Types of metal corrosion chemistry. General information about metal corrosion
Corrosion– spontaneous oxidation of metals, harmful to industrial practice (reducing the durability of products). This word comes from Latin corrodere- corrode. The environment in which a metal corrodes (corrodes) is called corrosive or aggressive. In this case, corrosion products are formed: chemical compounds containing metal in oxidized form. In cases where metal oxidation is necessary to carry out any technological process, the term “corrosion” should not be used. For example, we cannot talk about corrosion of a soluble anode in a galvanic bath, since the anode must oxidize, sending its ions into the solution, for the desired process to occur. It is also impossible to talk about corrosion of aluminum during the aluminothermic process. But the physical and chemical essence of the changes occurring with the metal in all such cases is the same: the metal is oxidized. Consequently, the term “corrosion” has not so much a scientific as an engineering meaning. It would be more correct to use the term "oxidation" regardless of whether it is harmful or beneficial to our practice. In the standardization system (GOST 5272-68), metal corrosion is defined as the destruction of metals due to their chemical and electrochemical interaction with a corrosive environment. In the ISO system ( international standardization) this concept is somewhat broader: a physical and chemical interaction between a metal and the environment, as a result of which the properties of the metal change, and often the functional characteristics of the metal, the environment or the technical system that includes them deteriorate.
Objects affected by corrosion– metals, alloys (solid solutions), metal coatings, metal structures of machines, equipment and structures. The corrosion process is represented as a corrosion system consisting of metal and a corrosive environment. A corrosive environment contains one or more substances that react with the metal. It can be liquid or gaseous. A gaseous medium that oxidizes a metal is called oxidizing gas environment. A change in any part of a corrosion system caused by corrosion is called corrosive effect. Corrosive effect, worsening functional characteristics metal, coating, medium or including them technical systems, are regarded as damage effect or how corrosive damage(according to the ISO system). As a result of corrosion, new substances are formed, including oxides and salts of the corroding metal, these are - corrosion products. Visible products of atmospheric corrosion, consisting mainly of hydrated iron oxides, are called rust, gas corrosion products – scale. The amount of metal converted into corrosion products over a certain time is referred to as corrosion losses. Corrosion losses per unit of metal surface per unit of time characterize corrosion rate. The effect of damage associated with loss of mechanical strength of the metal is defined by the term - corrosion damage, its depth per unit time is called corrosion penetration rate. The most important concept is corrosion resistance. It characterizes the ability of a metal to resist the corrosive effects of the environment. Corrosion resistance is determined qualitatively and quantitatively - by the rate of corrosion under given conditions, by a group or resistance score on an accepted scale, using optical instruments. Metals with high corrosion resistance are called corrosion resistant. Factors influencing the rate, type, distribution of corrosion and related to the nature of the metal (composition, structure, internal stresses, surface condition) are called internal factors corrosion. Factors influencing the same corrosion parameters, but related to the composition of the corrosive medium and process conditions (temperature, humidity, medium exchange, pressure, etc.) are called external corrosion factors. In some cases, it is advisable to divide corrosion factors in accordance with Table 4.
Table 4
Corrosion factors
2. Classification of metal corrosion processes
It is customary to classify corrosion according to the mechanism, conditions of the process and the nature of destruction. According to the mechanism of occurrence, corrosion processes, according to GOST 5272-68, are divided into two types: electrochemical And chemical. Electrochemical corrosion includes the process of interaction of a metal with a corrosive environment, in which the ionization of metal atoms and the reduction of oxidizing agents of the environment occur in more than one act and depend on the electronic potential (the presence of conductors of the second type). Let's consider several types of electrochemical corrosion:
1) atmospheric– characterizes the process in humid air conditions. This is the most common type of corrosion, since most structures are operated in atmospheric conditions. It can be divided as follows: in the open air, with the possibility of precipitation falling on the surface of the vehicle, or with protection from it in conditions of limited air access and in a confined air space;
2) underground– destruction of metal in soils and soils. A type of this corrosion is electrochemical corrosion under the influence of stray currents. The latter arise in the ground near sources of electric current (electricity transmission systems, electrified transport routes);
3) liquid corrosion, or corrosion in electrolytes. Its special case is underwater corrosion– destruction of metal structures immersed in water. According to the operating conditions of metal structures, this type is divided into corrosion during complete and partial immersion; in case of partial immersion, the process of corrosion along the waterline is considered. Aquatic environments may differ in corrosive activity depending on the nature of the substances dissolved in them (sea, river water, acidic and alkaline solutions chemical industry and so on.). With underwater corrosion, corrosion processes of equipment are possible in non-aqueous liquid media, which are divided into non-electrically conductive and electrically conductive. Such environments are specific to chemical, petrochemical and other industries. Chemical corrosion refers to a process in which the oxidation of the metal and the reduction of the environment represent a single act (the absence of conductors of the second type). Chemical corrosion– is the destruction of metals in oxidizing environments during high temperatures Oh. There are two types: gas(i.e. oxidation of the metal when heated) and corrosion in non-electrolytes:
a) a characteristic feature of gas corrosion is the absence of moisture on the metal surface. The rate of gas corrosion is influenced primarily by the temperature and composition of the gaseous medium. In industry, cases of this corrosion are often encountered: from the destruction of parts of heating furnaces to corrosion of metal during heat treatment.
b) corrosion of metals in non-electrolytes, regardless of their nature, comes down to a chemical reaction between the metal and the substance. Organic liquids are used as non-electrolytes.
IN special group It is necessary to distinguish types of corrosion under conditions of exposure to mechanical stress (mechanical corrosion). This group includes: actual stress corrosion, characterized by the destruction of metal under simultaneous exposure to a corrosive environment and constant or variable mechanical stresses; corrosion cracking– with simultaneous exposure to a corrosive environment and external or internal mechanical tensile stresses with the formation of transgranular cracks.
Distinguish independent species corrosion:
1) friction corrosion– metal destruction caused by the simultaneous influence of a corrosive environment and friction;
2) fretting corrosion– destruction during oscillatory movement of two surfaces relative to each other under exposure to a corrosive environment;
3) corrosion cavitation– destruction due to impact of the environment;
4) corrosion erosion– under abrasive influence of the environment;
5) contact corrosion– destruction of one of two metals that are in contact and have different potentials in a given electrolyte.
A distinction must be made between corrosion and erosion. Erosion O Latin word erodere(destroy) - gradual mechanical destruction of metal, for example, during abrasion of rubbing parts of mechanisms.
An independent type of corrosion - biocorrosion– this is the destruction of metal, in which a biofactor acts as a significant factor. Bioagents– microorganisms (fungi, bacteria) that initiate or stimulate the corrosion process.
According to the nature of destruction, corrosion is divided into continuous (or general) and local (local). Continuous corrosion covers the entire surface of the metal, and it can be uniform or uneven. Local corrosion occurs with the destruction of individual areas of the metal surface. The types of this corrosion are: pitting, spot corrosion and through corrosion.
Subsurface corrosion begins at the surface, but develops primarily below it in such a way that the corrosion products are concentrated inside the metal. Its variety is layer-by-layer corrosion, propagating predominantly in the direction of plastic deformation of the metal.
Structural corrosion is associated with the structural heterogeneity of the metal. Its variety is intergranular– destruction of the metal along the boundaries of crystallites (grains) of the metal; intragranular– destruction of metal along crystallite grains. It is observed during corrosion cracking that occurs under the influence of external mechanical loads or internal stresses.
Knife corrosion– localized destruction of metal in the fusion zone of welded joints in liquid environments with high corrosive activity.
Crevice corrosion– intensification of the process of metal destruction in the gaps between two metals.
Selective corrosion– destruction of one structural component or one component of the metal in highly active environments. There are a number of varieties: graphitization of cast iron (dissolution of ferritic or pearlite components) and dezincification (dissolution of the zinc component) of brass.
3. Types of corrosion damage
Corrosion, depending on the nature of the metal, the aggressive environment and other conditions, leads to various types of destruction. Figure 13 shows sections through a corroded metal sample, showing possible changes in surface topography as a result of corrosion.
Rice. 11. Schematic illustration various types corrosion: a – uniform corrosion; b – spot corrosion; c, d – corrosion by ulcers; d – pitting (pitting); e – subsurface corrosion; НН – initial metal surface; CC – surface relief changed due to corrosion.
Sometimes corrosion occurs at a uniform rate over the entire surface; in this case, the surface becomes only slightly rougher than the original (a). Different corrosion rates are often observed in individual areas: spots (b), ulcers (c, d). If the ulcers have a small cross-section, but relatively greater depth(e), then they talk about pitting. In some conditions, a small ulcer extends deeper and wider below the surface (e). Uneven corrosion is much more dangerous than uniform corrosion. Uneven corrosion, with a relatively small amount of oxidized metal, causes a large reduction in the cross-section in some places. Pitting or pitting corrosion can lead to the formation of through holes, for example, in sheet material, with little metal loss.
The above classification is, of course, conditional. Numerous forms of failure are possible, falling between the characteristic types shown in this figure.
Some alloys are subject to a peculiar type of corrosion that occurs only along the boundaries of crystallites, which are separated from each other by a thin layer of corrosion products (intercrystalline corrosion). Here the loss of metal is very small, but the alloy loses strength. This is a very dangerous type of corrosion that cannot be detected during an external inspection of the product.
4. Corrosion protection methods
To weaken the corrosion process, it is necessary to influence either the metal itself or the corrosive environment. The main areas for combating corrosion are:
1) alloying the metal, or replacing it with another, more corrosion-resistant one;
2) protective coatings (metallic and non-metallic) of organic or inorganic origin;
3) electrochemical protection; there are cathodic, anodic and sacrificial protection as a variant of cathodic protection.
For example, for atmospheric corrosion, coatings of organic and inorganic origin are used; Electrochemical protection is effective against underground corrosion;
4) introduction of inhibitors (substances that slow down the reaction rate).
Corrosion of metals can manifest itself in various forms, the main ones being:
1. General corrosion, also known as uniform corrosion. General corrosion is the most common type of destruction of metals and is caused by chemical or electrochemical reactions. General corrosion results in deterioration of the entire metal surface, but is considered one of the safest forms of corrosion because it is predictable and controllable.
2. Local (localized) corrosion. Unlike general corrosion, this type of corrosion is focused on one area of the metal structure.
Localized corrosion is classified into three types:
2.1 Pitting: corrosion in the form of a small hole or cavity in metal. It usually occurs as a result of depassivation of a small area of the surface. The affected area becomes the anode and some of the remaining metal becomes the cathode, resulting in localized galvanic reactions. This form of corrosion can often be difficult to detect due to the fact that the affected area is usually relatively small and may be hidden beneath the surface.
2.2 Crevice: Like pitting, crevice corrosion is localized to a specific location. This type of corrosion is often associated with a stagnant micro-zone of aggressive media, such as under gaskets, washers and clamps. An acidic environment, or lack of oxygen in narrow crevices, can lead to this type of corrosion.
2.3 Filament Corrosion: Occurs under painted or metallized surfaces when water or a humid environment disturbs the coating. Filiform corrosion begins with small defects in the coating and spreads, causing structural damage.
3. Galvanic corrosion begins when two different metals are placed together in a corrosive electrolyte environment. A galvanic couple is formed between two metals, one of the metals is the anode, and the other is the cathode. In this case, metal ions move from the anodized material to the cathode metal.
In the presence of an electrochemical effect, the anodic site is destroyed much more severely than the cathode. Without a flow of charged particles, both metals corrode equally. For galvanic corrosion to exist, three conditions must be present: electrochemically dissimilar metals, direct contact of these metals, and exposure to an electrolyte.
4. Destruction of metal from influence environment may be the result of a combination of environmental conditions affecting the material, or from one of the factors. Chemical exposure, temperature and conditions associated with mechanical stress (especially tensile forces) can lead to the following types of corrosion: corrosion fatigue cracking, stress corrosion cracking, hydrogen cracking, liquid metal embrittlement in contact with liquid metal.
5. Erosion-corrosion wear occurs when exposed to aggressive particles and environmental flow, cavitation, as a result of which the protective oxide layer on the metal surface is constantly removed, and the base metal corrodes.
6. Intergranular corrosion is chemical or electrochemical destruction at the grain boundaries of a metal. This phenomenon often occurs due to impurities in the metal, which are usually concentrated at the grain boundaries.
7. Selective leaching (or alloy failure) is the corrosion of one of the elements in the alloy. The most common type is zinc leaching from brass. Corrosion results in porous copper.
8. Frictional corrosion occurs as a result of wear and/or vibration on an uneven, rough surface. As a result, depressions and grooves appear on the surface. Frictional corrosion often occurs in rotating machine parts, in bolt assemblies and bearings, and on surfaces subject to vibration during transportation.
9. High temperature corrosion most often occurs in gas turbines, diesel engines and other machines containing vanadium or sulfates, which can form compounds with a low melting point when burned. These compounds are very corrosive to metal alloys, including stainless steels.
High temperature corrosion can also occur at high temperatures as a result of oxidation, sulfidation and carbonization of the metal.
DEFINITION
When in contact with the environment, many metals, as well as metal-based alloys, can be subject to destruction due to chemical interaction (ORR with substances in the environment). This process is called corrosion.
A distinction is made between corrosion in gases (gas corrosion), which occurs at high temperatures in the absence of moisture on metal surfaces, and electrochemical corrosion (corrosion in electrolyte solutions, as well as corrosion in a humid atmosphere). As a result of gas corrosion, oxide, sulfide, etc. are formed on the surface of metals. films. Furnace fittings, parts of internal combustion engines, etc. are subject to this type of corrosion.
As a result of electrochemical corrosion, metal oxidation can lead to both the formation of insoluble products and the transition of the metal into solution in the form of ions. This type of corrosion affects pipelines located in the ground, underwater parts of ships, etc.
Any electrolyte solution is an aqueous solution, and water contains oxygen and hydrogen that are capable of reduction:
O 2 + 4H + +4e = 2H 2 O (1)
2H + +2e=H 2 (2)
These elements are oxidizing agents that cause electrochemical corrosion.
When writing about the processes occurring during electrochemical corrosion, it is important to take into account standard electrode potentials (EP). Thus, in a neutral environment, the EC of process 1 is equal to 0.8B, therefore, metals whose EC is less than 0.8B (metals located in the activity series from its beginning to silver) are subject to oxidation by oxygen.
The EP of process 2 is -0.41V, which means that only those metals whose potential is lower than -0.41V (metals located in the activity series from its beginning to cadmium) are subject to oxidation with hydrogen.
The rate of corrosion is greatly influenced by impurities that a particular metal may contain. Thus, if a metal contains non-metallic impurities, and their EC is higher than the EC of the metal, then the corrosion rate increases significantly.
Types of corrosion
There are several types of corrosion: atmospheric (corrosion in humid air at no.), corrosion in the soil, corrosion due to uneven aeration (oxygen access to different parts metal product in solution is not the same), contact corrosion (contact of 2 metals with different EP in an environment where moisture is present).
During corrosion, electrochemical reactions occur on the electrodes (anode and cathode), which can be written by the corresponding equations. Thus, in an acidic environment, electrochemical corrosion occurs with hydrogen depolarization, i.e. Hydrogen is released at the cathode (1). In a neutral environment, electrochemical corrosion occurs with oxygen depolarization—water is reduced at the cathode (2).
K (cathode) (+): 2H + +2e=H 2 - reduction (1)
A (anode) (-): Me – ne →Me n + – oxidation
K (cathode) (+): O 2 + 2H 2 O + 4e → 4OH - - reduction (2)
In the case of atmospheric corrosion, the following electrochemical reactions occur on the electrodes (and at the cathode, depending on the environment, various processes can occur):
A (anode) (-): Me→Me n + +ne
K (cathode) (+): O 2 + 2H 2 O + 4e → 4OH - (in alkaline and neutral environments)
K (cathode) (+): O 2 + 4H + + 4e → 2H 2 O (in acidic medium)
Corrosion protection
The following methods are used to protect against corrosion: the use of chemically resistant alloys; protection of metal surfaces with coatings, which most often use metals that are coated in air with oxide films that are resistant to external environment; treatment of corrosive environments; electrochemical methods ( cathodic protection, protector method).
Examples of problem solving
EXAMPLE 1
EXAMPLE 2
| Exercise | The part consists of an alloy of iron and nickel. Which metal will corrode faster? Write down the equations of the anodic and cathodic processes during atmospheric corrosion. The values of standard electrode potentials are E(Fe 2+ /Fe) = - 0.444V, E(Ni 2+ /Ni) = -0.250V. |
| Solution | First of all, active metals (those with the most negative values standard electrode potentials), in this case it is iron. Chemical corrosion is a process consisting of the destruction of metal when interacting with an aggressive external environment. The chemical type of corrosion processes has no connection with the effects of electric current. With this type of corrosion, an oxidative reaction occurs, where the destroyed material is at the same time a reducer of environmental elements.
The classification of types of aggressive environments includes two types of metal destruction:
Gas corrosionThe most common type of chemical corrosion, gas corrosion, is a corrosion process that occurs in gases during elevated temperatures. This problem is typical for many types of work. technological equipment and parts (furnace fittings, engines, turbines, etc.). In addition, ultra-high temperatures are used when processing metals under high pressure (heating before rolling, stamping, forging, thermal processes, etc.). The peculiarities of the state of metals at elevated temperatures are determined by two of their properties - heat resistance and heat resistance. Heat resistance is the degree of stability of the mechanical properties of a metal at ultra-high temperatures. Stability of mechanical properties refers to maintaining strength over a long period of time and resistance to creep. Heat resistance is the resistance of a metal to the corrosive activity of gases at elevated temperatures.
The rate of development of gas corrosion is determined by a number of indicators, including:
The corrosion process is more influenced by the properties and parameters of the oxide film that appears on the metal surface. Oxide formation can be chronologically divided into two stages:
The first stage is characterized by the appearance of an ionic bond, as a consequence of the interaction of oxygen and surface atoms, when the oxygen atom takes a pair of electrons from the metal. The resulting bond is exceptionally strong - it is greater than the bond of oxygen with the metal in the oxide. The explanation for this connection lies in the action of the atomic field on oxygen. As soon as the metal surface is filled with an oxidizing agent (and this happens very quickly), at low temperatures, thanks to the van der Waals force, the adsorption of oxidizing molecules begins. The result of the reaction is the appearance of a thin monomolecular film, which becomes thicker over time, complicating the access of oxygen. At the second stage, a chemical reaction occurs, during which the oxidizing element of the medium takes valence electrons from the metal. Chemical corrosion - final result reactions.
Characteristics of the oxide filmThe classification of oxide films includes three types:
However, a truly effective film must meet a number of characteristics:
One of the above conditions - a continuous structure - is especially important. The continuity condition is the excess of the volume of the oxide film molecules over the volume of metal atoms. Continuity is the ability of the oxide to cover the entire metal surface with a continuous layer. If this condition is not met, the film cannot be considered protective. However, there are exceptions to this rule: for some metals, for example, magnesium and alkaline earth elements (except beryllium), continuity is not a critical indicator.
Several techniques are used to determine the thickness of the oxide film. The protective qualities of the film can be determined at the time of its formation. To do this, the rate of metal oxidation and the parameters of the rate change over time are studied. For already formed oxide, another method is used, which consists of studying the thickness and protective characteristics of the film. To do this, a reagent is applied to the surface. Next, experts record the time it takes for the reagent to penetrate, and based on the data obtained, they draw a conclusion about the thickness of the film.
Rate of corrosion developmentThe intensity with which chemical corrosion develops depends on temperature regime. At high temperatures, oxidative processes develop more rapidly. Moreover, reducing the role of the thermodynamic factor in the reaction does not affect the process. Cooling and variable heating are of considerable importance. Due to thermal stress, cracks appear in the oxide film. Through the holes, the oxidizing element reaches the surface. As a result, new layer oxide film, and the previous one peels off.
The components of the gaseous environment also play an important role. This factor is individual for different types of metals and is consistent with temperature fluctuations. For example, copper corrodes quickly if it comes into contact with oxygen, but is resistant to this process in a sulfur oxide environment. For nickel, on the contrary, sulfur oxide is destructive, and stability is observed in oxygen, carbon dioxide and an aqueous environment. But chromium is resistant to all of the above environments.
The rate of the oxidation reaction is also affected by the components of the alloy. For example, manganese, sulfur, nickel and phosphorus do not contribute in any way to the oxidation of iron. But aluminum, silicon and chromium make the process slower. Cobalt, copper, beryllium and titanium slow down the oxidation of iron even more. Additions of vanadium, tungsten and molybdenum will help make the process more intense, which is explained by the fusibility and volatility of these metals. Oxidation reactions occur most slowly with an austenitic structure, since it is most adapted to high temperatures.
Another factor on which the corrosion rate depends is the characteristics of the treated surface. A smooth surface oxidizes more slowly, and an uneven surface oxidizes faster. Corrosion in non-electrolyte liquidsNon-conducting liquid media (i.e. non-electrolyte liquids) include organic substances such as:
In addition, non-electrolyte liquids are not considered a large number of inorganic liquids such as liquid bromine and molten sulfur. It should be noted that organic solvents themselves do not react with metals, however, in the presence of a small volume of impurities, an intensive interaction process occurs. Sulfur-containing elements in oil increase the corrosion rate. Also, high temperatures and the presence of oxygen in the liquid intensify corrosion processes. Moisture intensifies the development of corrosion in accordance with the electromechanical principle. Another factor in the rapid development of corrosion is liquid bromine. At normal temperatures it is especially destructive to high-carbon steels, aluminum and titanium. The effect of bromine on iron and nickel is less significant. Lead, silver, tantalum and platinum show the greatest resistance to liquid bromine.
Molten sulfur reacts aggressively with almost all metals, primarily with lead, tin and copper. Sulfur has less effect on carbon steels and titanium and almost completely destroys aluminum. Protective measures for metal structures located in non-electrically conductive liquid environments are carried out by adding metals that are resistant to a specific environment (for example, steels with a high chromium content). Also, special protective coatings are used (for example, in environments where there is a lot of sulfur, aluminum coatings are used). Methods of protection against corrosionCorrosion control methods include:  The choice of a specific material depends on the potential efficiency (including technological and financial) of its use. Modern principles of metal protection are based on the following methods:
The above methods are divided into two groups:
One of the most commonly used methods of metal protection - galvanic anti-corrosion coating - is not economically profitable for large surface areas. The reason is the high costs of the preparatory process.
The leading place among protection methods is occupied by coating metals with paints and varnishes. The popularity of this method of combating corrosion is due to a combination of several factors:
At the same time, the use of widely available materials is not without its disadvantages:
And yet, the painted surface protects the metal from corrosive processes even with fragmentary damage to the film, while imperfect galvanic coatings can even accelerate corrosion. Organosilicate coatings
Chemical corrosion practically does not apply to organosilicate materials. The reasons for this lie in the increased chemical stability of such compositions, their resistance to light, hydrophobic properties and low water absorption. Organosilicates are also resistant to low temperatures, have good adhesive properties and wear resistance. The problems of metal destruction due to corrosion do not disappear, despite the development of technologies to combat them. The reason is the constant increase in metal production volumes and increasingly difficult operating conditions for products made from them. It is impossible to completely solve the problem at this stage, so the efforts of scientists are focused on finding ways to slow down corrosion processes. physical-chemical or chemical interaction between a metal (alloy) and the environment, leading to deterioration of the functional properties of the metal (alloy), environment or technical system incorporating them.The word corrosion comes from the Latin “corrodo” “to gnaw” (Late Latin “corrosio” means “corrosion”). Corrosion is caused chemical reaction metal with environmental substances flowing at the boundary of the metal and the environment. Most often, this is the oxidation of the metal, for example, by atmospheric oxygen or acids contained in solutions with which the metal is in contact. Metals located in the voltage series (activity series) to the left of hydrogen, including iron, are especially susceptible to this. As a result of corrosion, iron rusts. This process is very complex and includes several stages. It can be described by the summary equation: Fe + 6 H 2 O (moisture) + 3 O 2 (air) = 4 Fe (OH ) 3Iron hydroxide( III ) is very unstable, quickly loses water and turns into iron oxide ( III ). This compound does not protect the iron surface from further oxidation. As a result, the iron object can be completely destroyed.Many metals, including quite active ones (for example, aluminum), when corroded, become covered with a dense, well-bonded oxide film, which does not allow oxidizing agents to penetrate into deeper layers and therefore protects the metal from corrosion. When this film is removed, the metal begins to interact with moisture and oxygen in the air. Aluminum under normal conditions is resistant to air and water, even boiling water, but if mercury is applied to the surface of aluminum, the resulting amalgam destroys the oxide film pushes it from the surface, and the metal quickly turns into white flakes of aluminum metahydroxide: 4Al + 2H 2 O + 3O 2 = 4AlO(OH)Amalgamated aluminum reacts with water to release hydrogen: Al + 4 H 2 O = 2 AlO (OH) + 3 H 2Some rather inactive metals are also susceptible to corrosion. In humid air, the surface of copper becomes covered with a greenish coating (patina) as a result of the formation of a mixture of basic salts. Sometimes when metals corrode, it is not oxidation that occurs, but the reduction of some elements contained in the alloys. For example, at high pressures and temperatures, carbides contained in steels are reduced by hydrogen. The destruction of metals in the presence of hydrogen was discovered in the mid-nineteenth century. The French engineer Sainte-Claire Deville studied the causes of unexpected ruptures of gun barrels. During their chemical analysis, he found hydrogen in the metal. Deville decided that it was hydrogen saturation that was the reason for the sudden drop in the strength of steel. Hydrogen has caused a lot of trouble for designers of equipment for one of the most important industrial chemical processes ammonia synthesis. The first devices for this synthesis lasted only tens of hours, and then shattered into small parts. Only adding titanium, vanadium or molybdenum to steel helped solve this problem. Corrosion of metals can also include their dissolution in liquid molten metals (sodium, lead, bismuth), which are used, in particular, as coolants in nuclear reactors. In terms of stoichiometry, the reactions that describe the corrosion of metals are quite simple, but in terms of their mechanism they belong to complex heterogeneous processes. The corrosion mechanism is determined primarily by the type of aggressive environment. When a metal material comes into contact with a chemically active gas, a film of reaction products appears on its surface. It prevents further contact between metal and gas. If counter diffusion of reacting substances occurs through this film, then the reaction continues. The process is facilitated at high temperatures. During corrosion, the product film continuously thickens and the metal is destroyed. Metallurgy and other industries that use high temperatures suffer heavy losses from gas corrosion. Corrosion is most common in electrolyte environments. In some technological processes metals come into contact with molten electrolytes. However, most often corrosion occurs in electrolyte solutions. The metal does not have to be completely immersed in the liquid. Electrolyte solutions can be present in the form of a thin film on the surface of the metal. They often permeate the environment surrounding the metal (soil, concrete, etc.). During the construction of the metro bridge and the Leninskie Gory station in Moscow, large amounts of sodium chloride were added to the concrete to prevent freezing of the concrete that had not yet set. The station was built in the shortest possible time (in just 15 months) and opened on January 12, 1959. However, the presence of sodium chloride in the concrete caused the destruction of the steel reinforcement. 60% of reinforced concrete structures were subject to corrosion, so the station was closed for reconstruction , lasting almost 10 years. Only on January 14, 2002, the metro bridge and the station, called Vorobyovy Gory, were re-opened. Using salts (usually sodium or calcium chloride) to remove snow and ice from roads and sidewalks also causes metals to degrade faster. Suffer greatly vehicles and underground communications. It is estimated that in the United States alone, the use of salts to combat snowfall and ice leads to losses of about $2 billion per year due to engine corrosion and $0.5 billion in additional repairs of roads, underground highways and bridges. In electrolyte environments, corrosion is caused not only by the action of oxygen, water or acids on metals, but also by electrochemical processes. Already at the beginning of the 19th century. Electrochemical corrosion was studied by English scientists Humphry Davy and Michael Faraday. The first theory of electrochemical corrosion was put forward in 1830 by the Swiss scientist De la Rive. It explained the occurrence of corrosion at the point of contact between two different metals. Electrochemical corrosion leads to the rapid destruction of more active metals, which in various mechanisms and devices come into contact with less active metals located to the right in the electrochemical voltage series. The use of copper or brass parts in iron or aluminum structures that operate in seawater significantly increases corrosion. There are known cases of destruction and sinking of ships whose iron plating was fastened with copper rivets. Individually, aluminum and titanium are resistant to sea water, but if they come into contact in one product, for example, in a box for underwater photographic equipment, the aluminum is very quickly destroyed and the box leaks. Electrochemical processes can also occur in a homogeneous metal. They are activated if there are differences in the composition of the metal grain in the bulk and at the boundary, inhomogeneous mechanical stress, microimpurities, etc. In developing general theory Many of our compatriots participated in electrochemical corrosion of metal materials, including Vladimir Aleksandrovich Kistyakovsky (1865-1952) and Alexander Naumovich Frumkin (1895-1976). One of the reasons for the occurrence of electrochemical corrosion is stray currents, which appear due to the leakage of part of the current from electrical circuits into the soil or aqueous solutions, where they fall on metal structures. Where the current exits these structures, the dissolution of the metal begins again into the soil or water. Such zones of destruction of metals under the influence of stray currents are especially often observed in areas of ground electric transport (tram lines, electric railway transport). These currents can reach several amperes, which leads to large corrosion damage. For example, the passage of a current of 1 A for one year will cause the dissolution of 9.1 kg of iron, 10.7 kg of zinc, 33.4 kg of lead. Corrosion can also occur under the influence of radiation, as well as waste products of bacteria and other organisms. The development of bacteria on the surface of metal structures is associated with the phenomenon of biocorrosion. Fouling of the underwater part of ships with small particles marine organisms also affects corrosion processes. When the metal is simultaneously exposed to the external environment and mechanical stresses, all corrosion processes are activated, since this reduces the thermal stability of the metal, destroys oxide films on the metal surface, and intensifies electrochemical processes in places where cracks and irregularities appear. Corrosion leads to huge irreversible losses of metals; about 10% of the produced iron is completely destroyed every year. According to the Institute of Physical Chemistry of the Russian Academy of Sciences, every sixth blast furnace in Russia works in vain all the smelted metal turns into rust. The destruction of metal structures, agricultural and transport vehicles, and industrial equipment causes downtime, accidents, and deterioration in product quality. Taking into account possible corrosion leads to increased metal costs in the manufacture of devices high pressure, steam boilers, metal containers for toxic and radioactive substances, etc. This increases overall corrosion losses. Considerable amounts of money have to be spent on anti-corrosion protection. The ratio of direct losses, indirect losses and costs for corrosion protection is estimated as (34):1:1. In industrial developed countries Corrosion damage reaches 4% of national income. In our country it amounts to billions of rubles a year. Corrosion problems are constantly getting worse due to the continuous increase in metal production and the tightening of their operating conditions. The environment in which metal structures are used is becoming more and more aggressive, including due to its pollution. Metal products used in technology operate under increasingly high temperatures and pressures, powerful streams gases and liquids. Therefore, the issues of protecting metal materials from corrosion are becoming increasingly relevant. It is impossible to completely prevent metal corrosion, so the only way to combat it is to find ways to slow it down. The problem of protecting metals from corrosion arose almost at the very beginning of their use. People tried to protect metals from atmospheric influences with the help of fat, oils, and later by coating with other metals and, above all, low-melting tin (tinning). In the works of the ancient Greek historian Herodotus (5th century BC) and the ancient Roman scientist Pliny the Elder (1st century BC) there are already references to the use of tin to protect iron from rusting. Currently, the fight against corrosion is being carried out in several directions at once: they are trying to change the environment in which a metal product operates, influence the corrosion resistance of the material itself, and prevent contact between the metal and aggressive substances of the external environment. Corrosion can be completely prevented only in an inert environment, for example, in an argon atmosphere, but in the vast majority of cases it is impossible to actually create such an environment during the operation of structures and mechanisms. In practice, to reduce the corrosive activity of a medium, they try to remove the most reactive components from it, for example, they reduce the acidity of aqueous solutions and soils with which metals may come into contact. One of the methods of combating corrosion of iron and its alloys, copper, brass, zinc, and lead is the removal of oxygen and carbon dioxide from aqueous solutions. In the energy sector and some branches of technology, water is also freed from chlorides, which stimulate local corrosion. To reduce soil acidity, liming is carried out. The aggressiveness of the atmosphere strongly depends on humidity. For any metal there is some critical relative humidity, below which it is not subject to atmospheric corrosion. For iron, copper, nickel, zinc it is 50-70%. Sometimes, to preserve items of historical value, their temperature is artificially maintained above the dew point. In closed spaces (for example, in packaging boxes), humidity is reduced using silica gel or other adsorbents. The aggressiveness of the industrial atmosphere is determined mainly by fuel combustion products ( cm. ENVIRONMENTAL POLLUTION). Reducing losses from corrosion helps to prevent acid rain and elimination of harmful gas emissions. Destruction of metals in aquatic environments can be slowed down using corrosion inhibitors, which are added in small quantities (usually less than 1%) to aqueous solutions. They promote passivation of the metal surface, that is, the formation of a thin and dense film of oxides or other poorly soluble compounds, which prevents the destruction of the main substance. For this purpose, some sodium salts (carbonate, silicate, borate) and other compounds are used. If razor blades are immersed in a solution of potassium chromate, they will last much longer. Organic inhibitors are often used, which are more effective than inorganic ones. One of the methods of corrosion protection is based on the development of new materials that have higher corrosion resistance. The search for substitutes for corrosive metals is ongoing. Plastics, ceramics, glass, rubber, asbestos and concrete are more resistant to environmental influences, but in many other properties they are inferior to metals, which still serve as the main structural materials. Noble metals are practically resistant to corrosion, but for wide application They are too expensive, so they are used only in the most critical parts, such as non-corrosive electrical contacts. Nickel, aluminum, copper, titanium and alloys based on them have high corrosion resistance. Their production is growing quite quickly, but even now the most accessible and widely used metal remains quickly rusting iron. Alloying is often used to impart corrosion resistance to iron-based alloys. This is how stainless steel is obtained, which, in addition to iron, contains chromium and nickel. The most common stainless steel in our time, grade 188 (18% chromium and 8% nickel), appeared in 1923. It is completely resistant to moisture and oxygen. The first tons of stainless steel in our country were smelted in 1924 in Zlatoust. Many grades of steel have now been developed, which, in addition to chromium and nickel, contain manganese, molybdenum, tungsten and others chemical elements. Surface alloying of inexpensive iron alloys with zinc, aluminum, and chromium is often used. To resist atmospheric corrosion, thin coatings of other metals that are more resistant to moisture and atmospheric oxygen are applied to steel products. Chromium and nickel platings are often used. Because chrome platings often contain cracks, they are usually applied over less decorative nickel platings. Protecting tin cans from corrosion by organic acids found in food products requires a significant amount of tin. For a long time for covering kitchen utensils used cadmium, but it is now known that this metal is hazardous to health and cadmium coatings are used only in technology. To slow down corrosion, varnishes and paints, mineral oils and lubricants are applied to the metal surface. Underground structures are covered with a thick layer of bitumen or polyethylene. The internal surfaces of steel pipes and tanks are protected with cheap cement coatings. To make the paintwork more reliable, the metal surface is thoroughly cleaned of dirt and corrosion products and subjected to special treatment. For steel products, so-called rust converters containing orthophosphoric acid (H 3 PO 4) and its salts are used. They dissolve residual oxides and form a dense and durable film of phosphates, which can protect the surface of the product for some time. Then the metal is coated with a primer layer, which should fit well on the surface and have protective properties(usually red lead or zinc chromate is used). Only after this can varnish or paint be applied. One of the most effective methods The fight against corrosion is electrochemical protection. To protect drilling platforms, welded metal bases, and underground pipelines, they are connected as a cathode to an external current source. Auxiliary inert electrodes are used as an anode. Another version of such protection is used for relatively small steel structures or additionally insulated metal objects (for example, pipelines). In this case, a protector is used - an anode made of a relatively active metal (usually magnesium, zinc, aluminum and their alloys), which gradually collapses, protecting the main object. With the help of one magnesium anode, up to 8 km of pipeline is protected. Tread protection widespread; for example, in the USA, about 11.5 thousand tons of aluminum are spent annually on the production of protectors. Protection of one metal by another, more active metal located in the voltage series to the left is effective without imposing a potential difference. The more active metal (for example, zinc on the surface of iron) protects the less active metal from destruction. Electrochemical methods of combating corrosion also include protection against destruction of structures by stray currents. One of the ways to eliminate such corrosion is to connect a metal conductor to the section of the structure from which the stray current flows with the rail along which the tram or electric train moves. Elena Savinkina LITERATURE Fremantle M. Chemistry in action. In 2 parts. M., Mir, 1991Stepin B.D., Alikberova L.Yu. Chemistry book for home reading. M., Chemistry, 1994 Popular Latest articles |
