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Corrosion characterization of Al 6061/Zircon metal matrix composites in acid chloride mediums by open circuit potential studies.

Introduction

Metal matrix composites (MMCs) essentially consist of a non-metallic reinforcement incorporated into a metallic matrix. These have higher attainable strength and stiffness properties as compared to conventional alloys of the metal [1]. Thus, reinforcing Al 6061 alloy with Zircon particles resulted in MMCs with high specific strength for numerous weight sensitive applications [2]. Al-based MMCs are used widely in the aerospace and automobile industry, as well as in architectural applications. Particles reinforced MMCs has been the most popular over the last three decades. Among the MMCs, ceramic reinforced Al MMCs have most popular families being represented by aluminium reinforced with ceramic particulate. Although the incorporation of the second phase into a matrix material can enhance the physical and mechanical properties of the base metal, it could also significantly change the corrosion behavior [3]. MMCs with particulate reinforcement can result in an increase in strength at much lower additional costs than those of continuous reinforcements [4].

Materials and experimental procedure

Material selection

The matrix selected is Al 6061, which exhibits excellent casting properties and reasonable strength. This alloy is best suited for mass production of lightweight metal castings. The chemical composition of the Al 6061 alloy is given in table 1.

Zircon (ZrSi[O.sub.4]) is used for reinforcement in the form of particulates. Zircon is a very hard material, its hardness is 7.5 on Mohr's scale and specific gravity is 4.1. The principal structural unit of zircon is a chain of alternating edge-sharing Si[O.sub.4] tetrahedral and Zr[O.sub.8] triangular decahedra. The XRD spectra of zircon is given in figure: 1. The composition of zircon is given in table 2.

[FIGURE 1 OMITTED]

Composite preparation

The liquid-metallurgy Vortex method was employed to prepare the composites. The reinforcement material (Zircon) used is of size 40-50[micro]m. The SEM micrograph of the zircon is given in figure: 2. The composites containing 3, 5 and 7 weight percentage of Zircon were prepared. The matrix alloy was also cast under identical processing condition for comparison. Zircon is added in to the molten Al 6061 alloy melt by creating a vortex using a stirrer. The composite melt was thoroughly stirred and subsequently degassed and poured into preheated split-type permanent moulds [5]. Samples for microscopic examinations were prepared by standard metallographic procedures, etched with Keller's agent and examined under optical microscope.

[FIGURE 2 OMITTED]

Specimen preparation

Rectangular specimen of 2cm length and 1cm breath and 1mm thickness was prepared by adopting standard metallographic procedure using an abrasive cutting wheel [6]. They were polished and degreased in acetone and dried.

Open circuit potential test

The open circuit potential (also referred to as the equilibrium potential, the rest potential, or the corrosion potential) is the potential at which there is no current. These experiments are based on the measurement of the open circuit potential (OCP) or these are potentiometric experiments. Although such measurements are very simple, they have many important applications. The equipment contains a multimeter where resistance, alternate current and direct current can be measured with two outlet wires given and it runs with a cell of 9 volt capacity. An aluminium wire is used to hold the specimen. One sq.cm area of the specimen is exposed to corrosive medium. Specimen is connected to the wire suitably covered by Teflon tape so that aluminium is not exposed to electrolyte medium. The specimen is made anode and the cathode will be the reference electrode that is standard calomel electrode. Before testing each specimen was cleaned in acetone for five minutes and air dried. Both electrodes are connected to multimeter and the same is switched on to measure the DC voltage developed after dipping them in different concentrations of hydrochloric acid, which is used as electrolyte solution. The voltage developed and displayed by the multimeter is noted for every hour for a period of 24 hours. The procedure is repeated for all the four specimens. Then the graphs are plotted by taking exposure time versus the potential developed.

Results and Discussion

Microscopy

It is observed from the microstructures of unreinforced matrix alloy and composites that, there is invariably a uniform particulate distribution and that the average particulate size was 40-50[micro]m no matter what the weight percentage of reinforcement was. This is the same size as that of the original zircon particulates, indicating that the zircon particulates were not noticeably damaged or affected during preparation of the composite.

Effect of test duration

The potential measurements as a function of exposure time in the OCP test are shown in the figures 3, 4 and 5. In all the cases it is observed that the potential decreases in the beginning with increase in test duration and remains constant towards the end due to passivation. It is clear from the graph that the resistance of the composite to corrosion increases as the exposure time increases. The phenomenon of gradually decreasing potential indicates the possible passivation of the matrix alloy. Visual inspection of the specimens after the tests revealed that the presence of a black film covering the surface, which might have retarded the corrosion. De Salazar et al, who studied alumina reinforced Al-based MMC, observed that a protective black film is formed on the surface consisting of hydrogen hydroxyl chloride, which apparently retards the corrosion [7]. Castle et al, who studied SiC particulate-reinforced Al-based MMCs, believe that the black film formed on the surface consists of an aluminium hydroxide compound which protects the bulk material from further corrosion in the acid medium [8].

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

Effect of zircon content

From figures 3, 4 and 5 it can be clearly observed that, for both alloy and composite potential decreases with increase in zircon content. In case of base alloy the strength of the media used induces crack formation on the surface, which eventually leads to the formation of pits, there by causing the loss of material. The presence of cracks and pits on the base alloy surface was observed clearly on visual inspection of the specimens after the test. Since there is no reinforcement added in any form, the base alloy fails to provide any sort of resistance to the corrosion medium. Hence, the decrease in potential in case of unreinforced alloy is higher when compared to composites.

The SEM micrographs of the typical corroded surfaces of the 0% and 7% zircon reinforced composite specimens are presented in figures 6 and 7, which show the corroded surface morphology of the tested specimens. It was observed that in zircon reinforced aluminium composites pitting depends on the distribution of zircon.

[FIGURE 6 OMITTED]

[FIGURE 7 OMITTED]

Zircon being the ceramic material remains inert and is hardly affected by corrosion medium during the test and is not expected to affect the corrosion mechanism of the composites. The results indicates that there is an improvement in corrosion resistance as the percentage of zircon particulates increased in the composites which shows that zircon particulates directly or indirectly influence the corrosion property of the composites. Zircon particulates act as a physical barrier to the initiation and development of corrosion pits and also modifies the micro structure of the matrix material and hence reduces the potential.

The corrosive attack was extensive with deep pits on the surface of the unreiforced matrix alloy extending and connecting in such a way so as to cause cracking as shown. Cracks can be seen originating from the deep pits and traversing on to the surface. However no definite crack path can be clearly visualized. These connected cracks cause progressive removal of structure on the surface commonly called as flaking. SEMs of the flakes that were formed from the corroded sample and as well as the flakes still remaining on the sample were taken.

From the graphs it can be clearly seen that the ceramic reinforcement particles acts as insulator and remain inert in the acidic medium during the test. Hence the potential decreases with increase in zircon content in MMCs, which may decrease the area of exposure of the alloy with increase in the reinforcement. Less exposure of the MMCs area to aggressive acid environments in corrosion testing led to lesser pitting as well as corrosion than that of the matrix alloy. The pits on the matrix alloy were more when compared with those of MMCs. This may be due to the exposure of less matrix alloy surface in MMCs than matrix alloy, by the addition of reinforcement.

Conclusion

Corrosion behaviors of Al 6061/Zircon MMCs can be tested by OCP method. The zircon content in Al 6061 alloys plays a significant role in the corrosion resistance of the material. Increase in the percentage of zircon will be advantageous to reduce the density and increase the strength of the alloy. However, there is a significant reduction in the potential and corrosion rate. Al 6061 MMC reinforced with zircon of weight percentage from 0-7% could be successfully produced by liquid melt metallurgy technique. The corrosion rate of both the alloy and composite decreased with increase in exposure time in acid chloride solution. The potential developed for the composites was lesser than that of the corresponding matrix alloy in acid chloride solution. The extent of corrosion damage was decreased with increase in reinforcement from 0-7% in MMCs. The use of MMCs in bearing applications in acidic environment is more suitable than matrix alloy.

Acknowledgement

The authors express their humble gratitude to the management and Dr. N.C. Prasannakumar, Principal, EWIT, Bangalore for providing facilities and Dr. T.R. Shashishekar, Asst. Prof., Department of Chemistry, EWIT, Bangalore for cooperation.

[1] Suma A Rao, Padmalatha, Nayak J. and Shetty A.N., 2006, "Glycyl Glycine as corrosion inhibitor for aluminium silicon carbide composites in hydrochloric acid", Transactions of the SAEST, 41, pp.01-04.

[2] Aylor D.M., 1987, "Corrosion of Metal Matrix Composites", Metals Hand Book, Ninth Edition, ASM 859.

[3] Seah K.H.W., Krishna M., Vijayalakshmi V.T. and Uchil J., 2002, "Effects of temperature and reinforcement content on corrosion characteristics of LM 13/ albite composites", Corros. Sci, 44, pp.761-772.

[4] Terry, B. and Jones, G., 1990, "Metal Matrix Composites: Current Developments and Future Trends in Industrial Research and Applications", Elsevier Advanced Technology/Elsevier Science Publishers Ltd, Amsterdam.

[5] Sharma S.C., Satish B.M., Girish B.M., Rathnakar Kamat and Asanuma H.,1998, Tribology International, 31 (4), pp.183-188.

[6] Pruthviraj R.D., 2006, "Open circuit potential studies of Za-27/SiC metal matrix composites in acid chloride mediums", Transactions of the SAEST, 41, pp.94-96.

[7] De Salazar J.M.G., Urefia A., Manzanedo S. and Barrena M.I., 1996, "Corrosion behaviour f AA 6061 and AA 7005 reinforced with Al2O3 particulates in aerated 3.5% chloride solutions : Potentiodyanamic measurements and microstructure evaluation", Corros. Sci, 41, pp.529-545.

[8] Castle J.E., Sun L., Yan H., 1994, "The use of scanning auger microscopy to locate cathodic centers in SiCp/Al 6061 MMC and to determine the current density at which they operate", Corros. Sci, 36 (6), 1093-1110.

A. Abdul Jameel (1), H.P. Nagaswarupa (2) *, P.V. Krupakara (3) and K.C. Vijayamma (2) #

(1.) Dept. of Chem., Jamal Mohamed College, Tiruchirappalli-620020, Tamil Nadu, India. E-mail: jameelchem2001@yahoo.com

(2.) Dept. of Chem., East West Institute of Technology, Bangalore-560091, Karnataka, India.

* swarup_naga@yahoo.com, # kcv610@gmail.com

(3.) Dept. of Chemistry, Auden Technology and Management Academy, Bangalore--562112, Karnataka, India krupakarapv@yahoo.co.in
Table 1 : Composition of Al 6061

Element       Mg    Si     Fe     Cu     Zn      Cr
Weight (%)   1.0    0.6   0.5    0.25   0.2     0.15

Element       Mn    Ti     Pb     Sn     Ni      Al
Weight (%)   0.15   0.1   0.05   0.05   0.05   Balance

Table 2: Composition of Zircon

Silica     32.8
Zirconica  67.2
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Title Annotation:aluminum
Author:Jameel, A. Abdul; Nagaswarupa, H.P.; Krupakara, P.V.; Vijayamma, K.C.
Publication:International Journal of Applied Chemistry
Article Type:Report
Date:Jan 1, 2009
Words:1963
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