Stainless Steel Electropolishing with Clinox
Cloud Gate, Chicago
The electrolytic polishing is the process used to polish a metal surface through the use of electric current and of a chemical solution, through the use of a container provided with electrodes. This process allows to obtain a mirror surface through a selective removal of the surface of the steel.
This selective removal is generated by controlled electric current and by special electrolytic solutions. The electrical parameters are configured by the use of INVERTER technology embedded in our Clinox products, while the electrolytic solution, called E-polishing Bomar, is used with our E-polishing Box made of plastic material resistant to acids and carbon fiber electrodes to ensure the best performance and complete safety.
Thanks to this combination of products can be obtained the following results:
Coffee machine components
Coffee machine components
Coffee machine components
Electropolishing: Tecnical aspects
The electrolytic polishing is the process by which you can polish anodically a metal surface. Think of replacing the mechanical cleaning with this process is wrong; It can be understood as a completion process for small objects and irregular and complex shapes. The polishing may constitute an excellent anchorage for deposits since it determines a crystal structure suitable to weld in the most effective way to work of lattice forces. Under this point of view the process is said “glossy etching.” Like all anodic processes, electrolytic polishing is closely related to the metal-based structure. If this contains the defects and impurities, the effect of electrolytic polishing can have spots, dimples and pads.
Fig.1 Electrolytic Cell
The electrolytic cell shown in Figure 1 explains how occurs the electropolishing process. The metal piece used to to obtain the mirror surface is defined anode, the cathode may be a metal such as lead, copper, etc … .During the process, thanks to the passage of current and particular electrolytic solutions, it happens a selective anodic dissolution over the stainless steel surface, making the latter progressively smoother. The parameters governing the electropolishing process are:
- Current density
- Type of electrolytic solution
- Stirring of the liquid
- Cathodic material
- Size and shape of the electrodes
- Distance between anode and cathode
- Arrangment of the pieces
All these parameters affect the process of life and appearance of the steel surface. For example, the temperature should be kept constant and the agitation must be such as not to cause local heating.
Fig. 2 Current- Voltage diagram during electropolishing process
As you can see in Figure 2, to achieve a correct electropolishing, the electrical parameters must coincide in Vc-Vb range. For voltage values lower it generates an anodic corrosion, the pieces typically become opaque and corrode. For higher values than Vc it is generated the development of gaseous substances that alter the process of dissolution and cause an irregular attack on the metal surface. The curve in question changes depending on the resistivity of the electrolytic solution. Over the resistivity is high, the more the polishing straight portion (polishing) is tight until it is reduced to a point.
Clinox and Inverter technology
The best results should be maintained in well-defined ratios of the current density and the voltage. This ratio is defined in our Clinox machines that thanks to the inverter technology make it possible to control the electrical parameters, increasing the electrical efficiency and process reliability. Agitation is not always adopted. It is often used to prevent uncontrolled heating and localized turbulence in the electrolytic bath with high resistivity. The agitation must not be too vigorous and can be realized with the unassailable material or via insufflation of air or nitrogen. The “useful life” of the electrolyte is rather limited. When the bath reaches a certain amount of metal ions, its polishing effect decreases or disappears. So it resort to the partial or total replacement of the exhausted fluid.
Usually the duration of the process involves the removal of 0.5 – 2 microns of metal depending on the surface conditions. Such removal involves the use of substances that quickly dissolve in the bath the anodic attack products; among the most effective substances they are to be highlighted the phosphoric and sulfuric acid. Substances that are contained in our solution E-polishing Bomar. The cathode material can be made of lead, copper, or carbon fiber. The latter is used in our E- polishing Box to ensure a longer life of the electrodes and decrease the electrical dissipations. The distance between the anode (mechanical part) and the cathode (carbon fiber) can vary from 1 to 15 cm, and during the process is kept constant; decreasing the distance increases the ion exchange and decreases the electropolishing time . Furthermore it is essential a perfect contact of the electrodes to the respective bus bars, otherwise it can occurs secondary electrical actions inside defective contact points.
Electropolishing is used to ensure an optimum combination of aesthetic beauty and very high passivation values. When the stainless steel has been electropolished, the surface is free from impurities and has a very low roughness. In these conditions the stainless steel is full of pure chromium over the surface. The chromium binds with the oxygen present in the environment, creating what “passive” layer which allows to considerably slow the corrosion process.
Fig.3 Microstructure of stainless steel: 1 Microstructre after mechanical polishing. 2 Microstructure after electrolytic polishing
As can be seen in Fig. 3, the microstructure 1 it is generated after a mechanical cleaning treatment. As you can see the microstructure appears with diverse layers ferritic (number 2, 3 and 4) and layers austenitic (number 1, 5 and 6) of different morphology because they have been deformed following the mechanical process. The microstructure 2 is generated after an electrolytic cleaning treatment. As seen in figure 3 the microstructure appears uniform, with grains of the same size and with the same austenitic nature. The thickness of the passive layer varies greatly depending on the type of microstructure. The thickness of the mechanically polished sample (1) is lower than electropolished sample (2) because it is contaminated by foreign particles (abrasive residues and impurities) which prevent to create a uniform layer of chromium oxide. Finally, we show a series of images electropolished objects with our Nitty Gritty system.