nM nt

Received 12 June 2013; revised 16 F

Composites;

NaCl immersion corrosion tests of Al/SiC MMCs in 3.5 wt.% NaCl aqueous solution at several temper-

atures showed that, at ambient temperature, the Al/SiC MMCs have better corrosion resistance

composites exhibited higher corrosion rates at 50 C and 75 C than the pure Al matrix.

Aluminum alloys reinforced with ceramic particulates have

tion potential of composites as structural materials [5].

tle systematic studies have been done to study the corrosion

MMCs in comparison with the corrosion of the respective mono-lithic matrix alloys. Crevice attack at the metal/reinforcementinterfaces and preferred localized attack on structural and com-positional inhomogeneities can occur within the matrix. Since

corrosion decreases the load-bearing capacity resulting in cata-strophic failures, corrosion can limit the application of MMCsin corrosive environments especially in the presence of stresses.

Previous corrosion studies conducted of Al matrix compos-ites have been focused on the corrosion susceptibility in NaCl

* Tel.: +20 111 000 7708.

E-mail address: zhossam@hotmail.com.

Peer review under responsibility of Ain Shams University.

Production and hosting by Elsevier

Ain Shams Engineering Journal (2014) xxx, xxxxxx

Ain Shams

Ain Shams Engin

www.elsevier.cowww.sciencesignicant potential for structural applications due to their

high specic strength and stiffness as well as low density [13]. These properties have made particle-reinforced metal ma-trix composites (MMCs) an attractive candidate for the usein weight-sensitive and stiffness-critical components in aero-

space, transportation and industrial sectors [4]. Corrosionbehavior is very important parameter for assessing the applica-

behavior of Al MMCs [610]. Reinforcement particulates may

interact electrochemically, chemically, or physically with the ma-trix leading to accelerated corrosion [46]. In addition, galvanicinteractions between the reinforcement andmatrix can also accel-

erate corrosion. Preferential corrosion along a particle matrixinterface can lead to rapid penetration along the large interfacialareas in composites. This can result in enhanced corrosion of1. Introduction While considerable work has been done on the physical,mechanical and tribological characteristics of AlMMCs, very lit- 2014 Production and hosting by Elsevier B.V. on behalf of Ain Shams University.than the pure Al matrix. Reducing the SiC particles size and/or increasing the volume fraction of

the SiC particulates reduce(s) the corrosion rate of the Al/SiC MMCs. In contrast, the Al/SiC20

ht

P(2KEYWORDS

Corrosion;

Al/SiC;

Matrix;90-4479 2014 Productiontp://dx.doi.org/10.1016/j.asej

lease cite this article in press014), http://dx.doi.org/10.10Tand hosti

.2014.03.0

as: Zaka16/j.asej., Shoubra Faculty of Engineering, Benha University, Cairo, Egypt

ebruary 2014; accepted 16 March 2014

Abstract Several Al/SiC MMCs having several volume fractions up to 15 vol.% and different SiC

particulates average sizes, typically, 11, 6 and 3 lm were fabricated using conventional powdermetallurgy (PM) route. The effect of the size and volume fraction of SiC particulates on the

microstructural and corrosion behavior of Al/SiC metal matrix composites (MMCs) were studied.

he results revealed that the Al/SiC MMCs exhibited higher density than pure Al matrix. The staticechanical Engineering DepartmeMECHANICAL ENGINEERING

Microstructural and corrosiometal matrix composites

H.M. Zakaria *ng by Elsevier B.V. on behalf of A

03

ria HM, Microstructural and corr2014.03.003behavior of Al/SiC

University

eering Journal

m/locate/asejdirect.comin Shams University.

osion behavior of Al/SiC metal matrix composites, Ain Shams Eng J

solution, as well as pitting potential and pit morphology [1115]. Researchers reported that pits initiate at the secondaryparticles within the matrix; therefore, composites generally

have more pits than the monolithic matrix. For example,improvement in corrosion resistance has been declared withdecreasing volume fraction of Al2O3 particles in Al4 wt.%

Mg alloy matrix composites [11]. Kiourtsidis and Skolianos[12,13], explained the progress of corrosion by two anodicreactions; namely corrosion of the a-phase adjacent to inter-metallic regions, and pitting of the dendrite cores, instead ofgalvanic corrosion developed between the reinforcing particlesand the matrix. Additionally, they supported this argument bya further report that the pitting potential is unaffected by the

SiC particles. Since the processing method can heavily alterthe microstructure, the contradictory results of corrosion testsconducted on Al MMCs, may arise from processing methods

[14,15].The aim of the current investigation is to study the static

immersion corrosion behavior of Al/SiC MMCs in 3.5 wt.%

NaCl solution at both ambient and elevated temperatures.The Al/SiC MMCs were fabricated using the conventionalpowder metallurgy (PM) route. The effect of the SiC particu-

press having a capacity of 500 kN. The compaction pressure

2 H.M. Zakarialates size and volume fraction on the corrosion characteristicswas extensively studied.

2. Experimental procedures

Commercially pure aluminum powder having minimum purityof 99.8% was used as a matrix material. The aluminum pow-ders have an average size of 60 lm. The SiC ceramic partic-ulates were used as reinforcement. The SiC particulates havethree different average sizes, typically, 11, 6 and 3 lm. TheSiC particulates were dispersed in the Al matrix with 5, 10

and 15 vol.% using conventional PM route as follows: BothAl powder and the SiC particulates in addition to 11.5 wt.% parafn lubricant wax were placed into a blender,

mechanically mixed until a homogeneous mixture is achieved,and then placed into containers. The mixed Al/SiC powderswere cold compacted in a tool steel die shown schematically

in Fig. 1. The powders were then pressed using a hydraulic

Figure 1 A schematic illustration of the cold compaction die

used for preparation of Al/SiC composites.Please cite this article in press as: Zakaria HM, Microstructural and corr(2014), http://dx.doi.org/10.1016/j.asej.2014.03.003applied was about 400 MPa. The Al/SiC composites producedfrom the cold compaction step were subjected to sintering at

600 C for 120 min. The sintering process was performed un-der argon inert gas atmosphere. After sintering, the Al/SiCcomposites were subjected to hot extrusion. The Al/SiC com-

posites billets were extruded at 490 C. The extrusion reduc-tion ratio was 2:1 by area. The nal Al/SiC compositesamples had cylindrical shape of 8 mm diameter and about

12 mm length. The Al/SiC composites cylindrical extrudedrods were cut in the transverse directions for microstructuralexaminations using optical and scanning electron microscopes(SEM).

Specimens were ground under water on a rotating diskusing abrasive disks of increasing grade up to 1000 grit. Thenthey were polished using 3 lm alumina paste and 1 lm dia-mond paste, then cleaned with acetone. The density of theAl/SiC composites was calculated using water displacementapproach (Archimedean density) according to ASTM B311-

08. The theoretic density of Al/SiC composites was calculatedusing the rule of mixtures. The cylindrical sample was weighedin air (Wa), then suspended in distilled water and weighed

again (Ww). The actual density was calculated according thefollowing equation:

qa Wa

Wa Ww qw 1

where qa is the actual density,Wa is the mass of the cylindricalsample in air, Ww is the mass in distilled water and qw is thedensity of distilled water. The sample was weighed using a dig-

ital balance with an accuracy of 0.1 mg. Vickers hardness testmeasurements were carried out using a load of 10 kg. A mini-mum of ten readings were taken for each sample and the aver-

age value was determined.Static immersion corrosion tests were carried out at three

different temperatures, typically, room temperature, 50 and

75 C. Weight loss was measured to determine the corrosionrate of Al/SiC composites using a digital accuracy with anaccuracy of 0.1 mg. Each specimen was rst weighed beforebeing immersed in 3.5 wt.% NaCl solution and later taken

out after 24, 48, 72, 96 and 120 h, respectively. After dryingthoroughly, the specimens were weighted again. The weightloss was measured and converted into corrosion rate expressed

in mm penetration per year (mm/year). The corroded surfaceswere examined using SEM. Corrosion tests were carried out bysuspending the Al/SiC composite samples in a still solution of

3.5 wt.% NaCl aqueous solution. To avoid crevice corrosion,the specimens were suspended in the solution with a plasticstring. The results of corrosion tests were evaluated using

weight loss measurements, performed following the ASTM-G31 recommended practice [16]. Before immersing in3.5 wt.% NaCl aqueous solution, the Al/SiC composite sam-ples were ground to 1000 grit and then cleaned with deionized

water followed by rinsing with methanol and dried. For theelevated temperatures corrosion tests (i.e. accelerated tests), a3.5 wt.% NaCl solution was prepared, and heated to 50 1

and/or 75 1 C using an electric heater. The specimens wereput into the warm solution and a glass cover was put on thetop of the vessel to prevent evaporation.

The corrosion rate CR (from the mass loss) was calculatedusing the following equation [5]:osion behavior of Al/SiC metal matrix composites, Ain Shams Eng J

CR K WA D T 2

where CR is the corrosion rate (mm/year), K is a constant(8.766 104), T is the time of exposure (h) to the nearest0.01 h, A is the area (cm2), W is the weight loss in the nearest1 mg and D is the density of the material (g/cm3).

3. Results and discussion

3.1. Microstructural characteristics of Al/SiC composites

Fig. 2 shows SEM micrographs of the fabricated Al/SiC com-posites having a constant volume fraction of 10 vol.% but with

different sizes of the SiC particulates. Although some agglom-eration of SiC particulates could be observed, the distributiongenerally appeared to be fairly homogeneous throughout the

aluminum matrix. Fig. 3 shows typical optical micrographsof Al/SiC composites having a constant SiC particulates sizeof 11 lm but with different volume fractions of the SiC partic-ulates. It has been observed that increasing the volume fraction

increases the agglomerations of the SiC particulates. Suchobservation has been reported also by many workers [24].The agglomerations size was found varying between 10 and

35 lm.

3.2. Density of the Al/SiC composites

The variation in the measured (actual) density of the Al/SiCcomposites with the volume fraction at different SiC particles

size is illustrated in Fig. 4. The Al/SiC composites exhibitedhigher densities than the pure Al matrix. The Al/SiC compos-ites exhibited actual densities of about 9798% of the theoret-

ical density. The Al/SiC composites (6 lm) contain 5, 10, and15 vol.% of SiC particulates exhibited densities 2.682, 2.695,and 2.7195 g/cm3, respectively. The Al matrix alloy has density

2.685 g/cm3. It has been found that increasing the volume frac-tion of SiC particulates increases the density of the composites.The increase in the density of aluminum alloys due to the addi-

tion of ceramic particulates was reported by many investiga-tors [17,18]. The results revealed that the reinforcementsenhance the density of the MMCs. Moreover, the density ofthe composites increased with the increase in particulate vol-

ume fraction. The increase in the density can be attributed tothe higher density of the reinforcement particulates.

3.3. Corrosion behavior of the Al/SiC composites

Fig. 5 shows the variation in the corrosion rate of Al/SiC com-posites with the exposure duration in 3.5 wt.% NaCl solution

at room temperature. Generally, it has been found that theAl/SiC composites showed better corrosion resistance whencompared with the pure Al matrix. Increasing the volume frac-

tion of the SiC particulates increasing the corrosion resistanceof the Al/SiC composites. Moreover, reducing the SiC particlessize improved signicantly the corrosion resistance of the SiCcomposites. It has been found that the increasing the duration

exposure reduces the corrosion rate. Such observation imply-ing that the corrosion resistance of the materials under inves-tigation increases as the exposure duration is increased. The

rce

e S

Microstructural and corrosion behavior of Al/SiC metal matrix composites 3Figure 2 SEM micrographs of Al/10 wt.% SiC composites reinfo

and (d) Higher magnication micrograph of (c) showing clearly thPlease cite this article in press as: Zakaria HM, Microstructural and corr(2014), http://dx.doi.org/10.1016/j.asej.2014.03.003d with SiC particulates have size of (a) 11 lm; (b) 6 lm; (c) 3 lm;iC particulates.osion behavior of Al/SiC metal matrix composites, Ain Shams Eng J

4 H.M. Zakariaphenomenon of decreasing the corrosion rate with respect to

the exposure duration indicates some passivation of the matrixalloy.

The improvement of the corrosion resistance of theAl-MMCs

due to the increase in the SiC volume fraction was reported bymany workers [19,20]. For example, Feng et al. [19] examinedthe effects of the volume fraction of SiC particulate reinforce-

ments and the concentration of chloride ions in solution on thelocalized corrosion characteristics of SiCp/2024 Al-MMCs. They

Figure 3 Optical micrographs of Al/SiC composites reinforced with S

and (c) 15 vol.% of SiC partculates.

Figure 4 Variation in the density with the volume fraction of the

SiC particulates having different average sizes.

Please cite this article in press as: Zakaria HM, Microstructural and corr(2014), http://dx.doi.org/10.1016/j.asej.2014.03.003iC particulates having size of 11 lm and (a) 5 vol.%, (b) 10 vol.%reported that the increase in the volume fraction of SiCp rein-

forcement in the SiCp/2024Al composites resulted in a signicantdecrease in pitting potential. Candan [20] studied the effect of SiCparticle size on corrosion behavior of Al60 vol.% SiC particle

composites. Experimental results showed that the weight loss ofthe composites increased with increasing particle size and expo-sure time. The results showed also that intermetallics as a result

of reaction between Al and SiC particle have a benecial effecton corrosion resistance of the composites due to interruption ofthe continuity of the matrix channels within the pressure inl-trated composites. Moreover, the weight loss of the composites

in still 3.5 wt.%NaCl solutions increasedwith increasing particlesize.

Figs. 6 and 7 show typical variation in the corrosion rate of

the Al/SiC composites with the temperature after exposure in3.5 wt.% NaCl solution for 24 and 120 h. In contract to the re-sults obtained at room temperature, it has been found that the

Al/SiC composites have higher corrosion rates when comparedwith the pure Al matrix at elevated temperatures. Increasingthe volume fraction and/or the SiC particles size reduce(s)

the corrosion rates of the Al/SiC composites. The corrosionrates of the pure Al as well as the Al/SiC composites werefound to increase linearly with the temperature. At xed expo-sure duration, the Al/SiC composites always exhibited higher

corrosion rates at 75 C than at 50 C.Fig. 8 shows typical SEM micrographs of the corroded sur-

face for both pure Al matrix and Al/15 vol.% SiC (3 lm)composites after exposure in 3.5 wt.% NaCl solution for96 h at room temperature. It is clear that severe damage isfound on the surface of the pure Al matrix. Large pits were

osion behavior of Al/SiC metal matrix composites, Ain Shams Eng J

Figure 5 Variation in the corrosion rate with the exposure

duration for Al/SiC composites after static immersion in 3.5 wt.%

NaCl aqueous solution in laboratory room temperature; (a) Al/SiC

(11 lm), (b) Al/SiC (6 lm) and (c) Al/SiC (3 lm).

Microstructural and corrosion behavior of Al/SiC metal matrix composites 5

Please cite this article in press as: Zakaria HM, Microstructural and corr(2014), http://dx.doi.org/10.1016/j.asej.2014.03.003visible on the surface, indicating susceptibility of the materialtoward pitting corrosion in NaCl medium. In contrast, theAl/15 vol.% SiC (3 lm) composites corroded surfaces exhib-ited less damage then the pure Al matrix. It is clear fromFig. 8 that the surface of the unreinforced Al matrix underwentsevere degradation, especially along the grain boundaries.

These grain boundaries provide preferential corrosion initia-tion sites because of the discontinuity in the surface due tothe change in structure. In the case of Al/SiC composites, inaddition to grain boundary attack, pitting occurred at the sites

where the SiC particulates agglomerate. Fig. 9 shows highmagnication SEM micrograph of the corroded surface forAl/15 vol.% SiC (3 lm) composites after exposure in3.5 wt.% NaCl solution for 24 h at room temperature. It isclear that the pitting occurred at the sites where the SiC partic-ulates agglomerate.

The better corrosion resistance at room temperature ofAl/SiC composites compared with the pure Al matix may attri-bute to the fact that SiC particulates are being ceramics and

Figure 6 Variation in the corrosion rates with the temperature

for Al/SiC composites after immersion for 24 h in 3.5 wt.% NaCl

solution; (a) Al/5 vol.% SiC and (b) Al/15 vol.% SiC.

osion behavior of Al/SiC metal matrix composites, Ain Shams Eng J

6 H.M. Zakariaremain inert in the NaCl solution. They are hardly affected bythe NaCl aqueous medium. Although the corrosion rate of the

Al/SiC composites is lesser than that of the Al matrix metal,the Al/SiC composites showed also the formation of pits onthe surface. However, the number of pits gets decreased withthe addition of SiC particulates compared to that in the pure

Al matrix metal. It has been found that increasing the volumefraction and/or reducing the size of the SiC particulatesreduces the number of pits. The SiC particulates resist the

severity of the medium attack to a certain extent. Moreover,there is an evidence for the presence of grain boundarycorrosion and pitting corrosion in the Al/SiC composites (see

Fig. 9).It has been reported that the corrosion resistance of Al-

MMCs depends on many factors such as processing technique;type and characteristics of the matrix alloy (cast or wrought

and heat treating condition); type, size, shape and amount ofthe reinforcement; the type of corrosive media, and the envi-ronmental factors [1922]. The fabrication and processing of

Figure 7 Variation in the corrosion rates with the temperature

for Al/SiC composites after immersion for 120 h in 3.5 wt.% NaCl

solution; (a) Al/5 vol.% SiC and (b) Al/15 vol.% SiC.

Figure 8 SEM micrographs of pure Al matrix (a) and Al/

15 vol.% SiC (3 lm) composites and (b) after exposure in3.5 wt.% NaCl solution for 96 h at room temperature.

Figure 9 SEM micrograph of the corroded surface for Al/

15 vol.% SiC (3 lm) composites after exposure in 3.5 wt.% NaClsolution for 24 h at room temperature.

Please cite this article in press as: Zakaria HM, Microstructural and corrosion behavior of Al/SiC metal matrix composites, Ain Shams Eng J(2014), http://dx.doi.org/10.1016/j.asej.2014.03.003

MMCs sometimes lead to the formation of an interphase

rosion rate increases more rapidly at higher temperatures thanlower temperatures for both Al matrix and Al/SiC composites.

4. Conclusions

Based on the results presented, the following conclusions canbe drawn:

1. At room temperature, the Al/SiC composites exhibitedbetter corrosion resistance than the pure Al matrix in

3.5 wt.% NaCl aqueous solution.2. Increasing the volume fraction of the SiC particulates

increased the corrosion resistance of the Al/SiC com-

posites. Moreover, reducing the SiC particles sizeenhanced signicantly the corrosion resistance of theSiC composites. Increasing the duration exposurereduces the corrosion rate.

Figure 10 SEM micrographs of pure Al matrix (a) and Al/

15 vol.% SiC (3 lm) composites (b) after exposure in 3.5 wt.%NaCl solution for 96 days at 50 C.

Microstructural and corrosion behavior of Al/SiC metal matrix composites 7between the matrix and the reinforcements, which can alsoinuence corrosion. For example, it has been reported that

Al4C3 reaction product can be formed at the SiC particles/ma-trix interface during fabrication of the Al/SiC MMCs using li-quid metallurgy techniques. It has been reported [23] that if the

melt temperature of Al/SiC composite materials rises above710 C (in low Si-containing alloys), Al4C3 forms which can re-sult in a severe loss of corrosion resistance. Some investigators

have concluded that the increased corrosion rate is due to theformation of Al4C3 at the reinforcement/matrix interface[23,24]. It has been reported that the reinforcing phase, SiC,can also affect the corrosion behavior of MMCs, by modifying

the distribution of intermetallic phases in Al alloys [23].Increasing the number of intermetallic precipitates results inincreased corrosion rates. For example, Kiourtsidis and Skoli-

anos [12] reported that no detrimental galvanic corrosion wasobserved between the SiC reinforcing phase and the 2024 Almatrix. Instead, corrosion was described by two anodic reac-

tions: corrosion of the a-phase adjacent to Al2Cu interdendrit-ic regions, and pitting of the dendrite cores.

In the present investigation, the Al/SiC MMCs were fabri-

cated using PM route, which is a solid-state fabrication pro-cess. Accordingly, the formation of Al4C3 at the SiCparticles/matrix interface during fabrication is not likely toform. Moreover, the matrix used in the present investigation

is a pure Al that means no intermetallic phases in Al matrixmay exists. It is believed that the SiC particulates play animportant role as a physical barrier. A particle acts as a rela-

tively barrier to the initiation and development of corrosionpits. Rodriguez [25] reported that the interface between thebase matrix and the reinforcement is the weakest part of par-

ticulate composites. Hence the nature of the interfacial bond,whether weak or strong, is critical in the corrosion process.It is believed that the improvement of the corrosion resistance

of the Al matrix, observed in the present investigation, maydue to the strong interfacial bonding between the aluminumand SiC part. The increase in corrosion resistance with increas-ing the volume fraction of SiC particulates also support this.

Fig. 10 shows typical SEM micrographs of the corrodedsurface for both pure Al matrix and Al/15 vol.% SiC (3 lm)composites after exposure in 3.5 wt.% NaCl solution for

96 days at 50 C. It can be concluded from these gures thatthe amount of surface degradation increased with temperature.It clearly seen that increasing the NaCl solution temperature

increases the severity damage of both pure Al matrix as wellas the Al/SiC composites. However, the Al/SiC compositesexhibited more severe damage surfaces than the pure Almatrix. Generally, it has been observed that the severity of

damage in the Al/SiC composites and the pure Al matrixincreases with the increase in exposure duration and the NaClsolution temperature. The number per unit area and the size of

pits are seen to increase with the temperature of the NaCl solu-tion. It is important to mention that there is hardly any infor-mation available in the literature about the corrosion behavior

of the composites in 3.5 wt.% NaCl media at elevatedtemperatures.

The effect of temperature on the corrosion rate of Al/SiC

MMCs depends on the energy activation of corrosion. Thecorrosion rate increases with activation energy [26]. Accordingto the results obtained from the current investigation, the cor-Please cite this article in press as: Zakaria HM, Microstructural and corr(2014), http://dx.doi.org/10.1016/j.asej.2014.03.003osion behavior of Al/SiC metal matrix composites, Ain Shams Eng J

3. At elevated temperature, the Al/SiC composites exhib-

ited lower corrosion resistance than the pure Al matrixin 3.5 wt.% NaCl aqueous solution. However, increas-ing the volume fraction and/or the SiC particles size

reduce(s) the corrosion rates of the Al/SiC composites.The corrosion rates of the pure Al as well as theAl/SiC composites were found to increase linearly withthe temperature.

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Hossam El-din Mohamed Zakaria is an Asso-

ciate Professor, Department of Mechanical

Engineering, Shoubra Faculty of Engineering,

Benha University, Egypt. His eld of research

and supervision of Masters and PhDs are

Composites, friction stir processing, friction

stir welding, wear and corrosion.metal matrix composites. J Mater Sci 1998;33:563742.tribological behavior of particulate reinforced aluminum metal

matrix composites a review. J Miner Mater Charact Eng 2011;osion behavior of Al/SiC metal matrix composites, Ain Shams Eng J

Microstructural and corrosion behavior of Al/SiC metal matrix composites1 Introduction2 Experimental procedures3 Results and discussion3.1 Microstructural characteristics of Al/SiC composites3.2 Density of the Al/SiC composites3.3 Corrosion behavior of the Al/SiC composites

4 ConclusionsReferences