Corrosion resistance. Corrosion resistance of metals

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Corrosion resistance- the ability of materials to resist corrosion, determined by the rate of corrosion under given conditions. Both qualitative and quantitative characteristics are used to assess the corrosion rate. Change appearance metal surface, changes in its microstructure are examples of a qualitative assessment of the corrosion rate. For quantification, you can use:

the time elapsed before the appearance of the first corrosion focus;
the number of corrosion centers formed over a certain period of time;
decrease in the thickness of the material per unit of time;
change in the mass of metal per unit surface per unit time;
the volume of gas released (or absorbed) during the corrosion of a surface unit per unit of time;
current density corresponding to the rate of the given corrosion process;
change in some property over a certain time of corrosion (for example, electrical resistance, reflectivity of the material, mechanical properties).

Different materials have different corrosion resistance, to improve which special methods are used. Thus, an increase in corrosion resistance is possible by alloying (for example, stainless steels), applying protective coatings (chrome plating, nickel plating, aluminizing, zinc plating, product painting), passivation, etc. The resistance of materials to corrosion, typical for marine conditions, is studied in salt fog chambers.

Sources

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See what "Corrosion resistance" is in other dictionaries:

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The ability of materials to resist corrosion. For metals and alloys, it is determined by the corrosion rate, i.e., by the mass of material converted into corrosion products, from a surface unit per unit of time, or by the thickness of the destroyed layer in mm per year. ... ... Big encyclopedic Dictionary

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corrosion resistance- atsparumas korozijai statusas T sritis Standartizacija ir metrologija apibrėžtis Metalo gebėjimas priešintis korozinės aplinkos poveikiui. atitikmenys: engl. corrosion resistance vok. Korrosionswiderstand, m; Rostbeständigkeit, f; … Penkiakalbis aiskinamasis metrologijos terminų žodynas

corrosion resistance- korozinis atsparumas statusas T sritis chemija apibrėžtis Metalo atsparumas aplinkos medžiagų poveikiui. atitikmenys: engl. corrosion resistance. corrosion resistance... Chemijos terminų aiskinamasis žodynas

corrosion resistance- the ability of a material, such as metals and alloys, to resist corrosion in a corrosive environment; assessed by the corrosion rate; See also: resistance chemical resistance relaxation resistance... Encyclopedic Dictionary of Metallurgy

Metals, the ability of a metal or alloy to resist the corrosive effects of an environment. K. s. determined by the corrosion rate under given conditions. The corrosion rate is characterized by qualitative and quantitative indicators. To the first ... ... Great Soviet Encyclopedia

Books

Corrosion resistance of materials in aggressive environments of chemical industries, G. Ya. Vorobieva. The book summarizes data on the properties and corrosion resistance of metallic and non-metallic materials. It provides tables and diagrams of the corrosion resistance of metals and alloys, ...
Corrosion resistance and corrosion protection of metal, powder and composite materials, Vladimir Vasiliev. This manual is devoted to describing the corrosion resistance of the most commonly used in modern technology and technology of structural materials: iron, steels, cast irons, aluminum, ...

Corrosion resistance- the ability of materials to resist corrosion, determined by the rate of corrosion under given conditions.

Both qualitative and quantitative characteristics are used to assess the corrosion rate. A change in the appearance of a metal surface, a change in its microstructure are examples of a qualitative assessment of the corrosion rate.

For quantification, you can use:

the number of corrosion centers formed over a certain period of time;
the time elapsed before the appearance of the first corrosion focus;
change in the mass of metal per unit surface per unit time;
decrease in the thickness of the material per unit of time;
current density corresponding to the rate of the given corrosion process;
the volume of gas released (or absorbed) during the corrosion of a surface unit per unit of time;
change in some property over a certain time of corrosion (for example, electrical resistance, reflectivity of the material, mechanical properties)

Different materials have different corrosion resistance, to improve which special methods are used. An increase in corrosion resistance is possible by alloying (for example, stainless steels), applying protective coatings (chrome plating, nickel plating, aluminizing, zinc plating, painting products), passivation, etc. The resistance of materials to corrosion, typical for marine conditions, is studied in salt fog chambers.

Most mild form corrosive effect is a change in color and loss of gloss, which in principle is hardly noticeable from afar. By refinishing the surface, it is usually possible to return the steel to its former attractive appearance.

smallpox corrosion

smallpox corrosion(pitting corrosion) is a type of corrosive attack caused by chlorides.

Usually, small dots of a dark red color appear first, and only in very difficult cases can they grow to such an extent that corrosion passes into a new stage, continuous surface corrosion. The risk of corrosion increases if foreign materials (lacquer, etc.) remain on the surface after welding, if particles of another corroded metal get on the surface, if the tint color has not been removed after heat treatment.

stress corrosion cracking

stress corrosion cracking- this is the destruction of the metal due to the occurrence and development of cracks with the simultaneous action of tensile stresses and a corrosive environment. It is characterized by the almost complete absence of plastic deformation of the metal.

This type of corrosion appears in environments with a high content of chlorides, for example, in swimming pools.

crevice corrosion

crevice corrosion- occurs at junctions due to design or operational requirements.

The degree of corrosion attack will be influenced by the geometry of the joint and the type of materials in contact. The most dangerous are narrow joints with small gaps and the connection of steel with plastics. If it is not possible to avoid joints, then we recommend using stainless steels alloyed with molybdenum.

Intergranular corrosion

Intergranular corrosion- this type of corrosion currently occurs on steels after sensitization in combination with use in acidic environments.

During sensitization, chromium carbides are released, which accumulate along the grain boundaries. Accordingly, there are areas with a low content of chromium and more corroded. This happens, for example, during welding in the heat affected zone.

All austenitic steels are resistant to intergranular corrosion. They can be welded (sheet up to 6 mm, rod up to 40 mm) without the risk of ICC.

Bimetallic or galvanic corrosion

Bimetallic corrosion- occurs during the operation of a bimetallic corrosion element, i.e. a galvanic cell in which the electrodes are made of different materials.

Very often it is necessary to use inhomogeneous materials, whose mating under certain conditions can lead to corrosion. When two metals are paired, bimetallic corrosion is of galvanic origin. In this type of corrosion, the less alloyed metal suffers, which under normal conditions, not being in contact with a more alloyed metal, is not subject to corrosion. The consequence of bimetallic corrosion is at least a change in color and, for example, loss of tightness of pipelines or failure of fasteners. Ultimately, these problems can lead to a sharp reduction in the life of the structure and the need for premature overhaul. In the case of stainless steels, less alloyed metal is exposed to bimetallic corrosion.

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Basic concepts, terms, definitions

Corrosion resistance - the ability of a material to withstand the action of aggressive environments (corrosion).

Corrosion (from lat. sorrosio - corrosion) - the destruction of materials due to chemical or electrochemical interaction with the environment.

Building materials, and primarily their surfaces, during long-term operation are destroyed mainly as a result of two types of impact: corrosive, associated with the influence of an external, aggressive environment on the material, and erosive, caused by mechanical action.

Erosive destruction proceeds intensively with a relatively fast movement of the medium or material. Erosion reaches a particularly large value when the material comes into contact with melted metals and slags, as well as with gaseous oxidizers, etc.

The phenomena of corrosion and erosion often accompany each other, and therefore it is not always possible to separate them. In building materials science, these phenomena are considered separately. Erosion processes are considered when studying the performance properties of floor coverings, road surfaces, etc.

Types of corrosion building materials

Corrosion of building materials differs in the type of corrosive environment, the nature of destruction and the processes occurring in them:

corrosive environment:

gas: (inert gas; reactive gas);

liquid: (acid; saline; alkaline, marine; river; in the melt of metals, silicates)

nature of destruction: (uniform, saline, uneven, selective, surface, cracking, local, intergranular);

types of influences (processes):(chemical; electrochemical; biological).

Gas corrosion is corrosion in a gaseous environment in the complete absence of moisture condensation on the surface of the material. Materials operating at high temperatures in a dry gas environment (ceramics) are susceptible to this type of corrosion. Gas corrosion refers to chemical destruction processes. Its speed depends on the nature of the material, its structure and the properties of neoplasms on its surface.

Liquid corrosion natural and artificial stone materials, occurring under the action of electrolyte and non-electrolyte solutions, as well as various melts, is mainly chemical in nature, although, depending on the type and properties of the liquid, it differs in a number of features. The most important feature of liquids is the presence of forces of intermolecular interaction in them. This is due to two properties of the liquid state: molecular pressure and the surface tension associated with it. The surface tension of a liquid has a great influence on the intensity of material destruction, which is also determined by the wetting properties of the liquid.

uniform corrosion occurs as a result of the action of an aggressive environment with a sufficient thickness of the product and a uniform distribution of compressive, bending or tensile stresses. Corrosion of this type, unlike others, is significantly lesser degree affects the strength properties of the material.

Uneven or localized corrosion(spots, ulcers, streaks) occurs at different concentrations of the aggressive medium in individual areas or the heterogeneity of the material itself (its composition and structure). So, as a result of the uneven distribution of the crystalline and vitreous phases in the ceramic material corrosive destruction in its individual sections proceeds at different speeds. In this case, the process develops much faster in the glassy phase than in the crystalline phase. The presence of heterogeneous porosity in the material also contributes to the formation of uneven corrosion in it.

Selective corrosion characteristic of materials in which one of the components during the formation of the structure forms easily soluble compounds. During operation, these compounds can go into solution, forming the so-called "efflorescence" on the surface of the material.

Intergranular corrosion It arises as a result of the destruction of the material along the grain boundaries and quickly spreads into the depth of the material, sharply reducing its properties. This type of corrosion is inherent in some firing materials, during the sintering of which new phases, solid solutions, etc. are formed, and, consequently, interfaces.

Corrosion action in the general case can have two fundamentally different mechanisms: chemical interaction and dissolution.

Chemical interaction is reduced to a reaction between the medium and the material with the formation of new compounds. In the presence of impurities in aggressive media, and in the material - additives, chemical reactions can occur between all elements of interaction. Since stone materials are dielectrics and their interaction with an aggressive environment is not accompanied by the appearance of electric currents, the process of destruction of materials is called chemical corrosion.

When metals are exposed to corrosive media, an electrochemical process of electron transfer from a metal layer with a lower electric potential to a layer with a higher potential takes place and electropositive ions are restored, followed by destruction of the surface layer. This destruction process is called electrochemical corrosion.

biological corrosion- the destruction of the material under the direct influence of plant and animal organisms, as well as microorganisms. Higher plant organisms (root system, stems, leaves, seeds, etc.) produce different kinds substances, most of which are aggressive in relation to building materials. Animal organisms cause biodamage of materials both directly by their mechanical action (rodents, birds, etc.) and by the products of their vital activity. Lower plant organisms and microorganisms (algae, lichens, mosses, fungi, bacteria, etc.) destroy the surface layers of concrete and create conditions for decay of wood structures.

Corrosion resulting from the impact on building materials of products of technological processing of organic substances, both biogenic (fruits, vegetables, vegetable oils, blood, juices, fats, etc.), and non-biogenic origin (oil, coal, shale, shell limestone, exhaust gases, soot, etc.), is commonly called organogenic corrosion.

Factors affecting the corrosion resistance of building materials

Corrosion resistance of building materials depends on many factors, which are divided into external and internal.

External factors determine the aggressiveness of the medium and its effect on the material. These include the pH of the medium, the temperature and its difference, as well as the intensity of the impact of the medium on the material.

The pH of the electrolyte solution, which characterizes the activity of hydrogen ions in it, is very an important factor affecting the process chemical corrosion. The corrosion rate of silicates in electrolyte solutions largely depends on the nature of the solutions and proceeds differently in acidic, alkaline, or neutral media.

Water as a participant technological process It is considered in two aspects: as a neutral component, which serves to give the mixture the necessary properties, and as a solvent and ion carrier.

The reason for the corrosion of many building materials in water or in other electrolytes is the thermodynamic instability of the compounds contained in these materials, which is associated with the development of hydration processes accompanied by exothermic or endothermic effects.

The exothermic effect indicates a creative process in the material, for example, during the hydration of cement, and the endothermic effect indicates a destructive process, for example, during the hydration of a ceramic shard.

Behavior chemical elements in solutions largely depends on the size of the radii of the ions and their valence, or rather, on the ratio of the valence of the ion to its radius, called the ionic potential:

where PI - ionic potential, Å-1;

V - valency, units;

R - ionic radius, Å..

The lower the ionic potential, the stronger the basic properties of the elements are manifested, the larger it is - acidic. For example, K and Na, characterized by low ionic potentials, respectively 0.75 and 1.02, have pronounced alkaline properties. Elements with an ionic potential in the range of 4.7 ... 8.6 have amphoteric properties, and at pH> 8.6 acidic properties

Comparing the activity of elements by ionic potential, we obtain the following distribution of cations in descending order:

SiO2 → TiO2 → MgO → Fe → Cu

The high ionic potential of the silicon cation causes the formation of strong anionic groups with oxygen ions.

Temperature- one of the most important variables affecting corrosion and erosion resistance. An increase in temperature, as a rule, contributes to an increase in the corrosion effect due to an increase in the limiting solubility, diffusion rate and intensity chemical reactions.

temperature fluctuations thermal mass transfer is caused in the system, which can make it unsuitable to use a material that under normal conditions has little solubility.

The intensity of the impact of the environment affects the rate of corrosion processes. Increasing the volume of media in contact with the material can increase the corrosive effect by increasing average speed material dissolution.

Internal factors - this is the composition, structure of the material and its properties.

Due to the structural features various materials influence on them external factors is not the same, and therefore the corrosion resistance of roasted, fused, hydration materials, as well as metals and wood, is considered separately. And we will start studying the properties of specific materials with the next lecture.

General principles for improving corrosion resistance

Corrosion resistance is determined by the mass of the material converted into corrosion products per unit time per unit area, which is in interaction with an aggressive environment, as well as the size of the destroyed layer in mm per year.

The main principles of increasing the corrosion resistance of building products and structures are:

Selection of the composition of the compositions, characterized by low activity in aggressive environments;

The use of special coatings for chemical, thermal and mechanical protection of products and structures from the effects of aggressive environments.

It should be noted that the main criterion that determines the performance properties of building materials is time. Therefore, such characteristics of the material as water resistance, frost resistance and corrosion resistance are not true. physical properties, but only conditional indicators of changes in the state of its structure during prolonged constant or cyclic exposure of the material to an aggressive environment.

Maintaining performance over time is commonly referred to as durability of building materials .

1. Basic concepts, terms and definitions

Corrosion resistance- the ability of the material to withstand the action of aggressive environments (corrosion).

Corrosion (dt lat, corrosio - corrosive) - the destruction of materials due to chemical or electrochemical interaction with the environment.

Erosive destruction proceeds intensively at a relatively fast
rum moving a medium or material. Erosion reaches a particularly large value when the material comes into contact with melted metals and slags, as well as with gaseous oxidizers, etc.

2. Types of corrosion of building materials

Corrosion of building materials differs according to the type of corrosive environment, the nature of destruction and the processes occurring in them:

Corrosive environment:

Gas

inert gas;

Reactive gas;

Liquid:

Acid;

Salt;

alkaline;

Marine;

in melt:

metals;

silicates;

2) the nature of the destruction:

Uniform;

Uneven:

Selective;

superficial;

Cracking;

local;

Intergranular;

3) types of impacts (processes);

Chemical;

Electrochemical;

biological;

Organogenic.

Types of corrosive environment:

Gas corrosion refers to chemical destruction processes. Its speed depends; on the nature of the material, its structure and properties of neoplasms on its surface.

Liquid corrosion natural and artificial stone materials, occurring under the action of solutions of electrolytes and non-electrolytes, as well as various melts, is mainly chemical in nature, although, depending on the type and properties of the liquid, it differs in a number of features.

The most important feature of liquids is the presence of forces of intermolecular interaction in them. This is due to two properties of the liquid state: molecular pressure and the surface tension associated with it.

The surface tension of a liquid has a great influence on the intensity of material destruction, which is also determined by the wetting properties of the liquid.

The nature of the destruction:

uniform corrosion occurs as a result of the action of an aggressive environment with a sufficient thickness of the product and a uniform distribution of compressive, bending or tensile stresses. Corrosion of this type, unlike others, affects the strength properties of the material to a much lesser extent.

Uneven or localized corrosion(spots, ulcers, streaks) occurs at different concentrations of the aggressive environment in certain areas or the heterogeneity of the material itself (its composition and structure). Thus, as a result of the uneven distribution of the crystalline and vitreous phases in the ceramic material, corrosion destruction in its individual sections proceeds at different rates. In this case, the process develops much faster in the glassy phase than in the crystalline phase. The presence of inhomogeneous porosity in the material also contributes to the formation of uneven corrosion in the material.

Selective corrosion characteristic of materials in which one of the components during the formation of the structure forms easily soluble compounds. During operation, these compounds can go into solution, forming the so-called "efflorescence" on the surface of the material.

Intergranular corrosion It arises as a result of the destruction of the material along the grain boundaries and quickly spreads deep into the material, sharply reducing its properties. This type of corrosion is inherent in some firing materials, during the sintering of which new phases, solid solutions, etc. are formed, and, consequently, interfaces.

Corrosion action in the general case can have two fundamentally different mechanisms: chemical interaction and dissolution.

Chemical interaction is reduced to the reaction between the medium and the material with the formation of new compounds. In the presence of impurities in aggressive media, and in the material - additives, chemical reactions can occur between all elements of interaction.

Since stone materials are dielectrics and their interaction with an aggressive environment is not accompanied by the appearance of electric currents, the process of destruction of materials is called chemical corrosion.

Under the influence of corrosive media on metals, an electrochemical process of electron transfer from a metal layer with a lower electric potential to a layer with a higher potential takes place and electropositive ions are restored with subsequent destruction of the surface layer. This destruction process is called electrochemical corrosion.

biological corrosion- the destruction of the material under the direct influence of plant and animal organisms, as well as microorganisms.

1. Higher plant organisms (root system, stems, leaves, seeds, etc.) in the process of life produce various types of substances, most of which are aggressive in relation to building materials.

2. Living organisms cause biodamage of materials both directly by their mechanical action (rodents, birds, etc.) and by the products of their vital activity.

3. Lower plant organisms and microorganisms (algae, lichens, mosses, fungi, bacteria, etc.) destroy the surface layers of concrete and create conditions for decay of wood structures.

Lab #8

The purpose of the work: familiarization with the mechanisms and rates of corrosion destruction of metals.

1. Guidelines

Corrosion destruction of metals is a spontaneous transition of a metal to a more stable oxidized state under the action of environment. Depending on the nature of the environment, chemical, electrochemical and biocorrosion are distinguished.

Electrochemical corrosion is the most common type of corrosion. Corrosion of metal structures in natural conditions - in the sea, in the ground, in groundwater, under condensation or adsorption films of moisture (in atmospheric conditions) is of an electrochemical nature. Electrochemical corrosion is the destruction of a metal, accompanied by the appearance of an electric current as a result of the work of many macro- and microgalvanic pairs. The mechanism of electrical corrosion is divided into two independent processes:

1) anodic process - the transition of a metal into a solution in the form of hydrated ions, leaving an equivalent amount of electrons in the metal:

(-)A: Me + mH 2 O → 1+ + ne

2) the cathode process is the assimilation of excess electrons in the metal by some depolarizers (molecules or ions of the solution that can be reduced at the cathode). During corrosion in neutral media, the depolarizer is usually corrosion into oxygen dissolved in the electrolyte:

(+)K: O 2 + 4e +2H 2 O →4OH¯

During corrosion in acidic environments - hydrogen ion

(+)K: H H 2 O + e → 1/2H 2 +H 2 O

Macrogalvanic pairs are produced when different metals come into contact. In this case, the metal having a more negative electrode potential is the anode and undergoes oxidation (corrosion).

The metal with the more positive potential serves as the cathode. It acts as a conductor of electrons from the anode metal to the particles of the environment that are capable of receiving these electrons. According to the theory of microcouples, the cause of electrochemical corrosion of metals is the presence on their surface of microscopic short-circuited galvanic cells that arise due to the heterogeneity of the metal and its contact with the environment. Unlike galvanic cells specially made in the technique, they spontaneously appear on the metal surface. O 2 , CO 2 , SO 2 and other gases from the air are dissolved in a thin layer of moisture that always exists on the metal surface. This creates conditions for the contact of the metal with the electrolyte.

On the other hand, various areas of the surface given metal have different potentials. The reasons for this are numerous, for example, the potential difference between differently processed parts of the surface, different structural components of the alloy, impurities and the base metal.

Areas of the figurative surface with a more negative potential become anodes and dissolve (corrode) (Figure 1.1).

Part of the released electrons will pass from the anode to the cathode. The polarization of the electrodes, however, prevents corrosion, since the electrons remaining on the anode form a double electric layer with the positive ions that have passed into the solution, and the dissolution of the metal stops. Therefore, electrical corrosion can occur if electrons from the anode sites are continuously withdrawn at the cathode and then removed from the cathode sites. The process of removing electrons from the cathode sites is called depolarization, and the substances or ions that cause depolarization are called depolarizers. If there is a contact of any metal with an alloy, the alloy acquires a potential corresponding to the potential of the most negative metal in its composition. When brass (an alloy of copper and zinc) comes into contact with iron, brass will begin to corrode (due to the presence of zinc in it). When the medium changes, the electrode potential of individual metals can change dramatically. Chromium, nickel, titanium, aluminum, and other metals whose normal electrode potential is sharply negative, are strongly passivated under normal atmospheric conditions, covered with an oxide film, as a result of which their potential becomes positive. In atmospheric conditions and fresh water, the following will work galvanic cell:

(-) Fe | H 2 O, O 2 | Al 2 O 3 (Al) +

(-)A: 2Fe - 4e = 2Fe 2+

(+)K: O 2 + 4e + 2H 2 O \u003d 4OH¯

As a result: 2Fe 2 + 4OH¯ \u003d 2Fe (OH) 2

4Fe(OH) 2 + O 2 + 2H 2 O = 2Fe(OH) 3

However, in an acidic, alkaline or neutral environment containing chlorine ions (for example, in sea water), which destroy the oxide film, aluminum in contact with iron becomes an anode and undergoes a corrosion process. The following galvanic cell will work in NaCl solution and sea water:

(-) Al | H 2 O, O 2 , NaCl | Fe(+)

(-)A: Al - 3e = Al 3+

(+)K: O 2 +4e + 2H 2 O \u003d 4OH¯

4Al 3 + 12OH¯ \u003d 4Al (OH) 3

Very often, electrochemical corrosion occurs as a result of different aeration, that is, unequal access of air oxygen to individual sections of the metal surface. In Fig.1.2. a case of corrosion of iron and a drop of ox is depicted. Near the edges of the drop, where it is easier for oxygen to penetrate, cathode regions appear, and in the center, where the thickness of the protective water layer is greater and it is more difficult for oxygen to penetrate the anode region.

The occurrence of corrosive galvanic cells is influenced by the difference in the concentration of the dissolved electrolyte, the difference in temperature and illumination, and other physical conditions.

Corrosion protection

The reasons causing corrosion destruction of metals are numerous. There are various methods of protection against corrosion:

processing of the external environment;

protective coatings;

electrochemical protection;

production of specially corrosion-resistant alloys.

Treatment of the external environment is to remove or reduce the activity of some of the corrosive substances present in it. For example, the removal of oxygen dissolved in iodine (deaeration). Sometimes special corrosion-retarding substances are added to the solution, which are called retarders or INHIBITORS (urotropine, thiourea, aniline, and others).

Parts that are protected in atmospheric conditions are placed together with inhibitors in a container or wrapped in paper, the inner layer, which is impregnated with an inhibitor, and the outer layer, with paraffin. The inhibitor, evaporating, is adsorbed on the surface of the part, causing the inhibition of electrode processes.

The role of protective coatings is reduced to isolating the metal from the effects of a protective environment. This is achieved by applying varnishes, paints, metal coatings to the metal surface.

Metal coatings are divided into anodic and cathodic. In the case of ANODE coating, the electrode potential of the coating metal is more negative than the potential of the protected metal. In the case of a CATHODE coating, the electrode potential of the coating metal is more positive than that of the base metal.

Bye protective layer completely isolates the base metal from the environment, there is no fundamental difference between the anode and cathode coatings. When the integrity of the coating is violated, new conditions arise. A cathodic coating, for example, tin on iron, not only ceases to protect the base metal, but also enhances the corrosion of iron by its presence (in the resulting galvanic cell, iron is the anode).

At electrochemical protection reduction or complete cessation of corrosion is achieved by creating a high electronegative potential on the protected metal product. To do this, the product to be protected is either connected to a metal that has a more negative electrode potential, capable of more easily giving up electrons (protective protection) or with the negative pole of an external current source (cathodic electrical protection).

An anode coating, for example, zinc on iron, on the contrary, if the integrity of the coating layer is violated, will itself be destroyed, thereby protecting the base metal from corrosion (zinc is the anode in the resulting galvanic cell).

Production of special corrosion-resistant alloys, stainless steels, etc. is reduced to the introduction of additives of various metals into them.

These additives affect the microstructure of the alloy and contribute to the emergence in it of such microgalvanic cells, in which the total EMF approaches zero due to mutual compensation. Such useful additives, especially for steel, are chromium, nickel and other metals.

1. Getting the job done

Exercise 1

Carrying out high-quality chemical reactions that make it possible to detect metal ions that have passed into solution during the anodic corrosion process.

Instruments and reagents: solutions of ZnSO 4 , FeSO 4 and K 3 , a set of test tubes.

Progress of work: Pour 1-2 ml of salt solution into test tubes:

a) ZnSO 4 and a few drops of K 3 ;

b) FeSO and a few drops of K 3 .

Note the precipitation. Write the corresponding reactions in molecular and ionic form.

Task 2

Study of the mechanism of metal corrosion in direct contact in a neutral environment.

The experiment is carried out on the setup shown in Fig. 1.7

Pour 5-10 ml of an aqueous solution of NaCl into a U-shaped tube. Metal plates are lowered into it, interconnected with clamps.

The metal plates must be carefully cleaned with an emery cloth, and the place of contact between the plate and the clamp is out of the solution. When performing the experiment, it is necessary to note the change in the color of the solution at the cathode and anode.

Write:

1) anodic and cathodic corrosion processes

2) the corresponding reactions by which the metal ion was found in solution

3) a diagram of a galvanic cell.

1. Zn and Fe plates are lowered.

In the solution where the zinc electrode is located, add a few drops of K 3, where the iron electrode is located, a few drops of phenolphthalein.

2. Fe and Cu plates are lowered,

In the solution where the iron electrode is located, add a few drops of K 3, where the copper electrode is located, a few drops of phenolphthalein.

Compare the behavior of iron in both cases, draw the appropriate conclusions.

Task 3

Study of the mechanism of corrosion of metals in their direct contact in an acidic environment.

The experiment is carried out on the installation shown in Figure 1.8.

Pour 10% HCl solution into a porcelain cup. Dip two metals Al and Cu into the solution and observe the behavior of the metals. What metal produces hydrogen bubbles? Write appropriate responses. Bring two metals into contact with each other. On which metal do hydrogen bubbles form when the metals come into contact? Draw a diagram of a galvanic cell and electrode processes on its electrodes. Write summary equation reactions.

3. Examples of problem solving

Example 1

Consider the corrosion process in the contact of iron with lead in an HCl solution

In an electrolyte solution (HCl), this system is a galvanic cell, in the internal circuit of which Fe is the anode (E°=0.1260). iron atoms, passing two electrons to lead, go into solution in the form of ions. Electrons on lead restore hydrogen ions that are in solution, tk.

HCl = H + + Cl¯

Anode process Fe 0 - 2e \u003d Fe 2+

Cathodic process 2H + + 2e = 2H 0

Example 2

Corrosion process upon contact of Fe with Ph in NaCl solution. Since the NaCl solution has a neutral reaction (salt formed by a strong base and a strong acid), then

Anode process Fe - 2e \u003d Fe 2+,

Cathodic process O 2 + 4e + 2H 2 O = 4OH¯

Sodium chloride (NaCl) does not participate in corrosion processes; it is shown in the diagram only as a substance capable of increasing the electrical conductivity of the electrolyte solution.

Example 3

Why is chemically pure iron more resistant to corrosion than commercial iron? Compose the electronic equations of the anode and cathode processes occurring during the corrosion of technical iron.

Solution

The process of corrosion of technical iron is accelerated due to the formation of micro and submicrogalvanic elements in it. In microgalvanic pairs, the base metal, as a rule, serves as an anode; iron. The cathodes are inclusions in the metal, for example, grains of graphite, cement. At the anode sites, metal ions go into solution (oxidation).

A: Fe - 2e = Fe 2+

At the cathode sites, the electrons that have passed here from the anode sites are bound either by oxygen in the air dissolved in water, or by hydrogen ions. In neutral environments, oxygen depolarization occurs:

K: O 2 + 4e + 2H 2 O \u003d 4OH¯

In acidic environments (high concentration of H - ions), hydrogen depolarization

K: 2H + + 2e = 2H 0

Example 4

Name, cathodic, or anodic is zinc and a coating on an iron product? What processes will take place if the integrity of the coating is violated and the product is in humid air?

Solution

The electrode potential of zinc is lower in its algebraic value than the electrode potential of iron, so the coating is anodic. In case of violation of the integrity of the zinc layer, a corrosive galvanic couple is formed, in which zinc is the anode, and iron is the cathode. The anodic process consists in the oxidation of zinc:

Zn 2+ + 2OH \u003d Zn (OH) 2

The cathodic process takes place on the iron. In moist air, oxygen depolarization occurs predominantly.

K(Fe): O 2 + 4e + 2H 2 O = 4OH¯

Example 5

Cadmium and nickel plates, being immersed in dilute sulfuric acid, dissolve in it with the release of hydrogen. What will change if both of them are lowered simultaneously into a vessel with acid, connecting the ends with a wire?

Solution

If you connect the ends of the cadmium and nickel plates with a wire, cadmium is formed, a nickel galvanic cell in which cadmium, as a more active metal, is the anode. Cadmium will oxidize:

A: Cd - 2e \u003d Cd 2+,

Excess electrons will go to the nickel plate, where the process of reduction of hydrogen ions will take place:

K(Ni): 2H + 2e =2H 0 .

Thus, only cadmium undergoes dissolution, nickel will only become an electron conductor and will not dissolve itself. Hydrogen will be released only on the nickel plate.

Example 6

How does the pH of the environment affect the rate of aluminum corrosion?

Solution

Reducing the pH of the environment, i.e. an increase in the concentration of H-ions sharply increases the rate of nickel corrosion, - since an acidic environment prevents the formation of protective films of nickel hydroxide, active oxidation of nickel occurs in an acidic environment

A: Ni - 2e = Ni 2+

Reducing the concentration of H-ions, i.e. an increase in OH concentration promotes the formation of a layer of nickel hydroxide:

Ni 2+ - 2OH¯ \u003d NI (OH) 2

Aluminum hydroxide has amphoteric properties, i.e. soluble in acids and alkalis:

Al(OH) 3 + 3HCl = AlCl 3 + 3H 2 O

Al (OH) 3 + NaOH \u003d Na AlO 2 + 2H 2 O

More precisely, this reaction proceeds as follows:

Al(OH) 3 + NaOH = Na

Thus, the lowest corrosion rate of nickel is in an alkaline environment, aluminum - in a neutral one.

4. Tasks

1. An iron plate immersed in hydrochloric acid releases hydrogen very slowly, but if you touch it with a zinc wire, it is immediately covered with hydrogen bubbles. Explain this phenomenon. What metal goes into solution in this case?

2. There are nickel parts in the iron product. How will this affect the corrosion of iron? Write the corresponding anodic and cathodic processes if the product is in a humid atmosphere.

3. In what medium is the rate of iron destruction greater? What environment favors the anodic oxidation of zinc? Write appropriate responses.

4. How does atmospheric corrosion of tinned iron and tinned copper occur when the integrity of the coating is violated? Compose the electronic equations of the anode and cathode processes.

5. Copper does not displace hydrogen from dilute acids. Why? However, if a zinc plate is touched to a copper plate, then a rapid evolution of hydrogen begins on the copper. Explain this by writing the electronic equations for the cathode and anode processes.

6. A zinc plate and a zinc plate partially covered with copper were lowered into an electrolyte solution containing dissolved oxygen. In which case does the zinc corrosion process occur more intensively? Compose the electronic equations of the cathode and anode processes.

7. What can happen if a product in which technical iron is in contact with copper is left in the air at high humidity? Write the equations of the corresponding processes.

8. Aluminum riveted with iron. Which metal will corrode? What processes will take place if the product gets into sea water?

9. Why, when iron products come into contact with aluminum, do iron products undergo more intense corrosion, although aluminum has a more negative standard electrode potential?

10. Iron plates omitted:

a) distilled water

b) sea water

In which case is the corrosion process more intense? Motivate your answer.

11. Make the equations of the processes occurring during the corrosion of aluminum immersed in a solution:

a) acids

b) alkalis

12. Why does industrial zinc interact with acid more intensively than chemically pure zinc?

13. A plate is lowered into the electrolyte solution:

b) copper, partially covered with tin

In which case is the corrosion process more intense?

Motivate the answer

14. Why, when iron products are nickel-plated, are they coated first with copper and then with nickel?

Compose the electronic equations for the reactions that occur in the corrosion processes when the nickel coating is damaged.

15. An iron product was coated with cadmium. What kind of coating is it - anode or cathode?

Motivate your answer. What metal will corrode if the protective layer is damaged? Compose the electronic equations of the corresponding processes (neutral medium).

16. Which of the metals:

b) cobalt

c) magnesium

can be a protector to an iron-based alloy. Compose the electronic equations of the corresponding processes (acid medium).

17. What processes will occur on zinc and iron plates if each is immersed separately in a solution of copper sulphate? What processes will take place if the outer ends in the solution of the plates are connected with a conductor? Write electronic equations

18. Aluminum plate lowered

a) distilled water

b) in a solution of sodium chloride

In which case is the corrosion process more intense? Make equations for the anodic and cathodic corrosion processes of technical aluminum in a neutral environment.

19. If a nail is driven into a damp tree, then the part that is inside the tree is covered with rust. How can this be explained? Is this part of the nail anode or cathode?

20. In Lately cobalt was coated with other metals to protect against corrosion. Is cobalt coated steel anodic or cathodic? What processes take place in moist air when the integrity of the coating is violated?