Aluminum etching is carried out in alkaline or acidic environment. A widely used etchant consists of concentrated H 3 PO 4 (76%), glacial acetic acid (15%), concentrated nitric acid (3%) and water (5%) by volume. According to research, the process consists of two stages - the formation of Al 3+ and the formation of AlPO 4, controlled by the rates of the corresponding reactions:
Al 2 O 3 slow Al -3е HNO3 Al 3+ fast fast Film slow Soluble AlPO 4 . (40)
Water in phosphoric acid prevents the dissolution of Al 2 O 3, but it promotes the dissolution of the secondary product AlPO 4. The current strength is proportional to the etching speed. If current is applied to aluminum, then anisotropy of etching is observed.
The activation energy for etching Al in H3PO4/HNO3 is 13.2 kcal/mol, which suggests that the process is limited by the rate of dissolution of Al2O3 in H3PO4. The gas released is a mixture of H 2, NO and NO 2. Adsorption of gases onto Al surfaces is a constant problem when using viscous etchants. Bubbles can slow down etching - islands of unetched metal are formed under them, which can short-circuit nearby conductors.
Rice. 17.
The preferential adsorption of gaseous products on the side wall limits lateral etching.
An unexpected application of bubble adsorption was its use for smoothing profile edges when etching iron-nickel films in HNO 3 (Fig. 17). As soon as the etching process begins, nitrogen oxide bubbles collect along the side edge. The adsorbed NO 2 intermediate acts as a strong oxidizing agent in metal etching, and lateral etching is accelerated. Adsorption of gases on the side wall (Fig. 17) was also used to reduce the lateral etching of Al during its etching in H 3 PO 4. Reducing the pressure in the etching chamber from 10 5 to 10 3 Pa led to a decrease in etching from 0.8 to 0.4 μm. As a result of the adsorption of small hydrogen bubbles on the side wall, an effective diffusion barrier was formed on it. To reduce the lateral etching of Al from 1.0 to 0.25 µm, several etchants (Table 9) containing additives of sucrose (polyalcohol) and surfactants were proposed.
Table 9. Etchants for aluminum.
1) AK - cyclo rubber with azides, KTFR type resists; DCN - novolac with quinone diazides, resists type AZ-1350.
Poor etching of Al is caused by several factors:
- 1) underdeveloped resist;
- 2) uneven thickness;
- 3) stress in the films over the steps;
- 4) galvanic acceleration of etching due to the presence of Al-Cu precipitates;
- 5) uneven oxide thickness;
- 6) temperature instability (>1 o C).
These factors lead to over-etching and short-circuiting.
Chromium is the second most commonly etched metal after aluminum. It is widely used in the manufacture of photo masks. Cerium sulfate/HNO 3 is used as an etchant.
Due to the induction effect (formation of the upper layer of Cr 2 O 3), the etching of the film is nonlinear, and therefore the moment of the end of etching cannot be determined by its initial thickness.
Etching of aluminum (products made from this metal) is carried out in order to clean its surface from the top, unnecessary layer or from rust. There is also another variation of it - artistic etching, when it is necessary to engrave a design on the surface of a metal part.
Types of etching
There are two main types of etching of metals in general and aluminum in particular: chemical and galvanic. The last method is just artistic.
For chemical use: the product is placed in a container into which a solution of salt or salt has been previously poured. In the same way, the aluminum workpiece is etched with an alkali, for example, caustic soda.
And galvanic (otherwise - electrolytic or electrochemical) occurs thanks to the process itself is carried out in special bath, where there is an anode and a cathode.
Acid etching of aluminum
Due to the fact that very strong acids are used in this process, it is first of all necessary to take increased precautions when working with them. The operator must wear gloves, a mask, and an apron. It is important that the room where the process itself takes place is well ventilated. Without certain skills and without certain protective equipment, it is not recommended to work with acids.
As noted above, the aluminum product is placed in a container with acid. The most common reagents used for chemical etching of aluminum with acid are hydrochloric or sulfuric acid. When they interact with metal, hydrogen is released. Externally, it looks like this: the surface of the product is covered with small bubbles. But, in principle, this can be prevented if you add a special ingredient to the container in advance. This way the metal will be protected from bubbles by a thin film.
Very important point: all operations for etching an aluminum product with acid must be performed intensively so that the surface of the metal itself remains intact.
This method is not used very often in practice.
Aluminum etching with alkali
Most often this method is used water solution caustic soda (possible with or without additives).
And it is used to clean the surface of an aluminum product from oxide or unnecessary lubricant and obtain a smoother (matte or glossy) surface.
Why is it necessary to clean so thoroughly? In order to ready product(for example, decorative architectural elements, signs) had perfect surface. This method is also used for deep engraving.
The method of etching aluminum with alkali, on the one hand, is quite inexpensive, but it is very labor-intensive.
Features of this method
The solutions used contain from four to ten percent sodium. The temperature when etching with alkali is approximately 40-90 degrees Celsius.
If necessary, a moisturizer or special additive is used to obtain a light foamy coating on the workpiece.
The average temperature at the height of the process is sixty degrees. It is at these thermal parameters that high-quality surface cleaning occurs.
The optimal purity of aluminum is 99.5%, and the concentration of caustic soda solution is 10, 15, or 20%.
Thus, during the reaction, aluminum dissolves in sodium hydroxide, releasing hydrogen. As a result, a composite aluminate is formed, and it exists only in an alkali solution.
Further processes occurring during etching with alkali
During this process gradually the amount of caustic soda becomes less. And thus the speed of the process itself decreases, but the viscosity increases.
Provided that no sodium hydroxide was added to the container at all, the reaction can slow down very much. But eventually the brownish or clear aluminum etching solution turns white.
And from this moment on, the speed of the process increases.
As a result of the reaction, aluminum oxide hydrate precipitates, which looks like a suspension. Caustic soda is also released, which is also necessary for the etching process to continue.
Results with the method under consideration
It has been experimentally established that a solution of caustic soda, when used intensively during the etching process, begins to “absorb” aluminum. And this happens until the amount of caustic soda decreases to one-fourth of the original volume. And after this, the process will continue with free caustic soda, fluctuating in its quantity. And this, in turn, depends on temperature, frequency of use and intensity of stops (pauses).
In this case, the hydrate will slowly settle into sediment or form crystals at the bottom and/or sides of the container. The resulting hydrate will be quite dense and will not be easy to remove. Sometimes it tries to settle right on the surface of the heating coils.
There is another important point regarding the aluminum content. When etching products made of this metal in caustic soda, it is necessary to strictly observe the ratio of the amount of aluminum and soda. Because the more aluminum there is, the slower the process itself will occur. From a practical point of view, it becomes clear that it is necessary to constantly increase the amount of caustic soda as the amount of aluminum in the container increases.
Thus, the process of etching aluminum with alkali can be continued continuously. And losses will occur only due to its being carried away with the steam.
This method is really applicable from a practical point of view. But there are several nuances that should not be forgotten: remove the hardened hydrate sediment from time to time; clean the filter; remember that the container in which the process is carried out, with constant use, can last no more than two years.
Otherwise, there are no complications regarding the use this method was not identified.
In total, after chemical etching of an aluminum workpiece, it is necessary to thoroughly rinse its surface, neutralize and lighten it with a 15-20% solution of nitric acid. This process is called pickling.
Galvanic method
The second method of etching is galvanic. It is simpler and takes place much faster. And the result is a very high-quality surface of the product, clear contours of the design (with an artistic method, like a type of galvanic).
The peculiarity of this method is that it uses a source of electrical energy (4-5 V).
You will also need a bathtub of a size that will accommodate an aluminum product. The material from which the bath is made must be dielectric. The composition of the bath for etching aluminum is a solution copper sulfate and table salt.
Before starting the process, the workpiece must be cleaned and degreased. Next, solder copper wire to the product with tin and lower it into a solution of caustic soda, and then into a solution of sulfuric acid. After 2 minutes, remove and rinse under stream hot water. It is prohibited to touch the product with your hands at this moment.
If some areas of the workpiece do not need to be etched, mastic is applied to them. After this, you can begin the process itself.
This method uses two so-called supports, which must be connected to the anode (positive charge) and cathode (negative charge) of the electricity source. It is important that these supports are positioned across the bathtub. A workpiece made of aluminum is attached to the support with the anode, and a workpiece made of another metal is attached to the second one.
All this is lowered into the bath and kept for a certain amount of time. After this, it is washed with turpentine and further processed by grinding and polishing.
Artistic etching
This type of galvanic method is quite popular nowadays. With its help, you can make original drawings, engravings, artistic prints, and ornaments on any metal workpiece.
And the result is a very clear, beautiful drawing. So to speak, an original work that you can keep for yourself or give as a gift.
You can draw the original image yourself or print it (using laser printer) on paper. Next, stick the tape on the surface and wash off the paper. hot water. As a result, the image should remain on the tape. Leave to dry. In the meantime, it is necessary to prepare the metal surface on which the design will be applied - degrease it with alcohol.
Then glue the tape with the pattern onto the surface of the workpiece, while releasing air bubbles from under it. Excess glue and everything unnecessary, except the image itself, is removed with a hot awl.
Etching is carried out using the method already described above - galvanic.
Attention: this process may release harmful gases, so it is better for people to leave the room.
Thus, etching aluminum at home is quite feasible. Just be sure to follow all the most important precautions!
The boss once set me a task. It is necessary to make a duplicate keyboard to control the machine controller, since the factory one quickly became unusable because it was made of transparent self-adhesive film, onto which a design is applied at the factory.
I work at a small enterprise that produces spices. I am engaged in servicing packaging machines, electrical equipment, local network etc., in short, all the technology, smart and not so smart.
So there you go! After much thought and debate with the boss, I finally convinced him that for our lammer-operators the keyboard case would be best suited from “alloy gun steel”, :cool:, but in the absence of it we decided to use a high-strength aluminum case type 203-125B , dimensions 121x66x35 mm from Pros Kit.
Idea
The milling machine ordered aluminum buttons. The case was purchased from a store. And then the question arose of how to make indelible inscriptions on the buttons and body. I tried to scratch it and fill it with paint. It came out completely “meh”! Can be engraved! So I don’t have a Dremel, but I can’t help but search through friends.Laziness, my friends, is the most powerful engine of progress. After some thought, I remembered that I had once accidentally dripped on aluminum radiator ferric chloride. While I wiped away the drop, there was a stain on the radiator and a small indentation. Yeah...
What if you make a stencil from photoresist and then etch it? The guinea pig was a piece of duralumin plate. Everything turned out great!
Preparing the surfaces
Let's start with preparing the surfaces. First we sand dry with sandpaper No. 80-100, spreading it on a flat substrate, then we remove large scratches with an emery sponge No. 180-200, moistening the surface and sponge with water. From time to time we rinse everything with water.
Rice. 1. Surface preparation.
I was quite happy with this roughness. If desired, you can polish it.
Rice. 2. Case and buttons after polishing.
Rolling photoresist
Next, we measure out the photoresist for the body and buttons.
Rice. 3. Film photoresist.
I can't say anything about the photoresist. I bought it from an online store. All that was indicated: “Film negative indicator photoresist.”
We measure a little with a margin around the edges so that it is convenient to roll. Film photoresist consists of 4 layers: the bottom one (it’s matte) is polyethylene, then thin layer glue, then, in fact, the photoresist itself, and on top the glossy protective layer(lavsan). Carefully pry up the matte layer with a needle or scalpel, tear off a strip 5-8 millimeters wide and glue it to the body. It is easier to roll the photoresist along the length of the body.
Yes! One more nuance. It is better to heat the housing over gas to a temperature of approximately 40 degrees. Then the photoresist sticks better. Gradually tearing off the base, we roll the photoresist onto the surface with a hard photo roller, or, at worst, with your finger. We cut off the protruding edges of the photoresist with a file to the body or a sharp knife.
Make sure that no dust particles or air bubbles get under the photoresist. In this place, ferric chloride may get in and there will be a problem. If air bubbles do occur, you can carefully pierce them with a sharp needle and roll them firmly with a photo roller.
We do not remove the top protective layer yet, because the photomask may stick to the photoresist (there have been cases).
Rice. 4. Rolled photoresist.
Making a photo template
Next, use any convenient program to prepare a photo template and print it on transparent film for printers. When printing, we indicate the maximum contrast and minimum brightness, but here you have to try. I have an Epson RX610. The settings are as follows: print quality " Best photo", "Shades of gray", paper type "Epson Matte", brightness: -25, contrast +25.Photoresist is negative! That is, where there is no paint on the template, the photoresist will glow and will not wash off during development! Be careful.
Rice. 5. Photo template. I use film sparingly. That's why I'm typing various projects there is still space left on one sheet.
Illuminate with a UV lamp
We apply a photomask and press it with glass onto the photoresist.
Rice. 6. Preparing for exposure.
Hide the buttons before exposing them. If they light up, you will need to re-roll the photoresist.
We illuminate the photoresist with a UV lamp. Exposure time is about 1 min.
Rice. 7. Photoresist exposure
Rice. 8. After illumination, the contours of the drawing appear.
We illuminate the buttons in the same way. Now you can remove the top protective film photoresist.
We show
Next is development. We prepare a solution for development from: a household glass jar 0.5 l - 1 piece, soda ash (not baking soda) - 0.5 teaspoon, hot tap water - 0.5 l (full jar).Stir the solution until the soda is completely dissolved. Then we take a not very hard clothes brush, dip it into the solution more often and brush it almost without pressing over the photoresist. The unexposed photoresist is gradually washed off and the following picture is obtained:
Rice. 9. Developed photoresist.
We poison in ferric chloride
We cover exposed areas of metal that do not need to be etched (for example, the ends) with colorless nail polish (you can steal it from your wife, like I did). Now we take a photo bath, pour in ferric chloride and throw the body and buttons there with the image DOWN.
Rice. 10. Etching.
The solution immediately begins to bubble. Aluminum displaces iron from the solution and it settles right there, at the site of etching. It should be removed with a soft, unnecessary toothbrush approximately once every 30 seconds. In this case, you need to be careful: chips of the photoresist may appear at the edges of the image. If this happens, immediately rinse, dry and correct the chip with a waterproof marker or the same nail polish. However, the varnish can corrode the photoresist, so be careful.
I etched for about 5 minutes. After etching, I get indentations about 0.5 mm deep.
We remove the photoresist. During production printed circuit boards photoresist can be removed with a solution of caustic soda (caustic soda) or slightly diluted “Mole” for cleaning sewer pipes. But this is not suitable for aluminum. It darkens on contact with caustic. If the etched recesses are deep, then you can remove the photoresist with an emery sponge and water, if not very deep, then you can throw it in a bowl with acetone or solvent No. 646 or 647 for 15-20 minutes.
Rice. 11. After etching and removal of photoresist.
Final operations
Next we cut out the holes for the buttons.
Rice. 12. The holes are ready.
We seal the outline around the inscription with construction tape. I didn’t have construction tape, so I sealed it with aluminum.
The most commonly used agent for etching aluminum is an aqueous solution of caustic soda with or without additives. It is used for general cleaning where oxide, grease or sub-surface debris must be removed with longer etching times to achieve a glossy or matte finish. This is used in the production of nameplates or decorative architectural elements, for deep engraving or chemical etching. This etching method is quite cheap, but at the same time it can become too complex to perform.
Solutions for decorative etching can contain from 4-10% or more caustic soda, working temperature will be 40-90ºC, and it may also be necessary to use a wetting agent to disperse the grease and to obtain a light foam coating, as well as the use of other additives. The normal operating temperature for cleaning and decorative processing is 60ºC. The figure shows the metal removal rate at various concentrations and temperatures during a 5-minute etching of 99.5% aluminum sheet. These curves apply to a freshly prepared solution, with lower values referring to the period after aluminum is immersed in the solution. Springe and Schwall published data regarding the etching rates of 99.5% pure aluminum sheet extruded 6063 in 10, 15, 20% sodium hydroxide solutions at temperatures ranging from 40 to 70ºC. Chaterjee and Thomas also conducted a detailed study of caustic soda etching of extrusion 6063 and sheets 5005, 3013.
Etching rate of 99.5% aluminum in caustic soda.
Aluminum dissolves in caustic soda, releasing hydrogen and forming a compound aluminate, which exists only in an alkaline solution. The reaction occurring in this case can be written in two ways:
The amount of free caustic soda decreases as the reaction proceeds, along with this the etching rate decreases, electrical conductivity decreases, and viscosity increases. If no caustic soda is added to the bath at all, the reaction proceeds very slowly, but eventually the clear or brownish solution becomes milky white, from which point the etching rate begins to increase again, and grows to a value slightly less than initial etching speed. The reaction observed at this stage can be written as follows:
The formed aluminum oxide hydrate or Gibsite has the form of a suspension, and during the reaction, caustic soda is also released, which is so necessary for the continuation of etching.
Ionic structure of aluminate in solutions having high level pH is sufficient complex issue, fortunately, this problem does not actually affect the operator. Moolenaar, Evans and McKeever conducted studies of the infrared and Raman spectra of solutions of sodium aluminate in water and deuterium oxide (heavy water), and they also studied the nuclear resonance spectrum of Na and Al. For aluminum concentrations below 1.5 M, they derived 4 vibration zones, two of which were infrared active at 950 and 725 cm-1, as well as 3 Raman zones active at 725, 625 and 325 cm-1. For aluminum there was also a thin resonance line. All these facts are quite easy to correlate with the existence of tetrahedral Al(OH)4-, which is the main carrier of aluminum in solution.
When the aluminum concentration exceeds 1.5M, a new vibration zone appears at 900 cm-1 for the infrared zone and the Raman zone at 705 and 540 cm-1, while the nuclear resonance zone for aluminum will be significantly expanded without changing position. All these observations can be explained in terms of condensation of Al(OH)4-, with increasing concentration and the formation of Al2O(OH)62-, and in solutions of 6 M sodium aluminate these two forms coexist in parallel. It was found that caustic soda solution, when used continuously, would absorb aluminum until the volume of free caustic soda was reduced to approximately one-quarter of the original volume, after which etching would continue with free caustic soda fluctuating at approximately the same level with amplitude , which depends on temperature, intensity of use and pause period. The hydrate will then slowly settle or crystallize on the bottom and sides of the tank to form a very hard hydrate which is very difficult to remove and unfortunately tends to settle on the surface of the heating coils. Here we observe the third reaction, i.e. dehydrogenation reaction of aluminum hydroxide to form aluminum oxide:
The nature of this transformation is shown in Fig. 4-10, where different quantity aluminum are dissolved in a 5% (wt) solution of caustic soda, and measurements are carried out on free caustic soda immediately after each addition, as well as after three weeks. Up to 15 g/l of aluminum remains completely in solution without changing the amount of free caustic soda, but as soon as the precipitation of aluminum oxide begins, which occurs shortly before the appearance of a clearly visible precipitate, the free caustic soda is reduced to 4%, i.e. up to 80% of its initial value. With prolonged use, this value for such a solution can range from 1 to 1.5%, sometimes increasing to 2.5% in case of downtime lasting several hours. A similar ratio corresponds to a higher concentration of sodium hydroxide, and these values are virtually independent of temperature.
Effect of dissolved aluminum on free caustic soda.
Another important influence of aluminum is that as the aluminum content increases, the etching rate drops, quite clearly, this is reflected in the figure. In practice, this means that if it is necessary to maintain a constant etching rate, it is necessary to increase the free caustic soda content as the amount of aluminum in the bath increases.
The final reaction in this case will occur between aluminum and water with the release of hydrogen and aluminum. In theory, etching can thus continue indefinitely, with loss of caustic soda occurring only through entrainment. This method of working with a pickling tank is indeed applicable in practice, but one must remember the need to periodically remove solid hydrate sediment. According to current experience, when operating in this mode, the service life of the tank can be up to 2 years. Filtration of caustic soda solutions has not been as successful due to the fact that very fine sediment tends to clog the filter very quickly, but otherwise no problems have been identified with this technique.
Etching rate in sodium hydroxide 50 g/l, sodium nitrate 40 g/l at 60ºС depending on aluminum concentration.
Chemical control of a solution, applied before precipitation or in a stable state after sedimentation, involves determining total number soda and free caustic soda. The content of the latter can be calculated with sufficient accuracy for practical application by titration with hydrochloric acid, which is produced until the phenolphtolein indicator loses its color. As an alternative, potentiometric titration can also be proposed. To compensate for losses due to entrainment, it is enough to only maintain the total content of caustic soda at a fixed level, since it is not possible to control the fluctuations of free caustic soda in the solution. For an accurate determination, in which carbonate and dissolved aluminum are also taken into account, a more complex calculation method is used, which is given in the table.
One of the most common problems with caustic soda etching is the tendency to cause pitting or "burning" of part or all of the part, which is accompanied by an increase in etching speed of up to 300%. This usually occurs in heavily loaded solutions that are used so intensively that they have no possibility of recovery. In this case, the hydrate crystallizes on the part, which leads to an increase in the intensity of local etching, an increase in temperature and an effect on grain boundaries, which has the properties of acid etching. It is sometimes quite difficult to avoid pitting in this type of solution when attempting to remove the anodic film. If this happens, then it is necessary to lower the temperature.
Thus, it can be seen that, despite the apparent simplicity of the etching process, in practice there can be many competing reactions that must be recognized in order to obtain good result. The main factors responsible for etching are the content of free caustic soda in the solution, the presence and amount of additives in the bath, the temperature of the solution, as well as the aluminum content in the solution. The influence of solution composition has been discussed previously, but the temperature of the solution has a strong influence on the etching rate. This factor can usually be easily controlled, but in practice, due to the exothermic nature of this reaction, it is often necessary to cool the pickling baths, especially when they are in continuous use. Most pickling baths are used at temperatures between 55 and 65ºC, since at more high temperatures Contamination due to etching during transfer may occur, especially for sheet materials.
