Characterization of TiO2 substrate samplesPhysical properties of TiO2 substrate samplesThe physical properties are indicated in terms of bulk density and apparent porosity. The effect of different sintering temperatures at 900, 1000, and 1100 °C on the apparent porosity and bulk density of the TiO2 substrate sample are clearly shown in Fig. 2. As demonstrated in Fig. 2, the apparent porosity reduced, and bulk density increased by increasing the sintering temperature. The apparent porosities at 900, 1000, and 1100 °C are 48.98, 30.76, and 10.20%, respectively. The bulk densities are 1.18, 2.55 and 3.69 g/cm3 at 900, 1000 and 1100 °C. In this study, it is important to prepare titania substrate structure in a porous form to allow penetration of nano-coating solution through the pores of the substrate to facilitate adhesion to its body followed by complete covering of its surface by nano-alumina layers to increase the corrosive resistance of the coated titania substrates to a high extent27. Accordingly, the TiO2 substrate samples fired at 1000 °C were selected as the optimal substrates to be applied for the coating procedures.Figure 2The effect of sintering temperatures on the bulk density and apparent porosity of TiO2 substrate.Phase composition of TiO2 substrate sampleFigure 3 depicts the sintered TiO2 substrate sample’s powder XRD pattern. The anatase and rutile polymorph-related peaks’ locations were extracted from PDF #86–1157 and #89–0555, respectively. Anatase and rutile, which are both TiO2 phases, are combined. The primary peak is at 25.3°, which corresponds to the crystal plane (101) for the TiO2 anatase phase and the crystal plane (110) for the TiO2 rutile phase at 27.4°28. Generally, the synthesis process used in preparing the titania powder precursors significantly impacts the phase transition from anatase to rutile, varying from 850 °C in the sol–gel approach to 1100 °C in the solid-state reaction29.Figure 3XRD pattern of TiO2 substrate sintered at 1000 °C.IR analysis of TiO2 substrate sampleFigure 4 shows the FTIR spectra of the substrate TiO2 samples sintered at 1000 °C according to XRD results. Signals between 400 and 1000 cm−1 correspond to Ti–O–Ti vibration30. Abroad absorption band between 450 and 800 cm−1 regions is ascribed to the vibration assigned to absorption of the Ti–O-Ti linkages in TiO2 particles30. For the pure TiO2, the peak at 420 cm−1 is related to the presence of anatase titania. According to the standard spectra of TiO2, the peak at 499 and 850 cm−1 may be or was attributed to the vibration of the Ti–O bond in the TiO2 (rutile titania) lattice. The small IR absorption band at 870 cm−1 is attributed to the Ti–O-Ti stretching vibrations31.Figure 4FTIR spectra spectrum of TiO2 substrate sintered at 1000 °C.SEM of TiO2 substrate sampleThe morphology of the TiO2 substrate is seen in Fig. 5a after calcination of the sample at 1000 °C. The SEM images demonstrate the TiO2 particles appeared in spherical and rod shapes. Rod-like anatase particles were covered by small spherical particles of rutile32. The particle sizes of rod TiO2 shapes ranged between 4 to 15 µm. The spherical particles of TiO2 ranged from 100nm to 2µm. EDS spectra spectrum shown in Fig. 5b showed that only Ti and O elements were detectable, and no other elements were identified for the samples studied. The uniformity of the synthesized TiO2 consists of well-interconnected crystallite. The percentage of the surface porosity of the prepared titania substrate was calculated to be about (30%). Thus, some pinholes can be observed in the sample surface.Figure 5SEM image and corresponding EDS spectra of TiO2 substrate sintered at 1000 °C.TEM analysis of TiO2 substrate sampleTEM of powder TiO2 material fired at 1000 °C is shown in Fig. 6. It is indicated by the well crystallization of TiO2 particles with octahedral geometry; as shown in Fig. 6, the geometry of the rutile form of titanium dioxide is twisted hexagonal with a size ranging between 0.10 to 0.17µm.Figure 6TEM photomicrograph of TiO2 substrate sintered at 1000 °C.Characterization of Al2O3 powder prepared via polymeric and microwave techniquesPhase composition of Al2O3 powderXRD of the fired alumina powders prepared by the polymeric method and microwave combustion method at different temperatures is shown in Fig. 7. It was observed that the alumina powders calcined at 800 °C that were prepared by both methods are presented in their gamma phase (γ-Al2O3) as indicated by card No. PDF #10-0425. Upon increasing the temperature to 1000 °C, the small amount of corundum phase (α-Al2O3) is indicated according to card No. PDF# 46-1212 beside the main gamma alumina phase (γ-Al2O3) for the alumina powder prepared by polymeric method (Poly). On the contrary, alumina’s main corundum phase (α-Al2O3) is obtained using the microwave combustion method (MW). Thus, the microwave method is more favored in increasing the crystallinity degree of the prepared alumina, which is indicated by a decrease in the γ-Al2O3 phase peak intensity23.Figure 7XRD patterns of Al2O3 powder prepared by polymeric (Poly) and microwave methods (MW) at 800 °C and 1000 °C.IR analysis of Al2O3 powderThe FTIR spectra of prepared alumina powders calcined at 800 and 1000 °C and prepared by polymeric method (Poly) and microwave method (MW), respectively, are shown in Fig. 8. It was observed that alumina powders calcined at 800 °C showed an appearance of γ-Al2O3. The calcined alumina powder prepared by polymeric method showed sharp bands corresponding to the stretching vibration of Al-O appearing at 720 and 513 cm–1 while they appeared at 812, 740, and 436 cm–1 for the alumina calcined powder prepared by microwave method16. Bands for α-Al2O3 appear at 639, 576, and 437 cm–1 for powders calcined at 1000 °C and prepared by microwave method. Meanwhile, bands around 1084 and 1095 cm–1 are assigned to the symmetric bending of Al–O for calcined powders at 800 and 1000 °C, respectively16. From these results, it was observed that the band intensity of the alpha alumina at 1000 °C is high and appeared at 437 cm–1 compared with the calcined samples at 800 °C that were prepared by both polymeric and microwave methods; these results are confirmed by the XRD results.Figure 8FTIR spectra of Al2O3 powder prepared by polymeric (Poly) and microwave methods (MW) at 800 °C and 1000 °C.TEM analysis of Al2O3 powderTEM images of alumina powder prepared by the polymeric method at different temperatures, 800 and 1000, are shown in Fig. 9a, b, respectively. The alumina prepared by microwave method is seen in Fig. 9c, d after firing at 800 and 1000 °C, respectively. It was observed that the prepared alumina powders by the microwave combustion method are more crystalline than those prepared by the polymeric method.Figure 9TEM images of Al2O3 powder prepared by polymeric method at temperature (a) 800 °C, (b) 1000 °C and alumina prepared by microwave method at temperature (c) 800 °C, (d) 1000 °C.Alumina particles synthesized by microwave technique are presented in a nanosphere shaped at 800 °C and changed into nanoplate forms by raising the temperature to 1000 °C, while the alumina particles prepared via polymeric method at 800 °C still retained the shape of the amorphous polymeric structure that included some nanosphere grains compared to microwave method.These findings are consistent with other studies that show a rise in calcination temperature is accompanied by a sequence of transformations, including—γ-Al2O3 → δ-Al2O3 → θ-Al2O3 → α-Al2O324. All alumina phases that originated at low temperatures changed into α -Al2O3 at high temperatures. Low activation energies are required for transformations from one phase to another, whereas transformations proceed through nucleation, growth, and increased temperature17,33,34. The interaction of microwaves with the reactants at the molecular level, where this electromagnetic energy is transferred and transformed to heat by rapid kinetics through the motion of the molecules, can be used to explain why microwave heating causes the acceleration of the crystallinity and formation of alpha alumina at 1000 °C. Thus, the microwave combustion method causes the development of nanoparticles within a short period, the development of nanoparticles, early phase formation, and various morphologies35,36,37.Characterization of TiO2 substrate after coated with alumina coating solutionPhase composition of coated TiO2 substrateThe XRD patterns of the TiO2 substrate fired at 1000 °C and coated with Al2O3 by utilizing polymeric and microwave combustion methods, then firing at 800 and 1000 °C, are shown in Fig. 10. Through the XRD of TiO2-Al2O3 and its comparison with pure titanium substrate, which is seen in Fig. 3, an enhancement in the crystallization of TiO2 was observed by increasing the firing temperature. By firing the samples at 800 °C, the main anatase phase is indicated with small amounts of rutile phase. In comparison with pure substrate, the rutile peak intensities decrease. This can be attributed to the reaction between some γ-Al2O3 particles and rutile particles, leading to a small amount of aluminum titanate phase. However, the presence of the anatase phase at 800 °C as the main phase is due to some γ-Al2O3 particles surrounding the anatase phase. It prevents the growth of anatase crystals and their transformation to rutile at this temperature38. However, upon increasing the temperature to 1000 °C, the anatase phase disappears for the coated samples prepared by polymeric and microwave is observed. The rutile phase is the only phase obtained at this firing temperature compared with the pure substrate, as presented in Fig. 3. This can be attributed to the transformation of γ-Al2O3 to α-Al203 at 1000 °C, leading to the stability of the anatase phase. This work is conceded with Young Cheol Ryu39 observed that the metal cations diffusing from the substrate into the TiO2 layer might retard the Anatase–Rutile phase transformation of TiO2. The suppressing effect on the Anatase–Rutile transformation of TiO2 by mixed cations seems much stronger than that of single cations. In addition, it was observed that the Anatase –Rutile transformation in the TiO2 substrate deposited by α-alumina might proceed more easily, and the TiO2 substrate deposited by α-alumina has a higher rutile fraction39.Figure 10XRD patterns of TiO2 substrate coated with Al2O3 powder using polymeric (Poly) and microwave (MW) methods fired at 800 and 1000 °C.It was reported that the phase transformation of TiO2 depends on various parameters such as the initial particle size, impurity (doping) concentration, starting phase, and reaction atmosphere39,40. This work illustrates that the phase transformation of TiO2 from anatase to rutile is influenced by the metal ions (Al3+) diffused from the coated layer into TiO2 substrate as well as calcination temperature. Eskelinen et al.41, who studied the effects of heat treatment on the surface composition of TiO2 thin film in TiO2-phlogopite and in TiO2-muscovite system by X-ray photoelectron spectroscopy technique, claimed that the surface composition was dependent on the calcination temperature and the substrate components diffusing through the TiO2 film39,41.IR analysis of coated TiO2 substrateThe IR patterns of the TiO2 substrate after being coated with Al2O3 using the polymeric method and microwave combustion method fired at 800 and 1000 °C are seen in Fig. 11a, b, respectively. In comparison, the IR analysis of the pure TiO2 substrate that fired at 1000 °C with TiO2 substrate coated with alumina, it was found that the band at 420 cm−1 is corresponding to anatase after firing at 800 °C as seen in Fig. 11a, while the bands at 499 and 850 cm−1 that showed in Fig. 11b after firing at 1000 °C refers to rutile phase. Bands related to Ti–O bond vibrations are found in the 493–579 cm−1 and 594–639 cm−1 ranges, as seen in Fig. 11a42. Bands caused by the stretching vibrations of the Al-O bonds of the octahedrally coordinated Al were seen in the 500–750 cm−1 range, whereas bands caused by the vibrations of the Al-O bond in AlO4 units are presented in the 750–900 cm−1 range, as seen in Fig. 11a42Figure 11FTIR patterns of TiO2 substrate coated with Al2O3 powder by using polymeric (Poly) and microwave (MW) methods and fired at (a) 800 and (b) 1000 °C.Physical properties of coated TiO2 substrateFigure 12 depicts the physical characteristics of bulk density (BD) and apparent porosity (AP) of coated TiO2 substrate samples coated with alumina, created using polymeric and microwave techniques, and fired at 800 and 1000 °C temperatures. The apparent porosity increases and the bulk density decreases as the firing temperature rises, as was seen. Overall, all of the samples show pore characteristics. Porosity may result from several different factors, according to Yang43, including (1) gas formation during the sintering process and the subsequent expansion, entrapment, or escape of those gases, (2) shrinkage related to the sintering reaction (i.e., products with a higher specific volume than the starting materials), or (3) residual initial porosity of the powder due to partial sintering. When comparing titania-coated substrates with alumina made using the polymeric approach to those made using the microwave method, it was discovered that the microwave method produced coated substrates with greater porosity than the polymeric method. This is attributed to the absorption of Al2O3 into TiO2 samples using the polymeric method, which is higher than the microwave method and is indicated by the decrease in porosity. Another reason for this phenomenon is the higher crystallinity of the alumina produced by microwave as opposed to the polymeric method enhances the formation of some secondary aluminum titanate phase (Al2TiO5), which is difficult to sinter and has a lower density of 3.7 g/cm3 compared to that of titania at 4.23 g/cm344. Due to the presence of an Al2TiO5 phase that is less to be detected in XRD (Fig. 10), high-temperature firing could not improve the densification percentage44. Additionally, the open spinel structure of γ-Alumina, a metastable phase known to include more different doping elements Ti4+ than α-alumina does45, which affects the grain boundaries (GB) diffusion properties, may be the cause of the decrease in porosity of the coated samples at 800 °C. γ → α Phase transition is described as a type of nucleation and growth transformation that affects the porosity quantities in the samples, according to S. Lartigue-Korinek et al.45.Figure 12Physical properties of the coated TiO2 substrate with Al2O3 synthesized by polymeric (Poly) & microwave (MW) techniques and fired at 800 and 1000 °C.SEM of the uncoated and coated TiO2 substrates with nano-Al2O3
The SEM images of TiO2 coated substrate with Al2O3 synthesized via polymeric and microwave techniques and fired at 800 °C at different magnifications are shown in Fig. 13. It was observed that the surface of the coated TiO2 substrate by alumina prepared via microwave method has larger pinholes than the coated substrate with alumina prepared by polymeric method. In addition, the EDX analysis shows more absorption of Al ion for coated samples prepared by the polymeric method than the microwave method. This is attributed to the Al2O3 nanoparticles entering the TiO2 substrate’s pores and filling them. At the same time, the rest of the nano-Al2O3 particles covered the TiO2 substrate surface well, increasing the Al ion absorption and decreasing the Ti ion absorption to a high extent. This is due to the crystallinity of prepared alumina by microwave (α-alumina) being higher than that prepared by polymeric method (γ-alumina), as shown previously in TEM result Fig. 9, as the metal ions with large atomic radii diffused less readily than those with smaller atomic radii46.Figure 13SEM of coated TiO2 substrates with Al2O3 synthesized via (a) polymeric and (b) microwave techniques and fired at 800 °C at different magnifications.The SEM images of TiO2 substrates uncoated and coated with nano-Al2O3 synthesized via polymeric technique and fired at 800 °C after they were exposed to 0.5 M solution of H2SO4 are shown in Fig. 14a, b, respectively. Figure 14a illustrates the shape of the uncoated TiO2 substrate after the PDP was carried out in the aggressive medium 0.5 M H2SO4, It was clear that the surface of the sample contains many holes and roughness, and these characteristics are due to the effect of the solution.Figure 14SEM image and EDX analysis after the PDP process of (a) the uncoated TiO2 substrate in 0.5 M H2SO4 solution and (b) the coated TiO2 substrate with nano-Al2O3 prepared by polymeric method in 0.5 M H2SO4 solution.EDX analysis and its data tabulated in Table 2 found that the weight percentage of titanium in the substrate before the PDP was executed was about (54%). In comparison, it became (44%) after the PDP was achieved. This is due to the presence of sulfur element from the (SO42−) group, as its weight percentage is recorded at about 3%; thus, it penetrated the surface of the substrate, which led to a reduction in the weight percentage of titanium. Also, the effect of the penetration of sulfur ions to the surface of the substrate is demonstrated by increasing the porosity of the uncoated TiO2 substrate from 30% before PDP to 33% after PDP, as was established in Table 3, which means that the surface of the sample had been damaged and has many pores.
Table 2 Element composition of uncoated and coated TiO2 substrate by alumina prepared by (Poly) method at 800 °C before and after the PDP process in 0.5 M H2SO4 solution.Table 3 Effect of the porosity percentage on the corrosion rate of the TiO2 substrate uncoated and coated by alumina by polymeric (Poly) and microwave (MW) methods after tested in 0.5 M H2SO4 solution.Figure 14b represents the coated TiO2 substrate with nano-Al2O3 prepared by the polymeric method and calcined at 800 °C after being exposed to the aggressive medium 0.5 M H2SO4 solution and tested by the PDP method. It can be seen that the surface of the substrate no longer has pinholes, and the percentage of the surface porosity was calculated; it was found to be about 23% (Table 3); this means that the presence of the nano-Al2O3 helped to coat the surface well, which stimulated the reduction of pores. In addition, it can be noticed that the surface of the sample became free from cracks and gaps without scratches and also smoother. This result proves the alumina’s effect in protecting the sample’s surface47. The presence of a few pores in the range of nano-scale on the sample’s surface is a good factor in forming an oxygen barrier as a protective layer for the substrate in the corrosive medium48. The data obtained from EDX analysis showed that the percentage weight of the oxygen is about 70%, and this result explains the formation of an oxygen barrier, and the high percentage weight of oxygen may be related to the reduction of sulfate ions49.Compressive strength for the uncoated and coated TiO2 substrateUpon calcination at 800 °C, a few selected coated samples are examined for compressive strength and contrasted with uncoated substrate samples. Unlike microwave-coated samples and uncoated substrates, the TiO2-coated substrates with alumina via polymeric approach exhibit the highest strength among the remaining samples, with a maximum strength of about 65 MPa. This can be attributed to some of the Al2O3 nanoparticles entering the titania substrate’s pores and filling them while the rest of the nano-Al2O3 particles well covered the titania substrate surface, as discussed previously in the SEM results; this behavior led to an increase in the strength of the TiO2 coated substrates compared to uncoated TiO2 substrates. The polymeric approach Al2O3 nanoparticles’ tiny particle size with appropriate distribution produces the maximum reinforcing effect compared to the uncoated TiO2 substrate samples50. They are absorbed by the titania substrates, which reduces the titania substrate porosity compared to the alumina prepared via the microwave process, which has the opposite effect on strength values. Additionally, it has been found that treating the uncoated titania substrates with sulfuric acid decreased their compressive strength values to approximately 28 MPa, while treating the coated titania substrates with alumina prepared via microwave and polymeric method with sulfuric acid decreased their compressive strength value from 55 and 65 to be 45 and 52 MPa, respectively. As presented in Fig. 15.Figure 15Compressive strength values for uncoated and coated titania substrates before and after exposure to sulfuric acid treatment.Corrosion test estimationOpen circuit potential measurements. The open circuit potential of the uncoated and coated TiO2 substrate with alumina (Al2O3), which was prepared by polymeric (poly) and microwave (MW) methods at 800 °C, was measured in the aggressive medium 0.5M solution of H2SO4. The experimental results of the OCP were presented in Fig. 16 as three curves that have analogous shapes with an approximate difference: (a) the substrate without coating, (b) the substrate coated with alumina by the (MW) method, and (c) the substrate coated with alumina by the (poly) method. It can be seen from Fig. 16 that the OCP was shifted in a positive direction, from about − 0.382 V for the uncoated TiO2 substrate to about − 0.363 V for the coated TiO2 substrate with alumina, which was prepared by (MW) method and − 0.321 V for the coated TiO2 substrate with alumina which prepared by (poly), method against silver/silver chloride. This result was attributed to forming a protective coating layer on the surface of the TiO2 substrate. As well as this result also confirms that the polymeric method for coating was better than the microwave method, and this result agrees with the above discussion.Figure 16Open circuit potential of TiO2 substrate in 0.5 M H2SO4 solution (a) the substrate without coating (b) the substrate after coated by alumina prepared by (MW), method (c) the substrate after coated by alumina prepared by (poly), method examined in 0.5M solution of H2SO4 at 25 °C.Potentio-dynamic polarization method. The PDP experiment is one of the successful methods used to determine the corrosion rate of the substrate exposed to aggressive media like sulfuric acid, nitric acid, hydrochloric acid, and sodium chloride and the inhibition efficiency of the inhibitor or the coating which used to hinder the corrosion process51. The coating process is considered one of the important methods to increase the efficiency of some materials by changing some of their physical properties to protect the metals and their alloys from the undesirable effects of the corrosion process52. Among the materials used in this field are ceramic materials such as Al2O3, ZrO2, and TiO253. According to the data and the results of the PDP experiments for the TiO2 substrates uncoated and coated with alumina (Al2O3), after exposing them to a 0.5 M solution of sulfuric acid as an aggressive medium, it was found that the corrosion rate of the uncoated substrate was decreased from the value (67.71 to 16.30 and 29.75 mm/year), after it was coated with Al2O3 by polymeric (poly), and microwave (MW), methods and calcined at 800 °C, respectively. By comparing the corrosion rate values of the two coating methods, it can be noticed that the polymeric method was better than the microwave method. This result may be attributed to the shape of the phase formed by alumina on the substrate after it was coated, as was reported in previous studies54,55. The analysis and characterization of the TiO2 substrate explained that, after the substrate was coated with alumina, which was prepared by the microwave method at a temperature of 800 °C, the coating layer was formed in the alpha form of aluminum oxide crystallized in the corundum structure, which has some properties such as low surface area and is almost non-porous56. The coating layer of alumina prepared by the polymeric method at the same temperature was formed in the gamma alumina phase, which has some excellent properties, such as a large surface area with some porosity, which resulted in decreasing the corrosion rate of the substrate-coated by polymeric method more than the substrate coated by microwave method at 800 °C, as shown in Table 457.
Table 4 The parameters of the PDP process of uncoated and coated TiO2 substrates with Al2O3 prepared by microwave method (MW) and polymeric method (Poly) in 0.5 M solution of H2SO4 at 25 °C.Equation (1) was used to deduce the percentage of the inhibition efficiency of the substrate after it was coated with nano-Al2O3 to prevent the corrosion process, and all the parameters are established in Tables 4 and 5.$$IE\%=\frac{{i}_{corr}^{o}-{i}_{corr}}{{i}_{corr}^{o}} \times 100$$
(1)
where \({i}_{corr}^{o}\) and \({i}_{corr}\) are the corrosion current density of the uncoated and coated substrate, respectively.
Table 5 The electrochemical impedance data of the TiO2 substrate uncoated and coated by nano-Al2O3 prepared by microwave method (MW) and by polymeric method (Poly) tested in 0.5 M H2SO4 at 25 °C.Good values of the inhibition efficiency (56% and 78%) of the working electrode (TiO2 disc), after it was coated with alumina, which was prepared at 800 °C by microwave and polymeric methods and tested in the corrosive medium (0.5 M H2SO4), can be interpreted by reducing the active area on the substrate due to the formation of mix film of Al2O3, TiO2 phase58 and TiAl phase; which it has excellent mechanical properties and corrosion resistance at temperatures (over 600 °C)59. The appearance of smooth polarization curves in Fig. 17a, b and only one peak at the corrosion potential is evidence that the formed film was electrochemically inactive60. This result agrees with the previous characterization of the surface. It was clear from the results reported in Table 4 that the value of the corrosion potential (Ecorr), of the uncoated substrate (TiO2 disc), recorded − 409.13 mV, shifts to more noble value in a positive direction after coating with Al2O3 to reach − 289.73 mV at (800 °C), for the sample coated with alumina by polymeric method, while the corrosion potential (Ecorr), recorded − 382 mV, when the sample coated with Al2O3 by microwave method at (800 °C); this result verifies that the use of the polymeric method (Poly), for coating was better than the microwave method (MW). Also, this result confirms the formation of a good coating layer of the nano-composite formed of alumina and titanium (Al2O3 TiO2 phase and TiAl phase) that exhibits good resistance against the corrosion process61. As mentioned by M. Sabzi et al., the potentiodynamic polarization diagram for galvanized steel in seawater environment electrolyte shows that the galvanized layer’s resistance to corrosion is greater than that of the steel underlying. Moreover, the impedance resistance of galvanized steel decreased as the surrounding temperature rose62.Figure 17Tafel plots of uncoated and coated TiO2 substrate by the nano-Al2O3 after PDP process (a) coating by microwave method (MW) (b) coating by polymeric method (Poly), examined in 0.5 M solution of H2SO4 at 25 °C.Through Table 4, it can also be noted that the corrosion current density (\({i}_{corr}^{o}\)), was decreased from the value of 712.62 μA cm−2 of the uncoated TiO2 substrate to reach a value of about 313.15 and 152.79 μA cm−2 of the coated substrate by the nano alumina that, synthesized by microwave and polymeric methods, respectively. This result indicates that the formed film increases the ability of the substrate to resist the corrosion process2.Electrochemical impedance spectroscopy method. The EIS diagrams of the uncoated and coated substrate (TiO2 disc), after tested in 0.5 M H2SO4 solution, are depicted in Figs. 18, 19, 20 and 21. It can be seen from Fig. 18a, b that the Nyquist plots deviated from the ideal semicircle, meaning that it appeared as a depressed semicircle. This phenomenon, the effeteness of the frequency dispersion, has been attributed to forming a passive film with microporous, roughness and irregularity of the surface of the substrate, and random distribution of the active sites63,64.Figure 18Nyquist plots (a) TiO2 substrates uncoated and coated with nano-Al2O3 prepared by microwave method (MW), (b) TiO2 substrate uncoated and coated with nano-Al2O3 prepared by polymeric method (Poly) tested in 0.5 M H2SO4 at 25 °C.Figure 19The modal of the equivalent circuit used to fit the impedance data.Figure 20Bode phase of the impedance data (a) TiO2 substrates uncoated and coated with (Al2O3), formed by microwave method (MW), (b) TiO2 substrates uncoated and coated with (Al2O3), formed by polymeric method (Poly), tested in 0.5 M solution of H2SO4 at 25 °C.Figure 21Bode modulus of the impedance data (a) TiO2 substrates uncoated and coated with (Al2O3), formed by microwave method (MW), (b) TiO2 substrates uncoated and coated with (Al2O3), formed by polymeric method (Poly) tested in 0.5 M solution of H2SO4 at 25 °C.Increasing the diameter size of the semicircle of the substrate (TiO2 disc) after it was coated with alumina (Al2O3) pointed to the formation of a good passive layer on the surface of the sample attributed to the presence of alumina, which exhibited good stability in various media; as it allows improving the corrosion resistance of the sample surface65. In addition, the comparison between the diameter size of the two capacitive loops of the substrate (TiO2 disc), after it was coated with alumina, which was prepared by microwave and polymeric methods Fig. 18a, b, respectively, showed that; the diameter size of the capacitive loop Fig. 18b, was larger more than that of Fig. 18a, this result agrees with the above discussion that the polymeric method, was better for coating than the microwave method. According to previous studies, the large diameter of the semicircle of the sample also means more protection against the effects of the corrosion process66.The percentage of the inhibition efficiency (IE%) of the coating material (Al2O3) was calculated using. \({R}_{ct}\) according to the following equation:$$IE\%=\frac{{{R}_{ct}^{o}-R}_{ct}}{{R}_{ct}^{o}} \times 100$$
(2)
Abbreviation \({R}_{ct}^{o}\) and \({R}_{ct}\) refers to the charge transfer resistance of coated and uncoated samples after the EIS test. It was cleared by reading the results reported in Table 5 that the charge transfer resistance. \({R}_{ct}\) increases after the substrate (TiO2 disc) is coated with nano-(Al2O3); this result indicates that the corrosion rate is reduced due to the formation of a protective film of the coating materials on the surface of the sample.The equivalent circuit used for the analysis of EIS data was [R(QR)(QR)], shown in Fig. 19, where (Rs) is the solution resistance, (Rct) the charge transfers resistance and (Rf) the film resistance. Due to the presence of the phenomenon frequency dispersion; the constant phase element (CPE), was used in the equivalent circuit instead of the double-layer capacitance (Cdl), to be more suitable for the results of the impedance data67.The constant phase element (CPE), was determined from the following equation:$${Q}_{CPE}={\text{Y}}_{\rm o}^{-1}({\text{j}} \upomega)^{-{\text{n}}}$$
(3)
According to the above equation, the constant phase element (CPE) consists of a constant Yo, j is an imaginary number equal (− 1)1/2, ω is the angular frequency in rad/s, and a component n is the exponent of CPE expresses phase shift where n is between (− 1) and (+ 1), i.e. (− 1 ≤ n ≤  + 1). So, if a component n = 0, the constant phase element acts as a resistor, while n =  − 1, the CPE appears as an inductor, and if n =  + 1, the constant phase element performs as a capacitor68. Shifting of the phase angle or the component n to more positive value after the substrate (TiO2 disc), coated with nano-Al2O3, by microwave method from (0.443 to 0.525 at 800 °C) and after the substrate coated by polymeric method from (0.443 to 0.983 at 800 °C), this result refers to that, the irregularity of the sample surface was decreased69, as be reported in Table 5, and can also be seen in Fig. 20a, b.Also, it can be observed from Table 5 that the value of n is less than 0.5 for the substrate without coating; this result means that the corrosion rate was controlled by the diffusion process, while after the substrate was coated, the value of n increased than 0.5 indicates that the corrosion rate controlled by the charge transfer resistance \({(R}_{ct}),\)70. Figure 21a, b shows the Bode modulus impedance Z of the TiO2 substrate without and with coating by nano-Al2O3. It was cleared by looking at Fig. 21a, b, that; the Bode modulus increased from (1.8 to 2.4 Ω cm2) after the sample was coated, and this is explained by the good effect of the coating material (nano-Al2O3), to reduce the corrosion rate by slowing down the charge transfer resistance \({(R}_{ct}),\)71. It can be noticed that the two electrochemical techniques, PDP and EIS, used in this study agree with each other.
Study of microstructure and corrosion behavior of nano-Al2O3 coating layers on TiO2 substrate via polymeric method and microwave combustion
