Electrochemical investigation of the corrosion susceptibility of hybrid reinforced Al6063 with SiC and PKSA in 1.0 M sulfuric acid environment

The recycling of agro‐waste as complementary reinforcements has received significant recognition in the development of aluminium matrix composites. Hence, this study examines the corrosion behavior of Al6063 reinforced with hybrid SiC/PKSA (palm kernel shell ash) particles. Through various ratios of SiC and PKSA particles in Al6063 alloy, composites were fabricated by double stir casting. Samples were cut and metallographically prepared for 1 M H2SO4 solution corrosion experiments. Gravimetric, potentiodynamic polarization and electrochemical impedance spectroscopic analyses were employed. The composites corroded initially at relatively high rates, gradually declining during long immersion times in the acidic solution. The intersection of reinforcements at the general surfaces of the composites where flawed oxide skins predominate acted as active sites for corrosion initiation. From potentiodynamic polarization studies, the corrosion currents increased with time for all specimens, with A9 being 1075.65 μA/cm2 at 72 h as against 857.99 μA/cm2 at 24 h of measurement. The corrosion potentials for all the specimens hovered around −654.00 to −647.22 mV. Bode plots revealed similar electrochemical reactions over all the substrates' surfaces. The relative corrosion resistance by the specimen depends on the oxide films' nature as the cathodic interfacial reinforcements dropped off into the acidic environment.

composite materials. 5][8][9] Aluminium is the principal metal that has been extensively utilized as a matrix due to its excellent physical and mechanical properties, such as good weight-to-strength ratio, good wear resistance, toughness, high specific strength, and so on. 8,10,11When Al matrix is reinforced with a single ceramic reinforcement, they are called monolithic composites. 1,128][19] The utilization of two or more ceramic particles and/or agro-waste/industrial waste ash is termed hybrid reinforced composites.1][22] However, for the production of MMCs, the stir casting procedure is more favored because it is cheap, parameters are easily controlled, and probably the best method of producing MMCs.Through it, about 30% volume fraction of reinforcement can be utilized to produce composites. 1,23ncluding reinforcements in MMCs primarily initiates pits around the secondary particles within the matrix.A second phase inclusion into a metal alloy can improve the physico-mechanical properties of the alloy, which significantly alters its corrosion behavior.Various interactions such as chemical, electrochemical, or physical interaction of reinforcements with the matrix could cause accelerated corrosion. 4The load-bearing capacity of MMCs can be reduced by corrosion, which could result in catastrophic failures; hence, limiting MMC's application in corrosive environments.The following are frequent factors that determine the MMCs' corrosion behavior; alloy composition, the microstructure of the matrix, the metal alloy and dispersoid, and the method adopted for composite production. 24The corrosion behavior of MMCs is a pertinent parameter in assessing the potential areas of utilization of MMCs as structural materials. 4Considerable works have been done on the physicomechanical and wear properties of MMCs; however, further attention should be given to the corrosion susceptibility of hybrid MMCs, most notably those produced with derivatives of agro-wastes in partial or total replacements of synthetic reinforcements in the matrix alloy.
Although many successes have been recorded with improvement in the mechanical properties of MMCs, corrosion is a pertinent challenge that tends to hamper the complete utilization of MMCs.Cheng et al. 25 explored the corrosion behavior of MMCs produced using AA7075, which was reinforced with Si particulate via the multilayer spray deposition method.The electrochemical study determined the effect of SiC inclusion on the corrosion conduct of the MMCs in a saline environment.The study reported an increase in resistance to corrosion due to SiC addition.Using multiwall carbon nanotube (MWCNT) at various percentages as reinforcement in aluminium alloy AA5083, which was produced via the compo-casting method, Ratna Kumar et al. 26 found that there was an increase in corrosion resistance due to the MWCNT inclusions.
The influence of red mud addition to Al6061 as a reinforcement on its corrosion behavior in seawater was examined by Krupakara and Ravikumar. 3The MMCs (AA6061/Red mud) were produced using the vortex method.A decline in the corrosion rate of the sample in seawater was reported as exposure time increased.Zakaria 4 investigated the corrosion behavior of aluminium metal composites (AMCs) produced using a powdery metallurgy route using Al powder and SiC.The produced AMCs were immersed in 3.5 wt% NaCl environment at both ambient and elevated temperatures.The influence of SiC reinforcing particulate size and volume fraction on the surface morphology and corrosion behavior of the substrates were investigated.The resistance to corrosion by the composite was better compared to the unreinforced matrix alloy.SiC particle size reduction as well as increases in volume fraction yielded decreased corrosion rate.The MMCs production of composites with SiC and rice husk ash (RHA) incorporation into Al6063 matrix with corrosion behavior investigation was done by Haridass et al. 5 The study utilized the liquid metallurgy route (stir casting) for the synthesis of the composites, while AlCl 3 solution was prepared as the aggressive media.In the study, a rise in the hybrid reinforcing particulates addition in the matrix alloy led to the better corrosion resistance of the substrates compared to the unreinforced alloy.
In practical applications, corrosion is unavoidable and may lead to catastrophic failure.Both safety hazards and economic damage are experienced as a result of this worst failure process. 27,28Therefore, there is a necessity to investigate the relationship between reinforcing particles and the corrosion behavior of the MMCs.The corrosion behavior of MMCs developed using TiB 2 and TiC reinforcing particles in the Al matrix was investigated by Jinfeng Nie et al. 29 It was discovered that corrosion initiation of composites occurred at the interface between the matrix and the reinforcements.Prabhu and Rao 30 investigated the corrosion behavior of Al6063 alloy under three different concentrations and temperatures in phosphoric acid and sodium hydroxide media.The impact of the corrosion in NaOH solution was more severe than that in the phosphoric acid solution.The corrosion rate of matrix alloy showed a direct relationship with the concentration of both solutions; that is, the greater the concentration of the solution, the more the rate of corrosion.The corrosion mechanism of the dissolution of the Al alloy in the phosphoric solution and sodium hydroxide solution was highlighted.MMCs developed with the hybrid combination of Al-Mg-Si alloys and reinforcing particles of rice husk ash and alumina were subjected to corrosion tests.It was revealed that with single particles reinforced sample (Al-Mg-Si/10 wt% Al 2 O 3 ), there was a superior corrosion resistance than the hybrid composites in 3.5% NaCl medium. 31Ao et al. 32 studied the mechanism of degradation of Al6063 matrix composites with TiC and Al 2 O 3 reinforcing particles in NaCl solution using different experimental techniques.There was grain refinement of the composite structure due to the incorporation of the reinforcements.This increased the AlFeSi phase content at the grain boundary.The result revealed easier Cl − adsorption on the AlFeSi phase surface than on TiC particles and the AlMgSiCu phase.This adsorption created an intense absorption energy between the AlFeSi phase and Cl − which could disrupt the oxide film leading to pitting corrosion around the phase.Hence, the corrosion resistance of the MMCs reduces with the inclusion of the reinforcement particulates.
Literature on the production of advanced materials with hybrid reinforcements of synthetic materials (SiC) and agro-waste derivatives (palm kernel shell ash [PKSA]) for metal matrix composites is rare.Recent works of Edoziuno et al. 33 and Aigbodion and Ezema 34 have been limited to using PKS and PKSA nanoparticles as a single reinforcing particulate in the matrix for AMCs production.The tribological and physical properties of Al/SiC/PKSA have been previously published by Ikubanni et al. 8 Recently, Ikubanni et al. 35 reported the corrosion behavior of hybrid AMCs produced using SiC and PKSA as reinforcing particulates in 3.5% NaCl environment.Hence, there is a need to gather more information on the effects of utilizing PKSA and SiC as supporting particulates and how the incorporation in matrix alloy would affect the corrosion susceptibility of the composites.Studying the corrosion resistance of AMCs is germane, especially for parts that will be utilized in corrosion-prone media like acidic and saltwater environments or systems.Therefore, this current study aimed to investigate the corrosion behavior and vulnerability of synthesized AMCs (Al6063/SiC/PKSA) in a 1.0 M H 2 SO 4 solution.The gravimetric method for weight loss determination of the composites in the solution and electrochemical investigations were carried out.The corroded surfaces of the specimens were examined via photography.

Materials
The materials used to produce the metal matrix composites are Al6063, SiC, and PKSA.The metal matrix used in this study is Al 6063, while the reinforcements utilized are SiC and PKSA.The Al6063 was purchased from an aluminium industry in Lagos, Nigeria.SiC (99% purity) was obtained from a chemical vendor, Phemtech Scientific, in Akure, Nigeria.

Preparation and synthesis of PKSA sample
The palm kernel shell (PKS) was purchased in a palm oil-producing area in Osogbo and sorted to remove foreign materials.Later, the PKS was washed to remove unwanted dirts and dried under normal atmospheric conditions for 3 days.The PKS was put in a crucible placed in a muffle furnace, which was set at 900 • C for 4 h for calcination.The elemental compositions of PKSA thus obtained were determined using x-ray fluorescence (XRF) analysis (TEFA ORTEC automatic x-ray F) as previously reported. 2The PKSA was characterized using fourier transform infrared (FTIR) spectroscopy (PerkinElmer 1725× model) for functional group determination, x-ray diffractometer (XRD; PANalytical Empyrean diffractometer) for the phases, and scanning electron microscope-energy dispersive spectroscopy (SEM-EDS; JEOL-JSM 7600F model) for the microstructural analysis. 2he chemical constituents of both the Al6063 alloy and PKSA are displayed in Tables 1 and 2, respectively.The mean particle sizes of the SiC and the PKSA reinforcements were 30 and 40 m, respectively.Abbreviation: PKSA, palm kernel shell ash.

Synthesis of the metal matrix composites
Table 3 displays the designations of the composites produced.The two-stir casting route was used to create the composites, as demonstrated by several authors. 10,36The reinforcing particulates needed for the production of each composite were determined using the charge calculation method.To remove moisture and improve the wettability of reinforcements in the matrix, the reinforcements were preheated at 250 • C. To ensure the matrix alloy is wholesomely melted using a gas-fired crucible, the matrix alloy ingots was subjected to a high temperature of 750 • C, which is above its liquidus temperature.After, pre-heated reinforcing particulates were poured into the molten alloy in its semi-solid state.Manual stirring was done for 10 min before superheating the slurry to 800 • C. After, there was mechanical stirring action for another 10 min at a stirring speed of 400 rpm.After the homogenous mixing, the obtained slurry was poured into a freshly prepared sand mold cavity for solidification.Cylindrical-shaped products of 150 mm long and 40 mm diameter were obtained and kept for further experimentation activities.

Substrate preparation
According to Table 3, each sample was cut to dimensions (∅ 30 × 3 mm 2 ) from the cylindrical-shaped products.Different grit sizes of SiC paper, ranging from 240 to 1000, were used to grind the surfaces of the samples.After, acetone was utilized as a cleansing agent before being washed with water.The cleansed samples were dried and weighed with a digital electronic balance before being kept in a desiccator to prevent contamination before usage for the corrosion experiment.

Gravimetric method
The weight loss was evaluated to determine the corrosion rates of the substrates using a digital weighing scale with 0.1 mg accuracy.The solution used as the corrosion environment was 1.0 M H 2 SO 4 solution prepared from a laboratory-grade stock product from BDH Chemicals Limited.Before immersion into the aggressive media, the samples were initially weighed (W B ).After the pre-determined immersion time, the substrates were removed, cleansed, and dried before reweighing (W A ).These were performed at room temperature for 15 days.Measurements were taken at daily intervals and done in triplicates for data reliability and reproducibility.The average values were recorded as final readings.
From weight loss measurements, corrosion rates were determined in mm/year.The corroded surfaces were observed using photography.The weight loss measurement was determined using Equation (1) in line with ASTM G31-72, 37 while the corrosion rate was evaluated using Equation (2).
where CR, T, A, W, and D are the corrosion rate (mm/year), exposure time (h), sample area (cm 2 ), weight loss (mg), and density of the material (g/cm 3 ), respectively, while K is a constant (8.766 ×10 4 ).

Electrochemical investigation
Accelerated electrochemical investigations were performed on the fabricated composites using electrochemical impedance spectroscopy (EIS).The experimentations were executed in 1.0 M H 2 SO 4 solution at 25 • C. The specimens embedded in resin, which have 1 cm 2 active surfaces, were cleaned and polished before being placed in an electrochemical cell, served as working electrodes, platinum as the counter electrode, and silver/silver chloride (Ag/AgCl) served as the reference electrode.The cell was initially left for 30 min to attain an open circuit potential (OCP).The impedance of the specimens was measured using a test frequency range from 10 5 to 0.01 Hz with a 10 mV applied AC voltage amplitude using Autolab PGSTAT 302 N equipment.Using a scan rate of 1.6 mV/s, the range of values for which the polarization data were accomplished was between −1.5 and +1.5 V.The immersion periods of the substrate were 24 and 72 h in 1.0 M H 2 SO 4 solution using the equipment set-up.The Tafel plot was utilized to determine the corrosion potential (Ecorr) and corrosion current (Icorr) from the logarithm graph of current versus potential.The EIS data were obtained via the potentiodynamic equipment.The EIS data were utilized in plotting the Nyquist and Bode plots.Nova software was used to obtain the fitting EIS data.

FTIR analysis of the PKSA particulate
The FTIR spectrum of the PKSA particulate was compared with that of the raw PKS, as shown in Figure 1.The PKS revealed the hydroxyl group O-H bonding at 3430 and 3301.25 cm −1 peak range.This assignment was due to the O-H stretching of moisture in cellulose, as previously reported. 38,39At 2945.74 cm −1 , an assignment of C-H stretching in the methyl group in cellulose was observed. 38,40Other peaks obtained from the spectra are presented in Table 4, in which a detailed discussion has been fully reported elsewhere. 2With the peaks detected in the raw PKS, the functional groups observed made it categorically clear that the raw PKS is a bio-material (biomass material).However, due to the transformation caused by thermal energy on the raw PKS to become PKSA, the PKSA particulate revealed a different spectrum than the raw PKS (Figure 1B), as in Table 4.In the raw PKS, C=O, C-O, C-O-C, and C-H stretch bands were present since the material was a biomaterial.However, these bands were absent in the PKSA due to the high thermal energy the PKS was subjected to in synthesizing the PKSA. 41This phenomenon is linked to the decomposition and degradation of the constituents such as cellulose, hemicellulose, and lignin of the raw PKS. 41,42Si-O and Si-C detection in the spectrum of the PKSA indicate that a refractory reinforcement material was formed.This reinforcement particulate can improve wear, assist in the indirect strengthening mechanism, and reduce the thermal expansion coefficient between the matrix-reinforcement interface.

Analysis of the PKSA particles via XRD technique
The XRD spectra of the raw PKS and PKSA particles are displayed in Figure 2. As Ikubanni et al. 2 reported earlier, the predominant crystalline structure observed in both samples is quartz (SiO 2 ), having a hexagonal crystal system.Other peaks detected are cubic crystal system, titanomagnetite (TiFeO 4 ), hexagonal crystal system, hematite (Fe 2 O 3 ), and tetragonal crystal-shaped rutile (TiO 2 ).Although both samples present quartz as dominant, the quartz in the PKSA was more than the raw PKS.The abundance of SiO 2, as observed from the XRD results, confirms the XRF analysis carried out on the samples.Silica was the principal compound in the raw PKS and the thermally generated PKSA.In addition, the FTIR  analysis confirmed the presence of Si, SiC, and SiO 2 in the PKSA sample.The implication of these structural constituents is to assist in the strengthening capacity of the reinforcement in the matrix alloy.

Microstructural analyses of PKS and PKSA
The morphologies of the raw PKS and PKSA were characterized using SEM (Figure 3A,C).In the SEM of the raw PKS, porous surface features and variously oriented structured materials were observed.As a result of the thermal treatment of the raw PKS, honeycomb features and enlarged pores were observed in the PKSA particles.The predominance of silica in the particles of the raw PKS and PKSA was confirmed with the EDX spectra (Figure 3B,D), where the strongest elemental intensity of approximately 60% was obtained.The propensity for getting SiO 2 is high due to the association of silicon (Si) and oxygen, which the EDX detected. 43The formation of the oxides of Ca, Fe, and Mg is possible with oxygen.These oxides in the PKSA are hardeners and strengtheners, making the PKSA particulates useful in MMCs development.

Corrosion behavior of the substrate in 1.0 M H 2 SO 4 environment via gravimetric analysis
The variation of mass changes with exposure time is shown in Figure 4.The observation of PKSA-SiC hybrid reinforced composites immersed in 1.0 M H 2 SO 4 solution showed that mass loss for all samples increased with exposure time.The relatively high mass losses were indications of the high dissolution rates of the samples in the acidic solution.This might be attributed to the unstable and/or flawed films formed on the samples, which did not provide adequate protection against corrosion.This was also reported by Alaneme and Bodunrin. 44However, after 12 days of exposure, there was a gradual decline in mass loss for the specimens.Aluminium matrix is generally known to develop resistance to corrosion through the formation of passive oxides.These passive oxides mitigate further disintegration of the material in corrosive media.
The difference in corrosion rates with the time of exposure is shown in Figure 5.The curves for corrosion rates show that the films formed on the samples were unstable due to the intermittent formation of a nonprotective film of corrosion products.The mechanism of corrosion based on the observed corroded samples, as seen in Figure 6A,B, is pitting coupled with removing particulate reinforcements, likely due to galvanic corrosion, reinforcements usually cathodic to the matrix.The removal of the particulates led to the uneven topography observed on the specimens.The pits can be seen dispersed within the troughs and ridges, which decorated the surfaces of the specimens.These align with the studies of Alaneme and Bodunrin, 44 who reported similar physical observations on composite samples after exposure to acidic environments.
Interestingly, the PKSA-SiC hybrid reinforcements in MMCs of this study did not confer improved corrosion resistance on the composites in sulfuric acid.However, Alaneme et al. 12 reported enhanced corrosion resistance of the composites when groundnut shell ash and SiC were used as reinforcements in the Al6063 matrix.This observation may have arisen due to short exposure periods during instantaneous electrochemical measurements, which was observed for some of the samples in this study at 24 h of exposure.When pure Al 2 O 3 was used as reinforcement in Al-Mg-Si alloy, numerous but shallow pits were formed in the samples. 44A high rate of dissolution of the substrates in the H 2 SO 4 solution was observed, which increased with exposure time.
The presence of silica and alumina, by implication, did not inhibit the adverse effect of Al 4 C 3 phase formation on the resistance of the composites to corrosion.On the other hand, their inclusions led to galvanic corrosion between the matrix and reinforcing particulates.These inclusions usually aid corrosion by being cathodic to the matrix.From the results obtained for corrosion rates in this study, the composites will be helpful in engineering parts only by applying additional corrosion control measures like inhibitors/coatings.
The pattern with which the inclusions were removed from the composites, Figure 6, significantly pointed to their relatively even distribution within the matrix.Thus, confirming the double stir casting method as a veritable route for composite production.For instance, if some parts of the composites did not experience dissolution, it would have been said that the reinforcement inclusions were not evenly dispersed within the matrix alloy.From Figure 5, a general trend in reducing corrosion rates can be observed.It is generally known that corrosion usually starts initially rapidly at a rate that decreases with the time of exposure of specimens.This observation is due to several factors, including the development of corrosion products adhering to the surface of the substrate that limit the corrosive contact with the substrate, increase in pH of the solution, and reduction in the concentration of corrodent. 45he introduction of reinforcing particles in a matrix alloy hampers the corrosion behavior of the MMCs. 4 In this study, the corrosion resistance of the composite was reduced in the SO 4 2− containing environment.Although, the aluminium surface acted as a barrier layer due to the oxide film developed to protect the matrix alloy from corrosion. 46,47The occurrence of pitting corrosion in 1 M H 2 SO 4 is ascribed to the interruption of the flawed film of the composite.The presence of SO 4 2− led to the local destruction of the oxide film, which resulted in pits initiation.The pits have small surface holes owing to the highly localized anodic reaction sites.The pits' appearance on the surface of the composites is displayed in Figure 6.Pit initiation occurs at localized areas due to physical discontinuities or microstructural phase heterogeneity, including secondary phases.As reported by Ao et al., 32 the primary corrosion type on aluminium alloy is pitting, which has the tendency to cause more catastrophic corrosion events.It has been reported that different oxidation rates of the matrix and reinforcement usually resulted in thickness inhomogeneity of the oxide film. 32,48These further result in many anodic and cathodic reactions at the localized sites.
Pitting corrosion typically occurs in four stages involving initiation by anions.For the pitting corrosion mechanism, metal oxidation is always coupled with the hydrolysis of corrosion products that bring about localized acidity.This localized acidity is sustained through the substantial separation of the half-reactions of the cathode and anode.There is gradual acidification of the electrolyte in the pit, which is assisted by insufficient oxygen penetration.The acidity encourages the anions' electromigration in the direction of the pit.The pits formed can gradually be blocked by corrosion products, which may eventually lead to the termination of pits. 35nodic reactions begin at imperfections on the surface of the substrate in contact with the aggressive medium (electrolyte), while the passivated surrounding surface acts as the cathode.Hence, with time there will be an emergence of the second phase particles (reinforcement inclusions) on the surface of the substrate.The formation of initial pits and the development of localized galvanic corrosion could result in local anodes forming when the particles are precipitated along the grain boundaries.The dislocations that evolved on the surface of the substrate formed localized stresses, which may become anodes for pits initiation.The exposure of the metal to an oxygen-rich electrolyte, for instance, ensures the substrate surface acts as a cathode while the tips of pits form the anodes.The metal cations produced in the pits generate excess positive charges, which attract anions from the electrolyte to form salts.The subsequent hydrolysis of the metal compound formed metal hydroxide and acids of the offending anion, which further accelerated the dissolution rate at the tip of the pit.The mechanisms of dissolution of the MMCs in the acidic medium are illustrated following Equations ( 3)- (6).
Hence, the dissolution of the metal can be linked to the soluble complex ion. 30

Electrochemical investigation of the samples via potentiodynamic polarization
Figures 7 and 8 display the PDP plots for the exposed samples of unreinforced Al6063 alloy (A0) and reinforced aluminium composites (A1, A5, A6, and A9) in 1 M H 2 SO 4 for 24 and 72 h, respectively.As observed in Figure 7, all the samples showed similar polarization and passivation trends in the aggressive media.At 24 h of exposure, the corrosion potential (−0.628 V) and corrosion current (423.81μA/cm 2 ) revealed lower corrosion activities than the reinforced substrates (Table 5).This implied that the unreinforced substrate was less susceptible to corrosion attack in the aggressive media because Al easily forms oxides that prevent further corrosion.More so, Al6063 is alloyed with Cr, which aided the formation of passive oxide layers against corrosion.The initiation of corrosion occurred at the active sites brought about by including reinforcements in the metal matrix.Furthermore, flawed oxide layers occur at any point of the reinforcements' intersection with the composites' surface.Thus, these regions are potential stress raisers where corrosion reactions can initiate. 31Sample A9 revealed the highest corrosion resistance at 72 h of exposure time, with the corrosion current being 1075.65 μA/cm 2 , the least amongst similarly exposed samples.This observation may be linked to the increase in the pH value of the solution which invariably lowers the concentration of oxygen in the corrosion bath owing to the initial fast rate of corrosion experienced 35 by the specimen.From the generic knowledge in corrosion science, sulphates and chlorides (extraneous ions) usually develop soluble complexes with passivating oxide films on aluminium.Therefore, active region expansion and passive region contraction are on the E/pH diagram (Pourbaix diagram).With this, the rate of corrosion increased after prolonged exposure.the curves in the Nyquist plots.In Nyquist plots, the larger the diameter of the curve, the greater the corrosion resistance of the sample in the aggressive environment.This implies that with a larger diameter, there is much difficulty for electron loss or gain on the anode and cathode for material dissolution to occur. 49or the samples immersed in aggressive solutions for 24 and 72 h, respectively (Figures 9 and 10), the presence of the semicircle formed by each sample was their response to the frequency perturbation indicating charge transfer activity on the surface of the electrodes.As observed in Figures 9 and 10, the Nyquist profiles showed imperfect capacitive loops.This implied that the charge shift route regulated the dissolution of the metal composite. 50The distorted semicircles, as observed in the figures, are partly attributable to the non-uniformity of the current as a consequence of the non-uniformity of their surface features.A distorted high-frequency area capacitive loop a minor low-frequency inductive loop phenomena associated with Faradic processes in the samples.A time-continuous double electric layer and charge transfer process are usually associated with the high-frequency area capacitive loop.However, the lower-frequency area inductive loop may be related to the dissolution of the oxide films 50 and, in particular, ascribed to the recreations of the intermediates, which control the corrosion processes, such as metal ion adsorption and desorption on the surface of the electrodes.

Electrochemical investigation of the samples via EIS analysis
At 24 h immersion time in the acidic solution (Figure 9), the capacitive loop diameters of the reinforced sample A9(Al/8% SiC/2% PKSA) and unreinforced sample A0(Al6063) were the largest.The capacitive loops of the other reinforced samples (samples A1, A6, and A5) were smaller compared to the unreinforced sample A0.At 72 h immersion time in aggressive media (Figure 10), sample A9 displayed a larger capacitive loop diameter than other specimens.
The Bode plots in Figure 11A,B represent the Bode profiles and Bode phase profiles of the samples when immersed in the sulfuric solution for 24 h, respectively.In the Bode profiles, a single time constant was detected.These demonstrated that the reactions on the samples were predominantly through charge transfer processes.The impedance and the phase angle concerning the frequency are displayed in Figure 12A,B, respectively.A similar observation of the single time constant can be observed in Figure 12A,B when the samples were immersed in the solution for 72 h.
The EIS fitting results of the samples in the 1.0 M H 2 SO 4 solution are displayed in Table 6.Rct denotes the corrosion resistance of the samples in the solution.The composite substrates' corrosion resistance depends on the formation of thin film oxide on the composite substrate surface.This may have limited the electronic and ionic conductivity and decreased the electrochemical rate of reactions on the surface of the samples. 50rom Table 6, it was observed that the values of Rct are reduced.For instance, Rct values for sample A0 reduced from 51.81 to 9.502 Ω cm 2 when the exposure time increased from 24 to 72 h.More so, the Rct value for sample A1 reduced from 22.51 to 18.26 Ω cm 2 when the immersion time increased from 24 to 72 h.All other samples exhibit similar characteristics as observed for samples A0 and A1.The decrease in the Rct values is ascribed to the disruption of corrosion activity on the degraded oxide film on the composites.The degree of disorderliness (n1 and n2) variation with Rct, in Table 6, is attributable to the different compositions and percentage volume fractions of the reinforcement particulates in the composites.This confirms that the reinforcement particulates integrated into the metal matrix served as good locations for the initiation of corrosion, thus, leading to fluctuations in the degree of orderliness in the different composites.
2. The primary mechanism of corrosion when the substrates were exposed to the H 2 SO 4 solution was pitting corrosion with noticeable loss in reinforcement into the aggressive environment.3. The unreinforced alloy revealed better corrosion resistance than the reinforced samples.4. The corrosion initiation was credited to the reinforcement integration into the matrix alloy, which served as active points for initiation.Hence, stress raisers that are prone to corrosion initiation of pits resulted.5. Al6063 and its composites are susceptible to pitting corrosion in H 2 SO 4 .6.It is recommended that the composites should be protected against corrosion to find great utilization in areas less prone to corrosion attack.

F I G U R E 1 2 TA B L E 4
Functional groups detection in (A) raw palm kernel shell (PKS) and (B) palm kernel shell ash (PKSA) via fourier transform infrared (FTIR) spectra.FTIR band and functional group assignment for PKS and PKSA.

F I G U R E 4
Mass loss against the time of exposure in 1.0 M H 2 SO 4 solution.

F I G U R E 5
Corrosion rate against the time of exposure in 1.0 M H 2 SO 4 solution.F I G U R E 6 Photographs of corroded samples of (A) Al6063/2% palm kernel shell ash (PKSA) and (B) Al6063/2% SiC after 15 days of immersion in H 2 SO 4 solution.

7 F I G U R E 8
Potentiodynamic polarization (PDP) plots for the substrates in the corrosive media at 24 h.Potentiodynamic polarization (PDP) plots for the substrates in the corrosive media at 72 h.

F I G U R E 9 F
Nyquist plots for samples in an acidic solution for 24 h.I G U R E 10 Nyquist plots for samples in an acidic solution for 72 h.

Figures 9
Figures 9 and 10 show the Nyquist plots of the specimens immersed in acidic media at 24 and 72 h, respectively.The charge transfer resistance, Rct, in response to the frequency perturbation of the surface of the electrodes, is dictated by

F
I G U R E 12 (A) Bode profiles, (B) Bode phase profile of samples in 1.0 M H 2 SO 4 for 72 h.
Sample identification of composite produced.
TA B L E 2 PKSA chemical constituents (%).Abbreviations: LOI, loss on ignition; PKSA, palm kernel shell ash.TA B L E 3 TA B L E 5 PDP corrosion parameters for the samples.
EIS spectra fitting parameters for samples in 1.0 M H 2 SO 4 media at 24 and 72 h.
TA B L E 6