Y. SHI et al.: STUDY OF THE CORROSION PROCESS OF A PEO-TREATED ALUMINUM ALLOY ...
407–414
STUDY OF THE CORROSION PROCESS OF A PEO-TREATED
ALUMINUM ALLOY IN DIFFERENT CONCENTRATIONS
OF NaCl
[TUDIJ PROCESA KOROZIJE S PEO OBDELANE ALUMINIJEVE
ZLITINE Z RAZLI^NO KONCENTRACIJO NaCl
Yuanji Shi
1,2*
, Yunzhong Dai
1,4
, Guoqiang Gao
2
, Cheng Cheng
3
, Yunyun Song
1
1
Key Laboratory of Modern Agricultural Equipment and Technology, Jiangsu University, Zhenjiang, China
2
Industrial Perception and Intelligent Manufacturing Equipment Engineering Research Center of Jiangsu Province,
Nanjing Vocational University of Industry Technology, Nanjing, China
3
College of Material Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing, China
4
Department of Modern Manufacturing Engineering, Yibin Vocational and Technical College, Yibin, China
Prejem rokopisa – received: 2022-03-21; sprejem za objavo – accepted for publication: 2022-06-28
doi:10.17222/mit.2022.452
Plasma electrolytic oxidation (PEO) treated ceramic coatings were formed in silicate-based electrolytes without and with the ad-
dition of Al2O3, on an aluminum alloy. It was found that the coating produced in an electrolyte containing 7 g/L of Al2O3 exhib-
ited the most superior corrosion properties. The corrosion properties of the coatings in 0.5 M and 1 M NaCl solutions were stud-
ied by means of potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). The results proved that the
coating can protect the substrate from the corrosion due to aggressive Cl
–
in 0.5 M NaCl after 384-h immersion. However, it
cannot protect the substrate after 384-h immersion in 1 M NaCl solution. The potentiodynamic polarization results matched well
with the EIS test results.
Keywords: aluminum alloy, plasma electrolytic oxidation, ceramic coatings, electrochemical impedance spectroscopy, corrosion
Avtorji v prispevku opisujejo kerami~ne prevleke izdelane s plazemsko elektrolitsko oksidacijo (PEO). Te so nastale na izbrani
aluminijevi (ZL101A) zlitini v silikatnem elektrolitu brez in z dodatkom Al2O3. Avtorji so ugotovili, da je prevleka z najbolj{o
odpornostjo proti koroziji nastala v elektrolitu, ki je vseboval 7 g/l Al2O3. Nadalje so {tudirali korozijske lastnosti izdelanih
prevlek s potencio-dinami~no polarizacijo in elektrokemijsko impedan~no spektroskopijo (EIS) v 0,5 molarni (M) in 1 M
raztopini NaCl. Rezultati testov so pokazali, da prevleka lahko za{~iti substrat (Al zlitino) pred korozijsko zelo agresivnimi ioni
klora (Cl
–
), ~e je le ta potopljen v 0,5 M raztopino NaCl tudi do 384 ur. Vendar pa ga ne {~iti pred korozijo, ~e je le-ta potopljen
384urv1Mraztopini NaCl. Rezultati potencio-dinami~ne polarizacije so se dobro ujemali z rezultati EIS testov.
Klju~ne besede: aluminijeva zlitina, plazemska elektrolitska oksidacija, kerami~ne prevleke, elektrokemijska impedan~na
spektroskopija, korozija
1 INTRODUCTION
Aluminum and its alloys are widely used in industrial
engineering due to their superior properties, that is, low
density, great specific strength and good ductility. Never-
theless, their shortcomings, i.e., low surface hardness
and poor corrosion resistance, have extremely limited
their wide applications in many industrial fields.
1,2
In recent years, various surface-modification methods
have been proposed to prevent aluminum alloys from
corrosion in a severe industrial environment. Among
these great techniques, plasma electrolytic oxidation
(PEO) seems to be the most favorable surface-treatment
method since the year 2000.
3,4
It is a technique used at
high voltages to grow an oxide coating on an aluminum
alloy surface. During a PEO process, the substrate alloy
is the anode, while the gas layer enshrouding the surface
of the alloy consists of oxygen. Specifically, when the di-
electric gas layer completely covers the anode surface,
numerous sparks appear, accompanied by the gas
bubbles of the PEO treatment, provided that the applied
voltage is constantly higher than the breakdown voltage.
Then, a ceramic coating can be formed on the metal sur-
face during the chemical reactions in a plasma environ-
ment. The coatings produced with the PEO treatment are
of good corrosion resistance.
5–8
Therefore, the PEO
method has been popularly used in these years to pro-
duce coatings with superior corrosion properties on alu-
minum alloys.
According to previous researches, it is acknowledged
that the PEO treatment is a multifactor-controlled pro-
cess.
9
The properties of PEO coatings are determined by
many influencing factors, such as the substrate mate-
rial,
10
electrical parameters,
10–13
oxidation time,
14,15
additives
8,16–18
and the main electrolyte composition.
19
Generally, silicate-based electrolytes are the most widely
employed electrolytes. However, there are only a few
studies of the corrosion of the coatings formed on alumi-
num alloys with the PEO method. Thus, in this study, we
want to comprehensively study the corrosion properties
Materiali in tehnologije / Materials and technology 56 (2022) 4, 407–414 407
UDK 669.715.018.8 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 56(4)407(2022)
*Corresponding author's e-mail:
yuanji_shi@hotmail.com (Y. J. Shi)

of PEO coatings formed on ZL101A aluminum alloy in
silicate-based electrolytes.
In this work, PEO coatings were formed on ZL101A
aluminum alloy in silicate-based electrolytes with the ad-
dition of Al
2
O
3
. Then, potentiodynamic polarization and
electrochemical impedance spectroscopy (EIS) were car-
ried out to study the long-term immersion corrosion pro-
cess of the coatings in both 0.5 M and 1 M NaCl solu-
tions. X-ray photoelectron spectroscopy (XPS) was used
to study the corrosion products of the coatings after im-
mersion in the NaCl solution.
2 EXPERIMENTAL PART
The material was ZL101A aluminum alloy cut form
an Al sheet. Its nominal composition is as follows: Mg
(0.45–0.7 w/%), Si (6.5–7.5 w/%), Zn (0.07 w/%), Fe
(0.19 w/%), Cu (0.05 w/%) and balance Al. Cylinder
specimens (with a thickness of 3 mm and diameter of
40 mm) were used as the substrate in this study. To en-
sure their smoothness, the raw specimens were ground
with 200, 500, 800, 1200 grit alumina waterproof abra-
sive paper. After that, they were ultrasonically degreased
in ethanol for 20 min, rinsed in distilled water and dried
in ambient air.
For the PEO process, a specimen was registered as
the anode and a stainless-steel barrel measuring 200 mm
in diameter and 400 mm in length was used as the cath-
ode. The specimen was set at the center of the circle in
the barrel with the electrolyte. A silicate-based aqueous
solution (Na
2
SiO
3
(18 g/L) + KOH (3 g/L) + NaF
(4 g/L)) was used as the electrolyte. The concentration of
additive Al
2
O
3
particles (< 20 nm) in the electrolyte solu-
tion was varied from 3 g/L to 9 g/L with +2 g/L inter-
vals. PEO coatings were formed under a constant current
density of 150 mA/cm
2
over 40 min. Next, the barrel was
placed in cold water with a water-cooled system to keep
the temperature below 35 °C all the time during the PEO
treatment. After this treatment, the PEO-treated samples
were rinsed in deionized water and dried in hot air.
Electrochemical experiments were performed in the
NaCl aqueous solution (pH of app. 7.0) at room tempera-
ture (app. 25 °C) using an electrochemical system
(CS350, Wuhan Corrtest, China). Along with the system,
a three-electrode cell was used, composed of a weight-
saving platinum electrode, a saturated calomel electrode
(SCE) and the uncoated or coated specimens. They were
used as the auxiliary electrode, reference electrode and
working electrode, respectively. The working electrode
was made by inlaying pieces of uncoated and coated
square samples (about 1 cm
2
) into an in-house-made Tef-
lon plate, full of resin. The testing electrolyte used was
0.5 M or 1 M NaCl aqueous solution. Prior to the elec-
trochemical test, the testing specimens were immersed in
the NaCl solution for about 30 min to get a stabilized
open circuit potential. The potentiodynamic polarization
curves of the samples were obtained using an applied
scanning rate of 1.0 mV/s and a scanning region of
–1.0 V to 0 V with respect to the open circuit potential.
CorShow was used to deal with the data of the
potentiodynamic polarization test. Electrochemical im-
pedance spectroscopy (EIS) was applied in the
potentiostatic mode within a frequency range of 10
5
Hz
to 10
–1
Hz with a perturbation amplitude signal of
10 mV. The specimens were exposed to the NaCl solu-
tion for different durations, i.e., (0.5, 24, 48, 96, 192 and
384) h. The EIS plots obtained were analyzed using
ZSimpWin3.10. All the tests were repeated at least three
times to ensure reproducibility and reliability.
The surface and cross-sectional morphology of the
coatings were observed using scanning electron micros-
copy (SEM, ISM-6510). The thicknesses of the coatings
were determined by measuring the cross-sections of the
coatings. In addition, X-ray photoelectron spectra (XPS)
were obtained with a Thermo Escalab 250Xi photoelec-
tron spectrometer using an AlK
 source with energy of
1486.6 eV. The surfaces of the coatings were etched for
1 min with an argon-ion beam before the XPS analysis to
reduce the carbon contamination. Binding energies were
corrected relative to the C 1s signal at 284.6 eV. The
analysis of the obtained data was processed by the
Xpspeak 4.1 software.
To evaluate the effect of the Al
2
O
3
concentration on
the corrosion resistance of the PEO coatings formed in
silicate-based electrolytes, potentiodynamic polarization
tests were carried out, identifying the optimum Al
2
O
3
concentration for the PEO process. The obtained polar-
ization curves of the PEO coatings are depicted in Fig-
ure 1. This figure shows that the coating fabricated in the
electrolyte with 7 g/L of Al
2
O
3
had the lowest corrosion
current density (i
corr
) and highest corrosion potential
(E
corr
) among the five examined Al
2
O
3
concentrations.
Therefore, further investigations were focused on this
concentration.
Y. SHI et al.: STUDY OF THE CORROSION PROCESS OF A PEO-TREATED ALUMINUM ALLOY ...
408 Materiali in tehnologije / Materials and technology 56 (2022) 4, 407–414
Figure 1: Potentiodynamic polarization curves of the aluminum alloy
with PEO coatings formed in electrolytes containing (0, 3, 5, 7 and
9) g/L Al
2
O
3
and immersed in 0.5 M NaCl for 0.5 h

3 RESULTS AND DISCUSSION
3.1 Morphology of PEO coatings
Figure 2 shows the surface and cross-sectional mor-
phologies of the PEO coatings formed in the sili-
cate-based electrolyte under the current density of
150 mA/cm
2
for 40 min. The surface morphology of the
PEO coating formed in 7 g/L Al
2
O
3
containing electro-
lyte is shown in Figure 2a. Numerous discharge chan-
nels and regions resulting from the rapid cooling of mol-
ten materials are presented on the surface (see
Figure 2a). At the discharge channel sites, the tempera-
ture could suddenly increase to approximately 10
4
K,
20
which was high enough for the coating to melt. The mol-
ten alumina flowed out of the channels and the anionic
components such as SiO
3
2 −
enter the channels making
sharp and distinctly visible boundaries at the same time.
And due to the rapidly cooling system, the molten alu-
mina solidified immediately. The above process resulted
in a coating with numerous micropores on its surface.
The pore size ranged from 1–6 μm. The surface view ob-
tained in this study shows that the coating has a rela-
tively dense structure without considerable cracking. The
presence of some pores is expected because of the nature
of the PEO process.
The cross-sectional morphologies of the PEO coat-
ings produced in the silicate-based electrolyte with (0, 5
and 9) g/L of Al
2
O
3
are shown in Figures 2b, 2c and 2d.
The thickness of the coatings formed in (0, 3, 5, 7 and
9) g/L Al
2
O
3
containing electrolytes is approximately
(26, 32, 35, 39 and 38) μm, respectively. The link be-
tween the coating and the substrate is without any crack-
ing, which indicates good adhesion of the coating to the
substrate.
3.2 XPS analysis before corrosion
The effects of additive Al
2
O
3
on PEO coatings was
estimated on the basis of the chemical state of the sur-
face of the coating. The XPS analysis of the PEO coat-
ings formed in the silicate-based electrolyte without and
with the addition of 7 g/L Al
2
O
3
is shown in Figure 3.
This figure presents the Al 2p core level spectra of the
PEO coatings formed in the Al
2
O
3
free and 7 g/L Al
2
O
3
containing electrolytes. The binding energy of Al 2p of
the PEO coatings formed in these two electrolytes is
74.28 eV and 74.36 eV, respectively. The difference in
the binding energy of the coatings may be owing to the
incorperation of additive Al
2
O
3
. The fact that the Al 2p
spin-orbit components of the coating produced in 7 g/L
Y. SHI et al.: STUDY OF THE CORROSION PROCESS OF A PEO-TREATED ALUMINUM ALLOY ...
Materiali in tehnologije / Materials and technology 56 (2022) 4, 407–414 409
Figure 2: PEO coating morphology: a) surface (7 g/L Al
2
O
3
containing electrolyte), b) cross-section (without Al
2
O
3
), c) cross-section (5 g/L
Al
2
O
3
), d) cross-section (9 g/L Al
2
O
3
)

Al
2
O
3
containing electrolyte are shifted towards higher
binding energy than the coating produced in the Al
2
O
3
free electrolyte suggests that some of the additive Al
2
O
3
was incorporated into the coating. Figure 3b shows the
high-resolution spectrum of Al 2p. It is presented that the
single Al 2p peak at 74.36 eV, typical for  -Al
2
O
3
and
 -Al
2
O
3
, can be decomposed into two peaks at 74.70 eV
and 74.20 eV.
3.3 Corrosion behavior of PEO coatings
3.3.1 Potentiodynamic polarization
The potentiodynamic polarization curves of the
PEO-coated specimen formed in 7 g/L Al
2
O
3
containing
electrolyte after the immersion times of (0.5, 24, 48, 96,
192 and 384) h in 0.5 M and 1 M NaCl aqueous solu-
tions are shown in Figure 4. In general, a more noble
corrosion potential (E
corr
) and lower corrosion current
density (i
corr
) represented a lower corrosion rate and good
corrosion properties. Figure 4 shows that the corrosion
potential (E
corr
) first decreased before the immersion for
48 h, then increased after 48 h and decreased again after
the immersion for 96 h. Nevertheless, the corresponding
corrosion current density (i
corr
) first increased, then de-
creased and increased in 0.5 M NaCl aqueous solution.
Meanwhile, the same changes occurred to the coating,
i m m e r s e di n1MN a C la q u e o u ssolution during the
potentiodynamic polarization test. This indicated that the
corrosion resistance of the PEO-coated specimen first
decreased, then increased and finally decreased in both
0.5 M and 1 M NaCl solutions. Furthermore, in both
0.5 M and 1 M NaCl solutions, the corrosion current
density of the PEO coating formed in 7 g/L Al
2
O
3
con-
taining electrolyte was lower by about 30 orders than the
Y. SHI et al.: STUDY OF THE CORROSION PROCESS OF A PEO-TREATED ALUMINUM ALLOY ...
410 Materiali in tehnologije / Materials and technology 56 (2022) 4, 407–414
Figure 4: Potentiodynamic polarization curves of PEO-coated alumi-
num alloy formed in 7 g/L Al
2
O
3
containing electrolyte after different
immersion times of 0.5–384 h in the solutions: a) 0.5 M NaCl, b) 1 M
NaCl
Figure 3: XPS analysis of PEO coatings formed in silicate-based elec-
trolyte without and with the addition of 7 g/L Al
2
O
3
: a) Al 2p core
level, b) high-resolution spectrum of Al 2p corresponding to the PEO
coating formed in 7 g/L Al
2
O
3
containing electrolyte

PEO coating formed in 0 g/L Al
2
O
3
containing electro-
lyte.
3.3.2 EIS study
3.3.2.1 EIS spectra
To obtain additional information on the properties of
the coating as well as on the corrosion process at the
metal/electrolyte interface, EIS spectra were recorded
during the immersion in 0.5 M and 1 M NaCl aqueous
solutions at room temperature for a prolonged duration
of up to 384 h. The corrosion behavior of the PEO coat-
ing produced in 7 g/L Al
2
O
3
containing electrolyte and
immersed in the above solutions for (0.5, 24, 48, 96, 192
and 384) h was studied with EIS tests. The EIS spectra
(Nyquist plots) for the immersed coating are shown in
Figures 5a and 5b. In 0.5 M aqueous NaCl, the radius of
the capacitive loop decreased in 0.5–384 h immersion
process (see Figure 5a). After 384-h immersion, the ra-
dius of the loop only became smaller. However, in the
1M NaCl solution, the radius of the loop greatly de-
creased after 24-h immersion (see Figure 5b). After
384 h, an inductive loop appeared. In both 0.5 M and
1 M NaCl aqueous solution, the radius of the loop
changed homogeneously. That is, in 0.5 M NaCl aqueous
solution, the radius of the loop decreased before the im-
mersion time of 48 h, then increased after 48 h and again
decreased after 96 h; in 1 M NaCl, the radius of the ca-
pacitive loop decreased before the immersion time of
24 h, then increased after 24 h and again decreased after
48 h.
3.3.2.2 EIS analysis
To quantitatively analyze the EIS spectra of the coat-
ings in the NaCl aqueous solution, two equivalent cir-
cuits were proposed as shown in Figure 6. To improve
the accuracy of the simulation, the surface inhomo-
geneity factor and possible diffusional factor were in-
cluded. A more general constant phase element (CPE)
21
,
annotated by symbol Q, was used instead of a rigid ca-
pacitive element. The capacity element is expressed with
the following Equation (1):
22
Z
Y
j
n
Q
=
−
1
0
()  (1)
In the Equation (1), j is the imaginary unit (j
2
= –1)
and  is the angular frequency ( =2  f). Coefficient Y
0
or n (–1  n  1) is the parameter of the CPE.
In the equivalent circuits presented in Figure 6, R
s
is
the solution resistance between the specimen and refer-
ence electrode, R
p
is the resistance of the porous
layer/coating, paralleled with Q
p
(the constant phase ele-
ment indicating dispersion of the porous coating/layer
capacitance) and R
b
is the resistance of the inner compact
layer paralleled with Q
b
. L in Figure 6b is the induc-
tance, which is paralleled with R
L
. Figure 6a was used to
fit the EIS data of the coating in diluted 0.5 M NaCl so-
lution up to 384 h and in concentrated 1 M NaCl solution
Y. SHI et al.: STUDY OF THE CORROSION PROCESS OF A PEO-TREATED ALUMINUM ALLOY ...
Materiali in tehnologije / Materials and technology 56 (2022) 4, 407–414 411
Figure 6: Equivalent circuits for fitting the impedance data for
PEO-coated aluminum alloy: a) without inductance, b) with induc-
tance models
Figure 5: Nyquist plots of PEO-coated aluminum alloy formed in 7
g/L Al
2
O
3
containing electrolyte and immersed in solutions for differ-
ent immersion times: a) 0.5 M NaCl, b) 1M NaCl

up to 192 h. Figure 6b was used to fit the EIS data of the
coating immersed in 1 M NaCl solution for 384 h.
Tables 1 and 2 clearly show that high resistance val-
ues of R
p
and R
b
were registered, i.e., 27.89 k ·cm
2
and
259.87 k ·cm
2
, at the immersion time of 0.5 h in 0.5 M
NaCl aqueous solution. Then, with the immersion time
increased, the R
p
and R
b
gradually decreased. Neverthe-
less, the values of R
p
and R
b
increased at the immersion
time of 96 h, being 17.54 k ·cm
2
and 165.3 k ·cm
2
. Af-
ter that, R
p
and R
b
decreased eventually. After 384 h of
immersion, R
b
dropped to a relatively higher value of
53.24 k ·cm
2
compared to the R
p
value of 7.51 k ·cm
2
.
Simulation results exhibited a similar trend in 1 M
NaCl aqueous solution. After 384 h of immersion, due to
the adsorbed Al
3+
ions on the surface, inductance oc-
curred. The simulated circuit is changed in Figure 6b.
The L and R
L
are 2.5 × 10
3
H·cm
–2
and 310.4 k ·cm
2
,re-
spectively. These results demonstrate that the coating
could not protect the aluminum alloy substrate in 1 M
NaCl aqueous solution after 384 h of immersion. The
Y. SHI et al.: STUDY OF THE CORROSION PROCESS OF A PEO-TREATED ALUMINUM ALLOY ...
412 Materiali in tehnologije / Materials and technology 56 (2022) 4, 407–414
Table 1: EIS simulated data of PEO-coated specimen formed in silicate-based electrolytes with 7 g/L Al
2
O
3
in 0.5 M NaCl solution after differ-
ent times of immersion
Immersion time
(h)
R
s/( ·cm
2
) R
p/(k ·cm
2
) Qp/(F·cm
–2
) np R
b/(k ·cm
2
) Qb/(F·cm
–2
) nb
0.5 14 27.89 6.82 × 10
–6
0.58 259.87 7.24 × 10
–7
0.74
24 19 19.36 4.52 × 10
–6
0.67 184.52 6.27 × 10
–7
0.85
48 21 12.46 5.68 × 10
–5
0.45 118.64 6.69 × 10
–6
0.81
96 15 17.54 3.57 × 10
–5
0.51 165.3 4.58 × 10
–6
0.92
192 16 9.460 5.39 × 10
–5
0.61 72.16 5.79 × 10
–6
0.70
384 18 7.51 8.64 × 10
–5
0.72 53.24 7.25 × 10
–6
0.69
Table 2: EIS simulated data of PEO-coated specimen formed in silicate-based electrolytes with 7 g/L Al
2
O
3
in 1 M NaCl solution after different
times of immersion
Immersion time
(h)
R
s/( ·cm
2
) R
p/(k ·cm
2
) Qp/(F·cm
–2
) np R
b/(k ·cm
2
) Qb/(F·cm
–2
) nb
0.5 11 26.16 6.46 × 10
–6
0.61 252.17 7.18 × 10
–7
0.83
24 21 14.34 2.86 × 10
–6
0.60 138.92 5.71 × 10
–7
0.79
48 17 15.48 5.67 × 10
–5
0.69 158.96 7.22 × 10
–6
0.84
96 22 10.45 4.29 × 10
–5
0.52 98.76 5.64 × 10
–6
0.69
192 23 5.49 6.52 × 10
–5
0.78 34.78 2.29 × 10
–6
0.88
384 15 2.64 9.74 ×1 0
–4
0.66 9.21 6.69 × 10
–5
0.71
NaCl concentration 0.5 M 1 M
Immersion time (h) Macro Micro Macro Micro
24
96
384
Figure 7: Macroscopic and SEM morphology of corroded surfaces after 24, 96, 384 h of exposure/EIS testing in 0.5 M and 1 M NaCl solution

variation trend of the EIS results match well with the
potentiodynamic polarization results.
3.4 Corrosion morphology of PEO coatings
Macroscopic and SEM morphologies of corroded
surfaces after (24, 96 and 384) h exposure/EIS testing in
0.5 M and 1 M NaCl solutions are shown in Figure 7.
The aqua dash circles marked in macro-morphology
show the exposure area (1 cm
2
) during the test. In 0.5 M
NaCl aqueous solution, there was no evidence of any
corrosion damage across the surface of the coating after
384-h EIS testing. However, in 1 M NaCl aqueous solu-
tion, the coating was corroded by a gradual Cl
-
infiltra-
tion. After 384 h of immersion in 1 M NaCl aqueous so-
lution, a large pit (of about 30 μm in diameter) appeared
on the surface of the coating. This evidently proved that
the aluminum alloy substrate was corroded by the ag-
gressive Cl
–
at that time. The corrosion morphology of
the coating matches well with the corrosion study above
covering the whole immersion process.
3.5 XPS analysis after corrosion
Figure 8 shows the XPS analysis of the high-resolu-
tion Al 2p spectrum of the coating formed in 7 g/L Al
2
O
3
containing electrolyte after the immersion time of 384 h
in 1 M NaCl solution. The purple square in Figure 7 in-
dicates the analyzed area. From Figure 8, it can be seen
that the single Al 2p peak at 74.32 eV, typical for Al
2
O
3
,
and unstable corrosion products Al(OH)
3
can be decom-
posed into two peaks at 74.40 eV and 74.20 eV.
3.6. Corrosion mechanism
At the early stage, chloride ion easily penetrates into
the pores of the porous layer through discharge channels.
Therefore, the corrosion resistance of the coating de-
creases. Then, with the immersion time prolonged, un-
stable corrosion products Al(OH)
3
can be generated in
the pores as shown below:
NaCl  Na
+
+Cl
–
(2)
H2O  H
+
+OH
–
(3)
Al
2
O
3
+ 6Cl  2AlCl
3
+3O
2–
(4)
H
+
+O
2–
 OH
–
(5)
AlCl
3
+ 3(OH)
–
 Al(OH)
3
+ 3Cl
–
(6)
Then, with the corrosion continued, corrosion prod-
ucts are easily attached to the surface of the coating. To
some extent, Cl
–
is hard to infiltrate into the coating at
this time due to the prevention of corrosion products. So,
the corrosion resistance of the coating increases. How-
ever, some of the Cl
–
can go through the porous layer and
react with the barrier layer. Meanwhile, the reaction con-
tinues as follows:
Al
3+
+3H
2
O  Al(OH)
3
+3H
+
(7)
H
+
+Cl HCl (8)
Finally, some of the barrier layer is completely cor-
roded. The aggressive Cl
–
is in direct contact with the
aluminum alloy substrate. The hydrogen evolution reac-
tion occurs as follows:
2Al+6H
+
 2Al
3+
+3H
2
 (9)
The coating can no longer provide protection to the
aluminum alloy substrate. Thus, the corrosion mecha-
nism discussed above is completely in accord with the
corrosion test results.
4 CONCLUSIONS
The objective of this study was to investigate the cor-
rosion of a PEO coating, formed in an electrolyte con-
taining 7 g/L of Al
2
O
3,
in 0.5 M and 1 M NaCl solutions
through electrochemical methods. The summary can be
drawn as follows:
1) Long-term potentiodynamic polarization and EIS
tests showed that the corrosion resistance of the coating
first decreased, then increased and finally decreased.
2) The coating can protect the substrate in 0.5 M
NaCl solution after 384-h immersion, but it cannot pro-
tect the substrate after 384-h immersion in 1 M NaCl so-
lution.
3) The potentiodynamic polarization results match
well with the EIS test results.
Acknowledgments
This work was supported by the open project founda-
tion of High-Tech Key Laboratory of Agricultural Equip-
ment and Intelligence of Jiangsu Province (No.
MAET202104), the open foundation of Industrial Per-
ception and Intelligent Manufacturing Equipment Engi-
neering Research Center of Jiangsu Province (No.
ZK22-05-07) and the open project foundation of Intelli-
Y. SHI et al.: STUDY OF THE CORROSION PROCESS OF A PEO-TREATED ALUMINUM ALLOY ...
Materiali in tehnologije / Materials and technology 56 (2022) 4, 407–414 413
Figure 8: XPS analysis of high-resolution Al 2p spectrum of the PEO
coating formed in 7 g/L Al
2
O
3
containing electrolyte after the immer-
sion time of 384 h in 1M NaCl solution

gent Terminal Key Laboratory of Sichuan Province (No.
SCITLAB-1013). Dr. Yuanji Shi is indebted to the Qing
Lan Project of Jiangsu Province, China, for its financial
support.
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414 Materiali in tehnologije / Materials and technology 56 (2022) 4, 407–414