Metal corrosion is known to cause a lot of trouble. Isn’t it up to you, dear car owners, to explain what it threatens: give it free rein, and the car will only be tires. Therefore, the sooner the fight against this disaster begins, the longer the car body will live.
To be successful in the fight against corrosion, you need to find out what kind of “beast” it is and understand the reasons for its occurrence.
Today you will find out
Is there hope?
The damage caused to humanity by corrosion is enormous. According to various sources, corrosion “eats” from 10 to 25% of global iron production. Turning into brown powder, it is irrevocably scattered throughout the white world, 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 as such is lost, no, bridges, cars, roofs, and architectural monuments are destroyed. Corrosion spares nothing.
The same Eiffel Tower, the symbol of Paris, is terminally ill. Made from ordinary steel, it inevitably rusts and breaks down. The tower has to be painted every 7 years, which is why its weight increases by 60-70 tons each time.
Unfortunately, it is impossible to completely prevent metal corrosion. Well, unless you completely isolate the metal from the environment, for example, place it in a vacuum. 🙂 But what is the use of such “canned” parts? Metal must “work”. Therefore, the only way to protect against corrosion is to find ways to slow it down.
In ancient times, fat and oils were used for this, and later they began to coat iron with other metals. First of all, low-melting tin. In the works 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 that 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 sun temples on the coast near Konarak. And despite the fact that these temples were flooded by the sea for some time, the iron beams were perfectly preserved. So even in those distant times, people knew a lot about fighting 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” - corroding. But anyway:
Corrosion is the process of metal destruction as a result of chemical and electrochemical interaction with the environment.
Although corrosion is most often associated with metals, concrete, stone, ceramics, wood, and plastics are also subject to it. In relation to polymeric materials, however, the term destruction or aging is more often used.
Corrosion and rust are not the same thing
In the definition of corrosion in the paragraph above, it is not for nothing that the word “process” is highlighted. The fact is that corrosion is often identified with the term “rust”. However, these are not synonyms. Corrosion is 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, does that mean 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, when corroded, becomes covered with a beautiful greenish color (patina). Silver tarnishes when exposed to air—a sulfide deposit forms on its surface, a thin film of which gives the metal its characteristic pinkish color.
Patina is a product of corrosion of copper and its alloys
The mechanism 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 occurrence of corrosion cases. But despite this, all corrosion processes have not only a common result - the destruction of the metal, but also a single chemical essence - oxidation.
Simplified, oxidation can be called the process of electron exchange. When one substance is oxidized (donates electrons), another, on the contrary, is reduced (receives electrons).
For example, in the reaction... 
... the zinc atom loses two electrons (oxidizes), and the chlorine molecule gains them (reduces).
Particles that donate electrons and oxidize are called restorers, and particles that accept electrons and are reduced are called oxidizing agents. These two processes (oxidation and reduction) are interrelated and always occur simultaneously.
Such reactions, which in chemistry are called redox, underlie any corrosion process.
Naturally, the tendency to oxidize is different for different metals. To understand which ones have more and which have less, let’s remember the school chemistry course. There was such a concept as an electrochemical series of voltages (activities) of metals, in which all metals are arranged from left to right in order of increasing “nobility”.
So, metals located to the left in a row are more prone to losing electrons (and therefore to oxidation) than metals located to the right. For example, iron (Fe) is more susceptible to oxidation than the more noble copper (Cu). Certain metals (for example, gold) can only give up electrons under certain extreme conditions.
We’ll 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. Thus, corrosion is distinguished by the type of distribution (continuous, 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 allows us to most fully explain all the subtleties of this insidious process, is classification according to the mechanism of its occurrence.
Based on this criterion, two types of corrosion are distinguished:
- chemical
- electrochemical
Chemical corrosion
Chemical corrosion differs from electrochemical corrosion in that it occurs in environments that do not conduct electrical current. Therefore, with such corrosion, the destruction of the metal is not accompanied by the emergence of an electric current in the system. This is the usual redox interaction of a metal with its environment.
The most typical example of chemical corrosion is gas corrosion. Gas corrosion is also called high-temperature corrosion, since it usually occurs 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 electric heater elements or rocket engine nozzles.
The rate of chemical corrosion depends on temperature; as it increases, corrosion accelerates. Because of this, for example, during the production of rolled metal, fiery splashes fly in all directions from the hot mass. This is when scale particles break off from 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, and 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 drugs, chemical corrosion is sometimes called “dry”, and electrochemical corrosion is sometimes called “wet”. However, chemical corrosion can also occur in liquids. Only, unlike electrochemical corrosion, these liquids are non-electrolytes (i.e., non-conducting electric current, for example alcohol, benzene, gasoline, kerosene).
An example of such corrosion is the corrosion of iron parts of a car engine. The sulfur present in gasoline as an impurity interacts with the surface of the part, forming iron sulfide. Iron sulfide is very brittle and flakes off easily, freeing up a fresh surface for further interaction with sulfur. And so, layer by layer, the part is gradually destroyed.
Electrochemical corrosion
If chemical corrosion is nothing more than simple oxidation of a metal, then electrochemical corrosion is destruction due to galvanic processes.
Unlike chemical corrosion, electrochemical corrosion occurs in environments with good electrical conductivity and is accompanied by the generation of current. To “start” electrochemical corrosion, two conditions are necessary: galvanic couple And electrolyte.
Moisture on the metal surface (condensation, rainwater, etc.) acts as an electrolyte. What is a galvanic couple? To understand this, let us return to the activity series of metals.
Let's see. More active metals are located on the left, less active ones are on the right.
If two metals with different activities come into contact, they form a galvanic couple, and in the presence of an electrolyte, a flow of electrons occurs between them, flowing from the anode to the cathode sites. 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.
Galvanic cell diagram
For clarity, let's look at a few simple examples.
Let's say a steel bolt is secured with a copper nut. Which will corrode, iron or copper? Let's look at the activity row. Iron is more active (positioned to the left), which means 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: aluminum (Al), as a more active metal, will lose electrons and collapse.
Thus, contact of a more active “left” metal with a less active “right” metal increases the corrosion of the first.
As an example of electrochemical corrosion, we can cite cases of destruction and sinking of ships whose iron plating was fastened with copper rivets. Also noteworthy is the incident that occurred in December 1967 with the Norwegian ore carrier Anatina, traveling 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 couple: copper concentrate + steel hull of the ship. After some time, the steel hull of the ship began to soften and it soon sent out a distress signal. Fortunately, the crew was rescued by a German ship that arrived in time, and the Anatina itself 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 the iron from interaction with the external environment. This means that 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 arises 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, people still have legends about the supposedly “eternal” tin-plated bodies of the “Victory”. The roots of this legend are as follows: when repairing emergency vehicles, craftsmen used blowtorches for heating. And suddenly, out of the blue, tin begins to flow “like a river” from under the flame of the burner! This is where the rumor began that the body of the Pobeda was completely tinned.
In fact, everything is much more prosaic. The stamping equipment of those years was imperfect, so the surfaces of the parts were uneven. In addition, the steels of that time were not suitable for deep drawing, and the formation of wrinkles during stamping became common. The welded but not yet painted body had to be prepared for a long time. The bulges were smoothed out with sanding wheels, and the dents were filled with tin solder, especially a lot of which was near the windshield frame. That's all.
Well, you already know whether a tinned body is “eternal”: it is eternal until the first good blow from a sharp stone. And there are more than enough of them on our roads.
But with zinc the picture is completely different. Here, in essence, we are fighting electrochemical corrosion with its own weapons. The protecting metal (zinc) is to the left of iron in the voltage series. This means that if damaged, it will no longer be the steel that will be destroyed, but the zinc. And only after all the zinc has corroded will the iron begin to deteriorate. But, fortunately, it corrodes very, very slowly, preserving 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 - " dangerous" Safe coatings, in particular galvanizing, have long been successfully used as a method of protecting automobile bodies from corrosion.
Why zinc? Indeed, in addition to zinc, several other elements are more active in the activity series relative to iron. 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 automobile bodies, where in addition to good protection of the metal it is important to achieve a long service life of this protection, galvanizing is ideal. Moreover, zinc is available and inexpensive.
By the way, what happens if you cover the body, for example, with gold? Firstly, it will be oh so expensive! 🙂 But even if gold became the cheapest metal, this cannot be done, since it would do our hardware a disservice.
Gold, after all, stands very far from iron in the activity series (farthest), and with the slightest scratch, iron will soon turn into a pile of rust covered with a gold film.
The car body is exposed to both chemical and electrochemical corrosion. But the main role is still assigned to electrochemical processes.
After all, let’s be honest, there are a lot of galvanic couples in a car body and a small cart: these are welds, and contacts of dissimilar metals, and foreign inclusions in rolled sheets. All that is 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 technically pure iron, their corrosion resistance would be impeccable. But unfortunately, and maybe fortunately, this is impossible. Firstly, such iron is too expensive for a car, and secondly (and more importantly) it is not strong enough.
However, let's not talk about high ideals, but return to what we have. Let's take, for example, 08KP steel, which is widely used in Russia for stamping body parts. When examined under a microscope, this steel appears as follows: small 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 microgalvanic 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 the action of 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.
It is also for this reason that some metal objects of antiquity have remained virtually unaffected by corrosion over their centuries-old history. Take a look, for example, at this iron column that is located in the courtyard of the Qutub Minar in Delhi.
It has been standing for 1600 (!) years, and no matter what. Along with the low air humidity in Delhi, one of the reasons for such amazing corrosion resistance of Indian iron is precisely the low sulfur content in the metal.
So in reasoning along the lines of “before, the metal was cleaner and the body did not rust for a long time,” there is still some truth, and a considerable one.
By the way, why then do stainless steels not rust? But because chromium and nickel, used as alloying components of these steels, stand next to iron in the electrochemical voltage series. In addition, upon contact with an aggressive environment, they form a strong oxide film on the surface, protecting the steel from further corrosion.
Chromium-nickel steel is the most typical stainless steel, but there are other grades of stainless steel. For example, lightweight stainless alloys may include aluminum or titanium. If you have been to the All-Russian Exhibition Center, you have probably 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 technologies
The thickness of the sheet steel from which body parts of a modern passenger car are made is, as a rule, less than 1 mm. And in some places of the body this thickness is even less.
A feature of the stamping process of body panels, and indeed of any plastic deformation of metal, is the occurrence of unwanted residual stresses during deformation. These stresses are negligible if the stamping equipment is not worn out and the strain rates are adjusted correctly.
Otherwise, a sort of “time bomb” is placed in the body panel: the arrangement of atoms in the crystalline grains changes, so the metal in a state of mechanical stress corrodes more intensely than in its normal state. And, characteristically, the destruction of the metal occurs precisely in the deformed areas (bends, holes) that play the role of the anode.
In addition, when welding and assembling the body at the factory, many cracks, overlaps and cavities are formed in it, in which dirt and moisture accumulate. Not to mention the welds, which form the same galvanic couples with the base metal.
Environmental influence during operation
The environment in which metal structures, including cars, are used is becoming more aggressive every year. In recent decades, the content of sulfur dioxide, nitrogen oxides and carbon in the atmosphere has increased. This means that cars are no longer washed with just water, but with acid rain.
Since we're talking about acid rain, let's return once again to the electrochemical series of voltages. An observant reader will notice that hydrogen is also included in it. A 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, located 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 series to the left of hydrogen.
It has been noticed and proven that in a big city, bodies live less. In this regard, data from the Swedish Corrosion Institute (SCI) is indicative, establishing that:
- in rural 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. Thus, in a marine climate, corrosion is approximately twice as active.
Humidity and temperature
We can understand how great the influence of humidity on corrosion is by the example of the previously mentioned iron column in Delhi (remember the dry air as one of the reasons for its corrosion resistance).
Rumor has it that one foreigner decided to reveal the secret of this stainless iron and somehow broke off a small piece from the column. Imagine his surprise when, while still 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 very badly affected by corrosion.
The rate of corrosion at relative humidity up to 65% is relatively low, but when the humidity increases 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.
This is why the main foci of corrosion are always found in 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 beneficial. Mixed with melted snow and ice, deicing salts form a very strong electrolyte that can penetrate anywhere, including into hidden cavities.
As for 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
Still, this corrosion is an interesting thing. As interesting as it is, it is also insidious. For example, do not be surprised that a shiny steel cable, seemingly completely untouched by corrosion, may turn out to be rusty inside. This happens due to uneven air access: in 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 element. In this case, the area intensively supplied with oxygen remains unharmed, while the area poorly supplied with it corrodes.
A striking example: a drop of water falling on the surface of a metal. The area located under the drop and therefore less well 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 a 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 gradually crystallizes.
Crystallization begins inside the rust layer, while the outer shell of the gel, which in the dry state is very loose and fragile, peels off, and the next layer of iron is exposed. And so on until all the iron is destroyed or all the oxygen and water in the system are gone.
Returning to the principle of differential aeration, one can imagine how many opportunities there are for the development of corrosion in hidden, poorly ventilated areas of the body.
They rust... everything!
As they say, statistics know everything. Previously, we mentioned such a well-known center for the fight against corrosion as the Swedish Corrosion Institute (SCI), one of the most authoritative organizations in this field.
Every few years, the institute’s scientists conduct an interesting study: they take the bodies of well-worked cars, cut out the “fragments” most favored by corrosion (sections of thresholds, wheel arches, door edges, etc.) and assess the degree of their corrosion damage.
It is important to note that among the bodies under study there are both protected (galvanized and/or anti-corrosive) and bodies without any additional anti-corrosion protection (simply painted parts).
So, CHIC claims that the best protection for a car body is only the 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 refusing additional anti-corrosion treatment often refer to factory galvanization: with it, they say, the car is not in danger of any corrosion. 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, which are also not subject to mechanical attacks. And on edges, edges, joints, as well as places regularly exposed to sand and stones, galvanization succumbs to corrosion.
In addition, not all cars have completely galvanized bodies. Most often, only a few panels are coated with zinc.
Well, we must not forget that although zinc protects steel, it 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 “Anti-corrosion protection” section.
Do you think that rust is a problem for owners of 15-year-old Zhiguli cars? Alas, cars under warranty also become covered with red spots, even if the body is galvanized. Let's figure out how to properly care for metal and whether it is possible to protect it from corrosion once and for all.
What is a body? The structure is made of thin sheet metal, with different alloys and with many welded joints. And we must not forget that the body is used as a “minus” for the on-board network, that is, it constantly conducts current. Yes, it simply must rust! Let's try to figure out what is happening to the car body and how to deal with it.
What is rust?
Corrosion of iron or steel is the process of metal oxidation with oxygen in the presence of water. The output is hydrated iron oxide - a loose powder that we all call rust.
Destruction of a car body is considered a classic example of electrochemical corrosion. But water and air are only part of the problem. In addition to ordinary chemical processes, galvanic pairs that arise between electrochemically inhomogeneous pairs of surfaces play an important role in it.
I can already see a bored expression appearing on the faces of humanities readers. Don’t be alarmed by the term “galvanic couple” - we are not going to present complex formulas at a chemistry lecture. This very pair in a particular case is just a connection of two metals.
Metals, they are almost like people. They don't like it when someone else clings to them. Imagine yourself on a bus. A rumpled man pressed against you, who yesterday celebrated some kind of High-Rise Fitter's Day with friends. In chemistry this is called an unacceptable galvanic couple. Aluminum and copper, nickel and silver, magnesium and steel... These are “sworn enemies”, which in a close electrical connection will very quickly “devour” each other.
In fact, no metal can withstand close contact with a stranger for long. Think for yourself: even if a curvy blonde (or a slender brown-haired woman, depending on your taste) is pressed up to you, it will be pleasant at first... But you won’t stand like that all your life. Especially in the rain. What does the rain have to do with it? Now everything will become clear.
There are many places in a car where galvanic couples are formed. Not unacceptable, but “ordinary”. Welding points, body panels made of different metals, different fasteners and assemblies, even different points on the same plate with different mechanical surface treatments. There is always a potential difference between them all, which means that in the presence of an electrolyte there will be corrosion.
Wait, what is an electrolyte? An inquisitive motorist will remember that this is some kind of caustic liquid that is poured into batteries. And he will be only partly right. An electrolyte is generally any substance that conducts current. A weak acid solution is poured into the battery, but it is not necessary to pour acid on the car to speed up corrosion. Ordinary water performs the functions of an electrolyte perfectly. In its pure (distilled) form it is not an electrolyte, but pure water is not found in nature...
Thus, in each formed galvanic couple, under the influence of water, the destruction of the metal begins on the anode side - the positively charged side. How to overcome this process? We cannot prevent metals from corroding from each other, but we can exclude the electrolyte from this system. Without it, “permissible” galvanic couples can exist for a long time. Longer than the car lasts.
How do manufacturers fight rust?
The simplest method of protection is to cover the metal surface with a film through which the electrolyte will not penetrate. And if the metal is also good, with a low content of impurities that promote corrosion (for example, sulfur), then the result will be quite decent.
But don't take the words literally. The film is not necessarily polyethylene. The most common type of protective film is paint and primer. It can also be created from metal phosphates by treating the surface with a phosphating solution. The phosphorus-containing acids in its composition will oxidize the top layer of metal, creating a very strong and thin film.
By covering the phosphate film with layers of primer and paint, you can protect the car body for many years; it was according to this “recipe” that bodies were prepared for decades, and, as you can see, quite successfully - many cars produced in the fifties and sixties were able to survive to this day.
But not all, because over time the paint is prone to cracking. At first the outer layers fail, then the cracks reach the metal and phosphate film. And in case of accidents and subsequent repairs, coatings are often applied without maintaining the absolute cleanliness of the surface, leaving small points of corrosion on it, which always contain a little moisture. And under the film of paint a new source of destruction begins to appear.
You can improve the quality of the coating, use more and more flexible paints, the layer of which may be a little more reliable. Can be covered with plastic film. But there is better technology. Coating steel with a thin layer of metal that has a more resistant oxide film has been used for a long time. The so-called tinplate - sheet steel coated with a thin layer of tin - is familiar to everyone who has seen a tin can at least once in their life.
Tin has not been used to coat car bodies for a long time, although there are stories about tinned bodies. This is an echo of the technology for straightening defects during stamping with hot solders, when part of the surface was manually covered with a thick layer of tin, and sometimes the most complex and important parts of the car body actually turned out to be well protected.
Modern coatings to prevent corrosion are applied at the factory before body panels are stamped, and zinc or aluminum are used as “saviors”. Both of these metals, in addition to having a strong oxide film, have another valuable quality - lower electronegativity. In the already mentioned galvanic couple, which is formed after the destruction of the outer paint film, they, and not the steel, will play the role of an anode, and as long as a little aluminum or zinc remains on the panel, they will be destroyed. This property can be used in another way by simply adding a little powder of such metals to the primer with which the metal is coated, which will give the body panel an additional chance for a long life.
In some industries, when the task is to protect metal, other technologies are used. Serious metal structures can be equipped with special protector plates made of aluminum and zinc, which can be changed over time, and even with electrochemical protection systems. Using a voltage source, such a system transfers the anode to some parts of the structure that are not load-bearing. These things don't happen on cars.
A multi-layer sandwich consisting of a layer of phosphates on the surface of steel or zinc, a layer of zinc or aluminum, anti-corrosion primer with zinc and several layers of paint and varnish, even in a very aggressive external environment such as ordinary city air with moisture, dirt and salt, allows you to keep body panels intact ten or two years.
In places where the paint layer is easily damaged (for example, on the bottom), thick layers of sealants and mastics are used, which additionally protect the paint surface. We used to call this “anticorrosive”. Additionally, compounds based on paraffin and oils are pumped into the internal cavities; their task is to displace moisture from surfaces, thereby further improving protection.
None of the methods alone provides 100% protection, but together they allow manufacturers to provide an eight- to ten-year guarantee against through-corrosion of the body. However, we must remember that corrosion is like death. Its arrival can be slowed down or postponed, but cannot be completely excluded. In general, what do we say to rust? Correct: “Not today.” Or, to paraphrase a modern classic, “not this year.”
a href=”http://polldaddy.com/poll/8389175/”Have you had to deal with rust on the body?/a
Metal corrosion is a widespread cause of deterioration of various metal parts. Metal corrosion (or rusting) is the destruction of metal under the influence of physical and chemical factors. Factors that cause corrosion include natural precipitation, water, temperature, air, various alkalis and acids, etc.
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Metal corrosion is becoming a serious problem in construction, at home and in production. Most often, designers provide protection for metal surfaces from rust, but sometimes rust occurs on unprotected surfaces and on specially treated parts.
Metal alloys form the basis of human life; they surround him almost everywhere: at home, at work, and during leisure. People don’t always notice metal things and parts, but they constantly accompany them. Various alloys and pure metals are the most produced substances on our planet. Modern industry produces various alloys 20 times more (by weight) than all other materials. Even though metals are considered to be some of the strongest substances on Earth, they can break down and lose their properties through rusting processes. Under the influence of water, air and other factors, the process of oxidation of metals occurs, which is called corrosion. Despite the fact that not only metal, but also rocks can corrode, processes associated specifically with metals will be discussed below. It is worth paying attention to the fact that some alloys or metals are more susceptible to corrosion than others. This is due to the speed of the oxidation process.
Metal oxidation process
The most common substance in alloys is iron. Corrosion of iron is described by the following chemical equation: 3O 2 +2H 2 O+4Fe=2Fe 2 O 3. H 2 O. The resulting iron oxide is that red rust that spoils objects. But let's look at the types of corrosion:
- Hydrogen corrosion. It practically does not occur on metal surfaces (although theoretically possible). In this regard, it will not be described.
- Oxygen corrosion. Similar to hydrogen.
- Chemical. The reaction occurs due to the influence of the metal with some factor (for example, air 3O 2 +4Fe = 2Fe 2 O 3) and occurs without the formation of electrochemical processes. So, after exposure to oxygen, an oxide film appears on the surface. On some metals, such a film is quite strong and not only protects the element from destructive processes, but also increases its strength (for example, aluminum or zinc). On some metals, such a film peels off (destroys) very quickly, for example, sodium or potassium. And most metals deteriorate quite slowly (iron, cast iron, etc.). This is how, for example, corrosion occurs in cast iron. More often, rusting occurs when the alloy comes into contact with sulfur, oxygen, or chlorine. Due to chemical corrosion, nozzles, fittings, etc. rust.
- Electrochemical corrosion of iron. This type of rusting occurs in environments that conduct electricity (conductors). The destruction time of different materials during electrochemical reactions is different. Electrochemical reactions are observed in cases of contact between metals that are located at a distance in a series of tensions. For example, a product made of steel has copper soldering/fastenings. When water hits the connections, the copper parts will be the cathodes and the steel will be the anode (each point has its own electrical potential). The speed of such processes depends on the amount and composition of the electrolyte. For reactions to occur, the presence of 2 different metals and an electrically conductive medium is required. In this case, the destruction of alloys is directly proportional to the current strength. The greater the current, the faster the reaction; the faster the reaction, the faster the destruction. In some cases, alloy impurities serve as cathodes.
Electrochemical corrosion of iron
It is also worth noting the subtypes that occur during rusting (we will not describe it, we will just list it): underground, atmospheric, gas, with different types of immersion, continuous, contact, caused by friction, etc. All subspecies can be classified as chemical or electrochemical rusting.
2
Corrosion of reinforcement and welded structures often occurs during construction. Corrosion often occurs due to non-compliance with the rules for storing the material or failure to perform work on processing the rods. Corrosion of reinforcement is quite dangerous, since reinforcement is laid to strengthen structures, and as a result of the destruction of the rods, a collapse is possible. Corrosion of welds is no less dangerous than corrosion of reinforcement. This will also significantly weaken the seam and may lead to tearing. There are many examples where rust on power structures leads to the collapse of premises.
Other common cases of rusting in everyday life are damage to household tools (knives, cutlery, tools), damage to metal structures, damage to vehicles (both land, air and water), etc.
Perhaps the most common rusty things are keys, knives and tools. All these items are subject to rust due to the fact that friction removes the protective coating, which exposes the base.
The base is subject to destruction processes due to contact with aggressive environments (especially knives and tools).
Destruction due to contact with aggressive media
By the way, the destruction of things that are often used in everyday life can be observed almost everywhere and regularly, at the same time, some metal objects or structures can remain rusty for decades and will perform their functions properly. For example, a hacksaw, which was often used to saw logs and left for a month in a shed, will quickly rust and may break during the work, and a pole with a road sign can stand for ten or even more years rusty and not collapse.
Therefore, all metal items should be protected from corrosion. There are several methods of protection, but they are all chemical. The choice of such protection depends on the type of surface and the destructive factor acting on it.
To do this, the surface is thoroughly cleaned of dirt and dust in order to eliminate the possibility of the protective coating not reaching the surface. It is then degreased (for some types of alloy or metal and for some protective coatings this is necessary), after which a protective layer is applied. Most often, protection is provided by paints and varnishes. Depending on the metal and factors, different varnishes, paints and primers are used.
Another option is to apply a thin protective layer of another material. This method is usually practiced in production (for example, galvanizing). As a result, the consumer practically does not need to do anything after purchasing the item.
Applying a thin protective layer
Another option is to create special alloys that do not oxidize (for example, stainless steel), but they do not guarantee 100% protection; moreover, some things made from such materials oxidize.
Important parameters of protective layers are thickness, service life and rate of destruction under active adverse influences. When applying a protective coating, it is extremely important to accurately fit into the permissible layer thickness. Typically, manufacturers of paints and varnishes indicate it on the packaging. So, if the layer is larger than the maximum allowable, this will cause excessive consumption of varnish (paint), and the layer may be destroyed under strong mechanical stress, a thinner layer may wear off and shorten the protection period of the base.
A correctly selected protective material and correctly applied to the surface guarantees 80% that the part will not be subject to corrosion.
3
Many people in everyday life do not think about how to protect their things from rye. And they get a problem in the form of a damaged item. How to properly solve this problem?
Removing rust from a part
In order to restore a thing or part from rust, the first step is to remove all the red coating to a clean surface. It can be removed with sandpaper, files, or strong reagents (acids or alkalis), but drinks like Coca-Cola have earned particular fame for this. To do this, the item is completely immersed in a container with a miracle liquid and left for some time (from several hours to several days - the time depends on the item and the damaged area).
Red spots on steel products
According to the UN, each country loses from 0.5 to 7-8% of its gross national product per year due to corrosion. The paradox is that less developed countries lose less than developed countries. And 30% of all steel products produced on the planet are used to replace rusted ones. Therefore, it is highly recommended that you take this issue seriously.
Why does iron rust?
If you leave an iron object in a damp and damp place for several days, it will become covered with rust, as if it had been painted with reddish paint.
What is rust? Why does it form on iron and steel objects? Rust is iron oxide. It is formed as a result of the “combustion” of iron when combined with oxygen dissolved in water.
This means that in the absence of moisture and water in the air, there is no oxygen dissolved in the water at all and rust does not form.
If a drop of rain hits a shiny iron surface, it remains transparent for a short period of time. The iron and oxygen in the water begin to interact and form an oxide, that is, rust, inside the drop. The water turns reddish and rust floats in the water in the form of small particles. When the drop evaporates, the rust remains, forming a reddish layer on the surface of the iron.
If rust has already appeared, it will grow in dry air. This happens because the porous rust stain absorbs moisture in the air - it attracts and holds it. This is why it is easier to prevent rust than to stop it once it appears. The problem of rust prevention is very important, since iron and steel products must be stored for a long time. Sometimes they are covered with a layer of paint or plastic. What would you do to keep warships from rusting when not in use? This problem is solved with the help of moisture absorbers. Such mechanisms replace humid air in the compartments with dry air. Rust cannot appear in such conditions!
The phrase “metal corrosion” contains much more than the name of a popular rock band. Corrosion irreversibly destroys metal, turning it into dust: of all the iron produced in the world, 10% will be completely destroyed in the same year. The situation with Russian metal looks something like this: all the metal smelted in a year in every sixth blast furnace in our country becomes rusty dust before the end of the year.
The expression “costs a pretty penny” in relation to metal corrosion is more than true - the annual damage caused by corrosion is at least 4% of the annual income of any developed country, and in Russia the amount of damage is ten figures. So what causes corrosion processes in metals and how to deal with them?
What is metal corrosion
Destruction of metals as a result of electrochemical (dissolution in a moisture-containing air or aqueous medium - electrolyte) or chemical (formation of metal compounds with highly aggressive chemical agents) interaction with the external environment. The corrosion process in metals can develop only in some areas of the surface (local corrosion), cover the entire surface (uniform corrosion), or destroy the metal along grain boundaries (intercrystalline corrosion).
Metal under the influence of oxygen and water becomes a loose light brown powder, better known as rust (Fe 2 O 3 ·H 2 O).
Chemical corrosion
This process occurs in environments that are not conductors of electric current (dry gases, organic liquids - petroleum products, alcohols, etc.), and the intensity of corrosion increases with increasing temperature - as a result, an oxide film is formed on the surface of metals.
Absolutely all metals, both ferrous and non-ferrous, are susceptible to chemical corrosion. Active non-ferrous metals (for example, aluminum) under the influence of corrosion are covered with an oxide film, which prevents deep oxidation and protects the metal. And such a low-active metal as copper, under the influence of air moisture, acquires a greenish coating - patina. Moreover, the oxide film does not protect the metal from corrosion in all cases - only if the crystal-chemical structure of the resulting film is consistent with the structure of the metal, otherwise the film will not help in any way.
Alloys are subject to another type of corrosion: some elements of the alloys are not oxidized, but are reduced (for example, in a combination of high temperature and pressure in steels, carbides are reduced by hydrogen), and the alloys completely lose the necessary characteristics.
Electrochemical corrosion
The process of electrochemical corrosion does not necessarily require immersing the metal in an electrolyte - a thin electrolytic film on its surface is sufficient (often electrolytic solutions permeate the environment surrounding the metal (concrete, soil, etc.)). The most common cause of electrochemical corrosion is the widespread use of household and industrial salts (sodium and potassium chlorides) to remove ice and snow on roads in winter - cars and underground communications are especially affected (according to statistics, annual losses in the USA from the use of salts in winter are 2.5 billion dollars).
The following happens: metals (alloys) lose some of their atoms (they pass into the electrolytic solution in the form of ions), electrons replacing the lost atoms charge the metal with a negative charge, while the electrolyte has a positive charge. A galvanic couple is formed: the metal is destroyed, gradually all its particles become part of the solution. Electrochemical corrosion can be caused by stray currents that occur when part of the current leaks from an electrical circuit into aqueous solutions or into the soil and from there into a metal structure. In those places where stray currents exit metal structures back into water or soil, metal destruction occurs. Stray currents occur especially often in places where ground electric transport moves (for example, trams and electric railway locomotives). In just one year, stray currents with a force of 1A are capable of dissolving 9.1 kg of iron, 10.7 kg of zinc, and 33.4 kg of lead.
Other causes of metal corrosion
The development of corrosion processes is facilitated by radiation and waste products of microorganisms and bacteria. Corrosion caused by marine microorganisms causes damage to the bottoms of seagoing vessels, and corrosion processes caused by bacteria even have their own name - biocorrosion.
The combination of the effects of mechanical stress and the external environment greatly accelerates the corrosion of metals - their thermal stability decreases, surface oxide films are damaged, and in those places where inhomogeneities and cracks appear, electrochemical corrosion is activated.
Measures to protect metals from corrosion
An inevitable consequence of technological progress is the pollution of our environment - a process that accelerates the corrosion of metals, as the external environment shows them more and more aggression. There are no ways to completely eliminate the corrosive destruction of metals; all that can be done is to slow down this process as much as possible.
To minimize the destruction of metals, you can do the following: reduce the aggression of the environment surrounding the metal product; increase metal resistance to corrosion; eliminate interaction between the metal and substances from the external environment that exhibit aggression.
Over thousands of years, mankind has tried many methods of protecting metal products from chemical corrosion, some of them are still used today: coating with fat or oil, other metals that corrode to a lesser extent (the most ancient method, which is more than 2 thousand years old, is tinning (coating tin)).
Anti-corrosion protection with non-metallic coatings
Non-metallic coatings - paints (alkyd, oil and enamels), varnishes (synthetic, bitumen and tar) and polymers form a protective film on the surface of metals, excluding (while intact) contact with the external environment and moisture.
The advantage of using paints and varnishes is that these protective coatings can be applied directly at the installation and construction site. The methods for applying paints and varnishes are simple and amenable to mechanization; damaged coatings can be restored “on the spot” - during operation; these materials have a relatively low cost and their consumption per unit area is small. However, their effectiveness depends on compliance with several conditions: compliance with the climatic conditions in which the metal structure will be operated; the need to use exclusively high-quality paints and varnishes; strict adherence to the technology of application to metal surfaces. It is best to apply paints and varnishes in several layers - their quantity will provide better protection against weathering on the metal surface.
Polymers - epoxy resins and polystyrene, polyvinyl chloride and polyethylene - can act as protective coatings against corrosion. In construction work, reinforced concrete embedded parts are coated with coatings made from a mixture of cement and perchlorovinyl, cement and polystyrene.
Protection of iron from corrosion by coatings of other metals
There are two types of metal inhibitor coatings - protective (zinc, aluminum and cadmium coatings) and corrosion-resistant (silver, copper, nickel, chromium and lead coatings). Inhibitors are applied chemically: the first group of metals has greater electronegativity with respect to iron, the second has greater electropositivity. The most widespread in our everyday life are metal coatings of iron with tin (tinplate, cans are made from it) and zinc (galvanized iron - roofing), obtained by pulling sheet iron through a melt of one of these metals.
Cast iron and steel fittings, as well as water pipes, are often galvanized - this operation significantly increases their resistance to corrosion, but only in cold water (when hot water is supplied, galvanized pipes wear out faster than non-galvanized ones). Despite the effectiveness of galvanizing, it does not provide ideal protection - the zinc coating often contains cracks, the elimination of which requires preliminary nickel plating of metal surfaces (nickel plating). Zinc coatings do not allow paint and varnish materials to be applied to them - there is no stable coating.
The best solution for anti-corrosion protection is aluminum coating. This metal has a lower specific gravity, which means it consumes less, aluminized surfaces can be painted and the paint layer will be stable. In addition, aluminum coating is more resistant to aggressive environments than galvanized coating. Aluminizing is not very common due to the difficulty of applying this coating to a metal sheet - aluminum in the molten state is highly aggressive towards other metals (for this reason, molten aluminum cannot be kept in a steel bath). Perhaps this problem will be completely solved in the very near future - an original method of performing aluminization has been found by Russian scientists. The essence of the development is not to immerse the steel sheet in molten aluminum, but to raise liquid aluminum to the steel sheet.
Increasing corrosion resistance by adding alloying additives to steel alloys
The introduction of chromium, titanium, manganese, nickel and copper into the steel alloy makes it possible to obtain alloy steel with high anti-corrosion properties. The steel alloy is given special resistance by its large proportion of chromium, due to which a high-density oxide film is formed on the surface of structures. The introduction of copper into the composition of low-alloy and carbon steels (from 0.2% to 0.5%) makes it possible to increase their corrosion resistance by 1.5-2 times. Alloying additives are introduced into the steel composition in compliance with Tamman's rule: high corrosion resistance is achieved when there is one atom of alloying metal for every eight iron atoms.
Measures to counteract electrochemical corrosion
To reduce it, it is necessary to reduce the corrosive activity of the environment by introducing non-metallic inhibitors and reducing the number of components that can start an electrochemical reaction. This method will reduce the acidity of soils and aqueous solutions in contact with metals. To reduce corrosion of iron (its alloys), as well as brass, copper, lead and zinc, it is necessary to remove carbon dioxide and oxygen from aqueous solutions. The electrical power industry removes chlorides from water that can affect localized corrosion. By liming the soil you can reduce its acidity.
Stray current protection
It is possible to reduce electrical corrosion of underground communications and buried metal structures by following several rules:
- the section of the structure serving as a source of stray current must be connected with a metal conductor to the tram rail;
- heating network routes should be located at the maximum distance from the rail roads along which electric vehicles travel, minimizing the number of their intersections;
- the use of electrically insulating pipe supports to increase the transition resistance between the soil and pipelines;
- at inputs to objects (potential sources of stray currents), it is necessary to install insulating flanges;
- install conductive longitudinal jumpers on flange fittings and gland expansion joints to increase longitudinal electrical conductivity on the protected section of pipelines;
- In order to equalize the potentials of pipelines located in parallel, it is necessary to install transverse electrical jumpers in adjacent areas.
Protection of metal objects equipped with insulation, as well as small steel structures, is carried out using a protector that functions as an anode. The material for the protector is one of the active metals (zinc, magnesium, aluminum and their alloys) - it takes on most of the electrochemical corrosion, breaking down and preserving the main structure. One magnesium anode, for example, protects 8 km of pipeline.
Rustam Abdyuzhanov, specially for rmnt.ru
