D. BEKHITI et al.: IMPROVEMENT OF THE CORROSION BEHAVIOR OF ALUMINUM ALLOY 6061-T6 ...
329–334
IMPROVEMENT OF THE CORROSION BEHAVIOR OF
ALUMINUM ALLOY 6061-T6 WITH YTTRIUM AND
LANTHANUM CONVERSION COATINGS
IZBOLJ[ANJE KOROZIJSKE ODPORNOSTI ALUMINIJEVE
ZLITINE 6061-T6 S KONVERZIJSKIMI PREVLEKAMI NA OSNOVI
ITRIJA IN LANTANA
Djillali Bekhiti
1
, Juan Creus
2
, Nadir Mesrati
3
, Abderrezak Abdi
1
, Hiba Messaoudi
4
1
Polytechnic Military School, Bordj el Bahri, 1611 Algiers, BP17, Algeria
2
University of La Rochelle, Laboratory of Engineering Science for the Environment, UMR 7356 CNRS, La Rochelle, France
3
Polytechnic of Algiers, National High School, 1611 Algiers, Algeria
4
University of 20 August 1955 of Skikda, Faculty of Science, Department of Chemistry, Algeria
bekhitidjillali@gmail.com
Prejem rokopisa – received: 2017-09-26; sprejem za objavo – accepted for publication: 2017-11-03
doi:10.17222/mit.2017.160
In this study, yttrium- and lanthanum-based conversion coatings were deposited on a 6061-T6 aluminum alloy with cathodic
electrochemical deposition using Y- and La-nitrate precursors. The optimum [La
3+
]/[Y
3+
] ratio was evaluated with
cyclic-voltammetry (CV) curves. The influence of the conversion-treatment time was studied by performing the experiment for
periods of 10 and 20 minutes. Optical microscopy was used to determine the coating morphology and the anticorrosive
properties were studied using linear polarization (LP) and electrochemical impedance spectroscopy (EIS) in a 5-g/L NaCl
solution. The electrochemical results revealed an obvious improvement in the corrosion resistance against the aggressive media
after an extended imersion test for 48 h, indicating a good efficiency of the Y- and La-conversion coating.
Keywords: aluminum alloy, 6061-T6, conversion coating, rare earth elements, lanthanum, yttrium
V pri~ujo~em ~lanku avtorji opisujejo nanos konverzijske (pretvorbene) prevleke na osnovi itrija in lantana na Al zlitino
6061-T6 s katodno elektrokemijsko depozicijo (nanosom) z uporabo Y in La-nitratnih prekurzorjev (izhodnih surovin). S
cikli~nimi voltametri~nimi (CV; angl.: Cyclic Voltammetry) krivuljami so ovrednotili optimalno razmerje [La
3+
]/[Y
3+
]. [tudirali
so vplive ~asa na pretvorbo z izvedbo 10 in 20 minutnih preizkusov. S pomo~jo svetlobnega mikroskopa so podali morfologijo
prevleke. S pomo~jo linearne polarizacije (LP) in elektrokemi~ne impedan~ne spektroskopije (EIS) v 5 g/L NaCl pa so {tudirali
korozijsko odpornost prevleke. Rezultati elektrokemijskih preiskav so pokazali jasno izbolj{anje protikorozijske za{~ite v
agresivnem mediju po podalj{anem 48 urnem potopnem preizkusu. To ka`e na dobro u~inkovitost izbrane obdelave za izdelavo
Y in La pretvorbenih prevlek.
Klju~ne besede: zlitina na osnovi aluminija, 6061-T6, pretvorbena prevleka, redki elementi zemlje, lantan, itrij
1 INTRODUCTION
Aluminum alloy 6061-T6 is widely used due to its
significant characteristics such as low cost, high
strength-to-weight ratio and easy workability.
1
For this
reason, it is commonly used in modern construction,
automotive and aerospace industries.
2
Nevertheless, the
presence of multiple heterogeneities in Al-alloy classes
originates from the hardening as heat treatments signi-
ficantly reduce the corrosion resistance. These hetero-
geneities, mainly associated with the presence of inter-
metallic precipitations (IMs), are the source of localized
corrosion when in contact with a corrosive environment.
3
Localized corrosion generally occurs in the form of
pitting corrosion; it is initiated at discrete sites, in the
vicinity of Fe-containing intermetallic phases of 6xxx
aluminum alloys.
4–8
Metal impurities, even in small
amounts, are can cause the formation of new phases,
acting as cathodes in relation to the aluminum matrix.
Indeed, it was shown that intermetallic particles in 2xxx
aluminum alloys, for example, are initially anodic to the
matrix and progressively become cathodic due to the
selective corrosion of their less noble constituents that
provoke the corrosion of the adjacent matrix.
9,10
Rare-
earth conversion coating is one of the highly attractive
techniques used for improving the corrosion resistance of
aluminum and its alloys, including also chemical vapor
deposition (CVD), physical vapor deposition (PVD),
thermal spray coating, anodizing technique, organic
coating and micro-arc oxidation (MAO).
Since chrome (VI) is considered as a health and envi-
ronmentally hazardous compound,
11
rare-earth elements
(REEs) are considered as good alternatives to chromium
(VI)-based chemical-conversion coatings, commonly
used for the final finish pretreatment.
12
Among the REEs,
lanthanum and yttrium attract much attention owing to
their properties, creating a remarkable inhibition to the
localized corrosion of aluminum alloys.
13–15
Indeed, they
exhibit a strong ionic character enabling them to react
with hydroxyl anions in aqueous media to produce insol-
Materiali in tehnologije / Materials and technology 52 (2018) 3, 329–334 329
UDK 620.193:669.715:669.058.4 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 52(3)329(2018)

uble and adherent hydroxide/oxide deposits, increasing
the blocking effect and providing a high stability against
mechanical abrasion and catalytic activity.
16
The aim of this work was to evaluate the efficiency of
yttrium- and lanthanum-conversion coatings, made with
cathodic electrodeposition, for the corrosion protection
of the 6061-T6 aluminum alloy used as the substrate in
chloride-containing media.
2 EXPERIMENTAL PART
2.1 Lanthanum- and yttrium-conversion coating proce-
dures
The samples used were disks of aluminum alloy
6061-T6 with a 30-mm diameter, encapsulated in epoxy
resin. The nominal composition (% by weight) is given
in Table 1. Before the conversion-coating treatment, they
were grinded with (600, 800, 1200, 2400 and 4000) grit
SiC paper, using demineralized water as the lubricant.
Then they were washed with ethanol and demineralized
water and dried. Lanthanum and yttrium conversion
coatings were performed by means of the galvanostatic
technique at room temperature, using La(NO
3
)
3
·6H
2
O
(0.1 M) and Y(NO
3
)
3
·6H
2
O (0.1 M) as the precursors
(Prolabo, 99 %). The experiments were conducted in a
standard three-electrode electrochemical cell in an
aerated solution and under moderate stirring. All the
electrochemical measurements were performed using an
Autolab 302 N potentiostat/galvanostat coupled to a PC
and controlled with the GPES software. The aluminum-
alloy samples served as the working electrodes, a Pt-wire
as the counter electrode and a saturated calomel electro-
de (SCE) as the reference electrode (0.242 V vs. SHE).
Electro-deposition was performed at a cathodic current
of 2.5 mA for 10 min and 20 min. Preliminary cyclic-
voltammetry (CV) curves were plotted from -1 V to
0.2 V (vs. SCE) around the open circuit potential (OCP)
at a 0.1-mV/s scan rate, in different solutions based on
La(NO
3
)
3
(0.01 M) and Y(NO
3
)
3
(0.01 M) as the single
precursors, and in a mixture with the [La
3+
]/[Y
3+
] ratios
of 1/1, 1/3 and 3/1. The total concentration of
([La
3+
]+[Y
3+
]) was kept to be 0.01M.
Table 1: Nominal composition of the 6061 aluminum alloy
Elements (w/%)
Al Mg Si Ti Cr Mn Fe Ni Cu Zn Ga
97.78 1.16 0.01 0.01 0.18 0.11 0.22 0.02 0.28 0.20 0.01
2.2 Characterization methods
The coating morphologies were determined with
optical microscopy (OM) using a Leica DM 2500M
microscope, controlled by the Leica software. To provide
more information about the corrosion behavior and the
efficiency of RREs conversion coatings, electrochemical
experiments were performed at room temperature in a
standard three-electrode cell in an aerated NaCl solution
(5 g/L) under magnetic stirring (200 min
–1
), using an
Autolab 302 N PGSTAT. A Pt-foil served as the counter
electrode and the SCE as the reference electrode. The po-
larization curves were recorded from –150 mV to + 150 mV
around OCP, with a 1 mV/s scan rate. Electrochemical
impedance spectroscopy (EIS) was performed in the
AC-frequency domain in a range of 1 MHz–0.25Hz, with
a perturbation amplitude of 10 mV. Each experiment was
conducted three times for the testing reproducibility.
3 RESULTS AND INTERPRETATION
3.1. Lanthanum- and yttrium-conversion coatings
The rare-earth metal hydroxides La(OH)
3
( G°=
–1272,8 kJ/mol) and Y(OH)
3
( G°= –1299,0 kJ/mol)
were deposited and grown on the aluminum surface
owing to the local pH increase through the electro-
generation of the hydroxyl ions (OH
–
) from the reduction
of both dissolved O
2
and H
2
O molecules, following
Equations (1) and (3), in agreement with
3
:
O
2,aq
+2H
2
O+4e
–
 4OH
–
(1)
E
O/ O H
2
–
(V vs. ENH) = 0.32 – 0.06 pH (2)
2H
2
O+2e
–
 H
2
+ 2OH
–
(3)
E
HO/ H
22
(V vs. ENH) = –0.06 pH (4)
For high cathodic-potential values, the current is
mainly associated with the hydrogen production because
of the low dissolved-oxygen content in the aqueous me-
dia, evaluated to be 8.26 mg/L (~2.6×10
–4
)at25°C.
17
According to some authors,
18
the cathodic reduction
of nitrate ions may also produce an additional quantity of
OH
–
ions according to Equation (5):
NO
3
–
+H
2
O+2e
–
↔ H
2
+ 2OH
–
(5)
E
NO /NO
3
–
2
–
(V vs. ENH) = 0.01 – 0.06 pH (6)
It is well known that for the weakly soluble metallic
hydroxides such as lanthanum and yttrium hydroxides,
the pH is one of the most critical parameters. Indeed, the
D. BEKHITI et al.: IMPROVEMENT OF THE CORROSION BEHAVIOR OF ALUMINUM ALLOY 6061-T6 ...
330 Materiali in tehnologije / Materials and technology 52 (2018) 3, 329–334
Figure 1: CV curves plotted from –1 V
SCE
to 0.2 V
SCE
, in different
solutions based on La(NO
3
)
3
(0.01 M) and Y(NO
3
)
3
(0.01 M) at a scan
rate of 0.1 m s
–1

solid-phase precipitation for both La(OH)
3
(Ks =
2×10
–22
) and Y(OH)
3
( K s=1 ×10
–22
) occurs when
[La3+]×[OH-]
3
and ]Y
3+
]×[OH
–
]
3
are higher than theirs
corresponding solubility products. La
3+
and Y
3+
are con-
sidered as stronger Lewis acids due to their strong ionic
characters. They interact, in an aqueous solution, with
H
2
O molecules in the hydrolysis process.
Figure 1 shows CV curves plotted around the OCP,
from –1 to 0.2 V
SCE
, in different solutions based on
La(NO
3
)
3
(0.01 M) and Y(NO
3
)
3
(0.01 M). A significant
change in the current intensity was observed for
solutions based on the mixture of La- and Y-nitrate salts,
leading to an evident hysteresis in the reverse scan, in
agreement with the previous research on rare-earth
conversion coatings.
3,19
Additionally, we obtain the best
current density with the [La
3+
]/[Y
3+
] ratio of 3/1 since
oxygen- and water-reduction reactions provide for higher
amounts of OH
-
ions necessary for obtaining a thicker
deposit with a more compact structure, able to stop the
diffusion of oxygen dissolved to reduce or prevent corro-
sion.
3.2 Microstructural characterization of rare-earth con-
version coatings
In Figure 2, the obtained ceramic-coating morphol-
ogy for the specimen treated for 20 min is presented.
Optical microscopy revealed a typical compact
La/Y-conversion coating deposited over the entire sub-
strate surface (Figure 2, the insert). The microstructure
appears as homogenous agglomerates of particles with
no particular orientation and with grain sizes between
10 μm and 150 μm.
3.3 Electrochemical characterization
3.3.1 Polarization measurement
Figure 3 shows semi-logarithmic plots lg i (E
SCE
)) for
the treated and untreated specimens in the 0.5-% aerated
NaCl solution. The treated specimens show an obvious
decrease in the corrosion-current density, indicating an
improvement in the corrosion resistance. Many
authors
3,20
indicate that the REE deposit starts to grow
over the cathodic sites formed by intermetallic particles
of the aluminum matrix, where the local alkalization
occurs, due to the generation of hydroxyl (OH
–
) ions. In
Table 2, we see the potential corrosion (E
corr
) and the
kinetic values such as the corrosion current density (i
corr
),
the Tafel parameters (b
c
and b
a
) and the inhibition
efficiency ( ). The specimen treated for 20 minutes
generates more protective coating than that treated for 10
minutes. Indeed, specimen 6061 treated for longer times
(20 min) had the lowest i
corr
values, 4.92 μA/cm
2
and
2.97 μA/cm
2
, respectively. The polarization resistance
(R
p
) is given as a function of the corrosion current
density and the Tafel parameters b
a
(>0) and b
c
(< 0) are
expressed in the V/current decade, according to Relation
(4):
21
Ri
bb
pc o r r
().  cm
ca
2
1
2303
11
=×+
⎛ ⎝ ⎜ ⎞ ⎠ ⎟ ⎡ ⎣ ⎢ ⎤ ⎦ ⎥ −
(7)
For the treatment time of 20 minutes, the R
p
increases
by one order of magnitude more than for the untreated
sample. In addition, from Table 2, it can be clearly seen
that (b
a
< b
c
) indicates a cathodic controlling polariza-
tion of the electrochemical systems.
D. BEKHITI et al.: IMPROVEMENT OF THE CORROSION BEHAVIOR OF ALUMINUM ALLOY 6061-T6 ...
Materiali in tehnologije / Materials and technology 52 (2018) 3, 329–334 331
Figure 2: Morphology of the Y/La-conversion coating performed at
2.5 mA for 20 min
Figure 3: Lg i versus the potential characteristic of 6064-T6
Table 2: Electrochemical parameters obtained from polarization curves
Time
(min)
−E
corr
(mV vs. SCE)
icorr
(μAcm
–2
)
−β c
(mV/dec)
 a
(mV/dec)
R p
(Ω.cm
2
)
 (%) Porosity
Untreated
10
20
580
602
620
162
14.7
2.97
351
181
71
84
65
45
296
2996
17966
–
90
98
–
0.054
0.007

The inhibiting efficiency ( ), calculated using the
corrosion current densities of the treated (i
corr
) and un-
treated (i
corr
0
) specimens according to equation (5),
increases with the time treatment.
22
We obtained 98 % for the 20-min treatment time, in-
dicating a good inhibiting efficiency.
 (%)=−
⎛ ⎝ ⎜ ⎞ ⎠ ⎟ × 1 100
0
i
i
corr
corr
(8)
The total coating porosity, P, was evaluated using the
empirical equation (9), assuming that the conversion-
coating deposit is inert under a low anodic-bias poten-
tial
23
:
P
R
R
E
b
=×
−
p,s
p
corr
a
10
Δ
(9)
where R
p,s
and R
p
are the polarization resistances of the
treated and untreated aluminum-alloy specimens,
respectively.  E
corr
is the free-corrosion potential diffe-
rence between the coated and blank aluminum-alloy
substrates, and ba is the anodic Tafel slope associated
with the substrate.
Since the corrosion resistance of a coating is directly
related to its porosity, aggressive species of the environ-
ment, in the first instance, O
2
and Cl
–
, are better inhibited
and their diffusion towards the metallic subsurface is
substantially reduced with a low porosity. The obtained
porosity values of the samples treated for 10 min (P =
0.054) and 20 min (P = 0.007) indicate that the prolong-
ation of the treatment time allows a development of a
more compact deposit, reducing the porosity by nearly
13 times.
3.3.2 Electrochemical impedance spectroscopy
Figure 4 shows the Nyquist diagrams recorded after
48 h of contact with the 5-g/L NaCl solution over a freq-
uency range of 1 MHz–10 mHz. It clearly shows
depressed semicircles observed in a large frequency
domain, revealing a significant rise in the cell impedance
for the treated specimens in comparison with the un-
treated ones, owing to the presence of a compact
protective layer. Indeed, it can be clearly seen that the
apparent diameter of the non-ideal semicircles increases
with the treatment time. This result confirms those
obtained with linear polarization.
Figure 5 establishes a distorted phase versus the
lg (f) plot for both treated specimens in contrast to the
untreated one, indicating two time constants. The best
equivalent-circuit model describing the electrochemical
behavior of the treated materials (Figure 6a), combines
the internal resistance (R
i
) in the series with two RQ
loops; the first loop is associated with the ionic charge-
transfer resistance (R
ct
), in parallel with the constant
phase element (CPEdl) related to the double-layer
capacitance; the second loop is connected with the fara-
dic resistance (Rf) and the constant phase element
(CPEb) related to the bulk capacitance. However, the
experimental data of the untreated specimen are well
fitted with the equivalent circuit model in Figure 6b.
The CPE, given with Equation (10), is used to avoid
the data dispersion due to both porosity and surface-
roughness:
24
Z
CPE
= [Q(j )
 ]
-1
(10)
D. BEKHITI et al.: IMPROVEMENT OF THE CORROSION BEHAVIOR OF ALUMINUM ALLOY 6061-T6 ...
332 Materiali in tehnologije / Materials and technology 52 (2018) 3, 329–334
Figure 6: Equivalent circuit models obtained by fitting the EIS data:
a) for untreated and b) treated specimens
Figure 4: Nyquist diagram recorded after the 48-h contact with the
5 g/L NaCl solution, over a frequency range of 1 MHz–10 mHz
Figure 5: Bode diagram lg(Z) versus lg(f) after immersion in the 5 g/L
NaCl solution for 48 h

where Q (Fs
 –1
) is the admittance and  (–1 1)
is the dispersion factor expressing the deviation from
the ideal behavior.
The EIS parameters obtained by fitting the experi-
mental data are summarized in Table 3. The internal-
resistance value, R
i
, remains in the same order of mag-
nitude whereas the faradic resistance, R
f
, is improved by
more than 2 and 2.5 times for the specimens treated for
10 min and 20 min, respectively. The obtained
chi-squares ( 2
) are of the order of 10
–3
, indicating a
good fitting of the experimental data.
Table 3: EIS parameters for the aluminum-alloy specimens
Untreated error
(%)
10 min error
(%)
20 min error
(%)
R
i
( ) 47.6 0.55 42.6 2.03 37.7 1.56
R
tc1
( ) - - 569 39.1 531 37.9
Q
dl
(μF.s
–1
) - - 44.9 16.4 68.9 15.3
 dl
- - 0.64 2.68 0.68 2.47
R
f
( ) 3176 2.04 6695 5.18 8119 3.67
Q
b
(μF.s
–1
) 7.96 3.88 8.03 5.85 10.5 4.05
 b
0.92 0.63 0.92 2.90 0.99 2.19
 2
(×10
3
) 3.53 - 1.34 - 1.88 -
3.3.3 Immersion test
In Figure 8, we see the variation in the polarization
resistance (R
p
) versus time during the immersion in the
5 g/L NaCl solution, at room temperature. The treated
aluminum alloy exhibits a significant enhancement in the
corrosion behavior in the presence of chloride ions.
Indeed, after 48 h, the polarization-resistance value in-
creases nearly 2 times. These results are in agreement
with the explanation given by A. Pardo et al.
25
after
studying the Ce-conversion and electrolysis surface treat-
ments applied to A3xxx alloys and A3xxx/SiCp compo-
sites; they argue that chloride ions favor the initial pitting
attack producing Ce-oxides/hydroxides deposits which
block the cathodic sites, and therefore increase the pro-
tection. In addition, the micrograph in Figure 8 indicates
that the specimen is weakly affected by the aggresivity
of the chloride-contining media, and no apparent
corrosion product is detected with optical microscopy.
The obtained results indicate a good efficiency of the
applied yttrium- and lanthanum-conversion coating treat-
ment against the corrosion of the 6061-T6 aluminum
alloys.
4 CONCLUSION
In the present work, yttrium- and lanthanum-conver-
sion coatings for the 6061-T6 aluminum alloy were
applied with cathodic electrodeposition using Y- and
La-nitrate aqueous solutions, for the corrosion protection
in chloride-containing media. Cyclic-voltammetry (CV)
curves indicate that the optimal [La
3+
]/[Y
3+
] ratio, pro-
viding the necessary amounts of OH
–
ions for the elec-
trodeposition is 3/1. A typical compact La/Y-conversion
coating deposited over the entire substrate surface was
revealed using optical microscopy. The microstructure
appears as homogenous agglomerates of particles with
no particular orientation and grain sizes in a range of
10–150 μm. The images of the film surfaces after the
immersion in the 5-g/L NaCl solution, obtained with
micrography, reveal that the 6061 specimen is poorly
affected by the aggresivity of the chloride-contining
media after 48 h. The linear polarization and EIS tests
show an improvement in the corrosion resistance,
indicating a good efficiency of the applied yttrium- and
lanthanum-conversion coating. The inhibiting efficiency
( ) increases with the treatment time; it was calculated to
be 98 % for the time treatment of 20 min.
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Materiali in tehnologije / Materials and technology 52 (2018) 3, 329–334 333
Figure 7: Variation in the polarization resistance versus time during
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