Chemical corrosion - this is a type of corrosion destruction of a metal associated with the interaction of a metal and a corrosive medium, in which the metal is simultaneously oxidized and the corrosion medium is restored. Chemical is not associated with the formation, as well as 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 \u003d Reaction product

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

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

Δ G T< 0, то процесс химической коррозии возможен;

Δ G T\u003e 0, then the process of chemical corrosion is impossible;

Δ G T \u003d 0, then the system is in equilibrium.

Chemical Corrosion:

Gas Corrosion — Corrosion damage due to gases at high temperatures;

Corrosion in non-electrolyte liquids.

Gas corrosion

Gas corrosion - The most common type of chemical corrosion. At high temperatures, the surface of the metal under the influence of gases is destroyed. 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 a metal with oxygen. The process proceeds according to the reaction:

Me + 1 / 2O 2 - MeO

The direction of this reaction (oxidation) is determined by the partial pressure of oxygen in the gas mixture (pО2) and the dissociation pressure of oxide vapor at a certain temperature (pMeO).

This chemical reaction can proceed in three ways:

1) pO 2 \u003d pMeO, the reaction is equilibrium;

2) pO 2\u003e pMeO, the reaction is shifted towards the formation of oxide;

3) pО 2< рМеО, оксид диссоциирует на чистый металл и оксид, реакция протекает в обратном направлении.

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

Gas Corrosion Rate it is determined by several factors: ambient temperature, the nature of the metal or alloy composition, the nature of the gaseous medium, the contact time with the gaseous medium, and the properties of 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:

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

The metal interacts with the gas to form a chemical compound.

At the first stage, an ionic bond arises between surface atoms and oxygen: an oxygen atom takes two electrons from a metal. In this case, a very strong bond arises, much stronger than the bond of oxygen with the metal in the oxide. Perhaps this phenomenon is observed due to the action on the field of oxygen created by metal atoms. After the surface is completely saturated with an oxidizing agent, which occurs almost instantly, at low temperatures due to the bath-der-waltz forces, physical adsorption of oxidizing molecules can also be observed.

As a result, a very thin monomolecular protective film forms, which thickens over time, making oxygen difficult to approach.

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

If the formed 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 (give the color discolouration);

Thick (clearly visible).

In order for the oxide film to be protective, it must meet certain requirements: not have pores, be continuous, adhere well to the surface, be chemically inert with respect to its environment, have high hardness, and be wear-resistant.

If the film is loose and porous, in addition, it still 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 - the ability of the oxide to cover in a continuous layer the entire surface of the metal.

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 in technical terms.

Many methods are used to determine the thickness of the oxide film formed on the surface of the oxide and to study its protective properties. The protective ability of the film can be determined during its formation, by the rate of metal oxidation and the nature of the change in speed over time. If the oxide has already formed, it is advisable to study the thickness and its protective properties by applying to the surface some reagent suitable for this case (for example, a solution of Cu (NO3) 2, which is used for iron). By the time of penetration of the reagent to the surface, you can determine the thickness of the film.

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

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

The rate of chemical corrosion is very strongly influenced by temperature. With its increase, the oxidation processes go much faster. In this case, a decrease in the thermodynamic possibility of a reaction does not matter.

Particularly strongly affected by variable heating and cooling. Cracks are formed in the protective film due to 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 is gradually peeling off.

A large role in the corrosion process is played by the composition of the gaseous medium. But this is individual for each metal and varies with temperature fluctuations. For example, copper corrodes very quickly in an oxygen atmosphere, but is stable in an environment containing SO 2. Nickel, on the contrary, intensely corrodes upon contact with the atmosphere of SO 2, but is stable in O 2, CO 2, and H 2 O media. Chrome is relatively stable in all four media.

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

The oxidation rate depends on the composition of the alloy. Take iron, for example. Additives of sulfur, manganese, phosphorus and nickel do not affect its oxidation. Silicon, chromium, aluminum - slow down the process. And beryllium, cobalt, titanium and copper inhibit oxidation very strongly. At high temperatures, tungsten, molybdenum, and also vanadium can intensify the process. This is due to the volatility or fusibility of their oxides.

Observing the rate of iron oxidation at various 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 influenced by the nature of the 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 fluids - These 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 is sharply accelerated. For example, if the oil contains sulfur or sulfur-containing compounds (hydrogen sulfide, mercaptans), the process of chemical corrosion is accelerated. If in addition the temperature rises, dissolved oxygen will be in the liquid - chemical corrosion will intensify.

The presence of moisture in non-electrolyte liquids provides an intensive course of corrosion already by the electrochemical mechanism.

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

The approach of the oxidizing agent to the surface of the metal;

Chemisorption of the reagent on the surface;

The reaction of an oxidizing agent with a metal (formation of an oxide film);

Desorption of oxides with metal (may be absent);

Diffusion of oxides into 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.

The phrase "metal corrosion" contains much more than the name of a popular rock band. Corrosion irreversibly destroys the metal, turning it into dust: from 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 the metal smelted per year in every sixth blast furnace in our country becomes a rusty mess 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 a ten-digit figure. So what causes the corrosive processes of metals and how to deal with them?

What is metal corrosion

The destruction of metals as a result of electrochemical (dissolution in a moisture-containing air or water medium - an electrolyte) or chemical (the 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 parts of the surface (local corrosion), cover the entire surface (uniform corrosion), or destroy metal along grain boundaries (intergranular corrosion).

The 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 media that are not conductors of electric current (dry gases, organic liquids - petroleum products, alcohols, etc.), and the intensity of corrosion increases with increasing temperature - as a result, an oxide film forms on the surface of metals.

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

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

Electrochemical corrosion

The process of electrochemical corrosion does not require 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 - vehicles and underground utilities are particularly affected (according to statistics, the annual losses in the USA from the use of salts in winter are 2.5 billion dollars).

The following happens: metals (alloys) lose some of the atoms (they go into the electrolytic solution in the form of ions), electrons that replace 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 part of the current from an electric 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 the water or into the soil, metals are destroyed. Especially often stray currents occur in places of movement of ground electric vehicles (for example, trams and railway locomotives with electric traction). In just a year, wandering currents with a force of 1A are able to dissolve iron - 9.1 kg, zinc - 10.7 kg, lead - 33.4 kg.

Other causes of metal corrosion

The development of corrosive processes contribute to radiation, waste products of microorganisms and bacteria. Corrosion caused by marine microorganisms causes damage to the bottoms of ships, and the corrosion processes caused by bacteria even have their own name - biocorrosion.

The combination of the effects 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

The inevitable consequences of technological progress is the pollution of our living environment - a process that accelerates the corrosion of metals, as the external environment is increasingly aggressive towards them. There are no ways to completely eliminate the corrosion 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; to exclude the interaction between metal and substances from the external environment, showing aggression.

Over thousands of years, mankind has tried many ways to protect metal products from chemical corrosion, some of which are still used today: coating with grease or oil, other metals that are less corrosive (the oldest method, which has been tinning for more than 2 thousand years (coating tin)).

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, which excludes (with 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 to the installation and construction site. Methods for applying paints and varnishes are simple and mechanized, damaged coatings can be repaired “on the spot” - 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 climatic conditions in which the metal structure will be operated; the need to use exclusively high-quality paints and varnishes; strict adherence to the technology of application to metal surfaces. Paints and varnishes are best applied in several layers - their amount will provide better protection against weathering on a metal surface.

Polymers such as epoxy resins and polystyrene, polyvinyl chloride and polyethylene can act as protective coatings against corrosion. In construction work, embedded parts made of reinforced concrete are covered with coatings of a mixture of cement and perchlorovinyl, cement and polystyrene.

Protection of iron from corrosion by coatings of other metals

There are two types of metal coatings-inhibitors - tread (coatings with zinc, aluminum and cadmium) and corrosion-resistant (coatings with silver, copper, nickel, chromium and lead). Inhibitors are applied chemically: the first group of metals has a large electronegativity with respect to iron, the second - a large electropositivity. The most widespread in our everyday life are metallic coatings of iron with tin (tinplate, cans are made from it) and zinc (galvanized iron - roofing), obtained by drawing sheet metal through a melt of one of these metals.

Often galvanized are cast iron and steel fittings, as well as water pipes - this operation significantly increases their resistance to corrosion, but only in cold water (when conducting hot water, galvanized pipes wear faster than non-galvanized pipes). Despite the effectiveness of galvanizing, it does not provide ideal protection - the zinc coating often contains cracks, the elimination of which requires preliminary nickel plating of metal surfaces (nickel coating). Zinc coatings do not allow to apply paint and varnish materials on 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 that it is less consumed, aluminized surfaces can be painted and the paint coat will be stable. In addition, the aluminum coating in comparison with the galvanized coating has greater resistance in aggressive environments. Aluminization is poorly distributed due to the complexity of applying this coating to a metal sheet - aluminum in the molten state is highly aggressive to other metals (for this reason, aluminum melt cannot be contained in a steel bath). Perhaps this problem will be completely resolved in the very near future - an original method of performing aluminization was found by Russian scientists. The essence of the development is not to immerse the steel sheet in molten aluminum, but to raise liquid aluminum to the steel sheet.

Improving corrosion resistance by adding alloying alloys to steel alloys

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

Electrochemical Corrosion Measures

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

Stray Current Protection

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

• the construction site, which serves as a source of stray current, must be connected by a metal conductor to the rail of the tram road;
• heating network routes should be located at the maximum distance from the railways along which electric vehicles move, to minimize the number of intersections;
• the use of electrical insulating pipe supports to increase the transition resistance between the soil and pipelines;
• at the inputs to the objects (potential sources of stray currents) it is necessary to install insulating flanges;
• install conductive longitudinal jumpers on flange valves 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.

Protection of metal objects equipped with insulation, as well as small-sized steel structures, is performed using a tread that acts as an anode. The material for the tread 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, provides protection for 8 km of the pipeline.

Abdyuzhanov Rustam, specially for RMNT.ru

Electrochemical corrosion - The most common form of corrosion. The electrochemical occurs when the metal contacts the surrounding electrolytically conductive medium. In this case, the oxidation component of the corrosive medium is not restored simultaneously with the ionization of metal atoms and their velocities depend on the electrode potential of the metal. The root cause of electrochemical corrosion is the thermodynamic instability of metals in their environments. Rusting of a pipeline, upholstery of the bottom of a marine vessel, various metal structures in the atmosphere are, and much more, examples of electrochemical corrosion.

Electrochemical corrosion includes such types of local damage as pitting, intergranular corrosion, crevice. In addition, processes electrochemical corrosion occur in soil, atmosphere, sea.

Electrochemical corrosion mechanism can proceed in two ways:

1) Homogeneous mechanism of electrochemical corrosion:

The surface layer is met. regarded as homogeneous and homogeneous;

The reason for the dissolution of the metal is the thermodynamic possibility of the occurrence of cathodic or anodic acts;

K and A sections migrate over the surface in time;

The rate of electrochemical corrosion depends on the kinetic factor (time);

A homogeneous surface can be considered as a limiting case, which can be realized in liquid metals.

2) The heterogeneous mechanism of electrochemical corrosion:

For hard metals, the surface is not homogeneous, because different atoms occupy different positions in the crystal lattice in the alloy;

Heterogeneity is observed in the presence of foreign inclusions in the alloy.

Electrochemical corrosion has some features: it is divided into two simultaneously proceeding processes (cathodic and anodic), which are kinetically dependent on each other; in some parts of the surface, electrochemical corrosion can take on a local character; dissolution of the main met. occurs precisely on the anodes.

The surface of any metal consists of many microelectrodes short-circuited through the metal itself. Contacting with the corrosive medium, the resulting galvanic cells contribute to its electrochemical destruction.

The causes of local galvanic cells can be very different:

1) alloy heterogeneity

Inhomogeneity met. phases due to heterogeneity of the alloy and the presence of micro- and macroinclusions;

Non-uniformity of oxide films on the surface due to the presence of macro- and micropores, as well as uneven formation of secondary corrosion products;

The presence of crystal grain boundaries on the surface, dislocation exit to the surface, and crystal anisotropy.

2) heterogeneity of the medium

The area with limited access of the oxidizing agent will be the anode in relation to the area with free access, which accelerates electrochemical corrosion.

3) heterogeneity of physical conditions

Irradiation (irradiated area - anode);

The impact of external currents (the input point of the stray current is the cathode, the output point is the anode);

Temperature (in relation to cold areas, heated are anodes), etc.

When a galvanic cell operates, two electrode processes occur simultaneously:

Anodic - metal ions pass into solution

Fe → Fe 2+ + 2e

An oxidation reaction takes place.

Cathode - excess electrons are assimilated by molecules or atoms of the electrolyte, which are then restored. A reduction reaction takes place at the cathode.

O 2 + 2H 2 O + 4e → 4OH - (oxygen depolarization in neutral, alkaline media)

O 2 + 4H + + 4e → 2H 2 O (oxygen depolarization in acidic media)

2 H + + 2e → H 2 (with hydrogen depolarization).

Braking of the anode process also leads to inhibition of the cathode.

Corrosion of metal occurs precisely on the anode.

When two electrically conductive phases come into contact (for example, met. - medium), when one of them is positively charged and the other is negative, a potential difference arises between them. This phenomenon is associated with the appearance of a double electric layer (DEL). Charged particles are located asymmetrically at the phase boundary.

The potential jump in the process of electrochemical corrosion can occur due to two reasons:

With a sufficiently high hydration energy, metal ions can detach and pass into solution, leaving an equivalent number of electrons on the surface that determine its negative charge. A negatively charged surface attracts meth cations to itself. from solution. So at the phase boundary there is a double electric layer.

Electrolyte cations are discharged on the metal surface. This leads to the fact that the surface is met. acquires a positive charge, which with the anions of the solution forms a double electric layer.

Sometimes a situation arises when the surface is not charged and, accordingly, there is no DES. The potential at which this phenomenon is observed is called the zero charge potential (φ N). Each metal has its own zero charge potential.

The magnitude of the electrode potentials has a very large effect on the nature of the corrosion process.

The potential jump between the two phases cannot be measured, but using the compensation method it is possible to measure the electromotive force of the element (EMF), which consists of a reference electrode (its potential is arbitrarily taken as zero) and the electrode under study. As a reference electrode, a standard hydrogen electrode is taken. The EMF of a galvanic cell (standard hydrogen electrode and the cell under study) is called the electrode potential. Silver chloride, calomel, saturated copper sulfate can also serve as reference electrodes.

1953 International Convention in Stockholm when recording, it was decided to always put the reference electrode on the left. In this case, calculate the EMF as the potential difference of the right and left electrodes.

E \u003d Vp - Vl

If a positive charge inside the system moves from left to right, the emf of the element is considered positive, while

E max \u003d - (ΔG T) / mnF,

where F is the Faraday number. If the positive charges move in the opposite direction, then the equation will look like:

E max \u003d + (ΔG T) / mnF.

In corrosion in electrolytes, the most common and significant are adsorption (adsorption of cations or anions at the phase boundary) and electrode potentials (transition of cations from metal to electrolyte or vice versa).

The electrode potential at which the metal is in equilibrium with its own ions is called equilibrium (reversible). It depends on the nature of the metal phase, the solvent, the temperature of the electrolyte, the activity of meth ions.

The equilibrium potential obeys the Nernst equation:

E \u003d E ο + (RT / nF) Lnα Me n +

where, E ο - standard potential met .; R is the molar gas constant; n is the degree of oxidation of the ion met .; T is the temperature; F is the Faraday number; α Me n + is the activity of met.

At the established equilibrium potential, electrochemical corrosion is not observed.

If an electric current passes through the electrode, its equilibrium state is violated. The electrode potential varies depending on the direction and current strength. A change in the potential difference, leading to a decrease in current strength, is usually called polarization. A decrease in the polarizability of the electrodes is called depolarization.

The rate of electrochemical corrosion is lower, the greater the polarization. Polarization is characterized by the magnitude of the overvoltage.

There are three types of polarization:

Electrochemical (when slowing down the anode or cathode processes);

Concentration (observed when the speed of approach of the depolarizer to the surface and removal of corrosion products is low);

Phase (associated with the formation of a new phase on the surface).

Electrochemical corrosion is also observed upon contact of two dissimilar metals. In the electrolyte, they form a galvanic pair. The more electronegative of these will be the anode. The anode in the process will gradually dissolve. In this case, there is a slowdown or even complete cessation of electrochemical corrosion at the cathode (more electropositive). For example, upon contact with duralumin in nickel in seawater, duralumin will be intensely dissolved.

Chemical corrosion

External factors of gas corrosion. Gas corrosion is a special case of chemical corrosion and is possible only under conditions that exclude the occurrence of electrochemical processes. A characteristic feature of gas corrosion is the absence of moisture on the surface of the metal. Therefore, in most cases, we are talking about corrosion at elevated temperatures, at which water is in the gas phase. However, based on the definition, one can imagine gas corrosion at room temperature, but under conditions of a high degree of dryness, natural or artificially created. So, when drying with silica gel to the dew point - 30 ° C moisture capacity of air will be 0.333 g / m 3. At + 20 ° С this corresponds to air humidity only 2 % . In such conditions

under these conditions, the occurrence of electrochemical corrosion is practically excluded. In industry, cases of gas corrosion are quite common - from the destruction of parts, heating furnaces to metal corrosion during its heat treatment.

A number of factors affect the rate of gas corrosion, and especially such as temperature and composition of the gas medium.

An increase in temperature significantly increases the rate of corrosion. In a first approximation, this connection can be described

on the Arrhenius equation known from physical chemistry

ln K \u003d A -

where TO - speed reaction; AND and AT - constants; T - absolute temperature (° K).

It follows from the equation that the logarithm of the corrosion rate is linearly related to the reciprocal of the absolute temperature. This dependence in some cases (for example, for copper in the temperature range 700 - 900 ° C) is fully confirmed, but more often it is more complex in nature, which is associated with the influence of secondary reactions, the nature and properties of corrosion products, etc.

In a clean air environment, corrosion is reduced to the interaction of a metal with oxygen. Iron is already at a temperature 300 ° C covered in air by scale, i.e., an oxide film, distinguishable to the naked eye. Magnetite is part of the scale F 3 O 4 and hematite Fe 2 O 3. With increasing temperature, up to 575 ° Ccorrosion rate remains approximately constant, but starting from 575 0 Cincreases dramatically. This fact is associated with the appearance of wustite (iron oxide FeO).

On the surface of carbon steel in the process of corrosion two groups of reactions proceed in parallel: oxidation of iron to oxides with the formation of scale and decarburization reaction with the participation of iron carbide (cementite) according to the following equation:

Fe 3 C + O 2 → 3Fe + CO 2.

Thus, the surface layer of metal is depleted of cementite. With prolonged heating, the depth of the decarburized layer can be several millimeters. This noticeably affects the properties of the metal, and above all, its hardness and strength. Decarburization is also observed in the presence of carbon dioxide in the gaseous medium, water vapor or other oxidizing agents and proceeds according to similar reactions:

Fe 3 C + СО 2 → 3Fе + 2СО,

Fe 3 C + H 2 O → 3Fe + CO + H 2.

The increase in gas pressure, ceteris paribus, also greatly accelerates gas corrosion.

Hydrogen specifically affects the corrosion resistance of steel, causing the so-called hydrogen embrittlement at elevated temperature and pressure, i.e., a sharp decrease in strength. Hydrogen brittleness is explained not only by decarburization of the steel due to the restoration of cementite with hydrogen, but also by such phenomena as the molization of atomic hydrogen in the crystal lattice of steel and the formation of water and methane vapors at the grain boundaries of the metal. Each of the processes leads to the generation of gas, which creates tremendous pressure in a closed volume of metal. This in turn causes the appearance of numerous microcracks that lower the strength of the metal.

Many colored are also very susceptible to gas corrosion.

metals, although each of them refers to different gases differently. This can be illustrated by the data in table. 2, in which the corrosion rate of metals for clarity is given in relative units, while the corrosion rate of iron in oxygen is taken as 100.

Tab. 9 convincingly demonstrates the effect of the nature of the metal on the corrosion rate. For example, if during the transition from oxygen to water vapor, tungsten corrosion slows down approximately 20 times, and copper - in 3,5 times, then the corrosion rate of iron in this case, on the contrary, increases.

Table 9

Gas corrosion of some metals in some environments

(temperature 800 ° C, duration 24 h)

Oxide films.The formed corrosion products, their physicochemical and mechanical properties have a significant effect on the rate of gas corrosion. In most cases, corrosion occurs in an oxidizing environment; at the same time, an oxide film forms on the metal surface as a corrosion product. However, a thin oxide film on a metal usually appears even at room temperature. The properties of the formed oxide film have a decisive influence on the further course of the corrosion process. In the case of a sharp deceleration of the process up to the complete cessation of corrosion, they indicate the onset of passivity of the metal surface.

Thermodynamics of gas corrosion. The thermodynamic capability of the gas corrosion process with the formation of an oxide film is determined by the magnitude of the change in the free energy of the system. There is a convenient form for determining the thermodynamic possibility of corrosion due to metal oxidation, which reduces to comparing the dissociation elasticity of the resulting oxidation reaction product with the partial pressure of oxygen in the gas phase.

Indeed, if the partial pressure of oxygen P oand elasticity of dissociation of oxide P MeO in a metal oxidation reaction mMe + nO 2 Me m O 2 n will be equal, then the reaction will be in equilibrium. If P o> P MeO, then the reaction proceeds from left to right in the direction of the formation of oxide. If P o< P MeO, then the oxide spontaneously dissociates into oxygen and metal. Therefore, a comparison of the dissociation elasticity of a given oxide at a given temperature, for example, with the partial pressure of atmospheric oxygen ( P O 0.2 at at atmospheric pressure) allows you to find the boundary with thermodynamics

probability of the process of metal oxidation in air. So, judging by the data in table. 10, silver already at 400 ° C not able to oxidize. For copper, this boundary lies in the region 2000 ° C.

Table 10

Resilience of dissociation of silver and copper oxides

depending on temperature

Process	300 0 K	400 0 K	500 0 K	800 0K	1200 0 K	1600 0 K
Ag 2 O 2Ag + O 2	8,4 ∙ 10 -5	6,9 ∙ 10 -1		-	-	-
Cu 2 O 2 Cu + O 2	-	-	0,56 ∙ 10 - 30	3,7 ∙ 10 - 16	2 ∙ 10 - 8	1,8 ∙ 10 - 4

Properties of oxide films. Depending on the conditions of formation, oxide films can have a thickness from monomolecular to several millimeters. Thin, medium and thick films are distinguished. Thin films range in thickness from a few angstroms to 400 Å. They are invisible and can be detected and measured by the so-called optical method of reflection of polarized light.

Middle films have a thickness 400 - 5000 Å and are visible to the naked eye due to the appearance of runaway colors (the phenomenon of light interference, known from physics). Their thickness can be measured by various methods, among which the most accessible are gravimetric (weight) and electrometric (cathodic reduction method).

Thicker films 5000 Å (i.e. thicker 0.5 microns) are determined by the gravimetric method or by the method of cathodic reduction, as well as using a microscope, micrometer or other similar measuring instruments. Usually they are easily detected with the naked eye.

It should be noted that when studying the phase composition and

structures of oxide films are widely used electron microscopy, electron-graphic and radiographic methods of research.

In the table. 11 gives examples of oxide films on iron. The clear dependence of the film thickness on the conditions of its formation is noteworthy, as well as the thickness range itself, on 15 Å before 0.6 mm.

It would be a mistake to assume that the thicker the oxide film, the more it protects the metal from corrosion. In reality, the situation is rather the opposite, namely, thin films have the best protective properties. However, the film thickness, strictly speaking, is not yet a criterion of protective ability.

In order for an oxide film to have protective properties, it must first of all be continuous, non-porous. The condition for the continuity of the oxide film was formulated by Pilling and Bedworth: if the volume of metal oxide is less than the volume of the metal from which the film was formed, then the film

formed non-continuous; if the volume of metal oxide is greater than the volume of metal, then the film can be non-porous, compact.

The above can be explained by the following inequalities:

< 1 the film cannot be continuous; at > 1 the film may be continuous.

In turn

V me \u003dand V Me O \u003d,

where AND - the atomic weight of the metal (i.e., the gram is considered - a metal atom); - metal density; M - molecular weight

metal oxides; n - the number of metal atoms in the oxide molecule; D - density of the oxide.

Table 11

Iron oxide thickness

depending on conditions

The continuity condition is necessary and essential, but not the only one for the characterization of protective

properties of the oxide film. For too large values V Me O / V Me the film experiences such high internal stresses that it collapses, losing continuity. For example, with respect to V WO / V W \u003d 3.35 tungsten oxide film has very weak protective properties.

The film must have good adhesion to the metal, should be strong enough and flexible. The coefficients of thermal expansion of the film and metal should be fairly close. Finally, the film must be chemically resistant under the influence of a corrosive environment.

An important condition is the need for orientation.

the corresponding correspondence of the resulting film to the metal. The essence of orientational correspondence is reduced to the requirement of maximum similarity of the crystal lattices of the metal and the oxide formed with a minimum displacement of atoms. Most often, in the presence of a crystalline structure of an oxide close to the structure of a metal, the protective properties of such a film are better than that of an oxide that is not oriented with respect to the metal.

The laws of growth of oxide films. If a non-continuous oxide film forms as a result of corrosion, oxygen gains free access to the metal surface. In this case, the corrosion rate should be constant:

where y - thickness of the oxide film. After integration, we obtain the equation

y \u003d k + A,

expressing a linear dependence of the film thickness on time. Constant AND indicates the presence of some oxide film at the time of the onset of oxidation ( y \u003d A at = 0 ) As follows from the equation, the film growth rate in this case does not depend on its thickness. Corrosion can occur at a constant rate until the complete conversion of the metal into an oxide, as is the case with the oxidation of magnesium in oxygen.

However, often the actual rate of oxidation, while maintaining constancy, is lower than the theoretical rate of the chemical reaction of metal oxidation. This discrepancy is explained by the presence of the thinnest, up to several monomolecular layers, continuous film of pseudomorphic oxide at the metal-oxide interface. The pseudomorphic oxide has a high degree of orientational correspondence to the metal and is, therefore, a kind of crystallographic continuation of the lattice of the oxidized metal, while at the same time differing in parameters from the lattice of the metal oxide. Being non-porous, it impedes the penetration of oxygen to the surface of the metal.

Thus, even if a thick and friable oxide film is formed on the metal, the corrosion rate will ultimately be limited not by the rate of the oxidation reaction, but by the rate of oxygen diffusion through the compact pseudomorphic oxide.

If an oxide is formed in the corrosion process that possesses fairly good protective properties, then the corrosion rate will depend on the ratio of the rates of mutual diffusion through the film of oxygen atoms to the metal surface and metal atoms to the oxide-gas interface. It can be shown that in this case, as the film thickness increases, the corrosion rate will slow down according to the equation

After integrating and combining the constants, we obtain a parabolic dependence of the thickness of the oxide film on the duration of corrosion:

y 2 \u003d k + A.

Such a dependence is observed during the oxidation of copper, nickel, and tungsten. Having a parabolic curve of the dependence of corrosion on time, it is possible to determine the corrosion rate at any point in the curve. It will be expressed as the tangent of the angle of inclination of the tangent passing through this point, since

tg \u003d.

Finally, under certain conditions, the inhibition of the rate of metal oxidation with increasing thickness of the oxide film occurs more intensively than is required by the parabolic law. In these cases, the oxidation rate is related to the film thickness exponentially

After integration, we arrive at the logarithmic equation

y \u003d ln (k).

The logarithmic law of film growth has experimental evidence for the oxidation of aluminum and zinc in air in the temperature range 20 - 255 ° C, copper - up 100 ° Ciron up 385 ° C.

It is important to emphasize that the patterns of film growth on a metal can vary depending on conditions. So, the oxidation of iron at a temperature below 385 ° C obeys the logarithmic law, in the region above this temperature and up to 1000 ° C - parabolic, and at an oxygen pressure below 1 mmHg Art. and temperature 700 - 950 ° C - linear.

The destruction of the films. During the growth of the oxide film, significant internal stresses arise in it. Therefore, if the resulting film is not strong enough or has poor adhesion to the metal, or is too inelastic, or for other reasons mentioned above (for example, the difference in the coefficients of thermal expansion of the metal and the film), it is destroyed. The nature of the destruction is associated with the cause that caused it. If the strength of the film is large and the adhesion to the metal is not good enough, bubbles form. Large bubbles usually lead to ruptures (Fig. 68, and), and protective

a B C D E

Figure 68. Types of destruction of oxide films.

and- bubble with a gap; b - microbubbles in the oxide layer (vacuum porosity); at - peeling; g - cracking at

shear; d - cracking at corners and ribs.

film properties are sharply reduced. In other cases, small bubbles form in the oxide layer (Fig. 68, b), and then the protective properties of the film can even increase, since such a “vacuum porosity” prevents the diffusion of reacting atoms or ions and thus inhibits the corrosion process. Peeling of the oxide may be observed (Fig. 68, at), as well as cracking on the surface (Fig. 68, g) or at corners and ribs (Fig. 68, d).

Methods of protection against gas corrosion.The main method of protection against gas corrosion is reduced to the use of alloyed alloys with the so-called heat resistance. To reduce the rate of oxidation of iron at 900 ° C twice is enough to enter 3,5 % aluminum, and four - about 5,5 % . The concentration of the alloying component may be negligible. So, molten magnesium is so energetically oxidized in air that it is able to spontaneously ignite. However, with the introduction of only 0,001 % beryllium magnesium oxidation rate decreases sharply.

The action of the alloying elements is explained by the formation of protective films on the metal surface. They are either formed only from the alloying component, or consist of mixed oxides of the alloying component and the base metal. Spinel-type oxides have the best protective properties. The spinel structure of the oxide is characterized by a high degree of compactness of ions in the lattice and the practical absence of vacant sites; this determines their high thermodynamic stability. Oxides are an example of oxides. FeO ∙ Cr 2 O 3 on the surface of chrome steel or NiO ∙ Cr 2 O 3 on the surface of chromium - nickel steel.

The second method of combating gas corrosion is the use of a protective atmosphere. Depending on the nature of the metal, the gas medium should not contain oxidizing agents (for steel) or, conversely, reducing agents (for copper). In some cases, inert gases are used - nitrogen, argon. In practice, this method is found only in special cases: during heat treatment and welding. So, annealing of steel is carried out in an atmosphere containing a mixture of nitrogen, hydrogen and carbon monoxide. Welding of aluminum-magnesium and titanium parts proceeds successfully in an argon atmosphere.

The third method of reducing the rate of gas corrosion is to protect the metal surface with special heat-resistant

covered In some cases, a surface, for example, a steel part, is thermally diffused coated with an alloy of iron - aluminum or iron - chromium. Both alloys have high protective properties, and the process itself is called

respectively alitirovanie and thermochromizing. In other cases, the surface is protected by a layer of cermet - a mixture of metal with oxides. Ceramic - metal coatings (cermets) are interesting in that they combine refractoriness, hardness and heat resistance of ceramics with ductility and conductivity of the metal. Refractory oxides are used as a non-metallic component Al 2 O 3, MgO and compounds — such as carbides and nitrides. The metal component is the metals of the iron group, as well as chromium, tungsten, molybdenum.

Corrosion of metals (from late Lat. Corrosio - erosion) is the physicochemical interaction of a metal material and a medium, leading to a deterioration in the operational properties of the material, medium or technical system of which they are parts.

Corrosion of metals is based on a chemical reaction between a material and a medium or between their components, proceeding at a phase boundary. This process is spontaneous and also a consequenceredox reactions with environmental components. Chemicals that destroy building materials are called aggressive. Aggressive environment can be atmospheric air, water, various solutions of chemicals, gases. The process of destruction of the material is enhanced when there is even a small amount of acids or salts in the water, in soils in the presence of salts in the soil water and groundwater level fluctuations.

Corrosion processes classify:

1) under the conditions of corrosion,

2) by the mechanism of the process,

3) by the nature of corrosion failure.

By corrosion conditions, which are very diverse, distinguish several types of corrosion.

Corrosive media and the destruction caused by them are so characteristic that the corrosion processes occurring in them are classified by the name of these media. So, emit gas corrosioni.e. chemical corrosion under the influence of hot gases (at temperatures well above the dew point).

Some cases are characteristic electrochemical corrosion (mainly with cathodic oxygen reduction) in natural environments: atmospheric - in clean or contaminated air at a humidity sufficient to form an electrolyte film on the metal surface (especially in the presence of aggressive gases, such as CO 2, Cl 2, or aerosols of acids, salts, etc.); sea \u200b\u200b- under the influence of sea water and underground - in soils and soils.

Energized corrosion develops in the zone of action of tensile or bending mechanical loads, as well as residual deformations or thermal stresses and, as a rule, leads to transcrystalline corrosion cracking, which is exposed, for example, steel cables and springs in atmospheric conditions, carbon and stainless steels in steam power plants, high strength titanium alloys in seawater, etc.

With alternating loads, it may occur corrosion fatigue, expressed in a more or less sharp decrease in the fatigue limit of the metal in the presence of a corrosive medium. Corrosion erosion (or friction corrosion) represents accelerated metal wear with the simultaneous action of mutually reinforcing each other corrosion and abrasive factors (sliding friction, flow of abrasive particles, etc.).

The cavitation corrosion related to it occurs during cavitation regimes of a metal flowing around an aggressive medium, when the continuous occurrence and “collapse” of small vacuum bubbles creates a flow of destructive microhydraulic shocks acting on the metal surface. A close variety can be considered fretting corrosionobserved at the contact points of tightly pressed or rolling parts one by one, if microscopic shear displacements occur as a result of vibrations between their surfaces.

The leakage of electric current through the boundary of the metal with the aggressive medium causes, depending on the nature and direction of the leak, additional anodic and cathodic reactions that can directly or indirectly lead to accelerated local or general destruction of the metal ( stray current corrosion) Similar damage, localized near the contact, may cause the contact in the electrolyte of two dissimilar metals forming a closed galvanic cell, - contact corrosion.

In the narrow gaps between the parts, as well as under a lagging coating or growth, where the electrolyte penetrates, but the access to oxygen necessary for passivation of the metal is difficult, it can develop crevice corrosionin which the dissolution of the metal mainly occurs in the gap, and the cathodic reactions partially or completely occur next to it on an open surface.

Accepted also biological corrosion that is influenced by the waste products of bacteria and other organisms, and radiation corrosion- when exposed to radioactive radiation.

1 . Gas corrosion- corrosion of metals in gases at high temperatures (for example, oxidation and decarburization of steel when heated);

2. Atmospheric corrosion- corrosion of metals in the atmosphere of air, as well as any moist gas (for example, rusting of steel structures in the workshop or in the open air);

Atmospheric corrosion is the most common type of corrosion; about 80% of metal structures are operated in atmospheric conditions.
The main factor determining the mechanism and rate of atmospheric corrosion is the degree of wetting of the metal surface. According to the degree of moisture, three main types of atmospheric corrosion are distinguished:

• Wet atmospheric corrosion - corrosion in the presence of a visible film of water on the metal surface (film thickness from 1 μm to 1 mm). Corrosion of this type is observed at a relative humidity of about 100%, when there is drip condensation of water on the metal surface, as well as when water directly hits the surface (rain, surface hydrotreatment, etc.);
  
• Wet atmospheric corrosion - corrosion in the presence of a thin invisible film of water on the metal surface, which is formed as a result of capillary, adsorption or chemical condensation at a relative humidity of less than 100% (film thickness from 10 to 1000 nm);
  
• Dry atmospheric corrosion - corrosion in the presence of a very thin adsorption film of water on the metal surface (of the order of several molecular layers with a total thickness of 1 to 10 nm), which cannot yet be considered as continuous and possessing the properties of an electrolyte.
  

It is obvious that the minimum corrosion periods occur during dry atmospheric corrosion, which proceeds by the mechanism of chemical corrosion.

With an increase in the thickness of the water film, a transition of the corrosion mechanism from chemical to electrochemical occurs, which corresponds to a rapid increase in the corrosion process speed.

It can be seen from the above dependence that the boundary of regions II and III corresponds to the maximum corrosion rate, then there is some slowdown in corrosion due to the difficulty of oxygen diffusion through the thickened layer of water. Even thicker layers of water on the metal surface (section IV) lead only to a slight slowdown in corrosion, since they will affect oxygen diffusion to a lesser extent.

In practice, it is not always possible to so clearly distinguish between these three stages of atmospheric corrosion, since, depending on external conditions, a transition from one type to another is possible. So, for example, a metal structure that corroded by the mechanism of dry corrosion, with an increase in air humidity, will begin to corrode by the mechanism of wet corrosion, and with precipitation, wet corrosion will already take place. When the moisture dries, the process will change in the opposite direction.

A number of factors influence the rate of atmospheric corrosion of metals. The main of them should be considered the duration of surface wetting, which is determined mainly by the relative humidity. Moreover, in most practical cases, the corrosion rate of a metal increases sharply only when a certain critical critical value of relative humidity is reached, at which a continuous film of moisture appears on the metal surface as a result of condensation of water from the air.

The effect of relative humidity on the atmospheric corrosion rate of carbon steel is shown in the figure. The dependence of the increase in the mass of corrosion products m on the relative humidity W was obtained by exposing steel samples in an atmosphere containing 0.01% SO 2 for 55 days.

Impurities of SO 2, H 2 S, NH 3, HCl, etc. contained in air greatly affect the rate of atmospheric corrosion. When dissolved in a film of water, they increase its electrical conductivity and

Solid particles from the atmosphere falling on the metal surface can dissolve, act as harmful impurities (NaCl, Na 2 SO 4), or in the form of solid particles facilitate condensation of moisture on the surface (coal particles, dust, abrasive particles, etc. )

In practice, it is difficult to identify the influence of individual factors on the corrosion rate of a metal in specific operating conditions, but it can be approximately estimated based on the generalized characteristics of the atmosphere (the estimate is given in relative units):

dry continental - 1-9
sea \u200b\u200bnet - 38
marine industrial - 50
industrial - 65
industrial, heavily polluted - 100.

3 . Fluid corrosion- corrosion of metals in a liquid medium: in non-electrolyte(bromine, molten sulfur, organic solvent, liquid fuel) and in the electrolyte (acid, alkaline, salt, marine, river corrosion, corrosion in molten salts and alkalis). Depending on the conditions of interaction of the medium with the metal, liquid corrosion of the metal is distinguished during complete, incomplete and variable immersion, corrosion along the waterline (near the boundary between the part of the metal immersed and not immersed in the corrosive environment), corrosion in non-mixed (quiet) and stirred (moving) corrosive medium ;

Fluid corrosion

4. Underground corrosion - corrosion of metals in soils and soils (for example, rusting of underground steel pipelines);

Underground corrosion

By its mechanism is an electrochem. corrosion of metals. Underground corrosion is caused by three factors: the corrosiveness of soils and soils (soil corrosion), the action of stray currents and the vital activity of microorganisms.

Corrosive aggressiveness of soils and soils is determined by their structure, granulometric. composition, beats. electric resistance, humidity, breathability, pH, etc. Typically, the corrosiveness of the soil with respect to carbon steels is evaluated by beats. electric soil resistance, average cathode current density when the electrode potential is displaced by 100 mV, more negative than the corrosion potential of steel; in relation to aluminum, the corrosion activity of the soil is estimated by the content of chlorine and iron ions in it, the pH value, and in relation to lead, the content of nitrate ions, humus, and pH.

5. Biocorrosion - corrosion of metals under the influence of the vital activity of microorganisms (for example, increased corrosion of steel in soils by sulfate-reducing bacteria);

Biocorrosion

The biocorrosion of underground structures is mainly due to the vital activity of sulfate-reducing, sulfur-oxidizing and iron-oxidizing bacteria, the presence of which is determined by bacteriological. studies of soil samples. Sulfate-reducing bacteria are present in all soils, but with a noticeable rate, biocorrosion occurs only when the water (or soil) contains 105-106 viable bacteria in 1 ml (or 1 g).

6. WITHstructural corrosion - corrosion associated with the structural heterogeneity of the metal (for example, acceleration of the corrosion process in H 2 S0 4 or HCl solutions by cathode inclusions: carbides in steel, graphite in cast iron, CuA1 3 intermetallic in duralumin);

Structural corrosion

7. External current corrosion - electrochemical corrosion of metals under the influence of current from an external source (for example, dissolution of the steel anode grounding of the cathodic protection station of an underground pipeline);

External current corrosion

8. Stray current corrosion- electrochemical corrosion of a metal (for example, an underground pipeline) under the influence of stray current;

The main sources of stray currents in the earth are electrical circuits. direct current railways, trams, subways, mine electric vehicles, direct current power lines through the wire-to-ground system. The stray currents cause the greatest damage in those places of the underground structure where the current flows from the structure to the ground (the so-called anode zones). Iron losses from corrosion by stray currents are 9.1 kg / A · year.

On underground metallic. structures can leak currents of the order of hundreds of amperes and, if there is damage to the protective coating, the current density flowing down from the structure in the anode zone is so high that through damage occurs in the walls of the structure in a short period. Therefore, in the presence of anode or alternating zones on underground metallic. Corrosion by stray currents is usually more dangerous than soil corrosion.

9. Contact corrosion - electrochemical corrosion caused by the contact of metals having different stationary potentials in a given electrolyte (for example, corrosion in seawater of parts made of aluminum alloys in contact with copper parts).

Contact corrosion

Contact corrosion in electrolytes with high electrical conductivity can occur in the following particular cases:

upon contact, low-alloy steel of various grades, if one of them is alloyed with copper and (or) nickel;


when these elements are introduced into welds in the process of welding steel not alloyed with these elements;


when exposed to structures made of steel not alloyed with copper and nickel, as well as galvanized steel or aluminum alloys, dust containing heavy metals or their oxides, hydroxides, salts; the listed materials are cathodes with respect to steel, aluminum, metal protective coatings;


when water flows from corrosive copper parts get onto structures made of the listed materials;


when exposed to the surface of structures made of galvanized steel or aluminum alloys, graphite or iron ore dust, coke chips;


when aluminum alloys come into contact with each other, if one alloy (cathode) is alloyed with copper and the other (anode) ¾ not;

10. crevice corrosion - increased corrosion in cracks and gaps between metals (for example, in threaded and flange joints of steel structures in water), as well as in places of loose contact of metal with non-metallic corrosion-inert material. Inherent in stainless steel structures in aggressive liquid environments in which materials outside narrow slots and gaps are stable due to a passive state i.e. due to the formation of a protective film on their surface;

11. Energized corrosion - corrosion of metals under the influence of a corrosive environment and mechanical stresses. Depending on the nature of the loads, there can be corrosion at constant load (for example, corrosion of metal of steam boilers) and corrosion at variable load (for example, corrosion of axles and rods of pumps, springs, steel ropes); simultaneous exposure to a corrosive environment and alternating or cyclic tensile loads often causes corrosion fatigue - lowering the fatigue limit of the metal;

Energized corrosion

12. Corrosion Cavitation - the destruction of metal caused by the simultaneous corrosion and impact of the external environment (for example, the destruction of the propeller blades of ships);

Corrosion Cavitation

Cavitation - (from lat. cavitas - emptiness) - the formation in a liquid of cavities (cavitation bubbles, or caverns) filled with gas, steam or a mixture thereof. Cavitation occurs as a result of a local decrease in pressure in the liquid, which can occur with an increase in its velocity (hydrodynamic cavitation). Moving with the flow to a region with a higher pressure or during a half-period of compression, the cavitation bubble closes, emitting a shock wave.

Cavitation is in many cases undesirable. On devices, such as screws and pumps, cavitation causes a lot of noise, damages their components, causes vibrations and reduced efficiency.

When cavitation bubbles collapse, the energy of the fluid is concentrated in very small volumes. Thus, places of elevated temperature are formed and shock waves arise that are sources of noise. The destruction of the caverns releases a lot of energy, which can cause major damage. Cavitation can destroy almost any substance. The consequences caused by the destruction of the cavities lead to large wear of the components and can significantly reduce the service life of the screw and pump.

To prevent cavitation

• select material resistant to this type of erosion (molybdenum steels);
  
• reduce surface roughness;
  
• reduce flow turbulence, reduce the number of turns, make them smoother;
  
• do not allow direct impact of an erosive jet into the apparatus wall using reflectors, jet dividers;
  
• purify gases and liquids from solid impurities;
  
• do not allow the operation of hydraulic machines in cavitation mode;
  
• conduct systematic monitoring of material wear.
  

13. friction corrosion(corrosive erosion) - the destruction of metal caused by the simultaneous influence of a corrosive environment and friction (for example, the destruction of the shaft neck during friction against a bearing washed by sea water);

14. Fretting corrosion- corrosion of metals during the oscillatory movement of two surfaces relative to each other under the influence of a corrosive environment (for example, the destruction of two surfaces of metal parts of a machine tightly bolted due to vibration in an oxidizing atmosphere containing oxygen).

Fretting corrosion

By process mechanism distinguish between chemical and electrochemical corrosion of metals:

1. chemical corrosion- the interaction of metal with a corrosive environment, in which the oxidation of the metal and the restoration of the oxidative component of the corrosive environment occur in one act. Examples of this type of corrosion are reactions that occur when metal structures come in contact with oxygen or other oxidizing gases at high temperatures (above 100 ° C):

2 Fe + O 2 \u003d FeO;

4FeO + 3O 2 \u003d 2Fe 2 O 3.

If, as a result of chemical corrosion, a continuous oxide film is formed that has sufficiently strong adhesion to the surface of the metal structure, then the access of oxygen to the metal is impeded, corrosion slows down, and then stops. Porous, poorly adhered to the surface of the structure of the oxide film does not protect the metal from corrosion. When the volume of the oxide is greater than the volume of the reacted oxidation of the metal and the oxide has sufficient adhesion to the surface of the metal structure, such a film protects the metal well from further destruction. The thickness of the protective film of oxide varies from several molecular layers (5-10) x10 –5 mm to several microns.

The oxidation of the material of metal structures in contact with the gaseous medium occurs in boilers, chimneys of boiler rooms, gas-fired water heaters, and heat exchangers running on liquid and solid fuels. If the gaseous medium did not contain sulfur dioxide or other aggressive impurities, and the interaction of metal structures with the medium occurred at a constant temperature along the entire plane of the structure, then a relatively thick oxide film would serve as a fairly reliable protection against further corrosion. But due to the fact that the thermal expansion of metal and oxide is different, the oxide film exfoliates in places, which creates conditions for further corrosion.

Gas corrosion of steel structures can occur as a result of not only oxidative, but also reduction processes. With strong heating of steel structures under high pressure in a hydrogen-containing medium, the latter diffuses into the bulk of the steel and destroys the material according to a double mechanism - decarburization due to the interaction of hydrogen with carbon

Fe 3 OC + 2H 2 \u003d 3Fe + CH 4 O

and imparting brittleness to steel due to the dissolution of hydrogen in it - “hydrogen brittleness”.

2. Electrochemical corrosion- the interaction of a metal with a corrosive medium (electrolyte solution), in which the ionization of metal atoms and the restoration of the oxidizing component of a corrosive medium do not occur in one, the act and their speed depend on the electrode potential of the metal (for example, steel rusting in sea water).

Upon contact with air, a thin film of moisture appears on the surface of the structure, in which impurities in the air, such as carbon dioxide, dissolve. In this case, solutions are formed that promote electrochemical corrosion. Different parts of the surface of any metal have different potentials.

The reasons for this may be the presence of impurities in the metal, different processing of its individual sections, unequal conditions (environment), in which various sections of the metal surface are located. In this case, metal surface sections with a more electronegative potential become anodes and dissolve.

Electrochemical corrosion is a complex phenomenon, consisting of several elementary processes. The anode process proceeds at the anode sites - metal ions (Me) pass into the solution, and the excess electrons (e), remaining in the metal, move to the cathode site. On the cathode sections of the metal surface, excess electrons are absorbed by ions, atoms or electrolyte molecules (depolarizers), which are restored:

e + D → [De],

where D is the depolarizer; e is an electron.

The intensity of the corrosive electrochemical process depends on the rate of the anodic reaction, at which the metal ion passes from the crystal lattice to the electrolyte solution, and the cathodic one, which consists in the assimilation of electrons released during the anodic reaction.

The possibility of the transition of a metal ion into an electrolyte is determined by the strength of the bond with electrons in the interstices of the crystal lattice. The stronger the bond between electrons and atoms, the more difficult the transition of a metal ion into an electrolyte. In electrolytes there are positively charged particles - cations and negatively charged - anions. Anions and cations attach water molecules to themselves.

The structure of water molecules determines its polarity. An electrostatic interaction occurs between charged ions and polar water molecules, as a result of which the polar water molecules are oriented in a certain way around anions and cations.

During the transition of metal ions from the crystal lattice to the electrolyte solution, an equivalent number of electrons is released. Thus, a double electric layer is formed at the metal-electrolyte interface, in which the metal is negatively charged, and the electrolyte is positive; there is a jump in potential.

The ability of metal ions to enter an electrolyte solution is characterized by the electrode potential, which is the energy characteristic of a double electric layer.

When this layer reaches the potential difference, the transition of ions into solution stops (an equilibrium state sets in).

Corrosion diagram: K, K ’- cathodic polarization curves; A, A ’- anode polarization curves.

By nature of corrosion failure The following types of corrosion are distinguished:

1. solidor general corrosioncovering the entire surface of the metal under the influence of this corrosive environment. Continuous corrosion is characteristic of steel, aluminum, zinc and aluminum protective coatings in any environments in which the corrosion resistance of a given material or coating metal is not high enough.

This type of corrosion is characterized by a relatively uniform gradual penetration into the depth of the metal over the entire surface, i.e., a decrease in the thickness of the section of the element or the thickness of the protective metal coating.

In case of corrosion in neutral, slightly alkaline and slightly acidic environments, structural elements are covered with a visible layer of corrosion products, after mechanical removal of which to pure metal the surface of the structures is rough, but without obvious ulcers, corrosion points and cracks; during corrosion in acidic (and for zinc and aluminum and in alkaline) environments, a visible layer of corrosion products may not form.

The areas most susceptible to this type of corrosion, as a rule, are narrow crevices, gaps, surfaces under the heads of bolts, nuts, other areas of dust and moisture accumulation, because the actual corrosion duration in these areas is longer than on open surfaces.

Continuous corrosion happens:

* uniform which flows at the same rate over the entire metal surface (for example, corrosion of carbon steel in H 2 S0 4 solutions);

* uneven which proceeds with unequal speed in different parts of the metal surface (for example, corrosion of carbon steel in sea water);

* selective in which one structural component of the alloy (graphitization of cast iron) or one component of the alloy (dezincification of brass) is destroyed.

2. local corrosioncovering individual sections of the metal surface.

Local corrosion it happens:

* stain corrosion it is typical for aluminum, aluminum and zinc coatings in environments in which their corrosion resistance is close to optimal, and only random factors can cause local disturbance of the state of stability of the material.

This type of corrosion is characterized by a small depth of corrosion penetration in comparison with the transverse (in the surface) dimensions of corrosion lesions. Affected areas are covered with corrosion products as with continuous corrosion. When identifying this type of corrosion, it is necessary to establish the causes and sources of temporary local increases in the aggressiveness of the medium due to the ingress of liquid media (condensate, atmospheric moisture during leaks, etc.) onto the surface of the structure, local accumulation or deposition of salts, dust, etc.

* corrosion ulcers typical for carbon and low-carbon steel (to a lesser extent for aluminum, aluminum and zinc coatings) when operating structures in liquid media and soils.

Ulcerative corrosion of low alloy steel under atmospheric conditions is most often associated with an unfavorable metal structure, i.e., with an increased number of nonmetallic inclusions, primarily sulfides with a high content of manganese.

Peptic corrosion is characterized by the appearance on the surface of the structure of individual or multiple lesions, the depth and transverse dimensions of which (from fractions of a millimeter to several millimeters) are comparable.

It is usually accompanied by the formation of thick layers of corrosion products covering the entire metal surface or its significant areas around individual large ulcers (typical for corrosion of unprotected steel structures in soils). Peptic corrosion of sheet structures, as well as structural elements from thin-walled pipes and rectangular elements of a closed section with time passes into through with the formation of holes in the walls up to several millimeters thick.

Ulcers are acute stress concentrators and can be the initiators of the initiation of fatigue cracks and brittle fractures. To assess the rate of ulcerative corrosion and predict its development in the next period, the average corrosion penetration rates in the deepest ulcers and the number of ulcers per surface unit are determined. These data should be further used in the calculation of the bearing capacity of structural elements.

* pitting corrosion typical for aluminum alloys, including anodized, and stainless steel. Low alloy steel is extremely rare to corrode this type.

Almost a prerequisite for the development of pitting corrosion is the effect of chlorides, which can reach the surface of structures at any stage, from metallurgical production (pickling of rolled products) to operation (in the form of salts, aerosols, dust).

If pitting corrosion is detected, it is necessary to identify the sources of chlorides and the possibility of eliminating their effects on the metal. Pitting corrosion is destruction in the form of separate small (not more than 1 - 2 mm in diameter) and deep (depth greater than the transverse dimensions) sores.

* through corrosion, which causes the destruction of the metal through (for example, pitting or peptic corrosion of sheet metal);

* filamentous corrosionspreading in the form of threads mainly under non-metallic protective coatings (for example, on carbon steel under a varnish film);

* subsurface corrosionstarting from the surface, but predominantly extending beneath the surface of the metal in such a way that failure and corrosion products are concentrated in some areas within the metal; subsurface corrosion often causes the metal to swell and delaminate (for example, the formation of bubbles on the surface
poor-quality rolled sheet metal during corrosion or etching);

* intergranular corrosion characteristic of stainless steel and hardened aluminum alloys, especially in welding areas, and is characterized by a relatively uniform distribution of multiple cracks in large areas of the surface of structures. The depth of the cracks is usually less than their size on the surface. At each site of development of this type of corrosion, cracks almost simultaneously arise from many sources, the connection of which with internal or operating stresses is not mandatory. Under an optical microscope on transverse sections made of selected samples, it can be seen that cracks propagate only along the grain boundaries of the metal. Separate grains and blocks may crumble, resulting in ulcers and surface peeling. This type of corrosion leads to a rapid loss of metal strength and ductility;

* knife corrosion- localized metal corrosion, which has the form of a notch with a knife in the fusion zone of welded joints in highly aggressive environments (for example, cases of corrosion of welded joints of X18H10 chromium-nickel steel with a high carbon content in strong HN0 3).

* corrosion cracking - a type of quasi-brittle fracture of steel and high-strength aluminum alloys under the influence of static tensile stresses and aggressive media; characterized by the formation of single and multiple cracks associated with the concentration of the main working and internal stresses. Cracks can propagate between crystals or over the body of grains, but with a greater speed in the plane normal to the acting stresses than in the surface plane.

Carbon and low alloy steel of ordinary and increased strength undergoes this type of corrosion in a limited number of environments: hot solutions of alkalis and nitrates, mixtures of СО - СО 2 - Н 2 - Н 2 О and in environments containing ammonia or hydrogen sulfide. Corrosion cracking of high-strength steel, such as high-strength bolts, and high-strength aluminum alloys can develop in atmospheric conditions and in various liquid media.

When establishing the fact of damage to the structure by corrosion cracking, it is necessary to make sure that there are no signs of other forms of quasi-brittle fracture (cold brittleness, fatigue).

* corrosion brittlenessacquired by metal as a result of corrosion (for example, hydrogen embrittlement of pipes from high-strength steels in the conditions of hydrogen sulfide oil wells); fragility should be understood as the property of a material to collapse without a noticeable absorption of mechanical energy in an irreversible form.

Quantification of corrosion. The rate of general corrosion is estimated by the loss of metal per unit area of \u200b\u200bcorrosion , for example, in g / m 2 ․ hor by the rate of penetration of corrosion, i.e., by a one-sided decrease in the thickness of an intact metal ( P), for example, in mm / year.

With uniform corrosion P = 8,75K / ρwhere ρ - metal density in g / cm 3. For uneven and local corrosion, maximum penetration is assessed. According to GOST 13819-68, a 10-point scale of general corrosion resistance is established (see table). In special cases, K. can be evaluated by other indicators (loss of mechanical strength and ductility, increase in electrical resistance, decrease in reflectivity, etc.), which are selected in accordance with the type of K. and the purpose of the product or design.

10-point scale for assessing the overall corrosion resistance of metals

Resilience Group	Metal corrosion rate, mm / year.	Score
Completely resistant	| Less than 0.001	1
Very resistant	Over 0.001 to 0.005	2
	Over 0.005 to 0.01	3
Persistent	Over 0.01 to 0.05	4
	Over 0.05 to 0.1	5
Low resistant	Over 0.1 to 0.5	6
	Over 0.5 to 1.0	7
Low resistant	Over 1.0 to 5.0	8
	Over 5.0 to 10.0	9
Unstable	Over 10.0	10

When selecting materials that are resistant to various aggressive environments in specific conditions, they use reference tables of corrosion and chemical resistance of materials or conduct laboratory and full-scale (directly on the spot and in the future use) corrosion tests of samples, as well as entire semi-industrial units and apparatuses. Tests in conditions more stringent than operational are called accelerated.

The use of various methods of protecting metals from corrosion allows to some extent minimize the loss of metal from corrosion. Depending on the causes of corrosion, the following protection methods are distinguished.

1) Processing the environment in which corrosion occurs. The essence of the method is either to remove from the environment those substances that play the role of a depolarizer, or to isolate the metal from the depolarizer. For example, special substances or boiling are used to remove oxygen from water.

Removing oxygen from a corrosive environment is called deaeration.. It is possible to slow down the corrosion process by introducing special substances into the environment - inhibitors. Volatile and vapor-phase inhibitors are widely used that protect ferrous and non-ferrous metal products from atmospheric corrosion during storage, transportation, etc.

Inhibitors are used for descaling steam boilers, for descaling from waste parts, and for storing and transporting hydrochloric acid in steel containers. As organic inhibitors, thiourea (chemical name is carbon sulfide diamide C (NH 2) 2 S), diethylamine, urotropin (CH 2) 6 N 4) and other amine derivatives are used as organic inhibitors.

As inorganic inhibitors, silicates (metal compounds with silicon Si), nitrites (compounds with nitrogen N), alkali metal dichromates, etc. are used. The mechanism of action of inhibitors is that their molecules are adsorbed on the surface of the metal, preventing the occurrence of electrode processes.

2) Protective coatings. To isolate the metal from the environment, various kinds of coatings are applied to it: varnishes, paints, metal coatings. The most common are paint coatings, but their mechanical properties are much lower than that of metal. The latter by the nature of the protective action can be divided into anodic and cathodic.

Anode Coatings. If a metal is coated with another, more electronegative metal, then in the event of conditions for electrochemical corrosion, the coating will break down, because it will act as an anode. An example of an anode coating is chromium deposited on iron.

Cathode Coatings. In the cathode coating, the standard electrode potential is more positive than in the protected metal. As long as the coating layer isolates the metal from the environment, electrochemical corrosion does not occur. If the cathode coating is broken, it ceases to protect the metal from corrosion. Moreover, it even intensifies the corrosion of the base metal, because in the resulting galvanic pair, the anode is the base metal, which will be destroyed. An example is the tin coating on iron (tinned iron).

Thus, when comparing the properties of anode and cathode coatings, we can conclude that anode coatings are most effective. They protect the base metal even in case of violation of the integrity of the coating, while cathode coatings protect the metal only mechanically.

3) Electrochemical protection. There are two types of electrochemical protection: cathodic and protective. In both cases, conditions are created for the appearance of a high electronegative potential on the protected metal.

Tread protection . The product protected from corrosion is connected to scrap metal from a more electronegative metal (tread). This is equivalent to creating a galvanic cell in which the protector is the anode and will be destroyed. For example, to protect underground structures (pipelines), scrap metal (tread) is buried at some distance from them, connecting it to the structure.

Cathodic protection differs from the tread one in that the protected structure located in the electrolyte (soil water) is connected to the cathode of an external current source. A piece of scrap metal is placed in the same medium, which is connected to the anode of an external current source. Scrap metal undergoes destruction, thereby protecting the protected structure from destruction.

In many cases, the metal protects against corrosion a stable oxide film formed on its surface (for example, Al 2 O 3 is formed on the surface of aluminum, which prevents further oxidation of the metal). However, some ions, such as Cl -, destroy such films and thereby increase corrosion.

Corrosion of metals causes great economic harm. Humanity suffers huge material losses as a result of corrosion of pipelines, machine parts, ships, bridges, marine structures and technological equipment.

Corrosion reduces the reliability of equipment: high-pressure apparatuses, steam boilers, metal containers for toxic and radioactive substances, turbine blades and rotors, aircraft parts, etc. Given the possible corrosion, it is necessary to overestimate the strength of these products, which means to increase the consumption of metal, which leads to additional economic costs. Corrosion leads to production downtime due to the replacement of failed equipment, to the loss of raw materials and products (oil, gas, water leakage), to energy costs to overcome the additional resistance caused by a decrease in the flow cross sections of pipes due to deposition of rust and other corrosion products . Corrosion also leads to contamination of the product, and therefore to a decrease in its quality.

The costs of recovering losses associated with corrosion are estimated at billions of rubles per year. Experts estimate that in developed countries the cost of losses associated with corrosion is 3 ... 4% of gross national income.

Over a long period of intensive work of the metallurgical industry, a huge amount of metal was smelted and converted into products. This metal is constantly corroding. Such a situation has developed that the loss of metal from corrosion in the world is already about 30% of its annual production. It is believed that 10% of corroded metal is lost (mainly in the form of rust) irretrievably. It is possible that in the future a balance will be established at which approximately as much metal will be lost from corrosion as it will be smelted again. From the foregoing it follows that the most important problem is the search for new and improvement of old methods of protection against corrosion.

Bibliography

Kozlovsky A.S. Roofing. - M.: "Higher School", 1972

Akimov G.V., Fundamentals of the doctrine of corrosion and metal protection, M., 1946;

Tomashov N. D., The theory of corrosion and metal protection, M., 1959;

Evans, Yu. P., Corrosion and oxidation of metals, trans. from English., M., 1962;

Rosenfeld I. L., Atmospheric corrosion of metals, M., 1960;