Chrome parts. Drawing, description

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Chrome parts. Tips for a modeler

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An extremely important part of the technology in the manufacture of engines is the coating of rubbing elements with antifriction alloys or metals. This thin layer, in addition to anti-friction properties, must have high wear resistance, and the appearance must be appropriate.

As practice has shown, such requirements are fully satisfied by coating with chromium or alloys based on it. Good results can be obtained, in particular, electroplating - an electrochemical process of coating parts in a galvanic bath filled, for example, with a dilute sulfate electrolyte. As a bath, any glass container (say, a jar) of a suitable size can be used so that the chrome object is freely placed in it and at the same time is not too close to the anode plates. The electrolyte is recommended with the following composition: CrO3 - 150 g/l and H2So4 - 1,5 g/l.

It is preferable to prepare it with distilled water. You can also use tap water, but only after boiling and settling.

In any case, the water is heated to 60-70°C and CrO2 is dissolved in 3/3 of the volume. Then water is added and mixed.

It is desirable to analyze the solution for the content of ions in it, since in chromium dioxide (especially in technical) they are usually present in the form of impurities. Otherwise, it will be difficult to count on a high-quality coating due to the fact that it is extremely difficult to achieve the required concentration of SO4 ions in the electrolyte when sulfuric acid is poured into it "by eye".

After adding the required amount of H2SO4, the electrolyte must be "worked out". This operation is carried out at a temperature of 45-50°C. The cathodic current density is from 4 to 6 A/dm2. The processing time is 4-6 hours, which is quite enough for the accumulation of the required amount of Cr ions in the solution. The electrolyte changes color from dark red to dark brown. A steel plate is used as the cathode. The anode is made of lead.

Behind the study is the process of sedimentation of the electrolyte. And only after a day you can start trial chrome plating. The electrolyte is heated to 50-52°C, maintained at this temperature for 2-3 hours. Then a test piece (usually brass) is hung. The peculiarity here is that the hanging of brass, as well as aluminum parts, is carried out only under current.

Chrome parts
Scheme and equipment for chromium plating: 1 - glass container (jar), 2 - anode (s), 3 - chrome-plated part (cathode), 4 - electrolyte solution

Trial chromium plating is usually carried out for an hour, after which the quality of the coating is certainly checked. First of all visually. The main criteria are the fine-grained structure of chromium deposits, the uniformity of the coating. The crystals should be shiny, not "milky". And the mechanical properties of the coating are such that if you take a tool made of high-speed steel and try, without pushing through the applied layer, to scratch the chrome-plated surface with it, no trace will remain on the latter. Well, if the coating turned out to be soft, it is necessary to carry out an additional study of the electrolyte for two hours. Naturally, with subsequent repeated trial chrome plating of the part.

Experience shows that in order to select the best parameters of chromium plating (density, temperature), it is practically impossible to do without 5-6 trial coatings with the duration of each process 30-40 minutes. It is desirable that these tests be made for different modes, allowing you to quickly reach the best option.

Everyone here is convinced: each design of the mandrel corresponds strictly to its own, optimal current density. For internal combustion engine liners, 1,5 cm3, for example, it is 45 A/dm2 at 50°C. The deposition rate of pure chromium in this mode is approximately 0,04 mm/h.

After chromium plating, the sleeves are subject to mandatory boiling for 1-1,5 hours in a large volume (2-3 liters) of water, preferably distilled. Then they are placed in an oven for 2-3 hours, where at this time the temperature is maintained at 120-130°C. The latter is important for cartridge cases made of BrB-2 and aluminum alloys. And especially - for steel parts: piston pins, crankshafts, spools.

At the end of chromium plating, machining, grinding and final finishing of the part (product) usually follow.

As for anodes, which, as practice shows, provide the best, most stable chromium plating processes, we can recommend their manufacture from the following alloy: Pb=81-86%, Sn=10-15%, Sb=4%. Moreover, these anodes work even better than pure lead anodes.

And further. The "clear cleanliness" and "fresh-metal" brilliance of anodes are not at all a guarantee of superiority over browned ones. Rather the opposite. Anodes also need to be worked out. And you can’t keep them in air for a long time before hanging them in an electrolyte. To avoid oxidation, it is better to lower the anodes into heated water. So store it in it until the device is assembled. This is especially true for pure lead anodes. If this recommendation was suddenly not observed for some reason, it is necessary to remove the formed crust from the surface of the anodes before chromium plating, lowering for 30-40 minutes into the electrolyte of the following composition: 100 g/l Rochelle salt and 80 g/l NaON (with subsequent wiping the anodes with a cloth).

Author: V.Kriger

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