INTRODUCTION:

Pure Aluminum alloy (PuAl) has superior corrosion resistance, machinability, weldability, and anodizing response due to the alloying elements used^1,2 It is widely employed in a variety of applications, including home structures³, cell phone cases, and automobile^4,5. Even though PuAl has high corrosion resistance⁶, corrosion sometimes occurs due to intermetallic particles in the alloy⁷. As a result, varieties of surface treatments are often utilized to preserve aluminium from corrosion⁸. One of them is anodizing. Which electrochemical processes produce anodic aluminium oxide (AAO) coating on the surface during the anodizing process⁹. The anodic aluminium oxide (AAO) film has two layers: an inner barrier layer and an outer porous layer. Because of its compact dense structure, the inner barrier layer plays an essential role in corrosion resistance^10,11.

The outer porous layer is composed of hexagonal cells with a single cylindrical pore in the middle of each cell, resulting in its absorption characteristic¹². As a result, numerous qualities, such as good interface adhesion, good surface appearance, and enhancement of mechanical and anti-corrosive capabilities, maybe enhanced after anodizing^13,14. In general, the anodizing process mainly consists of three parts. The first step is surface preparation, which is used to manage the surface's quality. The second stage is anodizing, which creates an anodic oxide coating, and the last step is sealing, which closes up oxide pores. major processes have been patented to achieve an anodized aluminum surface¹⁵. the nanoparticle incorporated by adding it with anodizing solution, Soaking anodized aluminum into a suspension mixture containing nanoparticles this method provides pore sealing layer distributed with nanoparticles^16,17. The precise control of the size of nanoparticles in the suspension was needed and aggregation of nanoparticles must be avoided. In this work, we have proposed a new method to modify the surface of PuAl by using the electrochemical method of Anodizing and incorporating nanoparticles simultaneously into the porous layer of Anodized Aluminum PuAl¹⁸. This process is carried out during the anodizing process. Study the Effects of Anodized Pure Aluminum in the presence of Fe₃O₄ (APuAl+Fe₃O₄).

MATERIALS AND METHODS:

Anodizing of Aluminum:

Before anodization, Aluminium specimens have a diameter of 2.5cm₂ and a thickness of 0.5mm were produced by press cutting of commercially high purity PuAl sheet. The samples were polished with a different emery paper with grit 600, 800, 1000, 1200, and 2000, and then it was polished to a semi-mirror. After polishing the samples were washed with distilled water and placed in an ultrasonic bath with acetone for five minutes, then rinsed with distilled water, dried, and finally kept in a desiccator.

Then the aluminium samples were treated by immersing them in 10% NaOH for 30 seconds to oxidize the surface, followed by a rinse with distilled water to remove the sodium hydroxide remnants, and the oxidized layer was removed by immersing in 50% HNO₃ until the black oxide layer was removed. Finally, the samples were rinsed with distilled water to remove the acid residue and prepare them for anodizing

The anodizing process was carried out in the anodizing cell; the anodizing process was performed under a constant voltage (20 volts) for 30 minutes. Using a DC power supply, in which the large titanium foil counter electrode was employed. The electrodes were immersed in a 20% sulfuric acid solution with an inter-electrode spacing of up to 4 cm. To incorporate nanoparticles (F_e3O₄) in Aluminum oxide nanopores formed during the anodizing process, 0.1 wt. percentage nanoparticles were added to sulfuric acid¹⁹.

After the anodizing process with and without nanoparticles finished, samples were removed, rinsed with cold distilled water then immersed in boiled water for two minutes (to seal the nanopores), dried, and placed in containers to be tested later.

Corrosion Study:

For corrosion study: threeelectrodes as cells including (working electrode PuAl, modified, or non-modified), reference electrode (SCE) and auxiliary electrode (platinum electrode), Anodic and cathodic polarization for corrosion of PuAl were performed under potentiostat conditions in saline media for modified and non-modified PuAl at temperatures ranging from 298 K to 328 K.

Surface Characterizations:

Some techniques have been used to characterize the oxide film (AAO) formed on PuAl. The morphologies of the treated aluminium were observed by field emission scanning electron microscopy (FE-SEM), (EDX) and the chemical composition of samples was measured by X-ray diffraction (XRD).

RESULT AND DISCUSSION:

Chemical structure and morphology of oxide layer:

The FE-SEM and XRD techniques showed the morphology and chemical structures of the oxide layer formed on PuAl with and without nanoparticles incorporated in the nanopore, respectively (Figure 1 and Figure 2). During the anodizing processes of all samples (PuAl, and PuAl+Fe₃O₄) at the applied 20 voltage and kept constant for 30min. In the absence of nanoparticles, a layer of aluminium oxide with non-uniform micropores was formed on the surface of the anodized sample (PuAl), where the diameter of formed pores was 45 nm while the average particle size of particle oxide was 19 nm, (Figure 1a). It was observed by adding Fe₃O₄ nanoparticles to the electrolytes of the PuAl+ Fe₃O₄ samples (Figure 1b).

Nanoparticles were distributed throughout the surfaces of these oxide layers, filling the majority of the microspores. Furthermore, as can be shown, the majority of these integrated nanoparticles are not recognizable and they are detected with a size bigger than their main diameter. Indeed, the electrophoretic force attracts nanoparticles to the surface of the sample (anode) and causes them to cover the surface and fill the micropores of the coatings.

(A)

(B)

Figure 1. Surface morphologies as seen via a FESEM of A) APuAl, and B) APuAl+Fe3O4 samples in ×3000 and ×135000 magnifications.

X-ray diffraction (XRD):

The X-ray diffraction features of the Anodized aluminium oxide and the incorporated ZnO and Fe₃O₄ formed on PuAl by Anodizing method were shown in Figure 2. The phases may be seen in all of the XRD patterns. Furthermore, due to the thinness of the oxide layer, the matching peaks to the aluminium substrate were identified in all XRD spectrums. However, peaks of Al₂O₃ (the oxide of the PuAl substrate) were found in these patterns.

The average particle size of incorporated nanoparticles induced using the current electrochemical process may be calculated using Debye–approximation Scherrer's from the peaks' full width at half maximum (FWHM). (Eq. 1)^{20, 21}.

d= kλ/βcosθ………………………………………… (1)

Where d is the crystallite size, k is the CuKa radiation wavelength (k = 1.542A°), β is the FWHM for the diffraction peak in concern (in radians), θ is the diffraction angle, and k is the broadening constant (k=0.9).

The small peaks at 2θ (38°, 42°, and 65°) are referred to γ-Al₂O₃. The crystallite size has been detected for Al₂O₃ formed on anodized PuAl without nanoparticles and has an average size equal to (19.8 nm) (Figure 2). The incorporation of nanoparticles slightly affects the peak positions are slightly shifted to higher 2-theta (2θ) values compared to that anodized PuAl without nanoparticles.

Figure 2: The XRD spectra of the anodized samples.

Corrosion Measurements:

The influence of Anodizing and Ferric Oxide Incorporation on the anodic and cathodic polarization curves for the corrosion of pure aluminium alloy in saline media was studied at temperatures ranging from 298 K to 328 K. Figure (3) represents the Tafel curves for the corrosion test on PuAl with Anodizing and oxide nanoparticles incorporated at various temperatures. The corrosion potential of blank PuAl is shown by the polarization curves (-763.9 to -959.5 mV). After anodizing in sulfuric acid solution, the potential shifted to (-718.6 mV) at 298K, and the ECorr shifted to 613.4 after PuAlwas incorporated with Fe₃O₄ respectively. The corrosion current density significantly raised as the temperature raised for Blank Aluminum, but after anodizing, the corrosion current density decreased from 44.66 microamperes to 6.17 microamperes, and the ICorr value decreased further to 211 Nanoampere with the Nanoparticle incorporated. These results indicate that anodizing and incorporation improve the corrosion resistance of PuAl. Using the equation below, the Protection Efficiency (% PE) may be computed ^{22, 23}.

% PE= ((I_Corr)_b -( I_Corr)_t )/( I_Corr)_b x 100………………(2)

Where (I_Corr)_b and (I_Corr) _t are the corrosion current density (μA.cm^-2) for PuAl (Blank) and treated PuAl after anodizing respectively.

Figure 3: Polarization curves for a) Blank PuAl (Blank), b) Anodizing PuAl c) Anodizing + Fe₃O₄

The best %PE was observed after PuAl anodizing, which gave a %PE of 86.18% at 298K; nevertheless, table (1) data reveals that %PE decreased as temperature increased, illustrating that ICorr for Anodized PuAl was affected by temperature. While the best %PE was obtained after PuAl was incorporated with Fe₃O₄, which gave a %PE of 99.15% at 298K, The PE% for incorporated PuAl slightly decreased with temperature increasing, indicating that I_Corr for incorporated PuAl was not affected by temperature change.

Near the corrosion potential E_Corr, there is a little amount of polarization. The following equation, known as the Stern–Geary equation²⁴, was used to determine corrosion resistance:

Rp =∆E/∆i= (b_a * b_c)/ (2.303*( b_a + b_c))*1000 … (3)

Rp is the system's polarization resistance, ΔE denotes the difference between the polarization potential E and the corrosion potential E_Corr, ΔI denotes the difference between the observed current density I and the corrosion current density I_Corr, and ba and bc denote the anodic and cathodic Tafel coefficients, respectively.

Table 1: Corrosion kinetic parameters for Protective Pure Aluminum alloy in seawater at the temperatures range (298-328) K.

Temp. (K)		E_Corr (mV)	I_Corr (µA/cm)	-Bc (mV/Dec)	Ba (mV/Dec)	W.L (g/m².day)	P.L (Mm/year)	R_p (Ω.cm²)	PE%
un treated PuAl	298	-763.9	44.66	172.9	77	3.59	0.486	518	-
	308	-795.8	54.53	188.4	55.8	3.97	0.531	343	-
	318	-838.2	67.93	157.3	78.8	4.25	0.581	336	-
	328	-959.5	78.75	158.7	99.4	5.26	0.654	337	-
After Anodizing	298	-718.6	6.17	98.4	88	0.497	0.067	3269	86.18
	308	-817.4	9.22	94	79.8	0.778	0.0775	2163	83.09
	318	-903.7	12.8	135	106.6	0.978	0.139	2021	81.16
	328	-905.8	16.39	141.9	158.9	0.569	0.0769	1986	79.19
Anodizing with Fe₃O₄	298	-613.4	0.376	74.4	94.9	0.0321	0.00434	48162	99.15
	308	-715.9	0.546	90.5	97.2	0.044	0.00595	37270	98.99
	318	-723.5	0.801	45.6	97.5	0.0645	0.00871	17822	98.82
	328	-732.7	0.967	49.2	85.6	0.0681	0.00895	14029	98.77

The measurement of entire polarization curves is required when discussing polarization resistance and it is especially useful in diagnosing corrosion problems and initiating reconditioning action¹⁸.

The Thermodynamic Studies:

The Arrhenius equation might be used to calculate the change in entropy (ΔS*) for the transition, state of corrosion process²³:

Log I_Corr/T=log R/Nh +∆S*/2.303R - ∆H*/2.303RT …….(4)

Where I_Corr is the PuAl corrosion current density calculated from the Tafel plot, h is the Plank's constant, N is the Avogadro number, ΔS* is the activation entropy, and ΔH* is the activation enthalpy. The slope of (-ΔH* / 2.303 R) and the intercept of [(log (R/Nh) + (ΔS*/2.303 R)] were derived from the plot of log I_Corr/T versus 1/T, from which the values of ΔH* and ΔS*, respectively, were estimated.

The data in the table (2) shows that after treating the surface of PuAl, the values of entropy changed slightly. Figure (4) shows the plot of log I_Corr/T against 1/T.

Figure 4: Kinetic and thermodynamic parameters for the corrosion Protective of Pure Aluminum alloy in 3.5% NaCl at the temperatures range (298-328) K

The following equation may be used to compute the freeenergy change of the electrochemical corrosion process transition state²⁵:

ΔG* = ΔH* – TΔS* ……………………………….. (5)

ΔG* values for PuAl handled with Anodizing and incorporation increased from (63.570 kJ.mol^-1) to (68.414 kJ.mol^-1), and (75.428 kJ.mol^-1) with Anodizing, and Anodizing+ Fe₃O₄incorporation" respectively at 298K, indicating a rise in free energy of activation after PuAl surface modified. Anodizing with Fe₃O₄incorporation resulted in a larger change in enthalpy, indicating that corrosion requires more energy in this case, and these results are in excellent agreement with the protection efficiency results ²⁶.

Table 2: Kinetic and thermodynamic parameters for the corrosion Protective of Pure Aluminum alloy in seawater at the temperatures range (298-328) K.

Temp.(K)		-ΔG/ kJ.mol^-1	ΔH/ kJ.mol^-1	ΔS/J.K¹.mol^-1	Ea/ kJ. mol^-1	A Molecules .cm^-2.S^-1 X10²⁸
APuAl	298	63.570	13.035	-0.170	15.633	1.48
	308	65.266
	318	66.962
	328	68.658
Anodizing	298	68.414	23.955	-0.149	26.553	1250.54
	308	69.906
	318	71.398
	328	72.890
Anodizing with Fe₃O₄	298	75.428	27.197	-0.162	29.795	3.75
	308	77.046
	318	78.665
	328	80.283

Kinetic parameters for the corrosion process:

Figure. (5) Represents the Arrhenius plot of (blank aluminium, anodized aluminium, and anodized aluminium + Fe₃O₄ NPs). With the logarithm of current density (log I_Corr) plotted against reciprocals temperature.

The data is obtained in the table (2). In addition, it shows the apparent activation energy (E_a) increased after anodizing Aluminum alloys without and Fe₃O₄, which changed from (16.633 kJ.mol^-1) to (26.553 kJ.mol^-1), and (29.795 kJ.mol^-1) respectively. the activation energy for Aluminum, anodized aluminium was lower than anodized nanoparticles because of the presence of Fe₃O₄ NPS on Aluminum anodized surface, the number of corrosion sites can be indicated by Arrhenius factor^{27, 28}.

Figure 5: Arrhenius Plot of log I_Corr. Versus 1/T for the corrosion for Protective Pure Aluminum, alloy in seawater at the temperatures range (298-328) K

CONCLUSION:

In conclusion, the electro-modification of the PuAl surface with anodizing and nanoparticle incorporation provided a better barrier film on aluminium. Anodizing aluminium changes the corrosion potential but reduces the current density to 86.18 % at 298K, and it has significantly raised as the temperature raised. The protection efficiency % PE was observed after incorporation, which gave a % PE of 99.15% at 298K.

The apparent activation energy of PuAl is15.633kJ/mol while these increases in activation energy result increase in the activation sites and are indicated by the Arrhenius factor and this increase in the Arrhenius factor is due to a decrease in corrosion rate.

The free energy change ΔG* values for PuAl that were modified with (Anodizing and incorporation); increased from (63.570 kJ.mol^-1) to (68.414 kJ.mol^-1), and (75.428 kJ.mol^-1), with Anodizing, and "Anodizing + Fe₃O₄incorporation" respectively at 298K, indicating a rise in free energy of activation after PuAl surface modified. The average partials size of the diameter of formed pores was 45 nm while the average. From XRD data was (19.8 nm). These results qualify our work to be applied in the field of surface modification.

ACKNOWLEDGEMENT:

The authors are thankful to everyone who supported us during our work on this research.

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Received on 01.07.2022 Modified on 04.09.2022

Research J. Pharm. and Tech 2023; 16(6):2935-2940.

DOI: 10.52711/0974-360X.2023.00484