - destruction of the surface of the material under the influence of mechanical stress.
Erosion is caused by rain, winds, sand dust and other natural factors. Ideal corrosion protection is 80% ensured by proper preparation of the surface for painting and only 20% by the quality of the paints and varnishes used and the method of their application (ISO).

Corrosion process

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

The environment in which metal corrodes (corrodes) is called a corrosive or aggressive environment. In the case of metals, when talking about their corrosion, we mean the undesirable process of interaction of the metal with the environment.

Stages of the corrosion process:

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

Classification of corrosion processes

Based on the nature of destruction, the following types of corrosion are distinguished:

Chemical corrosion is a process in which the oxidation of a 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 is observed in cases where the corrosive environment is not an electrolyte (gas corrosion, corrosion in non-conducting organic liquids).

Electrochemical corrosion 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. In 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 sea water is enhanced by the fouling of the surface by marine organisms.
Electrocorrosion is an increase in corrosion under the influence of anodic polarization caused by an external electric field (for example, during welding work 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 corrosion processes occurring in them are also 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 so-called mixed corrosion.

Gas corrosion is corrosion in a gas environment at high temperatures.

Atmospheric corrosion - corrosion of metal in atmospheric conditions with humidity sufficient to form an electrolyte film on the metal surface (especially in the presence of aggressive 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 resulting corrosion products.

Liquid corrosion is corrosion in liquid media.

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

Corrosion is classified according to the nature of its destruction

Continuous - Covers the entire surface of the metal
Local - Covers individual areas of corrosion
Uniform - Flows at approximately the same speed over the entire surface
Dotted (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 spots with a diameter of more than 50 mm and a depth of up to 2 mm
Subsurface - Causes delamination of the metal and swelling of layers
Subfilm - Flows under the protective coating of the metal
Intergranular - In the form of destruction of grain boundaries
Selective (selective) - In the form of dissolution of individual components of the alloy
Crevice - Develops in cracks and narrow gaps

Source: http://obrabotka.ru/articles/obshhie_svedeniya_o_korrozii_metalla.html

Classification of types of corrosion and methods of protection against it

Corrosion is a dangerous process of degradation (i.e. destruction) of the top layer of metals under the influence of various environments. Corrosion is usually caused by the chemical environment that surrounds the metal.

Regardless of the type of structure and its operation, the simplest and most understandable method for combating this phenomenon is to “protect against corrosion” using protective paints and special varnishes.

Main types of corrosion

Speaking about the mechanism of the corrosion process, we can notice 2 main types of corrosion: chemical and electrochemical.

Chemical is a clear result of the passage of reactions during which, after the destruction of the metal bond, parts of the metal and all the atoms that are included in the oxidizing agents create a strong bond. An electric current cannot occur between different parts of a metal surface. This type of corrosion may be inherent in chemical media that are not able to transmit electric current. This includes gases and non-electrolytes.

It is important to remember that the cause of corrosion also affects the rate of corrosion. Electrochemical corrosion is a process of degradation of metals. This process occurs along with the emergence of electricity in the system.

Classification of corrosion according to the significance of the process itself

Bridge arch, with more severe rust in places where the paint layer has been destroyed

Corrosion processes can be divided:

• by the type of mutual influence of metals with the surrounding atmosphere;
• by the type of corrosive atmosphere and the conditions of the process itself;
• according to the characteristics of degenerative effects;
• The “corrosion rate” is greatly influenced by the type of surrounding atmosphere.

Types of corrosion and process description

• Chemical is a type of mutual influence of a metal with the environment, during which the oxidation and further reduction of part of the environment occur in a single act. Products of mutual influence have no separation in space.
• Electrochemical is a type of mutual influence of a metal with a corrosive space in which the ionization reaction of the corrosive environment takes place in several acts.
• Gas corrosion is corrosion of metal surfaces with low water content (usually there is no more than 0.2% moisture) or at maximum operating temperatures. In the modern chemical and gas industry, this type of corrosion can occur more often than others.
• Atmospheric is a type of corrosion in an air atmosphere or in a humid gas environment.
• Biocorrosion is a biological type of metal corrosion that occurs under the influence of the vital activity of microbes and various microorganisms.
• Contact corrosion is a type of corrosion that is provoked by the contact of several types of metals with different stationary potentials.
• Radiation is a type of metal corrosion that is caused by the influence of radioactive radiation.

Corrosion by external or stray electric current also exists.

Another type of corrosion is stress corrosion, which is provoked by the simultaneous influence of a corrosive environment and the flow of mechanical stress. It is important to consider that this type of corrosion is very harmful, especially for systems subject to severe physical stress.

Features of anti-corrosion compounds

Rust is visible in places where the paint is chipped, but there is no corrosion on the axles covered with lubricant

What are corrosion inhibitors? These are substances and elements that, when present in an environment subject to the dangerous influence of corrosion, are able to reduce and generally stop the corrosive effect on the metal. A corrosion inhibitor can be either a single chemical compound or a mixture of many.

Inhibitors are most often surfactants, as well as all kinds of organics.
When affecting the product, they further improve the protective characteristics of the oxide film on the metal. For this reason, you can conclude that the presence of oxygen in the environment favors an increase in the protective effect against corrosion. However, if the oxide film has poor stability, the adsorption of the inhibitor on the top layer of metal worsens.

• Scale inhibitor IOMS-1 (solution) g 200 rub/kg. Macroflex.
• Corrosion inhibitor Protectogen (protectogen). C Aqua.
• Complexonate is a solution of a zinc complex. Ectoscale.
• GALAN. Tread. Corrosion inhibitor (5 l). Protects pipelines, radiators and other heating systems from corrosion.

Copper corrosion

Copper corrosion is its destruction under the influence of the environment. Copper in varying concentrations in other metals is widely used in many industries around the world. Copper is used in construction due to the maximum corrosion resistance of this metal and its high degree of thermal conductivity.

Iron corrosion

The oxidation reaction during iron corrosion occurs by the passage of electrons to the oxidizing agent. The corrosion product of this type of metal is rust. In practice, you can see rust with a fairly diverse range of shades - from dark red to light orange, or almost black.

Effect of corrosion on aluminum

Corrosion of aluminum occurs in many environments and may only persist in cases where a protective coating has formed on the top of the metal.

https://www.youtube.com/watch?v=u7Dej6IV_28

It is important to remember that alkalis can quickly dissolve the protective film of aluminum, which begins its reaction with water. As a result, the metal usually dissolves in the environment, releasing a large amount of hydrogen. This is how “corrosion of aluminum and its alloys” occurs in nature.

Ways to protect metal from corrosion

Anti-corrosion protection can be carried out using the following methods:

• Increasing the overall corrosion resistance of a metal by changing its chemical properties. structures;
• Insulation of the metal surface using special coatings can also be used;
• Reducing the aggressiveness of the place where the metal is used;
• The use of currents that increase resistance to the effects of corrosion.

Protective compounds are quite easy to use, inexpensive and quickly updated if desired, and also create a more decorative appearance for a metal product.

Protection against corrosion is determined by the physical insulation of the metal, or the powerful electrical-chemical effect of the anti-corrosion coating on the surface. The main disadvantages of most paints and varnishes are their poor water permeability and sometimes poor temperature stability.

Protective film as a barrier to destruction

Coating the top of the metal with a special paint layer serves as an excellent barrier to the formation of corrosion, and therefore can reduce the spread of corrosion. Of particular importance in this matter is the quality of the coating (anti-corrosion paint is used) - the thickness of the paint layer and its porosity are taken into account.

Also, the quality of the metal coating will depend on the level of surface preparation and the method of working with the anti-corrosion coating. Any rust must be cleaned off.

Anti-corrosion paints and varnishes

Anti-corrosion paint for metal is a substance that protects metal from exposure to a hazardous external environment.

• The composition is lubricated on the damaged surface.
• This material creates a more weather-resistant coating.
• Maximum resistance to rust.

Rust paint can also be used. Rust-resistant paint, when applied to metal in a timely and correct manner, guarantees long-term protection of pipelines and metal systems. This paint can protect elements of mechanisms and industrial devices, as well as machine parts from the harmful effects of rust. The paint is used to treat technological systems and other equipment, including hydraulic mechanisms and bridges.

• Elakor-PU - enamels for processing metal parts and paint for metal.
• Thick-layer anti-corrosion coating based on alkyd resins. FEIDAL Coatings.
• Enamel PF, PF 115 and enamel PF-1189. "Sigma paints"
• Anti-corrosion paint NOR-MAALI. Used as a surface paint. Manufacturer CARBOLINE - Norway. The line of Nor-Maali paints is quite wide - about a hundred types. They are mainly used for industrial purposes: shipbuilding, bridge building, construction

However, not every paint can guarantee the best protection. It is worth considering the types of anti-corrosion enamels. Specialists use paints and varnishes to reduce the cost of work. In addition, this product speeds up the coating process.

Source: https://accorel.ru/tsvetnaya-zhizn/klassifikatsiya-vidov-korrozii-i-sposoby-zashhity-ot-nee

Metal corrosion and its types

→ Theory → Metal corrosion and its types

Chemical and physicochemical reactions that occur during the interaction of the environment with metals and alloys, in most cases lead to their spontaneous destruction. The process of self-destruction has its own term - “corrosion”.

The result of corrosion is a significant deterioration in the properties of the metal, as a result of which products made from it quickly fail. Every metal has properties that allow it to resist destruction.

Corrosion resistance, or, as it is also called, chemical resistance of a material, is one of the main criteria by which metals and alloys are selected for the manufacture of certain products.

Depending on the intensity and duration of the corrosion process, the metal can be subjected to either partial or complete destruction. The interaction of a corrosive environment and metal leads to the formation of phenomena such as scale, oxide film and rust on the metal surface. These phenomena differ from each other not only in appearance, but also in the degree of adhesion to the surface of metals.

For example, during the oxidation of a metal such as aluminum, its surface is covered with a film of oxides, which is characterized by high strength. Thanks to this film, destructive processes are stopped and do not penetrate inside. If we talk about rust, then the result of its influence is the formation of a loose layer.

The corrosion process in this case very quickly penetrates the internal structure of the metal, which contributes to its rapid destruction.

Indicators by which the classification of corrosion processes is carried out:

• type of corrosive environment;
• conditions and mechanism of occurrence;
• nature of corrosion damage;
• type of additional effects on metal.

According to the mechanism of the corrosion process, both chemical and electrochemical corrosion of metals and alloys are distinguished.

Chemical corrosion is the interaction of metals with a corrosive environment, during which the simultaneous oxidation of the metal and the reduction of the oxidizing component of the environment are observed. Products interacting with each other are not separated spatially.

Electrochemical corrosion is the interaction of metals with a corrosive environment, which is an electrolyte solution. The process of ionization of metal atoms, as well as the process of reduction of the oxidizing component of a given corrosive environment, occur in different acts. The electrode potential of the electrolyte solution has a significant impact on the rate of these processes.

Depending on the type of aggressive environment, there are several types of corrosion

Atmospheric corrosion is the self-destruction of metals in an air atmosphere or in a gas atmosphere characterized by high humidity.

Gas corrosion is the corrosion of metals that occurs in a gas environment in which the moisture content is minimal. The absence of moisture in a gaseous environment is not the only condition that contributes to the self-destruction of a metal. Corrosion is also possible at high temperatures. This type of corrosion is most common in the petrochemical and chemical industries.

Radiation corrosion is the self-destruction of a metal under the influence of radioactive radiation of varying degrees of intensity.

Underground corrosion is corrosion that occurs in soils and various soils.

Contact corrosion is a type of corrosion, the formation of which is facilitated by the contact of several metals that differ from each other in stationary potentials in a particular electrolyte.

Biocorrosion is the corrosion of metals that occurs under the influence of various microorganisms and their activity.

Corrosion by current (external and stray) is another type of corrosion of metals. If the metal is exposed to current from an external source, then this is corrosion by external current. If the effect is carried out through stray current, then this is stray current corrosion.

Corrosive cavitation is a process of self-destruction of metals, the occurrence of which is promoted by both shock and corrosive effects of the external environment.

Stress corrosion is metal corrosion caused by the interaction of a corrosive environment and mechanical stress. This type of corrosion poses a significant danger to metal structures that are subject to severe mechanical stress.

Fretting corrosion is a type of metal corrosion that is caused by a combination of vibration and exposure to a corrosive environment. To minimize the likelihood of corrosion due to friction and vibration, it is necessary to carefully approach the choice of structural material. It is also necessary to use special coatings and, if possible, reduce the coefficient of friction.

Based on the nature of destruction, corrosion is divided into continuous and selective.

Complete corrosion completely covers the metal surface. If the rate of destruction over the entire surface is the same, then this is uniform corrosion. If the destruction of metal in different areas occurs at different rates, then corrosion is called uneven.

Selective corrosion involves the destruction of one of the alloy components or one structural component.

Local corrosion , manifested in the form of spots scattered separately on the surface of the metal, consists of depressions of different thicknesses. The lesions may be shells or points.

Subsurface corrosion forms directly on the surface of the metal, after which it actively penetrates deeper. This type of corrosion is accompanied by delamination of metal products.

Intergranular corrosion manifests itself in the destruction of metal along grain boundaries. It is quite difficult to determine by the appearance of the metal. However, the strength and ductility of the metal change very quickly. Products made from it become fragile. This type of corrosion is most dangerous for chromium and chromium-nickel steels, as well as for aluminum and nickel alloys.

Crevice corrosion forms in those areas of metals and alloys that are located in threaded fasteners, various gaps and under all kinds of gaskets.

Source: https://steel-master.ru/teoria/korrozia-metallov-i-ee-vidy/

Corrosion of metal. Why it occurs and how to avoid it

11/14/14

Today our article is devoted to the most terrible enemy of iron. Metal corrosion, or rust, as it is often called, can almost completely destroy such a seemingly strong and solid material in the shortest possible time. Moreover, this process occurs every day and every minute all over the world.

According to statistics, 10% of the iron produced in the world turns into orange dust within a year. In Russia this figure reaches 16-17%. The material damage caused by the corrosion process accounts for about 4% of annual government income in developed countries. In our country, as you understand, the situation is even worse.  

So why does metal corrosion occur?

 Causes and types of metal corrosion

 Metal corrosion most often occurs:

— electrochemical (being in an excessively humid environment);

— chemical (interaction with aggressive chemical elements and alloys);

— biocorrosion (under the influence of bacteria (for example, marine microorganisms on the bottom of a ship)).

 According to the degree of damage they are also distinguished:

— local corrosion (only certain areas of the surface are affected);

— uniform corrosion (the entire surface is covered);

— intergranular corrosion (destruction of metal along grain boundaries).

 The most common cause of electrochemical corrosion is improper (excessively wet) storage or operating conditions of metal structures. Cars, as well as above-ground and underground communications, often suffer due to the active use of household and industrial salt to combat ice cover on roads (US losses due to this amount to about $2.5 billion per year).

Chemical corrosion, in turn, occurs under the influence of an environment that is not capable of conducting electrical current. These can be dry gases, as well as various petroleum products, alcohols and much more. At the same time, with increasing exposure temperature, the intensity of corrosion processes also increases.

 How to protect metal from corrosion?

 As practice shows, it is impossible to completely protect against corrosion processes. However, there are a number of techniques that will slow them down as much as possible.

Let's list the most effective ones:

 1. Protection with a layer of non-metallic coating.

Paint, varnish and some other polymers that block the access of moisture are perfect for this.

 2. Protection with a layer of another metal.

Commonly used materials are zinc, aluminum, cadmium, silver, copper, nickel, chromium or lead.

 3. Addition of alloying elements to alloys

Steel with high anti-corrosion properties is obtained by adding manganese, copper, nickel, chromium or titanium to the alloy.

Source: http://ntc-bulat.ru/korrosia-matalla

Lipaeva M. | Metal corrosion | Journal "Chemistry" No. 4/2004

Corrosion - a red rat, Gnawing on scrap metal.V.ShefnerMetal exemplifies strength. It’s not for nothing that when they want to emphasize this property, they say: “Strong as steel.” The concept of “metal” is associated with the idea of ​​something unchanging, durable, and solid. But it is not always the case. Under the influence of the external environment, metals oxidize and, as a result, are destroyed. This is called corrosion. What is the reason for the destruction of metals? All of them, with the exception of gold, silver and platinum, occur in nature in the form of compounds that form minerals and rocks. The existence of metals in a free state is energetically less favorable. To obtain them in their pure form, it is necessary to expend energy, mainly thermal energy. They are transferred from their natural state to metal. Metals, when corroded, return to the energetically favorable state of oxides (Fig. 1). Corrosion is of two types: chemical and electrochemical. Chemical corrosion Oxygen in the air interacts with the surface layer of the metal, and an oxide film is formed. It is formed in dry air conditions at room temperature and when heated. This type of corrosion is called chemical corrosion. The film can be durable and prevent further corrosion. Such durable films are formed on the surface of aluminum and zinc. But there are also loose films that do not protect the metal from destruction, as, for example, with alkali metal oxides. Thus, the surface of a freshly cut piece of sodium becomes cloudy before our eyes, a loose film with cracks is formed, which freely allows air oxygen, as well as other gases and water vapor, to pass to the metal surface. The rapid oxidation of sodium or calcium metal in air is an example of chemical corrosion. Let's conduct an experiment to study films. Let's clean the surface of the metal plate and start heating. Gradually, so-called tarnish colors appear on the surface, i.e. the surface of the plate is painted in all the colors of the rainbow. These are films of different thicknesses that refract light differently. The redox processes that occur during chemical corrosion are carried out by the direct transfer of electrons to the oxidizing agent. Examples of chemical corrosion are reactions of metals with oxygen, chlorine, and sulfur oxides. As a result of such corrosion, many important parts of engineering structures (gas turbines, rocket engine nozzles, furnace fittings, etc.) are severely destroyed.

Electrochemical corrosion

This type of corrosion occurs in an electrically conductive environment. Many metal objects that we use in everyday life do not undergo visible corrosion, while a lost key quickly rusts. Consequently, electrochemical corrosion depends on external conditions (composition and concentration of the electrolyte). The rate of destruction of different metals is different.

Electrochemical corrosion is also observed in the case when metals that are in a series of voltages at some distance from each other come into contact. Thus, if copper rivets are used in the manufacture of a product from sheet iron, then in the presence of moisture they will act as a cathode, and the iron sheet will become an anode and, therefore, will be destroyed.

Corrosive destruction of metal in electrolyte solutions can be considered as the result of the work of a large number of microscopic galvanic cells, in which foreign impurities in the metal serve as cathodes, and the metal itself serves as an anode.

For the operation of a galvanic cell, it is necessary to have two metals of different chemical reactivity and a medium that conducts electric current - an electrolyte. In this case, the strength of the manifested current is greater, the further the metals are in the voltage series from each other. The flow of electrons goes from the more active metal to the less active one, which is the cathode. In these cases, the chemical energy of the redox processes of the galvanic cell is converted into electrical energy.

Examples of corrosion

1. Tin can (tinned iron) is an iron-tin galvanic couple. As long as the can is hermetically sealed, the contact pair is not in an electrically conductive environment, and the can can remain free of corrosion for a long time. There is a known case when a tin can lay in the ground for about a hundred years and did not corrode. But as soon as the can is opened, the all-consuming process of corrosion immediately begins.

In this case, electrons from iron, as a more active metal, are transferred to tin. A potential difference arises between the surface of the tin coating and the solution. Hydrogen ions from water or acid collect on the surface of the low-active metal, where they are reduced with the help of electrons coming from the dissolving iron. The more acidic the solution, the more intense the corrosion. In this case, the galvanic element “works”.

Iron serves as the soluble anode, and tin serves as the cathode:

anode: Fe0 – 2e = Fe2+,
cathode: 2H+ + 2e = H2.

Rusting of iron is a complex process that results in the formation of rust on the surface. A simplified composition of rust, a loose mass of red-brown color, is iron(III) hydroxide:

Fe2+ ​​+ 2OH– = Fe(OH)2,
4Fe(OH)2 + O2 + 2H2O = 4Fe(OH)3.

2. The base of the galvanized bucket is iron, the surface is a silvery-white shiny metal. As you use it, the surface of the bucket becomes covered with brownish-whitish spots and streaks.

Zinc in this galvanic couple, as a more active metal, will act as an anode and will be destroyed in the presence of a medium conducting electric current, while iron does not rust. Therefore, galvanized buckets are relatively inexpensive and last a long time.

The composition of the white-brown spots is mainly zinc hydroxide. Process chemistry:

anode: Zn0 – 2e = Zn2+,
cathode: 2H+ + 2e = H2.

Zn2+ + 2OH– = Zn(OH)2.

“Ringworm of zinc coatings” is a picture of one of the types of zinc corrosion, very similar in appearance to the well-known fungal disease. It has been noticed that zinc corrodes faster indoors than on a galvanized roof exposed to winds and rain. This happens because corrosion products (zinc oxide, hydroxide and carbonate) are not washed away by rain.

The resulting “white rust” deposits absorb moisture, and light spots grow on the galvanized surface.
Another interesting circumstance: soft water causes more severe corrosion of this metal than water containing large amounts of hardness salts. Hard water is less aggressive because...

carbonate sediment forms a fairly durable protective coating on the zinc surface.

Based on the position of an element in the periodic table, we can say in advance what chemical, and therefore corrosive properties it has. The term “corrosive properties” refers to how easily a given element is oxidized, what is the stability of the resulting oxides in relation to water, salt solutions and various gases. Thus, in group I, in the secondary subgroup, there are metals that are very resistant to corrosion.

These are copper, silver, gold, and their corrosion resistance increases with increasing atomic mass. In group II, the metals of the secondary subgroup are also more stable: zinc, cadmium, mercury. On their surface, in the presence of oxygen, a thin, fairly strong film of oxides is formed, which protects against further destruction. In group III of the technically important metals is aluminum - it is a chemically active metal.

It is easily oxidized by atmospheric oxygen, as a result of which a thin glassy film is formed on its surface. But this film has high protective properties. Group IV contains corrosion-resistant metals - tin, lead, the resistance of which is also explained by the formation of durable protective films. Metals located in even rows of large periods, in groups V, VI and VII, have a high ability to passivate, and therefore, high corrosion resistance.

These are vanadium, chromium, cobalt, etc. The most corrosion-resistant metals are in group VIII, and the greater their atomic mass, the greater their stability. Consequently, of the Group VIII metals, the most corrosion-resistant are osmium, iridium and platinum. Corrosion leads to man-made pollution of our planet. Currently, more than 0.7 billion tons of steel are smelted annually on the globe, and 10 to 25% of this amount is destroyed by corrosion.

The average lifespan of steel products is about 15 years. The same is the average age of products made from many non-ferrous and ferrous metals. The famous Eiffel Tower (7.3 thousand tons of metal structures) would have long been destroyed by corrosion if it had not been covered with paint every 7 years. More money has already been spent on painting than the cost of the structure itself. Corrosion, like an insatiable dragon, devours billions of tons of iron and causes enormous damage in all countries.

No protective coatings, lubricants, varnishes, paints, or the use of alloys can prevent metal diseases - oxidation, sputtering, rusting.

All applied methods of protecting metals can be divided into three groups:

The article was prepared with the support of VtorMetal Plus LLC. If you need to get rid of a cast iron bathtub or other scrap metal, then the best solution would be to contact the VtorMetal Plus company. On the website located at “www.Vivoz-Loma.

Ru", you can, without leaving the monitor screen, find out where the scrap metal collection points are located, as well as order the dismantling and removal of scrap metal.

The VtorMetal Plus company employs only highly qualified specialists who will carry out the required work on dismantling and removal of scrap metal in the shortest possible time.

1) various coatings; 2) processing the environment in which metals are located;

3) electrochemical protection methods.

Experience.
A solution of table salt is poured into a U-shaped tube to half its volume.
A few drops of phenolphthalein are added to each elbow of the tube. A pre-cleaned copper plate is immersed in one elbow of the tube, and a zinc plate in the other. Both plates are connected with copper wire (Fig. 2). Soon, the copper-plated elbow exhibits first a pink and then a crimson coloration, indicating the presence of excess free hydroxide groups (OH–).

In this experiment, when connecting a zinc plate to a copper plate with a wire, excess electrons, formed as a result of the transition of zinc in the form of ions into solution, flow from the zinc plate to the copper one. A galvanic element appears. From the copper plate, electrons move to hydrogen ions.

An excess of hydroxide ions is formed in the solution, which gives an alkaline reaction. In other words, a redox reaction occurs, resulting in an electric current. The flow of electrons moves from the zinc (anode) to the copper (cathode).

Zinc oxidizes (dissolves), and on copper there is a reduction of hydrogen ions (bubbles are visible).

M.A. LIPAEVA, chemistry teacher

(Elektrostal)

Source: https://him.1sept.ru/article.php?ID=200400409

A Comprehensive Guide to Types of Corrosion

Corrosion is the spontaneous destruction of metals under the influence of environmental factors in its chemical or physicochemical expression. In a broader sense, it can be argued that in addition to metals, other materials are also susceptible to corrosion: plastic, rubber, ceramic products, concrete mixtures, etc. Types of corrosion are divided based on the main factor, but they can overlap.

It’s not difficult to understand what corrosion is, because there are more than enough examples around. The degree of corrosion can be expressed in direct and collateral damage.

Incidental losses include losses due to the failure of equipment that has become inoperable in a corrosive environment, temporary downtime, loss of time for replacement of parts and repairs, damage to goods of parallel production due to contamination of products with visible effects of corrosion, additional costs for electricity, water and resources.

Direct losses include damaged pipelines, machinery and equipment.

Main types of corrosion

Corrosion processes are classified according to several parameters, and specific types of corrosion are distinguished according to certain characteristics. Below you will learn by what factors corrosion is classified, and what, for example, chemical corrosion is.

Classification according to the mechanism of processes

Chemical corrosion is the process of contact of a metal element with a medium that promotes the development of corrosion, during which the reaction of oxidation of the metal element and the reduction of the oxidizing component of the medium itself occurs simultaneously.

Chemical corrosion is when reaction products are not separated in space.

Electrochemical corrosion is a mechanism of reaction contact of an electrolyte solution with metal, the essence of which, as opposed to the chemical type of rusting, is the process when metal atoms are ionized, the oxidizing component of the medium is not restored in a single spatial and temporal interval: the rate of reactions is determined by the electrode potential.

Classification by type of corrosive environment and conditions

Gas corrosion is the rusting reaction of metals in a gas environment with a minimum permissible water content (coefficient not higher than 0.1%) or using extremely high temperatures. Gas corrosion is popular in industrial areas: petrochemical and chemical industries.

Example: isolation of the “bread” of the chemical industry – sulfuric acid – by carrying out the oxidation reaction of the element’s dioxide; splitting oil to produce derivatives of lower molecular weight.

Underground corrosion is rusting in a soil environment.

Atmospheric corrosion is the rusting of metals in air or moist gas.

Biocorrosion is a reaction with the appearance of rye under the influence of microorganisms.

Contact corrosion - in such a reaction several metals with different electrolyte potentials are involved.

Radiation corrosion is the formation of rye under the influence of radioactive rays.

Electrical corrosion – the corrosion process occurs under conditions of exposure to external or stray current.

Stress corrosion is the rusting of metal in a corrosive environment under mechanical stress. The chemistry of this type of rye is unsafe, mainly for supporting structures exposed to mechanical loads (turbines, springs, driving axles of buildings). An important nuance with this type of rusting is the potential for corrosion fatigue - the cumulative effect occurs during periodic tensile stress.

Similar cyclic rusting is characteristic of rolling mill rolls, automobile springs, and similar structures.

Corrosive cavitation is the destructive effect on metal of a corrosive environment and impact force.

Fretting corrosion is the destruction of metal surfaces by the simultaneous influence of rust-friendly environment and vibration. Based on the manifestation of the results of the process, it is possible to eliminate the consequence; to do this, you will need to clearly select the structural material, reduce the level of friction, apply a covering film, or perform other appropriate actions in this case.

Intercrystalline corrosion is the manifestation of rye along the edges of inclusions. The so-called secretive fault, during the period of activity of which external signs are not noticeable, but the metal in a short time loses its properties of strength and elasticity. Alloys that most often suffer from this type of external interference include nickel, aluminum, and chromium.

Crevice corrosion causes damage to metal in threaded fasteners, between gaskets and similar areas.

Source: https://GidPoKraske.ru/spetsialnye-materialy/rzhavlenie/vidy-korrozii.html

Classification of metal corrosion according to various criteria

Corrosion is the spontaneous destruction of metals under the influence of environmental factors in its chemical or physicochemical expression. In a broader sense, it can be argued that in addition to metals, other materials are also susceptible to corrosion: plastic, rubber, ceramic products, concrete mixtures, etc. Types of corrosion are divided based on the main factor, but they can overlap.

It’s not difficult to understand what corrosion is, because there are more than enough examples around. The degree of corrosion can be expressed in direct and collateral damage.

Incidental losses include losses due to the failure of equipment that has become inoperable in a corrosive environment, temporary downtime, loss of time for replacement of parts and repairs, damage to goods of parallel production due to contamination of products with visible effects of corrosion, additional costs for electricity, water and resources.

Direct losses include damaged pipelines, machinery and equipment.

Corrosion of metals. Types and features. Protection and principle of operation

Metal corrosion is the process of destruction of a metal surface as a result of adverse environmental influences. Its reason is the thermodynamic instability of the material to the influence of various substances that come into contact with it.

Types of corrosion

Destruction of surfaces occurs due to chemical or electrochemical interaction of an unfavorable environment. Both varieties are equally harmful to metal products.

Chemical corrosion

This process is carried out in an environment that does not transmit electrical current. It is observed, for example, when heated, resulting in the formation of chemical compounds such as sulfides, as well as various types of films.

Often the formed continuous films become impenetrable and preserve the surface, so subsequent corrosion of metals stops. This protective layer can be found on surfaces made of aluminum, chromium, nickel and lead.

The film on steel or cast iron is fragile, so its presence does not stop the further progress of destruction deeper into the material.

Chemical corrosion can be of two types:

Gas occurs as a result of the action of an aggressive gas environment or steam on the surface of the metal, which is accompanied by elevated temperatures. Thanks to the hot environment, there is no condensation on the surface. The gas can be oxygen, sulfur dioxide, water vapor, hydrogen sulfide, and so on. Such a corrosive influence can cause complete destruction of the active metal, except in cases where a protective impermeable film is formed.

Liquid corrosion of metals occurs in liquid media that are not capable of transmitting electricity. It is primarily observed when metals come into contact with crude oil, petroleum products or lubricating oils. If such substances contain a small proportion of water, corrosion becomes electrochemical.

In both types of chemical corrosion, the rate of destruction is proportional to the chemical reaction with which the oxidizing agent penetrates the created oxide film on the surface.

Electrochemical corrosion of metals

This type of metal surface failure occurs in an environment that can transmit electrical current. As a result of this process, a change in the composition of the metal is observed. Atoms are removed from the crystal lattice by anodic or cathodic action.

With anodic influence, metal ions pass into the solution of the liquid that surrounds it. With cathodic influence, the electrons obtained during the anodic process are associated with the oxidizing agent. The most common is electrochemical corrosion under the influence of hydrogen or oxygen.

The process of influence of electrochemical corrosion on metals depends on the level of their activity. According to this criterion, they are divided into 4 groups:

1. Active.
2. Moderate activity.
3. Inactive.
4. Noble.

Active ones have high instability. They are characterized by the occurrence of corrosion even in a neutral aqueous environment, which is devoid of dissolved oxygen or oxidizing agents. A prominent representative of such a metal is cadmium.

Intermediate activity metals are located on the table of chemical elements between cadmium and hydrogen. They are not subject to the onset of destruction in a neutral liquid environment devoid of oxygen, but begin to intensively succumb to corrosion when exposed to acids.

Low-active metals are located in the periodic table between hydrogen and rhodium. They are not affected by corrosion when in contact with neutral liquids and acidic environments. To activate the process of their destruction, the presence of oxygen or other oxidizing agents is necessary.

Noble metals are stable, due to which they are susceptible to corrosion only when exposed to an acidic environment and subject to contact with strong oxidizing agents. The list of noble metals includes platinum, gold, palladium and iridium.

Electrochemical corrosion of metals is the most common, since the natural conditions in which metal products are stored and operated are often influenced by a humid environment.

The following types of electrochemical corrosion are distinguished:

• Electrolytic – observed upon contact with solutions of salts, acids, bases, including ordinary water.
• Atmospheric - observed in atmospheric conditions that contain water evaporation. This type is the most common; it affects almost all metal products.
• Soil - observed as a result of exposure to moist soil, which may contain various chemical elements that accelerate the process of metal destruction. When exposed to acidic soils, the corrosion process is most aggressive. Soils with sand act the slowest.
• Aeration - is more rare and is observed in cases where there is uneven air access to different metal surfaces. As a result of the heterogeneous impact, the transition lines between such sections begin to collapse.
• Marinemetal corrosion implies destruction from the influence of sea water.

  It is classified as a separate group, since this liquid has a high content of salts and dissolved organic substances. This makes her more aggressive.

• Biocorrosion – this type of destruction occurs when the surface of the metal is exposed to bacteria, which, as a result of their vital activity, produce carbon dioxide and other substances.

• Electrocorrosion - this type of metal destruction is observed when it is exposed to stray currents, which is typical for underground structures, in particular subway rails, grounding rods, tram lines, etc.

Corrosion protection methods

The bare surface of the vast majority of metals is prone to rapid corrosion, so various methods of protection are used to reduce the destructive effects.

Coating with insulating layers:

• Another metal.
• Cement mortar.
• Varnishes.
• With paints.
• Bitumen.

One of the most effective ways to protect against corrosion is to coat the surface of one metal with another that is less prone to corrosion. An example of such a technical solution is galvanizing, when the steel is protected by a layer of zinc.

The inner metal is completely isolated until the zinc, as a result of natural corrosion, which occurs very slowly, is completely destroyed, exposing the steel. This method of protection is one of the most effective, since the covering metal of the sheet is held on the base, so it cannot be torn off in layers.

The disadvantage of this method is that mechanical action can scratch the thin protective film.

Coating metal with protective cement mortar, bitumen, varnishes and paints is also a very common solution, which is still inferior to galvanizing. This is due to the heterogeneity of the compositions of the base and coating. As a result of poor paint adhesion, the finished coating will peel off. Such protection may become cracked, allowing moisture to enter.

Corrosion of metals can be stopped by using a chemical coating:

• Oxidation.
• Phosphating.
• Nitriding.
• Blueing.
• Cementation.

The metal surface is exposed to various substances, phosphates, nitrogen or oxides, resulting in the creation of films that, due to their impermeability, prevent destruction. Such methods are applicable primarily to steels.

Another common solution is steel bluing, when the surface of the metal interacts with organic substances. Surfaces treated in this way acquire a dark color, reminiscent of a raven's wing, which is why this method got its name.

One of the most effective chemical coating options is carburization, which involves exposing the surface to carbon, resulting in a crust of reacted metal.

To protect ferrous metals from corrosion, technology can be used to change their composition. The addition of various compounds produces alloys that are more resistant to corrosion. An example of such a connection is stainless steel.

The most unusual is tread protection, which involves covering structures made of one metal with plates of a more active metal, the so-called tread. Since it has a more negative potential, it acts as an anode.

The protected surface is used as a cathode. They are connected to each other by a current conductor, which creates unfavorable conditions for the tread. As a result, it is he who is susceptible to destruction, while the valuable structure remains intact.

A rarer solution is to change the composition of the environment. Under such conditions, corrosion of metals slows down or does not occur. This method involves cleaning the composition of a liquid or gas from acids and salts that cause destruction. This method is not applicable in all cases, since it is characterized by technical difficulties and a certain high cost. It is used in various mechanisms. For example, they can use in certain environments only those metals for which they are not aggressive.

Related topics:

Source: https://electrosam.ru/glavnaja/jelektrotehnika/korroziia-metallov/