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Microstructure, Phase Occurrence, and Corrosion Behavior of As-Solidified and As-Annealed Al-Pd Alloys Libor Dˇurisˇka

, Maria´n Palcut, Martin Sˇpota´k, Ivona Cˇernicˇkova´, Ja´n Gondek, Pavol Priputen, Roman Cˇicˇka, Dusˇan Janicˇkovicˇ, and Jozef Janovec (Submitted September 20, 2017; in revised form December 7, 2017)

In the present work, we studied the microstructure, phase constitution, and corrosion performance of Al88Pd12, Al77Pd23, Al72Pd28, and Al67Pd33 alloys (metal concentrations are given in at.%). The alloys were prepared by repeated arc melting of Al and Pd granules in argon atmosphere. The as-solidified samples were further annealed at 700 °C for 500 h. The microstructure and phase constitution of the as-solidified and as-annealed alloys were studied by scanning electron microscopy, energy-dispersive x-ray spectroscopy, and x-ray diffraction. The alloys were found to consist of (Al), en ( Al3Pd), and d (Al3Pd2) in various fractions. The corrosion testing of the alloys was performed in aqueous NaCl (0.6 M) using a standard 3electrode cell monitored by potentiostat. The corrosion current densities and corrosion potentials were determined by Tafel extrapolation. The corrosion potentials of the alloys were found between 2 763 and 2 841 mV versus Ag/AgCl. An active alloy dissolution has been observed, and it has been found that (Al) was excavated, whereas Al in en was de-alloyed. The effects of bulk chemical composition, phase occurrence and microstructure on the corrosion behavior are evaluated. The local nobilities of en and d are discussed. Finally, the conclusions about the alloyÕs corrosion resistance in saline solutions are provided. Keywords

Al-Pd alloys, corrosion resistance, intermetallic, microstructure characterization, potentiodynamic polarization

1. Introduction Among metallic systems comprising structurally complex phases (SCPs), the Al-TM systems (TM stands for one or more transition metals) have been the most widely studied (Ref 1-7). The SCPs have specific transport properties, e.g., an anomalous relation between electrical and thermal conductivities has been found (Ref 7-10). Moreover, surfaces of complex metallic alloys (CMAs) provide a rich variety of different adsorption sites (Ref 11-13). As such, potential applications of Al-based CMAs are reported in catalysis (Ref 14), solar light absorption (Ref 15), polymer–matrix (Ref 2), and metal–matrix (Ref 16) composites. Some of the applications are now commercialized. For instance, the Al-based coatings comprising SCPs have been used as scratch-resistant films, offering a lowered adhesion to some polymers or food (Ref 2). The maraging steel comprising nano-sized quasicrystalline particles has also been used in electric shavers (Ref 2). The Al-based alloys exposed to corrosive environments form a compact protection layer composed of Al2O3 and

ˇ ernicˇkova´, ˇ urisˇka, Maria´n Palcut, Martin Sˇpota´k, Ivona C Libor D ˇ icˇka, Faculty of Ja´n Gondek, Pavol Priputen, and Roman C Materials Science and Technology in Trnava, Slovak University of Technology in Bratislava, Ja´na Bottu 2781/25, 917 24 Trnava, Slovak Republic; Dusˇan Janicˇkovicˇ, Institute of Physics, Slovak Academy of Sciences, Du´bravska´ 9, 845 11 Bratislava, Slovak Republic; and Jozef Janovec, Slovak University of Technology in Bratislava, University Science Park, Vazovova 5, 812 43 Bratislava, Slovak Republic. Contact e-mail: [email protected].

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AlO(OH) (Ref 17, 18). Chlorine anions, often present in aqueous solutions, weaken the passive films. As such, these materials are susceptible to pitting corrosion (Ref 19-22). By adding TM elements (e.g., Cr, W, Mo, Ta), the Al pitting potential can be increased (Ref 23-25). Nevertheless, electrochemically noble intermetallic phases formed in these alloys may play the role of cathodes during electrochemical corrosion. As such, their corrosion stability may result in an anodic dissolution of the surrounding metal matrix. The matrix depletion may have fatal consequences for the materials integrity. The electrochemical properties of SCPs were reported to be different from those of Al (Ref 26-38). It has been argued that metal composition of CMAs plays a major role in determining the alloyÕs corrosion resistance (Ref 26). In corrosion protection, the Al-based CMAs could be utilized as coatings to coat either Al alloys or low-alloyed steels. The passivation stability of classical Al alloys is often very poor when a pH is increased above 8-9. Since the alloying elements, such as Cr or Cu, are passivating at different pH, a mixed passivation layer can be formed on these materials. The chemical composition of the passive layer could be further modified by specific TM elements. The recent study of Al-CrFe CMAs (Ref 30) revealed a high stability of these materials in alkaline solutions. The corrosion resistance of these alloys thus could make them attractive for technologically important applications taking place in alkaline environments. Al-Pd alloys have been studied for their catalytic properties (Ref 39). Pd exhibits a strong interaction with Al resulting in the formation of noble metal-like electronic structure of Al-Pd alloys. In the Al-Pd system, a number of SCPs have been found. Altogether five structures of e-phases (binary e6, e28 (Ref 40-42), ternary e16, e22, and e34) were found and classified as approximants of decagonal quasicrystal (Ref 5, 7). The structures differ in the length of lattice parameter c. The CMAs comprising e-phases are diamagnetic. Their electrical resistivity is moderate and shows a weak temperature dependence at 4-

300 K. An interesting feature of the e-phases is their low thermal conductivity. These phases exhibit the electrical conductivity typical of metallic alloys combined with a low thermal conductivity (Ref 43). The corrosion performance of Al-Pd alloys in 1 M NaCl has been studied in the form of rapidly solidified ribbons previously (Ref 44). Recently, the corrosion behavior of bulk alloys has also been investigated in HCl and NaOH solutions (Ref 45). The results showed a selective dissolution of Al from en ( Al3Pd) (Ref 44, 45). This phenomenon is known as electrochemical de-alloying (Ref 46). De-alloying is a corrosion process during which an alloy is decomposed by selective dissolution of the most electrochemically active element. This process results in the formation of nano-porous metal networks composed of the nobler alloy constituents. As such, de-alloying is an efficient tool for creating nano-porous noble metal networks with a three-dimensional bi-continuous inter-penetrating ligament–channel structure, being applicable in catalysis (Ref 47-51). Recently, nano-porous metal ribbons of Au, Pd, Pt, Ag, and Cu have been fabricated through chemical de-alloying of rapidly solidified Al-based alloys under free corrosion conditions (Ref 47). Interactions between co-existing phases in double- and triple-phase alloys may play an important role during corrosion. These interactions are governed by Pd diffusivity, curvature-dependent undercritical potential dissolution as well as chemical interactions between Pd and chlorine anions (Ref 44). In the present work, the effects of both microstructure and phase constitution on the alloyÕs corrosion behavior were investigated. This study is complementary to previous studies (Ref 44, 45), as altogether four bulk alloys, Al88Pd12, Al77Pd23, Al72Pd28, and Al67Pd33 (metal concentrations are given in at.%), already studied in as-solidified conditions (Ref 45), have been further analyzed in both as-solidified and as-annealed (700 C/500 h) conditions in this paper. The metal concentrations of the alloys were chosen to obtain different microstructure constituents and to study the effect of Pd bulk content on their corrosion behavior in the 0.6 M NaCl solution.

2. Experimental Methods The Al88Pd12, Al77Pd23, Al72Pd28, and Al67Pd33 alloys were prepared by arc melting of Al and Pd lumps (> 99.95 wt.%) in argon atmosphere using a compact MAM-1 arc melter from Edmund Bu¨hler GmbH. Subsequently, the melts were rapidly solidified on a water-cooled Cu mold. The melt homogeneity was improved by repeated re-melting. The as-solidified alloys were cut into smaller samples and sealed up in silica capsules. Each capsule was rinsed with argon before sealing. Subsequently, the Al77Pd23, Al72Pd28, and Al67Pd33 alloys were annealed at 700 C for 500 h. After annealing, the samples were water-quenched to retain their high-temperature microstructures. The Al88Pd12 alloy was not annealed since it was expected to be molten at 700 C. The samples were mounted in epoxy resin, ground with SiC paper grade no. 1200 and polished with diamond suspension down to 1 lm surface roughness. The polished samples ( 1 cm2) were subject to an electrochemical testing in a standard three-electrode cell. The Ag/AgCl electrode suspended in a saturated KCl solution (Ag/AgCl) was used as the reference electrode. A platinum sheet (4 cm2) was used as the

counter electrode. The corrosion experiments were carried out in an aerated aqueous NaCl (0.6 M) at room temperature (21 ± 2 C). The cell was controlled by a PGU 10 V-1A-IMP-S potentiostat/galvanostat from Jaissle Electronic Ltd. (Waiblingen, Germany). The open circuit potential (Eoc) was measured during first 600 s of sample immersion in the electrolyte. After the Eoc measurement, the electrode polarization was conducted. Each measurement started at potentials more negative than Eoc and continued in positive direction by using a sweeping rate of 1 mV/s. The potential range covered at least 500 mV on both cathodic and anodic sides. This corresponded to potentials between  1200 and 0 mV (Ag/AgCl) for most of the alloys. The susceptibility of the alloys toward pitting was also studied by cyclic polarization. In this experiment, the polarization direction was reversed after reaching the potential maximum and continued with negative polarization down to the starting potential. Both as-solidified and as-annealed samples before and after corrosion experiments were characterized by means of a JEOL JSM-7600F scanning electron microscope (SEM) operating at 20 kV accelerating voltage in regimes of secondary electrons (SEI) and backscattered electrons (BEI). The microscope was equipped with an Oxford Instruments X-max 50 spectrometer for energy-dispersive x-ray spectroscopy (EDX) and operated by INCA software. To obtain the mean values of metal concentrations, a minimum of ten point EDX measurements per constituent were taken. The x-ray diffraction (XRD) was carried out using a Panalytical Empyrean PIXCel 3D diffractometer with Bragg–Brentano geometry using iron filtered CoKa1,2 radiation. The scattering angle 2h was varied between 20 and 60, the step size was 0.0131, and the exposure time was 98 s per step. To explain the phase occurrence in the assolidified Al77Pd23 alloy, this alloy was annealed at 500 C for 3000 h and investigated by differential scanning calorimetry (DSC). The measurement was taken by using a NETZCH STA 409 CD simultaneous thermal analyzer in Ar. The measurement was taken between room temperature and 950 C with heating/cooling rate 5 C/min. A sample mass was approximately 10 mg. The surface topography of the corroded samples was analyzed by Zeiss LSM 700 confocal laser scanning microscope (CLSM). The three-dimensional topographical resolution was achieved using the ZEN 2009 software.

3. Results and Discussion 3.1 AlloyÕs Microstructures and Phase Occurrences Before Corrosion Testing Microstructures of the investigated alloys are documented in Fig. 1. The as-solidified microstructures are illustrated in Fig. 1(a), (b), (d), and (f), while the as-annealed microstructures are documented in Fig. 1(c), (e), and (g), respectively. The corresponding XRD patterns are shown in Fig. 2 and 3. The alloys were found to contain various combinations of (Al), en ( Al3Pd), and d (Al3Pd2). The metal concentrations of microstructure constituents determined by EDX/SEM and their phase assignments are summarized in Table 1. The microstructure of the as-solidified Al88Pd12 alloy (Fig. 1a) was found to consist of two constituents. Peaks of Al, observable in Fig. 2a, indicate that the dark-gray constituent was (Al). The visually and chemically homogeneous bright-

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Fig. 1 BEI/SEM images of microstructure constituents in Al88Pd12 (a), Al77Pd23 (b, c), Al72Pd28 (d, e), and Al67Pd33 alloys (f, g). Columns [(a), (b), (d), (f)] and [(c), (e), (g)] correspond to as-solidified and as-annealed alloys, respectively. Phases assigned to particular constituents are also marked

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Table 1 Metal concentrations and phase assignments of microstructure constituents observed in as-solidified Al88Pd12, Al77Pd23, Al72Pd28, and Al67Pd33 alloys and as-annealed Al77Pd23, Al72Pd28, and Al67Pd33 alloys Element content, at.% Alloy Alloy composition, denotation at.% Pd

Alloy condition

Al88Pd12

12.2 ± 0.2

As-solidified

Al77Pd23

23.2 ± 0.4

As-solidified

23.3 ± 0.3

Annealed at 700 C for 500 h

Al72Pd28

27.8 ± 0.3

As-solidified

Al67Pd33

27.6 ± 0.1 33.4 ± 0.3

Annealed at 700 C for 500 h As-solidified

33.2 ± 0.2

Annealed at 700 C for 500 h

Microstructure constituent Bright-gray Dark-gray Bright-gray Near-uniform dark-gray Bright-gray Mixed Dark-gray Bright-gray Dark-gray Dark-gray Bright-gray Dark-gray Bright-gray

Identified phase/ structure

Al

Pd

Volume fraction, %

en/e6 + e28 (Al) en/e6 + e28 (Al)

73.6 ± 0.1 26.4 ± 0.1 99.7 ± 0.1 0.3 ± 0.1 73.2 ± 0.3 26.8 ± 0.3 99.4 ± 0.4 0.6 ± 0.4

48.1 ± 0.1 51.9 ± 0.1 86.4 ± 0.4 13.6 ± 0.4

en/e6 + e28 Solidified liquid (Lt) en/e6 + e28 d en/e6 + e28 en/e6 + e28 d en/e6 + e28 d

72.5 ± 0.1 85.4 ± 2.8 72.5 ± 0.1 59.1 ± 0.2 72.4 ± 0.1 72.3 ± 0.2 58.9 ± 0.1 71.6 ± 0.2 61.1 ± 0.3

79.3 ± 0.4 20.7 ± 0.4 97.7 ± 0.3 2.3 ± 0.3 100 58.7 ± 0.2 41.3 ± 0.2 53.8 ± 0.3 46.2 ± 0.3

27.5 ± 0.1 14.6 ± 2.8 27.5 ± 0.1 40.9 ± 0.2 27.6 ± 0.1 27.7 ± 0.2 41.1 ± 0.1 28.4 ± 0.2 38.9 ± 0.3

Phases were identified by XRD; metal concentrations of alloys and phases were determined by EDX/SEM

gray constituent was identified as en. This constituent was a mixture of e6 and e28 structures. To index peaks of e6 and e28, the data published in (Ref 52) and the data derived from the preliminary model of e28 (Ref 53) were used, respectively. For simplicity, en is considered as a single microstructure constituent in the present work. In the as-solidified Al77Pd23 alloy, a uniform bright-gray constituent and near-uniform dark-gray constituent, consisting of the dark matrix and fine areas of a lighter appearance, were found (Fig. 1b). In the corresponding XRD pattern, peaks of e6, e28, and Al were found (Fig. 2b). Since the near-uniform darkgray constituent was chemically homogeneous (Table 1), it was considered to consist of the (Al) only. The fine areas of a lighter appearance are probably related to sequential changes resulting from the solidification history. The microstructure of the Al77Pd23 alloy annealed at 700 C (Fig. 1c) comprises a homogeneous bright-gray constituent (en), and a mixed constituent formed by eutectic, (Al) + en, as follows from the XRD pattern (Fig. 3a). In the microstructure of the as-solidified Al72Pd28 alloy, two constituents have been observed (Fig. 1d). Both constituents can be considered as homogeneous. The major dark-gray constituent has been assigned to en (Fig. 2c), whereas the minor bright-gray constituent corresponded to d phase. The distribution of d was uneven. The microstructure of the Al72Pd28 alloy after annealing (Fig. 1e) was formed by en only, as follows from the diffraction pattern shown in Fig. 3b. The as-solidified microstructure of the Al67Pd33 alloy was composed of dark- and bright-gray constituents (Fig. 1f), corresponding to en and d (Fig. 2d), respectively. The same phases were identified in the Al67Pd33 alloy after annealing (Fig. 1g and 3c). The above presented observations show that the phase constitutions of as-solidified alloys were different from those expected from the phase diagram. Namely, the absence of k and c phases was investigated by a separate DSC measurement. For this investigation, the Al77Pd23 alloy annealed at 500 C for 3000 h was chosen. As follows from Fig. 4, a sharp peak with maximum at 597.0 C, corresponding to decomposition of k,

appeared in the first heating run, whereas an opposite peak was missing in the subsequent cooling run indeed. This peak corresponds to k phase decomposition. Therefore, if k (Fig. 4b) lost its stability, it has not been repeatedly formed during the continuous heating/cooling. This shows that the kinetics of kformation was rather slow, which influenced both courses and products of the phase transformations observed in the alloy. For instance, the eutectic reaction, LM(Al) + en (Ref 40, 42), specified to occur at about 616 C in the phase diagram (maxima at 617.9 and 607.3 C on the heating and cooling curves in Fig. 4a, respectively), is not followed by the expected peritectoid reaction (Ref 40, 42), (Al) + enMk, which results in the formation of (Al) and en after cooling (see Fig. 4c). The slow kinetics of k-formation could be a consequence of a transformation of two complex structures, en fi k, without any identical structural features such as clusters or crystallographic planes. It could also be affected by a lower diffusivity of Pd in comparison with Al. The above presented results can also be related to the phase occurrence in the as-solidified conditions, since the cooling rate in the ‘‘technological’’ solidification is higher than that in the DSC measurement. Therefore, the microstructure of both Al88Pd12 and Al77Pd23 alloys is formed by (Al) + en, instead of (Al) + k and k + c for the former and latter alloy, respectively. A similar effect can also be suggested for c-phase formation.

3.2 Corrosion Behavior The open circuit potentials of as-solidified and as-annealed alloys are presented in Fig. 5. The open circuit potentials of the alloys increase with increasing Pd concentration. This observation is in agreement with expectations since Pd is electrochemically nobler compared to Al (Ref 22). The open circuit potentials increase in the following manner: Al < Al88 Pd12  Al77 Pd23 < Al72 Pd28  Al67 Pd33

ðEq 1Þ

The corrosion behavior of both as-solidified and as-annealed samples was comparable. The open circuit potentials of Al72Pd28 and Al67Pd33 alloys were nearly identical in both

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Fig. 2

XRD patterns of as-solidified Al88Pd12 (a), Al77Pd23 (b), Al72Pd28 (c), and Al67Pd33 (d) alloys

as-annealed and as-solidified conditions (Fig. 5). While the asannealed Al72Pd28 alloy is a single-phase alloy composed of en, the remaining three alloys are double-phase alloys composed of en and d, respectively. As the open circuit potentials of these alloys were nearly identical, the corrosion probably concerned mostly en.

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The open circuit potentials of the as-solidified Al88Pd12 alloy and both as-solidified and as-annealed Al77Pd23 alloys were also comparable (Fig. 5). The as-solidified Al88Pd12 and Al77Pd23 alloys were composed of en and (Al), while the asannealed Al77Pd23 alloy was composed of en and solidified liquid. The open circuit potentials of both alloys were less

Fig. 3

XRD patterns of as-annealed Al77Pd23 (a), Al72Pd28 (b), and Al67Pd33 (c) alloys

negative compared to Al but more negative compared to Al72Pd28 and Al67Pd33 alloys. As such, Eoc of these alloys was not only determined by the bulk chemical composition, but was also influenced by a combination of structural factors including phase occurrence. A further insight into the corrosion behavior of the Al-Pd alloys could be obtained by investigating the polarization behavior. The cyclic polarization curves of the alloys in 0.6 M NaCl are presented in Fig. 6. The forward curves are characterized by a single corrosion minimum followed by an increase in the current density at potentials higher than the corrosion potential. This current density increase is further followed by stabilization or even a decrease in the current density indicating a possible passivation behavior. The observed transient behavior is further followed by an increase in the current density at potentials greater than  600 mV (Ag/AgCl). This final current density increase is observed at higher potentials for Al72Pd28/

Al67Pd33 alloys compared to Al88Pd12/Al77Pd23 alloys. Because of their distinct corrosion behavior, the Al-Pd alloys were merged into two different groups in this paper: group I (Al88Pd12, Al77Pd23) and group II (Al72Pd28, Al67Pd33). The corrosion behavior of these two alloy groups will be further investigated in the following discussion. The forward curves presented in Fig. 6 have been analyzed by Tafel extrapolation (Ref 22). The electrochemical parameters are presented in Table 2. The corrosion potentials of the assolidified alloys were nearly identical. This behavior is in contrast to previous Eoc measurements where major differences between alloy groups I and II were found. The Eoc and Ecorr values of alloys Al88Pd12 and Al77Pd23 (group I) differ of more than 100 mV. A similar difference between Eoc and Ecorr values has also been found for Al (Fig. 5). This difference, however, is attributable to a definite sweeping rate used during the polarization experiment (Ref 44).

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A major difference between Eoc and Ecorr of more than 400 mV for group II alloys (Al72Pd28 and Al67Pd33) has been found. The open circuit potentials of these alloys are, in fact, comparable to their breakdown potentials (Table 2). This

Fig. 4 Partial heating and cooling curves corresponding to first run of DSC measurement on Al77Pd23 alloy (heating and cooling rate 5 C/min) (a), initial microstructure of sample after annealing at 500 C for 3000 h (b), and final microstructure of sample after first run (c). Phases assigned to particular constituents are also marked

Fig. 5

indicates that these alloys were probably in a localized corrosion stage (pitting) already during the Eoc measurement. This is also manifested by the oscillations of Eoc indicating a possible breakdown behavior. The bursts of dissolution at localized corrosion sites require the generation of bursts of cathodic current from the surrounding boldly exposed surface. This increased demand causes a decrease in the Eoc value. Localized corrosion sites are typically very small. However, during transient bursts, the current densities inside the cavities can be in the order of 1 A/m2 (Ref 54). These rates are possible because of the extremely aggressive environments developing inside the localized corrosion sites. Although the sites have small dimensions, they may influence the electrochemical potential of the much larger boldly exposed surface, where the kinetics is far slower. This difference in relative current densities on separated anodes and cathodes accounts for the ability to detect the electrochemical noise associated with localized corrosion. Since Al alloys are prone to localized corrosion in chlorinecontaining electrolytes, their susceptibility toward pitting has been further investigated by cyclic polarization (Fig. 6). A positive hysteresis loop was found upon reverse polarization in 0.6 M NaCl for all alloys. This observation indicates localized corrosion (Ref 54). The positive hysteresis can be explained by the combination of the critical crevice solution (CCS), the stability of pits, and the actual competition between diffusion and dissolution at localized corrosion sites (Ref 54). The driving force for dissolution is decreasing during the initial portion of the reverse scan. Nevertheless, the CCS has not had a sufficient time to diffuse away. Therefore, by continuing the reverse scan, the driving force for dissolution continues to decrease, and eventually the CCS cannot be maintained against diffusion. At this point, the localized corrosion sites repassivate, and the anodic current drop is observed (Fig. 6). Throughout the scan, the surface areas not dissolved at high rate have thick passive films covering them. Thus, the repassivation potential can be expressed as a convolution of the loss of the CCS from the localized corrosion site and the raising of the corrosion potential of the surrounding passive surface (ennoblement) due to a decrease in anodic kinetics (Ref 54). As such, the re-passivation potential, Erp, is nobler than the corrosion potential (Table 2).

Open circuit potentials of as-solidified (a) and as-annealed (b) Al-Pd alloys

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Fig. 6 Cyclic polarization curves of Al88Pd12 (a), Al77Pd23 (b, c), Al72Pd28 (d, e), and Al67Pd33 (f, g) alloys (scan rate 1 mV/s). Columns [(a), (b), (d), (f)] and [(c), (e), (g)] correspond to as-solidified and as-annealed alloys, respectively

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Table 2 Electrochemical parameters of both as-solidified and as-annealed Al-Pd alloys Eoc, mV versus Ag/AgCl As-solidified alloys Al  1088  668 Al88Pd12  662 Al77Pd23  360 Al72Pd28  373 Al67Pd33 As-annealed alloys  660 Al77Pd23  371 Al72Pd28  393 Al67Pd33

Ecorr, mV versus Ag/AgCl

Eoc 2 Ecorr, mV

icorr, A/m2

 1038  794  809  797  798

 50 126 147 437 425

0.015 0.885 0.817 0.628 0.615

    

493 247 261 288 277

60.8 63.0 90.9 51.0 79.0

    

 763  841  783

103 470 390

0.753 0.683 0.723

 303  256  268

54.7 120 63.2

 316  304  548

3.3 AlloyÕs Microstructures After Corrosion Testing More information about the specific corrosion attack of different intermetallic phases could be obtained by investigating the alloyÕs microstructures after electrode polarization. Microstructures of the alloys after corrosion testing are documented in Fig. 7. Metal compositions of phases after corrosion testing are summarized in Table 3. A dissolution of (Al) in the Al88Pd12 alloy has been found (Fig. 7a). Simultaneously, the Al concentration in en decreased from 73.6 to 32.4 at.% (compare data in Tables 1 and 3). This was probably a result of selective dissolution of Al from en (dealloying). The excavating of (Al) was also observed in both assolidified and as-annealed Al77Pd23 alloys (Fig. 7b and c). Moreover, intermittent inter-penetrating channels have been observed in en. As with the Al88Pd12 alloy, both as-solidified and as-annealed Al77Pd23 alloys were found to consist of en with lower Al content (53.0 at.% for the as-solidified condition and 56.9 at.% for the as-annealed condition). The homogeneity of the de-alloyed en is decreased in the as-annealed alloy (Table 3). However, the corrosion attack in the as-annealed alloy was less pronounced compared to the as-solidified alloy (Table 2). The three-dimensional CLSM-image of the microstructure of the as-annealed Al77Pd23 alloy after corrosion testing is presented in Fig. 8a. The irregular pits seen in the figure correspond to the original eutectic [(Al) + en] present in the microstructure before corrosion (Fig. 1c). It can be assumed that the preferential (Al) dissolution from the original eutectic caused the collapse of the whole eutectic. The d phase was not subject to de-alloying (Table 3). The microstructures of both as-solidified and as-annealed Al72Pd28 alloys were found to be comparable after corrosion testing (Fig. 7d and e). Intermittent inter-penetrating channels and decreased Al concentrations were observed in en in both alloys; however, de-alloying was more pronounced in the assolidified condition. en can be considered homogeneous in both microstructures (Table 3). The formation of intermittent interpenetrating channels may lie in the combination of de-alloying and pitting during electrochemical testing. A pit was found in the inter-connection between two channels. As such, the pits were probably the initiation points of the channels. The threedimensional CLSM-image of the microstructure of the asannealed Al72Pd28 alloy after corrosion testing showing the presence of pits and intermittent inter-penetrating channels is illustrated in Fig. 8b. The corrosion testing resulted in comparable microstructures for both as-solidified and as-annealed Al67Pd33 alloys (Fig. 7f and g). en as the less noble phase in

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bc, mV/ decade

ba, mV/ decade

Ebd, mV versus Ag/AgCl

730 579 577 338 334

Erp, mV versus Ag/AgCl

    

750 669 610 436 430

 439  433  639

both Al67Pd33 conditions corroded preferentially, whereas d was retained. Intermittent inter-penetrating channels as well as decreased content of Al in en were observed in both alloys. Therefore, the electrochemical activity of phases in 0.6 M NaCl solution decreases in the following manner ðAlÞ < en ð Al3 PdÞ < d ðAl3 Pd2 Þ

ðEq 2Þ

The comparison of en chemical composition in both assolidified and as-annealed alloys reveals a higher Al content in as-annealed alloys, see Table 3. Thus, the annealing positively influenced the de-alloying of Al in en. This fact is more significant for group II alloys where the difference was 15.6 and/or 16.5 at.% Al for the Al67Pd33 and Al72Pd28 alloys, respectively. The as-solidified and as-annealed Al77Pd23 alloys (group I) differ by 3.9 at.% Al only. The more pronounced dealloying observed in the as-solidified alloys could be caused by a higher concentration of crystallographic defects. This is probably a result of their lower thermodynamic stability leading to a higher Al diffusivity when compared to as-annealed alloys. As follows from the above presented observations, group I alloys were subject to the combination of (Al) dissolution and de-alloying of Al from the nobler phase. In group II alloys, the de-alloying of Al from the less noble phase combined with the retaining of the nobler phase was observed.

4. Conclusions In the present work, the microstructure, phase occurrence, and corrosion performance of four alloys, Al88Pd12, Al77Pd23, Al72Pd28, and Al67Pd33 were studied. Both as-solidified and asannealed alloys were investigated. The annealing was conducted at 700 C for 500 h in Ar. Subsequently, the alloys were subject to potentiodynamic polarization in aqueous 0.6 M NaCl. The investigations led to the following conclusions: 1. The alloys were found to contain various combinations of (Al), en ( Al3Pd), and d (Al3Pd2). 2. The kinetics of k- and c-formation was rather slow which influenced both courses and products of phase transformations in the alloy. 3. The open circuit potentials of both as-solidified and asannealed alloys were found to increase with increasing bulk Pd content. 4. Al88Pd12 and Al77Pd23 alloys were subject to the combi-

Fig. 7 SEI/SEM images of microstructure constituents in Al88Pd12 (a), Al77Pd23 (b,c), Al72Pd28 (d,e), and Al67Pd33 (f,g) alloys after corrosion testing. Columns [(a), (b), (d), (f)] and [(c), (e), (g)] correspond to as-solidified and as-annealed alloys, respectively. Phases assigned to particular constituents are also marked

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Table 3 Metal concentrations of microstructure constituents observed in as-solidified Al88Pd12, Al77Pd23, Al72Pd28, and Al67Pd33 alloys and as-annealed Al77Pd23, Al72Pd28, and Al67Pd33 alloys after corrosion testing. Metal concentrations were determined by EDX/SEM Element content, at.% Alloy denotation

Alloy condition

Al88Pd12

As-solidified

Al77Pd23

As-solidified

Al72Pd28

Annealed at 700 C for 500 h As-solidified

Al67Pd33

Annealed at 700 C for 500 h As-solidified Annealed at 700 C for 500 h

Fig. 8

Identified phase/structure

Al

Pd

en/e6 + e28 (Al) en/e6 + e28 (Al) en/e6 + e28 Solidified liquid (Lt) en/e6 + e28 d en/e6 + e28

32.4 ± 1.1 98.2 ± 0.9 53.0 ± 2.0 99.1 ± 0.6 56.9 ± 2.6 37.4 ± 3.1 35.8 ± 1.9 59.2 ± 0.7 52.3 ± 0.8

67.6 ± 1.1 1.8 ± 0.9 47.0 ± 2.0 0.9 ± 0.6 43.1 ± 2.6 62.6 ± 3.1 64.2 ± 1.9 40.8 ± 0.7 47.7 ± 0.8

en/e6 + e28 d en/e6 + e28 d

34.9 ± 0.4 59.9 ± 0.4 50.5 ± 0.3 60.1 ± 0.2

65.1 ± 0.4 40.1 ± 0.4 49.5 ± 0.3 39.9 ± 0.2

Three-dimensional CLSM-images of as-annealed Al77Pd23 (a) and Al72Pd28 (b) alloys after corrosion experiment

nation of (Al) dissolution and de-alloying of Al from the nobler phase (en). 5. In Al72Pd28 and Al67Pd33 alloys, the de-alloying of Al from en combined with the retaining of the nobler phase (d) was observed. 6. The de-alloying of Al in en was more pronounced in the as-solidified alloys.

Acknowledgements The authors wish to thank the European Regional Development Fund (ERDF) for financial support of the project ITMS:26220120048 ‘‘Center for Development and Application of Advanced Diagnostic Methods in Processing of Metallic and Non-metallic Materials’’ funded within the Research & Development Operational Programme, the Grant Agency VEGA for the financial support under contracts 1/0018/15 and 1/0465/15, and the

Journal of Materials Engineering and Performance

Slovak Research and Development Agency for the financial support under the contract APVV-15-0049.

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