corrosion performance based on the microstructural array of al-based monotectic alloys in a nacl...

Corrosion Performance Based on the MicrostructuralArray of Al-Based Monotectic Alloys in a NaCl Solution

Wislei R. Osorio, Emmanuelle S. Freitas, and Amauri Garcia

(Submitted July 3, 2013; in revised form September 22, 2013; published online October 17, 2013)

The aim of this study is to compare the electrochemical behavior of three monotectic Al-based alloys (Al-Pb,Al-Bi, and Al-In) in a 0.5 M NaCl solution at room temperature. Two distinct microstructure arrays wereexperimentally obtained for each Al monotectic alloy by using a water-cooled unidirectional solidificationsystem. Results of electrochemical impedance spectroscopy (EIS) plots, potentiodynamic polarizationcurves, and impedance parameters obtained by an equivalent circuit analysis are discussed. It was foundthat the Al-Pb alloy has lower corrosion current density, higher polarization resistance, lower relativeweight, and cost than the corresponding values of Al-Bi and Al-In alloys. It is also shown that the elec-trochemical behavior of the three alloys examined are intimately correlated with the scale of the corre-sponding microstructure, with smaller droplets and spacings (i.e., cell and interphase spacings) beingassociated with a decrease in the corrosion resistance.

Keywords EIS, monotectic alloys, polarization, relative cost,relative weight, solidification

1. Introduction

It is known that some immiscible Al-based alloys have a highpotential for self-lubricated sliding bearing applications. Al-Pb,Al-Bi, and Al-In alloys have important bearing requirements suchas reasonable self-lubricant behavior due to the formation of aninterfacial film of lubricant (Ref 1-3). These Al-based monotecticbinary alloys have also significant potential for many otherindustrial applications, such as high temperature superconductors,electronic, and optical components (Ref 1-7). The microstructuralarray of these monotectic alloys is commonly constituted by softand homogeneously dispersed fiber-like or droplet-like particlesof solute into the Al matrix (Ref 3, 4). These alloys also have alimited solubility in the liquid state (Ref 3-5). The Al-Pb, Al-Bi,and Al-In alloys have their corresponding monotectic tempera-tures and compositions given by: 659 �C and 1.2 wt.% Pb,658 �C and 3.2 wt.% Bi, and 640 �C and 17.5 wt.% In (Ref 6).At their corresponding monotectic temperatures, during equilib-rium solidification, an Al-rich liquid phase L1 is decomposed intoan Al-rich solid phase S1 and a liquid phase L2 (Ref 6).

Considering tribological characteristics, mechanicalstrength, and electrochemical behavior, the comprehension of

the monotectic microstructures is essential for the design ofcomponents with well-balanced properties. Although Al-Pb,Al-Bi, and Al-In alloys are considered promising slidingbearing alloys, investigations on interrelations between micro-structure array and electrochemical behavior are scarce in theliterature. Some studies have shown that Al-Pb alloys havebetter friction and wear properties with lower cost whencompared with Al-Sn alloys (Ref 8). The mechanism ofsolidification and the resulting microstructures of these mono-tectic alloys are reported in the literature, and relate the scale ofthe minority phase, having fibers or droplets morphologies,with both the growth and cooling rates (Ref 6, 9, 10).

The existing experimental investigations suggest that thecontrol of the microstructural array of these monotectic alloyscan be used as an alternative way for the design of theirelectrochemical corrosion behavior. Some investigations basedon the potentiodynamic polarization technique of monotecticalloys have been reported (Ref 11, 12). Investigations interre-lating the microstructural array, characterized by the interphaseor cell spacings, with EIS, equivalent circuit, and polarizationparameters have also been reported (Ref 13, 14). It has beenshown that the size of the droplet-like particles associated withcell spacings are the driving force determining the electro-chemical behavior of these monotectic alloys (Ref 13, 14).

Finer cell spacings and more homogeneous distribution ofthe soft droplets tend to decrease the corrosion resistance,represented by the corrosion current density. This is reported tobe intimately associated with localized strains between Al andsoft Pb, Bi, or In boundaries (Ref 13, 14). The electrochemicalbehavior of these monotectic alloys was shown to be verysimilar to a number of Al-based alloys (Ref 14-19). It has alsobeen reported (Ref 13-19) that the anodic/cathodic areavariation has been correlated with the microstructural arrayand that it has also an important role on the corrosionresistance. Krasovskii et al. (Ref 11) and Gundersen et al.(Ref 12) have shown that increased solute content andsegregation at the surface of samples have a significant effect,decreasing the corrosion resistance in sodium chloride solution.

Wislei R. Osorio, School of Applied Sciences (FCA), University ofCampinas (UNICAMP), P.O. Box 1068, 1300, Pedro Zaccaria St., Jd.Sta Luiza, Limeira, SP 13484-350, Brazil; and Department of MaterialsEngineering, University of Campinas (UNICAMP), P.O. Box 6122,Campinas, SP 13083-970, Brazil and Emmanuelle S. Freitas andAmauri Garcia, Department of Materials Engineering, University ofCampinas (UNICAMP), P.O. Box 6122, Campinas, SP 13083-970,Brazil. Contact e-mails: [email protected] and [email protected].

JMEPEG (2014) 23:333–341 �ASM InternationalDOI: 10.1007/s11665-013-0741-6 1059-9495/$19.00

Journal of Materials Engineering and Performance Volume 23(1) January 2014—333

Although experimental investigations (Ref 11-14) havereported the electrochemical behavior of some monotecticAl-based alloys, a comprehensive analysis on the roles of themicrostructural array (correlated with the applied cooling rate),distinct electrochemical behavior of the solute (lead, bismuth,or indium) and anode/cathode area ratios on the electrochemicalbehavior of these monotectic Al alloys has not been carried out.

The present experimental investigation focus on the effect ofthe microstructural array associated with electrochemicalfeatures provided by each concentration of Pb, Bi, and In onthe resulting EIS and potentiodynamic polarization plots ofmonotectic Al-Pb, Al-Bi, and Al-In alloys in a stagnant andnaturally aerated sodium chloride solution at room temperature.

2. Experimental Procedure

The monotectic Al alloys were prepared using commerciallypure (c.p.) elements, as shown in Table 1. The correspondingmain impurities are also shown in Table 1. Both the x-rayfluorescence technique and energy dispersive x-ray spectros-copy (EDS) were used in order to determine the average valuesof the chemical compositions shown in Table 1.

The experimental analysis was carried out in samplesextracted from the directionally solidified casting of eachmonotectic alloy, selected at distances (positions) of 10 mm(±0.5) and 15 mm (±0.5) from the cooled bottom of thecasting. These directionally solidified castings were obtained ina water-cooled solidification set-up, which promotes a verticalupward directional solidification with heat extracted onlythrough the water-cooled bottom, as detailed in previous studies(Ref 9, 10, 18). The as-cast alloys samples were selected as theworking electrodes for the electrochemical corrosion tests. Thesamples were positioned at the glass corrosion cell kit, leaving acircular 1 cm2 (±0.05) alloy (metal) surface in contact with anaturally aerated and stagnant 0.5 M NaCl solution at 25 �C(±3 �C) with neutral pH range between 6.8 and 7.3.

These samples were further ground, polished, and etched inorder to reveal the resulting microstructure. More details of themetallographic procedures can be found in previous articles(Ref 9, 10). An image processing system Neophot 32 and ascanning electron microscope (SEM) were used to characterizethe microstructure.

EIS tests were carried out using a potentiostat coupled to afrequency analyzer system, a glass corrosion cell kit with aplatinum counter-electrode and a saturated calomel referenceelectrode (SCE). The selected working electrodes (w.e.)constituted by Al-based monotectic alloys samples werepositioned at the glass corrosion cell kit permitting a circular1 cm2 surface contact with the electrolyte. It is important to

remark that all samples were ground to a 1200 grit surfacefinish using silicon carbide paper, followed by distilled waterwashing and air drying before electrochemical measurements.These samples after polished were immediately positioned inthe cell kit and the solution (electrolyte) was carefully pouredinto the cell. This procedure was adopted for all the samplesexamined. EIS tests were initiated after an initial delay of about15 and 20 min. This procedure was taken in order tostandardize a steady-state condition for all samples examined,which is considered enough for stabilization of the potentialsince preliminary measured EIS tests under open-circuitpotential have not shown distortions.

In the EIS measurements, a potential amplitude of 10 mVwith peak-to-peak (AC signal) with 5 points per decade andfrequency ranging between 100 mHz and 100 kHz wereadjusted.

The potentiodynamic polarization tests were carried outimmediately after the EIS measurements, at the same positionsand using the same electrolyte. A potential scan rate of 0.1667mV/s from �250 mV (SCE) to +250 mV (SCE) was also set.In order to confirm the tendency of the alloys samplesexamined and to obtain an average value, duplicate tests forboth EIS and polarization curves were performed. In order toobtain the impedance parameters (polarization resistances andcapacitances), a proposed model for equivalent circuit quanti-fication (ZView�) has also been used.

3. Results and Discussion

3.1 Interphase/Cell Spacings and Diameters of Droplets

The three examined monotectic Al alloys samples wereobtained using a water-cooled unidirectional solidificationsystem. This solidification set-up imposes higher cooling ratesclose to the casting cooled surface. The samples were selectedat positions P10 and P15, i.e., at 10 and 15 mm from the cooledsurface, in order to parameterize the distance from the bottomof the casting for all alloys examined considering a similarrange of cooling rates (i.e., between 4 and 9 �C/s). In this rangeof cooling rates, similar microstructural parameters (e.g.,interphase and cell spacings and diameter of droplets) areobtained for all monotectic alloys examined, which are shownin Table 2. The cell and interphase spacings as a function of thecooling rate have been previously reported (Ref 9, 10, 13, 14).It can be seen that the smallest microstructure parameters wereattained for the Al-In alloy with cell/interphase spacings atpositions 10 mm (P10) and 15 mm (P15) of about 20 (±8) and25 (±5) lm under cooling rates of ±5 and ± 4.5 �C/s,respectively. Besides, the Al-In alloy samples have alsoexhibited about 56% of the droplets diameters in the range

Table 1 Chemical compositions of the three examined monotectic alloys

Chemical composition, wt.%

Alloy Fe Sb Pb Zn Sn Al

Al-Pb * <0.001 1.25 (±0.3) 0.002 <0.001 BalanceAl-Bi <0.001 … 2.12 (±0.2) 0.001 <0.001 BalanceAl-In * * 5.23 (±0.4) * <0.002 Balance

*Less than 50 ppm

334—Volume 23(1) January 2014 Journal of Materials Engineering and Performance

between 0.6 and 0.9 lm (600 and 900 nm). The Al-Bi alloysamples had about 47% of their droplet-like bismuth particlesbetween a diameter range of about 3.5 and 6.5 lm and cellspacings of about 30(±8) and 41(±11) lm at positions P10 andP15, respectively. On the other hand, the examined Al-Pb alloyexhibited interphase spacings of about 24(±6) and 32(±5) lmwith 45% of the droplet-like Pb particles with diameter between0.9 and 2.5 lm.

Figure 1 shows the range of diameter of droplets for all thethree examined Al-Pb, Al-Bi, and Al-In alloys at position 10mm from the cooled surface of the castings. There aretendencies to bimodal distributions with respect to the evolu-tion of the diameter of the droplet-like particles. It can clearlybe seen that the Al-In alloy has the smallest microstructuralscale (both cell/interphase spacings and droplets diameters)when compared with the Al-Pb and Al-Bi alloys. As previouslyreported, this Al-In alloy experienced an increase in the dropletsize with the increase in distance from the cooled surface of thecasting (Ref 13, 20). It has also been reported the dependencesof the cell/interphase spacings on the experimental cooling rateof these three monotectic Al alloys (Ref 9, 10, 13).

3.2 Potentiodynamic Polarization Results

Figure 2 shows the experimental potentiodynamic polariza-tion curves from �1.5 V (SCE) to �0.5 V (SCE) in order todepict the three curves of the Al-Pb, Al-Bi, and Al-In alloys atpositions 10 mm (position P1) and 15 mm (position P2) fromthe bottom of the casting (cooled surface). These polarizationcurves were obtained in a stagnant and naturally aerated 0.5 MNaCl solution at room temperature. The average values for theexperimental corrosion current densities (icorr) and their corre-sponding corrosion potentials (Ecorr) for each alloy and sampleexamined are also shown in Fig. 2. These values were obtainedusing Tafel�s extrapolation.

Considering the three examined alloys at positions P1 andP2, the highest values (more positive) of Ecorr (i.e., displacedtoward the more noble-side potential) are that of Al-Pb alloysamples (Ecorr = �750 mV, SCE), followed by the intermediatevalues of the Al-Bi alloy samples (Ecorr = �960 mV, SCE). Thelowest Ecorr measurements (more negative) (i.e., displacedtoward the less noble-side potential) are that of Al-In alloysamples (Ecorr = �1170 mV, SCE). Besides, it is clearlyobserved that the highest range of corrosion current densities(icorr) is that of the Al-In alloy samples, i.e., of about 5 and 6lA/cm2. The intermediate values of icorr are that of Al-Bi alloysamples (between 1 and 2 lA/cm2). The lowest experimental

icorr measurements are that of Al-Pb alloy samples (i.e., ofabout 0.86 and 1.12 lA/cm2) associated with the noblestexperimental corrosion potential (�750 mV, SCE).

Previous investigations have reported (Ref 13, 14) that theexperimental ‘‘icorr’’ decreases with the increase in the cell/interphase spacings. This has been associated with the mod-ification in the ratio of cathode/anode areas as a function of thecell size and distribution of the immiscible droplet-like particlesin the Al matrix. Additionally, the strain generated at theborders between Al and droplet particles are associated with alocal higher level of energy, and consequently a highersusceptibility to corrosion is also induced (Ref 13, 14).

Another interesting observation in the polarization curves ofthe examined alloys is that the Al-Bi alloy has clearlyevidenced a trend to transient passivity regions and turbulentpassivity platforms. This is reasonably represented when theanodic current density stabilized about 9 lA/cm2 at �915 mV(SCE).

A number of investigations (Ref 11-21) have pointed-outthat the understanding of the corresponding microstructurearray is a vital factor in order to understand the corrosionresistance performance of a number of Al-based alloys (Ref 11-21). In this sense, based on the present results, it can be saidthat Pb, Bi, and In, which are immiscible particles disseminatedinto the Al matrix, have distinct electrochemical characteristicscontributing to the kinetics of corrosion action. From theelectrochemical point of view, a less-noble particle willconstitute an anode site increasing drastically the corrosion,while more noble regions become cathode sites (Ref 22-24).

The standard electrode potentials (Ref 22) of aluminum(Al+3/Al0), indium (In+3/In0), lead (Pb+2/Al0), and bismuth(Bi+3/Bi0) are of about �1.67 V (SHE), �0.34 V (SHE),�0.13 V (SHE), and +0.32 V (SHE), respectively. From theseaforementioned standard electrode potentials, the highestgalvanic couple is that formed by Al//Bi (±199 mV, SHE)followed by Al//Pb (±154 mV, SHE), and Al//In (±133 mV,SHE), which is confirmed by the level of oscillations verified inthe polarization curves of the Al-Bi, Al-Pb, and Al-In alloys, asshown in Fig. 2. However, this decreased sequence of galvaniccouples (Al//Bi>Al//Pb>Al//In) is not coincident with theobserved trends for icorr and Ecorr, which have shown adecreasing sequence (i.e., Al-In>Al-Bi>Al-Pb) of currentdensity and more active potential (less-noble), respectively.This sequence is inverted with that of the microstructuralparameters (Al-In<Al-Bi<Al-Pb), i.e., icorr increases withthe decrease in the scale of microstructural parameters (cell/interphase spacings). This evidences that the electrochemical

Table 2 Microstructural parameters and cooling rate for Al-Pb, Al-Bi, and Al-In alloys at positions 10 and 15 mm fromthe bottom of the cooled surface

Parameters AlPb AlBi AlIn

P10Interphase spacing 24 (±6) lm … …Cell spacing … 30 (±8) lm 20 (±8) lmDiameter of droplet 45% 0.9 to 2.5 nm 47% 3.5 to 6.5 nm 56% 0.6 to 0.9 nmCooling rate (dT/dt) 9 (±1.5) �C/s 6 (±2) �C/s 5 (±1.2) �C/sP15Interphase spacing 32 (±5) lm … …Cell spacing … 41 (±5) lm 25 (±5) lmDiameter of droplet 45% 0.9 to 2.5 nm 47% 3.5 to 6.5 nm 56% 0.6 to 0.9 nmCooling rate (dT/dt) 7 (±1.5) �C/s 4 (±1.8) �C/s 4.5 (±1.6) �C/s


characteristics (noble or active behavior) associated with theratio of cathode/anode areas, which is described by the resultingmicrostructural array, will define the electrochemical corrosionresistance.

Figure 3 shows the binary micrographs of the examinedAl-Pb, Al-Bi, and Al-In alloys samples. It is well known thatthese monotectic alloys have Al and second immiscibleelements (i.e., Pb, Bi, and In) forming droplets and/or cellular

microstructures and consequently galvanic cells are formedbetween the droplets and the Al matrix and/or second phasessegregated at cell boundaries. Aluminum/droplets interfaces orcell boundaries are regions of higher energy, which is a result ofdistortions at the limits between adjacent phases, which canconstitute galvanic couples. Considering the white regions aselectrochemically more noble regions than the Al regions(black), the Al-Pb alloy has a ratio of anode/cathode areas (a/cAR) of about 1:70 followed by Al-Bi and Al-In alloys withratios of anode/cathode areas of about 1:18 and 1:20, respec-tively, as shown in Fig. 3. These ‘‘a/c AR’’ values have beendetermined by the ImageJ� software analyzing binary images(Ref 24). It can also be seen that, despite the similar ‘‘a/c AR’’values (i.e., 1:18 and 1:20) observed for Al-Bi and Al-In alloyssamples, their corresponding corrosion current densities (icorr)are favoring the Al-Bi alloy with lower icorr (up to 4 times)when compared with that of the Al-In alloy. This seems to beassociated with both the smallest and homogeneously distrib-uted droplets (Fig. 1) and the more active (negative) potential,exhibited by the Al-In alloy (Fig. 2).

Although from the standard potential, the Pb droplets areless noble than Bi droplets, the Al-Bi alloy has evidenced bothhigher corrosion potential and current density than the corre-sponding values of the Al-Pb alloy. This seems to be intimatelyassociated with the ‘‘a/c AR’’ which is of about 3.5 times lowerfavoring the Al-Pb alloy, constituting lower number of galvaniccouples.

It is also important to remark that during corrosion thedroplets detach from the Al matrix lattice and are incorporatedinto complex Al oxides (e.g., Al(OH)x and AlCl�y) (Ref 14).With this, the ratio anode/cathode areas decreases significantly(the cathode area decreases) and corrosion action is minimized.Considering that the droplets initiate the anodic oxidation athigher potentials (Ref 24-27) (i.e., Pb at �0.55 V, Bi at +0.32V, and In at �0.34 V), the complex Al oxide is not exactly thatmore stable Al2O3 film, which is formed at higher potentials.Ekilik et al. (Ref 27) reported that bismuth is passive at apositive potential (of about +0.3 V, SCE) leading to theformation of Bi2O3, which can be developed in five polymorphoxides (Ref 28).

0

10

20

30

40

28%

Per

cent

age

/ %

Range of diameters of droplets / μm

P10 of AlPb alloy

17%

(a)

0

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20

30

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20%

Per

cent

age

/ %


P10 of AlBi alloy

13%

(b)

0.6...0.9 1.2...1.5 1.8...2.1 2.4...2.7 3.5...4.5 5.5...6.5 7.5...8.5 9.5...10.5

0.6...0.9 1.2...1.5 1.8...2.1 2.4...2.7 3.5...4.5 5.5...6.5 7.5...8.5 9.5...10.5

0.6...0.9 1.2...1.5 1.8...2.1 2.4...2.7 3.5...4.5 5.5...6.5 7.5...8.5 9.5...10.50

20

40

60

Per

cent

age

/ %


P10 of AlIn alloy

10%

56%

(c)

Fig. 1 Typical bimodal distribution of diameters of droplets atpositions P10 for: (a) Al-Pb, (b) Al-Bi, and (c) Al-In alloys

10-9 10-8 10-7 10-6 10-5 10-4 10-3 10-2-1.5

-1.4

-1.3

-1.2

-1.1

-1.0

-0.9

-0.8

-0.7

-0.6

-0.5

AlIn P1; 6.1 (0.6)μA/cm2; -1169mV AlIn P2; 4.7 (0.5)μA/cm2; -1171m

AlBi P1; 2.2 μA/cm2; -983mV AlBi P1; 1.2 μA/cm2; -927mV

Al Bi

Pot

entia

l (

E )

[ V

, SC

E ]

Current density ( i ) [ A cm-2 ]

AlPb P1; 1.12 μA/cm2; -753mV AlPb P2; 0.86 μA/cm2; -755mV

Al In

Al Pb

Fig. 2 Typical experimental potentiodynamic polarization curves ofAl-Pb, Al-Bi, and Al-In alloys samples in a 0.5 M NaCl solution atroom temperature at positions P1 (10 mm) and P2 (15 mm)


3.3 EIS and Equivalent Circuits Results

Figure 4 shows the experimental EIS plots for as-castmonotectic Al-Pb, Al-Bi, and Al-In alloys samples at positionsP10 and P15 carried out in a 0.5 M NaCl solution at roomtemperature. Figure 4(a) compares EIS plots of the

Al-1.2wt.%Pb alloy with that of the Al-2wt.%Bi alloy. Atlow frequency (±10�1 Hz), the Bode plots show the experi-mental modulus of impedance (|Z|) for the Al-Pb alloy samplesbetween 29 104 and 39 104 X cm2, while |Z| for Al-In isabout 59 103 X cm2. At frequencies between 100 and 103 Hz,

(a) Al-Pb (1:70); -0.13V

1 2 3 4 5 6 7 8 9 10 11 12 13 14 150.0

0.5

1.0

1.5

2.0

Are

a pe

rcen

tage

of P

b ric

h pa

rtic

les

/ %

Number of measurements

AlPb

1.4 %

(b) Al-Bi (1:18); +0.32V

1 2 3 4 5 6 7 8 9 10 11 12 13 14 154.0

4.5

5.0

5.5

6.0

6.5

7.0

Are

a pe

rcen

tage

of B

i par

ticle

s /

%


AlBi

5.4 %

(c) Al-In (1:20); -0.34V

1 2 3 4 5 6 7 8 9 10 11 12 13 14 153.0

3.5

4.0

4.5

5.0

5.5

6.0

Are

a pe

rcen

tage

of I

n pa

rtic

les

/ %


AlIn

4.5 %

Fig. 3 Typical binary micrographs with corresponding area percentage of droplets for the examined: (a) Al-Pb, (b) Al-Bi, and (c) Al-In alloyssamples


the double layer characteristic is favoring the Al-Pb alloy (slopelines).

The maximum phase angles (hmax) are also favoring theAl-Pb alloy with hmax between 70� and 75� at ±10 Hz), whilefor the Al-Bi alloy hmax is between 65� and 70� at 20 Hz. Athigh frequency range, the Bode plots of both Al-Pb and Al-Inalloys samples clearly characterize an electrolyte resistance of20 X cm2 . This comparison clearly evidences a better

corrosion resistance exhibited by the Al-Pb alloy. Figure 4(b)shows a comparison of EIS plots for Al-Pb alloy and Al-Bialloy samples. It can also be seen that a better corrosionresistance is attained by the Al-Pb alloy in terms of |Z| and hmax.The Al-Pb alloy has |Z| of about 2 or 3 times higher than that ofAl-Bi alloy with hmax also favoring slightly the Al-Pb alloy.Although the experimental data of Bode and Bode-phase ofAl-Bi and Al-In alloys are unclear, it seems that the Al-Bi alloyhas a slight advantage over the examined Al-Bi alloy, asdepicted in Fig. 4(c).

Figure 5 shows the experimental and simulated Nyquistplots evidencing capacitive semiarcs for all alloys samplesexamined. From these Nyquist plots, it is clearly observed thatthe semiarcs increased in the following sequence: Al-In<Al-Bi<Al-Pb. Besides, it is also confirmed that the corrosionresistance increased with the increase in distance from thebottom of the casting, i.e., with the coarsening of themicrostructural array. The impedance parameters have beenobtained using a previously proposed equivalent circuit (Ref13-19, 23, 26) and simulated by the ZView� software. Nyquistplots were chosen in order to compare the simulated andexperimental results, as shown in Fig. 5. The fitting qualitydetermined between simulated and experimental plots was

10-2 10-1 100 101 102 103 104 105 106100

101

102

103

104

105

θAl-In

θAl-Pb

ZAl-Pb P10 for Al-In

P15 for Al-In

P10 for Al-Pb P15 for Al-Pb

20 Ωcm2

Pha

se A

ngle

( θ

) [ d

egre

es ]

Mod

ulus

of I

mpe

danc

e (

Z )

[ Ω

cm

2 ]

Frequency ( F ) [ Hz ]

ZAl-In

10-2 10-1 100 101 102 103 104 105 106

0

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(a)

10-2 10-1 100 101 102 103 104 105 106100

101

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θAl-Bi

θAl-Pb

ZAl-Pb P10 for Al-Bi

P15 for Al-Bi

P10 for Al-Pb P15 for Al-Pb

20 Ωcm2

Pha

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ngle

( θ

) [ d

egre

es ]

Mod

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mpe

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e (

Z )

[ Ω

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ZAl-Bi

10-2 10-1 100 101 102 103 104 105 106

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(b)

10-2 10-1 100 101 102 103 104 105 106100

101

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θAl-Bi

θAl-In

ZAl-In

P10 for Al-Bi P15 for Al-Bi

P10 for Al-In P15 for Al-In

20 Ωcm2

Pha

se A

ngle

( θ

) [ d

egre

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Mod

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of I

mpe

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Z )

[ Ω

cm

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ZAl-Bi

10-2 10-1 100 101 102 103 104 105 106

0

10

20

30

40

50

60

70

80

90

(c)

Fig. 4 Comparison of the experimental Bode and Bode-phase plotsin a 0.5 M NaCl solution at room temperature at positions P10 (10mm) and P15 (15 mm) for: (a) Al-Pb vs. Al-In, (b) Al-Pb vs. Al-Bi,and (c) Al-In vs. Al-Bi alloys

0 10k 20k 30k 40k0

10k

20k

30k

40k

0.26 Hz

1Hz

0.26 Hz

1Hz

P10, Al-Pb P15, Al-Pb P10, Al-Bi P15, Al-Bi P10, Al-In P15, Al-In Simulated

Z Real

[ Ω cm2 ]

ZIm

agin

ary [

Ω c

m2 ]

(a)

0 2k 4k 6k 8k 10k0

2k

4k

6k

8k

10k

0.26 Hz

1 Hz

0.26 Hz

1 Hz

P10, Al-Pb P15, Al-Pb P10, Al-Bi P15, Al-Bi P10, Al-In P15, Al-In Simulated

Z Real

[ Ω cm2 ]

ZIm

agin

ary [

Ω c

m2 ]

(b)

Fig. 5 Experimental and simulated Nyquist diagrams at positionsP10 and P15 in a 0.5 M NaCl solution at room temperature for thethree examined monotectic alloys samples between: (a) 0 and40 kX cm2; and (b) 0 to 10 kX cm2


evaluated by Chi-squared (v2) values, as shown in Tables 3 and4. A good fitting quality is expressed by lowest values of v2

and in this investigation these values are between 41910�4

and 1109 10�4, as shown in Tables 3 and 4.The physical significance of the elements in the proposed

equivalent circuit has been extensively reported in previousinvestigations (Ref 13-19, 23, 26). Summarizing, Rel is theresistance of the electrolyte, R1 and R2 are the resistances ofthe porous and barrier layers, respectively, associated withthe charge transfer resistance through the porous layer and theparticipation of adsorbed intermediates. The capacitance of theporous and barrier layers are represented by CPE(1) and CPE(2),respectively. More details can be found in previous articles (Ref13-19, 23, 26).

From the results of Table 3, it can be said that the Al-Inalloy has its corrosion resistance controlled by the outer porouslayer since its corresponding R1 is considerably higher than R2

(inner and barrier layer) associated with the highest capaci-tances CPE(1) and CPE(2). The capacitance increases caused bythe increase in the film dieletric constant or when the thicknessof the oxide film thickness decreases associated with a decreasein the corresponding polarization resistance, in this case R2

(inner and barrier layer resistance). When comparing thecapacitances of the barrier layers (R2) of the three examinedalloys associated with their corresponding polarization resis-tances (R2), a trend favoring the Al-Pb alloy followed by theAl-Bi and Al-In alloys, can be clearly observed.

Analyzing the impedance parameters (R1 + R2) of Al-Pbversus Al-Bi alloys, it can be said that the polarizationresistance and corrosion current density of the Al-Pb alloyare about 6 and 2 times higher, respectively, than those of theAl-Bi alloy. (R1 + R2) and icorr favor the Al-Pb alloy of about 8and 6 times, respectively. When the comparison between Al-Bi

versus Al-In alloys is made, an icorr of about 4 times favors theAl-Bi alloy and only 30% differentiates (R1 + R2) for these twoalloys. A similar discussion can also be made when comparingthe impedance parameters obtained at positions P15 for allalloys examined, as shown in Table 4. However, these valuesindicate that coarser microstructure arrays provide betterelectrochemical corrosion resistance. For instance, the highest(R1 + R2) of about 90 kX cm2 is that of the Al-Pb alloy

Table 3 Impedance parameters of monotectic Al-Pb,Al-Bi, and Al-In alloys at position P10

Alloy Al-Pb Al-Bi Al-In

Rel, X cm2 18.6 18.2 19.75CPE(1), lF/cm2 10.32 (±0.85) 10.2 (±1.5) 33.5 (±0.6)n1 0.91 0.94 0.85R1, X cm2 1159 (±158) 159 (±35) 4.6k (±0.22k)CPE(2), lF/cm2 3.1 (±0.5) 17.1 (±0.6) 611 (±341)n2 0.93 0.75 0.85R2, kX cm2 46.5 (±2.8) 8.2 (±0.75) 1.2 (±0.2)v2 699 10�4 41910�4 1109 10�4

Table 4 Impedance parameters of monotectic Al-Pb,Al-Bi, and Al-In alloys at position P15

Alloy Al-Pb Al-Bi Al-In

Rel, X cm2 19.1 17.8 18.66CPE(1), lF/cm2 11.88 (±0.6) 8.6 (±1.2) 30.2 (±0.3)n1 0.90 0.94 0.87R1, X cm2 800 (±74) 178 (±13) 5.7k (±0.15k)CPE(2), lF/cm2 1.7 (±0.5) 13.8(±0.8) 608 (±282)n2 0.94 0.75 0.85R2, kX cm2 89.1 (±2.8) 10.9 (±0.9) 2.7 (±0.5)v2 449 10�4 589 10�4 509 10�4 Fig. 6 SEM micrographs after corrosion test (EIS + polarization;

2 h) of: (a) Al-Pb, (b) Al-Bi, and (c) Al-In alloys at P10


followed by the 11 and 9 kX cm2 corresponding to the Al-Biand Al-In alloys, respectively. Another interesting observationis associated with the parameter ‘‘n’’, for which values lowerthan 1 suggest a non-uniform distribution of current as a resultof roughness and possible oxide surface defects (Ref 13-19, 23,26). This is particularly observed for the Al-Bi alloy samples(Tables 3, 4) corroborating the oscillations of the polarizationcurves shown in Fig. 2. This indicates higher electrochemicalactivity caused by Bi droplets or their cations incorporated intothe inner oxide layer since bismuth has a positive potential(+0.32 V, SHE). Talbot and Ransley (Ref 29) demonstrated thatsmall bismuth additions (20 to 200 ppm) into Al-Mg alloysprevent detrimental effects of sodium on hot cracking (embrit-tlement). From the electrochemical point of view, the recentinvestigation developed by Krasovskii et al. (Ref 11) evidencedthat bismuth additions has a dual effect on the electrochemicalproperties of Al-based alloys, i.e., the corrosion currentdecreases with bismuth content up to 0.05 wt.% and withhigher bismuth amounts the corrosion resistance decreases.

A number of experimental investigations (Ref 30-34)reported the activation role of indium on localized dissolutionand pitting of aluminum in a sodium chloride solution undercertain electrochemical characteristics. Gudic et al. (Ref 33, 34)stated that corrosion of Al-In alloys occurs due to the formationof Al//In microgalvanic cells, where indium droplets constitutemore noble regions with respect to aluminum and corrosion issevere surrounding the Al-rich phase. The present experimentalresults are in same direction of these previous investigations,evidencing galvanic couples between Al and droplets of theminority phase resulting in pitting dissolution of the Al matrix,as shown in Fig. 6. These SEM images, under high magnifi-cations, show the predominance of pits in all the three alloysexamined. These images can be correlated with the corre-sponding polarization curves shown in Fig. 2. This permits toassume that due to high polarization resistances (R1 + R2 ofabout 90 kX cm2) associated with lowest capacitances (up to12 lF/cm2), the examined Al-Pb alloy has evidenced the lowestcorrosion current density. The observed values of (R1 + R2) andcapacitances of the Al-Pb alloy in the present investigation arehigher and lower, respectively, than the corresponding param-eters previously determined for four distinct Al-based alloys[i.e., Al-Fe (Ref 16), Al-Ni (Ref 17, 19) Al-Si and Al-Cu (Ref18) alloys], which have attained up to 90 kX cm2 and between20 to 80 lF/cm2 of polarization resistance and capacitance

values, respectively. This suggests that the Al-Pb has acorrosion protection provided by the inner (barrier) oxidelayer. Gundersen et al. (Ref 12) concluded that the activationprovided by indium is more effective than that of Pb and Bi,which is also supported by this experimental investigation, i.e.,the Al-Bi alloy samples exhibited the highest corrosion currentdensities and lowest polarization resistances.

From the point of view of applications, in particular for thecase of manufacture of aluminum sacrificial anodes (Ref 35),these three elements (i.e., Pb, Bi, and In) are consideredpotential alternatives for a relatively toxic element as mercuryor expensive as gallium and silver.

Table 5 shows the relatives cost and weight with respect tothe c.p. (commercially pure) aluminum (Al) and the Al-Pballoy. The range of average prices in U.S. dollars per kilogramof each c.p. metals are also indicated. Melting points andliquidus temperatures are also indicated in Table 5. The relativecorrosion resistance based on the corrosion current density(icorr) of each alloy examined with respect to the Al-Pb alloy isalso demonstrated in Table 5.

The results of Table 5 show that the Al-Pb alloy examined ischeaper, lighter and more corrosion resistant than the Al-Bi andAl-In alloys. Its relative cost with Al is very similar and itsrelative weight is only 4% higher than c.p. Al, while Al-Bi andAl-In are 6 and 11% higher, respectively. Although the meltingpoint of lead, bismuth, and indium are relatively different, theconsumption of electricity in the elaboration of these alloys arevery similar due to their corresponding liquidus temperatures.

4. Conclusions

Based on the experimental results from polarization curves,EIS parameters, equivalent circuit, and relative costs andweight of three distinct monotectic Al-based alloys (Al-Pb,Al-Bi, and Al-In), the following conclusions can be drawn:

1. The experimental corrosion current density of the threeexamined Al-Pb, Al-Bi, and Al-In alloys, depends stronglyon the resulting microstructure array (i.e., cell or interphasespacings). Smaller droplets and spacings are associatedwith a decrease in the corrosion resistance when comparedwith that of samples with coarser microstructures.

Table 5 Average price, relative cost, weight and corrosion resistance, and melting point and liquidus temperature for allmonotectic alloys and commercially pure metals

MaterialAverage

price, $US/kg*

Relativecost

Relativeweight

Relativecorrosion resistance

Meltingpoint, �C

Liquidustemperature, �C(Al) (AlPb) (Al) (AlPb)

Lead (Pb) 1-2 327Bismuth (Bi) 100-150 271Indium (In) 220-300 157Aluminum (Al) 20-30 1 660Al-Pb 29.67** 0.99 1 1.04 1 1 659Al-Bi 32.40** 1.08 1.09 1.06 1.02 1.4-1.9 652Al-In 44.85** 1.50 1.51 1.11 1.06 5.5 658

*Values based on Brazilian metal market**Values estimated from the individual prices of metals composing the alloy


2. The corrosion resistance in terms of the polarizationresistance is as follows Al-Pb>Al-Bi>Al-In.

3. The Al-Pb alloy has a relative weight that is about 4%higher than that of the c.p. Al.

4. Although Al-Bi and Al-In show similar ‘‘a/c AR’’, thecorrosion current density favors the Al-Bi alloy (i.e., low-er up to 4 times).

5. Thus, it should be given attention in the solidificationprocess through the cooling rate in order to achieve ade-quate microstructural array.

Acknowledgments

The authors acknowledge the financial support provided by,CNPq (The Brazilian Research Council), and FAEPEX-UNICAMP.

References

1. K. Lepper, M. James, J. Chashechkina, and D.A. Rignry, SlidingBehavior of Selected Aluminum Alloys, Wear, 1997, 46, p 203–204

2. X. Liu, M.Q. Zeng, Y. Ma, and M. Zhu, Wear Behavior of Al-SnAlloys with Different Distribution of Sn Dispersoids Manipulated byMechanical Alloying and Sintering, Wear, 2008, 265, p 1857–1863

3. I. Kaban, M. Kohler, L. Ratke, W. Hoyer, N. Mattern, J. Eckert, andA.L. Greer, Interfacial Tension, Wetting and Nucleation in Al-Bi andAl-Pb Monotectic Alloys, Acta Mater., 2011, 59, p 6880–6889

4. L. Ratke and A. Muller, Intensity of Different Pixels on the Line CCDCamera as a Function of Time for Monotectic Al-Bi, Scripta Mater.,2006, 54, p 1217–1220

5. H. Yasuda, I. Ohnaka, B.K. Dhindaw, S. Fujimoto, N. Takezawa, A.Tsuchiyama, and T. Nakano, Fabrication of Aligned Pores in Alumi-num by Electrochemical Dissolution of Monotectic Alloys SolidifiedUnder a Magnetic Field, Scripta Mater., 2006, 54, p 527–532

6. J. Zhu, T. Wang, F. Cao, H. Fu, Y. Fu, H. Xie, and T. Xiao, Real-TimeObservation on Evolution of Droplets Morphology Affected by ElectricCurrent Pulse in Al-Bi Immiscible Alloy, J. Mater. Eng. Perform.,2013, 22(5), p 1319–1323

7. S.Y. Hu, Y.C. Lee, Z.C. Feng, and S.H. Yang, Investigation of Defect-Related Optical Properties in AlxInyGa1-x-yN quaternary alloys withdifferent Al/In ratios, J. Lumin., 2012, 132, p 1037–1040

8. J. An, Y.B. Liu, Y. Lu, J. Wang, and B. Ma, Friction and WearCharacteristics of Hot-Extruded Leaded Aluminum Bearing Alloys, J.Mater. Eng. Perform., 2002, 11, p 433–443

9. A.P. Silva, A. Garcia, and J.E. Spinelli, Microstructure MorphologiesDuring the Transient Solidification of Hypomonotectic and MonotecticAl-Pb, J. Alloys Compd., 2011, 509, p 10098–10104

10. A.P. Silva, J.E. Spinelli, and A. Garcia, Microstructural EvolutionDuring Upward and Downward Transient Directional Solidification ofHypomonotectic and Monotectic Al-Bi Alloys, J. Alloys Compd.,2009, 480, p 485–493

11. M.O. Krasovskii, V.O. Lavrenko, and L.M. Kostenkon, AnodicBehavior of Al-Bi and Al-Sb Alloys in 3% NaCl Solution, PowderMetall. Met. Ceramics, 2010, 49, p 347–350

12. J.T.B. Gundersen, A. Aytac, J.H. Nordlien, and K. Nisancioglu, Effectof Heat Treatment on Electrochemical Behaviour of Binary AluminiumModel Alloys, Corros. Sci., 2004, 46, p 697–714

13. W.R. Osorio, E.S. Freitas, and A. Garcia, EIS and PotentiodynamicPolarization Studies on Immiscible Monotectic Al-In Alloys, Electro-chim. Acta, 2013, 102, p 436–445

14. W.R. Osorio, E.S. Freitas, and A. Garcia, EIS Parameters and CellSpacings of an Al-Bi Alloy in NaCl Solution, Electrochim. Acta, 2013,108, p 781–787

15. W.R. Osorio, C.M.A. Freire, and A. Garcia, Effects of the Longitudinaland Transversal Structural Grain Morphologies Upon the CorrosionResistance of Zinc and Aluminium Specimens, Revista de Metalurgiav.Extr., 2005, p 176–180

16. W.R. Osorio, L.C. Peixoto, P.R. Goulart, and A. Garcia, Electrochem-ical Corrosion Parameters of As-Cast Al Fe Alloys in a NaCl Solution,Corros. Sci., 2010, 52, p 2979–2993

17. W.R. Osorio, J.E. Spinelli, C.R.M. Afonso, L.C. Peixoto, and A.Garcia, Electrochemical Corrosion Behavior of Gas Atomized Al-NiAlloy Powders, Electrochim. Acta, 2012, 69, p 371–378

18. W.R. Osorio, L.C. Peixoto, D.J. Moutinho, L.G. Gomes, I.L. Ferreira,and A. Garcia, Corrosion Resistance of Directionally Solidified Al-6Cu-1Si and Al-8Cu-3Si Alloys Castings, Mater. Des., 2011, 32, p3832–3837

19. W.R. Osorio, L.C. Peixoto, M.V. Cante, and A. Garcia, MicrostructureFeatures Affecting Mechanical Properties and Corrosion Behavior of aHypoeutectic Al-Ni Alloy, Mater Des., 2010, 31, p 4485–4489

20. Y. Liu, J.J. Guo, Y.Q. Su, H.S. Ding, and J. Jia, Microstructures ofRapidly Solidified Al-In Immiscible Alloy, Trans. Nonferrous Met.Soc. China, 2001, 11, p 84–89

21. Y. Li, Z. Liu, S. Bai, L. Lin, and L. Gao, Effects of Pre-strain onExfoliation Corrosion Behavior in Al-Cu-Mg Alloy, J. Mater. Eng.Perform., 2012, 21, p 1479–1484

22. P. Vanysek, Electrochemcial Series, Handbook of Chemistry andPhysics: 93rd Edition, W.M. Haynes, Ed., Chemical Rubber Company,2012/2013, p 5–80, ISBN 9781439880494

23. W.R. Osorio, L.C. Peixoto, and A. Garcia, Comparison of Electro-chemical Performance of As-Cast Pb-1 wt.% Sn and Pb-1 wt.% SbAlloys for Lead-Acid Battery Components, J. Power Sources, 2010,195, p 1726–1730

24. K. Nisancioglu, Ø. Sævik, and Y. Yu, Passivation of Metal andSemiconductors, and Properties of Thin Oxide Layers, P. Marcus andV. Maurice, Ed., 2006, p 621

25. D. Pavlov, M. Bojinov, T. Laitinen, and G. Sundholm, ElectrochemicalBehaviour of the Antimony Electrode in H2SO4 Solutions-II. Forma-tion and Properties of the Primary Anodic Layer, Electrochim. Acta,1991, 36, p 2087–2092

26. W.R. Osorio, D.M. Rosa, and A. Garcia, Electrochemical Behaviour ofa Pb Sb Alloy in 0.5 M NaCl and 0.5 M H2SO4 Solutions, Mater. Des.,2012, 34, p 660–665

27. V.V. Ekilik, E.A. Korsakova, and A.G. Berezhnaya, Inhibition ofBismuth Dissolution in 0.1 M Sodium Chloride Solution, Int. J.Corros. Scale Inhib., 2013, 2, p 30–38

28. A.J. Salazar-Perez, M.A. Camacho-Lopez, R.A. Morales-Luckie, V.Sanchez-Mendieta, F. Urena-Nunez, and J. Arena-Alatorre, StructuralEvolution of Bi2O3 Prepared by Thermal Oxidation of Bismuth Nano-particles, Superficies y Vacio, 2005, 18, p 4–8

29. D.E.J. Talbot and C.E. Ransley, The Addition of Bismuth toAluminum-Magnesium Alloys to Prevent Embrittlement by Sodium,Metall. Trans., 1977, 8, p 1149–1154

30. H.A. El Shayeb, F.M. Abd Wahab, and S.Z. El-Abedin, Role of IndiumIons on the Activation of Aluminium, J. Appl. Electrochem., 1999, 29,p 601–609

31. S.B. Saidman and J.B. Bessone, Activation of Aluminium byIndium Ions in Chloride Solutions, Electrochim. Acta, 1997, 42,p 413–420

32. C.B. Breaslin and W.M. Carroll, The Effects of Indium Precipitates onthe Electrochemical Dissolution of Al-In Alloys, Corros. Sci., 1993,34, p 1099–1109

33. S. Gudic, I. Smoljko, and M. Kliskic, Electrochemical Behaviour ofAluminium Alloys Containing Indium and Tin in NaCl Solution,Mater. Chem. Phys., 2010, 121, p 561–566

34. S. Gudic, I. Smoljko, and M. Kliskic, The Effect of Small Addition ofTin and Indium on the Corrosion Behavior of Aluminium in ChlorideSolution, J. Alloys Compd., 2010, 505, p 54–63

35. M. Karaminezhaad, A.H. Jafari, A. Sarrafi, and G. Safi, Influence ofBismuth on Electrochemical Behavior of Aluminum Anode, Anti-Corros. Methods Mater., 2006, 53, p 102–109


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