Electrochemical studies of Aluminium 6061 / Titanium Diboride Metal matrix Composites

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1 Electrochemical studies of Aluminium 6061 / Titanium Diboride Metal matrix Composites V.Latha 1, Dr.H.R.Radha 2, Dr.P.V.Krupakara 3, R.Lakshmi 1 1 Research Scholar, Bharathiyaar University, Coimbatore & Department of Chemistry, T John Institute of Technology, Bangalore 2 Professor and Head, Department of Chemistry, T John Institute of Technology, Bangalore 3 Vice Principal, Professor and Head, Department of Chemistry, Cambridge Institute of Technology-North Campus, Bangalore latha@tjohngroup.com, radha.sudhakar99@gmal.com, drkrupakara@gmail.com,lakshmir@tjohngroup.com Abstract- Metal matrix composites are engineering combination of two or more materials. They are the combinations of matrix and reinforcements where their properties can be tailored. Composites are material system consisting of a mixture of two or more constituents which is insoluble in each other and differing in form and material compositions and characteristics not available from any discrete material. They are heterogeneous materials which consisting of two or more phases which are in intimate contact with each other on a microscopic scale. They are also homogeneous material in the sense that any part of will have the same physical properties on a microscopic scale. They find applications in civil engineering, automobile, toys and sports. In this research paper an attempt is made to study the electro chemical behaviour of metal matrix composites consisting of Aluminium 6061 alloy as matrix material and Titanium diboride particulates as reinforcement. The composite materials are manufactured by liquid melt metallurgy technique using vortex method. The tests conducted were open circuit potential tests and potentiodynamic polarization tests in different concentrated solutions of hydrochloric acid. In all the test composite materials exhibited increased corrosion resistance irrespective of concentration of hydrochloric acid. Hence composite materials are more suitable than Matrix alloy. Keywords Diboride, Liquid melt metallurgy, vortex, open circuit, potentiodynamic I. INTRODUCTION Metal matrix composites (MMC) are important class of materials, with non-metallic reinforcements incorporated in metal matrices. Al alloy matrices, mainly 2024, 5052, 6061 and 7071 have been widely used as matrix materials to manufacture composites with various reinforcements. These materials have received increased attention due to their increases mechanical properties. Aluminium based metal matrix composites find potential applications in high-temperature environments, especially in automobile engine parts such as drive shafts, cylinders, pistons and brake rotors, and in aerospace applications. 1 Aluminium is the most popular matrix for the manufacture of metal matrix composites. Aluminium alloys which are quite attractive due to their improved properties like low density, good corrosion resistance, high thermal and electrical conductivity and high damping capacity. Aluminium alloy composites are becoming popular engineering matrix offering excellent combination of properties with respect to mechanical and thermal properties. They also show improved wear r e s i s t a n c e. Page No:398

2 Because of their excellent combination of properties, aluminium based composites are being used in varieties of applications in automobile, mining and mineral, aerospace, defence and other related sectors 2 H.R.Radha et al studied the static weight loss corrosion properties of Aluminium 6061 / quartz metal matrix composites in alkaline solutions and report that as the weight quartz particulates increases corrosion resistance also increases. 3 Gireesha C 4 et al studied the weight loss corrosion properties of Aluminium 6061 reinforced with red mud particulates in hydrochloric acid and found that as the reinforcement of red mud particulates increases corrosion resistance increases. Abdul Jameel 5 et al characterized the corrosion properties of Aluminium 6061 composites containing red mud particulates as reinforcement in various corrosion mediums and report that composite materials exhibit improved corrosion resistance. From the works by others mentioned above it is clear that research work with respect to titanium diboride reinforced Aluminium 6061 composites is very scares. II. MATERIAL SELECTION In the present study the matrix alloy used is Al6061. The chemical compositions of the Al6061 alloy are given in Table I. Table I: Composition of Al6061 Mg Si Fe Cu Ti Pb Zn Mn Sn Ni Al Bal The reinforcement used is TiB 2 particulates. It is available commercially micro meters size particulates of TiB 2 are used in this study. Titanium diboride (TiB2) is well known as a ceramic material with relatively high strength and durability as characterized by the relatively high values of its melting point, hardness, strength to density ratio, and wear resistance The corrosion mediums used to characterize the composites are 0.25M, 0.5M and 1M solutions of hydrochloric acid. III. COMPOSITE PREPARATION By using vortex method composites were prepared through liquid melt process used by Krupakara. 6 Since this method is suitable for all foundries hence many researchers used this procedure. The matrix material Aluminium6061 is heated to its melting temperature and a vortex is created by using an impeller made of stainless steel stirrer coated with aluminite (to prevent migration of ferrous ions from the stirrer material to the molten aluminium 6061 alloy). The stirrer was rotated at a speed of 450 rpm to create the necessary vortex. TiB 2 particulates of size 50-80micro meters preheated to 400 C in a muffle furnace and added in to the vortex of liquid melt at a rate of 100 g/m. The composite was melt thoroughly, stirred and immediately degasification was carried out by the addition of degasifying tablets made up of hexa chloro ethane to the melt. Castings were taken in heated permanent moulds made up of cast iron in the form of cylindrical rods with dimensions 30mmdiameter and length 150mm. Aluminium 6061 containing 2,4 and 6 weight percentages were casted in this way. The matrix alloy was casted under identical conditions for comparison. The tests were conducted in 0.025, 0.05 and 0.1 molar solutions of hydrochloric acid. Page No:399

3 IV. SPECIMEN PREPARATION For open circuit potential and potentiodynamic polarization tests specimens of dimension 20 mm x 10 mm x 1mm size were machined from the castings. Dimensions of all specimens were noted down using vernier gauge. V. CORROSION TEST For open circuit potential test the specimen and a calomel electrode were connected to a multi meter then immersed in corrodent solutions and the potential developed was noted down every hour up to 35 to 40 hours till potential developed becomes constant. The setup of this experiment is shown in the figure 1 below. Fig 1: Set up for open circuit potential test. Four sets of these arrangements were made and four specimen of matrix, 2%, 4% and 6% composites were immersed in the corrodents and readings were taken. The experimental arrangement was similar to the one used by Krupakara et al 7 for the open circuit potential studies of ZA-27 / quartz metal matrix composites. Electrochemical measurements like potentiodynamic polarization were carried out using electrochemical work station model CH 608E series manufactured by CH Instruments, USA which connected to a cell with a reference electrode, counter electrode and a provision for connecting the manufactured specimen as working electrode. Figure 2 given below shows the electrochemical work station. The electrochemical investigations were performed in open to air using 100ml beaker as cell with an Ag/AgCl electrode as the reference electrode and a platinum wire as the counter electrode (CE). Magnetic stirring was employed at the bottom of the cell to increase the mass transfer at the electrode. One square centimetre area of the specimen was exposed to the corrosive environment. Figure 3 given below shows the cell used for the test. Page No:400

4 VI. RESULT AND DISCUSSION Fig. 4 to 6 show the simulation curves for open circuit potential metal matrix composites of Aluminium 6061 alloy reinforced with different percentages of Titanium diboride particulates 0.025, 0.05 and 0.1 molar solutions of hydrochloric acid. In all the graphs it is clearly observed that as the percentage of reinforcement increases the open circuit potential developed decreases in all the three concentrations of hydrochloric acid. Same type of results obtained by Jayaprakash et al 8 in the open circuit potential studies of ZA-27 / Red mud particulates reinforced metal matrix composites in sodium chloride solutions. Fig. 7 to 9 are the simulation curves for potentiodynamic polarization tests of the Aluminium 6061 / Titanium Diboride metal matrix composites in comparison with matrix in three different concentrated solutions of hydrochloric acid. From the polarization curves shown above corrosion current values of matrix and composite materials is automatically given by the software in the personal computer attached to electrochemical work station. The results are given in the table 2 given below. Page No:401

5 Weight percentage of reinforcement Matrix alloy Al 6061 Al % TiB 2 Al % TiB 2 Al % TiB 2 Concentration Corrosion rate in mpy of HCl 0.025N N N Table 2: Corrosion Current (I corr ) Values of Al6061 / TiB 2 composites From the table it is clear that as the concentration of hydrochloric acid increases the corrosion current increases in matrix and composites. But it is also observed that in each concentration of hydrochloric acid corrosion current value decreases with increase in reinforcement content. Effect of exposure time In open circuit potential tests (Fig. 4-6), where the potential developed for metal matrix composites and matrix, decreasing with exposure time and become constant after thirty hours of exposure. The potentials experienced by metal matrix composites are less when compared to that of metal matrix. In this test there will be a black deposit on the specimens. The phenomenon of gradually decreasing corrosion rate indicates a passivation of the matrix alloy. De Salazar et al 9 explained that the protective black film consists of hydroxy chloride film, which retards the forward reaction. Castle et al 10 pointed out that the black film consists of hydroxide compound. This layer protects further corrosion in acid media. But the exact chemical nature of such passive film is still not established. Effect of concentration of the medium on the Rate of Corrosion The concentration had a marked effect on the corrosion rate of all the samples. With the increase in concentration of hydrochloric acid solution corrosion rate also has increased for matrix as well as composite materials. The corrosion current is found to be directly proportional to the rate of hydrogen gas evolution. It is well known that the chemical reaction depends on the concentration of solution, area of their action surfaces etc. Effect of Reinforcement From the results, it is observed that corrosion rate of aluminium 6061 matrix and composites decrease with increase in reinforcement content and that the rates of metal matrix composites were less than that of the matrix alloy. This trend is attributed to the fact that reinforcement particulates are ceramic in nature and hence remains inert in aggressive chloride media. Reinforcement particulates also reduce the active exposure area of metal matrix composites to the corrosive media. Page No:402

6 VII. CONCLUSIONS Metal matrix composites of Aluminium 6061 matrix reinforced with titanium diboride particulates were manufactured by liquid melt metallurgy technique using vortex method. Composite materials containing 2, 4 and 6 weight percentages of titanium diboride were casted. Matrix and composites were subjected to open circuit potential test and potentiodynamic polarization tests in 0.025, 0.05 and 0.1 molar solutions of hydrochloric acid. In all concentrations of hydrochloric acid corrosion rate decreased with increase in exposure time in open circuit potential test. In potentiodynamic polarization test also it was found that the corrosion rate decreases with increase in reinforcement content. ACKNOWLEDGMENT Authors wish to thank continuous encouragement given by Management and principal of T.John Institute of Technology, Bannerghatta road, Bangalore and Cambridge Institute of Technology-North campus, Devanahalli, Bangalore. REFERENCES 1. P.D. Reena Kumari, Jagannath Nayak, A. Nityananda Shetty,Corrosion behavior of 6061/Al-15 vol. pct. SiC(p) composite and the base alloy in sodium hydroxide solution, Arabian Journal of Chemistry, 9, 2016, S1144 S Muna K. Abbass, Khairia S. Hassan, and Abbas S. Alwan, Study of Corrosion Resistance of Aluminum Alloy 6061/SiC Composites in 3.5% NaCl Solution, International Journal of Materials, Mechanics and Manufacturing, 3(1), 2015, Radha H R., Krupakara, P. V., Somashekariah B.V., Vinutha K, Corrosion Inhibition Studies of Al6061 / Quartz Metal matrix Composites in Alkaline Solutions International journal of Chemical Sciences, 7(3), 2011, Gireesha. C, Krupakara P.V.,, Jayaprakash, H.V, Weight loss corrosion studies of Al6061 Red Mud metal matrix composites in 1N Hydrochloric acid solution International Journal of Chemical Sciences, Journal of Ultra Chemistry, 3(2), 2007, Abdul Jameel, Krupakara P V, Jayaprakash H V, Veeraiah M K, Corrosion studies of Al6061/ Red mud Metal Matrix Composites. J. Electrochem. Soc. India, 60(2011) Krupakara, P. V., Corrosion Characterization of Al6061/Red Mud Metal Matrix Composites,Portugaliae Electrochimica Acta 2013, 31(3), Krupakara P.V., Jayaprakash, H.V, Gireesha. C, Open Circuit Potential Studies of ZA-27 / Quartz MMC in 0.1n Acid Chloride Medium International Journal of Physical Sciences, Ultimate Scientist of Physical Sciences, 19(3), (2007) 8. Jayaprakash H V, Krupakara P V, Veeraiah M K, Open circuit potential studies of ZA-27 / Red mud metal matrix composites in different concentrations of sodium chloride J. Electrochem. Soc. India, 59 (4), (2010), J.M.G.DeSalazar, A.Urefia, S.Mazanedo and M.Barrens Corrosion behaviour of AA6061 and AA7075 reinforced with Al 2O 3 particles in aerated 3.5%chloride solution potentiodynamic measurements and microstructure evaluation, Corrosion Science, vol.. 41, 1999, pp J.E.castle, L.Sun and H.yan, The use of scanning auger microscopy to locate cathodic centers in SiC/Al6061 MMC And to determine the current density at which they operate Corrosion Science, vol. 36(6), 1994, pp Page No:403