Preparation of FeSA₃/PDA composite coating on N80 steel and enhancement of corrosion resistance via surface hydrophobic modification with ODT, ODA, and DTMS-SiO₂

2.1. Materials

In this study, N80 steel, commonly used in oil and gas drilling, was modified using low surface energy modifying substances such as ODA and ODT, and all reagents used in the experiment were of analytical grade.

2.2. Sample preparation

In this study, the surface of the N80 waste steel sheet was first pretreated. The steel sheet was ground step by step by using 360 #, 800 #, 1500 #, and 2500 # water sandpaper in turn until the surface was smooth and without obvious scratches. Subsequently, ultrasonic cleaning was performed using deionized water and anhydrous ethanol for 10 min, aiming to achieve the complete elimination of surface contaminants. After cleaning, the steel sheet was dried in a blast drying oven at 85°C in preparation for subsequent surface modification.

In the process of surface modification, stearic acid was dissolved in methanol, and heated and stirred by the constant temperature heating magnetic stirrer until stearic acid was completely dissolved. The effect of different conditions on the hydrophobic coating was studied by immersing the treated steel sheet in stearic acid solution for 6 h when the solution temperature was controlled in the range of 30-60°C. After modification, the steel sheet was washed with ethanol and dried to obtain a hydrophobic steel sheet surface [12].

Tris (hydroxymethyl) aminomethane hydrochloride buffer solution of 10 mM was prepared. The pH value was adjusted to 8.5 with 0.5 mol/L NaOH solution. Hydrogen peroxide and copper sulfate were used as oxidants. The FeSA₃-N80 steel sheet was soaked in the solution after the substance was dissolved, and the modified steel sheet covered with PDA film was obtained after cleaning and drying.

ODA, ODT, and DTMS-SiO₂ were used to carry out a secondary reaction on the PDA film. The prepared coating was then cleaned with anhydrous ethanol to keep its surface clean.

2.3. Testing of membrane hydrophilicity-hydrophobicity and characterization of surface microstructure

The water static contact angle of the film surface was measured to determine the hydrophilicity and hydrophobicity of the film surface, and then the microscopic morphology of the film surface was observed by optical electron microscope, SEM and energy dispersive spectroscopy (EDS), the surface state of the film was measured, and the center point of the sample and the vertex of the square with a side length of 5 mm were selected as five test points. The surface morphology was observed, and the surface element composition was detected.

2.4. Electrochemical testing and corrosion morphology of modified steel coatings

Some modified ODT/PDA/FeSA₃-N80 steel coating, ODA/PDA/FeSA₃-N80 steel coating, and DTMS-SiO₂/PDA/FeSA₃-N80 steel coating samples were put into a 3.5% NaCl aqueous solution, and the polarization curves and impedance spectra were tested, respectively. The working electrode was a modified N80 steel sample, the auxiliary electrode was a carbon rod, and the reference electrode was a saturated calomel electrode. Several modified SFSS-B44660 samples were immersed in 3.5% NaCl solution for 30 days, and the corrosion products on the surface of the samples were removed at the end of the immersion period. The micro-morphology of the surface of each sample was observed by SEM [12].

3.1. Microstructure and chemical composition analysis

From the micro-morphology analysis results of Figures 1(a) and 1(b), it can be seen that the FeSA₃/N80 coating formed on the surface of the N80 steel sheet is flat and uniformly covered; the height difference between the lowest and highest points of the coating surface is relatively small, and the surface exhibits a composite morphology characterized by the coexistence of flaky structures, particles and pits. The synthesis mechanism of the coating is illustrated in Figure 1(c). Energy-dispersive spectroscopy (EDS) analysis results of Figure 2(a) and 2(b) show that the main elements of the coating are carbon (C), oxygen (O) and iron (Fe), which confirms that the surface of the N80 steel sheet is completely and uniformly covered by the FeSA₃ coating.

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Figure 1.

Microstructure analysis and synthesis mechanism of FeSA₃-N80 steel coating (a) Morphology analysis, (b) SEM analysis, (c) FeSA₃ synthesis mechanism.

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Figure 2.

EDS analysis of FeSA3-N80 steel coating surface (a) Elemental signal identification (b) Elemental content.

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Figure 3 shows the PDA/FeSA₃-N80 steel coating microstructure analysis. The microstructure analysis in Figure 3(a) shows that the overall structure of the coating surface is complete, with no obvious holes, fractures, or peeling. It shows that PDA forms a dense and continuous organic protective layer on the surface of iron stearate. The maximum fluctuation of the coating surface is about 35.645 μm, and the coating thickness increases, which further verifies the PDA deposition-induced surface reconstruction effect. SEM characterization results of Figure 3(b) demonstrate that PDA has completely covered the surface of the ferric FeSA₃- N80 steel coating; no obvious structural defects, cracks, or large particle agglomeration were observed across the entire coating, indicating that the coating possesses favorable compactness and stable quality [13], with the detailed synthesis procedure illustrated in Figure 3(c). EDS measurement results of Figures 4(a) and 4(b) confirm that a PDA-based organic coating rich in C, O, and N elements was successfully introduced onto the surface of ferric stearate. In the EDS spectra, the relatively high signal intensities of C and O elements suggest that the coating surface is abundant in C-O backbone structures, which is consistent with the molecular composition of polydopamine; the presence of N element further verifies the effective deposition and polymerization of PDA molecules on the substrate surface. In contrast, the relatively weak Fe element signal implies that most areas of the metal substrate have been covered by the PDA coating, with only localized regions detectable by the electron beam. Taken together, these EDS results are highly consistent with the structural characteristics of PDA molecules, validating the formation of a uniform and dense organic protective film on the substrate surface.

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Figure 3.

Microstructure analysis and synthesis mechanism of PDA/FeSA3-N80 steel coating (a) Morphology analysis, (b) SEM analysis, (c) PDA synthesis mechanism.

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Figure 4.

EDS analysis of PDA/FeSA₃-N80 steel coating surface. (a) Elemental signal identification (b) Elemental content.

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Figure 5(a) reveals that the ODT coating uniformly coats the surface of the PDA/FeSA₃-modified N80 steel coating. The height difference between the highest and lowest points of the coating surface reaches 41.437 μm, without obvious voids or peeling areas, presenting a continuous undulating morphology as a whole, and the maximum thickness of the coating is 35.645 μm. SEM characterization results of Figure 5(b) demonstrate that the coating surface is dominated by a large number of spherical-like or lamellar structures with a typical ordered arrangement, indicating that ODT molecules form an ordered monolayer or multilayer aggregation state on the surface of the PDA substrate. The specific synthesis procedure is illustrated in Figure 5(c). EDS analysis results of Figure 6 show that distinct characteristic peaks corresponding to C, O, N and S elements are observed in the spectrum of Figure 6(a). Among these elements, C, N and O are the fundamental components of PDA, while the presence of S element verifies that octadecyl mercaptan is grafted onto the PDA surface, confirming the successful binding of the thiol group (–SH) to PDA. The elemental content histogram in Figure 6(b) further displays the mass fraction and atomic fraction of S element, which provides additional evidence for the effective modification of octadecyl mercaptan molecules.

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Figure 5.

Microstructure analysis and synthesis mechanism of ODT/PDA/FeSA₃-N80 steel coating (a) Morphology analysis, (b) SEM analysis, (c) ODT synthesis mechanism.

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Figure 6.

EDS analysis of ODT/PDA/FeSA₃-N80 steel coating surface. (a) Elemental signal identification (b) Elemental content.

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From the three-dimensional topography map in Figure 7(a), the coating surface exhibits a certain degree of micro-fluctuations, with a maximum thickness of 63.551 μm. SEM characterization results of Figure 7(b) reveal that the ODA/PDA/FeSA₃-N80 steel coating surface is relatively flat, with only irregular flaky protrusions distributed in local areas, which are thin-edged and slightly tilted. Following the synthesis procedure depicted in Figure 7(c), the coating was subjected to EDS characterization, with the corresponding results presented in Figures 8(a) and 8(b). In Figure 8(a), no Fe signal was detected via EDS on the ODA-modified PDA/FeSA₃-N80 steel coating surface, indicating that octadecylamine has formed a continuous and dense covering layer on the pristine coating surface. The variation trends of elemental mass and atomic percentages in Figure 8(b) further confirm the stable adhesion of octadecylamine onto the PDA/FeSA₃-N80 steel coating.

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Figure 7.

Microstructure analysis and synthesis mechanism of ODA/PDA/FeSA₃-N80 steel coating (a) Morphology analysis, (b) SEM analysis, (c) ODA synthesis mechanism.

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Figure 8.

EDS analysis of ODA/PDA/FeSA₃-N80 steel coating surface. (a) Elemental signal identification (b) Elemental content.

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From the three-dimensional topography provided by Figure 9(a), the coating surface has an obvious undulating microstructure, and the maximum height difference in the Z-axis direction is about 41.714 μm, indicating that the surface has a certain roughness. The SEM analysis in Figure 9(b) shows that the DTMS-SiO₂/PDA/FeSA₃-N80 steel coating is uniformly deposited on the surface, forming a dense granular coating. The detailed synthesis procedure is illustrated in Figure 9(c). The EDS analysis of Figure 10 reveals that the coating surface is mainly composed of four elements, C, O, N, and Si, of which the Si element dominates in the atomic ratio. DTMS-SiO₂/PDA/FeSA₃-N80 steel coating was successfully prepared.

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Figure 9.

Microstructure analysis and synthesis mechanism of DTMS-SiO₂/PDA/FeSA₃-N80 steel coating (a) Morphology analysis, (b) SEM analysis, (c) DTMS-SiO₂ synthesis mechanism.

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Figure 10.

EDS analysis of DTMS-SiO₂/PDA/FeSA₃-N80 steel coating surface. (a) Elemental signal identification (b) Elemental content.

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3.2. Hydrophobicity and infrared spectroscopy analysis

Figure 11(a) shows the untreated N80 steel plate, which exhibits strong hydrophobicity. After stearic acid treatment, a FeSA₃-N80 coating is formed on its surface, and Figure 11(b) demonstrates a further improvement in hydrophobicity. As can be seen from Figure 11(c), the hydrophilicity of the PDA/FeSA-N80 steel coating is significantly enhanced after PDA)modification.

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Figure 11.

Contact angle measurement of N80 steel sheets under different treatments (a) Bare N80 steel sheet, (b) FeSA₃-N80 coating. (c) PDA/FeSA₃-N80 coating.

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Figure 12(a) shows the infrared spectrum curve of the PDA/FeSA₃-N80 coating. The double peaks at 1600 cm⁻¹ and 1485 cm⁻¹ originate from the C=C stretching vibration of the aromatic ring, and are also affected by the contribution of N-H bending vibration; the overlap of the two enhances or broadens the peaks. The absorption peaks at 1386 cm⁻¹ and 1348 cm⁻¹ are generated by the bending vibration of aromatic O-H bonds and the stretching vibration of aromatic primary amine C-N bonds, respectively. This part mainly comes from catechol produced by the self-polymerization of dopamine. The absorption peak at 1246 cm⁻¹ is attributed to the stretching vibration of C-N bonds, the peak at 725 cm⁻¹ is the out-of-plane stretching vibration of C-H, and the peak at 559 cm⁻¹ is the out-of-plane bending vibration of C-N in the indole structure.

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Figure 12.

FT-IR of stearic acid powder and surface coatings of metal samples (a) PDA/FeSA₃-N80 coating. (b) SA powder. (c) FeSA₃-N80 coating.

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Figure 12(b) presents the infrared spectrum curve of stearic acid powder, and Figure 12(c) shows that of the FeSA3-N80 coating. The absorption peak at 1711 cm⁻¹ is caused by the free carbonyl group in stearic acid molecules. The peaks in the range of 1500 cm⁻¹-1200 cm⁻¹ correspond to the bending vibration of -CH₂, the bending vibration of α-CH₂, and the stretching vibration of C-O, respectively. The absorption peaks at 1232 cm⁻¹ and 1198 cm⁻¹ belong to the bending vibration of COOH. The peak at 725 cm⁻¹ is the in-phase rocking vibration of -CH₂, while the peaks at 683 cm⁻¹ and 543 cm⁻¹ are the bending vibrations of -C-OH and C-C=O, respectively.

Compared with the infrared spectrum of the iron stearate coating formed on the N80 steel sheet, there is no obvious change in the stretching vibration absorption peaks of methyl and methylene groups. However, the original free carbonyl absorption peak at 1711 cm⁻¹ shifts to a lower wavenumber, and asymmetric and symmetric stretching vibration absorption peaks of coordinated carbonyl groups appear at 1589 cm⁻¹ and 1530 cm⁻¹. This change indicates that the carboxyl group coordinates with iron ions, resulting in a decrease in electron cloud density and a weakening of the C=O bond strength, thereby reducing the wavenumber of its characteristic absorption peak. The above results confirm the presence of long-chain alkane skeletons in iron stearate, and the appearance of coordinated carbonyl absorption peaks in the low-frequency region indicates the formation of iron stearate.

The ODT/PDA/FeSA₃ composite was characterized by FT-IR, with results presented in Figure 13. Notably, the successful grafting of PDA onto the surface of iron stearate has been confirmed in our previous experiments. In the FT-IR spectrum of pure ODT, two distinct absorption peaks corresponding to the symmetric and asymmetric stretching vibrations of methylene (-CH₂-) groups were observed at 2922 cm⁻¹ and 2848 cm⁻¹ in the high-frequency region, while a weak peak at 2641 cm⁻¹ was attributed to the stretching vibration of the S-H bond; in the low-frequency region, the absorption peak at 1466 cm⁻¹ corresponded to the stretching vibration of the C-S bond, and the peak at 725 cm⁻¹ was assigned to the rocking vibration of long alkyl chains (-CH₂ₙ-). Correspondingly, significant changes occurred in the FT-IR spectrum after the reaction between ODT and PDA: the absorption peak at 3429 cm⁻¹ was ascribed to the stretching vibrations of N-H bonds (from amino groups) and O-H bonds (from phenolic hydroxyl groups) in the high-frequency region, the S-H stretching vibration peak at 2641 cm⁻¹ completely disappeared, attributed to the chemical bonding between thiol groups and quinone groups in PDA, which led to the formation of more stable C-S bonds, and the absorption peaks at 1485 cm⁻¹ and 754 cm⁻¹ were respectively assigned to the stretching vibration of C-S bonds and the characteristic vibration of long alkyl chains, collectively confirming the successful immobilization of ODT on the PDA/iron stearate coating surface.

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Figure 13.

FT-IR of ODT (a) ODT/PDA/FeSA₃ coating, (b) ODT powder, (c) PDA/FeSA₃ coating.

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Figure 14 shows the FT-IR spectrum of the PDA/FeSA₃ coating surface modified with ODA. For pure ODA, the absorption peak at 3380 cm⁻¹ is assigned to the stretching vibration of N-H bonds; the peaks at 2916 cm^-1 and 2852 cm⁻¹ correspond to the asymmetric stretching vibration of methyl (-CH₃) groups and symmetric stretching vibration of methylene (-CH₂-) groups, respectively. Additionally, the peak at 1600 cm⁻¹ is attributed to the bending vibration of N-H bonds, while the absorption at 1469 cm⁻¹ arises from the asymmetric stretching vibration of C-H bonds. The peak at 1059 cm⁻¹ is associated with the stretching vibration of C-N bonds in aliphatic amines, and the absorption at 727 cm⁻¹ corresponds to the rocking vibration of long aliphatic alkyl chains (-CH₂ₙ-).

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Figure 14.

FT-IR of ODA (a) ODA/PDA/FeSA₃ coating, (b) ODA powder, (c) PDA/FeSA₃ coating.

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After modifying the PDA/FeSA₃ surface with ODA, distinct changes were observed in the FT-IR spectrum, accompanied by the emergence of several new characteristic absorption peaks. Specifically, a broad peak at 3419 cm⁻¹ is generated by the stretching vibrations of O-H (from PDA) and N-H (from ODA and PDA) bonds following the grafting of ODA onto PDA. The peaks at 2920 cm⁻¹ and 2850 cm⁻¹ are respectively assigned to the C-H stretching vibrations of methyl and methylene groups. The absorption at 1580 cm⁻¹ originates from the C=C stretching vibration of the aromatic ring skeleton, while the peak at 721 cm⁻¹ corresponds to the bending vibration of C-H bonds on the benzene ring. A broad peak appearing at 1635 cm⁻¹ is attributed to the overlapping stretching vibrations of C=O (from aromatic ring structures) and C=N bonds. The presence of the C=N stretching vibration peak indicates the formation of a Schiff-base adduct during the modification of the PDA/FeSA₃ coating with ODA, which involves a nucleophilic addition reaction between the quinone or catechol moieties in PDA and the amino groups in ODA [14]. Furthermore, the absorption peak at 1473 cm⁻¹ is ascribed to the bending vibration of aliphatic methylene groups, the peak at 1397 cm⁻¹ corresponds to the symmetric stretching vibration of methyl groups, the absorption at 1255 cm⁻¹ arises from the C-N stretching vibration of aromatic rings, and the peak at 719 cm⁻¹ is associated with the rocking vibration of long alkyl chains.

Notably, the intensity of the broad peak in the high-frequency region of the ODA/PDA/FeSA₃ spectrum is reduced and its width narrowed. This phenomenon can be attributed to the covalent binding of ODA to PDA via Schiff-base formation, which reduces the number of hydrophilic active groups in PDA and thereby weakens the infrared characteristic peaks of PDA’s polar groups. Collectively, these results confirm the successful conjugation of ODA to PDA and its subsequent immobilization on the PDA/FeSA₃ coating surface.

Figure 15 shows the FT-IR spectra of the PDA/FeSA₃ coating surface before and after adsorption of DTMS-SiO₂. For pure DTMS-SiO₂, a broad peak at 3425 cm⁻¹ corresponds to the stretching vibration of -OH groups. The absorption peaks at 2924 cm⁻¹ and 2859 cm⁻¹ are attributed to the asymmetric stretching vibration of methyl groups and symmetric stretching vibration of methylene groups, respectively. The peaks at 1466 cm⁻¹ and 710 cm⁻¹ are assigned to the in-plane bending vibration and out-of-plane bending vibration of methylene groups, while the absorption bands at 1107 cm⁻¹ and 801 cm⁻¹ are characteristic of Si-O-Si stretching vibrations. Additionally, the peak at 482 cm⁻¹ arises from the bending vibration of Si-O-Si bonds. These spectral features collectively confirm the successful preparation of hydrophobic silica.

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Figure 15.

FTIR of DTMS-SiO₂ (a) DTMS-SiO₂/PDA/FeSA₃ coating, (b) DTMS-SiO₂ powder, (c) PDA/FeSA₃ coating.

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In the FT-IR spectrum of the DTMS-SiO₂/PDA/FeSA₃ coating, all characteristic peaks of DTMS-SiO₂ are retained, with the emergence of a new peak at 1640 cm⁻¹, which is primarily attributed to the overlapping C=C stretching vibration of aromatic rings and N-H bending vibration from PDA. Furthermore, the stretching vibration peak of hydroxyl groups at 3446 cm⁻¹ exhibits reduced intensity and increased width compared to pure DTMS-SiO₂. This phenomenon is ascribed to the hydrogen bond formation between the phenolic hydroxyl groups in PDA and the hydroxyl groups on the DTMS-SiO₂ surface after grafting of hydrophobic silica (with hydrophobic moieties) onto the PDA layer. Taken together, these results provide compelling evidence for the successful immobilization of DTMS-SiO₂ on the PDA/FeSA₃ coating surface.

The contact angle of the coating surface has been characterized in Figure 16. The contact angle of the coating treated with hydrophobic silica was the largest, and the hydrophobicity was the strongest.

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Figure 16.

Contact angle (a) ODT/PDA/FeSA₃, (b) ODA/PDA/FeSA₃, and (c) DTMS-SiO₂/PDA/FeSA₃ coating.

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3.3. Study on corrosion resistance and electrochemistry of N80 steel substrate modified with composite coatings in NaCl solution

Combining the potentiodynamic polarization curves in Figure 17 and fitting data in Table 1, it is concluded that the ODT-modified ODT/PDA/FeSA₃-N80 composite coating retains excellent electrochemical corrosion resistance following 30-day immersion in 3.5% NaCl solution. The untreated steel substrate exhibits anodic active dissolution, with a corrosion potential (E_corr) of -0.60897 V and a high corrosion current density (I_corr) of 4.0738×10⁻⁴ A/cm²; the anodic (ba) and cathodic (|bc|) polarization slopes are 40.42 mV and 82.57 mV, respectively, indicating the absence of effective surface barrier protection and kinetic-dominated corrosion reactions.

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Figure 17.

Potentiodynamic polarization curves of ODT coating-steel after 30 days of immersion in 3.5% NaCl solution.

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Modification with varying ODT concentrations significantly optimizes the coating’s polarization behavior. For the 0.08 mol/L ODT sample, E_corr shifts positively to -0.36575 V, while I_corr decreases to 1.5824×10⁻⁷ A/cm²; concurrently, ba and |bc| increase to 124.82 mV and 178.69 mV, respectively. These results demonstrate stronger inhibition of charge transfer and interfacial reactions, significantly enhanced polarization resistance, and effective retardation of corrosion—corresponding to a low corrosion rate of 0.008006 mm/a and a high protection efficiency of 99.92%.

Notably, although the 0.10 mol/L ODT sample shows a slightly higher I_corr than the 0.08 mol/L counterpart, its I_corr remains at a low level, confirming the good protective performance of its barrier layer. Thus, the coating prepared with 0.08 mol/L ODT exhibits more stable corrosion resistance in long-term corrosive environments, attributed to the formation of a dense hydrophobic film on the metal surface that effectively inhibits the penetration of corrosive media and migration of reactants, thereby achieving long-lasting protection.

Figure 18 presents the electrochemical polarization curves of octadecylamine-coated steel after 30-day immersion in 3.5% NaCl solution. Overall, all modified samples exhibit a significant positive shift in E_corr and I_corr. The anodic and cathodic polarization slopes of octadecylamine-treated steel sheets are greater than those of untreated N80 steel, indicating a reduced corrosion tendency for the coated specimens.

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Figure 18.

Electrochemical polarization curves of ODA coating after 30 days of immersion in 3.5% NaCl solution.

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Table 2 shows the electrochemical fitting data of coatings treated with different concentrations of ODA after 30-day immersion. Comparative analysis of various concentrations reveals that with increasing octadecylamine concentration, both the anodic polarization slope (ba) and cathodic polarization slope (|bc|) generally increase, suggesting that the coating imposes a certain degree of restriction on both anodic metal dissolution and cathodic oxygen reduction reactions. Specifically, at a concentration of 0.2 mol/L, the polarization slopes reach the maximum values (140.56 mV for anodic slope and 131.73 mV for |bc|), corresponding to the lowest corrosion current density (6.3178×10⁻⁸ A/cm²) and the highest corrosion inhibition efficiency (99.97%). These results indicate that the hydrophobic coating formed at this concentration has the densest structure and possesses the optimal barrier protection capability [14,15].

Based on the potentiodynamic polarization curves shown in Figure 19 and the electrochemical parameters listed in Table 3, the corrosion inhibition behavior of PDA/FeSA₃-N80 composite coatings modified with different mass concentrations of DTMS-SiO₂ after 30 days of immersion in 3.5% NaCl solution was systematically analyzed.

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Figure 19.

Potentiodynamic polarization curves of DTMS-SiO₂ coating-steel after 30 days of immersion in 3.5% NaCl solution.

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The results demonstrate that the untreated steel sheet exhibits a typical active dissolution tendency, with a corrosion potential (E_corr) as low as -0.63897 V and a corrosion current density (I_corr) as high as 4.0738×10⁻⁴ A/cm², indicating the absence of an effective barrier layer at the interface and rapid progression of corrosion reactions. In contrast, the DTMS-SiO₂-modified sample shows significant optimization in polarization characteristics, with a positive shift in E_corr and a notable reduction in I_corr, effectively lowering the overall corrosion rate. This reflects the stable corrosion-resistant properties of the coating during mid-term service. Among them, the sample with a concentration of 20 g/L performs most prominently, with its E_corr increasing to -0.36775 V, I_corr decreasing to 5.6751×10⁻⁸ A/cm², and corrosion rate dropping to 0.00020808 mm/a, achieving a protection efficiency (η) as high as 99.99%, compared with traditional organic-inorganic corrosion inhibitors, it exhibits higher corrosion inhibition efficiency [15]. Additionally, the anodic polarization slope (ba) and cathodic polarization slope (|bc|) of this sample reach 167.84 mV and 141.82 mV, respectively, significantly higher than those of the bare steel sample, indicating a substantial enhancement in electrode reaction resistance and effective suppression of the corrosion process.

The DTMS-SiO₂ coating demonstrates excellent corrosion resistance in long-term anti-corrosion applications, exhibiting optimal comprehensive performance particularly under 20 g/L conditions, with outstanding polarization resistance, interfacial stability, and long-term protective potential.

According to the EIS shown in Figure 20 and the equivalent circuit fitting parameters in Table 4, the corresponding physical model is given by the equivalent circuit in Figure 21, where Rs is the solution resistance, CPE_f is the coating capacitance, and R_c is the coating resistance, CPE_dl is the electric double layer capacitor, and R_ct is the charge transfer resistance. The corrosion resistance of ODT-modified coatings after immersion in 3.5% NaCl solution for 30 days was systematically evaluated. The Nyquist diagram of the untreated steel sheet shows only a short open arc, the impedance modulus in the low frequency region is low, and the peak phase angle in the Bode diagram is also obviously small, indicating that the charge transfer resistance is limited and the interface lacks an effective barrier structure, the corrosion reaction is mainly controlled by thermodynamic versus kinetic reaction control [16].

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Figure 20.

Electrochemical impedance diagrams of ODT coating-steel after 30 days of immersion in 3.5% NaCl solution (a) Nyquist diagram, (b) Bode diagram impedance, (c) Bode diagram phase angle, (d) Nyquist diagram, (e) Bode diagram impedance, (f) Bode diagram phase angle.

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Figure 21.

Equivalent circuit diagram after 30 days of immersion.

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The increase of ODT concentration significantly improved the overall impedance performance of the coating, especially at the concentration of 0.08 mol/L, the capacitance arc radius in the Nyquist diagram reached the maximum, and the charge transfer resistance R_ct increased to 97990.42Ω·cm², it is much higher than the 11.21 Ω·cm² of the unmodified steel sheet, indicating that the coating formed at this concentration has a good barrier effect on the penetration of corrosive media. The low frequency |Z|_0.01Hz modulus is maintained at the level of 104Ω·cm², the Bode diagram has a wide phase angle platform, and the peak angle is obviously improved, which reflects that the charge transfer is effectively suppressed, the system is in a relatively ideal capacitive behavior dominant state [16].

The CPE_f and CPE_dl of the sample treated with 0.08 mol/L were 3.5156×10^-6 F· cm^-2 and 9.4318×10^-3 F·cm^-2, respectively, which were significantly lower than those of the control group. The N value was close to 0.77, indicating that the surface of the sample was dense and uniform. Good capacitance response characteristics are beneficial to improve the stability of the coating and reduce the risk of charge accumulation and leakage [17]. It was found that the ODT modified coating showed significant corrosion resistance in long-term anti-corrosion applications, especially with 0.08 mol/L as the optimal treatment concentration, high polarization impedance, and stable interface structure. It has good long-term protective performance and engineering application prospects.

Figure 22 shows the Nyquist and Bode plots of ODA-modified and unmodified, Figure 22 shows the Nyquist and Bode plots of ODA modified and unmodified steel sheets soaked in a 3.5% NaCl solution for 30 days. Except for the 0.2 mol/L ODA/PDA/FeSA₃-N80 steel coating, all other coatings exhibit double capacitive arcs. Therefore, the experimental group of 0.2 mol/L ODA/PDA/FeSA₃-N80 steel coating was fitted using the equivalent circuit diagram shown in Figure 23(a), while the remaining experimental groups were fitted using the equivalent circuit diagram shown in Figure 23(b). In Figure 22(d) Nyquist diagram, the pure steel sheet shows only a small capacitance arc, and the diffusion characteristics of Warburg impedance show that the system is mainly controlled by rapid corrosion reaction, and the interface protection ability has been greatly reduced, however, the phase angle Figure 22(e) and Bode diagram phase angle Figure 22(f) of the pure steel sheet decreased significantly, indicating that with the increase of immersion time, the pure steel sheet gradually entered the stage of accelerated corrosion, and the Bode diagram phase angle decreased significantly, the maximum phase angle is less than 40°, which indicates that the system gradually changes from capacitance-dominated behavior to resistance-dominated mechanism, the interface structure has been significantly damaged, and the charge storage capacity of the interface is weakened. In Figure 22(a) Nyquist plots, the ODA-coated sample exhibits a distinct double-capacitive arc structure as a whole, especially in the 0.10 mol/L and 0.15 mol/L samples, where the arc top radius is large, and the ODA-coated sample exhibits a double-capacitive arc structure [18], the phase angle of the coating with double capacitive arcs in (b) and (c) in Figure 22 shows a second time constant in the low frequency region, it indicates that the integrity of the coating is destroyed and the protective performance of the coating is degraded. In addition, the coating treated by 0.02 mol/L Oda has an impedance value (|Z|_0.01Hz) of 10⁵ Ω· cm² in the low frequency region, the maximum phase angle is maintained above 70°, and exhibits a wide frequency band distribution, the comprehensive performance of the coating is far better than that of the coating and the pure steel sheet obtained at other concentrations, indicating that the coating prepared at this concentration is relatively dense, the long-term immersion in the corrosive medium solution can greatly slow down the corrosion of metals by the corrosive medium.

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Figure 22.

Electrochemical impedance diagrams of ODA coating-steel after 30 days of immersion in 3.5% NaCl solution (a) Nyquist diagram, (b) Bode diagram, (c) Bode diagram phase, (d) Nyquist diagram, (e) Bode diagram impedance, (f) Bode diagram phase angle.

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Figure 23.

(a,b) Equivalent circuit diagram after 30 days of immersion.

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According to the EIS fitting data in Table 5, it was found that the solution resistance (Rs) and coating resistance (Rct) of the system showed an overall increasing trend as the ODA concentration increased from 0.05 to 0.20 mol/L. At 0.20 mol/L, Rs reached 35.758 Ω·cm², which was significantly higher than that of the unmodified steel sheet (2.18 Ω·cm²), indicating that a high concentration of ODA helps to build a stable and dense barrier layer at the interface; thus, the penetration and diffusion of the corrosive medium can be effectively limited. At the same time, the charge transfer resistance (Rct) also increases with the increase of concentration, and the maximum value can reach 92160.32 Ω·cm², which reflects that the charge transfer process at the interface is limited, and the system is obviously in a high impedance state; the corrosion reaction was significantly inhibited. In addition, the steel sheet treated at 0.2 mol/L concentration still maintained high impedance and good capacitive response after soaking in 3.5% NaCl solution for 30 days, which was beneficial to the improvement of the corrosion resistance of the steel sheet, the results show that the coating formed under the modified conditions has good electrochemical stability and corrosion resistance persistence, and shows strong sustained-release long-term protection ability [19].

Based on the EIS shown in Figure 24 and the fit in Table 6, the corrosion resistance of DTMS-SiO₂ modified coatings at different mass concentrations after immersion in 3.5% NaCl medium for 30 days can be systematically evaluated, The 20 g/L DTMS-SiO₂/PDA/FeSA₃-N80 steel coating was fitted using the equivalent circuit diagram shown in Figure 25(a), while the remaining experimental groups were fitted using the equivalent circuit diagram shown in Figure 25(b).

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Figure 24.

Electrochemical impedance diagrams of DTMS-SiO₂ steel after 30 days of immersion in 3.5% NaCl solution (a) Nyquist diagram, (b) Bode diagram impedance, (c) Bode diagram phase angle, (d) Nyquist diagram, (e) Bode diagram impedance, (f) Bode diagram phase angle.

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Figure 25.

Equivalent circuit diagram after 30 days of immersion.

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From the Nyquist diagram, the DTMS-SiO₂ modified samples show a significantly enlarged capacitance arc radius compared with the bare steel, indicating that the interfacial charge transfer impedance is significantly improved. Among them, the coating sample with a mass concentration of 20 g/L showed the best performance, and the fitted charge transfer resistance (Rct) reached 307540.98 Ω·cm², which was much higher than the 11.21 Ω · cm² of the unmodified control group. It is shown that the coating constructed at this concentration can effectively inhibit the occurrence of corrosion reactions. The |Z| value of the DTMS-SiO₂ modified coating is generally higher than 10⁴Ω· cm² in the low frequency region, which reflects its excellent electrochemical stability and dielectric barrier ability. At the same time, the peak angle of the coating sample in the phase angle diagram is obviously increased, and the platform area is widened, indicating that the system is mainly dominated by capacitive behavior, and the degree of charge transport limitation is further enhanced [20].