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Electrochimica Acta
jou rn al hom ep age: www.elsev ier .com/ locate /e lec tac ta
IS and potentiodynamic polarization studies on immiscibleonotectic Al–In alloys
islei R. Osórioa,b,∗, Emmanuelle S. Freitasb, Amauri Garciab
School of Applied Sciences/FCA, University of Campinas, UNICAMP, Campus Limeira, P.O. Box 1068, 1300, Pedro Zaccaria Street, Jd. Sta Luiza,3484-350 Limeira, SP, BrazilDepartment of Materials Engineering, University of Campinas, UNICAMP, P.O. Box 6122, 13083-970 Campinas, SP, Brazil
a r t i c l e i n f o
rticle history:eceived 29 January 2013eceived in revised form 8 April 2013ccepted 9 April 2013vailable online 17 April 2013
a b s t r a c t
The electrochemical behavior of monotectic Al–In alloys is experimentally investigated. Electrochemicalimpedance spectroscopy (EIS), potentiodynamic anodic polarization techniques and an equivalent circuitanalysis were used to evaluate the corrosion response in a stagnant and naturally aerated 0.5 M NaClsolution at 25 ◦C. It was found that a better galvanic protection can be provided for microstructures havingindium droplets of larger diameters and larger interphase spacings. From five samples extracted along the
eywords:l–In alloys
mmiscible alloysolidification
length of a directionally solidified Al–In casting, that having smallest interphase spacing (� = 18 �m) anddroplet diameter (d = 0.7 �m) had its corrosion resistance significantly decreased (about 2 and 3 times interms of the current density and polarization resistance) when compared with that of the sample havingthe coarsest microstructure (� = 60 �m and d = 2.5 �m). Such behavior is attributed to both localized
m an
lectrochemical impedance spectroscopyolarization
strains between aluminu
. Introduction
A number of Al-based monotectic binary alloys having limitedolubility in the liquid state have significant potential for manyndustrial applications, such as electronic components, high tem-erature superconductors, self-lubricated bearings [1–5] andptical devices [6]. Serious problems can be associated with con-entional melting and casting techniques of these alloys, caused byevere gravity segregation in casting due the large densities differ-nces between the liquid phases [2,5]. It is important to remark thatl-based monotectic alloys have an Al-rich liquid phase L1, at theonotectic temperature, which is decomposed into an Al-rich solid
hase S1 and a liquid phase L2. During cooling, a continuous Al-richatrix is formed with the liquid minority phase being retained in
discontinuous way within the solid matrix in the form of isolatedockets [5,7]. The competition between the growth of the minorityhase and the rate of displacement of the solidification front willetermine if the prevalent morphology of the microstructure will
e characterized by fibers or droplets [7].
Kamio et al. [8] reported the development of the microstruc-ure of a monotectic Al–17.5 wt.% In alloy during the steady-state
∗ Corresponding author at: School of Applied Sciences/FCA, University of Campi-as, UNICAMP, Campus Limeira, 1300, Pedro Zaccaria Street, Jd. Sta Luiza, 13484-350imeira, SP, Brazil. Tel.: +55 19 3521 3320; fax: +55 19 3289 3722.
growth in an unidirectional solidification set-up at various growthrates and temperature gradients. The resulting microstructureswere shown to be formed either by small indium droplets or byfine and regularly aligned fibers along the growth direction dissem-inated in the Al matrix, which depended on the range of thermalgradients and growth rates imposed during solidification. Theyreported that the droplet morphology prevailed when a growth rateof about 1.1 × 10−6 m/s was attained. Kamio et al. [8] have also com-pared their experimental results of interphase spacing and growthrate with those reported in studies by Grugel and Hellawell [9,10]. Itwas shown that the interphase spacing decreases with the increasein the growth rate [8–10]. Yasuda et al. [4] used a static magneticfield in order to control the monotectic solidification of Al–In alloys.They observed the presence of both indium rods and droplets seg-regated close to the bottom of the casting [4]. Liu et al. [2] haveshown that the size of indium particles increased with the increasein distance from the chilled surface.
Despite the potential for the use of Al–In alloys in a number ofindustrial applications, the literature is scarce on studies interrelat-ing microstructural effects such as the morphology of the indiumparticles and the interphase spacing to the corresponding electro-chemical corrosion behavior. It is expected that the control of theresulting microstructural array of a monotectic Al–In alloy wouldpermit to prescribe guidelines with a view to preprogramming a
required corrosion resistance.
The present study focus on the effect of the scale of the inter-phase spacing of a microstructure, formed by indium droplet-likeparticles embedded in an Al-matrix, on the resulting EIS and
Table 1Chemical composition of metals used to prepare the Al–In alloy and average composition of the examined samples at position P10, P15, P50, P60 and P70.
Metal Fe Si Sn Zn In Al
Chemical composition (wt.%)In a – 0.005 0.02 Balance –Al 0.05 0.03 a a – Balance
Metal P10 P15 P50 P60 P70 Impurities
0.2)
ce
pwa
2
2
cmtflvs
Fm
In 5.6 (±0.1) 5.5 (±0.2) 5.4 (±Al Balance Balance Balan
a Less than 50 ppm.
otentiodynamic polarization plots of a monotectic Al–In alloy,ith tests performed into a naturally stagnant 0.5 M NaCl solution
t 23 (±3) ◦C.
. Experimental procedure
.1. Specimens preparation
Al–5.5 wt.% In alloy samples were prepared using commer-ially pure (c.p.) Al (99.8 ± 0.13 wt.%) and In (99.9 ± 0.01 wt.%). Theain impurities were determined by Energy Dispersive X-ray Spec-
roscopy (EDS) coupled with SEM, and confirmed through an X-rayuorescence technique. The results, shown in Table 1, are averagealues, which are based on measurements carried out in differentamples.
ig. 1. (a) Macrostructure and position of each sample extracted from the casting: P10icrostructures of Al–5.5 wt.% In samples P15 and P50.
In order to obtain directionally solidified Al–5.5 wt.% In alloysamples, a water-cooled transient solidification set-up was used,in which heat was extracted only through the water-cooled bot-tom promoting vertical upward directional solidification. Someselected specimens were extracted from longitudinal sections ofthe vertically solidified casting at five different positions (10,15, 50, 60 and 70 mm) along the casting length from the bot-tom of the casting, as shown in Fig. 1. These five positions wereselected in order to permit the effect of a range of interphase spa-cings between indium droplets (which are disseminated in theAl-matrix) on the electrochemical behavior to be investigated.Since segregated areas of indium located close to the bottom
of the casting have been reported in previous study [5], in thepresent experimental investigation the samples were extractedat positions farther than 10 mm from the chilled surface of thedirectionally solidified casting. This will permit the analysis to be
, P15, P50, P60 and P70; (b) examples of typical transversal and (c) longitudinal
ig. 2. Typical optical microstructures of Al–In alloy samples from five different pof) Experimental cell spacing and interphase spacing as a function of position in cas
ocused only on the effects of morphology and scale of the Indiumhase.
In order to reveal the microstructures, the samples wereround, polished and etched with a solution of 0.5% HF in water.
s from the cooled bottom of the casting: (a) 10, (b) 15, (c) 50, (d) 60, (e) 70 mm and
Microstructural characterization was performed by an opticalmicroscope linked to an image processing system Neophot 32 anda scanning electron microscope (SEM), as previously reported inrecent studies [11–16]. Image processing systems were used to
ig. 3. (a) and (b) Typical SEM images evidencing the cellular morphology and inellular (�c) and interphase spacings (�).
easure the interphase spacing on longitudinal sections of theamples by averaging the horizontal distance between the centersf adjacent indium particles.
.2. Preparation of corrosion tests
The Al–In alloy samples for corrosion tests were extracted fromhe positions in the casting shown in Fig. 1. These selected samplesere positioned into a corrosion cell with a circular 1.0 (±0.02) cm2
urface immersed in a naturally aerated and stagnant 500 mL of a.5 M NaCl solution at 25 ◦C, with a pH of 6.95 (±0.5). Before thelectrochemical measurements, the Al–In samples were previouslyround to a 1200 grit surface finish using silicon carbide paper, fol-owed by distilled water washing and air drying. It is also importanto remark that the samples after polished were immediately pos-tioned in the cell kit and the electrolyte was poured into the cell.his procedure was adopted for all the samples examined.
EIS measurements began after an initial delay of about 15 minor the sample to reach a steady-state condition. This period ofime was considered enough for stabilization of the potential sincehe EIS tests were carried out under open-circuit potential and annsteady-state condition could provide distortions on measure-ents. A potentiostat coupled to a frequency analyzer system, a
lass corrosion cell kit with a platinum counter-electrode and aaturated calomel reference electrode (SCE) were used to performhe EIS tests. The potential amplitude was set to 10 mV (SCE); peak-o-peak (AC signal) in open-circuit, with 5 points per decade andhe frequency range was set from 100 mHz to 100 kHz.
Immediately after the EIS measurements, the polarization testsere also carried out at the same positions in a 0.5 M NaCl solution
t 25 ◦C. These tests were conducted by stepping the potential atpen-circuit with a scan rate of 0.1667 mV s−1 from −250 mV (SCE)o + 250 mV (SCE). Using an automatic data acquisition system, theotentiodynamic polarization curves were plotted and the corro-
ion current density, pitting potential and corrosion potential werestimated. Duplicate tests for both EIS and potentiodynamic polar-zation curves were carried out. An appropriate model (ZView® ver-ion 2.1b) for equivalent circuit quantification has also been used.
droplets (white particles) and (c) a schematic representation of measurements of
3. Results and discussion
3.1. Microstructure array
Fig. 2(a)–(e) shows typical microstructures of Al–In alloy sam-ples from five different positions in the casting (10, 15, 50, 60 and70 mm from the cooled bottom). It can be seen that from the cast-ing surface (cooled bottom) up to a position (P) of about 40 mm, themicrostructure is characterized by a mixture of cells and indiumdroplets and for P > 40 mm only In-droplets prevail. The cellular(�c), and interphase spacings (�) were measured according to theschematic representation shown in Fig. 3(c).
Typical microstructural SEM images constituted by an Al-richcellular matrix and In-droplets located in the intercellular spa-cings are shown in Fig. 3(a) and (b). The white regions are formedby the minority phase which was segregated during solidification(±99.8 wt.% In as determined by an EDAX technique) with the largerdroplets having diameters (d) between 2.5 �m and 4 �m besidesa regular network of nanometer-sized indium droplets disposedalong the cells boundaries. During cooling from the melt, the L2droplets, as similarly reported for other Al-based monotectic alloys,grow originally by diffusion and soon become large enough fortransportation by Marangoni motion [17–20]. Some droplets willthen interact and coalesce forming larger droplets. During growththe monotectic front will push the indium droplets toward the cellboundaries where they will remain entrapped [17,20].
Fig. 4 shows a bimodal distribution with respect to the evolu-tion of the diameter of indium droplets with the position in casting.Despite the bimodal distribution, it is clearly observed that the par-ticle size increases with the increase in distance from the cooledsurface of the casting. A similar tendency has also been reported byLiu et al. [2].
It can be seen that at position the P10 (10 mm from the cooledsurface) about 60% of the particles have sizes ranging between
0.6 �m and 0.9 �m (600 nm and 900 nm) coexisting with a cellspacing (�c) of about 20 (±8) �m, i.e. for a position that is associatedwith a relatively high growth and cooling rates during solidifica-tion (about 1.3 mm/s and 5 ◦C/s, respectively). On the other hand,
440 W.R. Osório et al. / Electrochimica
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Fig. 4. Typical bimodal distribution of diameters of the indium droplets.
Acta 102 (2013) 436– 445
at the position P70 (70 mm from the bottom of the casting), 70%and 25% (bimodal distribution) of the droplets have sizes between2.1 �m and 2.7 �m and 6 �m and 8 �m, respectively, which is adiameter distribution associated with lower growth and coolingrates (about 0.9 mm/s and 0.5 ◦C/s, respectively) when comparedwith the corresponding values of P10.
Considering sea water galvanic series tables, from the electro-chemical point of view it can be said that indium is nobler thanaluminum [20,21]. Saidman and Bessone [22,23] have shown thatat a pH between 2 and 8, indium has a corrosion potential of about−0.6 V (SCE). Venugopal and Raja [24,25] have also shown thatthe addition of small amounts (0.03–0.05 wt.%) of indium into Aldisplaced the corrosion potential toward the more noble potential-side. Gudic et al. [26] have demonstrated that indium alloying of0.1 wt.% in Al caused activation of Al shifting the open corrosionpotential in a 2 M NaCl solution. These same authors [26,27] havealso reported that microgalvanic cells are established between Aland In particles, with In displaying a cathodic behavior with respectto the Al matrix. It has also been recently reported [28] that indiumcan easily be dissolved into an acid electrolyte, exhibiting corrosionpotential of about −680 mV (SCE). These observations, give indi-cations that possible galvanic couples between Al and In dropletswere formed. Besides, it should also be considered that due tothe formation of galvanic couples between Al//In, some complexadsorbed intermediates species can also be formed, as will be dis-cussed in next sections.
3.2. ElS results and microstructure array
Experimental results of EIS diagrams of the as-cast monotecticAl–In alloy samples are shown in Fig. 5. Fig. 5(a) shows the exper-imental Bode plots describing the modulus of impedance (/Z/) vs.frequency, which depict three distinct regions: (i) at low frequen-cies between 100 Hz and 10−1 Hz, which represents the polarizationresistances of the alloy samples, where the highest value of /Z/ withfrequency is attained for position P70 (±14 k� cm2), followed byP60, P50 and P15 (±10, 8 and 6 k� cm2, respectively). The lowest/Z/ is associated with the sample at position P10 (±5 k� cm2); (ii)at intermediate frequencies, between 100 Hz and 103 Hz, similardouble layer characteristics are observed while the phase angleswith frequency (� = f(F)), i.e. Bode-phase plots, at about 20 Hz show� decreasing from 70 mm to 10 mm; and (iii) between 103 Hz and105 Hz frequencies a constant 20 � cm2 can be observed, whichrepresents the electrolyte resistance.
Fig. 5(b) shows the Nyquist plots for the five examined alloysamples, which evidence the experimental results of EIS withcapacitive semi-arcs clearly increasing with the increase in posi-tion (i.e. distance from the bottom of the casting). It is clearlyobserved that from 105 Hz to about 7 Hz the plots are similar, andbetween 7 Hz and 10−2 Hz, it can be seen that both componentsZReal (in-phase) and ZImaginary (out-of-phase) are increasing withthe increase in distance from the bottom of the casting for anysample examined. With these observations, it can be said that theelectrochemical corrosion resistance increased from positions P10to P70, i.e. the corrosion resistance increased with the increase inboth the diameter and interphase spacing of indium droplets. Thisis in agreement with the previous discussion based on the Bode andBode-phase plots shown in Fig. 5(a).
A comparison between experimental and simulated Nyquistplots of the five examined alloy samples is shown in Fig. 5(b). Inorder to provide “quantitative support” to the experimental EISresults, impedance parameters were obtained by the ZView® soft-
ware, adopting a well-known equivalent circuit [11–16,29–37],which is shown in Fig. 5(c). The agreement between experimen-tal and simulated plots indicates that the experimental results arewell fitted to the proposed equivalent circuit. The fitting quality
Fig. 5. (a) Experimental Bode and (b) Nyquist plots for five examined Al–5.5 wt.% In alloy samples, and (c) the proposed equivalent circuit used to obtain impedanceparameters.
Fig. 6. Experimental potentiodynamic polarization curves for the examined monotectic Al–5.5 wt.% In alloys samples at: (a) −1.5 and −0.9 V (SCE), and (b) −1.5 and −1.1 V(SCE).
Table 2Impedance parameters of each of the five examined Al–5.5 wt.% In alloy samples.
Fig. 7. Post-mortem SEM/BSD (backscattered electrons analysis) images of the five examined Al–5.5 wt.% In alloys samples after EIS + polarization tests in a 0.5 M NaCls
wi
e[NraltZ
olution at 25 ◦C.
as evaluated by Chi-squared (�2) values of about 10−3, as shownn Table 2.
The physical significance of the elements of the proposedquivalent circuit has been intensively reported in the literature11–16,29–37], where Rel corresponds to the resistance of a 0.5 MaCl solution (at high frequency limit, F > 1 Hz), R1 and R2 are the
esistances of the porous and barrier layers, respectively, which
re associated to the charge transfer resistance through the porousayer and the participation of adsorbed intermediates. The capaci-ances of the porous and barrier layers correspond to the values ofCPE(1) and ZCPE(2), respectively.
For simplicity, a constant-phase element representing a shiftfrom an ideal capacitor was used instead of the capacitance itself.The impedance of a phase element is defined as ZCPE = [C(jω)n]−1,where C is the capacitance; j is the current; ω is the frequency and−1 ≤ n ≤ 1. The value of n seems to be associated with the non-uniform distribution of current as a result of roughness and possibleoxide surface defects [11–16,29–37].
Comparing the impedance parameters of the five examinedAl–In alloy samples, it can be seen that the values of ZCPE(2) (barrierlayer) are higher (of about 20 times) than the corresponding val-ues of ZCPE(1) for all the samples examined. Besides, it can also be
ig. 8. Post-mortem SEM/SE (secondary electrons analysis) micrographs of the fiolution at 25 ◦C.
learly verified that both capacitances ZCPE(1) and ZCPE(2) are veryimilar. This observation is in accordance with the previous resultsoncerning the EIS Bode and Bode-phase plots of Fig. 5, which givendications that the double layer mechanisms are similar for allamples examined.
With the exception of position P10, the values of R2 are aboutalf of the corresponding R1 values, indicating that the corrosionrotection is predominantly provided by the outer porous layerR1). It can also be seen that both R1 and R2 increased with thencrease in distance from the bottom of the casting.
With these observations, it can be said that the mechanismf corrosion is significantly dependent on the resulting micro-tructural array, which is also intimately associated with theroplet-like indium particles. Although the Bode and Bode-phaselots in this investigation refer to a total immersion time of about
h, it seems that the observed trends can be considered adequateor the assessment of the electrochemical behavior of this mono-ectic Al–In alloy. In order to permit the current density of eachample examined to be obtained and correlations to be established
mined Al–5.5 wt.% In alloys samples after EIS + polarization tests in a 0.5 M NaCl
with the resulting microstructure array (mainly the interphasespacing (�) and the diameter (d) of droplets), potentiodynamicpolarization curves were carried out in a 0.5 M NaCl solution atroom temperature, as previously described in the experimentalprocedure section.
3.3. Potentiodynamic polarization and microstructure array
Fig. 6 shows a comparison of experimental potentiodynamicpolarization curves of the monotectic Al–5.5 wt.% In alloy sam-ples. Polarization curves between −1.5 V and −0.9 V (SCE) withsimilar shapes for all the samples examined can be observed inFig. 6(a). On the other hand, the corrosion current densities (iCorr)were obtained from the polarization curves by Tafel’s extrapola-tion, using both cathodic and anodic branches of the polarization
curves, and considering the potential ranging from −1.2 V to −1.1 V(SCE).
By comparing the corrosion potential of all five examined Al–Inalloy samples, it can be seen that there exists a difference of about
4 imica Acta 102 (2013) 436– 445
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44 W.R. Osório et al. / Electroch
7 mV (SCE) between more-noble and less-noble potentials. A high-st current density (i) of 6.1 (±0.6) �A cm−2 associated with a corro-ion potential (ECorr) of about −1169 mV (SCE) is observed for posi-ion P10, as shown in Fig. 6(b). The lowest current densities werettained for samples corresponding to positions P60 and P70, i.e..8 (±0.6) �A cm−2 and 3.5 (±0.2) �A cm−2, respectively. Consid-ring that Al exhibits a corrosion potential between −800 mV and900 mV (SCE) and that indium has a potential about −700 mV
SCE) [20–28], some perturbations can be clearly observed in thenodic branches of all samples examined, which are intrinsicallyssociated with the formation of Al//In galvanic couples.
Figs. 7 and 8 depict typical SEM images provided by both BSDbackscattered electrons analysis) and SE (secondary electronsnalysis) techniques of the Al–In alloy samples at positions P10,5 and 50–70 mm, respectively. These SEM images were obtainedfter the potentiodynamic polarization tests, which were carriedut after EIS tests in a NaCl solution at room temperature. BSD/SEMmages highlight the light (white) droplet-like indium particleshighest atomic mass) throughout all the Al–In alloy samples, ashown in Fig. 7.
Figs. 7 and 8(a) and (b) show clearly the resulting cellular mor-hology with indium droplets located in the cells boundaries foramples from positions 10 to 15 mm from the cooled surface. Withhese images, it can be said that due to galvanic couples consti-uted by “Al” and “In”, the indium droplets (more noble potential)ocated at these boundaries have accelerated the dissolution (corro-ion) of the Al matrix (less-noble potential). The nano-sized indiumroplets, previously discussed and shown in Figs. 2 and 3, are dis-ersed throughout the corrosion by-product or forming adsorbedarticles, as can be seen in Figs. 7 and 8(a) and (b).
Fig. 8 indicates that the flat undissolved planes of the Al-ellular matrix at positions P10 and P15 are better organized thanhose of other positions, which seems to be associated with sim-lar crystallographic characteristics of the cellular array. Althoughhese aforementioned undissolved planes can also be observed atositions P50–P70, it seems that they are randomically dissolvedcorroded), as shown in Fig. 8(c)–(e). The mentioned flat undis-olved planes correspond to the Al phase and it is likely that thesere {1 0 0} planes, as reported in the literature [38–41]. It can alsoe observed that the planes which are not {1 0 0}, correspond to
ower atomic density planes.The aforementioned observations also agree with the previous
xperimental results of corrosion current density and polariza-ion resistances (R1 and R2), which from positions P10 to P70,ncreased considerably. A correlation between the experimentallectrochemical corrosion response (R1, R2 and i) and the diam-ter of indium droplets (d) are shown in Fig. 9. It can be seen that1 and R2 (polarization resistances) increased with the increase inhe diameter of the indium droplets, while the current density (i)xhibits a non-linear decrease with the increase in droplet size (d).
From the electrochemical point of view, indium is nobler thanl in neutral or alkaline solutions [20,21,26,27]. Besides, it has alsoeen suggested that indium particles grows in a faceted manner2], while Al has a dissimilar growth behavior with surfaces thatre rough [15]. Because of these different growth mechanisms alu-inum/indium boundaries will not be perfectly conformed, but
ather will be subjected to certain strain in the atomic level, mainlyn the Al side of the interface. Thus, it is believed that the Al–In sam-le having smallest indium droplets (closer to the bottom of theasting, P10), could be more susceptible to corrosion than thoseith coarser droplets at positions closer to the top of the cast-
ng (e.g. position P70 mm), in which the aforementioned localized
eformation could be less intense.
Considering the resulting microstructure arrays and the afore-entioned reasoning, it can be concluded that the microstructural
rrangement has a paramount role on the electrochemical
Fig. 9. Correlation of impedance parameters (R1, R2) and corrosion current density(i) with the diameter of the indium droplets (d) for Al–5.5 wt.% In alloy samples in a0.5 M NaCl solution at room temperature.
corrosion behavior of the examined monotectic Al–In alloy sam-ples. Additionally, it can also be said that the Al–5.5 wt.% In alloysamples have corrosion resistance responses that are very similarto previous experimental observations for commercially pure (c.p.)Al [35,36] and various Al-based alloys [15,40–43]. For instance,in the case of the c.p. Al samples [35,36] and dilute Al–Fe alloys[43], the corrosion behavior is intimately associated with thereduction in grain size (c.p. Al) and cell size (Al–Fe), respectively.The localized strain between aluminum and silicon particles wasalso considered as a driving force to the increase in the corrosionaction of finer microstructures [15,41,42].
4. Conclusions
The following conclusions can be drawn from the present exper-imental investigation:
1. The experimental results have shown that for regions closerto the casting surface (higher cooling rate, about 5 ◦C/s), thedroplet-like indium particles have the lowest diameters (i.e.between 600 nm and 900 nm coexisting with a average cellspacing (�c) of about 20 (±8) �m). The size of these dropletsis associated with the driving-force determining the corrosionresistance (current density). This is attributed to localized strainsbetween aluminum and indium boundaries and the corrosionpotential of the indium particles.
2. It was found that a galvanic protection can be provided whenthe microstructure is characterized by indium droplets of largerdiameters (i.e. about 70% of the droplets have sizes between2.1 �m and 2.7 �m associated with a lower cooling rate about0.5 ◦C/s).
3. The interphase spacing increases with the increase in the dropletsize, which is connected with the applied cooling rate dur-ing solidification. More finely and homogeneously distributeddroplets (i.e. 60% of droplets with diameter between 600 nm and900 nm) are associated with Al–In samples closer to the cooledsurface of the casting (when the cooling rate is about 5 ◦C/s),which tend to have lower corrosion resistance (i.e. a corrosioncurrent density of about 12 �A cm−2 against about 7 �A cm−2 forthe indium droplets of larger diameters).
4. The potentiodynamic polarization results have shown anincrease in the current density (i.e. from ±7 �A cm−2 to12 �A cm−2) and a decrease in the polarization resistance (i.e.displaced to less-noble side potential about 30 mV, SCE) with
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the decrease in both the interphase spacing and droplet diam-eter size (±3 or 3.5 times). Although, from the electrochemicalpoint of view, indium has a nobler potential (standard poten-tial about −0.34 V, In3+/In0) than Al (standard potential about−1.7 V), the examined sample with smallest interphase spacing(�) and droplets (d), i.e. (� = 18 �m and d = 0.7 �m), had its cor-rosion resistance significantly decreased (about 2 and 3 times interms of the current density and polarization resistance) whencompared with that of the coarsest sample having � = 60 �m andd = 2.5 �m.
cknowledgements
The authors acknowledge the financial support provided byAEPEX-UNICAMP, CNPq (The Brazilian Research Council) andAPESP (The Scientific Research Foundation of the State of Sãoaulo, Brazil).
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