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Journal of The Electrochemical Society, 162 (9) C442-C448 (2015)
0013-4651/2015/162(9)/C442/7/$33.00 © The Electrochemical Society
The Role of Intermetallics on the Corrosion Initiation of Twin Roll
Cast AZ31 Mg Alloy
S. Pawar,a,z X. Zhou,a G. E. Thompson,a,∗ G. Scamans,b,c and Z. Fanb
a School of Materials, The University of Manchester, Manchester M13 9PL, United Kingdom
b The EPSRC Centre – LiME, BCAST, Brunel University, Uxbridge, London UB8 3PL, United
c Innoval Technology Ltd., Beaumont Close, Banbury, Oxon OX16 1TQ, United Kingdom
Kingdom
The micro-galvanic coupling between the microconstituent phases and the α-Mg matrix in the twin roll cast AZ31 Mg alloy sheet
of 6 mm thickness has been investigated using scanning Kelvin probe force microscopy (SKPFM) and electron microscopy. The
β-Mg17 (Al,Zn)12 phase at the interdendritic spaces, along with the Al8 Mn5 particles with rosette/flower-shaped morphologies and
Fe-particles have been observed in the surface/near-surface region of the alloy sheet. Volta potential differences exhibited by the
microconstituents relative to the α-Mg matrix followed the order of Fe-particles > Al8 Mn5 intermetallics > β-Mg17 (Al,Zn)12 phase.
The susceptibility to localized corrosion initiation by the intermetallics predicted from the SKPFM was confirmed by immersion
testing conducted in naturally aerated 3.5% NaCl solution at ambient temperature.
© 2015 The Electrochemical Society. [DOI: 10.1149/2.0291509jes] All rights reserved.
Manuscript submitted May 4, 2015; revised manuscript received June 4, 2015. Published June 12, 2015.
Magnesium alloys have gained significant attention from the automotive sector as attractive light weight structural materials that exhibit
desirable properties including high specific strength, good damping
capacity, castability, weldability and machinability.1–3 However, the
poor corrosion resistance of most magnesium alloys limits their use
in many applications.4–8 Therefore, the need for the development of
magnesium alloys with improved corrosion performance has become
increasingly important.
Mg-Al-Zn based (AZ series) magnesium alloys are widely used
for a range of engineering applications. The effect of aluminum addition (<10%) on the corrosion behavior of these magnesium alloys
has been extensively studied.9,10 The improvement in the corrosion
resistance in the Mg-Al based magnesium alloys was attributed to
the formation of the β-phase (Mg17 Al12 ) in the alloys. However, the
β-phase has been reported to exhibit a dual behavior, depending on its
morphology and distribution. Specifically, if the volume fraction of
β-phase is small, it serves as a micro-galvanic cathode, which further
accelerates the corrosion process of the α-Mg matrix and decreases the
corrosion resistance. However, a high volume fraction of the β-phase
may act as barrier to the progressing corrosion front, thus increasing
the corrosion resistance and inhibiting the overall corrosion of the
alloy.11–18
Twin roll casting (TRC), which combines casting and hot rolling
in a single-step process,19,20 has been successfully employed for the
production of magnesium alloy sheet at reduced costs. The TRC process is associated with relatively high cooling rates ranging from 102
to 103 K/s, resulting in fast solidification rates.21 However, the rapid
solidification in the TRC process also influences the formation of
intermetallics in the microstructure. Importantly, the electrochemical
behavior and the corrosion resistance of magnesium alloys largely
depend on the cathodic or anodic behavior of such intermetallics.
Scanning Kelvin probe force microscopy (SKPFM), which is a
non-destructive technique, provides topographic information simultaneously along with the Volta potential distribution of the metallic
surfaces, with a spatial resolution close to the sub-micron scale.22–28
The Volta potential values measured in air on pure metals were reported to be linearly related to the open-circuit potentials measured
in aqueous solutions.22 Also, a linear correlation has been reported
between the Volta potential measured with the Kelvin probe on the
samples covered with a thin layer of electrolyte and their corrosion potentials determined by a reference electrode in the electrolyte layer.24
However, this relationship is not universal and therefore debated. As
considered in the discussions presented by Rohwerder et al.,27 the
overall corrosion behavior also depends on factors including pH, the
electrolyte, the concentration of the electrolyte, etc. where the same
∗
z
Electrochemical Society Fellow.
E-mail: [email protected]
alloy surfaces may exhibit different behavior under different pH conditions, resulting in variable corrosion behaviors. Hence, SKPFM can
be considered as an indicative tool to predict the role of an intermetallic under free corrosion conditions (OCP). The findings reported by
Frankel et al.,22,32 on the Volta potential measurements using SKPFM
on a variety of metallic surfaces demonstrated the relevance of the
technique for corrosion related research.
Considering the local electrochemistry of the metallic surfaces,
scanning Kelvin probe force microscopy (SKPFM) has been successfully employed to investigate the Volta potential differences on various
metal surfaces including pure Al,23 Al alloys,29–34 pure Mg35 and Mg
alloys.34–47
Several magnesium alloys have been characterized using the
SKPFM, including AZ91D,36–43 AM50,42,44,45 Mg-Al-Ca based Mg
alloy,47 AZ8038,42,48 and AZ31.42,49–51 The usual microconstituents reported in Mg-Al based Mg alloys include the β-Mg17 Al12 phase and
the Alx Mny intermetallics. The Volta potential difference values exhibited by the β-phase and the Alx Mny intermetallic phase relative to
the Mg matrix are reported to be ∼10–260 mV and ∼130–450 mV
respectively, which provide an estimation of the local nobility of the
β-Mg17 Al12 phase and the Alx Mny intermetallics with respect to the
alloy matrix. A summary of the Volta potential measurements on different intermetallics in magnesium alloys has recently been reported
by Hurley et al.52
The present study is focused on the micro-electrochemical characterization of the complex dendritic microstructure in the TRC AZ31
Mg alloy sheet using SKPFM, which is complemented with detailed microstructural investigations by scanning electron microscopy
(SEM). The information regarding the nobility of the constituent
phases in the alloy is further correlated to the corrosion behavior
of the alloy in 3.5% NaCl solution.
Experimental
Material.— AZ31 magnesium alloy sheets of 6 mm thickness were
produced by the twin roll casting (TRC) process. The composition
of the TRC AZ31 Mg alloy was determined using a Perkin-Elmer
Optima 5300 dual view inductively coupled plasma atomic emission spectroscope (ICPAES), which detected 70 ppm Fe along with
2.9 wt% Al, 0.88 wt% Zn and 0.34 wt% Mn and remainder Mg.
Specimen preparation and surface characterization.— The sample preparation included sequential mechanical grinding using successive grades of SiC abrasive papers from P240 to P4000, followed
by polishing using 6 μm to 14 μm diamond pastes with a non-aqueous
lubricant, and subsequent ultrasonic cleaning for 15–20 minutes. The
metallographic specimens were etched using an acetic-picral etchant,
comprising 5 ml acetic acid + 4.2 g picric acid + 10 ml distilled
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Journal of The Electrochemical Society, 162 (9) C442-C448 (2015)
Figure 1. Polarized light optical micrograph revealing the etched surface of
the twin roll cast AZ31 Mg alloy.
water + 100 ml (95%) ethanol for up to 10 s, to reveal the microstructural features. Optical microscopy was carried out using a
Zeiss light microscope fitted with a digital camera. Scanning electron
microscopy (SEM) was undertaken using a Philips XL30 field emission gun instrument, equipped with an energy dispersive X-ray (EDX)
analysis facility, at an accelerating voltage of 20 kV. Phase structure
was investigated by X-ray diffraction (XRD), using a Philips X’Pert
diffractometer (Cu Kα = 1.54056 Å)
Scanning Kelvin probe force microscopy (SKPFM).— The Volta
potential difference measurement was conducted using a Nanoscope
III Dimension 3100 atomic force microscope, equipped with an Extender TM electronic module (Digital Instruments Nanoscope). The
Volta potential measurements were simultaneously performed along
with the surface topography, using electrically conductive Si cantilevers with a 20 nm Pt coating and a tip radius of 15 nm. The
fundamental aspects of the SKPFM technique and the detailed procedure for the Volta potential measurements have been briefly explained
by Guillaumin et al.28 In this study, the Volta potential difference values were obtained in laboratory air relative to the potential of the tip.
The measurement was performed at a scan height of 100 nm and a
scan frequency of 0.2 Hz. The alloy specimens were polished with
1
μm diamond paste prior to the Volta potential mapping; hence,
4
the alloy surfaces were expected to be relatively flat. However, the
rate of removal of the Mg matrix was expected to be higher compared to the harder microconstituents present in the microstructure.
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The specimens were immediately mounted on the AFM stage after
polishing, to limit oxidation of the alloy surfaces. The work function
of the cantilever tip, used for the Volta potential measurement, was
obtained by calibration with respect to freshly cleaved, highly oriented
pyrolytic graphite (HOPG). A freshly cleaved HOPG (0001) surface
was chosen because it possesses a good electrical conductivity, which
is constant over a scale of several hundreds of micrometres due to the
crystalline nature of the sample and its flatness.54 The Volta potential
was referenced to the tip. During the data analysis, the Mg matrix was
arbitrarily set to zero, using plane fit, as the relative potential difference of the intermetallics with respect to the Mg matrix was analyzed.
Each area was marked prior to the Volta potential measurements in
order to identify the compositional details of the respective microconstituents using EDX later. The darker regions represent the Mg
matrix, while the individual bright entities indicate the intermetallics.
The final conclusions were made after analyzing at least 50 different
locations on the alloy surface to ensure reproducibility of the results.
The data processing of the Volta potential maps was performed using
WSxM software.55
Immersion testing in 3.5% NaCl solution.— The immersion testing was conducted at ambient temperature in naturally aerated, nearneutral 3.5% NaCl solution, according to ASTM-G31-72.56 The tested
specimens were analyzed using SEM-EDX before and after chemical
cleaning for 1 min in a boiling solution containing 20% CrO3 + 1%
AgCrO4 . After removal of the corrosion products, the specimen surfaces were rinsed in deionized water and acetone, followed by drying
in a cool air stream.
Results
General microstructure.— The optical micrograph of Fig. 1 displays the surface of the TRC AZ31 alloy, which comprises coarse
dendritic grains of average size of ∼600 μm. These dendritic Mg
grains revealed an average secondary dendrite arm spacing (SDAS) of
∼7 μm, which was measured using the line intercept method. The
SEM image and the corresponding EDX elemental maps in Fig. 2
show the distributions of Mg, Al, Zn, Mn and Fe elements in the
TRC AZ31 Mg alloy. The distributions of the fine Mg-Al-Zn and AlMn intermetallic phases on the interdendritic spaces along with the
α-Mg matrix between the dendrite arms are evident from the compositional maps. The β-phase particles revealed an average particle size
of 2–5 μm, and the EDX analyses confirmed an average composition
of 62 at.% Mg, 31 at.% Al and 7 at.% Zn. Interestingly, the distribution of zinc in the β-phase particles is nonunifom, with zinc-rich
regions being clearly revealed in the particles, as shown in Figs. 2a
and 2d. The interdendritic regions show significant aluminum and zinc
Figure 2. (a) SEM micrograph of the TRC AZ31 Mg
alloy, showing the general microstructure of the alloy
surface considered for SKPFM analysis and (b-f) EDX
maps showing the Mg, Al, Zn, Mn and Fe elemental
distributions respectively.
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Journal of The Electrochemical Society, 162 (9) C442-C448 (2015)
Scanning Kelvin probe force microscopy (SKPFM).– Feparticles.— The 3-D surface topography map (Fig. 4a) and the 2-D
Volta potential map (Fig. 4b) show a representative area from the
polished alloy surface. The Fe-particle is clearly evident, revealing a
bright contrast and indicating a Volta potential difference relative to
the α-Mg matrix. The high magnification contoured Volta potential
map of the Fe-particle (Fig. 4c) displays a particle size of ∼500 nm.
The line profile A-B across the Fe-particle shows a height difference of 100 nm (Fig. 4d) and a Volta potential difference of +650
± 95 mV relative to the matrix (Fig. 4e). EDX analysis confirmed the
composition of the particles in bright contrast to be pure Fe.
Figure 3. Backscattered scanning electron micrograph showing the circled
areas of the Al8 Mn5 intermetallics distributed on the surface.
contents (Figs. 2c and 2d), suggesting non-homogeneous distributions
of Al and Zn across the microstructure. Fe particles were also observed
occasionally in the alloy (Fig. 2f).
The backscattered electron micrograph of the etched surface in
Fig. 3 shows the distribution of the rosette/flower shaped intermetallic
particles, highlighted in the circled areas, revealing an average particle
size of 0.2–1.5 μm and a population density of 4 × 107 particles/cm2 .
EDX analyses performed on such intermetallic particles revealed an
average composition of 58 at.% Al and 37 at.% Mn, which is consistent
with the stoichiometric composition of Al8 Mn5 . A minor Mg content
is detected due to the surrounding magnesium matrix as a result of
the relatively small particle size and the typical rosette shaped morphology of the Al8 Mn5 intermetallic particle compared with the X-ray
interaction volume. Transmission electron microscopy performed on
the Al8 Mn5 intermetallic particles has been reported earlier by the
authors.53 It is important to note that the morphologies of the Al8 Mn5
intermetallic particles observed in the current study are significantly
different from those in cast Mg-Al based magnesium alloys.57 Such
rosette/flower-shaped particle morphologies and the relatively fine
sizes can be attributed to the relatively high cooling rates associated
with the TRC process, particularly on the alloy surface.
Al8 Mn5 intermetallic particles.— Figure 5 shows the 3-D surface
topography map and the 3-D Volta potential map from a representative area of the alloy surface. The Al8 Mn5 intermetallic particles are
evident from the bright peaks at the interdendritic spaces, which are
identical to the particle distribution shown in the SEM image of Fig. 3.
The peaks highlight the Volta potential difference between the Al8 Mn5
particles and the Mg matrix. The high magnification contoured Volta
potential map of an individual Al8 Mn5 particle (Fig. 5c) reveals a
particle size of ∼600 nm and the line profile A-B across the Al8 Mn5
particle shows a height difference of 25 nm (Fig. 5d) and a Volta
potential difference of +250 ± 85 mV relative to the matrix (Fig. 5e).
However, the rosette/flower-shaped intermetallic morphology is difficult to capture on the Volta potential map due to the sub-micron
particle size and the low resolution of the scanning cantilever tip.
Again, the composition of the Al8 Mn5 particle was confirmed using
EDX analysis.
β-Mg17 (Al,Zn)12 phase.— The 3-D surface topography map (Fig.
6a) and the 3-D Volta potential map (Fig. 6b) show a representative
area at the grain boundary region. The contoured Volta potential map
of the β-phase is displayed in Fig. 6c, where the line profile (A-B)
reveals a height difference of 100–130 nm (Fig. 6d) and a Volta
potential difference of +120 ± 25 mV relative to the matrix (Fig. 6e).
The bright contrast on the edges of the β-phase, evident in the Volta
potential map, indicates the presence of zinc. The compositions of the
β-phase and the zinc-rich regions were confirmed using EDX analysis.
The average particle sizes and Volta potential differences of the
microconstituents relative to the Mg matrix from the current study are
presented in Table I. It is clear that a relatively large standard deviation
is observed in most of the microconstituents from the Volta potential
Figure 4. (a) 3D surface topography map of the twin
roll cast AZ31 Mg alloy. (b) 2D Volta potential map.
(c) 2D contour plot for the Fe particle. (d) Line (A-B)
indicating the height profile across the Fe particle and
the α-Mg matrix and (e) corresponding Volta potential
difference between the Fe-particle and the α-Mg matrix.
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Journal of The Electrochemical Society, 162 (9) C442-C448 (2015)
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Figure 5. (a) 3D surface topography map of the twin
roll cast AZ31 Mg alloy. (b) 3D Volta potential map.
(c) 2D contour plot of an individual Al8 Mn5 particle.
(d) Line (A-B) indicating the height profile across the
Al8 Mn5 particle and (e) corresponding Volta potential
difference between the Al8 Mn5 particle and the α-Mg
matrix.
Figure 6. (a) 3D surface topography map of the twin roll
cast AZ31 Mg alloy. (b) 3D Volta potential map. (c) 2D
contour plot of the β-phase. (d) Line (A-B) indicating the
height profile across the β-phase and (e) corresponding
Volta potential difference between the β-phase and the
α-Mg matrix.
measurements. This could be attributed to the particle sizes and the
lateral resolution of the SKPFM technique. The rate of removal of
the Mg matrix, during the polishing is higher than the intermetallics,
which are harder. Accordingly, proper care was taken so that a minimal
effect of polishing would be seen and a relatively smooth surface finish
is achieved. The influence of the lateral resolution of SKPFM on the
Volta potential measurements was evident, where particles with larger
Table I. Average particle sizes and the Volta potential differences
(V) of the microconstituents, relative to the Mg matrix.
Microconstituents
Particle sizes
Average V (mV)
Al8 Mn5
Mg17 Al12
Fe particles
0.2–1.5 μm
2–5 μm
< 1 μm
250 ± 85
120 ± 25
650 ± 95
sizes showed higher Volta potential differences, relative to the adjacent
Mg matrix, compared to the smaller particles.
The Volta potential difference values of the Al8 Mn5 particles in the
present study are in good agreement with values reported previously
for AlxMny intermetallic particles in different Mg alloys.36–45,48–51
However, the literature shows a wide range of the Volta potential differences exhibited by the β-phase relative to the Mg matrix, ranging
from 10–260 mV. The relatively large difference in these values was
attributed to the high aluminum content (8–9 wt%) in the alloys. On
the contrary, the twin roll cast AZ31 magnesium alloy contains only 3
wt% Al, but exhibited a Volta potential difference of +120 ± 25 mV
between the β-phase and the magnesium matrix. Interestingly, specific mention of the Volta potential differences for the β-phase in
AZ31 magnesium alloys has not been reported in the earlier literature. As revealed by the EDX analyses, the β-phase also contains
7 at.% zinc. Therefore, the presence of excess zinc in the β-phase
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Journal of The Electrochemical Society, 162 (9) C442-C448 (2015)
Figure 7. Backscattered electron micrographs show (a) localized corrosion initiation (Fe particle), after exposure for 15 minutes in naturally aerated 3.5% NaCl
solution and (b) localized corrosion initiation at the Al8 Mn5 intermetallic site after immersion in naturally aerated 3.5% NaCl solution for 15 minutes.
likely resulted in the increased Volta potential difference and, consequently, the increased nobility of the β-phase.
Immersion testing in 3.5% NaCl solution.— Visual observations
during the immersion testing showed localized sites with cathodic
hydrogen evolution evident from the gas bubble formation on the
alloy surface. Such localized corrosion sites were further examined
using scanning electron microscopy. Figure 7 shows the as-corroded
alloy surface of the twin roll cast AZ31 Mg alloy, after immersion in
3.5% NaCl solution (naturally aerated condition) for 15 minutes. The
as-corroded alloy surface reveals a typical corrosion site (Fig. 7a),
with the presence of an Fe particle evident in bright contrast, which
was confirmed by EDX analysis. The EDX analysis revealed the oxide
formation on the circular part of the corrosion site. The spherical corrosion morphology of the typical site suggests that the corrosion initiation occurred by micro-galvanic corrosion between the Fe-particle
and the magnesium matrix. The localized corrosion site in Fig. 7b
shows the presence of an Al8 Mn5 particle that was confirmed by
EDX analysis. XRD analysis performed on the as-corroded specimen
(Fig. 8) mainly revealed characteristic peaks for α-Mg and Mg(OH)2
(brucite) on the specimen surface.
Additionally, the immersion tested specimens were subjected to
corrosion product removal prior to the microscopic examination. The
corrosion morphologies developed after immersion for 30 minutes
are shown in the secondary electron and backscattered electron micrographs of Fig. 9a and Fig. 9b, respectively. The localized sites
mostly revealed identical shapes to the rosette/flower morphology of
the Al8 Mn5 intermetallics, as shown in Fig. 3, thus providing evidence
Figure 8. XRD analysis of the as-corroded alloy surface in naturally aerated
3.5% NaCl solution after 1 h immersion.
of removal of the Al8 Mn5 intermetallics. At the same time, the β-phase,
which is highlighted in the circled areas also suffered from localized
corrosion attack. However, if the intensity of localized micro-galvanic
corrosion is to be compared, the Mg matrix surrounding the Al8 Mn5
particles suffered from a very severe corrosion attack compared to
the β-phase, which is evident from the immersion testing results. This
resulted in the undermining of the Al8 Mn5 particles, which eventually detached from the alloy. This finding clearly indicates that the
Al8 Mn5 particles generated a much stronger micro-galvanic coupling
with the adjacent magnesium matrix, compared to the β-phase, which
is in agreement with the predictions from the SKPFM analyses. Also,
it needs to be mentioned that the development of the corrosion pits at
the intermetallic sites was locally restricted and had a limited influence
on the corrosion propagation, resulting in the localized dissolution of
the magnesium matrix (Fig. 9).
Discussion
It is well known that in the solidification of cast Mg-Al based
magnesium alloys, the size, shape and distribution of the microconstituents in the resultant microstructure are directly influenced by the
alloy composition and the cooling rates.58,59 The TRC process which
is associated with relatively high cooling rates, particularly on the
surface reduces the solidification intervals available for the formation
of the microconstituent phases. The influence of the reduced solidification intervals was clearly evident from the small particle sizes of
the Al8 Mn5 intermetallics (Fig. 3) and the Fe-particles shown in the
SKPFM data (Fig. 4).
It is also known that the corrosion behavior of magnesium alloys is influenced by the alloy microstructure and, specifically, the
distribution of microconstituents, which are noble compared to the
adjacent Mg matrix.36–45,47–52 Consequently, it has been well documented that the Alx Mny intermetallics and the β-phase present in
Figure 9. Secondary electron micrograph (a) and the corresponding backscattered electron micrograph (b) of the as-corroded surface in naturally aerated
3.5% NaCl solution after corrosion product removal. The micrographs show
evidence of removal of Al8 Mn5 particles at sites (1) and localized corrosion at
the β-phase, indicated by the circled sites (2), after immersion for 30 minutes.
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Journal of The Electrochemical Society, 162 (9) C442-C448 (2015)
the Mg-Al based magnesium alloys behave as potential cathodes that
are responsible for driving the corrosion initiation.36–45,47–52 SKPFM
investigation performed in the current study revealed positive Volta
potential differences of the microconstituents (Figs. 4–6). The average particle sizes and the Volta potential difference values of the
microconstituents relative to the Mg matrix from the current study are
presented in Table I. The Volta potential differences for the Al8 Mn5
intermetallics and the β-phase are in a similar range to the values
reported earlier.36–45,47–52 However, the Volta potential difference for
the β-phase relative to the adjacent magnesium matrix in the twin roll
cast AZ31 magnesium alloy with 3 wt% Al, appears slightly high,
which is possibly due to the presence of excess zinc. It was therefore
clear that the microconstituents, namely the Fe-particles, the Al8 Mn5
intermetallics and the β-phase, were noble compared to the adjacent
magnesium matrix. Consequently, such microconstituents are potential cathodic sites, which are anticipated to preferentially initiate the
corrosion in the magnesium matrix.
As determined from the SKPFM analyses, the Fe-particles and
the Al8 Mn5 intermetallics exhibited an average Volta potential difference of +650 ± 95 mV and +250 ± 85 mV, respectively, relative to the magnesium matrix. The immersion testing results showed
that the Volta potential differences were sufficient to generate microgalvanic coupling that dictates the preferential localized dissolution
of the magnesium matrix. Subsequently, microscopic investigation of
the immersion tested alloy surfaces in 3.5% NaCl solution showed
significant susceptibility to localized micro-galvanic corrosion
(Fig. 7). The observation of individual corrosion initiation sites, after
the removal of the corrosion products, revealed that the corrosion occurring at local sites with the Fe particles and Al8 Mn5 intermetallics
was much more pronounced compared with the local sites with the
β-phase (Fig. 9). It has been well documented that the small atomic
radius of hexagonally closed-packed (HCP) magnesium results in reduced solid solubility of the alloying elements, which consequently
leads to the formation of different intermetallics, namely Alx Mny ,
Mg17 Al12, etc. as well as impurities like Fe, scattered in the alloy
microstructure.58 These intermetallics/impurities lead to the subsequent deterioration in the corrosion resistance, which was also reported by Sudholz et al., using microeletrochemical measurement of
different phases that are usually present in magnesium alloy.46 The immersion in 3.5% NaCl has shown the direct nobility of the different
particles and their role on the corrosion initiation. However, the long
term behavior of the alloy both in immersion and under atmospheric
conditions might be different.
Conclusions
1.
2.
3.
4.
The rapid solidification rates in the TRC process showed a direct influence on the particle sizes and morphologies of the constituent phases formed in the AZ31 magnesium alloy. The Al8 Mn5
intermetallics have an average particle size of 0.2–1.5 μm,
with rosette/flower shaped morphologies while the β-phase exhibited a particle size of 2–5 μm.
The average Volta potential difference values exhibited by the
microconstituent phases relative to the alloy matrix, are listed
as follows: Fe-particles +650 ± 95 mV, Al8 Mn5 intermetallics
+250 ± 85 mV and the β-Mg17 (Al,Zn)12 phase +120 ± 25 mV.
The wide range in the standard deviation of the Volta potential
values is attributed to the variation in the particle sizes of the
microconstituents.
The average Volta potential difference values for the β-phase are
within the range of Volta potential difference values reported
earlier. However, for the twin roll cast AZ31 magnesium alloy
with 3 wt% Al, the Volta potential difference exhibited by the
β-phase is relatively high, which is possibly due to the presence
of excess zinc.
The immersion testing of the alloy in 3.5% NaCl solution confirmed that corrosion initiation at local sites with Fe particles and
Al8 Mn5 intermetallics occurred at an apparently higher rate, while
C447
the β-phase acted as a relatively ineffective cathode, consistent
with the Volta potential difference values.
Acknowledgments
The authors are grateful to the Engineering and Physical Sciences
Research Council UK for support of the TARF-LCV grant and the
LATEST2 Programme Grant.
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