Why aluminum does not rust - Metals and their processing

Corrosion of aluminum and methods of its protection

Aluminum and its alloys are characterized by excellent resistance to various types of damage. However, despite this, corrosion of aluminum is not such a rare phenomenon. Various forms of corrosion represent the main cause of deterioration of these materials. To combat destructive processes, it is imperative to understand the factors that cause their occurrence.

Aluminum corrosion is a reaction that takes place between the metal and its environment. This process can be of either natural or chemical origin. The most common form of metal destruction is the appearance of rusting processes on its surface.

A special feature of all types of metals is their ability to react with water and the environment. The only difference for each type of metal is the intensity of this process. For example, for noble metals such as gold, the rate of such a reaction will not be too fast, but iron, including aluminum, will react to influences of this nature quite quickly.

Two factors can be distinguished that have a direct impact on the intensity of the corrosion process. One of them can be called the degree of aggressiveness of the environment, and the second is the metallurgical or chemical structure. The atmosphere that surrounds us is always characterized by a certain level of humidity. In addition, it is characterized by a certain level of pollution and waste.

If we consider that the properties of the atmosphere are often determined by the region and the degree of industrialization, today we can distinguish:

rural areas (low pollution and average humidity);
coastal areas (moderate pollution and high humidity);
urban area (average level of humidity and average level of decay products of liquid fuel, sulfur and carbon oxides);
industrial and industrial areas (large amounts of sulfur, carbon oxides and acids, as well as average humidity levels)

For most cases, inorganic acids, even at low concentrations, will be able to dissolve aluminum. And even a natural aluminum oxide film cannot provide sufficient protection against the occurrence of corrosive processes.

The most powerful solvents are fluorine, potassium and sodium. In addition, aluminum is characterized by rather low resistance to chlorine and bromine compounds. Lime and cement mortars are very aggressive to various aluminum metal alloys.

There are several types of corrosion of aluminum and its alloys:

Superficial. This type of destruction occurs most often and is the least harmful. It is easiest to notice on the surface. This makes it possible to use safety precautions in a timely manner. Surface damage is very common on anodized profiles for construction.
Local. Such damage appears in the form of shapes, depressions and spots. This type of corrosion can be of the surface or intercrystalline type. Destructions of this nature are especially dangerous because they are quite difficult to detect. Such corrosion very often destroys hard-to-reach parts of structures and assemblies.
Thread-like or filigree. This type of aluminum failure often appears under organic-type coatings, as well as at the boundary surfaces of processing. Filament-like corrosion occurs at weakened areas of damaged organic coating or at the edges of holes;

Quite often, the natural anti-corrosion abilities of aluminum and its alloys are not enough to protect against destruction. And a long period of operation of products made from these metals will necessarily require the use of additional protection methods. The most common methods of protecting metals from corrosion include:

anodizing oxidation (research by German specialists shows that this type of protection is used by 15% of the total production of building profiles in the world);
coating of metal surfaces with powder compositions;
protection against contact with other metals

Anodizing

Anodized coating is a coating that creates a durable aluminum oxide film on the surface of aluminum that is resistant to aggressive environments. This treatment makes it possible to create a layer of film on the surface of the metal that simply does not allow aluminum to come into contact with the external environment and protects it from oxidation processes.

Anodized coating of various colors of aluminum profiles

When in contact with other metals, aluminum and its alloys can form a galvanic couple. Such contact often causes corrosion. In order to avoid the occurrence of such processes, it is necessary to use fasteners on products made of this metal that are made only of stainless and galvanized steel.

Polymer coatings

One of the most effective ways of anti-corrosion protection of structures made of aluminum and its alloys is considered to be coating surfaces using various paints and polymer compounds. The constant growth in demand for metal products and huge color products made from this material is the reason that the technology and methods of applying such coatings are constantly improving and becoming more technologically advanced.

Modern materials used to apply a protective coating to aluminum consist of solvents, dyes and binders. Paints that do not contain a solvent are called powder paints, and those that do contain a solvent are called wet paints.

The dyeing methods used by modern manufacturers can be divided into:

“on a wet surface” coating is carried out using two-component paint with a hardener, which in the technical information for the material is often called DD paint and PUR-Lack.
powder-type coatings, applied by conventional spraying in one layer or dry.

It can be noted that aluminum itself has excellent resistance to corrosion. But upon contact with electricity or other metals, it is still subject to various destructive processes.

The best ways to protect this metal and its alloys are anodization and powder coating.

Source: https://kraska.guru/specmaterialy/korroziya/korroziya-alyuminiya.html

Does aluminum rust in water?

In recommendations for preparing and storing various products at home, you can often find the phrase that you should use enamel, glass or stainless steel utensils. It is noted that aluminum is not suitable. To understand this relationship, it is worth turning to chemistry and metal corrosion. They are the ones who will tell you why alkaline solutions cannot be stored in aluminum containers.

Basic facts about the substance

Element occupying cell No. 13 of the Periodic Table of Chemical Elements D.I. Mendeleev, belongs to the main subgroup of the third group. It is intermediate between metals and non-metals. Therefore, it combines the properties of both the first and second. Its characteristic valence in compounds is III, and its oxidation state is: 0 for a simple substance, and +3 for compounds.

It is a strong but soft metal, silvery-white in color. Low density makes it easy to give it any shape. Hence, the widespread use of aluminum, including for domestic purposes. Suffice it to recall the bent forks and spoons in public canteens.

What will chemistry tell us about aluminum?

From a chemical point of view, aluminum is a reactive substance.
Usually, in all interactions it behaves as a reducing agent, and itself is oxidized, that is, it easily gives up all its 3 valence electrons. Therefore, it does not exist in nature in its pure form. The whole secret of the extraordinary behavior of this metal lies in its duality or, as it is also called, amphotericity.

It consists in the manifestation of acidic or basic properties depending on the environment, basic or acidic, respectively. Thus, aluminum (Al), as a simple substance, reacts with dilute acids and releases hydrogen molecules (H2) from them. With alkalis in solution, it forms beautiful complex compounds.

With the same alkalis, but in the melt it forms salts of aluminum acid (H3AlO3) - aluminates.

Corrosion resistance of aluminum

Aluminum metal is a favorite material in production. In addition to its advantages listed above: softness and strength, you can add high corrosion resistance under normal conditions. Corrosion is the destruction of substances without external mechanical influence. This concept is familiar to metals and alloys, although in fact it is applicable to any other material too.

Pure metals or alloys react with environmental substances and oxidize, compromising the integrity of the product. The most common example is the formation and peeling of rust on an iron surface. Corrosion on aluminum parts appears as dark spots, scratches and holes.

Aluminum's resistance to corrosion is due to the presence of an oxide film (Al2O3) on the surface.
The film is thin and durable, almost invisible in appearance. Completely covers the surface of the metal, thereby protecting it from the negative effects of external factors. Al2O3 is easily formed by interaction with atmospheric oxygen.

It turns out that in the atmosphere aluminum protects itself from corrosion processes. It is resistant even in environments with high contents of hydrogen sulfide, ammonia, hydrogen chloride and other gases.

This justifies the use of aluminum as a material for equipment in the chemical industry or storage tanks in agriculture.

If we talk about dishes, then destruction processes occur in them under the influence of aqueous solutions. Desalted (distilled) water, like hot steam, will not have any effect on the aluminum surface. Corrosion can be caused by mineral salts or alkalis in the water if the oxide film loses its integrity (for example, scratched with a spoon) and exposes the molecular aluminum. In this case, it will react with water to form white hydroxide (Al(OH)3) and release hydrogen (H2).

What do you mean by alkaline solutions?

Alkalis are compounds that contain metals of the main subgroup of the first group and a hydroxide ion (OH-). They are soluble in water, where they exist in the form of the corresponding ions. These solutions are alkaline.

For example, sodium hydroxide (NaOH), potassium hydroxide (KOH), etc. However, alkaline solutions include those whose medium has a pH value above 7.

This parameter depends on the concentration of hydrogen ions (H+) in the solution, and shows the reaction of the medium: acidic, neutral or alkaline.

An alkaline environment is found in: baking soda solution (NaHCO3), ammonia (NH4OH) and others. Soap solutions show a high pH value.

What happens to aluminum cookware in alkaline solutions?

Under the influence of alkalis, the oxide film on the surface of aluminum dissolves. The exposed metal reacts with water, forming, as mentioned earlier, aluminum hydroxide.

For example, if caustic soda (NaOH) is poured into an aluminum container, a blue color of the solution will appear due to sodium tetrahydroxyaluminate (Na[Al(OH)4]). The color may disappear if excess water or an acid such as vinegar is added.

The reaction also releases molecular hydrogen, which can be observed in the form of bubbles.

The baking soda solution will also react violently with aluminum cookware, producing a lot of foam. Afterwards, the container can be washed with water, and the surface will shine like new. This can be explained by the fact that sodium bicarbonate (soda) will react with aluminum oxide, forming a complex salt and carbonic acid (H2CO3), which is unstable and decomposes with the release of gas, dioxide carbon (CO2). A similar reaction is observed in cooking when extinguishing soda with vinegar.

Thus, aluminum is an excellent material for making containers. It is corrosion-resistant where other metals and alloys fail. Even aggressive environments are not able to destroy it. But it turns out that alkaline solutions, even as harmless as a mixture of baking soda and water, can have a destructive effect on products made of pure aluminum and its alloys.

Source: https://respect-kovka.com/rzhaveet-li-alyuminiy-v-vode/

What causes aluminum to oxidize?

Like any other metal, aluminum is subject to corrosion, one of the manifestations of which is rust. Unlike steel, on which corrosion leaves characteristic marks, aluminum rusts differently. It has high corrosion resistance, flexibility, non-toxicity, silver color, due to which it is used in many industries and households.

September 19, 2016

Does aluminum rust: reasons

If iron alloys rust relatively quickly, then under normal conditions aluminum practically does not deteriorate. A protective film of aluminum oxide appears on its surface. It has a thin layer of approximately 5-10 mm, but is highly durable. This layer does not allow moisture and air to destroy the structure of the metal.

As soon as the integrity of the oxide film is compromised, the metal corrodes. The causes of damage to the protective layer may be interaction with acids, solvents and alkalis, or mechanical effects (for example, friction forces).

In industrial areas and in urban environments, the oxide film is damaged due to fuel breakdown products, interaction with sulfur and carbon oxides.

Elements such as fluorine, chlorine, sodium, and bromine compounds intensively dissolve the passivating layer. Construction mortars with the addition of cement also lead to rapid deterioration of the metal. Sea water also causes intense destruction of pure aluminum, so in practice an alloy with copper and manganese, called duralumin, is used.

Galvanic couples can cause electrochemical corrosion. Rust appears most noticeably at the junctions of two dissimilar metals. In this case, only one metal undergoes corrosion, and the second acts as a source. Therefore, it is not recommended to use aluminum bodies in contact with iron.

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.

The oxidation state of aluminum characterizes the valence of a chemical element and reflects its ability to form compounds. This property is taken into account when exploring ore deposits rich in valuable components, technologies for their enrichment, purification of minor compounds from impurities and use in various industries.

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.

Source: https://l2rv.ru/info/ot-chego-okisljaetsja-aljuminij/

Does aluminum rust in water - Metalist's Handbook

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.

Is it susceptible to corrosion?

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.

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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.

Types of corrosion

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:

Chemical. In this case, rusting occurs in a gas environment without water. In this case, the surface of the aluminum product is destroyed evenly over the entire area.
Electrochemical. Corrosion of aluminum in this case occurs in a humid environment.
Gas. This type of corrosion occurs when aluminum comes into direct contact with some chemically aggressive gas.

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.

Alloys

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.

Use of insulators

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.

Source: https://ssk2121.com/rzhaveet-li-alyuminiy-v-vode/

Corrosion of aluminum: how to protect against electrochemical rusting in sea water or stop its speed, how to coat the metal and its resistance

Climatic factors and the degree of atmospheric pollution act together.

For example, increased air pollution can reduce the critical value of relative air humidity at which corrosion begins to develop.

Atmospheric corrosion parameters

The resistance of aluminum and its alloys to atmospheric corrosion depends on:

on the climatic conditions in which they are located:

humidity;
duration and intensity of rains;
temperature;
number of sunny days per year;

on the degree of air pollution, that is, concentration:

sulfur dioxide (SO2);
nitrogen oxides (NOx);
quantity and chemical composition of dust.

These factors can also have the opposite effect: rain increases air humidity, but also washes away dust and corrosion products, which can reduce the rate of corrosion

Relative humidity

The rate of atmospheric corrosion depends on the relative humidity of the air, and not simply on the amount or intensity of rain in a given area. Rain is one, but not the only factor on which relative air humidity depends.

Relative humidity level is the relationship between the actual water vapor pressure and the maximum water vapor pressure at a given temperature. This ratio is expressed as a percentage.

At normal room temperature, air is considered:

dry if the relative humidity is no more than 30%;
normal if the relative humidity is between 50 and 60%;
humid if the relative humidity is above 80%;
saturated with moisture if the relative humidity is about 100%.

In deserts and arid zones, relative humidity levels rarely exceed 10-20%, while in temperate climates it is generally between 40 and 60%. During a rainstorm it can reach 90-95%, and during tropical rains it can approach 100% [2].

Dew point

The dew point is the temperature at which moisture will begin to condense. For a given level of relative humidity, this is the temperature to which the air must be cooled so that it becomes saturated with moisture, and its precipitation begins on nearby surfaces.

Duration of humidification and sulfate electrolyte

Corrosion of metals in the open air depends on the so-called duration of humidification and the chemical composition of surface electrolytes.

The duration of moisture is the period during which there is enough moisture on the metal surface for corrosion to occur.

The duration of humidification is usually defined as the time during which the relative air humidity exceeds 80% and, at the same time, the temperature on the metal surface is above 0 ºС. Under these conditions, moisture condensation may occur on the metal surface.

Critical relative humidity

Atmospheric corrosion of metals, including aluminum, occurs in thin films of moisture that are located on the surface of the metal. There is a critical threshold of relative humidity below which aluminum and its alloys do not corrode.

This occurs because when there is insufficient humidity, there is not enough moisture to create a continuous electrolytic film on the surface of the metal.

No sulfates - no corrosion

In normal rural areas and in atmospheres with moderate sulfate pollution, aluminum's environmental resistance is very high. In an atmosphere with a high sulfate content and high humidity, pitting (pitting) corrosion can occur on aluminum products. In such conditions, aluminum may require corrosion protection.

Chlorides

The presence of salts (especially chlorides) in the air reduces the durability of aluminum, but to a lesser extent than for most other building materials.

The maximum depth of corrosion pits is usually only a small fraction of the thickness of the aluminum part.

Unlike carbon steel, the strength properties of aluminum parts that have been subjected to corrosion remain virtually unchanged.

Corrosion of aluminum in soil

The corrosion behavior of aluminum in soil is a very important practical issue. Electrical and telecommunications cables, water and gas distribution networks, as well as the bases of road signs, street lamps and various road structures are all very often made from aluminum and aluminum alloys.

Soil acidity-alkalinity

Assessing the corrosion resistance of metals, including aluminum, in contact with soils is very difficult.

The soil is characterized by its pH value, which is closely related to the type and content of salts dissolved in it, the amount of carbon dioxide (CO2), as well as possible pollution from industrial and domestic wastewater.

Soil electrical resistance

The corrosive aggressiveness of soil is associated with its electrical resistivity, which depends not only on the composition of the soil, but on the water content and the concentration of inorganic salts.

Forms of aluminum corrosion in soils

Unprotected aluminum in soil can exhibit the following forms of corrosion [2]:

pitting corrosion;
galvanic corrosion (in contact with other metals);
corrosion from stray currents.

Protection of aluminum in soil

For aluminum that works in soil, corrosion protection in the form of a bitumen coating, as well as cathodic protection, is most often used.

Physical chemistry of water

Water is a strong solvent that can dissolve:

many inorganic and organic compounds,
liquids if they are polar and contain a hydroxyl group;
gases.

Therefore, any water has a variable content:

inorganic salts;
dissolved gases;
solids in the form of a suspension and
organic substances.

However, not all of these dissolved elements affect aluminum corrosion. The main influence on aluminum corrosion is exerted by the following dissolved in water:

chlorides;
heavy metal ions.

Effect of chloride concentration

It is generally accepted that among all anions, chloride ions have the highest ability to penetrate the natural oxide film on the surface of aluminum. This is because these ions are very small and very mobile.

It is known that chlorides, as well as fluorides, bromides and iodides, are anions that activate corrosion of aluminum in water, while sulfates, nitrates and phosphates activate such corrosion less (Figure 5) or do not activate it at all.

The peculiarity of chlorides is that they can replace oxygen atoms in the aluminum oxide film. This leads to a weakening of the oxide film's resistance to corrosion.

Pitting corrosion

In natural fresh water and tap water, aluminum can be subject to pitting corrosion. However, with regular cleaning and drying, the risk of serious corrosion is very small.

Aluminum pots, kettles and pans, as well as soldiers' aluminum bowls, spoons and mugs, served faithfully for decades without any signs of corrosion.

The likelihood of corrosion increases if water is standing and aluminum is kept wet for long periods.

Effect of copper

The presence of copper in aluminum alloys significantly reduces their corrosion resistance. Such alloys are used only if they have reliable corrosion protection.

Chlorides in seawater

Typically seawater contains about 35 g/l of dissolved inorganic salts, of which chlorides make up about 19%. This is associated with the increased corrosiveness of sea water.

pH value of sea water

The pH value of sea water near the surface of seas and oceans is very stable and is about 8.2. This pH value is within the stability range of the natural oxide film. This explains the good corrosion resistance of aluminum in seawater.

Aluminum alloys for sea water

In seawater, aluminum-magnesium alloys (AlMg) with a magnesium content of no more than 2.5% exhibit particularly high durability. Ship hulls and other supporting structures are made from these alloys. For deck superstructures, the corrosion resistance of aluminum alloys of the 6xxx series (AlMgSi alloys) is quite sufficient.

Aluminum in contact with concrete

The use of aluminum in the construction industry forces it to come into contact with most materials used in construction: concrete, gypsum, polymers, etc.

Impact of concrete

Aluminum resists the effects of concrete and cement mortar well, despite their high alkaline properties with a pH value of about 12.

When the concrete begins to set, there is always a slight etching of the aluminum to a depth of no more than 30 microns. This effect, however, slows down after a few days of contact.

This leads to a very localized decrease in pH to 8 units and the formation of a protective film of calcium aluminate on the aluminum surface.

Concrete has a similar effect on aluminum castings. This increases adhesion between these materials. Once the concrete has hardened (dried), corrosion usually no longer occurs. However, where moisture accumulates and remains, corrosion can develop. The increased volume of corrosion products can cause cracks in the concrete.

Protecting aluminum from concrete

Therefore, splashes of wet alkaline building materials such as mortar and concrete leave superficial but clearly visible stains on aluminum surfaces. Since these stains are difficult to remove, visible aluminum surfaces must be protected, for example on construction sites.

Source: https://xn----8sbna6aihebzq3cl.xn--p1ai/sposoby-borby-s-korroziej/kak-borotsya-s-korroziej-alyuminiya.html

Does aluminum rust? Should I be afraid of corrosion of this metal?

However, scientists have proven through scientific experiments that, in fact, aluminum cookware is not so harmless to human health.

But the point is this: when in contact with food, soft metal easily breaks down at the molecular level and ends up in food. This usually happens when cooking food that contains a large amount of acid.

As a result, this metal ends up in the human body and can cause severe food poisoning.

In addition, metal salts, and aluminum salts in this case are no exception, can accumulate in internal organs, as well as in body tissues, which is the cause of the development of many ailments. Most often, cancer occurs for this very reason.

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It would seem that the harm of aluminum cookware is obvious. However, before you throw away your favorite ladle or fork, remember that even food well seasoned with vinegar absorbs a completely harmless dose of aluminum in a short time.

The body can easily get rid of such a small amount of this metal in its natural way, which, unfortunately, cannot be said about lead or strontium.

That is why a dish cooked in an aluminum pan, which you immediately consumed as food, does not pose a particular health hazard.

But the situation changes dramatically if the product is prepared and/or stored in aluminum containers for more than eight to twelve hours. In this case, the food is saturated with aluminum in much larger quantities and, what is most dangerous, due to the oxidation of the metal, toxic compounds appear in it. That is why, after opening an aluminum can of canned food, you should immediately transfer it to another, safer container (for example, glass or ceramic).

It takes quite a long time, years and even decades, for the amount of aluminum in the human body to reach a dangerous level, but you should know that it is this metal that provokes the development of such a disease as Alzheimer's disease. In the cerebral cortex of people suffering from this disease, there is several times more aluminum than normal.

What is aluminum

Aluminum is a lightweight metal that lends itself well to casting and machining. It is malleable, conducts heat well and does not rust, since an oxide film forms on the surface of the aluminum product.

In the old days, aluminum was a very valuable metal. Only rich people could afford to wear jewelry made of this light silver-white metal. Now it is used in the food industry for the production of cookware and baking foil. It has long been popular in the aviation industry, construction, and heating engineering, as it is lightweight and cannot be oxidized.

Harm of aluminum to humans

There has been a lot of conflicting information floating around aluminum lately. Some say that, accumulating in the body, it destroys the nervous system, others that it causes Alzheimer's disease. However, does this have anything to do with aluminum cookware?

Aluminum is indeed toxic to humans in large quantities. Ingestion of more than 50 mg of this metal per day is considered hazardous to health. Why didn't you like the dishes so much? The media spread information that this silvery-white metal:

destroys nerve cells;
leads to Alzheimer's disease;
destroys the brain and impairs its activity;
promotes the growth of tumors;
leads to kidney dysfunction;
impairs the metabolism of vitamins and minerals;
inhibits the production of hemoglobin.

In the 70s of the last century, experiments were conducted in Canada aimed at discovering the cause of Alzheimer's disease. Researchers began to sound the alarm, since all patients with this disease were found to have an increased aluminum content compared to a healthy person.

However, scientists have not been able to establish a connection between this fact and the etiology of this serious disease. The nature of senile dementia of this type has not been identified to this day, but one thing is certain - aluminum cookware does not contribute to the manifestation of this disease.

This and much more is attributed to this natural metal. This cannot be said that these accusations are groundless - an excess of any substance in the body leads to malfunctions. But, let me note that the dishes have nothing to do with it.

Can I use aluminum cookware?

Our grandparents did not have the opportunity to eat out of beautiful gold-plated stainless steel dishes. Even cupronickel silver cutlery was worth its weight in gold. Nevertheless, the older generation, which used aluminum cookware, is for the most part healthier and stronger than today's youth.

The fact is that even if you cook food in an aluminum pan, transfer it to an aluminum bowl, eat from it with an aluminum spoon, and then wash it all down from an aluminum mug, more than 2 g of aluminum will not enter the body. This is a completely normal indicator - such an amount of this substance will not affect human life and health in any way.

In addition, small doses of aluminum are needed by humans to restore bone tissue, regenerate epithelium, and regulate the secretion of food enzymes. It is found in tap water, as it is purified with aluminum sulfate, in dry antiperspirants and even in medications, such as Aspirin. In addition, having healthy kidneys, you do not have to worry about high concentrations of aluminum in the body - it is quickly eliminated by the urinary system.

Source: https://zdorovo.live/otravleniya-i-yady/rzhaveet-li-alyuminij-stoit-li-boyatsya-korrozii-etogo-metalla.html

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

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.

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 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:

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.

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.

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.

Under what conditions does aluminum deteriorate in air?

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: https://FB.ru/article/458332/rjaveet-li-alyuminiy-svoystva-materiala-prichinyi-korrozii-i-sposobyi-zaschityi

Aluminum does not rust: reasons - www

In industrial areas and in urban environments, the oxide film is damaged due to fuel breakdown products, interaction with sulfur and carbon oxides.

Manifestation of aluminum corrosion

The following types of corrosion of aluminum and its alloys are distinguished:

Superficial – the most common, causes the least harm, is easily noticeable and can be quickly eliminated.
Local – destruction is observed in the form of depressions and spots. A dangerous type of corrosion due to its invisibility. Found in hard-to-reach parts and assemblies of metal structures.
Thread-like, filigree - observed under organic coatings, on weakened areas of the surface.

Any type of corrosion of aluminum structures causes destruction.

This shortens the life of the products. In a galvanic couple, aluminum can corrode while protecting the other metal.

The natural anti-corrosion properties of aluminum and its alloys are not enough. Therefore, mechanisms, assemblies, structures and metal products need additional protection.

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.

Stamping and embossing

The cheapest products are made from extruded aluminum. The technique involves the use of ready-made sheets - blanks are stamped from them by mechanical action, the deformation causes the desired shape of the product. During production, the metal can be thickened; this method is common when producing tableware.

Stamping by embossing comes down to extruding a blank from a single sheet of aluminum. As a result, the structure of the metal changes and parts may lose shape when heated. The technology is very simple and does not require large expenses, which determines the low cost of products (mainly tableware).

Application of aluminum in construction

Lightness, corrosion resistance and resistance to alternating loads have long attracted the attention of aircraft designers and space industry scientists to aluminum.

Rolled duralumin: what it is and where it is needed

Products made from aluminum alloys are widely used in the construction of lightweight, wear-resistant structures, in the manufacture of instruments, household and artistic objects, and in the food industry.

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Application of aluminum in house construction

A material that is very often used in construction these days is aluminum. This material has high ductility, has a relatively long service life, is resistant to corrosion, lightweight, multifunctional and practical.

Why are cookware so often made of aluminum?

The production of aluminum cookware began only in the last century. But it quickly gained popularity, since aluminum is cheap, non-corrosive - does not rust, is lightweight, and has high thermal conductivity. It is not surprising that today almost all major types of cookware are made from aluminum, from forks to pressure cookers.

Aluminum cookware can be stamped or cast. The first one is cheaper. The second one is more reliable.

Corrosion resistance is determined by the film that forms on the surface of the cookware when interacting with oxygen. The metal is specially oxy-coded during the manufacture of products from it in order to make this film thicker. However, now in the production of dishes they often use aluminum alloys or make dishes with different types of coating.

Different types of frying pans: what you need to know about their features

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Technologies for the production of aluminum cookware

Aluminum pans can be divided into cast and stamped (sheet aluminum). Sheet technology is simpler and cheaper. This is the most budget segment on the market.

A mixture of glue and baking soda will help get rid of carbon deposits.

Don’t be upset that the pan has lost its appearance over time, and it’s impossible to get rid of the old carbon deposits. For this you need:

large bowl;
baking soda – 100 grams;
hot water – 10 l;
office glue – 100 grams.

So that not a trace remains of the soot, you need to pour water into a bowl and dissolve all the ingredients in it. Dip the vessel into the resulting solution and leave for three hours. Then rinse and wipe dry with a cloth.

Recommendations for cooking in aluminum cookware

You can safely cook in pans and other aluminum containers with a protective non-stick coating. The metal is protected from contact with food by a durable composite or other layer and does not interact chemically with products. Such dishes can also be used to store ready-made dishes.

If there is no protective layer on the inner surface, prepared food cannot be stored in such a container.

I use my pots and pans for cooking according to the following rules:

I don’t cook very acidic foods in them, such as sauerkraut cabbage soup, cranberry jelly or fruit juice;
I use a wooden spoon or plastic spatula for mixing;
I immediately place the finished dish on plates or transfer it to a glass or plastic container for storage.

After freeing yourself from food, the dishes should be washed immediately. To avoid damaging the metal surface, I only use soft foam sponges and regular cleaning product. It is impossible to achieve the same shine from aluminum pans as stainless steel cookware, and special shine products can react chemically and damage the surface.

Source: https://GolovaNeBoli.ru/otravleniya/alyuminij-ne-rzhaveet-prichiny-www.html

Does aluminum rust?

There is a huge selection of smokehouses.

Many people make smoking installations from wood or brick, and equip old refrigerators, metal barrels and buckets in smokehouses.

The most common option is a stainless steel smokehouse. You can make it yourself or purchase it at a specialized store.

A stainless steel smokehouse can be used in a summer cottage, on a picnic or even at home

Advantages

Stainless steel smokehouses have a number of advantages, thanks to which they are favorite smoking devices among summer residents and owners of their own plots. The advantages of such devices:

strength;
durability;
low susceptibility to soot formation;
the ability to prepare products using hot and cold smoking methods;
mobility;
design safety;
high resistance to rust;
ease of maintenance and operation, etc.

We also advise you to study the drawings of a mini smokehouse in addition to this article.

Important! Stainless steel is highly machinable, which is why homemade smokehouses are often made from it. In this case, we are not talking about magnetic stainless steel, but about food grade.

A steel smokehouse is easy to transport and assemble, which is why it is often purchased by lovers of homemade outdoor smoked meats

Readers found these materials useful:

How to make a smokehouse from a bucket or pan with your own hands
Do-it-yourself smokehouse from a gas cylinder: design, assembly and operation

Factory smokehouses made of stainless steel are usually produced in the form of a small horizontal box.

Fasteners are welded to the inner walls of the container for installing grates on which meat and fish preparations are laid out.

There are models with several rows of stands for food; such devices are called two- and three-tier smokehouses.

A lid with a hole to allow smoke to escape is put on the box.

There are designs in which the chimney is already welded to the top cover. We have already written about how to make a smokehouse with a hydraulic lock earlier and recommended bookmarking the article.

: review of a stainless steel smokehouse with a water seal

Important! If you plan to use a stainless steel smokehouse indoors, then in order to avoid excessive smoke, attach a hose to the pipe and lead it out into the ventilation/window, etc. The hose must be made of heat-resistant material - plastic, metal.

Standard models of factory-made steel smokehouses have a fat collection tray, a stand or shelf, and sliding/removable legs.

Thanks to its collapsible design, the metal box can be used on a stove, on a barbecue, on an open fire, etc.

Explore also: creating a smokehouse from a saucepan on our website.

In specialized stores, customers are presented with a wide range of stainless steel smokehouses. However, such a device can be made with your own hands at home.

Stainless steel smokehouse with water seal

A smoking device with a water seal deserves special attention.

This installation is a metal box with a U-shaped groove in the upper part of the container.

It is located around the entire perimeter of the box. Pour a small amount of water into it and close the lid.

A “water seal” is needed to ensure that air from the street does not get inside the smokehouse, since it is the flow of fresh oxygen that can cause the fire of wood chips or sawdust. The water seal ensures that smoke from the smokehouse only goes into the chimney.

In other words, it is responsible for the tightness of the smoking device.

Stainless steel smokehouses with a water seal are an improved version of the standard smoking device. The gutter around the perimeter of the smokehouse is filled with water and prevents smoke from passing into the unit.

A stainless steel smokehouse with a water seal has a number of advantages over the standard design. As a rule, it is stronger and more durable. This installation is less subject to deformation.

Diagram of a smokehouse with a water seal. Water will prevent smell and smoke from leaking into the room

The disadvantages of stainless steel smokehouses include their high cost and considerable weight. Below is a comparison table of smokehouses by parameters, weight and price.

Smokehouse parameters with a water seal, cm	Steel thickness, mm	Weight, kg	Approximate cost, rub.
402020	1,5	7	4400
402520	1,5	8	4700
402525	1,5	9	5000
403020	1,5	9	5000
502520	1,5	11	5400
452525	1,5	9	5400
503025	2	15	6700
503030	2	16	6800
603030	2	23	7700
604030	2	25	8700

Note! When choosing a stainless steel smokehouse in a store, you should pay attention to the brand and manufacturer.

The Russian market offers a huge selection of smoking devices from local factories.

Stainless steel smokehouses do not have to be in the shape of a rectangular box - a cylindrical smokehouse is no less popular.

This material will be perfectly complemented by the following publications:

Smokehouse with hydraulic lock (hydraulic seal)
How to make a wooden smokehouse with your own hands

To make a box-shaped smoking installation yourself, you will need the following tools and materials:

sheet of stainless food steel 2 mm thick;
sheet of metal 1 cm thick for the lid;
metal saw;
jigsaw;
Bulgarian;
grinding wheel;
square and rectangular pipes 1.5 cm thick;
metal legs;
fire resistant paint.

Assembling a stainless steel smokehouse is a sequential process, so for convenience it is recommended to sketch the future structure on paper with marks and dimensions.

Stages of work:

Before you start building a smokehouse, you should make a sketch of the future device with the dimensions of all the pieces.
According to the diagram, all the parts are cut out of a sheet of metal using a jigsaw.

Experts advise preparing two sheets of metal of different thicknesses; the thicker one is suitable for the body of the smokehouse, the thinner one for the lid.

A hole is made in the workpiece (front part) for a hopper with sawdust.
All edges and irregularities are cut off with a grinder wheel.
The cut parts are welded together in the form of a box. The sides are attached to the back, then the bottom.

A stainless steel smokehouse can be made of any size.

However, it is better to use ready-made diagrams, since the smoking process is affected not only by the selected temperature, the quality of sawdust and the amount of raw materials, but also by the size of the device. Existing schemes on the Internet are proposed taking into account all the features of this technological process.

To make the structure reliable, stiffeners made of a square pipe are welded to the back side. They are placed on top and in the middle of the workpiece.
Two more pipes are welded to the lower part to move the box with wood chips.

The sawdust box should fit snugly into the box body. For convenience, it is “placed” on a sled, which is made from a square pipe.

Next, the front wall is welded, a place for the “sled” is cut out on it, and stiffeners are attached.

In a homemade smokehouse, you can use either a ready-made grate or one made from a metal rod.

In accordance with the size of the resulting hole, a rectangular hopper is made for storing sawdust and wood chips.
To improve traction, small holes are made in the lower part of the smokehouse.
To make the smokehouse stable, steel legs are welded to it. The optimal height is 30 cm.
A lid for the smokehouse is cut out of thinner metal.

A stainless steel smokehouse can be coated with fire-resistant compounds. But this is not at all necessary, it all depends on the desire and imagination of the master.

Next, the limiters and handle are welded.
An additional shelf is attached to the legs.
The resulting smokehouse is painted with fire-resistant paint on the outside.

We previously wrote about the construction of a two-tier smokehouse and recommended bookmarking the article.

Rectangular stainless steel smokehouses can be a worthy replacement for stationary cold or hot smoked smokehouses. With proper care, they will serve their owner for many years.

You can make a homemade stainless steel smokehouse with your own hands in a few hours.

However, experienced craftsmen who have skills in working with a welding machine and an angle grinder can cope with this task.

A self-made smoking device will cost the owner much less than a factory installation.

Source: https://steelfactoryrus.com/rzhaveet-li-alyuminiy/

Aluminum corrosion

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.

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:

4Al + 3O2 → 2Al2O3.

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.

Corrosion of aluminum in water

Aluminum corrosion is almost not observed when interacting with clean, fresh, distilled water. Increasing the temperature to 180 °C does not have any special effect. Hot water vapor also has no effect on aluminum corrosion. If you add a little alkali to water, even at room temperature, the corrosion rate of aluminum in such an environment will increase slightly.

The interaction of pure aluminum (not covered with an oxide film) with water can be described using the reaction equation:

2Al + 6H2O = 2Al(OH)3 + 3H2.

When interacting with sea water, pure aluminum begins to corrode, because... sensitive to dissolved salts. To use aluminum in seawater, a small amount of magnesium and silicon is added to its composition. The corrosion resistance of aluminum and its alloys when exposed to sea water is significantly reduced if the metal contains copper.

Corrosion of aluminum in acids

As the purity of aluminum increases, its resistance to acids increases.

Corrosion of aluminum in sulfuric acid

Sulfuric acid (has oxidizing properties) in medium concentrations is very dangerous for aluminum and its alloys. The reaction with dilute sulfuric acid is described by the equation:

2Al + 3H2SO4(dil) → Al2(SO4)3 + 3H2.

Concentrated cold sulfuric acid has no effect. And when heated, aluminum corrodes:

2Al + 6H2SO4(conc) → Al2(SO4)3 + 3SO2 + 6H2O.

In this case, a soluble salt is formed - aluminum sulfate.

Al is stable in oleum (fuming sulfuric acid) at temperatures up to 200 °C. Due to this, it is used for the production of chlorosulfonic acid (HSO3Cl) and oleum.

Corrosion of aluminum in hydrochloric acid

Aluminum or its alloys quickly dissolve in hydrochloric acid (especially when the temperature rises). Corrosion equation:

2Al + 6HCl → 2AlCl3 + 3H2.

Solutions of hydrobromic (HBr) and hydrofluoric (HF) acids act similarly.

Corrosion of aluminum in nitric acid

A concentrated solution of nitric acid has high oxidizing properties. Aluminum in nitric acid at normal temperatures is extremely resistant (resistance is higher than that of stainless steel 12Х18Н9). It is even used to produce concentrated nitric acid by direct synthesis.

When heated, corrosion of aluminum in nitric acid proceeds according to the reaction:

Al + 6HNO3(conc) → Al(NO3)3 + 3NO2 + 3H2O.

Corrosion of aluminum in acetic acid

Aluminum is quite resistant to acetic acid of any concentration, but only if the temperature does not exceed 65 °C. It is used to produce formaldehyde and acetic acid. At higher temperatures, aluminum dissolves (with the exception of acid concentrations of 98 - 99.8%).

Aluminum is stable in bromic and weak solutions of chromic (up to 10%), phosphoric (up to 1%) acids at room temperature.

Citric, butyric, malic, tartaric, propionic acids, wine, and fruit juices have a weak effect on aluminum and its alloys.

Oxalic, formic, and organochlorine acids destroy metal.

The corrosion resistance of aluminum is greatly influenced by vapor and liquid mercury. After a short contact, the metal and its alloys intensively corrode, forming amalgams.

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).

2Al + 2NaOH + 6H2O → 2Na[Al(OH)4] + 3H2;

2(NaOH•H2O) + 2Al → 2NaAlO2 + 3H2.

Aluminates are formed.

Also, the oxide film is destroyed by mercury, copper and chlorine ions.

Source: https://www.okorrozii.com/korrozia-aliuminiya.html