Cathodic corrosion protection. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Automobile. Electronic devices
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Many car enthusiasts know that it is enough to have a small scratch - and rust begins to absorb the car downright. And it is very difficult to deal with it. Whatever tricks motorists come up with - various coatings, mastics, anticorrosives ... But the trouble is: in order to process all the most affected places with proper quality, you sometimes have to disassemble the entire car. Such an operation takes a lot of time, and requires constant monitoring. In addition, during operation, the gradual destruction of the coatings occurs. Due to vibrations during movement, microcracks appear, under the impact of stones or sand, the paint breaks off. Therefore, the desire of motorists to purchase a miracle device is quite understandable: it was spent once and forever protected the body from rust.
The method of cathodic corrosion protection has long been used in a wide variety of objects. For example, special protectors are installed on ships, which, dissolving in sea water, provide protection for the ship's hull. Underground pipelines before laying are treated with anti-corrosion compounds and wrapped with a special tape. At a certain distance from the pipeline, an anode (electrode) is buried - a metal blank, to which the "plus" of a direct current source is connected, and the "minus" is connected to the pipe itself. Due to the potential difference between the electrode and the protected metal, a current passes in the circuit of the resulting electrolyte (moisture, salt, etc.). At the anode, electrons are released - an oxidation reaction, and the self-dissolution of the cathode stops [1, 2].
With cathodic polarization, the metal must be given such a negative potential at which its oxidation becomes thermodynamically improbable. For iron and its alloys, complete protection against corrosion is achieved at a potential of 0,1 ... 0,2 V. A further shift in potential has little effect on the degree of protection. The protective current density must be within 10...30 mA/m2.
In addition, over time, on the metal, due to the concentration polarization with respect to oxygen, an additional potential shift to the negative side is observed, which makes it possible to periodically turn off the device (when repairing a car, charging a battery, etc.) [3].
The corrosion protection device consists of an electronic unit and protective electrodes. A light indication of device operation is placed on the body of the electronic unit.
The device allows to maintain the value of the potential of the wet areas of the surface of the body at the level necessary for a complete stop and termination of corrosion processes due to the destruction of the protective electrodes.
As protective electrodes (anodes), both destructible materials (stainless steel, aluminum) requiring replacement after 4 ... 5 years, and non-destructive ones can be used. Carboxyl, magnetite, graphite or platinum can be used as non-destructive electrodes. Protective electrodes are made in the form of rectangular or round plates with an area of 4...9 cm2.
The figure shows a diagram of a simple anti-corrosion device that can successfully cope with corrosion phenomena. Of course, in its simplest form, a cathodic protection device can consist of protective electrodes and wires connected directly to the "positive" terminal of the battery. However, here it is difficult to control the possible short circuit of the electrodes with the car body and its operation as a whole. To do this, in the device, the VD1 LED is included in the voltage divider circuit R2, R3, R1, which glows with a steady light in operating mode, consuming a small current from the battery (about 2 mA).
If suddenly one of the protective electrodes closes on the car body, the VD1 LED stops glowing. In this case, it is necessary to find and eliminate the short circuit. With increased body humidity, the VD1 LED can change its glow to a small extent, which indicates the operation of cathodic protection. In addition, this device has high reliability, since it gives an overload current of not more than 25 ... 30 mA in case of a short circuit of the output with a body.
When installing and mounting the device, remember that:
- one protective electrode protects the area with a radius of about 0,25 ... 0,35 m;
- protective electrodes are installed only in places protected by a paint and varnish coating;
- you can use only epoxy glue or putty based on it;
- the outer side of the protective electrodes (where there is no soldering) must not be covered with mastic, paint, glue or other electrically insulating coating.
The electronic unit is installed anywhere in the car and is connected to the general electrical circuit of the car. At the same time, it is necessary that the electronic unit remains on even when the general electrical equipment of the car is turned off.
In general, the device consumes no more than a car clock and guarantees long-term efficient operation even with a heavily discharged battery.
Literature
1. Krasnoyarsky V.V., Zobov E.V. Electrochemical protection of structures and equipment against corrosion. -1981.
2. Lyublinsky E.Ya. Electrochemical protection against corrosion. -1987.
3. Tershin V., Bondarenko A. Current protects against corrosion. Behind the wheel.-1993.-N 12.-S.23.
Author: P.Belyatsky, Novosibirsk region, Berdsk; Publication: N. Bolshakov, rf.atnn.ru
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