Materials Sciences and Applicatio ns, 2011, 2, 244-250
doi:10.4236/msa.2011.24031 Published Online April 2011 (
Copyright © 2011 SciRes. MSA
Effect of Scandium Doping on the Corrosion
Resistance and Mechanical Behavior of Al-3Mg
Alloy in Neutral Chloride Solutions
Zaki Ah mad1, Abdul Aleem B. Jabbar1, Kachalla Abdullahi1, Mohamma d Abbas2
1Mechanical Engineering Department, King Fahd University of Petroleum & Minerals, Dhahran, Saudi Arabia; 2Cathodic protection
Engineer, Arabial Petrochemical Company, Jubail, Saudi Arabia.
E-mail:{ahmadz, abaleem},
Received July 24th, 2010; revised January 31st, 2011; accepted February 28th, 2011.
Scandium addition significan tly alters th e corro sion resistance and mechan ica l streng th of Al-3 Mg alloys. The add ition
of 0.3% - 0.4% scandium with 0.14% zirconium has a beneficial effect on the corrosion resistance of the alloy under
smoothly stirred condition. Addition of 0.3% Sc significantly suppresses corrosion under dynamic flow conditions. It
also creates an optimal strengthening effect on the alloys. The corrosion resistance is attributed to the strong passive
layer of Sc2O3 formed on the ultrafine coherent precipitates of Al3Sc. A strong evidence of the pinning of grain bounda-
ries by coherent nano Al3Sc precipitates is responsible for the strengthening effect.
Keywords: Corrosion, Polarization, Passive, Pitting, Strengthening Effect, Pinning, Grai n Bou ndari es, Precipitates,
Films, Passive Layers
1. Introduction
Scandium addition has a strong influence on the proper-
ties of aluminum and aluminum alloys. Aluminum alloys
doped with scandium have shown improved mechanical
strength, hot cracking resistance, weld strength and age
hardening response [1-4].
Scandium is a strong modifier of a cast grain structure
which makes it possible to obtain continuously cast
billed with a non-dendrite structure. The effect of Al3
(Sc1-xZrx) is similar to that of Al3 Sc because grains can
be refined permanently by Al3(Sc1-xZrx) [5]. The im-
proved hardness is attribution to decreasing dendrites and
dispensed secondary phase what inhibit movement of
dislocation [6-8]. The optimal concentration of Sc has
been reported to be 0.3 wt% with Al-2.5Mg [9]. In the
presence of zirconium a non dendritic structure is formed
with lower scandium contents. The formation of coherent
Al3Sc ultra fine precipitates controls the mechanical
properties and corrosion resistance of Al-Mg alloys. The
creation of ultra fine precipitates thermodynamically is
essential to bring about a significant improvement in
high temperature mechanical strength and inhibition of
re-crystallization of aluminum alloys. Addition of scan-
dium and zirconium increases the tensile strength by pin-
ning of grain boundaries by Al3(Sc1-xZrx) precipitates
[10]. In recent years scandium doping has also been ex-
tended to Al MMC’s. In aluminum based MMC material
with minor additions of Sc was found to be less suscepti-
ble to corrosion than the base alloys particularly in neu-
tral and alkaline solutions. At high temperature the com-
posite was characterized by an extended passive region
accompanied by lower corrosion rate [11]. The Al3Sc
precipitates only affect the mechanical strength but sig-
nificantly suppresses the corrosion rate of Al-Mg alloys.
Whereas most work has been focused on mechanical
strength there is a lack of published data on the corrosion
behavior of Sc doped aluminum alloys. It has been
shown that resistance to stress corrosion cracking can be
increased by overaging due to un-crystallized grain struc-
ture together with non-interconnected nature of grain
boundary precipitates [12].
Despite significant progress made in recent decades,
not much has been reported to establish a relationship
between micrstructure, mechanical strength and corro-
sion resistance. This paper describes the critical role of
ultrafine Al3(Sc1-xZrx) precipitates on the mechanical
mincrostructural and corrosion resistance of Al-2.5Mg
alloys and a correlation between the above three parame-
Effect of Scandium Doping on the Corrosion Resistance and Mechanical Behavior of 245
Al-3Mg Alloy in Neutral Chloride Solutions
2. Experimental
2.1. Fabrication of Alloys
Five Al-Mg (2.87 - 2.96 Mg) alloys were doped with
scandium (0.15 - 0.9 wt%) scandium and 0.14 wt% zir-
connium. They were made by induction melting in a re-
crystalized aluminum crucible under an argon atmos-
phere. The scandium powder was prevented by air con-
tact by covering with an aluminum foil before dipping in
the melt covered by argon. The alloy was chill cast in
copper mold. Strips of 2 mm were obtained by extrusion.
The alloy was fabricated by Light Aluminum Metalle,
Germany in consultation with the principle author. The
chemical composition of the alloy is given in Table 1.
2.2. Experimental Technique
2.2.1. Specimen Preparation
Specimens in the form of 16 mm circular discs were used
for electrochemical investigations. For recirculation loop
test, specimens measuring 70 × 100 mm, 58 × 100 mm,
48 × 100 mm and 40 × 100 mm were prepared. All
specimens were polished with 400 and 600 μm SiC paper
using de-ionized water as lubricant. Final polishing was
done with 0.05 micron aluminum powder. The specimens
were rinsed with acetone and washed with de-mineralized
water prior to commencement of experiment. Samples in
triplicates were used for experiments.
2.2.2. Weight Loss Studies
These studies were conducted in accordance with ASTM
G5-72 [13]. The samples were exposed for 1360 hours
before being re-weighed. The solution was smoothly
stirred by a magnetic stirrer. The solution was exposed to
open air.
2.3.3. Electrochemical Studies
Potentiodynamic polarization, polarization resistance and
cyclic polarization studies were conducted to determine
the corrosion behavior of the experimental alloys.
1) Potentiodynamic studies were conducted in accor-
dance with ASTM standard G5-87. A software Softcorr
III was used to obtain the polarization plots, electro-
chemical parameters and the corrosion rates [14].
2) The Polarization resistance plots were obtained by
applying a controlled potential over a small range of po-
tential ( 25 mV vs SCE) with respect to corrosion po-
tential (Ecorr). The experiments were conducted in accor-
dance with ASTM standards G 59-91 [15].
3) Cyclic polarization measurements were made in ac-
cordance with ASTM practice G61-78 [16]. The speci-
men were polarized for 2 hours at –1200 mV vs SCE.
Polarization was commenced at a scan rate of 100
mV/min and continued in a noble direction until a sharp
rise in the current occurred (Ep). At this point the scan
was reversed until the current reached a very small value
(Epp). The protection potential was determined by the
intersection of reverse anodic with forward anodic po-
larization curve.
2.2.4. Recirculating Loop Studies
A high velocity PVC loop was constructed to study the
effect of velocity. It was comprised of a centrifugal pump
(20 m3/hour), entry and exit valves, manometer, mag-
netic flow meters and different sizes of specimen holders
to create different velocities. A maximum velocity of 3.8
ms–1 was achieved in the loop.
2.3. Microstructure Examination
The surface morphology was investigated by low vacuum
scanning electron microscope (LVSEM) with and EDS
system. A Philips Tecnai F 20 S Twin TEM was used to
conduct surface texture studies. It was fitted with an
EDAX energy dispersive X-ray detector with a S-UTW
window with 30 mm2 active area. The samples were cut
by FIB. The lamella were positioned on a carbon film
and TEM examination was made.
3. Results and Discussion
As shown by the weight loss studies (Figure 1), all ex-
perimental alloys exhibited low rates of corrosion. In
particular, alloys 3 and 4 containing (0.3 and 0.6 wt% Sc)
showed the lower corrosion rates of (0.65 mpy and 0.69
mpy) respectively at the end of 1360 hours of exposure.
The rate of corrosion tends to stabilize after 600 hours of
exposure after rapid initial fluctuations because of break-
down and repair of oxide film of Al2O3, Sc2O3. After 850
hours, a maximum rate of corrosion (0.529 mpy) was
shown by alloy 1 (0% Sc) whereas alloy containing 0.15
and 0.3 wt% Sc showed relatively low rates of corrosion
( 0.45 mpy and 0.4 mpy respectively). The corrosion
rate of alloys rises during initial exposure, but decreases
with increased period of exposure. Alloy 3 and 4 showed
lower corrosion rate.
Figure 2 shows a plot of the effect of Sc concentration
on the corrosion rates of alloys in 3.5% NaCl. Corrosion
rates were determined by Polarization resistance tech-
niques. Alloy 3 containing 0.3 wt% Sc showed the mini-
mum rate of corrosion.
The results obtained were principally from weight loss
studies are in agreements with the potentiodynamic po-
larization resistance studies. From the studies of weight
loss studies conducted it may be concluded that alloying
addition of 0.3 and 0.6 wt% Sc exercises a beneficial
influence on the corrosion rate. As shown by Figure 3
optimum lowering of corrosion rates is shown by alloy 3
(0.3 wt% Sc), increase in addition of Sc tend to increase
the corrosion rate. One of the major concerns of alumi
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Effect of Scandium Doping on the Corrosion Resistance and Mechanical Behavior of
Al-3Mg Alloy in Neutral Chloride Solutions
Copyright © 2011 SciRes. MSA
Table 1. Composition of Al-Mg-Sc-Zr Alloys.
No. Alloy Name Si Fe Cu MnMgCr ZnTi Zr Sc
1 AlMgZr 0.0-Sc 0.0 0.0870.1660.0020.0032.960.0020.00250.03
2 AlMgZr 0.14-Sc 0.15 0.14 0.15
3 AlMgZr 0.14-Sc 0.3 0.14 0.3
4 AlMgZr 0.14-Sc 0.6 0.14 0.6
5 AlMgZr 0.14-Sc 0.9 0.0920.160.0010.0042.870.0010.0070.028 0.14 0.9
02004006008001000 12001400 1600
Time (Hours)
Corros ion R ate (mpy)
Alloy 1
Alloy 2
Alloy 3
Alloy 4
Alloy 5
Figure 1. Variation of corrosion rate of Al-Mg-Sc-Zr alloy with exposure time—The corrosion rate of the alloys decreases
with increased exposure time.
Scandi um (%)
Corrosion Rate (mpy)
Figure 2. Corrosion rates of Al-Mg-Sc-Zr alloys vs Scandium concentration—The lowest rate of corrosion is shown by alloy 3.
num alloys is the ability to withstand pitting. Cyclic po-
larization studies were conducted to obtain pitting poten-
tials of the alloy, however small variations in pitting po-
tentials did not allow to conclusively establishing the
relative pitting resistance of the alloys as shown by Ta-
ble 2.
Studies under dynamic flow conditions in a recirculat-
ing loop showed that alloy without scandium exhibited
the lowest resistance to flow induced corrosion. In-
creased velocity accelerates the rate of corrosion. Alloy
containing 0.3 wt% Sc and 0.9 wt% Sc show higher re-
sistance to increasing velocity.
Effect of Scandium Doping on the Corrosion Resistance and Mechanical Behavior of 247
Al-3Mg Alloy in Neutral Chloride Solutions
Velocity (m/sec)
Corrosion Rate (mpy)
Alloy 1 (0% Sc )
Alloy 3 (0.3%Sc-0.14% Zr)
Alloy 5 (0.9 Sc-0.14% Zr)
Figure 3. Variation of corrosion rates for alloys 1, 3 and 4 with velocity in recirculating loop exposed to 3.5% NaCl solution.
The corrosion rate increased with an increase in velocity. Alloy 3 showed a maximum resistance to velocity.
All the different techniques used to determine the cor-
rosion rate showed that an optimum corrosion resistance
is shown by alloy 3 containing 0.3 wt% Sc. It appears
that the size of Al3Sc precipitate and its distribution pri-
marily controls the corrosion rate as the thickness and
uniformity of the protective films of boehmite (
H2O) and bayerite (
-3Al2O3, H2O) would depend on
how uniformly the passivating layer is formed on the
surface of the alloy. The rate limiting factor is, however
the dissolution of the oxide film formed on the surface.
The study of the kinetics of film formation was beyond
the scope of this work. The evidence of formation of a
needle shaped protective film of boehemite on alloy 2
(0.3 wt% Sc) is shown in Figure 4.
Protective film of Boehmite and Bayerlite are known
to reduce the corrosion rate of aluminum alloys. The
SEM photomicrograph shows the formation of needle
shape film of Boehmite in alloy 4. It has been suggested
earlier that the corrosion potential of the alloy covered
Figure 4. SEM photomicrograph showing the formation of
a protective film of boehmite on alloy 4. The needle shaped
structure is seen on the alloy surfaces.
with dense low defective layers of aluminum oxy-
hydroxide and scandium oxide (Sc2O3) increases and the
reduction rate of the depolarizer and accordingly the cor-
rosion rate of scandium containing alloy decreases [17]
which is consistent with our observations on corrosion
rates. It is also reported that whereas aluminum oxy-
hydroxids (bayerite or boehmite) is formed in the defec-
tivee surface layer because of increased thermodynamic
activity, Sc2O3 which accumulates in the upper part of
this layer [18].
From the studies reported above it is established that
small concentration of scandium (0.3 - 0.6 wt%) have a
beneficial effect on the corrosion resistance, and the pro-
tective films of boehemite and Sc2O3 plays an important
role in corrosion resistance. The precipitates of AlSc
(Sc1-xZrx) are in the nanorange whereas the precipitates in
aluminum alloys like 6061 and 6013 are in the micron
range. This has been confirmed by the FEG-SEM micro-
analytical studies [9]. These particles in contrast offer a
lower resistance to corrosion as in bulk alloys. This is
Table 2. Electrochemical parameter obtained from cyclic
polarization curves.
E1 =
Ep – Ecorr
Al-Mg-Zr 0.0
-Sc 0.0 0 –0.715 –0.783 –0.84 0.127
Al-Mg-Zr 0.15
-Sc 0.15 0.15 –0.721 –0.795 –0.786 0.065
Al-Mg-Zr 0.15
-Sc 0.3 0.3 –0.701 –0.812 –0.758 0.057
Al-Mg-Zr 0.15
-Sc 0.6 0.6 –0.755 –0.803 –0.826 0.071
Al-Mg-Zr 0.15
-Sc 0.9 0.9 –0.766 –0.811 –0.848 0.082
opyright © 2011 SciRes. MSA
Effect of Scandium Doping on the Corrosion Resistance and Mechanical Behavior of
Al-3Mg Alloy in Neutral Chloride Solutions
confirmed by recent studies on nanoprecipitates [9,19].
Mechanical Properties and Microstructures
The mechanical properties of the five experimental alloys
are shown in Figure 5. Alloy 1 without Sc addition
shows the least strength whereas all Sc doped alloys ex-
hibit higher strengths. The results confirm the role of
scandium as a potent strengthener. A maximum increase
in strength is obtained on adding 0.3 wt% Sc as shown
by alloy 3. Addition of Sc beyond 0.6 wt% does not cre-
ate any appreciable gain in the strength. Zirconium addi-
tion increases the tensile strength over the zirconium free
alloy as shown by Figure 5. It has been suggested that
earlier the alloy with Sc contains a higher volume frac-
tion of coherent particles Al3(ZrSc) particles and the sub
grain size of the alloy with scandium is much smaller
than the Sc free alloy [9,18]. The presence of Al3Sc pre-
cipitates (FCC) of nano dimensions are responsible for
age hardening response, strengthening effects and corro-
sion resistance as shown by this work.
The tensile strength of the alloys shows optimum
strength on addition of 0.3 wt% Sc, with higher addition,
the strength begins to decrease as shown in the figure.
The strengthening effect by scandium has been attributed
to the Orwan dislocation looping around the nanosize
coherent precipitates of Al3Sc2 (r = 15 - 20 nm). The pre-
cipitates of Al3(Sc1-xZrx) pin down the grain and sub
grain boundaries and maintain a fine grain size. The
TEM bright image field shows the different types of
small precipitate (Figure 6) of Al3Sc.
Al3 (Sc1-xZrx) are observed on the surface of speci-
mens. The nano precipitates and sub grains are observed
in the figure. An EDS profile taken shows the peaks of
scandium and aluminum. The grain, sub grains and ultra
fine spherical and square precipitates are clearly shown
in the dark field and inverted signal images in Figure 7.
Figure 8 shows a very clear evidence of pinning of the
grain boundaries by the nanosize precipitates of Al3Sc as
shown by our investingations. The pinning of grain
boundaries is a major contribution of Al3Sc precipitates
to the strengthening mechanism of Sc doped alloy.
The ultra fine coherent precipitates shown in Figures
8 and 9 above would allow the formation of a relatively
more uniform oxide film of Sc2O3 with boehemite in con-
trast to the larger and incoherent precipitates of boehmite
and bayerite observed in Al-Mg alloys. The size of the
precipitates not only influences the corrosion behavior
but also the mechanical strength of the scandium doped
aluminum alloys. More work is needed to explore the
properties of the nanosize precipitates and their influence
on the mechanical properties and corrosion behavior of
scandium doped Al-Mg alloys.
4. Conclusions
Based on the investigations conducted following is a
summary of conclusion.
1) Scandium addition has a beneficial influence on the
corrosion resistance of Al-Mg alloys as shown by weight
loss and electrochemical and microanalytical studies.
2) The amount of alloying addition controls the corro-
sion rate as shown by beneficial effect of 0.3 and 0.4
wt% Sc addition on the corrosion rates under smoothly
stirred conditions. Under dynamic conditions, maximum
resistance to corrosion is shown by the alloy containing
0.3 wt% Sc.
3) A passive layer of boehemite is observed to be
formed on the alloy surface with a film of Sc2O3.
4) A maximum improvement in mechanical strength is
obtained by adding 0.3 wt% Sc.
5) The increase in the strength of Al-Mg-Sc alloys is
Figure 5. Combined variation of Yield strength and Ulti-
mate strength of Al-Mg-Sc-Zr alloy with %Sc. The maxi-
mum strength is shown by alloy 3 contains 0.3 wt% Sc.
Figure 6. Different ty pe s of pr eci pitate s ar e clear ly show n in
HAADF STEM image. Al3Sc precipitates on the grain
boundaries and a homogenous distributes of nano precipi-
ates is observed. t
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Effect of Scandium Doping on the Corrosion Resistance and Mechanical Behavior of
Al-3Mg Alloy in Neutral Chloride Solutions
Copyright © 2011 SciRes. MSA
Figure 7. An EDS profile of the precipitates showing the peaks of Aluminum and Scandium thereby indicating the formation
of Al3 (Sc1-xZrx) precipitates.
caused by pinning of Al3Sc precipitates on the grain
conventional Al-Mg alloys by offering higher strengths
and better corrosion resistance.
6) The Al-Mg-Sc alloys offer an improvement on the 5. Acknowledgements
The authors appreciate the support and encouragement
provided by KFUPM in conducting the above investiga-
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