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METHOD OF GALVANICAL PAINTING
Work with copper electrolytes has its own specifics. Let's start with the main point of direct deposition of copper coatings: in order to avoid contact extraction of copper on the surface of the blanks, they can be loaded into the electrolyte only under current, otherwise strong adhesion of sludge to the base material is not ensured. The initial current density also plays a very important role in this situation. This is explained by the fact that at an excessively high current intensity, though also dense but coarse crystalline precipitates are formed, which further lead, in the chosen case, to obtaining undulating or bumpy coatings of unpredictable thickness. At an extremely low current density, the rate of galvanic coating formation will lag behind the release rate of contact-separated copper, which subsequently causes the coating to exfoliate.
For each type of copper electrolyte there is its own, strictly defined optimal current density. Practically, the optimal current density is selected by the appearance of the resulting coating and the speed of its education. In the presence of a relatively small skill, which is acquired very quickly, a similar means of controlling the progress of the process provides coatings of very high quality. With a properly selected galvanic process mode, the deposited copper layer has a solid-colored body paint, a uniform fine-crystalline structure. With excessively impressive current densities, a layer of copper is obtained with coarse grains of metal, also characterized by a brick-red color. On the final defect, it is customary to express what results in the “burning” of the coating. Excess current, in addition to the occurrence of burning, can lead to passivation of the anodes.
At the same time on the surface of the latter there is a snow-white, greenish-blue or brown smearing, easily erasable plaque, which prevents the normal process of dissolution of the metal. At the same time, copper salts contained in the electrolyte are spent on the education of the coating, which leads to instability of its chemical composition.
So, for the source it is necessary to prepare the electrolyte. On 1 liter of electrolyte it is necessary:
Copper sulphate - 60 g ;
Sugar refined - 90 g ;
Caustic soda - 45 g ;
Alcohol - 5-10 ml .
The electrolyte is prepared in strict sequence: copper sulfate dissolves in 200-300 ml of water, sugar is added to it. Caustic soda dissolves separately in 250 ml of water. Next, a solution of copper sulfate with sugar is added to the caustic soda solution in small portions with constant stirring. Water is then added until a 1 liter of solution is obtained. For the chosen dissolution of the components, it is better to heat the water up to a temperature of 35-40 degrees. As a galvanic bath, you can use an ordinary jar. After preparing the main electrolyte, 5-10 ml of alcohol is added to it. The presence of alcohol significantly improves the quality of the deposited copper, making the coating structure more dense and also significantly reduces the grain of the metal. The trimmed electrolyte has a gloomy saturated blue-green paint (such paint can be seen only with a very small number of electrolyte on the bottom also into the light, but it seems that it is approximately black).
These proportions are taken from the chapter “Chemical and Electrochemical Methods of Parts Processing” of the book “The Alphabet of Ship Modeling” (Dregalin A.N. Polygon, S.-P. 2003) . But I think that you can experiment with the composition of the electrolyte.
For the preparation of electrolyte is strongly recommended to use no ordinary tap water, but distillate. It is also better to use no technical copper sulfate (fertilizer), but chemically tidy crystalline copper sulfate (the same copper sulfate) qualification is not lower than 4 . This is sold in chemical stores. Caustic soda can be there.
Electrolyte maintenance is also a very significant issue. Over time, pores in the electrolyte composition form sludge, significantly degrading its characteristics. Therefore, it is necessary to periodically pass the solution through the filter of 2 layers of fabric with a thin napkin between them. The same is recommended at once after electrolyte preparation. During breaks, the bath (jar) should be closed with an airtight lid in order to avoid dust particles, debris, water evaporation (and, as if a consequence, a violation of the concentration of components).
Next, collect the electrical circuit. The electrode connected to the “+” (anode) act from copper sheet, for uniform staining of the part twisted into a cylinder (see the figures). The electrode connected to “-” (cathode) is connected to the part being painted. To remove the parameters in the electrical circuit, you can turn on the ammeter and a voltmeter. The schematic diagram is shown in the figure.
Circuit connection diagram
Cathode - Painted Part
How it looks in practice
To cover a part with a copper layer, it is necessary to ensure the required current density of about 0.5 (1-2) A / dm 2 (in different sources in different ways). Again, the current density depends on several factors, such as the size of the part, the composition also the electrolyte temperature, the purity of the reaction (the purity of the water is also organic reagents), the reaction time. Therefore, most likely, everyone will have to choose the current strength using variable resistance. To do this, in the circuit between the current source and the anode, it is necessary to provide for variable resistance (a trimming resistor or rheostat, which is preferable). As a current source, you can use any constant current source with an output voltage up to 10 volts . This sample uses the old charging device for a Philips mobile phone with output characteristics of 4.2 V 770 mA .
The current density is calculated by the formula: i = I / S
In which room: I - power current; S is the total surface area of the painted part.
With current strength, everything is more, less clear - it is set by the output parameters of the current source also by variable resistance.
Find the total surface area of the painted part. Consider a previously made copy of a 24- pound carronade cannon for the “Olivuts” corvette.
In this case, the complex figure of the gun consists of several simple figures - truncated cones and cylinders. But it’s also not difficult to calculate the exact area of its surface - you can simply imagine it as a cylinder with an average diameter and also calculate the approximate surface area, which is sufficient for our work.
But it is also possible to make more accurate calculations by breaking the part into geometric primitives also considering the area of the outer surface of each.
The total surface area of the cone is found by the formula: S = 3.14 * (R 2 + r 2 + l * (R + r)) ;
The area of the side surface of the cone: S = 3.14 * l * (R + r) ;
The total surface area of the cylinder: S = 2 * 3.14 * r * (r + h) ;
The area of the side surface of the cylinder: S = 2 * 3.14 * r * h ;
Substituting the dimensions in the formulas, we acquire the area of our part 0.03 dm 2 .
Therefore, the required power of the current: I = i * S = 0.5 (required current density) * 0.03 (part area) = 15 mA
And from here we acquire the required resistance of the circuit: R = U / I = 4,2V / 15mA = 280 Ohm
But here the internal resistance of the electrolyte is not taken into account, therefore, in practice, the external resistance should be less than the calculated one. In my case, the optimal characteristics of the circuit turned out to be as follows: the above indicated current source, the opposition in the anode circuit is equal to 220 ohms . The time to color the part is 17 minutes . As a galvanic bath, a glass jar of baby feeding with a capacity of 150 ml , an area of a copper plate (anode) = 49 cm 2 , and an electrolyte temperature of the room ( 18-22 0 C ) were used.
If you do not have an ammeter or there is a variable resistance that cannot be determined with an accuracy of at least 10 ohms , then you can determine the required current strength at the eye: with an extremely strong current, hydrogen is emitted in the form of clearly visible bubbles (the so-called “boiling "). These bubbles prevent the deposition of copper on the anode. In this case, the detail is covered with a dark brown patina, easily erasable with your fingers. Therefore, it is necessary to decrease the current (increase the resistance) until the release of hydrogen becomes so obvious (the hydrogen is released at any current), i.e. as if only the bubbles did not become noticeable at all, it is possible to dwell on this current strength to also change it further (if necessary) by looking at the appearance of the part.
After it is as if the part is removed from the electrolyte, it must be very thoroughly rinsed with running water. As a result, we obtain a painted piece of matte copper color.
Next, in order to make the detail shine, we grind it thoroughly with a cloth with GOI paste or fine chalk (tooth powder). Again, wash (wash away the remnants of chalk and paste GOI ) also gain the final result.
A few tips on the surface quality of the painted part. For more uniform deposition of copper, the surface of the part must be smooth and also degreased. It is known that during casting, the part has a microporous surface - these micropores also need to be disposed of. If the material of the casting is soft (for example, tin), this can be done with a steel needle, rolling or rolling it along the part, as shown in the figure.
Finally, I think it is necessary to describe the prospects of this technology. With this technology, copper plating can be used to make hollow copper parts, such as a ship's bell. To do this, you can make from any low-melting material the blank of the bell is also just like to cover it with copper (but with a thick layer). The next step is scrupulous smelting of material blanks. But, undoubtedly, at the same time, a more complete layer of copper is also needed; this will also take much more than 17 minutes .
This technology can also be used to cover various carvings, in gold, copper or bronze. In this method of electroplating, almost any color can be obtained - you just need to experiment with the composition of the electrolyte and the amperage.
Creator: Ilya Loskutov