Z. YAN et al.: IMPROVING THE CORROSION RESISTANCE OF THE AZ61 MAGNESIUM ALLOY ...
803–808
IMPROVING THE CORROSION RESISTANCE OF THE AZ61
MAGNESIUM ALLOY WITH A HOMOGENIZATION TREATMENT
BEFORE THE EXTRUSION-SHEAR PROCESS
IZBOLJ[ANJE KOROZIJSKE ODPORNOSTI MAGNEZIJEVE
ZLITINE AZ61 S HOMOGENIZACIJO PRED POSTOPKOM
STRI@NEGA IZTISKOVANJA
Zhi Yan
1
, Hong Xing
1
, Hu Hongjun
1*
, Zhang Dingfei
2
, Ou Zhongwen
3
, Gou Yinning
1
,
Chai Linjiang
1
1
Chongqing University of Technology, Chongqing Key Laboratory of Mold Technology, College of Materials Science and Engineering,
no. 69 Hongguang Road, Banan District, Chongqing 400050, China
2
Chongqing University, National Engineering Research Center for Magnesium Alloys, no. 174 Shazhengjie, Shopingba District,
Chongqing 400044, China
3
Army logistics University of PLA, Chemical and Materials Engineering, no. 20 North City Road, Shopingba District, 401311, China
hhj@cqut.edu.cn
Prejem rokopisa – received: 2018-05-16; sprejem za objavo – accepted for publication: 2018-07-20
doi:10.17222/mit.2018.102
The corrosion resistance of the AZ61 magnesium alloy with and without a homogenization treatment fabricated using the
extrusion-shear (ES) process was investigated and evaluated using electrochemical tests. The influences of NaCl concentrations
on the corrosion behavior of the samples were studied. The corrosion morphologies of the corrosion products were analyzed by
SEM and X-ray diffraction. Electrochemical measurements of the magnesium alloy in NaCl solutions with different concen-
trations were conducted. The research results show that homogenization treatments before the ES process can improve the
corrosion resistance of the AZ61 magnesium alloy significantly. The influences of the homogenization treatments on the
corrosion resistance were found to be associated with the uniformity of the grain sizes and the re-distributions of intermetallic
particles within the microstructures.
Keywords: corrosion resistance, AZ61 magnesium alloy, extrusion-shear, homogenization treatment
Avtorji prispevka so raziskali in z elektrokemi~nimi testi ovrednotili korozijsko odpornost homogenizirane in nehomogenizirane
Mg zlitine AZ61. Raziskovali so vplive koncentracije NaCl na korozijsko obna{anje vzorcev. Morfologijo korozijskih produktov
so analizirali z vrsti~nim elektronskim mikroskopom (SEM) in rentgensko difrakcijo (XRD). Izvedli so elektrokemi~ne meritve
Mg zlitine v raztopinah z razli~no koncentracijo NaCl. Rezultati raziskav so pokazali, da lahko homogenizacija pred stri`no
ekstruzijo (ES angl.: Extrusion-Shear process) znatno izbolj{a korozijsko obstojnost Mg zlitine AZ61. Izbolj{anje je posledica
pove~anja homogenosti mikrostrukture (enovitosti velikosti kristalnih zrn) in redistribucije delcev intermetalnih faz znotraj
le-te.
Klju~ne besede: odpornost proti koroziji, magnezijeva zlitina AZ61, stri`na ektruzija, homogenizacijsko `arjenje.
1 INTRODUCTION
Magnesium (Mg) alloys are used in a variety of
industries,
1,2
such as communications, transportation,
aerospace and electronics, due to their characteristics,
especially their high strength-to-weight ratios compared
to other superalloys,
3
such as stainless steel,
4,5
titanium
alloys
6,7
and nickel-based superalloys.
8,9
Nowadays, the
large-scale production of wrought Mg alloy products
produced by plastic processing, such as plates, rods and
tubes, are in great need as they have good comprehensive
properties and can meet various application require-
ments.
10–12
However, Mg alloys have a poor corrosion
resistance, because secondary phases or impurity ele-
ments can cause galvanic corrosion and oxidation films
on the surface of the Mg alloys, which have a porous
structure and cannot protect the Mg alloys from corro-
sion effectively, compared to other alloys,
13–16
especially
in special environments, such as high chlorine-ion
contents. The compositions of Mg alloys also have signi-
ficant influences on the microstructures, which also
affect the corrosion resistance.
The corrosion properties of Mg alloys could be im-
proved by purifying the alloy compositions, developing
new alloys by adding rare-earth elements,
17–19
surface
treatments that could cover the surface of the metal using
a layer of protective film,
20–23
or optimizing the pro-
cessing methods.
24
One example is the severe plastic
deformation (SPD) techniques, such as equal channel
angular pressing (ECAP). They have attracted growing
interest from specialists in materials science.
25,26
Figueiredo et al.
27
utilized a new processing procedure to
extrude a cast Mg-9 % Al alloy, which involves the
sequential applications of extrusion (EX) and equal
channel angular pressing (ECAP). They found that the
grain size of the Mg-9 % Al alloy reduced from an initial
50 μm (casting) to 0.7 μm after one extrusion process
Materiali in tehnologije / Materials and technology 52 (2018) 6, 803–808 803
UDK 620.1:669.018.8:669.721.5 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 52(6)803(2018)

and two ECAP processes. However, as EX-ECAP
usually includes more than 2 steps, the materials have to
endure an intricate diversification of the forming
environments, which can lead to oxidization of the
material due to the process temperature. Matsubara et
al.
28
demonstrated the feasibility of combining
conventional EX and ECAP into a single process. The
extruded material exhibited an excellent balance of
strength and tensile ductility. But SPD technologies are
difficult to promote and industrialize, the processes are
very complicated, and the costs are high.
A new SPD method named the extrusion-shear (ES)
process has been developed to fabricate fine-grained Mg
alloys.
29–31
The composite extrusion for the Mg alloy
combines direct extrusion and two steps of ECAP. The
ES process not only makes continuous forming possible,
but also allows the various kinds of rods to be produced
with the desired final dimensions. Even though nume-
rous studies have focused on the corrosion properties of
the Mg alloy, none of them considered factors including
manufacturing routes and heat treatments. In this study,
an ES die with a horizontal extruder was designed and
manufactured to investigate the effects of the homo-
genization treatment before the ES process on the corro-
sion properties of the final AZ61 Mg alloy products by
immersion and electrochemical performance tests. The
microstructures of the samples from the as-received and
ES-formed rods were observed and analyzed to deter-
mine the deformation mechanisms of the ES process,
respectively.
2 EXPERIMENTAL PART
2.1 Sample preparation
Based on X-Ray fluorescence spectroscopy, the
as-cast AZ61 Mg alloy samples had compositions of
6.11 w/% Al, 0.87 w/% Zn, 0.25 w/% Mn, 0.018 w/% Si
with a balance of Mg. The homogenization heat treat-
ment was performed at a temperature of 400 °C with a
holding time of 24 h for half of the samples. Figure 1
shows a schematic diagram of the ES process, which
includes direct extrusion and two steps of ECAP. The
punch can move horizontally left and right in the X-axial
direction. The oblique angle# between the two channels
is called the die channel angle, which is 120° in this
study. The preheated temperatures for the billets and the
ES die are 400 °C and 380 °C, respectively. The ES pro-
cesses for the as-cast and as-homogenizrd billets were
executed with an extrusion ratio of 12 at an extrusion
speed of 10 mm/s.
All the specimens used in this study had a size of
20 mm × 15 mm × 5 mm. They were cut from the longi-
tudinal sections of the center ES-processed rods by line
cutting to avoid the possible effects of location diffe-
rence. Standard metallographic procedures were adopted
to prepare the samples, which includes mounting,
grinding, and polishing with a diamond suspension down
to 0.5 μm. For the microstructures characterization, they
were etched using a picric acid solution, composed of
5.5 g picric acid, 100 mL alcohol, 5 mL acetic acid, and
10 mL distilled water. The microstructures were revealed
by SEM. In addition, an AD5000X X-ray diffractometer
was used to identify the phase compositions of the
specimens.
2.2 Corrosion tests
The tafel polarization curves and AC impedance in
NaCl solutions with concentrations of 1 %, 3.5 %, 5 %,
7 % and 10 %, respectively, at room temperature were
obtained with an electrochemical work station and the
corresponding analysis software, in which the specimens
were used as the working electrode, and Hg/Hg
2
Cl
2
-
saturated potassium chloride was used as the reference
electrode.
3 RESULTS AND DISCUSSION
3.1 Microstructures of Mg alloy fabricated using the
ES process
X-ray diffraction patterns of the ES-processed
samples with the as-cast and the homogenized initial
state are shown in Figure 2. According to the difference
in the diffraction-peak intensities, the deformation
microstructures of the Mg alloy include a main phase,
a-Mg, and a small amount of secondary phases, such as
Mg
17
Al
12
. With the homogenizing treatment, the percent-
ages of the composition phases were changed.
Figure 3a and 3b show SEM images for the
ES-extruded AZ61 Mg alloy sample in the as-cast and
homogenized initial state, respectively. Most of the
massive second phases (Mg
17
Al
12
) with white polygonal
Z. YAN et al.: IMPROVING THE CORROSION RESISTANCE OF THE AZ61 MAGNESIUM ALLOY ...
804 Materiali in tehnologije / Materials and technology 52 (2018) 6, 803–808
Figure 1: Schematic diagram of the ES die

shapes are distributed among the grain boundaries in
Figure 3a. But in Figure 3b, the secondary phases are
small and rounded, which indicates that the homogeni-
zation treatment would decrease the elements’ segre-
gation between the grain boundaries. In addition, the
homogenization treatment before the ES process would
make the distributions of the secondary phases more
even.
3.2 Electrochemical measurements
The impedance in the NaCl solution with various
concentrations are high-frequency capacitance arcs with
some scatter in the low-frequency regions, as shown in
Figure 4. The radius of the arc indicates the quality of
the corrosion resistances for the Mg alloy matrix. Gener-
ally, the radii of the capacitance arcs decrease with the
concentration of the NaCl solution increasing from 1 %
to 10 %. In addition, the modulus of the NaCl solutions
reduce as the concentration of the NaCl solution in-
creases from1%to10%.
In the same corrosion periods, where the concentra-
tion of NaCl increases from1%to5%,thecorrosion
areas increased, and the corrosion holes deepened, which
indicates an increase of the corrosion degree. At a con-
centration of 7 %, the surface of the AZ61 Mg alloy was
damaged severely, and the corrosion developed from
Z. YAN et al.: IMPROVING THE CORROSION RESISTANCE OF THE AZ61 MAGNESIUM ALLOY ...
Materiali in tehnologije / Materials and technology 52 (2018) 6, 803–808 805
Figure 2: X-ray diffraction patterns for ES-extruded AZ61 Mg alloy
with as-cast and homogenized states
Figure 4: Impedance of AZ61 Mg alloy fabricated using the ES
process in NaCl solutions with different concentrations, a) as cast, b)
homogenization treatment
Figure 3: SEM micrographs of ES-extruded AZ61 Mg alloy samples,
a) as cast, b) homogenized initial state

pitting corrosion to affect the entire surface of the sample
(general corrosion), which would lead to an increase of
the model values.
32,33
EIS plots of the as-cast and homogenization samples
show two capacitance arcs (Figure 4), which indicate
that there are at least two time constants in the process of
the electrode reaction. The capacitance arc at high
frequency represents the influence of the charge-transfer
resistance R3 and the double-layer capacitance. The
appearance of the capacitance arc at low frequency
explains that the barrier layer, which blocks the diffusion
and transfer of ion, has formed. It represents R2 and the
film capacitance when the ion diffuses in the corrosion
product film. Figure 5 is the equivalent circuits (EC) of
EIS. R1 and R2 represent the solution resistance and the
charge-transfer resistance, respectively. R3 represents the
resistance that is caused by the ion diffusing in the corro-
sion product film on the Mg alloy surface. CPE is the
constant-phase element. Table 1 and 2 show the EIS
fitting results in different conditions.
Table 1 shows the fitting results of the as-cast AZ61
in different NaCl solution concentrations. The radius of
the capacitance arc at high frequency diminishes with the
increasing solution concentration, which explains that
the high solution concentration accelerates the anodic
active dissolution process. The value of R2 decreases
from 1743 ’ to 943.7 ’, which can prove this method.
The capacitance arc at low frequency indicates that the
corrosion product Mg(OH)2 has formed on the surface
of sample, and corrosion product increases with the
increasing of the corrosive media concentration. The
value of R3 increases from 0.013084’ to 312’, which
can prove this method. The inductance arc appears at low
frequency when the concentration of the corrosive media
increases to 10 %, which indicates that the pitting induc-
tion period is appearing, the Cl
–
starts to cause the pitting
on the Mg(OH)
2
deposition film, and R3 decreases from
Z. YAN et al.: IMPROVING THE CORROSION RESISTANCE OF THE AZ61 MAGNESIUM ALLOY ...
806 Materiali in tehnologije / Materials and technology 52 (2018) 6, 803–808
Figure 6: Potentiodynamic curves for AZ61 Mg alloy fabricated by
ES process in NaCl solutions with different concentrations, a) homo-
genization treatment, b) as-cast state
Figure 5: Equivalent circuit based on impedance spectrum fitting
Table 1: Parameters obtained by the fitting of the impedance spectra demonstrated in Figure 4a
NaCl %
R1
(’·cm
2
)
R2
(’·cm
2
)
R3
(’·cm
2
)
CPE1-T
(’
–1
·cm
–2
·S
n
)
n1
CPE2-T
(’
–1
·cm
–2
·S
n
)
n2
1 % 70.43 1743 0.013084 1.2597E-5 0.89189 0.00025157 0.090165
3.5 % 22.87 288.2 196.3 1.0735E-5 0.94515 3.4243E-5 0.63592
5 % 17.02 2365 251 1.8634E-5 0.87328 0.00090487 0.10777
7 % 12.04 2130 312 1.8164E-5 0.87109 0.00088419 0.099244
10 % 9.545 743.7 102.2 8.7511E-6 0.95444 3.7945E-5 0.63
Table 2: Parameters obtained by the fitting of the impedance spectra demonstrated in Figure 4b
NaCl %
R1
(’·cm
2
)
R2
(’·cm
2
)
R3
(’·cm
2
)
CPE1-T
(’
–1
·cm
–2
·S
n
)
n1
CPE2-T
(’
–1
·cm
–2
·S
n
)
n2
1 % 16.64 2631 123 2.3468E-5 0.84336 0.0024413 0.66221
3.5 % 21.46 2231 213 1.1782E-5 0.91933 0.00055903 0.11003
5 % 68.17 1081 472 1.539E-5 0.87172 0.0016064 0.56608
7 % 12.53 850 1058 2.8031E-5 0.82657 0.0020704 0.14753
10 % 9.16 850 1287 2.1731E-5 0.85408 0.0012385 0.08905

312’ to 102.2’. Figure 4b and Table 2 show that the
EIS results of homogenized AZ61 have the same trend
with an increasing concentration of corrosive media.
The polarization curves of the AZ61 Mg alloy
samples with and without the homogenization treatment
in the NaCl solution with different concentrations (1 %,
3.5 %, 5 %, 7 % and 10 %) are in compliance with the
tafel rule, as shown in Figure 6. With an increase of the
chlorination concentration, the corrosion degrees of the
samples become serious, and the anode current densities
increase. However, the slope of B
c
tafel only exhibits
small changes.
3.3. Analysis of causes for an improvement of the cor-
rosion resistance
In the NaCl solution the corrosion resistance of the
Mg alloy is lower than that of other metals, because the
Mg alloy has lower potentials than other metals due to
the existence of porous oxide films. The electrochemical
corrosion mechanisms can be explained by the following
chemical reaction:
34–35
At the anode: Mg + H
2
O®Mg
2+
+OH
–
+ 1/2H
2
+e
–
At the cathode: H
2
O+e
–
®OH
–
+ 1/2H
2
The possible overall reaction is:
Mg+H
2
O ® Mg(OH)
2
+H
2
Corrosion product: Mg
2+
+2 OH
–
® Mg(OH)
2
In addition, the potentials of the Mg alloy surface are
inhomogeneous as a lot of micro regions on the surface
have uneven potentials. An anode reaction and a cathode
reaction would occur, and the reaction occurs in the
region of low and high potentials, respectively. This
intensifies the corrosion of the Mg alloy in solution.
The corrosion resistance of the AZ61 Mg alloy
samples with the homogenization treatment is superior to
that of the as-cast ones. The main compositions of the
alloy phases are the Mg matrix and the b-Mg
17
Al
12
phase
that was observed in the inter-regional grain boundaries
in the form of continuation or semi-continuation, as
shown in Figure 2 and 3. In the homogenization treat-
ment, dynamic recrystallization occurred and secondary
phases transformed from semi-continuous precipitation
to continuous precipitation, which would lead to the
decreased corrosion rates of the Mg alloy. The potential
differences between the precipitation of the b -Mg
17
Al
12
phase and non-equilibrium phases of the AZ61 Mg alloy
increase in the homogenization heat-treated samples,
which might lead to galvanic corrosion due to the
increased driving force. This results in an increase of the
corrosion rate of the alloy. The distributions of the
secondary phases in the homogenization-treated samples
are uniform, which leads to a non-concentrated corrosion
and a dispersed corrosion current. Therefore, the pitting
corrosion is changed into a uniform corrosion. This
results in a remarkably decreased corrosion current.
4 CONCLUSIONS
In this study, to investigate the effects of the homo-
genization treatment on the corrosion resistance of the
final ES-fabricated AZ61 Mg alloy samples, half of the
received as-cast billets were homogenization-treated and
then processed using the ES process. Their corrosion
resistances were studied and evaluated by comparing to
those without heat-treated samples using immersion and
electrochemical performance tests. In comparison with
the ES-processed samples without homogenization treat-
ment, the ES-processed samples with homogenization
treatment have a higher corrosion resistance as they
show higher corrosion potentials in different concen-
trations of NaCl solution, a smaller corrosion current,
and a higher capacitance arc radius of impedance. The
ES-processed AZ61 Mg alloy samples with a homo-
genization treatment have a better corrosion resistance
than those without homogenization treatment billets
prepared by the ES process. Homogenization treatment
billets before the ES process could improve the corrosion
resistance of the Mg alloy. The corrosion resistance of
the AZ61 Mg alloys with the homogenization treatment
fabricated by the extrusion-shear process is superior to
that of the as-cast state AZ61 Mg alloy for composition
homogenization.
Acknowledgment
This work was supported by Scientific and Techno-
logical Research Program of Chongqing Municipal
Education Commission (KJ1600924), Chongqing Natu-
ral Science Foundation Project of cstc2018jcyjAX0249
and cstc2018jcyjAX0653 and cstc2016jcyjA0434,
National Science Foundation of China (51771038 and
51571040).
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Z. YAN et al.: IMPROVING THE CORROSION RESISTANCE OF THE AZ61 MAGNESIUM ALLOY ...
808 Materiali in tehnologije / Materials and technology 52 (2018) 6, 803–808