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

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

Metal + Oxidizing component of the medium = Reaction product

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

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

ΔG T< 0, то процесс химической коррозии возможен;

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

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

Chemical corrosion is:

Gas corrosion - corrosion destruction under the influence of gases at high temperatures;

Corrosion in non-electrolyte liquids.

Gas corrosion

Gas corrosion- the most common type of chemical corrosion. At high temperatures, the metal surface 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 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 (pO2) and the dissociation pressure of the 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 > pMeO, the reaction is shifted towards oxide formation;

3) pO 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 is thermodynamically possible.

Gas corrosion rate is determined by several factors: ambient temperature, the nature of the metal or alloy composition, the nature of the gaseous medium, the time of contact 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 appearance of an oxide film on the surface can be conditionally divided into two stages:

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

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

At the first stage, an ionic bond arises between the surface atoms and oxygen: the oxygen atom takes two electrons from the 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 effect on oxygen of the field created by metal atoms. After complete saturation of the surface with an oxidizing agent, which occurs almost instantly, at low temperatures due to van der Waals forces, physical adsorption of oxidant molecules can also be observed.

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

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

If the resulting oxide film has good protective properties, it will slow down 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 tint colors);

Thick (clearly visible).

In order for the oxide film to be protective, it must meet certain requirements: it must 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, besides it 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 the entire surface of the metal with a continuous layer.

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

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

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

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

Influence of external and internal factors on the rate of chemical corrosion.

Temperature has a very strong influence on the rate of chemical corrosion. With its increase, the oxidation processes go much faster. In this case, the decrease in the thermodynamic possibility of the reaction does not matter.

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

The composition of the gaseous medium plays an important role in the corrosion process. But this is individual for each metal and changes with temperature fluctuations. For example, copper corrodes very quickly in an oxygen atmosphere, but is stable in an environment containing SO 2 . Nickel, on the contrary, corrodes intensively upon contact with an SO 2 atmosphere, but is stable in O 2 , CO 2 and H 2 O. Chromium is relatively stable in all four media.

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

The rate of oxidation depends on the composition of the alloy. Let's 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 very strongly inhibit oxidation. 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 different temperatures, we note that with increasing temperature, the slowest oxidation is observed with an austenitic structure. It is the most heat resistant, compared to others.

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

Chemical corrosion in non-electrolyte liquids

Non-electrolyte liquids are liquid media that are not conductors of electricity. These include: organic (benzene, phenol, chloroform, alcohols, kerosene, oil, gasoline); inorganic origin (liquid bromine, molten sulfur, etc.). Pure non-electrolytes 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 oil contains sulfur or sulfur-containing compounds (hydrogen sulfide, mercaptans), the process of chemical corrosion is accelerated. If, in addition, the temperature increases, dissolved oxygen will appear in the liquid - chemical corrosion will increase.

The presence of moisture in liquids-non-electrolytes 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 metal surface;

Chemisorption of the reagent on the surface;

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 the non-electrolyte (may be absent).

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

The phrase "corrosion of metal" contains much more than the name of a popular rock band. Corrosion irrevocably destroys the metal, turning it into dust: of all the iron produced in the world, 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 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 figures. So what causes the corrosion processes of metals and how to deal with them?

What is metal corrosion

The destruction of metals as a result of electrochemical (dissolution in moisture-containing air or aquatic 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).

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 metal surface.

Absolutely 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 air moisture, acquires a greenish coating - patina. Moreover, the oxide film protects the metal from corrosion not 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 in any way.

Alloys are subject to a different type of corrosion: some elements of alloys do not oxidize, but are reduced (for example, in a 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 thin electrolytic film on its surface is sufficient (electrolytic solutions often impregnate the environment surrounding the metal (concrete, soil, etc.)). The most common cause of electrochemical corrosion is the widespread use of household and technical salts (sodium and potassium chlorides) to remove ice and snow on roads in winter - cars and underground utilities are especially affected (according to statistics, annual losses in the United States from the use of salts in winter are 2.5 billion dollars).

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 couple is formed: the metal is destroyed, gradually all its particles become part of the solution. Electro chemical corrosion can cause stray currents that occur when part of the current leaks from an electrical circuit into aqueous solutions or into the soil and from there into a metal structure. In those places where stray currents exit the metal structures back into the water or into the soil, the destruction of metals occurs. Especially often, stray currents occur in places where ground electric vehicles are moving (for example, trams and electric railway locomotives). In just a year, stray currents with a power of 1A are able to dissolve iron - 9.1 kg, zinc - 10.7 kg, lead - 33.4 kg.

Other causes of metal corrosion

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

The combination of exposure to 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, it is activated electrochemical corrosion.

Measures to protect metals from corrosion

An inevitable consequence of technological progress is the pollution of our environment, a process that accelerates the corrosion of metals as the external environment becomes 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 the metal to corrosion; eliminate the interaction between the metal and substances from the external environment that exhibit aggression.

Mankind has tried many methods of protection for thousands of years. metal products from chemical corrosion, some of them are used to this day: coating with grease or oil, other metals that are less corrosive (the oldest method, which is more than 2 thousand years old, is tinning (tin coating)).

Anti-corrosion protection with non-metallic coatings

Not metal coatings- paints (alkyd, oil and enamel), varnishes (synthetic, bituminous and tar) and polymers form a protective film on the surface of metals, excluding (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 at the assembly and construction site. Methods for applying paints and varnishes are simple and amenable to mechanization; damaged coatings can be restored "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 the climatic conditions in which it will be operated metal structure; the need to use exclusively high-quality paints and varnishes; strict adherence to the technology of application to metal surfaces. Paintwork materials are best applied in several layers - their quantity will provide the best protection against atmospheric action on the metal surface.

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

Protection of iron against corrosion by coatings from other metals

There are two types of metal inhibitor coatings - protective (zinc, aluminum and cadmium coatings) and corrosion resistant (silver, copper, nickel, chromium and lead coatings). Inhibitors are applied by chemical means: 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 is metal coatings of iron with tin (tinplate, tin cans are made from it) and zinc (galvanized iron - roofing), obtained by pulling sheet iron through a melt of one of these metals.

Often cast iron and steel fittings, as well as water pipes are subjected to galvanizing - this operation significantly increases their resistance to corrosion, but only in cold water(when hot water is connected, galvanized pipes wear out faster than non-galvanized ones). Despite the effectiveness of galvanizing, it does not provide perfect 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 on them - there is no stable coating.

The best solution for corrosion protection is aluminum coating. This metal has less specific gravity, which means less consumption, aluminized surfaces can be painted and the paint layer will be stable. In addition, the aluminum coating, compared to galvanized coating, is more resistant to aggressive environments. Aluminizing is not very common due to the difficulty of applying this coating to a metal sheet - aluminum in the molten state exhibits high aggression towards other metals (for this reason, aluminum melt cannot be kept in a steel bath). Perhaps this problem will be completely solved in the very near future - the original way of performing aluminization 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.

Improving corrosion resistance by adding alloying additives to steel alloys

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

Measures to counter electrochemical corrosion

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 starting 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 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 being removed from the water, which can affect localized corrosion. Liming the soil can reduce its acidity.

Stray current protection

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

• the section of the structure that serves as a source of stray current must be connected with a metal conductor to the tram rail;
• heating network routes should be located at the maximum distance from the railroads on which electric transport moves, to minimize the number of their intersections;
• the use of electrically insulating pipe supports to increase the transient 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 flanged fittings and stuffing box compensators - to increase the 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 provided with insulation, as well as steel structures small size performed using 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, provides protection for 8 km of pipeline.

Abdyuzhanov Rustam, especially for rmnt.ru

Electrochemical corrosion is the most common form of corrosion. Electrochemical occurs when the metal comes into contact with the surrounding electrolytically conductive medium. In this case, the reduction of the oxidizing component of the corrosive medium does not proceed simultaneously with the ionization of metal atoms, and their rates depend on the electrode potential of the metal. The root cause of electrochemical corrosion is the thermodynamic instability of metals in their environments. Corrosion of pipelines, upholstery of the bottom of a sea vessel, various metal structures in the atmosphere are, and many more, examples of electrochemical corrosion.

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

Mechanism of electrochemical corrosion can proceed in two ways:

1) Homogeneous mechanism of electrochemical corrosion:

Surface layer met. regarded as homogeneous and homogeneous;

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

K and A regions 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 also be realized in liquid metals.

2) Heterogeneous mechanism of electrochemical corrosion:

In hard metals, the surface is inhomogeneous, 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 occurring processes (cathodic and anodic), which are kinetically dependent on each other; in some areas of the surface, electrochemical corrosion can take on a local character; dissolution of the main met. occurs at the anodes.

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

The reasons for the occurrence of local galvanic cells can be very different:

1) alloy heterogeneity

Heterogeneity met. phases due to the inhomogeneity of the alloy and the presence of micro- and macro-inclusions;

Unevenness 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, the appearance of a dislocation on the surface, the anisotropy of crystals.

2) inhomogeneity of the medium

The area with limited access to the oxidizer will be an anode 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 place of entry of the stray current is the cathode, the place of exit is the anode);

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

During the operation of a galvanic cell, two electrode processes occur simultaneously:

Anodic- metal ions go 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 reduced. 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 environments)

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

The inhibition of the anodic process leads to the inhibition of the cathodic process as well.

Corrosion of metal takes place at 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 negatively charged, a potential difference arises between them. This phenomenon is associated with the appearance of a double electric layer (EDL). Charged particles are located asymmetrically at the phase boundary.

Potential jumps in the process of electrochemical corrosion can occur due to two reasons:

At a sufficiently high hydration energy, metal ions can detach and go into solution, leaving an equivalent number of electrons on the surface, which determine its negative charge. A negatively charged surface attracts meth cations to itself. from a solution. Thus, a double electric layer appears at the phase boundary.

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

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

The magnitude of the electrode potentials has a very great influence on the nature of the corrosion process.

The potential jump between 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 conventionally taken as zero) and the electrode under study. A standard hydrogen electrode is taken as the reference electrode. The EMF of a galvanic cell (a standard hydrogen electrode and the element under study) is called the electrode potential. Reference electrodes can also be silver chloride, calomel, saturated copper sulfate.

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

E = 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 positive charges move in the opposite direction, then the equation will look like:

E max =+(ΔG T)/mnF.

During 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, solvent, electrolyte temperature, activity of met ions.

The equilibrium potential obeys the Nernst equation:

E=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 met ion; T - temperature; F - Faraday number; α Me n+ - activity of met ions.

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

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

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

Polarization is of three types:

Electrochemical (when slowing down the anodic or cathodic processes);

Concentration (observed when the rate 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 when two dissimilar metals come into contact. In the electrolyte, they form a galvanic couple. The more electronegative of these will be the anode. The anode will gradually dissolve in the process. In this case, there is a slowdown or even complete cessation of electrochemical corrosion at the cathode (more electropositive). For example, when duralumin and nickel come into contact with sea water, it will be duralumin that will intensively dissolve.

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 metal surface. 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 and 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 °С the moisture content of the air will be 0.333 g/m3. At + 20 °С it corresponds to the humidity of the air only 2 % . In such conditions

viah flow of electrochemical corrosion is virtually eliminated. In industry, cases of gas corrosion are quite common - from the destruction of parts, heating furnaces to metal corrosion during its heat treatment.

The rate of gas corrosion is affected by a number of factors, primarily such as the temperature and composition of the gaseous medium.

An increase in temperature markedly increases the rate of corrosion. In the first approximation, this relationship can be described

on the well-known physical chemistry Arrhenius equation

log K = A -

where TO- speed reaction; A and V- constants; T- absolute temperature (°C).

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 °С) is fully confirmed, but more often it is more complex, 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 metal with oxygen. Iron already at temperature 300 °С covered in air with scale, i.e., an oxide film visible to the naked eye. The composition of the scale includes magnetite F 3 O 4 and hematite Fe2O3. With increasing temperature, up to 575 °С, the corrosion rate remains approximately constant, but starting from 575 0 C, increases sharply. This fact is associated with the appearance of wustite (iron oxide FeO).

Two groups of reactions proceed in parallel on the surface of carbon steel during corrosion: oxidation of iron to oxides with the formation of scale and decarburization reactions 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 the metal is depleted in cementite. With prolonged heating, the depth of the decarburized layer can be several millimeters. This significantly affects the properties of the metal, and above all, its hardness and strength. Decarburization is also observed in the presence of carbon dioxide, water vapor or other oxidizing agents and proceeds according to similar reactions:

Fe 3 C + CO 2 → 3Fe + 2CO,

Fe 3 C + H 2 O → 3Fe + CO + H 2.

An increase in gas pressure, other things being equal, also greatly accelerates gas corrosion.

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

Gas corrosion is highly susceptible and many non-ferrous

metals, although each of them refers to certain gases in different ways. 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 influence of the nature of the metal on the corrosion rate. For example, if the corrosion of tungsten slows down in the transition from oxygen to water vapor by about 20 times, and copper - in 3,5 times, then the corrosion rate of iron, on the contrary, increases.

Table 9

Gas corrosion of a number of metals in certain environments

(temperature 800 °С, duration 24 hours)

oxide films. Significant influence on the rate of gas corrosion is exerted by the resulting corrosion products, their physico-chemical and mechanical properties. In most cases, corrosion occurs in an oxidizing environment; in this case, an oxide film is formed on the metal surface as a corrosion product. However, a thin oxide film on the metal usually appears already at room temperature. The properties of the resulting 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, one speaks of the onset of passivity of the metal surface.

Thermodynamics of gas corrosion. The thermodynamic possibility of the process of gas corrosion 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 is reduced to comparing the elasticity of dissociation of the resulting oxidation reaction product with the partial pressure of oxygen in the gas phase.

Indeed, if the partial pressure of oxygen P O and elasticity of oxide dissociation P MeO in the metal oxidation reaction mMe + nO 2 Me m O 2 n are 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 oxide formation. If P O< P MeO, then the oxide spontaneously dissociates into oxygen and metal. Therefore, a comparison of the elasticity of dissociation of a given oxide at a given temperature, for example, with the partial pressure of oxygen in air ( P O 0.2 at at atmospheric pressure) allows you to find the boundary by thermodynes

cal probability of the process of metal oxidation in air. So, judging by the data in Table. 10, silver already at 400 °K unable to oxidize. For copper, this boundary lies in the region 2000 °K.

Table 10

Elasticity of dissociation of silver and copper oxides

temperature dependent

Process	300 0 K	400 0 K	500 0 K	800 0 K	1200 0 K	1600 0 K
Ag 2 O 2 Ag + 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 formation conditions, oxide films can have a thickness from monomolecular to several millimeters. There are thin, medium and thick films. Thin films have thicknesses ranging from a few angstroms to 400 Å. They are invisible and can be detected and measured by the so-called optical method of reflecting polarized light.

Medium films have a thickness 400 - 5000 Å and are visible to the naked eye due to the appearance of tint colors (a 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 (cathode reduction method).

Film thickness above 5000 Å(i.e. thicker 0.5 microns) are determined by the gravimetric method or by the cathodic reduction method, as well as using a microscope, micrometer or other similar measuring instruments. They are usually easily visible to the naked eye.

It should be noted that when studying the phase composition and

The structures of oxide films are widely used by electron microscopic, electron graphic and X-ray methods of investigation.

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

It would be erroneous to assume that the thicker the oxide film, the more reliably 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 for the protective ability.

In order for the 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 discontinuous; if the volume of metal oxide is greater than the volume of metal, then the film can be non-porous, compact.

This can be explained by the following inequalities:

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

In turn

V Me = and V Me O = ,

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

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

Table 11

The thickness of the oxide film on iron

depending on conditions

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

properties of the oxide film. When too large values V Me O / V Me The film experiences such high internal stresses that it collapses, losing its continuity. For example, in relation VWO / VW = 3.35 tungsten oxide film has very weak protective properties.

The film must have good adhesion to the metal, must be sufficiently strong and elastic. The coefficients of thermal expansion of the film and metal should be close enough. Finally, the film must be chemically resistant when exposed to a corrosive environment.

An important condition is the need for orientation

ionic 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 resulting oxide with a minimum displacement of atoms. Most often, in the presence of an oxide crystal structure close to the metal structure, the protective properties of such a film are better than those of an oxide that is not oriented with respect to the metal.

Laws of growth of oxide films. If a non-continuous oxide film is formed as a result of corrosion, oxygen gets free access to the metal surface. In this case, the corrosion rate should be a constant value:

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

y = k + A,

expressing the linear dependence of the film thickness on time. Constant A indicates the presence of some oxide film by the time the oxidation starts ( y = a at = 0 ). As follows from the equation, the film growth rate in this case does not depend on its thickness. Corrosion can proceed at a constant rate up to the complete transformation of the metal into oxide, as occurs during the oxidation of magnesium in an oxygen environment.

However, often the actual rate of oxidation, while remaining constant, turns out to be 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-metal oxide interface. Pseudomorphic oxide has a high degree orientation correspondence to the metal and is, thus, a kind of crystallographic continuation of the lattice of the oxidized metal, differing at the same time in parameters from the metal oxide lattice. Being non-porous, it makes it difficult for oxygen to penetrate the metal surface.

Thus, even in the case of the formation of a thick and loose oxide film 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 with sufficiently good protective properties is formed during corrosion, 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 oxide film thickness on the corrosion duration:

y 2 \u003d k + A.

This dependence is observed during the oxidation of copper, nickel, and tungsten. Given a parabolic corrosion versus time curve, one can determine the corrosion rate at any point on the curve. It will be expressed as the tangent of the slope of the tangent passing through the given point, since

tg = .

Finally, under certain conditions, the deceleration of the rate of metal oxidation with an increase in the thickness of the oxide film occurs more intensely than required by the parabolic law. In these cases, the oxidation rate is related to the film thickness by the exponential dependence

After integration, we arrive at the logarithmic equation

y \u003d ln (k).

The logarithmic film growth law is experimentally confirmed during the oxidation of aluminum and zinc in air in the temperature range 20 - 255 °C, copper - up to 100 °С, iron up 385 °С.

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

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

a B C D E

Figure 68. Types of destruction of oxide films.

a- bubble with rupture; b- microbubbles in the oxide layer (vacuum porosity); v- flaking; G- cracking at

shift; d- cracking at the 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 may 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, v), as well as cracking on the surface (Fig. 68, G) or on the corners and edges (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 iron oxidation at 900 °С twice is enough to enter 3,5 % aluminum, and four times - about 5,5 % . The concentration of the alloying component may be negligible. So, molten magnesium oxidizes so vigorously in air that it is capable of spontaneous combustion. However, with the introduction of only 0,001 % beryllium, the rate of oxidation of magnesium is sharply reduced.

The action of alloying elements is explained by the formation of protective films on the metal surface. They are either formed from the alloying component only 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 virtual absence of vacant sites; this is what determines their high thermodynamic stability. An example of spinels are the oxides FeO ∙ Cr 2 O 3 on the surface of chromium 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 coatings.

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

by aluminizing and thermochromizing, respectively. In other cases, the surface is protected with a layer of cermet - a mixture of metal and oxides. Ceramic-metal coatings (cermets) are interesting in that they combine the refractoriness, hardness and heat resistance of ceramics with the ductility and conductivity of metal. Refractory oxides are used as a non-metallic component Al2O3, MgO and compounds - such as carbides and nitrides. The metal components are the metals of the iron group, as well as chromium, tungsten, molybdenum.

Corrosion of metals (from late Latin corrosio - corrosion) - physical and chemical interaction metal material and environment, leading to a deterioration in the operational properties of the material, environment or technical system, of which they are parts.

The corrosion of metals is based on a chemical reaction between the material and the medium or between their components, which occurs at the interface. This process is spontaneous and is also a consequenceredox reactionswith environmental components. Chemicals that destroy building materials are called aggressive. An aggressive medium can be atmospheric air, water, various solutions of chemicals, gases. The process of destruction of the material is enhanced in the presence of even a small amount of acids or salts in water, in soils in the presence of salts in soil water and fluctuations in the level of groundwater.

Corrosion processes are classified:

1) according to the conditions of corrosion,

2) according to the mechanism of the process,

3) by the nature of corrosion damage.

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

Corrosive media and the destruction they cause are so characteristic that the names of these media are used to classify the corrosion processes occurring in them. Yes, allocate gas corrosion, i.e. chemical corrosion under the influence of hot gases (at a temperature much higher than the dew point).

Some cases are typical electrochemical corrosion(mainly with cathodic oxygen reduction) in natural environments: atmospheric- in clean or polluted 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.); marine - under the influence of sea water and underground - in soils and soils.

stress corrosion develops in the area of ​​action of tensile or bending mechanical loads, as well as permanent deformations or thermal stresses and, as a rule, leads to transgranular stress corrosion cracking, which, for example, steel cables and springs are subject to atmospheric conditions, carbon and stainless steels in steam power plants, high-strength titanium alloys in sea water, etc.

Under alternating loads, it can manifest itself corrosion fatigue, which is expressed in a more or less sharp decrease in the fatigue limit of the metal in the presence of a corrosive environment. Corrosive erosion(or friction corrosion) is an accelerated wear of the metal under the simultaneous action of mutually reinforcing corrosive and abrasive factors (sliding friction, the flow of abrasive particles, etc.).

Cavitation corrosion related to it occurs during cavitation modes of flow around a metal with an aggressive medium, when the continuous occurrence and “collapse” of small vacuum bubbles creates a stream of destructive microhydraulic shocks that affect the metal surface. A close variety can be considered fretting corrosion, observed at the points of contact of tightly compressed or rolling parts one over the other, if as a result of vibrations between their surfaces microscopic shear displacements occur.

Leakage of electric current through the boundary of a metal with an aggressive environment 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 destruction, localized near the contact, can cause contact in the electrolyte of two dissimilar metals forming a closed galvanic cell - contact corrosion.

In narrow gaps between parts, as well as under a loose coating or build-up, where the electrolyte penetrates, but the access of oxygen necessary for metal passivation is difficult, crevice corrosion, at which the dissolution of the metal mainly occurs in the gap, and the cathodic reactions partially or completely proceed next to it on the open surface.

It is also customary to single out biological corrosion, going under the influence of 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 a 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. There are three main types of atmospheric corrosion according to the degree of moisture:

• Wet atmospheric corrosion– corrosion in the presence of a visible water film on the metal surface (film thickness from 1 µm to 1 mm). Corrosion of this type is observed at a relative air humidity of about 100%, when there is a droplet 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 relative air humidity below 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 having the properties of an electrolyte.
  

It is obvious that the minimum terms of corrosion occur with dry atmospheric corrosion, which proceeds according to the mechanism of chemical corrosion.

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

It can be seen from the above dependence that the maximum corrosion rate corresponds to the boundary of regions II and III, then some slowing down of corrosion is observed due to the difficulty of oxygen diffusion through the thickened water layer. 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 distinguish these three stages of atmospheric corrosion so clearly, since, depending on external conditions, a transition from one type to another is possible. So, for example, a metal structure that corroded by the dry corrosion mechanism, with an increase in air humidity, will begin to corrode by the wet corrosion mechanism, and with precipitation, wet corrosion will already take place. When moisture dries, the process will change in the opposite direction.

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

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

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

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

In practice, it is difficult to identify the influence of individual factors on the metal corrosion rate under 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 ​​clean - 38
marine industrial — 50
industrial - 65
industrial, heavily polluted - 100.

3 .Liquid corrosion- corrosion of metals in a liquid medium: in non-electrolyte(bromine, molten sulfur, organic solvent, liquid fuel) and in the electrolyte (acid, alkali, salt, sea, 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 with 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 medium), corrosion in an unmixed (calm) and mixed (moving) corrosive medium ;

Liquid corrosion

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

underground corrosion

According to its mechanism, it is electrochemical. metal corrosion. Underground corrosion is caused by three factors: the corrosive aggressiveness of soils and soils (soil corrosion), the action of stray currents, and the vital activity of microorganisms.

The corrosive aggressiveness of soils and soils is determined by their structure, granulometric. composition, ud. electric resistance, humidity, air permeability, pH, etc. Usually, the corrosive aggressiveness of the soil in relation to carbon steels is evaluated by beats. electric soil resistance, average cathode current density at a displacement of the electrode potential by 100 mV more negative than the corrosion potential of steel; in relation to aluminum, the corrosive activity of the soil is estimated by the content of chlorine and iron ions in it, by the pH value, in relation to lead, by the content of nitrate ions, humus, by the pH value.

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

Biocorrosion

Biocorrosion of underground structures is caused in the main. vital activity of sulfate-reducing, sulfur-oxidizing and iron-oxidizing bacteria, the presence of which is established by bacteriological. soil sampling studies. Sulfate-reducing bacteria are present in all soils, but biocorrosion proceeds at a noticeable rate only when waters (or soils) contain 105-106 viable bacteria per 1 ml (or 1 g).

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

Structural corrosion

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

Corrosion by external current

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

The main sources of stray currents in the earth are electrified cir. DC railways, trams, subways, mine electric transport, DC power lines using the wire-ground system. Stray currents cause the greatest damage in those places of the underground structure where the current flows from the structure into the ground (the so-called anode zones). The loss of iron from corrosion by stray currents is 9.1 kg / A·year.

On underground metal Structures can leak currents of the order of hundreds of amperes, and if there are damages in the protective coating, the current density flowing from the structure in the anode zone is so high that through damages form in the walls of the structure in a short period. Therefore, in the presence of anode or alternating zones on underground metal. structures 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 sea water 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 special cases:

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


when these elements are introduced into welds during welding of steel not alloyed with these elements;


when exposed to steel structures 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 in relation to steel, aluminum, metal protective coatings;


when structures made of the listed materials get water drips from corroding copper parts;


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


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

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

11. stress corrosion- corrosion of metals with simultaneous exposure to a corrosive environment and mechanical stresses. Depending on the nature of the loads, there may be corrosion under constant load (for example, corrosion of the metal of steam boilers) and corrosion under 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 - a decrease in the metal fatigue limit;

stress corrosion

12. Corrosive cavitation- destruction of metal caused by simultaneous corrosion and impact of the external environment (for example, the destruction of the propeller blades of marine vessels);

Corrosive cavitation

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

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

When cavitation bubbles collapse, the energy of the liquid is concentrated in very small volumes. As a result, hot spots are formed and shock waves are generated, which are sources of noise. When the caverns are destroyed, a lot of energy is released, which can cause major damage. Cavitation can destroy almost any substance. The consequences caused by the destruction of cavities lead to great wear constituent parts and can significantly shorten the life of the propeller and pump.

To prevent cavitation

• select a 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 a direct impact of an erosive jet into the wall of the apparatus, 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) - metal destruction caused by simultaneous exposure to a corrosive environment and friction (for example, the destruction of a shaft journal during friction against a bearing washed by sea water);

14. Fretting corrosion- corrosion of metals during the vibrational 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 connected by bolts as a result of vibration in an oxidizing atmosphere containing oxygen).

Fretting corrosion

By process mechanism There are chemical and electrochemical corrosion of metals:

1. chemical corrosion- interaction of a metal with a corrosive medium, in which the oxidation of the metal and the reduction of the oxidizing component of the corrosive medium occur in one act. Examples of this type of corrosion are reactions that occur when metal structures come into contact with oxygen or other oxidizing gases at high temperatures (over 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, which has a sufficiently strong adhesion to the surface of the metal structure, then the access of oxygen to the metal is hindered, corrosion slows down, and then stops. A porous, poorly bonded oxide film to the surface of the structure does not protect the metal from corrosion. When the volume of the oxide is greater than the volume of the metal that has entered into the oxidation reaction 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 oxide protective film ranges from several molecular layers (5-10) x 10 -5 mm to several microns.

Oxidation of the material of metal structures in contact with the gaseous medium occurs in boilers, chimneys of boiler houses, water heaters operating on gas fuel, heat exchangers operating 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 over the entire plane of the structure, then a relatively thick oxide film would serve sufficiently reliable protection from further corrosion. But due to the fact that the thermal expansion of metal and oxide is different, the oxide film peels off 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 medium containing hydrogen, the latter diffuses into the volume of steel and destroys the material by a double mechanism - decarburization due to the interaction of hydrogen with carbon

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

and imparting brittle properties to steel due to the dissolution of hydrogen in it - "hydrogen embrittlement".

2. Electrochemical corrosion- the interaction of a metal with a corrosive medium (electrolyte solution), in which the ionization of metal atoms and the reduction of the oxidizing component of the corrosive medium do not occur in one act and their speed depend on the electrode potential of the metal (for example, rusting of steel 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, various processing its individual sections, unequal conditions (environment) in which there are various sections of the metal surface. In this case, the areas of the metal surface with a more electronegative potential become anodes and dissolve.

Electrochemical corrosion is a complex phenomenon, consisting of several elementary processes. An anode process takes place in the anode sections - metal ions (Me) pass into the solution, and excess electrons (e), remaining in the metal, move towards the cathode section. On the cathode sections of the metal surface, excess electrons are absorbed by ions, atoms or electrolyte molecules (depolarizers), which are reduced:

e + D → [De],

where D is a depolarizer; e is an electron.

The intensity of the corrosion 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 reaction, 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 the metal ion into the electrolyte. Electrolytes contain 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 polar water molecules are oriented around anions and cations in a certain way.

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 positively charged; there is a potential jump.

The ability of metal ions to pass into the electrolyte solution is characterized by the electrode potential, which is the energy characteristic of the electrical double layer.

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

Corrosion diagram: K, K' - cathode polarization curves; A, A' - anodic polarization curves.

By nature of corrosion damage There are the following types of corrosion:

1. solid, or general corrosion covering the entire metal surface exposed to a given corrosive environment. Continuous corrosion is typical for steel, aluminum, zinc and aluminum protective coatings in any environment in which the corrosion resistance of this material or coating metal is not high enough.

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

During 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 turns out to be 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 cracks, gaps, surfaces under the heads of bolts, nuts, other areas of accumulation of dust, moisture, for the reason that in these areas the actual duration of corrosion is longer than on open surfaces.

Solid corrosion happens:

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

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

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

2. local corrosion, covering certain areas of the metal surface.

localized corrosion happens:

* stain corrosion characteristic of aluminum, aluminum and zinc coatings in environments in which their corrosion resistance is close to optimal, and only random factors can cause a local violation of the stability of the material.

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

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

Pitting corrosion of low-alloy steel under atmospheric conditions is most often associated with an unfavorable metal structure, i.e., with an increased amount of non-metallic inclusions, primarily sulfides with a high manganese content.

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

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

Pits are sharp stress concentrators and can be the initiators of fatigue cracks and brittle fractures. To assess the rate of pitting corrosion and predict its development in the subsequent period, the average corrosion penetration rates in the deepest pits and the number of pits per unit surface are determined. These data should be used in the future when calculating the bearing capacity of structural elements.

* pitting (pitting) corrosion characteristic of aluminum alloys, including anodized, and stainless steel. Low-alloy steel is subject to corrosion of this type is extremely rare.

An almost obligatory condition for the development of pitting corrosion is the impact of chlorides, which can get on the surface of structures at any stage, from metallurgical production (pickling of rolled products) to operation (in the form of salts, aerosols, dust).

When pitting corrosion is detected, it is necessary to identify sources of chlorides and ways to exclude their effect on the metal. Pitting corrosion is a destruction in the form of individual small (no more than 1–2 mm in diameter) and deep (depth greater than transverse dimensions) ulcers.

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

* filiform corrosion, propagating in the form of filaments mainly under non-metallic protective coatings (for example, on carbon steel under a varnish film);

* subsurface corrosion, starting from the surface, but mainly propagating under the surface of the metal in such a way that the destruction and corrosion products are concentrated in some areas inside the metal; subsurface corrosion often causes metal swelling and delamination (for example, blistering on the surface
low-quality rolled sheet metal during corrosion or pickling);

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

* knife corrosion- localized corrosion of the metal, which has the form of a cut with a knife in the fusion zone of welded joints in highly aggressive environments (for example, cases of corrosion of welds of chromium-nickel steel Kh18N10 with a high carbon content in strong HN0 3).

* stress corrosion cracking— type of quasi-brittle fracture of steel and high-strength aluminum alloys under simultaneous action of static tensile stresses and corrosive 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 along the body of grains, but at a higher rate in the plane normal to the acting stresses than in the surface plane.

Carbon and low-alloy steel of ordinary and increased strength is subjected to this type of corrosion in a limited number of media: hot solutions of alkalis and nitrates, mixtures of CO - CO 2 - H 2 - H 2 O and in media containing ammonia or hydrogen sulfide. Stress 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 brittleness, acquired by the metal as a result of corrosion (for example, hydrogen embrittlement of pipes made of high-strength steels in conditions of hydrogen sulfide oil wells); brittleness should be understood as the property of a material to break down without appreciable 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 ​​corrosion , for example, in g/m 2 ․ h,or by the rate of penetration of corrosion, i.e., by a unilateral decrease in the thickness of the intact metal ( P), for example, in mm/year.

With uniform corrosion P = 8,75K/ρ, where ρ - metal density in g/cm3. For uneven and localized corrosion, the maximum penetration is evaluated. According to GOST 13819-68, a 10-point scale of general corrosion resistance is established (see table). In special cases, K. can also be evaluated according to other indicators (loss of mechanical strength and plasticity, 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 structure.

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

Resistance 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 resistance	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 media in certain specific conditions, they use reference tables of corrosion and chemical resistance of materials or conduct laboratory and full-scale (directly on site and in conditions of future use) corrosion tests of samples, as well as entire semi-industrial units and devices. Tests under conditions more severe than operational are called accelerated.

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

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

The removal of oxygen from a corrosive environment is called deaeration.. It is possible to slow down the corrosion process as much as possible by introducing special substances into the environment - inhibitors. Volatile and vapor-phase inhibitors are widely used, which protect articles made of ferrous and non-ferrous metals from atmospheric corrosion during storage, transportation, etc.

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

Silicates (compounds of metal with silicon Si), nitrites (compounds with nitrogen N), alkali metal dichromates, etc. are used as inorganic inhibitors. 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 types of coatings are applied to it: varnishes, paints, metal coatings. The most common are paint coatings, but their mechanical properties are much lower than those of metal ones. The latter, according to the nature of the protective action, can be divided into anode and cathode.

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

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

Thus, when comparing the properties of anodic and cathodic coatings, it can be concluded that anodic coatings are the most effective. They protect the base metal even if the integrity of the coating is compromised, while cathodic 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 occurrence of a high electronegative potential on the protected metal.

Protective protection . The product protected from corrosion is combined with a metal scrap from a more electronegative metal (tread). This is equivalent to creating a galvanic cell in which the protector is an anode and will be destroyed. For example, to protect underground structures (pipelines), scrap metal (protector) is buried at some distance from them, attaching 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 is subjected to destruction, thereby protecting the protected structure from destruction.

In many cases, the metal is protected from corrosion by 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. Mankind suffers huge material losses as a result of corrosion of pipelines, machine parts, ships, bridges, offshore structures and technological equipment.

Corrosion leads to a decrease in the reliability of equipment operation: high pressure apparatuses, steam boilers, metal containers for toxic and radioactive substances, turbine blades and rotors, aircraft parts, etc. Taking into account 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 losses of raw materials and products (leakage of oil, gases, water), to energy costs to overcome additional resistance caused by a decrease in the flow sections of pipelines due to the deposition of rust and other corrosion products. . Corrosion also leads to contamination of products, and hence to a decrease in its quality.

The cost of compensating for losses associated with corrosion is estimated at billions of rubles a year. Experts have calculated that developed countries the cost of losses associated with corrosion is 3...4% of the 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. There is such a situation that the loss of metal from corrosion in the world is already about 30% of its annual production. It is believed that 10% of the corroded metal is lost (mainly in the form of rust) irretrievably. Perhaps in the future a balance will be established in which about the same amount of metal will be lost from corrosion as it will be smelted again. From all that has been said, it follows that the most important problem is to find new and improve old methods of corrosion protection.

Bibliography

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

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

Tomashov N. D., Theory of corrosion and protection of metals, M., 1959;

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

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