Corrosion Behavior of Spark Plasma Sintered Alumina and Al 2 O 3 -SiC-CNT Hybrid Nanocomposite

2020 The use of ceramic-based materials has become more common in many applications because of their unique characteristics and properties. Design of alumina hybrid nanocomposites‎ achieved by incorporating two nanoreinforcements, with different morphologies and/or attributes, such as CNTs and SiC, is a new approach that has been adopted to enhance the properties of alumina. The microstructural, mechanical, electrical, and thermal properties‎ of Al 2 O 3 -SiC-CNT hybrid nanocomposites were investigated and reported in the literature. However, the corrosion behavior was not considered. The present paper reports‎ the electrochemical corrosion behavior of pure Al 2 O 3 and Al 2 O 3 -5SiC-2CNT hybrid nanocomposite in acidic (2.34M HCl) and alkaline (6.5M NaOH) environments at room temperature. Ball milling (BM) and spark plasma sintering (SPS) were used for preparation of samples. The microstructure of sintered samples was investigated through field emission scanning electron microscopy (FE-SEM). Potentiodynamic polarization (PDP) technique was used to investigate the corrosion behavior. The corrosion rate of the Al 2 O 3 -5SiC-2CNT nanocomposite increased 96 and 178% in ‎HCl and NaOH solution, respectively, compared to alumina.‎ Possible corrosion mechanisms and factors effecting corrosion were


Introduction
Alumina is the most widely used and studied ceramic because of its good mechanical properties, high temperature stability, wear and corrosion resistance, along with electrical and thermal insulation nature 1,2 . Due to these properties, alumina is widely used in applications involving critical conditions 3 , such as high pressure, temperature and corrosive environments, i-e in cutting tools, dental implants, gas laser turbines, high temperature bearings, mechanical seals in nuclear reactors, and wear resistant parts 1,[4][5][6][7][8] . Lower fracture toughness is the only restraint 1,9 , restricting the use of alumina in wide range of applications. In order to overcome this limitation, micro/nano sized reinforcements such as CNT, CNF, SiC, and graphene are added to alumina matrix [10][11][12] , resulting in the formation of nanocomposites with improved fracture toughness 13 . Recently, further enhancements in properties of alumina were made possible through the hybrid microstructural design 14 , by reinforcing alumina with two reinforcements simultaneously. For instance, 117% increase in fracture toughness along with 44% increase in bending strength was reported by Ahmad and Pan 14 , for Al 2 O 3 -SiC-CNT hybrid nanocomposites in comparison to monolithic alumina.
Generally, ceramics are known for their good corrosion resistance 15 . However, they are susceptible to corrosion in presence of corrosive environment. In case of monolithic alumina, corrosion investigations under different corrosive environments such as hydrothermal 4,16 , acidic 3,17-20 and alkaline 21,22 media were reported. Oda and Yoshio 4 investigated hydrothermal corrosion at 300°C and 8.6 MPa pressure, using alumina with variable purity levels (99%, 99.9% and 99.99%), for 1 to 10 days. The authors observed intergranular corrosion and attributed it to the dissolution of grain-boundary impurities, which was confirmed from the presence of Si, Na, and Al ions in the solution. The corrosion weight loss and resulting strength, of the corroded samples, were found to be dependent on purity level, with the highly pure sample showing the lowest corrosion and highest strength. Ono et al. 16 and Sato et al. 21 , in similar investigations, showed the dependency of corrosion on the purity of alumina. The 92% alumina sample showed higher corrosion rate of 3.48 mg/cm 3 , as compared to 0.145 mg/cm 3 of the 99.5% alumina sample, in demineralized water at 275°C after 100 hours of exposure 16 3 , HF, HCl) and alkaline (NaOH) media within a low temperature range of up to 180°C. Lidija et al. 20 studied the effect of different concentrations (2, 10 and 20wt.%) of HCl and H 2 SO 4 on corrosion behavior of alumina. Corrosive attack on alumina was found decreasing with increased acidic concentrations. However, in case of alkaline caustic solution, an opposite trend of increased corrosive attack with increase in concentration of NaOH was reported by Sato 21 . All these studies suggest that corrosion attack starts from grain boundaries, by the dissolution of grain boundary impurities. It was found that the intensity of corrosion attack depends on the purity of alumina, nature and concentration of the corrosive media, time of exposure, and temperature of the medium.
With growing applications of alumina, especially in the fields of biomaterials and bio-pharmaceuticals [23][24][25] , efforts were made to enhance the corrosion resistance of alumina while enhancing or retaining the other properties. Shikha et al. 23 reported the enhancement of corrosion resistance of alumina through surface modification using N + ions implantation at 60 keV. Corrosion current density (i corr ) of the un-implanted alumina was found to be 4.62 µA/cm 2 in Ringer solution, this was reduced to 1.11 µA/cm 2 for the sample implanted with ion beam dose of 1x10 16 ions/cm 2 . Also, nanohardness was found to increase from 10 to 37.5 GPa, when alumina was implanted with N + beam dose of 1x10 16 ions/cm 2 . This enhancement in corrosion resistance and nanohardness was attributed to the formation of hard AlN and AlON compounds on the surface of the implanted sample. Wu et al .4 reported that addition of Y 2 O 3 in alumina led to increased corrosion resistance in 12 wt% HCl +3 wt% HF acid, due to the formation of Y 3 Al 5 O 12 and CaAl 12 O 19 acid resistant phases within alumina. Lekatou et al. 25 studied the corrosion and tribological properties of alumina reinforced with 0-50wt.%Ni in 3.5wt.% NaCl. Both corrosion and wear rate of the reinforced alumina decreased, as compared to pure alumina, up to 30wt.% of Ni reinforcement.
It was reported that the addition of reinforcements alter the mechanical, electrical, thermal and corrosion behavior of alumina. Literature is available on mechanical 26-33 , electrical [34][35][36][37][38][39] and thermal [39][40][41][42][43][44] behavior of alumina-based nanocomposites. However, corrosion behavior of alumina hybrid nanocomposites was not considered. Due to the fact that alumina-based-nanocomposites are mostly designed for applications with extreme working conditions, knowledge of corrosion behavior is equally important. In previous works, the authors investigated the effect of SiC and CNT reinforcement on microstructural and mechanical properties 45 as well as electrical and thermal properties 39 of Al 2 O 3 -SiC-CNT hybrid nanocomposites. In this work, the effect of SiC and CNT on corrosion behavior of spark plasma sintered alumina was investigated in both acidic and alkaline media, using potentiodynamic polarization method. Corrosion mechanism was discussed, corrosion rates were calculated and factors effecting corrosion behavior were explained.

Materials and Methods
Pure alumina powder, having 99.85% purity (particle size of 150 nm), procured from ChemPUR Germany; SiC β (45-55nm) with 97.5% purity supplied by Nanostructured and Amorphous Material and locally manufactured functionalized carbon nanotubes (CNTs), produced through chemical vapour deposition (CVD) were used in this study. The hybrid nanocomposite was synthesized via two stages. In the first stage, Al 2 O 3 -10Si nanocomposite powder was prepared by sonication and ball milling. In the second stage, the required amount of functionalized CNTS was added to the slurry and further sonicated for 2 h using a high-energy probe sonicator to obtain Al 2 O 3 -10SiC-2CNT nanocomposite. Pure Al 2 O 3 and Al 2 O 3 -5SiC-2CNT samples were sintered through spark plasma sintering (SPS) at 1500°C for 10 minutes holding time. Detailed methodology for nanocomposite powder preparation and sintering is explained somewhere else 39,45 . Disc shaped specimens (a diameter of 20 mm and a height of 10 mm) were prepared using a graphite die. Tescan Lyra-3 FE-SEM was used to analyze the microstructure of spark plasma sintered samples.
Corrosion testing was conducted at room temperature (25°C), within electrochemical three-electrode cell. The tested specimen was used as working electrode. A platinum wire and a saturated calomel electrode (SCE) were used as counter and reference electrodes, respectively. Detailed working of the polarization cell is can be found in the literature 46 . Gamry potentiostat (Reference 3000) was used for potentiodynamic polarization. Samples were ground and polished prior to corrosion testing. A special 3M tape with an exposed working area of 0.22 cm 2 was used on specimen surface. Electrochemical testing was conducted in both acidic (2.34 mol/l HCl) and alkaline (6.5 mol/l NaOH) solution mediums, using a scan rate of 0.2 mV/s. In built software of potentiostat, DC105, was used to interpret the Tafel region of potentiodynamic curves and to determine the values of corrosion potential (E corr ), corrosion current (i corr ), and corrosion rate. Figure 1 shows FE-SEM images of surfaces of Al 2 O 3 and Al 2 O 3 -5SiC-2CNT nanocomposite samples. The black arrows show CNTs. Reduction in grain size of alumina due to addition of SiC and CNT reinforcements can be observed. This reduction in grain size, also reported by Saheb and Khwaja 45 , is primarily due to the pinning effect of the reinforcements, which restricts the grain boundary motion during densification 11 . The mode of fracture changed from intergranular for pure alumina to intragranular for Al 2 O 3 -5SiC-2CNT nanocomposite. These mechanisms of grain refinement and change of fracture mode are few important factors contributing towards enhanced mechanical properties of Al 2 O 3 -SiC-CNT nanocomposites 14,45 .

Results and Discussion
The potentiodynamic polarization response of Al 2 O 3 and Al 2 O 3 -5SiC-2CNT hybrid nanocomposite in 2.34M HCl and 6.5M NaOH solutions is shown in Figure 2. The curves of Al 2 O 3 -5SiC-2CNT nanocomposite are present towards higher current density, in both acidic and alkaline solutions, showing relatively high corrosion as compared to monolithic alumina. Table 1 shows the values of corrosion parameters of Al 2 O 3 and Al 2 O 3 -5SiC-2CNT nanocomposite in 2.34 M HCl and 6.5M NaOH solutions. In the case of HCl solution, corrosion rate and i corr of pure alumina were found to be 5.07 x 10 -3 mpy and 1.12 nA that increased to 0.488 mpy and 105 nA for Al 2 O 3 -5SiC-2CNT nanocomposite, respectively. Similarly, in case of NaOH solution, corrosion rate and i corr of pure alumina increased from 12.8 x 10 -3 mpy and 2.82 nA to 2.282 mpy and 494 nA for Al 2 O 3 -5SiC-2CNT nanocomposite. This clearly shows that the addition of SiC and CNT to alumina increased its corrosion in both acidic and alkaline environments.
Corrosion results of Al 2 O 3 and Al 2 O 3 -5SiC-2CNT nanocomposite in acidic and alkaline environments, presented in Table 1, shows that corrosion in 6.5 M NaOH alkaline solution was severe as compared to 2.34 M HCl solution. This is due to the fact that acids are more corrosive at lower concentrations 20 , while alkaline bases are more corrosive at higher concentrations 21    Oda and Yoshio 4 reported that corrosion in alumina is generally intergranular in nature, where corrosive attack starts preferentially on grain boundaries, resulting in dissolution of SiO 2 and NO 2 grain boundary impurities. Other researcher 20 attributed the corrosion of alumina, in HCl and H 2 SO 4 acid solutions, to the dissolution of MgO, Na 2 O, CaO, SiO 2 and Fe 2 O 3 grain boundary impurities. This intergranular nature of corrosive attack in alumina has also been reported in other studies 3,21,22 . Due to the fact that grain boundaries are the high energy sites, they are more susceptible to corrosive attack. For instance, Mikeska et al. 3 found that single-crystal alumina had higher corrosion resistance than polycrystalline alumina in aqueous hydrofluoric acid. After corrosive dissolution of grain boundaries, grains get exposed to corrosive media and eventually they wash out of the sample 18 . Hence, corrosion resistance in alumina is primarily dependent on microstructure and impurities present within grain boundaries 19 . It is believed that in the case of Al 2 O 3 -5SiC-2CNT nanocomposite, the presence of large number of grain boundaries due to grain refinement, Figure 1, the increased number of interfaces, and possible agglomeration of the reinforcements have contributed to corrosive attack. Similar behavior of increased corrosion with increase in reinforcement content was reported by Lekatou et al. 25 for sintered alumina-Ni nanocomposites. The authors found that, in 3.5wt.% NaCl solution, the corrosion current density value of 0.89 mA/cm 2 for pure alumina increased to 2.52 and 2.44 mA/cm 2 for Al 2 O 3 -40wt.%Ni and Al 2 O 3 -50wt.%Ni nanocomposites, respectively. This increased corrosion current density, in case of Al 2 O 3 -Ni nanocomposites, was attributed to Ni agglomeration, increase in grain boundaries and increased number of interfaces. Moreover, corrosion in alumina is highly sensitive to impurities present within grain boundaries 4 . SiC and CNTs in the nanocomposite behave like impurities and increases the corrosion rate. Influence of impurity content on corrosion behavior of alumina has been reported by Oda and Yoshio 4 . The authors found that alumina with 99% purity suffered relatively higher corrosion, as compared to alumina with 99.9% purity. In another similar study 16 , sintered alumina with 99.5% purity displayed higher corrosion resistance, in demineralized water at 275°C, compared to alumina with 92% purity. The corrosion rates of 92 and 99.5% alumina were found to be 3.48 and 0.145 mg/cm 3 , respectively.
Corrosion behavior is known to depend on the electrical conductivity of the material 47 . Due to its electrical insulating nature, corrosion of alumina is generally attributed to chemical corrosive attack 24 . For the Al 2 O 3 -5SiC-2CNT nanocomposite, addition of SiC and CNT resulted in the formation of interconnected network of semiconducting phases within the grain boundaries of alumina, which enhanced the electrical conductivity. An electrical conductivity of 8.85 S/m was reported for Al 2 O 3 -5SiC-2CNT nanocomposite 39 , as compared to 6.87 x 10 -10 S/m for alumina. The high electrical conductivity provided more electronic flow during polarization, resulting in intensifying the corrosive attack of the electrolyte, thus increasing the corrosion of Al 2 O 3 -5SiC-2CNT nanocomposite. Similar behavior was overserved by Sydow et al .8 in their investigation on two types of SiC ceramics with different electrical resistivity values. They found that SiC with high electrical conductivity showed high corrosion. Also, Liu and co-authors 49 , reported corrosion current densities of 0.70 and 0.82 mA/cm 2 , for alumina nanocomposite containing 0.33 wt.% carbon, in 1M HCl and 1M NaOH solutions, respectively. The increase in carbon content to 0.62 and 0.82 wt.% resulted in increased corrosion. This increase in corrosion rate was attributed to the increase in the electrical conductivity due to the formation of nano-carbon interconnected network within alumina grain boundaries.
It is known that alumina with relatively low porosity is more resistant to corrosive attack 18 . Therefore, the presence of small fraction of porosity in Al 2 O 3 -5SiC-2CNT nanocomposite might have contributed towards high corrosion. The pure alumina used in this study was almost fully dense (99.8%), while the Al 2 O 3 -5SiC-2CNT nanocomposite had 97.2% relative density 39 . Because of porosity, relatively larger interface area of Al 2 O 3 -5SiC-2CNT nanocomposite was exposed to the electrolyte, as compared to alumina, resulting in higher corrosion. However, because of the very low porosity in Al 2 O 3 -5SiC-2CNT nanocomposite, it is believed that its contribution in increasing corrosion, if any, is very small. It is worth mentioning here that in their investigation on the corrosion behavior of MgO-doped alumina, with different MgO contents in acidic solutions (HCl, H 2 SO 4 , HNO 3 ), Genthe and Hausner 19 concluded that porosity and composition of grain boundaries are defining factors for corrosion behavior. This influence was also observed by Oda and Yoshio 4 , where high purity alumina with relatively high density showed more corrosion resistance as compared to less pure alumina with low density.
The corrosion behavior of metal matrix composites and nanocomposites has been thoroughly investigated and reported [50][51][52][53][54] , however, it was not considered for hybrid ceramic matrix nanocomposites. Therefore, the corrosion rates of the alumina and Al 2 O 3 -5SiC-2CNT nanocomposite samples were compared with those of some metals and metal matrix nanocomposites 50,55-61 and presented in Table 2. It can be clearly seen that the Al 2 O 3 and Al 2 O 3 -5SiC-2CNT nanocomposite have relatively low corrosion rates. It is worth mentioning here that although the Al 2 O 3 -5SiC-2CNT nanocomposite showed higher corrosion rate than alumina, its corrosion resistance remained high compared with many metals and metal matrix nanocomposites. This corrosion behavior coupled with mechanical 45 as well as electrical and thermal properties 39 make these alumina hybrid nanocomposites potential materials in many applications such as cutting tools, dental implants, chemical and electrical insulators, and armouries 62 .

Conclusion
The corrosion behavior of spark plasma sintered alumina and Al 2 O 3 -5SiC-2CNT hybrid nanocomposite, in acidic and alkaline conditions, was investigated. From this work, the authors concluded the following: (1) The Al 2 O 3 -5SiC-2CNT hybrid nanocomposite had a higher corrosion rate than pure alumina in both acidic and alkaline solutions . (2) In the HCl solution, the corrosion rates of pure alumina and Al 2 O 3 -5SiC-2CNT nanocomposite were 5.07 x 10 -3 and 0.488 mpy, respectively. (3) In the NaOH solution, the corrosion rates of pure alumina and Al 2 O 3 -5SiC-2CNT nanocomposite were 12.8 x 10 -3 and 2.282 mpy, respectively. (4) The high corrosion rate of Al 2 O 3 -5SiC-2CNT hybrid nanocomposite was attributed to the increased number of grain boundaries due to grain refinement, presence of impurities on grain boundaries, and increased electrical conductivity of the nanocomposite.