CORROSION OF METALS
spontaneous physical and chemical destruction and transformation of a useful metal into useless chemical compounds. Most environmental components, whether liquids or gases, contribute to the corrosion of metals; constant natural influences cause rusting of steel structures, damage to car bodies, the formation of pittings (etching pits) on chrome coatings, etc. In these examples, the surface of the metal is visibly destroyed, but the concept of corrosion includes cases of internal destructive action, for example, at the interface between metal crystals. This so-called structural (intercrystalline) corrosion proceeds imperceptibly from the outside, but can lead to accidents and even accidents. Often, unexpected damage to metal parts is associated with stresses, in particular, those associated with corrosion fatigue of the metal. Corrosion is not always destructive. For example, the green patina often seen on bronze sculptures is copper oxide, which effectively protects the metal beneath the oxide film from further atmospheric corrosion. This explains the excellent condition of many ancient bronze and copper coins. Corrosion control is carried out by methods of protection developed on the basis of well-known scientific principles, but it remains one of the most serious and challenging tasks modern technology. OK. 20% of the total amount of metals is lost annually due to corrosion, and huge amounts of money are spent on corrosion protection.
Electrochemical nature of corrosion. M. Faraday (1830-1840) established a connection between chemical reactions and electric current, which was the basis of the electrochemical theory of corrosion. However, a detailed understanding of corrosion processes came only at the beginning of the 20th century. Electrochemistry as a science arose in the 18th century. thanks to the invention of A. Volta (1799) of the first galvanic cell (voltaic column), with the help of which a continuous current was obtained by converting chemical energy into electrical energy. A galvanic cell consists of a single electrochemical cell in which two different metals (electrodes) are partially immersed in an aqueous solution (electrolyte) capable of conducting electricity. The electrodes outside the electrolyte are connected by an electrical conductor (metal wire). One electrode ("anode") dissolves (corrodes) in the electrolyte, forming metal ions that go into solution, while hydrogen ions accumulate on the other electrode ("cathode"). The flow of positive ions in the electrolyte is compensated by the passage of an electron current (electrical current) from the anode to the cathode in an external circuit.
Metal ions, passing into the solution, react with the components of the solution, giving corrosion products. These products are often soluble and do not prevent further corrosion of the metal anode. So, if two adjacent areas, for example, on the surface of steel, even slightly differ from each other in composition or structure, then in a suitable (for example, humid) environment, a corrosion cell is formed in this place. One area is the anode to the other, and it is this area that will corrode. Thus, all small local inhomogeneities of the metal form anode-cathode microcells; for this reason, the metal surface contains numerous areas potentially susceptible to corrosion. If steel is immersed in ordinary water or almost any water-containing liquid, then a suitable electrolyte is already ready. Even in a moderately humid atmosphere, moisture condensate will settle on the metal surface, leading to the appearance of an electrochemical cell. As already noted, an electrochemical cell consists of electrodes immersed in an electrolyte (i.e., two half-cells). The potential (electromotive force, EMF) of an electrochemical cell is equal to the potential difference between the electrodes of both half-cells. The electrode potentials are measured relative to the hydrogen reference electrode. The measured electrode potentials of metals are summarized in a series of voltages, in which noble metals (gold, platinum, silver, etc.) are at the right end of the series and have a positive potential value. Ordinary, base metals (magnesium, aluminum, etc.) have strongly negative potentials and are located closer to the beginning of the row to the left of hydrogen. The position of the metal in the series of stresses indicates its resistance to corrosion, which increases from the beginning of the series to its end, i.e. from left to right.
See also ELECTROCHEMISTRY; ELECTROLYTES.
Polarization. The movement of positive (hydrogen) ions in the electrolyte towards the cathode with subsequent discharge leads to the formation of molecular hydrogen on the cathode, which changes the potential of this electrode: the opposite (stationary) potential is set, which reduces the total cell voltage. The current in the cell drops very quickly to extremely small values; in this case the cell is said to be "polarized". This condition suggests a reduction or even cessation of corrosion. However, the interaction of oxygen dissolved in the electrolyte with hydrogen can negate this effect, so oxygen is called a "depolarizer". The effect of polarization sometimes manifests itself as a reduction in the rate of corrosion in stagnant water due to lack of oxygen, although such cases are not typical, since the effects of convection in the liquid medium are usually sufficient to supply dissolved oxygen to the cathode surface. An uneven distribution of the depolarizer (usually oxygen) over the metal surface can also cause corrosion, since this forms an oxygen concentration cell in which corrosion occurs in the same way as in any electrochemical cell.
Passivity and other anode effects. The term "passivity" (passivation) was originally used in relation to the corrosion resistance of iron immersed in a concentrated solution of nitric acid. However, this is a more general phenomenon, since under certain conditions many metals are in a passive state. The passivity phenomenon was explained in 1836 by Faraday, who showed that it was caused by an extremely thin oxide film formed as a result of chemical reactions on the metal surface. Such a film can be restored (changed chemically), and the metal becomes active again upon contact with a metal that has a more negative potential, for example, iron in the vicinity of zinc. In this case, a galvanic couple is formed, in which the passive metal is the cathode. The hydrogen released on the cathode restores its protective oxide film. Oxide films on aluminum protect it from corrosion, and therefore anodized aluminum resulting from the anodic oxidation process is used both for decorative purposes and in everyday life. In a broad chemical sense, all anodic processes occurring on the metal are oxidative, but the term "anodic oxidation" implies the targeted formation of a significant amount of solid oxide. A film of a certain thickness is formed on aluminum, which is the anode in the cell, the electrolyte of which is sulfuric or phosphoric acid. Many patents describe various modifications of this process. The initially anodized surface has a porous structure and can be painted in any desired color. The introduction of potassium bichromate into the electrolyte gives a bright orange-yellow tint, while potassium hexacyanoferrate(II), lead permanganate, and cobalt sulfide color the films blue, red-brown, and black, respectively. In many cases, water-soluble organic dyes are used and this imparts a metallic sheen to the painted surface. The resulting layer must be fixed, for which it is enough to treat the surface with boiling water, although boiling solutions of nickel or cobalt acetates are also used.
Structural (intergranular) corrosion. Various alloys, in particular aluminum, increase their hardness and strength during aging; the process is accelerated by subjecting the alloy to heat treatment. In this case, submicroscopic particles are formed, which are located along the boundary layers of microcrystals (in the intergranular space) of the alloy. Under certain conditions, the area immediately adjacent to the boundary becomes an anode with respect to the inner part of the crystal, and in a corrosive environment, the boundaries between crystallites will be predominantly subject to corrosion, with corrosion cracks penetrating deeply into the metal structure. This "structural corrosion" seriously affects the mechanical properties. It can be prevented either with the help of properly selected heat treatment modes, or by protecting the metal with a corrosion-impervious coating. Cladding is a cold coating of one metal with another: a high-strength alloy is rolled between thin strips of pure aluminum and compacted. The metal included in such a composition becomes corrosion-resistant, while the coating itself has little effect on the mechanical properties.
See also METAL COATINGS.
Corrosion prevention. During electrochemical corrosion, the resulting products often dissolve (pass into solution) and do not prevent further destruction of the metal; in some cases, a chemical compound (inhibitor) can be added to the solution, which reacts with the primary corrosion products to form insoluble and protective compounds that are deposited on the anode or cathode. For example, iron easily corrodes in a dilute solution of ordinary salt (NaCl), however, when zinc sulfate is added to the solution, sparingly soluble zinc hydroxide is formed at the cathode, and when sodium phosphate is added, insoluble iron phosphate is formed at the anode (examples of cathodic and anodic inhibitors, respectively). Such protection methods can only be used when the structure is wholly or partially immersed in a liquid corrosive medium. Cathodic protection is often used to reduce the rate of corrosion. In this method, an electrical voltage is applied to the system in such a way that the entire structure to be protected is the cathode. This is done by connecting the structure to one pole of a rectifier or DC generator while an external chemically inert anode such as graphite is connected to the other pole. For example, in the case of corrosion protection of pipelines, an insoluble anode is buried in the ground near them. In some cases, additional protective anodes are used for this purpose, for example, suspended inside water storage tanks, the water in the tank acting as an electrolyte. Other methods of cathodic protection provide sufficient current to flow from some other source through the structure, which becomes the entire cathode and contains possible local anodes and cathodes at the same potential. To do this, a metal with a more negative potential is connected to the protected metal, which in the formed galvanic pair plays the role of a sacrificial anode and is destroyed first. Zinc protector anodes have been used since 1825, when the famous English chemist H. Davy suggested using them to protect copper sheathing. wooden cases ships. Anodes based on magnesium alloys are widely used to protect the hulls of modern ships from corrosion in sea water. Protector anodes are more commonly used than anodes associated with external sources current, since they do not require energy consumption. Surface painting is also used to protect against corrosion, especially if the structure is not completely immersed in liquid. Metallic coatings can be applied by metal spraying or electroplating (eg chromium plating, zinc plating, nickel plating).
Types of specific corrosion. Stress corrosion is the destruction of metal under the influence of the combined action of static load and corrosion. The main mechanism is the initial formation of corrosion pits and cracks, followed by structural failure caused by stress concentrations in these cracks. The details of the corrosion mechanism are complex and not always understood, and may be related to residual stresses. Pure metals, as well as brass, are not prone to stress corrosion. In the case of alloys, cracks appear in the intergranular space, which is an anode in relation to the internal regions of the grains; this increases the likelihood of corrosion action along the intergranular boundaries and facilitates the subsequent process of cracking along them. Corrosion fatigue is also a consequence of the combined action of mechanical stress and corrosion. However, cyclic loads are more dangerous than static ones. Fatigue cracking often occurs in the absence of corrosion, but the destructive effect of corrosion cracks that create stress concentrations is obvious. It is likely that all so-called fatigue mechanisms involve corrosion, since surface corrosion cannot be completely excluded. Liquid metal corrosion is a special form of corrosion that does not involve an electrochemical mechanism. Liquid metals are of great importance in cooling systems, in particular, nuclear reactors. Liquid potassium and sodium and their alloys, as well as liquid lead, bismuth and lead-bismuth alloys are used as coolants. Most structural metals and alloys, when in contact with such a liquid medium, undergo destruction to one degree or another, and the corrosion mechanism in each case may be different. First, the material of the container or pipes in the heat transfer system may dissolve to a small extent in the liquid metal, and since the solubility usually changes with temperature, the dissolved metal may precipitate out of solution in the cooled part of the system, thereby clogging the channels and valves. Secondly, intergranular penetration is possible liquid metal, if there is its selective reaction with alloying additives structural material. Here, as in the case of electrochemical intergranular corrosion, the mechanical properties deteriorate without visible manifestations and without changing the mass of the structure; however, such cases of destructive impact are rare. Thirdly, liquid and solid metals can interact with the formation of a surface alloy, which in some cases serves as a diffusion barrier in relation to further exposure. Erosive corrosion (impact, cavitation corrosion) refers to the mechanical action of liquid metal flowing in a turbulent regime. In extreme cases, this leads to cavitation and erosional destruction of the structure.
See also CAVITATION. The corrosive effects of radiation are being intensively studied in connection with the development of nuclear energy, but there is little information on this issue in the open press. The common term "radiation damage" refers to all changes in the mechanical, physical or chemical nature of solid materials that are caused by exposure to radiation of the following types: ionizing radiation (X-ray or g), light charged particles (electrons), heavy charged particles (a-particles) and heavy uncharged particles (neutrons). It is known that the bombardment of metal by high-energy heavy particles leads to the occurrence of disturbances at the atomic level, which, under appropriate circumstances, can be the sites of electrochemical reactions. However, a more important change occurs not in the metal itself, but in its environment. Such indirect effects arise as a result of the action of ionizing radiation (for example, g-rays), which does not change the properties of the metal, but in aqueous solutions causes the formation of highly reactive free radicals and hydrogen peroxide, and such compounds contribute to an increase in the corrosion rate. In addition, a corrosion inhibitor such as sodium dichromate will recover and lose its effectiveness. Under the action of ionizing radiation, oxide films are also ionized and lose their corrosion-protective properties. All of the above features are highly dependent on the specific conditions associated with corrosion.
Oxidation of metals. Most metals react with atmospheric oxygen to form stable metal oxides. The rate at which oxidation occurs is highly dependent on temperature, and at normal temperature only a thin oxide film is formed on the metal surface (on copper, for example, this is noticeable by the darkening of the surface). At higher temperatures, the oxidation process proceeds faster. Noble metals are an exception to this rule, as they have a low affinity for oxygen. It is assumed that gold does not oxidize at all when heated in air or in oxygen, and the weak oxidation of platinum at temperatures up to 450 ° C stops when heated to higher temperatures. Ordinary structural metals, on the other hand, oxidize to form four types of oxide compounds: volatile, dense, protective, or non-porous. A small number of refractory metals, such as tungsten and molybdenum, become brittle at high temperatures and form volatile oxides, so a protective oxide layer is not formed and at high temperatures the metals should be protected by an inert atmosphere (inert gases). Ultralight metals form, as a rule, too dense oxides, which are porous and do not protect the metals from further oxidation. For this reason, magnesium oxidizes very easily. Protective oxide layers form on many metals, but they usually have a moderate protective ability. An oxide film on aluminum, for example, completely covers the metal, but cracks develop under compressive stresses, apparently due to changes in temperature and humidity. The protective effect of oxide layers is limited by relatively low temperatures. Many "heavy metals" (eg copper, iron, nickel) form non-porous oxides which, although they do not crack, do not always protect the base metal. Theoretically, these oxides are of great interest and are being actively studied. They contain less than a stoichiometric amount of metal; missing metal atoms form holes in the oxide lattice. As a result, atoms can diffuse through the lattice, and the thickness of the oxide layer is constantly increasing.
The use of alloys. Since all known structural metals are prone to oxidation, structural elements that are at high temperatures in an oxidizing environment should be made from alloys that contain an oxidizing-resistant metal as an alloying element. Chromium meets these requirements - a fairly cheap metal (used in the form of ferrochromium), which is present in almost all high-temperature alloys that meet the requirements for oxidation resistance. Therefore, all stainless steels alloyed with chromium have good oxidation resistance and are wide application in household and industry. Nichrome alloy, which is widely used as a wire for spirals electric ovens, contains 80% nickel and 20% chromium and is quite resistant to oxidation at temperatures up to 1000 ° C. Mechanical properties are important as well as oxidation resistance, and it is often found that certain alloy elements (such as chromium) give the alloy and high temperature strength, and resistance to oxidation, so that the problem of high temperature oxidation did not introduce serious difficulties until the use (in gas turbine engines) of fuel oil containing vanadium or sodium as a fuel. These contaminants, together with the sulfur in the fuel, produce combustion products that are extremely corrosive. Attempts to solve this problem have culminated in the development of additives that, when burned, form harmless volatile compounds with vanadium and sodium. Fretting corrosion does not include electrochemical corrosion or direct oxidation in the gas phase, but is mainly a mechanical effect. This is damage to articulated metal surfaces as a result of abrasion at their small multiple relative displacements; observed in the form of scratches, ulcers, shells; is accompanied by jamming and reduces resistance to corrosion fatigue, as the resulting scratches serve as starting points for the development of corrosion fatigue. Typical examples are damage in the grooves of the turbine blades due to vibration, abrasion of the compressor impellers, wear of the gear teeth, threaded connections etc. At small repeated displacements, the protective oxide films are destroyed, rubbed into powder, and the corrosion rate increases. Fretting corrosion of steel is easily identified by the presence of red-brown oxide particles. The fight against fretting corrosion is carried out by improving designs, using protective coatings, elastomeric gaskets, and lubricants.
see also
Great Soviet Encyclopedia
Corrosion of metals- - destruction of metals due to their chemical or electrochemical interaction with a corrosive environment. 1. For the corrosion process, the term "corrosive process" should be used, and for the result of the process - "corrosive ... ... Encyclopedia of terms, definitions and explanations of building materials
metal corrosion- Destruction of metals due to their chemical or electrochemical interaction with a corrosive environment. Application 1. For the corrosion process, the term "corrosive process" should be used, and for the result of the process ... ... Technical Translator's Handbook
Surface destruction of metals under the action of chemical. or electrochem. factors: exposure to gases (oxygen, carbon dioxide, hydrogen sulfide, etc.), soluble salts(of which the most dangerous are chlorides), mineral and organic acids ... Technical railway dictionary
CORROSION OF METALS- spontaneous destruction of metals caused by chemical. and electrochem. processes on their surface when interacting with the external environment, as a result of which the appearance surface, then plasticity is lost, mechanical ... Great Polytechnic Encyclopedia
Corrosion of metals- 1. Corrosion of metals The destruction of metals due to their chemical or electrochemical interaction with a corrosive environment 1. For the corrosion process, the term "corrosive process" should be used, and for the result of the process "corrosive ... ... Dictionary-reference book of terms of normative and technical documentation
- (from late Latin corrosio corrosive), physical. chem. metal interaction. material and environment, leading to a deterioration in operational performance. St. in material, environment or tech. systems that they are parts of. At the heart of K. the m is chemical. ration between the material and ... ... Chemical Encyclopedia
metal corrosion- metalų korozija statusas T sritis chemija apibrėžtis Metalų, jų lydinių ir metalinių gaminių irimas dėl aplinkos poveikio. atitikmenys: engl. corrosion of metals; metal corrosion; metallic corrosion rus. metal corrosion... Chemijos terminų aiskinamasis žodynas
Corrosion of metals- Corrosion: a physical-chemical interaction between a metal and a medium that changes the properties of the metal and often results in deterioration of the functional characteristics of the metal, the medium or the medium containing them. technical system...
Corrosion is the destruction of metal, ceramic, wood and other materials as a result of chemical or physico-chemical interaction. As for the causes of such an undesirable effect, they are different. In most cases, this is a structural instability to the thermodynamic effects of the environment. Let's take a closer look at what corrosion is. Types of corrosion must also be considered, and it will not be superfluous to talk about protection against it.
Some general information
We are accustomed to hearing the term "rusting" which is used in the case of corrosion of metal and alloys. There is also such a thing as "aging" - it is characteristic of polymers. Essentially, they are one and the same. A vivid example is the aging of rubber products due to active interaction with oxygen. Besides this, some plastic elements are destroyed under the influence. The rate of corrosion directly depends on the conditions in which the object is located. So, rust on a metal product will spread the faster, the higher the temperature. Humidity also affects: the higher it is, the faster unsuitable for further exploitation. It has been empirically established that approximately 10 percent of metal products are irretrievably written off, and corrosion is to blame. Types of corrosion are different and are classified depending on the type of media, the nature of the flow, and the like. Let's look at them in more detail.
Classification
Currently, there are more than two dozen rusting options. We present only the most basic types of corrosion. Conventionally, they can be divided into the following groups:
- Chemical corrosion is a process of interaction with a corrosive environment, in which the reduction of an oxidizing agent takes place in one act. Metal and oxidizing agent are not spatially separated.
- Electrochemical corrosion - the process of metal interaction with the ionization of atoms and the reduction of the oxidizing agent take place in different acts, but the rate largely depends on the electrode potential.
- Gas corrosion - chemical rusting of metal at a minimum moisture content (not more than 0.1 percent) and / or high temperatures in a gaseous environment. Most often this species found in the chemical and oil refining industries.
In addition, there is still a huge number of rusting processes. All of them are corrosion. Types of corrosion, in addition to those described above, include biological, radioactive, atmospheric, contact, local, target rusting, etc.
Electrochemical corrosion and its features
With this type of destruction, the process proceeds when the metal comes into contact with the electrolyte. The latter can be condensate or rainwater. The more salts and acids are contained in the liquid, the higher the electrical conductivity and, consequently, the rate of the process. As for the places of the metal structure most susceptible to corrosion, these are rivets, welded joints, places of mechanical damage. If the structural properties of the iron alloy make it resistant to rust, the process slows down somewhat, but still continues. A prime example is galvanization. The fact is that zinc has a more negative potential than iron. For this simple reason, the iron alloy recovers and the zinc corrodes. However, the presence of an oxide film on the surface greatly slows down the destruction process. Of course, all types of electrochemical corrosion are extremely dangerous and sometimes it is even impossible to fight them.
Chemical corrosion
Such a change in the metal is quite common. A striking example is the appearance of scale as a result of the interaction of metal products with oxygen. High temperature in this case acts as an accelerator of the process, and liquids such as water, salts, acids, alkalis and salt solutions can participate in it. If we talk about materials such as copper or zinc, then their oxidation leads to the formation of a film that is resistant to further corrosion. Steel products form iron oxides. Further lead to the appearance of rust, which does not provide any protection against further destruction, but rather contributes to this. Currently, all types of chemical corrosion are eliminated by galvanizing. Other means of protection may also apply.
Types of concrete corrosion
Changing the structure and increasing the fragility of concrete under the influence of the environment can be of three types:
- The destruction of parts of cement stone is one of the most common types of corrosion. It occurs if the concrete product is systematically exposed to precipitation and other liquids. As a result, calcium oxide hydrate is washed out and the structure is disturbed.
- interaction with acids. If a cement stone will come into contact with acids, then calcium bicarbonate is formed - an aggressive chemical element for a concrete product.
- Crystallization of sparingly soluble substances. In fact, this refers to biocorrosion. The bottom line is that microorganisms (spores, fungi) enter the pores and develop there, as a result of which destruction occurs.
Corrosion: types, methods of protection
Billions in annual losses have led people to fight it harmful effects. It is safe to say that all types of corrosion lead to the loss not of the metal itself, but of valuable metal structures, the construction of which costs a lot of money. It is difficult to say whether it is possible to provide 100 percent protection. However, at proper preparation surfaces that are abrasive blast cleaned can achieve good results. Reliably protects the paintwork from electrochemical corrosion if it is applied correctly. And a special surface treatment will reliably protect against the destruction of metal underground.
Active and passive methods of struggle
The essence of active methods is to change the structure of the double electric field. To do this, use a constant current source. The voltage must be chosen in such a way that the product to be protected increases. Another extremely popular method is the "sacrificial" anode. It collapses, protecting the base material.
Passive protection involves the use of paintwork. The main task is to completely prevent the ingress of moisture, as well as oxygen, onto the protected surface. As noted above, it makes sense to use zinc, copper or nickel coating. Even a partially destroyed layer will protect the metal from rusting. Of course, these types of metal corrosion protection are effective only when the surface does not have visible defects in the form of cracks, chips, and the like.
Galvanizing in detail
We have already considered the main types of corrosion, and now I would like to talk about best practices protection. One of these is galvanizing. Its essence lies in the fact that zinc or its alloy is applied to the treated surface, which gives the surface certain physical and chemical properties. It is worth noting that this method is considered one of the most economical and efficient, and this despite the fact that about 40 percent of the world production of this element is spent on zinc plating. Steel sheets, fasteners, as well as appliances and other metal structures can be galvanized. Interestingly, with the help of plating or spraying, you can protect a product of any size and shape. Zinc does not have a decorative purpose, although with the help of some special additives it becomes possible to obtain shiny surfaces. In principle, this metal is able to provide maximum protection in aggressive environments.
Conclusion
So we told you about what corrosion is. Types of corrosion were also considered. Now you know how to protect the surface from premature rusting. By and large, it is extremely simple to do this, but where and how the product is operated is of considerable importance. If it is constantly subjected to dynamic and vibration loads, then there is a high probability of cracks in the paintwork, through which moisture will enter the metal, as a result of which it will gradually collapse. However, the use of various rubber gaskets and sealants in metal-to-metal contact areas can slightly extend the life of the coating.
Well, that's all for this topic. Be aware that premature structural failure due to corrosion can lead to unforeseen consequences. At the enterprise, large material damage and human casualties are possible as a result of rusting of the supporting metal structure.
Corrosion lends itself to many materials, such as metal, ceramic, wood, as a result of exposure to them. As a rule, this effect is achieved due to the instability of the structure, which is affected by the thermodynamics of the environment. In the article we will understand in detail what metal corrosion is, what types it has, and also how you can protect yourself from it.
Some general information
Among the people, the word "rust" is quite popular, which refers to the process of corrosion of metal and various alloys. For polymers, people use the concept of “aging”. In fact, these words are synonymous. A striking example is the aging of rubber products that actively interact with oxygen. Some plastic products can quickly become unusable due to precipitation. How quickly the corrosion process will occur depends entirely on the conditions in which the product is placed. The humidity of the environment is especially affected. The higher its value, the faster the metal will become unusable. Experimentally, scientists have found that about 10% of products in production are simply written off due to corrosion. The types of this process are different, their classification depends on the type of environment in which the products are located, the speed and nature of the flow. Next, we consider in more detail the types of corrosion. Now every person should understand what metal corrosion is.
artificial aging
The corrosion process is not always destructive and renders certain materials unusable. Often, due to corrosion, the coating has additional properties that a person needs. That is why it became popular artificial aging. Most often it is used when it comes to aluminum and titanium. Only with the help of corrosion it is possible to achieve increased strength of materials. In order to complete the destruction process correctly, it is necessary to use heat treatment. Given that the natural aging of materials under certain conditions is a rather slow process, there is no need to clarify that when using this method, the material must have a special hardening. You also need to understand all the risks that are associated with this method. For example, although the strength of the material increases, but the ductility decreases as much as possible. With ease, now the reader will be able to answer the question of what is the corrosion of an artificial type of metal.
Heat treatment reviews
This method densifies the molecules of the material, respectively, the structure changes. Often, thermal protection is necessary to strengthen pipelines, as it allows you to protect the material from rust, as well as minimize the pressure that is exerted on the structure if it is underground. Users of this technique leave reviews in which they describe that this method of protection is as effective as possible and really shows nice results. Such processing is desirable to apply only in the industrial sector. Due to the fact that the chambers for firing and performing other processes necessary to obtain reliable protection are expensive, the method is not popular. Such protection of metal from corrosion is quite effective.
Classification
At the moment, there are more than 20 rust options. The article will describe only the most popular types of corrosion. Conventionally, they are divided into the following groups, which will help to understand in more detail what metal corrosion is.
Chemical corrosion is the interaction with a corrosive environment. In this case, the oxidation of the metal and the reduction of the oxidizing agent occur simultaneously in one cycle. Both materials are not separated by space. Consider other types of metal corrosion.
Electrochemical corrosion is the interaction of a metal with an electrolyte. The atoms are ionized, the oxidizing agent is reduced, and these two processes occur over several cycles. Their speed is completely dependent on the potential of the electrodes.
Gas corrosion is the rusting of metal with a small amount of liquid. Moisture should not exceed 0.1%. Also, this type of corrosion can occur in a gaseous environment at high temperatures. Most often this species is found in the industry associated with the chemical industry and oil refining.
In addition to the above, there are many more types of corrosion of materials. There are biological, target, contact, local and other types of rusting.
Electrochemical corrosion and its features
In electrochemical corrosion, the destruction of the material occurs due to its contact with the electrolyte. As the last substance, there may be condensate, rainwater. It should be noted that the more salts in the liquid, the higher the electrical conductivity. Accordingly, the corrosion process will proceed quite quickly. If we talk about the most popular places that are susceptible to corrosion, it should be noted rivets in a metal structure, welded joints, as well as simply places where the material is damaged. It happens that an iron alloy during its creation is coated with special substances that have anti-corrosion properties. However, this does not prevent the rusting process, but only slows it down. A fairly striking example is galvanization. Zinc has a negative potential when compared to iron. Because of this, the last material will be restored, and the zinc will be damaged. If there is an oxide film on the surface, the destruction process will become lengthy. Electrochemical corrosion has several types, but it should be noted that all of them are dangerous and, as a rule, it is impossible to stop this type of metal corrosion.
Chemical corrosion
Chemical corrosion is quite common. For example, if a person notices scale, then he must understand that it appeared as a result of metal combination, that is, interaction, with oxygen. As a rule, if the ambient temperature is high, the corrosion process will be markedly accelerated. A liquid can participate in rusting, that is, water, salt, any acid or alkali, salt solutions. When it comes to chemical corrosion of metals such as copper or zinc, their oxidation leads to a stable corrosion process of the film. The rest form iron oxide. Further, all the chemical processes that will occur will lead to the appearance of rust. It will not provide protection in any way, but, on the contrary, contributes to the occurrence of corrosion. With the help of galvanizing at the moment it is possible to protect many materials. Other means of protection against chemical corrosion of metals have also been developed.
Types of concrete corrosion
The brittleness of concrete can be caused by one of three types of corrosion. Quite often there is a change in the structure of this material. Let's take a look at why this is happening.
The most common type of corrosion should be called the destruction of cement stone. As a rule, this occurs when liquid and atmospheric precipitation constantly act on the material. Because of this, the structure of the material is destroyed. There are more below detailed examples metal corrosion:
- interaction with acids. If the cement stone is constantly exposed to these materials, then a rather aggressive element is formed, which is harmful to the coating. This is calcium bicarbonate.
- Crystallization of sparingly soluble substances. This is about corrosion. Due to the fact that fungi, spores and other substances enter the pores, the concrete coating begins to quickly collapse.
Corrosion: ways to protect
Manufacturers often suffer huge losses due to corrosion, so a lot of work is being done to avoid this process. Moreover, it should be noted that most often corrosion does not lend itself to the metal itself, but to huge metal structures. Manufacturers spend a lot of money on their creation. Unfortunately, it is almost impossible to provide 100% protection. However, if you properly protect the surface, that is, carry out abrasive blasting, you can delay the corrosion process for several years. They also fight with paintwork. It reliably protects the material. If the metal is underground, then it must be treated with special materials. This is the only way to achieve maximum protection of the metal from corrosion.
Measures to prevent aging
As mentioned above, the corrosion process cannot be stopped. But you can maximize the time during which the material will collapse. Also, in production, as a rule, they try to get rid of the factors that affect the aging process as much as possible. For example, in factories, each structure is periodically treated with solutions and polishes. It is they who save the material from the negative impact on the metal from the mechanical, temperature and chemical conditions. In order to understand this in more detail, it is necessary to study the definition of corrosion of metals. If we talk about slowing down the effect of aging, then it should be noted that heat treatment can be used for this. Under normal operating conditions, this method will avoid the rapid destruction of the material as much as possible. Welders, in order to prevent the seams on the product from opening, use firing at a temperature of 650 degrees. This technique will reduce the intensity of aging.
Active and passive methods of struggle
Active anti-corrosion methods act by changing the structure of the electric field. To do this, you need to use direct current. The voltage must be such that the product has enhanced characteristics. A fairly popular method would be to use a “sacrificial” anode. It protects the material by its own destruction. The conditions for corrosion of metals are described above.
As for passive protection, a paintwork is used for this. It completely protects the product from the ingress of liquid, as well as oxygen. Thanks to this, the surface is maximally protected from destruction. Zinc, copper, nickel coating should be used. Even if the layer is severely destroyed, it will still protect the metal from rusting. Of course, you need to understand that passive protection methods will only be relevant if the surface does not have cracks or chips.
Reviews about the paint and varnish protection of metals
At the moment paint protection enjoys particular popularity. It is efficient, flexible to use, and inexpensive. However, if long-term use of a metal structure is necessary, then this method of protection will not work. More than 7-8 years paint and varnish coatings will not be able to protect the material. Accordingly, they will have to be updated. Most likely, it will be necessary to carry out restoration and replace the surface of the material. Among other disadvantages of this coating, limitations in terms of use should be noted. If it is necessary to strengthen pipes that are underground or water, then paint protection will not work. Therefore, it should be understood that if it is necessary for the structure to be used for more than 10 years, other methods of protection should be resorted to.
Galvanizing in detail
Having considered the main types of corrosion, it is also necessary to discuss the most effective methods protection. One of these is galvanizing. It allows you to protect the material from severe damage by changing the physical and chemical properties. At the moment, this method is considered economical and efficient, given that almost 40% of all mined material on Earth is spent on zinc processing. It is important to treat the material with an anti-corrosion coating.
Galvanizing is carried out for steel sheets, fasteners, appliances and huge metal structures. In general, with the help of such spraying, products of any size and shape can be protected. Zinc has no decorative purpose, although it may occasionally be added to the alloy to give it a sheen. In general, you need to understand that this metal will provide maximum protection against corrosion even in the most aggressive conditions.
Rust protection features
When working with metal, any person understands that before applying protective materials, it is necessary to prepare the surface. Often all the difficulties lie precisely in this stage. In order to create a special barrier that will allow rust to reach the metal, it is necessary to introduce the concept of a compound. Thanks to him, the kit will form protection against corrosion. In this case, electrical insulation takes place. It is usually quite difficult to protect against corrosion of ferrous metals.
Due to the nature of the use various means for protection, it is necessary to understand the operating conditions of the material. If the metal will be located underground, then it is necessary to use multi-layer coatings that will have not only anti-corrosion properties, but also enhanced protection against mechanical damage. If we are talking about communications that actively interact with oxygen and gases, you should use a tool that minimizes the effects of water and oxygen. Accordingly, increased attention on the part of the manufacturer will be given to insulation from moisture, steam and low temperatures. In this case, additives and special plasticizers should be added, because the causes of metal corrosion are different and all types should be protected.
Mix "Urizol"
The Urizol mixture should be considered separately, as it is used to coat the pipeline. It is also suitable for fittings, fittings, valve assemblies and those products that are constantly in contact with oil or gases. This composition is needed in order to get rid of the influence of underground and atmospheric influences. Often this mixture is also used for the insulation of concrete materials. This substance is applied very simply, without any difficulty. In order to treat the surface, it is necessary to use a sprayer. This is the only way to avoid corrosion of metals and alloys of similar products. As soon as the components are combined, the reaction begins. This results in polyurea. After that, the mixture passes into a gel-like and non-fluid state, and after some time becomes solid. If the polymerization rate is slow, smudges will begin to form. They are harmful, because they make it difficult to increase the thickness of the coating. It should be noted that this mixture retains a sticky state for a long time. Due to this, all layers will be as uniform as possible, and intermediate thickness measurements will be equal to each other. If the polymerization process is too fast, then the adhesion of the composition will decrease. In this case, the thickness of the resulting layer for insulation will be uneven. By the way, the spray gun will quickly clog if the coating speed is too fast. Metal corrosion factors will not appear if everything is done correctly. In order to prevent such situations, it is necessary to carefully select the components and follow the manufacturing rules.
Paints and enamels
Protection of metal-plastic structures can be carried out using three methods.
Coatings have already been described. They are simple, have a variety color solution, and also with the help of them you can easily handle huge surfaces. Since the process of metal corrosion is quite fast, then you should immediately think about coating with materials.
The second type is plastic coatings. As a rule, they are made of nylon, PVC. This coating will provide maximum protection against water, acids and alkalis.
The third type is rubber coating. Often it is used to protect tanks and other structures from the inside.
Phosphating and chromating
The metal surface must be properly prepared for the protection process. Which methods will be used depends entirely on the type of surface. For example, ferrous metals are protected by phosphating. Non-ferrous metals can be processed by both methods. In general, if we talk about chemical preparation, it is necessary to clarify that it takes place in several stages. To begin with, the surface is degreased. Then it is washed with water. Next, a conversion layer is applied. After that, it is washed again with two types of water: drinking and demineralized, respectively. The next thing to do is passivation. Chemical treatment should be carried out by spraying, immersion, steam jet and water jet methods. The first two methods must be applied using special units that will fully prepare the surface for work. Which method to choose, it is necessary to decide depending on the size, configuration of the product, and so on. In order to better understand this issue, one should know the equations for the reactions of corrosion of metals.
Conclusion
The article described what corrosion is and what types it has. Now any person after reading this article will be able to understand how to protect any material from aging. By and large, this is quite easy to do, knowing everything necessary instructions. The main thing is to understand all the characteristics of the environment in which the material is used. If the products are located in a place where constant vibrations occur, as well as there are strong loads, then cracks will occur in the paintwork. Because of this, moisture will begin to get on the metal, respectively, the corrosion process begins immediately. In such cases, it is better to additionally use rubber sealants and gaskets, then the coating will last a little longer.
In addition, it must be said that the design, with premature deformation, will quickly deteriorate and age. Accordingly, this can lead to completely unforeseen circumstances. This will bring material damage and may result in the death of a person. Accordingly, special attention should be paid to corrosion protection.
Corrosion of metals, as you know, brings a lot of trouble. Is it not for you, dear car owners, to explain what she threatens: give her free rein, so only tires will remain from the car. Therefore, the sooner the fight against this disaster begins, the longer the car body will live.
To be successful in the fight against corrosion, it is necessary to find out what kind of "beast" it is and understand the reasons for its occurrence.
Today you will know
Is there any hope?
The damage done to mankind by corrosion is colossal. According to various sources, corrosion "eats" from 10 to 25% of the world's iron production. Turning into a brown powder, it is irretrievably scattered over the white light, as a result of which not only we, but also our descendants are left without this most valuable structural material.
But the trouble is not only that metal is lost as such, no - bridges, cars, roofs, architectural monuments are destroyed. Corrosion spares nothing.
The Eiffel Tower, the symbol of Paris, is terminally ill. Made of ordinary steel, it inevitably rusts and collapses. The tower has to be painted every 7 years, which is why its mass increases by 60-70 tons each time.
Unfortunately, it is impossible to completely prevent the corrosion of metals. Well, except to completely isolate the metal from the environment, for example, place it in a vacuum. 🙂 But what is the use of such "canned" parts? The metal must "work". Therefore, the only way to protect against corrosion is to find ways to slow it down.
In ancient times, fat, oils were used for this, later they began to cover iron with other metals. First of all, low-melting tin. In the writings of the ancient Greek historian Herodotus (5th century BC) and the Roman scientist Pliny the Elder, there are already references to the use of tin to protect iron from corrosion.
An interesting incident occurred in 1965 at the International Symposium on Corrosion Control. An Indian scientist spoke about a society for the fight against corrosion, which has existed for about 1600 years, and of which he is a member. So, one and a half thousand years ago, this society took part in the construction of temples of the Sun on the coast near Konarak. And despite the fact that these temples were flooded by the sea for some time, the iron beams are perfectly preserved. So even in those distant times, people knew a lot about the fight against corrosion. So, not everything is so hopeless.
What is corrosion?
The word "corrosion" comes from the Latin "corrodo" - to gnaw. There are also references to the late Latin "corrosio - corrosive". But anyway:
Corrosion is the process of metal destruction as a result of chemical and electrochemical interaction with the environment.
Although corrosion is most commonly associated with metals, it also affects concrete, stone, ceramics, wood, and plastics. Applied to polymer materials However, the term destruction or aging is more often used.
Corrosion and rust are not the same
In the definition of corrosion in the paragraph above, the word “process” is not in vain highlighted. The fact is that corrosion is often identified with the term "rust". However, these are not synonyms. Corrosion is precisely a process, while rust is one of the results of this process.
It is also worth noting that rust is a corrosion product exclusively of iron and its alloys (such as steel or cast iron). Therefore, when we say “steel rusts”, we mean that the iron in its composition rusts.
If rust only applies to iron, then other metals don't rust? They don't rust, but that doesn't mean they don't corrode. They just have different corrosion products.
For example, copper, corroding, is covered with a beautiful greenish coating (patina). Silver tarnishes in air - this is a deposit of sulfide on its surface, whose thin film gives the metal a characteristic pinkish color.
Patina is a corrosion product of copper and its alloys.
The mechanism of the course of corrosion processes
The variety of conditions and environments in which corrosion processes occur is very wide, so it is difficult to give a single and comprehensive classification of the occurring corrosion cases. But despite this, all corrosion processes have not only a common result - the destruction of the metal, but also a single chemical entity - oxidation.
Simplified, oxidation can be called the process of electron exchange of substances. When one substance is oxidized (donates electrons), the other, on the contrary, is reduced (receives electrons).
For example, in a reaction... 
… a zinc atom loses two electrons (is oxidized), and a chlorine molecule adds them (is reduced).
Particles that donate electrons and are oxidized are called reducing agents, and particles that accept electrons and are reduced are called oxidizers. These two processes (oxidation and reduction) are interrelated and always occur simultaneously.
Such reactions, which are called redox reactions in chemistry, underlie any corrosion process.
Naturally, the tendency to oxidation in different metals is not the same. To understand which ones have more and which ones have less, let's remember the school chemistry course. There was such a thing as an electrochemical series of voltages (activity) of metals, in which all metals are arranged from left to right in order of increasing “nobility”.
So, the metals located in the row to the left are more prone to donating electrons (and hence to oxidation) than the metals to the right. For example, iron (Fe) is more susceptible to oxidation than more noble copper(Cu). Some metals (for example, gold) can donate electrons only under certain extreme conditions.
We will return to the activity series a little later, but now let's talk about the main types of corrosion.
Types of corrosion
As already mentioned, there are many criteria for the classification of corrosion processes. So, corrosion is distinguished by the type of distribution (solid, local), by the type of corrosive medium (gas, atmospheric, liquid, soil), by the nature of mechanical effects (corrosion cracking, Fretting phenomenon, cavitation corrosion) and so on.
But the main way to classify corrosion, which makes it possible to most fully explain all the subtleties of this insidious process, is classification according to the mechanism of flow.
According to this criterion, two types of corrosion are distinguished:
- chemical
- electrochemical
Chemical corrosion
Chemical corrosion differs from electrochemical corrosion in that it occurs in media that do not conduct electric current. Therefore, with such corrosion, the destruction of the metal is not accompanied by the appearance of an electric current in the system. This is the usual redox interaction of the metal with the environment.
The most typical example of chemical corrosion is gas corrosion. Gas corrosion is also called high-temperature corrosion, since it usually proceeds at elevated temperatures when the possibility of moisture condensation on the metal surface is completely excluded. This type of corrosion can include, for example, corrosion of elements of electric heaters or nozzles of rocket engines.
The rate of chemical corrosion depends on temperature - as it rises, corrosion accelerates. Because of this, for example, during the production of rolled metal, fiery splashes scatter in all directions from the hot mass. It is scale particles that are chipped off the surface of the metal.
Scale is a typical product of chemical corrosion, an oxide resulting from the interaction of hot metal with atmospheric oxygen.
In addition to oxygen, other gases can have strong aggressive properties towards metals. These gases include sulfur dioxide, fluorine, chlorine, hydrogen sulfide. For example, aluminum and its alloys, as well as steels with a high chromium content (stainless steels), are stable in an atmosphere that contains oxygen as the main aggressive agent. But the picture changes dramatically if chlorine is present in the atmosphere.
In the documentation for some anti-corrosion preparations, chemical corrosion is sometimes called "dry", and electrochemical - "wet". However, chemical corrosion can also occur in liquids. Only in contrast to electrochemical corrosion, these liquids are non-electrolytes (i.e., non-conductive, such as alcohol, benzene, gasoline, kerosene).
An example of such corrosion is the corrosion of iron parts of a car engine. Sulfur present in gasoline as an impurity interacts with the surface of the part, forming iron sulfide. Iron sulfide is very brittle and easily peels off, leaving a fresh surface for further interaction with sulfur. And so, layer by layer, the detail is gradually destroyed.
Electrochemical corrosion
If chemical corrosion is nothing more than a simple oxidation of a metal, then electrochemical corrosion is destruction due to galvanic processes.
Unlike chemical corrosion, electrochemical corrosion proceeds in media with good electrical conductivity and is accompanied by the appearance of a current. To "start" electrochemical corrosion, two conditions are necessary: galvanic couple and electrolyte.
Moisture on the metal surface (condensate, rainwater, etc.) acts as an electrolyte. What is a galvanic couple? To understand this, let's go back to the activity series of metals.
We look. On the left are the more active metals, on the right are the less active ones.
If two metals with different activity come into contact, they form a galvanic pair, and in the presence of an electrolyte, a flow of electrons occurs between them, flowing from the anode to the cathode sections. In this case, the more active metal, which is the anode of the galvanic couple, begins to corrode, while the less active metal does not corrode.
Diagram of a galvanic cell
For clarity, let's look at a few simple examples.
Let's say a steel bolt is secured with a copper nut. What will corrode, iron or copper? Let's look at the activity row. Iron is more active (to the left), which means that it will be destroyed at the junction.
Steel bolt - copper nut (steel corrodes)
What if the nut is aluminum? Let's look at the activity row again. Here the picture changes: already aluminum (Al), as a more active metal, will lose electrons and break down.
Thus, the contact of a more active "left" metal with a less active "right" metal enhances the corrosion of the first.
As an example of electrochemical corrosion, one can cite the cases of destruction and flooding of ships, the iron skin of which was fastened with copper rivets. Also noteworthy is the incident that occurred in December 1967 with the Norwegian ore carrier Anatina, en route from Cyprus to Osaka. In the Pacific Ocean, a typhoon hit the ship and the holds were filled with salt water, resulting in a large galvanic pair: copper concentrate + steel hull of the ship. After some time, the steel hull of the ship began to soften and it soon gave a distress signal. Fortunately, the crew was rescued by a German ship that came to the rescue, and Anatina herself somehow made it to the port.
Tin and zinc. "Dangerous" and "safe coatings
Let's take another example. Let's say the body panel is covered with tin. Tin is a very corrosion-resistant metal, in addition, it creates a passive protective layer, protecting iron from interaction with the external environment. So the iron under the tin layer is safe and sound? Yes, but only until the tin layer gets damaged.
And if this happens, a galvanic couple immediately appears between tin and iron, and iron, which is a more active metal, will begin to corrode under the influence of galvanic current.
By the way, there are still legends about the supposedly “eternal” tinned bodies of the “Victory” among the people. The roots of this legend are as follows: when repairing emergency vehicles, the craftsmen used blowtorches for heating. And suddenly, for no apparent reason, tin begins to flow from under the flame of the burner! Hence the rumor that the body of the "Victory" was completely tinned.
In fact, everything is much more prosaic. The stamp equipment of those years was imperfect, so the surfaces of the parts turned out to be uneven. In addition, the then steels were not suitable for deep drawing, and the formation of wrinkles during stamping became business as usual. A welded but not yet painted body had to be prepared for a long time. Bulges smoothed out emery circles, and the dents were filled with tin solder, especially a lot of which was near the windshield frame. Only and everything.
Well, you already know whether a tinned body is “eternal”: it is eternal until the first good hit with a sharp stone. And there are more than enough of them on our roads.
But with zinc, the picture is quite different. Here, in fact, we beat electrochemical corrosion with its own weapon. The protective metal (zinc) is to the left of iron in the voltage series. This means that in case of damage, it will not be steel that will be destroyed, but zinc. And only after all the zinc has corroded, the iron will begin to break down. But, fortunately, it corrodes very, very slowly, keeping the steel for many years.
a) Corrosion of tinned steel: when the coating is damaged, the steel is destroyed. b) Corrosion of galvanized steel: when the coating is damaged, the zinc is destroyed, protecting the steel from corrosion.
Coatings made from more active metals are called " safe", and from the less active ones -" dangerous". Safe coatings, in particular galvanizing, have long been successfully used as a way to protect car bodies from corrosion.
Why Zinc? After all, in addition to zinc, in the series of activity relative to iron, several more elements are more active. Here's the catch: the farther two metals are from each other in the activity series, the faster the destruction of the more active (less noble). And this, accordingly, reduces the durability of anti-corrosion protection. So for car bodies, where, in addition to good metal protection, it is important to achieve a long service life of this protection, galvanizing is the best fit. Moreover, zinc is available and inexpensive.
By the way, what will happen if you cover the body, for example, with gold? First, it will be oh so expensive! 🙂 But even if gold would become the cheapest metal, this cannot be done, since it will do our “piece of iron” a disservice.
After all, gold is very far from iron in the activity series (furthest), and at the slightest scratch, iron will soon turn into a pile of rust covered with a golden film.
The car body is exposed to both chemical and electrochemical corrosion. But the main role is still assigned to electrochemical processes.
After all, it’s a sin to hide, galvanic couples in a car body and a small truck: these are welds, and contacts of dissimilar metals, and foreign inclusions in sheet metal. The only thing missing is an electrolyte to “turn on” these galvanic cells.
And the electrolyte is also easy to find - at least the moisture contained in the atmosphere.
In addition, under real operating conditions, both types of corrosion are enhanced by many other factors. Let's talk about the main ones in more detail.
Factors Affecting Car Body Corrosion
Metal: chemical composition and structure
Of course, if car bodies were made of commercially pure iron, their corrosion resistance would be impeccable. Unfortunately, or perhaps fortunately, this is not possible. Firstly, such iron is too expensive for a car, and secondly (more importantly) it is not strong enough.
However, let's not talk about high ideals, but let's get back to what we have. Take, for example, steel grade 08KP, widely used in Russia for stamping body parts. When examined under a microscope, this steel is as follows: fine grains of pure iron mixed with grains of iron carbide and other inclusions.
As you may have guessed, such a structure gives rise to many microvoltaic cells, and as soon as an electrolyte appears in the system, corrosion will slowly begin its destructive activity.
Interestingly, the corrosion process of iron is accelerated by sulfur-containing impurities. Usually it gets into iron from coal during blast-furnace smelting from ores. By the way, in the distant past, not stone, but charcoal containing practically no sulfur.
Including for this reason, some metal objects of antiquity during their centuries-old history practically did not suffer from corrosion. Take a look, for example, at this iron pillar, which is located in the courtyard of the Qutub Minar in Delhi.
It has been standing for 1600 (!) years, and at least something. Along with the low humidity in Delhi, one of the reasons for such an amazing corrosion resistance of Indian iron is, just the same, the low content of sulfur in the metal.
So, in reasoning in the manner of “before, the metal was cleaner and the body did not rust for a long time,” there is still some truth, and a lot of it.
By the way, why don't stainless steels rust then? But because chromium and nickel, used as alloying components of these steels, stand next to iron in the electrochemical series of voltages. In addition, upon contact with an aggressive environment, they form a strong oxide film on the surface, which protects the steel from further corrosion.
Chrome nickel steel is the most typical stainless steel, but there are other grades of stainless steel besides it. For example, light stainless alloys may include aluminum or titanium. If you have been to the All-Russian Exhibition Center, you must have seen the obelisk "To the Conquerors of Space" in front of the entrance. It is lined with titanium alloy plates and there is not a single speck of rust on its shiny surface.
Factory body technology
The thickness of sheet steel, from which the body parts of a modern car are made, is usually less than 1 mm. And in some places of the body, this thickness is even less.
A feature of the process of stamping body panels, and indeed, any plastic deformation of the metal, is the occurrence of unwanted residual stresses during deformation. These stresses are negligible if the punching equipment is not worn and the strain rates are set correctly.
Otherwise, a kind of “time bomb” is laid in the body panel: the arrangement of atoms in crystal grains changes, so the metal in a state of mechanical stress corrodes more intensively than in a normal state. And, characteristically, the destruction of the metal occurs precisely in the deformed areas (bends, holes), which play the role of the anode.
In addition, when welding and assembling the body at the factory, a lot of cracks, overlaps and cavities are formed in it, in which dirt and moisture accumulate. Not to mention the welds that form the same galvanic pairs with the base metal.
Influence of the environment during operation
The environment in which metal structures are operated, including cars, is becoming more and more aggressive every year. In recent decades, the content of sulfur dioxide, nitrogen oxides and carbon has increased in the atmosphere. This means that cars are no longer washed with water, but with acid rain.
Since we are talking about acid rain, let's return once again to the electrochemical series of voltages. The observant reader will notice that it also includes hydrogen. Reasonable question: why? But why: its position shows which metals displace hydrogen from acid solutions, and which do not. For example, iron is located to the left of hydrogen, which means it displaces it from acid solutions, while copper, which is to the right, is no longer capable of such a feat.
It follows that acid rain is dangerous for iron, but not for pure copper. But this cannot be said about bronze and other copper-based alloys: they contain aluminum, tin and other metals that are in the row to the left of hydrogen.
It has been observed and proved that under the conditions big city bodies live less. In this regard, the data of the Swedish Institute of Corrosion (SHIK) are indicative, which found that:
- in countryside Sweden, the rate of destruction of steel is 8 microns per year, zinc - 0.8 microns per year;
- for the city, these figures are 30 and 5 microns per year, respectively.
The climatic conditions in which the car is operated are also important. So, in a marine climate, corrosion is activated approximately twice.
Humidity and temperature
How great is the effect of moisture on corrosion, we can understand the example of the previously mentioned iron column in Delhi (remember the dryness of the air as one of the reasons for its corrosion resistance).
Rumor has it that a foreigner decided to reveal the secret of this stainless iron and somehow broke off a small piece from the column. What was his surprise when, on the ship on the way from India, this piece became covered with rust. It turns out that in the humid sea air, stainless Indian iron turned out to be not so stainless after all. In addition, a similar column from Konarak, located near the sea, was hit very hard by corrosion.
The corrosion rate at relative humidity up to 65% is relatively low, but when the humidity rises above the specified value, corrosion accelerates sharply, since at such humidity a layer of moisture forms on the metal surface. And the longer the surface remains wet, the faster corrosion spreads.
That is why the main centers of corrosion are always found in the hidden cavities of the body: they dry much more slowly than open parts. As a result, stagnant zones form in them, a real paradise for corrosion.
By the way, the use of chemical reagents to combat ice corrosion is also on hand. Mixed with melted snow and ice, anti-icing salts form a very strong electrolyte that can penetrate anywhere, including hidden cavities.
With regard to temperature, we already know that increasing it activates corrosion. For this reason, there will always be more traces of corrosion near the exhaust system.
Air access
Interesting all-??? thing this corrosion. As interesting as it is insidious. For example, do not be surprised that a shiny steel cable, seemingly completely untouched by corrosion, may turn out to be rusted inside. This is due to the uneven access of air: in those places where it is difficult, the threat of corrosion is greater. In corrosion theory, this phenomenon is called differential aeration.
The principle of differential aeration: uneven access of air to different parts of the metal surface leads to the formation of a galvanic cell. In this case, the area intensively supplied with oxygen remains unharmed, and the area poorly supplied with oxygen corrodes.
A striking example: a drop of water that has fallen on the surface of a metal. The area under the drop and therefore less supplied with oxygen plays the role of an anode. The metal in this area is oxidized, and the role of the cathode is played by the edges of the drop, which are more accessible to the influence of oxygen. As a result, iron hydroxide, a product of the interaction of iron, oxygen, and moisture, begins to precipitate at the edges of the drop.
By the way, iron hydroxide (Fe 2 O 3 nH 2 O) is what we call rust. A rust surface, unlike the patina on a copper surface or an aluminum oxide film, does not protect the iron from further corrosion. Initially, rust has a gel structure, but then it gradually crystallizes.
Crystallization begins within the rust layer, while the outer shell of the gel, which is very loose and brittle when dry, peels off and the next layer of iron is exposed. And so on until all the iron is destroyed or the system runs out of oxygen and water.
Returning to the principle of differential aeration, one can imagine how many opportunities exist for the development of corrosion in hidden, poorly ventilated areas of the body.
Rust ... everything!
As they say, statistics know everything. Earlier, we mentioned such a well-known center for the fight against corrosion as the Swedish Corrosion Institute (SHIK) - one of the most authoritative organizations in this field.
Once every few years, scientists of the institute conduct an interesting study: they take the bodies of well-worked cars, cut out the “fragments” most beloved by corrosion from them (sections of thresholds, wheel arches, door edges, etc.) and evaluate the degree of their corrosion damage.
It is important to note that among the studied bodies there are both protected (galvanized and / or anticorrosive) and bodies without any additional anticorrosion protection (simply painted parts).
So, SHIK claims that the best protection for a car body is only a combination of “zinc plus anticorrosive”. But all other options, including “just galvanizing” or “just anticorrosive”, according to scientists, are bad.
Galvanization is not a panacea
Proponents of the refusal of additional anti-corrosion treatment often refer to factory galvanization: with it, they say, no corrosion threatens the car. But, as Swedish scientists have shown, this is not entirely true.
Indeed, zinc can serve as an independent protection, but only on smooth and smooth surfaces, moreover, not subject to mechanical attacks. And on the edges, edges, joints, as well as places regularly exposed to "shelling" with sand and stones, galvanizing gives in to corrosion.
In addition, not all cars have fully galvanized bodies. Most often, only a few panels are coated with zinc.
Well, we must not forget that zinc, although it protects steel, is inevitably consumed in the process of protection. Therefore, the thickness of the zinc "shield" will gradually decrease over time.
So the legends about the longevity of galvanized bodies are true only in cases where zinc becomes part of the overall barrier, in addition to regular additional anti-corrosion treatment of the body.
It's time to finish, but the topic of corrosion is far from exhausted. We will continue to talk about the fight against it in the following articles under the heading "Anti-corrosion protection".
All of us in our lives are faced with various types corrosion. There are corrosions of metal, concrete and some types of plastics. In order to learn how to properly deal with corrosion, it is first necessary to understand what corrosion is.
Corrosion is destruction solids caused by chemical and electrochemical processes that develop on the surface of the body when it interacts with the external environment. Even the word corrosion itself comes from the Late Latin corrosio - corrosive. Corrosion of metals causes particular damage. The most common and most familiar type of corrosion to all of us is the rusting of iron. The term "corrosion" applies to metals, concrete, some plastics, and other materials. In addition to corrosion, metal (in particular, building) structures are subject to erosion - the destruction of the surface of the material under the influence of mechanical stress. Erosion is provoked by rains, winds, sand dust and other natural factors. Therefore, arches of bridges, construction trusses and other structures must be protected comprehensively. Thus, corrosion is a physical and chemical interaction of a metal with a medium, leading to the destruction of the metal. As a result of corrosion, metals turn into stable compounds - oxides or salts, in the form of which they are found in nature. Corrosion eats up to 10 percent of the metal produced in the country. It is difficult to take into account higher indirect losses from downtime and reduced productivity of equipment that has undergone corrosion, from disruption of the normal course of technological processes, from accidents caused by a decrease in the strength of metal structures, etc.
Why is corrosion called corrosion?
The word corrosion comes from the Latin "corrodo" - "to gnaw." Some sources refer to the late Latin "corrosio" - "corrosion". The concepts of "corrosion" and "rust" should not be confused. If corrosion is a process, then rust is one of its results. This word applies only to iron, which is part of steel and cast iron. In the following, the term "corrosion" will mean the corrosion of metals. According to international standard ISO 8044 Corrosion is a physical-chemical or chemical interaction between a metal (alloy) and a medium, leading to a deterioration in the functional properties of the metal (alloy), the medium or the technical system that includes them. RUST is a layer of partially hydrated iron oxides that forms on the surface of iron and some of its alloys as a result of corrosion. Concrete is also susceptible to corrosion building stone, wood, other materials; corrosion of polymers is called degradation.
The environment in which the metal undergoes corrosion (corrodes) is called a corrosive or aggressive environment. In the case of metals, speaking of their corrosion, they mean the undesirable process of interaction of the metal with the environment.
The physical and chemical essence of the changes that a metal undergoes during corrosion is the oxidation of the metal. Any corrosion process is multi-stage:
- It is necessary to supply a corrosive medium or its individual components to the metal surface.
- Interaction of the medium with the metal.
- Full or partial removal of products from the metal surface (into the liquid volume, if the medium is liquid).
It is known that most metals (except Ag, Pt, Cu, Au) occur in nature in the ionic state: oxides, sulfides, carbonates, etc., usually called metal ores. The ionic state is more favorable, it is characterized by lower internal energy. This is noticeable when obtaining metals from ores and their corrosion. The absorbed energy during the reduction of metal from compounds indicates that the free metal has a higher energy than metal connection. This leads to the fact that the metal in contact with the corrosive medium tends to move into an energetically favorable state with a lower energy reserve. That is, we can say that the root cause of corrosion is the thermodynamic instability of a system consisting of metal and components of the surrounding (corrosive) medium. The measure of thermodynamic instability is the free energy released during the interaction of the metal with these components. But free energy by itself does not yet determine the rate of the corrosion process, i.e., the value most important for assessing the corrosion resistance of a metal. In some cases, adsorption or phase layers (films) that appear on the metal surface as a result of the onset of the corrosion process form such a dense and impenetrable barrier that corrosion stops or is very strongly inhibited. Therefore, under operating conditions, a metal with a high affinity for oxygen may turn out to be not less, but more stable (for example, the free energy of oxide formation in Cr or Al is higher than that of Fe, and they often surpass Fe in stability).
Classification of corrosion processes
According to the type (geometrical nature) of corrosion damage on the surface or in the volume of the metal.
Corrosion that has captured the entire surface of the metal is called solid. It is divided into uniform and uneven, depending on whether the depth of corrosion damage is the same in different areas. At local corrosion lesions are local and leave a significant (sometimes overwhelming) part of the surface practically unaffected. Depending on the degree of localization, there are corrosion spots, ulcers and points (pitting). Spot lesions can give rise subsurface corrosion that spreads laterally under a very thin (for example, riveted) layer of metal, which then blisteres or peels off. The most dangerous types of local corrosion - intercrystalline (intercrystalline), which, without destroying the grains of the metal, moves inward along their less stable boundaries, and transcrystalline, which cuts the metal with a crack right through the grains. Leaving almost no visible marks on the surface, these lesions can lead to a complete loss of strength and destruction of a part or structure. close to them in character. knife corrosion, like a knife cutting the metal along the weld during the operation of some alloys in especially aggressive solutions. Sometimes specially allocated superficial filiform corrosion developing, for example, under non-metallic coatings, and layer by layer corrosion proceeding predominantly in the direction of plastic deformation. specific electoral corrosion, in which even individual components of solid solutions can be selectively dissolved in the alloy (for example, dezincification of brass).
According to the mechanism of reactions of metal interaction with the environment (chemical and electrochemical corrosion).
Corrosion is chemical if, after breaking the metal bond, the metal atoms are directly connected by a chemical bond with those atoms or groups of atoms that are part of the oxidizing agents that take away the valence electrons of the metal. Chemical corrosion is possible in any corrosive environment, but most often it is observed in cases where the corrosive environment is not an electrolyte (gas corrosion, corrosion in non-conductive organic liquids). Its rate is most often determined by the diffusion of metal and oxidant particles through the surface film of corrosion products (high-temperature oxidation of most metals by gases), sometimes by the dissolution or evaporation of this film (high-temperature oxidation of W or Mo), its cracking (oxidation of Nb at high temperatures) and occasionally - convective delivery of an oxidizing agent from the environment (at very low concentrations).
Corrosion is electrochemical if, when leaving the metal lattice, the resulting cation enters into contact not with the oxidizing agent, but with other components of the corrosive medium; the oxidizing agent, on the other hand, receives electrons that are released during the formation of a cation. Such a process is possible in those cases when there are two types of reagents in the environment, of which one (solvating or complexing) is able to combine stable bonds with a metal cation without the participation of its valence electrons, while others (oxidizers) can add valence electrons of the metal without holding cations around. Solutions or melts of electrolytes have similar properties, where solvated cations retain significant mobility. Thus, during electrochemical corrosion, the removal of an atom from a metal lattice (which is the essence of any corrosion process) is carried out as a result of two independent, but conjugate, interconnected by electrical balance, electrochemical processes: anodic - the transition of solvated metal cations into solution, and cathodic - binding an oxidizing agent for the released electrons. It follows that the process of electrochemical corrosion can be slowed down not only by directly inhibiting the anode process, but also by influencing the speed of the cathodic one. The two most common cathodic processes are the discharge of hydrogen ions (2 e+ 2H + = H 2) and the reduction of dissolved oxygen (4 e+ O 2 + 4H + = 2H 2 O or 4 e+ O 2 + 2H 2 O \u003d 4OH -), which are often called hydrogen and oxygen depolarization, respectively.
The anodic and cathodic processes, with one or another probability and in one sequence or another, proceed at any points of the metal surface where cations and electrons can interact with the components of the corrosive medium. If the surface is homogeneous, then cathodic and anode processes are equally probable over its entire area; in such an ideal case, corrosion is called homogeneous electrochemical (thus noting the absence of any inhomogeneity in the distribution of the probability of electrochemical processes at any point on the surface, which, of course, does not exclude the thermodynamic heterogeneity of the interacting phases). In fact, on metal surfaces there are areas with various conditions delivery of reacting components with different energy states of atoms or with various impurities. In such areas, either anodic or cathodic processes can proceed more vigorously, and corrosion becomes heterogeneous-electrochemical.
By type of corrosive environment
Some corrosive media and the destruction caused by them are so characteristic that the corrosion processes occurring in them are also classified by the name of these media.
As a rule, metal products and structures are exposed to many types of corrosion - in these cases they speak of the action of the so-called mixed corrosion.
Gas corrosion– corrosion in a gas medium at high temperatures.
atmospheric corrosion– metal corrosion in atmospheric conditions with humidity sufficient for the formation of an electrolyte film on the metal surface (especially in the presence of aggressive gases or aerosols of acids, salts, etc.). A feature of atmospheric corrosion is the strong dependence of its rate and mechanism on the thickness of the moisture layer on the metal surface or the degree of moisture of the formed corrosion products.
Liquid corrosion- corrosion in liquid media. According to the conditions of the impact of a liquid medium on the metal, this type of corrosion is also characterized as corrosion with full immersion, with partial immersion, with variable immersion, which have their own characteristic features.
underground corrosion– corrosion of metal in soils and soils. characteristic feature underground corrosion is a large difference in the rate of oxygen delivery (the main depolarizer) to the surface of underground structures in different soils(tens of thousands of times).
By the nature of additional influences
Stress corrosion develops in the area of action of tensile or bending mechanical loads, as well as permanent deformations or thermal stresses, and, as a rule, leads to transgranular corrosion cracking to which, for example, steel cables and springs are exposed in atmospheric conditions, carbon and stainless steels in steam power plants, high-strength titanium alloys in sea water, etc. Under alternating loads, corrosion fatigue can manifest itself, which is expressed in a more or less sharp decrease in the fatigue limit metal in the presence of a corrosive environment. Corrosive erosion(or friction corrosion) is an accelerated wear of the metal with the simultaneous action of mutually reinforcing corrosive and abrasive factors (sliding friction, the flow of abrasive particles, etc.). related to her cavitation Corrosion occurs during cavitation modes of flow around a metal with an aggressive medium, when the continuous occurrence and “collapse” of small vacuum bubbles creates a stream of destructive microhydraulic shocks that affect the metal surface. A close variety can be considered fretting- corrosion observed at the points of contact of tightly compressed or rolling parts one over the other, if microscopic shear displacements occur between their surfaces as a result of vibrations.
Leakage of electric current through the boundary of a metal with an aggressive environment causes, depending on the nature and direction of the leak, additional anodic and cathodic reactions that can directly or indirectly lead to accelerated local or general destruction of the metal (corrosion). stray current). Similar destruction, localized near the contact, can cause contact in the electrolyte of two dissimilar metals forming a closed galvanic cell, - contact corrosion. In narrow gaps between parts, as well as under a loose coating or build-up, where the electrolyte penetrates, but the access of oxygen necessary for metal passivation is difficult, slotted corrosion, in which the dissolution of the metal mainly occurs in the gap, and the cathodic reactions partially or completely proceed next to it on the open surface.
It is also customary to single out biological corrosion under the influence of the waste products of bacteria and other organisms, and radiation corrosion - when exposed to radioactive radiation.
Corrosion rate index
To establish the corrosion rate of a metal in a given medium, one usually monitors the change in time of some characteristic that objectively reflects the change in the properties of the metal. Most often in corrosion practice, the following indicators are used.
Mass change rate
The mass change indicator is the change in the mass of the sample as a result of corrosion, related to the unit of the metal surface S and to the unit of time (for example, g/m h).
Depending on the conditions of corrosion, there are:
1. negative weight change
K-m=
where m is the loss of metal mass during corrosion after the removal of corrosion products.
2. positive indicator of mass change K+m=
where m is the increase in the metal mass over time due to the growth of a film of corrosion products.
If the composition of corrosion products is known, then you can recalculate from K to K and vice versa K-m \u003d K + m (nok A Me / n Me Aok)
where A and M are the atomic and molecular masses of Me and the oxidizing agent, respectively; n and n valency of the metal and oxidizing agent in an oxidizing environment.
Volumetric corrosion index
K - the volume of gas absorbed or released in the process V, referred to the unit of the metal surface and the unit of time (for example, cm / cm h).
K = vol. V/s
volume of gas usually lead to normal conditions.
With regard to electrochemical corrosion, when the process of cathodic depolarization is carried out due to the discharge of hydrogen ions, for example, according to the scheme 2Н + 2 e= H, or ionization of oxygen molecules O + 4 e+2HO = 4OH; the oxygen (K) and hydrogen (K) indicators are entered, respectively.
The hydrogen index of corrosion is the volume of H released during the corrosion process, referred to Su.
The oxygen index of corrosion is the volume of O absorbed in the process, referred to Su.
Resistance indicator
The change in the electrical resistance of a metal sample over a given test time can also be used as an indication of corrosion (K).
KR = (R/Ro) 100% in time t
where Ro and R are the electrical resistance of the sample before and after corrosion, respectively.
This method has a certain drawback. The thickness of the metal must be the same during the entire time of testing, and for this reason, the resistivity is most often determined, i.e. change in electrical resistance per unit area of the sample (cm, mm) with a length equal to one. This method has application limitations (for sheet metal no more than 3mm). The most accurate data is obtained for wire samples. This method is not suitable for welded joints.
Mechanical index of corrosion
A change in some property of a metal during corrosion. Relatively often use the change in tensile strength. The strength index is expressed as follows:
Ko = (in / in) 100% in time t
where c is the change in tensile strength after sample corrosion over time; in - tensile strength to corrosion.
Depth corrosion index
K - the depth of destruction of the metal P per unit of time (for example, mm / year).
The depth of corrosion damage P can be medium or maximum. The depth index of corrosion can be used to characterize both uniform and non-uniform corrosion (including local corrosion) of metals. It is convenient for comparing the corrosion rate of metal with different densities. The transition from mass, current and volume to deep is possible with uniform corrosion.
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