Metal corrosion - causes and methods of protection. Information and construction portal oldmix

The phrase "metal corrosion" contains much more than the name of a popular rock band. Corrosion irrevocably destroys the metal, turning it into dust: of everything produced in the world of iron, 10% will completely collapse in the same year. The situation with Russian metal looks something like this - all metal smelted in a year in every sixth blast furnace in our country becomes rusty dust before the end of the year.

The expression "costs a pretty penny" in relation to metal corrosion is more than true - the annual damage caused by corrosion is at least 4% of the annual income of any developed country, and in Russia the amount of damage is calculated in ten digits. So what causes corrosive processes in metals and how to deal with them?

What is metal corrosion

Destruction of metals as a result of electrochemical (dissolution in a moisture-containing air or water environment - electrolyte) or chemical (formation of metal compounds with chemical agents of high aggression) interaction with the external environment. The corrosion process in metals can develop only in some areas of the surface (local corrosion), cover the entire surface (uniform corrosion), or destroy the metal along the grain boundaries (intergranular corrosion).

Metal under the influence of oxygen and water becomes a loose light brown powder, better known as rust (Fe 2 O 3 · H 2 O).

Chemical corrosion

This process occurs in environments that are not conductors of electric current (dry gases, organic liquids - oil products, alcohols, etc.), and the intensity of corrosion increases with increasing temperature - as a result, an oxide film forms on the metal surface.

All metals, both ferrous and non-ferrous, are subject to chemical corrosion. Active non-ferrous metals (for example, aluminum) under the influence of corrosion are covered with an oxide film that prevents deep oxidation and protects the metal. And such a low-active metal as copper, under the influence of moisture in the air, acquires a greenish bloom - a patina. Moreover, the oxide film does not protect the metal from corrosion in all cases - only if the crystal-chemical structure of the resulting film is consistent with the structure of the metal, otherwise the film will not help.

Alloys are susceptible to another type of corrosion: some elements of the alloys are not oxidized, but are reduced (for example, a combination of high temperature and pressure in steels is the reduction of carbides with hydrogen), while the alloys completely lose the necessary characteristics.

Electrochemical corrosion

The process of electrochemical corrosion does not require the mandatory immersion of the metal in the electrolyte - a sufficiently thin electrolytic film on its surface (often electrolytic solutions impregnate the environment surrounding the metal (concrete, soil, etc.)). The most common cause of electrochemical corrosion is the widespread use of household and industrial salts (sodium and potassium chlorides) to remove ice and snow on roads in winter - cars and underground utilities are particularly affected (according to statistics, annual losses in the United States from the use of salts in winter are $ 2.5 billion).

The following happens: metals (alloys) lose some of their atoms (they pass into the electrolytic solution in the form of ions), electrons replacing the lost atoms charge the metal with a negative charge, while the electrolyte has a positive charge. A galvanic pair is formed: the metal is destroyed, gradually all its particles become part of the solution. Electrochemical corrosion can be caused by stray currents arising from the leakage of a part of the current from the electric circuit into aqueous solutions or into the soil and from there into a metal structure. In those places where stray currents go out of metal structures back into water or soil, the destruction of metals occurs. It is especially common for stray currents to occur in places where ground electric transport (for example, trams and railway locomotives powered by electric traction) moves. In just a year, wandering currents of 1A are capable of dissolving iron - 9.1 kg, zinc - 10.7 kg, lead - 33.4 kg.

Other causes of metal corrosion

The development of corrosive processes is facilitated by radiation, waste products of microorganisms and bacteria. Corrosion caused by marine microorganisms damages the bottoms of ships, and corrosive processes caused by bacteria even have their own name - biocorrosion.

The combination of the impact of mechanical stresses and the external environment accelerates the corrosion of metals many times - their thermal stability decreases, surface oxide films are damaged, and in those places where inhomogeneities and cracks appear, electrochemical corrosion is activated.

Corrosion protection measures for metals

An inevitable consequence of technological progress is the pollution of our environment - a process that accelerates the corrosion of metals, since the external environment is increasingly aggressive towards them. There is no way to completely eliminate the corrosive destruction of metals; all that can be done is to slow down this process as much as possible.

To minimize the destruction of metals, you can do the following: reduce the aggression of the environment surrounding the metal product; increase the resistance of metal to corrosion; exclude the interaction between the metal and substances from the external environment that show aggression.

For thousands of years mankind has tried many methods of protection metal products from chemical corrosion, some of them are used to this day: coating with fat or oil, other metals that corrode to a lesser extent (the most ancient method, which is more than 2 thousand years old, is tinning (tin coating)).

Anti-corrosion protection with non-metallic coatings

Non-metallic coatings - paints (alkyd, oil and enamels), varnishes (synthetic, bituminous and tar) and polymers form a protective film on the surface of metals, excluding (in its integrity) contact with the external environment and moisture.

The use of paints and varnishes is advantageous in that these protective coatings can be applied directly on the assembly and construction site... Methods of applying paints and varnishes are simple and amenable to mechanization, damaged coatings can be restored "on site" - during operation, these materials have a relatively low cost and their consumption per unit area is small. However, their effectiveness depends on compliance with several conditions: compliance with the climatic conditions in which the metal structure will be used; the need to use exclusively high-quality paints and varnishes; strict adherence to the technology of application on metal surfaces. It is best to apply paints and varnishes in several layers - their quantity will provide the best protection against weathering on the metal surface.

In the role protective coatings polymers can act from corrosion - epoxy resins and polystyrene, polyvinyl chloride and polyethylene. In construction work, reinforced concrete embedded parts are covered with coatings from a mixture of cement and perchlorovinyl, cement and polystyrene.

Iron protection against corrosion by coatings of other metals

There are two types of metal inhibitor coatings - tread (zinc, aluminum and cadmium coatings) and corrosion resistant (silver, copper, nickel, chromium and lead coatings). Inhibitors are applied chemically: the first group of metals is highly electronegative to iron, the second is highly electropositive. The most widespread in our everyday life are metal coatings of iron with tin (tinplate, from which they produce cans) and zinc (galvanized iron - roofing), obtained by pulling sheet iron through a melt of one of these metals.

Cast iron and steel fittings, as well as water pipes are often galvanized - this operation significantly increases their resistance to corrosion, but only in cold water(with wire hot water galvanized pipes wear out faster than non-galvanized ones). Despite the effectiveness of galvanizing, it does not provide ideal protection - the zinc coating often contains cracks, which require preliminary nickel plating of metal surfaces (nickel plating) to eliminate them. Zinc coatings do not allow the application of paints and varnishes to them - there is no stable coating.

The best solution for corrosion protection is an aluminum coating. This metal has a lower specific gravity, which means it is less consumed, aluminized surfaces can be painted and the paint layer will be stable. In addition, the aluminum coating, in comparison with the galvanized coating, is more resistant to aggressive environments. Aluminum is not widely used due to the difficulty of applying this coating to a metal sheet - aluminum in a molten state exhibits high aggression to other metals (for this reason, the aluminum melt cannot be contained in steel bath). Perhaps this problem will be completely solved in the very near future - the original method of performing aluminizing was found by Russian scientists. The essence of the development is not to immerse the steel sheet in the aluminum melt, but to raise the liquid aluminum to the steel sheet.

Increasing corrosion resistance by adding alloying additives to steel alloys

The introduction of chromium, titanium, manganese, nickel and copper into a steel alloy makes it possible to obtain an alloy steel with high anti-corrosion properties. The steel alloy is particularly resistant to a large proportion of chromium, due to which a high-density oxide film forms on the surface of structures. The introduction of copper into the composition of low-alloy and carbon steels (from 0.2% to 0.5%) makes it possible to increase their corrosion resistance by 1.5-2 times. Alloying additives are introduced into the steel composition in compliance with Tamman's rule: high corrosion resistance is achieved when there is one alloying metal atom for every eight iron atoms.

Anti-corrosion measures

To reduce it, it is necessary to reduce the corrosive activity of the medium by introducing non-metallic inhibitors and to reduce the number of components capable of initiating an electrochemical reaction. This method will reduce the acidity of soils and aqueous solutions in contact with metals. To reduce the corrosion of iron (its alloys), as well as brass, copper, lead and zinc, carbon dioxide and oxygen must be removed from aqueous solutions. In the electric power industry, chlorides are removed from water, which can affect localized corrosion. By liming the soil, you can reduce its acidity.

Protection against stray currents

It is possible to reduce the electrocorrosion of underground utilities and buried metal structures by observing several rules:

• the section of the structure serving as a source of stray current must be connected with a metal conductor to the rail of the tramway;
• heating network routes should be located at the maximum distance from the railways along which electric transport moves, to minimize the number of their intersections;
• the use of insulating pipe supports to increase the transition resistance between the soil and pipelines;
• at the inputs to objects (potential sources of stray currents), it is necessary to install insulating flanges;
• install conductive longitudinal jumpers on flange fittings and stuffing box expansion joints - to increase longitudinal electrical conductivity on the protected section of pipelines;
• in order to equalize the potentials of pipelines located in parallel, it is necessary to install transverse electrical jumpers in adjacent sections.

The protection of insulated metal objects and small steel structures is accomplished with a protector that acts as an anode. The material for the protector is one of the active metals (zinc, magnesium, aluminum and their alloys) - it takes on most of the electrochemical corrosion, collapsing and preserving the main structure. One magnesium anode, for example, protects an 8 km pipeline.

Abdyuzhanov Rustam, specially for RMNT.ru

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

1

Corrosion means the destruction of materials caused by physical and chemical or purely chemical influence Wednesday. First of all, corrosion is divided by type into electrochemical and chemical, by nature - into local and continuous.

Local corrosion is knife, intergranular, through (through corrosion is known to car owners who do not monitor the state of the body of their vehicle), pitting, subsurface, filamentary, ulcerative. It also appears brittle, cracked and stained. Continuous oxidation can be selective, uneven, and uniform.

The following types of corrosion are distinguished:

• biological - due to the activity of microorganisms;
• atmospheric - destruction of materials under the influence of air;
• liquid - oxidation of metals in non-electrolytes and electrolytes;
• contact - formed by the interaction of metals with different values ​​of stationary potentials in an electrolytic medium;
• gas - becomes possible at elevated temperatures in gaseous atmospheres;
• white - often found in everyday life (on objects made of galvanized steel, on radiators);
• structural - refers to the heterogeneity of materials;
• crevice - occurs exclusively in cracks and gaps present in metal products;
• soil - observed in soils and grounds;
• fretting corrosion - is formed when two surfaces move (oscillatory) in relation to each other;
• external current - the destruction of the structure caused by the effect of an electric current coming from any external source;
• stray currents.

In addition, there is the so-called corrosion erosion - rusting of metals during friction, stress corrosion caused by mechanical stress and the influence of an aggressive environment, cavitation (a corrosion process plus shock contact of a structure with the external atmosphere). We have given the main types of corrosion, some of which will be discussed in more detail below.

2

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

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

In fact, fretting corrosion occurs due to the influence of an active corrosive environment in combination with mechanical wear.

The mechanism of this process is as follows:

• on the surface of contacting materials under the influence of a corrosive environment, corrosion products appear (oxide film);
• the specified film is destroyed by friction and remains between the contacting materials.

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

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

• The use of lubricating viscous compositions. This technique works if the products are not subject to excessive loads. Before applying the lubricant, the surface of metals is saturated with phosphates (slightly soluble) of manganese, zinc or ordinary iron. This method protection against fretting corrosion is considered temporary. It remains effective until the protective compound is completely removed due to slipping. Lubricants, by the way, are not used to protect structures from.
• Competent choice of materials for the manufacture of structures. Fretting corrosion is extremely rare when the object is made of hard and soft metals. For example, steel surfaces are recommended to be coated with silver, cadmium, tin, lead.
• The use of additional coatings with special properties, gaskets, cobalt alloys, materials with a low coefficient of friction.

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

3

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

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

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

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

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

• Decrease in humidity (relative) air. Relatively uncomplicated and at the same time very effective method, which consists in dehumidification and heating of rooms where metal structures are used. Atmospheric corrosion with this technique is greatly slowed down.
• Coating surfaces with non-metallic (varnishes, paints, pastes, lubricants) and metallic (nickel and zinc) compounds.
• Alloying metals. Atmospheric corrosion becomes less violent when phosphorus, titanium, chromium, copper, aluminum, nickel are added to the metal in small quantities. They suspend the anodic process or transfer the steel surfaces to a passive state.
• Use of inhibitors - volatile or contact. Volatiles include dicyclohexylamine, benzoates, carbonates, monoethanolamine. And the most famous contact type inhibitor is sodium nitrite.

4

Gas corrosion is usually noted at elevated temperatures in an atmosphere of dry vapors and gases. Enterprises of the chemical, oil and gas and metallurgical industries suffer the most from it, as it affects containers where chemical compounds and substances are processed, engines of special machines, chemical plants and units, gas turbines, equipment for heat treatment and melting of steel and metals.

Gas corrosion occurs during oxidation:

• carbon dioxide (carbon dioxide corrosion);
• hydrogen sulfide (hydrogen sulfide corrosion);
• hydrogen, chlorine, various halogens, methane.

Gas corrosion is most commonly caused by exposure to oxygen. The destruction of metals with it proceeds according to the following scheme:

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

After that, oxygen ions penetrate deep into the film, where they come into contact with metal cations. Gas corrosion caused by the influence of other chemical compounds follows a similar principle.

The phenomenon of hydrogen corrosion of steel is noted in technological equipment that operates in hydrogen atmospheres at high (from 300 MPa) pressures and temperatures above +200 ° C. Such corrosion is formed due to the contact of carbides included in steel alloys with hydrogen. Visually, it is poorly visible (the surface of the structure has no obvious damage), but at the same time the strength indicators of steel products are significantly reduced.

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

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

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

Chemical or electrochemical metal materials environment undergo destruction, which is called corrosion. Corrosion of metals caused by which metals are converted to an oxidized form and lose their properties, which renders metallic materials unusable.

There are 3 features that characterize corrosion:

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

Types of metal corrosion

The most common are the following types of metal corrosion:

1. Uniform - covers the entire surface evenly
2. Uneven
3. Electoral
4. Local spots - corrode certain areas of the surface
5. Ulcerative (or pitting)
6. Point
7. Intergranular - propagates along the boundaries of the metal crystal
8. Cracking
9. Subsurface

Main types of corrosion

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

Chemical corrosion of metals

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

Chemical corrosion of metals is gas and liquid.

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

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

With chemical corrosion the rate of destruction of the metal is proportional to the rate at which the oxidant penetrates through the metal oxide film covering its surface. Oxide films of metals may or may not exhibit protective properties, which is determined by the continuity.

Continuity such a film is estimated by the value the Pilling-Badwards factor: (α = V ok / V Me) the ratio of the volume of the formed oxide or any other compound to the volume of the metal consumed for the formation of this oxide

α = V ok / V Me = M ok ρ Me / (n A Me ρ ok),

where V ok is the volume of the formed oxide

V Me - the volume of metal consumed for the formation of oxide

M ok - molar mass of the formed oxide

ρ Me - metal density

n is the number of metal atoms

A Me - atomic mass metal

ρ ok - density of the formed oxide

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

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

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

Below are the values α for some metal oxides

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

Electrochemical corrosion of metals

Electrochemical corrosion of metals- This is the process of destruction of metals in a variety of environments, which is accompanied by the emergence of an electric current inside the system.

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

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

The very process of removing electrons from the cathode sections is called depolarization, and substances that facilitate the removal of electrons are called depolarizers.

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

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

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

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

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

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

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

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

1. Active metals (high thermodynamic instability) - these are all metals in the range of alkali metals - cadmium (E 0 = -0.4 V). Their corrosion is possible even in neutral aqueous media in which oxygen or other oxidizing agents are absent.
2. Medium activity metals (thermodynamic instability) - located between cadmium and hydrogen (E 0 = 0.0 V). In neutral environments, in the absence of oxygen, they do not corrode, but corrode in acidic environments.
3. Low-activity metals (intermediate thermodynamic stability) - are between hydrogen and rhodium (E 0 = +0.8 V). They are resistant to corrosion in neutral and acidic environments in which oxygen or other oxidizing agents are absent.
4. Noble metals (high thermodynamic stability) - gold, platinum, iridium, palladium. They can corrode only in acidic environments in the presence of strong oxidants.

Electrochemical corrosion can take place in a variety of environments. Depending on the nature of the environment, the following types of electrochemical corrosion are distinguished:

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

For example, when iron interacts with environmental components, some of its sections serve as an anode, where iron is oxidized, and others as a cathode, where oxygen is reduced:

A: Fe - 2e - = Fe 2+

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

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

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

Methods of protection against metal corrosion

The main way to protect against metal corrosion is creation of protective coatings- metallic, non-metallic or chemical.

Metallic coatings.

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

For example, when a zinc layer is applied to iron, if the integrity of the coating is broken, the zinc acts as an anode and will be destroyed, and the iron is protected until all the zinc is consumed. Zinc coating is in this case anode.

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

Non-metallic coatings.

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

Chemical coatings.

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

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

phosphating - receiving protective film phosphates (Fe 3 (PO 4) 2, Mn 3 (PO 4) 2);

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

blueing - the metal surface interacts with organic substances;

cementation - obtaining on the surface of the metal its compound with carbon.

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

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

Electrochemical protection is based on attaching the protected structure to the cathode of an external direct current source, as a result of which it becomes the cathode. The anode is metal scrap, which, when destroyed, protects the structure from corrosion.

Protective protection - one of the types of electrochemical protection - is as follows.

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

Categories ,

Many materials are susceptible to corrosion, for example, metal, ceramic, wood, as a result of exposure to them. As a rule, this effect is achieved due to the instability of the structure, which is affected by the thermodynamics of the environment. In the article, we will understand in detail what metal corrosion is, what types it has, and also how you can protect yourself from it.

Some general information

The word "rust" is quite popular among the people, which refers to the process of corrosion of metal and various alloys. People use the term “aging” to refer to polymers. In fact, these words are synonymous. A prime example is the aging of rubber products that actively interact with oxygen. Some plastic products due to precipitation, they can quickly deteriorate. How quickly the corrosion process will occur depends entirely on the conditions in which the product is placed. The humidity of the environment is especially affected. The higher its indicator, the faster the metal will become unusable. Empirically scientists have found that about 10% of products in production are simply written off due to corrosion. Views this process different, their classification depends on the type of environment in which the products are located, the speed and nature of the flow. Next, we will consider in more detail the types of corrosion. Now everyone should understand what metal corrosion is.

Artificial aging

The corrosion process is not always destructive and renders certain materials unusable. Often, due to corrosion, the coating has additional properties that a person needs. That is why artificial aging has become popular. Most often it is used if it comes about aluminum and titanium. Only with the help of corrosion can the increased strength of the materials be achieved. In order to complete the destruction process correctly, you must use heat treatment... Considering that the natural aging of materials in some conditions is a rather slow process, there is no need to clarify that when using this method, the material must have a special hardening. You also need to understand all the risks that are associated with this method. For example, although the strength of the material increases, the plasticity decreases as much as possible. Now the reader will be able to easily answer the question of what is the corrosion of an artificial metal.

Heat treatment reviews

This method compresses the molecules of the material, respectively, the structure changes. Often, thermal protection is necessary to strengthen pipelines, as it protects the material from rust, and also minimizes the pressure that is exerted on the structure if it is underground. Users of this technique leave reviews describing that this method protection is as effective as possible and really shows good results. It is advisable to apply such processing only in the industrial field. Because the chambers for firing and other processes required to obtain reliable protection are expensive, the method is not popular. This protection of the metal against corrosion is quite effective.

Classification

At the moment, there are more than 20 rust options. Only the most popular types of corrosion will be described in the article. Conventionally, they are divided into the following groups, which will help to understand in more detail what metal corrosion is.

Chemical corrosion is interaction with a corrosive environment. In this case, the oxidation of the metal and the reduction of the oxidant occur simultaneously in one cycle. Both materials are not separated by space. Consider other types of metal corrosion.

Electrochemical corrosion is the interaction of a metal with an electrolyte. Atoms are ionized, the oxidant is reduced, and these two processes take place in several cycles. Their speed depends entirely on the potential of the electrodes.

With gas corrosion, metal rusting occurs with a small liquid content. Moisture should not exceed 0.1%. Also given view corrosion can occur in a gaseous environment at high temperatures. Most often this type is found in the industry related to the chemical sphere and oil refining.

In addition to the above, there are many more types of material corrosion. There are biological, targeted, contact, local and other types of rusting.

Electrochemical corrosion and its features

In case of electrochemical corrosion, the destruction of the material occurs due to its contact with the electrolyte. The last substance can be condensate, rainwater... It should be noted that the more salts there are in the liquid, the higher the electrical conductivity. Accordingly, the corrosion process will proceed fairly quickly. If we talk about the most popular places that are susceptible to corrosion, rivets in metal structures, welded joints, and simply places where the material is damaged should be noted. It so happens that the alloy of iron, when it is created, is coated with special substances that have anti-corrosion properties. However, this does not prevent the rusting process, but only slows it down. A rather striking example is galvanizing. Zinc has a negative potential when compared to iron. Because of this, the latter material will regenerate and the zinc will be damaged. If there is an oxide film on the surface, then the destruction process will become long. Electrochemical corrosion has several types, but it should be noted that they are all dangerous and, as a rule, it is impossible to stop this type of metal corrosion.

Chemical corrosion

Chemical corrosion is quite common. For example, if a person notices dross, then he must understand that it appeared as a result of the combination of metal, that is, interaction with oxygen. As a rule, if the ambient temperature is high, then the corrosion process will be noticeably accelerated. Liquid can participate in rusting, that is, water, salt, any acid or alkali, salt solutions. When it comes to chemical corrosion of metals such as copper or zinc, their oxidation leads to a stable corrosion process of the film. The rest form iron oxide. Further, all chemical processes that will occur will lead to the appearance of rust. It will not provide protection in any way, but, on the contrary, promotes corrosion. At the moment, many materials can be protected with the help of galvanizing. Other means of protection against chemical corrosion of metals have also been developed.

Types of concrete corrosion

The brittleness of concrete can be caused by one of three types of corrosion. Changes in the structure of a given material are quite common. Consider why this is happening.

The most common type of corrosion is the destruction of cement stone. As a rule, this happens when liquid and precipitation are constantly acting on the material. Because of this, the structure of the material is destroyed. Below there are more detailed examples corrosion of metals:

• Interaction with acids. If cement stone will be constantly under the influence of these materials, then a rather aggressive element is formed, which is harmful to the coating. This is calcium bicarbonate.
• Crystallization of sparingly soluble substances. This is about corrosion. Due to the fact that fungi, spores and other substances enter the pores, the concrete coating begins to deteriorate quickly.

Corrosion: methods of protection

Due to corrosion, manufacturers often suffer huge losses, so a lot of work is being done to avoid this process. Moreover, it should be noted that most often it is not the metal itself that lends itself to corrosion, but huge metal structures. Manufacturers spend a lot of money on their creation. Unfortunately, it is almost impossible to provide 100% protection. However, if the surface is properly protected, that is, by abrasive blasting, the corrosion process can be delayed for several years. Also, paint and varnish are fought with it. It reliably protects the material. If the metal is underground, then it must be processed with special materials. This is the only way to achieve maximum protection of the metal against corrosion.

Anti-aging measures

As mentioned above, the corrosion process cannot be stopped. But you can maximize the time it takes for the material to break down. Also, in production, as a rule, they try to get rid of the factors that affect the aging process as much as possible. For example, in factories, every structure is periodically treated with solutions and polishes. It is they who relieve the material of the negative impact on the metal from mechanical, temperature and chemical conditions... In order to understand this in more detail, you should study the definition of metal corrosion. If we talk about slowing down the effect of aging, then it should be noted that heat treatment can be used for this. Under normal operating conditions, this method will maximize the avoidance of rapid material destruction. Welders, so that the seams on the product do not disperse, use firing at a temperature of 650 degrees. This technique will reduce the intensity of aging.

Active and passive methods of struggle

Active methods of combating corrosion work by changing the structure of the electric field. For this, a direct current must be used. The voltage must be such that the product has increased characteristics. A rather popular method would be to use a “sacrificial” anode. It protects the material by its own destruction. The corrosion conditions for metals are described above.

As for passive protection, a paint-and-lacquer coating is used for this. It completely protects the product from liquid and oxygen ingress. Thanks to this, the surface is maximally protected from destruction. Spraying of zinc, copper, nickel should be used. Even if the layer is severely destroyed, it will still protect the metal from rusting. Of course, you need to understand that passive protection methods will be relevant only if the surface does not have cracks or chips.

Reviews about paint protection of metals

At the moment, paint protection is very popular. It is efficient, flexible and inexpensive. However, if long-term use of a metal structure is required, then this method of protection will not work. For more than 7-8 years, paint and varnish coatings will not be able to protect the material. Accordingly, they will have to be updated. Most likely, you will have to carry out restoration and replace the surface of the material. Other disadvantages of this coating include limitations in terms of use. If it is necessary to reinforce pipes that are underground or water, then paint protection will not work. Therefore, it should be understood that if it is necessary for the structure to be used for more than 10 years, you should resort to other methods of protection.

Galvanizing in detail

Having considered the main types of corrosion, it is also necessary to discuss the most effective methods protection. One of these can be called galvanizing. It allows you to protect the material from severe damage by changing the physical and chemical properties. At the moment, this method is considered economical and effective, given that almost 40% of all extracted material on Earth is spent on zinc processing. It is important to treat the material with an anti-corrosion coating.

Galvanizing is carried out for steel sheets, fasteners, devices and huge metal structures. In general, such spraying can protect articles of any size and shape. Zinc has no decorative purpose, although it can occasionally be added to the alloy to obtain a shine. In general, you need to understand that this metal will provide maximum protection against corrosion even in the most aggressive conditions.

Features of rust protection products

When working with metal, any person understands that before applying protective materials, it is necessary to prepare the surface. Often all the difficulties lie in this stage. In order to create a special barrier that will allow rust to get to the metal, it is necessary to introduce the concept of a compound. Thanks to him, a corrosion protection will be formed in the kit. In this case, electrical insulation takes place. It is usually quite difficult to protect against corrosion of ferrous metals.

Due to the specifics of the use of various means for protection, it is necessary to understand the operating conditions of the material. If the metal will be located underground, then it is necessary to use multi-layer coatings, which will have not only anti-corrosion properties, but also enhanced protection against mechanical damage... If we are talking about communications that actively interact with oxygen and gases, you should use a tool that minimizes the effect of water and oxygen. Accordingly, increased attention from the manufacturer will be paid to insulation from moisture, steam and low temperatures... In this case, additives and special plasticizers should be added, because the causes of metal corrosion are different and should be protected from all types.

Mixture "Urizol"

The Urizol mixture should be considered separately as it is used to coat the pipeline. It is also suitable for fittings, connecting parts, valve assemblies and those products that are constantly in contact with oil or gases. This composition is needed in order to get rid of the influence of underground and atmospheric influences. Often this mixture is also used for insulation concrete materials... This substance is applied very simply, without any difficulty. In order to treat the surface, it is necessary to use a spray gun. This is the only way to avoid corrosion of metals and alloys of similar products. As soon as the components combine, the reaction begins. This creates polyurea. After that, the mixture turns into a gel-like and non-flowing state, and after some time it becomes solid. If the polymerization rate is slow, smudges will begin to form. They are harmful because they make it difficult to build up the thickness of the coating. It should be noted that this mixture remains sticky for a long time. Due to this, all layers will be as uniform as possible, and intermediate thickness measurements will be equal to each other. If the polymerization process is too fast, the adhesion of the composition will decrease. In this case, the thickness of the resulting layer for insulation will be uneven. By the way, the spray gun quickly clogs up if the coating speed is too fast. Corrosion factors for metals will not appear if everything is done correctly. In order to prevent such situations, it is necessary to carefully select the components and follow the manufacturing rules.

Paints and enamels

Protection metal-plastic structures can be done in three ways.

Paints and varnishes have already been described. They are simple, come in a variety of colors and can handle huge surfaces with ease. Since the metal corrosion process is quite fast, you should immediately think about coating with materials.

The second type is plastic coatings. They are usually made from nylon, PVC. This coating will provide maximum protection against water, acids and alkalis.

The third type is rubber coating. It is often used to protect tanks and other structures from the inside.

Phosphating and chromating

The metal surface must be properly prepared for the protection process. Which methods are used depends entirely on the type of surface. For example, ferrous metals are protected by phosphating. Non-ferrous metals can be processed by both methods. In general, if we talk about chemical preparation, it is necessary to clarify that it takes place in several stages. To begin with, the surface is degreased. Then it is washed with water. Next, a conversion layer is applied. After it is washed again with two types of water: drinking and demineralized, respectively. Next, it remains to passivate. Chemical treatments should be carried out by spraying, immersion, steam and water jet methods. The first two methods must be used with the help of special units that will completely prepare the surface for work. It is necessary to decide which method to choose depending on the size, configuration of the product, and so on. In order to better understand this issue, one should know the reaction equations for metal corrosion.

Conclusion

The article described what corrosion is and what types it has. Now, after reading this article, any person will be able to understand how to protect any material from aging. By and large, it is quite easy to do this, knowing everything necessary instructions... The main thing is to understand all the characteristics of the environment in which the material is used. If the products are located in a place where constant vibrations occur, and there are also strong loads, then in paintwork cracks will appear. Because of this, moisture will enter the metal, respectively, the corrosion process begins immediately. In such cases, it is better to additionally use rubber sealants and gaskets, then the coating will last a little longer.

In addition, it must be said that the structure, with premature deformation, will quickly deteriorate and age. Accordingly, this can lead to completely unforeseen circumstances. This will cause material damage and may result in the death of a person. Accordingly, special attention should be paid to protection against corrosion.

Corrosion of metals is known to cause many troubles. Is it not for you, dear car owners, to explain what it threatens: give it free rein, so only tires will remain from the car. Therefore, the sooner the fight against this scourge begins, the longer the car body will live.

To be successful in the fight against corrosion, you need to find out what kind of "beast" it is and understand the reasons for its occurrence.

Today you will find out

Is there any hope?

The damage caused to humanity by corrosion is colossal. According to various sources, corrosion "eats up" from 10 to 25% of the world's iron production. Turning into brown powder, it is irretrievably scattered in the white light, as a result of which not only we, but also our descendants are left without this most valuable structural material.

But the trouble is not only that metal as such is lost, no - bridges, cars, roofs, and architectural monuments are destroyed. Corrosion spares nothing.

The same Eiffel Tower - the symbol of Paris - is incurably ill. Made from regular steel, it will inevitably rust and deteriorate. The tower has to be painted every 7 years, which is why its weight increases by 60-70 tons each time.

Unfortunately, metal corrosion cannot be completely prevented. Well, except that completely isolate the metal from the environment, for example, place it in a vacuum. 🙂 But what is the use of such "canned" parts? The metal must "work". Therefore, the only way to protect against corrosion is to find ways to slow it down.

In time immemorial, fat and oils were used for this, later they began to cover iron with other metals. First of all, low-melting tin. In the writings of the ancient Greek historian Herodotus (5th century BC) and the Roman scientist Pliny the Elder, there are already references to the use of tin to protect iron from corrosion.

An interesting case occurred in 1965 at the International Corrosion Control Symposium. A certain Indian scientist spoke about a society for combating corrosion, which has existed for about 1600 years, and of which he is a member. So, one and a half thousand years ago, this society took part in the construction of the temples of the Sun on the coast near Konarak. And despite the fact that these temples were flooded by the sea for some time, the iron beams are perfectly preserved. So, even in those distant times, people knew a lot about the fight against corrosion. This means that not everything is so hopeless.

What is Corrosion?

The word "corrosion" comes from the Latin "corrodo" - to gnaw. There are also references to the late Latin “corrosio - erosion”. But one way or another:

Corrosion is the process of destruction of metal as a result of chemical and electrochemical interaction with the environment.

Although corrosion is most often associated with metals, concrete, stone, ceramics, wood, and plastics are also exposed to it. Applied to polymeric materials however, the term destruction or aging is more often used.

Corrosion and rust are not the same thing

In the definition of corrosion, the paragraph above is not in vain highlighted the word "process". The fact is, corrosion is often equated with the term "rust". However, these are not synonymous. Corrosion is precisely a process, while rust is one of the results of this process.

It is also worth noting that rust is a corrosion product exclusively of iron and its alloys (such as steel or cast iron). Therefore, when we say "steel rusts," we mean that the iron in its composition rusts.

If rust only refers to iron, then other metals do not rust? They do not rust, but this does not mean that they do not corrode. It's just that their corrosion products are different.

For example, copper, corroding, becomes covered with a beautiful greenish bloom (patina). Silver dims in air - this is a sulphide coating on its surface, whose thin film gives the metal its characteristic pinkish color.

Patina is a product of corrosion of copper and its alloys

Mechanism of Corrosion Processes

The variety of conditions and environments in which corrosion processes take place is very wide, therefore it is difficult to give a unified and comprehensive classification of the encountered cases of corrosion. But despite this, all corrosion processes have not only a common result - the destruction of the metal, but also a single chemical essence - oxidation.

In simpler terms, oxidation can be called the process of electron metabolism. When one substance is oxidized (gives up electrons), the other, on the contrary, is reduced (receives electrons).

For example, in reaction ...

... the zinc atom loses two electrons (is oxidized), and the chlorine molecule adds them (is reduced).

Particles that donate electrons and oxidize are called restorers, and the particles that accept electrons and recover are called oxidants... These two processes (oxidation and reduction) are interconnected and always occur simultaneously.

These are the reactions, which are called redox reactions in chemistry, underlie any corrosion process.

Naturally, the tendency to oxidation is not the same for different metals. To understand which ones have more and which ones have less, let us recall the school chemistry course. There was such a concept as an electrochemical series of voltages (activities) of metals, in which all metals are located from left to right in order of increasing "nobility".

So, the metals located in the row to the left are more prone to donating electrons (and hence to oxidation) than the metals to the right. For example, iron (Fe) is more susceptible to oxidation than more noble copper (Cu). Certain metals (for example, gold) can donate electrons only under certain extreme conditions.

We will return to a number of activities a little later, but now let's talk about the main types of corrosion.

Types of corrosion

As already mentioned, there are many criteria for the classification of corrosion processes. So, corrosion is distinguished by the type of propagation (continuous, local), by the type of corrosive medium (gas, atmospheric, liquid, soil), by the nature of mechanical effects (corrosion cracking, Fretting phenomenon, cavitation corrosion), and so on.

But the main method of classification of corrosion, which makes it possible to most fully explain all the subtleties of this insidious process, is the classification according to the mechanism of its occurrence.

According to this criterion, two types of corrosion are distinguished:

• chemical
• electrochemical

Chemical corrosion

Chemical corrosion differs from electrochemical corrosion in that it occurs in environments that do not conduct electric current. Therefore, with such corrosion, the destruction of the metal is not accompanied by the appearance of an electric current in the system. This is the normal redox interaction of a metal with its environment.

The most common example of chemical corrosion is gas corrosion. Gas corrosion is also called high-temperature corrosion, since it usually occurs at elevated temperatures, when the possibility of moisture condensation on the metal surface is completely excluded. This type of corrosion can include, for example, corrosion of elements of electric heaters or nozzles of rocket engines.

The rate of chemical corrosion depends on temperature - as it rises, corrosion accelerates. Because of this, for example, during the production of rolled metal, fiery sprays scatter in all directions from the hot mass. It is scale particles that are chipped off the metal surface.

Scale is a typical product of chemical corrosion, an oxide that occurs as a result of the interaction of a hot metal with oxygen in the air.

In addition to oxygen, other gases can be highly corrosive to metals. These gases include sulfur dioxide, fluorine, chlorine, hydrogen sulfide. So, for example, aluminum and its alloys, as well as steels with a high chromium content (stainless steels) are stable in an atmosphere that contains oxygen as the main aggressive agent. But the picture changes dramatically if chlorine is present in the atmosphere.

In the documentation for some anticorrosive agents, chemical corrosion is sometimes referred to as “dry” and electrochemical as “wet”. However, chemical corrosion can also occur in liquids. Only in contrast to electrochemical corrosion, these liquids are non-electrolytes (that is, they do not conduct electric current, for example, alcohol, benzene, gasoline, kerosene).

An example of such corrosion is the corrosion of the iron parts of a car engine. Sulfur present in gasoline as impurities interacts with the surface of the part, forming iron sulfide. Iron sulfide is very fragile and peels off easily, freeing up a fresh surface for further interaction with sulfur. And so, layer by layer, the detail is gradually destroyed.

Electrochemical corrosion

If chemical corrosion is nothing more than a simple oxidation of a metal, then electrochemical corrosion is destruction due to galvanic processes.

Unlike chemical corrosion, electrochemical corrosion occurs in environments with good electrical conductivity and is accompanied by the occurrence of current. To "start" electrochemical corrosion, two conditions are required: galvanic couple and electrolyte.

Moisture on the metal surface (condensate, rainwater, etc.) acts as an electrolyte. What is Galvanic Vapor? To understand this, let us return to the series of metal activities.

We look. More active metals are located on the left, less active ones on the right.

If two metals with different activity come into contact, they form a galvanic pair, and in the presence of an electrolyte, a flow of electrons arises between them, flowing from the anode sections to the cathodic ones. In this case, the more active metal, which is the anode of the galvanic couple, begins to corrode, while the less active metal does not corrode.

Galvanic cell diagram

For clarity, consider a few simple examples.

Let's say a steel bolt is secured with a copper nut. Which will corrode, iron or copper? We look in a row of activity. Iron is more active (it is to the left), which means that it will be destroyed at the junction.

Steel bolt - copper nut (corrodes steel)

And if the nut is aluminum? Looking again at the activity row. Here the picture changes: already aluminum (Al), as a more active metal, will lose electrons and collapse.

Thus, the contact of the more active “left” metal with the less active “right” metal increases the corrosion of the former.

As an example of electrochemical corrosion, one can cite cases of destruction and flooding of ships, the iron skin of which was fastened with copper rivets. Also noteworthy is the case that occurred in December 1967 with the Norwegian ore carrier "Anatina", en route from Cyprus to Osaka. In the Pacific Ocean, the ship was hit by a typhoon and the holds were filled with salt water, resulting in a large galvanic pair: copper concentrate + steel hull of the ship. After some time, the steel hull of the ship began to soften and it soon gave a distress signal. Fortunately, the crew was rescued by a German ship that arrived in time, and the Anatina herself somehow made it to the port.

Tin and zinc. "Dangerous" and "safe coatings

Let's take another example. Let's say the body panel is tin plated. Tin is a very corrosion-resistant metal, in addition, it creates a passive protective layer, protecting iron from interaction with the external environment. Does this mean that the iron under the tin layer is safe and sound? Yes, but only until the tin layer is damaged.

And if this happens, a galvanic pair immediately appears between tin and iron, and iron, which is a more active metal, will begin to corrode under the influence of a galvanic current.

By the way, there are still legends among the people about the supposedly "eternal" tinned bodies of "Victory". The roots of this legend are as follows: repairing emergency vehicles, the craftsmen used blowtorches for heating. And suddenly, for no apparent reason, from under the flame of the burner, tin begins to pour like a "river"! Hence the rumor began that the body of the "Victory" was completely tinned.

In fact, everything is much more prosaic. The stamping equipment of those years was imperfect, so the surfaces of the parts turned out to be uneven. In addition, the steels of the time were not suitable for deep drawing, and the formation of wrinkles during stamping became common. The welded, but not yet painted body had to be cooked for a long time. The bulges were smoothed out with emery wheels, and the dents were filled with tin solder, especially a lot of which was near the windshield frame. That's all.

Well, and whether the tinned body is so "eternal", you already know: it is eternal until the first good blow with a sharp stone. And there are more than enough of them on our roads.

But with zinc, the picture is completely different. Here, in fact, we are beating electrochemical corrosion with its own weapon. The protective metal (zinc) in the series of voltages is to the left of iron. This means that when damaged, it will no longer be steel, but zinc. And only after all zinc has corroded, iron will begin to break down. But, fortunately, it corrodes very, very slowly, preserving the steel for many years.

a) Corrosion of tinned steel: if the coating is damaged, the steel will be destroyed. b) Corrosion of galvanized steel: if the coating is damaged, the zinc is destroyed, protecting the steel from corrosion.

Coatings made of more active metals are called " safe", And from the less active -" dangerous". Safe coatings, in particular galvanizing, have been successfully used for a long time as a method of protection against corrosion of car bodies.

Why zinc? Indeed, in addition to zinc, several other elements are more active in the series of activities with respect to iron. Here's the catch: the further in the line of activity two metals are from each other, the faster the destruction of the more active (less noble)... And this, accordingly, reduces the durability of anti-corrosion protection. So for car bodies, where, in addition to good metal protection, it is important to achieve a long service life of this protection, galvanizing is the best fit. Moreover, zinc is available and inexpensive.

By the way, what happens if you cover the body, for example, with gold? First, it will be oh, how expensive! 🙂 But even if gold would become the cheapest metal, this cannot be done, since it will do our piece of hardware a disservice.

Gold, after all, is very far from iron in the series of activity (farthest), and at the slightest scratch the iron will soon turn into a pile of rust covered with a golden film.

The car body is exposed to both chemical and electrochemical corrosion. But the main role is still assigned to electrochemical processes.

After all, what a sin to conceal, there is a cart and a small cart of galvanic vapors in a car body: these are welded seams, and contacts of dissimilar metals, and foreign inclusions in sheet metal. Only the electrolyte is lacking to "turn on" these galvanic cells.

And the electrolyte is also easy to find - at least the moisture contained in the atmosphere.

In addition, under real operating conditions, both types of corrosion are exacerbated by many other factors. Let's talk about the main ones in more detail.

Factors Affecting Automotive Body Corrosion

Metal: chemical composition and structure

Of course, if car bodies were made of commercially pure iron, their corrosion resistance would be impeccable. But unfortunately, and perhaps fortunately, this is not possible. Firstly, such iron is too expensive for a car, and secondly (more importantly) - not strong enough.

However, let's not talk about lofty ideals, but return to what we have. Take, for example, 08KP steel, which is widely used in Russia for stamping body parts. When viewed under a microscope, this steel is as follows: fine grains of pure iron are mixed with grains of iron carbide and other inclusions.

As you may have guessed, such a structure generates many microvoltaic cells, and as soon as an electrolyte appears in the system, corrosion will slowly begin its destructive activity.

Interestingly, the process of iron corrosion is accelerated by the action of sulfur-containing impurities. It usually gets into iron from coal during blast furnace smelting from ores. By the way, in the distant past, for this purpose, not coal, but charcoal, practically containing no sulfur, was used.

Including for this reason, some metal objects of antiquity have practically not suffered from corrosion during their centuries-old history. Take a look, for example, at this iron column, which is located in the courtyard of the Qutub Minar minaret in Delhi.

It has been standing for 1600 (!) Years, and at least that. Along with the low humidity in Delhi, one of the reasons for the amazing corrosion resistance of Indian iron is precisely the low sulfur content in the metal.

So, in reasoning in the manner of "before the metal was cleaner and the body did not rust for a long time", there is still some truth, and considerable.

By the way, why then don't stainless steels rust? But because chromium and nickel, used as alloying components of these steels, are in the electrochemical series of voltages next to iron. In addition, when in contact with an aggressive medium, they form a strong oxide film on the surface, which protects the steel from further corrosion.

Chromium-nickel steel is the most typical stainless steel, but there are other grades of stainless steel besides it. For example, lightweight stainless alloys can include aluminum or titanium. If you have been to the All-Russian Exhibition Center, you must have seen the obelisk to the Conquerors of Space in front of the entrance. It is lined with titanium alloy plates and on its shiny surface not a single speck of rust.

Factory body technology

The thickness of the sheet steel from which the body parts of modern passenger car, is usually less than 1 mm. And in some places of the body, this thickness is even less.

A feature of the process of stamping body panels, and indeed any plastic deformation of the metal, is the occurrence of unwanted residual stresses during deformation. These stresses are negligible if the padding equipment is not worn out and the deformation rates are set correctly.

Otherwise, a kind of "time bomb" is put into the body panel: the order of arrangement of atoms in crystal grains changes, so the metal in a state of mechanical stress corrodes more intensely than in a normal state. And, which is characteristic, the destruction of the metal occurs precisely in the deformed areas (bends, holes) that play the role of the anode.

In addition, when welding and assembling the body at the factory, many cracks, overlaps and cavities are formed in which dirt and moisture accumulate. Not to mention the welds that form the same galvanic pairs with the base metal.

Influence of the environment during operation

The environment in which metal structures are used, including cars, is becoming more and more aggressive every year. In recent decades, the content of sulfur dioxide, nitrogen oxides and carbon has increased in the atmosphere. This means that cars are washed not just by water, but by acid rain.

Since we are already talking about acid rain, let us return once again to the electrochemical series of voltages. An observant reader has noticed that hydrogen is also included in it. A reasonable question: why? But why: its position shows which metals displace hydrogen from acid solutions, and which do not. For example, iron is located to the left of hydrogen, which means it displaces it from acid solutions, while copper, standing to the right, is no longer capable of such a feat.

It follows that acid rain is dangerous for iron, but not for pure copper. But this cannot be said about bronze and other copper-based alloys: they contain aluminum, tin and other metals located in the row to the left of hydrogen.

It has been noticed and proven that bodies live less in a big city. In this regard, the data of the Swedish Institute of Corrosion (SHIK) are indicative, which established that:

• in rural areas of Sweden, the rate of destruction of steel is 8 microns per year, zinc - 0.8 microns per year;
• for the city, these figures are 30 and 5 microns per year, respectively.

The climatic conditions in which the car is operated are also important. So, in the conditions of a maritime climate, corrosion is activated approximately twofold.

Humidity and temperature

How great is the influence of humidity on corrosion, we can understand on the example of the previously mentioned iron column in Delhi (remember the dry air as one of the reasons for its corrosion resistance).

Rumor has it that one foreigner decided to reveal the secret of this stainless iron and somehow broke a small piece from the column. Imagine his surprise when, while still on the ship on the way from India, this piece was covered with rust. It turns out that in the humid sea air, the stainless Indian iron turned out to be not so stainless. In addition, a similar column from Konarak, located near the sea, was severely corroded.

The corrosion rate at a relative humidity of up to 65% is relatively low, but when the humidity rises above the specified value, corrosion accelerates sharply, since at such humidity a layer of moisture forms on the metal surface. And the longer the surface remains wet, the faster the corrosion spreads.

That is why the main centers of corrosion are always found in hidden body cavities: they dry out much more slowly than open parts. As a result, stagnant zones form in them - a real paradise for corrosion.

By the way, the use of chemical reagents to combat icy corrosion also plays into the hands. Mixed with melted snow and ice, deicing salts form a very strong electrolyte that can penetrate anywhere, including into hidden cavities.

With regard to temperature, we already know that an increase in temperature activates corrosion. For this reason, there will always be more traces of corrosion near the exhaust system.

Air access

This corrosion is an interesting thing. As interesting as it is insidious. For example, do not be surprised that a shiny steel cable, seemingly completely untouched by corrosion, may turn out to be rusted inside. This is due to the uneven access of air: in those places where it is difficult, the threat of corrosion is greater. In corrosion theory, this phenomenon is called differential aeration.

Differential aeration principle: uneven air access to different areas of the metal surface leads to the formation of a galvanic cell. In this case, the area that is intensively supplied with oxygen remains unharmed, and the area that is worse supplied with it corrodes.

A striking example: a drop of water hitting the surface of a metal. The area under the drop and therefore less well supplied with oxygen plays the role of an anode. The metal in this area is oxidized, and the droplet edges, which are more accessible to the influence of oxygen, play the role of the cathode. As a result, iron hydroxide, a product of the interaction of iron, oxygen and moisture, begins to precipitate at the edges of the drop.

By the way, iron hydroxide (Fe 2 O 3 · nH 2 O) is what we call rust. A rusty surface, unlike a patina on a copper surface or an aluminum oxide film, does not protect the iron from further corrosion. Initially, rust has a gel structure, but then it gradually crystallizes.

Crystallization begins inside the rust layer, with the outer shell of the gel, which is very loose and brittle when dry, flakes off and the next layer of iron is exposed. And so on until all the iron is destroyed or the system runs out of oxygen and water.

Returning to the principle of differential aeration, one can imagine how many opportunities there are for the development of corrosion in hidden, poorly ventilated areas of the body.

Rust ... everyone!

As they say, statistics know everything. Earlier, we mentioned such a well-known center for the fight against corrosion as the Swedish Corrosion Institute (SHIK) - one of the most reputable organizations in this field.

Every few years, the scientists of the institute conduct an interesting study: they take the bodies of well-worked cars, cut out the most favorite corrosion "fragments" from them (sections of thresholds, wheel arches, door edges, etc.) and assess the degree of their corrosion damage.

It is important to note that among the investigated bodies there are both protected (galvanized and / or anticorrosive) and bodies without any additional anti-corrosion protection (just painted parts).

So, CHIK claims that the best protection of an automobile body is only a combination of "zinc plus anticorrosive". But all other options, including "just galvanizing" or "just anticorrosive," according to scientists, are bad.

Galvanizing is not a panacea

Supporters of refusal from additional anti-corrosion treatment often refer to factory galvanizing: with it, they say, no corrosion threatens the car. But, as Swedish scientists have shown, this is not entirely true.

Indeed, zinc can serve as an independent protection, but only on smooth and smooth surfaces, moreover, not subject to mechanical attacks. And at the edges, edges, joints, as well as places that are regularly exposed to "shelling" with sand and stones, galvanized steel fails before corrosion.

In addition, not all cars have fully galvanized bodies. Most often only a few panels are coated with zinc.

Well, one should not forget that although zinc protects steel, it is inevitably consumed in the process of protection. Therefore, the thickness of the zinc "shield" will gradually decrease over time.

So the legends about the longevity of galvanized bodies are true only when zinc becomes part of a common barrier, in addition to regular additional anti-corrosion treatment of the body.

It's time to end, but the topic of corrosion is far from exhausted. We will continue to talk about the fight against it in the following articles of the heading "Anti-corrosion protection".