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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Anodizing aluminum parts. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Ham Radio Technologies Usually, anodizing is carried out in a 20% sulfuric acid solution, which presents some inconvenience, since this requires the presence of sulfuric acid itself, which is not always easy to get, it is necessary to have a hydrometer to measure the density of the solution, and finally, the aggressiveness of the solution entails the need to comply with certain security measures . However, anodizing can be carried out without sulfuric acid, using chemical compounds that are always present in the household, such as sodium hydrogen carbonate (baking soda) and sodium chloride (table salt). To prepare the electrolyte, two saturated solutions of baking soda and table salt are prepared separately in boiled water at room temperature. To obtain saturated solutions, the amount of soda and salt is taken in excess, the dissolution is carried out for at least half an hour, stirring the solutions with a glass rod from time to time. Then the solutions are allowed to settle for ten minutes and drained from the excess of undissolved soda and salt, after which it is advisable to filter them. The electrolyte is prepared from nine volume parts of a soda solution and one volume part of a salt solution with thorough mixing. The preparation of the electrolyte is carried out in glassware. When manufacturing a part to be anodized, it is necessary to leave a small area on it. This is the so-called technological contact tab, which is removed after anodizing. A hole with a diameter of 3,3 mm is drilled in it for the M10 screw. The part is carefully cleaned with a fine sandpaper, degreased in any washing powder and washed in running tap water, after which you should not touch its surface with your hands. A wire is attached to the petal of the part with a screw with a nut, designed to connect it to the positive pole of the current source. The petal, the screw with the nut and the end of the wire are covered with a layer of plasticine to prevent their interaction with the electrolyte. After that, the whole part is wiped with cotton wool moistened with acetone, and suspended in a bath. For suspension, you can use an insulating rod made of textolite or plexiglass, placed on the sides of the bath. The bath must be made of aluminum and is connected to the minus of the current source through an ammeter connected in series (you can use an avometer in ammeter mode) and a variable resistor to regulate the current. The suspended part should not touch the bath, and the minimum distance between them should be about XNUMX mm. The electrolyte is poured into the bath to such a level that the entire part is covered with it, and the part is connected to the plus of the current source. During the anodizing process, it can be seen that the entire surface of the part begins to be covered with gas bubbles and a slight gray coating, which indicates the beginning of the process. Estimated current density is 10...20 mA/cm2. This current density must be multiplied by the surface area of the part, expressed in square centimeters, and the resulting current value is maintained by a variable resistor across the ammeter. The duration of anodizing is from one to one and a half hours, it can also be determined visually. When the entire part is covered with an even bluish-gray coating, the anodizing process can be considered complete. As a current source, you can use a car battery or a rectifier designed for a voltage of 12 ... 15 V and a current of about 1,5 A. If an adjustable current source is used, there is no need for a variable resistor. After anodizing is completed, the part is washed in running water, and then, using a cotton swab moistened with a warm solution of potassium permanganate, it is cleaned from the products of the electrochemical reaction. The surface of the part then becomes smooth and acquires a light gray tint. The solution of potassium permanganate should be thickly dark, but there should be no undissolved grains in it. Then the part is washed again in running water and dried. The dried part can be covered with a thin layer of colorless varnish. After anodizing and washing, the part can also be painted in a variety of colors. To do this, it is immersed in a ten percent solution of aniline dye heated to a temperature of 50 ... 60 ° C. Saturation of the color depends on the residence time of the part in the dye, but should not exceed 20 minutes. Aniline dyes (powders for dyeing woolen fabrics) are sold in hardware or household chemical stores. After painting, the part is washed in running water, dried and coated with a colorless varnish. Publication: cxem.net
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