Ž .Materials and Design 23 2002 173�180

Degradation of aluminum metal matrix composites in saltwater and its control

Zaki Ahmad�, B.J. Abdul AleemDepartment of Mechanical Engineering, King Fahd Uni�ersity of Petroleum & Minerals, Dhahran- 31261, Saudi Arabia

Received 24 March 2001; accepted 12 May 2001

Abstract

Alloy Al 6013-20 Si in tempers O, F and T4 showed good resistance to corrosion in salt spray tests. The corrosion rate of theŽ .alloy decreased with increased exposure time due to the formation of boehmite AlO �OH . Studies conducted in 3.5 wt.% NaCl

solution containing suspended particles of polystyrene showed a linear increase in the erosion�corrosion rate with velocity. Thelocalized attack was concentrated mainly on the Al 6013�SiC interface. A high dislocation density was observed at the Al6013�SiC interface, which interfered with the formation of a homogeneous protective film of boehmite on the alloy surface.Addition of cerium chloride drastically suppressed the rate of corrosion. Sodium molybdate offered a lesser degree of protectioncompared to cerium chloride. � 2002 Elsevier Science Ltd. All rights reserved.

Keywords: Composites; Corrosion; Inhibitors

1. Introduction

Attention in the Gulf region has shifted considerablyfrom conventional to newly emerging engineering ma-terial in recent years. Aluminum alloys, such as Al5052, Al 5054, Al 6061 and Al 6063, remained the focusof attention in the late 1970s and early 1980s forseawater applications because of their good combina-tion of mechanical strength, formability, corrosion re-

� �sistance and cost advantages 1�4 . One major reasonthat led to intensified research activity was the un-precedented growth of desalination plants in SaudiArabia, which has the highest population density ofdesalination plants in the world. In recent years, high-performance polymers, advanced ceramics and metalmatrix composites have been the focus of attention

� Corresponding author. Fax: �96-63-860-2949.Ž .E-mail addresses: ahmadz@kfupm.edu.sa Z. Ahmad ,

Ž .abeleem@kfupm.edu.sa B.J. Abdul Aleem .

because of their unique mechanical and physicalproperties and promising application potential inaerospace, defense, construction and automotive indus-tries. Alloys Al 6061 and Al 6013 reinforced withsilicon carbide belong to a new generation of metalmatrix composites which have been developed forweight-critical functions in aerospace, defense, and

� �structural applications 5,6 . Alloy Al 6013�SiC wasdeveloped to obtain more attractive mechanicalproperties than its competitor Al 6016�SiC. Alloy Al6013 reinforced with 20 vol.% SiC showed a higherstrength�weight ratio than alloy 6061 reinforced with

� �SiC 1 . Research efforts have largely been concen-trated on the physical and mechanical characteristicsand thermo-mechanical treatment of the alloy, withoutany recourse to its corrosion behavior, which is a vitalparameter in assessing its application potential as a

� �structural material 5,7 . This paper summarizes theresults of research studies on the degradation of Al6013�SiC in salt water and its control using selectedinhibitors.

0261-3069�02�$ - see front matter � 2002 Elsevier Science Ltd. All rights reserved.Ž .PII: S 0 2 6 1 - 3 0 6 9 0 1 0 0 0 6 6 - 8

( )Z. Ahmad, B.J. Abdul Aleem � Materials and Design 23 2002 173�180174

2. Experiment

2.1. Material

ŽThe alloy Al 6013�20SiC in three tempers: F as. Žfabricated , T4 solution treated at 525�C for 30 min,

. Žwater-quenched and naturally aged and O annealed.for 3 h were used. The compositions of alloys Al 6013

and Al 6061 are shown in Table 1.

2.2. Sample preparation

The samples were ground with 320-, 400- and 600-gritsilicon carbide paper and polished with 6-�m diamondpaste on a struers cloth. They were rinsed in demineral-ized H O and ethyl alcohol, followed by polishing with20.04-�m silica. The specimens were placed in a desicca-tor before use.

2.3. Test media

All tests were conducted in 3.5 wt.% NaCl solution.

2.4. Experimental procedure

Weight loss studies were conducted in accordance� �with ASTM standard G31-72 8 . All electrochemical

measurements were made in accordance with ASTMŽstandard G5-87 standard reference method for making

. � �potentiostatic and potentiodynamic measurements 9Žand G61-78 standard reference method for cyclic po-

� �tentiodynamic measurement 10 .All polarization measurements were made with ref-

erence to a standard Ag�AgCl reference electrode.The corrosion potential was measured by switching theinstrument to the corrosion potential mode. Polariza-tion was commenced at a scan rate of 10 mV s�1 andcontinued until the pitting potential was reached.

( )2.5. Salt spray fog testing

A Singleton salt-spray cabinet was used to investigateŽ .the corrosion resistance of Al 6013�20 SiC P . It is

comprised of a basic chamber, bubble tank, twin opticŽ .fog assembly and atomizers Fig. 1 . Salt spray was

introduced into the cabinet from the optic fog assem-bly, in which a constant level of salt solution wasmaintained. The compressed air supply was maintainedat 130 kPa m�2 and the pH at 7.4 by adjustment withHCl solution. All experiments were conducted at 25�1�C.

2.6. Recirculation loop studies

A high-density polyvinyl chloride loop was con-structed to study the effect of suspended particles onthe erosion�corrosion of Al 6013. A schematic of theloop is shown in Fig. 2. It consists of entry and exitvalves, a manometer, a water pump, a flow meter andseveral specimen holders to accommodate flat speci-mens. Each specimen holder contained up to fourspecimens, which were housed in an outside container.Velocity was varied by varying the diameter of thespecimen holder.

Micro-analytical studies were conducted under alow-vacuum scanning electron microscope fitted with

Ž .an energy-dispersive spectroscopy EDS system con-taining a QUANT MAP software package. For transmis-

Ž .sion electron microscopy TEM studies, samples werethinned by a dimpler prior to final thinning in an ionmill.

Ž .Measurements of ultimate tensile strength UTSŽ .and yield strength YS were carried out in a tensile

testing machine. Specimens were exposed in a 3.5 wt.%sodium chloride solution prior to testing.

3. Results and discussion

3.1. En�ironmental studies

Results of corrosion rate studies on the three tem-Ž .pers of alloy Al 6013�20SiC F, O and T4 in a salt

spray chamber are described in Fig. 3. Temper O ofalloy 6013 showed the highest rate of corrosion in the

Ž �1 .salt spray chamber 10.23 ml year , followed by tem-Ž �1 . Ž �1 .pers F 8.42 ml year and T4 7.12 ml year in

decreasing order after an exposure of 200 h. The rateof corrosion, however, showed a steady decline withincreasing exposure time. This was exemplified by adecrease in the corrosion rate from 10.23 to 4.27 ml

Table 1Composition of the alloys

Ž .Alloy Composition wt.% Impurities

Si Fe Cu Mn Mg Cr Zn Ti Each Total

Al 6013 0.6�1.0 0.5 0.6�1.0 0.2�0.8 0.8�1.2 0.10 0.25 0.1 0.05 0.55Al 6061 0.4�0.5 0.7 0.15�0.4 0.15 0.8�1.2 0.04�0.35 0.25 0.15 0.05 0.15

( )Z. Ahmad, B.J. Abdul Aleem � Materials and Design 23 2002 173�180 175

Fig. 1. Singleton salt-spray fog corrosion-test cabinet.

Fig. 2. Modified PVC recirculating loop for velocity studies.

( )Z. Ahmad, B.J. Abdul Aleem � Materials and Design 23 2002 173�180176

Ž .Fig. 3. Variation of corrosion rate of Al 6013�20SiC P with time inŽ .salt spray fog testing.

year�1 for temper O, 7.12 to 4.27 ml year�1 for F and7.12 to 4.27 ml year�1 for T4 after 1200 h of exposurein the chamber. A decrease in the corrosion rate withincreased exposure period is generally associated withthe build-up of protective layers of aluminum hydrox-

Ž . Ž .ide Al OH , bayerite Al O �3H O and boehmite3 2 3 2Ž . � �Al O �H O 11 . In the investigations conducted,2 3 2the presence of boehmite was established by Fourier-transform infrared spectroscopy. The drying up of

Ž .Al OH gel and its transformation to AlO � OH3Ž .boehmite is shown in Fig. 4. It may be recalled thatgelatinous boehmite can be formed, but it would be

Ž .converted to Al OH by reaction of the solid with the3mother liquor:

�� Ž . Ž .AlO �OH�H O�OH �Al OH �Al OH4 32

�OH�

and

� �Ž . Ž . Ž .Al OH �Al OH �2Al OH �OH3 4 3

Increased exposure time allows a steady state to be

Ž .Fig. 4. Formation of crystals of boehmite by drying up of Al OH .3

Fig. 5. Photomicrograph showing a shallow hemispherical pit on theŽ .surface of Al 6013�20SiC T4 .

reached, as it allows an equilibrium in the kinetics ofdissolution of the outer porous layer and build-up of aninner compact layer to be reached after an interval oftime. Pitting is the predominant form of localized cor-rosion. Fig. 5 shows shallow hemispherical pits on the

Ž .surface of Al 6013�20SiC T4 after exposure to thesalt spray test.

3.2. Erosion�corrosion

A range of aluminum alloys and aluminum metalmatrix composites have been tested in erosive�corro-

� �sive slurry environments 12�16 . Two mechanisms,namely matrix cutting and reinforcement fracture, have

� �been identified 17 . The reduced erosion�corrosionresistance of the matrix has been attributed to the lackof ductility and matrix constraint caused by reinforcing

� �particles in MMCs 18 . It was observed that the com-posites corroded faster than the monolithic aluminumalloys, even though the attack was mainly confined tothe interface, resulting in the creation of crevices orpits. The corrosion product which accumulated at theinterface acted as the cathode and increased the cath-ode�anode area ratio, which caused an increase in the

Žrate of corrosion. Polystyrene particles 100 �m, 2.vol.% were used as suspended particles in a solution

Ž .of 3.5 wt.% NaCl solution de-aerated . All experi-ments were conducted in the recirculation loop de-scribed earlier. The results of the investigation arepresented in Table 2.

The erosion�corrosion rate of Al 6013�20SiC variedlinearly with velocity in the presence of SiC particles. Itwas further observed that temper T4 of the alloy showedthe best resistance to erosion�corrosion. A slightly

Ž .lower resistance was shown by temper O annealedŽ .and F as-fabricated . Upon increasing the temperature

from 30 to 50, 75 and 90�C, the erosion�corrosion rate

( )Z. Ahmad, B.J. Abdul Aleem � Materials and Design 23 2002 173�180 177

Table 2Ž .Erosion�corrosion behavior of Al 6013�20 SiC P in a solution of 3.5

wt.% NaCl �2 vol.% polystyrene at 30�C

�1Ž .Velocity Corrosion rate ml year�1Ž .m s O F T4

1.0 11.8 9.9 9.61.9 12.6 10.4 10.12.7 12.9 10.8 10.83.8 13.6 11.3 11.4

increased, as shown in Tables 2 and 3. The rate oferosion�corrosion for temper T4 increased from 9.6 to9.9, 11.7 and 12.3 ml year�1 on increasing the tempera-ture from 30 to 50, 75 and 90�C, respectively, at avelocity of 1 m s�1. The magnitude of the erosion�cor-rosion rate also increased on increasing the velocityfrom 1.0 to 3.8 m s�1.

The results presented above showed that the ero-sion�corrosion rate of alloy Al 6013�20SiC increasedwith increasing velocity and temperature. The relativelybetter resistance of the T4 temper is due to less cluster-ing of SiC particles and a more homogeneous distribu-

� �tion of secondary phases 19 . The localized corrosionattack on aluminum alloy is confined to the Al6013�SiC matrix interface and the predominant attackis pitting. Fig. 6 shows the localized corrosion attack tobe confined predominantly to the Al 6013�SiC inter-face in 3.5 wt.% NaCl at a velocity of 1 m s�1. A largenumber of secondary phases were observed at theAl�SiC interface. EDS studies at the interface showed

Ž . Ž .a high concentration of copper 3.55% , Fe 1.77% ,Ž . Ž .Mg 1.71% and some Cl 0.32% . Larger numbers of

secondary-phase particles are present in compositesthan in the matrix alloy surface, and the Al�SiC inter-

� �face is the preferred site for these particles 20 .High dislocation densities have been observed at the

Al�SiC matrix interface as a result of differential ther-

mal contraction between SiC particles and the Al ma-trix. These dislocations are associated with secondary-phase particles. A high dislocation density at the Al6013�SiC surface, as revealed by transmission electron

Ž .microscopy TEM studies, makes the surface morereactive and introduces inhomogeneity into the protec-

Ž . Ž .tive film of Al O �3H O and Al OH �3H O Fig. 7 .2 3 2 3 2The formation of a coherent film is rendered moredifficult by the protrusion of particles, such as SiC, onthe alloy surface of Al 6013�20SiC. The factors men-tioned above add synergistically to the erosion�corro-sion caused by the polystyrene particles.

The effect of corrosion on the mechanical properties� �has previously been investigated 21 . All samples suf-

fered a loss in the tensile strength of between 12 and15%. The loss in strength of Al 6013�20SiC did notexceed 15%, which is an improvement over alloy 2024,which showed a greater loss. Table 4 shows the effectof corrosion on the ultimate tensile strength of thealloys.

3.3. Corrosion inhibition

Ž .It has been shown that Al 6013�20SiC P -T4 is sensi-tive to localized corrosion in sodium chloride solutionsflowing at higher velocities. The addition of K Cr O2 2 7�NaHCO remarkably suppressed the localized corro-3sion of the alloy. Because of the toxic effects associatedwith the above inhibitor, further investigations wereconducted with cerium chloride and sodium molybdate.The effect of cerium chloride and sodium molybdateon the corrosion inhibition of Al 6013�20SiC in 3.5wt.% NaCl is shown in Fig. 8. Sodium molybdate provedrelatively less effective than cerium chloride. The effectof pre-exposure time prior to polarization in 3.5 wt.%NaCl containing cerium chloride and sodium molyb-date is shown in Fig. 9. A remarkable decrease in the

Table 3Ž .Erosion�corrosion behavior of Al 6013�20SiC P in a 3.5 wt.% NaCl solution in the presence of 2 vol.% polystyrene at different temperatures

�1Ž .Temperature Velocity Erosion corrosion rate ml year�1Ž . Ž .�C m s O F T4

50 1.0 12.1 10.3 9.91.9 13.6 11.2 10.12.7 14.2 12.1 11.43.8 14.9 13.6 12.3

75 1.0 14.8 11.9 11.71.9 16.3 15.5 14.12.7 19.5 17.2 14.83.8 20.1 19.6 15.9

90 1.0 14.1 13.3 12.31.9 17.1 15.1 13.62.7 20.0 17.8 16.33.8 21.0 19.7 17.6

( )Z. Ahmad, B.J. Abdul Aleem � Materials and Design 23 2002 173�180178

Fig. 6. Corrosion attack at the Al 6013�SiC matrix interface.

corrosion rate was observed after 72 h. It was observedby the authors that cerous ions act as efficient cathodic

� � Ž .inhibitors 22 . When the protective films of Al OH ,3Ž . Ž .bayerite Al O �3H O and boehmite Al O �H O2 3 2 2 3 2

are dissolved, they are replaced by cerium oxide�hy-Ž .droxide film. Under conditions of high pH, Al OH 3

dissolves, whereas Ce O remains insoluble. The2 3growth of cerium oxide crystals is shown in Fig. 10. Ithas been shown by the above studies that CeCl effec-3tively inhibits the localized corrosion of Al 6013�20SiCunder dynamic conditions in 3.5 wt.% NaCl. The abovefindings have addressed the concern of designers infinding a solution to the decreased sensitivity of Al

Ž .6013�20SiC P to increasing velocity.

Fig. 7. Transmission electron micrograph showing dislocations at theAl 6013�SiC interface.

Table 4Effect of corrosion on the ultimate tensile strength of Al 6013�20

Ž .SiC P in 3.5 wt.% NaCl solution

Ž .Type UTS MPa

Before Afterexposure exposure

Ž .Sheet O 290 210T4 531 436T6 559 480

4. Conclusion

On the basis of the investigations described above,the following conclusions are drawn.

� The corrosion rate of Al 6013�20SiC in a salt sprayenvironment decreased with increasing exposure

Ž �1 .period. The lowest rate of corrosion 2.55 ml yearwas exhibited by temper T4, followed by temper FŽ �1 . Ž �1 .3.68 ml year , and temper O 4.27 ml year .

� The corrosion rate of Al 6013�20SiC in a salt spraychamber decreased slightly on increasing the tem-perature from 50 to 100�C because of the formationof boehmite Al O .H O Temper T4 of Al 60132 3 2showed a good resistance to corrosion in salt sprayenvironment.

Ž .� Surface treatment of Al 6013�20SiC P by ceriumchloride drastically reduced the corrosion rate of O,F, and T4 tempers of the alloy in the temperaturerange 30�80�C. Sodium molybdate offered lowerprotection than cerium chloride.

Fig. 8. Effect of cerium chloride and sodium molybdate on theŽ .corrosion of Al 6013�20SiC P .

( )Z. Ahmad, B.J. Abdul Aleem � Materials and Design 23 2002 173�180 179

Ž .Fig. 9. Variation of the corrosion rate of Al 6013�20SiC P -T in open air conditions and de-aerated 3.5 wt.% NaCl.4

Ž .Fig. 10. Particles of CeCl observed on exposing Al 6013�SiC P to33.5 wt.% NaCl containing 1000 ppm CeCl at 100�C.3

Acknowledgements

The authors appreciate the support, help and en-couragement given by KFUPM.

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