Journal of Minerals & Materials Characterization & Engineering, Vol. 10, No.6, pp.535-551, 2011
jmmce.org Printed in the U S A. All rights reser ved
535
Degradation Behavior of High Chromium Sodium-Modified
A356.0-Type Al-Si-Mg Alloy in Simulated Seawater Environment
M. Abdulwahab*1, 2, I.A. Madugu1, S.A. Yaro1, A.P.I. Popoola2
1Department of Metallurgical and Materials Engineering, Ahmadu Bello University, Zaria,
Nigeria
2Department of Chemical and Metallurgical Engineering, Tshwane University of Technology,
Pretoria, Republic of South Africa
*Corresponding Author:
ABSTRACT
The effect of multiple-step therm al ag eing treatm ent (MSTAT) on th e corrosion characteris tics of
A356.0-type Al-Si-Mg alloy in simulated seawater has been studied. The MSTAT treatment also
consists of Double Thermal Ageing (DTAT- T7), Single Thermal Ageing (STAT- T6), Step-
Quenching and Ageing (SQA). The corrosion of the thermal treated samples was characterized
by electrochemical Potentiodynamics polarization techniques consisting of linear polarization
and chronopotentiometric method using the fit Tafel plot. Generally, from the linear
polarization, the corrosion rate decreases at all temperatures with the ageing time. The
corrosion behavior of the DTAT and SQA Al-Si-Mg alloy in the simulated seawater showed
better resistance than the STAT Al-Si-Mg alloy. Samples in the SQA-STAT have improved
corrosion resistance than the SQA-DTAT one. The chronopotentiometric corrosion study of some
selected samples indicates a decrease in the corrosion resistance with open circuit potential
exposure time. Consequently, the form of corrosion in the studied Al-Si-Mg alloy are slightly
uniform and predominantly pitting corrosion as obtained from the SEM study. The pits diameter
were found to range from 30-50µm.
Key words: pits diameter, MSTAT, degradation behavior, corrosion rate.
1. INTRODUCTION
Foundry aluminium alloys based on the Al-Si system are widely used in the automobile field
since they provide excellent fluidity and castability, good resistance to corrosion and mechanical

536 M. Abdulwahab, I.A. Madugu, S.A. Yaro, A.P.I. Popoola Vol.10, No.6
properties [1-4]. Al-Si alloys are characterized by a wide temperature range in the semi-solid
region [1,5]. Al-Si alloys, which comprise 85% to 90% of the total aluminium cast parts
produced, exhibit excellent castability, mechanical and physical properties [6,7]. Aluminium
alloys has successf ully pen etrated t he auto motiv e market, largely (> 75%) in the form of castings
[8]. Addition of Mg to Al-Si (A356 alloys) is the main solid solution strengthener to aluminium
alloys and responsible to precipitation hardening (PH) to yield higher strength [9-12]. A356
alloys contains silicon but also magnesiu m as main alloying ele ments. It shows response to heat
treatment [1,3,13-16] and increase strength by precipitation of Mg2Si in aluminium matrix [1,17-
24].
Aluminium and its alloys are considered to be highly corrosion resistant under the majority of
available service condition [25]. The various grades of pure aluminium are most resistance,
followed closely by the Al-Mg, Al-Mn alloys. Next in order is Al-Mg-Si and then Al-Si alloy.
The alloy containing copper are the least resistant to corrosion; but this can be improved by
coating each sid e of the copper containing alloy with a thin lay er of high purity aluminium, thus
gaining a three ply metal, i.e. Alclad. This claddi ng acts as a mechanical shield and also protects
the material by sacrificial [26]. When aluminium surface are exposed to the atmosphere, a thin
invisible oxide (Al2O3) skin forms, which protects the metal from corrosion in many
environments [25]. This film protect the metal from further oxidation unless this coating is
destroyed, the material remains fully protected against corrosion [27,28]. The composition of an
alloy and its thermal treatment are of important for susceptibility of alloys to corrosion [29].
In the development and processing of aluminium alloys, various works have been reported on the
degradation of the alloys at various environment and operating conditions. Several methods of
controlling and preventing or minimizing corrosion attach in aluminium alloys have been
demonstrated by different authors. Alloy ing addition have been reported to successfully reduced
corrosion attack in aluminium alloy [30,31]. Other methods of corrosion control include
inhibitors addition [32-34]. Heat treatment (PH) (including DTAT) [31,35-40]. Corrosion
resistance of Al-Si-Mg alloy is much better than the aluminium-copper alloys [41]. On the
microstructural and submicroscopic scales, the electroche mical properties develop point-to-point
non-uniformities that account for changes in resistance of the alloy [42]. Abdulrahman and
Agbodion [43] studied the effect of ageing time and temper on the corrosion of Al-Si-Cu alloy in
varying HCl concentrated acid solution. They noticed that the rate of corrosion of the alloy
increased with increase in concentrations of the acid and d e creased sharply with time.
Khoshnaw and Gardi [44] studied the effect of ageing time and temperature on exfoliation
corrosion of aluminium alloys 2024-T3 and 7075-T6. They observed that with increase in the
ageing time for aluminium alloy type 2024-T3, the susceptibility to exfoliation corrosion
increase while for the 7075-T6 decreased. The intermetallic compounds formed such as CuAl2
and MZn2 phase increase with increase in ageing time for both alloys. Aluminium alloys has

Vol.10, No.6 Degradation Behavior of Sodium-Modified Hi gh Chromium A356.0-Ty pe A l-Si-Mg All oy 537
been used for along time because of it essential properties. New demanding applications are
developed continuously. To be able to make the alloys competitive in future applications, assess
to powerful tools and methods for materials or/ properties development is essential. The interest
in this work for DTAT and SQA treatments comes from the possibility to i mprove the corrosion
resistance of Al-S i-Mg alloy in the T6 te mper. It is on this note that an electroche mical corrosion
test consisting of linear polarization and chronopotentiometric as a means of corrosion
assessment of this group of alloy was carried out after these novel thermal treatments.
2. MATERIALS AND METHODS
The A356.0-type Al-Si-Mg alloy with chemical composition (see Table 1) was produced
according to the methods described elsewhere [30,45]. Some seconds to pouring of the molten
alloy into the mould, 0.01% elemental sodium was added and stirred thoroughly. The cast
samples were machined to specified electrochemical corrosion dimensions at the Centre for
Advanced Manufacturing Technology (CAMT), Tshwane University of Technology,
Soshanguve, Pretoria, Republic of South Africa.
Table 1. Chemical composition of the produced A356.0-type Al-Si-Mg alloy (wt %).
Al Si Mg Fe Mn Cr Pb+SnZn Cu Ti Ni Na
92.14 7.00 0.30 0.08 0.03 0.200.03 0.05 0.03 0.11 0.03 0.01
The electrochemical potentiodynamics technique was used to characterize the corrosion rate
(current densities) consisting of linear polarization and chronopotentiometric or open circuit
potential (OCP). A potentiostat coupled to a computer system, a glass corrosion cell kit with a
platinum counter electrode and a saturated Ag/Ag reference electrode were used. The working
electrodes consist of thermally aged alloys. The samples were positioned at the glass corrosion
cell kit, leaving a 3.803cm2 alloy surfaces in contact with the solution. Polarization test were
carried out in a 3.5wt%NaCl solution at room temperature (RT) using a potentiostat. The
polarization curves were determined by stepping the potential at a scan rate of 0.003V/sec. The
polarization curves were plotted using Autolab data acquisition system (Autolab model:
AuT71791 and PGSTAT 30), and both the corrosion rate and potential were estimated by the
Tafel extrapolation method (Tafel plot or corrosion rate analysis) using both the anodic and
cathodic branches of the polarization curves. The chronopotentiometric (OCP) was also used to
assess the corrosion behavior of some alloys samples in order to determine whether there was
prolong or short passive or active condition. The electrochemical corrosion test was done at
Department of Chemical and Metallurgical Engineering, Tshwane University of Technology,
Pretoria, Republic of South Africa.

538 M. Abdulwahab, I.A. Madugu, S.A. Yaro, A.P.I. Popoola Vol.10, No.6
3. RESULTS AND DISCUSSIONS
3.1 Results
The corrosion r ate analysis fro m the linear polarizations using the data from the Tafel plots have
been represented in the normal form; shown as Figures 1-6 and the OCP cur ves are presented as
Figure 7. The SEM of the surface morphology of some treated as-corroded samples can be found
in Plates 1-5.
0
0.05
0.1
0.15
0.2
0.25
0.3
1234518
Agei n g ti me (h r)
Corrosion rate (mm/year)
DTA T-T7
STAT-T6
Figure 1: Variation of corrosion rate with ageing time for DTAT-T7 and
STAT-T6 A356.0-t ype Al -Si-Mg alloy at 150oC from the Tafel
plot data in simulated seawater environment.
0
0.05
0. 1
0.15
0. 2
0.25
0. 3
123420
Agei n g time (hr )
Corrosion rate (mm/year)
DTA T-T7
STAT-T6
Figure 2: Variation of corrosion rate with ageing time for DTAT-T7 and
STAT-T6 A356.0-t ype Al -Si-Mg alloy at 180oC from the Tafel
plot data in simulated seawater environment.

Vol.10, No.6 Degradation Behavior of Sodium-Modified Hi gh Chromium A356.0-Ty pe A l-Si-Mg All oy 539
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
123420
Agei ng time (h r )
Co rrosio n rate (m m/year)
DTA T-T7
STAT-T6
Figure 3: Variation of corrosion rate with ageing time for DTAT-T7 and
STAT-T6 A356.0-t ype Al -Si-Mg alloy at 210oC from the Tafel
plot data in simulated seawater environment.
0
0.02
0.04
0.06
0.08
0.1
0.12
1020 30
Ste p-que nchi ng tim e (Se c)
Co rrosio n rate (m m/year)
DTAT-T7, 180oC/ 2hr
S TA T-T6, 180oC/2hr
Figure 4: Variation of corrosion rate with ageing time for DTAT-T7 and
STAT-T6 A356.0-t ype Al -Si-Mg alloy at 220oC SQ temperature and aged
180oC/2hr from the Tafel plot in simulated seawater environment.

540 M. Abdulwahab, I.A. Madugu, S.A. Yaro, A.P.I. Popoola Vol.10, No.6
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
1020 30
Ste p-que nching tim e (Se c)
Corrosion rate (m m/year)
DTAT-T7, 180oC/ 4hr
S TA T-T6, 180oC/4hr
Figure 5: Variation of corrosion rate with ageing time for DTAT-T7 and
STAT-T6 A356.0-t ype Al -Si-Mg alloy at 220oC SQ temperature and aged
at 180oC/4hr from the Tafel plot in simulated seawater environment.
0
0.05
0. 1
0.15
0. 2
0.25
0. 3
0.35
0. 4
0.45
123420
1234518
Agei ng tim e (hr)
C orrosion rate (mm/year)
DTAT-T7 150oC
STAT-T6 150oC
DTAT-T7 180oC
STAT-T6 180oC
DTAT-T7 210oC
STAT-T6 210oC
Figure 6: Comparative study of corrosion rate for DTAT-T7 and STAT-
T6 A356.0-type Al-Si-Mg alloy at 150oC, 180oC and 210oC at various
ageing time from the Tafel plot data in simulated Seawater environment

Vol.10, No.6 Degradation Behavior of Sodium-Modified Hi gh Chromium A356.0-Ty pe A l-Si-Mg All oy 541
-1
-0 .95
-0 .9
-0 .85
-0 .8
-0 .75
-0 .7
-0 .65
-0 .6
05001000 1500 200025003000 3500
O pen cir cui t pot enti al Ti m e (Sec. )
Corr osi on pot ent ia ls ( V)
A
B
C
E
F
G
H
D
Figure 7: Chronopotentiometric curve for some selected DTAT and STAT A356.0-type alloys
(A) STAT-T6 150oC/18hr (B) STAT-T6,180oC/20hr (C)STAT-T6, 210oC/20hr (D)DTAT-T7,
180oC/20hr (E)DTAT-T7 210oC/20hr (F)SQA DTAT-T7, 220oC/3Osec., 180oC/4hr (G)DTAT-
T7 150oC/18hr (H)SQA STAT-T6, 220oC/3Osec., 180oC/4hr
3.2 Discussion of Results
3.2.1 Corrosion characteristics
3.2.1.1 Potentiodynamics linear polarization and MSTAT treatment
From Figures 1-6, the potentiodynamics polarization curves for A356.0-type Al-Si-Mg alloy
subjected to different MSTAT treatment in a 3.5wt% NaCl are presented. However, the
corrosion rate decreases with increasing ageing time (see Figures 1-6) for all the DTAT, STAT,
SQA; DTAT and STAT samples considered. This is in agreement to those reported [43,44,46-
49]. The corrosion rate of DTAT and STAT samples at 150oC, 180oC and 210oC decreases with
increasing ageing time (1-20hr). The DTAT treatment has demonstrated a remarkable decrease
in the corrosion rate than STAT samples at all ageing time. For example, at 150oC/1hr DTAT
and STAT, the corrosion rates are 0.09766 and 0.284mm/year respectively. While at 18hr same

542 M. Abdulwahab, I.A. Madugu, S.A. Yaro, A.P.I. Popoola Vol.10, No.6
temperature, DTAT and STAT corrosion rates are 0.0002171 and 0.03093mm/year respectively.
However, similar results [50] and trends [48] have been reported. On the other hand, since the
grain boundaries are often more susceptible to corrosion than the grain interiors because of the
microstructural heterogeneity associated with grain boundaries [38,51,52]. It is expected that
samples with higher have pronounced grain boundaries, as such higher hardness is expected for
such sample. For all the DTAT samples investigated, the corrosion rate has been observed to be
significantly lower than that obtained at the STAT condition for all ageing temperatures and ti me
comparatively . One of the reasons for this behavior is that the pre-ageing treat ment at 105oC/5hr
in the DTAT is responsible for these decreases in the corrosion rate compare to those observed
with the STAT samples. The phases formed which enhanced hardness may probably have some
deleterious effect on the physicochemical properties of the alloy in the electrochemical
polarization condition.
However, the SQA; DTAT-T7 at 180oC/2hr and 4hr have higher corrosion rate than those of
SQA; STAT-T6 at 180oC/2hr and 4hr for all the step-quenching time considered (see figure 4
and 5). For example the current densities at SQA; DTAT-T7 at 180oC/2hr and 4hr
(220oC/30sec.) are 2.031 and 0.5µA/cm2 as compared to those at SQA; STAT at 180oC/2hr and
4hr (220oC/30sec), the current densities are 1.419 and 0.1696µA/cm2 respectively. This results
showed a very wide difference in the corrosion rate of the two conditions indicating that SQA;
DTAT-T7 are more susceptible to corrosion attach than SQA; STAT-T6 samples within the
studied conditions.
It is also believ ed that after qu enching, particles of the se cond phase precipitates occurred which
affect the physicochemical properties of the alloy. The ageing temperatures and the degree of
supersaturation play a major role in the final properties of the alloy. However, as the ageing
progresses, the inter-atomic spacing and the bond between the molecul es changed w hich account
for the electrochemical behavior of these samples. Such tendency agreed with the results
obtained in previous study [29, 46]. The corrosion resistances of A356.0-type Al-Si-Mg alloy
studied seems to be strongly associated not only with the distribution of eutectics which is
dependent on the interdendritic spacing, but also with the fineness of the eutectic inter-phase
spacing.
Comparatively , samples aged at 150oC for 18hr at DTAT exhibit th e highest corrosion resistance
in 3.5wt%NaCl solution simulated environment considered in this work. The sequence below
presents the decreasing o rder of corrosion resistance of the sample at various ageing temperature
and time:

Vol.10, No.6 Degradation Behavior of Sodium-Modified Hi gh Chromium A356.0-Ty pe A l-Si-Mg All oy 543
DTAT-T7, 150oC/18hr
DTAT-T7, 210oC/20hr
DTAT-T7, 180oC/20hr
STAT-T6, 180oC/20hr
STAT-T6, 150oC/18hr
STAT-T6, 210oC/20hr
and
SQA 180oC/4hr; STAT-T6 SQA 180oC/4hr; DTAT-T7
SQA 180oC/2hr; STAT-T6 SQA 180oC/2hr; DTAT-T7.
The results above motiv ated the interest in examining the samp les of best corrosion resistance at
each ageing temperature considered using the chronopotentiometric analysis which enables the
evaluation of the corrosion kinetic of the selected sample through corrosion potentials with
prolong time.
3.2.1.2 Chronopotentiometric and MSTAT treatments
From the OCP study of the selected samples of higher corrosion resistance, the corrosion
potential, E (V), decreases with the electrochemical exposure time (see Figure 7). This becomes
stable (passive) throughout the study time. This shows that the resistances of the thermally
treated alloy to corrosion attach decreases with increasing electrochemical exposure time and
that the samples later developed pas sivity (protection against corrosion). At this ‘stable-level’, it
is assumed that the s ample has some level of passivity and immunity as a result of the corroding
products which covers the corroding surface and retard/stabilizes corrosion attach. These
observations have been reported [47,50]. It can also be observed, for example, that sample with
highest corrosion resistance still have the best corrosion potential (T7150oC/18hr) indicating
higher corrosion resistance in 3.5wt%NaCl solution.

544 M. Abdulwahab, I.A. Madugu, S.A. Yaro, A.P.I. Popoola Vol.10, No.6
3.2.2 Microstructures and degradation behavior after corrosion study of A356.0-type Al-Si-Mg
alloy
The microstructure of some selected as-corroded samples after electrochemical corrosion study
using SEM indicates some traces of cracks and pronounced pits; showing that the samples have
suffered pitting corrosion attach (Plates 1-5). The exposure surface shows evidence of localized
attach at the location of the intermetallic caused by the dissolution of the matrix. There was
evidence of corroding products of intermetallic compounds in all the samples examined. Besides,
several pits are visible in all samples examined at different magnifications; x300, x1000, x10000.
In Plates 1 and 3 there seems to be uniform surface pits formations which are less deep as
compared to those in Plates 2, 4 and 5. More pronounced deeper pit were seen in T6 150oC/3hr
(Plate 4) and SQA T7 220oC/20sec.180oC/2hr (see Plate 5).
T7 180oC/20hr x300 T7 180oC/20hr x1000
T7 180oC/20hr x10,000
Plate 1: SEM Secondary Electron Image of the damaged surface Morphology of as- corroded T7
180/20hr in 3.5wt%NaCl Solution. Microstructures at different magnifications indicates several
and severe pits.

Vol.10, No.6 Degradation Behavior of Sodium-Modified Hi gh Chromium A356.0-Ty pe A l-Si-Mg All oy 545
T6 180oC/2hr x300 T6 180oC/2hr x1000
T6 180oC/2hr x10, 000
Plate 2: SEM Secondary Electron Image of the damaged surface Morphology of as-corroded T6
180/2hr in 3.5wt%NaCl Solution. Microstructures at different magnifications indicate several
and severe pits higher than those observed in plate 10a.
T7 150oC/4hr x300 T7 150oC/4hr x1000
T7 150oC/4hr x10, 000
Plate 3: SEM Secondary Electron Image of the damaged surface Morphology of as-corroded T7
150/4hr in 3.5wt%NaCl Solution. Microstructures at different magnifications indicates several
and severe pits. Indicating traces of cracks.

546 M. Abdulwahab, I.A. Madugu, S.A. Yaro, A.P.I. Popoola Vol.10, No.6
T6 150oC/3hr x300 T6 150oC/3hr x1000
T6 150oC/3hr x10, 000
Plate 4: SEM Secondary Electron Image of the damage surface Morphology of as-corroded T6
150/3hr in 3.5wt%NaCl solution. Microstructures at different magnifications indicate severe pits
deeper and wider than those observed in plate 10a, b, c.
SQA T7 220oC/20sec.180oC/2hr x300 SQA T7 220oC/20sec.180oC/2hr x1000
SQA T7 220oC/20sec.180oC/2hr x10,000
Plate 5: SEM Secondary Electron Image of the damaged surface Morphology of as-corroded
SQA T7220oC/20sec.180oC/2hr in 3.5wt%NaCl solution. Microstructures at different
magnifications indicate severe pits deeper and wider than those observed in plate 10a, b, c.

Vol.10, No.6 Degradation Behavior of Sodium-Modified Hi gh Chromium A356.0-Ty pe A l-Si-Mg All oy 547
The resultants pits diameter have been determined from the SEM surface morphology to range
between 30-50µm. Specifically in T6 180oC/2hr (see Plate 2) the pit diameter are averages of
44.4445 and 50µm. It is probable that the pits are formed by intermetallic dropping out from the
surface due to the dissolution of the surrounding matrix. However, it is also possible that the pits
are caused by selective dissolution of the intermetallic/or particles of the second phase
precipitates. Consequently, the form of corrosion in the studied samples A356.0-type Al-Si-Mg
alloy are slightly uniform and predominantly pitting corrosion as obtained by the SEM.
4. CONCLUSIONS
(1) From the lin ea r p ol ar i za ti o n and Tafel extrapolation plot, the corrosion rate de creases
at all temperatures with the ageing time.
(2) The corrosion of the DTAT and SQA A356.0-type Al-Si-Mg alloy in the simulated
Seawater showed better resistance than the STAT A356.0-type Al-Si-Mg alloy.
(3) Those samples in the SQA-STAT have improved cor rosion resistance than th e SQA
DTAT samples. The chronopotentiometric corrosion study of some selected samples
indicate a decrease and stability in there corrosion resistance with electrochemical
exposure time.
(4) Consequently, the forms of corrosion in the studied A356.0-type Al-Si-Mg alloy are
uniform pitting corrosion as obtained from the SEM study with pits diameter ranging
from 30-50µm.
ACKNOWLEDGEMENTS
The authors would like to thank the Department of Chemical and Metallurgical Engineering,
TUT, South Africa for the equipment support. The technical assistance of Mrs. Adams Feyisayo
at Department of Chemistry, University of Johannesburg, Johannesburg, Republic of South
Africa during the corrosion test is highly appreciated.
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