Investigation of Alumina-Doped Prunus domestica Gum Grafted Polyaniline Epoxy Resin for Corrosion Protection Coatings for Mild Steel and Stainless Steel

Eco-friendly inhibitors have attracted considerable interest due to the increasing environmental issues caused by the extensive use of hazardous corrosion inhibitors. In this paper, environmentally friendly PDG-g-PANI/Al2O3 composites were prepared by a low-cost inverse emulsion polymerization for corrosion inhibition of mild steel (MS) and stainless steel (SS). The PDG-g-PANI/Al2O3 composites were characterized by different techniques such as X-ray diffraction (XRD), UV/Vis, and FTIR spectroscopy. XRD measurements show that the PDG-g-PANI/Al2O3 composite is mostly amorphous and scanning electron micrographs (SEM) reveal a uniform distribution of Al2O3 on the surface of the PDG-g-PANI matrix. The composite was applied as a corrosion inhibitor on mild steel (MS) and stainless steel (SS), and its efficiency was investigated by potentiodynamic polarization measurement in a 3.5% NaCl and 1 M H2SO4 solution. Corrosion kinetic parameters obtained from Tafel evaluation show that the PDG-g-PANI/Al2O3 composites protect the surface of MS and SS with inhibition efficiencies of 92.3% and 51.9% in 3.5% NaCl solution, which is notably higher than those obtained with untreated epoxy resin (89.3% and 99.5%). In particular, the mixture of epoxy/PDG-g-PANI/Al2O3 shows the best performance with an inhibition efficiency up to 99.9% on MS and SS. An equivalent good inhibition efficiency was obtained for the composite for 1M H2SO4. Analysis of activation energy, formation enthalpy, and entropy values suggest that the epoxy/PDG-g-PANI/Al2O3 coating is thermodynamically favorable for corrosion protection of MS and exhibits long-lasting stability.


Introduction
Many metals and steels are very susceptible to corrosion. As a result, huge quantities of metals are scrapped every year, causing enormous losses to the global economy. Protecting metals from corrosion is therefore essential to mitigate the limitations in industrial use. In this context, organic coatings have attracted much attention due to their excellent lowcost production, improved corrosion inhibition, excellent mechanical properties, high functionality, and environmental friendliness [1]. Among organic coatings, resins are widely used, especially water-based epoxy resins, which are applied for anti-corrosion coatings due to their excellent adhesion, corrosion-inhibiting properties, and mechanical durability [2]. Mechanical damage in organic coatings such as age-related brittleness, fractures, and external mechanical stress can compromise the integrity of the coating and reduce its long-term usability. The damage is difficult to detect and repair, and thus provides opportunities for oxygen and water to penetrate through cracks and cause and N-methyl-2-pyrrolidone (NMP) were purchased from (Sigma Aldrich, St. Louis, MI, USA). Magic ® epoxy glue (resin 100 g, hardener 80 g, 7 Mart Ltd., Punjab, Pakistan) with a mean viscosity of 16.500 cps and mean specific gravity of 1.10 was used without further treatment. Doubled distilled water was used for all the experiments.

Synthesis of PDG Grafted Polyaniline/Aluminium Oxide (PDG-g-PANI)/Al 2 O 3 Composites
For the synthesis of the (PDG-g-PANI)/Al 2 O 3 composite, 30 mL of distilled water was placed in a flask and then 0.1 g of Al 2 O 3 powder was added and dispersed by ultrasonication for 30 min. Then, 0.1 g PDG powder was added under constant stirring. After 30 min, 1 mL of DBSA, 30 mL diesel and 0.4 mL of aniline were added to the suspension. For initializing polymerization, 1 g of BPO oxidant was added and the whole mixture was stirred at room temperature for 24 h. Over this time, a dark green product was precipitated. The product was washed with distilled water several times to remove water soluble impurities; a proper amount of acetone was added to disperse the product. The dispersion was poured into a ceramic tray and dried at room temperature. After drying, the product was washed with n-hexane for the removal of non-polar impurities. The obtained product was dried at 40 • C for 2 h.

Composite Characterisation Methods
UV/Vis spectra of PGD-g-PANI/Al 2 O 3 were collected in chloroform in a spectral range from 200 to 1100 nm with a Lamda 1050 spectrometer (Waltham, MA, USA). Fourier transformed infrared spectroscopy (FTIR) was performed on an Affinity-1S FT-IR spectrometer from Shimadzu (Kyoto, Japan), scanning over an effective range of 400 to 4000 cm −1 , with a 2 cm −1 resolution. X-ray diffraction (XRD) analysis was carried out using an X-ray diffractometer from Rigaku (Tokyo, Japan) with Cu Kα radiation (λ = 1.54 A) at 40 kV and 35 mA current with 2θ ranging from 10 • to 80 • , step width of 0.0164 • , and a step rate of 1 s −1 . Imaging and structural analysis of the composites were performed using a scanning electron microscope (JSM-5910, JEOL, Tokyo, Japan).

Preparation of the Anti-Corrosion Coatings
Epoxy resin and hardener (1:1 v/v, total 0.5 g) were thoroughly mixed and applied together. Then PDG-g-PANI/Al 2 O 3 (4.5 wt%) was suspended in 5 mL chloroform and mixed with the epoxy adhesive. The mixture was kept at room temperature for 30 min. Steel electrodes were polished using different sized sandpapers with 120 to 800 grit sizes and smoothed with a polishing pad containing alumina oxide suspension (0.05 µm), followed by washing with acetone, and double distilled water. Then the mixture 1 mL of the solution was drop casted on the MS or SS surface and dried at room temperature in an open atmosphere.

Electrochemical Characterization
Electrochemical characterizations of PDG-g-PANI/Al 2 O 3 samples were carried out by using Reference 600 ZRA potentiostat/galvanostat (Gamry, Warminster, PA, USA), in three electrode assembly and in 1 M H 2 SO 4 solution using gold sheet as the working electrode, gold wire as the counter, and saturated calomel electrode as the reference electrode. Cyclic voltammetry measurements were performed in the potential range −0.2 to 0.9 V at scan rates ranging from 10 mV s −1 to 500 mV s −1 .
Anti-corrosion analysis of the composite was performed in a corrosive solution of 3.5% NaCl and 1M H 2 SO 4 . Corrosion experiments for blank and coated MS and SS disks were performed in a customized cell with three electrodes using the Reference 600 ZRA potentiostat/galvanostat (Gamry, Warminster, PA, USA). MS and SS disks were used as the working electrodes, saturated calomel electrodes (SCE) as the reference electrodes, and stainless-steel plates as the counter electrodes. Cathodic/anodic potentiodynamic polarization measurements were performed and corrosion kinetic parameters such as i corr (corrosion current density), E corr (corrosion potential) and the corrosion rate C R (mm/year) for MS and SS were calculated by extrapolating Tafel plots using Gamry Echem Analyst software. The inhibition efficiency was determined by using Equation (1).
where i 0 corr is the corrosion current density of blank steel electrodes, while i corr is the corrosion current density of coated steel electrodes.
The corrosion rate was calculated using the weight loss per time. To ensure an accurate measurement, two comparable electrodes were polished, then thoroughly cleaned with ethanol, acetone, and distilled water, dried in an oven to constant weight and weighed. Then, one of the electrodes was coated with the composite and tested for corrosion resistance under saline and acidic conditions. An uncoated electrode was used as a reference sample. The corrosion rate was then calculated according to Equation (2): where W is weight loss in g, D is density in g cm −3 , A is area in cm 2 , and t is time of exposure in seconds (s).

Solubility Study
The composite solubility is an important property from an application point of view since both the processability and the resistance in different areas of the application are key criteria. To investigate the solubility of the PDG-g-PANI/Al 2 O 3 composite, various organic solvents were explored ( Figure 1). It was found that the composite is soluble in alcohols, especially methanol and ethanol/propanol, which is due to the presence of polar components such as -NH and -OH, which are responsible for the solubility in polar solvents. Such solubility can be attributed to the formation of hydrogen bonds between composites and alcohols [20]. The same phenomenon occurred when the composite approached NMP and THF, albeit these solvents differ significantly from alcohols. The presence of the amide and ether functional groups in NMP as well THF may form hydrogen bonds with −OH of PDG and −NH of PANI in the composite particles. and stainless-steel plates as the counter electrodes. Cathodic/anodic potentiodynamic polarization measurements were performed and corrosion kinetic parameters such as icorr (corrosion current density), Ecorr (corrosion potential) and the corrosion rate CR (mm/year) for MS and SS were calculated by extrapolating Tafel plots using Gamry Echem Analyst software. The inhibition efficiency was determined by using Equation (1).
where is the corrosion current density of blank steel electrodes, while is the corrosion current density of coated steel electrodes.
The corrosion rate was calculated using the weight loss per time. To ensure an accurate measurement, two comparable electrodes were polished, then thoroughly cleaned with ethanol, acetone, and distilled water, dried in an oven to constant weight and weighed. Then, one of the electrodes was coated with the composite and tested for corrosion resistance under saline and acidic conditions. An uncoated electrode was used as a reference sample. The corrosion rate was then calculated according to Equation (2): where W is weight loss in g, D is density in g cm −3 , A is area in cm 2 , and t is time of exposure in seconds (s).

Solubility Study
The composite solubility is an important property from an application point of view since both the processability and the resistance in different areas of the application are key criteria. To investigate the solubility of the PDG-g-PANI/Al2O3 composite, various organic solvents were explored ( Figure 1). It was found that the composite is soluble in alcohols, especially methanol and ethanol/propanol, which is due to the presence of polar components such as -NH and -OH, which are responsible for the solubility in polar solvents. Such solubility can be attributed to the formation of hydrogen bonds between composites and alcohols [20]. The same phenomenon occurred when the composite approached NMP and THF, albeit these solvents differ significantly from alcohols. The presence of the amide and ether functional groups in NMP as well THF may form hydrogen bonds with −OH of PDG and −NH of PANI in the composite particles.
The solubility of the composite in dichloromethane can be attributed to a dipole-dipole interaction, since the composite is doped with DBSA, which contains both a nonpolar alkyl chain and a polar sulfonate group. The solubility of the composite in nonpolar solvents such as acetone and chloroform are due to the nonpolar interaction between the solvent and the nonpolar components present in the composite [24,25].  The solubility of the composite in dichloromethane can be attributed to a dipole-dipole interaction, since the composite is doped with DBSA, which contains both a nonpolar alkyl chain and a polar sulfonate group. The solubility of the composite in nonpolar solvents such as acetone and chloroform are due to the nonpolar interaction between the solvent and the nonpolar components present in the composite [24,25].

UV/Visible Spectroscopy
The absorption spectra of neat PDG and PANI, PDG-g-PANI, and PDG-g-PANI/Al 2 O 3 in chloroform were recorded by UV/Vis (200-1100 nm, see Figure 2). The arbinogalactan components of PDG showed absorption at 272 nm. The spectrum of PANI revealed peaks at 276 nm, 402 nm, and a broadened characteristic at 600 nm. These peaks were due to the π-π*-transition of the benzoid ring, the polaron π*-transition of the quinoid ring, and the π-polaron transition [26]. The PDG-g PANI spectrum exhibited peaks at 259 nm, 301 nm, 407 nm, and a broadened feature at 556 nm. The region at 556 nm is attributed to the π-polaron transition, while the peak at 407 nm is attributed to the polaron π*-transition of the quinoid ring. Peaks at 301 nm and 261 nm are attributed to the π-π*-transition of the benzoid ring and the arabinogalactose component of PDG, respectively [20]. The UV/Vis spectra of PDG-g-PANI/Al 2 O 3 gave three sharp peaks at 251 nm, 290 nm, and 403 nm, while there was one broad peak at 573 nm. The occurrence of sharp peaks is due to the incorporation of Al 2 O 3 into the PDG-g-PANI backbone. The peak at about 251 nm is attributed to the arbinogalactan components of PDG, while the sharp peak at 290 nm originates from the benzoid ring of PANI. A clear blue shift is observed in both peaks, which can be attributed to the incorporation of Al 2 O 3 into the polymer chain. The peak at about 407 nm is attributed to the polaron-π* transition of the quinoid ring, while a broad absorption peak at 573 nm is attributed to the exiton transition in the composite. All these peaks indicate the formation of PDG-g-PANI/Al 2 O 3 .

UV/Visible Spectroscopy
The absorption spectra of neat PDG and PANI, PDG-g-PANI, and PDG-g-PANI/Al2O3 in chloroform were recorded by UV/Vis (200-1100 nm, see Figure 2). The arbinogalactan components of PDG showed absorption at 272 nm. The spectrum of PANI revealed peaks at 276 nm, 402 nm, and a broadened characteristic at 600 nm. These peaks were due to the π-π*-transition of the benzoid ring, the polaron π*-transition of the quinoid ring, and the π-polaron transition [26]. The PDG-g PANI spectrum exhibited peaks at 259 nm, 301 nm, 407 nm, and a broadened feature at 556 nm. The region at 556 nm is attributed to the π-polaron transition, while the peak at 407 nm is attributed to the polaron π*-transition of the quinoid ring. Peaks at 301 nm and 261 nm are attributed to the π-π*transition of the benzoid ring and the arabinogalactose component of PDG, respectively [20]. The UV/Vis spectra of PDG-g-PANI/Al2O3 gave three sharp peaks at 251 nm, 290 nm, and 403 nm, while there was one broad peak at 573 nm. The occurrence of sharp peaks is due to the incorporation of Al2O3 into the PDG-g-PANI backbone. The peak at about 251 nm is attributed to the arbinogalactan components of PDG, while the sharp peak at 290 nm originates from the benzoid ring of PANI. A clear blue shift is observed in both peaks, which can be attributed to the incorporation of Al2O3 into the polymer chain. The peak at about 407 nm is attributed to the polaron-π* transition of the quinoid ring, while a broad absorption peak at 573 nm is attributed to the exiton transition in the composite. All these peaks indicate the formation of PDG-g-PANI/Al2O3.  Figure 3 shows the FTIR spectra of PDG, PANI, PDG-g-PANI, and PDG-g-PANI/Al2O3, and the peak assignments concerned are given in Table 1 [20]. In the spectrum of PDG-g-PANI/Al2O3, the bands at 2917 cm −1 and 2847 cm −1 are due to coupled stretching vibrations of the aliphatic C-H of PDG and C-H of aniline in the polymer chain. The typical peak of -COOH occurring at 1716 cm −1 due to the stretching vibration of -C=O confirms the grafting of PDG into the polymer chain. The peak at 1490 cm −1 and 1451 cm −1 is assigned to the C=C stretching of quinolines and benzene rings, respectively [27]. The shift in quinoid stretching indicates the doping of the polymer chain with metal oxide. The peak at 1372 cm −1 is due to C-N stretching, while the peak at about 1003 cm −1 is assigned to C-H stretching vibration. The peak at 826 cm −1 was due to out-of-plane C-H deformation. The band at 753 cm −1 is assigned to the C-H bending vibration, while the occurrence of the peak at 1003 cm −1 is due to -SO3H of DBSA, confirming the doping of DBSA into the polymer chain [26]. The peak at 689 and 580 cm −1 is attributed to Al2O3, confirming the incorporation of Al2O3 into the PDG-g-PANI chain [28].  Figure 3 shows the FTIR spectra of PDG, PANI, PDG-g-PANI, and PDG-g-PANI/Al 2 O 3 , and the peak assignments concerned are given in Table 1 [20]. In the spectrum of PDGg-PANI/Al 2 O 3 , the bands at 2917 cm −1 and 2847 cm −1 are due to coupled stretching vibrations of the aliphatic C-H of PDG and C-H of aniline in the polymer chain. The typical peak of -COOH occurring at 1716 cm −1 due to the stretching vibration of -C=O confirms the grafting of PDG into the polymer chain. The peak at 1490 cm −1 and 1451 cm −1 is assigned to the C=C stretching of quinolines and benzene rings, respectively [27]. The shift in quinoid stretching indicates the doping of the polymer chain with metal oxide. The peak at 1372 cm −1 is due to C-N stretching, while the peak at about 1003 cm −1 is assigned to C-H stretching vibration. The peak at 826 cm −1 was due to out-of-plane C-H deformation. The band at 753 cm −1 is assigned to the C-H bending vibration, while the occurrence of the peak at 1003 cm −1 is due to -SO 3 H of DBSA, confirming the doping of DBSA into the polymer chain [26]. The peak at 689 and 580 cm −1 is attributed to Al 2 O 3 , confirming the incorporation of Al 2 O 3 into the PDG-g-PANI chain [28].

X-ray Diffraction
The XRD pattern of pristine Al 2 O 3 , PDG, PDG-g-PANI, and PDG-g-PANI/Al 2 O 3 is shown in Figure 4. The XRD pattern of Al 2 O 3 exhibits characteristic diffraction peaks at 37.5 • , 45.7 • , 66.6 • corresponding to (311), (400), and (440) lattice planes of γ-Al 2 O 3 [29]. Both the pristine PDG and the PDG-g-PANI show broad peaks in the range of 17 • and 21 • , indicating the amorphous nature of the natural rubber and the polymer. The incorporation of Al 2 O 3 in the PDG-g-PANI matrix as a nanofiller did not change the mainly amorphous structure [30].

Energy Dispersive X-ray Analysis
Energy dispersive X-ray (EDX) analysis was performed to verify the incorpor Al2O3 into the PDG-g-PANI matrix. The result is shown in Figure 5, and the corresp elemental composition is listed in Table 2. The high carbon content originates from and PDG in the composite, since it is the main content in both compounds. Th content of 4.65 wt% indicates an efficient doping of the PANI backbone by DBSA the presence of alumina confirms the incorporation of Al2O3 into the polymer ma

Energy Dispersive X-ray Analysis
Energy dispersive X-ray (EDX) analysis was performed to verify the incorporation of Al 2 O 3 into the PDG-g-PANI matrix. The result is shown in Figure 5, and the corresponding elemental composition is listed in Table 2. The high carbon content originates from aniline and PDG in the composite, since it is the main content in both compounds. The sulfur content of 4.65 wt% indicates an efficient doping of the PANI backbone by DBSA, while the presence of alumina confirms the incorporation of Al 2 O 3 into the polymer matrix.

Energy Dispersive X-ray Analysis
Energy dispersive X-ray (EDX) analysis was performed to verify the incorporation of Al2O3 into the PDG-g-PANI matrix. The result is shown in Figure 5, and the corresponding elemental composition is listed in Table 2. The high carbon content originates from aniline and PDG in the composite, since it is the main content in both compounds. The sulfur content of 4.65 wt% indicates an efficient doping of the PANI backbone by DBSA, while the presence of alumina confirms the incorporation of Al2O3 into the polymer matrix.

Scanning Electron Microscopy Analysis
SEM images of PDG, PANI, PDG-g-PANI, and PDG-g-PANI/Al2O3 are shown in the Figure 6. PANI exhibits a porous morphology while the PDG shows irregular surface characteristics. The image of PDG-g-PANI displays a porous surface of PANI having a PDG network surrounding the porous network of PANI [20]. For PDG-g-PANI/Al2O3 it can be seen that all three components including PANI, PDG, and Al2O3 particles are present in the composite. PANI forms the principal matrix surrounded by the fibrous   network surrounding the porous network of PANI [20]. For PDG-g-PANI/Al 2 O 3 it can be seen that all three components including PANI, PDG, and Al 2 O 3 particles are present in the composite. PANI forms the principal matrix surrounded by the fibrous network of PDG. It is noticed that Al 2 O 3 particles are distributed along the whole network of the composite. The images show that PDG and alumina particles block the pores in the matrix of the polymer and thus restrict the movement of corrosive ions to reach the metal surface, protecting it from corrosion. This results in an increase in corrosion protection capability.   Figure 7 displays the cyclic voltammogram of PDG-g-PANI/Al2O3 at different scan rates from 10 to 90 mV s −1 obtained in 1 M aqueous H2SO4 solution in the potential range of −0.2 to 0.9 V. The CV exhibits the two characteristic oxidation and reduction peaks of PANI. The first peak at ESCE = 0.19 V is assigned to the conversion of neutral leucoemeraldine to the partially oxidized emeraldine form of PANI, while the second oxidation peak at 0.60 V is attributed to the redox transition of the emeraldine to pernigraniline. In the reverse scan, the conversion of pernigraniline to emeraldine is at 0.68 V, while the peak at −5 mV shows the conversion of the emeraldine form of PANI back to the fully reduced leucoemeraldine [31]. The observed redox peaks are particularly associated with the PANI composite, indicating that the polymerization of aniline in the presence of PDG and Al2O3 nanoparticles generates an electroactive reversible composite.   The first peak at E SCE = 0.19 V is assigned to the conversion of neutral leucoemeraldine to the partially oxidized emeraldine form of PANI, while the second oxidation peak at 0.60 V is attributed to the redox transition of the emeraldine to pernigraniline. In the reverse scan, the conversion of pernigraniline to emeraldine is at 0.68 V, while the peak at −5 mV shows the conversion of the emeraldine form of PANI back to the fully reduced leucoemeraldine [31]. The observed redox peaks are particularly associated with the PANI composite, indicating that the polymerization of aniline in the presence of PDG and Al 2 O 3 nanoparticles generates an electroactive reversible composite.   Figure 7 displays the cyclic voltammogram of PDG-g-PANI/Al2O3 at different scan rates from 10 to 90 mV s −1 obtained in 1 M aqueous H2SO4 solution in the potential range of −0.2 to 0.9 V. The CV exhibits the two characteristic oxidation and reduction peaks of PANI. The first peak at ESCE = 0.19 V is assigned to the conversion of neutral leucoemeraldine to the partially oxidized emeraldine form of PANI, while the second oxidation peak at 0.60 V is attributed to the redox transition of the emeraldine to pernigraniline. In the reverse scan, the conversion of pernigraniline to emeraldine is at 0.68 V, while the peak at −5 mV shows the conversion of the emeraldine form of PANI back to the fully reduced leucoemeraldine [31]. The observed redox peaks are particularly associated with the PANI composite, indicating that the polymerization of aniline in the presence of PDG and Al2O3 nanoparticles generates an electroactive reversible composite.

. Potentiodynamic Polarization Study of Mild Steel in NaCl Solution
Potentiodynamic polarization measurement is a suitable analysis technique for corrosion protection coating systems. Tafel curves of the uncoated, bare composite, bare epoxy, and epoxy/composite coated mild steel (MS) immersed in 3.5 wt% NaCl solution are shown in Figure 8. The corrosion kinetic parameters calculated from polarization curves by Tafel extrapolation are presented in Table 3. The corrosion current density and corrosion potential of uncoated MS are 20.2 µA and −777 mV, with a corrosion rate of 9.224 m/year. The PDG-g-PANI/Al 2 O 3 coating on MS reduces the current density to 1.56 µA, thus shifting the corrosion potential to −420 mV and reducing the corrosion rate to 0.711 m/year with an inhibition efficiency of 92.3%. The pristine epoxy coating on MS also shows an inhibition efficiency of 89.3%. Incorporation of the PDG-g-PANI/Al 2 O 3 composite into the epoxy coating inhibits the surface of MS by two simultaneous effects: (1) the inorganic Al 2 O 3 particles in polymer coating provides a physical barrier for the corrosive environment from reaching the metal surface by filling the nanopores and micropores in the coating [32,33]; (2) the PDG-g-PANI matrix enhances the corrosion protection of epoxy coating due to its electrical conductivity and the formation of a strong protective oxide layer on the MS surface [34]. The lowest current density and inhibition efficiency of 99.9% indicates that the presence of Al 2 O 3 has a great impact on epoxy/composite coating.

Potentiodynamic Polarization Study of Mild Steel in NaCl Solution
Potentiodynamic polarization measurement is a suitable analysis technique for corrosion protection coating systems. Tafel curves of the uncoated, bare composite, bare epoxy, and epoxy/composite coated mild steel (MS) immersed in 3.5 wt% NaCl solution are shown in Figure 8. The corrosion kinetic parameters calculated from polarization curves by Tafel extrapolation are presented in Table 3. The corrosion current density and corrosion potential of uncoated MS are 20.2 μA and −777 mV, with a corrosion rate of 9.224 m/year. The PDG-g-PANI/Al2O3 coating on MS reduces the current density to 1.56 μA, thus shifting the corrosion potential to −420 mV and reducing the corrosion rate to 0.711 m/year with an inhibition efficiency of 92.3%. The pristine epoxy coating on MS also shows an inhibition efficiency of 89.3%. Incorporation of the PDG-g-PANI/Al2O3 composite into the epoxy coating inhibits the surface of MS by two simultaneous effects: (1) the inorganic Al2O3 particles in polymer coating provides a physical barrier for the corrosive environment from reaching the metal surface by filling the nanopores and micropores in the coating [32,33]; (2) the PDG-g-PANI matrix enhances the corrosion protection of epoxy coating due to its electrical conductivity and the formation of a strong protective oxide layer on the MS surface [34]. The lowest current density and inhibition efficiency of 99.9% indicates that the presence of Al2O3 has a great impact on epoxy/composite coating.

Corrosion Study of Stainless Steel in NaCl Solution
The behavior of uncoated and composite, epoxy and epoxy/composite coated SS were also tested in 3.5% NaCl solution and the results are shown in Figure 9 and Table 4. Uncoated SS shows a corrosion current density of 11.9 μA and −960 mV for the corrosion overpotential, with a corrosion rate of 5.42 m/year. With coating, the corrosion rate decreases to 2.616 m/year with a lowered corrosion current density of 5.72 μA, a positive

Corrosion Study of Stainless Steel in NaCl Solution
The behavior of uncoated and composite, epoxy and epoxy/composite coated SS were also tested in 3.5% NaCl solution and the results are shown in Figure 9 and Table 4. Uncoated SS shows a corrosion current density of 11.9 µA and −960 mV for the corrosion overpotential, with a corrosion rate of 5.42 m/year. With coating, the corrosion rate decreases to 2.616 m/year with a lowered corrosion current density of 5.72 µA, a positive shift of corrosion potential value of −563 mV, and an inhibition efficiency of 52%. However, the PDG-g-PANI anti-corrosion behavior is still moderate, which can be attributed to the low adhesion on SS, which allows the salt solution to penetrate the surface of the steal and cause corrosion. For improvement, the composite was integrated into an epoxy matrix with strong adhesion and mechanical durability. The epoxy/composite coating on SS shows an excellent anti-corrosion behavior with a corrosion rate of 0.0051 m/year and efficiency of 99.9%. olymers 2022, 14, x FOR PEER REVIEW shift of corrosion potential value of −563 mV, and an inhibition efficiency of 5 ever, the PDG-g-PANI anti-corrosion behavior is still moderate, which can be to the low adhesion on SS, which allows the salt solution to penetrate the sur steal and cause corrosion. For improvement, the composite was integrated into matrix with strong adhesion and mechanical durability. The epoxy/composite SS shows an excellent anti-corrosion behavior with a corrosion rate of 0.0051 m efficiency of 99.9%.

Potentiodynamic Polarization Study of Mild Steel in H2SO4 Solution
The corrosion protection ability of PDG-g-PANI/Al2O3, pristine epoxy, and of epoxy with PDG-g-PANI/Al2O3 on MS were investigated in 1 M H2SO4 and w pared with uncoated MS (Figure 10 and Table 5). The results of the corrosion cu corrosion potential for uncoated MS are 1480 μA and −508 mV. By coatin PANI/Al2O3 on MS, the corrosion current density decreases to 11.80 μA and Ecor to −475 mV, showing excellent corrosion protection with an inhibition efficiency On the other hand, the coating of pristine epoxy on MS decreases the corrosion 2.05 μA, displaying an inhibition efficiency of 99.86%. However, the coating of e composite coating inhibits the surface of steel tremendously from corrosion th synergistic effect of all components present in a single coating. The coating re

Potentiodynamic Polarization Study of Mild Steel in H 2 SO 4 Solution
The corrosion protection ability of PDG-g-PANI/Al 2 O 3 , pristine epoxy, and the blend of epoxy with PDG-g-PANI/Al 2 O 3 on MS were investigated in 1 M H 2 SO 4 and were compared with uncoated MS (Figure 10 and Table 5). The results of the corrosion current and corrosion potential for uncoated MS are 1480 µA and −508 mV. By coating PDGg-PANI/Al 2 O 3 on MS, the corrosion current density decreases to 11.80 µA and E corr is shifted to −475 mV, showing excellent corrosion protection with an inhibition efficiency of 99.2%. On the other hand, the coating of pristine epoxy on MS decreases the corrosion current to 2.05 µA, displaying an inhibition efficiency of 99.86%. However, the coating of epoxy with composite coating inhibits the surface of steel tremendously from corrosion through the synergistic effect of all components present in a single coating. The coating reduces the corrosion current density to 0.473 µA, which is almost 1.5 times lower than for the epoxy resin, revealing an enhanced corrosion protection with perfect inhibition efficiency of 99.96% and corrosion rate of 0.216 m/year. The results conclude that PDG-g-PANI/Al 2 O 3 blended with epoxy possesses excellent behavior as a corrosion inhibition coating in strongly acidic medium.

Potentiodynamic Polarization Study of Stainless Steel in H2SO4 Solution
The corrosion behavior of stainless steel in H2SO4 solution is shown in the ( Figure 11 and Table 6). Firstly, blank SS was tested towards its corrosion prop corrosion rate is 2157 m/year with 4.72 mA corrosion current density and −491 sion potential. When coated with PDG-g-PANI/Al2O3 the corrosion current d creases to 96.8 μA and shifts to a corrosion potential positively with −458 mV crease in corrosion rate to 44.2 m/year (97.9% inhibition efficiency). By coatin epoxy resin the corrosion current density and voltage drop to 6.65 μA and −47 3.03 m/year corrosion rate and 99.85% inhibition efficiency. The combination resin and composite result in in an excellent corrosion rate of 0.014 m/year a inhibition. The behavior of epoxy/composite coating on SS is attributed to th combined epoxy/composite coating contains an optimal sticky epoxy resin with chanical strength, PDG with sticky behavior due to presence of electron rich groups and good mechanical properties, PANI with its redox reversible nature with its mechanical barrier property, self-healing ability, and its resistivity [18]   The corrosion behavior of stainless steel in H 2 SO 4 solution is shown in the following ( Figure 11 and Table 6). Firstly, blank SS was tested towards its corrosion properties. The corrosion rate is 2157 m/year with 4.72 mA corrosion current density and −491 mV corrosion potential. When coated with PDG-g-PANI/Al 2 O 3 the corrosion current density decreases to 96.8 µA and shifts to a corrosion potential positively with −458 mV and a decrease in corrosion rate to 44.2 m/year (97.9% inhibition efficiency). By coating pristine epoxy resin the corrosion current density and voltage drop to 6.65 µA and −470 mV with 3.03 m/year corrosion rate and 99.85% inhibition efficiency. The combination of epoxy resin and composite result in in an excellent corrosion rate of 0.014 m/year and 99.99% inhibition. The behavior of epoxy/composite coating on SS is attributed to the fact the combined epoxy/composite coating contains an optimal sticky epoxy resin with good mechanical strength, PDG with sticky behavior due to presence of electron rich functional groups and good mechanical properties, PANI with its redox reversible nature and Al 2 O 3 with its mechanical barrier property, self-healing ability, and its resistivity [18]. lymers 2022, 14, x FOR PEER REVIEW Figure 11. Tafel plot of uncoated, PDG-g-PANI/Al2O3, Epoxy, and Epoxy/P coated SS in 1 M H2SO4. The kinetic study of epoxy/composite coated MS was studied outdoo treatment by salt spray after 24 h for 41 days at room temperature. The plied to the polished surface of the MS and dried at room temperature. A anti-corrosion behavior of the coating was tested using the potentiodyna technique in 3.5% NaCl solution. After that, the samples were stored ou jected to constant salt spray treatment every 24 h, and the corrosion abil was checked at constant time intervals. The corrosion kinetic parameter the Tafel extrapolation are shown in Table 7 and the Tafel plots are sho The corrosion rate of the epoxy/composite coating decreased to a large first day of coating compared to uncoated MS. The behavior of the coatin at different time intervals, and it was found that the coating exhibited e rosion protection behavior to the surface of the MS for 41 days. After 41 lated inhibition efficiency was 99.3% with a corrosion rate of 0.0626 m/ye ficient inhibition over a long period of time.  The kinetic study of epoxy/composite coated MS was studied outdoors with constant treatment by salt spray after 24 h for 41 days at room temperature. The material was applied to the polished surface of the MS and dried at room temperature. After drying, the anti-corrosion behavior of the coating was tested using the potentiodynamic polarization technique in 3.5% NaCl solution. After that, the samples were stored outdoors and subjected to constant salt spray treatment every 24 h, and the corrosion ability of the coating was checked at constant time intervals. The corrosion kinetic parameters obtained from the Tafel extrapolation are shown in Table 7 and the Tafel plots are shown in Figure 12. The corrosion rate of the epoxy/composite coating decreased to a large extent after the first day of coating compared to uncoated MS. The behavior of the coating was observed at different time intervals, and it was found that the coating exhibited excellent anti-corrosion protection behavior to the surface of the MS for 41 days. After 41 days, the calculated inhibition efficiency was 99.3% with a corrosion rate of 0.0626 m/year, showing sufficient inhibition over a long period of time.   Quantification of the weight loss during the corrosion experiments were performed by immersing the uncoated and the epoxy/(PDG-g-PANI/Al2O3) coated MSs in a 3.5% NaCl solution for 25 days at room temperature. The corrosion rate was calculated for both the uncoated and coated electrodes based on the weight loss. The corresponding inhibition efficiencies were calculated from the corrosion rates for uncoated and coated MS; the results are shown in Table 8. The weight loss for uncoated MS was 129 mg after 25 days of immersion in 3.5% NaCl solution, whereas the weight loss of PDG-g-PANI-coated MS was 5 mg. According to the calculation, the corrosion rate of the uncoated MS was 2.41 m/year and that of the coated MS was 0.093 m/year with an inhibition efficiency of 96.1%. The results were in close agreement with the electrochemical data. Table 8. Weight loss of uncoated and coated MS before and after immersion with the corresponding corrosion rate and inhibition efficiency.

Sample
Weight before Weight after Weight Corrosion Inhibition

Weight Losses during Corrosion
Quantification of the weight loss during the corrosion experiments were performed by immersing the uncoated and the epoxy/(PDG-g-PANI/Al 2 O 3 ) coated MSs in a 3.5% NaCl solution for 25 days at room temperature. The corrosion rate was calculated for both the uncoated and coated electrodes based on the weight loss. The corresponding inhibition efficiencies were calculated from the corrosion rates for uncoated and coated MS; the results are shown in Table 8. The weight loss for uncoated MS was 129 mg after 25 days of immersion in 3.5% NaCl solution, whereas the weight loss of PDG-g-PANI-coated MS was 5 mg. According to the calculation, the corrosion rate of the uncoated MS was 2.41 m/year and that of the coated MS was 0.093 m/year with an inhibition efficiency of 96.1%. The results were in close agreement with the electrochemical data. For an in-depth explanation of the corrosion protection properties of the coating, the thermodynamic parameters such as activation energy, change in formation enthalpy and entropy were determined for uncoated and epoxy/PDG-g-PANI/Al 2 O 3 coated MS. The effect of temperature on the anticorrosion reaction of the coating is complex and leads to changes in the metal surface state by desorption of the inhibitor, rapid etching, and decomposition or restructuring of the coating material. Based on the potentiodynamic polarization curve, it was observed that the corrosion rates for both coated and uncoated MS in 3.5% NaCl solutions increase with the increasing temperature ( Figure 13). effect of temperature on the anticorrosion reaction of the coating is complex and leads to changes in the metal surface state by desorption of the inhibitor, rapid etching, and decomposition or restructuring of the coating material. Based on the potentiodynamic polarization curve, it was observed that the corrosion rates for both coated and uncoated MS in 3.5% NaCl solutions increase with the increasing temperature ( Figure 13). It is well understood that the activation energy depends on temperature, which can be calculated from Arrhenius Equation (3). The logarithm of the corrosion rate Cr is plotted against 1/T, as given below, where Ea is the activation energy, R is the universal gas constant, and T is the Kelvin temperature (Figure 14a,b). The Ea values are calculated from the slope of the graph and are given in Table 9. The activation energy for coated MS is higher than for uncoated MS in a 3.5% NaCl solution, indicating a strong inhibition behavior through the coating by reducing the reaction rate for the corrosion process. It confirms once more the strong adsorption of the epoxy/composite coating towards a mild steel surface, and thus a superficial anticorrosion behavior [35]. For a closer understanding of the thermodynamics behind the anti-corrosion coating, the change of formation enthalpy (∆ ) and entropy (∆ ) of the activation complex between metal surface and coating material in the transition state is given by the transitionstate Equation (4)  from which the values of enthalpy and entropy were calculated (Figure 15a,b). The values of enthalpy and entropy are given in Table 9. The higher and positive values of enthalpy for coated MS compared to uncoated MS suggested that the corrosion/dissolution process is more difficult and endothermic in coated MS. Additionally, entropy of activation ∆ for coated MS is less negative, showing an ordered arrangement of particles present in the coating on the surface of MS. This confirms the activated complex as an association step rather than a dissociation step in the rate determining step [37]. From this study it is also concluded that the epoxy/composite coating thermodynamically inhibits the surface of steel from the corrosion process as well. It is well understood that the activation energy depends on temperature, which can be calculated from Arrhenius Equation (3). The logarithm of the corrosion rate C r is plotted against 1/T, as given below, where E a is the activation energy, R is the universal gas constant, and T is the Kelvin temperature (Figure 14a,b). The E a values are calculated from the slope of the graph and are given in Table 9. The activation energy for coated MS is higher than for uncoated MS in a 3.5% NaCl solution, indicating a strong inhibition behavior through the coating by reducing the reaction rate for the corrosion process. It confirms once more the strong adsorption of the epoxy/composite coating towards a mild steel surface, and thus a superficial anti-corrosion behavior [35].   For a closer understanding of the thermodynamics behind the anti-corrosion coating, the change of formation enthalpy (∆H o a ) and entropy (∆S o a ) of the activation complex between metal surface and coating material in the transition state is given by the transitionstate Equation (4) (Figure 15a,b). The values of enthalpy and entropy are given in Table 9. The higher and positive values of enthalpy for coated MS compared to uncoated MS suggested that the corrosion/dissolution process is more difficult and endothermic in coated MS. Additionally, entropy of activation ∆S o a for coated MS is less negative, showing an ordered arrangement of particles present in the coating on the surface of MS. This confirms the activated complex as an association step rather than a dissociation step in the rate determining step [37]. From this study it is also concluded that the epoxy/composite coating thermodynamically inhibits the surface of steel from the corrosion process as well.

Conclusions
In this work, we successfully synthesized a green composite based on PDG-g-PANI/Al 2 O 3 by a low-cost inverse emulsion polymerization. Material characterization showed that all three components were incorporated into a stable matrix, while cyclic voltammetry proved that the redox activity of PANI was preserved. The composite was used as an anti-corrosion coating for mild and stainless steel in salty and acidic media. The corrosion kinetic parameters obtained from the Tafel analysis showed that the PDG-g-PANI/Al 2 O 3 composite protected the surface with a high inhibition efficiency of 92.3% and 51.9% for mild and stainless steel, respectively, in a 3.5% NaCl solution, which was significantly higher than the value without coating. When additionally dispersed in epoxy resin, the combination of epoxy and PDG-g-PANI/Al 2 O 3 showed the best performance, with an inhibition efficiency of 99.9% for both steels in saline as well as acidic solution. Moreover, the anti-corrosion coating was maintained in a long-term study without significant changes in the inhibition efficiency. The analysis of Arrhenius plots for uncoated and coated mild steel showed that the corrosion process is strongly endothermic when the coating is applied, resulting in high stability. Overall, the high anti-corrosion efficiency can be attributed to (i) the excellent adhesion with outstanding mechanical strength of the epoxy resin, (ii) the adhesive behavior due to the presence of electron-rich functional groups and the good mechanical properties of PDG, and (iii) the redox reversibility of PANI.