CHROMIUM THEMSELVES | MODEL CONSTRUCTION

An extremely important piece of technology in the manufacture of engines is the coating of the friction elements of the antifriction alloys or metals. This thin layer, except for the anti-friction properties, should have high wear resistance, and appearance to be appropriate.

As shown, such requirements are fully satisfied coating of chromium or alloys based on it. Good results allows to obtain, in particular, electroplating is an electrochemical process for the coating of parts in a galvanic bath filled, for example, dilute sulfate electrolyte. As baths can be used with any glass container (say a Bank) of a suitable size to chromium coated object freely placed in it and it would not be too close to the anode plates. The electrolyte is recommended the following composition: CrO₃ — 150 g/l H₂So₄ 1.5 g/l.

Cook it preferable to distilled water. You can apply water, but only after boiling and sludge.

In any case, the water is heated to 60-70°C and 2/3 of the volume of dissolved CrO₃. Then add water and mix.

It is desirable to carry out the analysis of the solution on the concentration of ions as chromium dioxide (especially in engineering) they are usually present as impurities. Otherwise it will be difficult to expect high-quality coating due to the fact that to achieve the desired ion concentration of SO₄ in the electrolyte, when it “by eye” poured sulfuric acid, is extremely difficult.

After adding the required amount of H₂SO₄ electrolyte must “work out”. This operation is carried out at a temperature of 45-50°C. Cathode current density — from 4 up to 6 A/DM². Time study — 4 to 6 hours that is sufficient for accumulation in the solution of the necessary amount of Cr ions. The electrolyte changes color from dark red to dark brown. As the cathode used here is the steel plate. The same anode is made of lead.

For study the process of sedimentation of the electrolyte. And only a day later, you can begin to test plating. The electrolyte is heated to 50-52°C, aged at this temperature for 2-3 hours. Then halt test component (usually brass). The peculiarity here is that the brass hanging loop, as well as aluminum parts is carried out only under current.

Scheme and equipment for chrome plating:

1 — glass bowl (the Bank), 2 — anode(s), 3 — chromium coated part (cathode), 4 — electrolyte solution.

Trial chromium usually are an hour, and then is sure to check the quality of the coating. First of all visually. The main criteria — grained structure of chromium precipitation, the uniformity of the coating. The crystals should be shiny, not “dairy”. And mechanical properties of the coating are such that if you take a HSS tool and try not pressing the applied layer, to scratch them threeround surface, no trace of the last will remain. Well, if the coating turned soft, it is necessary to conduct an additional study of the electrolyte for two hours. Of course, with subsequent re-trial chrome details.

Experience has shown that for selection of the best options chrome plating (density, temperature) practically can not do without 5-6 trial coatings with the duration of each process for 30-40 minutes. It is desirable that these samples were made for different modes, enabling faster access to the best option.

Everyone here realizes that any construction of the mandrel corresponds to the strictly its own, the optimal current density. Cartridges internal combustion engines, 1.5 cm³, for example, it is 45 A/DM² at a temperature of 50°C. the Rate of deposition of pure chromium in this mode is approximately 0.04 mm/h.

After chroming casings are subject to mandatory boiling for 1-1. 5 h in a large volume (2-3 l) of water distilled. Then for 2-3 hours put into an oven, where the temperature is maintained at 120-130°C. the Latter is important for shells of BRB-2 and aluminum alloys. Especially for steel parts: piston pins, crankshafts, spools.

After chroming, you should generally machining, grinding and final lapping parts (products).

As for the anodes to ensure that, as practice shows, the best, most stable processes of chrome plating, it is possible to recommend their production of the following alloy: Pb=81-86%, Sn=10-15%, Sb=4%. Moreover, the anodes of these work even better than pure lead.

And again. “Stainless purity” and “vegematarian” Shine anodes are not a guarantee of benefits compared to other, covered with brown patina. On the contrary. The anodes also need study. And long to hold them in the air not in front of the hanging loop to the electrolyte. To prevent oxidation of the anodes should drop in heated water. And store it to the Assembly fixture. This is especially true of anodes of pure lead. If this recommendation is suddenly, for whatever reason, was not observed from the surface of the anode before oxidation you need to remove the crust formed was lowered to 30-40 minutes in the electrolyte of the following composition: 100 g/l of Rochelle salt and 80 g/l NaON (followed by wiping with a cloth anodes).

V. KRIGER, head of the self-similar laboratory regional station of young technicians, Yaroslavl

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