What is anodized aluminum – Builder
The use of aluminum profiles for decorative finishing of facades and interiors has been used for more than half a century. A beautiful, ductile and very light metal in a humid atmosphere quickly became covered with a gray coating of oxides.
Preserving the silvery shine and expressiveness of the metal decor turned out to be possible only by applying a special coating.
The appearance of anodized aluminum remained virtually unchanged, the colors became brighter, and corrosion could be forgotten once and for all.
How Anodizing Works
To understand that this is anodized aluminum, you need to look a little more closely at how the protective film is formed. Most metals are protected either by protectors or insulators made of alloys and compounds that are more resistant to oxygen and moisture.
The anodized protective layer is ordinary oxidized aluminum Al2O3, but not in the form of a soft amorphous microfilm, which is always present on its surface, but as a crystalline structure, with properties reminiscent of corundum or spinel.
Anodized film has the following characteristics:
- Microcrystalline structure;
- The presence of a huge number of pores in the surface layer of the anodized film and a super-dense and durable structure at the base;
- Incredibly strong adhesion of the oxidized layer to the metal.
For your information! If the technological process is strictly followed, there is no clear boundary between the metal and the anodized film. A complex network of microcrystals smoothly transitions into metal without a clearly defined boundary.
What does this mean? This means that an anodized aluminum film will not peel off from the base under any load even after 40 years, while a nickel or paint coating slowly peels off from the aluminum matrix over time.
Depending on the selected conditions for obtaining an anodized surface, the technology allows you to obtain several options for the protective layer.
An ultra-thin oxidized film of an ordered structure with a thickness of 10-25 microns on the surface of an aluminum mirror is not even visible to the naked eye. However, the thinnest anodized layer on the aluminum mirror makes it possible to protect the metal from oxidation and simultaneously transmit up to 95% of the light flux.
Aluminum anodizing technology
The process of obtaining protective anodized coatings on the surface of aluminum is based on the anodic oxidation of aluminum in an electrolyte solution. Depending on the desired result, three types of electrolytes are used for anodized oxidation:
- Treatment with low currents at constant voltage in a weakly acidic electrolyte;
- Application of anodized coating on dichromate-acid electrolyte;
- Oxidation of aluminum in an alkaline electrolyte.
In all three cases, a protective film is formed due to oxidation, compaction and transformation of oxidized aluminum into a dense crystalline structure. The result is a coating reminiscent of glass microflakes.
For your information! In this case, the dimensions or external dimensions of the part do not change; the anodized metal coating seems to grow deeper into the aluminum until the resulting film breaks the electrical contact.
By changing the acidity and temperature of the electrolytic bath, the current and operating voltage at the anode and cathode, it is possible to obtain anodized aluminum films with very different properties.
With a small amount of current, an elusive patina is formed. It is difficult to feel even when touching the surface of anodized aluminum with your fingers.
The only sign of the presence of a protective film is the uniform color of the metal and the absence of the effect of getting your hands dirty.
Ordinary aluminum, under the influence of sweat and fat secretions from the skin of the fingers, can dissolve to form aluminates of organic acids. As a result, dark gray spots remain on the hands. Therefore, most aluminum products are protected by anodizing.
The essence of the anodizing process
The mechanism of formation of a protective coating on the surface of aluminum is based on the direct transformation of the metal into an oxide with a crystalline structure. If you simply fix the anode on an aluminum plate, fix the cathode on a carbon electrode, apply voltage and immerse it all in an acidic or alkaline electrolyte, then you will not get an anode film. The metal will simply dissolve in the electrolyte.
In order for a crystalline film to form on the surface of aluminum, high voltages and currents are required. The process of formation of the anodized layer itself is accompanied by a large release of heat, so the bath with electrolyte must be cooled to several degrees.
The process is so intense that microscopic plasma lights flash on the aluminum plate. The metal instantly melts, oxidizes, and the pressure firmly seals the electrolyte to the base. That's why in the photo the anodized film looks like crocodile skin.
A similar process can be relatively easily reproduced at home, but given the high voltage of more than 100V and high currents, the artisanal production of anodized aluminum is unsafe. In addition, effective ventilation will be required to remove evaporating electrolyte.
The operating modes of the installation for the production of anodized aluminum are not a secret and have long been published in the technical literature.
Practical Applications of Anodized Aluminum
Traditionally, the anodizing process is used to produce several types of oxidized films:
- Ultra-thin microcrystalline coatings with a thickness of 20-25 microns;
- Decorative films made of anodized aluminum;
- Electrical insulation based on crystalline Al2O3;
- Special protective films 1.5-2.0 mm thick.
Aluminum polished to a mirror reflects up to 98% of the light flux, but within a day, due to oxidation, a plaque forms, which turns into a gray film.
Most optical instruments equipped with polished aluminum reflectors are protected by an ultra-thin microcrystalline film of colorless corundum.
The dense, non-porous structure reliably blocks the access of oxygen and water vapor to easily oxidized aluminum, while maintaining 95-97% light transmission. Anodized aluminum film protects 99% of all headlights, high-power flashlights, reflectors and optical devices.
Decorative materials
The anodized aluminum coating has a rather interesting structure. The outer 35-50 micron films are a microporous, sponge-like surface with very narrow and deep pores.
Even a small amount of dye penetrates deeply into anodized aluminum, turning it into a very durable and at the same time bright coating. Colorless microcrystals refract the light falling on the anodized coating, as a result of which the colors become bright and saturated.
The applied paint coating does not fade or lose its intensity.
Most modern paints and varnishes with an iridescent effect are made by adding microscopic flakes coated with oxidized aluminum. The thinnest film of anodized metal ensures high resistance of the filler to ultraviolet radiation and organic solvents, so the paint does not lose saturation for decades.
The popularity of coatings has increased so much that metal is sprayed onto steel and even cast iron structural parts for subsequent oxidation and anodized protection.
Instead of unsafe zinc coating or very expensive alloy steels, anodized aluminum is widely used today.
For example, a metal facade made of double-glazed windows of a multi-story shopping center would have to be repaired within five years, but with anodized aluminum frames the structure could last for several decades.
Crystalline metal oxide coatings have seriously displaced the most resistant powder and ceramic paints, which were previously widely used to protect facades and structural elements made of aluminum alloys.
Special anodized aluminum films
In addition to high decorative qualities, anodized aluminum films have a number of very useful properties. For example, high hardness and wear resistance. The microcrystalline structure of corundum is practically not afraid of any abrasive. Sand and cement dust, and even metal carbides and silicides are not able to significantly damage the corundum protection.
Therefore, anodized parts cannot be cleaned with sandpaper or polishing or grinding paste. A thick layer of crystalline Al2O3 on the surface of rubbing parts increases the service life of any mechanism by two to three times. Protection made of oxidized aluminum is used for high-temperature painting of wheel rims, suspension elements of quarry machines and long-haul equipment.
Application of anodized aluminum coating:
- Does not degrade under the influence of frost, heat, ultraviolet radiation or chemically active substances, withstands direct contact with acids, alkalis, and organic solvents;
- Does not generate dust and does not wear out during repeated washing, cleaning, or under abrasive loads;
- There is no filamentary or gaseous form of corrosion; if the anodized aluminum layer is manufactured in compliance with the technology, then the service life of the coating can easily reach 60-80 years.
The second interesting quality of anodized aluminum film is low thermal conductivity. The processed metal is used to make foundry molds for casting copper alloys, despite the fact that the temperature of aluminum is several hundred degrees lower than that of copper. A thin, just a couple of millimeters oxidized coating reliably protects the aluminum mold from overheated liquid copper.
Heating radiators, pipeline fittings, boilers, stoves, fireplaces made of steel and cast iron are protected by anodized aluminum films according to modern standards. Even when walls, grates, and power fittings are heated to 500-600°C, steel and cast iron do not burn or corrode to rusty holes. The service life of a steel furnace has increased from 10 to 40 years of service.
Conclusion
The use of thin films of crystalline Al2O3 makes it possible to obtain coatings with completely new properties. The point is that most metal parts and structures, and even certain types of plastic, can be provided with almost “eternal” coatings.
Even if the film is damaged due to an impact or chip, it can be restored using a simple procedure.
So far, oxidized metal is more expensive than paint, so it is used as a decorative material and a way to protect the metal surface under extreme operating conditions.
Source: https://strojchik.ru/chto-takoe-anodirovannyj-alyuminij.html
About anodized aluminum
In terms of high consumer properties and unique characteristics, aluminum stands out quite strongly from other metals. It conducts heat and electricity well, has low strength, is ductile and resistant to corrosion. In order to significantly increase the strength of aluminum, the metal is anodized.
As a result of the electrochemical action, the surface of the metal is converted into porous aluminum oxide, due to which the metal becomes more resistant to mechanical stress, atmospheric influences and increases its service life. In order to compare the durability of anodized aluminum, we can say that the hardness of anodes and corundum is almost equal. Corundum, in turn, has 9 strength points, and diamond 10.
Thus, aluminum acquires characteristics previously unknown to it.
Anodized aluminum - clean hands
When aluminum without an anodic coating is used for the manufacture of stairs, chairs, railings, handrails, sports equipment, handles and other products that often come into contact with hands or light-colored clothing, you can often hear complaints that aluminum leaves gray marks - “gets dirty.” Anodizing completely solves this problem and is used, for example, for all aluminum parts in trains, buses, trolleybuses and trams. Aluminum knitting needles have gained popularity precisely because of anodization: they no longer stain fingers.
Anodized aluminum in thermal reflectors
Anodizing has long been used for aluminum heating reflectors - they can be found in every home. Their surface is easy to clean and can withstand even the humidity of bathrooms.
The effectiveness of anodized aluminum as a reflector of thermal radiation is ensured by the fact that the thickness of the anodic coating is only about one micron.
The heat-reflective properties of thicker anodic coatings are used in the manufacture of cooling radiators - “combs” in electronic devices, including every computer. To increase the thermal emissivity of the anodic coating, it is often painted black.
Anodized aluminum to combat friction and wear
The anodic coating is much harder than the base aluminum, so the wear resistance and “anti-marking” of the product are significantly increased. Before hydrothermal hydration of the anodic coating began to be used, its physical filling with oils, waxes and similar substances was widely used.
Filling the anodic coating with lubricating oils has found application in those engineering solutions where constant lubrication is required on given surfaces. This is widely used in aluminum pistons of gasoline and diesel engines. Filling of the anodic coating with graphite suspensions is also used.
Hard anodic coating with a typical thickness of 40 to 60 microns has been successfully used on various machine parts, for example, hydraulic and pneumatic cylinders.
In modern life, metal products are used almost everywhere. Naturally, preference is given to lightweight, plastic, impact-resistant metals that do not pose a danger to human health. According to these indicators, aluminum stands out quite noticeably among others.
Once this metal began to be produced on a large scale, the question arose about ways to modify it in order to improve its basic properties. In particular, there was a need to increase corrosion resistance and the ability to resist mechanical damage. The solution to this problem was anodizing aluminum.
How does the aluminum anodizing process work?
Anodized aluminum is aluminum that has been intentionally coated with an oxide film that is firmly bonded to the surface. Briefly, the anodizing process occurs as follows. The part is placed in a sulfuric acid solution along with the electrode. Anodizing is carried out with direct current up to 100 volts.
Depending on the required properties, the process takes place at different temperatures. If anodized aluminum then needs to be painted, the electrolyte temperature is maintained in the range from +10 to +20 degrees, since at these temperatures a colorless and thin film is obtained, which very easily adsorbs paint.
If it is necessary to obtain a stable coating of large thickness, the process is carried out at low temperatures (-20 degrees), but the color of the coating remains natural - dull golden.
Fixing is carried out by intensive steam treatment, as a result of which the molecules of the oxide film are hydrogenated and the metal surface is completely isolated.
Advantages of anodized aluminum
Anodized aluminum has versatile properties that make it widely used in manufacturing areas such as metal printing, tableware, decorative items, everyday products, reflective surfaces, and so on.
Briefly describing these properties, it can be noted that they are all associated with the oxide film obtained as a result of anodization. For example, direct fixation of the film to the surface eliminates peeling and cracking. This immediately ensures the possibility of long-term use in conditions of high humidity.
The film is uniform and has specific reflective properties, as a result of which anodized aluminum is widely used for aesthetic and decorative purposes, since its surface can be presented as a glossy or matte surface with a “noble” appearance.
Due to the stability of the film, there is no need to polish or renew the oxide layer.
Anodizing must meet certain requirements. Its reflective properties depend on how pure the metal is and the quality of the coating: color, purity of reflection, diffuse reflection coefficient.
It is worth noting that to achieve the desired result when anodizing, it is necessary to take into account many nuances. In a word, this process is complex and high-tech.
Anodizing aluminum is used for consumer, commercial and industrial purposes, as well as in the creation of works of art and jewelry.
Anodized aluminum is a fairly popular material, and its scope of application is truly extensive. Anodized aluminum is used to make:
- Anodized heating radiators MANDARIN
- lamps and spotlights in which it is necessary to eliminate glare;
- primary and secondary reflectors having a high reflectivity;
- railings and fences, anodized aluminum is ideal for these purposes, since precipitation and other weather conditions do not in any way affect the appearance of the structure and it retains its consumer properties for a long time;
- light guiding systems, such as blinds. Their slats are coated with glossy anodized aluminum, which allows them to achieve a reflectivity of about 93%;
- slatted shelves made of anodized aluminum, polished to a high shine. They can emit the color of copper, gold, silver and allow you to give the room a very luxurious, noble look;
- wall cladding and basement cladding;
- sandwich-type building panels, facade systems, windows and doors;
- decorative elements of microwave ovens, televisions, refrigerators and other household appliances;
- car interior trim and decorative trims that are installed on the outside;
- components for optical, measuring instruments and watches (hands, scales, dials);
- nameplates, profiles, pipes, locks, plugs, closures and caps used in aerosol bottles;
- stamped spare parts, which are obtained as a result of deep stamping;
- lithographic foil used to emboss and decorate the surface of various fabrics, allowing the color to become more fresh and sophisticated. Lithographic foil is considered a premium finishing material.
Thanks to anodization, aluminum gains many new properties that make it possible to use it in difficult conditions. For example, high resistance to corrosion determines the use of anodized aluminum in coastal areas. The anodizing layer is tightly adhered to the surface, which eliminates the formation of cracks and peeling of such a coating.
In addition, anodized aluminum has high aesthetic properties and is resistant to UV radiation and temperature changes. Products made from anodized aluminum can be successfully used for quite a long period of time, while surface restoration is not required, and the surface itself visually looks very clean and is slightly susceptible to further contamination.
The anodizing process does not harm the environment and is environmentally friendly, and anodized products are also resistant to abrasion.
Source: https://radiatormandarin.ru/ob_anodirovannom_alyuminii
What is anodized aluminum
Today, aluminum remains a very important and sought-after material for the manufacture of all kinds of parts, fakes, etc. You can list a lot of its advantages, for example, light weight, sufficient strength, does not corrode, and is easy to process for further use. But despite all this, many are not attracted by its appearance.
If you have ever tried to paint aluminum, then your attempts may have ended unsuccessfully, because paint sticks to aluminum very poorly. If you use it without paint, then very soon it will become covered with dark spots. To prevent all this, aluminum anodizing technology was developed.
We invite you to consider the question of what anodized aluminum is, what types of it exist, in what areas anodized aluminum is used and whether it is possible to anodize this material with your own hands.
Anodizing - what is it?
Anodizing means anodic oxidation. That is, this is a process as a result of which an oxide coating is formed or appears on the surface of aluminum. As a result of this process, oxidation of the metal occurs. As a result, aluminum becomes invulnerable to negative external influences. That is, the oxidized place becomes much stronger.
Why anodize?
As mentioned above, when aluminum interacts with oxygen, a film forms on its surface. It prevents oxidation. But there is an important nuance here: this natural oxide film is very thin. As a result, it can break through. And to eliminate this, it was decided to anodize the aluminum. As a result, the metal acquires much better technical characteristics.
Thus, anodized aluminum does not corrode. The resulting film is wear-resistant. Over time, this coating will not even peel off. Here it is important to understand one more nuance, why this became possible. Some metals are coated with chromium or zinc. In the case of aluminum, it is not coated with anything. This film forms directly on the metal itself.
So, this procedure is resorted to with the goal of giving the metal a more decorative appearance, for example, a particular shade. It is noteworthy that the anodizing color can be changed. To do this, you should use aniline dyes, which are used when dyeing clothes.
If we talk about industrial technologies, then aluminum is anodized in a 20 percent sulfuric acid solution. As for home conditions, this technology is unsafe, so it is necessary to use another technique.
Application of anodized aluminum
There are many areas of use to achieve completely different goals. Now let's look at them:
Anodizing techniques
Aluminum can be anodized in different ways, at least we will mention two:
Let's look at the important features of each technology.
Warm anodizing
This work is performed at room temperature from 15 to 20 degrees Celsius. The procedure is known to be easily repeatable. With simple manipulations you can get a beautiful result.
However, this method does not allow achieving excellent anti-corrosion protection. When the material comes into contact with an aggressive environment, corrosion may occur. Also, the workpiece will not have good mechanical protection.
For example, the coated material can be easily scratched even with a needle, and sometimes it can be wiped off by hand.
But on the other hand, this coating serves as an excellent basis for further processing of the material. The anodizing process takes place in the following sequence:
Cold anodizing
This means that the anodizing process occurs at temperatures from -10 to +10 degrees Celsius. Thanks to this, much better quality, hardness and strength of the anodic coating can be achieved. The cold process perfectly demonstrates the low rate of dissolution of the outer film. As a result, a thick layer is formed. The situation is completely opposite with a warm process.
So, to achieve such results, it is necessary to create forced cooling conditions. Without this, it will be impossible to create a beautiful and wear-resistant coating. If we talk about the disadvantage of this technology, it is as follows: the surface cannot be painted with organic dyes.
The technological process of cold anodizing of aluminum looks like this:
- The surface is thoroughly degreased.
- The workpiece is mounted in a hanger.
- The anodizing process takes place in the bath until a dense shade is formed.
- Washing is carried out in cold and hot water.
- Next, the process of cooking the workpiece in distilled water occurs. The product is also steamed. These actions allow you to consolidate all the resulting layers.
Think about safety
So, you can perform this process at home, but to do this you should be extremely prudent and follow safety precautions. It's best to do this outdoors. After all, acid is a very dangerous substance. And this is even despite the fact that you will be using a large concentrate of acid.
Important! If it gets on your skin, you will experience unpleasant itching. But if it accidentally gets into your eyes, it can lead to serious consequences.
So, you should use protective clothing, gloves and goggles for work. Plus, always have a solution of soda or a bucket of clean water nearby.
Conclusion
So, here we have learned what anodized aluminum is. We looked at the areas of its use and options on how to do similar work yourself. In addition to everything, we suggest watching a video that will consolidate all the knowledge gained from this article on how to anodize aluminum with your own hands. We are confident that you will cope with all the work yourself without outside help.
Source: http://obrawa.ru/anodirovannyj-alyuminij/
What is the difference between anodized aluminum and regular aluminum - Metalist's Handbook
The properties of the anodic oxide coating on aluminum are unique among other coatings. Therefore, they have found wide application in various aspects of human life.
Anodized aluminum as a base for painting
This was the first industrial application of anodic coatings since the invention of aluminum anodization (in chromic acid) in the twenties of the last century.
This was a standard surface treatment for aluminum (duralumin) aircraft parts and is still specified in standards, such as the modern British military standard DEF STAN 03-24/3.
This combination of an organic coating with a chrome anodic coating gives maximum life to the paint layer on the protective coating and provides protection to the metal even after paint damage.
Sulfuric acid anodic coatings with bichromate filling are also used as a protective layer and a base for applying organic paints. This protective coating has a long service life, including in sea water.
Anodized aluminum - corrosion protection
The unpainted chrome anodic coating has high corrosion resistance, including in salt environments. It is used to protect aluminum aircraft parts from corrosion when they cannot be painted.
Sulfuric acid anodic coatings with hydrothermal filling are widely used for corrosion protection of aluminum structures in marine and industrial atmospheres.
In recent decades, anodic coatings, colorless and colored, have been widely used for external and internal building materials and parts, including windows, doors, building facades, internal partitions and railings.
Military aluminum structures and machine parts, especially those that must withstand long periods of storage and operate in tropical and marine environments, are also most often protected with anodic coatings.
Anodized aluminum in design
Thanks to the ability of anodic coatings to absorb dyes, a wide range of “colored aluminum” is obtained. This method is called absorption and it is widely used for various aluminum products - cast, pressed, stamped. A more durable color coating - electrolytic - is obtained in various electrolytes, most of them in solutions of nickel, cobalt and tin salts. Its “range” of colors is much narrower than that of the adsorption one, but quite diverse.
Anodized aluminum - clean hands
When using aluminum without an anodic coating, for example, for the manufacture of stairs, chairs, railings or handrails, you can often hear complaints that aluminum leaves gray marks - “gets dirty”.
Anodizing completely solves this problem and is used, for example, for all aluminum parts in trains, buses, trolleybuses and trams. Aluminum knitting needles have gained popularity precisely because of anodization: they no longer stain fingers.
An important role in achieving this property of anodized aluminum is played by filling the pores of the anodic coating.
Anodized aluminum in floodlight reflectors
Sulfate anodizing is used to protect the surface of floodlight reflectors. An initial small loss in reflectivity is considered acceptable as this condition will persist for years, whereas unprotected aluminum will continually corrode and reduce its ability to reflect light. Additionally, anodized aluminum is much easier to clean than regular, bare aluminum.
Anodized aluminum in thermal reflectors
Anodizing has long been used for aluminum heating reflectors - they can be found in every home. Their surface is easy to clean and can withstand even the humidity of bathrooms.
The effectiveness of anodized aluminum as a reflector of thermal radiation is ensured by the fact that the thickness of the anodic coating is only about one micron.
The heat-reflective properties of thicker anodic coatings are used in the manufacture of cooling radiators - “combs” in electronic devices, including every computer. To increase the thermal emissivity of the anodic coating, it is often painted black.
Anodized aluminum to combat friction and wear
The anodic coating is much harder than the base aluminum, so the wear resistance and “anti-marking” of the product are significantly increased. Before hydrothermal hydration of the anodic coating began to be used, its physical filling with oils, waxes and similar substances was widely used.
Filling the anodic coating with lubricating oils has found application in those engineering solutions where constant lubrication is required on given surfaces. This is widely used in aluminum pistons of gasoline and diesel engines. Filling of the anodic coating with graphite suspensions is also used.
Hard anodic coating with a typical thickness of 40 to 60 microns has been successfully used on various machine parts, for example, hydraulic and pneumatic cylinders.
Anodized aluminum as an electrical insulator
Although the anodic coating is a good electrical insulator, the danger of local electrical breakdown due to minor defects limits the use of anodized aluminum wires.
However, anodized aluminum tape has been used for many years for some types of transformers where it is important to reduce their weight.
Anodic oxide coating resists heat much better than organic electrical insulating materials, so it is often chosen for high temperature applications.
Source: https://ssk2121.com/chem-otlichaetsya-anodirovannyy-alyuminiy-ot-obychnogo/
Application of anodized aluminum
The properties of the anodic oxide coating on aluminum are unique among other coatings. Therefore, they have found wide application in various aspects of human life.
Anodized aluminum – corrosion protection
The unpainted chrome anodic coating has high corrosion resistance, including in salt environments. It is used to protect aluminum aircraft parts from corrosion when they cannot be painted.
Sulfuric acid anodic coatings with hydrothermal filling are widely used for corrosion protection of aluminum structures in marine and industrial atmospheres.
In recent decades, anodic coatings, colorless and colored, have been widely used for external and internal building materials and parts, including windows, doors, building facades, internal partitions and railings.
Military aluminum structures and machine parts, especially those that must withstand long periods of storage and operate in tropical and marine environments, are also most often protected with anodic coatings.
Anodized aluminum - clean hands
When using aluminum without an anodic coating, for example, for the manufacture of stairs, chairs, railings or handrails, you can often hear complaints that aluminum leaves gray marks - “gets dirty”.
Anodizing completely solves this problem and is used, for example, for all aluminum parts in trains, buses, trolleybuses and trams. Aluminum knitting needles have gained popularity precisely because of anodization: they no longer stain fingers.
An important role in achieving this property of anodized aluminum is played by filling the pores of the anodic coating.
Products – Tekhmashholding – group of companies, official website
Deep anodization is the process of producing oxide films with a thickness of more than 40 microns, characterized by high hardness, wear resistance and good electrical insulating properties. With the help of such films, it is possible to increase the wear resistance of the rubbing surfaces of parts, for example gears, increase resistance to erosive wear, and provide heat-resistant electrical insulation. Deep anodizing of thin-walled parts increases structural rigidity.
Deep anodizing often has a negative effect on the mechanical properties of anodized products: the endurance limit, relative elongation and narrowing of the cross section decrease. This effect is insignificant at low film thickness and increases with its increase. After removing the oxide film, the original properties are restored (before anodizing).
The mechanical and electrical properties of thick oxide films and the degree of their influence on the characteristics of the metal depend on the oxidation conditions and the composition of the metal or alloy being electrochemically treated.
The role of thermal processes occurring in the oxide formation zone increases as the film thickness increases.
This is explained by the difficulty of heat removal from the metal surface deep in the pores, which can lead to an increase in the rate of film dissolution and etching of the film and metal.
The Institute of Physical Chemistry of the USSR Academy of Sciences researched and developed the process of deep anodizing at a low temperature of the electrolyte and workpieces. Electrolysis is carried out in a 20% solution of sulfuric acid at a temperature from -3 to 10 ° C.
To maintain the required temperature, intensive mixing of the solution is used with purified compressed air or mechanical mixers and cooling it using special units. The anodic current density during deep anodization is 2–2.5 A/dm2; in the case of intensive cooling, it can be increased to 5–10 A/dm2.
The voltage across the bath increases as the thickness of the oxide film increases and reaches 40–80 V.
During oxidation, a constant current density is maintained using rheostats connected to the bath power circuit. Depending on the required thickness of the oxide film, the rate of voltage increase is changed from 22-26 to 30-36 V in 15 minutes or up to 45-60 V in 45 minutes.
By cooling and stirring the electrolyte on aluminum and its alloys, oxide films with a thickness of 40-60 microns, and in some cases up to 120-150 microns, can be obtained. Good results are obtained if the electrolyte is circulated in the bath using acid-resistant pumps.
To obtain films with a thickness of 200-300 microns, such cooling is not enough and it is necessary to use internal cooling of the workpieces. This method is suitable for parts with internal non-oxidizing cavities through which a cooling solution can circulate.
Liquid refrigerants or water pre-cooled to -3+0° C are used as coolants. The circulation rate of the cooling solution should be such that the temperature difference at the inlet and outlet of the part does not exceed 1 degree. The oxidation mode is set taking into account the grade of the alloy from which the parts are made.
Oxide films obtained at the same current density and duration of electrolysis on technical aluminum and AB alloy have greater thickness and hardness than on other deformed alloys. Microhardness (kgf/mm2) of the oxide layer on technical aluminum is 500-520, on the AB alloy - 480-500, on the D16 alloy - 330-360, on the AJT9 alloy - 450-480.
The best results in terms of film quality are obtained by deep oxidation of aluminum and its alloys with magnesium and manganese. On cast alloys such as silumin, the breakdown voltage of oxide films is 2-3 times lower than on deformable alloys AB, AK4, V95, AMg-5VM. The wear resistance of wrought alloys is also relatively lower.
Electrical insulating oxide films are obtained in a 15-20% h3SO4 solution at a temperature from -5 to +2° C and an anodic current density of 5 A/dm2. The electrical mode of the process is set taking into account the material from which the workpieces are made. For parts made of A7 grade aluminum, the initial voltage on the bath is 20-25 V, the final voltage is 60-65 V, the oxidation duration is 75-90 minutes, the breakdown voltage of the film is 600-800 V.
For parts made of alloy D16, the initial voltage is 20-25 V, the final voltage is 60-65 V, the oxidation duration is 25-30 minutes, the breakdown voltage of the film is 380-450 V. Parts made of AMg alloy are oxidized for 60-90 minutes, the initial voltage on the bath is 20 -25 V, final 60-70 V, breakdown voltage 600-800 V.
Parts subjected to electrical insulating oxidation must have a surface finish corresponding to the ninth class, the radius of curvature must be at least 2-3 mm; reducing it leads to a decrease in the breakdown voltage of the film.
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