Preparation of a metal for coating

Preparations of a workpiece surface for coating is the main criteria for producing of high-quality galvanic coating. Contaminations of organic and inorganic nature and passivating films prevent the formation of high-quality galvanic coating on a backing. Backing's roughness is transmitted to a coating and penetrates into it to a considerable thickness. Preparation of a surface for coating is traditionally carried out by galvanic or electrochemical methods, requiring the use of concentrated toxic acid solutions and being resource and time consuming. Application of electrolyte-plasma technology is a good solution to these problems.

Electrolyte-plasma treatment is carried out in diluted solution of neutral salts, that is why the necessity of workpieces' washing falls away. Electrolyte-plasma treatment method not only polishes the surface of a product, but also degreases and activates it, thus eliminating additional process step in the surface preparation for coating. As a result, process productivity can be increased in 2-3 times in comparison with conventional processing techniques.

The effect of changing the wetting angle of a steel surface by distilled water depending on the treatment time is shown in the fig. 1.

Figure 1. The effect of changing a wetting angle of a steel surface by distilled water. a — initial state, b — 2 minutes after treatment, c — 4 minutes after treatment

Obviously, that by increasing the electrolyte-plasma treatment time, the wetting angle of a steel surface reduces, thus indicating the effect of metallic contaminants cleaning.

In the figure 2 you can see a sample of a friction disc with a half treated surface.

Figure 2. A friction disc, half treated by electrolyte-plasma method

When using electrolyte-plasma method, high quality treatment of discs from steel 65G is confirmed by the fact that the adhesion of the coating by using an intermediate copper backing was comparable to the coating adhesion without any additional backing.

Table 1 shows the data allowing to estimate the economic effect of electrolyte-plasma technology for cleaning discs compared with electrochemical technology.

Table 1. Comparative data on cleaning technologies under the assumption that the electricity cost is 300 rub./KW∙h

  Electrolyte-plasma method Electrochemical method
The specific consumption of electrolyte on processing 1 m2 of disk surfaces, kg/m2 0.16 5.43
Cost of chemical reactants at polishing 1 m2, thousand rub./m2 0.192 7.8
The specific consumption of the electric power at processing 1 m2, kWt∙h/m2 17,4–30,0 20,2–75,0
Electric power cost at processing of 1 m2, thousand rub/m2 5,22–9,0 6,12–22,5
Productivity of processing, m2/hour 1.88 0.32

In connection with the foregoing, it can be concluded that the method of electrolyte-plasma treatment has the best economic indicators and allows to increase productivity of line where products are prepared for coating, also it solves the problem of expensive and highly toxic chemicals' disposal.