Preparation and Research on 2-Methylimidazole-Lanthanum Nickel-Based Sol-Gel Conversion Coating for Oxide Scale Reinforcement Bars

Abstract

Corrosion induced by defective oxide scales severely compromises the durability of concrete structures. This study develops a dual-mechanism sol-gel protection strategy based on La³⁺/Ni²⁺/2-Methylimidazole (2-MI). First, 2-Methylimidazole-catalyzed epoxy ring-opening constructs defect-minimized Si–O–Si/C–O–C networks through 60 °C low-temperature curing, reducing microcrack formation and curing energy consumption compared to conventional 130 °C processing. Second, utilizing 400 °C waste heat from hot-rolled steel triggers pH-modulated La₂O₃/NiO co-deposition within oxide scale defects, enhancing corrosion resistance. After a 40-day immersion in SCP + 0.1 M NaCl, the coated reinforcement exhibits a low-frequency impedance modulus of 25.6 kΩ·cm², achieving a 10.4-fold increase over untreated steel. Specimens embedded in 3.5 wt% NaCl mortar demonstrate a 120-day impedance modulus of 74.63 kΩ·cm², exceeding the control by 8.03-fold. This strategy integrates efficient industrial waste heat utilization with energy-saving low-temperature curing, providing long-term corrosion protection for marine concrete structures.

1. Introduction

Reinforced concrete structures are widely used in marine infrastructure due to their corrosion resistance. The highly alkaline concrete matrix (pH ≥ 12.5) facilitates protective passive film formation on steel reinforcement, ensuring durability [1,2,3,4,5,6]. However, in marine settings, chloride ions (Cl⁻) readily penetrate the porous oxide scale formed during the hot-rolling of steel, disrupting the passive film and initiating localized corrosion, stress corrosion cracking, and concrete spalling, ultimately leading to catastrophic structural failure [7,8,9]. Consequently, enhancing the corrosion resistance of reinforced concrete in marine environments has emerged as a critical research focus.

Traditional strategies include the following: stainless steel reinforcement (enhanced by Cr/Ni/Mo passivating elements), though prohibitively costly for large-scale applications [10,11]; and protective coatings and cathodic protection, limited by complex implementation, poor interfacial adhesion, and high long-term maintenance [12,13,14,15]. Recent advances target chemical conversion coatings on steel oxide scales/rust layers [16,17]. Sol-gel technology offers a low-cost, facile route to form dense inorganic–organic hybrid films that penetrate the substrate, creating covalent Si–O–M bonds for enhanced adhesion [18,19,20,21] and corrosion resistance [22,23,24]. Previous work by our group demonstrated that rare-earth ion doping (e.g., Ce³⁺) repairs inherent oxide scale defects by generating nano-CeO₂ films at defect sites [24,25]. Further studies revealed that semi-coherent epitaxial growth of nanocrystalline CeO₂ on the oxide scale introduces grain boundary defects, which are mitigated by Ni-induced formation of a stable NiFe₂O₄ spinel phase, significantly improving defect remediation and corrosion resistance [26]. Lanthanum—a rare earth with high oxygen affinity—forms dense La(OH)₃/La₂O₃ layers on carbon steel, suppressing corrosion [27]. Thus, La³⁺-doped sol-gel systems hold promise for enhancing oxide scale remediation.

The current industrial practice primarily recovers high-grade waste heat (>650 °C) and medium-grade waste heat (230–650 °C) from hot-rolled rebars via water cooling for steam generation, rarely considering concurrent corrosion resistance enhancement. To address this gap, we propose a field-applicable innovation that efficiently utilizes medium-grade waste heat after high-grade heat recovery to integrate corrosion protection [28,29,30,31]. Our prior research on sol-gel coatings for steel reinforcement required a 130 °C curing temperature, where solvent evaporation induced microcracks that severely compromised coating performance [24,25,26]. Simultaneously, high-temperature curing increased production energy consumption, constraining large-scale application. Herein, we innovatively implement 60 °C low-temperature curing to minimize microcracking and energy consumption.

Industrial context: after 800–1000 °C water cooling, rebars enter the medium-grade waste heat zone (230–650 °C) with ~400 °C residual surface temperatures. Field implementation: direct immersion in La/Ni/2-Methylimidazole-modified sol-gel precursor for 30 s at this stage, followed by 60 °C curing, significantly enhances corrosion resistance through defect remediation. Lab simulation: (1) preheat rebars at 400 °C for 1 h in a muffle furnace; (2) immerse in precursor where 2-Methylimidazole catalyzes GPTMS epoxy ring-opening [32] and chelates La³⁺/Ni²⁺ [33,34]; and (3) cure at 60 °C for 4 h.

This strategy simultaneously reduces microcracks via crosslinking enhancement and low-temperature curing while upgrading waste heat utilization through corrosion protection. The study develops an energy-efficient sol-gel method for remediating oxide scale defects, improving concrete durability. Key innovations include the following: waste-heat-driven corrosion protection; 2-Methylimidazole-mediated chelation-crosslinking; and pH-modulated La₂O₃/NiO/SiO₂ co-deposition for defect repair. This work provides a novel solution for enhancing marine concrete durability.

2. Materials and Methods

2.1. Materials

Substrate: as-received HRB400 steel reinforcement with oxide scale (composition in

Table 1. The chemical composition of HRB400 steel.

Element	C	Si	Mn	V	Fe
Percent (wt%)	0.23	0.37	1.57	0.07	balance

).

Chemicals: ethanol (ETOH), sodium chloride (NaCl), tetraethyl orthosilicate (TEOS), Macklin Biochemical Technology Co., Ltd. (Shanghai, China); (3-glycidyloxypropyl) trimethoxysilane (GPTMS), ammonia solution (NH₃·H₂O), Meryer Chemical Technology Co., Ltd. (Shanghai, China); 2-Methylimidazole, calcium hydroxide (Ca(OH)₂), lanthanum nitrate hexahydrate (La(NO₃)₃·6H₂O), nickel nitrate hexahydrate (Ni(NO₃)₂·6H₂O), Aladdin Biochemical Technology Co., Ltd. (Shanghai, China). All reagents were employed as received without further purification.

2.2. Electrode and Electrolyte Preparation

Cylindrical steel samples (Ø 12 mm × 20 mm) with intact oxide scales served as working electrodes. Electrical contact was established by connecting one end to a copper wire, followed by sealing both ends with rosin-paraffin wax to expose a lateral surface area of 7.536 cm². The electrolyte was a simulated concrete pore SCP solution (saturated Ca(OH)₂ containing 0.1 M NaCl).

2.3. Sol-Gel Coating Deposition

A precursor solution was prepared by mixing TEOS, GPTMS, and ETOH (1:3:2 v/v) under magnetic stirring (600 rpm, 120 min) and aging for 3 days [26]. After adding specified volumes of 0.1 M La(NO₃)₃ and 0.5 M Ni(NO₃)₂, the pH was adjusted to 10.0 with 25 wt% NH₃·H₂O. To ensure safety during 400 °C immersion, operators wore flame-resistant lab coats and high-temperature resistant gloves, using titanium-coated extended tweezers (length ≥ 30 cm) to handle steel samples preheated at 400 °C for 1 h. Within a fume hood, samples were immersed in precursor solution for 30 s, during which the temperature dropped from 400 °C to 80 °C within 30 s. Operators maintained a safe distance (>30 cm) throughout the process. Immediately after immersion, samples were transferred to a 60 °C oven for 4 h of drying. This procedure ensured safety through engineering controls (ventilation system) and personal protective equipment, with solvent vapor concentration consistently <10% LEL (Lower Explosive Limit).

Coating nomenclature: LS: 1.5 mL La(NO₃)₃ + 0 mL Ni(NO₃)₂ (per 1 mL TEOS); 0.8LNS: 1.2 mL La(NO₃)₃ + 0.3 mL Ni(NO₃)₂; 0.2LNS: 0.75 mL La(NO₃)₃+ 0.75 mL Ni(NO₃)₂; 0.05LNS: 0.3 mL La(NO₃)₃ + 1.2 mL Ni(NO₃)₂; NS: 0 mL La(NO₃)₃ + 1.5 mL Ni(NO₃)₂. For 2-Methylimidazole-modified coatings, 0.52 wt% 2-MI (relative to GPTMS mass) was added during precursor preparation.

2.4. Mortar Specimen Fabrication

Portland cement (composition in

Table 2. Mass fraction of main chemical composition of cement.

Component	SiO2	MgO	Al2O3	Fe2O3	CaO	SO3	Loss
Content (%)	20.86	3.50	5.90	3.61	62.54	2.43	1.16

) and deionized water (water–cement ratio 0.35) were mixed for 10 min, cast into 70 × 70 × 70 mm molds containing steel reinforcement, and cured at room temperature for 24 h. After demolding, exposed steel was sealed with epoxy resin, and specimens were cured for 28 days (20 ± 1 °C, RH 95 ± 2%).

2.5. Electrochemical Measurements

Tests were performed on a Gamry Interface 1010E system using a three-electrode configuration.

SCP solution tests: working electrode: HRB400 steel (7.536 cm²); reference electrode: saturated calomel electrode (SCE); counter electrode: platinum foil (2.5 × 2.5 cm²). EIS: 10 mV amplitude, 10⁵–10⁻² Hz after OCP stabilization (≥1 h); potentiodynamic polarization: −0.3 to 1.2 V vs. OCP, 0.333 mV/s.

Mortar-embedded steel tests: electrolyte: 3.5 wt% NaCl; working electrode: steel reinforcement embedded in mortar specimens; reference electrode: saturated calomel electrode (SCE); counter electrode: titanium mesh (other parameters identical to above). Data were analyzed with ZsimpWin 3.6 software.

2.6. Material Characterization

Surface/cross-section morphology: SEM (Thermo Fisher Apreo 2S, Waltham, MA, USA); elemental distribution: EDS (Oxford Instruments, Oxfordshire, UK); chemical states: XPS (Shimadzu AXIS SUPRA+, Kyoto, Japan; Vantage™ 6.9.0 software); functional groups: FTIR (Nicolet 6700, 32 scans, 4 cm⁻¹ resolution, 400–4000 cm⁻¹, Waltham, MA, USA); crystalline structure: XRD (Rigaku Ultima IV, Tokyo, Japan).

3. Results

3.1. Effect of La³⁺/Ni²⁺ Ratios and 2-Methylimidazole Content on Coating Morphology

3.1.1. Surface Morphology Analysis

Surface morphologies of Blank HRB400 steel and sol-gel conversion coatings under varied conditions are shown in Figure 1. The native oxide scale on the Blank steel exhibits abundant micropores and microcracks (Figure 1a), which facilitate corrosive species penetration, accelerating substrate corrosion and compromising concrete durability. Coatings derived from sol-gel precursors with different La³⁺/Ni²⁺ ratios (pH 10) effectively seal inherent defects in the oxide scale (Figure 1b–g). Notably, precursors without metal ions yield coatings with pronounced defects (Figure 1b), whereas metal-ion-containing formulations reduce defect density. This is attributed to chemical crosslinking between inorganic ions and organic moieties, forming M–O–M bonds that enhance three-dimensional network formation [35,36]. La³⁺-only coatings demonstrate superior smoothness and reduced colloidal particles versus Ni²⁺-only counterparts (Figure 1c,d), consistent with reports that La³⁺ improves film-forming capability in alkaline sol-gels [37].

Optimal performance occurs at La³⁺:Ni²⁺ = 0.8 (concentration ratio), generating coatings with minimal surface defects (Figure 1e). Deviations from this ratio degrade coating quality (Figure 1f,g), corroborating findings that dual-ion incorporation optimizes microstructure and functionality [38].

Building on the optimal 0.8 La³⁺/Ni²⁺ concentration ratio, 2-Methylimidazole (0.5–2 wt% relative to GPTMS) was introduced (Figure 1h–j). 2-Methylimidazole catalyzes epoxy ring-opening in GPTMS, accelerating hydrolysis/condensation and increasing crosslinking density [30,31,32]. Consequently, coatings with 1 wt% 2-MI exhibit the smoothest morphology with negligible particle aggregation (Figure 1i), attributable to enhanced sol stability and controlled metal ion hydrolysis.

3.1.2. Cross-Sectional Morphology Analysis

Backscattered electron (BSE) imaging reveals distinct differences in oxide scale morphology (Figure 2a–c). Blank HRB400 steel exhibits extensive defects/cracks (Figure 2a), facilitating Cl⁻ ingress. The 0.8LNS treatment forms a dense hybrid film that reduces defects but retains microcracks/pores (Figure 2b) [28,29]. Incorporation of 1 wt% 2-Methylimidazole eliminates defects via a crosslinking-enhanced 3D network, forming a homogeneous barrier (Figure 2c) [34,35,36]. Oxide scale thickness metrics demonstrate the following: Blank HRB400 steel average 8.54 µm (variance 3.21), and 0.8LNS averages 8.01 µm (variance 3.73), while 1 wt% 2-MI—0.8LNS shows 9.25 µm with significantly reduced variance (0.0958), indicating remarkable uniformity improvement. Sol-gel coating thickness metrics show the following: 0.8LNS averages 49.38 µm (variance 2.93) and 1 wt% 2-MI—0.8LNS averages 31.88 µm (variance 2.77).

3.2. Compositional Analysis

3.2.1. XRD Characterization

XRD patterns of Blank HRB400 and sol-gel coatings are shown in Figure 3a. Key observations are as follows: peaks at 2θ = 44.51°, 49.15°, and 56.85° are assigned to Fe [25,39]. Iron oxides are identified by peaks at 33.21°, 35.57°, 46.48°, 53.79°, 57.52°, 62.45°, 63.99° (Fe₂O₃); 36.14° (FeO); and 29.34°, 35.32°, 43.07°, 56.94°, 62.81° (magnetite, Fe₃O₄) [40,41]. Additional phases (e.g., FeOOH) are present. Coatings exhibit a broad peak near 2θ = 21°, confirming amorphous SiO₂ formation [25].

3.2.2. FTIR Spectroscopy

FTIR spectra of precursor solutions and resultant coatings (Figure 3(b1,b2,d1,d2)) reveal the following:

Precursor solutions (Figure 3(b1)): peaks at 1170 and 965 cm⁻¹ correspond to CH₃ rocking vibrations; 1045 and 784 cm⁻¹ to Si–O–Si bending; 878 cm⁻¹ to Si–O–H stretching [42,43,44,45]; and 1258 and 911 cm⁻¹ to epoxy group signatures (GPTMS) [25,43,46]. To determine whether La³⁺/Ni²⁺ and 2-Methylimidazole incorporation promote epoxy ring-opening, quantitative FTIR peak area ratio analysis of epoxy group/siloxane bonds was performed using OMNIC 9.2 software. The normalized peak areas yielded ratios of 1 wt% 2-MI—0.8LNS (0.0290), 0.8LNS (0.0403), and Undoped sol-gel (0.0749). This confirms metal ions and 2-Methylimidazole synergistically catalyze epoxy ring-opening, enhancing crosslinking density [30,31,32]. In the expanded view (400–700 cm⁻¹, Figure 3(b2)), the absence of Ni–O (434–454, 558–570 cm⁻¹) or La–O (486–499 cm⁻¹) bands [47,48,49] confirms metal hydroxide predominance post-pH adjustment.

Coatings (Figure 3(d1)): similar profiles to precursors but with diminished O–H bands (3500–3150 cm⁻¹), indicating that high-temperature condensation enhanced Si–O–Si network formation [50]. Epoxy peak attenuation parallels solution trends: quantitative FTIR peak area ratio analysis yields values of 0.0224, 0.0616, and 0.0655, consistent with precursor solution trends. In the expanded view (400–700 cm⁻¹, Figure 3(d2)), the emergence of Ni–O (434–454/558–570 cm⁻¹) and La–O (486–499 cm⁻¹) bands [48,49] confirms La₂O₃/NiO formation during high-temperature deposition.

3.2.3. XPS Analysis

XPS spectra of Blank HRB400 steel and the optimized 1 wt% 2-MI—0.8LNS coating are presented in Figure 4. High-resolution spectra were deconvoluted using mixed Gaussian–Lorentzian functions (70:30 ratio) with Shirley background subtraction. All binding energies were referenced to adventitious carbon (C 1s = 284.80 eV).

For the Blank HRB400 steel (Figure 4(a1–a3)), the oxide scale composition is dominated by FeOOH (712.56 eV), Fe₂O₃ (710.53 eV), and FeO (709.28 eV) based on O 1s and Fe 2p deconvolution [51,52]. Synchronous La 3d and Ni 2p analysis confirms the absence of lanthanum and nickel signatures (Figure 4(a3)).

For the 1 wt% 2-MI—0.8LNS coating (Figure 4(b1–b6)), the survey spectra detected C, N, O, Si, La, and Ni, with attenuated Fe 2p signals (Figure 4(b3)). Key bonding states confirm successful coating formation: C 1s: O=C–O (287.35 eV), C–O–C (286.60 eV), C–O–Si (286.04 eV); O 1s: O–Si (532.55 eV); Si 2p: Si–O–C (103.30 eV), Si–OH (102.60 eV), Si–O–Si (101.92 eV) [49,50,51,52,53,54,55]; and N 1s spectra show nitrate-derived NO₃⁻ (407.31 eV), N–C (401.35 eV), and 2-Methylimidazole-associated N=C (399.73 eV) [56]. Metal oxide formation is evidenced by Ni 2p: NiO signatures at 852.81 eV (2p_3/2) and 873.78 eV (2p_1/2) with satellites (862.63, 880.91 eV) [57,58]; and La 3d: La₂O₃ peaks at 856.30 eV (3d_3/2) and 837.32 eV (3d_5/2) with satellites (852.81, 833.95 eV) [57,59]. These findings align with FTIR data confirming La₂O₃/NiO generation (Figure 3).

For the post-coating removal surface (Figure 4(c1–c3)), the exposed oxide scale retains FeOOH, Fe₂O₃, and FeO dominance [51,52], while residual O–Si bonding (Figure 4(c1)) indicates persistent Si–O–M covalent interfaces [54]. Crucially, La₂O₃ signatures persist at 856.30 eV (La 3d_3/2) with satellites (851.99, 835.26 eV) [57,59], confirming in situ defect remediation via lanthanum oxide formation.

3.3. Corrosion Behavior of Sol-Gel Coatings in SCP + 0.1 M NaCl

3.3.1. Sol-Gel Conversion Coating Prepared Without 2-Methylimidazole

Electrochemical impedance spectroscopy (EIS) assessed the corrosion resistance of Blank HRB400 steel and coated steel in SCP + 0.1 M NaCl solution (Figure 5,

Table 3. Electrochemical data from EIS fitting for sol-gel films constructed on mill-scale steel with different La³⁺/Ni²⁺ concentration ratios in simulated concrete pore solution containing 0.1 M NaCl.

Sample	Rs (Ω·cm2)	Q1 (×10−5 Ω−1·cm−2·sn)	n1	Rf (×103 Ω·cm2)	Q0 (×10−5 Ω−1·cm−2·sn)	n0	Ros (×103 Ω cm2)	Q2 (×10−5 Ω−1·cm−2·sn)	n2	Rct (×105 Ω·cm2)	Chi-Squared (×10−4)
Blank HRB400	52.56	45.00	0.56	/	/	/	0.44	26.76	0.75	0.12	4.0
Undoped sol-gel	111.10	0.89	0.76	1.42	11.98	0.76	9.40	22.29	0.54	1.32	6.2
LS film	116.60	9.67	0.78	0.96	34.85	0.54	11.20	4.60	0.89	1.17	1.7
0.8LNS film	77.09	4.35	0.89	1.58	12.30	0.69	25.10	0.05	0.85	4.26	9.0
0.2LNS film	99.94	9.53	0.73	0.57	7.60	0.64	22.59	3.30	0.92	3.93	7.3
0.05LNS film	65.88	1.54	0.88	1.09	32.22	0.60	13.42	13.35	0.52	1.26	0.8
NS film	77.37	14.37	0.62	1.48	29.35	0.62	8.97	32.00	0.68	0.53	0.6

) [16,17,60,61].

Nyquist analysis (Figure 5a): All sol-gel coatings exhibit larger capacitive loop diameters than untreated steel, indicating enhanced barrier properties against Cl⁻; ingress. The 0.8LNS sol-gel coating (La³⁺:Ni²⁺ = 0.8) shows the maximum loop diameter, signifying optimal corrosion resistance. For the low-frequency impedance |Z|_0.01Hz (Figure 5b), |Z|_0.01Hz follows the order: Blank HRB400 < NS < LS < 0.05LNS < Undoped sol-gel < 0.2LNS < 0.8LNS. The 0.8LNS sol-gel coating achieves the highest |Z|_0.01Hz (8.73 × 10⁴ Ω·cm²), confirming superior protective performance.

Equivalent Circuit Modeling Rationale: The inherent microdefects and porosity within the oxide scale of steel reinforcement impart distinct electrical resistance and interfacial capacitance characteristics. These defects facilitate the permeation of corrosive species (e.g., Cl⁻) to the metallic substrate, triggering electrochemical reactions. Consequently, the EIS data for untreated steel were modeled using the following equivalent circuit: (R_s(Q₁(R_os(Q₂R_ct)))). (Figure 5(g1)) Following sol-gel modification, the coated steel exhibits an additional organic-inorganic hybrid layer with intrinsic coating resistance (R_f) and capacitance. This necessitates the following expanded circuit [25,26]: (R_s(Q₀(R_f(Q₁(R_os(Q₂R_ct)))))). (Figure 5(g2)) The following are circuit element definitions: R_s: solution resistance; R_os: oxide scale resistance; R_f: sol-gel coating resistance; R_ct: charge transfer resistance; Q₀/Q₁/Q₂: constant phase elements (CPEs), accounting for non-ideal capacitive behavior arising from interfacial heterogeneity. CPE Mathematical Formulation: The impedance of each CPE is defined as [62,63,64]:

$$\mathit{Z} ={ \left(Y_0\right)}^{-1}{\left(jw\right)}^{-n}$$

(1)

where Y₀: CPE admittance coefficient (Ω⁻¹sncm⁻²); j: Imaginary unit; ω: angular frequency; and n: CPE exponent (0 ≤ n ≤ 1), where n = 0 is the pure resistor, and n = 1 is the ideal capacitor.

Fitted electrochemical parameters are presented in Table 2, with the total resistance R_tot (R_tot = R_os + R_f + R_ct) compared across all samples. This parameter collectively represents the barrier properties against charge transfer and ionic penetration.

Electrochemical Performance Analysis: the 0.8LNS sol-gel coating exhibits the highest total resistance (R_tot) among all specimens (Figure 5d), demonstrating superior chloride diffusion barrier properties. Critical correlations emerge from constant phase element analysis. Oxide scale defect density correlates with CPE exponent n₁ [26]. Blank HRB400 steel shows low n₁ (0.42), indicating high defect concentration. Sol-gel modified specimens display significantly increased n₁ values, confirming defect remediation. The 0.8LNS coating achieves maximum n₁ (0.89), reflecting optimal defect sealing. Charge transfer resistance (R_ct) of 0.8LNS coating exceeds Blank HRB400 steel by 35.5-fold, and oxide scale resistance (R_os) shows a 57.0-fold enhancement; these improvements confirm reduced exposed metallic area at coating-substrate interfaces and effective corrosion suppression.

Forty-day immersion tests in SCP + 0.1 M NaCl solution with daily electrochemical impedance spectroscopy (EIS) monitoring were conducted to assess the long-term corrosion resistance of sol-gel conversion coatings [60,61]. Figure 5e depicts the daily evolution of low-frequency impedance modulus |Z|_0.01Hz for all specimens during the 40-day immersion in SCP + 0.1 M NaCl. Figure 5f compares the day 40 |Z|_0.01Hz values. Critical observations are as follows: the Undoped sol-gel coating exhibited rapid degradation, reaching the lowest impedance among all sol-gel film samples. This confirms its poor long-term corrosion resistance and early failure in chloride environments. In contrast, the 0.8LNS coating maintained consistently high impedance throughout immersion. At day 40, it retained a |Z|_0.01Hz value 6.0-fold higher than the Blank HRB400 steel (Figure 5f), demonstrating superior durability.

3.3.2. Sol-Gel Coatings with 2-Methylimidazole Incorporation at Optimal La³⁺/Ni²⁺ Ratio

To enhance coating performance, sol-gel formulations incorporating 0.5–2 wt% 2-methylimidazole (relative to GPTMS mass) were applied to steel specimens at the optimal La³⁺:Ni²⁺ ratio of 0.8 (Figure 6). Following immersion in SCP + 0.1 M NaCl with daily EIS monitoring (Figure 6e,

Table 4. Electrochemical data from EIS fitting for sol-gel coatings constructed on mill-scale steel with varying 2-Methylimidazole content added to the base (La³⁺/Ni²⁺ concentration ratio = 0.8) in simulated concrete pore solution containing 0.1 M NaCl.

Sample	Rs (Ω·cm2)	Q1 (×10−5 Ω−1·cm−2·sn)	n1	Rf (×103 Ω·cm2)	Q0 (×10−5 Ω−1·cm−2·sn)	n0	Ros (×103 Ω cm2)	Q2 (×10−5 Ω−1·cm−2·sn)	n2	Rct (×105 Ω·cm2)	Chi-Squared (×10−4)
Blank HRB400	52.56	45.00	0.56	/	/	/	0.44	26.76	0.75	0.12	4.0
0.8LNS film	77.09	4.35	0.89	1.58	12.30	0.69	25.10	0.05	0.85	4.26	9.0
0.5 wt% 2-MI—0.8LNS film	66.30	4.37	0.90	1.05	15.95	0.77	27.80	0.11	0.92	0.52	7.2
1 wt% 2-MI—0.8LNS film	59.75	3.05	0.93	2.17	15.20	0.73	31.10	0.04	0.89	1.77	9.3
2 wt% 2-MI—0.8LNS film	79.32	1.86	0.91	1.73	10.49	0.75	26.55	0.33	0.92	1.49	7.3

), the 1 wt% 2-MI—0.8LNS coating demonstrated significantly superior corrosion resistance, exhibiting dominant |Z|_0.01Hz values beyond day 8. At day 40, it maintained 10.4-fold higher impedance than Blank HRB400 steel (Figure 6f).

Parameter analysis confirms the 1 wt% 2-MI—0.8LNS coating achieves optimal performance, exhibiting a maximized CPE exponent n₁ = 0.93 (Table 4), surpassing the 0.8LNS coating (n₁ = 0.89). This demonstrates superior defect remediation within the oxide scale, consistent with cross-sectional SEM observations (Figure 2). Concurrently, the oxide scale resistance (R_os) reaches 31.10 kΩ·cm²—a 70.68-fold increase over Blank HRB400 steel (0.44 kΩ·cm²)—validating exceptional electrochemical barrier integrity. Therefore, incorporating 1 wt% 2-Methylimidazole at the optimal La³⁺/Ni²⁺ ratio enhances the long-term corrosion resistance of sol-gel conversion coatings.

Electrochemical impedance analysis during 40-day immersion in SCP + 0.1 M NaCl, using equivalent circuits in Figure 5(g1) (Blank HRB400 steel) and Figure 5(g2) (1 wt% 2-MI—0.8LNS coating), reveals distinct corrosion resistance behaviors (Figure 7,

Table 5. Electrochemical data from EIS fitting for Blank HRB400 and 1 wt% 2-MI—0.8LNS film in 0.1 M NaCl + SCP solution after 40 days of immersion.

Sample	Time (Day)	Rs (Ω·cm2)	Q1 (×10−5 Ω−1·cm−2·sn)	n1	Rf (×103 Ω·cm2)	Q0 (×10−5 Ω−1·cm−2·sn)	n0	Ros (×103 Ω cm2)	Q2 (×10−5 Ω−1·cm−2·sn)	n2	Rct (×105 Ω·cm2)	Chi-Squared (×10−4)
Blank HRB400	1D	52.56	45.00	0.56	/	/	/	0.44	26.76	0.75	0.12	4.0
	5D	43.40	48.16	0.58	/	/	/	0.34	41.22	0.74	0.48	6.9
	10D	49.95	54.00	0.55	/	/	/	2.45	11.67	0.86	1.12	9.4
	15D	77.15	58.39	0.53	/	/	/	0.66	41.85	0.68	0.37	2.8
	20D	52.88	107.30	0.52	/	/	/	0.21	81.61	0.70	0.13	3.0
	25D	69.38	85.19	0.55	/	/	/	0.30	103.60	0.73	0.15	9.6
	30D	105.20	39.42	0.62	/	/	/	0.20	142.00	0.61	0.32	9.3
	35D	75.66	41.78	0.61	/	/	/	0.18	157.70	0.56	0.27	9.3
	40D	99.10	61.33	0.50	/	/	/	0.17	89.08	0.50	0.13	1.0
1 wt% 2-MI—0.8LNS film	1D	59.75	3.05	0.93	2.17	15.20	0.73	31.10	3.81	0.89	1.77	9.3
	5D	64.42	3.31	0.92	2.08	16.36	0.73	30.60	3.53	0.90	2.80	9.7
	10D	47.95	3.08	0.90	1.81	14.83	0.74	30.10	38.45	0.86	4.53	9.4
	15D	40.99	3.09	0.89	1.42	13.16	0.73	31.10	92.00	0.87	7.35	4.9
	20D	22.52	2.90	0.91	1.13	15.54	0.73	29.50	29.46	0.85	4.18	0.9
	25D	55.34	7.89	0.85	0.50	11.71	0.78	26.70	865.00	0.60	3.21	6.5
	30D	64.57	7.35	0.86	0.62	11.97	0.78	24.30	953.00	0.54	2.87	5.3
	35D	55.25	9.24	0.82	0.39	10.41	0.79	21.60	125.00	0.54	2.84	8.9
	40D	76.99	8.46	0.85	0.63	11.86	0.79	17.20	142.20	0.52	2.41	7.6

). Both samples demonstrated an initial increase followed by a progressive decline in low-frequency impedance modulus |Z|_0.01Hz. Critically, the Blank HRB400 steel exhibited rapid degradation (Figure 7b), while the 1 wt% 2-MI—0.8LNS coating maintained a gradual decline (Figure 7e), indicating superior barrier stability. At day 40, the 1 wt% 2-MI—0.8LNS coating demonstrated exceptional corrosion resistance, exhibiting a low-frequency impedance modulus of 25,600 Ω·cm²–10.4-fold higher than Blank HRB400 steel (2470 Ω·cm²). Critical electrochemical parameters confirm sustained barrier integrity: the coated sample maintained an oxide scale resistance of 17,200 Ω·cm² (exceeding the blank sample’s 170·Ω cm² by two orders of magnitude) and a charge transfer resistance of 2.41 × 10⁵ Ω·cm² (18.5-fold greater than the blank’s 0.13 × 10⁵ Ω·cm²). These results validate the coating’s outstanding long-term protective capability against chloride ingress.

3.3.3. Potentiodynamic Polarization Analysis

Potentiodynamic polarization curves measured in SCP + 0.1 M NaCl (Figure 8a,b,

Table 6. Electrochemical parameters from potentiodynamic polarization curves for sol-gel films constructed on mill-scale steel with different La³⁺/Ni²⁺ concentration ratios in simulated concrete pore solution containing 0.1 M NaCl.

Sample	icorr (µA/cm2)	Ecorr (V vs. SCE)	βa (mV/dec)	|βc| (mV/dec)
Blank HRB400	0.82779	−0.54041	69.08	87.44
Undoped sol-gel	0.71633	−0.48261	89.33	99.66
LS film	0.19144	−0.42969	112.53	136.46
0.8LNS film	0.13085	−0.43933	196.12	151.06
0.2LNS film	0.18628	−0.45639	150.58	122.77
0.05LNS film	0.12585	−0.48048	102.37	104.24
NS film	0.19911	−0.45290	102.48	100.86

and

Table 7. Electrochemical parameters from potentiodynamic polarization curves for sol-gel coatings constructed on mill-scale steel with varying 2-MI content added to the base (La³⁺/Ni²⁺ concentration ratio = 0.8) in simulated concrete pore solution containing 0.1 M NaCl.

Sample	icorr (µA/cm2)	Ecorr (V vs. SCE)	βa (mV/dec)	|βc| (mV/dec)
Blank HRB400	0.82779	−0.54041	69.08	87.44
Undoped sol-gel	0.71633	−0.48261	89.33	99.66
0.8LNS film	0.13085	−0.43933	196.12	151.06
0.5 wt% 2−MI—0.8LNS film	0.08300	−0.43142	94.11	125.67
1 wt% 2−MI—0.8LNS film	0.10870	−0.38397	105.39	200.94
2 wt% 2−MI—0.8LNS film	0.19502	−0.40967	95.18	176.67

) demonstrate that sol-gel coatings significantly enhance corrosion resistance. La³⁺/Ni²⁺-modified coatings exhibit positive shifts in corrosion potential (E_corr) and reduced corrosion current density (i_corr) compared to Blank HRB400 steel and undoped sol-gel. This improvement is attributed to La³⁺/Ni²⁺ forming dense passive films that block oxidant permeation to suppress anodic dissolution, while simultaneously precipitating as hydroxides/oxides at cathodic sites to cover active areas and inhibit oxygen reduction reactions.

Tafel slope analysis further quantifies these mechanisms: 0.8LNS coating demonstrates dominant anodic inhibition with a high β_a value (196.12 mV/dec vs. 69.08 mV/dec for bare steel), reflecting La³⁺-induced passivation impeding metal dissolution. Its β_c value (151.06 mV/dec) confirms cathodic suppression via hydroxide/oxide precipitation at cathodic sites. A total of 1 wt% 2-MI—0.8LNS achieves optimal cathodic control (β_c = 200.94 mV/dec), attributed to enhanced coverage by La₂O₃/La(OH)₃-based compounds and physical obstruction of oxygen reduction pathways by 2-MI coordination complexes. Its balanced β_a (105.39 mV/dec) maintains anodic blocking capability.

Data in Table 6 and Table 7 confirm the 0.8 La³⁺/Ni²⁺ ratio as optimal, exhibiting the highest β_a, lower i_corr, and higher E_corr. Incorporating 1 wt% 2-Methylimidazole further enhances performance, yielding the most positive E_corr and the second-lowest i_corr (0.11 µA/cm²), substantially outperforming the 0.8LNS film and achieving optimal cathodic control (β_c = 200.94 mV/dec).

3.4. Post-Immersion Morphological and Compositional Analysis

3.4.1. Surface Morphology After 40-Day Immersion

After a 40-day immersion in SCP + 0.1 M NaCl, Blank HRB400 steel exhibited severe oxide scale spallation with extensive cracking and defect exposure (Figure 9a). Elemental mapping revealed calcium salt accumulation within defects, indicating cathodic site activation during corrosion (Figure 9(a1–a3)).

In contrast, the 1 wt% 2-MI—0.8LNS coating developed surface cracks due to alkaline-induced silicic acid dissociation (Figure 9b). Calcium ions permeated the sol-gel layer, forming calcium silicate precipitates that altered the silica network structure, evidenced by homogeneous Ca distribution within the coating (Figure 9(b1–b6)) [65].

Following ultrasonic removal of residual sol-gel, the underlying oxide scale of the coated sample remained intact without spallation or cracks (Figure 9c). Uniform elemental distribution of Si, Ca, La, and Ni throughout the oxide scale (Figure 9(c1–c6)) confirmed enhanced electrochemical homogeneity, eliminating localized cathodic/anodic zones. This contrasts sharply with Blank steel’s heterogeneous Ca accumulation (Figure 9(a3) vs. Figure 9(c3)). Macroscopic comparison (Figure 9(d1,d2)) validated the protective mechanism: Blank steel showed pronounced corrosion traces, while the coated specimen maintained pristine oxide integrity.

3.4.2. Cross-Sectional Morphology After 40-Day Immersion

Cross-sectional analysis (Figure 10) reveals distinct degradation patterns:

Blank HRB400 steel: Severe spallation of oxide scale with extensive cracking/pore formation (Figure 10(a–a3)), consistent with surface deterioration in Figure 9a. Residual oxide scale thickness (measured at thickest preserved regions) averages 2.55 µm (variance 5.465).

The 1 wt% 2-MI—0.8LNS: Sol-gel layer develops through-thickness cracks and delamination (Figure 10b), with residual thickness averaging 22.45 µm (variance 2.177), severely compromising barrier function. Modified oxide scale retains structural integrity with only minor microcracking, significantly outperforming blank samples. Elemental mapping (Figure 10(b3)) confirms calcium enrichment confined exclusively to the sol-gel layer, mirroring Figure 9b surface patterns. Oxide scale thickness averages 7.08 µm (variance 2.492), reflecting sustained barrier functionality despite sol-gel degradation in alkaline chloride environments.

3.4.3. Post-Immersion Surface Chemistry Analysis

XPS characterization after 40-day immersion in SCP + 0.1 M NaCl reveals critical interfacial transformations (Figure 11): Blank HRB400 steel exhibited substantial calcium carbonate deposition (C 1s: 290.17 eV; O 1s: O-Ca at 531.57 eV) from carbonation reactions [26,66]. The Fe 2p spectrum showed metallic iron signatures (706.11 eV) alongside FeOOH (712.16 eV), Fe₂O₃ (710.91 eV), and FeO (709.46 eV), confirming oxide scale spallation and substrate exposure—consistent with a macroscopic deterioration in Figure 9a [51,52]. Conversely, the 1 wt% 2-MI—0.8LNS coating demonstrated: calcium carbonate deposition within the sol-gel matrix, but no metallic Fe detection—validating intact oxide scale integrity (Figure 9c). Silica network preservation (Si 2p: 104.78 eV), indicating calcium silicate formation from alkaline hydrolysis [53]. Dominant La₂O₃ is present with minor NiO (Figure 11(b6)), mirroring pre-immersion elemental distribution patterns (Figure 4(c3)). These findings confirm the coating’s dual functionality: chemical stabilization of the oxide scale and barrier retention against chloride penetration [57,58,59].

3.5. Long-Term Corrosion Resistance in Mortar Systems

To validate practical performance, 1 wt% 2-MI—0.8LNS-coated steel was embedded in mortar blocks (cured at 20 ± 2 °C, 95 ± 2% RH for 28 days) and subjected to simulated seawater immersion (3.50 wt% NaCl) with 120-day EIS monitoring (Figure 12a–g). Equivalent circuit modeling (Figure 12h) identified three time constants: high-frequency: mortar matrix properties (R_mQ_m); mid-frequency: oxide scale/coating behavior (R_fQ_f); and low-frequency: charge transfer process (R_ctQ_dl) [67,68,69,70,71,72]. Critical performance metrics at Day 120 are as follows: |Z|_0.01Hz: 74.63 kΩ·cm² (1 wt% 2-MI—0.8LNS) vs. 9.29 kΩ·cm² (Blank HRB400 steel), indicating that the impedance of the coated system is 8.0 times that of the blank steel.. This confirms the coated system’s exceptional barrier retention against chloride penetration in real-service conditions.

3.6. Corrosion Resistance Comparative Study of Oxide-Scaled Steel with Engineered Coatings

Compared to prior studies (

Table 8. Comparative analysis of engineered sol-gel coatings versus previous studies.

Research System	Steel Bar Temperature (°C)	Sol-Gel Solution Temperature (°C)	Curing Temperature (°C)	Immersion Time in SCP Solution (days)	Impedance Gains (-Fold)
Ce Sol-gel film [25]	25	25	130	30	3.0
Ce-Ni Sol-gel film [26]	25	35	130	30	5.8
ZIF-8—Ce sol-gel film [24]	25	25	130	30	5.0
1 wt%-2MI—0.8LNS	400	25	60	40	10.4

), this work demonstrates distinct advantages:

Implementation of a low-temperature curing process, reducing energy consumption and enabling industrial-scale production; superior corrosion resistance in simulated concrete pore solution despite extended immersion; and development of a novel waste heat utilization strategy.

4. Discussion

4.1. Multifunctional Hierarchical Defense for Oxide-Scaled Steel

This study establishes a multifunctional defense system through synergistic integration of La³⁺, Ni²⁺, and 2-Methylimidazole within sol-gel matrices, achieving hierarchical corrosion protection on oxide-scaled steel via two core mechanisms (Figure 13):

1.

Sol-Gel Network Optimization

Chemical Design: La³⁺ acts as a Lewis acid catalyst (accelerating TEOS hydrolysis by polarizing Si–O–C₂H₅ bonds [73]), while Ni²⁺ stabilizes intermediate silanol groups (Si–OH). 2-Methylimidazole promotes epoxy ring-opening in GPTMS.

Microstructural Outcome: sol-gel films form hybrid Si–O–Si/C–O–C networks (FTIR, Figure 3). Coating defects are minimized by 1 wt% 2-Methylimidazole and 60 °C low-temperature curing (Figure 1 and Figure 2). 2-Methylimidazole chelation ensures uniform metal ion distribution [74].

Protective Performance: the sol-gel has enhanced barrier integrity through increased crosslinking density and defect reduction, improving physical barrier properties.

2.

Interfacial Bonding and Defect Remediation

Chemical Design: pre-oxidation at 400 °C generates reactive Fe-OH groups, forming covalent Si-O-Fe bonds with sol-gel silanol. At pH 10, La³⁺/Ni²⁺ form colloidal La(OH)₃/Ni(OH)₂ via sol-gel homogeneity [26]. Upon steel immersion, surface heat promotes La₂O₃/NiO formation while simultaneously ammonia evaporation lowers pH, triggering SiO₂-Fe₂O₃reactions for defect transport. Additionally, Ni²⁺ reacts with hydroxylated iron oxides to form NiFe₂O₄ spinel (Figure 13b).

Microstructural Outcome: Defect Remediation and Sealing: adsorbed colloids dehydrate into La₂O₃(dominant)/NiO sealing layers at 400 °C at the interface, which deposit onto the oxide scale surface to remediate defects (Figure 13b). Validation is demonstrated with FTIR oxide peaks (Figure 3), XPS La₂O₃/NiO detection (Figure 4 and Figure 11), and surface/cross-section SEM (Figure 1, Figure 2, Figure 9 and Figure 10). Bonding networks correspond to Si-O-Fe covalent bonds (enhancing the coating/scale interfacial bonding strength) (XPS O 1s: 532.55 eV, Figure 4) and 2-Methylimidazole-coordinated Fe(MeIm)₂/Fe(MeIm)₃ protective complexes which adsorb on the steel surface, improving chloride resistance [75].

Protective Performance: Compared to Blank HRB400 steel, the 1wt%2-MI—0.8LNS film exhibits no spallation/cracking after 40-day immersion (Figure 9 and Figure 10), with persistent La₂O₃ signatures and absence of metallic Fe signals in XPS (Figure 11), confirming structural integrity and defect remediation efficacy. The impedance modulus |Z|_0.01Hz = 25.6 kΩ·cm² (10.4 × untreated) (Figure 5, Figure 6 and Figure 7).

4.2. Enhanced Corrosion Resistance in Concrete Structures

The 1 wt%-2MI—0.8LNS sol-gel-coated steel embedded in mortar blocks demonstrates exceptional corrosion resistance in simulated marine environments. After 120-day immersion in 3.50 wt% NaCl solution, the coated system maintains a low-frequency impedance modulus of 74.63 kΩ·cm²—8.03-fold higher than the uncoated control (9.29 kΩ·cm²). This performance aligns with our established corrosion inhibition mechanisms [26]:

C–S–H gel formation: sol-gel SiO₂ reacts with cement-derived Ca(OH)₂ to generate pore-filling calcium silicate hydrate (C–S–H), reducing chloride diffusion paths.

Pore refinement: silica particles and silanes physically seal micro-pores and capillaries within the mortar matrix, enhancing chloride barrier properties.

Stainless steel-like passivation: lanthanum incorporation promotes a chromium-equivalent passive film on the oxide scale [65,76,77], significantly improving anodic stability.

5. Conclusions

This study establishes a dual-mechanism strategy for marine concrete protection: catalytic crosslinking: 2-Methylimidazole-driven epoxy ring-opening constructs defect-minimized Si–O–Si/C–O–C networks via low-temperature curing (60 °C); and thermally activated repair: 400 °C steel interfaces enable pH-modulated La₂O₃/NiO co-deposition within oxide scale defects.

Quantitative outcomes: SCP + 0.1 M NaCl (40-day): |Z|_0.01Hz = 25.6 kΩ·cm² (10.4× untreated), and oxide scale integrity is preserved; and mortar in 3.5 wt% NaCl (120-day): |Z|_0.01Hz = 74.63 kΩ·cm² (8.03× control).

This design integrates low-temperature curing (60 °C vs. conventional 130 °C) with industrial waste heat utilization (400 °C), significantly reducing energy consumption during curing while efficiently harnessing medium-grade waste heat in industrial practice. The corrosion resistance of steel reinforcement is markedly enhanced, providing long-term protection for marine infrastructure. However, practical engineering applications still face the following challenges:

Scalability: requires synchronization with steel rolling line cooling rates to enable continuous production, alongside investigation into mortar-rebar interfacial behavior;

Long-term durability: necessitates > 24-month field exposure trials to validate performance degradation under tidal cycling and biofouling;

Cost-effectiveness: demands economic viability optimization through recycling process improvements.