Corrosion behaviour of maize husk reinforced aluminium (1070) metal matrix composites in chloride-sulphate solution

The effect of maize husk (MH) particulates on the corrosion resistance of 1070 aluminium alloy (AA70) matrix at 5% and 15% wt. content, and 150 μm and 300 μm particle size (AA70/MH composites) was studied in 3.5% NaCl, 0.0125 M H2SO4 and 3.5% NaCl/0.0125 M H2SO4 solutions. Potentiodynamic polarization technique, open circuit potential measurement, weight loss analysis, optical microscopy characterization, x-ray fluorescence and x-ray diffractometry were used for the investigation. Results show increase in MH %wt. content and particle size decreases the corrosion rate of AA70/MH composite. Data from potentiodynamic polarization and weight loss analysis shows AA70/MH composite at 15% wt. content and 300 μm particle size exhibited the lowest corrosion values of 0.135 mm y−1, 0.156 mm y−1 and 1.445 mm y−1, and −0.00043 mm y−1, 0.001 04 mm y−1 and 0.00218 mm y−1 in 3.5% NaCl, 0.0125 M H2SO4 and 3.5% NaCl/0.0125 M H2SO4 solutions. Optical representative images of the composites showed the presence of micro-pits on samples from NaCl solution, macro-pits and extensive surface deterioration from H2SO4 solution, and combined localized and total surface deterioration from NaCl-H2SO4 solution. Weight loss plots showed oxide formation significantly influenced the corrosion behavior of the alloys with respect to time. Open circuit potential plots showed AA70/MH composite at 5% wt. content and 150 um particle size was the most electronegative with the highest thermodynamic tendency to corrosion. Significant X-ray diffraction peaks from AA70/MH composite at 15% wt. content and 300 μm particle size showed the presence of corrosion resistant phase compounds of Al2O3 and Al(ZnS2) before and after corrosion.


Introduction
The difficulty in identifying pure metallic materials that meets the service requirements of industry and engineering applications necessitates the addition of metallic and non-metallic reinforcements and elements to improve their properties for weight decrease, increased efficiency and high performance. The physical, mechanical, electrical, thermal, thermo-mechanical, tribological and wear resistance properties of metallic alloys continually undergo improvement and modifications to satisfy operational and in-service requirements of consumers and industries [1,2]. Research into, and production of composites materials have been the major focus of attention in emerging materials industry for the production of products that combines excellent mechanical properties with corrosion resistance and aesthetic value [3]. Aluminium matrix composites are a class of lightweight aluminium alloys impregnated with specific materials responsible for the formation of complex intermetallic phases within the alloy matrix and heterogeneous microstructural properties [4]. The composite reinforcements could be in the form of continuous or discontinuous fibres, whisker or particulates in varying volume fractions integrated into a continuous phase. Research on aluminium matrix composites have proven beyond doubt the advantages of the composites over their base substrate metal [5]. The composites impart strength to the aluminium matrix that matches the strength of steel at half its weight. The resulting effect is the extensive application of aluminium matrix composites in the production of parts and components in agriculture, aeronautical, bicycle, automobile, mining, construction and marine industries [6][7][8]. Various composites reinforcements have been tested and used in aluminium alloy matrix [9]. Silicon carbide (SiC) and aluminium oxide (Al 2 O 3 ). enhances the tensile strength, compressive strength, wear resistance, hardness and density of aluminium alloy [10,11]. Impregnation of boron carbide (B 4 C) into the aluminium alloy matrix improves the hardness of the alloy [12]. Zirconium has been noted to increase the wear resistance of aluminium alloys [13]. Fly ash has also been used due to their economic value, availability and low density properties [14]. Other reinforcement such as rice husk, coconut husk, lignite ash has been used to achieve property requirements [15]. Research on graphene as a reinforcement material has been ongoing due to its higher dispersion and bonding properties with the matrix for application in automotive and aerospace industry [16][17][18]. Significant research on aluminium matrix composites has focused on their production and processing techniques, and their mechanical properties [19][20][21][22][23]. However, another major concern of reinforced matrix composites is the significant impact of the reinforcement on the corrosion rate of the matrix composites. Protective oxide film decreases with the corrosion rate in aluminum alloy-based composite [24]. The addition of reinforcement phase could cause discontinuities or flaws in the protective film. Composite reinforcements undergo electrochemical interactions with the matrix substrate, enhancing the microstructural deterioration of the composites [25][26][27]. Coupled with these galvanic reactions between the reinforcement and matrix also increase their rate of deterioration. This tends to be localized along the matrix interface leading to rapid penetration detrimental to structural stability of the composites in service. Localized attack on structural and compositional in-homogeneities can occur within the matrix leading to consequential failures. Corrosion resistance properties of aluminium matrix composites is a significant prerequisite for judging the performance and properties of the composites for structural applications [28]. Resistance of aluminium matrix composites to corrosion depends on its composite composition, manufacturing methods, dispersion of reinforcements in the continuous phase and environmental factors [29]. It must be noted that manufacturing processes enables uniform dispersion of reinforcement materials within the aluminium matrix invariably influencing the grain structure of the matrix composites [30][31][32]. Despite the shortcomings regarding the corrosion resistance of aluminium matrix composites, some investigations have shown that presence of some reinforcements in aluminium matrix do enhance the corrosion resistance of the matric composites. Obi et al studied the effect of fly ash reinforcement on the corrosion resistance of A535 aluminium alloy and its composites at 10 wt.% to 15 wt.% fly ash. Results show the general corrosion rate of the composite increased with increase in fly ash content while the critical pitting potential decreased with increasing fly ash content [33]. Corrosion behavior of the Al7075 composites prepared with carbon nanotubes as the potential reinforcements was studied. The composites were added to the base material in 2 wt.% to 6 wt.% in HCl solution. Results show decrease in corrosion rate with increase in carbon nano tube concentration [34]. The corrosion resistant properties of aluminium 6061 alloy reinforced with zircon particle composite, was studied in sodium hydroxide solution by open circuit potential methods. Results show potential decreases with respect to exposure time before attaining passivation [35]. Mamatha et al studied the corrosion resistance of aluminium 7075/ red mud metal matrix composites in neutral chloride solutions. Corrosion rate was observed to decrease with increase in the exposure time reinforcement content of the composites [36]. Narayanan et al evaluated the corrosion behaviour of nanographene oxide reinforced aluminium metal matrix composites. Results show corrosion rate decreased with increasing immersion time periods [37]. Mallireddy and Siva studied the corrosion resistance of aluminium matrix composite (7010-TiB) at 5-10 wt.% content. The matrix composite exhibited significant resistance to corrosion at 10% TiB shows [38]. Aluminum (Al6061) reinforced with ZrO 2 was investigated at 0% to 15 vol% of ZrO 2 for its corrosion resistance properties. Corrosion rate of substrate Al6061 was determined to be higher than its composite counterparts [39]. Shargh [40] studied the influence of post weld heat treatment on electrochemical and microstructural characteristics of explosive bonded interface of AA5083/ AA1050/SS 321 tubes at 350 and 450°C. Results show variation in thickness and concentration of alloying elements in locally melting zone. Rate of corrosion and electrical charge transfer was observed to decrease. The corrosion resistance of aluminium/copper joint fabricated through explosive welding process was studied by Jandaghi et al [41]. Results show that the higher the stand-off distance the greater the increase in corrosion potential at the interface. Consequent upon the above and in contribution to research on the effect of reinforcements on the mechanical, thermal and corrosion resistance properties of aluminium composites, application of maize husk a sustainable materials of plant origin was studied as particle reinforcement on aluminium matrix for possible replacement of the conventional inorganic reinforcements.

Experimental methods
Materials and preparation AA1070 aluminium metal (AA70) obtained from Aluminium Rolling Mills, at Ota town in Ogun State, Nigeria was analysed and determined to have the following (wt. %) composition presented in table 1. Maize husk (MH) was obtained from the open market in Lagos state, Nigeria. It was dried for two weeks in open air and processed in line with the steps detailed in [39], which includes manual cleaning, drying, chopping, and milling to attain a powdery texture. The MH was subsequently dried in an oven before being milled and sieve analysis was performed to filter out the coarse grains and retained powder with particle sizes of 150 μm and 300 μm. The texture is important for homogeneity of the resulting MH composite with focus on the particle size, mix ratios, and particle behavior due to their influence on the synthesis of aluminum based composites. The stir casting method is the process route used in this research. An inclined furnace of 20kg capacity with a graphite crucible within was employed to melt AA70 for approximately 45 min at 660°C. The temperature of the molten aluminium was observed with using K-type thermocouple. MH at particle sizes 150 μm and 300 μm, and at wt. % of 5% and 15% was added to the molten AA70 at vortex of 500 rpm (stirred mechanically) before being cast into prepared sand moulds and cooled for 24 h. The resulting AA70 composite (AA70/MH) at 5% and 15% MH wt. % content, and 150 μm and 300 μm MH particle size are of cylindrical dimension with diameter of 110 mm and height of 30 mm. The designation of AA70 composite with respect to MH particle size and wt.% content are shown in table 2. They were cut with power hacksaw and underwent metallographic preparation using abrasive emery papers (80, 320, 600, 800 and 1000 grit), washed with deionized water and acetone for subsequent electrochemical tests. Recrystallized NaCl obtained from Titan Biotech, India was formulated in percentage concentration of 3.5% electrolyte. Analar grade H 2 SO 4 was formulated in molar concentration of 0.0125 M, and a third electrolyte consisting of the admixture of 3.5% NaCl and 0.0125 M of H 2 SO 4 (3.5% NaCl/0.0125 M H 2 SO 4 ) was also prepared.

Potentiodynamic polarization test
Potentiodynamic polarization was performed on AA70 and AA70/MH specimens with MH particle sizes of 150 μm and 300 μm at 5% and 15% wt.%. Pt counter electrode, Ag/AgCl reference electrodes and acrylic casted AA70 and AA70/MH working electrodes were placed within a transparent beaker filled with 200 ml of each electrolyte (3.5% NaCl, 0.0125 M H 2 SO 4 and 0.0125 M H 2 SO 4 /3.5% NaCl). The electrodes were attached to Digi-Ivy 2311 potentiostat linked to a computer system. Polarization curves were plotted at sweep rate of 0.0015 V s −1 and set at potentials of −1.4 V to −0.55 V, −1.35 V to 0.08 V and −1.25 V to −0.60 V respectively for the NaCl, H 2 SO 4 and NaCl-H 2 SO 4 solutions. Corrosion current density, C d (A cm −2 ) and corrosion potential, C p (V) values were obtained from the crosslink of the anodic-cathodic polarization curves produced by Tafel extrapolation. Corrosion rate, C R (mm y −1 ) and inhibition efficiency, η (%) were determined from the expression below; ); E q indicates alloy equivalent weight (g). 0.00327 stands for corrosion rate constant. Open circuit potential (OCP) measurement and optical microscopy characterization OCP measurements were performed at step potential of 0.1 V s −1 for 2400 s in 3.5% NaCl, 0.0125 M H 2 SO 4 and 0.0125 M H 2 SO 4 /3.5% NaCl solution with the same electrode configuration as the potentiodynamic polarization test. Optical images of AA70 and AA70/MH (at 150 μm and 300 μm particle size, and 5% and 15% wt.% content) surface morphologies were obtained and analysed with three different microscopes (Omax trinocular metallurgical microscope, HD G1200 Digital Microscope and Professional HD USB Microscope).

Weight loss analysis
Weight of the specimens were recorded at 24 h interval with Ohaus PA114 measuring device with resolution of 0.0001g, maximum capacity is 110g, repeatability is 0.1 mg and linearity is±0.2 mg. Weight loss was determined from the subtraction between the initial weight of AA70 and AA70/MH (maintained for 264 h) and subsequent weight measured at 24 h interval for a total of 240 h. Corrosion rate of AA70 and AA70/MH was determined from the equation below; AT 2 R W denotes weight loss (g), ρ denotes density (g cm −2 ), A denotes area (cm 2 ), and T symbolizes time of exposure (h). Inhibition efficiency (η) was enumerated from the following equation; 1 ω 1 denotes weight loss of AA70 and AA70/MH from the acid solution without the distillates while ω 2 denotes weight loss of AA70 and AA70/MH at precise extract concentration.
X-ray diffractometry X-ray diffraction plots and pattern list of the phase compounds, precipitates and metallic complexes on AA70 and AA70/MH composites (samples A, B and E) surface were produced. The data was obtained after analysis with Rigaku D/Max-lllC x-ray diffractometer at scanning rate of 2°min −1 in the 10 to 140°at room temperature with a CuKa radiation set at 40 kV and 20 mA in 2θ. The diffraction data (relative intensity) obtained was matched with the standard data of minerals from the mineral powder diffraction file, ICDD which contained and includes the standard data of more than 3000 minerals.

Results and discussion
Potentiodynamic polarization studies and optical microscopy characterization  figure  1(b) exhibits corrosion potential at relatively higher values compared to the plots in figures 1(a) and (c) which signifies a higher influence of SO 4 2− anions on the redox electrochemical processes occurring on the samples. This is evidenced by the higher cathodic-anodic Tafel slopes on the plots. Shift in corrosion potential occurred on the polarization plot in figures 1(b). This shift is in proportion to the variation in MH %wt. content and particle size due to cathodic polarization responsible for the weakening of the inert oxide. This causes selective dissolution on the composite surface [42,43].      4 2− anions completely destroyed the protective oxide exposing the substrate alloy and composite fractions to corrosion. Beyond sample B, corrosion rate decreases progressively to 1.445 mm/y for sample E. Samples in NaCl solution exhibited the lowest corrosion rate values. At sample A corrosion rate is 0.276 mm/y. Corrosion rate increased to 0.321 mm/y for sample B. However, beyond sample B corrosion rate decreased similarly to the trend in NaCl-H 2 SO 4 solution to 0.135 mm/y for sample E. Generally, sample B exhibited the highest corrosion rate in all the solutions studied. This shows AA70/MH composite at MH 5% wt. content and 150 μm particle size is the most vulnerable to accelerated corrosion. The higher the MH % wt. content and particle size, the lower the corrosion rate of the Al composite. MH reinforcement at 5% wt. content and 150 μm particle size (sample B) causes inhomogeneity's on the Al composite surface resulting in penetration by the corrosive anions through the discontinuity on the inert protective oxide [49]. Previous research has reported on decrease in corrosion rate with increase in reinforcement %wt. content and particle size. probably due to improved interfacial bonding between the MH particles, thus hindering diffusion of corrosive anions [50,51].

Weight loss and optical macroscopy analysis
Data on corrosion rate results for samples A to E from weight loss analysis in 3.5% NaCl, 0.0125M H 2 SO 4 and 3.5% NaCl/0.0125M H 2 SO 4 solutions at 264 h of exposure are shown in table 4. Figures 3(a) to (c) shows the corresponding plots of corrosion rates versus exposure time throughout the exposure hours for the samples from the solutions earlier mentioned. Optical macroscopic images (mag. x5) of samples A to E are shown from figures 4(a) to (d). Figure 4(a) shows the representative etched sample image (mag. x5) before corrosion. The microstructural features are clearly visible. Figures 4(b)-(d) shows the representative images (mag. x5) of the samples from 3.5% NaCl, 0.0125M H 2 SO 4 and 3.5% NaCl/0.0125M H 2 SO 4 solutions. Figures 5(a) to (c) shows the shows the corresponding representative images at mag. x10 of the samples from 3.5% NaCl, 0.0125M H 2 SO 4 and 3.5% NaCl/0.0125M H 2 SO 4 solutions. Table 4 shows AA70 and AA70/MH composites (sample A to E) generally exhibited decrease in corrosion rate at 264 h of exposure in NaCl solution due to formation and growth of the inert oxide on the surface of the samples and the formation of complexes with adsorbed Cl − anions. However, it is observed that sample B exhibited the lowest amount of oxide formed on its surface with final corrosion rate of −0.00007 mm y −1 while sample E exhibited the highest amount of oxide complexes formed on its surface. Sample B also exhibited the highest corrosion rate in H 2 SO 4   solution showed dentritic microstructures with white spots caused by entrapment of moisture on the sample surface forming Al 2 O 3 . This proves the partial formation of protective oxide on the surface in addition to corrosion along the grain boundaries of the Al-composite surface. The corresponding higher resolution of the image in figure 5(a) shows corrosion along the grain boundaries and formation of micro-pits. The optical image in figure 4(c) relating to corrosion from H 2 SO 4 solution reveals the absence of the whitish protective oxide. Secondly, the dentritic microstructure is significantly more visible due to greater deterioration of surface properties of the composite along the grain boundaries. The corresponding higher resolution of the image in   Tables 5 and 6 shows the normalized x-ray fluorescence data for AA70 and AA70/MH composite samples (sample A, sample B and sample E) before and after corrosion in 3.5% NaCl/0.0125M H 2 SO 4 solution. The elements/compounds identified are shown in the tables consisting of TiO 2 , CaO, Na 2 O, SiO 2 , Al 2 O 3 and MgO. The two major observations from the tables are (i) the % composition of the elements (excluding Al 2 O 3 ) for the samples studied decreased significantly after corrosion. This is due to oxidation of the composites alloy in the presence of debilitating action of combined Cl − and SO 4 2− anions in the acid-chloride solution. (ii) Al 2 O 3 the principal compound responsible for corrosion increased in concentration due to the adsorption of O 2 atoms in the aqueous media despite corrosion of the composite. Sample A being the monolithin Al alloy exhibited the highest % concentration of Al 2 O 3 . However, after corrosion sample E which exhibited the least corrosion rate from weight loss and potentiodynamic polarization exhibited the highest concentration of Al 2 O 3 , representing a 59.87% increase in Al 2 O 3 formation on the composite and aiding in its corrosion resistance. A significant decrease in SiO 2 was also observed. SiO 2 is a major component of the corrosion resistance properties of Al composite alloys with sample B exhibiting the highest value after corrosion compared to sample A before corrosion.

Open circuit potential measurement
The open circuit potential plots for AA70 and AA70/MH composites (samples A-E) from 3.5% NaCl, 0.0125 M H 2 SO 4 and 3.5% NaCl/0.0125 M H 2 SO 4 solution are shown in from figures 6(a) to (b). Comparison of the plots in figures 6(a) and (b) shows the significant variation in the starting potential of the plots. Corrosion potential of the plots in figure 6(a) initiated at values between −0.800 V and −0.757 V. Significant potential transients are visible on the plots due to the electrochemical action of Cl − anions on AA70 and AA70/MH composite surfaces. The transients arise as the inert protective oxide on AA70 undergoes active-passive transition where the anions compete with O 2 for adsorption unto the surface. The transients are more visible on the plot for sample B which invariably had the highest corrosion rate from polarization test. There is the possibility of galvanic action which also contributes to the transients on the plots. The plots for sample D and E exhibited the relatively minimum potential transient. These samples (AA70/MH15%/150um and AA70/MH15%/300um) shows MH at higher %wt. and particle size significantly improves the corrosion resistance properties of the composite. Nevertheless, the plots generally showed thermodynamic stability in the presence of Cl − anions. At 2400s, the plots culminated at potentials between −0.679 V and −0.740 V.
Comparing the plot configuration in figures 6(a) to (b), it is visible that the plot configuration shows significantly less potential transients compared to the plots in figure 6(a). This shows nature of anion contents significantly impacts the electrochemical behavior of the composite. Firstly, the plots occurred at significantly more electropositive corrosion potentials between −0.348 V and −0.542 V. This signifies lower tendency to electrochemical activity in the presence of SO 4 2− anions compared to Cl − anions. Secondly, the potential transients in figure 6(b) fizzled out at around 700s, before which potential transients were significant and thirdly, the corrosion potential plots shifted from electropositive values to electronegative values before attaining thermodynamic stability at varying potentials and exposure time. The reason for the high potential transients in   X-ray diffractometry X-ray diffraction analysis was performed on AA70 and AA70/MH composite samples (sample A, sample B and sample E) before and after corrosion test in 3.5% NaCl/0.0125 M H 2 SO 4 solution. Figures 7(a) and (b) shows the diffraction plots for sample A before and after corrosion. Figures 8(a) and (b) shows the diffraction plots for sample B before and after corrosion while figures 9(a) and (b) shows the diffraction plots for sample E before and after corrosion. The diffraction peaks for sample A before corrosion (figure7(a)) at 2θ are 18 figure  7(b). The phases in figure 9(a) is dominated by Al 2 O3 and Al(ZnS 2 ). These compounds are significantly responsible for passivation of Sample E (AA70 Al at 15% MH wt. content and 300 μm particle size) as indicated by sample E exhibiting the highest corrosion rate. The passivation of the composite is influenced by the %wt. content and particle size of MH in sample E. The phase in sample E occurred at 2θ=20.280, 36.204, 37.302, 41.402, 63.181, 78.196, 80.282 and 115.214 corresponding to peak intensities of 770, 800, 750, 1300, 700, 1300, 800 and 800. After corrosion, the remaining dominant phase is Al(ZnS 2 ) an insoluble precipitate which occurred at 2θ=40.860 and 62.618 corresponding to intensities of 1400 and 1300. The insoluble precipitate provides insoluble pseudo protection of the composite surface thus limiting the redox electrochemical processes responsible for corrosion.

Conclusion
Corrosion resistance of 1070 aluminium matrix composites reinforced with maize husk particulates at 5% and 15% wt. content, and 150 μm and 300 μm particle size was studied. The following conclusions were obtained from the research.
• Aluminium matrix composite at 5% wt. content and 150 um particle size of maize husk exhibited the least resistance to localized and corrosion from potentiodynamic polarization and weight loss analysis in chloride, sulphates and chloride-sulphate solution compared to the monolithic aluminium alloy and other composite fractions.
• Corrosion occurred marginally along the grain boundaries in chloride solution in addition to formation of micro-pits and scattered whitish precipitates. • In sulphates solution, the extent of corrosion along the grain boundaries was much higher coupled with general surface deterioration. However, in the combined chloride-sulphate solution, total surface degradation was observed.
• Increase in maize husk weight content and particle size progressively decreased the corrosion vulnerability of the composite wherewith maize husk at highest weight content of 15% and 300 um particle size exhibited the lowest corrosion rate values obtained.
• Weight loss plots from chloride and sulphate solution showed significant increase in corrosion before relative stability compared to the plots from chloride-sulphate solution where significant decrease in corrosion was observed before relative stability.
• Open circuit potential plots showed significant potential transients in chloride and chloride-sulphate solution coupled with thermodynamic corrosion tendencies compared to the sulphate solution which where the transients were comparatively fewer.
• X-ray diffraction patterns showed significant variation between the corroded and non-corroded composites samples, and between the composite samples with respect to maize husk weight content and particle size in aluminium matrix.