metals

Article
Role of Chemical Composition in Corrosion of
Aluminum Alloys
Lenka Kuchariková 1, *, Tatiana Liptáková 1 , Eva Tillová 1 , Daniel Kajánek 1 and Eva Schmidová 2
1 Faculty of Mechanical Engineering, Department of Materials Engineering, University of Žilina, Univerzitná
8215/1, 010 26 Žilina, Slovakia; tatiana.liptakova@fstroj.uniza.sk (T.L.); eva.tillova@fstroj.uniza.sk (E.T.);
daniel.kajanek@fstroj.uniza.sk (D.K.)
2 Faculty of Transport Engineering, University of Pardubice, Studentská 95, 532 10 Pardubice, Czech Republic;
eva.schmidova@upce.cz
* Correspondence: lenka.kucharikova@fstroj.uniza.sk; Tel.: +421-41-513-2626




Received: 4 July 2018; Accepted: 23 July 2018; Published: 26 July 2018


Abstract: Aluminum alloys are the most important part of all shaped castings manufactured,
especially in the aerospace and automotive industries. This work focuses on the corrosion properties
of the heat-hardening aluminum alloys commonly used for production of automotive castings
AlSi7Mg0.3 and on self-hardening AlZn10Si8Mg. Iron is a common impurity in aluminum cast alloy
and its content increases by using secondary aluminum alloys. Therefore, experimental materials
were developed, with chemical composition according to standards (primary alloys) and in states
with an increasing content of Fe. The experimental aluminum alloys are briefly discussed in terms of
their chemical composition, microstructure, mechanical properties and corrosion behavior. Corrosion
properties were examined using three types of corrosion tests: exposure test, potentiodynamic tests,
and Audi tests. Corrosion characteristics of materials were evaluated using stereo, optical and
scanning electron microscopy, energy dispersive X-ray analysis, too. Correlation of pit initiation
sites with microstructural features revealed the critical role of iron-rich phases, silicon particles and
corresponding alloy matrix.

Keywords: aluminum alloys; iron; corrosion behavior; exposure test; potentiodynamic test; corrosion
Audi test

1. Introduction
The modern world requires the use of light structural materials to improve fuel economy,
energy consumption and emissions of gas in industrial application [1]. The properties (low density,
high strength stiffness to weight ratio, good formability and good corrosion resistance) make
aluminum alloys an ideal material for the manufacturing of components for automotive and aerospace
applications [2–5]. The main components of internal combustion engines such as cylinder head,
cylinder block, crankshaft and pistons are the main automotive components where aluminum cast
alloys are used. The importance is to achieve the requested properties, which do not depend only on
the casting condition and solidification rate, but they are also significantly influenced by their chemical
composition. The chemical composition leads to the formation of different microstructural features.
From this point of view the most important features are α-matrix (dendrite cell size, secondary dendrite
arm spacing-SDAS, grain size), eutectic silicon particles and intermetallic phases (size, morphology,
amount) and porosity [3,4,6,7]. Osório et al. [5,6] investigated that the dendrite fineness can be even
of more importance in the mechanical properties compared to the effect of grain size. It was also
reported that unmodified samples of as-cast and heat-treated conditions reached higher corrosion
resistance than the modified samples. On the other hand, research shows that the T4 heat treatment

Metals 2018, 8, 581; doi:10.3390/met8080581 www.mdpi.com/journal/metals

Metals 2018, 8, 581 2 of 13

provides a recovery on the corrosion resistance due to the spheroidizing effect on silicon particles.
Tahamntan et al. [8] explained morphological aspects of silicon phase as well as the area effect as
related to galvanic corrosion between silicon particles and eutectic aluminum phase.
The other microstructural features which significantly affect the properties of aluminum castings
are intermetallic phases. Intermetallic phases improve or decrease mechanical and physical properties
which depend on the morphologies, type and distribution of these phases that are in turn a function of
alloy composition and cooling rate [9,10]. Donatus et al. [11] showed that intermetallic particles such as
the Al2 CuMg, Al3 Mg2 , Mg2 Si, and MgZn2 are anodic to the Al matrix and corrode preferentially with
respect to the surrounding Al matrix. The intermetallic phases particles such as Al2 Cu (θ), AlFeMnSi,
AlCuFeMn, AlCuFeSi, and (Al,Cu)x (Fe,Mn)y Si particles which are mostly cathodic to the Al matrix
and cause peripheral trenches of the surrounding Al matrix adjacent to these intermetallic phase
particles. The element represented the major classes of intermetallic phases and caused a decrease in
aluminum alloy properties is Fe. Iron has to be considered in industrially processed alloys as well,
since it is usually presented as impurities, stemming from impurities in bauxite ore and contamination
with ferrous metals and oxides during handling and recycling. The most significant is the presence
of iron in aluminum cast alloys because of reducing adhesion to metal molds [12]. Due to their low
solubility (only 0.05% at 660 ◦ C), these can have a negative effect on formability by forming large
“constituent” particles (Al-Fe-Si) during eutectic solidification [4,13,14]. Removing Fe from the melts is
a very expensive process [15]. Therefore, it is important to study how these elements influence the
properties of Al-Si castings. Phases of Al-Fe-Si-Mg, which crystallize in the form of so-called “Chinese
script”, are iron-magnesium phases that also behave in a cathodic manner regarding the α-matrix,
although they are expected to be less detrimental than Al-Fe, Al-Fe-Si and Al-Fe-Si-Mn due to the
presence of magnesium. The strengthening phase of Mg2 Si is anodic regarding the aluminum matrix
and it may enhance localized corrosion [4,16,17]. Other microstructural features affect the properties of
final aluminum casts–e.g., porosity, the most common defect in Al-Si castings [9,13]. Taylor et al. [18]
reported the specific effect of Fe needle phases on porosity: the total porosity is minimized at 0.4 % Fe;
a localized shrinkage-porosity defect (termed the “extended defect”) develops at iron concentrations
greater than 0.4% under no optimum casting conditions; and there is a change from a discrete pore
morphology at 0.1% Fe content to zones of sponge-like interdendritic porosity at higher iron levels.
Samuel et al. [13] examined that further increase in the iron content, and hence the size of the β-Al5 FeSi
platelets, cause an increase in pore sizes, however the platelets also limit pore growth.
Mechanical and fatigue properties of aluminum alloys used in the automotive and aerospace
industry were examined in many works [2,5,6,12,19,20]. Corrosion is not considered a big problem for
aluminum castings due to their high material thickness. Therefore, corrosion properties of aluminum
castings were not investigated to the same extent as wrought aluminum alloys. For the future, we will
still need to think about decreasing vehicle weight, and economic demands for production in the
automotive industry. This can be achieved by reducing the material thickness. Thus, corrosion
properties will be of vital importance for the properties and a life-time of such components. On the
other hand, it is most likely that pitting corrosion or other forms of localized corrosion attack enhanced
fatigue crack initiation [21]. Properties of final aluminum products are more important from the point
of view of the usage of aluminum casts in industries. Therefore, the aim of the present study is to
contribute to the understanding of the microstructural arrangement role on the corrosion resistance of
different hypoeutectic secondary Al cast alloys.

2. Materials and Methods

Al-Si based alloys are the most significant commercial casting alloys (basic materials for cylinder
heads are especially AlSiMg and AlSiCu cast alloys) [22,23]. These materials are suitable for heat
treatment and can reach the required mechanical properties [9,24]. Thanks to its excellent foundry
properties, the hypoeutectic AlSi7Mg0.3 cast alloy was an object of the research. Experimental alloys,
with the chemical composition reported in Table 1, have been produced by gravity casting at Uneko,
Metals 2018, 8, 581 3 of 13

spol.s.r.o., Zátor, Czech Republic, and cast into sand molds. Such alloys are commonly used in the
production of components of the aerospace industry, automotive castings-wheels, engine parts, and so
on. However, with the increased usage of recycling this material was produced with different iron
concentrations. The alloy was produced according to the standard 0.123 wt. % of Fe (alloy A), with the
content of 0.454 wt. % Fe (alloy B) and with the content of 0.655 wt. % Fe (alloy C). The content of Fe
was defined by the company based on the input raw materials and economical point (Table 1).
Contemporary manufacturers would like to lower the economic demands of their manufacturing;
therefore, makes sense to use self-hardened (without heat treatments) alloys. This class of alloys has
a particular characteristic: they are subjected to a natural ageing phenomenon and after a period
of about 7 to 10 days can achieve good final mechanical properties without any further thermal
treatment [2,25,26]. This is a good opportunity to reduce the final production costs. Therefore, the
second experimental material was AlZn10Si8Mg (UNIFONT 90). The AlZn10Si8Mg material was
produced with a higher Fe content. Alloy D-AlZn10Si8Mg was prepared according to standard
(0.150 wt. % Fe), and E-AlZn10Si8Mg was produced with the content of Fe 0.559 wt. %.
Experimental alloys, with the chemical composition reported in Table 1, were produced by
gravity casting. According to the requirements, the material is not modified and grain refined.
The experimental material was observed in form of bars with a 20 mm diameter and 280–300 mm
length. Its mechanical properties (ultimate tensile strength, Brinell hardness, ductility), microstructure
and corrosion resistance of the prepared samples with different content of Fe were investigated.

Table 1. Chemical composition measured using arc spark spectroscopy, wt. %.

Alloy Si Fe Cu Mn Mg Zn Ti Na Al Other
A 7.028 0.123 0.013 0.009 0.354 0.036 0.123 0.002 92.253 balance
B 7.34 0.454 0.021 0.009 0.302 0.02 0.118 0.004 91.673 balance
C 7.315 0.655 0.03 0.01 0.292 0.028 0.12 0.005 91.486 balance
D 8.703 0.150 0.008 0.013 0.381 10.001 0.05 0.002 80.64 balance
E 8.831 0.559 0.008 0.019 0.319 9.335 0.049 0.002 80.828 balance

The test specimens with dimensions corresponding to the standards (ISO 6892-1:2009) for
measuring the ultimate tensile strength (UTS) and specimens for corrosion properties with 10 mm
diameter and 17.50 mm length. Changes in tensile properties (UTS and ductility) were measured
on INSTRON Model 5985 according to the standard ISO 6892-1:2009. Test rates and control are set
according to the A Method recommended ranges. The template is intended for specimens that produce
a clearly defined linear elastic region and homogenous deformation. The calculated results include
UTS and ductility. Hardness measurement for secondary aluminum alloy was performed by using a
Brinell hardness tester with load of 250 kp (1 kp = 9.80665 N), a 5 mm testing ball, and a dwell time
of 15 s. The evaluated UTS, ductility and Brinell hardness reflect the average values of at least six
separate measurements for each experimental material.
Three different pre-exposing environments were included in order to elucidate the important
relationship between the material and environment. The first was an exposure test in 3.5 wt. %
NaCl solution at 20 ◦ C for three weeks. Each specimen of the experimental material was degreased
in ethanol before testing, and then dried. Potentiodynamic (PP) testing was chosen to evaluate the
electrochemical corrosion characteristics. Each specimen of the experimental material was degreased
in ethanol, and then dried before testing. Measurements were performed in the 0.5 M NaCl at
20 ± 2 ◦ C, using laboratory potentiostat VSP Biologic SAS (Univesity of Žilina, Žilina, Slovakia).
The three-electrode cell system was used, including an experimental specimen with the exposed area
of 1 cm2 set as the working electrode, a platinum electrode was set as a counter and saturated calomel
electrode (SCE), which served as a reference electrode (+0.242 V vs. platinum electrode). The PP tests
started after 10 min of potential stabilization—between the experimental specimen and the testing
electrolyte. The applied potential ranged from −200 mV to +300 mV. The range of potentials was set
Metals 2018, 8, 581 4 of 13

with respect to the open circuit potential (OCP) and the scan rate was 0.2 mV/s. The measured data in
form of potentiodynamic curves were analyzed by the Tafel extrapolation method, and the values of
corrosion potential Ecorr and corrosion current density icorr we obtained using EC Lab V10.34 software
(Univerity of Žilina, Žilina, Slovakia). From the Ecorr and icorr values the corrosion rate rcorr was
calculated. The third corrosion test was carried out according to the Audi internal PV 11 13 standard
Metals 2018, 8, x FOR PEER REVIEW 4 of 13
used in the automotive industry [20]. Each of the experimental material specimens were degreased in
ethanol
standardbefore
usedtesting,
in the then dried with
automotive hot air,
industry [20].and
Eachimmersed in an Audi solution:
of the experimental 1 dm3 H2 O
material specimens + 20g
were
NaCl + 0.1 dm 3 25% HCl for 2testing,
h at 20then ◦
± 2dried
C. After
degreased in ethanol before with this test,and
hot air, specimens
immersed were
in anrinsed
Audi in distilled
solution: water,
1 dm 3

dried in hot air, and weighted. Weight losses were used to calculate corrosion
H2O + 20g NaCl + 0.1 dm 25% HCl for 2 h at 20 ± 2 °C. After this test, specimens were rinsed in
3 rates.
Corrosion
distilled water,surface analysis
dried in hot air, was carried out
and weighted. to identify
Weight lossestheweretype ofto
used corrosion
calculateattack by scanning
corrosion rates.
electron microscope
Corrosion (SEM)
surface and stereo
analysis microscopy
was carried out to after exposure
identify the type corrosion tests.
of corrosion The by
attack characteristics
scanning
and depthmicroscope
electron of corrosion(SEM)attack of studied
and stereo alloysafter
microscopy were examined
exposure using
corrosion a cross
tests. section of the
The characteristics
specimens—optical
and depth of corrosion microscopy.
attack of Thestudied
stereo alloys
microscope Olympus using
were examined SZX 16a withcross camera
section DP73
of thefor
specimens—optical
visual observation, and microscopy.
a computerThe stereo microscope
for photo Olympus SZX
documentation was16 with camera
used. DP73 for
The samples forvisual
optical
observation,
microscopy and a computer
observation using for photo documentation
a Neophot 32-microscopewas withused. The samples
a Nikon for optical
digital sight DS-U2microscopy
camera were
observation
prepared using a Neophot
by standard 32-microscope
metallographic with (wet
procedures a Nikon digital
ground onsight DS-U2 camera
SiC papers, were prepared
DP polished with 3 µm
by standard metallographic procedures (wet ground on SiC papers,
diamond pastes followed by Struers Op-S). Some specimens were also observed using DP polished with 3 μm diamond
a scanning
pastes microscope
electron followed by VEGA Struers LMU
Op-S).IISome specimens
(Univerity were also
of Žilina, observed
Žilina, usingequipped
Slovakia)., a scanning electron
with Energy
microscope VEGA LMU II (Univerity of Žilina, Žilina, Slovakia)., equipped with
dispersive X-rays analysis unit (EDX) in order to study the corrosion localization. The metallographic Energy dispersive
X-rays analysis unit (EDX) in order to study the corrosion localization. The metallographic
observation of microstructure changes as the effect of increased iron content was observed using an
observation of microstructure changes as the effect of increased iron content was observed using an
optical microscope. The samples were prepared by standard metallographic procedures (wet ground
optical microscope. The samples were prepared by standard metallographic procedures (wet ground
on SiC papers, DP polished with 3 µm diamond pastes followed by Struers Op-S, etched by ammonium
on SiC papers, DP polished with 3 μm diamond pastes followed by Struers Op-S, etched by
molybdate-MA and Dix Keller) (Univerity of Žilina, Žilina, Slovakia).
ammonium molybdate-MA and Dix Keller) (Univerity of Žilina, Žilina, Slovakia).
3. Results and Discussions
3. Results and Discussions
3.1. Mechanical Properties
3.1. Mechanical Properties
The results of mechanical properties show increasing or comparable properties with increasing
The results of mechanical properties show increasing or comparable properties with increasing
content of Fe in both types of experimental material (Figure 1). The first material type AlSi7Mg0.3
content of Fe in both types of experimental material (Figure 1). The first material type AlSi7Mg0.3
has maximum mechanical properties in state (B) with 0.454 wt. % of Fe (UTS = 150 MPa, HBW = 55,
has maximum mechanical properties in state (B) with 0.454 wt. % of Fe (UTS = 150 MPa, HBW = 55,
and
andductility
ductility= =1.91%).
1.91%).The
TheA-series
A-series specimens
specimens (in(inthe
thestate
stateaccording
accordingtotostandards)
standards) have
have thethe lowest
lowest
mechanical properties (UTS = 141 MPa, HBW = 52, and ductility = 1.45%). The differences
mechanical properties (UTS = 141 MPa, HBW = 52, and ductility = 1.45%). The differences of UTS and of UTS
and Brinell
Brinell hardness
hardness are are insignificant,
insignificant, because
because these these are about
are about 5–7% in5–7%
UTS,in
andUTS, and 3.5–5.7%
3.5–5.7% in HBW. in HBW.
More
More
important are the differences in ductility of about 9–30%. 30% was present in samples B (with 0.454% B
important are the differences in ductility of about 9–30%. 30% was present in samples
(with 0.454%
of Fe). of Fe).isThe
The above aboverelated
probably is probably related
to a higher to a higher
content contentfinessing
of Si causing of Si causing
of the finessing of the
alpha matrix
alpha
[5,6].matrix [5,6].

Figure 1. Mechanical properties of experimental materials.

Figure 1. Mechanical properties of experimental materials.
The second experimental material AlZn10Si8Mg showed a decrease in UTS, a little increase in
The hardness
Brinell second experimental material
and a comparable valueAlZn10Si8Mg
of ductility byshowed a decrease
comparing in UTS,
standards, a little
as well increase
as a higher
in content
Brinellofhardness
Fe (Figureand a comparable
1). The valuehardness
increase in Brinell of ductility by comparing
was caused by with a standards, as of
higher amount well as a
hard
and brittle Fe-needles phases. The difference was of 5%, therefore these changes in mechanical
properties are insignificant. The usage of the material with higher content is not impossible. The
influence of the α-matrix, as reported Osório [5], was confirmed. Mechanical properties change of the
same experimental materials properties with different content of Fe was just slightly different,
because of the α-matrix (finesses and content) being very similar for each material (Figures 2 and 3).
Metals 2018, 8, 581 5 of 13

higher content of Fe (Figure 1). The increase in Brinell hardness was caused by with a higher
amount of hard and brittle Fe-needles phases. The difference was of 5%, therefore these changes in
mechanical properties are insignificant. The usage of the material with higher content is not impossible.
The influence of the α-matrix, as reported Osório [5], was confirmed. Mechanical properties change
of the same experimental materials properties with different content of Fe was just slightly different,
Metals 2018, 8, x FOR PEER REVIEW 5 of 13
because of the α-matrix (finesses and content) being very similar for each material (Figures 2 and 3).
Regarding
Regardingthe the possibility
possibility of of replacing AlSi7Mg0.3 with
replacing AlSi7Mg0.3 withAlZn10Si8Mg,
AlZn10Si8Mg,it it wewecancansaysaythatthat
AlZn10Si8Mg
AlZn10Si8Mghas hasbetter
better mechanical propertiescompared
mechanical properties compared to to AlSi7Mg0.3
AlSi7Mg0.3 castcast
alloyalloy
(but (but this alloy
this alloy is
is in
in not
notheat-treated
heat-treatedstate).
state).
TheThe ultimate
ultimate tensile
tensile strength
strength was higher,
was 33% 33% higher,
BrinellBrinell
hardnesshardness
by 59%,by 59%,
and
and ductility
ductility by by about
about 38%38%in asincast
as cast
statestate of both
of both materials.
materials.

3.2. Microstructure
3.2. MicrostructureofofExperimental
Experimental Materials
Materials
Typical
Typicalhypoeutectic
hypoeutecticaluminum–silicon
aluminum–silicon alloys alloyspossess
possesstwo twomajor
majormicrostructural
microstructural components,
components,
namely,
namely, aluminum
aluminummatrixmatrixandandan analuminum-silicon
aluminum-silicon eutectic. eutectic.The Thewidewidevariety
variety ofof intermetallic
intermetallic phases
phases
in in aluminum
aluminum alloysoccurs
alloys occursbecause
becauseofofAlAlhigh highreactivity,
reactivity,caused causedby byitsitsnegative
negativestandard standardpotential
potential [9].
[9]. Most
Most negativenegative are iron-rich
are iron-rich intermetallic
intermetallic phases.In
phases. InFe-containing
Fe-containing aluminum aluminumcast castalloys,
alloys,Fe-rich
Fe-rich
intermetallic phases are formed, such as β-Al FeSi; Al FeSi , Al FeSi
intermetallic phases are formed, such as β-Al5 FeSi; Al9 FeSi2 , Al3 FeSi2 , Al4 FeSi2 , α-Al15 (FeMn)
5 9 2 3 2 , Al 4 FeSi 2 , α-Al 15(FeMn) 3Si2,Si ,
3 2
Al Fe Si, Al Fe MnSi, Al MnSi , Al Fe Si, π-Al Mg FeSi , and Al Si Mg
Al8 Fe2 Si, Al19 Fe4 MnSi, Al12 MnSi2 , Al12 Fe3 Si, π-Al8 Mg3 FeSi6 , and Al5 Si6 Mg8 Fe2 . From these phases
8 2 19 4 12 2 12 3 8 3 6 5 6 8Fe 2. From these phases
identifiedininAl-Si
identified Al-Sibase
basealloys,
alloys,thetheα-Al
α-Al15FeMn
FeMn3Si 2 and β-Al5FeSi phases are more important [12–15].
15 3 Si2 and β-Al5 FeSi phases are more important [12–15].
TheThe Chinese
Chinese script
script morphology
morphology of the
of the α-iron
α-iron phase
phase occurs
occurs duringduring eutectic
eutectic solidification.
solidification. TheThe
ironiron
phase
phase can also appear in the form of polyhedrons if it solidifies as a
can also appear in the form of polyhedrons if it solidifies as a primary phase. The β-phases crystallizeprimary phase. The β-phases
crystallize as thin plates looking like needles in their cross section. This phase is mostly associated
as thin plates looking like needles in their cross section. This phase is mostly associated with iron levels
with iron levels greater than 1 wt. %. From electrochemical point of view, the β-Al5FeSi phase is
greater than 1 wt. %. From electrochemical point of view, the β-Al5 FeSi phase is nobler than the matrix
nobler than the matrix in aqueous media, making the alloy system highly susceptible to localized
in aqueous media, making the alloy system highly susceptible to localized corrosion. The harmful
corrosion. The harmful effect of the β-iron phase can be neutralized by rapid solidification; addition
effect of the β-iron phase can be neutralized by rapid solidification; addition of neutralizers such as
of neutralizers such as Mn, Co., Cr, Ni, Sr, K, and Be can change the morphology of the phases or
Mn, Co., Cr,
enhance theNi, Sr, K, and Be
precipitation of can change
Fe-rich the morphology
particles, which are less of harmful
the phases than orneedles;
enhancemelt the superheat;
precipitation
ofstrontium
Fe-rich particles,
modification and non-equilibrium solution heat treatment [1,12–14,27,28]. Observation ofand
which are less harmful than needles; melt superheat; strontium modification
non-equilibrium solution heat
the basic microstructure of alltreatment
experimental [1,12–14,27,28].
materials showed Observation that it of the basic
consists microstructure
of α-phase dendrites,of all
experimental
eutectic (a mechanical mixture of α-phase and eutectic silicon) and different intermetallic phases of
materials showed that it consists of α-phase dendrites, eutectic (a mechanical mixture
α-phase
(Figures and eutectic
2 and 3). silicon) and different intermetallic phases (Figures 2 and 3).

(a) (b) (c)

Figure 2. Microstructure of AlSi7Mg0.3 cast alloys, etch. MA. (a) alloy A with 0.123 wt. % Fe; (b) alloy
Figure 2. Microstructure of AlSi7Mg0.3 cast alloys, etch. MA. (a) alloy A with 0.123 wt. % Fe; (b) alloy
B with 0.454 wt. % Fe; (c) alloy C with 0.655 wt. % Fe.
B with 0.454 wt. % Fe; (c) alloy C with 0.655 wt. % Fe.
The α-matrix precipitates from the liquid as a primary phase in the form of dendrites, and it
nominally comprises of Al and Si in AlSi7Mg0.3 cast alloy. The size of dendrites (fineness and
content) is similar for each state of our experimental materials, but the SDAS is slightly different. Si-
particles are like small-poorly rounded grains. However, thickened grains were observed on the
periphery of α-phase dendrites (Figure 2). Intermetallic phases in the microstructure of the
 Metals 2018, 8, 581 6 of 13

The α-matrix precipitates from the liquid as a primary phase in the form of dendrites, and it
nominally comprises of Al and Si in AlSi7Mg0.3 cast alloy. The size of dendrites (fineness and content)
is similar for each state of our experimental materials, but the SDAS is slightly different. Si-particles
are like small-poorly rounded grains. However, thickened grains were observed on the periphery of
α-phase dendrites (Figure 2). Intermetallic phases in the microstructure of the experimental materials
Metals 2018, 8, x FOR PEER REVIEW
were: Fe-rich intermetallic phases in needles form: Al5 FeSi, Fe-rich intermetallic phases in6 the of 13
form
of skeleton or Chinese script: Al15 (FeMg)2 Si2 and Mg-rich intermetallic phases: Mg2 Si. Studying the
intermetallic phases in the form of skeleton or Chinese script: Al15(FeMg)2Si2 and Mg-rich
microstructure confirmed formation of Fe-rich intermetallic phases mostly in form of skeleton-like
intermetallic phases: Mg2Si. Studying the microstructure confirmed formation of Fe-rich intermetallic
as needles in alloy A (chemical composition according to standards—0.123 wt. % of Fe). Needle,
phases mostly in form of skeleton-like as needles in alloy A (chemical composition according to
iron-rich intermetallic phases are smallest (alloy A) compared to an increased Fe content (alloys B
standards—0.123 wt. % of Fe). Needle, iron-rich intermetallic phases are smallest (alloy A) compared
and C). The increasing amount of Fe leads to formation of larger Fe-needles intermetallic phases
to an increased Fe content (alloys B and C). The increasing amount of Fe leads to formation of larger
(Figure 2) and the amount of these phases’ increases as well. The increasing content of Fe leads to
Fe-needles intermetallic phases (Figure 2) and the amount of these phases’ increases as well. The
formation
increasing of, especially,
content iron-rich
of Fe leads phasesof,
to formation in form of needles,
especially, iron-richthenphases
it in forms
in formintoofskeleton-like
needles, thenshapes.
it
in Skeleton-like shapes were
forms into skeleton-like not observed
shapes. in alloys
Skeleton-like B andwere
shapes C. These findings in
not observed correlate
alloys Bwith
andresearch
C. Thesework
of authors
findings [12–14].
correlate withThe authors
research confirmed
work of authors that[12–14].
the higher
The the iron content,
authors confirmed thethat
longer and wider
the higher the the
ironneedles.
content,The theMg-rich
longer phases
and widerweretheobserved
needles.inThe a smaller
Mg-rich volume
phasesinwere materials A and
observed inBacompared
smaller to
alloy A.
volume in materials A and B compared to alloy A.
TheThe microstructure
microstructure of AlZn10Si8Mg
of AlZn10Si8Mg castconsists
cast alloy alloy consists
of α-phase, of α-phase,
eutectic (dark eutectic (dark
gray Si gray Si
crystals
in crystals
α-phase)inand variousand
α-phase) typesvarious types of intermetallic
of intermetallic phases (Chinesephases (Chinese
script-Mg script-Mg2 Si, oval
2Si, oval round-like
round-like
particles
particles Al
Al2CuMg, Fe-needles-Al
2 CuMg, Fe-needles-Al
5FeSi, particlesFeSi, particles of AlFeMnSiNi, and ternary
5 of AlFeMnSiNi, and ternary eutectic Al-MgZn2-Cu). (Figure eutectic Al-MgZn 2 -Cu).
3) (Figure 3) [25,26].
[25,26]. The α-matrixThe α-matrixfrom
precipitates precipitates
the liquidfrom the
as the liquid phase
primary as theinprimary
the formphase in the and
of dendrites form of
dendritescomprises
nominally and nominallyof Al comprises
and Zn. The of Al and
size of Zn. The size
dendrites of dendrites
(fineness (finenessisand
and content) content)
similar for is similar
both
for both states of the experimental material. Si-particles, in the form of
states of the experimental material. Si-particles, in the form of small, poorly rounded, thickened small, poorly rounded, thickened
grains
grains were
were observed
observed on onthethe periphery
periphery of α-phase
of α-phase dendrites
dendrites (Figure
(Figure 3). Fe-containing
3). Fe-containing intermetallics,
intermetallics,
suchsuchas as
Al5Al
FeSi5 FeSi phases,
phases, areare formed
formed especially
especially between
between the the α-dendrites.
α-dendrites. Studies Studies confirmed
confirmed that that
withwith
increasing Fe content grows the amount and length of Fe-rich
increasing Fe content grows the amount and length of Fe-rich needle phases in the experimentalneedle phases in the experimental
material
material AlSi7Mg0.3.
AlSi7Mg0.3. Since
Since thisthis alloy,
alloy, despite
despite its high
its high zinczinc content,
content, belongsbelongs totypical
to the the typical
Al-SiAl-Si alloys,
alloys,
it could
it could be be
used used as replacement
as replacement for for AlSiMg
AlSiMg alloyalloy applications,
applications, because
because it has it ahas a similar
similar microstructure
microstructure
to to hypoeutectic
hypoeutectic Al-Si
Al-Si alloys
alloys [24–26].
[24–26].

(a) (b)

Figure 3. Microstructure of AlZn10Si8Mg cast alloys, etch. Dix-Keller. (a) alloy D with 0.150 wt. % Fe;
Figure 3. Microstructure of AlZn10Si8Mg cast alloys, etch. Dix-Keller. (a) alloy D with 0.150 wt. % Fe;
(b) alloy E with 0.559 wt. % Fe.
(b) alloy E with 0.559 wt. % Fe.

3.3. Corrosion Behavior

3.3.1. Exposure Test

The specimens of the experimental Al-alloys were immersed in the 3.5 wt. % NaCl solution at
 Metals 2018, 8, 581 7 of 13

3.3. Corrosion Behavior

3.3.1. Exposure Test

The specimens of the experimental Al-alloys were immersed in the 3.5 wt. % NaCl solution at
20 ◦ C for three weeks. Corrosion attack was evaluated visually, by light and electron microscopy.
Metals 2018, 8, x FOR PEER REVIEW 7 of 13
By first visual evaluation (Figure 4), differences in corrosion characteristics of the tested alloys were
Metals 2018, 8, x FOR PEER REVIEW 7 of 13
apparent. Pitting corrosion was observed preferentially in alloy AlSi7Mg0.3. The density and scale
corrosion pits grew with the Fe content. The effect of Fe on pitting corrosion of Al-alloys was
of corrosion
corrosion pitspits grew
grew with
with thethe Fecontent.
content.TheTheeffect
effectofofFeFeonon pitting
pitting corrosion
corrosion of of Al-alloys
Al-alloys was
confirmed earlier by Samuel [13]Feand Aziz [29]. Mingo et al. [30] and Arrabal et al. [4] mentioned was
in
confirmed
confirmed earlier
earlier by Samuel
by Samuel [13] and
[13] and Aziz [29].
Azizcorrosion Mingo
[29]. Mingo et al. [30]
et al.at[30] and Arrabal et al. [4] mentioned
their work that in A356 (AlSi7Mg0.3) alloy initiates theand Arrabal
interface et al. [4]
between thementioned
α-Al matrixin
in their
their work
work thatthat
in in A356
A356 (AlSi7Mg0.3)
(AlSi7Mg0.3) alloyalloy corrosion
corrosion initiates
initiates at theat the interface
interface between between
the α-Al the α-Al
matrix
and the Fe-rich intermetallic as a result of microgalvanic corrosion processes. Specimens of the
matrix and
and the Fe-rich the Fe-rich intermetallic
intermetallic as a
as abyresult result of microgalvanic
of microgalvanic corrosion
corrosion processes.
processes. Specimens of
AlZn10Si8Mg alloy were attacked general irregular corrosion and their surfaceSpecimens
was covered of by
the
the AlZn10Si8Mg
AlZn10Si8Mg alloy
alloy werewere attacked
attacked byby general
general irregular
irregular corrosionand
corrosion andtheir
theirsurface
surface was covered
covered by by
grey corrosion products. This correlates with the results by Khireche [31] stating that was
the impedance
grey corrosion
grey corrosionand products.
products. This correlates
This correlates with the results
with theconfirmed by Khireche
results by the
Khireche [31] stating
[31] stating that the
that the impedance
measurements the microscopic observations great activity of Al-Zn andimpedance
Al-Zn-Sn
measurements and
measurements and the
the microscopic
microscopic observations
observations confirmed the
confirmed the great
great activity
activity of
of Al-Zn and
Al-Zn and Al-Zn-Sn
Al-Zn-Sn
compared to pure Al to corrosion. The segregation at the grain boundaries leads to intergranular
compared to
compared to pure Al Al to
to corrosion. The segregation
segregation at at the
the grain boundaries leadsleads to intergranular
corrosion. The pure
assessment corrosion. The
of the AlSi7Mg0.3(alloy B) surfaces grain
afterboundaries
exposure test by to intergranular
using scanning
corrosion.
corrosion. The
The assessment
assessment of
of the
the AlSi7Mg0.3(alloy
AlSi7Mg0.3(alloy B)
B) surfaces
surfaces after
after exposure
exposure test
test by using
by using scanning
scanning
electron microscopy shows that on surface there are places with localized corrosion pits (Figure 5).
electron microscopy
electron microscopy shows shows that
that on
on surface
surface there
there are
are places
places with
with localized
localized corrosion
corrosion pitspits (Figure
(Figure 5).
5).
SEM observation of the AlZn10Si8Mg alloys was not sufficient.
SEM observation of the AlZn10Si8Mg alloys was
SEM observation of the AlZn10Si8Mg alloys was not sufficient. not sufficient.

Alloy A Alloy B Alloy C Alloy D Alloy E

Alloy A Alloy B Alloy C Alloy D Alloy E
Figure 4. Corrosion characteristics of the tested alloys after exposure test.
Figure 4. Corrosion characteristics of the tested alloys after exposure test.
Figure 4. Corrosion characteristics of the tested alloys after exposure test.

Figure 5. SEM observation of a corrosion attack on samples of alloy B after testing.

Figure 5. SEM observation of a corrosion attack on samples of alloy B after testing.
Figure 5. SEM observation of a corrosion attack on samples of alloy B after testing.
Corrosion pits start nearby cathodic Si particles, where the protective layer is diminished and
grain Corrosion
boundariespits arestart nearby
weaker as cathodic
well. This Si results
particles,
in where the protective
an agreement layer is diminished
with experimental resultsand
of
Corrosion
grain boundaries pits start nearby cathodic Si particles, where the protective layer is diminished and grain
Arrabal [4], Mingo are [30],weaker as well.
Davis [32], and This
Osório results in an
[33]. We agreement
suppose withFeexperimental
that the results
cathodic particles canof
boundaries
Arrabal [4], are weaker
Mingo asDavis
[30], well. [32],
This and
results in an[33].
Osório agreement
We with experimental
suppose that the Fe results ofparticles
cathodic Arrabalcan[4],
have a similar effect and can promote a higher density of corrosion pits in specimens with a bigger
Mingo
have [30],
a of
similarDavis [32],
effect and
and Osório [33].a We suppose thatofthe Fe cathodic particles can have
with aaphase.
similar
content Fe. According tocan
the promote higher
authors of [33], density
Si particles corrosion
disseminated pits in specimens
throughout the Al-rich bigger
effect and
content of can
of the promote
Fe. According a higher density
to the authors of corrosion
of [33], pits in specimens with a bigger content of Fe.
Because different growth mechanisms ofSieach
particles
phasedisseminated throughout
that was mentioned, theirthe Al-rich phase.
boundaries are
According
Because to
of the the authors of
different growth [33], Si particles disseminated throughout the Al-rich phase. Because of
not perfectly conformed, but are mechanisms
rather subjectedof each
to aphase
certain that was mentioned,
deformation at atomictheirlevel,
boundaries
mainly are
in
the different
notphase
perfectly growth mechanisms of each phase that was mentioned, their boundaries are not perfectly
the side conformed, but It
of the interface. are rather
seems subjected
that to a certain
these regions, due todeformation
their massive at localized
atomic level, mainly in
deformation,
conformed,
the phase butofare
side the rather subjected
interface. It to athat
seems certain
these deformation
regions, due attoatomic
their level, mainly
massive in the
localized phase side
deformation,
could be more susceptible to corrosion than a phase region that are not too close to the Si particles
could be6 more
(Figures and 7).susceptible to corrosion than a phase region that are not too close to the Si particles
(Figures 6 and 7).
 Metals 2018, 8, 581 8 of 13

of the interface. It seems that these regions, due to their massive localized deformation, could be more
susceptible
Metals 2018,
Metals 8, x8,
2018, FORxtoFOR
corrosion
PEER than a
REVIEW
PEER phase region that are not too close to the Si particles (Figures86ofand
REVIEW of 7).
8 13 13

Figure
Figure
Figure Energy
6. Energy
6.Energy
6. dispersive
dispersive
dispersive X-rays
X-rays
X-rays (EDX)
(EDX) analysis
analysis
(EDX) of corrosion
corrosion
of corrosion
analysis of localization
localization of alloy
alloy
of alloy
localization of BB after
after
B after testing.
testing.
testing.

(a) (a) (b) (b) (c) (c)

Figure 7. Corrosion
Figure attack of AlSi7Mg0.3 cast alloys, etch. MA. (a) alloy A with 0.123% Fe; Fe;
(b) alloy B
Figure 7.
7. Corrosion
Corrosion attack
attack of
of AlSi7Mg0.3
AlSi7Mg0.3 cast
cast alloys,
alloys, etch.
etch. MA.
MA. (a)
(a)alloy
alloyA
A with
with 0.123%
0.123% Fe; (b)
(b) alloy
alloy BB
with 0.454%
with
with 0.454%Fe;
0.454% Fe;(c) alloy
Fe; (c) C
(c) alloywith
alloy C 0.655%
C with Fe.
with 0.655%
0.655% Fe.
Fe.

In some
In someplaces
placesof researched
of researchedspecimens,
specimens,thisthis
pitpit
dissolution was
dissolution combined
was combined with inter-granular
with inter-granular
corrosion (Figure
corrosion 6). 6).
(Figure A similar attack
A similar was
attack observed
was observed by by
Gharavi [34][34]
Gharavi in their work.
in their TheThe
work. EDXEDXanalysis
analysis
also showed that corrosion products identified by a higher concentration of oxygen are
also showed that corrosion products identified by a higher concentration of oxygen are localized localized in in
around
aroundSi particles. Near
Si particles. Nearto the Fe Fe
to the particles, a slight
particles, accumulation
a slight accumulation of of
corrosion products
corrosion productswas also
was also
observed
observed(Figure 6). 6).
(Figure
Metals 2018, 8, 581 9 of 13

In some places of researched specimens, this pit dissolution was combined with inter-granular
corrosion (Figure 6). A similar attack was observed by Gharavi [34] in their work. The EDX analysis
also showed that corrosion products identified by a higher concentration of oxygen are localized in
around Si particles.
Metals 2018, Near
8, x FOR PEER to the Fe particles, a slight accumulation of corrosion products was9 also
REVIEW of 13
observed (Figure 6).
Metallographic analyses
Metallographic analyses of of the
the AlSi7Mg0.3
AlSi7Mg0.3alloy alloyconfirmed
confirmed(Figure
(Figure7)7)
that in in
that corrosion pitspits
corrosion the
matrix
the is dissolved
matrix is dissolvedandandthetheSiSiparticles
particlesare
areresistant
resistant to
to the chloride solution.
the chloride solution. TheThe beginning
beginning of of
intergranular corrosion was observed only on the specimen surface. With
intergranular corrosion was observed only on the specimen surface. With the higher Fe content,the higher Fe content, the
matrix
the major
matrix major amount
amount of of
cathodic
cathodic needles
needles forms
formsininthetheAl-alloy.
Al-alloy. This
This may
may create more places
create more places for
for
corrosionon
corrosion onthethesurface.
surface.The
Theabove
abovewas wasagreed
agreedby byvisual
visualdetection
detection(Figure
(Figure7).
7).
The specimens of AlZn10Si8Mg cast alloys featured intensive inter-granular
The specimens of AlZn10Si8Mg cast alloys featured intensive inter-granular corrosion corrosion (Figure
(Figure 8),
which can be expected in this type of Al-alloy [35]. Figures demonstrated a similar effect as theasone
8), which can be expected in this type of Al-alloy [35]. Figures demonstrated a similar effect the
one described
described by Tahamtan
by Tahamtan [8]. It [8].
shows It shows
galvanicgalvanic
corrosioncorrosion
betweenbetween
silicon silicon
particles particles
and theand the α-
α-matrix.
matrix. The results of exposure tests show that the susceptibility to inter-granular
The results of exposure tests show that the susceptibility to inter-granular corrosion increased by corrosion increased
by higher
higher Fe content.
Fe content.

(a) (b)

Figure8.8.Corrosion
Figure Corrosionattack
attackof
ofAlZn10Si8Mg
AlZn10Si8Mgcast
castalloys,
alloys,etch.
etch. Dix-Keller.
Dix-Keller. (a)
(a) alloy
alloy D
D with
with 0.150%
0.150% Fe;
Fe;
(b) alloy E with 0.559% Fe.
(b) alloy E with 0.559% Fe.

3.3.2.Potentiodynamic
3.3.2. PotentiodynamicTest
Test
Thecorrosion
The corrosioncharacteristics
characteristicsachieved
achievedbybypotentiodynamic
potentiodynamicmeasurement
measurementtest testmade
madein inchloride
chloride
solution reflect the Fe content in the AlSi7Mg0.3 alloy. The corrosion potential E corr of all AlSi7Mg0.3
solution reflect the Fe content in the AlSi7Mg0.3 alloy. The corrosion potential Ecorr of all AlSi7Mg0.3
alloywith
alloy withaadiffering
differingFe-drop
Fe-dropcontent
contentvery
veryslightly
slightlydiffer
differwith
withananincreasing
increasingFe Fecontent.
content. However,
However,
thedifferences
the differencesarearenegligible
negligible(Table
(Table2).
2).The
Thedecrease
decreasein inthe
thecorrosion
corrosionrate
rateisisvery
verywell
wellcomparable
comparablein in
Figure 9. The Fe up to 0.655 wt. % content shows that the corrosion rate decreased
Figure 9. The Fe up to 0.655 wt. % content shows that the corrosion rate decreased almost by a half. almost by a half.
ItItis
is interesting
interesting that
that the
the thermodynamic
thermodynamicstability
stabilitypresented
presentedby byEcorr decreased
Ecorr decreased with thethe
with Fe content, but
Fe content,
kinetics of the corrosion process expressed by the corrosion rate was
but kinetics of the corrosion process expressed by the corrosion rate was retained. retained.
Inthe
In theAlZn10Si8Mg
AlZn10Si8Mgalloy alloywith
withaahigher
higherFe FeEEcorr
corr content, the corrosion rate can be considered
content, the corrosion rate can be considered
equivalent. In
equivalent. In Figure
Figure 99 wewe can
can compare
compare thethe corrosion
corrosion resistivity
resistivity ofof the
the alloys
alloys AlSi7Mg0.3
AlSi7Mg0.3 andand
AlZn10Si8Mg. The alloys (A, C-D, E) with similar Fe contents, but with higher contents of Zna
AlZn10Si8Mg. The alloys (A, C-D, E) with similar Fe contents, but with higher contents of Zn have
different corrosion behavior. The Zn content decreases the corrosion resistance of Al cast alloys, but
negative influences elevated contents of Fe were not recorded.
 Metals 2018, 8, 581 10 of 13

have a different corrosion behavior. The Zn content decreases the corrosion resistance of Al cast alloys,
Metals 2018, 8, x FOR PEER REVIEW 10 of 13
but negative influences elevated contents of Fe were not recorded.
Table 2. Corrosion characteristics of the tested Al-alloys.
Table 2. Corrosion characteristics of the tested Al-alloys.
Alloy Ecorr (mV) icorr (µA/cm2) rcorr (mm/year)
Alloy
A Ecorr (mV)
−824.333 ±34 0.85367 (µA/cm2 ) 0.022rcorr
icorr ±0.0259 ±0.001(mm/year)

AB −833.333
−824.333±32 ±34 0.821 ±0.0137
0.85367 ± 0.0259 ±0.001 ±0.001
0.01333 0.022
BC − 833.333 ±32 0.821 ±0.0137 0.01333 ±0.001
−850.666 ±11 0.49133 ±00.161 0.008067 ±0.001
C −−984
850.666 ±11 0.49133 ±00.161 0.008067 ±0.001
D ±23 2.457 ±0.0567±0.0567 0.04 ±0.003 ±0.003
D −984 ±±38 23 2.457 0.04
E
E −1022.33
−1022.33 ±38
2.49133 ±0.0192
2.49133 ±0.0192
0.04 ±0.002
0.04 ±0.002

Figure 9. Potentiodynamic polarization curves of the experimental materials in 0.1 M NaCl solution.
Figure 9. Potentiodynamic polarization curves of the experimental materials in 0.1 M NaCl solution.

3.3.3. Audi Test

3.3.3. Audi Test
In practice, the Audi test is used to test corrosion behavior of Al alloys. The reason of its application
In practice, the Audi test is used to test corrosion behavior of Al alloys. The reason of its
in this work was to compare the results of this test with the ones usually applied in research works.
application in this work was to compare the results of this test 3with the ones usually applied in
The Audi test was carried out in an acid chloride solution (1 dm H2 O + 20 g NaCl + 0.1 dm3 25% HCl)
research works. The Audi test was carried out in an acid chloride solution (1 dm3 H2O + 20 g NaCl +
for 2 h. Characteristics of the corrosion attack are documented in Figure 10. In or to determine the
0.1 dm3 25% HCl) for 2 h. Characteristics of the corrosion attack are documented in Figure 10. In or
corrosion rates, experimental alloys were rinsed after testing in distilled water and dried, weighted
to determine the corrosion rates, experimental alloys were rinsed after testing in distilled water and
before and after testing. The corrosion rate was calculated from weight losses (Table 3).
dried, weighted before and after testing. The corrosion rate was calculated from weight losses (Table
3). Table 3. Corrosion rate of the tested alloys after Audi test.

Table 3. Corrosion rate of the

Alloy tested alloys
Corrosion after2 )Audi test.
rate (g/m
Alloy
A Corrosion rate (g/m2)
0.00768
A B 0.175957
0.00768
B C 0.132133
0.175957
C D 0.23014
0.132133
E 0.840229
D 0.23014
E 0.840229
Visual evaluation is shown in Figure 10, and no corrosion products are seen on the surface of
Visual evaluation
AlSi7Mg0.3 is shownwith
alloy specimens in Figure 10, Fe
differing andcontent.
no corrosion products
Corrosion are seen
pits were only on the surface
observed, of
especially
AlSi7Mg0.3 alloy specimens
on the specimen with differing
with a medium Fe content.
Fe content. OsórioCorrosion
[5,6] in hispits
workwere only observed,
demonstrated thatespecially
an increased
on the specimen with a medium Fe content. Osório [5,6] in his work demonstrated that an increased
Si content provoked a decreased corrosion resistance. This can be an explanation for the difference
between specimens B and C. The highest Si content was included in the experimental material labeled
B—one of the AlSi7Mg0.3 cast alloys (7.34%—Table 1).
Metals 2018, 8, 581 11 of 13

Si content provoked a decreased corrosion resistance. This can be an explanation for the difference
between
Metals 2018,specimens B and
8, x FOR PEER C. The
REVIEW highest Si content was included in the experimental material 11 labeled
of 13
B—one of the AlSi7Mg0.3 cast alloys (7.34%—Table 1).
Specimens
Specimens of of AlZn10Si8Mg
AlZn10Si8Mg werewere covered
coveredbybygrey
greycorrosion
corrosionproducts.
products.The
The corrosion
corrosion products
products on
on
thethe AlZn10Si8Mg
AlZn10Si8Mg alloy
alloy with
with a lower
a lower Fe content
Fe content werewere
not not
beenbeen continuous
continuous whenwhen compared
compared toones
to the the
ones onsurface
on the the surface
of theofsample
the sample
with with a higher
a higher Fe content.
Fe content. The corrosion
The corrosion rate calculated
rate calculated from weight
from weight losses
losses was nearly four times
was nearly four times higher. higher.
Under the above
Under the aboveconditions,
conditions, AlSi7Mg0.3
AlSi7Mg0.3 alloys
alloys withwith varying
varying Fe content
Fe content weremore
were much much more
resistant
resistant to corrosion.
to corrosion.

Alloy A Alloy B Alloy C Alloy D Alloy E

Figure 10. Corrosion characteristics of the tested alloys after the Audi test.
Figure 10. Corrosion characteristics of the tested alloys after the Audi test.

4. Conclusions
4. Conclusions
According to the experiments and analysis performed, we can conclude that:
According to the experiments and analysis performed, we can conclude that:
• Various chemical compositions greatly influence the studied properties of the tested aluminum
• alloys.
Various chemical compositions greatly influence the studied properties of the tested
• aluminum
In terms ofalloys.
the effect of higher Fe content, the mechanical properties of AlZn10Si8Mg and
• AlSi7Mg0.3
In terms ofcast the alloys
effect are
of higher Fe content,
not significantly the mechanical
influenced by it. Theproperties
presence of ofhard
AlZn10Si8Mg
and brittle andFe-
AlSi7Mg0.3 cast alloys are not significantly influenced by
needle phases leads to slightly improved mechanical properties. Therefore, the use it. The presence of hard andofbrittle
such
Fe-needledoes
materials phases not leads to slightly
significantly improved
influence mechanicalproperties
the mechanical properties.ofTherefore, the use of such
resulting alloys.
• materials does not significantly influence the mechanical properties
The microstructure of both types of experimental materials is typically hypoeutectic, involving of resulting alloys.
• an The microstructure
α-matrix, eutecticofandboth types of experimental
intermetallic phases. Thematerials
α-phase’sisfinesses
typically andhypoeutectic,
content were involving
similar
anthe
in α-matrix, eutectic and material.
same experimental intermetallicThephases. The α-phase’s
silicon present is in the finesses
form of and content
small grainswere similar
of poorly
rounded, thickened grains that were observed on the periphery of dendrites α-phase. Outpoorly
in the same experimental material. The silicon present is in the form of small grains of of the
rounded, thickened
intermetallic phases grains
were that were observedof
in microstructure onAlSi7Mg0.3
the periphery cast of dendrites α-phase.AlOut
alloys observed: 5FeSi,of
the intermetallic phases were in microstructure of AlSi7Mg0.3
Al15(FeMg)2Si2 and Mg2Si. The Mg2Si, Al2CuMg, Al5FeSi, AlFeMnSiNi and Al-MgZn2-Cu cast alloys observed: Al FeSi,
5 in
Al15 (FeMg)2 Sicast
AlZn10Si8Mg 2 and Mg2 Si.
alloy. The Mg
In both 2 Si, Al2 CuMg,
experimental Al5 FeSi,
materials wereAlFeMnSiNi and Al-MgZn
observed increase 2 -Cu
lengths andin
AlZn10Si8Mg
amounts cast alloy.
of Fe-needle phasesIn both experimental
as a reaction materials
to increased were observed
Fe content increase lengths and
in the microstructure.
• amounts of Fe-needle phases as a reaction to increased Fe content
Worse corrosion properties were documented in case of AlZn10Si8Mg cast alloys, compared to in the microstructure.
• experimental
Worse corrosion properties
AlSi7Mg0.3 by were documented
all carried in case ofDuring
out experiments. AlZn10Si8Mg cast alloys,
their exposure and Audicompared
tests,
ittoshowed
experimental AlSi7Mg0.3
that higher by alldecrease
Fe contents carried corrosion
out experiments.
resistance During their exposure
of experimental and Audi
materials, but
tests,regard
with it showedto Sithat higher
content. Fe contents
Based decrease
on the results of corrosion resistance
potentiodynamic of experimental
a test was found that materials,
higher
butamounts
Fe with regard to Si content.
in AlSi7Mg0.3 castsBased
alloys on the results
decelerate of potentiodynamic
corrosion kinetic. These results a test was foundwith
correlate that
higher Fe amounts in AlSi7Mg0.3 casts alloys decelerate corrosion
Osório’s work [33]. All applied tests showed better corrosion resistance in AlSi7Mg0.3 cast kinetic. These results correlate
with Osório’s
alloys. But it was work [33].
also All applied
found that the tests
aboveshowed
depends better corrosion
not only on the resistance
Fe content, in AlSi7Mg0.3
but also on cast the
alloys. But of
proportion it was
Fe andalsoSifound
in the that the above
Al alloys depends
as reported bynot only[6]
Osório onin thehisFework.
content, but also on the
proportion of Fe and Si in the Al alloys as reported by Osório [6] in his work.
Author Contributions: Conceptualization, L.K., T.L. and E.T.; Methodology, L.K., T.L. and E.T.; Software, L.K.,
D.K. and E.S.; Validation, L.K., T.L., E.T. and Z.Z.; Formal Analysis, L.K., T.L., D.K. and E.S.; Investigation, L.K.,
Author Contributions: Conceptualization, L.K., T.L. and E.T.; Methodology, L.K., T.L. and E.T.; Software, L.K.,
D.K.
D.K. and E.S.; Validation,
and E.S.; Resources,L.K.,
L.K.,T.L.,
T.L.E.T.
andandE.T.;
Z.Z.;Data Curation,
Formal L.K.,
Analysis, T.L.
L.K., and
T.L., D.K.;
D.K. and Writing-Original
E.S.; Investigation,Draft
L.K.,
D.K. and E.S.;L.K.
Preparation, Resources,
and T.L.;L.K., T.L. and E.T.; &
Writing-Review Data Curation,
Editing, L.K.,L.K.,
T.L.,T.L.
D.K.and D.K.;
and E.T.;Writing-Original DraftT.L.
Visualization, L.K., Preparation,
and E.T.
L.K. and T.L.; Writing-Review & Editing, L.K., T.L., D.K. and E.T.; Visualization, L.K., T.L. and E.T.
Funding: This research was funded by Scientific Grant Agency of Ministry of Education of Slovak republic
VEGA grants number [No1/0533/15] and [No1/0029/18].

Conflicts of Interest: The authors declare no conflicts of interest.

References
Metals 2018, 8, 581 12 of 13

Funding: This research was funded by Scientific Grant Agency of Ministry of Education of Slovak republic VEGA
grants number [No1/0533/15] and [No1/0029/18].
Conflicts of Interest: The authors declare no conflicts of interest.

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