What is metal corrosion called? The concept of corrosion of metals and classification. Chemical corrosion of metals
Corrosion is destruction solids caused by chemical and electrochemical processes that develop on the surface of the body during its interaction with the external environment. 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. Corrosion is the physical and chemical interaction of a metal with a medium, leading to the destruction of the metal.
It is difficult to take into account the higher indirect losses from downtime and the reduction in performance of corroded equipment from disruption of normal operation. technological processes, from accidents caused by a decrease in the strength of metal structures, etc. An accurate assessment of the damage from corrosion of iron and steel is, of course, impossible. However, based on some of the available data on the average annual replacement volume of corrugated metal roofs, wires, piping, steel cars and other iron and steel objects subject to corrosion, it can be concluded that due to improper protection, annual replacement costs can average up to 2 percent of the total volume of steel used.
About corrosion of metals
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 understood as 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.
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.
Ideal corrosion protection is 80% ensured by the correct preparation of the surface for painting and only 20% by the quality of the used paintwork materials and how they are applied (ISO).
Corrosion process
Corrosion of metals is called their spontaneous destruction due to chemical or electrochemical interaction with the environment.
The environment in which the metal corrodes (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.
Stages of the corrosion process:
- corrosive medium supply to the metal surface;
- interaction of the medium with the metal;
- complete or partial removal of products from the metal surface.
Classification of corrosion processes
According to the nature of destruction, the following types corrosion:
Chemical corrosion- this is a process in which the oxidation of the metal and the reduction of the oxidizing component of the medium proceed in one act.
Chemical corrosion is possible in any corrosive medium, but most often it is observed in cases where the corrosive medium is not an electrolyte (gas corrosion, corrosion in non-conductive organic liquids).
Electrochemical corrosion- this is the destruction of metals due to their electrochemical interaction with an electrolytically conducting medium, in which the ionization of metal atoms and the reduction of the oxidizing component of the medium do not occur in one act and their rates depend on the value of the electrode potential of the metal. This type of corrosion is the most common. During electrochemical corrosion, the chemical transformation of a substance is accompanied by the release of electrical energy in the form of direct current.
Biochemical corrosion- in the case when the corrosion of metal in sea water is enhanced by the fouling of the surface by marine organisms.
electrocorrosion- increased corrosion under the action of anodic polarization caused by an external electric field (for example, during welding afloat, in the presence of stray currents in the water area).
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 at a humidity sufficient to form 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.
underground corrosion— corrosion of metal in soils and soils. characteristic feature underground corrosion is a big difference in the rate of oxygen delivery to the surface of underground structures in different soils (tens of thousands of times).
According to the nature of destruction, corrosion is distinguished
solid- Covers the entire surface of the metal
local- Covers individual areas of corrosion
Uniform- Flows at approximately the same speed over the entire surface
Spot (pitting)— In the form of individual dots with a diameter of up to 2 mm
Ulcerative– In the form of ulcers with a diameter of 2 to 50 mm
Spotted- In the form of spots with a diameter of more than 50 mm and a depth of up to 2 mm
subsurface- Causes metal delamination and swelling of layers
Underfilm- Leaks under a protective coating of metal
intercrystalline— In the form of destruction of grain boundaries
Selective (selective)— In the form of dissolution of individual components of the alloy
slotted- Develops in crevices and narrow gaps
Corrosion(from Latin corrosio - corrosive) is the spontaneous destruction of metals as a result of chemical or physico-chemical interaction with the environment. In the general case, this is the destruction of any material, whether it be metal or ceramics, wood or polymer. The cause of corrosion is the thermodynamic instability of structural materials to the effects of substances in contact with them.
An example is the oxygen corrosion of iron in water: 4Fe + 6H2O + 3O2 = 4Fe(OH)3. Hydrated iron hydroxide Fe(OH)3 is what is called rust.
Mechanism of car corrosion
Before trying to protect yourself from corrosion, it is necessary to answer the question of what metal corrosion is. In everyday life, corrosion refers to the appearance of rust on the surface of a metal. What are the main mechanisms for the appearance of rust?
It must be admitted that so far there is no complete answer to this question, and the results of ongoing research show that the corrosion process is very complex, since a large number of factors influence its course - chemical composition metal environment in which it is located temperature pressure presence of gases, etc. For this reason, the book contains only the most basic information from the theory of corrosion, the knowledge of which is necessary for the proper protection of the car body. A more complete understanding of the mechanisms of corrosion the reader can draw from the recommended literature.
Corrosion of iron (namely, this process we will consider later) is carried out if there are additionally at least two more components of the electrolyte, which borders the iron, and another conductor, which also borders the electrolyte. The electrolyte under normal conditions is rainwater, atmospheric moisture, snow, road dirt. The second conductor, in relation to the car body, is most often the surface of the earth, the atmosphere, or some other external conductor located near the car. Two conductors (which in this case are called electrodes) immersed in an electrolyte form a so-called galvanic cell. The main property of a galvanic cell is that if the electrodes are made of different metals, then such an element is a voltage source. In this case, the positive, the electrode is called the anode, the negative - the cathode.
Do a simple experiment. Dissolve a spoon in a glass of warm water table salt and lower two plates - one copper the other steel. The simplest voltage source is ready. Using a voltmeter, you can easily verify that the galvanic cell creates a small voltage less than half a volt. If you continue the experiment for several days, you will notice how rust will begin to appear on the surface of the steel. This simple experiment clearly demonstrates the mechanism of metal corrosion. The explanation for this mechanism is as follows.
From the course of physics it is known that conductors are characterized by the ability to give electrons to the external environment. It can be visually imagined that each conductor is surrounded by a cloud of electrons, which, under the influence of thermal energy, fly out of it, and then, if nothing interferes with them, return to the conductor under the influence of electric forces. If a metal is placed in an electrolyte, then positive metal ions (i.e., those metal atoms whose electrons are in the external environment) will begin to pass into the electrolyte. As a result, the metal acquires some potential that can be measured. In practice, the potential of the metal is determined in relation to a special standard electrode, the potential of which is assumed to be zero. The resulting potential difference between the standard electrode and the metal is called the standard electrode potential (SEP).
Of greatest interest is the process of corrosion of iron in the electrolyte in the presence of a less active metal. In this case, iron, as the more active metal, is the anode, and the less active metal is the cathode. In a galvanic pair, the more active metal, the anode, always corrodes.
Corrosion of the anode is accompanied by two types of reactions - oxidative at the anode and reduction at the cathode. In the future, for definiteness, we will consider iron (Fe) as an anode, however, all the results regarding its corrosion are valid, at least qualitatively for any previously named metal.
The oxidative reaction can be represented as a process in which iron atoms donate two electrons and, as a result, turn into positively charged iron ions (Fe2+), which pass into the electrolyte solution at the point of contact with the anode. These two electrons impart a negative charge to the anode and thereby cause a current towards the cathode, where they are combined with positive ions. Simultaneously, the positive ions of the anode combine with negatively charged hydroxyl groups (OH), which are always present in the electrolyte solution.
Schematically, the reaction at the anode can be written as follows:
Fe + 20Н- = Fe2+ + 2е + 20Н- = Fe(OH)2 + 2е
Under the action of iron ions, hydrogen ions (H +) appear on the cathode, with which the anode electrons are connected. Schematically, this process is described as follows:
H+ + 2e = 2H = H2
those. hydrogen is released at the cathode.
If the anodic and cathodic reactions are combined, they lead to a general corrosion reaction:
Fe + 2H20 \u003d Fe (OH) 2 + H2
Thus, iron in combination with water and a less active metal turns into iron hydroxide, which is commonly called rust.
The presence of additional salt in water leads to an increase in the conductivity of the electrolyte and, as a consequence, to an increase in the rate of anode oxidation. In this case, ferric chloride and hydrochloric acid solution are additionally formed. These are the conditions our road builders create for motorists every winter. However, acid rain, which falls with precipitation, also does not contribute to the longevity of the car.
An important characteristic of corrosion is the corrosion rate, which is defined as the depth of penetration of corrosion into the metal per unit time. For iron, the most characteristic value is the corrosion rate in the range of 0.05-0.02 nm/year. From the given values of the corrosion rate, it follows that in case of violation of the paintwork over 5 years of operation of the car, the thickness of the metal can decrease by 0.25-1 mm, i.e., in fact, if special protection measures are not provided, the metal will rust, as they say, through.
The described corrosion mechanism also indicates the main ways to combat this phenomenon. The cardinal way is to eliminate the cathode or electrolyte, however, this method is the least suitable, since the car cannot be isolated from the environment and, in particular, from the surface of the earth. There are two ways left - to isolate the metal from the electrolyte using a coating or to turn the car body from an anode to a cathode.
The first method is known to all motorists and is widely used in practice, however, it does not stop corrosion as such, but only protects the metal from rusting. If the paint coating is damaged, corrosion begins to corrode the metal, and re-coating is associated with large time and material costs (Appendix 1, 2).
In this case, the most vulnerable parts of the car body are hidden cavities and slots, such as thresholds, internal beams, spars, pillars, inner surfaces of doors, the ceiling, and almost the entire car body (see Appendix 1). The complex shape of hidden cracks and cavities makes it difficult, and often impossible, to prepare the surface for painting and painting itself, and internal stresses metal bent in these places contribute to its intense corrosion. Under these conditions, the service life of a car body before its failure is 6 years.
At the same time, without denying the importance of regular restoration of the paintwork, the author draws attention to a fundamentally different method of protecting the car body from corrosion, namely, the complete cessation of the corrosion process itself by changing the potential of the body. This method is called cathodic protection in the literature.
Cathodic protection of metals is based on the fact that the corrosion rate is proportional to the activity of the metals forming a galvanic couple. Under normal conditions, the car body is an anode and therefore corrodes. If we change the potential of the hull with respect to external environment either with the help of an external voltage source or by bringing it into contact with a more active metal, then the car body itself will become a cathode and will not corrode at all (at least the corrosion rate will decrease hundreds of times), and the anode will begin to collapse. In accordance with the method of changing the potential of the protected metal, there are tread and electrochemical protection. However, before considering protection methods, it is advisable to describe the features of car corrosion in various operating conditions.
Corrosion of the car during operation and passive methods of dealing with it
The conditions of its storage have a special influence on the corrosion of the car body. This is due to the fact that most of the time the car is kept in the parking lot, in the garage and only a small part of the time is in motion. While driving, the car is intensively blown with fresh air, “ventilated”, which, ceteris paribus, reduces the rate of corrosion.
Storage conditions in the first approximation can be divided into car storage in an open parking lot (including under an awning) and car storage in a garage. Consider storage options.
Corrosion of a car in an open parking lot
In an open parking lot, the car is constantly exposed to moisture in the air and precipitation. In conditions of low and medium humidity in the warm season, when the air temperature changes (for example, in the evening or early morning), atmospheric moisture condenses over the entire surface of the car, both outside and inside the passenger compartment. Its greatest accumulation is observed in hidden cavities (thresholds, spars, pillars, on the inner surface of doors, ceiling under decorative upholstery). With an increase in temperature, moisture evaporates from open surfaces but remains in hidden cavities for a long time. As a result, it is these, as a rule, hard-to-reach parts of the body that suffer from corrosion more than others. At high air humidity or during precipitation, moisture is more or less evenly distributed over the entire outer surface of the car and, since it does not stagnate in this case, it causes the least corrosion process.
However, it should be noted that in this case, the accumulation of moisture in the passenger compartment is also possible. Thus, when storing a car in an open parking lot, the internal surfaces of its body are most susceptible to corrosion. External surfaces corrode only where the paintwork is broken.
Strange as it may seem, additional precautions must be taken when storing a car under a tarpaulin. An awning (for example, from a tarpaulin) reliably protects the car from dust, snow and partly from water, but does not protect the body from exposure to air moisture at all. Moreover, moisture condenses under the awning and stays on the car body for a long time. Thus, the car under the awning is, as it were, in a water bath, which contributes to the corrosion of the car in the summer, when the air temperature rises after the night coolness. The mechanism of rust occurrence in this case is clear from the previous presentation. The car body and moist air together form a galvanic couple in which the car body is the anode. If the cover touches the surface of the car, then even the paintwork does not save from corrosion and rust appears through the paint.
It usually takes a few summer morning fogs to turn a new car into a pile of rusty metal. Therefore, if you close the car with an awning, be sure to follow the rules:
1. Do not let the case come into contact with the car body;
2. provide air ventilation under the cover;
3. periodically, especially during periods of high humidity and temperature changes, remove the cover and ventilate the car.
These rules can be put into practice in a variety of ways.
From duralumin corners measuring 40x40 mm, a frame is made in the form of a ladder. The length of the frame corresponds to the length of the car, the width of the frame is slightly larger than the width of the car. The middle crossbars of the frame are fixed to the roof rack of the car with screws or ropes.
A rectangular tarpaulin is thrown onto the resulting frame. This design protects the car from rain and dirt, provides good ventilation (since the front and rear walls are missing) and is disassembled and assembled within a few minutes.
Corrosion of the car when stored in the garage
At first glance, the best conditions for long-term storage of the car are in the garage, since the garage protects the car from external precipitation. However, numerous studies have shown that this is true only at low air humidity. In conditions of high humidity (in the middle lane this period includes autumn and especially spring, i.e., almost half a year), the metal corrosion rate in an ordinary steel box with a concrete floor is 1 mm / year, which is 5-20 times higher than the rate in an open air. The reason for this, paradoxical at first glance, the phenomenon is that the metal walls of the garage are an example of an additional cathode, which increases the rate of corrosion. The presence of such a large additional cathode causes corrosion both from the inside and outside of the entire body. At the same time, those parts of the body that are in the more humid lower layers of the atmosphere, the floor, bottom, wheel rims, and transmission suffer to a greater extent.
In order to better preserve the car, the walls of the garage must be painted, and the floor must be reliably protected from groundwater. To this end, before laying concrete, asphalt or gravel, lay polyethylene sheets on the ground that will completely cover the floor surface. Thus, you will reliably protect your garage from moisture contained in the ground, which is especially important during autumn rains and spring floods. Some car enthusiasts upholster the walls and floor of the garage with wood. Such protection of the car, however, drastically reduces fire safety. Therefore, if possible, it is better to use asbestos coating or fiberglass for this purpose. When arranging a garage, be sure to provide ventilation. Garage ventilation promotes constant air exchange, reduces air humidity and thus slows down the rate of corrosion. The simplest way to ensure the ventilation of the garage is to use an asbestos pipe installed vertically at a height of 30-40 cm above the floor and rising 1 m above the roof of the garage.
The diameter of the pipe for a standard garage with a volume of 50-60 m3 should be at least 20 cm. To prevent rain from entering the garage through the pipe, decorate its top with a metal cone, which, in addition, must be grounded.
Moving car corrosion
As a rule, when driving, the rate of corrosion of the car body decreases. The reason for this phenomenon is that the oncoming air intensively blows the body of the car, and as a result, the humidity of the air decreases both outside and inside the body. However, when driving on a dirty or wet road, the impact on the body of the car of rain, snow, salt, which is sprinkled on roads in the winter, combined with the mechanical effects of sand of small stones, ice floes and vibration leads to aging and destruction of the coating. The most vulnerable places in this case are the inner surfaces of the front and rear fenders, the bottom, transmission and suspension of the car. Mechanical impacts in combination with moisture lead to the fact that it is these places of the car body that begin to corrode in the first place.
The most well-known ways to protect the body of a moving car are anti-corrosion treatment of the bottom and the use of fender liner. The best protective coating for the bottom is a coating based on rubber resins, which has excellent adhesion to metal and forms a thick, loose layer in which mechanical particles (sand, dirt) get stuck and do not reach the metal.
The fender liner perfectly protects the inner surfaces of the wings from the mechanical impact of dirt and sand. At the same time, a closed space is formed between the fenders and the surface they protect, which contributes to the accumulation of moisture. Therefore, when installing the fender liner, it is necessary to ensure free access of air for ventilation and it is advisable to remove the fender liner during long-term parking of the car.
The above facts, as well as the own observations of motorists, indicate a wide variety of conditions under which corrosion of the car body occurs. Among this variety, we single out two conditions that, in our opinion, have the greatest impact on the formation of local places of accumulation of moisture and condensation of moisture throughout both the inner and outer surfaces of the car body. It is for these cases that cathodic protection methods will be considered.
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 irretrievably scatters across 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". So the only way protection 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. In relation to polymeric materials, however, the term degradation 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 overall 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 the 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 electricity. 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, we 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 common. A welded but not yet painted body had to be prepared for a long time. The bulges were smoothed out with emery wheels, 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 suits perfectly. 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 cart: 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, which practically did not contain sulfur, was used for this purpose.
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
Thickness 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 noticed and proved that in the conditions of a big city, bodies live less. In this regard, the data of the Swedish Institute of Corrosion (SHIK) are indicative, which found that:
- in rural areas of 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 humidity on corrosion, we can understand by 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 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".
DEFINITION
In contact with the environment, many metals, as well as metal-based alloys, can be destroyed due to chemical interaction (OVR with substances in the environment). Such a process is called corrosion.
Distinguish between corrosion in gases (gas corrosion), which occurs at high temperatures in the absence of moisture on the metal surface, and electrochemical corrosion (corrosion in electrolyte solutions, as well as corrosion in a humid atmosphere). As a result of gas corrosion, oxide, sulfide, etc. are formed on the surface of metals. films. Furnace fittings, parts of internal combustion engines, etc. are exposed to this type of corrosion.
As a result of electrochemical corrosion, metal oxidation can lead both to the formation of insoluble products and to the transition of the metal into solution in the form of ions. Pipelines located in the ground, underwater parts of ships, etc. are exposed to this type of corrosion.
Any electrolyte solution is an aqueous solution, and water contains oxygen and hydrogen that can be reduced:
O 2 + 4H + + 4e \u003d 2H 2 O (1)
2H + +2e=H 2 (2)
These elements are oxidizing agents that cause electrochemical corrosion.
When writing the processes occurring during electrochemical corrosion, it is important to take into account the standard electrode potentials (EP). So, in a neutral environment, the EC of process 1 is 0.8V, therefore, metals whose EC is less than 0.8V (metals located in the activity series from its beginning to silver) undergo oxidation with oxygen.
The EP of process 2 is -0.41V, which means that only those metals whose potential is lower than -0.41V (metals located in the activity series from its beginning to cadmium) are subjected to hydrogen oxidation.
The corrosion rate is greatly influenced by the impurities that a particular metal may contain. So, if the metal contains impurities of a non-metallic nature, and their EC is higher than the EC of the metal, then the corrosion rate increases significantly.
Types of corrosion
There are several types of corrosion: atmospheric (corrosion in humid air at n.c.), corrosion in soil, corrosion with uneven aeration (access of oxygen to different parts of a metal product in solution is not the same), contact corrosion (contact of 2 metals, with different EPs in an environment where moisture is present).
During corrosion, electrochemical reactions occur on the electrodes (anode and cathode), which can be written by the corresponding equations. So, in an acidic environment, electrochemical corrosion proceeds with hydrogen depolarization, i.e. hydrogen is released at the cathode (1). In a neutral medium, electrochemical corrosion proceeds with oxygen depolarization—water is reduced at the cathode (2).
K (cathode) (+): 2H + + 2e \u003d H 2 - recovery (1)
A (anode) (-): Me - ne → Me n + - oxidation
K (cathode) (+): O 2 + 2H 2 O + 4e → 4OH - - reduction (2)
In the case of atmospheric corrosion, the following electrochemical reactions occur on the electrodes (moreover, on the cathode, depending on the environment, various processes can occur):
A (anode) (-): Me→Me n + +ne
K (cathode) (+): O 2 + 2H 2 O + 4e → 4OH - (in an alkaline and neutral environment)
K (cathode) (+): O 2 + 4H + + 4e → 2H 2 O (in an acidic environment)
Corrosion protection
The following methods are used to protect against corrosion: the use of chemically resistant alloys; protection of the surface of metals by coatings, which are most often used as metals that are covered in air with oxide films that are resistant to the action of the external environment; treatment of corrosive environment; electrochemical methods(cathodic protection, protector method).
Examples of problem solving
EXAMPLE 1
EXAMPLE 2
| Exercise | The part is made of an alloy of iron and nickel. Which metal will corrode faster? Write down the equations of anodic and cathodic processes during atmospheric corrosion. The values of standard electrode potentials E(Fe 2+ /Fe)= - 0.444V, E(Ni 2+ /Ni)= -0.250V. |
| Decision | First of all, active metals (having the most negative values of standard electrode potentials) undergo corrosion, in this case it is iron. Ministry of Education of the Russian Federation Pacific State Economic University ESSAY Discipline:Chemistry Subject: Corrosion of metals Completed: Group 69 student Krivitskaya Evgeniya Nakhodka Corrosion of non-metallic materials As the operating conditions become more severe (increase in temperature, mechanical stress, aggressiveness of the environment, etc.), non-metallic materials are also exposed to the action of the environment. In this connection, the term "corrosion" began to be applied to these materials, for example, "corrosion of concrete and reinforced concrete", "corrosion of plastics and rubbers". This refers to their destruction and loss of operational properties as a result of chemical or physico-chemical interaction with the environment. But it should be taken into account that the mechanisms and kinetics of processes for nonmetals and metals will be different. Corrosion of metals The formation of galvanic pairs is usefully used to create batteries and accumulators. On the other hand, the formation of such a pair leads to an unfavorable process, the victim of which is a number of metals - corrosion. Corrosion is understood as the electrochemical or chemical destruction of a metallic material that occurs on the surface. Most often, during corrosion, the metal is oxidized with the formation of metal ions, which, upon further transformations, give various corrosion products. Corrosion can be caused by both chemical and electrochemical processes. Accordingly, there are chemical and electrochemical corrosion of metals. Chemical corrosion Chemical corrosion - the interaction of the metal surface with (corrosion active) medium that is not accompanied by the occurrence of electrochemical processes at the phase boundary. In this case, the interactions of metal oxidation and reduction of the oxidizing component of the corrosive medium proceed in one act. For example, the formation of scale when iron-based materials are exposed to oxygen at high temperature: 4Fe + 3O 2 → 2Fe 2 O 3 During electrochemical corrosion, the ionization of metal atoms and the reduction of the oxidizing component of the corrosive medium do not occur in one act and their rates depend on the electrode potential of the metal (for example, rusting of steel in sea water). Electrochemical corrosion The destruction of metal under the influence of galvanic cells arising in a corrosive environment is called electrochemical corrosion. Not to be confused with electrochemical corrosion is the corrosion of a homogeneous material, such as the rusting of iron or the like. Electrochemical corrosion (the most common form of corrosion) always requires the presence of an electrolyte (condensate, rainwater, etc.) with which the electrodes are in contact - either different elements of the material structure, or two different contacting materials with different redox potentials. If ions of salts, acids, or the like are dissolved in water, its electrical conductivity increases, and the rate of the process increases. corrosive element When two metals with different redox potentials come into contact and are immersed in an electrolyte solution, such as rainwater with dissolved carbon dioxide CO 2 , a galvanic cell is formed, the so-called corrosion cell. It is nothing more than a closed galvanic cell. In it, a slow dissolution of a metallic material with a lower redox potential occurs; the second electrode in a pair, as a rule, does not corrode. This type of corrosion is especially characteristic of metals with high negative potentials. Thus, a very small amount of impurities on the surface of a metal with a high redox potential is already sufficient for the appearance of a corrosive element. Particularly at risk are places where metals with different potentials come into contact, such as welds or rivets. If the dissolving electrode is corrosion-resistant, the corrosion process slows down. This is the basis, for example, for the protection of iron products from corrosion by tinning or galvanizing - tin or zinc have a more negative potential than iron, therefore, in such a pair, iron is reduced, and tin or zinc must corrode. However, due to the formation of an oxide film on the surface of tin or zinc, the corrosion process is greatly slowed down. Hydrogen and oxygen corrosion If there is a reduction of H 3 O + ions or H 2 O water molecules, they speak of hydrogen corrosion or corrosion with hydrogen depolarization. The recovery of ions occurs according to the following scheme: 2H 3 O + + 2e − → 2H 2 O + H 2 2H 2 O + 2e - → 2OH - + H 2 If hydrogen is not released, which often occurs in a neutral or strongly alkaline environment, oxygen reduction occurs and is referred to as oxygen corrosion or oxygen depolarization corrosion: O 2 + 2H 2 O + 4e - → 4OH - A corrosive element can form not only when two different metals come into contact. A corrosive element is also formed in the case of a single metal, if, for example, the surface structure is inhomogeneous. Corrosion control Corrosion results in billions of dollars in losses every year, and solving this problem is an important task. The main damage caused by corrosion is not the loss of metal as such, but the enormous cost of products destroyed by corrosion. That is why the annual losses from it in industrialized countries are so great. True losses from it cannot be determined by evaluating only direct losses, which include the cost of a collapsed structure, the cost of replacing equipment, and the costs of measures to protect against corrosion. Even more damage is indirect losses. These are downtime of equipment when replacing corroded parts and assemblies, leakage of products, disruption of technological processes. Ideal corrosion protection is 80% ensured by proper surface preparation, and only 20% by the quality of the paints used and the way they are applied. . most productive and effective method surface preparation before further protection of the substrate is abrasive blasting . There are usually three areas of corrosion protection methods: 1. Structural 2. Active 3. Passive To prevent corrosion as structural materials used stainless steels , corten steels , non-ferrous metals . As protection against corrosion, the application of any coatings, which prevents the formation of a corrosive element (passive method). Oxygen corrosion of galvanized iron Oxygen corrosion of tin-plated iron Paint coating, polymer coating and enameling should, above all, prevent the access of oxygen and moisture. Often a coating is also applied, for example steel with other metals such as zinc, tin, chromium, nickel. The zinc coating protects the steel even when the coating is partially destroyed. Zinc has a more negative potential and corrodes first. Zn 2+ ions are toxic. In the manufacture of cans, tin coated with a layer of tin is used. Unlike galvanized sheet, when the tin layer is destroyed, iron begins to corrode, moreover, intensively, since tin has a more positive potential. Another possibility to protect the metal from corrosion is to use a protective electrode with a large negative potential, for example, made of zinc or magnesium. For this, a corrosion element is specially created. The protected metal acts as a cathode, and this type of protection is called cathodic protection. The soluble electrode is called, respectively, the anode of sacrificial protection. This method is used to protect against corrosion of ships, bridges, boiler plants, pipes located underground. To protect the ship's hull, zinc plates are attached to the outer side of the hull. If we compare the potentials of zinc and magnesium with iron, they have more negative potentials. But nevertheless, they corrode more slowly due to the formation of a protective oxide film on the surface, which protects the metal from further corrosion. The formation of such a film is called metal passivation. In aluminum, it is strengthened by anodic oxidation (anodizing). When a small amount of chromium is added to steel, an oxide film forms on the surface of the metal. The content of chromium in stainless steel is more than 12 percent. Cold galvanizing system The cold galvanizing system is designed to enhance the anti-corrosion properties of a complex multi-layer coating. The system provides complete cathodic (or galvanic) protection of iron surfaces against corrosion in various aggressive environments The cold galvanizing system is available in one, two or three packs and includes: binder - compositions on chlorinated rubber, ethyl silicate, polystyrene, epoxy, urethane, alkyd (modified) basis are known; · anti-corrosion filler - zinc powder ("zinc dust"), with a content of more than 95% of metallic zinc, having a particle size of less than 10 microns and a minimum degree of oxidation .; hardener (in two- and three-pack systems) One pack cold galvanizing systems are supplied ready to use and require only thorough mixing of the composition prior to application. Two- and three-pack systems can be supplied in multiple packages and require additional preparation steps before application (mixing binder, filler, hardener). Interesting articlesInteresting articles |