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Electrochimica Acta 88 (2013) 129 134

Contents lists available at SciVerse ScienceDirect

Electrochimica Acta

jou rn al hom epa ge: www.elsev ier .com/ locate /e lec tac ta

lectrochemical behavior of a discontinuously A6092/SiC/17.5p metal matrixomposite in chloride containing solution

bdel Salam Hamdya,, Feras Alfosailb, Zuhair Gasemb

Central Metallurgical Research and Development Institute, Cairo, EgyptCenter of Research Excellence in Corrosion, King Fahd University of Petroleum & Minerals, Dhahran, Saudi Arabia

r t i c l e i n f o

rticle history:eceived 6 October 2012eceived in revised form 18 October 2012ccepted 18 October 2012vailable online 26 October 2012

eywords:orrosion assessmentilicon carbide

a b s t r a c t

Aluminum alloys are used widely in automotive and aerospace industries. The demand for weight savingsmaterials of superior mechanical, thermal and electrical properties has focused attention on aluminummetal matrix composites (AMMCs). AMMCs have been applied in areas that can cost-effectively capi-talize on improvements in specific stiffness, specific strength, fatigue resistance, wear resistance, andcoefficient of thermal expansion. However, AMMCs typically have lower damage tolerance propertiesthan their unreinforced counterparts, and hence the extent of application in primary structures has beenlimited. In this paper, the corrosion resistance of ALCOA peak-aged Al6092/SiC/17.5p composite has beenevaluated by recording of impedance spectra during immersion over one week in an air-exposed 3.5%

luminum matrix compositeutomotive and aerospace materialslectrochemical impedance spectroscopy

NaCl. Results confirmed the occurrence of galvanic, crevice, intergranular corrosion and pitting attack dueto the inhomogeneous structure of the composite which is the main reason behind enhancing the cor-rosion susceptibility. This paper formulates a complex hypothesis concerning the corrosion mechanismsof A6092/SiC/17.5p metal matrix composite in chloride containing solution as a function of time. A four-stage corrosion mechanism has been proposed to explain the sequence and the reasons of occurrence ofeach corrosion form.

2012 Elsevier Ltd. All rights reserved.

. Introduction

Aluminum alloys are used widely in the aerospace, automotivend architectural industries. The increased demand for improvedechanical, electrical and physical performance and weight sav-

ngs has focused attention on a number of advanced materials, suchs nano-structured materials, ceramics and composites.

A composite material consists of two main constituents: aatrix and reinforcement materials. Composite materials are a

apidly growing class of tailor-making materials. They can be clas-ified, based on the matrix, to include organic matrix composites,eramic matrix composites and metal matrix composites [1].

Metal matrix composites are the most common class of com-osites used in aerospace and automotive industries and can beefined by the type of metal matrix, type of reinforcement, andeinforcement geometry. According to the type of metal matrix,

Corresponding author at: Central Metallurgical Research and Developmentnstitute, Cairo, 5 Egypt. Tel.: +20 2 1061607279; fax: +20 2 25010 639.

E-mail address: [email protected] (A.S. Hamdy).

013-4686/$ see front matter 2012 Elsevier Ltd. All rights reserved.ttp://dx.doi.org/10.1016/j.electacta.2012.10.079

According to type of the reinforcement materials, the reinforce-ments can be ceramics such as Al2O3, SiC, B4C, TiC, graphite toprovide a very beneficial combination of stiffness, strength, andrelatively low density. Or, in some cases, metallic materials such astungsten or steel fibers can be used as reinforcements.

According to the geometry of the reinforcement materials, thereinforcement can be in the form of continuous fiber, chopped fiberor whisker, and particulate. Typically, the selection of the reinforce-ment morphology is determined by the required property/costcombination. Generally, the reinforcements in the particulatesgeometry provide a comparatively more moderate but isotropicincrease in properties, and are typically available at the lowest cost[2].

Aluminum composite, AMMCs, materials are formed by theaddition of a second phase, generally a ceramic material, to analuminum matrix. AMMCs are used extensively in aerospace,automotive and military applications due to various reasons includ-ing weight reduction, corrosion and wear resistance, erosionresistance, high-temperature performance, thermal and acousticalinsulation, electrical performance and life cycle cost reductions [1].

130 A.S. Hamdy et al. / Electrochimic

Table 1The chemical composition of aluminum alloy 6092.

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Elements Si Fe Cu Mg Zn Al

Composition (wt%) 0.8 0.3 1.0 1.2 0.25 Balance

nd corrosion resistance. MMCs typically have lower damage tol-rance properties than their unreinforced counterparts, and hencehe extent of application in primary structures has been limited.owever, MMCs have been selected for some aircraft structures,

uch as F-16 aircraft, Airbus, and the Eurocopter rotor sleeve [2].Aluminum-based, particulate-reinforced AMMCs have been the

ost popular due to their low density and isotropic properties3]. Although the incorporation of a second phase into a matrix

aterial can enhance the physical and mechanical properties of anMC, such an addition could also significantly change its corrosion

ehavior [46].Several particulate such as alumina, graphite, SiC, have been

sed as reinforcements for AMMCs. However, SiC is one of the mostommon reinforcements used in Al-based MMCs [6]. SiC-AMMCsere used to replace aluminum tubes in the catamaran Stars and

tripes88 resulting in 20% weight savings in comparison to mono-ithic aluminum [7].

In contrast to aluminum alloys, relatively little is known abouthe corrosion behavior of aluminum MMC and the role of the rein-orcement in the corrosion process. Some researchers reported thathe corrosion resistance of composite materials decreases in NaClolution [17]. However, another research group investigated theorrosion behavior of Al 6092/17.5 SiC(p) and reported that theaterial offered a good resistance to corrosion in 3.5% NaCl aseasured by weight loss, salt spray chamber and electrochemi-

al studies [8]. Therefore, the objective of this work is to provide aetter understanding about the electrochemical corrosion behaviorf A6092/SiC/17.5p metal matrix composite in 3.5% NaCl solu-ion using EIS techniques. Surface examination will be performedy scanning electron microscopy (SEM), visual inspection, opticalicrography, and energy dispersive X-ray (EDS) analyses before

nd after immersion in NaCl corrosive solution.

. Experimental

.1. Materials and surface preparation

Aluminum alloy 6092 reinforced with 17.5 vol% SiC T6 metalatrix composite was supplied as sheets from ALCOA. Samples

ig. 1. Optical microscopic images of the composite samples before corrosion. (Theacrograph shows the surface defects and high surface roughness of the virgin

omposite samples before corrosion.)

a Acta 88 (2013) 129 134

were cut into 60 mm 30 mm and 3 mm thickness. Aluminumsamples were mechanically ground using SiC paper startingfrom coarse to fine with grades of 240, 320, 400, and 600 usingmetallographic techniques. Samples were degreased with acetone,washed with distilled water and dried for 5 min at 55 C. Table 1summarizes the chemical composition of aluminum alloy 6092.

3. Testing methods

3.1. Electrochemical impedance spectroscopy

Electrochemical impedance spectroscopy measurements werecarried out in aerated 3.5% sodium chloride (NaCl) solution at roomtemperature for up to seven days. The exposed surface area was2.30 cm2 and all data were normalized to 1 cm2.

A three-electrode set-up was used with impedance spectrabeing recorded at the corrosion potential ECorr. A saturated calomelreference electrode (SCE) was used as the reference electrode. Itwas coupled to a graphite rod serving as a counter electrode. EISwas performed between 0.01 Hz and 65 kHz frequency range usinga frequency response analyzer GAMRY Instruments Reference 3000ac/dc potentiostat. The amplitude of the sinusoidal voltage signalwas 10 mV.

3.2. SEMenergy-dispersive spectrometry

SEM images were obtained using a digital scanning electronmicroscope Model JEOL JSM 6460 Oxford Instruments, Japan, wasused to examine the corrosion products after immersion. Micro-probe analysis was performed using Oxford Instrument INCAenergy dispersive spectrometry, EDS.

3.3. Surface morphology

Corrosion morphology was examined with a metallographicmicroscope LEICA DMR with a Quips programming window, LEICAImaging Systems Ltd., Cambridge, UK.

4. Results and discussion

4.1. Surface examinations

4.1.1. Visual inspection and optical microscopic examinationMicroscopic examination of the as-polished virgin compos-

ite samples before corrosion revealed presence of many surfacedefects. The surface roughness was very high as shown in Fig. 1.

Fig. 2. Macro-image of virgin composite samples after one week in NaCl solution.

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Visual inspection and macrographs of the as-polished aftereven days of immersion in 3.5% NaCl solution showed severe pit-ing and crevice corrosion (Fig. 2). The average number of pitsormed on the surface of the as-polished samples was 10 pit/cm2.t was noticed that pits of different sizes are distributed randomlyt the surface. Some of them are tiny; while others are large, deepr wide.

.1.2. SEMEDS micrographsSurface examination using SEM micrograph of the as-polished

irgin composite samples before corrosion revealed the presencef many surface defects, high surface roughness and intermetallic

Localized corrosion initiated on a A6092-T6/SiC/17.5p in anir-exposed 3.5 wt% NaCl solution. Scanning electron microscopy

Fig. 3. SEMEDS of the composite samples bef

Acta 88 (2013) 129 134 131

indicated that corrosion mainly initiated around SiC reinforcementparticles. After seven days of immersion in corrosive NaCl, twoforms of corrosion have been identified; namely pitting corrosionand intergranular corrosion attack (Fig. 3b and c).

Bavarian et al. [9] found that the corrosion of AMMC-25%SiCpwas observed only when the breakdown of passive film occurred,which exposed active aluminum and noble SiC particles to the cor-rosive environment and initiated a galvanic corrosion. Ding andHihara [10] reported that although it is not necessary for Cl tobe present for corrosion initiation and propagation, the presence ofCl accelerated the corrosion process. Accordingly, we suggest thatgalvanic corrosion might be formed in the early stage of exposure

ore and after one week in NaCl solution.

132 A.S. Hamdy et al. / Electrochimica Acta 88 (2013) 129 134

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Fig. 4. Schematic representation (not to scale) showing the proposed mech

Galvanic corrosion was not the only mechanism proposed fororrosion of composite materials in chloride media. Other two pos-ible mechanisms have been proposed [7,11]. The first one wasuggested by Hihara et al. [7] and is based on crevice formation athe reinforcementmatrix interface either by debonding or somether mechanism. The second mechanism is hypothesized that

thin layer of Al4C3 formed at the SiCAl interface during therocessing of the Al/SiC MMC as reported by Iseki et al. [11]. In

l4C3 + 12H2O = 4Al(OH)3 + 3CH4 (11)

of corrosion at the interface between Al6092 alloy matrix and SiC particle.

The schematic diagram in Fig. 4 illustrates the possible forma-tion of Al4C3 interphase at the Al/SiC interface and the hydrolysis ofAl4C3 is likely the cause of a preferential dissolution of the interfacebetween the alloy matrix and the SiC reinforcing particles.

5. Electrochemical impedance spectroscopy

A series of electrochemical experiments was performed onsamples of A6092/SiC/17.5p in order to investigate potentialapplications for these promising materials in automotive andaircraft industries in presence of a corrosive chloride medium. The

A.S. Hamdy et al. / Electrochimica Acta 88 (2013) 129 134 133

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Fig. 5. Electrochemical impedance spectroscopy of aluminum meta

orrosion resistance of the AMMC material has been evaluated by

Ongoing research by our team showed that the cathodic cur-ent density for oxygen reduction in 3.5% NaCl increased, which

ix composite over one week immersion in corrosive NaCl solution.

was attributed to electrochemically active interfaces between the

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According to Nyquist plots (Fig. 5a), the corrosion started tooarly where a small tail representing the diffusion process wasbserved in the impedance spectra recorded after only 30 minf immersion in corrosive NaCl solution. This result is in agree-ent with the previous observations by Hihara and his coworkers

7,10] who noticed that the corrosion initiation and propagation ofhe AMMC started even before exposure to chloride media. Theyttributed such behavior to the galvanic action between SiC rein-orcements and the aluminum matrix.

Accordingly, we suggest that the whole corrosion process haseen done, as a function of time, and passed with four stages. Therst and early stage of corrosion was galvanic corrosion due to gal-anic action between the inert SiC reinforcements (act as cathode)nd the aluminum matrix (acts as anode) in presence of mois-ure. Then, crevice corrosion is the second stage of corrosion toake place due to the presence of the deposits (corrosion prod-cts resulted from the galvanic corrosion in the first stage) andhe inert SiC reinforcements themselves. This hypothesis matchesith the mechanism proposed by Iseki et al. [11] in which a thin

ayer of Al4C3 formed at the SiCAl interface during the processinghich can be hydrolyzed to form crevice corrosion at the AlSiC

nterfaces.The third stage is the initiation and propagation of pitting

orrosion due to the differential aeration and formation of micro-lectrochemical cells at the materials surface. This stage of pittingorrosion formation has been attributed to nucleation of precipi-ates at matrixreinforcement interfaces in the composite material13,14].

Parallel to this stage, a fourth stage in which intergranular cor-osion can also occur through the electrochemical dissolution ofhe active elements (such as Al and other minor alloying elements)fter relatively long immersion time in NaCl leaving the surfaceracked as mud-like structure. Moreover, increasing the corrosionround the SiC particulates can result in debonding of these parti-les from the surface leaving hole similar to pitting (Figs. 3c and 4).

As expected, the surface resistance of the AMMC to localizedorrosion decreased with increasing the exposure time in NaCl.he measured surface resistance after 30 min, one day and oneeek of immersion in NaCl solution was 15 103, 7.1 103, and

.8 103 cm2 respectively. This result confirms the previousbservations by SEM, optical microscope and visual inspectionhere the number and size of pits increased by increasing the

mmersion time in corrosive NaCl.The resistance and () angle spectra in Bode plots (Fig. 5b

nd c) provide further explanation of these observations wherehe samples exposed to corrosive NaCl solution for short time30 min) showed the highest polarization resistance (Rp) comparedith those exposed for longer immersion times (one day and oneeek). The Rp values were 20 103 cm2, 10 103 cm2 and

103 cm2 for the samples immersed in NaCl for 30 min, one daynd one week, respectively. These results confirm our hypothesisbout the four-stage mechanism of AMMC corrosion in corrosivehloride solution.

. Conclusions

A four-stage mechanism for corrosion of A6092/SiC/17.5petal matrix composite in NaCl solution has been proposed. The

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a Acta 88 (2013) 129 134

mechanism suggested that the time of immersion in corrosivemedia has a detrimental effect on the type of corrosion formed. Itwas hypothesized that galvanic corrosion is naturally occurred atthe surface in presence of moisture due to galvanic action betweenSiC reinforcements (act as cathode) and the aluminum matrix (actsas anode). Crevice corrosion was formed in the second stage dueto the deposition of a thin layer of Al4C3 at the SiCAl interface andthe formed corrosion products due to the galvanic corrosion in thefirst stage. In the third stage, pitting corrosion was enhanced dueto the differential aeration and formation of micro-electrochemicalcells at the materials surface in addition to the nucleation of pre-cipitates at matrixreinforcement interfaces. In the fourth stage,intergranular corrosion could also take place as a result of theelectrochemical dissolution of the active alloying elements segre-gated at the grain boundaries.

Increasing the exposure time in NaCl solution increases the cor-rosion around the SiC particulates and can result in debonding someof these particles leaving hole at the surface.

The proposed four-stage mechanism provides new insightstowards a better understanding of the practical electrochemi-cal behavior of A6092/SiC/17.5p metal matrix composite in Cl

containing media and hence, designing suitable coatings forcorrosion protection which is the main objective of our nextarticles.

References

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