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The role of cathodic current in plasma electrolytic
oxidation of aluminum: Phenomenological concepts
of the “soft sparking” mode
Aleksey B. Rogova,b,c *, Aleksey Yerokhina, Allan Matthewsa
a) School of Materials, The University of Manchester, Manchester, M13 9PL, UK
b) Nikolaev Institute of Inorganic Chemistry, Novosibirsk, Russia, 630090
c) Scientific and Technical Centre “Pokrytie-A”, Novosibirsk, Russia, 630015
Abstract
A comprehensive analysis of experimental data relating to so-called “soft sparking” mode of
plasma electrolytic oxidation (PEO) has been undertaken. The transition to the soft sparking
mode is accompanied by a number of characteristic effects, such as a decrease in anodic voltage,
acoustic and light emission, increase in hysteresis in transient current-voltage curves, improved
uniformity of the discharge distribution on the surface; disappearance of atomic lines and
development of continuous radiation in the optical emission spectra. An explanation of the main
features of PEO process operated under soft sparking conditions is proposed assuming the
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existence of a specific narrow region in the coating thickness, where the main anodic voltage
drops. Because of high electric field in this “active zone”, both anodic oxidation of the metal
substrate and high-energy processes may take place. According to this assertion, the soft
sparking mode of PEO is caused by cathodic polarization a) eliminating the potential barrier at
the oxide-electrolyte interface due to local acidification and b) increasing electric field at the
metal-oxide interface during subsequent anodic half-cycle due to narrowing of low-conductive
part within the active zone. Based on this consideration it is possible to account for the main
characteristic phenomena accompanying the PEO process on aluminum under alternating
polarization.
Keywords: plasma electrolytic oxidation, aluminum alloy, alternating current, soft sparking
mode.
* - corresponding author.
School of Materials, The University of Manchester, Manchester, M13 9PL, UK
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Introduction
Plasma electrolytic oxidation (PEO) is an electrochemical method of surface treatment at high
anodic potentials (up to 1000 V) 1–3, resulting in the appearance of confined current channels –
microdischarges, due to local dielectric breakdown in the oxide film. Thermal and electrical
conditions in the coating during the PEO process promote high temperature phase
transformations of the coating material. On the surface of aluminum alloys, such conditions lead
to the formation of the α-Al2O3 phase. As a result, PEO coatings often have high wear and
corrosion resistance.
Historically, surface treatments under sparking conditions were initially carried out under
anodic biasing at higher voltages 4–6. These conditions resulted in non-uniform porous coatings.
Later, interruption of anodic polarization and introduction of cathodic pulses were found to
achieve better coating quality. Finally, it was found that the most dense and hard layers on Al are
formed under alternating current, with the average cathodic current density exceeding the anodic
one. Detailed discussion of other reasons caused the cathodic current addition can be found
elsewhere 7.
An interesting phenomenon during PEO of aluminum alloys under alternating polarization is a
transition of the process to so-called “soft sparking” mode. This allows hard, thick and dense
coatings enriched with alpha alumina to be obtained at near ambient bulk substrate temperature.
Utilization of ecologically friendly electrolyte solutions in typical PEO processes is an additional
advantage of this technique. Currently, there is a large amount of dispersed experimental data
describing specific features of PEO treatments under the soft sparking mode. Unfortunately, the
absence of a fundamental understanding of the PEO process, particularly the soft sparking mode
(even on a phenomenological level) is a main limiting factor for its industrial application.
3
Literature analysis has shown that the main specific features accompanying the transition to the
soft PEO mode are as follows:
a) decrease of anodic voltage 8–13;
b) decrease of acoustic emission 12,14–16;
c) decrease of light emission from discharge 10,15,17,18;
d) a more uniform discharge distribution over the sample surface 9–11,15,19,20;
e) changes in spectral properties of the light emitted by discharges 15,18;
f) hysteresis in transient current-voltage curves 13,16,21,22.
At the same time, the following improvements can be achieved in PEO coatings due to the
suitable soft sparking mode:
a) roughness reduction 9,23;
b) increase in the relative thickness of the inner dense region 11,12,15,23–28;
c) increase in coating thickness uniformity 15,19,25,29;
d) increase in average microhardness 23,29–31;
e) porosity reduction 15,20,24,29,31;
f) increase in coating adhesion 32,33.
A few mechanisms have been suggested regarding the effects of cathodic current on the metal-
oxide-electrolyte (MOE) system during PEO of aluminum. Timoshenko et al., 22,34 assumed that
the cathodic current causes decrease of the space charge region, which was developed under
anodic polarization. This effect leads to an increase in coating electronic conductivity thus
promoting hydrogen evolution and alkalization of adjacent electrolyte regions. Subsequent
anodic discharges occur in regions enriched with hydrogen, which may cause an increase in
plasma temperature, promoting dehydration and phase transformation of hydrated alumina.
4
Malyshev 13 suggested that transition of the MOE system (which can be treated as an open
system in terms of thermodynamics) to the soft PEO mode is accompanied by self-organization
producing a “dissipative structure” with minimum entropy production. Terleeva et al. 35
suggested that cathodic current is responsible for formation of “…specific amorphous oxide-
hydroxide layer under initial coating”. Subsequent anodic discharge causes transformation of
polymeric chains in boehmite and hydrargillite into crystalline alumina mainly due to heating.
Duan et al. 36 assumed possible dissolution of oxide under cathodic polarization. Gebarowski et
al., carried out the most comprehensive experimental study of the soft sparking mode on
aluminium alloy 9,27. They suggested the “barrier layer” is destructed by cathodic hydrogen
evolution “…forming some defects in its continuity, which facilitates passing charge and mass
transfer towards metal/oxide interface during the anodic cycle”27. Fatkullin et al. 37 have shown
that cathodic current pulses are characterized by positive phase shifts in impedance spectra
(inductive response), which may reflect either cathodic deposition of electrolyte species or
cathodic gas evolution. Shashkov et al. 38 assumed that resistive heating during cathodic
polarization might increase diffusion of ions in the coating and assist formation of grains as well
as crystallization. Rogov et al. 7 suggested that the cathodic current causes an increase in coating
conductivity in the anodic direction due to cation incorporation into the alumina phase interior or
adsorption on its boundaries. It was also found that such a highly conductive state of the coating
has a life-time of at least 10 ms. Martin et al. 17 suggested that the cathodic current influences
mainly the structure of a “double layer” (separated H+ and OH-) at the oxide/electrolyte interface.
They also mentioned that degree of separation (accumulated charge) is affected by duration of
cathodic polarization and its amplitude. Therefore, subsequent anodic polarization has to
discharge the double layer first, only then a breakdown may occur.
5
Unfortunately, the mechanisms mentioned above, describing some particular behaviors, are not
consistent with each other and are unable to explain the main sets of experimental data
simultaneously.
In this work, an attempt was made to summarize already known information and present some
new data concerning the soft sparking mode. The main tenet of this discussion comprises a
systematic consideration leading to a self-consistent concept, which would enable dispersed data
to be linked together and represented in a coherent model.
Experimental
Substrates, in the form of disks 10 mm in diameter, were made of A2024 and A1050
aluminum alloys with thickness of 1 mm and 0.2 mm respectively. The sample holder had a
round hole 9 mm in diameter, providing working area of 0.635 cm2. Two electrolyte solutions
were prepared using the following reagents: Na2B4O7·10H2O, KOH, Na2SiO3·5H2O, acetic acid
glacial and distilled water. All reagents were chemically pure (>99.5%). The silicate-alkaline
electrolyte (“Si”) has been chosen as a typical solution for PEO of Al, it contained 0.017 M KOH
and 0.05 M Na2SiO3. Unfortunately, such electrolyte is not suitable for experiment with
acidifications due to decrease in pH may cause precipitation of silicic acid. Therefore, borate
buffer electrolyte (“B”) contained 0.025 M Na2B4O7 has been chosen for experiments with
acidification. Electrolyte acidification was carried out with dilute acetic acid solution (4.88 g /
100 ml). The electrochemical cell had volume of 250 ml and was made from stainless steel and
equipped by quartz window. The power supply provided three current modes (f = 50 Hz):
alternating current (AC) alone or with additional pulse trains of cathodic (C) and anodic (A) half-
cycles (Fig.1). The process parameters are listed in Table 1. A Rigol DS1054 4-channel
oscilloscope was used for synchronous acquisition of cell voltage, current and light emission
6
during the PEO process. Temporal dependencies of anodic and cathodic voltage amplitudes and
transient current-voltage curves (CVC) were recorded with PC based analog-to-digital converter.
The light emission was measured using a BPW21 photodiode operated in a current generator
mode. Microstructural investigations were conducted in dark field using a PMT-3 microscope
equipped with a 5Mpixels CMOS camera. Discharge appearance was captured using an Olympus
C4000 digital camera.
Before presenting and analyzing the experimental data, it should be noted that electrical and
optical parameters of the PEO process considered here reflect integrated characteristics obtained
over the whole sample surface. For instance, a gradual or smooth transition between two
different stages may be often associated with the simultaneous existence of these stages in
various locations on the sample surface caused by non-uniform current density distribution
determined by the cell geometry. Since local variations of processing conditions across the
sample surface cannot be discriminated, all subsequent diagrams, schematics and theoretical
considerations can be reduced to one-dimensional situation.
Table 1. Electrical parameters of PEO processes. R - negative to positive averages current
densities ratio; JAC, JC, JA – average current densities in AC, C and A pulses, respectively; tAC, tC,
tA – durations of AC, C and A pulse trains, respectively.
Mode # Electrolyte R JAC,
mA/cm2tAC, ms
JC, mA/cm2
tC, ms
JA, mA/cm2
tA,
msProcess
duration, min
1
2
3
4
B
B
Si
Si
1.000
0.000
1.045
1.042
100
-
100
100
-
-
140
3200
-
-
11
112
-
-
60
400
-
100
-
75
-
-
-
400
40
40
90
60
7
Fig.1. Schematic illustration of current mode containing symmetrical AC 50 Hz signal combined
with cathodic (C) and anodic (A) half-cycle trains. Notations are provided for mode #4.
Results and discussion
1. Factors influencing effective conductivity of coating in anodic direction. As discussed in the
Introduction, the transition to the soft PEO mode at certain coating thickness is accompanied by
an increase in effective conductivity, which causes either a reduction in anodic voltage amplitude
in quasi-galvanostatic mode (average current in the half-waves of every polarity is kept at the
same magnitude) or an additional increase in current if power supplies with no current
stabilization are used. This effect indicates increase of process efficiency and may bring
significant advantages in applications.
It has been shown by numerous researchers that the potential barrier for charge transfer during
the PEO process strongly depends on the presence of either passivating (e.g. sodium silicate 39,40)
or dissolving (e.g. Trilon-B, EDTA 20,41) components in the electrolyte composition as well as its
pH40,42–46. In those studies, a stable increase in anodic voltages and the appearance of light
emission may be used as indicators of potential barrier existence. It was found that the structure
of the electrical double layer (EDL) depends on pH value in respect to isoelectric point (IEP) of
the oxide material as well as on electrode polarity 47, particularly in PEO 46. Our recent work has
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demonstrated 7, that in alkaline solutions, water molecules, coordinated by singly charged
cations, can produce protons by dissociation in electric double layer under cathodic polarization,
which then influences the local pH in the vicinity of the electrode surface.
The isoelectric point of aluminum oxide is estimated at pH of about 9.1 47. It is known that
PEO treatments of Al alloys at pH < 9.1 usually result in weak sparking and/or significant
substrate etching. Thus these conditions are inappropriate for investigation of the soft sparking
PEO mode, the transition to which requires coating with a specific thickness 12,15,18,27,48. Therefore,
the influence of electrolyte acidification on charge transfer during PEO treatment was studied in
the following manner.
The PEO processing of A1050 alloy was started in the electrolyte containing 0.025 M Na2B4O7
with pH = 9.33 under AC (mode #1) or anodic half-wave (mode #2) polarization conditions.
After 20 min of treatment, without interrupting the process, a gradual electrolyte acidification
was performed by addition of appropriate portions of dilute acetic acid every one or two minutes,
Fig.2. Acidification steps and corresponding electrolyte pH values are marked 1 to 10 in the
Figure. For comparison, the voltage behavior in the reference PEO processes without electrolyte
acidification are also depicted.
It can be seen that the first few portions of acid have no noticeable effect in the anodic voltage
amplitudes both in the anodic half-wave (Fig.2a points #1,2) and in the AC current modes
(Fig.2b points #1,2,3). However, initial acidifications cause an increase in the cathodic voltage
amplitude in respect to the reference levels. Moreover, these acidifications result in increase of
characteristic hysteresis in CVCA under AC current mode, mainly due to increase of conductivity
on upward branch, that can be noticed between the time points t1, just before acidification, and t2,
after two portions of acetic acid have been added (Fig.2d). It should be noted that similar
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acidification in the absence of cathodic polarization did not lead to the hysteresis in CVCA
(Fig.2c, times t1-t3).
Further acidification in both AC and anodic modes causes steep voltage drops, increases in
anodic current in both upward and downward branches (Fig.2c times t4, t5; Fig.2d times t3-t5) as
well as discharge disappearance. Finally, after 40 min of PEO treatment, significant pitting
corrosion was observed for both coatings. Later destructive effect may be attributed to
depolarization of oxide surface due to specific adsorption of acetate anions. However, the effects
of initial acidification may be attributed to decreases of pH. The reasons for this assumption are
as follows. First, depolarization of electrode must reduce voltage drop, whereas increase or stable
value of voltage amplitudes can be clearly seen during initial acidification (in respect to
reference). Second, the depolarization is expected to affect both upward and downward branches
of CVCA, however, initial acidifications have noticeable effect only to the upward branch,
whereas downward branch is kept almost the same (Fig.2d time t2).
10
Fig.2. Electrical parameters of PEO treatments of A1050 alloy in 0.025 M Na2B4O7 electrolyte
with acetic acid additions. Voltage-time charts (a,b) and transient CVC (c,d) during treatments in
anodic half-wave, mode#2 (a,c) and AC, mode#1 (b,d) current modes. Reference curves (a, b)
correspond to the PEO treatment in the electrolyte without acid addition. The numbers 1 to 10
indicate moments of time when acid portions were added, corresponding pH values are shown in
the inset table.
To explain the influence of lowering pH on the PEO process we will refer to a schematic
structure of EDL (Fig.3). The figure illustrates that the structure of EDL compact layer strongly
11
depends on electrolyte chemical composition due to the extremely high electric field within the
compact layer (up to 1 MV/cm), whereas the properties of the diffuse region outside the outer
Helmholtz plane (OHP) are mainly controlled by the external polarization. Nomine et al. 46, have
shown that discharge phenomena during PEO treatments of Al occur mainly when either external
electric field (denoted by red arrows in Fig.3) is opposed to the internal field within the compact
layer (blue arrows in Fig.3) or in the case of surface charge neutralization at pH ≈ IEP.
The EDL structure of the PEO coating on aluminum in an electrolyte with pH > IEP (typical
for PEO alkaline media) is shown schematically in figures 3a and 3c under anodic and cathodic
polarization, respectively. Under such pH conditions, oxide surface tends to be deprotonated,
forming a negatively charged layer, which must be compensated by a positively charged layer of
cationic species (Cat+). Under anodic polarization (a), internal and external electric fields are
opposite therefore, at a certain voltage, breakdown may occur. Under cathodic polarization (c),
the field in the compact layer (internal field) and the field caused by external power supply
coincide and no discharges could be observed at typical levels of current densities in PEO.
Acidification of the electrolyte causes rearrangement of the EDL structure. Namely, the net
oxide surface charge is neutralized at pH ≈ IEP and further acidification (pH < IEP) leads to
positive surface charging (protonation), which is compensated by corresponding anion species.
As a result, we can see that the internal electric field becomes opposed to that in alkaline media.
A potential barrier appears under cathodic polarization (Fig.3d), but it disappears under anodic
polarization (Fig.3b).
12
Fig.3. Schematic representation of oxide surface under different polarization conditions and
electrolyte pH in respect to oxide IEP; based on data from 46,47. Blue arrows indicate the internal
field within the EDL compact layer, red arrows indicate field induced by external polarization.
All electrolyte species are implied as solvated. Detailed structure of diffuse layer is not shown.
Such behavior corresponds to the experimental results shown on Fig.2b. At the moment of
time t2, the cathodic voltage amplitude is higher than that in the reference process, which is in
agreement with appearance of potential barrier shown in Fig.3d. Under anodic polarization,
acidification results in lowering of the potential barrier as well as depolarization of the electrode,
manifested as a hysteresis in the CVCA curve (t2 in Fig.2d).
It is worth noting that both the prior cathodic polarization and the bulk electrolyte acidification
have similar effects on the anodic electrical characteristics of the PEO processes. It becomes
clear from comparison of CVCA in the acidified electrolyte (Fig.2b, t2) and during transition to
the soft sparking PEO mode (See Fig. 2d, “Na+” in ref. 7). Both curves have similar hysteresis
due to higher conductive state on upward anodic curve.
2. Cathodic reactions.
13
In general, cathodic polarization may cause both an increase and a decrease of local pH
depending on electrode reactions taking place. If the surface layer can support electronic current
of such a level that the hydrogen evolution reaction (HER) becomes limited by migration and
diffusion of reacting species from the bulk of electrolyte, the solution in the vicinity of the
electrode may be more basic due to the decrease in proton concentration (1) or increase of
hydroxyl concentration (2):
H+(ads.) + e = 1/2H2 (gas.) (1);
H2O(ads.) + e = 1/2H2(gas.) + OH-(solv.). (2);
If the electrode tends to be polarized in respect of HER (i.e. the electrode reaction is kinetically
limited), then external electric field will cause the appearance of the positive space charge
(diffuse layer) due to the increase in cation concentration (acidification) in the vicinity of the
electrode.
We may assume that until the coating thickness has reached a specific value it possesses
sufficiently high electronic conductivity and the solution in the vicinity of the electrode is
alkalized due to hydrogen gas evolution, so cathodic current would pass with no considerable
obstacle (Fig.3c). However, subsequent anodic pulse, which will occur in the alkalized medium,
would face a strong potential barrier characteristic to typical alkaline electrolytes in PEO
treatments (Fig.3a). Further, coating thickening may cause a decrease in electronic conductivity
(at least due to the geometry factor), which will hinder the cathodic HER and the adjacent
environment would become more acidic. As discussed above, electrolyte acidification caused
depolarization accompanied by an increase in CVCA hysteresis, which is a characteristic feature
of transition to the soft sparking PEO mode. We may conclude here that this transition should
occur when the electronic conductivity of the surface oxide layer changes and the HER is
14
replaced by proton accumulation in the vicinity of the working electrode. Therefore, prior to the
transition to the soft sparking mode, the anodic behavior of metal-oxide-electrolyte system
would be similar to that under DC (See Fig.1 in ref.39; Fig.1 in ref.49; Fig.1 in ref.50) or AC
conditions with various R (See Fig.5 in ref.35; Fig.2 in ref.12; Fig.2 in ref.9; Fig.1 in ref.17).
Unfortunately, it is impossible to account for the soft sparking mode as the only effect of oxide
surface acidification. For instance, whilst it is clear that the increase in the anodic current density
at the same R would lead to earlier transition to the soft sparking mode due to the earlier
achievement of the specific thickness, it is unclear why transition to the soft mode occurs earlier
at higher R-values at the same anodic current density. Local surface acidification is expected to
occur independent of sub-layers if they can sustain a suitable cathodic current. We consider that
local surface acidification may play an important, but not a sole role.
3. Non-barrier type growth. It is well known that conventional growth of anodic oxide films is
accompanied by corresponding increase in applied voltage 51 required to sustain a suitable
electric field driving the ion migration which provides formation of new oxide layers. The anodic
type of coating growth obeys a linear dependency between applied voltage and coating thickness
for barrier type films.
Formation of PEO coatings in DC or AC modes is also accompanied by gradual voltage
increase, although this does not obey a linear relationship between thickness and voltage. The
voltage behavior during PEO process can be divided into two main parts. An initial voltage
increase occurs for a short time period in the beginning of the process (0-1 min) without the
discharges. It should be noted that the coating thickness at the end of this dischargeless stage
usually does not exceed a few microns; however, the anodic voltage may reach 300 to 500 V
15
depending on electrolyte composition. The second part of the PEO process is associated with the
main coating growth under sparking conditions from microns to a few hundreds of microns. This
is usually accompanied by a gradual increase in anodic voltage by a few tens of volts or even by
a decrease in voltage if special process modes, such as soft sparking, are used. It is clear that the
main coating growth requires only slight voltage changes and can be regarded as a “non-barrier”
type of growth.
As discussed earlier, one of the specific features attributed to the transition to soft sparking is a
reduction of anodic voltage amplitude in quasi-galvanostatic modes at a specific coating
thickness. Such voltage behavior reflects an increase in effective conductivity of the metal-
oxide-electrolyte system.
Fig. 4 shows schematically anodic voltage behavior and corresponding coating thickness
evolution for PEO processes operated in typical microarcing (R < 1) and the soft sparking
(R > 1) modes 9,10,12,15. It is known that until a specific thickness HST (for a given set of initial and
boundary conditions) is reached the common voltage and thickness behaviors are independent of
the R value 12,15,18,27,48. Then if a current mode with appropriate R is applied, the anodic voltage
begins to decrease (ΔU < 0) at a certain moment of time (tST) and the PEO process undergoes a
transition to the soft mode. It was found that the increase in R causes an increase in ΔU
magnitude and a decrease in tST 9,10,12. We can see that the main growth of the coating thickness
(H0 – HARC or H0 – HSOFT) occurs in the specific region of cell voltage (U0 – UARC or U0 – USOFT)
including the process accompanied by the voltage reduction due to a transition to the soft
sparking mode at tST. In spite of significant voltage drop (up to 30%), the coating thickness
continues to increase up to the termination point of the process. It is worth noting that the anodic
16
voltage amplitude weakly correlates with current density 40. It is clear that conventional anodic
barrier film growth mechanism is insufficient to describe the above-mentioned case.
Fig.4. Schematic representation of anodic voltage amplitude and coating thickness evolution
during PEO of aluminum alloy; based on data of 9,10,12,15.
The initial sharp and subsequent gradual voltage changes appear to reflect formation of the
specific coating region, which supports the main voltage drop. This region is created during the
initial stage of PEO process (0 – t0) and then remains almost unchangeable or changes slightly up
to time tST (in comparison with total coating thickness). After the transition to the soft mode
occurrs, the properties of this region are affected by the value of R.
4. Active zone concept. We assume that in such a region the main electrochemical reactions
and other high energetic processes (luminescence, breakdown and dissociation) take place due to
17
the strongest electric field. Therefore, we call this region the “active zone”. Based on this
assumption we denote the rest of the coating as a “product zone” where the main coating
material is accumulated. We should define where the active zone could be situated within the
coating thickness.
It is known that primary reaction in any PEO process is electrochemical oxidation of substrate
resulting in oxide layer formation. Such a process requires high field conditions at the metal-
oxide interface, otherwise the oxidation will stop once the film thickness corresponds to that of
the native oxide (a few nm). Therefore, one boundary of the active zone should lie at the metal-
oxide interface, and the active zone would be situated beneath the main part of coating. Figure 5
shows schematic representation of the active zone location in the thick PEO coating.
Fig. 5. Schematic view of the active zone concept.
It is clear that aluminum enters the active zone directly through the metal-oxide interface, but
the source of anions is not so obvious. There should be a way of electrolyte species
transportation through the above product zone (see Fig.5). In order to describe a possible
transport mechanism, it is useful to compare specific volumes of solid substances participating in
the coating formation, taking into account the fact that primary anodic oxide mainly consists of
18
an amorphous alumina 52. Thermal or electrical conditions may favor its crystallization to
metastable or stable oxide phases. The transformation path (3) of aluminum into the dense oxide
layer includes formation of the following phases (with corresponding density in g/cm3 and
relative volume in respect to Al provided in the brackets): 1) aluminum substrate (2.7, 1.00); 2)
primary amorphous alumina a-Al2O3 (2.9, 1.76); 3) secondary metastable γ-Al2O3 (3.4, 1.50) or
stable α-Al2O3 (3.99, 1.28) phases.
2Al + “3O” → a-Al2O3 → γ-Al2O3 → α-Al2O3 (3);
It is also known that the anodic oxidation process occurs in a direction normal to the substrate
surface. Therefore, the primary aluminum oxide appears as a uniform film, and the surface
volume increase transforms into film thickening following a 1D-growth mechanism. In contrast,
phase transformations within the coating cause changes in density and corresponding specific
volume of the phases. Such processes are isotropic and therefore 3D shrinking may be expected.
The shrinking results in fragmentation of the coating, causing separation of fragments by gaps,
which may form a porous system, filled with either electrolyte or a loose material permeable to
electrolyte species. Evidence of fragmentation can be observed in the coating cross-section near
to the metal region (Fig.6a,b). It can be seen that the dense layer, dark colored on the A2024
alloy, has significant fragmentation. In addition, Curran et al. 53 have reported that the dense
layer of the PEO coating produced in the AC mode possesses a “fine, interconnected porous
network” that could provide electrolyte penetration into the deep coating layers as well as could
cause “discharges … across … thin barrier near the substrate”. Direct evidence of dense layer
permeability for electrolyte species was reported by Matykina et al. in 16, where 18O isotope
markers clearly showed the exchange of electrolyte species in the dense layer up to the metal-
oxide interface and electrolyte solution (Fig.4 in ref. 16). Moreover, Matykina et al. 54 have also
19
suggested that the dense intermediate layer is permeable to electrolyte solution and “…the
voltage is therefore supported mainly by the barrier layer composed of amorphous alumina next
to the substrate”.
Fig.6. PEO coating on A2024 aluminum alloy, silicate-alkaline electrolyte, mode #3: a)
vertical cross sectional view; b) beveled cross sectional view at 5º. Fragmentation of the dark
dense layer (inset). Scale bar is 50 μm.
5. Light emission from the inner coating layer. The concept of the active zone, particularly its
location near the metal-oxide interface, can be supported by additional experimental evidence.
Martin et al. 17 have recently shown that during pulsed bipolar PEO treatment of A2214 (Al-4Cu-
Mg-Mn) alloy in the soft sparking mode, the light emission under anodic polarization appears
with a specific delay in respect to the current pulse initiation. Our experiments on PEO
treatments of A2024 alloy in an alkaline-silicate electrolyte (mode #4) showed similar behavior
of light emission during the anodic pulse train applied after the cathodic one (Fig.7a). However,
when we applied the same experimental conditions to A1050 alloy the observed results were
different (Fig.7b). The light emission had local maximum at the beginning of the anodic pulse
20
train (A), then decreased to the lowest value (B) and after that appeared again with significant
amplitude fluctuations (C)
Fig.8 shows the surface appearance of the specimen at various moments of time within the
anodic pulse train. A uniform distribution of numerous fine discharges can be observed at the
initial stage (a) after which light emission almost disappeared (b) and then a few large bright
discharges appeared (c). In the case of A2024 alloy, only discharges similar to those depicted in
Fig.8b,c could be observed, whereas no light emission appeared in the initial period of anodic
pulse train (Fig.7a), which is similar that observed by Martin et al. 17. As a result, we can
distinguish two types of light emission during anodic polarization following the cathodic one.
Fig.7. Oscillograms of current, light emission and voltage during cathodic and anodic pulse
trains in PEO of A2024 (a) and A1050 (b) alloys, mode #4, between 59 and 60 min. Points A, B
and C indicate corresponding images in Fig.8.
21
Fig.8. Sample appearance at various stages of anodic pulse train after preliminary cathodic
pulse train corresponding to the moments of time denoted as A, B and C in Fig.7b. Substrate
alloy is A1050, shutter is 1/15 s.
A plausible explanation of observed behavior and its relation to the active zone concept may
be provided as follows. It is known 12,18 that thick PEO coatings on alloys with high copper
contents develop a dark-colored dense inner layer situated between the white thin intermediate
and loose outer layer (Fig.9a). Although the precise chemical nature of the coloration is still
unclear, it can be speculated that the light emission at the initial part of the anodic pulse train
occurs in the coating layer located beneath the dense layer (near to metal), therefore it is difficult
to detect the light emission in the case of A2024 alloy, for which the dense layer is dark colored
(Fig.9a), whereas in the case of A1050 alloy (Fig.9b), a white transparent material is capable of
transmitting light from the inner coating layer. In addition, light emission at the end of the anodic
pulse train (Fig.8c) occurs independently of alloy composition, therefore it must be generated in
the top part of the dense layer, or we may assume that such discharges are visible due to the
higher specific energy causing destruction of the upper layers 15.
5 mm
22
Fig.9. Cross-sectional optical micrographs of typical PEO coatings on aluminum alloys
containing 4 wt.% of copper (A2024) and no copper (A1050, 99.5% Al).
6. Mechanism of narrowing the low-conductive part within active zone. As discussed in the
Introduction, unlike PEO processes with R < 1 that are susceptible to large destructive
discharges, the soft sparking PEO mode enables growth of thicker coatings with inner dense
layer enriched with α-Al2O3. It is clear that cathodic polarization affects not only EDL and
diffuse layer structure, but also processes in the active zone near to the metal-oxide interface.
The following consideration can be proposed based on comprehensive analysis of the above
experimental data together with extensive data available in the literature.
Figure 10 shows the main processes taking place under anodic (A1-A3) and cathodic (C1, C2)
polarization. The process A1 represents conventional anodic oxidation of aluminum
accompanied by opposite migration of cationic and anionic species under high-field conditions.
Depending on electrolyte composition, such migration may result in oxide film formation and/or
substrate dissolution accompanied by formation of soluble aluminum complexes. The process A2
is a direct electron transfer from adsorbed electrolyte species to the anodically polarized metal
23
substrate. This process becomes dominant at certain film thickness, at which electric field across
the film cannot support sufficient ionic migration. As a result, an avalanche breakdown may
occur when anionic species with appropriate energy appear at the oxide-electrolyte interface.
There is a significant difference in spatial distribution between A1 and A2. The former is a bulk
process occurring over the whole sample surface if sufficient electric field is present. The latter is
a localised process governed by statistically defined probabilities of high-energy particles
appearing at the oxide-electrolyte interface.
Cathodic process C1 is a common process of electron injection from the metal into the anodic
oxide. It is coupled with discharge of electrolyte species at the oxide-electrolyte interface, which
depending on the electrolyte composition, results in evolution of gaseous hydrogen 55 and/or
electrodeposition of reduced metal species 56. Based on the above discussion, it would be
reasonable to assume that at certain coating thickness, the proton injection could make a separate
process C2. The hydrogen incorporation is considered to be responsible for rectification in such
systems 57. In addition, anodic alumina possesses proton conductivity which can account for up
to 5% of total current in acid solutions (pH = 1) 58. This can be significantly reduced by increase
in degree of cathodic polarization, resulting in domination by electronic current up to 99.9% of
total current.
In alkaline media, water may serve as a source of hydrogen ions due to high-field dissociation
within the EDL. Inward proton migration and opposite electron current in the primary
(amorphous) alumina are accompanied by recombination yielding atomic hydrogen which can be
stabilized by surrounding oxygen ions due to formation of hydrogen bonds 59. We assume that
due to the zero net charge, such complexes (denoted as [H•]OX) may be relatively stable and long-
living within the coating. On the other hand, hydrogen in the amorphous alumina may cause n-
24
type doping 59 resulting in conductivity increase in the region enriched with [H•]OX complexes.
Formation of molecular hydrogen seems to be limited mainly by diffusion of hydrogen atoms
similar to that in γ-Al2O3 phase considered in 60.
Moreover, it is known that significant excess in cathodic current density (R ≥ 2.0) results in
formation of bubble-like coating defects and, ultimately, in debonding of the oxide layer, which
may be due formation of molecular hydrogen at the metal-oxide interface under excessively high
cathodic polarization. Notably, recent observations made by Martin at al. 17 have shown that the
coating destruction during high-R PEO treatments starts at a specific process duration (e.g. 10
min at R = 2.0), rather than at the very beginning, which is in accordance with our hypothesis of
the change in main cathodic reaction with coating thickness (sec. 2.2).
The application of subsequent anodic polarization (A3) causes dissociation of [H •]OX
complexes due to higher electric field 57. This reaction and the external electric field provide
outward proton migration through the oxide-electrolyte interface, which would cause local
acidification, changing the EDL structure from one depicted in Fig.2a to that in Fig.2b and
lowering the potential barrier. Thus, the processes C2 and A3 become coupled.
Of course, the actual mechanism may be more complex. For instance, under anodic
polarization, hydroxyl in the hydrogenated alumina could migrate towards the substrate, react
with anion vacancies at the metal-oxide interface and the released protons would then migrate
outwards. However, this should be clarified in further investigations.
25
Fig.10. Mass and charge transfer schematic under anodic and cathodic polarization of coated
aluminum in alkaline solutions.
A combination of the active zone concept, the EDL considerations and the hydrogen
accumulation mechanism is depicted schematically in Fig.11. This figure illustrates electric field
redistribution under conditions of the same potential difference applied to the metal-oxide-
electrolyte interface with different pre-history (cathodic treatment). This form of presentation
was chosen because it was easier to demonstrate this conditions in the static figures in
comparison with more common quasi-galvanostatic mode. However, there is no difference in
understanding of the narrowing process within the active zone whether we work under voltage or
current control mode, when the system has the same state at the beginning of positive pulse.
26
Without prior cathodic polarization (Fig.11a), a potential barrier exists in the EDL at the oxide-
electrolyte interface (Fig.11c) and electric field appears within the active zone (green area in
Fig.11c). Depending on the field strength, current can pass via either ionic migration or an
avalanche breakdown mechanism (Fig.10. A1 and A2 respectively). In the soft sparking mode,
the EDL structure changes due to the local acidification during cathodic polarization and the
outer part of the active zone becomes enriched with neutral hydrogen complexes [H •]OX
(Fig.11b). Subsequent anodic polarization (Fig.11d) would lead to concentration of electric field
in the narrow non-conductive region within the active zone resulted from increased conductivity
in its outer part (due to presence of [H•]OX) and diminished potential barrier in the EDL (due to
local acidification caused by escaped protons, Fig.10. A3). The acidification at the oxide-
electrolyte interface suppresses the breakdown processes (Fig.3b). Increased electric field at the
metal-oxide interface promotes ion migration (substrate oxidation, see Fig.10, A1). Moreover,
additional processes can take place, including proton outflow from the active zone and oxide
formation at the metal-oxide interface. These both will widen the non-conductive region of the
active zone without [H•]OX complexes, where the main voltage drops; therefore the electric field
at the metal-oxide interface would decrease, oxide formation would stop and the EDL changes
its structure to the initial state (Figs. 11a, 3a). If anodic polarization is still applied at that
moment of time, breakdown may occur following the mechanism A2 in Fig.10. Thus, we would
have obtained dynamic rearrangement of both the high field conditions at the metal-oxide
interface and the structure of EDL at the oxide-electrolyte interface.
27
Fig.11. Schematic representation of charge (a,b), potential and electric field distribution (c,d)
in the active zone and EDL of PEO coating under anodic polarization without (a,c) and with
(b,d) prior cathodic polarization. The symbol [H•] denotes hydrogen enriched area.
7. Examples of experimental data explanation and additional interpretations. Taking into
account dynamically arranged high field conditions at the metal-oxide interface and EDL
structure at the oxide-electrolyte interface the experimental data concerning transition of the PEO
process to the soft sparking mode can be explained as follows:
a) A decrease in anodic voltage, total optical and acoustic emission are attributed to the
decrease in potential barriers and the corresponding numbers of breakdown events as well as the
increase in the effective electrical conductivity. More uniform discharge appearance and
28
distribution coating characteristics are associated with reduction in number of localized
discharge events caused by the stochastic nature of the dielectric breakdown.
b) With insufficient prior cathodic polarization, the anodic dielectric breakdown of EDL
affects electrolyte and surface species, resulting in an appearance of characteristic lines in
discharge spectra. On the other hand, sufficient prior cathodic polarization eliminates EDL
breakdowns and causes a field increasing in a narrowed region at the metal-oxide interface,
where the substrate may be anodically oxidized under high-field conditions. It is well known that
anodic polarization of valve metals is often accompanied by continuous optical emission known
as galvanoluminescence 61. Without discussing the exact nature of this phenomenon, we may
expect similar reasons to be responsible for continuous spectra of optical emission during PEO
treatments in the soft sparking mode.
c) Higher anodic conductivity caused by prior cathodic polarization and being responsible for
the hysteresis in anodic current voltage curves, is also in accordance with the active zone
concept. In addition to EDL elimination under cathodic polarization, narrowing down the low-
conductive part of active zone due to incorporation of hydrogen complexes results in a
conductivity increase until appropriate amounts of hydrogen complexes exist. Anodic current
promotes hydrogen complex dissociation with proton evolution thus the active zone becomes
wider until it reaches its maximum. Further anodic polarization applied to the low-conductive
coating will cause dielectric breakdown by an avalanche mechanism.
d) Transition to the soft sparking PEO mode may reflect changes in the ability of PEO coatings
to support a cathodic hydrogen evolution reaction (HER). We assume that the initial film
provides a higher rate of HER and environment in the vicinity of the working electrode becomes
more alkaline, then at a certain thickness, the electron transfer becomes obstructed and the field-
29
assisted acidification takes place. The later process causes changes in the EDL structure as well
as the thickness of low-conductive part within active zone, therefore a transition to the soft
sparking PEO mode becomes possible. In the case of alkalization, the following anodic
polarization occurs under typical conditions for alkaline electrolytes, thus the anodic behavior is
not influenced by the cathodic current. This is the reason for the similarity between the initial
stages of the PEO processes at different R, including the anodic DC mode.
e) A few points should be made in regard to the α-Al2O3 formation. It is well known that
during the soft PEO process a layer enriched with α-Al2O3 may be formed. In accordance with
our considerations, the phase transformation occurs due to Joule heating during the higher
conductive state. The sample surface surrounded by a thin conductive film (active zone) is
heated up to temperatures sufficient for the γ-Al2O3 → α-Al2O3 transformation. However, the
most effective heating occurs not continuously, but only in the narrow time interval when the
average resistance of the active zone is near to the effective output impedance of the power
supply. Such conditions are usually achieved at the initial part of anodic pulse following prior
cathodic polarization, therefore the formation of the dense inner layer is the most efficient under
certain process parameters. Moreover, from own experience in industrial application of PEO,
especially when massive (e.g. 5 kg, A2024 alloy, S = 2.5 dm2) substrates were coated by a thick
hard coating (dense layer thickness ~ 200 μm, JA = 10A/dm2, 50 Hz, R = 1.15, t = 4 hours) we
have observed that the temperature of substrate is much greater than electrolyte solution (e.g.
+120 °C and +40 °C, respectively). Such a phenomenon indicates that the heat source is inside
the coating and the average heat flux directed inwards to the substrate is more intense than the
outward one, until thermal equilibrium will be reached at significant temperature difference.
30
Conclusions
In this work, concepts concerning the main specific features of plasma electrolytic oxidation
on aluminum in soft sparking mode are considered. The soft sparking mode can be described in
terms of potential barriers in the specific regions at metal-oxide (active zone) and oxide-
electrolyte (EDL) interfaces. It was suggested that cathodic polarization at certain coating
thickness could cause local acidification (in respect of the oxide isoelectric point) that leads to
both a decrease in the EDL potential barrier due to surface recharging as well as increase of the
electric field at the metal-oxide interface due to narrowing of low-conductive part within active
zone in intermediate layer of the coating. Thus, the electrical conductivity of the coating under
anodic polarization becomes higher, and the probability of discharges appearance decreases. As a
result, it is possible to sustain high field conditions at the metal-oxide interface even when
coating has considerable total thickness (up to a hundreds of microns). It should be noted that
such high field conditions are dynamically generated by alternating substrate polarization.
Therefore, temporal process parameters (e.g. pulse duration, frequency and duty cycle) should be
taken into account in the future investigations of PEO kinetics. Moreover, hydrogen complexes,
responsible for the narrowing active zone, since they have zero charge and are expected to be
relatively stable, therefore highly conductive state of the coating may be long living in scale of
prior cathodic treatment duration. Thus, the proposed concept allows the main experimental
observations to be described conjointly. However, this concept is mainly phenomenological and
further investigation is required to clarify actual mechanisms of charge and mass transfer in the
soft sparking PEO mode.
Finally, several similarities between the soft sparking PEO process and the porous anodic film
formation can be noticed. A high field conditions exist at the metal-oxide interface within both
31
the barrier layer of the anodic oxide film and the proposed active zone of the PEO coating. Initial
increases and subsequent decreases in anodic voltage are observed due to the formation of the
porous layers in anodic oxide films and the dense inner layer within PEO coatings. Another
common feature for both coatings is permeability of the upper layer for electrolyte solution
leading to penetration of electrolyte species in the vicinity of the metal-oxide interface. Although
differences in typical electrolyte compositions and polarization conditions lead to the formation
of significantly different coatings, the processes seem to have common basic principles.
Acknowledgments
Financial support from the ERC Advanced Grant (#320879‘IMPUNEP’) is acknowledged with
thanks.
32
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