Corrosion occurs under the influence of chemically aggressive media - water, organic and inorganic acids. As a result, metal oxides form on the surfaces of parts. Corrosion not only spoils appearance surfaces, but also reduces the mechanical properties of metals.
The cause of corrosion is the thermodynamic instability of metals. All metals and alloys from which a car is made tend to transform into a more stable oxidized (ionic) state under operating conditions. The spontaneous transition of the metal to such a stable state is the essence of corrosion.
Many problems that have direct relation to the corrosion resistance of created products can be decided at the stage of their design and manufacturing. For example, if it is ensured that there are no narrow gaps, cracks or pockets in the product, and where this cannot be avoided, drainage holes are installed, then crevice corrosion will be eliminated. It is necessary to exclude the possibility, which is very dangerous in terms of corrosion, of contact between different metals and alloys capable of forming active galvanic couples and stimulating corrosion of one of them.
Losses from corrosion have become comparable to investments in the development of large industries. In the USA, for example, these losses currently significantly exceed $120 billion per year. A considerable part is made up of indirect losses associated with forced equipment downtime, a decrease in the capacity of existing equipment, and deterioration of working conditions. There are known cases where corrosion of vehicles caused serious accidents involving casualties.
Road transport is characterized by the use of aggressive agents, high temperatures and pressures, high flow rates, as well as conditions when products are operated under simultaneous exposure to an aggressive environment and large mechanical loads, i.e. factors promoting corrosion.
Due to corrosion, a large amount of metal is lost, the replenishment of which in the automotive industry consumes up to 50% of the metal produced annually.
Corrosion is diverse in its manifestations. The metal surface is not always subject to uniform destruction - the so-called general corrosion. More often the process focuses on separate areas, the destruction is local in nature.
The use of metals in a stressed state, the transition to high-strength steels and alloys characterized by high internal stresses have led to the fact that corrosion cracking has become one of the most dangerous types of corrosion. Stainless steels, alloys based on copper, aluminum, and magnesium are highly susceptible to it. The susceptibility to corrosion cracking is also determined by the composition corrosive environment. The presence of individual components serves a necessary condition for the occurrence of corrosion cracking. For stainless steels these are chlorides and alkalis, and for copper-based alloys - ammonia.
Welds are especially vulnerable to corrosion. By characteristic appearance Corrosion lesions of this type are called knife corrosion.
A particular type of corrosion cracking is corrosion fatigue, in which the appearance of cracks and its development are caused by simultaneous exposure to an aggressive environment and cyclic mechanical loads.
Iron-based alloys and high-strength alloys are prone to intercrystalline corrosion, in which destruction occurs along grain boundaries and mechanical strength is lost.
A very dangerous corrosion is pitting, which affects individual very small areas of the surface up to through perforation of products. Under certain conditions, iron, nickel, aluminum, magnesium, zirconium, copper, tin, zinc and especially stainless steels are susceptible to it.
For iron-based alloys, a common and dangerous type of local corrosion is crevice corrosion under all kinds of gaskets, build-ups, in crevices and narrow gaps. Metal areas in contact with non-metallic materials (wood, plastic, glass, concrete, asbestos, fabrics) are very prone to this type of corrosion.
For copper-based alloys, selective etching of certain components from them is dangerous (for example, dezincification of brass).
According to the mechanism of their occurrence, corrosion processes are divided into chemical, electrochemical and biochemical.
Chemical corrosion is a type of corrosion where the metal enters into direct chemical interaction with environmental components. Chemical corrosion occurs in gaseous environments at high temperatures, when the formation of a moisture film on the metal surface is impossible, as well as in solutions that do not conduct current.
An example of chemical corrosion is gas corrosion of the exhaust tract of a car engine by exhaust gases. Chemical corrosion of metals can occur in the engine fuel system due to their interaction with fuel impurities such as hydrogen sulfide, elemental sulfur and mercaptans. As a result of oil oxidation during engine operation, products can be formed that cause chemical corrosion of the metal of the bearing shells.
With high-temperature or gas corrosion, the composition of corrosion products depends on the composition of the gaseous environment, but most often these are metal oxides. The aggressive components of the gas environment are compounds of sulfur, chlorine, nitrogen, and most often oxygen and its compounds.
The rate of corrosion of ordinary steel increases in the presence of carbon dioxide, water vapor, sulfur dioxide and especially mixtures thereof. Combustion products liquid fuels reduce the protective properties of films of the resulting corrosion products. The ratio of CO and CO 2 in the exhaust gases has a significant influence on the corrosion rate of carbon and low-alloy steels. With increasing CO content, the corrosion rate decreases and at 14-18% it can stop. The resulting products, as a rule, create a film on the surface of the corroding metal, which inhibits the delivery of aggressive components directly to the metal, which reduces the corrosion rate. The protective properties of the resulting films primarily depend on its continuity, thickness (thin ones are more protective), adhesion to metal, strength, elasticity, etc. With increasing temperature, the protective properties of films in most cases deteriorate. Increasing the pressure and speed of movement of the gas medium increases the rate of corrosion. The corrosion process can be accompanied by erosive wear.
However, in the general process of corrosion destruction of a car, electrochemical corrosion is of primary importance, mainly due to its significantly higher speed compared to chemical corrosion. Electrochemical corrosion is only possible when there is an electrolyte on the metal surface, i.e. an aqueous solution of salts, acids, alkalis that have the ability to conduct electricity. Electrochemical corrosion occurs under normal atmospheric conditions, in solutions and melts that conduct current.
Numerous studies have established that a thin film of water forms on the surface of any metal in the atmosphere. The thickness of such a film can vary depending on temperature and humidity, as well as other atmospheric conditions. Gases in the air dissolve in the film of water and create an electrolyte on the metal surface. This creates conditions for electrochemical corrosion. Thus, conditions for this type of corrosion on unprotected metal surfaces almost always exist.
In the vast majority of cases, corrosion is electrochemical. In this case, numerous microgalvanic couples are formed on the surface of the metal, the operation of which leads to the destruction of the metal. In certain areas of the surface (impurities, additives), cathode areas are localized, where the reduction of oxidizing agents in solution occurs. Most often this is dissolved oxygen.
On the rest of the surface and especially on the protrusions and distortions of the crystal lattice, anodic areas are localized, where the metal dissolves. Thus, the entire process of electrochemical corrosion is modeled by the operation of a short-circuited galvanic cell.
Along with the formation of numerous corrosive micropairs on the surface of one metal, the formation of macropairs between mating parts made of different metals is possible. A metal with a more negative potential in such a macropair will be the anode, and its corrosion rate will increase.
With increasing temperature and electrical conductivity of the solution, the rate of electrochemical corrosion increases. Internal stresses and mechanical loads, especially alternating ones, lead to the appearance of corrosion fatigue, accompanied by a decrease in mechanical strength, and even more so the higher the electrical conductivity of the solution.
There is also biochemical corrosion, which occurs under the influence of microorganisms.
The overall process of iron corrosion in most cases is described by the following reaction equation:
and comes down to the formation of ferrous hydride or hydrated ferrous oxide 
.
A film forms on the outer surface, due to the access of oxygen, further oxidation occurs
with the formation of iron oxide hydrate or hydrous iron oxide 
.
Between the formed hydrated and often formed oxide - iron oxide 
. Rust films usually consist of these three layers. When iron comes into contact with copper, the true depth of corrosion destruction of iron increases due to the localization of the anodic process near the contact.
Stainless steels can be paired with copper and aluminum. Copper dissolves anodicly in most aqueous solutions to form a divalent ion
(3.6)
Copper in contact initiates corrosion of iron and aluminum, acting as a cathode in relation to them.
Aluminum under normal conditions oxidizes to form Al 2 O 3, which sharply inhibits further corrosion of aluminum.
Copper and iron significantly stimulate the dissolution of aluminum in limited areas.
Complete corrosion is less dangerous than local corrosion, which leads to the destruction of metal parts of the body and their loss of strength.
According to the conditions under which car corrosion occurs, the following types of corrosion are distinguished:
- gas (in the combustion chambers on the chamfers of the exhaust valve plates, exhaust pipe, in the muffler, etc.);
- in non-electrolytes (in fuel and oil systems);
- atmospheric (in natural conditions of storage, transportation and operation of the vehicle);
- in electrolytes (in places where moisture is retained in body pockets);
- structural (in areas of the car body subjected to gas-plasma or electric welding, which results in heterogeneity in the composition of the metals);
- slot (in narrow cracks and gaps under the influence of differences in the pH environment or different oxygen content in the electrolyte);
- under voltage (on the surface of parts, assemblies and structures under voltage);
- during friction (in friction units in the presence of a corrosive environment, accompanied by corrosion-mechanical wear);
- biological (occurs with the participation of products secreted by microorganisms).
Corrosion of a car body with untimely protection of the metal, considered as a combined result of chemical and electrochemical corrosion, occurs in next sequence:
- sublayer corrosion develops under the paintwork;
- peeling and swelling in areas damaged by corrosion;
- through corrosion of the body, especially at the joints;
- cracking of welds at the joints of floor parts, sills, fenders and, as a result, moisture, dust and dirt entering the body interior;
- the appearance of cracks in reinforcements, side members and cross members with loss of body rigidity;
- deformation of doorways due to loss of rigidity of pillars and body sills;
- violation relative position vehicle chassis units, leading to disruption of controllability and uniform braking of wheels;
- damage to the metal pipelines of the brake drive due to loss of rigidity at the base of the body due to corrosion of the mounting points;
- mechanical damage to the body floor at the attachment points of shock absorbers, springs and other vehicle components as a result of corrosion of their attachment points, especially during sudden braking and driving over rough terrain.
The effect of corrosive factors, such as humidity, the concentration of saline solutions and sulfur compounds formed from exhaust gases, is especially pronounced in places that are difficult to access for inspection and cleaning, in small gaps, as well as in flanges and bends of edges, where moisture periodically enters them may persist for a long time.
As the temperature increases, the corrosion rate increases (especially if there are aggressive impurities and moisture content in the atmosphere).
Destructive processes on the body are also often intensified by unfavorable storage conditions for the car. There is an increase in corrosive wear as a result of the use of sand-salt mixtures on roads to combat ice, as well as due to sudden temperature changes in the cabin and outside of the car.
Corrosion damage to the body also occurs as a result of contact of steel parts with parts made of some other materials (duralumin, rubbers containing sulfur compounds, plastics based on phenolic resins, etc.), as well as as a result of contact of metal with parts , made from material containing a noticeable amount of organic acids (in particular formic acid).
Now about the causes of corrosion caused by the impact of petroleum products on car parts. This is due, first of all, to the presence of water and aggressive chemical compounds in them. Water penetrates fuels, oils and lubricants during their production, storage and use. Aggressive chemical compounds arise, as a rule, during long-term storage of petroleum products, as a result of the aging processes occurring in them, as well as during engine operation.
Thus, among the reasons contributing to the intensive development of corrosion of cars, there are the main ones: incorrect design of the body, its parts and assemblies; technological shortcomings in the manufacture of the body; failure to comply with the rules for pre-sale storage and transportation of the vehicle; improper care of the body during operation.
PHYSICAL AND CHEMICAL BASES OR PROCESSES OF CHANGING THE TECHNICAL CONDITION OF VEHICLES IN OPERATION
Definition of corrosion and its causes
Corrosion is a spontaneous process of destruction of metals and alloys in the natural environment.
During corrosion, metals oxidize and products are formed, the composition of which depends on environmental conditions.
According to modern ideas, all major changes in the organic and inorganic world are associated with redox processes. Redox reactions also underlie corrosion processes.
The main cause of corrosion is the thermodynamic instability of metals and alloys in environment. The vast majority of metals in the earth's crust are in the form of oxides, sulfides and other compounds. When metals are produced in metallurgy, they are transferred from such a stable state to an elemental form that is unstable. When a metal comes into contact with an external oxidizing environment, a driving force appears that tends to transform them into stable compounds similar to those found in ores. An example of this is the corrosion of steel. As a result of this, elemental iron is converted into oxidized di- and trivalent iron, which corresponds to minerals such as magnetite (Fe 3 O 4) or limonite (Fe 2 O 3 ˙H 2 O).
Thermodynamic instability of metals is quantified by the sign and magnitude of the isobaric-isothermal potential ΔG (Gibbs energy). Those processes occur spontaneously that are accompanied by a decrease in the Gibbs energy, that is, for which ΔG<0. Металлы, стоящие в ряду напряжений до водорода, имеют по сравнению с водородом более отрицательный потенциал, их окисленное состояние термодинамически более устойчиво, чем восстановленное. Для металлов, расположенных после водорода, восстановленное состояние термодинамически более устойчиво, то есть для них ΔG>0. This group of metals includes corrosion-resistant gold, platinum, silver, etc.
Classification of corrosion processes. Chemical and electrochemical corrosion
Corrosion processes are classified:
1. According to the mechanism of reactions of interaction of the metal with the environment;
2. By type of corrosive environment;
3. By the nature of corrosion damage on the surface and in the bulk of the metal;
4. By the nature of the mechanical influences to which the metal is exposed simultaneously with the action of a corrosive environment.
Based on the first sign, two types of corrosion are distinguished – chemical and electrochemical.
Chemical corrosion
Chemical corrosion occurs when metals interact with oxidizing agents in environments that do not conduct electrical current. The mechanism of chemical corrosion can be represented as a one-stage process of metal oxidation, that is, the interaction of the metal surface with an oxidizing agent.
Chemical corrosion is the process of spontaneous destruction of a metal in an oxidizing gas (for example, oxygen) at elevated temperature. The rate of chemical corrosion depends on many factors, primarily determined by the nature of the corrosion products. During oxidation, a solid film of oxides forms on the surface of the metal. The rate of oxidation is determined by the condition and protective properties of the surface film. This depends on the ratio of the volumes of the oxide film V ok and the corroded metal V m from which it was formed. It has been established that for porous films that do not protect the metal from the access of aggressive air impurities. And for films with protective properties, .
The rate of chemical corrosion increases with increasing temperature due to an increase in the diffusion coefficient and a change in the protective properties of the film. Sudden temperature changes often cause rapid destruction of the protective film. This is due to different thermal expansion coefficients of the metal and film.
According to the conditions of the corrosion process, a distinction is made between gas corrosion (occurring in gases, vapors at high temperatures in the absence of water), and corrosion in liquids - non-electrolytes (oil, phenol, gasoline, benzene).
Electrochemical corrosion
In electrochemical corrosion, the process of interaction between a metal and an oxidizing agent consists of two coupled reactions: anodic dissolution of the metal and cathodic reduction of the oxidizing agent. This corrosion can occur in electrolytes, the atmosphere of any moist gas, and also in the soil.
The main difference between electrochemical corrosion and chemical corrosion is the presence of moisture on the surface of the metal, which leads to contact of two different metals through the electrolyte. In this case, short-circuited galvanic couples arise, resulting in an electric current. In this case, the corrosion process is caused by the operation of a galvanic couple, that is, an electrochemical reaction. For this reason, electrochemical corrosion is more aggressive towards metals than chemical corrosion.
The mechanism of electrochemical corrosion is that anodic oxidation of the metal occurs: M – ne = M n + and cathodic reduction of the oxidizer (Ox) Ox + ne = Red.
Oxidizing agents during corrosion are molecules of oxygen, chlorine, ions H +, Fe 3+, NO 3 –, etc. Most often during corrosion, ionization (reduction) of oxygen is observed in a neutral (alkaline) environment O 2 + 2H 2 O + 4e = 4OH – , in an acidic environment – reduction of hydrogen 2H + +2e=H 2.
Corrosion involving oxygen is called oxygen absorption corrosion or oxygen depolarization corrosion. Corrosion involving hydrogen ions is called hydrogen evolution corrosion or hydrogen depolarization corrosion.
In addition to primary reactions, secondary reactions occur in solution:
M x+ +xOH - =M(OH) x
As a result of the interaction of metal with oxygen, as with chemical corrosion, metal oxide is formed: M(OH) 2 = MO + H 2 O.
In addition to anodic and cathodic reactions, during electrochemical corrosion there is a movement of electrons in the metal and ions in the electrolyte. Electrolytes can be solutions of salts, acids and bases, sea and atmospheric water (containing oxygen, carbon dioxide, sulfur dioxide and other gases). The main difference between electrochemical corrosion and processes in a galvanic cell is the absence of an external circuit.
The equilibrium potentials of the hydrogen and oxygen electrodes depending on the pH of the medium are found based on the Nernst equation:
φ 2H + /H2 = -0.059рН;
φ O2/OH = 1.23-0.059 pH.
Corrosion of metals in various environments
Contact corrosion
Contact bimetallic corrosion is a type of electrochemical corrosion caused by the contact of metals having different electrode potentials in the electrolyte. In this case, the corrosion of metal with a more negative potential usually increases, and the destruction of metal with a positive potential slows down or stops completely. When designing, the possibility of contacts between different metals is taken into account.
Atmospheric corrosion
The rate of atmospheric corrosion is affected by the humidity and gas composition of the atmosphere. Humidity, temperature and the degree of atmospheric pollution affect the quality and composition of the films formed on the metal surface. The most aggressive environments are those heavily contaminated with industrial gases (CO 2 , SO 2 , NO 2 , NH 3 , HCl), salt particles and coal dust. In industrial areas, atmospheric corrosion can be intensified by so-called “acid rain”, the main aggressive components of which are sulfuric and nitric acids. Acid rain (pH<4) легко вызывают коррозию сплавов алюминия, железа и цинка.
Depending on the humidity of the atmosphere, several types of atmospheric corrosion are distinguished: wet, damp and dry. Wet atmospheric corrosion at relative humidity up to 100% is observed in the presence of an adsorption capillary or chemical film of moisture on the metal surface. Its thickness ranges from 0.1 mm to 1 mm. A decrease in temperature intensifies the condensation process and leads to the appearance of moisture droplets on the metal surface.
Wet corrosion occurs when atmospheric humidity is below 100%. Moisture film thickness from 100 A 0 to 0.1 mm. When air humidity is less than 60%, dry atmospheric corrosion (corrosion under the influence of atmospheric oxygen) is observed. The process of metal destruction obeys the laws characteristic of gas corrosion.
Underground corrosion
Corrosive destruction of metal structures in soils and soils is caused by underground corrosion. Pipelines (water, gas, oil), electrical contact network supports, etc. are susceptible to it. The rate of corrosion depends on the porosity and composition of the soil, pH value, and the presence of microorganisms. Underground corrosion proceeds through the mechanism of electrochemical corrosion. Soil moisture plays the role of an electrolyte and the corrosion process proceeds as follows:
Anodic reaction Fe-2e=Fe 2+
Cathode reaction O 2 +2H 2 O+4e=4OH –
Reactions in soil Fe 2+ +2OH - =Fe(OH) 2, 4Fe(OH) 2 +2H 2 O+O 2 =4Fe(OH) 3, 2Fe(OH) 3 +(n-3)H 2 O= Fe 2 O 3 nH 2 O.
The metal surface in places with limited access to oxygen acts as a cathode.
Ground corrosion of metal structures most often occurs under conditions characteristic of neutral environments, with the participation of oxygen as a depolarizer. In acidic soils, corrosion with hydrogen depolarization can occur.
A study of the corrosive activity of soils led to the conclusion that the most corrosive are swampy soils, peat bogs, and silt. Sand and limestone are practically non-corrosive. Soil pH has a significant influence on the rate of metal corrosion. In soils with a pH less than 6.5, corrosion activity towards steel increases. Soils with pH are the most corrosive<5,5. Нейтральные почвы с рН=6,5–7,5 и слабощелочные до рН=8,5 не коррозионно-активны.
The rate of corrosion is also affected by the electrical resistivity of the soil. Corrosion of metal underground structures depends on the content of various salts in the soil and soil. Thus, with an increase in the content of chlorides and sulfates, the corrosion rate increases. An increase in temperature also increases the rate of soil corrosion of metals.
Corrosion due to stray currents
Stray currents are electrical currents flowing in the ground when it is used as a conductive medium. When they get into metal structures located in the ground, they cause corrosion. The sources of stray currents in the soil are electrified DC railways, trams, and power lines.
Since the rails are not sufficiently insulated from the ground, and the soil is a conductor, part of the current goes into the ground, encountering underground metal structures on its way. Since the contact wire is connected to the positive pole of the traction substation, and the rail to the negative, an anodic zone is formed where the current exits the rail, where corrosion destroys the rail base and fasteners. At the same time, the lower the rail-ground transition resistance, the larger part of the current returns to the traction substation through the ground and the more intense the anodic zone on the rail. This type of corrosion is very dangerous, since stray currents often spread over several tens of kilometers and cause severe damage to metal structures.
Types of corrosion damage
Based on the type of corrosion destruction, corrosion is divided into the following types.
1. Complete or general corrosion. It can be uniform if the corrosion destruction front is distributed parallel to the plane of the metal, and uneven when the corrosion rate in different areas is not the same.
2. Selective corrosion. It is typical for alloys and solid solutions.
3. Local corrosion. It is associated with the formation and localization of areas affected by corrosion in the form of “shells” of different sizes.
4. Pitting - corrosion. The destruction of metal begins in depth, with the formation of pores; often leads to the formation of through holes.
5. Intergranular corrosion. Destruction occurs along the boundaries of metal crystals.
6. Intracrystalline corrosion. Observed during corrosion cracking under the influence of external mechanical loads or internal stresses.
Topic: Protection of metals from corrosion
All methods of protecting metals from corrosion are conventionally divided into the following groups: metal alloying, protective coatings, electrochemical protection, changing the properties of the corrosive environment, rational design of products.
Metal alloying
This is an effective method of increasing the corrosion resistance of metals. When alloying, components are introduced into the alloy that cause passivation of the metal. Chromium, nickel, tungsten and other metals are used as such components. Alloying is widely used to protect against gas corrosion. The introduction of certain additives into steel (titanium, copper, chromium and nickel) leads to the formation of a dense film of reaction products during corrosion, which protects the alloy from further corrosion. This ensures the heat resistance and heat resistance of the alloys.
Heat resistance is usually achieved by alloying metals and alloys (for example, steel with chromium, aluminum and silicon). At high temperatures, these elements oxidize more energetically than iron and form dense protective films of oxides, for example, SiO 2, Al 2 O 3, Cr 2 O 3. Chromium and silicon also improve the heat resistance of steels. Alloying is also used to reduce the rate of galvanic corrosion, especially hydrogen evolution corrosion. Corrosion-resistant alloys include stainless steels, in which the alloying components are chromium, nickel and other metals.
Protective coatings
Layers artificially created on the surface of metal products to protect them from corrosion are called protective coatings. Coatings used in technology are divided into metallic and non-metallic.
Metal coatings. Materials for metal protective coatings can be either pure metals (zinc, cadmium, aluminum, nickel, copper, tin, chromium, silver) or their alloys (bronze, brass, etc.). Based on the nature of the behavior of metal coatings during corrosion, they can be divided into anodic and cathodic.
Cathodic coatings include coatings whose potential in a given environment is greater than the potential of the base (coated) metal. Examples of cathode coatings for steel include copper, nickel, cadmium, tin, and silver. When the coating is damaged, a corrosion element appears in which the base material (steel) serves as an anode and dissolves, and the coating material acts as a cathode, on which hydrogen is released or oxygen is absorbed. Consequently, cathodic coatings can protect metal from corrosion only in the absence of pores and damage to the coating.
Anodic coatings have a lower potential than the potential of the base metal. An example of an anodic coating is zinc on steel. In this case, the base metal will be the cathode of the corrosion element, so it will not corrode.
To obtain metal protective coatings, various methods are used: electrochemical (electroplating), immersion in molten metal, thermal diffusion and chemical.
Non-metallic protective coatings. They can be either inorganic or organic. The protective effect of these coatings is mainly reduced to isolating the metal from the environment. Inorganic enamels, metal oxides, compounds of chromium, phosphorus, etc. are used as inorganic coatings. Organic coatings include paint and varnish coatings, coatings with resins, polymer films, and rubber.
Electrochemical protection
Electrochemical protection is used to prevent the destruction of underground pipelines, cables, ship hulls, tanks, submarines, etc.
Electrochemical protection is based on slowing down the cathodic and anodic reactions of microvoltaic cells. It is carried out by connecting a direct current source or an additional electrode to the structure.
Electrochemical protection is divided into cathodic and anodic.
Cathodic protection – the most common type of electrochemical protection. It is used to combat corrosion of metals and alloys such as steel, copper, brass, aluminum in not very aggressive environments. It is effective in preventing corrosion cracking, dezincification of brass, pitting of steels in soils and sea water. Cathodic protection is most widely used to combat corrosion of underground structures - pipelines, gas pipelines, cable installations.
Cathodic polarization can be accomplished by connecting the protected structure to the negative pole (cathode) of an external current source or to a metal having a lower electrode potential. The positive pole is connected to the auxiliary electrode, the anode. During the protection process, the anode is actively destroyed and must be periodically renewed. Scrap cast iron, steel, graphite, etc. is used as an anode material.
The protective effect can be assessed using the formulas:
, 
.
Here z is the protective effect, k 1 is an indicator of the corrosion rate of the metal without cathodic protection, k 2 is with cathodic protection, Δm 1 is a reduction in the mass of the metal without cathodic protection, Δm 2 is with cathodic protection, i k is the cathodic current density.
Tread protection. A more electronegative metal is attached to the protected structure - a protector, which, dissolving in the environment, sends electrons and cathodically polarizes the structure. After complete dissolution of the protector or loss of contact with the protected structure, the protector must be renewed. Alloys of magnesium and zinc are most often used as protectors. Aluminum is used less frequently, since it is quickly covered with a very dense oxide film, which passivates it and limits current output. The protector works effectively if its transition resistance (between it and the environment) is low. During operation, the protector can become covered with a layer of corrosion products, which isolate it from the environment and sharply increase the contact resistance. To combat this, the protector is placed in a filler (a mixture of salts), which facilitates the dissolution of corrosion products. The action of the protector is limited to a certain distance (range of action). Currently, tread protection is used to combat corrosion of metal structures in sea and river water, soil and other neutral environments. The use of tread protection in acidic environments is limited by the high rate of self-dissolution of the tread.
Anodic protection. The rate of electrochemical corrosion of a metal can also be reduced during its anodic polarization, if it shifts the potential of the protected metal to the passive region.
The anodic protection method has relatively limited application, since passivation is effective mainly in oxidizing environments in the absence of active ions (for example, chlorine ions for iron). In addition, anodic protection is potentially dangerous: if the current supply is interrupted, the metal may be activated and undergo intense anodic dissolution. Therefore, anodic protection requires a careful control system. The protective current density is quite low and the power consumption is low. Another advantage of anodic protection is its high dissipative ability, that is, the possibility of protection at a distance further from the cathode and in electrically shielded areas.
The anodic protection method is used for metals and alloys that are easily passivated by anodic polarization; in the chemical industry - to reduce the rate of corrosion of low-carbon steel in sulfuric acid and in solutions containing ammonia and ammonium nitrate.
Protection against corrosion by stray currents
The fight against corrosion by stray currents is to reduce them. This is achieved:
1) Maintaining contacts between rails in good condition;
2) Increasing the resistance between the rail and the ground (use of sleepers, use of crushed stone ballast);
3) Electrical drainage protection. It is provided by removing stray currents from a metal structure towards their source. To do this, the underground metal structure is connected through a drainage device to a negative bus or suction line;
4) Using down conductors. For this purpose, the anode zones (for example, on a pipeline) are connected to cast iron scrap (anode) using a copper conductor. As a result, stray currents cause corrosion of only this scrap - the anode.
Corrosion inhibitors
Corrosion of metal equipment, for example, in heat exchanger cooling tubes of diesel engines on diesel locomotives, can be reduced by introducing compounds into the aggressive environment that significantly reduce the corrosion process. This method of reducing the corrosion rate is called inhibition, and the substances introduced into the environment are inhibitors or corrosion retarders.
So, inhibitors are substances, the introduction of small quantities of which into a corrosive environment, packaging materials and temporary protective coatings reduces the rate of corrosion and reduces its harmful consequences. The protective effect of inhibitors is associated with changes in the state of the surface of the protected metal and in the kinetics of reactions underlying the corrosion process.
Thanks to the introduction of an inhibitor, the corrosion rate can be reduced by any desired number of times, and the degree of protection increased to almost 100%. The effectiveness of the inhibitor is determined both by its nature and the nature of the corroding metal, and depends on temperature.
Corrosion inhibitors can be classified according to various criteria.
1. Based on their composition, they are divided into two groups: inorganic and organic. Recently, metal and organosilicon inhibitors have been widely used.
2. According to the areas of application, inhibitors are: acid corrosion, alkaline corrosion and corrosion in neutral environments.
3. According to the conditions of use, there are low-temperature and high-temperature inhibitors.
4. According to the peculiarities of the mechanism of action of inhibitors, there are adsorption and passivating inhibitors.
Passivating corrosion inhibitors for neutral environments are divided into:
¨ Oxidative type inhibitors, which exhibit their effect even in the absence of atmospheric oxygen. Examples: sodium nitrite NaNO2, ammonium nitrite NH4NO2, potassium chromate K2CrO4, potassium dichromate K2Cr2O7, sodium molybdate Na2MoO4, etc.
¨ Inhibitors that do not have oxidative properties and require atmospheric oxygen to exert their action. Examples: ammonium hydroxide NH 4 NO 3, sodium hydroxide NaOH, sodium carbonate Na 2 CO 3, sodium silicate, orthophosphate and tetraborate Na 2 SiO 3, Na 3 PO 4 and Na 2 B 4 O 7.
Inhibitors of passivating action in neutral environments of the oxidizing type in the absence of chlorides and sulfates in relation to low-carbon steels are approximately one hundred times more effective than inhibitors that do not have oxidizing properties. The lowest protective concentration of oxidizing inhibitors is 10 –3 ¸ 10 –4%, and inhibitors that do not have oxidizing properties are 0.1 ¸ 0.05%. Corrosion inhibitors can be introduced into liquid media of any acidity and into solid materials: oils, fuels, various organic liquids, paints, polymers, phosphates, oxides and other coatings, as well as packaging materials. The most promising is the introduction of volatile inhibitors into packaging materials (ammonium benzoate, triethanolamine benzoate, methenamine mixed with sodium nitrite, dicyclohexylammonium nitrite), which, evaporating into the atmosphere inside the package and adsorbing on the surface of the metal, transfer it to a passive state.
The slowdown in the corrosion rate is primarily due to the exclusion of part of the surface from the corrosion process due to its shielding with an inhibitor. When choosing inhibitors, one should proceed not only from how they reduce the rate of metal transfer into the environment, but also from how they affect metallic properties. Surfactants with a predominant cationic function are preferred over those with an anionic function. The use of selected inhibitors can not only prevent the dissolution of the metal, but also improve its mechanical properties.
The protective effect of passivating inhibitors is based on shifting the potential of the metal in a positive direction and transferring it to a passive state. This effect can be achieved in various ways, but in all cases the reason for the reduction in corrosion rate is the formation of a surface protective layer. Inhibitors can directly participate in the formation of this layer.
Acid corrosion inhibitors are used for pickling products made of ferrous and non-ferrous metals to remove scale and rust from their surfaces, acid washing of thermal power equipment, and in the production of acids.
The effect of atmospheric corrosion inhibitors, as well as other types of inhibitors, is primarily reduced to changing the kinetics of electrochemical reactions underlying corrosion. The effectiveness of any inhibitors depends on their concentration in a corrosive environment, and at some minimum values it drops to zero. The volume of the air atmosphere surrounding us is practically limitless and maintaining a protective concentration of inhibitor in it seems economically senseless. the use of inhibitors to protect metals from atmospheric corrosion is therefore possible only if it is possible to limit the space in which the protected object is placed and separate it from the rest of the atmosphere. For this purpose, inhibitors are introduced into lubricants, polymer and other coatings; place the metal in the packaging material with the introduction of an inhibitor into the free space between the packaging material and the metal product, or into the packaging material itself (for example, paper).
Corrosion of metals is the spontaneous destruction of metals due to their chemical or electrochemical interaction with the external environment. The corrosion process is heterogeneous (inhomogeneous), occurs at the interface between metal and aggressive environment, and has a complex mechanism. In this case, the metal atoms are oxidized, i.e. they lose valence electrons, the atoms move across the interface into the external environment, interact with its components and form corrosion products. In most cases, corrosion of armhole metals spreads unevenly over the surface; there are areas where local damage occurs. Some corrosion products, forming surface films, impart corrosion resistance to the metal. Sometimes loose corrosion products that have weak adhesion to the metal may appear. The destruction of such films causes intense corrosion of the exposed metal. Metal corrosion reduces mechanical strength and changes its other properties. Corrosion processes are classified according to the types of corrosion damage, the nature of the interaction of the metal with the environment, and the conditions of its occurrence.
Corrosion can be continuous, general and local. Continuous corrosion occurs over the entire surface of the metal. With local corrosion, the lesions are localized in individual areas of the surface.
Rice. 1Nature of corrosion damage:
I – uniform; II - uneven; III - selective; IV - spots; V - ulcers ; VI - points or pittings; VII - end-to-end; VIII - threadlike; IX - superficial; X - intercrystalline; XI - knife; XII - cracking
General corrosion is divided into uniform, uneven and selective (Fig. 1).
Uniform corrosion occurs at the same rate over the entire surface of the metal; uneven - on different parts of the metal surface at unequal speeds. Selective corrosion destroys individual components of the alloy.
In case of spot corrosion, the diameter of the corrosion lesions is of great depth. Pit corrosion is characterized by deep damage to a limited surface area. As a rule, the ulcer is located above a layer of corrosion products. With pitting corrosion, individual pinpoint lesions on the metal surface are observed, which have small transverse dimensions and a significant depth. Through is local corrosion that causes destruction of a metal product through and through, in the form of fistulas. Filiform corrosion appears under non-metallic coatings and in the form of filaments. Subsurface corrosion begins at the surface and primarily spreads below the surface of the metal, causing it to swell and delaminate.
In intergranular corrosion, destruction is concentrated along the grain boundaries of the metal or alloy. This type of corrosion is dangerous because there is a loss of strength and ductility of the metal. Knife corrosion takes the form of a knife cutting along a welded joint in highly aggressive environments. Corrosion cracking occurs under simultaneous exposure to a corrosive environment and tensile residual or applied mechanical stresses.
Under certain conditions, metal products are subject to corrosion-fatigue failure, which occurs when the metal is simultaneously exposed to a corrosive environment and variable mechanical stresses.
Based on the nature of the interaction of the metal with the environment, chemical and electrochemical corrosion are distinguished. Chemical corrosion is the destruction of metal during chemical interaction with an aggressive environment, which is non-electrolytes - liquids and dry gases. Electrochemical corrosion is the destruction of metal under the influence of an electrolyte during the occurrence of two independent but interrelated processes - anodic and cathodic. The anodic process is oxidative and occurs with the dissolution of the metal; The cathodic process is a reduction process, caused by the electrochemical reduction of the components of the medium. The modern theory of metal corrosion does not exclude the joint occurrence of chemical and electrochemical corrosion, since in electrolytes, under certain conditions, metal mass transfer through a chemical mechanism is possible.
According to the conditions of the corrosion process, the most common types of corrosion are:
1) gas corrosion, occurs at elevated temperatures and the complete absence of moisture on the surface; a product of gas corrosion - scale has protective properties under certain conditions;
2) atmospheric corrosion, occurs in the air; There are three types of atmospheric corrosion: in a humid atmosphere - with a relative air humidity above 40%; in a wet atmosphere - with a relative humidity of 100%; in a dry atmosphere - with a relative air humidity of less than 40%; atmospheric corrosion is one of the most common types due to the fact that the majority of metal equipment is operated in atmospheric conditions;
3) liquid corrosion - corrosion of metals in a liquid medium; distinguish between corrosion in electrolytes (acids, alkalis, salt solutions, sea water) and in non-electrolytes (oil, petroleum products, organic compounds);
4) underground corrosion - corrosion of metals caused mainly by the action of salt solutions contained in soils and soils; the corrosive aggressiveness of soil and soils is determined by the structure and moisture of the soil, the content of oxygen and other chemical compounds, pH, electrical conductivity, and the presence of microorganisms;
5) biocorrosion - corrosion of metals as a result of the influence of microorganisms or their metabolic products; aerobic and anaerobic bacteria participate in biocorrosion, leading to the localization of corrosion lesions;
6) electrocorrosion, occurs under the influence of an external current source or stray current;
7) crevice corrosion - corrosion of metal in narrow crevices, gaps, m threaded and flanged connections of metal equipment,used in electrolytes, in places of loose contact metal with insulating material;
8) contact corrosion, occurs when dissimilar metals come into contact in the electrolyte;
9) stress corrosion, occurs when the metal is exposed to a combined aggressive environment and mechanical stresses - constant tensile (corrosion cracking) and variable or cyclic (corrosion fatigue);
10) corrosion cavitation - destruction of metal as a result of simultaneous corrosion and impact effects. In this case, the protective films on the metal surface are destroyed when gas bubbles burst at the interface between the liquid and the solid;
11) corrosion erosion - destruction of metal due to simultaneous exposure to an aggressive environment and mechanical wear;
12) fretting corrosion - local corrosion destruction of metals when exposed to an aggressive environment under conditions of oscillatory movement of two rubbing surfaces relative to each other;
13) structural corrosion, caused by the structural heterogeneity of the alloy; in this case, an accelerated process of corrosion destruction occurs due to the increased activity of any component of the alloy;
14) thermal contact corrosion, occurs due to a temperature gradient caused by uneven heating of the metal surface.
Metal corrosion is a widespread cause of deterioration of various metal parts. Metal corrosion (or rusting) is the destruction of metal under the influence of physical and chemical factors. Factors that cause corrosion include natural precipitation, water, temperature, air, various alkalis and acids, etc.
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Metal corrosion is becoming a serious problem in construction, at home and in production. Most often, designers provide protection for metal surfaces from rust, but sometimes rust occurs on unprotected surfaces and on specially treated parts.
Metal alloys form the basis of human life; they surround him almost everywhere: at home, at work, and during leisure. People don’t always notice metal things and parts, but they constantly accompany them. Various alloys and pure metals are the most produced substances on our planet. Modern industry produces various alloys 20 times more (by weight) than all other materials. Even though metals are considered to be some of the strongest substances on Earth, they can break down and lose their properties through rusting processes. Under the influence of water, air and other factors, the process of oxidation of metals occurs, which is called corrosion. Despite the fact that not only metal, but also rocks can corrode, processes associated specifically with metals will be discussed below. It is worth paying attention to the fact that some alloys or metals are more susceptible to corrosion than others. This is due to the speed of the oxidation process.
Metal oxidation process
The most common substance in alloys is iron. Corrosion of iron is described by the following chemical equation: 3O 2 +2H 2 O+4Fe=2Fe 2 O 3. H 2 O. The resulting iron oxide is that red rust that spoils objects. But let's look at the types of corrosion:
- Hydrogen corrosion. It practically does not occur on metal surfaces (although theoretically possible). In this regard, it will not be described.
- Oxygen corrosion. Similar to hydrogen.
- Chemical. The reaction occurs due to the influence of the metal with some factor (for example, air 3O 2 +4Fe = 2Fe 2 O 3) and occurs without the formation of electrochemical processes. So, after exposure to oxygen, an oxide film appears on the surface. On some metals, such a film is quite strong and not only protects the element from destructive processes, but also increases its strength (for example, aluminum or zinc). On some metals, such a film peels off (destroys) very quickly, for example, sodium or potassium. And most metals deteriorate quite slowly (iron, cast iron, etc.). This is how, for example, corrosion occurs in cast iron. More often, rusting occurs when the alloy comes into contact with sulfur, oxygen, or chlorine. Due to chemical corrosion, nozzles, fittings, etc. rust.
- Electrochemical corrosion of iron. This type of rusting occurs in environments that conduct electricity (conductors). The destruction time of different materials during electrochemical reactions is different. Electrochemical reactions are observed in cases of contact between metals that are located at a distance in a series of tensions. For example, a product made of steel has copper soldering/fastenings. When water hits the connections, the copper parts will be the cathodes and the steel will be the anode (each point has its own electrical potential). The speed of such processes depends on the amount and composition of the electrolyte. For reactions to occur, the presence of 2 different metals and an electrically conductive medium is required. In this case, the destruction of alloys is directly proportional to the current strength. The greater the current, the faster the reaction; the faster the reaction, the faster the destruction. In some cases, alloy impurities serve as cathodes.
Electrochemical corrosion of iron
It is also worth noting the subtypes that occur during rusting (we will not describe it, we will just list it): underground, atmospheric, gas, with different types of immersion, continuous, contact, caused by friction, etc. All subspecies can be classified as chemical or electrochemical rusting.
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Corrosion of reinforcement and welded structures often occurs during construction. Corrosion often occurs due to non-compliance with the rules for storing the material or failure to perform work on processing the rods. Corrosion of reinforcement is quite dangerous, since reinforcement is laid to strengthen structures, and as a result of the destruction of the rods, a collapse is possible. Corrosion of welds is no less dangerous than corrosion of reinforcement. This will also significantly weaken the seam and may lead to tearing. There are many examples where rust on power structures leads to the collapse of premises.
Other common cases of rusting in everyday life are damage to household tools (knives, cutlery, tools), damage to metal structures, damage to vehicles (both land, air and water), etc.
Perhaps the most common rusty things are keys, knives and tools. All these items are subject to rust due to the fact that friction removes the protective coating, which exposes the base.
The base is subject to destruction processes due to contact with aggressive environments (especially knives and tools).
Destruction due to contact with aggressive media
By the way, the destruction of things that are often used in everyday life can be observed almost everywhere and regularly, at the same time, some metal objects or structures can remain rusty for decades and will perform their functions properly. For example, a hacksaw, which was often used to saw logs and left for a month in a shed, will quickly rust and may break during the work, and a pole with a road sign can stand for ten or even more years rusty and not collapse.
Therefore, all metal items should be protected from corrosion. There are several methods of protection, but they are all chemical. The choice of such protection depends on the type of surface and the destructive factor acting on it.
To do this, the surface is thoroughly cleaned of dirt and dust in order to eliminate the possibility of the protective coating not reaching the surface. It is then degreased (for some types of alloy or metal and for some protective coatings this is necessary), after which a protective layer is applied. Most often, protection is provided by paints and varnishes. Depending on the metal and factors, different varnishes, paints and primers are used.
Another option is to apply a thin protective layer of another material. This method is usually practiced in production (for example, galvanizing). As a result, the consumer practically does not need to do anything after purchasing the item.
Applying a thin protective layer
Another option is to create special alloys that do not oxidize (for example, stainless steel), but they do not guarantee 100% protection; moreover, some things made from such materials oxidize.
Important parameters of protective layers are thickness, service life and rate of destruction under active adverse influences. When applying a protective coating, it is extremely important to accurately fit into the permissible layer thickness. Typically, manufacturers of paints and varnishes indicate it on the packaging. So, if the layer is larger than the maximum allowable, this will cause excessive consumption of varnish (paint), and the layer may be destroyed under strong mechanical stress, a thinner layer may wear off and shorten the protection period of the base.
A correctly selected protective material and correctly applied to the surface guarantees 80% that the part will not be subject to corrosion.
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Many people in everyday life do not think about how to protect their things from rye. And they get a problem in the form of a damaged item. How to properly solve this problem?
Removing rust from a part
In order to restore a thing or part from rust, the first step is to remove all the red coating to a clean surface. It can be removed with sandpaper, files, or strong reagents (acids or alkalis), but drinks like Coca-Cola have earned particular fame for this. To do this, the item is completely immersed in a container with a miracle liquid and left for some time (from several hours to several days - the time depends on the item and the damaged area).
Red spots on steel products
According to the UN, each country loses from 0.5 to 7-8% of its gross national product per year due to corrosion. The paradox is that less developed countries lose less than developed countries. And 30% of all steel products produced on the planet are used to replace rusted ones. Therefore, it is highly recommended that you take this issue seriously.
There are a lot of different factors that can significantly damage metal. At the same time, all metals corrode in one way or another and have some defects. For example, you should not join copper and aluminum if they are part of electrical wiring. This is because such a small electrolyzer is formed, which little by little eats away the metal. As a result, heating occurs, and ultimately an arc strike, which can lead to a fire. Some metals, like tin, tend to decay. This is the so-called tin plague. This can happen, for example, due to low temperature. But steel is most susceptible to corrosion. Steel, if it is not alloyed, that is, mixed with chromium, tends to rust. And rust is the most terrible enemy of steel and iron. It has several features that can simply destroy metal.
In fact, there are many different ways you can prevent metal corrosion. In some cases, this can help, but sometimes it is pointless. To prevent corrosion, there is a special anti-corrosion primer enamel, which prevents and prevents rust. But is it really that dangerous? Let's understand a little about this and how you can prevent rust.
Why is rust dangerous?
Rust is the decay of steel or iron. When iron and moisture come into contact, a chemical reaction occurs that turns the metal into corrosion. Because of this, it turns out that the metal loses its strength and becomes softer. This is dangerous for all metal structures as it becomes thinner. Long-term rusting can cause even very thick metal beams to collapse. In addition, it significantly spoils the appearance, especially if the metal has some decorative meaning.
Ways to prevent
For tools, regular oil or special lubricant is usually used. Thus, contact between metal and moisture is prevented. Thanks to this, no corrosion occurs. But large structures are difficult to coat with oil. That's why they are covered with enamel. You can buy them at the link http://www.untec.ru, where there is a large selection. The main point of enamel is that it applies like paint. It holds up much better and can be used on reinforced concrete or other materials. An excellent material that can even be used without a primer, as it adheres well even without it.
