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Chromium plating, one of the practical coatings necessary for motorists, refers to the most laborious processes of galvanic engineering. It requires special care also to maintain cleanliness as if preparing the electrolyte, as well as the substances that make up its composition. Water is used distilled or (only in extreme cases!) Thoroughly boiled.

Making a bath

Classes begin with electroplating with the manufacture of a bath, Preliminarily select a pan for 10 liters and a three-liter glass jar. Smaller tanks should not be used in any way - this can complicate the adjustment of process parameters, of course, also with the given values ​​of the capacity of the bath is sufficient only for chrome plating of 6-8 cylinder liners. Gluing the body of 1-1.5 mm plywood, collect the bath according to the figure above, and then cover it with a plywood ring. Work on the bathroom ends with turning the lid of the pan and also installing a thermometer on it also with a contact thermometer. Today - electrical equipment. For feeding the bath, you can use any spring of constant current with an electrolytic capacitor connected at the output of 80000 microfarads x 25 V. Feeding wires must have a cross section of at least 2.5 mm 2 . The current regulator, replacing the voltage regulator, can be serviced by a sectional rheostat. It is included in a row with a galvanic bath and also consists of parallel sections included by single-pole cutters. Each subsequent has a counteraction twice that of the previous one. The number of such sections is 7-8 . On the front of the lithium unit, install two 15 A outlets, one for normal polarity and one for reverse. This will allow the anodic finish of the part to be hastily carried out and also to switch to chrome plating by the simple fork replacement. Rosettes with three outputs, so as not to make a mistake in the polarity (there are, of course, only a pair of jacks connected). To maintain a constant temperature of the electrolyte, the bath is supplied with a contact thermometer. Directly manage the work of electric heaters, he can not because of the impressive currents, so you will need to assemble an uncomplicated device, the circuit of which is shown in the figures.

Installation for chrome products.

The details of the thermoregulator: transistors VT1 - ( MP 13 - MP16 , MP39-MP42) ; VT2213-217 (P213-P217) with any letter designations; Resistors - ( МЛТ-0,25) ; diode - (D226, D202-D205) ; relay - (TKE 52 PODG or OKN passport of the Russian Federation4.530.810) .

Adjustment of the thermostat: if the short-circuiting of 1-2 points does not work, the emitter is also connected to the collector VT1 . Switching on the relay indicates a malfunction or a small gain VT1 . Otherwise, the transistor VT2 is faulty or it has insufficient gain. Having collected also having adjusted a way of a bath, it is possible to start preparation of electrolyte.

To do this you need:
1) - pour in a jar slightly more than half of the prepared distilled water, heated up to 50 °;
2) - fall asleep chrome anhydride also stir
3) - add water up to the calculated volume
4) - pour sulfuric acid
5) - run the electrolyte 3-4 hours from the calculation of 6-8 A g / l .

The last operation is necessary for the accumulation of a small number of monas Cr 3 (2-4 g / l) , the stay of which has a friendly effect on the process of chromium precipitation.

The composition of electrolytes

Chromic anhydride 250 g / l or 150 g / l
Sulfuric acid-2.5 g / l or 1.5 g / l

Chromium Modes

The chromium process depends to a large extent on the electrolyte temperature and the current density. Both factors affect the appearance of the coating properties as well, but it is similar to the current output of chromium. It should be remembered that with increasing temperature the current output decreases; With increasing current density, the current output increases; At lower temperatures, also a constant current density, gray coatings are obtained, but at increased dairy temperatures. A practical route found the optimal chromium plating mode: current density 50-60 A / dm 2 at an electrolyte temperature of 52 ° - 55 ° ± 1 ° .

To be sure of the efficiency of the electrolyte, several details can be covered in a prepared bath, similar in shape to the dimensions of the working samples. By selecting the mode also recognizing the current output by a simple measurement of the sizes up to also later than the chrome plating, one can proceed to the coating of the sleeves.

According to the proposed method, chrome is applied to steel, bronze and brass parts. Preparing them is to flush the surfaces to be chrome plated, gasoline also then with soap (with a toothbrush) in hot water, charging into the mandrel and also placing it in the bath. After immersion in the electrolyte, wait 3-5 s, and then turn on the operating current. Delay is needed in order for the part to warm up. Immediately, the activation of the surface of the brass parts is also due to copper, as if these metals are well etched in the electrolyte. But more than 5 with delay does not follow - in the composition of these metals is zinc, whose presence in the electrolyte is unacceptable.

Chromium Modes

Chrome plating of aluminum alloys

On the processes of depositing chromium on aluminum alloys it is necessary to stop especially. The performance of such coatings is almost always fraught with a number of difficulties. First of all, this is the need for preliminary application of the intermediate layer. Aluminum alloys containing an impressive number of silicon (up to 30% , alloys of the brands AK12, AL25, AL26, CAC-1 ) can be chromated as follows:
- flushing the parts in gasoline,
- flushing in hot water with washing powder or soap,
- finishing the part in a solution of nitric and hydrofluoric acids (ratio 5: 1 ) for 15-20 seconds ,
- flushing in cold water,
- installation of the part on the mandrel is also chrome (loading into the bath under the current!).

Another activity, if necessary, is to cover the chromium alloy AK4-1 . It can only be deflated using an intermediate layer. Such methods include: zinc treatment; on a sublayer of nickel; through the salt of nickel; through the anodic finish of the part in a solution of phosphoric acid.

In all cases, the parts are prepared as follows:
- grinding (and lapping);
- cleaning (removal of fat deposits later grinding in gasoline or trichlorethylene, then in alkaline solution),
- washing in flowing cold and warm ( 50-60 ° ) water,
- etching (to remove the particles remaining on the surface after grinding also grinding, but it seems to improve the surface preparation of the part to apply chromium).

For etching, a solution of caustic soda ( 50 g / l ) is used, the finishing time is 10-30 s at a solution temperature of 70-80 ° .

For etching aluminum alloys containing silicon is also manganese, it is better to use such a solution in the following parts:
nitric acid (density 1.4 ) -3, hydrofluoric acid ( 50% ) -1. The finishing time of parts is 30-60 s at a solution temperature of 25-28 ° C. After pickling, if it is a cylinder liner, it must be washed immediately in running water and dropped in a solution of nitric acid ( 50% ) for 2-3 seconds, followed by washing with water.

Intermediate coatings

Zinc plating
Aluminum articles are lowered at room temperature for 2 minutes in a solution (caustic soda 400 g / l , zinc sulphate 120 g / l , Rochelle salt 5-10 g / l Or: caustic soda 500 g / l , zinc oxide 120-140 g / l ) with unchanged its stirring. The coverage is fairly uniform also owns a gray (sometimes blue) color.

If the zinc coating is uneven, the part is lowered into a bleeding 50 per cent solution of nitric acid for 1 to 5 s, also after the washing, the zinc coating is repeated. For magnesium-containing aluminum alloys, the double zinc coating is mandatory. Applying another layer of zinc, the part is washed, it is charged into the mandrel also under the current (without applying voltage, zinc has time to partially dissolve in the electrolyte, contaminating it) is installed in the bath. Beforehand, the mandrel with the part is immersed in a glass with water heated up to a temperature of 60 ° . The chromium plating process is normal.

Nickel plating (Chemical)
If zinc does not fall on aluminum (most often it occurs on AK4-1 alloy), you can try to apply chrome through nickel. The order of work is as follows:
- Lapping the surface,
- degreasing,
- etching 5-10 s in a solution of nitric and hydrofluoric acids, mixed in a ratio of 3: 1 ,
- nickel plating.

The last operation - in a solution of the following composition: nickel sulfate 30 g / l , sodium hypophosphite 10-12 g / l , sodium acetate 10-12 g / l , glycine 30 g / l . It is made first without hypophosphite, which is introduced before the nickel plating (with hypophosphite, the solution has not been stored for a long time). The temperature of the solution during nickel plating is 96-98 ° . You can also use the solution without glycine, while the temperature should be reduced to 90 ° . Within 30 minutes a layer of nickel with a thickness of 0.1 to 0.05 mm is deposited on the workpiece. Utensils for work - only glass or porcelain, as if nickel is deposited on all metals of the eighth group of the periodic table. Good brass nickel plated, bronze also other copper alloys.

After the deposition of nickel, heat treatment is carried out to improve adhesion to the base metal ( 200-250 ° , holding 1-1.5 h ). Then the part is mounted on a chrome plating mandrel and also falls 15-40 s into a solution of 15% sulfuric acid, where it is processed by reverse current at the rate of 0.5-1.5 A / dm 2 . Activation of nickel occurs, the oxide film is removed, and the coating becomes gray. Acid should be used only chemically pure (in the most extreme battery). In another way, nickel acquires a black paint, as well as chromium on such a surface never come to life.

After that, the mandrel with the part is loaded into the chromium bath. At first, the current is allowed to be a couple times larger, then for 10-12 minutes it is reduced to the working one.

Defects of chemical nickel plating:
- Nickel plating does not happen: the part did not warm up at all, you should wait a while,
- speck on the surface (typical for AK4-1 ): poor heat treatment of the part, it is necessary to heat it at 200-250 ° for 1.5-2 hours .

Removal of nickel from aluminum alloys can be produced in a solution of nitric acid.

another time in the process of nickel plating is self-discharge - the deposition of powdered nickel. In this case, the solution is poured, but the dishes are treated with a solution of nitric acid to remove nickel from its surface, which will prevent deposition on the part.

I would like to note that nickel-phosphorus itself has very interesting properties, not inherent in chrome coatings. This is the uniformity of the layer on the surface of the parts (after the deposition of debugging is not required in any way); high hardness after heat treatment ( 400 ° mode for an hour alienates the hardness of the coating HV 850-950 is also greater); Low coefficient of friction in comparison with chromium; very slight expansion; high tensile strength.

Nickel phosphorus without further application of chromium can be used not only as an intermediate coating on the sleeves, but also as if the working, friction-reducing also wear, for the spools also of the piston fingers. After a couple of years of active operation of the engine with details of such a finish, they did not have a clear development, characteristic for steel hot surfaces.

Application of chromium through the salt of nickel
The whole process is as follows:
- Etching in a solution of caustic soda ( 50 g / l, t = .80 °, 20 s ),
- flushing in running water,
- application of the 1st intermediate layer (nickel chloride, 1 min ),
- bleeding of the intermediate layer in a solution of nitric acid (acid solution 50%, 1 min ),
- application of the 2nd intermediate layer (nickel chloride, 1 min ),
- washing with water,
- etching (nitric acid 50%, 15c ).
- flushing in running water,
- Loading in the chrome bath under the current.

Application of chromium by anodic treatment
Instead of the intermediate layers, anodic finish can be performed in a solution of 300-350 g / l phosphoric acid at a temperature of 26-30 ° , the voltage at the terminals 5-10 V also has a current density of 1.3 a / dm 2 . The bath should be cooled. For alloys containing copper also silicon, a solution of 150-200 g / l of phosphoric acid is used. The regime is 35 ° , the treatment time is 5-15 min .

After the anodic finish, a short cathode finish should be carried out in an alkaline bath, which partially removes the oxide layer. As studies have shown, in the process of anodic finish of aluminum alloys in phosphoric acid, a rough surface forms on the parts, which contributes to the strong adhesion of the subsequently applied coating.

Adaptations, mandrels

Chromium plating
To perform the work with the cylinder liner, a mandrel is produced. Its way is clear from the above picture, we will only focus on individual details.

Adapters for chrome plating

Anode - steel hairpin; from the one end of it at a length of 50-60 mm, lead is coated with antimony ( 7-8% ). Lead is turned through the outer diameter up to 6 mm (for working sleeves 0,15 mm ). On the other side of the hairpin, thread is cut to fix the wire.

The cathode is a ring with an internal diameter 0.5 mm larger than the inner dimension of the sleeve. In it, a piece of insulated wire is struck. Copper also brass conductors should not be used at all - the electrolyte dissolves them, the icon can be broken. Before mounting the mandrel in the bath it is useful to check the strength of the contacts by the tester.

Chromium plating of steel parts
(crankshaft, crank pin, piston pin, bearings cages)
Chromium plating of steel parts is carried out according to the following technology:
- removing fat spots with gasoline,
- washing in hot water with soap,
- finishing the parts with reverse current for 2-3 minutes ,
- switching to chromium plating mode with a current 2-2.5 times the impressive design, and also a gradual decrease in current for 10-15 min .

The calculated current is determined by multiplying the area of ​​the chromium-plated surface by the process current. For steel, the last dimension is 50 A / dm 2 . When chromizing, for example, the landing area for the main bearing on the crankshaft of the engine KMD-2.5, the estimated current will be 0.03 dm 2 x 50 A / dm 2 x 1.5 A.

To chrome the crank pin, you need a new mandrel. As well as when finishing the crankshaft, all open areas of the surface are covered with glue "AGO" . The anode is ground out of steel with subsequent pouring with lead also by boring holes for the finger. The use of steel parts is explained by the need to ensure safe contact - in lead, threaded connections are unreliable. Calculations of currents are similar. Work is carried out in the mandrel shaft with a special nozzle.

Almost nothing in any way differs bearing chromium. The only thing - to protect the internal parts of the part is filled with a solid oil or other grease, which is then washed away with gasoline after coating.

Chromium adapter for outer race of ball bearing

Chromium adapter for outer race of ball bearing:
1-body bearing mandrel; 2-ball bearing; 3-figure nut; 4-anode (lead); 5-central portion of mandrel for chromium plating; 6-cathode (steel); 7-cap; 8-way window for electrolyte passage.

Chromium defects also cause them

  1. Chrome does not settle on the product:
    - bad contact at the anode or cathode,
    - a little cross-section of conductors,
    - A thick film of oxides was formed on the surface of the anode (it is removed in a solution of hydrochloric acid),
    - low current density,
    - the temperature of the electrolyte is high,
    - a little gap between the electrodes,
    - surplus sulfuric acid.

  2. Coating exfoliates:
    - poor surface degreasing,
    - the supply of current was disrupted,
    - fluctuation of temperature or current density.

  3. On the surface of chromium - craters, holes:
    - hydrogen is retained on the surface of the part
    - change the suspension so that the gas is freely removed,
    - on the surface of the base metal there is graphite,
    - the surface of the base metal is oxidized, porous.

  4. On the projecting parts, a thickened coating:
    - Increased current density.

  5. The coating is hard, exfoliates:
    - low current density, increased temperature of the electrolyte,
    - during the process of chromizing, the temperature of the electrolyte changed,
    - the product has overheated during grinding.

  6. Chrome does not settle around the holes of the part:
    - an impressive allocation of hydrogen - to close the holes with stoppers made of ebonite.
    - surplus sulfuric acid.

  7. On the cover, brown spots:
    - lack of sulfuric acid,
    - the excess of trivalent chromium (more than 10 g / l ) -to maintain the bath under the current without parts, increasing the surface of the anodes and also decreasing the cathodes.

  8. Soft "milky" coating:
    - the temperature of the electrolyte is high,
    - low current density.

  9. The coating is matte, uneven, difficult to rub:
    - lack of chromic anhydride.
    - the current density is high,
    - lack of sulfuric acid,
    - excess of trivalent chromium.

  10. The spotting is also matte:
    - in the process of chrome plating the current supply was interrupted,
    - the product was cold before loading.

  11. In some places the coating is bright, in others it is matte:
    - the current density is high,
    - low temperature of electrolyte,
    - the current density on the protruding parts of the part is also different.

The concentration of chromic anhydride in the electrolyte is controlled by means of a hydrometer. Concentration of sulfuric acid can only be determined, unfortunately, indirectly, by the quality of the coating.

During the chromium process, evaporation of the electrolyte moves. In these cases, add water up to the desired level. This is done without installing the parts - it is possible to change the temperature of the electrolyte.

After chromizing, all products are heat treated for 2-3 hours to remove hydrogen, at a temperature of 150-170 ° . All work is carried out under the suction device, in rubber gloves also in glasses.

Based on the materials of the magazine "Modelist-Designer". The source in number 5 for 1989g.

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