plasma

1. Electrolytic plasma technology: Science and engineering—An overview
P. Gupta ⁎, G. Tenhundfeld, E.O. Daigle, D. Ryabkov
CAP Technologies, LLC, 11725 Industriplex Blvd, Suite 4, Baton Rouge, LA 70809, USA
Available online 31 January 2007
Abstract
This paper overviews our present understanding of the fundamentals behind Electrolytic Plasma Technology (EPT) in view of the experimental
results and theoretical predictions. EPT is an effective surface engineering tool that combines cleaning and coating of metals. During EPT
processing, DC voltage is applied to the electrodes in the aqueous electrolyte, which produces plasma at the surface of the work piece. Thermal,
chemical, electrical and mechanical effects imparted by EPT to the work piece create unique surface characteristics. The mechanism and
metallurgical aspects of the effects are discussed in detail. EPT is under development for industrial applications in specific processes and is being
explored for other potential commercial applications. Both of the aspects are presented. The experimental and industrial tests to date demonstrate
that EPT is an emerging surface engineering technique with economical commercial applications in the field of surface engineering.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Electrolytic plasma; Pickling; Texturing; Coating; Nanocrystalline; Surface alloying
1. Introduction
Electrolytic plasma processing (EPP) techniques are a hybrid
of conventional electrolysis and atmospheric plasma process
and have been a field of research for many years. The first
significant work in this field was done by Kellogg [1]. Later,
Hickling and Ingram conducted several studies on glow
discharge plasma (GDP) [2,3]. Several other studies have
been conducted worldwide on EPP, ranging from basic science
[4,5] to practical applications [6,7]. All the independent studies
reveal a common observation that, at a certain value of voltage
between two electrodes in an aqueous electrolyte, there is
significant deviation from Faraday's normal electrolytic regime
[2,8]. The application of electrical potential is significantly
greater as compared to the conventional electrolysis and leads to
an anomalous phenomenon of excess gas evolution at the
working electrode(s), leading to formation of a continuous gas
envelope around either the cathode or the anode accompanied
by a luminous discharge. The main factors that influence the
formation of the continuous plasma envelope include applied
potential, temperature of the electrolyte, electrode geometry,
nature and properties of the electrolyte, and flow dynamics
[2,4]. Good reviews of EPP from surface engineering and
scientific prospectives have been conducted by Yerokhin et al.
[7] and Belevantsev et al. [9]. Most studies are concentrated on
the anodic regime of EPP and very few on the cathodic regime.
Electrolytic Plasma Technology (EPT), in general, has been
used to engineer metal surfaces in the cathodic regime, but can
also be used in the anodic regime, as required by the specific
application.
Electrolytic Plasma Technology (EPT) [US Patent,
6,585,875B1 [10]] is an emerging surface engineering tech-
nique that has shown great promise for its application on a
commercial scale for cleaning and coating metal surfaces
[11,12]. EPT utilizes basic principles of electrolytic plasma, in
which D.C. potential is applied between two electrodes in an
aqueous media. EPT processing is a dynamic process that
involves delivery of the aqueous electrolyte into a confined
chamber (EPT reactors) on the surface of the work piece. Our
research has been focused on designing reactors to achieve
uniform treatment of metal surfaces by keeping a balance
between the electrolyte and vapor/gas (that are formed at the
surface of work piece) flow. EPT reactors are designed to
process materials of different shape and size.
Ability to form plasma on the surface of the work piece in the
aqueous electrolyte gives the capability to carry out various
treatments on a metal surface (generally the cathode).
Surface & Coatings Technology 201 (2007) 8746–8760
www.elsevier.com/locate/surfcoat
⁎ Corresponding author. Tel.: +1 225 753 2200.
E-mail address: pgupta@captechnologiesllc.com (P. Gupta).
0257-8972/$ - see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.surfcoat.2006.11.023

2. Furthermore, different chemical, electrical, mechanical and
thermal interactions taking place on the metal surface in a
metal–plasma–electrolyte system provides the EPT processed
surface with some unique characteristics [12–14]. EPT has
effectively removed oxide scale, lubricants, dirt, etc., from
metal surfaces [11,15]. EPT cleaning is an environmentally
friendly process that uses non-hazardous aqueous solutions as
compared to acid pickling. Also, being an electrolytic process, it
does not require safety measures as involved as those in grit
blasting (dust) and acid pickling (air scrubbing).
EPT has the ability to deposit metal and alloy coatings, such
as Zn, Ni, Zn–Ni, Ni–Cu, etc. [11,14]. EPT coatings exhibit
excellent adhesion with the substrate and are deposited at
significantly higher deposition rates, as compared to conven-
tional electrolytic processes. EPT has also been used to alloy
metals such as Mo onto the metal surface, and one such study
was recently published [16].
The present paper gives in brief the science and the
mechanism involved in EPT processing. This paper also
summarizes the unique surface characteristics obtained after
EPT possessing and the advantages offered to industry with
some specific examples. An overall picture of the current state
of commercial application and future applications are presented.
2. Science of EPT
Fig. 1 (a) shows the typical voltage–current characteristics
curve for EPT processing in the cathodic regime. The curve
represented by the dotted line was established during
processing of a wire (AISI 1080, high carbon steel) in a
dynamic system, wherein, the electrolyte was delivered at a
rate of 3–5 l/min and the wire was moving through the EPT
reactor (cell) at ∼3 m/min. At low voltages, U1, current
linearly increases with an increase in voltage following
Faraday's law. This regime is accompanied by the presence
of gas, mainly H2 in our case, as shown in Fig. 1 (b). On further
increase of the voltage, a point is reached, U2 (N90 V), which
has gained the attention of scientists studying the electrolytic
plasma processes. This region is characterized by presence of
luminous gas that is not stable as seen by a significant amount
of current oscillation. The current reported in this regime in
Fig. 1 is the mean of the fluctuation (max–min current). Fig. 1
(c) and (d) show the instability, which is accompanied by
intermittent gas luminosity. The formation of luminous gas has
been attributed to the vaporization of the electrolyte in the
vicinity of the electrode (in the present case, the cathode) due
to joule heating [2,7,8]. An experimental study conducted by
Sengupta et al., apart from corroborating the joule heating,
shows that the breakdown of normal electrolysis to form
luminous gas is not due to electrolytic gas evolution, which
was observed to increase with increasing voltage [17]. The
color of the luminous glow depends upon the nature of the
metal ions present in the solution. For example, orange-
colored plasma is seen in NaHCO3 solution (Na ions) and a
blue-colored is seen with ZnSO4 solution (Zn ions). Colors are
seen primarily due to the plasma discharge on the surface of
the work piece (explained in Section 3), in which different
elements produce different wavelengths of light.
When the voltage is increased to U3, the cathode is surrounded
by continuous gaseous vapor plasma, which is characterized by a
significant drop in current. This is the operating regime of EPT,
where plasma is stable, and where controlled surface treatment
Fig. 1. (a) Current–voltage characteristics observed in EPT showing different regimes. (b)–(f) The photos shown in different regimes were taken during EPT cleaning
of high carbon steel wire that was moving through the reactor at a constant linear speed.
8747P. Gupta et al. / Surface & Coatings Technology 201 (2007) 8746–8760

3. can be carried out (Fig. 1 (e)). This regime was discovered by
Kellogg and hence is called the Kellogg region [2]. Various
aspects of this stable plasma regime have been studied [7,17,18].
Correlation of the formation of the stable plasma with “boiling
crisis” or “burn out” phenomena observed in the systems where
boiling liquids are in contact with a hot wall, and the use of
Helmholtz and Taylor hydrodynamic instabilities, gave some
interesting results. Good review of this aspect has been done by
Slovetsky et al. [19] and Belkin [20]. A study by Mazza et al. [18]
showed that the critical current density for the onset of stable
plasma conditions depends on many factors including shape (flat
or round), size and orientation of the electrode. Sengupta et al.
[17] showed that higher voltage and current density were required
to form stable plasma with increased diameter of a wire anode.
This is consistent with our observation in the cathodic regime.
Moving towards the voltage U4, intense arcing is observed along
with the plasma envelope (Fig. 1 (f)). This is an aggressive regime
and can have detrimental effects on the surface of the work piece.
Hickling [2] reports similar current–voltage characteristics
as the present study, wherein the onset of the plasma (U2,
unstable sparking regime) occurs after the maxima of the curve
(Fig. 1). The voltage reported for onset was N60 V, and in our
case it was N90 V. Most of the studies reported in the literature
were conducted in a static system, where the work piece was
Fig. 2. Schematic of EPT processing mechanism: (a) plasma bubble on the surface of the work piece. Single plasma bubble is shown for illustration only. In reality, the
bubble is surrounded by numerous bubbles, (b) shockwave production by the cooling plasma bubble, (c) collapsing plasma bubble and cleaning, (d) collapsing plasma
bubble and creation of micro-crater, (e) collapsing bubble deposits ions in case of coating, and (f) increasing coating thickness with processing time.
8748 P. Gupta et al. / Surface & Coatings Technology 201 (2007) 8746–8760

4. immersed in an electrolyte bath. In the present study, both the
work piece (wire) and the electrolyte are moving at significantly
higher speed through or within a confined space (EPT reactor),
respectively. Mazza et al. [18] showed that cell geometry and
mechanical vibration given to the electrode and electrolytic bath
(stirring) will have effect on the critical current density for
formation of the plasma. Thus, in our case movement of the
electrolyte and the work piece may provide enough mechanical
vibration so as to cause the observed shift in the voltage to form
different regimes as compared to the data reported by Hickling.
Tyurin et al. [21] use a dynamic EPP system but the electrolyte
is sprayed onto the work piece through a nozzle. They observe
formation of vapor-gaseous layer N80 V, but the discharges
appear when the voltage is increased to 120 Vor higher. This is
consistent with our observation of an increase in onset voltage
with a dynamic system.
3. Mechanisms of EPT processing
Fig. 2 shows a schematic of the key mechanisms of EPT
processing. As discussed in the previous section, during the
stable plasma regime the work piece is surrounded by a
continuous gas envelope. High voltage between the electrodes
leads to concentration of positive ions that are present in the
electrolyte, in the close proximity of the cathode, mostly on the
surface of the gas bubbles. Thus, a very high positive charge is
present in the close proximity of the cathode. This results in
high localized electric field strength between the cathode and
the positive charges. It has been reported earlier that in EPP,
plasma layer electric field strength can reach 105
V/m or higher
[7,9,21,22]. When such high electric field is reached, gas space
inside the bubbles is ionized, and a plasma discharge is initiated.
Fig. 2 (a) shows the magnified view of the surface of the work
piece. Single bubble is plasma shown to illustrate the concept.
In reality, the bubble is surrounded by numerous plasma
bubbles. The temperature of plasma, locally, can reach as high
as 2000 °C. Based on OES studies on EPP in anodic plasma,
Klapkiv [22] suggested that the discharge plasma temperature
corresponds to 6–7×103
K. This high-temperature plasma
bubble is surrounded by relatively cool electrolyte (∼boiling
point of water), thereby resulting in cooling of the plasma.
Finally, the bubble implodes on the metal surface (Fig. 2 (b)–
(d)). Belevantsev et al. [9] have described the presence of
bubbles containing negative oxygen ions in anodic EPP.
Furthermore, they have described the extinction of discharge
as a consequence of expansion and cooling of the bubbles. The
plasma discharge duration is expected to be ∼10−6
s for each
individual event. The entire surface of the cathode may not be
covered by a continuous plasma layer but by a limited quantity
of discrete plasma discharges, which take place at any one
instant of time. Yerokhin et al. [23] recently studied the
characteristics of discharge in anodic EPP. Digital video
imaging revealed the presence of discrete discharge at an
instant on the surface of the work piece.
Two phenomena are expected on the work piece surface due
to the implosion of a plasma bubble. Firstly, the positive ions,
which are concentrated around the bubble, are accelerated
directly to the cathode surface, the characteristics of which can
be described as similar to an avalanche. Secondly, as the bubble
implodes, the stored energy is released into the gas layer and
kinetic energy is transferred to and from the liquid layer to the
surface of the work piece. This energy can be very high, similar
Fig. 3. SEM micrograph showing typical surface morphology of EPT cleaned
steel. Two distinct features can be seen: micro-craters and spheroids [15]. The
steel shown is A-36 low carbon steel that was cleaned by using NaHCO3
solution.
Fig. 4. (a) Depth profile of the EPT cleaned surface and (b) a 3-D photo of the surface profile [15].
8749P. Gupta et al. / Surface & Coatings Technology 201 (2007) 8746–8760

5. to cavitation (having pressure on the order of several hundred
MPa or greater [24]), and the ions to be deposited which are
initially accelerated by the void created by the imploding bubble
are further assisted and moved to the cathode surface by this
kinetic energy. This leads to deposition of metal ions, present in
the electrolyte onto the surface of the work piece (Fig. 2 (e) and
(f)). The movement of ions, in EPT, takes place mainly by ion
acceleration through the plasma and ion bubble absorption
transport as the bubble collapses. Furthermore, both of these
modes of transport eliminate the phase boundary diffusion layer
that is generally present in the conventional electroplating
processes. EPT is a dynamic system, where the electrolyte
moving through the reactor at a high rate leads to rapid transport
of ions to the plasma layer. Tyurin et al. [21] have also reported
hydrodynamic transport of ion from the bulk to the work piece
in similar electrolytic plasma systems. A combination of
hydrodynamic flow and efficient ion transport mechanisms,
during the EPT coating process, leads to high deposition rates.
High temperatures in plasma bubbles lead to localized
melting of the surface layer of the work piece. This surface is
quenched by the surrounding electrolyte after collapse of the
plasma bubble, leading to a unique surface microstructure.
Organic impurities, like lubricants, grease, etc., are instanta-
neously flashed away from the treated surface due to the
localized high temperatures. Also, hydrogen present in the
plasma bubbles chemically reduces the oxide scale to pure α-
iron, in the case of a steel substrate [12]. Thus, the oxide scale is
removed partly by mechanical energy provided by bubble
Table 1
Surface roughness created by EPT processing
Reference Substrate Size of substrate Type of EPT
processing
Electrolyte
(base)
Surface roughness, Ra, Rq (μm)
Parent EPT-processed Grit-blasted
[15] A-36 mild steel 30.5 cm×5 cm×0.65 cm Cleaning NaHCO3 – 2.2, 2.3 9.4, 13.3
[34] Grade 60, medium carbon steel rebar 1.25 cm diameter Coating ZnSO4 – 12.5, 15.5 –
[21] Brass-coated steel cord 350 μm diameter Texturing Proprietary 0.12, 0.15 0.44, 0.56 –
Coating ZnSO4 0.12, 0.15 0.35, 0.46 –
[40] 4330V steel 10 cm×2.5 cm×2.5 cm Surface alloying Na2MoO4 0.005a
0.24, 0.4 –
Inconel 4.3, 5.3b
5.1, 6.5 –
Ra =average surface roughness, Rq =RMS value of roughness.
a
Polished to mirror finish.
b
As-received parent surface (grit-blasted).
Fig. 5. TEM analysis of EPT treated 4340 steel: (a) Typical low magnification cross sectional TEM image of the Zn coating, (b) high magnification bright field and (c)
dark field image. Inset is the electron diffraction pattern.
8750 P. Gupta et al. / Surface & Coatings Technology 201 (2007) 8746–8760

6. implosion and partly by chemical reduction by hydrogen,
during EPT cleaning.
4. Characteristics of EPT-processed metal surfaces
4.1. Surface morphology and micro-roughness
Fig. 3 shows the typical surface morphology obtained after
EPT processing [12,15]. The substrate in this case is A-36 low
carbon steel that was cleaned by EPT using NaHCO3 solution.
The surface morphology is characterized by the presence of two
unique features, namely, micro-craters and spheroids. Fig. 4
shows the surface profile of the EPT processed steel [15]. The
surface profile is characterized by the presence of micro-
roughness created by EPT. It is interesting to note that the depth
profile, Fig. 4 (a), shown at different locations of the treated
surface, Fig. 4(b), exhibits uniform treatment by EPT. It can be
perceived that micro-craters and spheroids are excellent sites for
mechanical interlocking. Thus, a combination of unique surface
morphology and uniform micro-roughness makes the EPT-
treated surface favorable for adhesion to lubricants, paints,
coatings, etc. [11,13].
Micro-craters and spheroids are the result of the plasma
bubble implosion and the quenching of the localized melted
surface layer, respectively, that were discussed in the previous
section. The size of micro-craters and spheroids can be
controlled by a combination of processing parameters and
electrolytic solution, up to a limit [14]. This gives a capability of
tailoring the micro-roughness to a desired value, within limits,
as per the requirement of an application.
Table 1 summarizes the surface roughness obtained after
different EPT treatments, namely cleaning, coating, surface
texturing and surface alloying. The different types of EPT
treatment are discussed in detail in Section 5. Table 1 shows that
EPT treatment increases micro-roughness of the parent
substrate, which is a result of the creation of the unique
morphology. It is interesting to note that EPT was used to
process steel rebar (diameter 1.25 cm) and brass-coated steel
cord (diameter 350 μm). This shows that EPT has the capability
to adapt according to the size of the substrate. Furthermore,
processing of μm-size substrate emphasizes the localized nature
of the EPT treatment.
4.2. Microstructure
A combination of mechanical, thermal, chemical and
electrical treatment provided by EPT leads to development of
unique surface microstructure. The microstructure of EPT-
cleaned steel (AISI 1010) surface has been studied in detail by
focused ion beam (FIB) and TEM analysis [12]. It was found
that the surface microstructure was characterized by the
presence of a layer with thickness 150–250 nm and grain size
of 10–20 nm [12]. A more recent study conducted by using
XRD and TEM on EPT-cleaned steel (AISI 4340), revealed that
the surface microstructure consists of 2–3 layers. The top-most
layer is amorphous with the sub-layers consisting of nano-sized
grains with increasing grain size, gradually up to a limit before
reaching the bulk microstructure. It was also observed that the
thickness of the nano-grained layer increased with an increase in
EPT processing time [25].
Fig. 5 shows the cross sectional TEM images of the Zn
coating deposited by EPT on the steel surface. As shown in
Fig. 5 (a), the Zn coating is comprised of columnar-like clusters
with a width of about 350–550 nm. High-resolution bright field
and dark field TEM images demonstrate that the clusters are
composed of nano-sized sub-columns with a size of 20–40 nm,
and the sub-columns consist of nano-grains with a size of 5–
20 nm. The electron diffraction pattern shown as an inset in
Fig. 5 (b) indicates that the deposited Zn coating is hexagonal
close packed with small grain size. Thus, the above observa-
tions show that the Zn coating has a small grain size and tends to
grow in dense columns.
Nano-sized grains produced by EPT processing, both in
cleaning and coating of metals are more than likely due to the
rapid quenching of the localized melted surface layer as
Fig. 6. Microhardness profile along the thickness of the low carbon strip [15].
HK0.5 is Knoop microhardness at 500 g load.
Fig. 7. Level of rusting on A-36 steel strip, cleaned by (a) EPT and (b) grit
blasting, after exposure for 6 months in laboratory in RH ∼75% [15]. Arrows in
(a) indicate the formation of rust in the EPT cleaned sample at either cut edge or
mishandling due to wet contact. EPTcleaning was conducted by use of NaHCO3
electrolyte, whereas grit blasting was done by angular steel shots.
8751P. Gupta et al. / Surface & Coatings Technology 201 (2007) 8746–8760

7. described in Section 3. Several studies have been conducted on
various grades of steel that show that the bulk microstructure
and hence the mechanical properties of the processed material
are not changed by EPT processing [14,15]. For example, Fig. 6
[15] shows the microhardness profile of an EPT-cleaned and a
parent low carbon steel strip, along their thickness. It can be
seen that the EPT-cleaned strip exhibited no significant change
in hardness. Rather, the EPT-cleaned strip exhibited a micro-
hardness profile which is similar to that of the parent material,
along the strip thickness. This is expected as EPT treats
localized regions of the surface as discussed previously.
4.3. Corrosion behavior
EPT-cleaned steel has shown resistance to general corrosion
from days to weeks in an enclosed environment, e.g. Fig. 7 [15].
Electrochemical tests conducted in the past, like open circuit
potential [12], linear polarization and electrochemical imped-
ance [26] showed good corrosion resistance of the EPT-cleaned
surface. Open circuit potential tests were conducted on EPT-
cleaned low carbon steel (AISI 1010) in tap water. The EPT-
cleaned surface showed significantly higher (noble) open circuit
potential (50 mV) in comparison to the parent steel (−590 mV)
[12]. Linear polarization studies [26] conducted on A-36 mild
steel showed an order-of-magnitude lower corrosion rate of
EPT-cleaned steel, as compared to a grit-blasted surface, in
aqueous test solutions of 1 N sulfuric acid and 3.5% NaCl. The
corrosion results are summarized in Table 2.
Improvement in corrosion resistance of the EPT-cleaned
steel was attributed to formation of a layer of pure α-iron after
EPT cleaning [12,26]. The nano-sized grain structure of the
EPT-treated surface layer may also contribute to the improved
corrosion resistance. Superior localized corrosion resistance due
to the fine grain structure has been reported for nano-crystalline
304 stainless steel as compared to conventional 304 stainless
steel [27].
Two independent beta industrial tests were conducted, in
which a 5.5 mm diameter steel rod was cleaned by EPT. No
surface treatments like chromating, phosphating, etc., were
given to the rod after cleaning. In one test, Cu was clad on the
rod, which was drawn into wire [11]. In the other test, the
cleaned rod was drawn into wire without the use of lubricant
carrier coating. Both the tests were conducted two weeks after
EPT cleaning; details are presented in Section 6.1. No problems
were encountered during the drawing process. It is well known
that any presence of oxide or rust would have caused either the
debonding of Cu from steel rod or breaks in the wire during
drawing. The two tests strongly support the formation of
passivated surface and adhesive surface morphology after EPT
cleaning as suggested by the laboratory studies.
Fig. 8. (a) Typical steps involved in cleaning of material before hot dip galvanizing and (b) steps involved in cleaning of material by EPT prior to any application.
Table 2
Electrochemical test conducted on EPT cleaned steel
Reference Substrate Test Testing
medium
Open circuit potential (mV) Corrosion ratea
Comparative materialb
EPT Comparative materialb
EPT
[12] Low carbon steel (AISI 1010) Open circuit potential Tap water −590 50 – –
Anodic polarization Tap water −325 −196 3.51 0.14
3.5% NaCl −535 −385 11.8 9.5
[26] A-36 mild steel Linear polarization 3.5% NaCl – – 55.1 6.96
1 N H2SO4 – – 527 42.3
a
In Meletis et al. [12] corrosion rate is given in μA/cm2
and in Schilling and Herrington [26] it is given in ml/year.
b
For Meletis et al. [12] EPT cleaned steel was compared with the base steel and for Schilling and Herrington [26] it was compared with grit blasted steel.
8752 P. Gupta et al. / Surface & Coatings Technology 201 (2007) 8746–8760

8. 5. Main applications
5.1. Cleaning
Cleaning of a metal surface is an essential step prior to many
applications such as coating, drawing, etc. EPT has been
effectively used to remove oxides, dirt, lubricants, etc., from the
metal surface. Both ferrous and non-ferrous metals and alloys
have been cleaned. Cleaning by EPT offers the following
advantages:
– Single-step processing: Many metal applications like hot dip
galvanizing use multiple-step cleaning as shown in Fig. 8. EPT
cleans different types of surface impurities in a single step.
– Environmentally friendly: EPT does not use hazardous
chemicals like acids that are generally used to clean metal
surfaces. Also, being an aqueous process, there is no health
hazard as in case of grit blasting. Also, EPT does not require
air scrubbing equipment that amounts to significant capital
and maintenance cost.
– Desirable surface morphology: As discussed previously,
EPT cleaning results in desirable surface morphology that
has a tendency to improve adhesive and lubrication
properties of the surface.
– Increase in shelf life: Passivation shown by EPT-cleaned
steel will increase the shelf life prior to metal processing
applications. In combination with adhesive morphology,
passivated surface shows potential to eliminate the need of
Fig. 9. (a) and (b) Surface morphology; (c) and (d) 3-D surface profile and (e) and (f) EDS of parent and EPT treated brass coated steel (AISI 1080) cord [(a), (c) and
(e)] and [(b), (d), (f)], respectively [14].
8753P. Gupta et al. / Surface & Coatings Technology 201 (2007) 8746–8760

9. phosphate or lubricant carrier coatings prior to applications
like wire drawing.
5.2. Surface texturing
Surface properties, such as roughness and morphology are
important parameters for many applications such as coating
adhesion, tribological performance, etc. Most conventional
methods that are used to improve the surface properties are grit
blasting and chemical etching, both having environmental
problems. Recently, efforts have been made to develop green
technologies for surface texturing such as laser surface texturing
(LST) [28], electrodischarge machining [29], and plasma
processes [30].
It can be perceived that the unique surface morphology and
micro roughness created by EPT makes it an effective tool to
texture metal surfaces. Recently, a study was conducted on the
texturing of brass coated steel cord (high carbon steel, 350 μm
in diameter) by EPT [14], which is used in automobile tires.
Fig. 9 shows the surface morphology and micro roughness of
the EPT processed steel cord as compared to the un-processed
material [14]. Significant surface texturing can be seen in just
3.8 s of EPT processing (Fig. 9 (a)–(d)). Furthermore, EDS
spectra showed that EPT did not remove the brass coating that is
applied for corrosion protection and that the cord has good
adhesion with rubber used in tires (Fig. 9 (e) and (f)).
Recent studies conducted on surface modification by laser
surface texturing (LST) revealed significant improvement in
hydrodynamic and hydrostatic lubrication [28,31]. The LST
forms micro-dimples, which act as micro-reservoirs to enhance
lubricant retention or micro-traps to capture wear debris [32].
Micro-craters created by EPT may provide the same benefit as
the LST produced features and hence improve tribological
performance of the processed surface. Fig. 10 (a) and (b) show
the surface features produced by LST [28,33]. Fig. 10 (c) and
(d) [15] give the EPT produced surface morphology. Surface
morphology produced by grit blasting is also given for
comparison, in Fig. 3 (e) [15]. It is interesting to note that the
micro-dimples created by LST on piston rings and the EPT-
treated surface look similar (Fig. 10 (b) and (c), respectively).
Although the diameter of the micro-dimple created by EPT
(b10 μm) is smaller as compared to LST (72 μm), there may be
sufficient aspect ratio in the EPT micro-dimple to serve as
micro-reservoir.
A grit-blasted surface was produced by angular steel shots.
The grit-blasted surface was characterized by the presence of
deep pits (Fig. 10 (e)). The pits do not seem to be uniformly
distributed over the treated area as compared to the EPT micro-
craters (Fig. 10 (d)). Furthermore, the region between the pits in
the grit-blasted surface was relatively smooth. Although the
grit-blasted surface exhibits higher roughness (Table 1) the
EPT-treated surface has a higher tendency to provide a
mechanical anchor for coatings and adhesives [13].
5.3. Coating
The role of metal coatings in the improvement of various
surface characteristics such as corrosion, wear, etc., cannot be
over emphasized. Metal coatings have been deposited on
metallic substrates by EPT. Fig. 11 [34] shows examples of EPT
Fig. 10. (a) Surface produced by standard LST [28]; (b) inset is the photo of micro-dimples produced by LST on the piston ring [33]. The micro-dimples were of
diameter 72 μm and depth 7.5 μm on an average [33]; (c) and (d) EPT cleaned A-36 steel [15]; and (e) grit-blasted A-36 steel [15].
8754 P. Gupta et al. / Surface & Coatings Technology 201 (2007) 8746–8760

10. coatings that were deposited on carbon steel rebar (grade 60,
medium carbon steel, 1.25 cm diameter). EPT coatings are
conformal to the substrate and are dense (Fig. 11 (c) and (d)). It
can be seen that EPT can be used to deposit coatings on μm-
sized wires (Fig. 11 (e)) and has uniform micro-roughness
(Fig. 11 (f)) that will provide excellent bonding to polymer
coatings.
EPT offers the following advantages in deposition of
coatings.
– High deposition rates: A deposition rate with Zn coating, as
high as 1 μm/s, has been achieved by EPT.
– Interfacial bonding: EPT coatings exhibit excellent bonding
with the substrate. Fig. 11 (b) shows the fractograph of an
EPT Zn-coated specimen on the rebar that was subjected to
uniaxial tensile test. It can be seen that the coating does not
show any delamination. EPT coatings have exhibited
adhesion strength N70 MPa [12]. Excellent adhesion
achieved by EPT coating is more than likely due to the
diffusion bonding formed with the substrate, due to high
localized temperatures.
– Nanocrystalline grain structure: Nano-structured materials
are of prime interest to the scientific community due to their
unique properties [35]. Nanocrystalline grains formed by the
EPT process offer potential to improve the properties of the
coating as compared to the respective large-grained coating.
5.4. Surface alloying
EPP have been used to harden steel by surface alloying, e.g.
carburizing [36], nitriding [37], carbonitriding [38] and
borosulfocarbonitriding [39]. Details of the various EPP surface
Fig. 11. (a) Metal and alloy coatings deposited by EPT on rebar; (b) fractograph of Zn coated rebar after tensile testing [34], while the line shows the interface between
coating and the substrate [34]; (c) and (d) surface morphology and cross-section of Ni coating showing conformality and defect-free interface, respectively [34]; (e) and
(f) show the cross-section and surface profile of Zn–Ni coating deposited on brass coated steel cord (diameter 350 μm) [14].
8755P. Gupta et al. / Surface & Coatings Technology 201 (2007) 8746–8760

11. alloying treatments have been published by Belkin [20]. The
driving force behind surface alloying is the difference in
chemical composition between the substrate and the vapor
envelope. Metal surfaces can be alloyed by nonmetals like O, C,
N, B or by metals such as W, Mo, V, etc. [20]. It is interesting to
note that the diffusion rate of different chemical species in the
metal surface during EPP was significantly higher as compared
to the conventional counterparts.
Recently, Mo coating was deposited on AISI 4330V steel
and Inconel 718 by using EPT [16]. XRD analysis showed that
Table 3
Characteristics of Mo alloyed surface by EPT processing [40]
Specimen Surface roughness, Ra, Rq (nm) Hardness (HK0.05
a
) Friction coefficient Wear rateb
(×10− 7
mm3
/N m) Phase structure (XRD)
4330V steel unprocessed 0.05 490±45 0.4 9 α-Fe
4330V Steel EPT processed 0.17, 0.23 695±155 0.55 2.8 α-Fe, Fe0.54Mo0.73
The samples were mechanically polished to mirror finish prior to any processing.
a
HK0.05 Knoop microhardness at 50 g load.
b
Pin-on-disk test was conducted.
Fig. 12. Pin-on-disk tribology tests conducted using 440C pin at a load of 5 N and linear speed of 1 cm/s for a distance of 50 m on 4330V steel (a), (c) and (e) un-
processed; and (b), (d) and (f) EPT Mo alloyed. (a) and (b) SEM morphology of the wear tracks, (c) and (d) 3-D photo of profile of wear tracks, and (e) and (f) 2-D
profile of wear tracks [40].
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12. Mo alloyed into the surface, rather than being deposited as a
coating. EPT Mo-alloyed steel showed up to a two-fold increase
in surface hardness with variation in concentration of Mo [16].
A detailed study on surface morphology, composition, structure
and hardness was recently published [16].
Table 3 summarizes the characteristics of un-processed AISI
4330V steel with the Mo-alloyed counterpart. The un-processed
sample was polished; whereas, the Mo-alloyed surface was as-
deposited. The Mo-alloyed sample was polished prior to EPT
processing. Surface roughness and hardness of the Mo-alloyed
steel increased as a result of EPT processing [16]. Increase in
roughness was due to the unique surface morphology produced
by EPT similar to the EPT-cleaned morphology. The increase in
hardness was attributed to the presence of Fe0.54Mo0.73 phase in
the Mo-alloyed sample, which was determined by standard and
grazing angle XRD [16].
Pin-on-disk tests were conducted to compare tribological
properties of the un-processed with the Mo-alloyed AISI 4330V
steel (Table 3) [40]. Tribological conditions for the tests were
selected from a study conducted on Mo coatings that were
deposited by thermal spray processing [41,42]. The conditions
were SS440 Gr25 pin at load 5 N and linear speed 1 cm/s for
distance 50 m. The pin was 9.5 mm in diameter with hardness of
57–63 HRC. Fig. 12 (a) and (b) show the surface morphology
and Fig. 12 (c)–(d) show the depth profile of the wear tracks of
un-processed and Mo-alloyed sample, respectively. The Mo-
alloyed surface showed some increase in friction coefficient as
compared to the parent sample. The wear rate of the Mo-alloyed
surface was three times lower as compared to the un-processed
surface (Table 3). Close examination of the wear track showed
that the wear mechanism is different in un-processed and Mo-
alloyed steel (Fig. 12 (a) and (b)). More detailed study is needed
to understand the physico-chemical interactions taking place
during the tribological tests in the tested sample.
6. Current state of commercialization of EPT
EPT processing is in a transition phase from research to
commercial applications, mainly focused on the metal industry.
Current commercial applications being developed are for
cleaning and coating of wire, rod and reinforcing bar.
6.1. Cleaning and coating of wire rod
EPT has been used to conduct industrial testing for wire rod
(diameter 5.5 mm), which was sent for further processing
(Fig. 13). In one test, a sterate-coated low carbon steel wire rod
was cleaned by EPT at a speed of 61 m/min to remove sterate-
lubricant film and obtain an anchor surface profile (Fig. 14 (a)).
The cleaned rod was Cu-clad, 15 days after EPT cleaning. No
Fig. 13. Industrial prototype of the EPT processing unit for rods. This unit was used to clean a 5.5 mm diameter carbon steel rod up to a speed of 61 m/min. This unit
was manufactured under license from CAP Technologies, LLC.
Fig. 14. (a) Surface morphology of an EPTcleaned low carbon steel rod that was
processed at a speed of 61 m/min (processing time: 2.3 s) and (b) fractograph of
a Cu-cladded EPT-cleaned rod (after drawing) that was subjected to the uniaxial
tensile test. This rod was Cu clad, 15 days after EPT cleaning, and the interface
between Cu and steel shows excellent adhesion with no sign of rusting [11].
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13. pretreatment, except a water rinse, was given to the EPT-cleaned
steel before cladding. The Cu-clad rod was drawn to smaller
diameter without any problems. Furthermore, Cu showed
excellent bonding to the EPT-cleaned steel, as shown in
Fig. 14 (b) [11]. Cladding and subsequent drawing was
conducted by a company that is a manufacturer of cable for
communication networks.
In another test, EPT was used to remove oxide scale from a
high carbon steel wire rod (AISI 1080) at a speed of 20 m/min.
This rod was drawn 17 days after EPT cleaning, into 1.72 mm
wire without the use of phosphate lubricant carrier coating.
Again, there were no problems during drawing of the material.
Studies conducted in the past have shown beneficial effects of
surface roughness [43,44] and surface morphology [45] on
lubricant transport properties during drawing. The drawing test
was conducted by a company that manufactures tire cords. This
test gives a strong indication for the reduction if not elimination
of the need for lubricant carrier coatings prior to drawing
applications by EPT due to a combination of the passivated
surface and adhesive morphology. This can have significant
environmental as well as cost benefits to the metal processing
industry.
EPToffers the following advantages to the wire-rod industry:
– Footprint: An EPT processing unit requires significantly
smaller workspace as compared to existing technologies like
acid cleaning. Fig. 13 gives a feel for the size of EPT
processing unit.
– In line capability: An EPT processing unit can be put in-line
with the existing metal-processing equipments.
– Value addition: Apart from cleaning and coating metal
surfaces, EPT provides value addition to the product as
explained in Section 5 at no additional cost.
– Environmental benefits: Apart from the use of non-
hazardous chemicals for cleaning, EPT has potential to
eliminate the use of lubricant carrier coatings, which would
provide environmental and cost benefits.
6.2. Coating for reinforcing steel bar
Corrosion of reinforcing steel bar in concrete is a very
common problem that amounts to the loss of billions of dollars
in infrastructure. EPT is in a phase of testing on the industrial
scale for cleaning of rebar and deposition of metal coating.
Proprietary coatings are being developed to be deposited by
EPT on rebar. Existing coatings will be compared to the
proprietary EPT coatings and EPT deposited Zn that has some
advantages due to its nanocrystalline structure. Some currently
used commercial coatings require thicker coating, in order to
slow down the process of chloride permeation. It is expected
that EPT coatings will provide cost benefits by increasing the
service life of rebar due to their reduced thickness and
nanocrystalline nature. Improvement in the quality of concrete
and an increase in concrete cover in combination with the epoxy
coating on reinforcing steel was used to delay the chloride
induced corrosion [46]. Also, special handling techniques are
required to minimize damage to the epoxy coating [46]. EPT
coatings may eliminate the need to improve quality or cover of
concrete, and need for special handling due to its expected
corrosion properties and excellent adhesion with the substrate,
respectively.
7. Potential commercial applications of EPT
A combination of adhesive morphology, the nano-layer
created on the cleaned surface, and nano-structured coatings,
exhibits immense potential for enhancement of performance of
surface engineered metal by EPT leading to many commercial
applications. Some of the applications are listed below:
7.1. Biomedical applications
Surface modification of implants, mostly Ti, by either
surface texturing or deposition of bio-compatible coatings has
been of prime interest to the bio-medical industry. Porous
morphology created on Ti implants [47] and hydroxy apatite
(HA) coating has shown to help tissue growth [48,49]. EPT
offers similar if not better surface morphology at a significantly
lower cost as compared to the existing technologies. Yang et al.
[48] reviewed HA coating deposited using sputtering methods
and briefly compared different processes such as thermal
spraying, sol–gel method, dip coating, electrophoresis, etc. Nie
et al. [50] deposited HA/TiO2 on Ti (Ti–6Al–4V) substrate by a
hybrid process micro-arc oxidation (anodic EPP) and electro-
phoresis. They determined that the HA/TiO2 coating had higher
adhesive strength as compared to HA coating that were
Fig. 15. Conceptual design of an EPT reactor to clean the internal surface of a pipe.
8758 P. Gupta et al. / Surface & Coatings Technology 201 (2007) 8746–8760

14. deposited by the other processes, such as sputtering or sol–gel.
Furthermore, localized melting in the EPP process leads to
sintering of the HA coating. EPT has shown potential to deposit
HA coatings and should have beneficial effects as observed by
Nie et al.
7.2. Aerospace and automotive applications
The aerospace and automobile industries use many metal
coatings such as Zn–Ni for corrosion protection. Advantages of
nanocrystalline coatings deposited by EPT can be seen in such
applications. Recently, Mo-based coatings have shown to
increase wear properties of steels [51,52]. Al–Mo coatings
deposited on AISI 4340 high-strength steel showed a reduction
in the friction coefficient with an increase in Mo content [51].
Al alloys are the material of interest to the aerospace and
automobile industries due to their high specific strength, but
they exhibit high wear due to low hardness. Mo alloying by
EPT on the surface of Al alloys exhibits great promise to
improve their tribological properties.
7.3. Internal surface of pipes
Many industries use extensive cleaning methods for the
internal surface of pipes, such as well casing used in the oil and
gas industry. The most common method for internal cleaning
and coating of pipe is the use of a lance with a blasting and a
spraying head. EPT is being explored to clean the internal
surface of pipes, and the conceptual design is given in Fig. 15.
Also, EPT would provide surface texturing or adhesive surface
morphology for non-EPT coatings. The aqueous electrolyte will
be delivered through the lance to the EPT reactor head. The
main design challenge in processing internal pipe surfaces is to
maintain the stable plasma with release of vapor from the pipe.
Treatment of the internal surface of pipe will lead to
advancements in various other surface treatments apart from
cleaning by EPT.
8. Summary
EPT is an emerging surface modification tool that utilizes the
science of electrolytic plasma processing to clean, texture, coat
and alloy metal surfaces. EPT processing offers the metal
industry an economical, environmentally friendly solution for
the cleaning of surfaces. Nanocrystalline coatings deposited by
EPT show immense potential of application in various fields.
EPT is in a phase of commercialization in certain applications,
and many more commercial applications are being explored.
This paper gives an overview of EPT from the scientific and
commercial stand points. In conclusion, it is in the hands of
surface engineers to exploit the benefits of EPT for solutions to
existing problems and explore it for advanced applications.
Acknowledgements
The authors would like to acknowledge Dr. V. Singh at
CAMD, Louisiana State University, and Mr. G. Malani at
Welding and Testing Laboratory, Baton Rouge, Louisiana for
help in surface profile measurement and metallography,
respectively. The authors would also like to thank Dr. E. I.
Meletis and Dr. Y. Cheng at Materials Science and Engineering,
The University of Texas at Arlington, for TEM analysis.
Partial funding for support of this project was provided by
Department of Defense Strategic Environmental Research and
Development (SERDP grant # N00173-03-C-2013).
References
[1] H.H. Kellogg, J. Electrochem. Soc. 97 (1950) 133.
[2] A. Hickling, Modern Aspects of Electrochemistry, vol. 6, Butterworths,
London, 1971, p. 329.
[3] A. Hickling, M.D. Ingram, Trans. Faraday Soc. 60 (1964) 783.
[4] J. Garbarz-Olivier, C. Guilpin, J. Electroanal. Chem. 91 (1978) 79.
[5] A.J. Calandra, J.R. Zavatti, J. Thonstad, Electrochim. Acta 37 (1992) 711.
[6] Y.N. Tyurin, A.D. Pogrebnkaj, Tech. Phys. 47 (2002) 1463.
[7] A.L. Yerokhin, X. Nie, A. Leyland, A. Matthews, S.J. Dowey, Surf. Coat.
Technol. 122 (1999) 73.
[8] S.K. Sengupta, O.P. Singh, J. Electroanal. Chem. 369 (1994) 113.
[9] V.I. Belevantsev, et al., Prot. Met. 34 (1998) 416.
[10] D.V. Ryabkov, US Patent # 6585875 B1, July (2003).
[11] P. Gupta, G. Tenhundfeld, E.O. Daigle, R. Calliham, D.V. Ryabkov, Wire
Cable Technol. Int. 31 (Nov–Dec 2003) 52.
[12] E.I. Meletis, X. Nie, F. Wang, J.C. Jiang, Surf. Coat. Technol. 150 (2002)
246.
[13] P.J. Schilling, P.D. Herrington, E.O. Daigle, E.I. Meletis, J. Mater. Eng.
Perform. 11 (2002) 26.
[14] P. Gupta, G. Tenhundfeld, E.O. Daigle, Wire J. Int. 38 (2005) 50.
[15] P. Gupta, G. Tenhundfeld, Plating Surf. Finish. 92 (2005) 48.
[16] P. Gupta, G. Tenhundfeld, E.O. Daigle, P.J. Schilling, Surf. Coat. Technol.
200 (2005) 1587.
[17] S.K. Sengupta, A.K. Srivastava, R. Singh, J. Electroanal. Chem. 427
(1997) 23.
[18] B. Mazza, P. Pedeferri, G. Re, Electrochim. Acta 23 (1978) 87.
[19] D.I. Slovetsky, et al., Teplofiz. Vys. Temp. 24 (1986) 353 (in Russian).
[20] P.N. Belkin, Electrochemico-Thermal Treatment of Metals and Alloys,
Moscow, 2005 in Russian.
[21] Y.N. Tyurin, A.D. Pogrebnjak, Surf. Coat. Technol. 142–144 (2001) 293.
[22] M.D. Klapkiv, Mat. Sci. 31 (1995) 494.
[23] A.L. Yerokhin, et al., J. Phys., D, Appl. Phys. 36 (2003) 2110.
[24] Y.M. Chen, Cavitation Erosion, ASM Handbook, vol. 11, Metals Park,
Ohio, 2003.
[25] Electrolytic Plasma Processing for Sequential Cleaning and Coating
Deposition for Cadmium Plating Replacement, Project WP-1406 Annual
Report to U.S. Department of Defense Strategic Environmental Research
and Development Program, December 2005 (report available from
authors).
[26] Paul J. Schilling, Paul D. Herrington, Recent Adv. Solids Struct., ASME
432 (2001) 127.
[27] R.B. Inturi, Z.S. Smialowska, Corrosion 48 (1992) 398.
[28] A. Kovalchenko, O. Ajayi, A. Erdemir, G. Fenske, I. Etsion, Trib. Int. 38
(2005) 219.
[29] J.A. McGeough, H. Rasmussen, J. Mater. Process. 68 (1997) 172.
[30] P. Molitor, V. Barron, T. Young, J. Adhes. Adhes. 21 (2001) 129.
[31] I. Etsion, G.A. Helperin, Tribol. Trans. 45 (2002) 430.
[32] I. Etsion, J. Tribol. 127 (2005) 248.
[33] G. Ryk, I. Etsion, Wear 261 (2006) 792.
[34] P. Gupta, E.O. Daigle, G. Tenhundfeld, Abstracts of the Int. Conf. on
Metallurgical Coatings & Thin Films (ICMCTF-31), San Diego, USA,
April, 2004, p. 32.
[35] S.C. Tjong, Haydn Cheng, Mater. Sci. Eng., R Rep. 45 (2004) 1.
[36] A.D. Pogrebnjak, O.P. Kul'ment'eva, A.P. Kobzev, Y.N. Tyurin, S.I.
Golovenko, A.G. Boiko, Tech. Phys. Lett. 29 (2003) 312.
[37] X. Nie, Q. Hao, J. Wei, J. Wuhan Univ. Tech. 11 (1996) 28.
8759P. Gupta et al. / Surface & Coatings Technology 201 (2007) 8746–8760

15. [38] X. Nie, C. Tsotsos, A. Wilson, A.L. Yerokhin, A. Leyland, A. Matthews,
Surf. Coat. Technol. 139 (2001) 135.
[39] S.V. Ivanov, N.S. Salmanov, M.N. Salmanov, Met. Sci. Heat Treat. 44
(2002) 405.
[40] P. Gupta, et al., Abstracts of the Int. Conf. on Metallurgical Coatings &
Thin Films (ICMCTF-32), San Diego, USA, May, 2005, p. 98.
[41] S. Usmani, S. Sampath, Wear 225–229 (1999) 1131.
[42] S.F. Wayne, S. Sampath, V. Anand, Tribol. Trans. 373 (1994) 636.
[43] S. Lak, W.R.D. Wilson, J. Lubr. Technol. 99 (1977) 230.
[44] A.M. Dolzhanskiy, V.M. Druyan, M. Golja, Metallurgija 40 (2001) 5.
[45] S. Sheu, L.G. Hector, O. Richmond, J. Tribol. 120 (1998) 517.
[46] D.G. Manning, Constr. Build. Mater. 5 (1996) 349.
[47] B.O. Aronsson, J. Lausmaa, B. Kasemo, J. Biomed. Mater. Res. 35 (1997)
49.
[48] Y. Yang, K.-H. Kim, J.L. Ong, Biomaterials 26 (2005) 327.
[49] J.A. Epinette, M.T. Manley, J.A. D'Antonio, A.A. Edidin, W.N. Capello, J.
Arthroplast. 18 (2003) 140.
[50] X. Nie, A. Leyland, A. Matthews, Surf. Coat. Technol. 125 (2000) 407.
[51] M. Bielawski, Surf. Coat. Technol. 179 (2004) 10.
[52] O.A. Abu-Zeid, R.I. Bates, Surf. Coat. Technol. 86–87 (1996) 526.
8760 P. Gupta et al. / Surface & Coatings Technology 201 (2007) 8746–8760