The essence of the anodization process is the buildup of an oxide coating, which on aluminum and its alloys has a protective function against environmental influences. Another name is anodic oxidation. In addition, oxidation is used to increase the aesthetics of the appearance of products.
Surface defects are eliminated - small scratches, small chips. You can simulate the coating with precious metals or increase the adhesive properties. The coating can be applied not only at the factory, but also at home.
Anodizing aluminum at home is very popular among home craftsmen. In products subjected to anodic oxidation, the resistance of the protective coating increases.
General Anodization Technology
Aluminum anodizing technology is similar to galvanic processing. The deposition of ions of solution oxides on the workpiece occurs in a liquid electrolyte at high or low temperatures. The use of a heated solution is possible in industrial plants, where there is the possibility of careful monitoring and regulation of voltage and current in automatic mode.
At home, they usually use the cold method. This method is quite simple, does not require constant monitoring, and equipment and supplies are available. To prepare the solution, you can use the electrolyte used in lead automotive batteries. It is sold in every car shop.
The high strength of the protective oxide film depends on its thickness, which at home is obtained by processing in a cold solution. The increase is made by stepwise regulation of the operating current.
Black oxidation of aluminum refers to color anodizing. Black color is obtained in two stages. First, a colorless film is applied by electrolysis, and then the workpiece is placed with a salt solution of acids. Depending on the acid, the color can range from pale brass to deep black. Black aluminum is widely used in construction and decoration.
To conduct high-quality anodizing at home is not difficult. It is safer and more convenient to do this work on the street or balcony. During the process, you will find several health hazards.
Acid is a very caustic thing. Although it is in a very dilute form and causes only slight itching when it comes into contact with the skin, it can cause serious injuries if it gets into your eyes! Therefore, it is advisable to work in safety glasses when anodizing and always have a bucket of water or a weak soda solution on hand.
During the anodizing procedure, oxygen is released at the anode, and hydrogen is released at the cathode. After mixing these gases, they form a known explosive gas, which, in principle, is the same dynamite. Therefore, when anodizing indoors, you can die from the first spark.
The anodizing process consists of several technological steps, including mechanical and chemical preparation, the direct creation of a coating and, if necessary, the correction and refinement of the product. Primary machining is carried out in order to eliminate scratches, scratches, dents and other defects on the surface that will not allow to perform the operation qualitatively. In particular, color anodizing of aluminum requires maximum smoothness of the surfaces of the workpiece, which will allow it to give a natural shine with reflux. Processing is done by grinding and polishing, but large abrasives should be discarded. The best option would be felt and felt circles. Already at this stage, it is possible to envisage elements of chemical preparation - for example, Viennese lime or the same aluminum oxide is used as polishing pastes. Sometimes chemical electro-polishing is also performed on special equipment.
As for direct chemical preparation, it is carried out through the operations of degreasing, etching and clarification. Already in the process of anodizing, coloring or compaction can be carried out. Again, in the domestic sphere, this processing method is more often used for decorative color changes. Anodizing aluminum allows you to endow the workpiece with light gray, black, red, blue and other shades. A separate category of palettes is occupied by imitating colors - for example, with the effect of aging or “under the bronze”.
At home, as already mentioned, due to the limited availability of chemical materials, it is necessary to use simplified methods of organizing the process. The main compromise is at the stage of electrolyte preparation. Most often, home craftsmen use a solution of sulfuric acid diluted in distilled water for such purposes. You can limit yourself to tap water, but color anodizing of aluminum in this case can turn out to be of poor quality - in particular, with obvious signs of uneven coating. Only distilled liquid will ensure the distribution of the optimal current density over the entire surface of the part.
Sulfuric acid can be purchased at car dealerships. It is sold openly for charging battery packs with electrolytic mixture. You should buy compositions for lead batteries, which are sold in a diluted state with a density of about 1.27 grams per 1 m3. Mix this acid with distilled water in a ratio of 1: 1. That is, when preparing an electrolyte with filling a 10-liter bath, a 5 liter canister of sulfuric acid and the same amount of clean water will be required. And already at the stage of diluting the two components, it is worth considering the safety technique with which the anodizing of aluminum is performed. At home, without special protection, you can get burns in the process of mixing water and sulfuric acid. A sharp addition of liquid to an aggressive environment will provoke boiling with the release of spray. Therefore, it is important to organize the addition of water to acid so that the water pours slowly and in a thin stream.
Preparing anodizing containers
Anodizing is carried out in a container with electrolyte, the preparation of which should be tackled in a separate order. For large structures or parts, you need to use a bathtub, also made of aluminum. Small-sized workpieces are processed in containers, pots or basins, which can be made of plastic. Sometimes containers that are unsuitable for their characteristics are additionally covered with sheets of this metal. It is optimal if the bottom and walls are hermetically covered with an aluminum layer. This will allow you to evenly distribute the current, covering all sides of the workpiece. Further, the container should be insulated already from the outside. The fact is that anodizing aluminum at home does not allow you to accurately control the thermal regime of the container body and its contents. Therefore, it is necessary to pre-warm the structure using foam plastic with a thickness of 2-3 cm. If you plan to regularly use anodizing, you can prepare a special bath with fixation on a profile sealed frame and fill with mounting foam.
At the final stage of preparation of the tank, a lead cathode is made, which will be connected to the bathroom. In this part, it should be borne in mind that the area of the electrical element should be twice as large as the target area of anodizing aluminum. With your own hands, the cathode can be made of sheet lead, removed from the protective sheath of a thick cable. Also, small holes should be provided in this element that will allow gas to be released during processing.
Before starting the anodizing process, the part must be firmly fixed on the suspension devices for a more dense electrical contact. It is recommended to use aluminum structures or alloys based on titanium with duralumin as suspension devices. The fastening itself can be done using screw or spring clamping mechanisms. For additional insurance, aluminum tight wire is often used. Parts of the structure that will not participate in the contact must be isolated. This can be done either through a tape made of polyethylene, or using acid-resistant varnish. The second method requires extra care, since color anodizing of aluminum at home provides for the maximum elimination of factors of third-party influence on the surface of the workpiece. That is, the treatment with varnish should be done in advance with the complete drying of the surface. It should also be borne in mind that loose contact of the suspension system with the target material can cause the latter to overheat. This effect, in turn, will lead to the destruction of the oxide coating and a decrease in the strength of the deposited film.
Optimum processing mode
The optimum temperature range for oxidation is in the range from -10 to 10 ° C. Going beyond these limits implies certain risks in terms of obtaining high-quality color coating. Thus, a low temperature will not allow the home electrical system to maintain sufficient current for processing. Conversely, an increase in heat will make it possible to form a dense layer, but the film may not be so expressive in its shade. However, anodizing aluminum at home in black or light gray colors may well be carried out in high thermal mode. Another thing is that already the mechanical state of the surface structure of the workpiece will differ in different areas. This is due to the fact that there is an uneven distribution of heat throughout the volume of the electrolyte. Attention should also be given to the current density at the anode. It is recommended to maintain the value within 1.6 - 4 Amperes per 1 dm 2. With this current supply, a densely colored, durable protective layer can be obtained. Lowering this indicator will provide a thin layer, and an increase will facilitate etching of the product.
By the time the process begins, the capacitance, the cathode with the power supply, the target part, the suspension structure and the electrolytic mixture should be prepared. To adjust the current strength, it is necessary to connect a variable resistor to the electrical circuit. Two objects should be in the tank - a prepared lead cathode and a billet. When current is supplied, the process of oxygen evolution will begin and the part will receive an increase in the protective layer. The effectiveness of the reaction can be judged by the intensity of the release of oxygen microbubbles, which will slowly depart from the surface of the workpiece. As for the processing time, the standard technology of anodizing aluminum for small elements provides 20-30 minutes. Large workpieces are processed within 30-60 minutes. When the part becomes a dark matte color, it can be immersed in a solution with aniline dye with the desired shade. On average, another 15-20 minutes are given for staining.
Fixing the result of anodizing
When the part acquires the necessary color, a new layer on the surface will need to be fixed. The need for additional strengthening is due to the fact that the anodized coating has a porous structure that is easily permeable to water and air. Therefore, immediately after staining, micropores should be closed. The simplest method to achieve this effect is the cooking process in distilled boiling water. This procedure should take about 30-40 minutes. But if the process of anodizing aluminum was carried out at low temperature conditions, then it is better to abandon this method in favor of a steam bath. The part is kept under the intense influence of the steam generator for 30 minutes, after which it is washed and dried.
In this case, the simplest method of oxidation with a sulfuric acid electrolyte was considered. But if you want to get a better coating, you can use the technology of anodizing aluminum in sulfosalicylic acid, which forms thin, but dense coating layers. This is due to the fact that the electrolyte has a minimal effect on the metal in terms of its solubility. But this effect, if necessary, can be made up by adding the same sulfuric or oxalic acid to the active mixture. Distributed and processing in two-component sulfosalicylate environments. On the contrary, due to a slow increase in the temperature of the anode, while maintaining a small current strength, it is possible to build up thick and durable coatings.
An alternative is the so-called cold oxidation method. In this case, the process occurs at temperatures below zero. Risks have already been noted when using boundary temperature values, but this option justifies itself when you need to get not a decorative, but a protective base on the surface. In other words, cold anodizing of aluminum at a temperature of the order of -10 ° C will not allow providing details with a pronounced decorative shade, but will make its external structure stronger. But, again, this method will require the use of electrical equipment that can stably maintain current under a voltage of more than 12V.
As already noted, special safety requirements should be observed in operations with electrolyte. It is advisable to work with gloves and goggles. In this case, the working air environment will also be dangerous, therefore, the process should be organized in a room with an active supply and exhaust ventilation system. All containers with explosive and combustible mixtures should be removed from the bathtub and electrical equipment that provides the process of anodizing aluminum. At home, it also makes sense to carry out an operation with the presence of manual fire extinguishing means. Special attention is paid to subsequent cleaning of equipment. The container and auxiliary equipment should be washed in special solutions, and the electrolyte residues should be disposed of.
Oxidation of metals with acids is mainly used as a processing procedure in a production environment. But recently, ordinary townsfolk have been increasingly looking at her. Why might such coverage be required in the domestic sphere? Anodizing aluminum allows you to change the aesthetic qualities of the material, but in most cases the technology is used for practical protection purposes. A high-quality oxidized layer on the metal surface minimizes corrosion processes. In the case of aluminum, these can be automobile parts, engineering equipment, roofing sheets and elements of other building structures. There are less complex methods of such protection, but electrochemical treatment by anodizing provides an increased degree of protection of the metal structure from external influences.
Remember that parts become larger after anodizing. The thickness of the protective anode layer is usually 0.05 millimeters. For example, the threads that used to be tightened tightly after the anodizing process will generally cease to twist, since the bolt in the nut in this case will become 0.2 mm closer. And grinding anodized is almost impossible.
It is useful to polish products to a mirror shine on a polishing wheel. Thus, the aesthetics of the part will greatly benefit and the likelihood of anodizing “burnout” will decrease. By the way, the anode layer does not mask surface defects - they will be noticeable on the processed product.
Before electroplating, aluminum must be well degreased. You should not keep the metal in hot caustic sodium or potassium, as recommended in factory technologies, because the surface cleanliness noticeably spoils. It is better to use a piece of laundry soap and a toothbrush, because you have to work with small details. Rinse the product in warm water first, then in cold.
Washing powder works very effectively: it must be dissolved in hot water in a plastic container. Then you should pour out the products there and shake the dish well. After washing, thoroughly dry the parts with hot air. Do not worry about small traces of fat: after degreasing, you can pick up the product in your hands, because a layer of fat from your fingers is oxidized with oxygen instantly.
The electrolyte for anodizing at home is a solution of sulfuric acid in distilled water. You can use ordinary tap water, but if you can take distilled water, it is better to choose it, since in the first case the uniformity of the process spoils a little - the distribution of current density on the surface of the part.
Серную кислоту глупо делать самостоятельно, а вот дистиллированную воду - очень просто! Если на улице нет снега или дождя, то лед в морозильнике найдется всегда. You can get distilled water and sulfuric acid at the local auto parts store, because these ingredients are used to service car batteries.
However, acid is sold there in diluted form to a density of 1.27 grams per cubic centimeter under the name “Electrolyte for a Lead Battery”. You need to mix this electrolyte with distilled water in a ratio of 1: 1.
If you take a standard 5-liter canister with electrolyte and the same amount of water, you will end up with 10 liters of anodizing solution. This is enough for small parts, and for large it is worth doubling this amount.
Remember that mixing acid with water will generate a lot of heat. If you pour water into acid, it will instantly boil, splashing it in your face! That is why it is recommended to pour electrolyte into a container of water with a thin stream, constantly stirring with a glass rod. And it's better to wear safety glasses! If acid gets on clothing or skin, wash it off immediately with a stream of water and rinse with a solution of soda.
The temperature of the metal anodizing process is -10 - +10 degrees Celsius. A growing layer below -10 is quite good, but there will not be enough voltage that is supplied by the power supply to maintain the required amperage. Above +10 degrees, a protective film, although it will be formed, but it will turn out to be solid and colorless.
However, it is recommended to stop the anodizing process already at 5 degrees above zero. But the thing is, in the corner of the bath and on the surface of the part there is a different temperature, and when anodizing a lot of energy is released in the form of heat.
But if forced electrolyte mixing is not provided, you cannot trust the thermometer! However, it is necessary to mix the electrolyte constantly, with a spoon, air, a pump, this is necessary to equalize the temperature on the surface of an aluminum product. Otherwise, parts of local overheating are formed on the parts, and then breakdowns and grinding of the parts.
The anode current density should be in the range of 1.6 - 4 Amperes per square decimeter. Within such limits, a beautiful, colored and dense protective anode layer will grow. It is best to contain current densities from 2 to 2.2 Ampere / dm2. At a lower current strength, the coating will grow slowly, not thick. At a current greater than 4 Amperes / dm2, an electrical breakdown may occur, and the product will quickly be etched.
The cathodic current density should be low. The lower this indicator, the better, because it provides a uniform and soft mode of the distribution of current density over the surface of the workpiece, especially if it is large. Therefore, remember that the area of the cathode of lead should be twice as large as the area of the part (anode).
The process of anodizing the aluminum profile does not specify the voltage values of the anode-cathode. However, if your circuit has a non-zero resistance, then you need a decent voltage of the power supply. And it is desirable that you use a power supply with several output voltages. And that's why.
The protective layer that grows on the product is dielectric. As it grows, its electrical resistance constantly grows. To maintain the required current density, it is necessary to adjust the current strength several times with a variable resistor throughout the process.
However, the voltage may not be enough when the anode layer becomes thick enough. In this case, you need to add voltage. Therefore, the power supply must provide at least two voltages at the output.
The purpose of aluminum anodizing and its further use
Anodizing aluminum profiles and other parts makes a lot of sense. It is important that all the characteristics of the metal remain unchanged, but the surface of the product itself acquires additional qualities:
- A mechanically strong oxide layer is formed over the entire surface, which does not allow the metal to break down under the influence of moisture and oxygen.
- Minor damage in the form of point defects or minor scratches hide under the layer, and the metal becomes more uniform.
- When applying paint coatings, the latter are distributed more evenly, fit well on aluminum.
- Parts made of anodized aluminum take on a presentable appearance, they look advantageous on various mechanisms.
- In the process of anodizing, it is possible to transfer a completely different shade to aluminum, for example, silver-plated or gilded it or made the tint shine with pearl.
Machined aluminum parts can be further launched into the production of various components, machinery mechanisms, frames.
Methods for anodizing aluminum
The most common anodizing method is the method of chemical oxidation, when a film is deposited on a surface of aluminum using a special electrolyte. In this case, solutions based on acids are used:
In addition to chemical, anodizing can be integrated, microarc, interference, and color oxidation is also used. When dye is added, any color of the film can be obtained, for example black.
This method of anodizing aluminum is used when it is necessary to paint the product afterwards. The film has a porous structure, which is a positive point for the adhesion of the coating with epoxy dye. A serious disadvantage can be considered insufficient mechanical and corrosive strength. Active metals and sea water can easily destroy the coating. This method of anodizing can be used at home.
There is no clearly established temperature at which conditions for the formation of crystalline oxide are created by the warm method of anodizing aluminum. It is known that it should leak in a room where the temperature is comfortable for the body or it is elevated, but not more than 50 ° C. The process proceeds in an electrolyte solution under the influence of voltage.
The pre-defatted and washed part undergoes anodization until visually the entire processed surface becomes milky white.
Cold anodizing involves the same process of creating crystalline oxide as with warm technology, but the temperature of the solution should not exceed 5 ° C. A feature of the method is the accelerated growth of the anode coating on the aluminum side relative to its dissolution on the electrolyte side.
What happens when cold anodized:
- The capacity is filled with electrolyte.
- The component is lowered into the electrolyte, hanging it, and connected to the anode.
- The cathode plate is also lowered into the solution and a constant voltage of 12 V is applied with a current density of 4–1.6 A / dm².
- When coating small products, they wait 30 minutes, large ones - 60 minutes, after which they remove the voltage from the electrodes.
The advantage of the cold method: a high-strength oxide film is obtained that is resistant to all types of exposure. The disadvantage is poor adhesion with dyes.
The part must be properly prepared before being chemically treated. At this stage:
- The surface of the product is cleaned from contamination.
- Grind, removing oxides, significant defects and irregularities.
- Degrease, getting rid of substances that interfere with obtaining a high-quality film.
The temperature of the electrolyte is important for the process of obtaining a crystalline oxide film by anodizing aluminum. It directly affects the strength and friability of the coating and its further properties.
The lower the temperature, the more dense, strong and not so loose the shell will be, but the rate of formation of the latter is lower than when using high temperatures.
Correct anodizing of aluminum metals and their alloys implies exposure to a certain current density. This is an indication of the current strength applied to the entire surface that will be exposed to the oxide coating. This parameter directly determines how fast the layer will form. The density of the electrolyte and its temperature are also taken into account.
General rules require the use of a density in the range of 2.5–1 A / dm², if the goal is to obtain a decorative-protective coating - a thickness of 20–6 microns, use a density in the range of 4–2 A / dm² if an insulating layer or a very hard one is needed coating - thickness 75–40 microns.
Suspension part contact
Achieving the result of high-quality coating of aluminum parts by anodizing also depends on their correct location in the electrolyte. They should be completely immersed in the solution, have excellent contact with the anode and not touch any other surfaces. This can be done using a special suspension. It can be played by an aluminum block, which is stably mounted on a tripod. In the bar, drill holes for bolted connections. Bolts fasten an aluminum wire on which details are already suspended. Also connect the anode to the bar.
A large area of contact between the part and the suspension should be avoided: at this point, the film will not form during oxidation.
Common anodizing errors
When oxidizing aluminum at home, the following errors should be avoided:
- The use of twists and low-quality clamps in an electrical circuit.
- The use of cathodes smaller than the workpiece. It is necessary that the cathode area be at least twice as large.
- Poorly selected anode current.
Anyone who is associated with electroplating and in practice knows how to anodize aluminum, share your experiences in the comments. Such knowledge is very important for beginners.
To obtain a smooth surface at the preparation stage, it is necessary to polish the workpiece. Using a felt or other polishing wheel, scratches are eliminated and large pores are tightened. The absence of microroughness reduces the likelihood of burnout. The anode film is not able to hide external defects.
Before anodizing aluminum, it is necessary to determine the size of the workpiece. The resulting layer has a thickness of 50 microns, so it will not be possible to screw a nut onto the machined thread. If the parts are connected by means of a fit, then you should not forget that after anodizing the parts are not subject to grinding.
Anodizing at home
For the process, capacities are needed. Capacities for anodizing should correspond to the dimensions of the parts, to be slightly larger. In this connection, they usually use several bathtubs. The container material is aluminum. But if the products are small, then plastic containers will do. Only on the bottom and along the walls is it necessary to lay aluminum sheets. This is necessary to create a current of uniform density throughout the volume.
The electrolyte needs to be isolated from external heat. When heated, it will have to be changed. To exclude heating, the tanks outside are covered with a layer of thermal insulation. It can be glued with foam up to 50 mm thick or, placed in a box, fill the free space with mounting foam.
A solution of sulfuric acid is obtained by diluting the electrolyte for car batteries with distilled water in a one to one ratio. Having bought a canister with a capacity of 5 liters, you can get a solution of 10 liters.
Mixing, when water is added to the acid, is accompanied by abundant heat dissipation, and it literally boils splashing. Therefore, for safety reasons, sulfuric acid is poured into a container of water.
Before starting anodizing aluminum, it is subjected to chemical preparation. Chemical preparation is a degreasing process. In industrial conditions, the treatment is carried out with sodium hydroxide or potassium. But at home it is better to use household soap. With a toothbrush and soapy solution, dirt is well removed from the surface. After that, first the blanks are washed with warm water, and then cold.
An alternative to laundry soap is washing powder. After dissolving it in a closed plastic container and placing the workpieces there, you need to shake vigorously. Then the parts are washed and dried with a stream of hot air. The active oxygen contained in the washing powder protects low-fat products, even if they are taken with bare hands.
Before work, it is necessary to prepare equipment for anodizing. Usually requires several baths: for processing small parts, short and long products. They must be made of aluminum. A suitable option is also polyethylene. As a small container, you can use a food container or a long plastic flower pot.
It is desirable to cover the bottom and walls of the plastic bath with aluminum sheets. You can cut out a pattern from an aluminum sheet and bend an impromptu “container”. The meaning of this is to ensure uniform current density on all sides of the product.
The bath should be distinguished by good thermal insulation of the body, otherwise, otherwise, the electrolyte will heat up too quickly in it, and it will have to be replaced more often. The simplest solution will be pasting the bathtub with a thick layer of polystyrene foam - 2-4 centimeters. You can also fix the bath inside the box and fill the gap with construction foam.
After that, a lead cathode should be made for the bath. It can be made from sheet lead by removing the latter from thick power cables. Recall that the cathode area should be twice the surface area of the workpiece. This does not take into account the surface of the cathode, which is leaning against the wall. In the cathode plate should be openings for the exit of gas.
You can assemble a cathode from several pieces of lead, if not one. Pieces are recommended to be soldered with a powerful soldering iron, a thick seam along the joints. Try to make the cathode repeat the configuration of the surface of the part facing it. Withdraw from the contact bath with a strip of the same material. Although it is also customary to use a thick copper wire in isolation. Seal the soldering point with silicone sealant.