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Chrome plating, one of the practical engine coatings needed, is one of the most labor-intensive processes in electroplating. It requires special care as well as maintaining cleanliness as if preparing an electrolyte, as well as the substances themselves. Water is used distilled or (only as a last resort!) Thoroughly boiled.

Making a bath

Starting the manufacture of a bath with model electroplating, just pick up a 10-liter pan and a three-liter glass jar. It is better not to use smaller tanks in any way - this may complicate the adjustment of process parameters, of course, also with the above values, the bath capacity is just enough for chrome plating of 6-8 cylinder liners. After gluing the body from 1-1.5 mm plywood, assemble the bath according to the picture below and close the whole with a plywood ring. The work on the bathroom ends by turning the lid of the pan by mounting the heating elements on it as well as a contact thermometer. Today - electrical equipment. To feed the bath, you can use every spring of constant current with an electrolytic capacitor connected to the output of 80000 microfarads X 25 V. Feeding wires must have a cross section of no less than 2.5 mm 2 . The current regulator, replacing the voltage regulator, can serve as a section rheostat. It is included in a row with a galvanic bath also consists of parallel sections that are turned on by unipolar knife switches. Each subsequent owns the opposition twice the previous one. The number of such sections is 7-8 . On the front panel of the litany unit, install two sockets for 15 A , one for normal polarity, and the other for reverse. This will make it possible to hastily carry out the anodic finishing of the part, and also to switch to chrome-plating with a ingenuous rearrangement of the plug. Sockets with three outputs, so as not to be mistaken in polarity (only a pair of jacks are clearly connected). To maintain a constant electrolyte temperature, the bath is supplied with a contact thermometer. He cannot control the labor of heating elements directly because of the impressive currents, so it will be necessary to assemble a simple device, the circuit of which is shown in the figures.

Installation for chrome plating products.

Details of the thermostat: transistors VT1 - ( MP 13 - MP16 , MP39-MP42) ; VT2213-217 (П213-П217) with any letter symbols; resistors - ( MLT-0,25) ; diode - (D226, D202-D205) ; relay - (TKE 52 PODG or OKN passport RF4.530.810) .

Adjusting the thermostat: if shorting points 1-2 turnips does not work, connect the emitter and collector VT1 . Turning on the relay indicates a malfunction or a small VT1 gain. Otherwise, the transistor VT2 is faulty or it has insufficient gain. Having collected also having adjusted a bath way, it is possible to begin to prepare the electrolyte.

For this you need:
1) - pour into the jar a little more than half of the prepared distilled water, heated up to 50 °;
2) - add chromic anhydride as well as stir
3) - add water up to the estimated volume
4) - pour sulfuric acid
5) - to work the electrolyte 3-4 hours at the rate of 6-8 A g / l .

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

Electrolyte composition

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

Chrome Modes

The chrome plating process is strongly dependent on the electrolyte temperature and current density. Both factors affect the appearance of the properties of the coating, but it is similar to the chromium current output. It must be remembered that with increasing temperature the current output decreases; with increasing current density, the current output increases; at lower temperatures, also constant current density, gray coatings are obtained, but at elevated temperatures dairy ones are obtained. The optimum chrome plating mode was found by a practical route: a current density of 50-60 A / dm 2 at an electrolyte temperature of 52 ° - 55 ° ± 1 ° .

To be sure that the electrolyte is working, you can cover several parts in the prepared bath, similar in shape and size to the working samples. Having selected the mode also having recognized the current output by ingenious measurement of dimensions up to and also later on chrome plating, you can press on to the coating of the sleeves.

According to the proposed method, chrome is applied to steel, bronze and brass parts. Their preparation consists in washing the surfaces to be chrome-plated, then with gasoline and then soap (using a toothbrush) in hot water, charging it in a mandrel also placing it in a bath. After immersion in the electrolyte you need to wait 3-5 seconds and then turn on the operating current. The delay is needed for the part to warm up. Immediately results in the activation of the surface of parts made of brass and copper, as if these metals are well etched in the electrolyte. But more than 5 seconds should not be delayed - there is zinc in the composition of these metals, whose stay in the electrolyte is unacceptable.

Chrome Modes

Chrome plating of aluminum alloys

On the processes of deposition of chromium on aluminum alloys need to dwell on. The implementation of such coatings prkticheski always associated with a number of difficulties. First of all it is the need to pre-apply the intermediate layer. Aluminum alloys containing an impressive number of silicon (up to 30% , alloys of the grades AK12, AL25, AL26, CAC-1 ) can be chromed as follows:
- washing parts in gasoline,
- rinse in hot water with detergent or soap,
- finishing parts in a solution of nitric acid and hydrofluoric acid (ratio 5: 1 ) for 15-20 s ,
- washing in cold water,
- Installation of parts on the mandrel is also chrome-plating (load in the bath under current!).

Another lesson, if you want to chrome alloy AK4-1 . It is possible to chrome only with the help of an intermediate layer. Such methods include: zinc processing; on nickel underlayer; through nickel salt; through the anodic finish parts in a solution of phosphoric acid.

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

For etching using a solution of caustic soda ( 50 g / l ), it is time to finish 10-30 s at a solution temperature of 70-80 ° .

For etching aluminum alloys containing silicon and manganese, it is better to use such a solution in parts by weight:
nitric acid (density 1,4 ) -3, hydrofluoric acid ( 50% ) -1. Finishing time details 30-60 with a solution temperature of 25-28 ° . After etching, if it is a cylinder liner, it should be immediately rinsed in running water also by 2-3 s in a solution of nitric acid ( 50% ), followed by rinsing with water.

Intermediate Coatings

Aluminum products at room temperature are dipped for 2 minutes into the 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 its constant mixing. The coating is quite uniform and also has a gray (sometimes blue) color.

If the zinc coating is laid unevenly, the part is lowered into the bleed 50% solution of nitric acid for 1–5 s and also later after washing, Zinc is repeated. For magnesium-containing aluminum alloys, double galvanizing is mandatory. Applying another layer of zinc, the part is washed, charged into the mandrel also under current (without voltage supply, zinc has time to partially dissolve in the electrolyte, contaminating it) is installed in the bath. Pre-mandrel with the part is immersed in a glass of water heated to a temperature of 60 ° . The process of chromium usual.

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

The last operation is in a solution of the following composition: nickel sulphate 30 g / l , sodium hypophosphite 10-12 g / l , sodium acetate 10-12 g / l , glycocol 30 g / l . At first, it is prepared without hypophosphite, which is introduced before nickel plating (the solution with hypophosphite is not stored for a long time). Solution temperature at nickel plating is 96-98 ° . You can also use the solution without glycocol, while the temperature should be reduced to 90 ° . A layer of nickel with a thickness from 0.1 up to 0.05 mm is deposited on the part in 30 minutes . Ware for works is only glass or porcelain, as if nickel is deposited on all metals of the eighth group of the periodic table. Brass can nickel plating well, bronze and other copper alloys.

After nickel deposition, heat treatment is carried out to improve adhesion with the base metal ( 200-250 ° , exposure 1-1.5 h ). Then the part is mounted on the chrome-plated mandrel also lowered to 15-40 s in a solution of 15% sulfuric acid, where it is treated with reverse current at the rate of 0.5-1.5 A / dm 2 . Nickel is activated, the oxide film is removed, and the coating also turns gray. Acid should be used only chemically pure (at the very least rechargeable). In another way, nickel acquires black paint, and chrome on such a surface will not fall into life.

After that, the mandrel with the detail is loaded into a chrome plating bath. First, the current is allowed to be a couple times larger, then within 10-12 minutes it is reduced down to the worker.

Defects of chemical nickel plating:
- Nickel plating does not occur: the part has not warmed up in any way, 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 .

Nickel removal from aluminum alloys can be made in a solution of nitric acid.

another time in the process of nickel plating occurs self-discharge - precipitation 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 interfere with the deposition on the parts.

I would like to note that nickel-phosphorus itself has very interesting properties that are in no way inherent in chromium coatings. This is the uniformity of the layer on the surface of the parts (after deposition, no adjustment is required); high hardness after heat treatment (the 400 ° mode for an hour alienates the hardness of the HV 850-950 coating is also greater); low friction coefficient compared to chromium; very slight expansion; high tensile strength.

Nickel-phosphorus without further deposition of chromium can be used not only as if the intermediate coating on the sleeves, but also as if working wear, friction-reducing and also wear, for spools as well as piston pins. After a couple of years of active operation of the engine with details of a similar finish, there was no clear development characteristic of steel hardened surfaces.

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

Chromium deposition through anodic treatment
Instead of intermediate layers, anodic finishing can be performed in a solution of 300–350 g / l of phosphoric acid at a temperature of 26–30 ° , a terminal voltage of 5–10 V, and a current density of 1.3 a / dm 2 . Bath should be cooled. For alloys containing copper and silicon, a solution of 150-200 g / l of phosphoric acid is used. Mode - 35 ° , processing time 5-15 min .

After anodic finishing, a short-term cathodic finish should be carried out in an alkaline bath, which partially removes the oxide layer. Studies have shown that in the process of anodic finishing of aluminum alloys in phosphoric acid, a rough surface is formed on the parts, which contributes to a strong adhesion of the subsequently applied coating.

Fixtures, arbors

Chrome plating sleeve
To perform work with the cylinder liner is made mandrel. Her way is clear from the above figure, we dwell only on certain details.

Chrome plating devices

Anode - steel stud; Lead and antimony ( 7–8% ) are melted from one end of it at a length of 50–60 mm . Lead is machined through the outer diameter up to 6 mm (for working sleeves 0.15 mm ). On the other hand, the studs are threaded to fix the wire.

The cathode is a ring with an internal diameter that is 0.5 mm larger than the internal dimension of the sleeve. A piece of insulated wire is poured into it. Copper and brass conductors are better not to use - the electrolyte dissolves them, and the contact icon can be broken. Before mounting the mandrel in the bath is useful to check the strength of the contacts tester.

Chrome plating steel parts
(crankshaft, crank pin, piston pin, bearing race)
Chrome plating of steel parts is carried out according to the following technology:
- removal of fatty spots with gasoline,
- rinse in hot water with soap,
- finishing of the part with reverse current for 2-3 minutes ,
- switching to the chrome-plating mode with a current, 2-2.5 times the impressive calculated value, also a gradual decrease in current for 10-15 minutes .

The calculated current is determined by multiplying the surface area of ​​the chromed surface by the process current. For steel, the last dimension is 50 A / dm 2 . When chrome plating, for example, the landing space under the main bearing on the crankshaft of the KMD-2.5 engine, the calculated current will be equal to 0.03 dm 2 x 50 A / dm 2 x 1.5 A.

For chrome finger crank need a new mandrel. As well as when finishing the crankshaft, all open surface areas are covered with AGO glue. The anode is machined from steel, followed by pouring with lead and bore holes for the finger. The use of steel parts due to the need to ensure safe contact - in lead threaded connections unreliable. Current calculations are similar. Labor is carried out in the shaft mandrel using a special nozzle.

Almost nothing is different chrome bearings. The only thing is to protect the internal parts of the parts it is filled with grease or other grease, which is later washed with gasoline.

Mandrel for chrome plating ball bearing outer race

Mandrel for chrome plating ball bearing outer race:
1-body bearing mandrel; 2-ball bearing; 3-figured nut; 4-anode (lead); 5 central lobe portion for chrome plating; 6-cathode (steel); 7-cap; 8-through window for the passage of electrolyte.

Chrome plating defects are also their cause

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

  2. The coating exfoliates:
    - poor surface degreasing,
    - disrupted the flow of current
    - temperature fluctuation 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 protruding parts of the thickened coating:
    - increased current density.

  5. Hard coating, exfoliate:
    - low current density, elevated electrolyte temperature,
    - in the process of chrome plating the temperature of the electrolyte changed,
    - in the process of grinding the product is overheated.

  6. Chrome does not settle around the holes of the parts:
    - impressive release of hydrogen - close the openings with ebonite stoppers.
    - excess sulfuric acid.

  7. Brown patches on the cover:
    - lack of sulfuric acid,
    - excess of trivalent chromium (more than 10 g / l ) - keep the bath under current without parts, increasing the surface of the anodes and also reducing - the cathodes.

  8. Soft dairy coating:
    - high electrolyte temperature,
    - low current density.

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

  10. The spotted coating 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 matte:
    - high current density
    - low electrolyte temperature,
    - uneven current density on the speakers also in-depth parts of the part.

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

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

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

According to the magazine "Model-Designer". Source in the number 5 for 1989.

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