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Electrochimica Acta 108 (2013) 781– 787

Contents lists available at SciVerse ScienceDirect

Electrochimica Acta

jo u r n al hom ep age: www.elsev ier .com/ locate /e lec tac ta

IS parameters and cell spacings of an Al–Bi alloy in NaCl solution

islei R. Osórioa,b,∗, Emmanuelle S. Freitasb, Amauri Garciab

School of Applied Sciences/FCA, University of Campinas, UNICAMP, P.O. Box 1068, 1300, Pedro Zaccaria St., Jd. Sta Luiza, 13484-350 Limeira, SP, BrazilDepartment of Materials Engineering, University of Campinas, UNICAMP, P.O. Box 6122, 13083-970 Campinas, SP, Brazil

r t i c l e i n f o

rticle history:eceived 23 May 2013eceived in revised form 3 July 2013ccepted 3 July 2013vailable online xxx

a b s t r a c t

This present study is focused on the evaluation of the effect of the cell spacing of as-cast Al–2 wt.% Bialloy formed by droplet-like bismuth particles distributed in the Al matrix, on the corrosion response. EISparameters and polarization curves obtained in a 0.5 M NaCl solution at room temperature are discussed.It was found that with the decrease in the cell spacing, the corrosion current density and polarizationresistance increases and decreases, respectively. This corrosion behavior was attributed to strains that

eywords:l–Bi alloysolidificationISolarizationmmiscible alloys

are generated at the borders between the Al-matrix and droplet-like Bi particles.© 2013 Elsevier Ltd. All rights reserved.

. Introduction

It is known that some immiscible Al-based alloys have impor-ant metallurgical characteristics for self-lubricated sliding bearingpplications. Al–Sn, Al–In, Al–Pb, and Al–Bi alloys can offer rea-onable self-lubricant behavior due to the presence of particles ofhe soft phase homogeneously dispersed into the Al matrix [1–3].esides, these alloys have also considerable wear and fatigue resis-ances and good load-carrying capacity [3]. During equilibriumolidification of a monotectic Al–Bi alloy, the Al-rich liquid phase1 decomposes into an Al-rich solid phase S1 and a liquid phase L2t the monotectic temperature [1]. During subsequent cooling, aontinuous Al-rich matrix is formed with the liquid minority phaseeing retained in a discontinuous way within the solid matrix in theorm of isolated pockets [1]. The competition between the growthf the minority phase and the rate of displacement of the solid-fication front will determine if the prevalent morphology of the

icrostructure will be characterized by fiber-like or droplet-likearticles distributed in the Al matrix [1]. It is also known that mono-

ectic Al–Bi alloys can be used for fabrication of porous materialsith smaller pore-size when compared with classical procedures oforous materials fabrication. This is pointed out as a considerable

∗ Corresponding author at: School of Applied Sciences/FCA, University of Cam-inas, UNICAMP, P.O. Box 1068, 1300, Pedro Zaccaria St., Jd. Sta Luiza, 13484-350imeira, SP, Brazil. Tel.: +55 19 3521 3320; fax: +55 19 3289 3722.

E-mail addresses: wislei.osorio@fca.unicamp.br, wislei@fem.unicamp.brW.R. Osório).

013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved.ttp://dx.doi.org/10.1016/j.electacta.2013.07.036

advantage since it can permit to hold fluid in their pores associatedwith high strength to weight ratio [4].

The comprehension of the evolution of monotectic micro-structures is essential for the design of components withappropriate tribological characteristics. Considering Al–In andAl–Pb alloys, some recent investigations reported [5,6] that the sizeof the droplet-like particles is associated with the driving-forcedetermining the resulting electrochemical corrosion resistance.This is attributed to localized strains between Al and In or betweenAl and Pb boundaries and the corrosion potential of these elements.It has also been reported [5,6] that the interphase spacing increaseswith the increase in the droplet size, which is connected with theapplied cooling rate during solidification. Finer and homogeneouslydistributed droplets are associated with examined specimens pos-itioned closer to the cooled surface of the casting, which tend tohave lower electrochemical corrosion resistance [5,6].

Krasovskii et al. [7] have investigated different Bi content of as-cast Al–Bi alloys. They have concluded that the corrosion resistancedecreased with the increase in the alloy bismuth content in a 3%NaCl solution [7]. Gundersen et al. [8] have shown that the elec-trochemical corrosion behavior of a dilute Al–0.2 wt.% Bi alloy isconsiderably activated due to surface segregation. This activationis only temporary lasting until the active surface layer is etchedaway [8]. They have also pointed-out that polished samples exhib-ited a higher corrosion resistance than heat treated samples [8].

Although these aforementioned studies reported results on theelectrochemical behavior of Al–Bi alloys based on potentiodynamicpolarization curves, the literature is scarce on studies interrelatingmicrostructural effects such as interphase or cell spacings with EIS,

7 imica Acta 108 (2013) 781– 787

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quivalent circuit and polarization parameters. This would permito prescribe guidelines with a view to preprogramming a desiredorrosion performance in terms of the required final properties ofearing components, based on the microstructural arrangement. It

s well-known that the corrosion susceptibility of Al-based alloys,nder various forms of localized corrosion attack, is one of the

mportant aspects to be considered in industrial applications.Sukiman et al. [9] stated that the appreciation of the result-

ng microstructure is vital in order to understand the corrosionerformance of Al-based alloys. They [9] have reported that allo-ing elements and thermomechanical processing play an importantole in dictating the type of microstructure produced. A hetero-eneous alloy can be constituted by second phases, precipitates,nsoluble or impurity elements, dispersoids and intermetallic par-icles, which present distinct electrochemical characteristics andct as sites for the corrosion attack [9]. Although the aforemen-ioned work [9] has described the general corrosion performancef different classes of Al alloys, the influences of alloy chemistriesnd microstructure on corrosion of Al alloys, the authors pointedut that, specialist topics have not been dealt with in their entirety9]. Considering the presence of insoluble phases, for instanceismuth in Al–Bi alloys, these authors highlighted the effect ofegregation on electrochemical destabilization in the presence ofhlorides.

This present study focus on the evaluation of the effect of theell spacing of a monotectic Al–2 wt.% Bi alloy formed by bismuthroplet-like particles disseminated in the Al matrix, on the electro-hemical corrosion behavior in a sodium chloride solution.

. Experimental procedure

A monotectic Al–2 wt.% Bi alloy was prepared usingommercially pure (c.p.) grade Al (99.7 ± 0.15 wt.%) and Bi99.9 ± 0.01 wt.%). The main impurities detected were: Fe0.03 wt.%), Cu (0.01 wt.%) and Zn (0.01 wt. %), besides otherlements found with concentrations less than 100 ppm. Thesehemical compositions were determined by both an X-ray fluores-ence technique and energy dispersive X-ray spectroscopy (EDS).ased on the measurements carried out in two different sampleshe results represent average values.

Selected specimens were extracted from a directionally solid-fied Al–2 wt.% Bi alloy casting in order to develop the presentnalysis [5,10]. These selected specimens were extracted fromransverse sections at three (03) distinct positions along the cast-ng length from the bottom of the casting: 10 mm (P1), 15 mm (P2)nd 20 mm (P3), as shown in Fig. 1. For each position the aver-ge Bi content (wt.%) was about: 2.5 (±0.2) wt.%, 2.2 (±0.3) wt.%,nd 2.1 (±0.5) wt.%, respectively. It was previously reported [10]hat cell arrays with droplet-like Bi particles characterize the result-ng microstructure. In order to reveal the resulting microstructure,he chosen transversal samples were ground, polished and etched.

ore details of the metallographic procedures can be found in arevious article [10].

An image processing system Neophot 32 (Carl Zeiss®, Esslin-en, Germany) and a scanning electron microscope (SEM, Jeol®

XA 840A) were used in order to characterize the resulting micro-tructure array. Three positions were selected in order to permithe effect of a range of cell spacings between Bi droplets on thelectrochemical corrosion responses to be examined.

A potentiostat (EG&G Princeton Applied Research®, model73A) coupled to a frequency analyzer system (FRA, Solartron®

odel 1255), a glass corrosion cell kit with a platinum counter-lectrode and a saturated calomel reference electrode (SCE) weresed to perform the EIS tests. In order to carry out the electro-hemical impedance spectroscopy (EIS) and polarization tests, the

Fig. 1. (a) Experimental cell spacing (�) as a function of cooling rate (dT/dt) alongthe casting length for an Al–2 wt.% Bi alloy.

selected working electrodes (w.e.) constituted by Al–Bi alloy sam-ples were positioned at the glass corrosion cell kit. A circular 1.0(±0.02) cm2 surface of the w.e. was put in contact with a naturallyaerated and stagnant 500 mL of a 0.5 M NaCl (±3% vol.) solution at27 ◦C (±2 ◦C), having a pH of 7.2 (±0.4). The w.e. Al–Bi alloy sampleswere ground up to a 1200 grit surface finish using silicon carbidepaper, followed by distilled water washing and air drying beforethe electrochemical measurements. It is also important to remarkthat the samples after polished were immediately positioned in thecell kit and the electrolyte was poured into the cell. This procedurewas adopted for all the samples examined, as similarly adopted ina previous investigation [5].

In order to parameterize a steady-state condition for all exam-ined samples, EIS measurements began after an initial delay ofabout 20 min. This period of time was considered enough for stabi-lization of the potential since preliminary measured EIS tests underopen-circuit potential have not shown distortions. The potentialamplitude was set to 10 mV with peak-to-peak (AC signal), with 5points per decade and the frequency range was set from 100 mHzto 100 kHz.

Immediately after the EIS measurements, and at the same pos-itions, the polarization tests were also carried out. A 0.5 M NaClsolution at room temperature (±25 ◦C) was also used in the polar-

ization tests. These tests were conducted by stepping the potentialat open-circuit with a scan rate of 0.1667 mV s−1 from −250 mV(SCE) to +250 mV (SCE). Duplicate tests for both EIS and polariza-tion curves were carried out in order to confirm the tendency of the

imica Acta 108 (2013) 781– 787 783

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W.R. Osório et al. / Electroch

xamined alloy samples and to obtain average values. A proposedodel for equivalent circuit quantification (ZView®) was also used

n the present analysis.

. Results and discussion

.1. Microstructure array

The water-cooled mold used in the present experimentationmposes higher cooling rates (of about 9 ◦C/s) [10] close to the cast-ng cooled-surface and a decreasing profile along the casting lengthp to cooling rates of about 0.5 ◦C/s [10]. This occurs due to the

ncreasing thermal resistance of the solidified shell with increas-ng distance in casting. Both smaller cell spacings and droplet-likei particles are formed at the bottom of the casting. On the otherand, larger ones characterize the microstructure of regions close tohe top of the casting. A previous investigation [10] described thevolution of the cell spacing as a function of cooling and growthates. It has been established that the cooling rate (dT/dt) as a func-ion of the distance (P) from the cooled surface of the casting, washaracterized by an experimental law given by: dT/dt = 38.9 × P−0.84

10].The experimental cooling rate dependence on the cellular spac-

ng (�) for the Al–2 wt.% Bi alloy is shown in Fig. 1. Typicalicrostructural arrays (binary images) of Al–2 wt.% Bi alloy samples

or the positions 10 mm and 20 mm (longitudinal sections) are alsohown in Fig. 1. From these binary images the average percentagesf the Al-rich matrix (black) and droplet-like bismuth particles andell boundaries (white regions) were determined. Thus, at positions0 mm and 20 mm the microstructure arrays of the Al–Bi alloy sam-les have about 7 (±0.85) % and 5 (±0.52) % of their surfaces formedy white regions (cell boundaries or Bi particles), respectively. This

s very helpful in order to determine the ratios of anode/cathodereas, which are approximately 1:15 and 1:20, respectively.

.2. ElS and equivalent circuit

The experimental results of EIS tests for as-cast monotecticl–2 wt.% Bi alloy samples are shown in Figs. 2 and 3. Fig. 2 showsode and Bode-phase plots evidencing three distinct regions: (i)

ow frequencies between 100 and 10−2 Hz (which represents theolarization resistances of the alloy samples), | Z | for all Al–Bi alloyamples examined: between 0.7 × 104 � cm2 and 1.5 × 104 � cm2;ii) similar double layer characteristics and maximum phase angles

ig. 2. Experimental Bode and Bode-phase diagrams for Al–2 wt.% Bi alloy samplesn a 0.5 M NaCl solution at room temperature corresponding to positions P1 (10 mm),2 (15 mm) and P3 (20 mm) from the casting surface.

Fig. 3. Experimental and simulated Nyquist plots for Al–2 wt.% Bi alloy samples in a0.5 M NaCl solution at room temperature at positions P1 (10 mm), P2 (15 mm) andP3 (20 mm).

(�max) represented in Bode-phase plots are characterized at inter-mediate frequencies between 100 and 104 Hz. The similar �max arebetween 70 and 80◦ at low frequency of about 20 Hz; and (iii)between 105 and 104 Hz is clearly characterized the solution (elec-trolyte) resistance of ±20 �.

Fig. 3 shows the experimental and simulated Nyquist plotsevidencing capacitive arcs, which have clearly increased with theincrease in distance from the bottom of the casting, i.e. with theincrease in the cell spacing. These Nyquist plots provide indica-tions that the corrosion resistance increased from position P1 to P3,i.e. both the ZReal (in-phase) and ZImaginary (out-of-phase) increasedwith the increase in the cell spacing.

In order to obtain impedance parameters such as polariza-tion resistances and capacitances, a well-known equivalent circuit[11–16] was selected by using the ZView® software, as shown inFig. 4. Nyquist plots were chosen in order to permit simulated andexperimental results to be compared. Such comparison betweenexperimental and simulated Nyquist plots of Al–2 wt.% Bi alloysamples is shown in Fig. 3. The fitting quality was evaluated by chi-squared (�2) values between 41 × 10−4 and 58 × 10−4, as shown inTable 1.

Considering the proposed equivalent circuit, the physical sig-nificance of the elements is that [5,6,11–16]: Rel is the resistanceof the 0.5 M NaCl solution and is expressed at high frequency

limit (F > 1 Hz). R1 and R2 are the resistances of the porous andbarrier layers, respectively, which are associated to the chargetransfer resistance through the porous layer and the participation of

Fig. 4. Equivalent circuit used to obtain impedance parameters.

784 W.R. Osório et al. / Electrochimica Acta 108 (2013) 781– 787

Table 1Impedance and polarization parameters of Al–2 wt.% Bi alloy samples in a 0.5 M NaCl solution at room temperature.

Parameters P1: 10 mm P2: 15 mm P3: 20 mm

Cell spacings 30 (±8) �m 41 (±11) �m 55 (±13) �mRel (� cm2) 18.2 17.8 18.2CPE(1) (�F cm−2) 10.2 (±1.5) 8.6 (±1.2) 11.8 (±0.9)n1 0.94 0.94 0.90R1 (� cm2) 159 (±35) 178 (±13) 317 (±25)CPE(2) (�F cm−2) 17.1 (±0.6) 13.8(±0.8) 7.8 (±1.2)n2 0.75 0.75 0.78R2 (� cm2) 8.2k (±0.75k) 10.9k (±0.9k) 14.6k (±0.9k)�2 41 × 10−4 58 × 10−4 47 × 10−4

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dsorbed intermediates [5,6,11–16]. CPE(1) and CPE(2) represent theapacitance of the porous layer and the capacitance of the barrierayer, respectively, which seems to correspond to double layer for-

ation. A CPE (constant phase element) representing a shift fromn ideal capacitor was used for simplicity instead of the capaci-ance itself [5,6]. The impedance of a phase element is defined asPE = [C (j ω)n]−1, where C is the capacitance; j is the current; ω ishe frequency and −1 ≤ n ≤ 1 [5,6,11–16]. The value of n seems to bessociated with the non-uniform distribution of current as a resultf roughness and possible oxide surface defects [11–16].

Comparing the capacitances CPE(1) (porous layer) of the exam-ned Al–Bi alloy samples, similar results can be observed, which areetween 9 and 12 �F cm−2. On the other hand, CPE(2) decreased tobout half of its initial value from P1 to P3 while both the resistances1 and R2 increased (i.e. duplicate) from positions P1 (10 mm) to P320 mm). Besides, R2 values corresponding to the barrier oxide layerre considerably lower than R1. This suggests that an oxide layeror similar corrosion by-product) inner barrier is responsible forhe corrosion protection of the Al–Bi alloy samples examined.

Although no large differences in the EIS plots can be observed,hich is expected as a function of the small difference among the

xamined cell spacings (from ±30 �m up to ±60 �m) of the threel–Bi alloy samples, distinct kinetic reactions of the adsorbed ionsnd a different kinetics and intensity of oxide film growth (cor-osion by-product) can be inferred. This can be observed whenmpedance parameters are analyzed (Table 1). With these obser-ations, it is believed that the mechanism of oxide film formationnd growth will also be significantly dependent on the resultingicrostructural array.In order to confirm the aforementioned and discussed exper-

mental and simulated EIS parameters, the potentiodynamicolarization curves of the monotectic Al–2 wt.% Bi alloy in NaClolution were also considered.

.3. Potentiodynamic polarization curves

Fig. 5(a) Fig. 5 shows experimental potentiodynamic polar-zation curves from −1.1 V (SCE) to −0.82 V (SCE) obtained in atagnant and naturally aerated 0.5 M NaCl solution at room tem-erature at a potential scan rate of 0.1667 mV s−1 for the Al–2 wt.%i alloy samples. Fig. 5(b)–(d) depicts the experimental potentio-ynamic polarization curves for the Al–Bi alloy samples at thehree examined positions (P1, P2 and P3), respectively. Both theathodic and anodic branches of the polarization curves were usedo obtain the corrosion current density (icorr) using Tafel’s extrap-lation. It is also important to remember that a potential scan rate

f 0.1667 mV s−1 was used in order to minimize distortions of Tafellopes and current density.

Considering the sample at position P1, an icorr between 1.6 upo 2.2 �A cm−2 with a corrosion potential (Ecorr) of about −983 mV

1.0 (±0.2) 0.75 (±0.2)−927 (±3) −952 (±2)

(SCE) can be observed. At positions P2 and P3, the experimental icorr

and Ecorr are between 0.8 up to 1.2 �A cm−2 with −927 mV (SCE)and 0.6 up to 0.9 �A cm−2 with −952 mV (SCE), respectively. Thesemeasurements are also shown in Table 1. All examined polarizationcurves have clearly evidenced oscillations in shape in both anodicand cathodic branches, which can be intimately associated withthe amount of droplets of bismuth and cell boundaries leading tothe formation of local electrochemical cells between the bismuthparticles/Al matrix.

Although some curves do not permit a clear identificationdue to oscillations, all the polarization curves examined depict arapid increase in the anodic current. However, it occurs, indeed,between −0.97 V (SCE) and −0.90 V (SCE). Furthermore, it can alsobe observed a trend to anodic oxidation peaks, however, no cleartendency for discrete oxidation peaks could be seen. This rapidincrease in the anodic current density seems to be directly asso-ciated with the breakdown potential. It can also be said that theseexamined polarization curves exhibit a transient passivity region.

For instance, the examined Al–Bi alloy sample at position P1shows anodic peaks of about −960 mV (SCE) and −940 mV (SCE)and subsequently at −915 mV(SCE) the anodic current densitytends to stabilize at about 9 �A cm−2 (0.9 × 10−5 A cm−2), whichmight represent the passivity platform at the anodic branch takingplace at this point.

For the sample at position P2 the anodic peaks occur at −920 mV(SCE) and −915 mV (SCE) and at −910 mV (SCE) the anodic currentdensity increases of about one decade (i.e. from 2.05 �A cm−2 to9.5 �A cm−2), and subsequently a turbulent passivity platform ischaracterized. At least three anodic peaks have occurred for thesample P3 and the two last peaks were at −915 mV (SCE) and−905 mV (SCE) where the anodic current density increased from2.2 �A cm−2 up to that similar value of 9.5 �A cm−2 (passivity plat-form) verified for the two other samples examined. These peaksarise from the potential dependent instability of the oxide, leadingto varying rate of aluminum oxidation with increasing potential[8]. Gundersen et al. [8] reported that the instability of the oxideis related to the presence of the activating element in metallicform at the metal–oxide interface. This element is constituted bythe droplet-like Bi particles, which can be detached from the Al-matrix. It seems that those oscillations verified in the polarizationcurves of the Al–Bi alloy samples are associated with reactionspredominantly involving the Al cations and solution anions result-ing in intermediate species (e.g. Al(OH)x and AlCl−y). The mixturebetween the intermediate species associated with surrounded bis-muth droplets is responsible for the transient passivity regionsshown in Fig. 5.

Krasovskii et al. [7] reported Ecorr and passivation current foran Al–2 wt.% Bi alloy, which are higher than these obtained in thepresent investigation. This can be explained based on the scan-ning potential rate (5 mV s−1) used and the resulting microstructure

W.R. Osório et al. / Electrochimica Acta 108 (2013) 781– 787 785

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ig. 5. (a) Typical three experimental potentiodynamic polarization curves of Al–2.5 M NaCl solution at room temperature.

rray. They have also demonstrated that the current density andorrosion potential have increased and displaced toward the morective potential-side with the increase in Bi content [8]. Hamptont al. [17] and Xia et al. [18] reported that the increase in Bi allo-ing accelerated the corrosion rate of Pb–Bi alloys. Song et al. [19]eported a corrosion potential, for a potential scan rate of 1 mV s−1,f about −410 mV (SCE) or −350 mV (Ag/AgCl) for a pure bis-uth specimen in a 3.5% NaCl solution. From the aforementioned

esults, it can be assumed that the important role of the micro-tructural array on the electrochemical response of as-cast Al–Billoys is intimately associated with the cell size and distributionf the immiscible droplet-like bismuth particles in the Al matrix.dditionally, during solidification strains are generated at the bor-ers between Al and Bi, which produce a higher level of energy. Athese boundaries the susceptibility to corrosion increases consid-rably.

The corrosion potentials of Al and Bi specimens in a sodium chlo-ide solution are between −800 and −900 mV (SCE) and between380 and −415 mV (SCE) [5,19,20], respectively. With this, Al//Bialvanic couples can be certainly expected. Besides, it can also beupposed that under the experimental potential range used in thisnvestigation, the oxidation of bismuth is not reached. The anodicxidation of Bi takes places above −350 mV (SCE) and after long

nough polarization a barrier Bi2O3 oxide film will be formed andill passivate the electrode surface. Under the experimental poten-

ial range shown in Fig. 5, some aluminum chloride, hydroxides andomplex intermediate species are formed on the electrode surface.

Bi alloy samples and polarization curves at positions: (b) P1, (c) P2 and (d) P3 in a

Figs. 6 and 7 show typical SEM images (SE–secondary elec-trons technique) of Al–2 wt.% Bi alloy samples after the corrosiontests (EIS + polarization) in a NaCl solution at room temperature.Fig. 6 depicts typical SEM images under a magnification of 50×,which clearly evidence that from sample at position P1 (10 mm)to that at position P3 (20 mm from the cooled surface of the cast-ing) the pitting intensity (number of pits) decreases. Fig. 7 showsSEM images with a magnification of 5000× of the microstructuresshown in Fig. 6. These images evidence oxide formation (corro-sion by-product) and dispersed nano-sized droplet-like Bi particles(between ±200 nm and 300 nm). A more compact corrosion by-product seems to be associated with that sample at position P3,which has a coarser cell spacing. These SEM images also help toexplain the impedance parameters, mainly the polarization resis-tance R2 (barrier oxide layer), which is higher (of about 2 times)than that of the sample positioned at the top of the casting (coarsecells). Besides, they also help to understand the verified oscilla-tions in the polarization curves, which can be associated with boththe pits formed (hydrogen evolution) and the cracks on the oxidecorrosion by-product layer.

Fig. 8 shows a typical SEM micrograph of the Al–2 wt.% Bi alloysample at position P3 evidencing three distinct spectra corre-sponding to EDAX results with the average chemical compositions

(table in Fig. 8). The spectrum #1 reveals the corrosion by-productconstituted by a complex concentration of O, Al and Cl, whichunder the applied potential used in this experimentation seems besome species corresponding to Al(OH)x and AlCl−y. The spectrum

786 W.R. Osório et al. / Electrochimica Acta 108 (2013) 781– 787

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ig. 6. SEM micrographs after corrosion tests of Al–2 wt.% Bi alloy samples under0x of magnification at positions: (a) P1, (b) P2 and (c) P3.

2 indicates droplet-like Bi particles possibly embedded in the Alatrix, which can justify the presence of some surrounding Al and O

mounts. The spectrum # 3 corresponds to the examined Al matrix.The aforementioned reasoning permits to conclude that the

lectrochemical parameters, mainly those corresponding to theicorr” and R2 decreased and increased with the increase in the cellpacing, respectively. It can also be summarized that a better cor-osion resistance is attained when a coarser Al–Bi microstructure isonsidered. A coarser microstructure array is also associated with

smaller (±40%) cathode/anode area ratio (1:20) than a finer cell

pacing array (1:15). Since droplet particles are detached from thel matrix lattice, this cathode/anode ratio is considerably modified

i.e. the cathode area decreases) inducing minimization or stabi-ization of the corrosion action.

Fig. 7. SEM images of micrographs shown in Fig. 6 under a magnification of 5000×.

In this sense, a control on the solidification process through thecooling rate in order to attain an adequate microstructural arrayshould be taken in order to achieve a balance between wear andcorrosion resistances. A recent study on wear resistance of a mono-tectic Al-In alloy has shown that the wear resistance increases withthe increase in the interphase spacing and diameters of indiumdroplets [21].

Furthermore, a similar electrochemical behavior evidenced forthe Al–Bi alloy samples examined has also been shown for a numberof Al-based alloys (e.g. dilute as-cast Al–Fe [22] and Al–Ni [14,23]alloys) and the c.p. Al grade samples [24]. Besides, some monotectic

Al–In [5] and Al–Pb [6] alloys have also demonstrated that a coarsermicrostructural cell array has better corrosion response than finerones. The reduction in cell size and the localized strain between Aland the second phase or particles of these alloys have also been

W.R. Osório et al. / Electrochimica Acta 108 (2013) 781– 787 787

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[

Fig. 8. Typical SEM micrograph at position P3 showing three di

ntimately associated with their corresponding electrochemicalorrosion behavior. The modification in the ratio of anodic/cathodicreas with the progress of corrosion, caused by the detachment ofroplets from the Al matrix lattice, tends to stabilize the corrosionction (Al-based oxide film formation). This anodic/cathodic areaariation is also intimately associated with the resulting micro-tructural array and has also an important role in the corrosionesistance.

. Conclusions

From the aforementioned discussions concerning the exper-mental results, i.e. EIS parameters, equivalent circuit andotentiodynamic polarization curves of a monotectic Al–2 wt.% Billoy, the following conclusions can be drawn:

. The experimental polarization results show a decrease in thecorrosion current density (about 2.5 times from P1 to P3) andan increase in the polarization resistance (about 2 times fromP1 to P3) with the increase in the cell spacing (of about 2 times,i.e. from 30 �m to 60 �m). The lowest cell spacings are that ofregions closer to the Al–Bi alloy casting surface (associated witha cooling rate of about 9 ◦C/s), which resulted in the highestcorrosion current density examined of about 2.2 × 10−6 A cm−2.

. The resulting electrochemical corrosion behavior of the Al–Bialloy samples examined is very similar to those of two monotec-tic Al-based alloys (Al–In and Al–Pb) and various Al-based alloys.The localized strains that are generated at the borders betweenthe Al matrix and dispersed droplets or fiber-like particles, asso-ciated with the ratio of cathodic/anodic areas, play an importantrole in the resulting electrochemical performance.

cknowledgements

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

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