Materials and Corrosion 2008, 59, No. 10
DOI: 10.1002/maco.200804169
819
The effect of phosphate on MAO of AZ91D
magnesium using AC power source
Q. Wen, F.-H. Cao*, Y.-Y. Shi, Z. Zhang and
J.-Q. Zhang
A rapid and convenient anodization technology with AC power
source to obtain the MAO films formed on magnesium alloy AZ91D
in phosphate bath (base electrolyte þ Na3PO4) with or without
aluminate and silicate was studied. The corrosion resistance of
the anodic films was studied by electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization techniques and the
microstructure and composition of films were examined by SEM
and XRD. The results show that Na3PO4 can promote the occurrence of sparking during the MAO process, while abundant heat
generated by sparking might enhance the formation of the glassy
phase of the compound when the electrolyte contains the additives
of NaAlO2 and Na2SiO3 simultaneously. The optimized MAO film
is ivory-white smooth by naked eye, while presents porous and
microcracks in microscopic scale. The anodic film formed in the
alkaline solution with optimized parameters possesses superior corrosion resistance by electrochemical test. The XRD pattern shows that
the components of the anodized film consist of MgO, MgAlO2, and
MgSiO3. No oxide crystal with P element can be found.
1 Introduction
propitious to increase the thickness of the anodic film and
improve the roughness of the film surface, while the effect of
phosphate in microstructure and corrosion resistance is not
obvious. However, there are few studies about mutual action
of these additives and how the action influences the
properties of MAO films on magnesium alloys. In our previous work, we have already obtained high corrosion resistance anodic coating with silicate and aluminates in alkaline
borate electrolyte. The effects of phosphate concentration
with or without aluminates and silicate on the morphology,
composition, and corrosion resistance of anodic coating
formed with AC power source are investigated in the present
work.
Magnesium and its alloys have gained considerable
interest as structural materials for computer, mobile, and
aerospace application due to high strength-to-weight ratio,
good electromagnetic shielding, and damping characteristics
[1]. However, the application of Mg alloys is mainly limited
by their poor properties in corrosion [2]. Some surface
modification techniques have been developed for the
protection of Mg alloys [3–5]. Microarc oxidation (MAO)
[6,7], as a new surface modification technology developed
from traditional anodic oxidation, has been applied in the
surface treatment of Mg alloys and has become a hotspot of
international researches. MAO treatment has been used
extensively to modify the surface of magnesium alloys with
higher corrosion resistance, higher hardness, and improved
wear-resistance properties, as well as enhanced paint
adhesion.
The properties of MAO coatings on magnesium can be
closely related to its microstructure and composition, which
depend on voltage/current mode and the composition of
electrolyte. Recently, most researchers applied the DC mode
and alkaline electrolyte on the anodic substance with
different additives, like aluminates [8], silicate [9], and
phosphate [10]. Both aluminate in electrolyte and aluminum
in the alloy have been shown to increase the aluminum
content in the anodic film on magnesium alloys anodized in
KOH-aluminate solutions. The addition of silicate is
F.-H. Cao, Q. Wen, Y.-Y. Shi, Z. Zhang
Department of Chemistry, Zhejiang University,
Hangzhou 310027 (P.R. China)
E-mail: [email protected]
J.-Q. Zhang
State Key Laboratory for Corrosion and Protection, Institute of
Metal Research, Chinese Academy of Sciences,
Shenyang 110016 (P.R. China)
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2 Experimental
Rectangular samples (with dimensions 30 mm 20 mm 5 mm) of a Mg-Al wrought magnesium alloy
and its chemical components shown in Table 1 are selected in
this experimental work. The surface of magnesium samples
are ground down to 800-grit alumina, degreased in acetone
and alcohol, and then laundered with distilled water. Two
magnesium alloy electrodes are anodized synchronously up
to 120 V (virtual value, sine wave, 50 Hz) AC voltage in
500 mL base electrolyte containing 50.0 g/L NaOH, 10.0 g/
L H3BO3, 20.0 g/L Na2B4O7 10H2O, and different
additives, and all reagents used are analytical grade. Two
magnesium electrodes are used due to AC signal, while the
distance of two electrodes is constant (2 cm). During the
anodizing experiments, the bath temperature was controlled
at 50 1 8C by rotating the electrolyte using a vermicular
pump equipment (model YT-8A, China) in order to decrease
the temperature of the electrolyte, the temperature change
being due to sparking discharge during anodization, and
ensure the reproducibility of the results [11]. The morphologies of the as-obtained film and corresponding cross-section
are observed using SIRION scanning electron microscope
ß 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
820
Wen, Cao, Shi and Zhang
Materials and Corrosion 2008, 59, No. 10
Table 1. Chemical composition of AZ91D magnesium alloy (wt%)
Al
8.3–9.7
Zn
0.35–1.0
Mn
0.15–0.50
Si
0.01
(manufactured by FEI). The structure and composition of the
film are examined by D/MAX-RA X-ray diffraction (Rigaku,
Japan).
Potentiodynamic polarization is carried out using
CHI660A Potentiostat (CH Instrument, Inc., USA) at
25 1 8C. A three electrode cell with pretreated ceramiclike film (sealed with epoxy resin leaving only the square
surface with an area of 2 cm 2 cm) as a working electrode,
saturated calomel electrode (SCE) as a reference electrode,
and platinum sheet as a counter electrode is employed in all
electrochemical tests. The ratio of volume of neutral 3.5 wt%
NaCl solutions (pH 7.03) to sample area is 50 mL/cm2. After
the working electrode is immersed in the NaCl solution about
15 min until the open circuit potential keeps relative steady,
potential scanning is conducted at a rate of 1 mV/s from
0.25 V versus open circuit potential (OCP) to 1.25 V (vs.
OCP). Electrochemical impedance spectroscopy (EIS) [12–
14] is measured using impedance measurement unit (VMP2,
USA) from 10 000 to 0.01 Hz with a voltage amplitude of
5 mVat the OCP in neutral 3.5 wt% NaCl solution to evaluate
the corrosion resistance of anodic coating.
3 Results and discussion
3.1 AC power source
The types of power source used in MAO treatment are
classified into mostly two groups which include DC and AC
source. Yerokhin et al. [11] show that the application of pulse
DC allows controlling the interruption of the process and
pulse form, which is beneficial to control the composition
and structure of film, but the additional polarization of the
electrode surface supported by the pulsed current may need
very high voltages (up to 1000 V), which will result in large
energy waste. When the AC power source is applied, the
additional polarization of the electrode can be avoided and
anodic coating can present the self-repairing process during
no sparking discharge.
Fig. 1. Typical plot of the output voltage signals of the AC
microarc oxidation power source
Cu
0.030
Ni
0.002
Fe
0.005
Mg
Balance
So AC power source with a maximal voltage value of
170 V is selected for magnesium anodization with 120 V
constant amplitude shown in Fig. 1. As shown in the previous
studies [11,15], the first step of the traditional anodization
treatment is to form passive film before voltage increases
to U1 (about 60 V) which corresponds to the breakdown
voltage of the film, and no sparking can be observed. Then in
the region U1–U2 (about 60–80 V), sufficient electric field
strength for the initiation of the ionization process brings a
large number of rapid discharge sparking to promote the
porous oxide film growing on the substrate. Beyond U2, as
voltage increases, the electric field strength in the oxide film
reaches a critical value to produce microarc across the
surface of the oxide film. In this case, the porous film is
broken through due to impact or tunneling ionization. The
electrolyte containing PO43 ions can penetrate through
the porous film and react with the substrate directly to form
the compact inner film because of the high energy generated
in sparking discharge channel. When the voltage decreases to
a certain value, diminishing energy leads to no sparking
discharges which redound to repair the porous film and form
more smooth surface [16]. Due to 50 Hz of AC power source,
two electrodes of our experimental condition present classic
anodization, sparking discharge, and MAO process, repairing process every 0.02 s alternatively, and repeat 1000 times
during whole anodization treatment. At the end of
anodization, two compact and smooth MAO films can be
obtained synchronously.
3.2 Morphologies and composition of MAO films
Figure 2 shows the morphologies of MAO films obtained
in solutions containing basic electrolyte with different
additives. As shown in Fig. 2a and 2b, there are more
micropores and cracks on the surface of P-film compaed with
the surface of Al-P-film; moreover the sintering phenomenon
is more obvious on the Al-P-film. It is estimated that these
micropores are formed by gas bubbles thrown out of sparking
discharge channel, meanwhile microcracks due to oxidation
films are suddenly solidified by the surrounding cold
electrolyte [17–19]. The sparking discharge phenomenon
becomes more obvious with the increase in the concentration
of Na3PO4, which indicates that PO43 ions may migrate
inward under the high electric fields and react with anodic
substance.
Compared with the P-film and the Al-P-film, the Si-P-film
(Fig. 2c) takes on fewer occurrences of micropores and
microcracks, which is comparatively more uniform. The
rather smooth surface may owe to the uniform sparking,
indicating that the presence of Na2SiO3 in electrolyte can
promote the occurrence of uniform sparking during the MAO
process in the experimental condition [20]. The similar effect
can be found in the SEM image of the Al-Si-P-film surface
morphology on which there are less micropores and cracks
than that of the Al-P-film and more sintering phenomenon
than that of the Si-P-film.
The cross-section morphologies (Fig. 2e–h) show that the
MAO films are integrated firmly, due to sintering effect, onto
magnesium substrate. Two-layer coatings are obvious, which
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Materials and Corrosion 2008, 59, No. 10
Effect of phosphate on MAO of AZ91D magnesium
821
Fig. 3. Surface morphology of MAO film of magnesium alloy
AZ91D formed in base electrolyte with 10 g/L NaAlO2 þ 10 g/L
Na2SiO3 additives
Fig. 2. Surface and cross-section morphologies of MAO films on
alloy AZ91D, respectively, treated in solutions containing base
electrolyte þ 5 g/L Na3PO4 (a), base electrolyte þ 10 g/L NaAlO2 þ
10 g/L Na3PO4 (b), base electrolyte þ 10 g/L Na2SiO3 þ 10 g/L Na3PO4
(c), base electrolyte þ 10 g/L NaAlO2 þ 10 g/L Na2SiO3 þ 10 g/L
Na3PO4 (d); (e–h) are their cross-sectional morphologies
are composed of the inner barrier layer and outer porous
layer [21] in all anodic films. Although there are also some
micropores and microcracks to be seen in the cross-sections
especially in the Al-P-film (Fig. 2f), these pores and cracks
neither connect with each other nor perforate into the entire
oxide films. From these cross-section morphologies, it can be
seen that the thickness of the P-film is lower than that of AlP-film, Si-P-film and Al-Si-P-film, and the thickness of the
Al-P film is 13.23–1571 mm, which is the maximal value.
This result also proves that the PO3
4 ions in solution can
increase the thickness of anodic film with AlO2 ions, while
the SiO32 ions increase the roughness and decrease the
thickness of film.
Comparing Fig. 2d with Fig. 3, it can be seen that there are
less microcracks and more large bulk sintered oxides in the
Al-Si-P-film. As shown in the literature [4], the extremely
high energy generated by sparking might assist the formation
of glassy phase of compound, which is associated with the
polymerization reaction between Na2SiO3 and NaAlO2 in
electrolyte. Because the additive of Na3PO4 can facilitate the
occurrence of sparking, durative enough energy is generated
to sinter the glassy phase of compound and form the stable
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and compact oxide film. The AC power source supplies the
half energy for one AZ91D electrode shown in Fig. 1,
indicating that the discharge sparking of one electrode only
can carry out half of whole anodizing time, while the
working electrode acts on a cathodic reaction place without
sparking at the left half time in one cycle. Discharge sparking
and no sparking arise alternatively, which can promote
sintering surface oxide film due to cooling of anodic film by
cold electrolyte without sparking.
The structure and composition of MAO films have been
identified by X-ray diffraction (Fig. 4) and EDX (Table 2).
The XRD spectra show that these films are mainly consisted
of MgO, MgSiO3, and MgAl2O4. It can be seen that the peaks
corresponding to MgO are the strongest in the spectra of AlSi-P-film and Al-P-film. Moreover, a few peaks of forsterite
MgSiO3 and spinel MgAl2O4 increase in the Al-Si-P-film,
which indicates that SiO2
3 and AlO2 ions in the electrolyte
have directly engaged in chemical reactions near the
microarc zone [22]. Since the thickness of the anodic film
is thin and the X-ray can penetrate through the oxidation
films easily [15], the intensity of peaks corresponding to the
Mg substrate is strong. There are no peaks associated with
phosphate which can be detected in the XRD patterns, which
explains that phosphorus exists in the form of non-crystal
phosphate type in the film [23], but EDX results show the P
elements can be detected in anodic film surface. Al content in
anodic film results also indicates that the AlO
2 ions in
electrolyte react with the substrate and form oxide through
discharge, while the Mg element in alloys migrates to the
surface, and Al element in alloy migrates to the substrate,
which is proved by that Al content in anodic film when the
electrolyte does not contain AlO
2 ions shown in Table 2.
3.3 Corrosion behavior of MAO films
The protectiveness evaluated through potentiodynamic
polarization techniques of different MAO films in 3.5% NaCl
solution is shown in Fig. 5. All curves of polarization indicate
that there is a significant improvement in the corrosion
resistance compared to the untreated material. The corrosion
current is dramatically reduced. The corrosion rates of these
MAO films are about 3–5 orders of magnitude lower than that
of the AZ91D without anodic film. But in other words, the
curves also display that there are finite improvement on
resistance against anodic polarization. There is almost no
822
Wen, Cao, Shi and Zhang
Materials and Corrosion 2008, 59, No. 10
Fig. 4. XRD pattern of MAO films on magnesium alloy AZ91D,
respectively, treated in solutions containing base electrolyte þ 10 g/
L Na3PO4 (a), base electrolyte þ 10 g/L NaAlO2 þ 10 g/L Na3PO4
(b), base electrolyte þ 10 g/L Na2SiO3 þ 10 g/L Na3PO4 (c), and
base electrolyte þ 10 g/L NaAlO2 þ 10 g/L Na2SiO3 þ 10 g/
L Na3PO4 (d)
passivation, and the current increases quite rapidly after
passing the free corrosion potential, which is similar to
Blawert et al.’s work [24]. No marked passivation limits the
development of anodization technology. The lowest corrosion current density of experimental test are 1.158 108A/
cm2 (a), 4.933 109A/cm2 (b), 9.355 108 A/cm2 (c),
and 4.353 109 A/cm2 (d), respectively, which indicates
that aluminates in the electrolyte help increase the corrosion
resistance (Fig. 5b and 5d), while the phosphates can also
promote the corrosion resistance compared with our previous
work [25].
For comparison, there is an obvious characteristic among
the anodic branches of potentiodynamic polarization curves
in Fig. 5c, which present a very high Tafel slope for the active
resolution of the magnesium surface during the polarization
in 3.5% NaCl solution. In other words, for the testing sample
anodized in the electrolyte, which contains Na2SiO3
and Na3PO4, when the anodic polarization curve enters into
the active dissolution zone, the anodic current increases
steeply with the increase in potential. It indicates that the
oxidation film appears as pitting corrosion and deeply
concave pits on the MAO film due to the corrosion products
dissolving into solution. In contrast, the anodic branches of
potentiodynamic polarization curves in Fig. 5d present a
lower Tafel slope and a comparatively obvious passivation zone (about 150 mV) as seen at line 3 (base electrolyte þ 10 g/L NaAlO2 þ 10 g/L Na2SiO3 þ 10 g/L Na3PO4),
which indicates that the oxidation film obtained from the
electrolyte containing NaAlO2, Na2SiO3, and Na3PO4 is
Table 2. EDAX analysis of MAO films on magnesium alloy
AZ91D treated in solutions containing different additives
Elements (at%)
Al-Si-P-film
Si-P-film
Al-P-film
P-film
O
Mg
Al
P
Si
57.13
62.76
69.39
66.50
35.17
32.12
26.06
31.05
4.52
1.11
3.16
0.83
0.50
0.31
0.30
0.39
1.36
1.34
–
–
effective to hold back the corrosive medium (Cl) to transfer
through the outer porous layer and reach the inner barrier
layer of MAO film slowly with increasing anodic potential
during the polarization. This effect is mainly due to the
compactification and stableness of this film which contains
sintered glassy phase of compound enforced by additive
of Na3PO4. The trait of curves shown in Fig. 5a and 5b is
similar, which presents comparatively low increasing rate of
current density except for the distinct passivation zone.
Figure 6 shows the Nyquist and Bode plots of anodic
films formed with different additives and nude AZ91D. The
time constants of the impedance spectra are determined
through the method developed by de Wit and coworkers
[26] and there are two time constants above the real axis of
each impedance plot. The high capacitive may originate
from the discrete oxide film formed in the air for bare
AZ91D and the oxide film for the anodized AZ91D,
respectively, while the medium frequency one may be
attributed to the corrosion reaction. According to the
special morphological structure and the studies of Mansfeld
[27] and Lopez et al. [28] about the porous aluminum oxide
film, EIS data above the real axis are analyzed using two
time constants equivalent circuit (Fig. 7) and Boukamp
program to obtain the charge transfer resistance, where Rs1
represents for solution resistance on the electrode surface,
Rs2 solution/products resistance in holes [29], CPE1 and
CPE2 for electrode surface capacitance and the solution/
metal interface capacitance in holes, Rct for charge transfer
resistance. The equivalent circuit is also used for the
impedance analysis of long time immersion of the anodic
film in the NaCl solution.
As shown in Fig. 6, the corrosion resistance of magnesium
electrodes covered with the MAO film is increased obviously
by 2–3 orders than that of naked AZ91D without film, which
indicates that the anodized AZ91 possesses higher anticorrosion performance than the naked AZ91D alloy. The
main difference in the Bode plot of the MAO films obtained
from the solution containing different additives appears in
the low frequency (LF) range of Bode plots, while LF range
impedance corresponds to the inner layer properties [30]
which contribute chiefly to the corrosion resistance of MAO
film [31]. By comparison, Si-P-film and Al-Si-P-film
represent higher corrosion resistance of inner barrier layer,
and the similar characteristic is shown in Fig. 5b. No marked
difference of the anodic film formed with different additives
is found by EIS results. From Fig. 6, it can be seen that both
Nyquist and Bode plots contain a low-frequency inductive
component, which has been interpreted as a manifestation of
pitting or has been attributed to the formation and
precipitation of a salt film. For the nude AZ91D alloy,
discrete and thin film formed in air presents pitting corrosion,
while, in the case of anodic film with 10 mm thickness
(Fig. 2e–h), the aggressive particles, such as Cl, must
diffuse into the film un-consecutive pores. The inductive
component originated from the anodic film and nude film is
different [29]. The long immersion corrosion EIS analysis
results are shown in Fig. 8. The reaction resistance of the SiP-film declines slightly for immersion in 3.5 wt% NaCl
solution for 25–100 h as shown in Fig. 8, while the reaction
resistance of the P-film reaches the minimum value after
immersion in NaCl solution for about 20 h. Different
corrosion processes of these MAO films may be due to their
different morphological structures and chemical composition. It can be seen from Fig. 2g and 2h, that the Si-P-film and
Al-Si-P-film are relatively more uniform and comparatively
compact. So the corrosive medium (Cl) is hard to penetrate
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Materials and Corrosion 2008, 59, No. 10
Effect of phosphate on MAO of AZ91D magnesium
823
Fig. 5. Potentiodynamic polarization curves of the anodized AZ91D obtained in different additives containing electrolyte [(a) base
electrolyte þ x g/L Na3PO4, (b) base electrolyte þ 10 g/L NaAlO2 þ x g/L Na3PO4, (c) base electrolyte þ 10 g/L Na2SiO3 þ x g/L Na3PO4,
and (d) base electrolyte þ 10 g/L NaAlO2 þ 10 g/L Na2SiO3 þ x g/L Na3PO4]
through the outer porous layer quickly and react with the
inner barrier layer as well as the substrate. On the other hand,
the reason for the Al-P-film reflecting much more low Rct
value than other films at the early stage of immersion could
be due to the existence of many micropores and cracks
(Fig. 2b and 2f). The Rct moves up and down in Fig. 6b when
the electrolyte contains aluminates, which maybe due to the
self-sealing process of the anodic film in the solution
containing Cl [25,29] but from pitting corrosion. In other
words, the magnesia which is the main component of the
MAO film from the XRD plot may be converted to hydrate
magnesium in the solution containing Cl ions. Because
the intermedium products (hydrate magnesium) adsorb
at the interface of the inner barrier layer and substrate
to block the discharge channel, it enhances the corrosion
resistance capability of Al-P-film and Al-Si-P-film until
the intermedium products are transferred and diffused
into the solution. The EIS results indicate that Si-P-film
and Al-Si-P-film represent comparatively good corrosion
resistance than Al-P-film and P-film in a long immersion test,
which is a little different compared with the polarization
results.
Fig. 6. EIS plots of anodic film formed with different additives
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Fig. 7. The equivalent circuit for nude and anodized AZ91D Mg
alloys in neutral 3.5 wt% NaCl solution (pH 7.03)
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Wen, Cao, Shi and Zhang
Materials and Corrosion 2008, 59, No. 10
5 References
Fig. 8. Rct of different anodic film simulated by Fig. 7 versus time
4 Conclusion
(1) Process of the MAO film on magnesium AZ91D in
alkaline solution with Na3PO4, NaAlO2, and Na2SiO3
was investigated. Use of AC power in positive and
negative current intervals significantly increased the
coating rate that was caused by increase in microdischarges number and intensification of plasma-assisted
thermochemical synthesis.
(2) The additive of Na3PO4 is effective to enhance the
occurrence of sparking and bring abundance energy
which is beneficial to the formation of glassy phase of
the compound in the alkaline solution with the additive
of NaAlO2 and Na2SiO3 simultaneously. Optimal values
of additive contents are found in the electrolyte containing 10 g/L Na3PO4, 10 g/L NaAlO2, and 10 g/L
Na2SiO3.
(3) Through microarc oxidation, ivory-white ceramic coating composed of outer porous layer and inner barrier
layer is formed on the surface of magnesium AZ91D, by
which, particularly the inner barrier layer section, the
corrosion resistance of magnesium AZ91D is greatly
improved. The electrochemical results including polarization and EIS indicate that the anodic film formed in
the base electrolyte with 10 g/L Na3PO4, 10 g/L NaAlO2,
and 10 g/L Na2SiO3 presents a good corrosion
resistance, and a passivation zone in the anodic branch
which will be distinguished from other anodic films.
Acknowledgements: This work was supported by
National Natural Science Foundation of China (grant no.
50671095 and 2007BAB27B04) and the Natural Science
Foundation of Ministry of Science and Technology of China
(2005DKA10400-Z5).
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(Received: November 12, 2007)
(Accepted: January 2, 2008)
W4169
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