Electrochemical processing can serve as a substitute for labour consuming mechanical polishing methods and allows to avoid undesirable structural changing in product surface layer. Etched surface is frequently more noncorrodible and has improved mechanical properties. Electrochemical polishing is a surface treatment method by which workpieces are immersed in a bath of acid electrolyte solution. During treatment process, a workpiece is connected to the positive terminal (anode) of a power supply under constant voltage from 10V to 20V.
Anodic dissolution provides microroughness smoothing at the level of the surface submicrorelief. This is connected with the passivation film presence on a metal. The microroughness reduction rate is determined by the electric charge, passing through the electrolyte. During the electrolysis, along with the reduction of microroughness height, the protuberances rounding and formation of wave-like surface microrelief take place. These processes are caused by uneven distribution of charges along the surface microrelief and by changes of the electrolyte concentration in anode layer. The performance of electrochemical treatment depends on the following aspects: the composition of metals or alloys, the residual deformation degree and thickness of a workpiece.
By electrochemical polishing, in order to obtain a good surface quality, it is necessary to make sure before treatment that deep grooves, nicks and blisters are lacking, because electrochemical treatment is not able remove them. The highest level of brightness can be achieved by processing of small workpieces, and flat surfaces are less workable than cylindrical ones.
Electrochemical polishing is carried out in ordinary galvanic baths with compulsory sealing contact between suspended device, electrode bar and workpieces provided. The most durable fixtures are made of titanium, which is undestroyable under the effect of electrolytes. To make sure that the electrical contact between the suspended device and workpieces is reliable, it is advisable to remove an oxide film from them using diluted sulfuric acid.
In order to increase noncorrodibility after electrochemical polishing, it is recommended to treat steel parts with NaOH solution for 15-20 minutes at temperature of 60–70оC. In case of galvanic coating, to provide better engagement of the coating with a workpiece, one should subject it to electrochemical treatment in 3-5% hydrochloric acid solution.
Despite the sufficiently wide range of applications, the electrochemical method has significant shortcomings, such as:
- High energy intensity rate
- Toxicity, fire and explosive risk
- Processing complexity
- Restriction of metal assortment for processing
- Equipment corrosion