How to paint metal with an electric current
Electrostatic painting is the application of paint to the surface using the forces of interaction between fixed point electric charges (Coulomb force). The paint material (most often water-based, but there are also variants with organic solvent) is applied with a special paint gun.
The electrostatic atomizer was first used in 1941 by the American inventor G. Ransburg. The technique involved the use of electric fields through which charged paint particles move. Liquid paint material interacts with an electrode located in the gun, as a result of which a high-voltage negative charge (60-100 kW) is transferred to the paint. The charged particles, coming out of the spray gun nozzle, are directed along the electrostatic field lines to the grounded product, on which the paint material is applied.
Painting torch arises due to mutual repulsion of charged particles of paint material. An important difference of this technology from other methods is that there is no need for a paint mist, as the particles are guided along defined lines. The paint transfer rate can range from 70 to 98 percent. The transfer rate depends on the conductivity of the material being painted, the shape of the product and other indirect factors.
The electrostatic method reduces paint consumption and makes the painting process easier. When painting metal pipes using the traditional method, it is necessary to turn the product several times. With an electrostatic gun, however, it is not necessary to turn the part, because the charged particles are guided through the lines of force and easily envelope the obstacles. Painting is very uniform because the paint repels the excess of incoming material at an already treated area.
Types of spraying
Two types of electrostatic spraying are used – classic and cascade. The classic involves the electrostatic sprayer being supplied with a direct current at high voltage through a high-voltage cable. The classic scheme has a number of significant drawbacks. First of all, we are talking about voltage instability in the pistol electrode. In addition, it is quite inconvenient to paint, because the large cable embarrasses in the actions, and to disconnect the power you need every time to get to the transformer.
In the cascade method, the high voltage is not generated externally, but in the gun itself. A voltage of only 12 V is sent to the gun via a low-voltage cable, and the high voltage is generated inside the device. The conversion takes place on the paint gun cascade. The cable used is thin and flexible, which makes it very convenient to work with.
The cascade method makes it possible to cut off the power supply independently of the generator, as well as to control the voltage level by selecting the appropriate one for the type of material. The voltage itself is highly stable, which allows you to significantly reduce the consumption of paint. The main disadvantage of cascade spraying is the high cost of the equipment. However, the costs are quickly recouped by the cost-effectiveness of this technology.
Electrostatic spraying has some limitations dictated by the following circumstances:
The properties of the paint material. In order for the paint to charge properly at the electrode, a resistance of at least 30 kOhm is necessary. Otherwise, the efficiency of painting in an electrostatic field is drastically reduced. An example of a paint material with a low resistance level are formulations with significant additions of metallic powder (such as metallic enamels). Until recently, electrostatic painting was not used for water-soluble paints because there was a high risk of short-circuits due to the electrical conductivity of the liquid. The latest models of electrostatic painting equipment allow to work with water-soluble paints.
Material properties. Non-conductive products such as plastics and wood are difficult to paint. Special conductive primers (in the case of plastic) or wetting (for wood) can facilitate the process.
The shape of the part to be painted. As it was mentioned above, the electrostatic method allows you to paint products of different shapes, but in a closed conductive circuit the voltage of the electrostatic field is equal to zero. Therefore, there is no electric field in the deep recesses, due to which no particles of paint material fall on such areas. Moreover, not getting into all sorts of hollows, the paint concentrates in other areas (e.g. on the edges), resulting in too thick a coating layer. To avoid such problems (they are called Faraday contours), the painting of hard-to-reach areas is done with a conventional spray gun – airless or pneumatic.
How to paint metal with an electric current
Products made of steel, brass, copper acquire a beautiful appearance and are not subject to corrosion if they are painted. There is a very simple method of electrochemical coloring of ferrous and non-ferrous metals, and with the help of one solution – electrolyte – you can get different colored oxide films depending on the mode.
The electrolyte is made up of marketable and cheap chemicals and is very stable in operation:
- copper sulfate – 60 g;
- refined sugar – 90 g
- caustic soda – 45 g;
Copper sulfate is dissolved in 200-300 ml of water, add the amount of refined sugar specified in the recipe and stir everything until completely dissolved. Separately, in 200-300 ml of water dissolve caustic soda and in small portions, stirring, add to it a solution of copper sulfate and sugar. Then the remaining water is added.
Brown, purple, blue, blue, lettuce, yellow, orange, red-purple, blue-green, green and red-pink colors can be obtained in this electrolyte.
Carefully sanded (or polished) and degreased parts are suspended in a bath, which may be a suitably sized enamel pan.
The anodes should be made of pure copper (current-carrying wire used for streetcar and trolleybus lines is suitable).
The working temperature of the electrolyte is 25-40°C.
The current source can be some kind of rectifier or a single element like ZT-U-ZO (dry). The plus of the source is connected to copper, and the minus to the workpiece. An ammeter and a rheostat should be connected in series in this circuit. The electrical contacts must be reliable, because short interruptions in current can produce undesirable shades.
One to two minutes after immersion of the parts, the current is switched on, and the rheostat on the device is set to the desired mode, which should be adhered to as accurately as possible.
When the current density is 0.01 a per square decimeter of the painted surface should be passed it for brown color – up to 2 minutes, purple – 2-3.5 minutes, blue – 3.5-5.3 minutes, blue – 5.3-6.3 minutes, pale green – 6.3-8.5 minutes, yellow – 8.5-12 minutes, orange – 12-13 minutes, red-purple – 13-15.5 minutes, blue-green – 15.5-17 minutes, green-17-21 minutes, pink – over 21 minutes. At lower current densities, the time to get the desired color increases.
As the electrolyte evaporates, it is necessary to add pure water, as the quality of color deteriorates as its concentration increases.
You can get more contrast colors if you add 20 g of sodium carbonate (anhydrous soda) to the solution. If the coloring is unsuccessful or another color is needed, it can be easily removed. To do this, the part is dipped for 1-2 minutes in a weak solution of ammonia.
After staining, the parts are washed in running water and covered with a thin film of colorless varnish.
Quite satisfactory results can also be obtained in a simpler way.
A battery from a pocket lamp (CBS-L-0,50) is connected plus side to copper and minus side to the detail. After this copper and then the part to be painted is dipped into the electrolyte and after a few seconds the battery is disconnected. Subsequently, the coloring process takes place without current. The part remains in the electrolyte until the desired color coating is obtained. To check the color, the part is taken out from time to time and dipped back into the electrolyte.
The preparation of the electrolyte and the preparation of the part for painting is done in the same way as in the previous method. The colors appear in the same order.
The simplest galvanic painting. Electrochemical painting of steel parts
For electrochemical painting of steel, brass or copper parts, it is necessary to assemble a galvanic bath and an electrical circuit as shown in the figure.
The electrode connected to the plus terminal of the element is made of sheet copper. The minus of the element is connected to the part to be painted. Care must be taken to ensure that the parts do not touch the copper plate. A special electrolyte is poured into the jar and the electric circuit is closed. After 2-3 minutes the coloring will start. First the part will turn brown, then purple, etc. Everything will depend on time: 2 min – brown, 3 min – violet, 3-5 min – blue, 5-6 min – blue, 8-12 min – yellow, 12-13 min – orange, 13-15 min – red, 17-21 min – green.
For 1 liter of electrolyte:
Prepare the electrolyte as follows. In a 200-300 ml solution of copper sulfate add 90 grams of sugar and stir thoroughly. Separately, 45 g of caustic soda is dissolved in 250 ml of water, and a solution of copper sulfate and sugar is added in small portions, while stirring constantly. Then add water to make a 1 liter solution.
Be careful when working with caustic soda! To make the colors more contrasting, 20 grams of anhydrous sodium carbonate salt are added to the prepared electrolyte. After coloring the part is washed with water, dried and covered with colorless lacquer.
Anodizing. Colouring technology with the application of direct current
In the early stages of the development of electrolytic dyeing, the use of direct current was considered essential, and despite the similarities to galvanic processes, it was thought impossible to produce dyeing with just direct current. The system closest to direct current was the one developed by Pechiney, which we have already mentioned when considering nickel-based electrolytes. This process used DC for staining, but required a cycle interruption to allow the workpieces to discharge. A somewhat similar system was described by Aiden, who used intermittent periods of DC followed by a low-voltage overcurrent for dyeing.
Symitomo, on the other hand, showed that pure DC current could be used for dyeing, and much effort was expended in developing the DC process. The first description of this system in the application very broadly covered the deposition of many famous metals, but the examples were limited mainly to deposition from nickel- and cobalt-based electrolytes. These were fairly conventional and usually contained:
- nickel sulfate 50 g/l
- ammonium chloride 25 g/l
- boric acid 25 g/l
A graphite anode was used and the staining was done at relatively high current densities (up to 2.5 A/dm 2 ), so that the rates could be very high. Voltages were low, usually in the range of 10-14 V. The declared advantages were fast dyeing times, low operating voltages, the ability to dye castings as well as sheet and profile material, and good color control.
More recent work, however, showed that one of the reasons that others had difficulty with dyeing post-current was the great sensitivity of the system to sodium contamination. This is shown in the figure results were obtained on a 9 micron anodic film on sheet 1100 stained in a solution of nickel sulfate 50 g/L and boric acid 30 g/L, at 0.5 A/dm 2 for 0.5 minutes. There is sensitivity to very low levels of sodium, potassium and ammonium ions, an upper limit of sodium of 6 ppm (parts per mille) has been stated. At this level, special measures are required to avoid sodium contamination, as a bath made up of common reagents will have a much higher level, and Sumitomo suggests cathodic exchange to keep sodium at the specified low level. The use of alternating cathodic and anodic cycles is also said to make the system less sensitive to sodium contamination, as is the use of DC followed by periodic reverse-percurrent.
Most of the work done by Stumitomo was with nickel-based electrolytes, and the pH of these baths was preferably kept between 3.0 and 4.5, a level rather lower than that chosen for staining with AC. As with the perc staining, the nickel baths gave a range of light bronze to black colors, and the other colors obtained in cobalt, iron, tin, and copper based electrolytes are given in Table 8-23.
Alcoa also investigated color treatment using post-current, and in addition to the accepted coloring metals like copper, silver, cadmium, tin, selenium, and tellurium, also produced coloring with arsenic, antimony, and bismuth salts. He pointed out the need for intermetallic particles such as Mg, Si, MgZn2, Cu-Cr, Cu or Mn overlapping the barrier layer of the anodic coating so as to serve as preferred sites for metal deposition.