Corrosion Characterization of Aluminium 356 Hybrid Composites in Neutral Chloride Medium

Hybrid composites are the composites which contain two different reinforcements in them. One of the reinforcements will be added in different quantities and another one will be added with same quantity. In this work hybrid composites of aluminium 356 are manufactured by adding fly ash and silicon carbide particulates. Composites are manufactured by liquid melt metallurgy technique using vortex method. Composites containing 2% fly ash and 2 weight percentages of SiC particulates were manufactured. Matrix alloy was also casted in the same way for comparison. Static weight loss corrosion test and potentiodynamic polarization tests were conducted in different concentrated solutions of neutral chloride like sodium chloride. In both the tests the hybrid composites exhibited decreased corrosion rate with increase in reinforcement content. Hence hybrid composites are more suitable than the matrix alloy for applications.


Introduction
Designers get many advanced benefits in designing the components for automobile and aircraft industry through metal matrix composites (MMCs). These materials maintain good strength at good structural rigidity, dimensional stability and light weight. [1][2][3][4][5]the trend is towards safe usage of the MMC parts in the automobile engine which work particularly at high temperature and pressure environments. [6][7][8]for the last two reinforced MMCs has been the most popular.
Aluminium MMCs have most popular families being represented by aluminium alloy reinforced with ceramic particulates. By the addition of second phase into matrix material enhances not only physica mechanical properties, but also changes the corrosion behaviour significantly. In industries aluminium is used extensively with respect to their excellent fluidity, castability and mechanical properties. Aluminium compositesexhibit very good tribolog Lo et.al [10] found that aluminium is having better properties than copper, and cast iron. Many  Designers get many advanced benefits in designing the components for automobile and aircraft industry through metal matrix composites (MMCs). These materials maintain good strength at high temperature, good structural rigidity, dimensional stability and light 5]the trend is towards safe usage of the MMC parts in the automobile engine which work particularly at high temperature and pressure 8]for the last two decades particles reinforced MMCs has been the most popular.
Aluminium MMCs have most popular families being represented by aluminium alloy reinforced with ceramic particulates. By the addition of second phase into matrix material enhances not only physical and mechanical properties, but also changes the corrosion behaviour significantly. In industries aluminium is used extensively with respect to their excellent fluidity, castability and mechanical properties. Aluminium compositesexhibit very good tribological properties. [9] Lo et.al [10] found that aluminium is having better properties than copper, and cast iron.Many researchers have studied the aluminium matrix reinforced with different reinforcement and reported that the composites of aluminium are more suitable for applications. [11][12] B. M. Viswanathaet al [17] studied tensile and hardness properties of A356 alloy hybridcomposites reinforced with silicon carbide and graphite particulates manufactured by liquid metallurgy technique. Silicon carbide particulates addition was varied and graphite addition was constant. Authors report that the properties mentioned above were improved with the increase in the addition of silicon carbide particulates.

II. Experimental procedure A. Material Selection
The alloy selected for the work is Aluminium 356 alloy which is commercially available. Its composition is given in table 1

B. Composite preparation
The liquid metallurgy route using vortex technique employed by P.V.Krupakara [18] was used to prepare the composites. A mechanical stirrer was used to create the vortex. The reinforcement materials fly ash and silicon carbide of size varying 50-80 µm were preheated to 400 0 C in a muffle furnace. The weight percentage of fly ash used was 2 and silicon carbide particulates addition was varied from 2 to 6 in the steps of 2%. Addition of reinforcements in to the molten aluminium alloy melt was carried out by creating a vortex in the melt using a mechanical stainless steel stirrer coated with alumina (to prevent migration of ferrous ions from the stirrer material to the aluminium alloy). The stirrer was rotated at a speed of 450 rpm in order to create the necessary vortex. The reinforcement particles added in to the vortex of liquid melt at a rate of 120 g/m. The composite melt was thoroughly stirred and subsequently degassed by passing nitrogen through the melt at a rate 2-3 l/min for three to four minutes.

C. Specimen preparation
The castings were cut into 20mmx20mm pieces using an abrasive cutting wheel for static weight loss corrosion test. The matrix alloy also cast under identical conditions for comparison. For potentiodynamic polarization tests the rectangular specimens of size 20mm x 10mm x1mm were machined from matrix alloy and composites. The specimens were successively ground using 240, 320, 400 and 600 SiC paper and were polished according to standard metallographic techniques and washed in acetone and dried. The samples were weighed up to fourth decimal place using electronic balance and also the specimen dimensions were noted down using Vernier gauze.

III. Static weight loss corrosion test
The corrosion behaviour of aluminium alloy was studied by immersion test. The static immersion corrosion method was adopted to measure the corrosion loss. The tests were conducted in 0.035, 0.35 and 3.5 percent sodium chloride solutions.
200 ml of the prepared solution was taken in a beaker. Samples were suspended in the corrosive medium for different time intervals up to 96 hours in the steps of 24 hrs. To minimize the contamination of the aqueous solution and loss due to evaporation, the beakers were covered with parafilm during the entire test period. After the specified time the samples were cleaned mechanically by using a brush in order to remove the heavy corrosion deposits on the surface. The corresponding changes in the weights noted. At least three samples were tested and average value was taken. Corrosion rates were computed using the equation Corrosion rate: 534 W/DAT mpy Where W is the weight loss in grams, D is density of the specimen gm/cc, A is the area of the specimen (inch 2 ) and T is the exposure time in hours.

IV. Potentiodynamic polarization test
These tests are conducted in electrochemical work station model 608E manufactured by CH instruments, USA. It has a USB port or serial port for data communication with the personal computer. The graphs are directly obtained as per requirement with the help of the software installed. Mixed potential theory forms the basis for Tafel extrapolation method used to determine corrosion rate. The Tafel plot is as an illustration of the Tafel equation. This equation is mainly used in electrochemical kinetics connecting the over potential rate to the electrochemical reaction rate. The software itself gives the values for corrosion rates.  From the above figures it is clearly observed that as the time of exposure increases the matrix as well as composites of Aluminium 356 exhibit decreased corrosion rate. As the exposure time increases exposed aluminium surface in matrix and composites develop a non porous layer slowly this leads to the decrease in corrosion rate. From the above figures it is also clearly understood that as the percentage of reinforcement increases the corrosion rate decreases. This is due to the fact that as reinforcement content increases, the exposure of the alloy to the corrosion medium decreases hence the corrosion rate decreases. Hanumanthe Gowda and P.Rajendraprasad 19 in their research studies of wear and corrosion resistance of Aluminium 356 alloy reinforced with rice husk ash and alumina particulates report that after immersing the matrix and alloy for 96 hours in 5% sodium chloride solution, 0.1 normal hydrochloric acid and sea water corrosion resistance and wear resistance of the composites increases when compared with that of matrix. The specimens containing 0-5 percent of reinforcements and aged for different period exhibited decrease in corrosion rate for composites when compared with that of matrix alloy. Hence composites are suitable than matrix alloy. As the concentration of sodium chloride increases corrosion rate increases from matrix to 6 percent hybrid composites. This is natural because as the concentration increases evolution of hydrogen increases and corrosion rate increases. The point of intersection between the cathodic and anodic curve gives Icorr. The corrosion rate in mpy is calculated by the software attached to electrochemical work station.  Table 2 shows the corrosion rates in mpy for the matrix and hybrid composites in different concentrated solutions of sodium chloride. In each concentration of sodium chloride the corrosion rate decreases with increase in reinforcement content. That is as the percentage of silicon carbide increases the corrosion rate decreases. Corrosion rate increases as the concentration of the medium increases due to the increase in liberation of hydrogen gas.

V. Results and Discussion
This is attributed to the clean surface of Al 356 alloy reaches to the passivity rapidly when exposed to oxygen containing environment forming protective oxide film (Al 2 O 3 ) which is good adherence to metal surface and poor conductor for charge transfer. But this film contains flaws and increased pitting in chloride. [20]

VI. Conclusions
Hybrid composites and matrix were manufactured by liquid metallurgy route by using vortex method. Both matrix and composites were subjected to static weight loss corrosion and potentiodynamic polarization in different concentrated solutions of sodium chloride. Composites exhibited excellent corrosion resistance when compared with that of matrix alloy in both the tests. Hence composites are more suitable than matrix alloy for applications in marine and saltish environment.