Electropolishing (EP) is normally used to remove a very thin layer of material on the surface of a metal part or component. The process is of interest because of its ability to enhance the material properties of a workpiece in addition to changing its physical dimensions. Therefore, by means of electropolishing, the machining stress produced by grinding and the surface particles resulting in fatigue can be removed, so that the fatigue state of the metal surface is changed. It is obvious that electropolishing increases the machine element life and its effective efficiency is higher than that of machine grinding. EP produces a number of favorable changes in a metal part, which are viewed as credible benefits. These include: brightening, burr removal, total passivation, oxide and tarnish removal, reduction in surface profile, removal of surface occlusions, increased corrosion resistance, increased ratio of chromium to iron, improved adhesion in subsequent plating, reduced buffing and grinding costs, removal of directional lines, radiusing of sharp edges, reduced surface friction, stress relieved surface and removal of hydrogen (Wu et al., 2007; Kalpakjian and Schmid, 2007). According to Durkee (2003), another advantage of electropolishing can be raised. He said, EP can be considered as the ultimate cleaning technique. What he called the combination of removal of imperfection like stains and surface corrosion, heat discoloration, oxide films, localized stresses, weld mark, scratches, particles of all sizes, organic films and biological debris. An EP treatment removes the surface and everything which is on, or in it. Naturally, any debris on the surface removed as well. That how the cleaning is done.
EP has many applications in industry. Long list of applications can be extract
form open literature (Landolt et al., 2003; Hollywood, 2006; MPC, 2007;
Kalpakjian and Schmid, 2007; Yu et al., 2007). As a summery from these
references, EP can provide services for a wide range of industries, including:
aerospace, biotechnology, cryogenics, food and beverage processing, foundries,
hydraulics, marines, medical, nuclear, petrochemical, pharmaceutical, semi-conductor,
waste water systems and vacuum technology. It can be used with a wide range
of systems, sizes and shapes that includes air-sampling canisters, bolts and
fasteners, clean-room equipment, filters, gas delivery systems, gears, hardware,
heat-exchangers, investment castings, medical implants, pumps, pressure vessels,
springs, stampings, surgical instruments, tanks, tubing and pipes, vacuum chambers,
valves and fittings, water purification systems and wire products (Hollywood,
Almost any metal can be electropolished (Lee and Lai, 2003; Hu et al.,
2003; Kao and Hocheng, 2003; Guo and Johnson, 2004; Jones, 2004; Andrade et
al., 2005; Wynick and Boehlert, 2005; Aspart et al., 2006; Fushimi
et al., 2006; Abbott et al., 2006; MPC, 2007). The metal can be
ferrous or non-ferrous. Typical listings of metals and alloys that can be electro
polished are shown in Table 1 with some practical limitations.
||List of metals and alloys that can be super-finished by EP
The aim of this research is to study of an electropolishing Cell (EPC). This study explain the theory of electropolishing process that includes the basic principles and the general steps of the process. A simplified view of the theory has been presented that includes the mechanisms, problems, quality control and costs. The third section deals with the experimental setup and how the rig has been prepared in order to get the required results. These results are explained in details in the fourth section with there performance curves and the optimum values of temperature and time. Finally, the conclusion and discussion are presented in the last section and some recommendations have been given.
THEORY OF ELECTROPOLISHING (EP)
EP is the electrolytic removal of metal in a highly ionic solution by means
of an electrical potential and current. EP is normally used to remove a very
thin layer of material on the surface of a metal part or component. EP is often
referred to as a reverse plating process. Electrochemical in nature, EP uses
a combination of rectified current and a blended chemical electrolyte bath to
remove flaws from the surface of a metal part. The typical EP installation is
deceptively similar to a plating line. A power source converts AC current to
DC at low voltages. A tank typically fabricated from steel, glass and rubber-lined
is used to hold the chemical bath. A series of lead, copper or stainless steel
cathode plates are lowered into the bath and installed to the negative (-) side
of the power source. A part or groups of parts are fixtured to a rack made of
titanium or copper or bronze. That rack in turn is fixtured to the positive
(+) side of the power source. As the adjoining illustration depicts, the metal
part is charged positive (anodic) and immersed into the chemical bath. When
current is applied, the electrolyte acts as a conductor to allow metal ions
to be removed from the part. While the ions are drawn towards the cathode, the
electrolyte maintains the dissolved metals in solution. Gassing in the form
of oxygen occurs at the metal surface, furthering the cleaning process. Once
the process is completed, the part is run through a series of cleaning and drying
steps to remove clinging electrolyte.
The amount of change to the metal is highly dependent upon the metal itself
and how it has been processed up to the point where it is electro polished.
The EP effect occurs because as the current is applied, the EP film at the surface
of the metal changes its characteristics.
||The variation of the surface texture before and after electropolishing
As the current is applied to the workpiece, the EP solution becomes thicker
and becomes an insulator or resister. During electrolysis, the thickness of
the salt film forming on the workpiece surface varies with the surface aspect,
i.e., the film at raised parts is thin, but is thick at dented parts. The surface
is smooth because the dissolve rate of raised parts is faster than that of dented
parts. At the same time, an oxide film is generated on the machined surface.
This film has certain stability and makes the surface be in the lightly passivating
state. Then, the machined surface is bright. The resultant surface is clean
and bright. The quantity of metal removed from the workpiece is proportional
to the amount of current applied and the time. Other factors, such as the geometry
of the workpiece, affect the distribution of the current and, consequently,
have an important bearing upon the amount of metal removed in local areas (Hocheng
and Pa, 2000, 2002, 2003). Figure 1 shows both high and low current density
areas of the same part and notes the relative effects of electropolishing in
these two areas (Roy et al., 2007; MPC, 2007). The principle of differential
rates of metal removal is important to the concept of deburring accomplished
by electropolishing. The optimum combination of quality, effectiveness and cost
can be found by the proper control of the electrolytic process parameters. Fine
burrs become very high current density areas and are, subsequently, rapidly
dissolved. Low current density areas receive lesser amounts of current and may
show negligible metal removal. The general relationship between applied current
and voltage for a typical electropolishing system is shown in Fig.
2. An understanding of the combined effects of current and voltage is a
key to the production of high quality electropolishing.
In the course of electropolishing, the workpiece is manipulated to control
the amount of metal removal so that polishing is accomplished and, at the same
time, dimensional tolerances are maintained. EP literally dissects the metal
crystal atom by atom, with rapid attack on the high current density areas and
lesser attack on the low current density areas. The result is an overall reduction
of the surface profile with a simultaneous smoothing and brightening of the
metal surface (MPC, 2007).
||General relationship between current and voltage in (MPC,
To obtain high quality electropolished finishes on most material, it is necessary to process the work through three major operations [more details can be found in (MPC, 2007). These three stages are including metal preparation, electropolishing and post treatment. The aim of the first stage is to remove surface oils, greases, oxides and other contaminants, which interfere with the uniformity of electropolishing. The second stage is designed to accomplish the desired smoothing, brightening and/or deburring of the metal, followed by recapture of the electrolyte to minimize waste treatment. The final stage is to remove residual electrolyte, to remove by products of the electropolishing reaction and to dry the metal to prevent staining.
EP is performed through few controlled mechanisms. These mechanisms work simultaneously in the electropolishing process. A change in any single mechanism can affect the results of the process (MPC, 2007). These mechanisms include; Chemical Saturation Effect, Lightning Rod Effect, The Viscosity Effect, The Osmosis Effect, Gas Mixing/Pump Effect and the Parabolic Mirror or Deep Cone Effect.
The principal chemical reaction occurring at the electrical anode, that is,
at the workpiece, is as follows:
The chemical reaction at the Anodic (+Electrode) is as follows:
And the reactions at the Cathodic (- Electrode) are:
||The designed electropolishing cell
||The dimensions and shape of the two types of work piece
These reactions state that metal is dissolved from the anodic electrode, passing into the solution to form a soluble salt of the metal.
The experimental rig has been designed in a way that allows the user to change
the process variable to elaborate their effect on the process in addition to
perform the process it self. The designed and manufactured EPC, as any standard
cell, consists of the following components (Fig. 3).
Work piece (Anode): This part is the work piece of metal that is being
electropolished. This piece of metal is connected to the positive pole of the
electrical power supply. The origin shape of this work piece is a vertical cylinder
of 30 mm diameter and of 40 mm height as shown in Fig. 4a
provided with a threaded hole in order to be fixed to the equipment via a threaded
rod. Some of these specimens have been cut, as shown in Fig. 4b,
in order to simplify the measurement of roughness.
Cathode: This part is connected to the negative pole of the power supply
to receive the metal ions that are machined from the work piece.
||The electrolyte that used with each material
The cathode is made out of a shaped in such way to provide even current densities
to the work piece surface.
Electrolytes: The Electrolyte is the ionized liquid that provides the
right environment to perform the electropolishing chemical reactions that transfer
material from the work piece. The different electrolytes that used with each
materiel is the recommend one and as indicated in Table 2.
Heating Tank (Container) and Heater: It makes of PVC material with a volume of 96 L (40x40x60 cm). The tank filled with pure water that used to heat the Beaker. A 1 KW heater is used to heat the water within the range 0-80°C. The power and control of this heater is controlled with a thermostat.
Beaker: This glass beaker (scaled from 0-5 L) is located inside the heating tank. Only this beaker has been filled by the electrolytes at this stage of experiments.
Power supply: This equipment supply the cell with a direct current (DC) in the range (0-27.5) Amperes and a voltage between (0-12).
Multi-meter and roughness-meter: These calibrated stranded measuring devices are used to monitor the different required parameters.
The results have been classified into three categories according to the material to be electropolished. Experiments have been carried out for three materials, namely, Brass, Steel and Aluminum. Each of these three materials has been tested under three different operated parameters namely the process time, temperature and concentration of electrolyte. The following is the main results from these runs.
Category 1: The Brass specimen: The brass specimens have been tested
under two parameters which are processing time and the temperature. The results
of these tests are shown in Fig. 5. and parts of Fig.
6 and 7. It noted at this stage that the surface
finish improved significantly with time until reaching 40 min which considered
as the optimum time. At the other hand, the optimum temperature, when it
the only variable, has been found to be around 50°C.
||Effect of applied voltage on the current density for Brass
(T = 30°C)
||Effect of the processing time on the metal removal quantity
for all materials (T = 30°C)
Category 2: The Aluminum specimen: Similar tests are performed for Aluminum.
The results of these tests are shown in Fig. 6-9.
It noted at this stage that the surface finish improved significantly
with time until reaching 20 min which considered as the optimum time. The optimum
temperature, in this case, is found to be around 60°C as shown in Fig.
8. The effect of the concentration of the electrolyte is tested and the
result is shown in Fig. 9 which found in the range of expectations.
||Effect of the temperature on the metal removal quantity for
all materia (Time = 15 min)
||Effect of the processing time on the metal removal quantity
for aluminum at different temperatures
Category 3: The Steel specimen: The steel has been tested under the
same parameters, also and the results obtained shown in Fig. 6,
7 and 10. It noted, here, that the surface finish improved
significantly with time until reaching 50 min which considered as the optimum
time. The optimum temperature, when it the only variable, is around 70°C.
In general and for all of the three materials, the measured roughness of the
surface improved significantly and decreased with around 50% with respect to
before electropolishing and the results are summarized in Table
3. Photographs of the three different specimens before and after the electropolishing
are shown in Fig. 11.
||Effect of the acid concentration on the metal removal quantity
for aluminum at (T = 30°C and time =15 min)
||Effect of applied voltage on the current density for steel
(T = 70°C)
||Effect of the electropolishing on roughness
||A photographs of the three different material before and after
Many lessons have been learned after carrying out the tests of electropolishing
on the (Aluminum, Brass and Steel) specimens with different parameters that
affecting the electropolishing process. These parameters include current density,
voltage, solution concentrations, time and temperature. These factors
affect the quantity and the quality of metal removal from the specimens. The
optimum conditions are the aim of this study. The first important conclusions
that extract from these comprehensive tests are shown in Table
The improvement of the surface roughness by the process has been improved many
mechanical properties of the material (Callister, 1997; Durkee, 2003). These
improvements include the decrease of friction coefficient, wear resistance,
bearing stresses of contact, fatigue life and stress concentration.
||Summary of the optimum results for the three materials
Many problems have been faced during the work and in all of its stages. Solving
these problems gives strength to the results and improves the technique of electropolishing.
Examples of these problems and not restricted to, are the cost of operation,
materials selecting, the shape of the work piece, roughness measurements, post
processing of the work piece and post processing of data. One important point
to be raised here, is that work with brass needs accurate procedure. It is noted
that the EP effect in the zinc more than that of copper. This type of depletion
may change the properties of surface if the procedure is not followed accurately.
Cost estimating of the process has been carried out and good results were obtained. For example, the cost of electropolishing of 1 m2 of Aluminum is around 60 JD (approximately 85$).
Finally, many lessons have been learned during the research time but other improvements could be added to the future work like, ventilation system, circulating pump for the heating tank, work piece rotating inside the cell, multi cathodes system and multi tank system.