Aluminum corrosion is the destruction of metal under the influence of the environment.
For the reaction Al3+ +3e → Al, the standard electrode potential of aluminum is -1.66 V.
The melting point of aluminum is 660 °C.
The density of aluminum is 2.6989 g/cm3 (under normal conditions).
Aluminum, although an active metal, has fairly good corrosion properties. This can be explained by the ability to passivate in many aggressive environments.
The corrosion resistance of aluminum depends on many factors: the purity of the metal, the corrosive environment, the concentration of aggressive impurities in the environment, temperature, etc. The pH of solutions has a strong influence. Aluminum oxide forms on the metal surface only in the pH range from 3 to 9!
The corrosion resistance of Al is greatly influenced by its purity. For the manufacture of chemical units and equipment, only high-purity metal (without impurities), for example, AB1 and AB2 aluminum, is used.
Corrosion of aluminum is not observed only in those environments where a protective oxide film is formed on the surface of the metal.
When heated, aluminum can react with some non-metals:
2Al + N2 → 2AlN – interaction of aluminum and nitrogen with the formation of aluminum nitride;
4Al + 3С → Al4С3 – reaction of interaction of aluminum with carbon with the formation of aluminum carbide;
2Al + 3S → Al2S3 – interaction of aluminum and sulfur with the formation of aluminum sulfide.
How is aluminum protected from corrosion?
Alloys of other metals are susceptible to rust. It manifests itself quite quickly. If you create certain conditions for aluminum, it will not deteriorate for many years. To protect aluminum from corrosion, a special film is formed on it. It lays down in a thin layer, which ranges from 3 to 30 nanometers. This coating consists of aluminum oxide.
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The film is durable and gives the metal additional protection from external negative influences. Thanks to this layer, air and moisture do not enter the structure of the material. If the integrity of the oxide coating is compromised, then the process of aluminum corrosion begins. The metal loses its properties.
Aluminum oxidation
Aluminum oxidation occurs at direct current under a voltage of 250 V. The protective film is grown at room temperature with water cooling. No pulse source required. The films are obtained dense and durable within 45-60 minutes.
The density and color of the oxide coating are affected by the temperature of the electrolyte:
- low temperature forms a dense film of bright color;
- increased - forms a loose film that requires further painting.
Aluminum can be protected from corrosion by an electrochemical reaction. The process is divided into several stages:
1. At the preparation stage, the aluminum product is degreased by immersing it in a solution of oxalic acid.
2. After washing with water, immerse in an alkaline solution to remove the unevenly formed oxide layer.
3. For additional coloring, aluminum products are immersed in appropriate salt solutions. To fill the formed pores, the metal material is treated with steam.
4. The product is then dried. Anodic oxidation can be carried out using alternating current.
To protect against corrosion, chemical oxidation is used - it is less expensive and does not require special electrical equipment and qualifications of performers. A simple chemical composition is used.
During the aluminizing process, the resulting oxide film 3 microns thick has a light green color, has high electrical insulating properties, is non-porous, and does not stain.
Corrosion of aluminum occurs due to nearby metals that have oxidized. Isolation helps prevent this process. These can be gaskets made of rubber, bitumen, paronite. When covering with rust, varnish and other insulating materials are used. There are no other ways to get rid of this problem yet.
Three ways to remove oxide film from the surface of aluminum (1 video)
Materials for processing aluminum (22 photos)
Will metal corrosion occur upon contact with alkali?
Do not allow aluminum to come into contact with various alkalis. They easily destroy the protective film on the top layer. The metal reacts with water, after which hydrogen begins to be released. The corrosion process occurs quickly in this case. Mercury and copper also have a detrimental effect on the protective layer of aluminum.
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So, we found out whether aluminum rusts. As you can see, it does not always have good corrosion protection.
Causes of corrosion
When the question arises whether aluminum rusts, you need to think about the reasons leading to corrosion. Various external factors can accelerate this process. The reasons for the appearance of rust on aluminum can be the following:
Corrosion of aluminum in alkalis
Alkalis easily dissolve the protective oxide film on the surface of aluminum, it begins to react with water, as a result of which the metal dissolves with the release of hydrogen (aluminum corrosion with hydrogen depolarization).
Also, the oxide film is destroyed by mercury, copper and chlorine ions.
Aluminum is a metal widely used in industry and everyday life. Aluminum does not oxidize in air. Its inertness is due to the thin oxide film that protects it. However, under the influence of certain environmental factors, this metal is still subject to destructive processes, and corrosion of aluminum is not such a rare phenomenon.
Natural oxide coating
The natural surface of aluminum, which is created during the manufacturing of an aluminum product, such as extrusion, rolling or casting, has high resistance to corrosion in most types of environments. This occurs because a fresh aluminum surface spontaneously and instantly forms a thin but very effective oxide layer that prevents further oxidation of the metal.
This oxide film is impermeable and, unlike the oxide films of other metals, such as iron, is very firmly “attached” to the base metal. In case of any mechanical damage, this film is instantly restored and healed.
The natural oxide layer is the main reason for aluminum's good resistance to corrosion. This coating is resistant in environments with acidity - pH value - from 4 to 9.
Garage Recipes
There are several garage recipes that will help you make anti-oxidation products.
Boiling soda
Ordinary boiled water will help remove new traces of oxide. The liquid must be boiled in a small saucepan in advance. Then the boiling water is poured into a basin in which all parts that need cleaning will be soaked. The soaking time lasts about 3-4 hours. Then all products are washed and wiped from any remaining water. If the oxides have not disappeared, you will have to use other, more effective means.
A dense oxidized film can be removed using a special pharmaceutical borax. To restore an aluminum product, perform the following steps:
- add 10-15 grams of borax to a glass of water;
- stir 3-4 drops of ammonia into the mixture;
- treat the metal coating with the prepared liquid;
- after forty minutes, the remaining composition is washed off from the surface.
Sodium hydroxide
You can remove a layer of stubborn dirt with caustic soda. The advantages of this procedure include the fact that it is easy to carry out at home. The product in an amount of 150-200 grams is added to 7-8 liters of water and mixed thoroughly. It is necessary to use heated water, the temperature of which is 60-80 degrees. The part should be washed with the prepared sodium solution.
Coca Cola
This carbonated drink has excellent cleaning properties that help restore contaminated aluminum coatings. Several liters of Cola are poured into a saucepan. Then the product is placed in it and soaked for about an hour. During this time, the darkening should completely disappear.
If Cola did not help cope with the oxide, then you will have to use a more effective remedy.
What is the best way to process aluminum?
Aluminum is one of those structural materials that can be processed quite easily mechanically. Machining of this material includes turning, milling, drilling, planing and many other operations.
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Corrosion of aluminum in air (atmospheric corrosion of aluminum)
Aluminum, when interacting with air, becomes passive. When pure metal comes into contact with air, a thin protective film of aluminum oxide instantly appears on the aluminum surface. Further, film growth slows down. The formula of aluminum oxide is Al2O3 or Al2O3•H2O.
The reaction of aluminum with oxygen:
The thickness of this oxide film ranges from 5 to 100 nm (depending on operating conditions). Aluminum oxide has good adhesion to the surface and satisfies the condition of continuity of oxide films. When stored in a warehouse, the thickness of aluminum oxide on the metal surface is about 0.01 - 0.02 microns. When interacting with dry oxygen – 0.02 – 0.04 microns. When heat treating aluminum, the thickness of the oxide film can reach 0.1 microns.
Aluminum is quite resistant both in clean rural air and in an industrial atmosphere (containing sulfur vapor, hydrogen sulfide, ammonia gas, dry hydrogen chloride, etc.). Because sulfur compounds do not have any effect on the corrosion of aluminum in gas environments - it is used for the manufacture of sour crude oil processing plants and rubber vulcanization devices.
Aluminum protective coating
The production of an aluminum product is accompanied by the formation of a natural protective surface. The coating has a high degree of corrosion resistance. It is the thin oxide layer that prevents aluminum from oxidizing.
The impermeable film firmly “adheses” to the aluminum. This feature distinguishes it from other metals. The layer also self-heals when damaged mechanically.
Due to its unique properties and the presence of a protective surface, aluminum has a very wide range of uses. Metal is used to produce packaging, wires, parts and even jewelry. Today it is found in almost every industrial sector.
Mechanical coating
How to protect aluminum from corrosion? The most commonly used mechanical method is applying a layer of paint.
Paint the product and you will see the effectiveness of this method. Painting can be wet and dry, or powder. These technologies will improve. In wet painting, paint layers are applied after protecting the aluminum with a composition containing zinc and strontium compounds. The metal base is carefully prepared: protected, polished, dried. The primer is applied in stages.
When the solvent from the primer mixture has completely disappeared, the surface can be coated with an insulating composition: oil or glyphthalic varnish.
Special compounds help stop corrosion and protect aluminum structures from chemicals, gasoline, and various types of oils. The choice of coating depends on the conditions of subsequent use of the metal product:
- hammer - used to obtain structures of various color shades used in decoration;
- bakelite - applied under high pressure, filling microcracks and pores.
Powder painting requires thorough cleaning of the surface from grease and various deposits. This is achieved by immersion in alkaline or acidic solutions with the addition of wetting agents. Next, a layer of chromate, phosphate, zirconium or titanium compounds is applied to the aluminum structures. After this it will not oxidize.
After drying the material, a protective polymer is applied to the oxidized area. The most commonly used are polyesters that are resistant to mechanical, chemical and thermal influences. Polymerized urethane, epoxy and acrylic powders are used.
Does aluminum rust?
Although aluminum is a non-ferrous metal and, in comparison with ordinary steel, is relatively expensive, it is used by people quite widely. This durable and lightweight material can be used in everyday life, in construction, and in production. The chemical formula of aluminum in the periodic table looks like this: Al.
Aluminum, as you know, rusts very slowly. At least iron and steel cannot compare with it in this regard. Aluminum's resistance to corrosion is explained primarily by the fact that under normal conditions a thin oxide protective film forms on its surface. As a result, the chemical activity of aluminum is sharply reduced.
Factors affecting rust resistance
Aluminum is resistant to corrosion, but in some cases it can still begin to deteriorate quite quickly due to oxidation. This usually happens when the film is damaged for some reason or is unable to form.
Most often, aluminum loses its external thin protection under the influence of acids or alkalis. Ordinary mechanical damage can also cause film destruction.
After the destruction of the film, Al and its alloys begin to rust, that is, self-destruct, like many other metals. In this case, aluminum can be subject to corrosion:
The equation for aluminum corrosion (oxidation by oxygen) in air is as follows: 4AI+3O2=2AL2O3.
The chemical formula of the oxide protective film is AL2O3.
The most corrosion-resistant variety is technical aluminum. That is, almost pure 90% metal. Aluminum alloys, unfortunately, are much more susceptible to rust. At the same time, it is believed that the corrosion resistance of this metal is least reduced by magnesium impurities, and most of all by copper.
Such materials are widely used in construction, food and chemical industries. They are also very often used in mechanical engineering. It is believed that such materials are also well suited for the construction of structures exposed to sea water.
If the alloy contains no more than 3% magnesium, its anti-corrosion properties will be almost the same as technical aluminum. Magnesium in such an alloy is in solid solution and in the form of Al8Mg5 particles, evenly distributed throughout the matrix.
If the alloy contains more than 3% of this metal, Al8Mg5 particles begin to fall out, for the most part, not inside the grains, but along their boundaries. And this, in turn, has an extremely negative effect on the anti-corrosion properties of the material. That is, the product becomes much less resistant to rust.
Alloys with magnesium and silicon
Such materials are most often used in mechanical engineering and construction. Mg2Si makes alloys of this variety very strong. Sometimes copper is also a component of such elements. It is also introduced into the alloy for strengthening.
However, copper is added to such materials in very small quantities. Otherwise, the anti-corrosion properties of the aluminum alloy may be greatly reduced. Intercrystalline rusting in them begins already with the addition of more than 0.5% copper.
Also, the susceptibility to corrosion of such materials may increase with an unjustified increase in the amount of silicon included in their composition. This substance is usually added to aluminum alloys in such proportions that after the formation of Mg2Si there is nothing left over. Only some materials of this variety contain silicon in its pure form.
Corrosion of aluminum and its alloys with zinc
Al, as already mentioned, rusts more slowly than its alloys. This also applies to materials from the Al-Zn group. Such alloys are in great demand, for example, in aircraft construction. Some varieties may contain copper, others may not. Moreover, the first type of alloys, of course, is more resistant to corrosion. In this regard, Al-Zn materials are comparable to magnesium-aluminum materials.
Alloys of this variety with the addition of copper show signs of some rust resistance. But at the same time, they are destroyed due to corrosion more slowly than those made using magnesium and Cu.
Basic ways to combat rust
Of course, it is also possible to reduce the corrosion rate of aluminum and its alloys artificially. There are only a few ways to protect such materials from rust.
For example, contact of this metal and its alloys with the environment can be prevented by painting paintwork materials. Electrochemical methods are also often used to protect aluminum from rust. In this case, the material is additionally coated with a layer of a more active metal.
Another way to protect Al from rust is high-voltage oxidation. Powder coating techniques can also be used to prevent corrosion of aluminum. Of course, rust inhibitors are also used to protect it.
How is oxidation done?
Using this technique, aluminum and its alloys are often protected from corrosion. Oxidation is performed under a voltage of 250 V. When using this technique, a strong oxide film is formed on the surface of the metal or its alloy.
In this case, the material is exposed to current using water cooling.
At low temperatures, due to stress, a film on the surface of aluminum is formed very strong and dense. If the procedure is carried out at high temperatures, it turns out to be quite loose.
Aluminum processed in such an environment requires additional protection from contact with air (painting).
When using this technology, the product is first degreased in a solution of oxalic acid. The aluminum or alloy is then dipped into alkali. Next, the metal is exposed to current. At the final stage, if the oxidation was carried out at a sufficiently high temperature, the material is additionally painted by immersion in salt solutions and then treated with steam.
This method, like oxidation, is used to protect aluminum from rust quite often. This material can be painted using dry, wet or powder methods. In the first case, aluminum is first treated with a composition containing zinc and strontium. Next, the paintwork itself is applied to the metal.
When using the powder method, the working surface is first degreased by immersion in alkaline or acidic solutions. Next, chromate, zirconium, phosphate or titanium compounds are applied to the product.
Very often, other metals become stimulants for the onset of corrosion processes in aluminum and its alloys. This usually happens when products or their parts come into direct contact.
To prevent aluminum from rusting, special insulators are used in this case. Such gaskets can be made from rubber, paronite, or bitumen. Also in this case, varnishes and paints can be used.
Another way to protect aluminum from corrosion when in contact with other materials is to coat its surface with cadmium.
It is especially important to ensure that aluminum parts in various mechanisms and assemblies are insulated from direct contact with copper. It is also believed that it is not only the actual parts made of Al that should be protected from contact with other metals.
In terms of resistance to corrosion, iron is much inferior to aluminum, like steel, for example. Therefore, such metals and some others are often protected in a special way. The materials are simply coated with a protective aluminum layer.
Of course, such products should also be protected from contact with copper or other metals.
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Is corrosion possible for aluminum, copper and other non-ferrous metals or their alloys? It is generally accepted that they are less sensitive to various types of destruction.
In principle, this is true, but this does not mean that these materials do not need additional protection.
Below you will find general information not only about what such destructive corrosion is, but also how to prevent it.
There's fire all around! I don’t know the meaning of the word, but I feel it’s burning
Let's study the characteristics of aluminum. The described metal melts at a temperature of 659 degrees Celsius. The density of the substance is 2.69*103 kg/cm3. Aluminum belongs to the group of active metals. Resistance to corrosion processes depends on a number of factors:
- The pH of the environment has a great influence. You need to know that aluminum oxide can form when the pH is in the range between 3 and 9. In an environment where an oxide film immediately appears on the surface of an aluminum sheet, corrosion processes will not develop.
1 What is corrosion of metals and alloys?
In general, this process manifests itself as the destruction of a material as a result of its interaction with the external environment. Moreover, both metals and non-metals (ceramics, wood, polymers, etc.) are susceptible to it. We can also include the aging of rubber and the destruction of plastic here. What about metal alloys, in this case the most obvious example of corrosion is the well-known rust.
The main reason for this phenomenon is the insufficient thermodynamic stability of a particular material to any substances that we can detect in the contacting medium.
For example, rubber coatings deteriorate due to interaction with oxygen, polymers are destroyed after numerous contacts with precipitation, and most metals and their alloys are detrimentally affected by excessive humidity.
In addition, the ambient temperature also significantly influences the speed of the process; basically, the higher this parameter, the faster the destruction occurs.
Types of corrosion
Aluminum oxidizes quickly in the atmosphere, but to a shallow depth. This is prevented by a protective oxide film. Oxidation accelerates above the melting point of aluminum. If the integrity of the oxide film is compromised, aluminum begins to corrode. The reasons for the thinning of its protective layer can be various factors, from exposure to acids, alkalis and ending with mechanical damage.
Aluminum corrosion is the self-destruction of metal under the influence of the environment. According to the flow mechanism, they are distinguished:
- Chemical corrosion - occurs in a gas environment without the participation of water.
- Electrochemical corrosion – occurs in humid environments.
- Gas destruction - but accompanies heating and hot processing of aluminum. As a result of the interaction of oxygen with metals, a dense oxide film appears. This is why aluminum does not rust, like all non-ferrous metals.
On video: electrochemical corrosion of metals and methods of protection.
Does aluminum rust: properties of the material, causes of corrosion and methods of protection
Aluminum is a material that people often use in industry and for their own needs. This metal is flexible and resistant to external influences. It is non-toxic and safe for human health. Silver color allows the metal to be used for various purposes. These are industry and household spheres.
When working in industry, people often wonder if aluminum rusts. Everyone knows that if damage appears on a sheet, corrosion can develop. You should learn why aluminum rusts differently than other alloys. It is necessary to find out the reasons why it is subject to corrosion. Read about all this and more in our article today.
Properties
There's fire all around! I don’t know the meaning of the word, but I feel it’s burning
Let's study the characteristics of aluminum. The described metal melts at a temperature of 659 degrees Celsius. The density of the substance is 2.69*103 kg/cm3. Aluminum belongs to the group of active metals. Resistance to corrosion processes depends on a number of factors:
- Purity of the alloy. For the production of various equipment, metal is taken that is distinguished by its purity. It should not contain various impurities. Aluminum grades AI1 and AB2 are widely used.
- Environment in which aluminum is located.
- What is the concentration of impurities in the environment surrounding aluminum.
- Temperature.
- The pH of the environment has a great influence. You need to know that aluminum oxide can form when the pH is in the range between 3 and 9. In an environment where an oxide film immediately appears on the surface of an aluminum sheet, corrosion processes will not develop.
- Interaction with any acid or alkali.
- Mechanical pressure. For example, friction or a strong impact, after which a scratch appears on the top layer of metal.
- There are industrial areas. In them, fuel breakdown products affect the oxide film and destroy it. The metal begins to deteriorate. A similar situation occurs in megacities, where fuel breakdown products will interact with sulfur, as well as carbon oxides. A similar process destroys the film on aluminum. After this kind of external exposure, aluminum undergoes corrosion.
- It should be remembered that chlorine, fluorine, as well as bromine and sodium can dissolve the protective layer of the metal.
- If construction mixtures come into contact with metal, it quickly begins to deteriorate. In this case, aluminum is adversely affected by cement.
- Does water rust aluminum? If it gets on the sheet, the metal may be subject to corrosion processes. It is important to clarify which liquid has an effect. Many people use a special alloy that is not susceptible to corrosion from water. It is called duralumin. A unique alloy is used together with copper, as well as manganese.
- In order for the corrosion-inhibiting properties of aluminum to be maintained, it is necessary to maintain the acid-base balance. The range should be between six and eight units.
- It is believed that pure metal, without impurities, better withstands aggressive environments. Scientists conducted experiments. Based on the results, we can say that alloys of pure aluminum (90%) are more susceptible to corrosion than an alloy containing 99% of this substance. In the first option, corrosion occurs 80 times faster than in the second alloy.
- To ensure that the metal does not lose its properties longer in an aggressive environment, it is treated with a special paint. You can use a polymer composition. After processing, an additional protective layer appears.
- If you add 3% manganese to the alloy during production, it will be possible to avoid corrosion of aluminum.
How is aluminum protected from corrosion?
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Alloys of other metals are susceptible to rust. It manifests itself quite quickly.
If you create certain conditions for aluminum, it will not deteriorate for many years. To protect aluminum from corrosion, a special film is formed on it. It lays down in a thin layer, which ranges from 5 to 10 millimeters.
This coating consists of aluminum oxide.
The film is durable and gives the metal additional protection from external negative influences. Thanks to this layer, air and moisture do not enter the structure of the material. If the integrity of the oxide coating is compromised, then the process of aluminum corrosion begins. The metal loses its properties.
Causes of corrosion
When the question arises whether aluminum rusts, you need to think about the reasons leading to corrosion. Various external factors can accelerate this process. The reasons for the appearance of rust on aluminum can be the following:
What is electrochemical corrosion and can it occur on an aluminum sheet?
What are "patricians"? Historical information
Most often, the appearance of electrochemical corrosion is provoked by galvanic couples. Damage occurs at the junction of two different alloys. In this case, the rust will be clearly visible.
The important point is that only one metal deteriorates, and the second is the source of the corrosion process. In order not to be afraid of electrochemical corrosion, you need to use a magnesium alloy.
Due to electrochemical rust, experts do not recommend using ordinary iron when in contact with an aluminum body.
What factors can slow down the process?
There are a number of factors that slow down the corrosion process of aluminum, and some of them stop this phenomenon. The following are distinguished:
Under what conditions does aluminum deteriorate in air?
Alternativeity is the presence of a choice of possibilities
Some people wonder if aluminum rusts when exposed to air. If the oxide film on the top layer of metal is destroyed, the corrosion process may begin. As a result, rust may appear. Film growth tends to slow down in fresh air. It should be remembered that aluminum oxide has good adhesion to the metal surface.
If the sheet is stored in a warehouse, the film will be from 0.01 to 0.02 microns. If the metal comes into contact with dry oxygen, then the thickness of the oxide film on the surface will be from 0.02 to 0.04 microns. If aluminum is subjected to heat treatment, the film thickness changes. It will be equal to 0.1 microns.
Aluminum is considered to be durable enough to be used outdoors. For example, it is used in rural areas, as well as in remote industrial areas.
How does water affect the metal being described?
Corrosion of aluminum in water can occur from damage to the top layer and protective film. The high temperature of the liquid contributes to the rapid destruction of the metal.
If aluminum is placed in fresh water, then corrosion processes will practically not be observed. If you increase the water temperature, you may not notice any changes.
When the liquid heats up to a temperature of 80 degrees or higher, the metal begins to deteriorate.
The corrosion rate of aluminum increases if alkali gets into the water. The described metal is hypersensitive to salt. That is why sea water is destructive for it.
To use this metal in seawater, it is necessary to add magnesium or silicon to the liquid.
If you use a sheet of aluminum that contains copper, then corrosion of the alloy will occur much faster than that of a pure substance.
Is sulfuric acid dangerous for aluminum?
People wonder whether aluminum rusts in sulfuric acid. Such acid is potentially dangerous for alloys. It has pronounced oxidizing properties. They destroy the oxide film and accelerate metal corrosion.
An interesting point is that concentrated cold sulfur does not affect aluminum. If aluminum is heated, then metal corrosion processes can begin. In this case, salt appears, it is called aluminum sulfate. It is soluble in water.
Resistance of aluminum in nitric acid
The described metal is characterized by increased resistance when exposed to a solution of nitric acid. It is often synthesized to produce concentrated nitric acid.
What substances have no effect on aluminum?
Do not be afraid of corrosion processes if aluminum comes into contact with citric acid. Malic acid and fruit juice will not change the properties of its alloy. Oil has little effect on alloys that contain aluminum.
Will metal corrosion occur upon contact with alkali?
Do not allow aluminum to come into contact with various alkalis. They easily destroy the protective film on the top layer. The metal reacts with water, after which hydrogen begins to be released. The corrosion process occurs quickly in this case. Mercury and copper also have a detrimental effect on the protective layer of aluminum.
So, we found out whether aluminum rusts. As you can see, it does not always have good corrosion protection.
Source
Corrosion of wrought aluminum alloys
Aluminum alloys Al-Mn
Al-Mn alloys have very good corrosion resistance and are used outdoors without any protection.
Aluminum alloys Al-Mg
In general, Al – Mg have the best corrosion resistance of all wrought aluminum alloys.
When the magnesium content is more than 4%, the corrosion resistance of products made from these alloys is greatly influenced by manufacturing technology. After prolonged exposure at temperatures above 60 °C, alloys with high magnesium content become susceptible to stress corrosion and subsurface corrosion.
Aluminum alloys Al-Cu
Alloys with a copper content of more than 0.25% - all duralumins (duralumin, duralumin duralumin) have low corrosion resistance and are not used in aggressive environments of sea air or industrial atmospheres without protective coatings. Previously, one of the most common mistakes was the use of duralumin in corrosive environments without adequate protective coating.
Aluminum alloys Al–Zn–Mg–Cu
Alloys containing zinc, magnesium and copper have corrosion behavior similar to Al-Cu alloys and also require protection in corrosive environments.
Aluminum alloys Al-Zn-Mg
For this family of alloys, manufacturing methods, especially heat treatment, as well as the chemical composition of the alloy are critical to corrosion resistance. Al-Zn-Mg aluminum alloys can be susceptible to stress corrosion cracking and subsurface corrosion.
Specific surface of aluminum scrap
The loss of aluminum due to its oxidation during remelting of any scrap load in a furnace is proportional to the specific area of this scrap. The specific area is expressed by the ratio
where m is the total mass of the batch of scrap, A is the total surface area of all pieces of scrap that make up this load.
The specific surface area of aluminum waste is a critical parameter. Its value increases with decreasing scrap particle size. So, a cube with a side of 10 cm has a surface area of 600 square meters. cm, and for equivalent in mass 1000 cubes with a side of 1 cm - 10 times more. Therefore, the oxidation rate of these cubes will be 10 times greater than that of a large cube.
How to clean with improvised means?
When using aluminum items, dark spots appear on their surface. Oxide can be removed using simple cleaning methods.
Lemon acid
To treat an aluminum container, it is necessary to prepare a solution in the following proportion: for each liter of water - 2 tablespoons of citric acid. The resulting solution must be boiled for a quarter of an hour (or a little longer if there is a lot of oxide). After this, the container is washed.
Apples
Fruit acid, which is found in fresh apples, copes well with oxide.
Procedure:
- The fruit is cut in half.
- Dark spots are rubbed with a cut.
- Leave to act for 30 minutes.
- The container is washed.
Coca Cola
The sweet carbonated drink contains orthophosphoric acid, which copes well with the oxide. The use of Cola is very simple:
- pour the drink into an aluminum container;
- leave for 1.5 hours;
- Rinse.
Sorrel
Fresh sorrel is required for processing. Application procedure:
- A bunch of leaves is placed in a container that needs to be processed.
- Fill the container with water.
- Boil over low heat for half an hour.
- The solution with the leaves is poured out.
- The container is washed.
Salt, vinegar and mustard powder
If the darkening from the oxide is on the outside of the container, you can use the following method:
- add 1 tablespoon of salt and mustard powder into a bowl;
- pour in 1 tbsp. vinegar;
- stir;
- using a sponge, apply the resulting slurry to areas with oxide;
- leave for a quarter of an hour;
- rinse with warm water.
Kitchen salt
Table salt diluted with water can also be used to eliminate the oxide. Work order:
- Pour 2 tbsp into a bowl. salt.
- Pour in 1 tsp. warm temperature water.
- Mix.
- Apply with a sponge to areas with darkening.
- Leave for 30 minutes.
- Rinse.
Soda solution
To eliminate oxidation from the surface of aluminum objects, you can use an aqueous solution of soda. This recipe is suitable not only for dishes, but also for removing darkening from various parts of mechanisms.
Pour water into a large container in which aluminum objects will be cleaned by immersion and add baking soda in the following ratio: for every liter of liquid - 2 tablespoons of powder.
Processing order:
- pour the soda solution into the container;
- the item to be processed is lowered into the container;
- bring to a boil;
- boil for at least 10 minutes, monitoring the effect.
It may not be possible to completely eliminate foci of oxidation the first time, so if necessary, the process can be repeated even several times.
Aluminum extraction technologies
Chemical element No. 13 is the most abundant in nature, its content in the earth's crust is about 9%. The metal is a component of more than 250 minerals, mainly aluminosilicates, that make up the earth's crust.
The product of destruction of formations is clay, consisting of kaolinite. It sometimes contains an admixture of iron, which gives it a brown color.
Despite the fact that there are many mineral formations in nature, not all of them are ore material for extracting a valuable component. Bauxite ores, which contain industrial concentrations of the metal, are used for mining.
Aluminum forms the mineral corundum, which is inferior in hardness to diamond. The content of impurities of chromium, titanium and iron oxide in the aluminum compound Al2O3 forms the precious minerals ruby and sapphire.
- The valuable component is extracted from enriched ore by electrolysis of a solution of the oxide in a molten compound of fluorine, sodium and aluminum (cryolite). This method allows electrolysis to be carried out at temperatures below 1000 °C.
- Due to the low melt density, the liquid compound sinks to the bottom, making removal easier. When electrolytically producing metal, pure oxide Al2O3 is first isolated from alumina.
- Before use, the ore is purified from impurities of iron, silicon, and calcium compounds. When bauxite is fired, the water contained in the minerals evaporates. The resulting material is separated by exposing the compound to carbon dioxide.
The chemical method is widely used in the production of pure aluminum. It consists of treating the ore with NaOH alkali at a temperature of 220 °C to produce Al (OH)2. As a result of hydrolysis of the solution, aluminum oxidizes and its compound precipitates.
Aluminum is produced chemically
Then, as a result of using carbon dioxide, soda and potash are obtained. To obtain a chemically pure material, the technical material is heated in AlF3 vapor followed by cooling. As a result of temperature changes, pure aluminum is released.
The production of high-purity metal involves the development of new technologies and the creation of conditions under which the metal can oxidize without additional energy consumption.
One of the new methods involves the synthesis of high-purity aluminum oxide by the catalytic oxidation of metal with oxygen in water using ultrasonic vibrations, the development of an autocatalytic method for producing submicron powder, followed by the formation of high-density briquettes.
What factors can slow down the process?
There are a number of factors that slow down the corrosion process of aluminum, and some of them stop this phenomenon. The following are distinguished:
- In order for the corrosion-inhibiting properties of aluminum to be maintained, it is necessary to maintain the acid-base balance. The range should be between six and eight units.
- It is believed that pure metal, without impurities, better withstands aggressive environments. Scientists conducted experiments. Based on the results, we can say that alloys of pure aluminum (90%) are more susceptible to corrosion than an alloy containing 99% of this substance. In the first option, corrosion occurs 80 times faster than in the second alloy.
- To ensure that the metal does not lose its properties longer in an aggressive environment, it is treated with a special paint. You can use a polymer composition. After processing, an additional protective layer appears.
- If you add 3% manganese to the alloy during production, it will be possible to avoid corrosion of aluminum.
Properties
Let's study the characteristics of aluminum. The described metal melts at a temperature of 659 degrees Celsius. The density of the substance is 2.69*103 kg/cm3. Aluminum belongs to the group of active metals. Resistance to corrosion processes depends on a number of factors:
- Purity of the alloy. For the production of various equipment, metal is taken that is distinguished by its purity. It should not contain various impurities. Aluminum grades AI1 and AB2 are widely used.
- Environment in which aluminum is located.
- What is the concentration of impurities in the environment surrounding aluminum.
- Temperature.
- The pH of the environment has a great influence. You need to know that aluminum oxide can form when the pH is in the range between 3 and 9. In an environment where an oxide film immediately appears on the surface of an aluminum sheet, corrosion processes will not develop.
Three main types of aluminum corrosion
The most common types of aluminum corrosion are:
- galvanic (contact) corrosion;
- pitting (pitting) corrosion;
- crevice corrosion.
Stress corrosion, which leads to cracking, is a more specific type of corrosion. It occurs primarily in high-strength aluminum alloys, such as AlZnMg alloys, when they are subjected to prolonged tensile stress in the presence of a corrosive environment. This type of corrosion does not usually occur in 6xxx series alloys, i.e. AlMgSi alloys.
What is electrochemical corrosion and can it occur on an aluminum sheet?
Most often, the appearance of electrochemical corrosion is provoked by galvanic couples. Damage occurs at the junction of two different alloys. In this case, the rust will be clearly visible. The important point is that only one metal deteriorates, and the second is the source of the corrosion process. In order not to be afraid of electrochemical corrosion, you need to use a magnesium alloy. Due to electrochemical rust, experts do not recommend using ordinary iron when in contact with an aluminum body.
Physical and chemical parameters of the element
Aluminum is a chemical element with atomic number 13 and is a silvery-white metal. Its name comes from the Latin word alumen - alum. In almost all compounds, the chemical element exhibits a valence of 3.
- Crystallization of a chemical element occurs in a face-centered cubic lattice. The metal can oxidize at room temperature. At the same time, its surface is covered with a thin oxide film, which performs a protective function.
- The melting point of chemically pure aluminum is 660 °C, boiling point is 2450 °C. The density of the metal under normal conditions is 2.6989 g/cm3.
- In air, aluminum oxidizes to form a thin film, which prevents further reaction with the metal. This protective compound is formed by placing aluminum in a nitric acid concentrate.
- The metal actively interacts with hydrochloric acid. When reacting with alkalis, the protective oxidized layer is first destroyed, and then a reaction occurs with the formation of sodium and potassium aluminates (depending on the type of alkaline compound).
- When heated, the chemical element reacts with bromine and chlorine. When interacting with sulfur, aluminum sulfide is formed, which easily dissolves in water. The metal reacts with hydrogen indirectly through the artificial synthesis of organic compounds. As a result, a powerful reducing agent is formed - polymer aluminum hydride.
- When a powdered metal is burned in air, a refractory oxide powder of a chemical element is formed, the compound of which has high strength. This property is used to reduce metals from their oxides.
- In laboratory conditions, aluminum compounds containing the hydroxyl group OH can be obtained as a result of exchange reactions or by adding soda or ammonia to a solution. The aluminum compound settles to the bottom in the form of a gel-like sediment.
Why doesn't aluminum rust?
The following factors slow down and even completely stop the process of metal deterioration:
1. To preserve the anti-corrosion properties of aluminum, the acid-base balance (pH) in the range from 6 to 8 units is important.
2. Metal without impurities copes better with aggressive environments. According to experiments, an alloy consisting of 90% pure aluminum corrodes 80 times faster than an alloy that consists of 99% pure metal.
3. An additional protective layer in addition to the natural one preserves the structure of the metal even in aggressive environments. To do this, anodize the protective layer, coat it with special paints and polymer compounds.
4. Adding 3% manganese during aluminum production helps prevent rust.
Aluminum corrosion is the destruction of metal under the influence of the environment.
For the reaction Al 3+ +3e → Al, the standard electrode potential of aluminum is -1.66 V.
The melting point of aluminum is 660 °C.
The density of aluminum is 2.6989 g/cm 3 (under normal conditions).
Aluminum, although an active metal, has fairly good corrosion properties. This can be explained by the ability to passivate in many aggressive environments.
The corrosion resistance of aluminum depends on many factors: the purity of the metal, the corrosive environment, the concentration of aggressive impurities in the environment, temperature, etc. The pH of solutions has a strong influence. Aluminum oxide forms on the metal surface only in the pH range from 3 to 9!
The corrosion resistance of Al is greatly influenced by its purity. For the manufacture of chemical units and equipment, only high-purity metal (without impurities), for example, AB1 and AB2 aluminum, is used.
