^. DONIK: INFLUENCE OF ARTIFICIAL AGING ON THE ELECTROCHEMICAL PROPERTIES ...
71–75
INFLUENCE OF ARTIFICIAL AGING ON THE
ELECTROCHEMICAL PROPERTIES OF THE ALUMINIUM
AA 6063 ALLOY
VPLIV UMETNEGA STARANJA NA ELEKTROKEMIJSKE
LASTNOSTI ALUMINIJEVE ZLITINE AA 6063
^rtomir Donik
Institute of Metals and Technology, Lepi pot 11, 1000 Ljubljana, Slovenia
crtomir.donik@imt.si
Prejem rokopisa – received: 2017-10-05; sprejem za objavo – accepted for publication: 2017-11-14
doi:10.17222/mit.2017.168
The influence of artificial ageing on the corrosion behaviour of the aluminium alloy AA 6063 in chloride-containing solutions
was studied to determine the correlation between aging parameters and corrosion through the electrochemical parameters. The
present study showed that LM and standard SEM imaging with EDS observation does not expose significant differences for the
artificially aged aluminium. Nevertheless, a special statistical technique with observation of the larger area showed a significant
increase of the smallest precipitates which are the initial sites for the corrosion. With electrochemical measurements, we
corroborated that increase of corrosion rate is directly connected to the number of precipitates and not with the total area of
exposed precipitates. Scanning electron microscopy observations combined with EDS measurements were used also to establish
the type of precipitates in the studied alloy.
Keywords: aluminium, artificial aging, SEM, electrochemical measurements
Raziskovali smo vpliv umetnega staranja na korozijske lastnosti aluminijeve zlitine AA 6063 z namenom ugotoviti soodvisnost
med staranjem in elektrokemijskimi parametri. V raziskavi smo dokazali, da zgolj standardni pregled s svetlobnim in vrsti~nim
elektronskim mikroskopom v kombinaciji z EDS-analizo na prvi pogled ne poka`e bistvenih razlik med razli~no toplotno obde-
lanimi vzorci. Ugotovili smo, da s pomo~jo preiskave na ve~jem podro~ju in s statisti~no obdelavo lahko odkrijemo povezave
predvsem med manj{imi izlo~ki in elektrokemijskimi parametri. S korozijskimi meritvami smo tudi potrdili, da je pove~anje
korozijske hitrosti direktno povezano s {tevilom majhnih izlo~kov in ne s se{tevkom povr{ine vseh izlo~kov. S SEM-analizo v
kombinaciji z EDS-analizo smo uporabili za dolo~itev vrst vklju~kov v preiskovani zlitini.
Klju~ne besede: aluminij, umetno staranje, SEM, elektrokemijske meritve
1 INTRODUCTION
AA 6063 is an aluminium alloy, with magnesium and
silicon as the main alloying elements and iron as im-
purity. It has generally good mechanical properties, is
heat treatable and weldable. AA 6063 is the most com-
mon aluminium alloy used for extrusion. It allows
complex shapes to be formed with very smooth surfaces
fit for further anodizing and so is popular for visible
architectural applications such as window frames, door
frames, roofs, and sign frames. These alloys find appli-
cation in the artificial aging condition, which makes it
possible to obtain excellent mechanical properties.1–12
The physical properties exhibited by aluminium alloys
are significantly influenced by the treatment of the sam-
ple. In all the above-mentioned applications, alloys are
prone to corrode, while also exposed to severe atmosphe-
ric environments.13–17
The physical properties of aluminium alloys are
significantly influenced by the sample treatment. A
standardized system has been developed to designate
these treatments. T1 thermal treatment means aluminium
alloy cooled from an elevated temperature shaping pro-
cess and naturally aged to a substantially stable condi-
tion. T1 tempered AA 6063 used in our study, has an
ultimate tensile strength of at least 120 MPa. Applica-
tions requiring higher strength typically use other alloys,
such as AA 6061 or AA 6082 instead.18,19 In our study,
we aged the alloy even further to evaluate the electro-
chemical properties correlated to the aging processes.
Corrosion of the aluminium alloys are directly linked to
the numbers of the impurities, inclusions, precipitates –
areas of different electrochemical potential as bulk
metal, while these spots are initial sites for pitting and
local, general corrosion. Corrosion mode evolution with
aging stage at aging temperature of 150 °C and 175 °C
follows the following order: pitting and local IGC (early
aging), general IGC (under-aged), local IGC with pitting
(near peak-aged), and pitting (over-aged). At elevated
aging temperature of 200 °C, the corrosion mode evolves
much faster.1,2,20–23 Therefore, we used little over 200 °C
in our study to intensify and accelerate the precipitation
in the AA6063 alloy. Aging of studied AA6063 alloy
exhibited characteristic precipitation hardening pheno-
mena. The precipitation hardening process requires that
the second component in the aluminium alloy, is suffi-
ciently soluble to allow extensive dissolution at an
Materiali in tehnologije / Materials and technology 52 (2018) 1, 71–75 71
UDK 620.199:544.653:669.715 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 52(1)71(2018)
elevated temperature (solubilisation treatment tempera-
ture) and that the solubility is reduced at lower tem-
peratures like Si and Fe in our case.2,24,25
The aim of the presented study was to determine the
aging effect on corrosion stability in differently artifi-
cially aged aluminium alloy AA6063.
2 MATERIAL AND METHODS
2.1 Material
The chemical compositions of the studied alloy was
determined using X-ray fluorescence spectrometer XRF
(Thermo Scientific Niton XL3t GOLDD+).
Table 1: Chemical composition in mass fractions, (w/%)
Fe Cr Si Mg Al
0.17 0.14 0.51 0.65 balance
2.2 Sample aging
Samples for the study were treated with normal T1
treatment (sample T1) and afterwards artificially aged on
204 °C for 1 h (sample T1+1 h) and others were treated
for additional 5 h, together 6 h (sample T1+6 h).
2.3 Light Microscopy (LM)
Used Nikon Microphot FXA optical microscope with
Olympus® DP73 3CCD video camera and Stream-
Motion® software for metallographic image analysis in
dark-field mode. The samples for dark-field mode was
prepared with modified Keller’s etching solution for
10 s.
2.4 Scanning Electron Microscopy (SEM)
Field Emission Scanning Electron Microscope
JEOL® JSM-6500F with attached Energy Dispersive
X-ray spectroscopy EDS (INCA ENERGY® 400)
Oxford, Wave Dispersive X-ray spectroscopy WDS
(INCA WAVE® 700) Oxford and Electron Back
Scattered Diffraction EBSD (HKL Channel5®) were
employed to investigate the morphology of the different
polished surfaces of AA6063 as well as the morphology
and distribution of the precipitates evaluation and EDS
measurements. The statistical lateral distribution of the
precipitates was determined by INCA ENERGY add-in
called Feature®, which shows and calculate the number
of precipitates etc., which were later statistically shown
as histogram of the number of the particles in correlation
with their size.
2.5 Electrochemical measurements
Electrochemical measurements were performed on
prepared specimens, ground with SiC emery paper down
to 4000 grit and polishing down to 1 μm with a diamond
suspension. The experiments were carried out at room
temperature in a 3,5 % NaCl from Merck, Darmstadt,
Germany, and stabilized at pH=7. The measurements
were performed using a three-electrode, flat BioLogic®
corrosion cell (volume 0.25 L). The test specimen was
employed as the working electrode (WE). The reference
electrode (RE) was a saturated calomel electrode (SCE,
0.242 V vs. SHE) and the counter electrode (CE) was a
platinum mesh. Electrochemical measurements were
recorded using a BioLogic® Modular Research Grade
Potentiostat/Galvanostat/FRA Model SP-300 with an
EC-Lab® software V11.10. The specimens were
immersed in the solution 1 h prior to the measurement in
order to stabilize the surface at the open-circuit potential
(OCP). The cyclic voltametry measurements were
recorded after 1 h sample stabilization at the open-circuit
potential (OCP), starting the measurement at 250 mV vs.
SCE more negative than the OCP. The potential was then
increased, using a scan rate of 1 mV s–1. All the measure-
ments were made at room temperature and were repeated
at least three times. The linear polarization measure-
ments were performed at ±25 mV according to the OCP,
using a scan rate of 0.01 mV s–1.
3 RESULTS AND DISCUSSION
3.1 Light microscopy images
Figure 1 shows the images with special mode in LM,
called dark-field, which best shows the grain boundaries
in aluminium alloys. Just from the observation of the LM
(Figure 1) and SEM BE images (Figure 2) the number
and distribution of the precipitates (in our case
Al-Si-Fe-phases), we cannot determine that the number
^. DONIK: INFLUENCE OF ARTIFICIAL AGING ON THE ELECTROCHEMICAL PROPERTIES ...
72 Materiali in tehnologije / Materials and technology 52 (2018) 1, 71–75
Figure 1: LM darkfield images with 100× magnification of: a) T1, b) T1+1 h and c) T1+6 h
is increasing with the thermal treatment – aging. With
the exquisitely exposed grain boundaries in the Figure 1,
also the grain size is neither increased or decreased with
aging. As known from 2,24, the last is known fact by the
aging. It is accomplished by nucleation and growth
process in the grains, while the size of the grains remains
unchanged.
3.2 SEM images
Figure 2 shows SEM backscatter images of the
polished aluminium alloy with lighter shade for heavier
elements – in our case Fe phases. As known, typical Fe
phases in aluminium alloys are in the Al–Fe–Si system,
the ternary phases that can be in equilibrium with alu-
minium are the 
-Al8Fe2Si (31.6 % Fe, 7.8 % Si) with a
Chinese script morphology, -Al5FeSi (25.6 % Fe,
12.8 % Si), -Al4FeSi2 (25.4 % Fe, 25.5 % Si) and

-Al3FeSi (33.9 % Fe, 16.9 % Si) phases.26 Since EDS
measurements of the precipitates (Figure 4 and Table 2),
with morphology not similar to Chinese script, we could
conclude that most likely 
-Al8Fe2Si phases are present
in the precipitates. On the other hand, similarly as from
LM images, with just observation, we cannot determine
that the number of the inclusions increase with aging.
This is the reason the special add-in called Feature® in
INCA® Oxford package was used for larger area obser-
vation. In our study 1 mm2 with 1500× magnification, to
determine also the particles smaller than 0.1 μm2 was
used. Figure 3 shows the histogram for all the analysed
samples. The particles are counted just on the bases of
the different shade of grey and not the EDS analysed
chemistry. The sum of all the particle analysed dras-
tically differ from each other: T1 2592 precipitates,
T1+1 h 3154 precipitates and T1+6 h 3472 precipitates.
The increase of the number of inclusions is more than
33 % between samples T1 and T1+6 h. On the other
hand, if you additionally weight this numbers with the
size and calculate the sum area of the inclusions, the
increase is merely 7 % for T1+6 h as anticipated. T1
treatment expose almost all the inclusions and with aging
bigger phases dissolve to smaller and the number
increases. For the corrosion rate, the number of the
precipitates and not the area of the impurities on the
surface, play the crucial role.
^. DONIK: INFLUENCE OF ARTIFICIAL AGING ON THE ELECTROCHEMICAL PROPERTIES ...
Materiali in tehnologije / Materials and technology 52 (2018) 1, 71–75 73
Figure 3: Histogram of precipitate distribution of variously aged
samples
Figure 2: BE images with 500× magnification of samples: a) T1, b) T1+1 h and c) T1+6 h
Figure 4: BE image with spots and areas of EDS analyses on polished
sample T1
Table 2: EDS analyses for spots and areas from Figure 4 in mass
fractions, (w/%)
Mg Al Si Fe
Spectrum 1 0.6 94.4 1.7 3.3
Spectrum 2 0.5 91.5 2.0 6.0
Spectrum 3 0.3 69.3 6.4 24.0
Spectrum 4 0.4 84.0 3.4 12.2
Spectrum 5 0.5 99.2 0.3 0.0
3.3 EDS measurements
Figure 4 shows the analysed area and spots of the
polished sample T1. EDS measurements were performed
only on T1, while all the samples have the same chemi-
stry and also the precipitates have the same composition.
Spectrum 1 and 2 (Table 1) shows the composition of
the smallest inclusion – these are 0.1 μm2 or less. The
EDS technique also measures a lot of the surrounding
area/analysing volume, so we can just speculate that the
chemical composition of small precipitates is probably
similar as in the biggest precipitates. The bigger precipi-
tates are measured in Spectrum 3 and 4, and are as we
described above most probably, 
-Al8Fe2Si. Spectrum 5
analysed the area of the base material, aluminium alloy
AA 6063 with just 0.3 % of mass fractions of Si and
0.5 % of mass fractions of Mg, similar as shown in the
chemical composition of the material.
3.4 Electrochemical measurements
Figure 5 shows cyclic voltamogramm of all three
samples. The curves of the voltamogramm shows that
the thermal treatment increase the corrosion current and
also the shape of the loop of the voltamogramme, which
is directly connected to the oxidation of the metal which
means increased general corrosion of the studied alloy.
Figure 6 shows linear polarization curves for the
AA6063 samples in 3,5 % NaCl solution at pH = 7. The
calculations from linear polarization measurements were
executed by using the Equation (1):
Rp = a c/(2.3 Icorr (a + c)) (1)
The polarization resistance, Rp, is evaluated from the
linear polarization curves by applying a linear least-
squares fit of the data around OCP, ± 10 mV. The corro-
sion current, Icorr, is calculated from Rp, the least-squares
slope, and the Tafel constants, a and c, of 120 mV
decade–1. The value of E(I=0) is calculated from the
least-squares intercept. The corrosion rates were
calculated by using the following conversion formula in
Equation (2):
vcorr = C(EW/d)(Icorr/A) (2)
where EW is the equivalent weight of the sample in g, A
is the sample area in cm2, d is its density in g/cm3, and
C is a conversion constant that is dependent upon the
required units.
The steeper linear polarization curves means lower
polarisation resistance (Rp) which is directly linked to
higher corrosion currents (Icorr). The corrosion parame-
ters calculated from the linear polarization with potentio-
dynamic measurement data in a 3.5 % NaCl solution
indicated that the corrosion stability is highest for the T1
sample followed by the sample treated by T1+1 h and
the lowest corrosion resistance belongs to T1+6 h sam-
ple. Higher corrosion rate corroborates with the pre-
viously described fact that more inclusions means more
corrosion initial points. This was proven with the higher
corrosion currents (Icorr), higher corrosion rates (vcorr) and
lower polarization resistances (Rp) (Table 3).
Table 3: Corrosion characteristics of studied materials calculated from
linear polarisation curves
Icorr
/μA
Ecorr
/mV
vcorr
/μm year–1
Rp
/k
T1 0.281 –820 2.35 24.9
T1+1 h 0.617 –830 5.29 11.8
T1+6 h 0.906 –785 7.65 3.02
4 CONCLUSIONS
The AA6063 alloy was used with different thermal
treatment times to determine the correlation between the
number and size of precipitates and corrosion characte-
ristics through electrochemical parameters in chloride-
containing solutions. Light-microscopy observations and
standard SEM imaging with EDS do not show the big
differences they should due to the big difference in the
corrosion rates. INCA Feature® uses the observation of
the larger area and helped us to explain the direct
correlation between the smallest precipitates, which are
initial sites for the pitting and general corrosion, and the
corrosion rate. With scanning electron microscopy
observations combined with EDS measurements we also
determined the type of precipitates in the studied alloy.
^. DONIK: INFLUENCE OF ARTIFICIAL AGING ON THE ELECTROCHEMICAL PROPERTIES ...
74 Materiali in tehnologije / Materials and technology 52 (2018) 1, 71–75
Figure 6: Linear polarization curves of the samples T1, T1+1h and
T1+6 h
Figure 5: Cyclic voltamogramm of the samples in NaCl
Acknowledgement
The authors acknowledge the financial support from
the Slovenian Research Agency (research core funding
No. P2-0132).
5 REFERENCES
1 H. Maeng, Y. Choi, S.-J. Lee, Aging parameter for evaluating age
hardening in Al alloys, Mater. Res. Express. 4 (2017), 76509,
http://stacks.iop.org/2053-1591/4/i=7/a=076509
2 R. R. R. Ambriz, D. Jaramillo, Mechanical Behavior of Precipitation
Hardened Aluminum Alloys Welds, W.A. Monteiro (Ed.), Light Met.
Alloy. Appl., InTech, Rijeka, 2014, doi:10.5772/58418
3 Z. Xu, H. J. Roven, Z. Jia, Effects of cryogenic temperature and pre-
stretching on mechanical properties and deformation characteristics
of a peak-aged AA6082 extrusion, Mater. Sci. Eng. A. 679 (2017),
379–390, doi:10.1016/j.msea.2016.10.049
4 B. Podgornik, I. Paulin, B. Zajec, S. Jacobson, V. Leskovsek, Peep
cryogenic treatment of tool steels, J. Mater. Process. Technol., 229
(2016), 398–406, doi:10.1016/j.jmatprotec.2015.09.045
5 D. Mandrino, I. Paulin, S. D. Skapin, Scanning electron microscopy,
X-ray diffraction and thermal analysis study of the TiH2 foaming
agent, Mater. Charact., 72 (2012), 87–93, doi:10.1016/j.matchar.
2012.07.005
6 I. Paulin, Synthesis And Characterization of Al Foams Produced by
Powder Metallurgy Route Using Dolomite and Titanium Hydride as
a Foaming Agents, Mater. Tehnol., 48 (2014), 943–947
7 X. Fan, L. Chen, G. Chen, G. Zhao, C. Zhang, Joining of 1060/6063
aluminum alloys based on porthole die extrusion process, J. Mater.
Process. Technol., 250 (2017), 65–72, doi:10.1016/J.JMATPROTEC.
2017.07.009
8 I. Paulin, Stability of Close-Cell Al Foams Depending on The Usage
of Different Foaming Agents, Mater. Tehnol., 49 (2015), 983–988
9 M. Imam, V. Racherla, K. Biswas, H. Fujii, V. Chintapenta, Y. Sun,
Y. Morisada, Microstructure-property relation and evolution in
friction stir welding of naturally aged 6063 aluminium alloy, Int. J.
Adv. Manuf. Technol., 91 (2017), 1753–1769, doi:10.1007/s00170-
016-9865-9
10 Y. Liu, C. Geng, Q. Lin, Y. Xiao, J. Xu, W. Kang, Study on hot
deformation behavior and intrinsic workability of 6063 aluminum
alloys using 3D processing map, J. Alloys Compd., 713 (2017),
212–221, doi:10.1016/j.jallcom.2017.04.156
11 X. Yao, Z. Zhang, Y. F. Zheng, C. Kong, M.Z. Quadir, J.M. Liang,
Y.H. Chen, P. Munroe, D.L. Zhang, Effects of SiC Nanoparticle
Content on the Microstructure and Tensile Mechanical Properties of
Ultrafine Grained AA6063-SiCnp Nanocomposites Fabricated by
Powder Metallurgy, J. Mater. Sci. Technol., 33 (2017), 1023–1030,
doi:10.1016/j.jmst.2016.09.022
12 Z. Liu, L. Li, J. Yi, S. Li, Z. Wang, G. Wang, Influence of heat treat-
ment conditions on bending characteristics of 6063 aluminum alloy
sheets, Trans. Nonferrous Met. Soc. China, 27 (2017), 1498–1506,
doi:10.1016/S1003-6326(17)60170-5
13 Y.-M. Zhang, X.-M. Chu, H. Wang, S.-Y. He, D.-P. He, Fabrication
of Al-Mg-Re foams and their corrosion resistance properties, 51
(2009), 1436–1440, http://www.sciencedirect.com/science/article/
B6TWS-4VYXMR1-2/2/7efe698d7b89719ab21d24eaf6746c57
14 L. de Lara, R. Jagdheesh, J. L. Ocana, Corrosion resistance of laser
patterned ultrahydrophobic aluminium surface, Mater. Lett., 184
(2016), 100–103, doi:10.1016/j.matlet.2016.08.022
15 I. Paulin, M. Godec, Surface oxidation of heating resistors made
from Kanthal AF: Increasing the lifetime of glow plugs, Vacuum,
138 (2017), 146–151, doi:10.1016/j.vacuum.2016.12.007
16 M. Torkar, A. Kocijan, R. Celin, J. Burja, B. Podgornik, Metallogra-
phic Investigation and Corrosion Resistance of Welds of Ferritic
Stainless Steels, Mater. Tehnol., 50 (2016), 829–834, doi:10.17222/
mit.2016.059
17 M. Conradi, A. Kocijan, Surface and Anticorrosion Properties of
Hydrophobic and Hydrophilic TiO2 Coatings on A Stainless-Steel
Substrate, Mater. Tehnol. 50 (2016), 967–970, doi:10.17222/mit.
2016.068
18 N. Serban, D. Raducanu, M. Butu, V.D. Cojocaru, Crystallite Size
and Lattice Strain Evolution in a Nanostructured 6063-T1 Aluminum
Alloy Processed by Equal Channel Angular Pressing, Mater. Plast.,
53 (2016), 179–183
19 N. Serban, M. Butu, V.D. Cojocaru, Sustainable Lightweight
6063-T1 Aluminium Alloy Processed by Equal Channel Angular
Pressing for Low Environmental Impact Applications, Mater. Plast.,
53 (2016), 617–622
20 Q. Liu, R.-H. Zhu, D.-Y. Liu, Y. Xu, J.-F. Li, Y.-L. Chen, X.-H.
Zhang, Z.-Q. Zheng, Correlation between artificial aging and
intergranular corrosion sensitivity of a new Al-Cu-Li alloy sheet,
Mater. Corros., 68 (2017), 65–76, doi:10.1002/maco.201609090
21 C. T. Kwok, K. H. Lo, W. K. Chan, F. T. Cheng, H. C. Man, Effect of
laser surface melting on intergranular corrosion behaviour of aged
austenitic and duplex stainless steels, Corros. Sci., 53 (2011),
1581–1591, doi:10.1016/j.corsci.2011.01.048
22 A. S. Khanna, Introduction to High Temperature Oxidation and
Corrosion, 2002
23 A. H. M. Sanchez, B. J. C. Luthringer, F. Feyerabend, R. Willumeit,
Mg and Mg alloys: How comparable are in vitro and in vivo corro-
sion rates? A review, Acta Biomater., 13 (2015), doi:10.1016/
j.actbio.2014.11.048
24 C. Li, G. Sha, J. Xia, Y. Liu, S. P. Ringer, Si-induced precipitation
modification and related age-hardening response of an
Al–4Mg–1Cu–0.5Si alloy, Mater. Chem. Phys., 193 (2017),
421–426, doi:10.1016/j.matchemphys.2017.01.041
25 Y. Li, Z. Shi, J. Lin, Y.L. Yang, Q. Rong, B. M. Huang, T. F. Chung,
C. S. Tsao, J. R. Yang, D. S. Balint, A unified constitutive model for
asymmetric tension and compression creep-ageing behaviour of
naturally aged Al-Cu-Li alloy, Int. J. Plast., 89 (2017), 130–149,
doi:10.1016/j.ijplas.2016.11.007
26 T. O. Mbuya, B. O. Odera, S. P. Ng’ang’a, Influence of iron on cast-
ability and properties of aluminium silicon alloys: literature review,
Int. J. Cast Met. Res., 16 (2003), 451–465, doi:10.1080/13640461.
2003.11819622
^. DONIK: INFLUENCE OF ARTIFICIAL AGING ON THE ELECTROCHEMICAL PROPERTIES ...
Materiali in tehnologije / Materials and technology 52 (2018) 1, 71–75 75