Materials 2015, 8, 6455-6470; doi:10.3390/ma8095314

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materials ISSN 1996-1944 www.mdpi.com/journal/materials Article

Electrochemical Behavior of Al-B4C Metal Matrix Composites in NaCl Solution Yu-Mei Han * and X.-Grant Chen Department of Applied Sciences, Université du Québec à Chicoutimi, 555 boul. de l’Université, Saguenay, QC G7H 2B1, Canada; E-Mail: [email protected] * Author to whom correspondence should be addressed; E-Mail: [email protected]; Tel.: +1-819-354-1098. Academic Editor: J. Paulo Davim Received: 21 August 2015 / Accepted: 15 September 2015 / Published: 21 September 2015

Abstract: Aluminum based metal matrix composites (MMCs) have received considerable attention in the automotive, aerospace and nuclear industries. One of the main challenges using Al-based MMCs is the influence of the reinforcement particles on the corrosion resistance. In the present study, the corrosion behavior of Al-B4 C MMCs in a 3.5 wt.% NaCl solution were investigated using potentiodynamic polarization (PDP) and electrochemical impedance spectroscopy (EIS) techniques. Results indicated that the corrosion resistance of the composites decreased when increasing the B4 C volume fraction. Al-B4 C composite was susceptible to pitting corrosion and two types of pits were observed on the composite surface. The corrosion mechanism of the composite in the NaCl solution was primarily controlled by oxygen diffusion in the solution. In addition, the galvanic couples that formed between Al matrix and B4 C particles could also be responsible for the lower corrosion resistance of the composites. Keywords: Al-B4 C metal matrix composite; pitting; galvanic corrosion; oxygen diffusion

1. Introduction Aluminum based metal matrix composites (MMCs) have received considerable attention in the automotive, aerospace and nuclear industries due to their light weight, as well as their superior thermal conductivity, high stiffness and hardness [1–3]. The common reinforcements added to commercial MMCs are silicon carbide (SiC), alumina (Al2 O3 ) and boron carbide (B4 C). Compared to traditional SiC

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and Al2 O3 reinforcements, B4 C possesses numerous advantages, specifically a density (2.51 g¨ cm´3 ) [4] that is significantly lower than that of SiC or Al2 O3 , an extremely high hardness (HV = 30 GPa) and wear resistance, a remarkable chemical inertness [4–7] and a special neutron absorption capacity [8]. These features make B4 C an excellent reinforcement for high performance MMCs. Its applications include hard disc substrates, brakes with a high wear resistance and armor plates with a high ballistic performance [9,10]. In recent years, due to the special capturing neutron ability of isotope B10 , Al-B4 C MMCs have been increasingly used as neutron shielding materials when fabricating storage containers for spent nuclear fuels in the nuclear industry [11–14]. One of the main challenges using Al-based MMCs is the influence of the reinforcement particles on the corrosion resistance [15–19]. Because adding reinforcement particles interrupts the continuity of the aluminum matrix and its protective surface oxide films, the number of sites where corrosion could be initiated increases, making the composite more susceptible to corrosion [20,21]. Singh et al. [22] studied the influence of SiC particles addition on the corrosion behavior of 2014 Al-Cu alloy in 3.5 wt.% NaCl solution. They found that addition of 25 wt.% SiCp to base alloy decreases corrosion resistances considerably. Zhu and Hihara [23] investigated the influence of alumina fiber on the corrosion initiation and propagation of the Al-2 wt.% Cu-T6 metal matrix composite. Results show that the MMC exhibited inferior corrosion resistance as compared to its monolithic matrix alloy. Bhat et al. [24] investigated the corrosion behavior of the 6061 Al-SiCp composite and its base alloy in seawater using the potentiodynamic polarization technique. It was found that the composite corroded faster than its base alloy and that composite corrosion was mainly confined to the interface as opposed to the uniform corrosion observed for the base alloy. Sun et al. [25] also studied the corrosion behavior of 6061 Al-SiCp MMCs in a NaCl solution. With the observation that the pitting degree rose with an increasing SiC content, it is presumed that the pitting corrosion depends on the local SiC distribution and the surface film integrity. Roepstorff et al. [26] reported that the corrosion resistance of metal matrix composites can be affected by three processes: (1) galvanic coupling of the metal and reinforcement; (2) crevice attack at the metal/reinforcement interface; and (3) preferred localized attack on the possible reaction products between the metal and the ceramic. In contrast to the many research works dedicated to the corrosion behavior of Al-SiC composites, few studies have focused on the corrosion of Al-B4 C composites. Ding and Hihara [27] investigated the effect of B4 C particles on the corrosion behavior of 6092-T6 Al MMCs with 20 vol.% B4 C in a 0.5 M Na2 SO4 solution at room temperature. Corrosion initiation and propagation are related to the formation of microcrevices, the localized acidification and alkalization of the solution, and to aluminum containing amphoteric oxides. Katkar et al. [28] evaluated the effect of the reinforced B4 C particle content in AA6061 on the formation of a passive film in sea water. They reported that the passive film formed on B4 C particle-reinforced AA6061 alloy because there was a shift in the corrosion potential toward the positive direction compared to the base alloy. In our previous study [29], the Al-B4 C composite was less corrosion resistant than the base alloy in the NaCl, K2 SO4 and H3 BO3 solutions. In another study [30], the B4 C particles exhibited a cathodic character relative to the aluminum alloy in the K2 SO4 solution, meaning that the B4 C particles could form galvanic couples with the peripheral aluminum matrix in the composite.

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To have a deep understanding of the corrosion behavior and corrosion mechanism of AA1100-B4 C metal matrix composites in a 3.5 wt.% NaCl solution, the present study was carried out in open-to-air and deoxygenated conditions. Electrochemical techniques, including potentiodynamic polarization (PDP), electrochemical impedance spectroscopy (EIS) and zero resistance ammetry (ZRA) were used. Besides, the effect of the B4 C particle volume fraction on the corrosion behavior of Al-B4 C composites was investigated, and the surface morphology of the composite before and after corrosion was characterized using an optical stereoscope and a scanning electron microscope (SEM). 2. Experimental Procedures 2.1. Preparation of Samples and Electrolytes The investigated composites were AA1100-16 vol.% B4 C and AA1100-30 vol.%. Both composites were supplied by Rio Tinto Alcan (Saguenay, QC, Canada) via an ingot metallurgy route [3,9]. The average particle size of boron carbide in the composites is 17 µm and the matrix is a standard AA1100 aluminum alloy except the Ti content. Approximately 1.0–2.5 wt.% titanium was added to both Al-B4 C composites during the composite fabrication process to reduce the interfacial reactions between the B4 C and the liquid aluminum [31]. The DC cast ingots were preheated and hot-rolled with multi-passes of cross-rolling to the final 4.3 mm thick sheets. To study the effect of B4 C particles on the corrosion behavior of the composite, an AA1100 alloy without B4 C was used as the base alloy. The chemical composition of the AA1100 base alloy is listed in Table 1. Table 1. Chemical composition of the AA1100 base alloy. Elements

Fe

Si

Cu

Mn

Mg

Zn

Composition (wt.%)

0.15

0.10

0.05

0.02

0.001 0.01

Al Bal.

Samples were cut into small pieces (20 mm by 20 mm) and unless otherwise stated, sanded with a 3M Scotch-Brite™ (3M, Saint Paul, MN, USA) MMM69412 surface conditioning disc (5 inches in diameter, extra-fine surface finish) before being degreased with acetone and rinsed with nanopure water (15.2 MΩ¨ cm). Finally, all specimens were dried with clean compressed air. Analytical reagent grade NaCl was used to obtain the 3.5 wt.% NaCl electrolyte. 2.2. Electrochemical Measurements The potentiostat employed in the present study was a Reference 600 instrument (Gamry Instruments, Warminster, PA, USA). The electrochemical investigations were performed using a 300 cm3 -EG&G PAR flat cell (London Scientific, London, ON, Canada) with an Ag/AgCl electrode (4M KCl as filled solution) as the reference electrode and a platinum mesh as the counter electrode (CE). All potentials given in this article are referred to the Ag/AgCl electrode. The corrosion cell had a 1-cm2 orifice as the working surface. Magnetic stirring was employed at the bottom of the cell to increase the mass transfer at the electrode surface. The potentiodynamic polarization tests were performed in open-to-air and deoxygenated conditions. The deoxygenation process began 1 h before the measurements by purging argon into the solution and

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continued until the the end end of of the the experiment. experiment. A A potential potential scan scan was was taken taken from from ´250 −250 mV the E Eocp continued until mV below below the to ocp to −2 −1 the potential at at which which aa 11 mA¨ mA·cm was recorded recorded at at aa scan scan rate rate of of11mV¨ mV·s the potential cm´2 current current density density was s´1 .. The The EIS EIS curves perturbation voltage voltage of of 10 10 mV mV rms rms around around the the E Eocp curves were were obtained obtained by by applying applying aa sinusoidal sinusoidal perturbation in ocp in the mHz frequency range. The the 100 100 kHz kHz to to 10 10 mHz frequency range. The detailed detailed polarization polarization and and impedance impedance measurements measurements were were described in aa previous previous study study [15]. [15]. Prior described in Prior to to the the polarization polarization and and impedance impedance tests, tests, all all samples samples were were immersed to ensure ensure aa steady steady open open circuit circuit potential potential (E (Eocp ). ocp). immersed in in the the 3.5 3.5 wt.% wt.% NaCl NaCl solution solution for for one one hour hour to During corrosion test, test, the the variations variations in in the the galvanic galvanic current current and and potential potential of of the the B B44C During the the galvanic galvanic corrosion C wafer wafer and alloy were were recorded recorded continuously continuously for for 24 24 h. h. In and the the AA1100 AA1100 base base alloy In all all cases, cases, the the tests tests were were duplicated duplicated to of the the results. results. to ensure ensure the the reproducibility reproducibility of 2.3. 2.3. Metallographic Metallographic Examination Examination The composite composite surface surface morphology morphology was was characterized scanning The characterized with with an an optical optical stereoscope stereoscope and and aa scanning electron IL, USA) electron microscope microscope (SEM, (SEM, Hitachi Hitachi SU-70, SU-70, Hitachi Hitachi Instruments, Instruments, Schaumburg, Schaumburg, IL, USA) equipped equipped with with an energydispersive dispersivespectrometer spectrometer(EDS). (EDS). understand the surface morphology the composite an energy To To understand the surface morphology of the of composite before before andcorrosion, after corrosion, the samples for the metallographic analysis were polishedtotoaa0.05 0.05 µm μm and after the samples usedused for the metallographic analysis were polished fine finish. fine finish.

3. Results and Discussion 3.1. Microstructure of Al-B44C Composites The microstructure microstructure of of the theAA1100-16 AA1100-16vol.% vol.%BB compositeis isillustrated illustrated Figure 1. general, In general, in in Figure 1. In the 4 Ccomposite 4C the B C particles were distributed uniformly in the Al matrix, and two common reaction-induced B4C particles were distributed uniformly in the Al matrix, and two common 4 intermetallic phases were observed in the composite composite and and randomly randomly dispersed dispersed in in the the Al Al matrix: matrix: AlB22 (brown, block-like phase) and Al33BC (grey phase) [32]. When fabricating the Al-B44C composites, the liquid aluminum reacted with B44C particles and produced AlB22 and Al33 BC intermetallic particles [3,9]. To limit the reaction between the B44C particles and the liquid aluminum, 1.0~2.5 wt.% Ti was added to Afterward, aa thin thin but but dense TiB22 layer (a third reaction product) was formed in situ at the composites. Afterward, liquid aluminum [9,12]. Consequently, all the Al/B Al/B44C interfaces, interfaces, isolating isolatingthe theBB particlesfrom fromthethe liquid aluminum [9,12]. Consequently, 4Cparticles 4C B4 CBsurfaces werewere surrounded withwith a TiB layer, as observed from from the SEM and theand X-ray surrounded a 2TiB as observed the micrographs SEM micrographs the all 4C surfaces 2 layer, elemental map in map Figure The microstructure of the AA1100-30 vol.% B4 C vol.% composite very similar X-ray elemental in2.Figure 2. The microstructure of the AA1100-30 B4Cwas composite was very to the vol.% AA1100-16 vol.% B4Cexcept composite, for the increased to thesimilar AA1100-16 B4 C composite, for theexcept increased B4 C amount.B4C amount.

Figure 1. 1. Optical Optical micrograph micrograph of of hot-rolled hot-rolled AA1100-16 AA1100-16 vol.% C composites. composites. Figure vol.% B B44C

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(a)

(b) Figure 2. (a) SEMimage imageofof one one B B44C elemental mapping of Figure 2. (a) SEM C particle particleand and(b)(b)X-ray X-ray elemental mapping of (a) showing that B C particles are protected by an in-situ TiB layer. 4 2 (a) showing that B4 C particles are protected by an in-situ TiB2 layer. 3.2. Corrosion Behavior of Al-B4C Composites

3.2. Corrosion Behavior of Al-B4 C Composites 3.2.1. Potentiodynamic Polarization

3.2.1. Potentiodynamic Polarization

The electrochemical behavior of Al-B4C composite and effect of B4C particle content on corrosion investigated using potentiodynamic and effect electrochemical impedance spectroscopy Thewere electrochemical behavior of Al-B4 C polarization composite and of B4 C particle content on corrosion (EIS). Figure 3a displays polarization curves of the composite with different B4C contents. The were investigated using potentiodynamic polarization and electrochemical impedance spectroscopy corrosion current density (jcorr) and corrosion potential (Ecorr) are obtained at the intersection point of (EIS).extrapolation Figure 3a displays polarization curves of the composite with different B C contents. The corrosion of the cathodic polarization branch and the Ecorr horizontal line, 4which is shown in the currentFigure density ) andwill corrosion potential obtained thea intersection extrapolation corryou corrit) are 3b.(jAs notice from Figure(E 3a, is difficult to at find linear region point near Eof corr on the of the anodic cathodic polarization branch the Ecorr line, the which is shown in shows the Figure polarization branch of theand AA1100 basehorizontal alloy. Besides, cathodic branch a long3b. andAs you defined linear behavior over 100tomV; in this near case,Eas by McCafferty [33],branch will notice from Figure 3a, itfor is difficult findtherefore, a linear region on the anodic polarization corrsuggested extrapolation of the cathodic method was used. All fittings at thelinear liner behavior part of of the the AA1100 base alloy. Besides,branch the cathodic branch shows a longwere anddone defined for cathodic branch for over 50 mV. The corrosion current density, corrosion potential and cathodic Tafel over 100 mV; therefore, in this case, as suggested by McCafferty [33], the extrapolation of the cathodic slopes values are summarized in Table 2. branch method was used. All fittings were done at the liner part of cathodic branch for over 50 mV. The It shows that the jcorr increases from 0.35 to 11.21 μA·cm−2 when B4C volume fraction increases corrosion corrosionthat potential and cathodic Tafel slopes values are summarized in Table 2. fromcurrent 0 to 30density, vol.%, suggesting the corrosion resistance of the composites decreases significantly ´2 It shows that the jcorr from 0.35 to µA¨ potential cm when volume fraction increases 4 Cthe when increasing theincreases B4C volume fraction. The11.21 corrosion shiftsBin positive direction, but the from 0 to 30shift vol.%, suggesting that increasing the corrosion of the composites significantly does not continue when the B4resistance C content. According to the mixeddecreases potential theory [28], the when potential of the composite is expected to shift in the noble direction when increasing the B C in shift 4 increasing the B4 C volume fraction. The corrosion potential shifts in the positive direction,level but the the continue composite. However, increasing B4C volume fractionto in composite increases does not when increasing the B4the C content. According thethe mixed potential theory the [28], the discontinuity of the protective surface oxide films, making the Al-30 vol.% B4C composite more potential of the composite is expected to shift in the noble direction when increasing the B4 C level in the vulnerable to the chloride ions and generating a less noble potential.

composite. However, increasing the B4 C volume fraction in the composite increases the discontinuity of the protective surface oxide films, making the Al-30 vol.% B4 C composite more vulnerable to the chloride ions and generating a less noble potential.

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(a)

(b) Figure (a) Potentiodynamicpolarization polarization curve curve recorded AA1100, AA1100-16 vol.%vol.% Figure 3. (a)3. Potentiodynamic recordedonon AA1100, AA1100-16 C and AA1100-30 vol.% B C in 3.5 wt.% NaCl in open to air condition; (b) B 4 B4 C and AA1100-30 vol.% B4 C in4 3.5 wt.% NaCl in open to air condition; (b) PlotPlot showing showing the jcorr was obtained at the intersection point of extrapolation of the cathodic the jcorr was obtained at the intersection point of extrapolation of the cathodic polarization polarization branch and the Ecorr horizontal line. branch and the Ecorr horizontal line. Table 2. Electrochemical parameters derived from polarization curves.

Table 2. ElectrochemicalOpen parameters derived from polarization curves. Argon Deaerated to Air Materials

Ecorr jcorr βc Ecorr jcorr bc Open to Air Argon Deaerated (VAg/AgCl) (μA·cm−2) (V/decade) (VAg/AgCl) (μA·cm−2) (V/decade) Materials AA1100 0.18 −0.93 0.33jcorr 0.16 bc E−0.70 j0.35 βc Ecorr corr corr AA1100-16 vol.% B4C −0.55 0.71 −0.94 0.99 (VAg/AgCl ) (µA¨8.39 cm´2 ) (V/decade) (V (µA¨ cm´2 ) 0.14 (V/decade) Ag/AgCl ) AA1100-30 vol.% B4C −0.59 11.21 0.51 −0.93 2.00 0.12

AA1100 ´0.70 0.35 0.18 ´0.93 0.33 0.16 The anodic part of the polarization curve of the base alloy AA1100 reveals an oscillation region AA1100-16 vol.% B4 C ´0.55 8.39 0.71 ´0.94 0.99 0.14 beyond which the current density increases quickly, indicating the onset of pitting corrosion. However, AA1100-30 vol.% B4 C ´0.59 11.21 0.51 ´0.93 2.00 0.12 for the composite, jcorr increases steeply even under low overpotential, implying that pitting is more easily provoked in composites than in the base alloy.

The anodic part of the polarization curve of the base alloy AA1100 reveals an oscillation region beyond which the current density increases quickly, indicating the onset of pitting corrosion. However, for the composite, jcorr increases steeply even under low overpotential, implying that pitting is more easily provoked in composites than in the base alloy.

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3.2.2. (EIS) 3.2.2. Electrochemical Electrochemical Impedance Impedance Spectroscopy Spectroscopy (EIS) To further confirm confirm the the polarization polarization results, results, EIS measurements were carried out for the composites and base alloy in 3.5 wt.% Prior to to the EIS measurement, the variation of Eocp as a wt.% NaCl NaCl solution. solution. Prior function of time was was measured measured and the graph is shown observed that the Eocp shown in in Figure Figure 4. 4. ItIt isis observed ocp is mV and and −607 ´607±˘55mV mVfor forAA1100, AA1100,AA1100-16 AA1100-16 vol.% vol.% B44C and stabled at at ´710 −710 ˘ ± 5 mV, ´575 −575 ±˘33 mV AA1100-30 vol.% AA1100-30 vol.%BB44C, C,respectively. respectively.

Figure 4. as a function of time measured 100 seconds before EIS tests, showing the E ocp Figure 4. E Eocp ocp as a function of time measured 100 seconds before EIS tests, showing the Eocp of AA1100, AA1100, AA1100-16 AA1100-16vol.% vol.%BB4C andAA1100-30 AA1100-30vol.% vol.% is stable before tests. 4 Cand of B4BC4 C is stable before EISEIS tests.

The in complex complex impedance impedance (Nyquist (Nyquist plot) plot) and and Bode Bode impedance impedance The impedance impedance spectra spectra obtained obtained in magnitude are displayed displayed in in Figure Figure 5.5. The common characteristic: characteristic: capacitive magnitude are The EIS EIS spectra spectra show show aa common capacitive semicircles in the the high highand andmedium mediumfrequency frequencyrange rangethat that related to the aluminum oxide layer semicircles in areare related to the aluminum oxide layer andand the the electrolyte The biggest HF semicircle is observed for the base and theofdiameter of electrolyte [28].[28]. The biggest HF semicircle is observed for the base alloy, andalloy, the diameter semicircle semicircle decreases when increasing the B4C volumein fraction in the composite. This observation decreases when increasing the B4 C volume fraction the composite. This observation confirms confirms that incorporating B4Cinto particles into AA6061 breaks theof continuity of thedecreasing oxide layer, that incorporating B4 C particles AA6061 breaks the continuity the oxide layer, its decreasing its corrosion resistance. The base alloy also has an additional capacitive semicircle in the corrosion resistance. The base alloy also has an additional capacitive semicircle in the low frequency low that maywith be the associated with theacross chargethetransfer across the interface alloy–electrolyte rangefrequency that may range be associated charge transfer alloy–electrolyte [34,35]. interface the Al-B4C composites show an inductive loop with a reduced charge However, [34,35]. the Al-BHowever, 4 C composites show an inductive loop with a reduced charge transfer resistance at transfer resistance at the low frequency revealing the occurrence of pittingsurface. on the composite surface. the low frequency range, revealing the range, occurrence of pitting on the composite The Bode impedance impedancemagnitude magnitudeplot plotisisdisplayed displayedininFigure Figure The impedance of the composite at The Bode 5b.5b. The impedance of the composite at the the frequency decreases increasing B4C content. the material shows low low frequency rangerange decreases when when increasing the B4 Cthecontent. BecauseBecause the material shows resistive resistive behavior at low frequencies (0.01~0.1 Hz), the impedance at low could be behavior at low frequencies (0.01~0.1 Hz), the impedance at low frequencies couldfrequencies be considered as the considered as the resistance. The corrosion resistancedecreases of the composite decreases increasing the resistance. The corrosion resistance of the composite when increasing thewhen B4 C content, which B which validates the previous 4C content, validates the previous polarization results. polarization results. Similar EIS spectra spectrawere wereobtained obtainedforfor B4C-reinforced AA6061 alloy in water; sea water; data Similar EIS thethe B4 C-reinforced AA6061 alloy in sea thesethese data were were interpreted the equivalent in Figure In the first equivalent, s is the interpreted using using the equivalent circuitscircuits shown shown in Figure 6 [28]. 6In[28]. the first equivalent, Rs is the Rsolution solution resistance, ox is the aluminum oxide layer resistance and Rct is the charge-transfer resistance resistance, Rox is theRaluminum oxide layer resistance and Rct is the charge-transfer resistance of the alloy. of the alloy. CPE 1 is the capacitance between the electrolyte and the alloy, and CPE2 is the capacitance CPE is the capacitance between the electrolyte and the alloy, and CPE is the capacitance at the interface 1

2

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of the the interface alloy andofoxide layer. and Theoxide equivalent shown in circuit Figure shown 7b is used to interpret the EIS at the alloy layer. circuit The equivalent in Figure 7b is used to spectra ofthe theEIS composites. In this circuit, there are two independent circuits. Theindependent first circuit interpret spectra of the equivalent composites. In this equivalent circuit, there are two circuits. The to first corresponds to the HF capacitive loop1 (double that is described with CPE (double corresponds thecircuit HF capacitive loop that is described with CPE layer capacitance of1the oxide layer capacitance interface) of the oxide interface) andofRox resistance the layer–electrolyte and layer–electrolyte Rox (charge-transfer resistance the(charge-transfer oxide layer). The second of circuit oxide layer). second circuit represents LF inductive loop; this circuit is described by RL represents theThe LF inductive loop; this circuit isthe described by RL (inductance resistance), L (inductance) (inductance resistance), of L (inductance) and CPE2 (capacitance of pit–electrolyte interface). and CPE2 (capacitance pit–electrolyte interface).

Figure 5. 5. (a) (a)Nyquist Nyquistand and (b) (b) Bode Bode plot plot of of AA1100, AA1100, AA1100-16 AA1100-16 vol.% vol.% B B44CC and and Figure AA1100-30 vol.% vol.% B B44C C obtained obtained after after 11 hh immersion immersion in in 3.5 3.5 wt.% wt.% NaCl NaCl solution. solution. AA1100-30

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9

(a) (a)

(b)

(b)

Figure 6. Equivalent circuits proposedfor for fitting EIS spectra obtained from (a) AA(a) 1100 Figure Equivalent circuits proposed Figure 6. 6. Equivalent circuits proposed for fitting fitting EIS EIS spectra spectra obtained obtained from from (a) AA AA 1100 1100 and (b) AA1100-16 vol.% B4C and AA1100-30 vol.% B4C in 3.5% NaCl solution. and and (b) (b) AA1100-16 AA1100-16 vol.% vol.% B B44C C and and AA1100-30 AA1100-30 vol.% vol.% B B44C C in in 3.5% 3.5% NaCl NaCl solution. solution.

Figure 7. Potentiodynamic polarization curve recorded on AA1100, AA1100-16 vol.% B4C and AA1100-30 vol.% B4C in 3.5 wt.% NaCl in argon deaerated condition. Because the working electrode deviated from the ideal capacitive behavior due to its surface

Figure 7.7.Potentiodynamic polarization curve recorded on current AA1100, AA1100-16 B4 C Figure Potentiodynamic polarization curve recorded on AA1100, vol.% roughness, heterogeneities, anion adsorption, non-uniform potential, profile, etc.AA1100-16 [36], thevol.% constant and AA1100-30 vol.% Bemployed wt.% NaCl in argon deaerated condition. (CPE) was pureNaCl capacitances in the equivalent circuits employed. 4 C in andelement AA1100-30 vol.% B43.5 Ctoinsubstitute 3.5 wt.% in argon deaerated condition. B4Cphase All parameters derived from the equivalent circuits are summarized in Table 3. The oxide layer 2 resistance Rox decreases from 23.74 to 0.89 kΩ·cm and ideal the Rctcapacitive or RL value decreases Because the the working working electrode deviated from ,the the behaviorfrom due54.26 to to its surface surface Because electrode deviated from ideal capacitive behavior due to its 2 0.96 kΩ·cm when the B4C content increases from 0 to 30 vol.%. These results confirm the polarization roughness, heterogeneities, adsorption, non-uniform potential, current profile,profile, etc. [36], constant roughness, heterogeneities,anion anion adsorption, non-uniform potential, current etc.the[36], the results that the corrosion resistance of the composite decreases when increasing the B4C content.

phase element was employed to substitute pure in the capacitances equivalent circuits employed. constant phase(CPE) element (CPE) was employed tocapacitances substitute pure in the equivalent Table 3. Electrochemical parameters derived from the equivalent circuits in Figure 6. All parameters circuits employed. derived from the equivalent circuits are summarized in Table 3. The oxide layer 2 R23.74 Rare R CPE L s 1 ox the Rct ct or RL resistance RoxMaterials decreases toCPE 0.89 kΩ·cm , and or2 RL value decreases from 54.26 to All parameters derivedfrom from the equivalent circuits summarized in α1 α2 Table 3. 2 The oxide layer 2 −2 (Ω cm ) (μF cm ) (kΩ cm2) (μF cm−2) (kΩ cm ) (H) 2 the Bfrom from 30 vol.%. These confirm the polarization 0.96 kΩ·cm 4C content resistance Roxwhen decreases 23.74 increases to20.83 0.89 kΩ¨ cm02 ,toand or RL results value from 54.26 to AA1100 14.79 0.85 23.74the Rct 30.93 0.96 decreases 54.26 2 results that the corrosion resistance of the composite decreases when increasing the B C content. vol.% B4C 12.1 1.0 0.900 to 1.850 0.89 4.001 4 the 2200 0.96 kΩ¨ cmAl-16 when the B4 C content increases from 30 vol.%. 13.0 These results confirm polarization Al-30 vol.% B4C 11.0 7.6 0.87 0.898 65.0 0.96 0.960 3200 results that the corrosion resistance of the composite decreases when increasing the B4 C content. Table 3. Electrochemical parameters derived from the equivalent circuits in Figure 6. Table 3. Electrochemical parameters derived from Rs Roxthe equivalent Rct or R6.L CPE1 CPE2 circuits in Figure

Materials

AA1100 Materials Al-16 vol.% B4C Al-30 vol.% B4C AA1100

2

−2

α1

2

−2

α2

2

(Ω cm ) (μF cm ) (kΩ cm ) (μF cm ) (kΩ cm ) Rs CPE1 Rox CPE2 Rct or RL 14.79 20.83 23.74 30.93 α20.96 54.26 α10.85 2 ´2 2) ´2 ) (Ω cm (µF cm13.0 cm2 ) 12.1) (µF cm1.0 ) 0.90(kΩ cm 1.850 0.89 (kΩ4.001 11.0 7.6 0.87 23.74 0.898 65.0 0.96 0.96 54.26 0.960 14.79 20.83 0.85 30.93

L (H) L (H) 2200 3200 -

Al-16 vol.% B4 C

12.1

1.0

0.90

1.850

13.0

0.89

4.001

2200

Al-30 vol.% B4 C

11.0

7.6

0.87

0.898

65.0

0.96

0.960

3200

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3.3. Corrosion Mechanism Investigation To study the corrosion mechanism of the composite, polarization tests were conducted in deaerated conditions. The polarization curves are shown in Figure 7 and derived parameters are listed in Table 2. Compared with polarization curves obtained in open-to-air condition, a passive region followed by a well-defined pitting potential at the current density approximately 1 µA¨ cm´2 is observed in the anodic branch for both the base alloy and the composites. Besides, it is found that three materials have almost the same Ecorr , which means it does not vary with the B4 C content as observed in open-to-air condition. However, it is more negative than that obtained in open-to-air condition for each material. As seen from Table 2, Ecorr decreased from ´0.70 to ´0.93 VAg/AgCl for the base alloy, from ´0.55 to ´0.92 VAg/AgCl for the composite with 16 vol.% B4 C and from ´0.59 to ´0.93 VAg/AgCl for the composite with 30 vol.% B4 C, respectively. More importantly, the corrosion current density jcorr is considerably lower than that obtained in open-to-air condition, i.e., jcorr is 0.99 µA¨ cm´2 for the composite with 16 vol.% B4 C, and 2.00 µA¨ cm´2 for the composite with 30 vol.% B4 C. This significant difference in current density shows that the corrosion kinetics of the base alloy (AA1100) and the Al-B4 C composite in the NaCl solution are limited by the oxygen reduction reaction [33]. In addition, the difference on cathodic part of polarization curves was also observed. As seen from Figure 3, the current density remains constant or exhibits very small increases despite the potential increase of the cathodic branch in open-to-air condition. However, this observation was not found in deaerated condition. Similar behavior was observed by Singh et al. [30] for 2014-SiCp composites and by Dikici et al. [31] for SiO2 and Fe/TiO2 coated A380-SiC composites in aerated 3.5 wt.% NaCl solution. They believed that this electrochemical behavior is an indicator of the corrosion mechanism, which is controlled by oxygen diffusion. 3.4. Galvanic Current Measurement Because Al-B4 C composites have junctions of two electrochemically dissimilar materials, galvanic corrosion between the Al alloy (matrix) and the reinforcing B4 C particles may occur, degrading the corrosion resistance. As it is technically difficult to conduct a galvanic coupling test using small B4 C particles, a hot-pressed 99.5% purity B4 C wafer produced by Ceradyne, Inc. was used. During the galvanic corrosion test, the base alloy AA1100 was used as working electrode #1 (W1) and the B4 C wafer was working electrode #2 (W2). A positive galvanic current means that working electrode #1 acts as an anode; otherwise, it acts as a cathode. The measured galvanic current and galvanic potential are shown in Figure 8. The galvanic current during the test is always positive, and the stable galvanic current is approximately 11.2 µA. This result suggests that the Al matrix and B4 C particles in the composite can form galvanic couples in NaCl solution, and the Al matrix acts as an anode and dissolves. A similar result was obtained by Schneider et al. [37], when they studied the galvanic corrosion between pure Nickel and sintered SiC in 3.5 wt.% NaCl solution, i.e., SiC ceramic particles are cathodic sites of the coupling. Abenojar et al. [16] also found that the incorporated amorphous Fe/B particles act as cathode and form a strong galvanic couple with the aluminum matrix.

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Figure andand galvanic potential measured between AA1100 and B4and C wafer C Figure 8.8. 8.Galvanic Galvaniccurrent current galvanic potential measured between AA1100 B4C Figure Galvanic current and galvanic potential measured between AA1100 and B 4 in 3.5 wt.% NaCl solution. wafer in in 3.5 3.5 wt.% wt.% NaCl NaCl solution. solution. wafer

3.5. Pitting Pitting Morphology Morphology 3.5. Pitting Morphology 3.5. To identify identify the the initiation initiation sites sites for for the the pitting, pitting, the the AA1100-16 AA1100-16 vol.% vol.% B B44C C composite composite sample sample was was To To identify the initiation sites for the pitting, the AA1100-16 vol.% B sample was 4C composite −2 shows that pitting initiated at at two two sites: sites: polished to to 0.05 0.05 µm μm and and polarized polarized to to 111 mA¨ mA·cm −2 .. Figure polished cm´2 Figure 999 shows shows that that pitting pitting initiated initiated at two . Figure sites: polished to 0.05 μm and polarized to mA·cm (1) the the Al–B Al–B44C C interfaces interfaces and and (2) (2) the the sites sites away away from from the the B B44C C particles particles in in Al Almatrix matrix where whereintermetallic intermetallic (1) C and (1) the Al–B interfaces (2) the sites away from the B in Al matrix where intermetallic 4C particles phases appeared. appeared. Pits Pits at the Al–B C interfaces have an irregular irregular shape, shape, whereas whereas pits pits in in the the Al Al matrix matrix phases Pits at at the the Al–B Al–B444C C interfaces interfaces have have an an irregular shape, whereas pits in phases appeared. the Al matrix are generally generallylarge largeand andhemispherical. hemispherical.Similar Similartypes typesofof ofhemispherical hemispherical pits were observed on AA5083 are pits were observed onon AA5083 in are generally large and hemispherical. Similar types hemispherical pits were observed AA5083 in aerated aerated chloride solutions by Abelle Abelle et[38] al. and [38]Katkar and Katkar Katkar et al. al.They [28].believed They believed believed that these pits aerated chloride solutions by Abelle et al.et et al. [28]. that these pits formed in chloride solutions by al. [38] and et [28]. They that these pits formed duesimple to the the simple simple detachment detachment of the the cathodic cathodic precipitates due to gravity. gravity. As mentioned mentioned in Section Section due to the detachment of the cathodic precipitates due todue gravity. As mentioned in Section 3.1, formed due to of precipitates to As in and Al BC) were dispersed in the Al-B C microstructure. 3.1, many small intermetallic particles (AlB 2 3 4 many small intermetallic particles (AlB AlAl were 2 and 3 BC) weredispersed dispersedininthe the Al-B Al-B44C C microstructure. 3.1, many small intermetallic particles (AlB 2 and 3BC) Those intermetallic particles might be cathodic relative to the surrounding Al matrix. Due to the the Those intermetallic be be cathodic relative to thetosurrounding Al matrix. Due to the galvanic intermetallicparticles particlesmight might cathodic relative the surrounding Al matrix. Due to galvanic effect, the Al Al matrix around those intermetallic intermetallic phases dissolves, dissolves, detaching the particles. particles. effect, theeffect, Al matrix around those intermetallic phases dissolves, detachingdetaching the particles. galvanic the matrix around those phases the

Figure 9. 9. SEM SEM images showing the pitting morphology and initiation places places of of the the Figure SEM images images showing showing the the pitting pitting morphology morphology and and initiation initiation places of Figure 9. the −2 AA1100-16 vol.% vol.% BB44CC after after being being polarized polarized to to 111 mA¨ mA·cm in 3.5 3.5 wt.% wt.% NaCl NaCl solution: solution: −2 in AA1100-16 cm´2 being polarized to mA·cm 4 (a) at at Al–B Al–B44C C interfaces interfaces and and (b) (b) the the Al Al matrix, matrix, where where intermetallic intermetallic phases phases appear. appear. (a) appear.

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The formation of pits at the Al–B4 C interfaces may occur for three reasons: (1) defects existing at the Materials 2015, 8 12 interface between aluminum and B4 C where Cl´ can easily penetrate and attack the Al matrix; (2) the protectiveThe TiB that could preferentially attacked by chloride ions 2 layer at formation of Al/B pits at4 C theinterfaces Al–B4C interfaces maybeoccur for three reasons: (1) defects existing at [39]; − (3) thethegalvanic effect between Al matrix the penetrate B4 C particles. Consequently, interfacecoupling between aluminum and B4the C where Cl canand easily and attack the Al matrix;the Al the interfaces protective TiB Al/B interfaces that Bcould be preferentially by chloride 2 layer atand 4C form matrix(2) at the dissolves, pits around Once aattacked pit is formed, the local 4 C particles. ions [39]; (3) the galvanic coupling effect between the Al matrix and the B 4C particles. Consequently, chemical environment is substantially more aggressive than the bulk solution, and therefore the matrix the Al matrix at the interfaces dissolves, and pits form around B4C particles. Once a pit is formed, is more severely corroded. Additionally, with increased B4 C particles in the composite, the cathodic and the local chemical environment is substantially more aggressive than the bulk solution, and therefore anodic area ratio (AC /AA ) increases due to the cathodic character of the B4 C particles; a large AC /AA the matrix is more severely corroded. Additionally, with increased B4C particles in the composite, C value is the Alarea matrix. Hamdy et al. [40] the occurrence and pitting thedetrimental cathodic andtoanodic ratio (A /AA) increases dueconfirmed to the cathodic character ofof thegalvanic B4C particles; C A attacksa when they corrosiontoresistance of ALCOA peak-aged AA6092-SiC in large A /A studied value isthe detrimental the Al matrix. Hamdy et al. [40] confirmed the occurrence of 17.5p composite galvanic 3.5 wt.% NaCl.and pitting attacks when they studied the corrosion resistance of ALCOA peak-aged composite 3.5 wt.% NaCl. AA6092-SiC 17.5p Figure 10 shows the surface in appearance of the three materials (the base alloy, AA1100-16 vol.% B4 C Figure 10 shows the surface appearance materials base alloy, and AA1100-30 vol.% B4 C) after immersionofinthe 3.5three wt.% NaCl (the solution for 10AA1100-16 days. Thevol.% corrosion B4C and AA1100-30 vol.% B4C) after immersion in 3.5 wt.% NaCl solution for 10 days. degree of the test area increases when increasing the B4 C volume fraction. To evaluate the corrosion of The corrosion degree of the test area increases when increasing the B4C volume fraction. To evaluate the materials with different B4 C levels, cross sections from three samples are examined and displayed the corrosion of the materials with different B4C levels, cross sections from three samples are in Figure 11. and displayed in Figure 11. examined

(a)

(b)

(c)

Figure 10. Stereoscope graphsshowing showing the the corroded morphology of (a) Figure 10. Stereoscope graphs corrodedsurface surface morphology of AA1100; (a) AA1100; (b) AA1100-16 vol.% B4C and(c) (c)AA1100-30 AA1100-30 vol.% immersed in 3.5 wt.%wt.% NaCl NaCl 4C C (b) AA1100-16 vol.% B4 C and vol.%BB immersed in 3.5 4 solution for 10 days. solution for 10 days. Uneven layers of corrosion products formed on the outermost surface of the base alloy and

Uneven layersThe of X-ray corrosion products formed that on these the outermost surface of theoxide baseand/or alloy and composites. elemental maps revealed layers consist of aluminum hydroxides. average corrosion product thickness (the corrosion productsofinaluminum the pits were notand/or composites. The The X-ray elemental maps revealed that these layers consist oxide included The in theaverage calculation of the corroded thickness above) is obtained using 20 hydroxides. corrosion productlayer thickness (the corrosion products in measurements the pits were not collected from the SEM images and is presented in Figure 12. It is found that the thickness of the included in the calculation of the corroded layer thickness above) is obtained using 20 measurements corroded layers increases linearly with the B4C content. The thickness of the corroded layer is collected from the SEM images and is presented in Figure 12. It is found that the thickness of approximately 1.3 μm in the base alloy and 25.8 μm in the AA1100-16 vol.% B4C composite, which is the corroded with the increasing B4 C content. The thickness the the corroded layer is nearly 20layers times increases that of the linearly base alloy. When the B4C content to 30 of vol.%, thickness approximately 1.3μm, µmwhich in theis base and 25.8that µm B4 C composite, reaches 58.7 more alloy than 40 times of in thethe baseAA1100-16 alloy. These vol.% observations indicate thatwhich adding B4C particles Al alloy. alloy can reduce the corrosion in to NaCl solution,the which is nearly 20 times that of to thethe base When increasing the Bresistance 30 vol.%, thickness 4 C content accords with the polarization Moreover, shows that only the that reaches 58.7 µm, which is moreand thanimpedance 40 timesmeasurements. that of the base alloy. Figure These11 observations indicate suffertosevere confirms the Al-B 4C composite is more sensitive toward addingcomposites B4 C particles the Alpitting, alloy which can reduce the that corrosion resistance in NaCl solution, which accords pitting than the base alloy in the NaCl solution. with the polarization and impedance measurements. Moreover, Figure 11 shows that only the composites suffer severe pitting, which confirms that the Al-B4 C composite is more sensitive toward pitting than the base alloy in the NaCl solution.

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(b)

(a)

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(c) (c) Figure 11. Cross-sectionalmorphology morphology of of (a) (a) AA1100; vol.% B4C Band Figure 11. Cross-sectional AA1100;(b) (b)AA1100-16 AA1100-16 vol.% Figure 11. Cross-sectional morphology of (a) AA1100; (b) AA1100-16 vol.% B44C C and and (c) AA1100-30 vol.% B C specimen immersed in 3.5 wt.% NaCl solution for 10 days. 4 (c) AA1100-30 vol.% B C specimen immersed in 3.5 wt.% NaCl solution for 10 days. (c) AA1100-30 vol.% B44C specimen immersed in 3.5 wt.% NaCl solution for 10 days.

Figure 12. Relationship of corrosion product thickness and B4C volume fraction after 10 days exposure in 3.5 wt.% NaCl solution.

Figure Relationship of of corrosion corrosion product product thickness thickness and and B B44C C volume volume fraction fraction after Figure 12. 12. Relationship after 4. Conclusions 10 days exposure in 3.5 wt.% NaCl solution. 10 days exposure in 3.5 wt.% NaCl solution. (1) The polarization and impedance results show that the Al-B4C composites are less corrosion-resistant

4. Conclusions 4. Conclusions than the base Al alloy and that the corrosion resistance of the composites decreases when

increasing the B4C particle volume fraction. The cross-sectional images demonstrate that the

(1) The The polarization polarization and impedance resultsresults show that the Al-B less corrosion-resistant 4C composites (1) and impedance show Al-Bfraction. are less 4 Carecomposites thickness of corrosion products increases linearly withthat the B4the C volume than the 4base Al than alloyare and thatAlthealloy resistance thesolution. composites decreases corrosion-resistant the base andcorrosion that theincorrosion resistance oftypes the of composites C composites susceptible tocorrosion pitting theof NaCl Two pits when (2) Al-B C the particle volume fraction. The cross-sectional demonstrate the increasing the Bincreasing 4on decreases when the Bsurface volume fraction. The cross-sectional images are observed composite after polarization in the NaClimages solution: (1) pits with that an 4 C particle thickness of corrosion linearly products with the Bincreases fraction. with the B4 C 4C volumelinearly demonstrate that the products thicknessincreases of corrosion (2) volume Al-B4Cfraction. composites are susceptible to pitting corrosion in the NaCl solution. Two types of pits are 4observed on the surface aftercorrosion polarization the NaCl solution: (1) pits an (2) Al-B C composites arecomposite susceptible to pitting in theinNaCl solution. Two types of with pits are observed on the composite surface after polarization in the NaCl solution: (1) pits with an irregular

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shape that are preferentially initiated at Al/B4 C interfaces and (2) hemispheric pits that initiate in the Al matrix where the intermetallic particles appeared. (3) The corrosion of Al-B4 C composites in 3.5 wt.% NaCl solution is mainly controlled by oxygen reduction in the solution. Moreover, the galvanic couples formed between B4 C particles and Al matrix is also responsible for the low corrosion resistance. Acknowledgments The authors are grateful for the financial support from the Natural Sciences and Engineering Research Council of Canada (NSERC), from Rio Tinto Alcan and from the NRC-Aluminium Technology Centre through the NSERC Industrial Research Chair in Metallurgy of Aluminium Transformation at the University of Quebec in Chicoutimi. The authors also wish to thank D. Gallant for his kind guidance, as well as H. Grégoire, A. Ruest, G. Simard and M. Côté of the NRC-Aluminium Technology Centre for their technical assistance. Author Contributions Yu-Mei Han designed and conducted the whole experiment, interpreted all data and prepared the manuscript, X.-Grant Chen supervised the research work. Conflicts of Interest The authors declare no conflict of interest. References 1. Pardo, A.; Merino, M.C.; Merino, S.; Viejo, F.; Carboneras, M.; Arrabal, R. Influence of reinforcement proportion and matrix composition on pitting corrosion behaviour of cast aluminium matrix composites (A3xx.x/SiCp). Corros. Sci. 2005, 47, 1750–1764. [CrossRef] 2. Miracle, D.B. Metal matrix composites—From science to technological significance. Compos. Sci. Technol. 2005, 65, 2526–2540. [CrossRef] 3. Chen, X.-G. Application of Al-B4 C Metal Matrix Composites in the Nuclear Industry for Neutron Absorber Materials. In Proceedings of the Symposium on Solidification Processing of Metal Matrix Composites, San Antonio, TX, USA, 12–26 March 2006. 4. Mohanty, R.M.; Balasubramanian, K.; Seshadri, S.K. Boron carbide-reinforced alumnium 1100 matrix composites: Fabrication and properties. Mater. Sci. Eng. A 2008, 498, 42–52. [CrossRef] 5. Hemanth, J. Tribological behavior of cryogenically treated B4Cp/Al-12% Si composites. Wear 2005, 258, 1732–1744. [CrossRef] 6. Thévenot, F. Boron carbide—A comprehensive review. J. Eur. Ceram. Soc. 1990, 6, 205–225. [CrossRef] 7. Lillo, T.M. Enhancing ductility of AL6061 + 10 wt.% B4 C through equal-channel angular extrusion processing. Mater. Sci. Eng. A 2005, 410–411, 443–446. 8. Abenojar, J.; Velasco, F.; Martínez, M.A. Optimization of processing parameters for the Al + 10% B4 C system obtained by mechanical alloying. J. Mater. Process. Technol. 2007, 184, 441–446. [CrossRef]

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