Corrosion and Hardness Behaviour of Al/GO Nanocomposites Processed by the Ultrasonic Gravitational Stir Casting Method

The objective of this work is to evaluate the corrosion behaviour of nanographene oxide reinforced aluminium (Al/GO) metal matrix composites with different immersion time periods using the immersion corrosion technique. The Al/GO composites were fabricated by the ultrasonic gravitational stir casting process. The corrosions of Al/GO were evaluated using a scanning electron microscope. The experimental results revealed that the corrosion rate decreased and weight losses increased with increasing immersion time periods. The nonimmersed Al/GO composites exhibited higher microhardness values compared to the immersed Al/GO composites.


Introduction
Aluminium-based metal matrix composites (AMMCs) are used mainly for manufacturing various engineering components in aerospace, automotive, defense, and other domestic applications because they have superior mechanical properties and are lightweight and have good dimensional stability. In particular, the LM 24 aluminium alloy has widely many engineering applications owing to the excellent corrosion resistance, good machinability, and excellent formability and also has hot tear resistance [1]. Therefore, LM 24 aluminium allows the usage of secondary processes such as hot extrusion, rolling, and forging [2]. The AMMCs were fabricated by the liquid state and solid state methods. The solid state method is used to fabricate a few shapes of composite components and is not suitable for the fabrication of complex-shaped components [3]. Therefore, this method is not used in many industries; however, the conventional stir casting method is widely used in many industries [4] because stir casting is a simple technique, has more flexibility, and also has more thermodynamic stability for the ceramic particles to be distributed uniformly in the matrix liquid at high temperatures producing a strong interfacial bond between the reinforcement and the matrix materials [5]. However, the conventional stir casting process is not suitable for mixing nanoparticles in the matrix liquid because of size variation, some practical issues in nanocluster formation, and inadequate wetting of nanoparticles with the matrix phase [6]. Recently, the ultrasonic cavitation assisted stir casting process replaced the conventional stir casting process, and the high intensity of ultrasonic waves breaks the cluster particles and distributes them uniformly in the matrix liquid [7][8][9]. Therefore, the ultrasonic stir casting technique is best suitable for the fabrication of nanocomposite materials.
In general, aluminium is a highly reactive material at high temperature [10] and has high corrosion resistance because the oxide layer present on the surface protects it from the environment. This oxide layer forms on the surface naturally when the aluminium alloy is manufactured or exposed to high temperature through the following reaction [11]: Al + 3H 2 O ⟶ AlðOHÞ 3 + 3H 2 ↑. The thickness of the oxide layer on the Al surface, which is stable in an aqueous medium of pH level 7.5, is about 2.5 nm. The corrosions are easily accelerated in the aluminium alloy when it is exposed to different environmental conditions, which are in contact with salt water, and when ceramic particles are added to the aluminium alloy [12]. This oxide layer dissolves when the aqueous medium level of pH is more than 9.0 or sea water. Therefore, corrosions occur in the aluminium-based composite materials due to the addition of both micro-and nanosize ceramic particles such as SiCp, TiCp, B 4 Cp, carbon nanotube (CNT), nanofiber, graphene, graphene oxide, and n-TiB 2 [13,14]. Trowsdale et al. [15] and Han et al. recommended the Al/SiCp and Al/B 4 C in 3.5 NaCl solutions to increase the susceptibility of pitting corrosion as compared to the unreinforced alloys. Abu-Warda et al. [16] reported that the addition of nano-TiB 2 did not reduce the corrosion resistance or susceptibility to pitting corrosion at the interfacial region. Recently, graphene oxide has received much attention among the nanoreinforcements due to its unique structure and outstanding mechanical properties and the presence of oxygen in graphene oxide to enhance the interfacial bond between the matrix and the reinforcement [17]. Furthermore, graphene oxide is mostly used as a coating material because of its chemical inertness and impermeability characteristics [18]. So far, investigators have mostly studied and evaluated only the mechanical behaviour of graphene oxide-based composite materials, and only a few authors concentrated on the corrosion behaviour of graphene oxide reinforced composite materials [19]. This research gap has encouraged researchers to study the corrosion behaviour of graphene oxide reinforced metal matrix composites. Different types of corrosion occur in AMMCs depending on the various manufacturing methods [20] and the physical and chemical interaction between the matrix and reinforcements at an ambient temperature level. The percentage of reinforcement plays an important role in deciding the mechanical behaviour of metal matrix composites. When increasing the percentage of nanographene oxide particles in the aluminium matrix, more nanoagglomerations are formed in the aluminium matrix. It exhibits more corrosion in the composite materials. A number of researchers have investigated the various corrosion behaviours by using different corrosion techniques. Corrosion can be divided into two categories based on the locations: one is uniform corrosion and another is localised corrosion. Uniform corrosion occurs on the surface of the composite due to the changes in environmental conditions [11], and localised corrosion mostly occurred in the matrix region and interfacial region in composite materials through chemical reactions. Localised corrosions are stress corrosion [21,22], pitting corrosion [23], crevice corrosion, galvanic corrosion [24], and intergranular corrosion [25].
The aim of the present work is to evaluate the corrosion behaviour of Al/GO composites at different time period exposures to ambient temperature levels. The Al/GO samples were prepared by the ultrasonic stir casting method, and the corrosion behaviour in the samples was analyzed using the immersion corrosion test. Immersion corrosion testing is the most reliable and simple method for analyzing the corrosion behaviour of composite materials. The hardness strength of the corrosion samples was evaluated by microhardness testing.

Experimental Work
In this study, the LM 24 aluminium alloy with a density of 2.68 g/cm 3 was used as the matrix material because it has excellent casting characteristics. The 1.5 percentage nanographene particles (GO) were used as reinforcement. The average size of the graphene particles is 52 nanomicrons. The chemical composition of the LM 24 aluminium alloy is shown in Table 1. 1000 grams of LM 24 aluminium alloy was placed in a crucible and heated to 700°C at the rate of 12°C per minute. The preheated graphene oxide nanoparticles were added into the molten liquid. The liquid metal was stirred at a speed of 400 rpm using a mechanical stirrer for 10 minutes to achieve a uniform distribution of nanoparticles in the liquid metal. During this stirring process, a number of nanoclusters were formed in the liquid due to the size variation of the nanoparticles. Therefore, the mechanical stirrer was removed from the molten metal, and the ultrasonic horn was inserted in the liquid metal to one-third the height of the liquid for the purpose of inducing ultrasonic waves throughout the liquid metal. The power and frequency of the ultrasonic transducer device were 2 kW and 20 kHz, and the processing temperature was maintained at 700°C. The transducer horn produced ultrasonic waves in the liquid metal. These generated waves spread throughout the liquid and produced mechanical vibrations. These vibrations served to break up the cluster of nano-GO particles and distribute them uniformly in the liquid matrix. The composite slurry was poured into the preheated mild steel die and allowed to solidify. The casting sample of the Al/GO composite is shown in Figure 1. The size of the Al/GO composite casting is 130 × 60 × 15 mm 3 . The corrosion test was carried out using a simple immersion test. The Al/GO composite samples were totally immersed in the 3.5% NaCl electrolyte for different periods of time, such as 24 hrs, 48 hrs, and 72 hrs. The corrosion samples were prepared from the composite materials and polished by a 500 grit sheet. The size of the corrosion sample was 10 mm length, 10 mm breadth, and 5 mm thickness. The weight of the Al/GO composites was measured using a weighing machine. The accuracy of the weighting machine is 0.001 grams. The specimens were immersed in 250 ml of 3.5% NaCl electrolyte solution. After completing the time period, the specimens were taken out from the solution and cleaned using acetone, and then, the mass losses were calculated. Figure 2 shows the corrosion and noncorrosion of the Al/GO composite. The structural changes on the sample surface were examined by a scanning electron microscope. The corroded surface strength was measured by the microhardness test.

Results and Discussion
3.1. Structural Analysis. The SEM micrographs of the Al/nanographene oxide composites are shown in Figure 3. The nanographene oxide particles were distributed almost uniformly in the Al matrix material. The uniformity in the distribution of the particles and the strong interfacial bonds were achieved using the ultrasonic stir casting method under optimum fabrication conditions. The corroded surfaces of the Al/GO composite samples are shown in Figure 4. The SEM micrograph clearly shows that the surface morphologies were damaged by dissolution of the aluminium layer in the NaCl solution under the immersion 2 International Journal of Corrosion conditions. Figure 5 shows that the surface damages were observed due to the prolonged contact period. Within the initial immersion period, more surface damages were not observed, and it is shown in Figure 4. During the immersion periods, the presence of carbon in the nanographene oxide reacts with the aluminium to form aluminium carbide at the interfacial region. Smith et al. [26] reported that nanographene oxide has a carbon element in the hexagonal structure. The EDS analysis was carried out in the interfacial region using a line scanning option method. This analysis confirmed the presence of the carbon element in the interfacial region, as shown in Figure 6. The aluminium carbide reaction takes place in the following manner: Al 2 + 3C ⟶ Al 2 C 3 . The presence of intermetallic elements of alumina carbide (Al 4 C 3 ) which forms at the interfacial region initiated localised corrosion in and around the reinforcement. It is expected that there will be increased susceptibility to corrosion at the interfacial region between the Al matrix and the nanographene oxide reinforcement. Under prolonged immersion periods, the intermetallic elements were diluted in the NaCl solution and initiated the debonding between the Al matrix and the graphene oxide particles. The interfacial microcracks formed on the surface and interfacial region as shown in Figure 7. This is a favourable mechanism for debonding and corrosion formation in the A/GO composites.

Weight Loss and Water Absorption of Al/GO Composites.
The weight losses in the Al/GO composite samples were measured after completion of the immersion period using a high accuracy weighing machine. The weight losses of the Al alloy and Al/GO samples are shown in Figure 8. The Al/GO composites have more weight losses than the Al alloy due to the dilution of intermetallic compounds at the interfacial region and surface deterioration in the composites. The weight measurement revealed that the weight losses increased with increases in the time period due to the surface corrosion, dissolving of the oxide layer, and pitting corrosion in the composite materials. The NaCl absorptions were calculated according to the following equation [27]: NaCl absorption ð%Þ = ðWt:of wet sample − Wt:of dry sampleÞ/ðWeight of dry sampleÞ × 100. Figure 9 shows the absorption curve of the immersion time periods. The NaCl absorption in the composites occurred by a diffusion process. The NaCl is diffused in the matrix and interfacial region of the composites through surface microcracks. The surface microcracks were formed during the fabrication of the composites and covered by an oxide layer [28]. The presence of microcracks in the Al/GO sample is shown in Figure 7. The surface microcracks were observed due to the dissolving of oxide layers when in contact with the NaCl solution. Therefore, the NaCl absorption rate increased with the increased immersion time period.

Corrosion Behaviours of Al/GO Composites.
The corrosion rate of the composite was calculated following the equation [29]: corrosion rate ðmm/yearÞ = 87:6W/DAT, where W is the weight loss of the exposed samples in grams, D is the density of specimen in g/cm 3 , A is the area of the exposed samples in cm 2 , and T is exposure time in hours. The corrosion rate of the Al/GO composites is shown in Figure 10. The corrosion rates of both Al alloy and Al/GO composites showed similar trends, but the Al alloy exhibited a lower corrosion rate compared to the Al/GO samples due to the absence of nanoreinforcements in the matrix. The corrosion rate was high in the initial period of immersion due to the adsorption of chloride ions on the surface of the Al/GO composites and as it was the first stage of localised corrosion attack on the samples. The localised corrosion formation and surface damage are shown in Figures 4 and 5. The previous investigator Falcon et al. [30] reported that the surface corrosion and pitting corrosion in the composite materials take place through the following steps: (a) adsorption and diffusion of chloride ions on the oxide surface, (ii) formation of hydroxychloride aluminium salts, and (iii) dissolution of the oxide. When the immersion period was increased, the chloride solution diffused in the composite material through the existing surface microcracks. The presence of surface microcracks on the composite sample is shown in Figure 7. A minimum amount of the chloride solution was diffused in the interfacial region and other boundary areas in the Al matrix to initiate the corrosion inside the composites. Therefore, the corrosion rate decreased with the increase in the immersion time periods. The corrosion pits observed in the composite sample were shown in Figure 11. The corrosion pits appear cup-shaped, hemispherical, flat-walled, or sometimes irregular-shaped depending upon the preferential corrosion area, adsorption and diffusion of the chloride ions, and immersion periods [31]. The irregular shapes of the corrosion pits were found at the interfacial region between the Al matrix and the GO particles. It is shown in Figure 11. These corrosion pits are weakly passivized or thermodynamically unstable. Therefore, the interfacial region can serve as nucleation sites for pit formation. Finally, the experimental  3 International Journal of Corrosion results revealed that the corrosion rates depend mainly on the immersion period, chloride ions, interfacial region, and surface conditions of the composite materials.

Effect of Hardness Strength on Al/GO Composites.
This analysis is used to study the effect of the hardness strength of both the noncorroded and corroded surfaces on the composite samples and the interfacial region between the Al matrix and GO nanoparticles. The microhardness measure-ment experiments were conducted before and after the corrosion analysis of the different periods of immersion tests. The microhardness values were measured on the specimens at different points at constant distance intervals. Figure 12 shows the different hardness variations depending on the surface morphology conditions and the distribution of nanographene oxide particles in the Al matrix. However, a comparison of the different samples with different immersion periods shows that the nonimmersed samples show higher microhardness values when compared to the corroded samples due to the presence of the oxide layer on the surface and the uniformity in the distribution of the nanoparticles. The nanoparticles bonded well with the Al matrix, as shown in Figure 1. The deformation of the Al matrix was restricted by the presence of nano-GO particles during the loading conditions. Therefore, the nonimmersed samples provided higher hardness strength. The initial immersion periods (24 hrs.), the surface damage, and surface corrosion occurred due to the adsorption of chloride ions in the Al/GO samples. Therefore, the microhardness gradually decreases in these samples compared to the nonimmersion sample. When the immersion periods were increased from 24 hrs. to 72 hrs., the chloride solutions diffused in the Al matrix and the interfacial region between the Al matrix and GO particles through the preexisting     3.5. Tensile Properties of Al/GO Composites. The property of Al/GO composites depends mainly on the distribution of the nanoparticles in the matrix. It is necessary to understand the particle distribution in the matrix in order to correlate the properties of the PRMMCs. The tensile test was conducted on a flat-shaped specimen, performed according to the ASTM standard (E-8 model) test methods. Figure 13 shows the variation in the ultimate tensile strength Al/GO composites. The Al/GO composites exhibit more tensile strength compared to the aluminium alloy. The overall strength of the composites is influenced by the distribution of the graphene nanoparticles in the Al matrix.

Conclusions
The corrosion behaviour of Al/GO composites was investigated by using immersion for different periods. The following results can be inferred from this experimental work: (1) From the SEM micrograph analysis, the nanographene oxide particles were seen distributed uniformly in the Al matrix (using ultrasonic stir casting method)

Data Availability
No data used to support the study.

Conflicts of Interest
The authors declare that they have no conflicts of interest.