How does metal corrosion occur?

What is corrosion? Types and methods of protection against corrosion on metal products

Metal corrosion is rust, primarily formed on the surface; the more rust, the deeper it penetrates and destroys the material of the element.

Any corrosion can be characterized by three signs:

• Firstly, this is a reduction-oxidation process.
• Secondly, this process is spontaneous, that is, it occurs under any conditions.
• Thirdly, the corrosion process most often occurs and spreads on the surface of the element, and sometimes penetrates deeper.

Metal corrosion is a process that takes place in chemical or electrochemical environments, it leads to damage to the upper layers of the material.

Not only metal products, but also concrete and ceramic ones are susceptible to corrosion.

Types of corrosion by nature of destruction

There are two types of corrosion that can occur on a material:

• Continuous – distributed over the entire surface of the product. This type is also divided into several subspecies:

• Uniform - rust appears in the same amount in all areas of the product.
• Uneven – rust appears at different rates in different areas.

• Selective - a specific component of the metal alloy is subjected to destruction.
• Local – corrosion forms in individual small areas on the surface of the part. It is expressed in single depressions, cavities and corroding points.

Types of metal corrosion by mechanism of occurrence

There are several reasons for the occurrence of metal corrosion; the chemistry of these processes has now been sufficiently studied, which helps to effectively combat the destruction of materials.

Chemical corrosion of metals - occurs between the metal and the environment, an oxidation-reduction reaction occurs. This type of corrosion is characteristic of an environment in which electric current cannot flow. Chemical corrosion according to the conditions of its occurrence can be:

• With gas corrosion, rust occurs as a result of exposure of metal to a gaseous environment, most often at high temperatures. The peculiarity of this type is that the effect of a gaseous environment on some metals leads to their complete destruction, but on some metals (zirconium, aluminum, chromium) the resulting reaction leaves only a protective film.
• Liquid surface corrosion occurs when exposed to liquid aggressive media, also without the possibility of electric current flowing.

Electrochemical corrosion - this reaction takes place only in environments where electric current can flow.

Electrochemical corrosion can occur in a variety of environments, but they are all divided into two types according to the conditions of occurrence:

• Corrosion involving electrolytes in solutions occurs in an environment of acids, salts, water, and bases.
• Corrosion in atmospheric conditions is the most common corrosion.

Types of corrosion according to conditions

As noted above, depending on the conditions of corrosion, corrosion can be gas, liquid, atmospheric or in electrolyte solutions. It is necessary to make this list more complete, so additional types of corrosion are covered below:

• Corrosion of metals occurring in soils and soils;
• Biocorrosion resulting from the activity of microorganisms on the surface of the material;
• Structural - occurs due to the heterogeneous structure of the metal;
• Contact corrosion occurs when metals come into contact with different potentials in the electrolyte for a long time;
• Friction of material in a corrosive environment;
• Corrosion resulting from friction of a material in a corrosive environment;
• Fretting corrosion occurs when vibrations occur in a corrosive environment;
• Cavitation occurs when exposed to an existing corrosive environment and external shock.

Result of corrosion

Lamellar corrosion of metal - a view of the ongoing process

Main types of atmospheric corrosion

It is customary to distinguish three main types of atmospheric corrosion: wet, wet, dry. Liquid and wet, due to their ability to conduct electric current, flow according to electrochemical laws, and dry according to chemical laws.

• Wet deep corrosion of metal will occur where a thin wet film can be observed on the metal. Depending on what is happening in the environment, condensation may form on the film, after which the process of corrosive destruction begins.
• Wet corrosion begins on a well-moistened surface, at a relative ambient humidity of about 100%. Droplets formed on the surface aid corrosive wear.
• Dry atmospheric corrosion is less aggressive because the destruction process occurs at low air humidity. The film formed on the product slows down the formation of rust.

Corroded ship

Types of concrete corrosion

Concrete is a strong stone building material consisting of cement, filler and binders. Since this material is used in open environments, and often in aggressive and hazardous environments, it is also susceptible to corrosive wear.

Corrosion pattern on concrete

There are several types of concrete corrosion:

1. As a result of interaction with the environment, easily soluble salts can form on the surface of concrete, which, when interacting with the internal components of the material, lead to its destruction.
2. A common problem is the separation of the constituent parts of the cement stone with water or the leaching of calcium hydroxide, which is formed during such a reaction or earlier.
3. Under environmental conditions, substances that have a sufficiently large volume penetrate into the concrete composition in comparison with the initial reaction products, which leads to mechanical and chemical damage to the integrity of the material, then these areas, under the influence of the environment, begin to corrode according to principle 1 or 2.

When concrete corrosion occurs, it is impossible to identify only one cause; often the resulting corrosion is the product of several factors combined.

Iron corrosion

It has long been discovered that corrosion (rust) on iron elements often occurs due to oxidation reactions with air or acids - redox reactions. As with any metal, rust captures the upper layers of the iron product and chemical corrosion, electrochemical or electrical, occurs.

If we consider each of these processes separately, it turns out that when rust occurs chemically, electrons transfer to the oxidizing agent, resulting in the formation of an oxide film, and the reaction looks like this:

3Fe + 2O2 = Fe3O4 (FeO•Fe2O3)

The resulting film does not protect the material from further occurrence of redox reactions; it allows air to pass freely, which contributes to the formation of new rust.

With electrochemical corrosion, which most often occurs with iron in the ground, a reaction occurs with the formation of free oxygen and water; if they remain on the iron element, this causes new corrosion products.

Fe + O2 + H2O → Fe2O3 xH2O

Electrical corrosion of iron is the most unpredictable, as it occurs due to stray currents that can reach the iron element from power lines, tram tracks, large electrical equipment, and more. The stray current starts the process of electrolysis of the metal, and it contributes to the formation of rust spots.

Copper corrosion

When operating copper elements, it is necessary to take into account the causes of corrosion; they are often caused by the environment where the element is located. For example, in such environments as atmospheric, sea water, in contact with halogen substances and in weak salt solutions, copper corrodes steadily slowly.

1)Cu+2H2SO4→CuSO4+SO2+2H2O

2)Cu+H2SO4→CuO+SO2+H2O

Copper is also susceptible to corrosion under normal atmospheric conditions:

2Cu+H2O+CO2+O2→ CuCO3*Cu(OH)2

Methods and methods for protecting metals from corrosion

Due to the fact that the corrosion process occurs on the upper layers of the metal of the structure, surface protection consists of creating an upper protective layer for the product, which removes traces of corrosion on the metal. Such protective coatings are metallic and non-metallic substances.

It is important to understand that protection against corrosion does not eliminate it, but only slows down processes that are already occurring. However, if you choose the right means of control, it is possible to slow down the process of corrosion formation for several years.

As the name suggests, metal coatings are substances based on metal. For example, to protect an iron structure from corrosion, layers of zinc, copper or nickel are applied to its surface.

Cleaning pipes from corrosion

Non-metallic coatings are special substances, the widest group of protective compounds. They are manufactured in the form of paints, enamels, lubricants, primers, bitumen and bitumen-polymer based compositions, etc.

The great popularity of non-metallic compounds in eliminating traces of corrosion lies in their wide selection, wide price range, ease of manufacture and good protective properties.

Chemical coatings have gained the least popularity due to the need to carry out complex chemical processes:

• Oxidation is the formation of oxide films on the surfaces of protected parts.
• Nitriding is the saturation of the upper layers of the material with nitrogen.
• Cementation is a reaction in which the upper layers combine with carbon, etc.

Also, in case of corrosion of metals, there are methods of protection in which, at the stage of fusion of metals, special compounds are introduced into them, which can increase the corrosion resistance of the future material.

A large group of protection is represented by methods of electrochemical and protective protection.

Electrochemical protection consists of the process of converting corrosion products in an electrolyte environment using a conducting electric current. Direct current is connected to the cathode (the material being protected), and a conductive metal source acts as the anode, which, when destroyed, protects the object from rust.

Electrochemical corrosion protection

Protective protection proceeds according to the same principle, however, together with the metal connecting product there are special products - protectors, which act as an anode. As a result of the ongoing reaction, the protector is destroyed, protecting the cathode (metal structure).

Thus, although corrosion is an irreversible process, people have now learned to effectively slow down its destructive effects.

Source: https://corprotect.ru/news/detail/korroziya-eto-vidy-i-sposoby-zashchity-ot-korrozii-na-metallicheskikh-izdeliyakh/

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.
• Marine corrosion of metals involves 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 metal surface 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/

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/

Iron corrosion

The process of iron corrosion most often comes down to its oxidation by atmospheric oxygen or acids contained in solutions and its transformation into oxides. Corrosion of metals (rusting) is caused by redox reactions occurring at the interface between the metal and the environment. Depending on the mechanism of occurrence, there are such types of iron corrosion as: chemical, electrochemical and electrical.

Chemical corrosion process of iron

Redox reactions in this case proceed through the transfer of electrons to the oxidizing agent. During this type of corrosion process, atmospheric oxygen interacts with the surface of the iron. This creates an oxide film called rust:

3Fe + 2O2 = Fe3O4 (FeO•Fe2O3)

Unlike tight oxide films that form during corrosion on alkali metals, aluminum, and zinc, a loose oxide film on iron freely allows air oxygen, as well as other gases and water vapor, to pass to the metal surface. This promotes further corrosion of the iron.

Electrochemical corrosion process

This type of corrosion occurs in a medium that conducts electrical current. Metal in the ground is subjected primarily to electrochemical corrosion. This type of corrosion process is the result of chemical reactions involving environmental components. Also, electrochemical corrosion occurs in the event of contact between metals located in a series of voltages at some distance from each other, resulting in the formation of a galvanic cathode-anode pair.

The atmospheric and ground corrosion process is expressed by the scheme:

Fe + O2 + H2O → Fe2O3 xH2O

As a result, rust of various colors is formed, which is due to the fact that various iron oxides are formed. What kind of substance is formed during the process of iron corrosion depends on the oxygen pressure, air humidity, temperature, duration of the process, composition of the iron alloy, condition of the surface of the product, etc. The rate of destruction of different metals is different.

The process of metal corrosion in electrolyte solutions is the result of the work of a large number of microscopic galvanic cells, in which impurities in the metal act as a cathode, and the metal itself acts as an anode. As a result, microscopic galvanic elements appear.

Also, iron atoms in different areas have different abilities to donate electrons (oxidize). The areas of the metal on which this process occurs act as the anode. The remaining sections are cathode sections, where water and oxygen reduction processes occur:

H2O + 2e– = 2OH– + H2

O2 + 2H2O + 4e– = 4OH–

The result is that iron (II) hydroxide is formed from iron (II) ions and hydroxide ions. Next comes its oxidation to iron (III) hydroxide, the main component of rust:

Fe2+ ​​+ 2OH– = Fe(OH)2
Fe(OH)2 + O2 + H2O → Fe2O3 xH2O

In order for a galvanic cell to work, it is necessary to have two metals of different chemical reactivity and a medium that conducts electric current - an electrolyte.

When iron and another metal (for example, zinc) come into contact, the corrosion of the iron slows down, and the corrosion of the more active metal (zinc) accelerates. This is due to the fact that the flow of electrons goes from a more active metal (anode) to a less active metal (cathode).

Thus, when iron comes into contact with a less active metal, corrosion of the iron accelerates.

Electrical corrosion process

This type of destruction of metal underground structures, cables and structures can be caused by stray currents emanating from trams, subways, electric railways and various electrical installations with direct current.

The current from metal structures exits into the ground in the form of positive metal ions - electrolysis of the metal occurs. The area where currents exit is the anode zone. It is in them that active processes of electrical corrosion of iron occur. Stray currents can reach 300 A and operate within a radius of several tens of kilometers.

Stray currents emanating from alternating current sources cause mild corrosion of underground steel structures, and strong corrosion of structures made of non-ferrous metals. Protecting metal structures from corrosion is a very important task, as it causes huge losses.  

Source: https://notehspb.ru/o_korrozii/korroziia_zheleza

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

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

Chemical corrosion

Chemical corrosion is a type of corrosion destruction of metal associated with the interaction of the metal and the corrosive environment, in which the metal is simultaneously oxidized and the corrosive environment is restored. Chemical corrosion is not associated with the formation or exposure to electric current.

The driving force (root cause) of chemical corrosion is the thermodynamic instability of metals. They can spontaneously transition to a more stable state as a result of the process:

Metal + Oxidizing component of the medium = Reaction product

In this case, the thermodynamic potential of the system decreases.

By the sign of the change in the thermodynamic potential, one can determine the possibility of spontaneous occurrence of chemical corrosion. The criterion is usually the isobaric-isothermal potential G. When a chemical process occurs spontaneously, a decrease in the isobaric-isothermal potential is observed. Therefore, if:

Δ GT < 0, then the process of chemical corrosion is possible;

Δ GT > 0, then the process of chemical corrosion is impossible;

Δ GT = 0, then the system is in equilibrium.

Chemical corrosion includes:

— gas corrosion — corrosion destruction under the influence of gases at high temperatures;

— corrosion in non-electrolyte liquids.

Gas corrosion

Gas corrosion is the most common type of chemical corrosion. At high temperatures, the metal surface is destroyed under the influence of gases. This phenomenon is observed mainly in metallurgy (equipment for hot rolling, forging, stamping, parts of internal combustion engines, etc.)

The most common case of chemical corrosion is the interaction of metal with oxygen. The process proceeds according to the reaction:

Me + 1/2O2 - MeO

The direction of this reaction (oxidation) is determined by the partial pressure of oxygen in the gas mixture (pO2) and the dissociation pressure of oxide vapor at a certain temperature (pMeO).

This chemical reaction can occur in three ways:

1) pO2 = pMeO, equilibrium reaction;

2) pO2 > pMeO, the reaction is shifted towards the formation of oxide;

3) pO2 < pMeO, the oxide dissociates into pure metal and oxide, the reaction proceeds in the opposite direction.

Knowing the partial pressure of oxygen in the gas mixture and the dissociation pressure of the oxide, it is possible to determine the temperature range at which this reaction is thermodynamically possible.

The rate of gas corrosion is determined by several factors: ambient temperature, the nature of the metal or alloy composition, the nature of the gaseous environment, the time of contact with the gaseous environment, and the properties of the corrosion products.

The process of chemical corrosion largely depends on the nature and properties of the oxide film formed on the surface.

The process of the appearance of an oxide film on the surface can be divided into two stages:

— oxygen molecules are adsorbed on the surface of the metal, which is in direct contact with the atmosphere;

- metal reacts with gas to form a chemical compound.

At the first stage, an ionic bond occurs between surface atoms and oxygen: the oxygen atom takes two electrons from the metal. This creates a very strong bond, much stronger than the bond between oxygen and the metal in the oxide.

Perhaps this phenomenon is observed due to the effect on oxygen of the field created by metal atoms.

After complete saturation of the surface with the oxidizer, which occurs almost instantly, at low temperatures, due to Van der Waltz forces, physical adsorption of oxidant molecules can be observed.

As a result, a very thin monomolecular protective film is formed, which thickens over time, making it difficult for oxygen to reach.

At the second stage, due to chemical interaction, the oxidizing component of the medium takes away valence electrons from the metal and reacts with it, forming a corrosion product.

If the resulting oxide film has good protective properties, it will inhibit the further development of the chemical corrosion process. In addition, the oxide film greatly affects the heat resistance of the metal.

There are three types of films that can form:

- thin (invisible to the naked eye);

- medium (gives tarnished colors);

- thick (clearly visible).

In order for the oxide film to be protective, it must meet certain requirements: have no pores, be continuous, adhere well to the surface, be chemically inert in relation to its environment, have high hardness, and be wear-resistant.

If the film is loose and porous, and also has poor adhesion to the surface, it will not have protective properties.

There is a continuity condition, which is formulated as follows: the molecular volume of the oxide film must be greater than the atomic volume of the metal .

Continuity is the ability of the oxide to cover the entire surface of the metal with a continuous layer.

If this condition is met, then the film is continuous and, accordingly, protective.

But there are metals for which the continuity condition is not an indicator. These include all alkaline, alkaline earth (except beryllium), even magnesium, which is important technically.

To determine the thickness of the oxide film formed on the surface and study its protective properties, many methods are used. The protective ability of a film can be determined during its formation, by the rate of metal oxidation and the nature of the rate change over time.

If the oxide has already formed, it is advisable to study the thickness and its protective properties by applying some reagent suitable for this case to the surface (for example, a solution of Cu(NO3)2, which is used for iron).

By the time of penetration of the reagent to the surface, the thickness of the film can be determined.

Even the already formed continuous film does not stop its interaction with the metal and the oxidizing environment.

The influence of external and internal factors on the rate of chemical corrosion.

Temperature has a very strong influence on the rate of chemical corrosion. When it increases, oxidation processes occur much faster. In this case, the decrease in the thermodynamic possibility of the reaction occurring has no significance.

Variable heating and cooling have a particularly strong effect. Cracks form in the protective film due to the appearance of thermal stresses. Through cracks, the oxidizing component of the medium has direct access to the surface. A new oxide film is formed, and the old one gradually peels off.

The composition of the gas environment plays a major role in the corrosion process. But this is individual for each metal and changes with temperature fluctuations. For example, copper corrodes very quickly in an oxygen atmosphere, but is stable in an environment containing SO2. Nickel, on the contrary, corrodes intensively upon contact with an SO2 atmosphere, but is stable in O2, CO2 and H2O environments. Chromium is relatively stable in all four environments.

If the oxide dissociation pressure is higher than the pressure of the oxidizing component, the oxidation of the metal stops and it becomes thermodynamically stable.

The rate of oxidation depends on the composition of the alloy. Let's take iron, for example. Additions of sulfur, manganese, phosphorus and nickel do not affect its oxidation. Silicon, chromium, aluminum slow down the process. And beryllium, cobalt, titanium and copper greatly inhibit oxidation. At high temperatures, tungsten, molybdenum, and vanadium can intensify the process. This is explained by the volatility or fusibility of their oxides.

Observing the rate of iron oxidation at different temperatures, we note that with increasing temperature, the slowest oxidation is observed with an austenitic structure. It is the most heat-resistant compared to others.

The rate of chemical corrosion is also influenced by the nature of surface treatment. If the surface is smooth, then it oxidizes a little slower than a bumpy surface with defects.  

Chemical corrosion in non-electrolyte liquids

Non-electrolyte liquids are liquid media that are not conductors of electricity. These include: organic (benzene, phenol, chloroform, alcohols, kerosene, oil, gasoline); inorganic origin (liquid bromine, molten sulfur, etc.).

Pure nonelectrolytes do not react with metals, but with the addition of even a small amount of impurities, the interaction process accelerates sharply. For example, if the oil contains sulfur or sulfur-containing compounds (hydrogen sulfide, mercaptans), the process of chemical corrosion accelerates.

If in addition the temperature increases, there will be dissolved oxygen in the liquid - chemical corrosion will intensify.

The presence of moisture in non-electrolyte liquids ensures intensive corrosion through the electrochemical mechanism.

Chemical corrosion in non-electrolyte liquids is divided into several stages:

— approach of the oxidizing agent to the metal surface;

— chemisorption of the reagent on the surface;

— reaction of the oxidizing agent with the metal (formation of an oxide film);

— desorption of oxides with metal (may be absent);

— diffusion of oxides into a non-electrolyte (may be absent).

To protect structures from chemical corrosion in non-electrolyte liquids, coatings that are stable in this environment are applied to its surface.

Source: https://www.okorrozii.com/ximichiskakorrozia.html

Corrosion process

In the modern world, a large number of products are made from metals of various types. Metal materials are present in various industries in the form of machine tools, machines, and tools. When producing any product, it is very important to ensure that the metals are covered with rust as little as possible or are resistant to its appearance.

Corrosion characteristics

Corrosion is popularly known as rust. It is a process of spontaneous formation of plaque on a metal surface as a result of environmental influences. Rust usually has a dark brown tint, which spoils the external quality of a product made of a particular metal.

Metal corrosion is quite common today. The reason for its appearance is that some types of metal materials are unstable to temperature changes and changes in humidity. Metal products often come into contact with various substances. They can affect them in different ways. As a result, corrosion of various types is formed.

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

Corrosion not only affects the external qualities of products and objects, but also contributes to the destruction of the metal material.

As a result, the structure that is created from it becomes unusable.

Not only metals, but also other materials are subject to corrosion. Today there are quite often cases when it appears on plastic. Rust formation is also inherent in concrete products.

The rate of corrosion depends on the size of the temperature. With every hundred degree increase in temperature, rust appears faster.

Types of corrosion

In the modern world, there are a large number of types of such a process as the formation of rust on the surface of certain types of materials.

The following types of corrosion are found today:

1. Electrochemical corrosion. This type of corrosion is characterized by the appearance of galvanic elements on the surface of metals, which cause rust. For this type of corrosion to occur, an electrolyte must be present. Its role is most often played by water. When in contact with condensate or water, electrodes or other metal elements change their redox potential.
2. Hydrogen corrosion. With this type of corrosion, hydrogen depolarization is observed. In this case, hydrogen is reduced.
3. Oxygen corrosion. There are situations when hydrogen in an alkaline environment is not able to be released. As a result, oxygen is released, which leads to the appearance of rust on the metal surface.
4. Chemical corrosion. With this type of corrosion, the surface of the metal comes into contact with a medium that provokes the appearance of rust.

Table. Types of electrochemical corrosion

No. Type of electrochemical corrosion Method of pipeline laying (type of equipment) Additional corrosion factors

1.	Atmospheric corrosion	External surfaces of above-ground and channel pipelines (at a level of flooding and siltation of the channel that does not reach the insulating structures). Surfaces of various metal structures and equipment that are not in contact with water and soil.	Internal stresses in the metal of pipelines and metal structures, shock-mechanical impact of drops from ceilings. Typical corrosion damage: uniform corrosion, spotty corrosion is possible in places where there is a drop.
2.	Underground corrosion	External surfaces of pipelines for channelless installation (in case of violation of the integrity of the insulation), channel installation (periodic flooding and silting of the channel, accompanied by moistening of the thermal insulation).	Internal stresses in metal, corrosion by external direct and alternating current, exposure to drops. Typical corrosion damage: uneven corrosion, spot corrosion, when exposed to stray currents, through damage to the pipeline wall is possible.
3.	Underwater corrosion	External surfaces of channel-laying pipelines. (Constant flooding of the channel in the absence of thermal insulation on the pipeline). Internal surfaces of pipelines and chemical water treatment equipment (deaerators, filters, etc.)	Internal stresses in metal, corrosion by external direct and alternating current. If the pipeline is not completely immersed, corrosion along the waterline is possible. Typical corrosion damage: uneven corrosion; when exposed to stray currents, through damage to the pipeline wall and ulcerative lesions in the waterline area are possible. On hot water supply pipelines, the process of microbiological corrosion by iron bacteria may occur. Typical corrosion damage: pitting corrosion (for internal surfaces of pipelines), pitting corrosion, uneven corrosion.

Corrosion inhibitors

Corrosion inhibitors are chemical compounds that are used to block or delay the process of rust formation. If they are present in an aggressive environment, the process of corrosion formation on metal surfaces will be reduced significantly.

Inhibitors form a thin protective film on the surface of metals, which prevents air and liquids from penetrating into the pores of metals, which could disrupt their integrity. They are one of the most effective methods of combating rust formation.

Corrosion on cars

Many modern motorists are faced with the fact that rust appears on their cars over time. Most often, the car body suffers from this. Car corrosion is one of the most common situations. It appears on those parts that are not made of stainless steel.

Today there are special products that prevent rust from appearing on car parts. They are represented by various compositions that are applied to the surface before application.

coloring.

Methods for protecting metals from corrosion

Corrosion each leads to a large number of losses. They number in the millions. The damage occurs not because corrosion destroys metals, but because the process deteriorates things made from metallic materials.

There is a large amount of equipment used in the world, most of which is made of a metal base. Its cost is quite high. After equipment failure, not every organization has the opportunity to purchase another one of the same level.

This is why corrosion protection is so necessary.

In the modern world, it is very important to choose the right means to combat the appearance of rust on metal products. It is necessary to carefully prepare the metal surface before applying paint.

Eighty percent of the resistance to corrosion depends on this. Paints and varnishes that are subsequently applied provide only twenty percent protection.

Today, to treat metal surfaces, you can use special rust converters, which act as both protection and primer.

Source: http://lkmprom.ru/clauses/issledovaniya/korroziya-ee-vidy-i-mery-zaschity-ot-poyavleniya-k/

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

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