Electrolytic plasma polishing of stainless steel | Electroplasma polishing

Electrolytic-plasma polishing of stainless steel (electroplasma polishing) is a process of treating the surface of stainless steel based on physical phenomena occurring on the surface of the metal being treated, immersed in an electrolyte, under the influence of direct current with a voltage of hundreds of volts, when a vapor-plasma shell is formed over the surface being treated, that is, a thin film of ionized vapor and gas formed as a result of local boiling of the electrolyte.

The process of electrolytic plasma polishing of stainless steel has only recently come into use in industry, but is becoming increasingly popular with both manufacturers and customers of finished products.

Plasma polishing technology in electrolyte is considered environmentally friendly. It is an alternative to the conventional electrochemical polishing process. No acids or toxic substances are used in electrolyte solutions, but environmentally friendly low-concentration aqueous solutions with various inorganic salts are used. The level of gloss of the treated surface of the stainless alloy depends on the concentration of the electrolyte, as well as on the voltage.

Due to the rapidly increasing demands of customers for the quality of surface treatment of stainless steel products, on the one hand, and increasingly stringent environmental and sanitary requirements, on the other, manufacturers are forced to close old production facilities and invest in new modern methods of surface treatment. Compared to classical electrochemistry, this is a qualitatively and quantitatively different process.

The essence of the electrolytic plasma polishing process

At first glance, the plasma polishing scheme may resemble conventional electropolishing. However, there are some significant differences. The metal part to be processed is immersed in an electrolyte and anodically polarized, i.e. connected to the positive pole of the electric current source. The counter electrode is connected to the negative pole of the power source. The main differences are in the voltage value and the chemical composition of the electrolyte. The highly concentrated acid mixture used in electropolishing is replaced by a low-concentration aqueous solution of chemically neutral salts. Due to the high voltage value, a thin gas film is formed between the electrodes over the entire surface being processed. The film is formed from evaporated water and gas (oxygen), does not produce electricity and separates the metal surface from the electrolyte. Thus, the electric current is interrupted and the electrical circuit is broken. However, if the voltage between the electrodes is high enough (several hundred volts), the gas film is ionized due to the high electric field of the thin film. Therefore, the electric current flows through this film in the form of a glow discharge. The surface of the metal is affected by the glow discharge columns. The discharge columns are always directed towards the peaks of the surface profile, since the distance between the metal surface and the electrolyte wall is the smallest here. Thus, the surface peak is quickly removed. When one surface peak is removed on the material, the discharge column moves to another surface peak, where the thickness of the vapor-gas film is smaller. Thus, the surface becomes smoother. With the correct chemical composition of the electrolyte and the corresponding process parameters, the treated surface becomes shiny.

Advantages of electrolytic plasma polishing of stainless steel

One of the advantages of this technology is the ability to process metal parts of complex and irregular shapes. At the same time, there is no shielding effect, as in electrochemical polishing - the entire surface of the metal is processed uniformly, including through holes. Another advantage of the plasma polishing process is the harmless composition of the electrolyte. Electrolytes are based on environmentally friendly solutions of various inorganic salts. Aqueous solutions with a concentration of 4-6% are prepared by dissolving the granulate in water without additional quality requirements.

The innovative plasma polishing process is a solution in areas such as surface polishing, deburring and cleaning thanks to numerous advantages such as:

• smoothing of micro-roughness (< 0.01 µm);
• minimal material removal;
• may process any contours;
• achievement of unprecedented gloss;
• no pre-treatment or cleaning of workpieces required;
• environmentally friendly electrolytes consist of 98% H2O;
• no use of environmentally harmful substances or highly concentrated acids;
• processed surfaces are more resistant to corrosion than in the original state;
• lowest thermal and mechanical stress on the surface (t < 100 °C);
• cytotoxic effects cannot be expected on plasma-polished surfaces.

Parameters affecting the level of gloss of the treated surface of stainless steel

There are several basic process parameters that affect the process properties as well as the properties of the treated surface. The process parameters that can be changed independently are: voltage, treatment time, temperature and electrolyte concentration. All these parameters determine the value of the electric current density, which is the main reliable parameter of the process. The value of the material removal rate in the process depends mainly only on the value of the current density, so the process parameters also indirectly affect the properties of the treated surface.

Voltage

The working voltage is the most important parameter determining the quality of the polished surface. It has been experimentally established that there is a minimum threshold voltage for each metal, at which it is possible to obtain a sufficiently high quality of surface treatment, but below which the surface begins to deteriorate. When processing stainless steels, it is possible to reduce the voltage to 230 V without losing the quality of treatment.

Increasing the voltage to more than 350 V is undesirable. At this value, it is already possible to enter the region of the electrohydrodynamic mode, when the plasma-gas layer ruptures and electrolytic-plasma processing becomes impossible.

Temperature

Temperature conditions in the near-electrode zone and electrolyte play a significant role in the polishing process of stainless metals and alloys. High surface quality can only be achieved in a certain range of electrolyte temperatures. The relationship of microrelief with heating of the solution is explained by the temperature dependence of such liquid properties as electrical conductivity, surface tension, wetting angle and viscosity, which affect the stability of the plasma-gas layer responsible for the mechanism of removing microroughness from the surface of stainless steel.

A sufficiently high quality of stainless steel surface treatment can only be achieved in a well-heated electrolyte. And lowering the electrolyte temperature below 70 ℃ leads to a noticeable deterioration in surface quality. For example, when polishing low-carbon grades of stainless steel, dark spots and small streaks may form on the surface. In addition, in some cases, when the electrolyte temperature is reduced to 40 ℃, instead of polishing, the samples are heated.

Excessive increase in the electrolyte temperature above 90 ℃ may lead to a decrease in the quality of the obtained surface. In addition, some electrolytes containing ammonium salts, chloric acid and other substances decompose when heated above 85 ℃, forming volatile products that require frequent adjustment of the solution.

Processing time

The quality of polishing is proportional to its duration. As the duration of processing increases, the surface roughness decreases and the reflectivity increases, which is a good result. However, a further increase in the polishing duration from 120 to 240 s leads to an insignificant decrease in the surface roughness and a smaller increase in the reflection coefficient. This does not significantly improve the polishing quality and does not justify the additional energy costs. The rapid decrease in roughness in the first 90-120 s is due to the fact that there is active local smoothing of the microrelief in places of the largest protrusions and depressions, while the area of ​​the real surface decreases by an order of magnitude. A further increase in the duration of processing does not give such a significant change in the surface roughness and does not lead to active metal removal, but only gives the surface a mirror shine.

By selecting the appropriate electrolyte composition, very high polishing quality can be achieved in 90-120 sec. The final surface treatment of stainless steel can be done in a bath with a special electrolyte to achieve good smoothing and using lower current density.

The shape of the product and its geometric dimensions

The technical, economic and quality indicators of electrolytic-plasma polishing of stainless steel are affected by the dimensions and shape of the surface being processed. The geometric characteristics of the part primarily affect such process parameters as energy costs, the speed of immersion of the product in the electrolyte and the heating temperature of the product.

The essential factors determining the quality of stainless steel polishing include the external size of the product, which is a limiting condition. The dimensions of the workpiece must be such that when fully immersed in the electrolyte, the product does not touch the walls and bottom of the working bath. Its surface area must be several times smaller than the surface of the bath. The maximum possible polishing area is determined by the volume of the working bath and the electric power of the transformer. Large-sized products can be processed by immersing up to half with subsequent turning over and re-processing, and a small brown coating remaining at the immersion limit is removed by additional short-term processing. The shape and thickness of the products largely determine the thermal and hydrodynamic properties of the processing. For the normal flow of the technological process, the thickness of the walls of the products should not exceed several millimeters, and the shape of the product should be flat. Inside deep holes (when the depth exceeds the diameter of the hole) stagnant zones are formed and polishing of the walls does not occur, therefore before finishing the holes must be closed with plugs, otherwise they introduce significant instability in the formation of hydrodynamic flows of electrolyte around the workpiece. The presence of deep holes, recesses, cavities and cracks up to 10 mm in size, as well as hanging small parts above each other at a distance of less than 100 mm leads to a sharp increase in the current consumption and disruption of the polishing process.

What is the difference between electrolytic plasma polishing and electrochemical polishing ?

The plasma polishing process is similar in machine design to electropolishing. The part to be polished is subjected to electrical contact to act as an anode and in both cases is immersed in an electrolytic bath.

There are three main differences from the electropolishing process:

1. First, the voltage of the electric bath must be higher than 200 volts to spark the plasma underwater. The resulting surface current density is comparable to electropolishing. However, the typical material removal during plasma polishing at a rate of 1 μm/min is 10-30 times less than during electropolishing.
2. The second important difference is the composition of the electrolyte, which requires only a small proportion of dissolved salts in water for the plasma polishing process.
3. The third difference is that non-toxic salt combinations have been discovered to create stable plasma and achieve a special polishing effect. Since plasma polishing uses only environmentally friendly chemicals, it avoids the problems associated with hazardous working conditions compared to traditional polishing methods. In addition, plasma polishing is environmentally friendly due to the use of non-toxic chemicals in low concentrations. Any development of additional electrolytes specific to a particular material is carried out taking into account the impact on the environment, possible contamination of materials and their disposal.

Environmental friendliness of electrolytic plasma polishing of stainless steel compared to other types of processing

In the process of manufacturing stainless steel products, water is used in many stages, such as washing, cleaning, rinsing, etc. Aqueous solutions with various chemical components are used for various chemical and electrochemical processes, such as digestion, galvanic processes, electrochemical polishing, etc. As a result, a large amount of industrial waste is generated, which must be disposed of properly to prevent environmental pollution. This is a very important stage of the production process, since such waste may contain pathogens, heavy metals, toxic residues, the presence of which, even in small concentrations, can have a significant impact on the environment.

In the process of chemical and electrochemical polishing, a multi-component, highly concentrated solution of acids and salts is used as an electrolyte in the first method, and various types of acids in the second method. The use of acid as the main component of the electrolyte requires a special method of disposal of waste from such production, since acids are considered hazardous to the environment and can damage the sewage system during normal disposal, since they are highly corrosive chemicals. There are many specially designed processes for the disposal of such waste, which require large amounts of energy, space for the cleaning cycle and storage of chemicals, as well as time for cleaning. This leads to a decrease in the cost-effectiveness of production, which can lead to the rejection of such disposal processes.

Unlike highly concentrated solutions of acids and salts, the electrolyte used in electrolytic plasma polishing of stainless steel is a low-concentration salt solution (up to 10%) with various impurities, the content of which does not exceed 1-3%. This means that improper disposal of this waste, although it poses a threat to the environment, does not pose a threat to the destruction of the sewage infrastructure. It is also important to note that the salts used in the electrolyte, such as ammonium sulfate and ammonium chloride, are fertilizers and food additives, the use of which is safe for humans. This means that after the process of cleaning the electrolyte from contaminants formed during electrolytic plasma treatment, aqueous solutions of these salts do not pose a threat to the environment in the same proportions as acid solutions.

This makes electroplasma polishing an environmentally friendly method compared to other stainless steel finishing methods. Although it requires energy and a special waste treatment facility, the untreated waste has a less negative impact on the environment.