Etching of aluminum (products from this metal) is carried out in order to clean its surface from the upper, unnecessary layer or from rust. There is also such a variation of it - artistic etching, when it is necessary to engrave a picture on the surface of a metal part.
Types of Etching
Etching of metals in general and aluminum in particular is of two main types: chemical and galvanic. The last method is carried out just artistic.
In chemical: the product is placed in a container into which a solution of hydrochloric or sulfuric acid is previously poured . In the same way, the aluminum preform is etched with alkali, for example sodium hydroxide.
And galvanic (otherwise - electrolytic or electrochemical) occurs due to the electric battery. The process itself is carried out in a special bath, where there is an anode and a cathode.
Next, each of the methods for etching aluminum will be discussed in more detail. We will also find out which method is the safest at home.
Acid Etching
Due to the fact that very strong acids are used in this process, it is first of all necessary to observe increased precautions when working with them. The operator must wear gloves, a mask, an apron. It is important that the room where the process takes place is well ventilated. Without certain skills and without specific protective equipment, working with acids is not recommended.
As noted above, an aluminum product is placed in an acid container. The most commonly used reagents for chemical etching of aluminum with acid are hydrochloric or sulfuric acid. When they interact with the metal, hydrogen is released. Outwardly, it looks as follows: the surface of the product is covered with small bubbles. But, in principle, this can be prevented if a special ingredient is added to the container in advance. Thus, the metal will be protected from bubbles by the thinnest film.
A very important point: all operations for etching an aluminum product with acid must be performed intensively so that the metal surface itself remains integral.
The described method is recommended to be carried out in containers made of wood or concrete. At the same time, its inner surface should be faced with acid-resistant tiles so that the walls of the container do not corrode.
This method is not used very often in practice.
Alkali etching
Most often, this method uses an aqueous solution of caustic soda (a variant with or without additives is possible).
And it is used to clean the surface of an aluminum product from oxide or unnecessary lubrication and get a smoother (matte or glossy) surface.
Why is it necessary to clean so thoroughly? In order for the finished product (for example, decorative architectural elements, plates) to have an ideal surface. And also this method is used for deep engraving.
The alkali etching method of aluminum, on the one hand, is quite inexpensive, but it is very laborious.
Features of this method
The solutions used contain from four to ten percent sodium. The temperature during alkali etching is approximately 40-90 degrees Celsius.
If necessary, to obtain a light foamy coating on the workpiece, a moisturizer or a special additive is used.
The average temperature in the midst of the process is sixty degrees. It is with such thermal indicators that high-quality surface cleaning occurs.
Optimum 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, and hydrogen is released. As a result, a composite aluminate is formed, and it exists only in an alkali solution.
Further alkali etching processes
During this process, the amount of caustic soda gradually becomes less. And thus the speed of the process itself decreases, but the viscosity increases.
Provided that no caustic soda was added to the container at all, the reaction can slow down very much. But in the end, a brownish or clear solution for pickling aluminum turns white.
And from this moment, the speed of the process increases.
As a result of the reaction, alumina hydrate precipitates, which looks like a suspension. Caustic soda is also released, which is also necessary for the etching process to continue.
The results of the method under consideration
It was experimentally fixed that sodium hydroxide solution, when used intensively during etching, begins to "absorb" aluminum. And this happens until the amount of caustic soda is reduced to one fourth of the original volume. And after this, the process will continue with free caustic soda, fluctuating in quantity. And this, in turn, depends on temperature, frequency of use and intensity of stops (pauses).
In this case, the hydrate will slowly sink into the precipitate or form crystals at the bottom and / or sides of the vessel. The resulting hydrate will be sufficiently dense, and it will not be easy to remove. Sometimes he tries to settle right on the surface of the heating coils.
There is another important point regarding the aluminum content. During etching of products from this metal in caustic soda, it is necessary to strictly observe the ratio of the amount of aluminum and sodium. Because the more aluminum there is, the slower the process will occur. From the point of view of practice, it becomes clear that it is necessary to constantly increase the amount of caustic soda as the amount of aluminum in the tank increases.
Thus, the process of etching aluminum with alkali can be continued continuously. And the loss of caustic soda will occur only due to the entrainment of it with steam.
This method is really applicable from the point of view of practice. But there are several nuances that should not be forgotten: from time to time to remove the hardened hydrate precipitate; clean the filter; remember that the capacity in which the process is carried out, with its constant use, can last no more than two years.
And the rest, no complications regarding the application of this method were detected.
In total, after chemical etching of an aluminum billet, it is necessary to thoroughly rinse its surface, neutralize and brighten with a 15-20% solution of nitric acid. This process is called decapitation.
Galvanic method
The second method of etching is galvanic. It is simpler and is much faster in time. And the result is a very high-quality surface of the product, clear contours of the picture (with the artistic method, as a variety of galvanic).
The peculiarity of this method is that it uses an electric energy source (4-5 V).
You will also need a bathtub of this size to fit an aluminum product. The material the bath is made of must be a dielectric. The composition of the aluminum pickling bath is a solution of copper sulfate and sodium chloride.
Before starting the process, the workpiece must be cleaned and degreased. Next, solder the copper wire with tin to the product and dip it in a solution of caustic soda, and then in a solution of sulfuric acid. After 2 minutes, remove and rinse under a stream of hot water. It is forbidden to touch the product at this time.
If some sections of the workpiece do not need to be etched, mastic is applied to them. After that, you can start the process itself.
In this method, two so-called supports are used, which must be connected to the anode (positive charge) and the cathode (negative) of the electric power source. It is important that these supports are located across the bath. A billet of aluminum is mounted on a support with an anode, and on the second - a billet of another metal.
All this is lowered into the bath and maintained for a certain amount of time. After that it is washed with turpentine and further processed by grinding and polishing.
Art Etching
This type of galvanic method is quite popular at present. With its help, you can make author's drawings, engraving, art prints, ornaments on any metal blank.
And the result is a very clear, beautiful pattern. So to say, the author's work, which you can leave yourself or give.
The original image itself can be drawn independently or printed (using a laser printer) on paper. Then stick tape on the surface and rinse the paper with hot water. As a result, the image should remain on the tape. Leave to dry. Meanwhile, it is necessary to prepare the metal surface on which the drawing will be applied - degrease it with alcohol.
Then stick adhesive tape with a pattern on the surface of the workpiece, while releasing air bubbles from under it. The excess glue and everything unnecessary except the image itself is removed with a hot awl.
Etching is carried out in the manner already described above - galvanic.
Attention: in this process, there is a chance of emission of harmful gases, so people are better off leaving the room.
Thus, pickling aluminum at home is feasible. It is only necessary to carry out all the most important precautions!