What is Electropolishing – Advantages and Disadvantages

Introduction to Electroposhing :

Electropolishing, also known as electrochemical polishing, anodic polishing or electrolytic polishing (especially in the metallography field), is an electrochemical process that removes material from a metallic workpiece. It is used to polish, passivate, and deburr metal parts. It is often described as the reverse of electroplating. It may be used in lieu of abrasive fine polishing in microstructural preparation.

Principle of electropolishing

Electropolishing is an electrochemical process in which the atoms of a work piece submerged in an electrolyte convert into ions and are removed from the surface as a result of a passage of an electric current.

In electropolishing the metallic work piece dissolves in the electrolyte in contrast to Electroplating where the metallic ions traveling through the electrolyte solution deposit on the work piece surface.

In an electropolishing cell (see the figure below) the work piece is anode. It is connected to the positive terminal of the direct current power supply. The negative terminal is connected to a cathode plate commonly made of stainless steel, copper or lead. The anode and the cathode are immersed into an electrolyte solution.
Principally electropolishing is similar to Electrochemical machining where the work piece material is removed due to the conversion of the atoms into ions, formation of an insoluble precipitate and its transfer from the surface. The DC power supply, the anodically connected work piece, the electrolyte and the cathode form an electric circuit.

According to Faraday’s law the amount of the removed material is directly proportional to the amount of electric charge, passed through the circuit. The amount of electric charge Q=I*t (I – electric current, t – time).

The amount of the metal removed from the work piece surface in an electropolishing process varies from 0.1 to 2.5 mil (2.5-64 μm).

The important feature of the electropolishing process is its ability to dissolve asperities (peaks) on the work surface much faster than the material in “micro-valleys”. Such selective dissolution is a result of different values of the electrical potential of the peaks and valleys. The positive charge of the anodically connected work piece is concentrated in the peaks where the current density is higher than average which causes a selective dissolution of the peaks and smoothening the surface.

Electropolishing Process :

Typically, the work-piece is immersed in a temperature-controlled bath of electrolyte and serves as the anode; it is connected to the positive terminal of a DC power supply, the negative terminal being attached to the cathode.
A current passes from the anode, where metal on the surface is oxidised and dissolved in the electrolyte, to the cathode. At the cathode, a reduction reaction occurs, which normally produces hydrogen. Electrolytes used for electropolishing are most often concentrated acid solutions having a high viscosity, such as mixtures of sulphuric acid and phosphoric acid.
Other electropolishing electrolytes reported in the literature include mixtures of perchlorates with acetic anhydride and methanolic solutions of sulphuric acid.
To achieve electropolishing of a rough surface, the protruding parts of a surface profile must dissolve faster than the recesses.
This process, referred to as anodic levelling, can be subject to incorrect analysis when measuring the surface topography.Anodic dissolution under electropolishing conditions deburrs metal objects due to increased current density on corners and burrs.
Most importantly, successful electropolishing should operate under diffusion limited constant current plateau, achieved by following current dependence on voltage (polarisation curve), under constant temperature and stirring conditions.

Benefits and disadvantages of electropolishing

Benefits of electropolishing:

Bright appearance;
Absence of abrasive scratches;
Improved fatigue strength due to stress relieving and defects free surface;
Lower coefficient of friction due to smoother surface (reduced microasperities);
Better corrosion resistance;
Allows processing fragile and delicate parts.

Disadvantages of electropolishing:

Rough surface defects can not be removed;
Electropolishing multiphase alloys may cause roughening due to selective dissolution of different phases.

Application Of Electropolishing :

Electropolishing has many applications in the metal finishing industry because of its simplicity and its ability to be used on irregularly-shaped objects, such as electropolished stainless steel drums of washing machines, stainless steel surgical devices, and copper semiconductors. Electropolishing is also commonly used to prepare thin metal samples for transmission electron microscopy and atom probe tomography because the process does not cause the mechanical deformation of surface layers observed with mechanical polishing. Ultra high vacuum (UHV) components are typically electropolished in order to have a smoother surface for improved vacuum pressures, out-gassing rates, and pumping speed.