Materials Transactions, Vol. 50, No. 3 (2009) pp. 671 to 678
#2009 The Japan Institute of Metals
EXPRESS REGULAR ARTICLE
Corrosion Resistance of Plasma-Anodized AZ91 Mg Alloy
in the Electrolyte with/without Potassium Fluoride
Duck Y. Hwang* , Yong M. Kim and Dong H. Shin
Department of Metallurgy and Materials Science, Hanyang University, Ansan 426-791, Korea
Plasma Electrolyte Oxidation (PEO) behavior of AZ91 Mg alloy was investigated in the electrolytes with/without potassium fluoride.
Growth rate of coating thickness in the electrolyte containing potassium fluoride (Bath B) was much higher than that in the electrolyte without
potassium fluoride (Bath A). The oxide layer formed on AZ91 Mg alloy in electrolyte with potassium fluoride and sodium silicate consisted of
MgO, MgF2 and Mg2 SiO4 . Corrosion current density of oxide layer coated from the electrolyte with potassium fluoride was much lower than
that of oxide layer coated from the electrolyte without potassium fluoride. From the result of EIS analysis, it was known that inner barrier layer in
the oxide layer coated from the electrolyte with potassium fluoride had a good influence of the corrosion resistance of Mg alloy. The corrosion
resistance curves of Bath B were similar to the thickness curves, indicating that the thickness of the oxide layer played an important role in
corrosion resistance of AZ91 Mg alloy. The oxide layer in the Bath B containing potassium fluoride was found to be a compact barrier-type
passive film in presence of fluoride ions. The existence of the dense MgO and MgF2 in the barrier layer had a favorable effect on the corrosion
resistance of the AZ91 Mg alloy formed from Bath B by PEO process. [doi:10.2320/matertrans.MER2008345]
(Received September 24, 2008; Accepted December 10, 2008; Published January 28, 2009)
Keywords: magnesium alloy, plasma electrolytic oxidation, corrosion, oxide layer
1.
Introduction
Magnesium alloys have a superior strength-to-weight
ratio, high dimensional stability, lower density and good
electromagnetic shielding than several other alloys but
relatively poor corrosion resistance, especially in acidic
environments and the saltwater conditions, because Mg is
electrochemically the most active metal.1–4) Therefore, it is
desirable to alter the surface properties of Mg and its alloys in
order to improve its corrosion resistance. Various surface
treatments such as electrochemical plating, conversion coating and anodizing have been used to increase the corrosion
resistance of magnesium alloys.1,5,6) However, conventional
surface treatment method has the disadvantages of low
corrosion resistance, complicated process of manufacture,
low productivity and waste problems. Therefore it is worth
developing new process of surface treatment to solve the
problems.
Plasma electrolytic oxidation (PEO) is one of the electrochemical surface treatment methods, which form the oxide
layer on magnesium alloys in plasma state generated by
applying extremely high voltage in a suitable electrolyte.7)
The structures of oxide layer on the Mg and Mg alloys
fabricated by PEO process depend on various processing
conditions, including chemical composition and concentration of electrolyte, electric parameters, alloy composition of
substrate, pretreatment and post treatment. Especially, the
chemical composition of the electrolyte exerts a considerable
influence on the property and formation of effective oxide
layer for Mg alloy.
Multi-component electrolytes such as phosphate, silicate,
and aluminate based on potassium hydroxide are used in PEO
process in order to improve the corrosion resistance of
magnesium alloys. In general, microstructures of oxide layer
in PEO process consist of an outer porous layer and an inner
barrier layer.8,9) It was reported that the composition and
*Corresponding
author, E-mail: [email protected]
quality of the barrier layer had a considerable influence on
the corrosion resistance of coated Mg alloy.10) Therefore,
it is very important to control composition and quality of
barrier layer during the PEO process. Therefore, it is
imperative to select proper electrolyte compositions to
increase the corrosion resistance of magnesium alloys.3,11)
Some researchers reported that the presence of F ion played
an important role in the formation of oxide layer.9,12)
However, there is a lack of systematic study on the effect
of F ion on corrosion resistance in Mg alloy coated by PEO
process. In this study, the influence of potassium fluoride in
the electrolyte on the structure of oxide layer of AZ91 Mg
alloy was reported and the corrosion resistance was also
evaluated by electrochemical analysis.
2.
Experimental Procedures
Commercial AZ91 ingot was used in this study. Prior to the
experiments, AZ91 Mg alloy plates with 30 50 2 mm3
were mechanically polished to 1000 grit emery paper finish,
rinsed with de-ionized water, ultrasonically cleaned in
ethanol, and finally dried in warm air. PEO process was
conducted with 20-kW equipment which had a glass-vessel
container with a sample holder as the electrolyte cell, a
stainless steel used as the cathode, stirring and cooling
system. Two kinds of electrolyte were used in this study.
Table 1 lists the chemical compositions and constituents of
electrolyte. Temperature of electrolyte was maintained at
20 to 30 C during PEO process. Applied current density is
controlled at 10 A/dm2 .
Table 1 Electrolyte compositions for PEO process in AZ91 Mg alloy.
Electrolyte
KOH (M/L)
KF (M/L)
Na2 SiO3 (M/L)
Bath A
0.08
—
0.04
Bath B
0.08
0.08
0.04
672
D. Y. Hwang, Y. M. Kim and D. H. Shin
Surface morphology and cross-sectional images of the
oxide layer were observed using a scanning electron microscope (HITACHI, S-4800) with energy-dispersive spectroscopy (EDS). Phase analysis of the oxide layer was analyzed
using X-ray diffraction with Cu-K radiation and excitation
source at a grazing angle 2 . Surface roughness of the oxide
layer was determined using a laser scanning microscope.
Thickness of oxide layer was measured from the crosssection morphologies. Cross-sectional TEM (TECHNAI G2
instrument) was used to characterize the oxide layer (30 nm
thick) formed on AZ91 Mg alloy. Samples for TEM
observation were prepared using focused ion beam (FIB)
milling.
After the PEO process, the corrosion resistance of the
specimen was subjected to a salt spray test for 720 ks. In
compliance with ASTM standard B117, the chamber temperature was held at 35 C, and a salt solution of 5 mass% NaCl
and pH ¼ 7:0 was used. Electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization were
utilized to evaluate the corrosion resistance of the coated
AZ91 Mg alloy and carried out in a 3.5 mass% NaCl solution
using a Reference 600 potentiostat (Gamry Instruments,
Warminster, PA, USA). After stabilization of the electrochemical testing system, the following parameters were used:
the scanning rate of polarization was 1 mV/s, the EIS signal
amplitude was 10 mV, and the frequency range was between
0.1 and 106 Hz. The electrochemical measurement was a
conventional three-electrode cell with the AZ91 Mg alloy
sample as the working electrode, a carbon plate as the counter
electrode, and a saturated calomel electrode (SCE) as the
reference electrode.
3.
Results and Discussions
3.1 Characteristics of oxide layer on AZ91 Mg alloy
Figure 1 shows voltage-time behaviors of AZ91 Mg alloy
in two different electrolytes. PEO process in AZ91 Mg alloys
was typically divided into three stages regardless of composition and concentration of electrolyte. In the first stage of
PEO process, the voltage increased linearly until it reached a
breakdown voltage as depicted in Fig. 1. In first region, no
apparent sparks were found on the metal surface, a thin
25
400
20
Coating thickness, t / µ m
500
Critical voltage
Voltage (V)
transparent passive film was formed and oxygen resulting
from oxidation of water and hydroxyl anions was evenly
absorbed on the anode surface. In the second stage, a large
number of small size sparks was observed as a white light
scanning over the metal surface rapidly and randomly but
distributed evenly over the whole metal surface. This point is
breakdown voltage. The cell voltage increased with a rate
slower than that in the initial linear stage after the breakdown
voltage. In the third stage, steady sparking was established on
the anode surface and the cell voltage reached a relatively
stable value. Gradually, the spark size was increased, and
density decreased. As shown in Fig. 1, it was observed that
cell voltage of Bath B containing potassium fluoride was
rapidly increased, compared to increasing rate of cell voltage
of Bath A after occurring breakdown voltage. Final voltage
of Bath B coated for 600 s was higher than that of Bath A.
The properties of PEO process such as breakdown voltage,
critical voltage, and final voltage strongly depended on
composition and concentration of electrolyte. It was reported
that the addition of compounds containing fluorine ion in the
electrolyte helped to increase electrical conductivity of
electrolyte.7) Verdier13) reported that the cell voltage of
PEO process was important factors in the process parameter,
especially influencing growth rate of oxide layer. Therefore,
it was considered that potassium fluoride in the electrolyte
played a considerable influence on growth and properties of
the oxide layer during PEO process.
The changes of coating thickness as a function of coating
time in electrolytes with and without potassium fluoride is
shown in Fig. 2. The growth rate of Bath B exhibited much
faster than that of Bath A. When the coating time was 600 s,
thickness of oxide layer coated in the Bath A and B was
measured to be 12:69, 18.75 mm, respectively. Growth rate
of oxide layer coated in the Bath B containing potassium
fluoride was about 1.9 mm/min. Guo11) reported that the cell
voltage and time corresponding to the appearance of sparks
on the anode surface was defined as breakdown voltage and
ignition time strongly depended on the concentrations and
constituents of the electrolyte and a strong influence on the
growth rate of coating. Therefore, type of electrolyte was one
of the important factors influencing growth rate of the oxide
layer on AZ91 Mg alloy.
300
200
Breakdown voltage
100
AZ91 Mg alloy
Current density : 10 A/dm2
Bath A
Bath B
Stage |||
Stage ||
Stage |
AZ91 Mg alloy
Bath A
BathB
15
10
5
0
0
0
0
100
200
300
400
500
600
700
100
200
300
400
500
600
700
Coating time, t /sec
Time, t/sec
Fig. 1
Voltage-time curves of PEO process in AZ91 Mg alloy.
Fig. 2 Change of coating thickness with coating time of PEO process in
AZ91 Mg alloy.
Corrosion Resistance of Plasma-Anodized AZ91 Mg Alloy in the Electrolyte with/without Potassium Fluoride
1.6
AZ91 Mg alloy
current density : 10 A/dm2
Bath A
Bath B
Surface roughness, Ra / µ m
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
0
100
200
300
400
500
600
700
Coating time, t / sec
Fig. 3 Variation of surface roughness with coating time of PEO process in
AZ91 Mg alloy.
The changes of the surface roughness (Ra ) as a function of
coating time in two different electrolytes were shown in
Fig. 3. There was no change of surface roughness during the
initial stage of PEO process. Surface roughness increased
with increasing coating time. When the coating time was
600 s, surface roughness of Bath B exhibited much rougher
than that of Bath A. It was considered that surface
morphology of Bath B exhibited more coarse pores due to
the faster growth rate of oxide layer in Bath B, abovementioned in Fig. 2. It was believed that the electrolyte
containing potassium fluoride had an influence of growth
rate of oxide layer on AZ91 Mg alloy.
Figure 4 shows surface morphology of oxide layers in
AZ91 Mg alloy coated in two different electrolytes. In case of
Bath A coated for 120 s, the traces of scratches which were
formed during mechanical polishing before PEO process
were observed and fewer pores were observed. Especially,
wide gaps were observed at the interface between -Mg and
673
phases and inside of phases (Fig. 4(a)). Such a big gaps
might be generated by the difference of oxide formation
during the PEO process for and phases.14,15) Distribution
of irregular and coarse pores of oxide layer coated from the
Bath A for 600 s could be related to non-uniform distribution
of and phase (Fig. 4(b)). Oxide layer coated from the
Bath B for 120 s showed the typical surface morphology for
PEO process (Fig. 4(c)). For the oxide layer coated from the
Bath B for 600 s, the oxide layer showed surface morphology
of dense crater-like microstructures with some round-shape
shrinkage pores observed in the crater centers. Oxide layer in
Bath A exhibited irregular tiny pores with less than 12 mm
in diameter and sometimes coarse pores are locally existed
(Fig. 4(b)). Bath B showed bimodal distribution of pores due
to coexistence of both relatively coarse pores with about 3 mm
and fine pores with less than 12 mm in diameter (Fig. 4(d)).
The results of the EDS analysis for the oxide layer were
shown in Fig. 5. The component of Bath A consisted of
oxygen, silicon, magnesium, and aluminum. The two later
elements came directly from the oxidation of substrate
(Fig. 5(a)). As shown in Fig. 5(b), fluorine ion was detected
in the Bath B in addition to having component of the Bath A.
Figure 6 shows cross-sectional images of oxide layers in
AZ91 Mg alloy coated in two different electrolytes. In
general, the oxide layer formed on the AZ91 Mg alloy during
the PEO process composed of two different layers; an outer
porous layer and an inner barrier layer, which was very thin
layer with the thickness of 100300 nanometers.8,16,17) The
pores only existed in the outer porous layer and interconnected with each other, but did not crossover through the
inner barrier layer to the AZ91 Mg alloy substrate. A lot of
pores in the porous layer were observed in both samples.
Figure 7 shows the EDS analysis on the oxide layer/substrate
interface in the Bath B coated for 10 min. There was a
difference of the composition between the barrier layer and
the outer layer, especially for the fluorine content (Fig. 7(d)).
Fig. 4 Surface morphology of oxide layer of AZ91 Mg alloy formed by PEO process at 10 A/dm2 in electrolyte with/without potassium
fluoride; (a)(b): samples coated from Bath A for 120 and 600 s, respectively, (c)(d): samples coated from Bath B for 120 and 600 s,
respectively.
674
D. Y. Hwang, Y. M. Kim and D. H. Shin
Mg
(a)
Intensity
1500
O
1000
Si
500
Al
Zn
0
0
1
2
3
4
5
6
2500
Mg
Intensity
2000
(b)
O
1500
1000
Si
500
F
Al
K
Zn
0
0
1
2
3
4
5
6
Fig. 5 Energy-dispersive spectra of AZ91 Mg alloy coated by PEO
process; (a) Bath A, (b) Bath B.
The EDS line scanning showed a fluoride enriched zone at
the oxide layer/substrate interface. It was considered that
fluorine ion in the electrolyte played an important role in the
formation of barrier layer during the initial stage of PEO
process. These observations were in agreement with the
results of Cai and Liang.9,12) It was also reported that the
barrier layer had an influence on the corrosion resistance of
magnesium alloys although the layer existed very thin.18)
In order to evaluate microstructure in the oxide layer in
detail, the oxide layer was analyzed using cross-sectional
TEM. Figure 8 shows cross-sectional TEM images of the
sample in Bath A and B coated for 300 s. According to TEM
observation in Bath A (Fig. 8(a), (b)), oxide layer of the
Bath A mainly consisted of amorphous structure. A lot of
pores in the oxide layer were observed on the whole.
Sometimes, nanocrystalline structure was locally observed in
the oxide layer (Fig. 8(a)). The SAD pattern in the oxide
layer in Bath A was dim, indicating amorphous structure. The
amorphous structure of outer layer might be caused by rapid
solidification of oxide layer near surface. The plasma, which
was generated as an arc-shape on the surface, could melt
down the oxide layer causing the craters in the surface. If
arcs fade out by depletion of ions in plasma, melted oxide
would be solidified rapidly by heat loss through convection
Fig. 6 Cross-section of oxide layer of AZ91 Mg alloy in electrolyte with and without potassium fluoride coated for 600 s; (a) sample
coated from Bath A, (b) sample coated from Bath B.
Fig. 7
Cross-section SEM micrograph (a) and EDS spectra of Mg (b), O (c) and F (d) of AZ91 Mg alloy in Bath B coated for 600 s.
Corrosion Resistance of Plasma-Anodized AZ91 Mg Alloy in the Electrolyte with/without Potassium Fluoride
675
Fig. 8 TEM micrograph of oxide layer of AZ91 Mg alloy formed by PEO process at 10 A/dm2 in electrolyte with/without potassium
fluoride; (a), (b): samples coated from Bath A for 300 s, respectively, (c), (d): samples coated from Bath B for 300 s, respectively.
3500
3000
Intensity, I / a.u.
and conduction to electrolyte. In this moment, oxide close to
surface would be solidified much faster than inner layer.
Therefore, outer layer could be formed as amorphous phase.
Also, gases, existed inside of melted oxide by turbulence,
could not escape, but they were trapped inside of oxide layer
causing the big voids. It was observed that a lot of pores were
existed in the inner layer on the substrate (Fig. 8(b)).
Therefore, it was difficult to explain the difference between
outer porous and inner barrier layer because of inner layer
with the non-dense structure. In contrast, in case of Bath B,
oxide layer consisted of porous and barrier layer (Fig. 8(c)).
There was a clear line between porous and barrier layer. The
barrier layer was very thin and dense layer without pores. The
volume fraction of nanocrystalline structure in Bath B was
much larger than that of Bath A (Fig. 8(c)). It was known
from Fig. 8(d) that the substrate and inner layer was
compatible each other and no pores was also observed. It
was observed that the inner barrier layer contained higher
amount of the fluorine ions, compared to the outer porous
layer above-mentioned in Fig. 7.
XRD patterns of the oxide layer of the AZ91 Mg alloy
coated from Bath B for 600 s are shown in Fig. 9. Oxide layer
coated from Bath B containing fluorine ion consisted of
MgO, Mg2 SiO4 and MgF2 . Liang12) showed that electrolyte
containing fluorine ion in the potassium hydroxide-silicate
solution played an important role during the initial film
formation. Therefore, it was indicated that fluorine ion in the
electrolyte participates in the reaction and were incorporated
into the oxide layer. It was considered that MgF2 might be
formed in the oxide layer due to the high chemical reactivity
with magnesium substrate during the PEO process. Especially, it was known that volume fraction of the MgF2 in the
inner barrier layer existed more higher than that of the porous
layer from the EDS line scanning.
Mg
MgO
Mg 2SOi 4
AZ91 Mg alloy
Bath B
2500
MgF2
2000
1500
1000
500
0
20
30
40
50
60
70
80
Scattering angle, 2Θ / degree
Fig. 9 XRD pattern of AZ91 Mg alloy in Bath B containing potassium
fluoride coated for 600 s.
3.2
Corrosion resistance behavior of oxide layer on
AZ91 Mg alloy
Corrosion resistance of the oxide layer was evaluated by
electrochemical potentiodynamic polarization in 3.5 mass%
NaCl solution. Figure 10 shows the potentiodynamic polarization curve of the samples in the electrolyte with and
without potassium fluoride. It was well known that corrosion
potential and current density of coated samples were often
used to characterize corrosion protective property of the
oxide layer.19) In general, it was reported that the oxide layer
with a high corrosion potential and low corrosion current
density exhibited a good corrosion resistance.20) In case of the
samples coated for 120 s, the corrosion current density of
Bath B containing fluorine ion was lower than that of Bath A
without fluorine ion. The tendency of the corrosion current
density for the samples coated for 600 s was similar to the
same as for 120 s. However, their corrosion potential
remained almost at a constant. As the coating time increased,
676
D. Y. Hwang, Y. M. Kim and D. H. Shin
4
2.0x10
AZ91 Mg alloy
coating time : 120 s
current density : 10 A/dm2
uncoated AZ91 Mg alloy
Bath A
Bath B
-1.4
AZ91 Mg alloy
current density : 10 A/dm2
coating time : 600 s
Bath A
Bath B
Fit
(a)
4
1.5x10
Z", mΩ·m2
Potential, E/ V vs SCE
-1.2
-1.6
4
1.0x10
3
5.0x10
-1.8
Bath A
Bath B
-2.0
uncoated AZ91 Mg alloy
0.0
0
4
1x10
4
2x10
4
3x10
4
4x10
4
5x10
2
-9
-7
10
-5
10
-3
10
-1
10
Current density, I / mA/dm2
-1.0
Potential, E / V vs SCE
-1.2
Z', mΩ·m
10
AZ91 Mg alloy
coating time : 600 s
current density : 10 A/dm2
uncoated AZ91 Mg alloy
Bath A
Bath B
Fig. 11 Nyquist plot of oxide layer coated from the electrolyte with and
without potassium fluoride for 600 s.
(b)
-1.4
uncoated AZ91 Mg alloy
-1.6
Fig. 12 Equivalent circuit used for impedance data fitting of oxide layer.
-1.8
Bath B
Bath A
-2.0
-2.2
1E-11
1E-9
1E-7
1E-5
1E-3
0.1
Current density, I / mA/dm2
Fig. 10 Potentiodynamic polarization curves of the coatings formed in two
different electrolytes; (a) AZ91 Mg alloy coated for 120 s, (b) AZ91 Mg
alloy coated for 600 s.
corrosion current density of AZ91 Mg alloy in both samples
decreased and polarization resistance increased. Such improvement of corrosion resistance with time was mainly
related with increasing of thickness of coated oxide layers.
Corrosion current density of an oxide layer coated from Bath
B for 600 s was slower than that of oxide layer coated from
Bath A for 600 s because the thickness of the oxide layer
coated from Bath B was only 18:75 mm, which was more
than thicker than that of the oxide layer coated from Bath A
for 600 s which was 12:69 mm as shown in Fig. 2. This
result clearly showed that the thickness of the oxide layer
played an important role in corrosion resistance of the oxide
layer in AZ91 Mg alloy. Compared to the uncoated AZ91 Mg
alloy, the corrosion current density in the Bath B with
potassium fluoride coated for 600 s decreased by approximately five times and polarization resistance also increased
by approximately three times. These data clearly showed that
the corrosion resistance of Bath B resulted in the better
corrosion protective property of AZ91 Mg alloy in solution
containing chloride ion.
In order to get more information about the corrosion
phenomenon and mechanism, AC impedance measurement
was carried out for oxide layer on AZ91 Mg alloy coated
from electrolyte with and without potassium fluoride. The
nyquist plot for the oxide layer coated for 600 s is shown in
Fig. 11. Similar with the potentiodynamic polarization
experiment, corrosion resistance of oxide layer coated from
the electrolyte with potassium fluoride was superior to that of
oxide layer coated from the electrolyte without potassium
fluoride.
The electrode processes were in series or parallel with
each other due to the complicated structure of oxide layer
on the Mg alloys coated by PEO process. So it was essential
to develop the appropriate models for the impedance which
could then be used to fit the experimental data and extract
the parameters which characterized the corrosion process.
The simplified equivalent circuit on the oxide layer coated
from electrolyte with and without potassium fluoride is
proposed as shown in Fig. 12. In the equivalent circuit, the
electrolyte resistance (Rs) was in series with the unit of the
oxide layer system. Rp was outer porous layer resistance
paralleled with constant phase element (CPEp ). The properties of the inner barrier layer were described by the
resistance Rb in paralleled with CPEb . In order to include
a surface inhomogeneity factor and a possible diffusional
factor, a more general constant phase element (CPE) was
used instead of a rigid capacitive element. Based on
equivalent circuit model in Fig. 12, the nyquist plot is best
fitted and the fitting result is shown in Fig. 11 as solid lines
passing through the testing results. The corresponding
values of the equivalent elements are listed in Table 2. It
was reported that low frequency range of impedance
diagram characterized the inner layer properties and the
high frequency range reflected outer layer.21,22) The oxide
layers obtained electrolyte with and without potassium
fluoride showed the different EIS behavior. The fitting
results really showed that the resistance of inner barrier
layer against relatively thin layer was higher than the
Corrosion Resistance of Plasma-Anodized AZ91 Mg Alloy in the Electrolyte with/without Potassium Fluoride
677
Table 2 Equivalent circuit data of oxide layer coated from the electrolyte with and without potassium fluoride for 600 s.
Rs (mm2 )
CPEp (mFm2 )
CPE1-P
Rp (mm2 )
Rb (mm2 )
CPEb (mFm2 )
CPE2-P
Bath A
13.66
8.49E-5
0.63
8.28E3
4.93E4
5.74E-4
0.86
Bath B
18.04
1.46E-4
0.35
4.67E3
8.08E5
2.73E-6
0.92
galvanic effect between the and the phases in the Mg–Al
alloys in 5 mass% NaCl solution. Above-mentioned in Fig. 4,
therefore, it was considered that corrosion of the sample in
Bath A was occurred at two-phase boundaries due to the
microgalvanic corrosion. In contrast, it was observed that
the surface of the AZ91 Mg alloy in Bath B was entirely not
corroded regardless of coating time. Only pitting corrosion
was locally observed in Bath B coated for 120 s. The
corrosion resistance curves of Bath B are similar to the
thickness curves, indicating that the thickness of the oxide
layer played an important role in corrosion resistance of the
oxide layer in AZ91 Mg alloy. It was believed that the
corrosion resistance of AZ91 Mg alloy depends on the
existence of fluoride compound in the oxide layer. Compact
barrier-type passive film in Bath B containing potassium
fluoride was formed due to the presence of fluorine ions.
4.
Fig. 13 Appearance of coated AZ91 Mg alloy after salt spray test (720 ks).
corresponding value of the outer porous layer. When
chloride ions penetrated through the outer porous layer
reached the inner barrier layer, penetration process of
chloride ion into barrier layer was a slow diffusion reaction
for its much compact structure layer. This showed that inner
barrier layer in the oxide layer coated from the electrolyte
with potassium fluoride had a good influence of the
corrosion resistance of Mg alloy. This layer mainly
composed of MgO and MgF2 . It was considered that
fluorine ion played an important role on the property of
barrier layer and corrosion resistance of AZ91 Mg alloy.
Although growth rate of oxide layer in the Bath B was
higher than that of Bath A due to the presence of potassium
fluoride ion in the electrolyte, most of the layer growth was
focus on the growth of porous layer as shown in TEM
results. Therefore, it was considered that corrosion resistance of the oxide layer on AZ91 Mg alloy was dominantly
improved due to the structure and composition of barrier
layer rather than thickness of oxide layer.
The results of salt spray test for 720 ks in this study are
shown in Fig. 13. The result of salt spray test was in
agreement with the corrosion resistance of potentiodynamic
polarization tests. Severe filiform corrosion was observed in
surface of AZ91 Mg alloy in Bath A coated for 120 s. In case
of the sample in Bath A coated for 10 min, pitting corrosion
was locally showed in the surface of the sample. It was
revealed that this kind of structure of oxide layer cannot
provide perfect protection for their substrate. Z. Shi23)
suggested that corrosion mechanism of AZ91 Mg alloy
coated by PEO process could be attributed to the micro-
Conclusions
(1) PEO coating behavior of AZ91Mg alloy in the different
electrolytes with and without potassium fluoride was
investigated. Growth rate of coating in Bath B exhibited
much higher than that of Bath A. The oxide layer
formed on AZ91 Mg alloy in electrolyte with potassium
fluoride and sodium silicate consisted of MgO, MgF2
and Mg2 SiO4 .
(2) The corrosion resistance of the sample coated from
electrolyte with potassium fluoride was superior to that
of the sample without potassium fluoride. As a result of
salt spray test, it was observed that the surface of the
AZ91 Mg alloy in Bath B was entirely not corroded
regardless of coating time. The corrosion resistance
curves of Bath B were similar to the thickness curves,
indicating that the thickness of the oxide layer played an
important role in corrosion resistance of the oxide layer
in AZ91 Mg alloy.
(3) The corrosion resistance of AZ91 Mg alloy fabricated
by PEO process depended on the existence of the
fluoride compound in the oxide layer. The oxide layer
formed in Bath B in electrolyte containing potassium
fluoride was found to be a compact barrier-type
passive film in presence of fluoride ions. The existence
of the dense MgO and MgF2 in the barrier layer might
have a favorable effect on the corrosion resistance
of the AZ91 Mg alloy coated from Bath B by PEO
process.
Acknowledgements
This research was supported by a grant from the Center
for Advanced Materials Processing (CAMP) of the 21st
Century Frontier R&D Program funded by the Ministry of
Knowledge Economy (MKE), Republic of Korea.
678
D. Y. Hwang, Y. M. Kim and D. H. Shin
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