Passivation performance of Nb microalloyed rebar in concrete carbonation environments with different pH

Zeyun Zeng; Shangjun Gu; Jie Wang; Fulong Wei; Xiang Xie; Zhiying Li; Hui Yang; Changrong Li

doi:10.1038/s41598-024-74763-0

. 2024 Oct 2;14:22919. doi: 10.1038/s41598-024-74763-0

Passivation performance of Nb microalloyed rebar in concrete carbonation environments with different pH

Zeyun Zeng ^1,², Shangjun Gu ³, Jie Wang ³, Fulong Wei ³, Xiang Xie ³, Zhiying Li ^1,², Hui Yang ^1,², Changrong Li ^1,^2,^✉

PMCID: PMC11447213 PMID: 39358494

Abstract

In this study, XPS sputtering depth, SEM and electrochemical tests (CV, EIS, M-S, i-t, DPP) were used to study the structural composition and formation mechanism of surface passive film of Nb microalloyed rebar in SCPS with different pH. The results showed that after passivation for 10 d in SCPS with different pH, compared with CS rebar, the stability and compactness of surface passive film of 34Nb rebar gradually increased with the decreases of pH. Firstly, with the decreases of pH, the outer layer of surface passive film of 34Nb rebar was composed of Fe oxides and Fe hydroxides, and the inner layer was composed of Fe oxides and Nb oxides, thus increasing the mass ratio of Fe³⁺/Fe²⁺ and Nb₂O₅/(NbO₂ + NbO). Secondly, with the decreases of pH, the addition of Nb promoted the formation of Fe oxides in 34Nb rebar, obtaining the excellent R_ct, E_corr and E_p. Finally, with the decreases of pH, the addition of Nb promoted the enrichment of Nb oxides in 34Nb rebar, inhibiting the degradation of Fe oxides, and the passive film exhibited the P-N type semiconductor, thus significantly decreasing the carrier density and enhancing the passivation rate of 34Nb rebar.

Subject terms: Materials science, Structural materials, Metals and alloys

Introduction

Reinforced concrete structure is one of the important structures with supporting skeleton in infrastructure construction, and the main core material is Nb microalloyed high-strength anti-seismic rebar, which is widely used in major construction projects such as construction engineering, bridge engineering and marine engineering^1–3. Since the reinforced concrete structure is a highly alkaline concrete environment, the stable passive film will be formed on the surface of rebar when the rebar is in service in the concrete, so that the reinforced concrete structure has long-term stability^4–7. With the development of reinforced concrete structures, many concrete structures will be carbonized or corroded in harsh working environments, resulting in corrosion of rebars and failure of concrete structures^8–11. However, from the long-term stability development of reinforced concrete structures, how to design the long-term stability and corrosion resistance of reinforced concrete structures, the core issue is the long-term passivation performance of high-strength anti-seismic rebar. Therefore, it is of great strategic significance to develop the microalloyed high-strength anti-seismic rebar with high performance and high corrosion resistance in the development of long-term stability and durability of reinforced concrete structures.

As is well known, the passivation performance of microalloyed high-strength anti-seismic rebar is mainly affected by the mineral composition of concrete and the content of microalloying elements, in which the synergistic effect of concrete composition and microalloying elements has a great influence^12–15. There are also reports on the influence of microalloying elements and concrete components on the structural stability and corrosion resistance of surface passive film of microalloyed rebar. In simulated concrete pore solution (SCPS), the microalloying elements (such as Cr, Ni, etc.) have a great influence on the formation and growth of surface passive film of microalloyed rebar. Firstly, with the decreases of pH, the addition of microalloying elements enhanced the stability of the passivation layer structure of microalloyed steel, and the outer layer of the passive film was mainly composed of Fe oxides and Fe hydroxides, and the inner layer of the passive film was mainly composed of Fe oxides and microalloying element oxides^16–19. Secondly, with the decreases of pH, the microalloying element oxides were enriched in the inner layer of surface passive film of microalloyed steel, which inhibited the degradation of Fe oxides and enhanced the passivation rate of microalloyed steel^20–23. Zhang et al., Shi et al., Zou et al., Liu et al. studied the passivation characteristics of Cr microalloyed rebar in simulated concrete carbonation environment, and indicating that with the decreases of pH, the addition of Cr promoted the formation of Cr oxides and Fe oxides in the passive film of Cr microalloyed rebar and inhibited the dissolution of Fe oxides, thus enhancing the corrosion resistance of Cr microalloyed rebar^24–27. Wang et al. studied the passivation properties of stainless rebar in simulated concrete environments, and indicating that the decreases of pH led to the gradual decrease of Cr and Mo in the passive layer of stainless rebar, which reduced the Fe³⁺/Fe²⁺, O²⁻/OH⁻, corrosion potential and oxygen absorption potential, thereby reducing the stability of the passive film^28,29. Ai et al. compared the passivation characteristics of CS rebar and Cr10Mo1 rebar in simulated concrete carbonation environment, and indicating that Cr10Mo1 rebar had higher passivation rate than CS rebar after passivation for 10 d in concrete, which was attributed to the formation of stable Cr oxides on the surface of Cr10Mo1 rebar, which inhibited the degradation of Fe oxides and reduced the carrier density of the passive film^30,31. Liu et al. systematically studied the passivation evolution of low-carbon steel in concrete environments, and indicating that the high pH led to the rapid formation of the dense oxide film of low-carbon steel, thus obtaining the excellent passivation performance³². Zhang et al. studied the passivation characteristics of Cr-modified rebar and predicted the durability by Monte Carlo Simulation, indicating that when the Cr content increased to 11 wt%, the passive film was formed on the surface of Cr-modified rebar, and the chloride critical value was 6 ~ 14 times higher than that of carbon rebar, and the Cr11 rebar had a service life of 50 years in concrete containing Cl⁻ ion³³.

However, in the simulated concrete carbonation environment, the microalloying element Nb has an important influence on the improvement of surface passivation performance of microalloyed rebar. The effect of Nb element on the surface passivation characteristics of microalloyed rebar has also been reported. Previous studies pointed out that in simulated alkaline solution, the addition of Nb formed the Nb oxides (NbO and NbO₂) in the oxide film of alloy materials, and the decreases of pH led to the further transformation of unstable Nb oxides into stable Nb oxides (Nb₂O₅), which enhanced the corrosion resistance of alloy materials^34–36. Liu et al. studied the corrosion resistance of Nb microalloyed HRB500 rebar in simulated carbonation solution, and indicating that the increases of Nb improved the stability of passivation layer of HRB500 rebars³⁷. Orjuela et al. studied the effect of NbC coating on the corrosion resistance of AISI 1045 low-alloy steel in 3.0 w.t% NaCl solution, and indicating that the increases of NbC coating thickness promoted the corrosion resistance of AISI 1045 low-alloy steel³⁸. Barroux et al. studied the effect of Nb on the passivation behavior of 17-4PH martensitic stainless steel, and indicating that the surface passive film of 17-4PH martensitic stainless steel was mainly composed of Fe oxides and Cr oxides, while the Nb oxides was mainly formed in the inner layer of the passive film, which enhanced the stability of the passive film³⁹. Ji et al. studied the effect of Nb on the corrosion resistance of Ti-Zr alloy, and indicating that with the increases of Nb content, the Nb oxides (such as NbO, NbO₂, Nb₂O₅, etc.) were gradually formed in the passive film of Ti-Zr alloy, which enhanced the corrosion resistance of Ti-Zr alloy⁴⁰. Guan et al. studied the passivation behavior of high-purity Nb plate in acidic solution, and indicating that with the increases of pH, more stable Nb oxides (Nb₂O₅) was formed in the passive film of high-purity Nb plate, which led to the decrease of point defect density of the passive film⁴¹. Therefore, the above research mainly focuses on the effect of Nb on the surface passivation behavior of microalloyed rebar, but lacks the action law and formation theory of Nb in the surface passivation process of microalloyed rebar and the evolution mechanism of the passive film layer, thus it is necessary to study the effect of Nb on the passivation evolution mechanism of microalloyed rebar.

In short, in the concrete carbonation environment, the corrosion resistance of microalloyed rebar mainly depends on the compactness and stability of surface passive film of microalloyed rebar, and the passivation performance of microalloyed rebar is mainly affected by concrete environment and microalloying element composition. Although a large number of studies have been reported on the exploration of surface passive film of microalloyed rebar, but there is a lack of theoretical research on the effect of Nb element on the passivation performance of microalloyed rebar in concrete carbonation environment. Therefore, starting from the long-term stability of microalloyed rebar in concrete, the main purpose of this study is to explore the effect of Nb element on the structure and formation kinetics of surface passive film of microalloyed rebar in simulated carbonation environment, studying the synergistic effect of concrete carbonation environment and Nb element on the passivation behavior of microalloyed rebar, analyzing the formation law of the passive film and the role of Nb element in the passivation layer, revealing the development law of passivation behavior, and clarifying the passivation mechanism. This study hopes to provide a theoretical reference for solving the long-term stability of microalloyed rebar in concrete structures.

In this study, the structure, composition and electrochemical properties of surface passive film of Nb microalloyed rebar were studied by XPS sputtering depth, SEM, electrochemical test (such as CV, EIS, DPP, M-S, i-t, etc.) after passivation for different times in SCPS with different pH, and exploring the effect of Nb on the formation process and corrosion resistance of surface passive film of rebar, and proposing the passivation mechanism. The results of this study will provide new insights to the design of microalloyed rebar with high corrosion resistance in concrete structures.

Experimental procedures

Material preparations

In this study, the raw materials used were ordinary carbon steel rebar (named CS rebar), and Nb microalloyed rebar were prepared on the basis of ordinary carbon steel rebar. According to the new national standard GB/T 1499.2–2018 steel for reinforced concrete (Part 2: hot rolled ribbed rebar), the Nb microalloyed rebar was designed. Nb microalloyed rebar was a steel ingot with sizes of diameter (50 mm) × height (100 mm), which was melted by vacuum induction melting furnace, and the alloy material used was the ferroniobium alloy material (Nb_66.78Fe_33.22 alloy). The chemical compositions of CS rebar and Nb microalloyed rebar were tested by direct reading spectrometer (ARL4460), carbon-sulfur analyzer (CS840) and nitrogen-hydrogen-oxygen analyzer (NHO445), and the chemical compositions of the main elements were given in Table 1. The microstructure and mechanical properties of CS rebar and Nb microalloyed rebar were given in Fig. 1; Table 2.

Table 1.

Chemical composition of Nb microalloyed rebar and CS rebar (wt%).

Rebar	C	Si	Mn	P	S	O	Nb	Ti	C_eq
CS	0.20	0.55	1.55	0.018	0.015	0.008	0	< 0.01	0.46
34Nb	0.21	0.58	1.58	0.019	0.016	0.008	0.034	< 0.01	0.47

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Fig. 1 — Microstructure of two rebars. (a) CS rebar, (b) 34Nb rebar.

Table 2.

Mechanical properties of CS rebar and nb microalloyed rebar.

Rebar	Tensile strength (R_m)	Yield strength (R_eL)	R_m/R_eL	Elongation after fracture (A)
CS	605 MPa	486 MPa	1.24	16%
34Nb	762 MPa	543 MPa	1.40	23%

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The calculation formula of carbon equivalent (C_eq) was as follows¹:

It can be seen from Fig. 1; Table 2 that the microstructure of CS rebar and 34Nb rebar were mainly composed of ferrite and pearlite, and the ferrite morphology was polygonal, the pearlite morphology was lamellar, and the pearlite was distributed at the ferrite grain boundary. Compared with CS rebar, the ferrite grain size and pearlite lamellar spacing of 34Nb rebar refined to 10.15 μm and 0.22 μm, respectively, thus the tensile strength and yield strength increased to 762 MPa and 543 MPa, respectively, and the elongation after fracture reached to 23%, this indicated that the addition of Nb played a role in fine grain strengthening and precipitation strengthening, so that the strength of 34Nb rebar greatly improved.

The size of two rebar samples for SEM test and XPS test was 10 mm × 10 mm × 10 mm and 3 mm × 3 mm × 3 mm, respectively. These rebar samples were sealed by epoxy resin and exposed to the working surface. Different types of metallographic sandpaper (100 mesh ~ 2500 mesh) were used to coarsely grind and finely grind the exposed surface of these rebar samples, and the diamond polishing agent (particle size of 0.5 μm) and distilled water were used to polish the surface of these rebar samples on the PI-1 A metallographic sample polishing machine. Finally, the ethanol and ultrasonic were used to clean the surface of these rebar samples for subsequent passivation experiments and tests.

Simulated solutions

Due to the microalloyed rebar were in a passive state in a highly alkaline concrete environment, this passive state will be changed by carbonization and corrosion of the concrete environment, resulting in the failure of the concrete structure and destroying the authenticity of the reinforced concrete structure^42–44. While, the concrete environment was mainly affected by the composition of minerals, the composition changes of these minerals led to the carbonization of the concrete environment, which degraded the passivation process of rebar. In this study, the simulated concrete pore solution (SCPS) was used to simulate the real concrete environment to study the passivation process of microalloyed rebar in concrete environment^{22–25,45–47}. The main component of SCPS was a highly alkaline solution of 0.03 mol·L^− 1 calcium hydroxide (Ca(OH)₂) + 0.5 mol·L^− 1 potassium hydroxide (KOH) + 0.2 mol·L^− 1 sodium hydroxide (NaOH). In order to further simulate the carbonization process of SCPS, the different proportions of sodium bicarbonate (NaHCO₃) powder were added to SCPS, and the pH values of SCPS were adjusted to 12.5,10.5 and 9.0, respectively, which mainly studied the passivation behavior of microalloyed rebar in SCPS with different pH. The main components of SCPS with different pH were given in Table 3. The reagents used in SCPS with different pH were analytically pure reagents (99.99% purity), and the solvent was distilled water with a resistivity of 18.2 MΩ·cm. The pH of SCPS was tested and corrected every 12 h. Therefore, the SCPS with different pH was used as the passivation experimental group, and the time of each passivation experiment was set to 3 d, 5 d, 7 d and 10 d, respectively. Three parallel experimental groups were taken in each passivation experiment to ensure the accuracy of the passivation experimental results.

Table 3.

Main components of SCPS with different pH.

Solution	Concentration (mol·L^− 1)
Solution	Calcium hydroxide (Ca(OH)₂)	Sodium hydroxide (NaOH)	Potassium hydroxide (KOH)	Sodium bicarbonate (NaHCO₃)
pH13.2	0.03	0.2	0.5	-
pH12.5	0.03	-	-	0.041
pH10.5	0.03	-	-	0.057
pH9.0	0.03	-	-	0.152

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Electrochemical tests

The electrochemical sample size of two rebars was 20 mm × 10 mm × 5 mm. The electrochemical sample of two rebars were welded together with the nickel sheet by spot welding machine as the connecting wire. The opposite surface of the connecting nickel sheet was the working surface, and the remaining surfaces were sealed completely by epoxy resin. Different types of metallographic sandpaper (100 mesh ~ 2500 mesh) were used to coarsely and finely grind the surface of the electrochemical sample of two rebars, then polishing on the PI-1 A metallographic sample polishing machine. Finally, the surface of the electrochemical sample of two rebars was cleaned with ethanol and ultrasonic, dried and sealed for subsequent electrochemical tests.

All the electrochemical tests of two rebars were carried out on the CHI760E electrochemical workstation. The electrochemical system used the classical three-electrode system, the two rebar samples were the working electrode, the platinum sheet was the auxiliary electrode, and the saturated calomel was the reference electrode. The solution of electrochemical test was SCPS with different pH, and the passivation experiment time was set to 3 d, 5 d, 7 d and 10 d. The two rebar samples were passivated for different times in SCPS with different pH, then different electrochemical tests were carried out. Firstly, the open circuit potential (OCP) test was carried out for 3600 s. After the OCP test was stable, the electrochemical impedance spectroscopy (EIS) test was carried out, the test frequency range was 10^− 2 Hz ~ 10⁵ Hz and the amplitude was 10 mV, and the EIS data were fitted and analyzed by ZView software, which studied the corrosion resistance of surface passive film of two rebars. For the cyclic voltammetry (CV) curve test, the test voltage range was − 1.1 V ~ 0.9 V, the scanning rate was 0.008 V/s, the scanning direction was forward scanning, and the scanning period was five cycles, which studied the electrochemical reaction process and phase formation of surface passive film of two rebars. For the Mott-Schottky (M-S) curve test, the test voltage range was − 1.6 V ~ 1.6 V, the amplitude was 10 mV, and the fixed frequency was 1000 Hz, which studied the semiconductor properties of surface passive film of two rebars. For the chronopotentiometric current density (i-t) curve test, the chronopotentiometric current density scan was carried out for 1000 s at different film formation potentials, which studied the formation kinetics of surface passive film of two rebars. Finally, for the dynamic potential polarization curve (DPP) test, the test voltage range was − 0.5 V ~ 1.0 V, and the scanning rate was 0.5 mV/s, and the Tafer extrapolation method was used to fit and analyze the DPP curve, which studied the reaction process and corrosion tendency of surface passive film of two rebars. All electrochemical tests were carried out at room temperature, and three parallel tests were taken under each group of electrochemical tests to ensure the accuracy of each group electrochemical tests.

Characterization methods

After the two rebars were passivated for 10 d in SCPS with different pH, the morphology and elemental composition of surface passive film of two rebars were characterized by Hitachi Regulus 8100 field emission scanning electron microscope (FE-SEM). The main parameters: the working voltage was 15 KV, the resolution was 0.05 nm, and the working distance was 5 mm.

After the two rebars were passivated for 10 d in SCPS with different pH, the phase composition and element valence state (such as Fe, O, Nb, etc.) of surface passive film of two rebars were characterized by Thermo ESCALAB 250 XI high-resolution X-ray photoelectron spectroscopy (XPS), and the argon ion deep etching of 2 nm and 6 nm were carried out to study the phase composition and element valence state of surface passive film of two rebars. Based on the binding energy of C peak (248.8 eV), the XPS data were fitted and analyzed by Avantage software.

Results and discussions

Analysis of passivation reaction peak

The cyclic voltammetry (CV) curve of two rebars after passivation for different times in SCPS with different pH were given in Fig. 2.

It can be seen from Fig. 2(a)~(h) that the CV curve of two rebars showed three anode peaks (A₁, A₂, A₃) and three cathode peaks (C₁, C₂, C₃) after passivation for different times in SCPS with different pH. In the forward scanning process, the A₁ anode peak appeared at -0.9 V ~ 0.85 V, indicating that the Fe²⁺ ions (such as FeO, Fe(OH)₂, etc.) were formed on the surface of two rebars^17,44,48,49. The A₂ anodic peak appeared at -0.8 V ~ 0.75 V, indicating that the Fe²⁺ ions were transformed into the Fe³⁺ ions (such as Fe₂O₃, Fe₃O₄, etc.) during the passivation reaction of two rebars, which indicated that the surface passive film of two rebars has been formed^20,22,23,50. The A₃ anode peak appeared at -0.6 V ~ 0.5 V, indicating that the stable Fe oxides and Fe hydroxyl oxides were formed on the surface of two rebars, which indicated that the passive film reached the stable state^25,37,43,50. In the reverse scanning process, the appearance of C₁ cathode peak was attributed to the formation of Ca compounds⁵¹, the appearance of C₂ cathode peak was attributed to the reduction of Fe oxides^44,50,52, and the appearance of C₃ cathode peak was attributed to the reduction of Fe^{[3+ [22,37,49,50}.

It can be seen from the CV curve in Fig. 2(a)~(h) that with the decreases of pH and the increases of passivation time, the current density of A₂ anode peak and A₃ anode peak in the two rebars significantly enhanced, and the C₂ cathode peak and C₃ cathode peak significantly reduced, which indicated that the surface passive film of two rebars formed the stable Fe oxides. After passivation for different times in SCPS with high pH13.2, the current density of A₂ anode peak and A₃ anode peak in the two rebars did not change significantly, which indicated that the formation of surface passive film of two rebars was unstable. After passivation for different times in SCPS with low pH9.0, the current density of A₂ anode peak and A₃ anode peak in the two rebars significantly enhanced, which indicated that the surface passive film of two rebars stably formed, and the width of passivation zone gradually widened. Compared with CS rebar, with the increases of passivation time, the A₂ anode peak and A₃ anode peak in 34Nb rebar obtained higher current density, and the passivation zone became wider, which indicated that the addition of Nb promoted the formation of more stable Fe oxides in 34Nb rebar and inhibited the degradation of Fe oxides, thus the stable passive film formed on the surface of 34Nb rebar.

Semiconductor properties of passive film

In order to further analyze the semiconductor properties of surface passive film of two rebars, the Mott-Schottky (M-S) Equation was used to evaluate the carrier density of surface passive film of two rebars. The Mott-Schottky Equation described the relationship between the capacitance (C) and the applied voltage (E) of SCPS, and Mott-Schottky Equation was as follows^{15,24,29,40–42}.

Where, C is the capacitance, E is the applied voltage, ε is the dielectric constant (the dielectric constant of Fe oxides is 12^24,26,53, ε₀ is the vacuum dielectric constant (ε₀ = 8.85 × 10^− 12 F·m^− 1), e is the charge (e = 1.602 × 10^− 19 C), N_D and N_A are the carrier density of donor and acceptor respectively, E_FB is the flat band potential (E_FB is the intercept of linear part of M-S curve), k is the Boltzmann constant (k = 1.38 × 10^− 23 J·k^− 1), T is the absolute temperature, S is the slope of linear part of M-S curve (when the slope is positive, the passive film is the N-type semiconductor; when the slope is negative, the passive film is the P-type semiconductor).

The M-S curve of surface passive film of two rebars after passivation for different times in SCPS with different pH were given in Fig. 3. It can be seen from Fig. 3(a)~(h) that after passivation for different times in SCPS with different pH, the surface passive film of CS rebar showed the N-type semiconductor with positive slope (zone I), while the surface passive film of 34Nb rebar showed the N-type semiconductor with positive slope (zone I) and P-type semiconductor with negative slope (zone II). Previous studies pointed out that the semiconductor properties of surface passive film of microalloyed steel were related to the structure and composition of the passive film, in which, the Fe oxides exhibited the N-type semiconductors, and the microalloying element (such as Cr, Nb, Ni, etc.) oxides exhibited the P-type semiconductors^6,15,24,31. Figure 3(a)~(h) also illuminated that after passivation for different times in SCPS with different pH, the surface passive film of CS rebar mainly contained the Fe oxides and showed the N-type semiconductors, while the surface passive film of 34Nb rebar mainly contained the Fe oxides and Nb oxides, and showed the N-type semiconductors and P-type semiconductors.

The carrier density (N_D and N_A) and flat band potential (E_FB) obtained by M-S curve were given in Fig. 3. It can be seen from the fitting value in Fig. 3(i)~(l) that with the decreases of pH and the increases of passivation time, the flat band potential of surface passive film of two rebars gradually shifted positively, which indicated that the Fermi level of surface passive film of two rebars gradually increased, resulting in the gradual degradation of Fe oxides^54,55. Previous studies pointed out that the conductive resistance of surface passive film of microalloyed steel depended on the carrier density of the passive film, and the lower the carrier density of the passive film, the stronger the conductive resistance of the passive film^21,24,26,55. The fitting values in Fig. 3(i)~(l) showed that with the decreases of pH and the increases of passivation time, the N_D value and N_A value of surface passive film of 34Nb rebar were significantly lower than that of CS rebar, which indicated that the point defect density of surface passive film of 34Nb rebar greatly reduced and the compactness of the passive film enhanced. After passivation for 10 d in SCPS with low pH 9.0, the N_D value of surface passive film of 34Nb rebar was about 3 ~ 4 times lower than that of CS rebar in SCPS with pH13.2, and the N_A value of surface passive film of 34Nb rebar was about 2 times lower than that of 34Nb rebar in SCPS with pH13.2, this indicated that compared with CS rebar, the surface passive film of 34Nb rebar had the lower N_D value and N_A value, resulting in the stronger conductive resistance of surface passive film of 34Nb rebar, thus the surface passive film of 34Nb rebar obtained the excellent protection.

According to the “PDM” theory of passive film⁵⁶, the point defects were mainly distributed in the entire passivation layer of microalloyed steel, which was mainly composed of cation vacancies and anion vacancies, so that the surface passive film of microalloyed steel exhibited the semiconductor behavior. It can be seen from the fitting values Fig. 3(i)~(l) that with the decreases of pH and the increases of passivation time, the N_D value and N_A value of surface passive film of 34Nb rebar were significantly lower than that of CS rebar, which was attributed to the stable existence of Fe oxides in 34Nb rebar, resulting in the significant reduction of cation vacancies in the passive film, which enhanced the compactness of surface passive film of 34Nb rebar.

Corrosion resistance of passive film

The electrochemical impedance spectroscopy (EIS) results of surface passive film of two rebars after passivation for different times in SCPS with different pH were given in Fig. 4.

It can be seen from the Nyquist in Fig. 4(a)~(d) that after passivation for different times in SCPS with different pH, the surface passive film of two rebars showed a similar semi-circular capacitive arc curve, and with the decreases of pH and the increases of passivation time, the radius of the capacitive arc of surface passive film of two rebars gradually increased. Compared with CS rebar, the surface passive film of 34Nb rebar showed a better semi-circular capacitive arc curve, which indicated that the surface passive film of 34Nb rebar had better stability. Previous studies also pointed out that the larger the capacitive arc radius of surface passive film of microalloyed steel, the better the stability of the passive film, and the stronger the corrosion resistance of microalloyed steel^8,21,28,35. After passivation for 10 d in SCPS with low pH9.0, the capacitive arc radius of surface passive film of 34Nb rebar was larger than that of CS rebar, which indicated that the surface passive film of 34Nb rebar had better stability and corrosion resistance.

It can be seen from the Bode in Fig. 4(e)~(h) that with the decreases of pH and the increases of passivation time, the impedance value of surface passive film of two rebars significantly increased. Compared with the CS rebar, the impedance value of surface passive film of 34Nb rebar had the higher value, which indicated that the surface passive film of 34Nb rebar had the higher resistance impedance, thus enhancing the corrosion resistance of the passive film. After passivation for 10 d in SCPS with low pH9.0, compared with CS rebar, the impedance value of surface passive film of 34Nb rebar reached 46.58 × 10⁴ Ω·cm², which indicated that the surface passive film of 34Nb rebar had the strong conduction resistance, thus enhancing the corrosion resistance of 34Nb rebar.

It can be seen from the Phase in Fig. 4(e)~(h) that with the decreases of pH and the increases of passivation time, the maximum phase angle of surface passive film of two rebars obviously increased. Compared with CS rebar, the surface passive film of 34Nb rebar showed the excellent maximum phase angle, which indicated that the surface passive film of 34Nb rebar had the better protection. Previous studies also pointed out that the larger the maximum phase angle value of the passive film, the smaller the surface roughness of the passive film, and the better the protection of the passive film^17,22,28,36. After passivation for 10 d in SCPS with low pH9.0, compared with CS rebar, the maximum phase angle of surface passive film of 34Nb rebar reached 88.2 degree, which indicated that the surface passive film of 34Nb rebar had the lower surface roughness, thereby the surface passive film of 34Nb rebar had the better protection.

In order to fit the EIS data in Fig. 4, the equivalent circuit in Fig. 5 was used to fit the EIS results in Fig. 4, and the equivalent circuit were given in Fig. 5.

In Fig. 5, R_S is the resistance of SCPS, R_f is the resistance of the passive layer, R_ct is the charge transfer resistance, CPE_f is the constant phase element of the double-layer capacitance at the interface between the passive film and SCPS, and CPE_ct is the constant phase element of the double-layer capacitance at the interface between the passive film and the rebar substrate. Therefore, the constant phase element CPE was a non-ideal capacitance loop, and the impedance value Z_CPE of CPE parameter of the constant phase element was defined as follows^{20,23,34,41–43}.

Where, Z₀ is the admittance coefficient, n is the fitting index (0 < n > 1, n = 0 indicates that the passive layer is a conductor, n = 1 indicates that the passive layer is an ideal capacitor), j is the imaginary unit, and w is the angular frequency. In order to further analyze the double-layer capacitance behavior of surface passive film of two rebars, the Brug formula and the Hsu formula and the Mansfeld formula were used to convert the CPE parameters into the effective capacitance (C_f) of the interface between the SCPS and the passive film layer and the effective capacitance (C_ct) of the interface between the passive film layer and the rebar substrate, and the effective capacitance C_f and C_ct were defined as follows^{21,24,40,57–59}.

The effective capacitance C_f and C_ct values of surface passive film of two rebars after passivation for different times in SCPS with different pH were calculated by Eqs. (6) and (7), and the EIS data in Fig. 4 were fitted using the equivalent circuit in Fig. 5, and the best fitting value and effective capacitance value were given in Table 4.

Table 4.

EIS fitting value and effective capacitance value of surface passive film of two rebars after passivation for different times in SCPS with different pH.

pH	Rebar	Time	R_s (Ω·cm²)	R_f (Ω·cm²·10⁴)	CPE_f		R_ct (Ω·cm²·10⁵)	CPE_ct		Chi-square value (χ²·10^− 5)	C_f (F·cm^− 2·10^− 5)	C_ct (F·cm^− 2·10^− 6)
pH	Rebar	Time	R_s (Ω·cm²)	R_f (Ω·cm²·10⁴)	Y_f (Ω⁻¹·cm²·sⁿ·10^− 5)	n_f	R_ct (Ω·cm²·10⁵)	Y_ct (Ω⁻¹·cm²·sⁿ·10^− 5)	n_ct	Chi-square value (χ²·10^− 5)	C_f (F·cm^− 2·10^− 5)	C_ct (F·cm^− 2·10^− 6)
13.2	CS	3 d	5.51	0.45	9.53	0.95	0.81	8.23	0.86	4.41	9.11	2.35
		5 d	6.04	0.73	9.36	0.93	2.57	7.43	0.86	4.51	9.09	2.12
		7 d	7.47	1.29	8.43	0.91	3.26	6.22	0.85	4.81	8.50	1.61
		10 d	9.34	2.65	7.82	0.94	4.81	5.54	0.85	4.06	8.19	1.46
	34Nb	3 d	7.43	0.75	9.27	0.93	1.42	7.09	0.86	4.61	9.02	2.07
		5 d	9.01	0.96	8.42	0.92	3.34	6.36	0.86	4.32	8.26	1.89
		7 d	12.66	1.78	7.83	0.91	4.19	5.52	0.85	4.15	8.09	1.53
		10 d	13.29	2.58	7.30	0.93	6.96	5.02	0.85	4.68	7.65	1.38
12.5	CS	3 d	7.37	1.04	9.09	0.91	1.71	7.82	0.86	4.51	9.03	2.32
		5 d	9.14	2.12	8.56	0.94	2.68	7.17	0.85	4.78	8.89	1.97
		7 d	11.61	3.32	7.92	0.94	4.47	6.58	0.83	4.89	8.42	1.51
		10 d	13.15	5.57	7.18	0.94	6.15	6.16	0.83	4.19	7.84	1.43
	34Nb	3 d	9.49	1.66	8.44	0.92	3.02	6.62	0.86	4.84	8.69	1.99
		5 d	12.96	3.37	8.01	0.95	5.89	6.01	0.85	4.81	8.21	1.70
		7 d	14.81	5.07	7.56	0.95	7.45	5.58	0.84	4.49	8.01	1.44
		10 d	16.33	6.55	7.01	0.95	9.82	5.06	0.84	4.95	7.59	1.31
10.5	CS	3 d	9.69	2.35	8.12	0.94	3.38	6.87	0.86	4.03	8.46	2.09
		5 d	12.95	3.69	7.42	0.92	4.68	6.07	0.85	4.67	8.09	1.72
		7 d	13.55	4.67	6.81	0.93	6.32	5.31	0.85	4.58	7.42	1.48
		10 d	15.54	5.91	6.24	0.94	8.56	4.66	0.85	4.87	6.78	1.30
	34Nb	3 d	11.66	2.71	7.35	0.94	4.68	5.99	0.86	4.35	7.68	1.84
		5 d	13.57	4.53	6.54	0.93	6.02	5.28	0.86	4.68	7.09	1.62
		7 d	16.86	6.39	5.47	0.93	8.23	4.86	0.85	4.58	6.01	1.39
		10 d	19.06	10.65	4.52	0.92	12.81	4.05	0.86	4.56	5.18	1.26
9.0	CS	3 d	11.97	7.39	7.45	0.94	10.15	5.51	0.87	4.36	8.31	1.84
		5 d	14.58	10.41	6.49	0.93	14.07	4.83	0.86	4.34	7.49	1.48
		7 d	17.79	13.25	5.26	0.92	17.31	3.91	0.86	4.38	6.22	1.20
		10 d	20.55	16.71	4.25	0.93	20.38	3.04	0.86	4.62	4.93	0.92
	34Nb	3 d	13.59	10.86	6.48	0.93	13.31	4.73	0.85	4.33	7.51	1.29
		5 d	16.98	14.21	5.53	0.92	16.44	3.64	0.85	4.31	6.62	0.98
		7 d	20.25	17.27	4.61	0.91	21.66	3.05	0.85	4.41	5.66	0.83
		10 d	23.98	20.13	3.47	0.93	24.22	2.63	0.86	4.39	4.02	0.79

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It can be seen from Table 4 that after passivation for different times in SCPS with different pH, the fitting χ value of surface passive film of two rebars was χ < 10^− 3, and the fitting index n was between 0.80 and 0.95, which indicated that the fitting effect was better. As the pH of SCPS decreased and the passivation time increased, the R_f value and R_ct value of surface passive film of two rebars increased significantly, and the effective capacitance C_f value and C_ct value decreased significantly. Compared with CS rebar, the surface passive film of 34Nb rebar had the excellent R_f value and R_ct value, and the effective capacitance C_f value and C_ct value greatly reduced, which indicated that the resistance of surface passivation film of 34Nb rebar significantly enhanced, thus improving the stability of the passive film. After passivation for 10 d in SCPS with low pH9.0, the R_f value and R_ct value of surface passive film of 34Nb rebar were about 7 ~ 8 times higher than that of CS rebar in SCPS with high pH13.2, and the effective capacitance C_f value and C_ct value were about 2 ~ 3 times lower than that of CS rebar in SCPS with high pH13.2, this showed that the surface passive film of 34Nb rebar had the higher resistance, which hindered the further dissolution of rebar substrate and enhanced the corrosion resistance of 34Nb rebar, which was consistent with the results reported in previous studies^{17,25,41–43}.

The dynamic potential polarization curve (DPP) and the best fitting value of surface passive film of two rebars after passivation for different times in SCPS with different pH were given in Fig. 6. It can be seen from Fig. 6(a)~(h) that after passivation for different times in SCPS with different pH, the DPP curves of surface passive film of two rebars showed the cathodic reaction zone, the anodic dissolution zone, the passivation reaction zone and the pitting zone. Previous studies also showed that E_corr represented the corrosion potential and mainly predicted the corrosion tendency of alloy steel, i_corr represented the corrosion current density and mainly predicted the corrosion rate of alloy steel, i_p represented the passivation current density and mainly predicted the passivation behavior of alloy steel, and E_p represented the pitting potential and predicted the corrosion resistance of alloy steel^{13,21,27–29,43}. The fitting results in Fig. 6(i)~(p) showed that as the pH of SCPS decreased and the passivation time increased, compared with CS rebar, the E_corr value of surface passive film of 34Nb rebar gradually shifted positively, and the i_corr value decreased significantly, this indicated that the surface passive film of 34Nb rebar had a small corrosion tendency and corrosion rate, which further hindered the corrosion electrochemical reaction and enhanced the corrosion resistance of 34Nb rebar.

Fig. 6 — DPP curve and fitting value of surface passive film of two rebars after passivation for different times in SCPS with different pH values. (a)~(h) DPP curve; (i)~(p) Fitting value; (a), (c), (e), (g), (i)~(l) CS rebar; (b), (d), (f), (h), (m)~(p) 34Nb rebar; (a), (b), (i), (m) pH13.2; (c), (d), (j), (n) pH12.5; (e), (f), (k), (o) pH10.5; (g), (h), (l), (p) pH9.0.

Previous studies pointed out that the anodic dissolution process on the surface of microalloyed steel during the passivation reaction was mainly controlled by the dissolution degree of Fe, and the cathodic reaction process was mainly controlled by the dissolved oxygen content^17,25,31,46. During the initial passivation process, a large amount of Fe in microalloyed rebar dissolved to form Fe²⁺. As the passivation reaction proceeded, Fe²⁺ on the surface of microalloyed rebar was further transformed into the initial product Fe oxides (such as FeO, etc.) and hydroxides (such as Fe (OH)₂, Fe(OH)₃, etc.). Under the action of dissolved oxygen, the initial product was further transformed into the stable Fe oxides (such as Fe₂O₃, Fe₃O₄, etc.), which inhibited the anodic dissolution process of Fe and enhanced the stability of surface passive film of microalloyed rebar, resulting in the significant positive shift of E_corr value and the significant reduction of i_corr value, thus the corrosion resistance of microalloyed rebar obviously enhanced. After passivation for 10 d in SCPS with low pH9.0, compared with CS rebar, the E_corr value of surface passive film of 34Nb rebar shifted positively to -0.18 V and the i_corr value decreased to 1.72 × 10^− 8 A·cm^− 2, which indicated that the surface passive film of 34Nb rebar obtained the noble E_corr value, the passive film had the higher corrosion resistance, and reduced the corrosion tendency of 34Nb rebar.

Previous studies pointed out that the lower the passivation current density of microalloyed steel, the higher the pitting potential, the lower the conductivity of anions and cations in the passive film, and the better the corrosion resistance of microalloyed steel^5,18,27,43. It can be seen from the fitting results in Fig. 6(i)~(p) that with the decreases of pH and the increases of passivation time, compared with CS rebar, the i_p value of surface passive film of 34Nb rebar decreased significantly, the E_p value increased, and the passivation zone gradually widened, this showed that the surface passive film of 34Nb rebar had the high resistance, thus enhancing the corrosion resistance of 34Nb rebar. After passivation for 10 d in SCPS with low pH9.0, the i_p value of surface passive film of 34Nb rebar was about 7 ~ 8 times lower than that of CS rebar in SCPS with high pH13.2, and the E_p value was about 0.3 V higher than that of CS rebar in SCPS with high pH13.2, this showed that the surface of 34Nb rebar had the higher passive layer resistance, inhibited the anodic dissolution of substrate, and enhanced the corrosion resistance of 34Nb rebar.

Formation process of passive film

Chronoamperometric current density curve (i-t) on the surface of two rebars after passivation for different times in SCPS with different pH were given in Fig. 7.

It can be seen from Fig. 7(a)~(h) that after passivation for different times in SCPS with different pH, the i-t curve on the surface of two rebars at different constant potentials (200 mV, 400 mV, 600 mV) was roughly the same with the increase of the timing time, and the maximum current density of anodic dissolution rapidly decayed to the passivation current density, which indicated that the surface passive film of two rebars begun to form. With the decreases of pH and the increases of constant potential, the maximum anodic dissolution current density and passivation current density on the surface of two rebars gradually increased, which indicated that the formation of surface passive film of two rebars reached the saturation stability at high constant potential. After passivation for 10 d in SCPS with low pH9.0, compared with CS rebar, when the constant potential was 200 mV, the anodic dissolution current density on the surface of 34Nb rebar rapidly decayed to the lowest passivation current density, which indicated that the surface passive film of 34Nb rebar has been basically formed stably.

When the constant potential was 200 mV, the i-t curve on the surface of two rebars after passivation for 10 d in SCPS with different pH were given in Fig. 8. It can be seen from Fig. 8(a) and (c) that when the constant potential was 200 mV, with the decreases of pH after passivation for 10 d in SCPS, the passivation current density on the surface of CS rebar decreased from 1.27 × 10^− 5 A·cm^− 2 to 1.17 × 10^− 5 A·cm^− 2, while the passivation current density on the surface of 34Nb rebar decreased from 1.16 × 10^− 5 A·cm^− 2 to 1.06 × 10^− 5 A·cm^− 2, this indicated that the surface of 34Nb rebar had the high passivation, which inhibited a large number of the anodic dissolution and hindered the migration resistance of metal cations from the passive film layer to SCPS, thus greatly improving the stability and compactness of surface passive film of 34Nb rebar.

Fig. 8 — i-t curve and lgi(t)-1/q(t) curve on the surface of two rebars after passivation for 10 d in SCPS with different pH (the constant potential was 200 mV). (a) and (b) CS rebar, (c) and (d) 34Nb rebar, (a) and (c) i-t cures, (b) and (d) lgi(t)-1/q(t) cures.

Previous studies showed that the formation kinetics process of surface passive film of microalloyed steel mainly included the anodic dissolution and passive film formation^24,28,33. Therefore, the relationship between the current i(t) and the charge flux (q(t)) formed by the total current flowing through the surface of microalloyed steel was derived by Faraday’s law.

Where, i(t) is the total current, q(t) is the total electric quantity, A and B are the correlation constants of the activation energy of the ions migrating in the passive film, c is the correlation constant of microalloyed steel, V is the potential drop through the passive film. In Eq. (8), there was a linear relationship between lgi(t) and q(t), and the slope was k = cBV. The smaller the k value, the faster the passivation efficiency on the surface of microalloyed steel^16,24,30,60. The i-t curves of Fig. 8(a) and (c) were converted into lgi(t)-1/q(t) curves by Eqs. (8) and (9), and the lgi (t)-1/q(t) curves were given in Fig. 8(b) and (d). According to the variation characteristics of lgi(t)-1/q(t) curves in Fig. 8(b) and (d), the formation process of surface passive film of two rebars was mainly composed of three different stages after passivation for 10 d in SCPS with different pH.

The first stage was the anodic dissolution control. In the initial passivation process, the anodic dissolution rate of rebar substrate was large, and this stage has been basically completed within 0 s ~ 30 s, and the lgi(t)-1/q(t) curve at this stage was not linear. When the large amount of dissolution of rebar substrate formed Fe²⁺, the transmission rate of Fe²⁺ from the substrate to SCPS was accelerated under the action of dissolved oxygen, resulting in the maximum total current flowing through the surface of rebar, which indicated that the initial passive film on the surface of rebar was difficult to form.

The second stage was the formation and growth of the passive film. With the acceleration of the passivation reaction, a large amount of Fe²⁺ was further transformed into the initial product under the action of dissolved oxygen, indicating that the initial passive film was formed on the surface of two rebars, which hindered the large dissolution of rebar substrate, thus the total current flowing through the surface of rebar rapidly decayed to the minimum passivation current, which was completed within 30 s ~ 200 s. The lgi(t)-1/q(t) curve was linear at this stage, which indicated that the surface passive film of two rebars has been formed. As the pH of SCPS decreased, the slope of the lgi(t)-1/q(t) curve on the surface of CS rebar increased from 3.89 mC·cm^− 2 to 4.56 mC·cm^− 2, while the slope of the lgi(t)-1/q(t) curve on the surface of 34Nb rebar increased from 6.78 mC·cm^− 2 to 9.24 mC·cm^− 2, this meant that compared with CS rebar, the passivation efficiency of 34Nb rebar significantly enhanced, which improved the surface passive film of 34Nb rebar and hindered the anodic dissolution of rebar substrate.

The third stage was the stable stage of the passive film. In the later passivation stage, the formation and growth of the initial passive film led to the total current flowing through the surface of two rebars decreased to the lowest value, this stage was basically completed after 200 s, and the lgi(t)-1/q(t) curve deviated from the linear relationship, which indicated that the surface passive film of two rebars formed stably, and the passive film gradually thickened, resulting in the formation of surface passive film of two rebars has been reached saturation. As the pH of SCPS decreased, compared with CS rebar, the lgi(t)-1/q(t) curve on the surface of 34Nb rebar deviated from the linear relationship to a lesser extent, which indicated that the stability and compactness of surface passive film of 34Nb rebar gradually enhanced, and the passive film gradually thickened, resulting in the significant increase in the protection of surface passive film of 34Nb rebar.

Previous studies pointed out that the total current (i_total) on the surface of microalloyed steel was mainly composed of dissolution current (i_diss) and film formation current (i_film)^24,28,30,33.

Where, i_bare is the maximum current value of i-t curve, A is the constant, b is the empirical value, Ɵ is the coverage coefficient, k is the constant of oxide nucleation, n is the morphology index of oxide nucleation, n is related to the dimension of oxide nucleation, n = 1 indicates that the surface passive film of metal is preferentially covered on the two-dimensional plane. In order to analyze the nucleation and growth process of the passive film, the i-t curve in 0 ~ 100 s in Fig. 8 was selected and fitted by Eqs. (10) and (11), and obtained the optimal fitting parameters k, A, b. Bring k, A, b into Eqs. (10) and (11) to calculate the i_total, i_diss, i_film were given in Fig. 9.

It can be seen from Fig. 9(a)~(h) that after passivation for 10 d in SCPS with different pH, when the constant potential was 200 mV, the surface of two rebars showed the anodic dissolution at Ɵ<1, and the current value at this stage showed the dissolution current (i_diss), which was mainly used for dissolution of rebar substrate. When Ɵ=1, the dissolution current flowing through the surface of two rebars rapidly decayed to the lowest passivation current, and the current value at this stage showed the film forming current (i_film), which was mainly used for the formation of the initial passive film on the surface of two rebars. When Ɵ>1, the film forming current flowing through the surface of two rebars almost unchanged, indicating that the surface passive film of two rebars has been stably formed, and the passive film gradually thickened. As the pH of SCPS decreased, the dissolution current and film forming current on the surface of two rebars gradually increased, which indicated that the increase of dissolution current led to a large amount of dissolution of rebar substrate, forming a large amount of Fe²⁺ released into SCPS, and under the action of dissolved oxygen, Fe²⁺ further formed Fe oxides on the surface of two rebars. After passivation for 10 d in SCPS with low pH9.0, compared with CS rebar, the film forming current on the surface of 34Nb rebar increased significantly, indicating that the oxides on the surface of 34Nb rebar stably formed, which improved the stability of the passive film, and hindered the degradation of the passive film.

In addition, with the decreases of pH, the initial nucleation time of surface passive film of CS rebar increased from 15.5 s to 32.3 s, while the initial nucleation time of surface passive film of 34Nb rebar increased from 19.4 s to 39.1 s, this showed that the nucleation rate of oxide film on the surface of 34Nb rebar gradually increased and thickened, and the thickness of the passive film reached a certain saturation, thus the surface passive film of 34Nb rebar had the higher stability. Previous studies showed that the decreases of pH led to the gradual increase of dissolution current and film formation current on the surface of microalloyed steel, and the microalloying element oxides and Fe oxides were gradually formed and covered on the surface of microalloyed steel, which inhibited the degradation of Fe oxides and the surface passive film of microalloyed steel gradually thickened^24,28,30,33.

In order to obtain the thickness variation of the passive film with time, the film forming current (i_film) was integrated within 0 ~ 100 s to obtain the amount of charge (q_film(t)) consumed by the oxide and the thickness of the passive film d(t) was calculated by Faraday’s law^24,28,33.

Where, M is the molar mass of Fe (M = 56 g·mol^− 1), z is the charge number of Fe²⁺ (z = 2), ρ is the density of Fe (ρ = 7.86 g·mol^− 3), F is the Faraday constant (F = 96485.34 C·mol^− 1).

The thickness variation of surface passive film of two rebars with time after passivation for 10 d in SCPS with different pH calculated by Eqs. (12) and (13) were in Fig. 10. It can be seen from in Fig. 10(a) and (b) that when Ɵ=1, with the decreases of pH, the initial thickness of surface passive film of CS rebar increased from 0.18 nm to 1.10 nm, while the initial thickness of surface passive film of 34Nb rebar increased from 0.34 nm to 2.08 nm, this indicated that the decreases of pH led to more film forming current flowing through the surface of 34Nb rebar for the formation and growth of the passive film, thus the initial passive film obtained a certain thickness. Previous studies showed that the growth rate of surface passive film of microalloyed steel depended on the slope of d-t curve, and the larger the slope of d-t curve, the faster the growth rate of surface passive film of microalloyed steel, and the thicker the passive film^24,30,33. It can be seen from Fig. 10(a) and (b) that compared with CS rebar, after passivation for 10 d in SCPS, as the pH decreased from 13.2 to 9.0, the slope of d-t curve of surface passive film of 34Nb rebar increased slowly from 0.01427 to 0.04301, indicating that the growth rate of surface passive film of 34Nb rebar increased slowly, and the thickness of the passive film gradually thickened. This was attributed to the addition of Nb promoted the formation of more stable Fe oxides and Nb oxides in the surface passive layer of 34Nb rebar, inhibiting the degradation of the passive film, maintaining the thickness stability of the passive film, and enhancing the passivation performance of 34Nb rebar.

Fig. 10 — d-t curve of surface passive film of two rebars after passivation for 10 d in SCPS with different pH (the constant potential was 200 mV). (a) CS rebar; (b) 34Nb rebar.

When the thickness of the initial passive film reached a stable state, with the decreases of pH, the thickness of the stable passive film on the surface of CS rebar increased from 0.95 nm to 2.90 nm, while the thickness of the stable passive film on the surface of 34Nb rebar increased from 1.50 nm to 4.70 nm, this indicated that the stable passive film was obtained on the surface of 34Nb rebar, which was attributed to the formation of Nb oxides on the surface of 34Nb rebar, inhibiting the degradation of Fe oxides, and enhancing the stability of the passive film. Previous studies also found that the initial passive film on the surface of microalloyed steel had the high stability after passivation for 10 d in SCPS with low pH, which indicated that the stable microalloying element oxides were formed on the surface of microalloyed steel, inhibiting the degradation of Fe oxides, and enhancing the passivation efficiency of microalloyed steel^16,24,28,30.

Surface analysis of passive film

The XPS sputtering depth results and semi-quantitative analysis of surface passive film of two rebars after passivation for 10 d in SCPS with pH13.2 and pH9.0 were given in Figs. 11 and 12.

It can be seen from Fe2p and O1s spectra in Fig. 11(a) and (d) and Fig. 12(a) and (e) combined with Table 5 that after passivation for 10 d in SCPS with pH13.2 and pH9.0, at different sputtering depths, the Fe2p spectra of surface passive film of two rebars were mainly divided into Fe⁰ (the binding energy was 706.78 eV^{15–17,21,22}, Fe²⁺ (the binding energy was 708.83 eV^{17–19,24,26}, Fe³⁺ (the binding energy was 709.63 eV^{16–18,26–28} and FeOOH (the binding energy was 712.33 eV^{27–29,31,32}, while the O1s spectrum were mainly divided into O²⁻ (the binding energy was 530.08 eV^{26–28,42,50,61,62} and OH⁻ (the binding energy was 531.23 eV^{15–18,27–29,61,62}. According to the semi-quantitative analysis in Fig. 11(c) and (f) and Fig. 12(d) and (h), with the increases of sputtering depth, compared with CS rebar, the mass ratios of Fe³⁺/Fe²⁺, Fe³⁺/(Fe²⁺+FeOOH) and O²⁻/OH⁻ in the surface passive film of 34Nb rebar reached 0.68,0.65, 0.7, respectively, after passivation 10 d in SCPS with pH13.2, while after passivation for 10 d in SCPS with pH9.0, the mass ratios of Fe³⁺/Fe²⁺, Fe³⁺/(Fe²⁺+FeOOH) and O²⁻/OH⁻ in the surface passive film of 34Nb rebar reached 0.86, 0.82, 0.88, respectively. This indicated that the decreases of pH led to the stable existence of Fe³⁺ oxides in the surface passive film of 34Nb rebar, which inhibited the degradation of the passive film, and the inner passive film of 34Nb rebar was mainly composed of Fe³⁺ oxides (such as Fe₂O₃, Fe₃O₄, etc.), the outer passive film was mainly composed of Fe²⁺ oxides (such as FeO, etc.) and Fe hydroxyl oxides (such as FeOOH, etc.).

Table 5.

Band position and band width of each element in two rebars after passivation for 10 d in SCPS with different pH.

Rebar	pH	Element	Valences	Band position (± 0.5 eV)	Band width (± 0.2 eV)
CS	13.2	Fe	Fe⁰	706.98, 716.49^15–17	1.64, 3.36
			Fe²⁺	725.88^17–19	3.36
			Fe³⁺	710.55, 719.87, 730.74^26–28	3.36, 3.13, 2.25
			FeOOH	713.36, 723.63^27–29	3.36, 3.31
		O	O²⁻	530.13^{26–28,61,62}	1.48
		O	OH⁻	531.44^{15–18,61,62}	1.85
	9.0	Fe	Fe⁰	706.74, 716.28^15–17	0.98, 3.36
			Fe²⁺	707.84, 726.25^17–19	1.45, 3.36
			Fe³⁺	710.59, 720.04, 731.59^26–28	3.06, 3.35, 3.36
			FeOOH	713.16, 723.85^27–29	3.36, 3.33
		O	O²⁻	530.09^{26–28,61,62}	1.19
		O	OH⁻	531.77^{15–18,61,62}	3.17
34Nb	13.2	Fe	Fe⁰	706.87, 715.80^15–17	1.70, 3.36
			Fe²⁺	726.77^17–19	3.36
			Fe³⁺	710.35, 720.85, 733.36^26–28	3.35, 3.36, 3.33
			FeOOH	712.95, 723.98^27–29	3.36, 3.34
		O	O²⁻	530.12^{26–28,61,62}	1.33
		O	OH⁻	531.01^{15–18,61,62}	2.38
		Nb	NbO	204.26^34–36	2.33
			NbO₂	205.55^34–36	0.61
			Nb₂O₅	206.65^34–36	0.71
	9.0	Fe	Fe⁰	706.63, 715.93^15–17	0.92, 3.15
			Fe²⁺	708.01, 726.65^17–19	2.48, 3.36
			Fe³⁺	710.93, 719.69, 729.73^26–28	2.96, 2.12,3.35
			FeOOH	713.15, 724.15^27–29	3.35, 3.12
		O	O²⁻	530.18^{26–28,61,62}	1.52
		O	OH⁻	531.53^{15–18,61,62}	1.55
		Nb	NbO	204.21^34–36	2.80
			NbO₂	205.54^34–36	1.19
			Nb₂O₅	206.64^34–36	0.51

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According to the standard Gibbs free energy of metal oxidation^63–65, at 298.15 K, the standard Gibbs free energy of NbO (Nb²⁺), NbO₂ (Nb⁴⁺) and Nb₂O₅ (Nb⁵⁺) were − 391.490 kJ·mol^− 1, − 740.923 kJ·mol^− 1, − 1764.585 kJ·mol^− 1, respectively, and the higher the valence of Nb oxides, the higher the stability of Nb oxides. According to the Nb3d spectrum analysis in Fig. 12(c) and (g) combined with Table 5, after passivation for 10 d in SCPS with different pH, the Nb3d spectrum in the surface passive film of 34Nb rebar were divided into NbO (the binding energy was 204.28 eV^{34–36,38–41}, NbO₂ (the binding energy was 205.53 eV^{34–36,38–41} and Nb₂O₅ (the binding energy was 206.63 eV^{34–36,38–41}. According to the semi-quantitative analysis in Fig. 12(d) and (h), with the increases of sputtering depth, the mass ratios of NbO₂/NbO and Nb₂O₅/(NbO₂ + NbO) in the surface passive film of 34Nb rebar reached 0.88 and 0.78, respectively, after passivation for 10 d in SCPS with pH13.2, while after passivation for 10 d in SCPS at pH 9.0, the mass ratios of NbO₂/NbO and Nb₂O₅/(NbO₂ + NbO) in the surface passive film of 34Nb rebar reached 1.12 and 0.96, respectively. This indicated that the decreases of pH promoted the enrichment of more stable Nb oxides in the inner passive film of 34Nb rebar, and the appearance of these Nb oxides inhibited the degradation of stable Fe oxides, thereby enhancing the stability and compactness of the passive film, which further confirmed the experimental results of CV, M-S, EIS, DPP.

The SEM morphology of surface passive film of two rebar after passivation for 10 d in SCPS with different pH were given in Fig. 13. It can be seen from Fig. 13(a)~(h) that after passivation for 10 d in SCPS with different pH, many nano-scale oxide particles and some calcium oxides were distributed on the surface of two rebars, which indicated that the surface passive film of two rebars has been stably formed. After passivation for 10 d in SCPS with high pH13.2, a large number of calcium oxides were distributed on the surface of CS rebar, indicating that the formation of surface passive film of CS rebar was unstable (see Fig. 13(a)), while some nano-scale oxide particles were distributed on the surface of 34Nb rebar (see Fig. 13(e)), indicating that the dense passive film on the surface of 34Nb rebar begun to form. After passivation for 10 d in SCPS with low pH9.0, compared with CS rebar, a large number of small and dense nano-scale oxide particles (see Fig. 13(h)) were distributed on the surface of 34Nb rebar, indicating that a dense passive film was distributed on the surface of 34Nb rebar, which improved the stability of the passive film and enhanced the protection of 34Nb rebar. Previous studies pointed out that with the decreases of pH, a large number of small and dense nano-scale oxide particles were distributed on the surface of microalloyed steel, which enhanced the corrosion resistance of microalloyed steel^18,23,41.

Passivation mechanism

Based on the experimental results of CV, M-S, EIS, DPP, i-t, XPS and SEM, the formation mechanism of surface passive film of Nb microalloyed rebar were given in Fig. 14.

It can be seen from Fig. 14(a)~(d) and (e)~(h) that in the initial passivation process of high alkalinity, when the surface of two rebars was in contact with SCPS, the anodic dissolution and cathodic oxygen absorption reaction occurred on the surface of two rebars (see Eqs. (14) and (15))^8,11,24. At this time, the surface of two rebars was dissolved under the action of high alkaline ions (such as Ca²⁺, OH⁻, etc.), and a large amount of Fe²⁺ was released into SCPS to participate in the formation of the passive film. As the passivation reaction proceeded, when Fe²⁺ accumulated in SCPS, Fe²⁺ was further transformed into the initial products FeO and Fe(OH)₂ (see Eq. (16)), which were covered on the surface of two rebars (see Fig. 14(a)~(b) and (e)~(f))^8,11,24,26. At this time, Fe(OH)₂ was further transformed into Fe(OH)₃ (see Eq. (16))^11,24,26,30 under the action of dissolved oxygen, and Fe(OH)₃ was decomposed into FeOOH (see Eq. (17)) and Fe₃O₄ (see Eq. (18))^8,24,30,33, and Fe₃O₄ was further oxidized to Fe₂O₃ (see Eq. (19))^11,26,33,44 under the action of dissolved oxygen. When the passivation time reached 10 d, the surface of two rebars was covered with more stable Fe oxides, which inhibited the anodic dissolution of rebar substrate, indicating that a stable passive film has been formed on the surface of two rebars (see Fig. 14(c)~(d) and (g)~(h)).

Due to the Nb element played an important role in improving the stability and compactness of the surface passive film of rebar during the passivation reaction, and the more stable the Nb oxides in the passive layer, the higher the stability of the passive film^{34–36,38–41}. In the initial passivation process of SCPS, an initial Nb oxide layer (NbO) (see Eq. (20))^{34–36,38–41} (see Fig. 14(e) and (f)) was formed on the surface of 34Nb rebar, which increased the initial passivation rate of 34Nb rebar, and the surface passive film of 34Nb rebar formed rapidly. With the increases of passivation time, NbO was further oxidized to stable NbO₂ (see Eq. (20))^{34–36,38–41} under the action of dissolved oxygen, and NbO₂ was further transformed into more stable Nb₂O₅ (see Eq. (20))^{34–36,38–41} (see Fig. 14(g) and (h)). When the passivation time reached 10 d, these stable Nb oxides were gradually enriched in the inner layer of the passive film, which enhanced the self-healing ability of the passive film and the passive film had the higher stability.

In SCPS with low pH9.0, due to the carbonization of SCPS, the stable Fe oxides (such as Fe₂O₃, Fe₃O₄, etc.) in the surface passive film of CS rebar gradually degraded at the initial stage of passivation (see Fig. 14(i) and (j)), which reduced the passivation rate of CS rebar and deteriorated the stability of the passive film. With the increases of passivation time, the stable Fe oxides on the surface of CS rebar will undergo a small amount of degradation (see Fig. 14(k) and (l)), and the dissolution rate of the passive film will decrease, thus the protection of the passive film gradually decreased. Compared with CS rebar, the dissolution rate of Fe oxides in the surface passivation film of 34Nb rebar decreased at the initial stage of passivation, and the formation of initial Nb oxides led to an increase in the stability of the passive film (see Fig. 14(m) and (n)). With the increases of passivation time, more stable Nb oxides (Nb₂O₅) were formed on the surface of 34Nb rebar (see Fig. 14(o) and (p)), the presence of these Nb oxides enhanced the self-healing ability of the passive film, inhibited the degradation of stable Fe oxides, and improved the passivation rate of 34Nb rebar, thus enhancing the corrosion resistance of 34Nb rebar.

According to the analysis of the passivation mechanism, the addition of Nb has a beneficial effect on the improvement of the stability and compactness of the passive film on the surface of rebar. After passivation for 10 d in SCPS, compared with CS rebar, with the decreases of pH, the addition of Nb promoted the formation of more stable Nb oxides in the surface passive film of 34Nb rebar, inhibited the dissolution of Fe oxides, and improved the mass ratio of Fe³⁺/Fe²⁺ and Nb₂O₅/(NbO + NbO₂), the maximum impedance value and the corrosion potential, thus enhancing the passivation rate of 34Nb rebar. Therefore, in the carbonation environment of reinforced concrete, the design of Nb microalloyed rebar provides a new method for the long-term stability of reinforced concrete structures.

Conclusions

In this study, XPS sputtering depth and SEM were used to analyze the morphology and composition of surface passive film of two rebars and the electrochemical tests (such as CV, EIS, M-S, i-t, DPP, etc.) were used to analyze the formation process of surface passive film of two rebars in SCPS with different pH, and the corresponding passivation mechanism was proposed. The main key conclusions of this study summarized as follows.

(1) After passivation for 10 d in SCPS with different pH, compared with CS rebar, with the decreases of pH, the outer layer of surface passive film of 34Nb rebar were gradually composed of Fe oxides and Fe hydroxides, and the inner layer were gradually composed of stable Fe oxides and Nb oxides, resulting in the increases of the mass ratio of Fe³⁺/Fe²⁺ and Nb₂O₅/(NbO₂ + NbO) in the passive film, which improved the stability of surface passive film of 34Nb rebar.

(2) After passivation for 10 d in SCPS with different pH, the Ca-oxide particles and nano-scale oxide particles were distributed on the surface of two rebars, and these oxide particles constituted the composition of the passive film. Compared with CS rebar, the surface of 34Nb rebar was distributed with more fine and dense nano-scale oxide particles, resulting in the improvement of the compactness of surface passive film of 34Nb rebar.

(3) After passivation for 10 d in SCPS with different pH, the surface passive film of 34Nb rebar showed the higher stability and compactness. Firstly, with the decreases of pH, the addition of Nb in 34Nb rebar promoted the formation of more stable Fe oxides, which obtained the excellent R_ct value, E_corr value and E_p value, and the effective capacitance and passivation current density greatly reduced. Secondly, with the decreases of pH, the addition of Nb in 34Nb rebar promoted the formation of more stable Nb oxides, inhibited the degradation of stable Fe oxides, resulting in a significant decrease in carrier density. Finally, with the decreases of pH, the existence of Fe oxides and Nb oxides in 34Nb rebar enhanced the self-healing ability of the passive film and improved the passivation rate of 34Nb rebar, thus improving the corrosion resistance of 34Nb rebar.

Acknowledgements

This work was supported by the National Nature Science Foundation of China [Grant number 52074095]; Supported by Guizhou Provincial Basic Research Program (Natural Science) (Grant number QKHJC-ZK [2023] YB072); Supported by Guizhou Provincial Key Technology R&D Program (Grant number QKHZC [2023] YB404); Supported by Guizhou Provincial Key Technology R&D Program (Grant number QKHZC [2022] YB053).

Author contributions

Z.Y.Z: Conceptualization, Writing - original draft, Writing - review & editing. S.J.G: Methodology, Investigation. J.W: Project administration, Supervision. F.L.W: Conceptualization, Supervision. X.X: Investigation. Z.Y.L: Data curation. H.Y: Conceptualization. C.R.L: Writing - review & editing, Supervision, Conceptualization.

Data availability

All data generated or analyzed during this study are included in this published article.

Declarations

Competing interests

The authors declare no competing interests.

Footnotes

Publisher’s note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

References

1.Zeng, Z. Y. et al. Effect of nb content and thermal deformation on the microstructure and mechanical properties of high-strength anti-seismic rebar. Mater. Sci. Eng. A. 840, 142929 (2022). [Google Scholar]
2.James, A. et al. Rebar corrosion detection, protection, and rehabilitation of reinforced concrete structures in coastal environments: a review. Constr. Build. Mater.224, 1026–1039 (2019). [Google Scholar]
3.Lundgren, K., Robuschi, S. & Zandi, K. Methodology for testing rebar-concrete bond in specimens from decommissioned structures. Int. J. Concr Struct. Mater.13, 38 (2019). [Google Scholar]
4.Tian, Y. et al. Corrosion of steel rebar in concrete induced by chloride ions under natural environments. Constr. Build. Mater.369, 130504 (2023). [Google Scholar]
5.Tang, Y. B., Wang, S. N., Xu, Y. P. & Ni, J. X. Influence of calcium nitrite on the passive films of rebar in simulated concrete pore solution. Anti-corros Method Mater.64, 265–272 (2017). [Google Scholar]
6.Lv, S. L. & Li, K. F. Semiconducting behaviour and corrosion resistance of passive film on corrosion-resistant steel rebars. Materials. 15, 7644 (2022). [DOI] [PMC free article] [PubMed] [Google Scholar]
7.Macdonald, D. D. et al. Corrosion of rebar in concrete. Part I: calculation of the corrosion potential in the passive state. Corros. Sci.177, 109018 (2020). [Google Scholar]
8.Diaz, B., Guitian, B. & Novoa, X. R. The effect of chlorides on the corrosion behaviour of weathered reinforcing bars. Electrochim. Acta. 336, 135737 (2020). [Google Scholar]
9.Poyet, S., Dridi, W., L’Hostis, V. & Meinel, D. Microstructure and diffusion coefficient of an old corrosion product layer and impact on steel rebar corrosion in carbonated concrete. Corros. Sci.125, 48–58 (2017). [Google Scholar]
10.Shao, W., Nie, Y. H. & Liang, F. Y. A novel comprehensive evaluation method for the corrosion initiation life of RC hollow piles in chloride environments. Constr. Build. Mater.249, 118801 (2020). [Google Scholar]
11.Li, K. Q. et al. Influence of partial rust layer on the passivation and chloride-induced corrosion of Q235b steel in the carbonated simulated concrete pore solution. Metals. 12, 1064 (2022). [Google Scholar]
12.Yoon, H. N., Seo, J., Kim, S., Lee, K. H. & Park, S. Hydration of calcium sulfoaluminate cement blended with blast-furnace slag. Construct Build. Mater.268, 121214 (2021). [Google Scholar]
13.Lv, D. Y. et al. Quantitative characterization of passivation process of steel reinforcement in concrete towards durability against anticorrosion based on electrochemical methods. Appl. Sci.14, 1297 (2024). [Google Scholar]
14.Beddoe, R. E., Mullauer, W. & Heinz, D. On leaching mechanisms of major and trace elements from concrete - carbonation, exposure to deicing salt and external sulphates. J. Build. Eng.45, 103435 (2022). [Google Scholar]
15.Zhu, M., Zhang, Q., Yuan, F. Y., Guo, S. Y. & Pan, J. Passivation behavior of 2507 super duplex stainless steel in simulated concrete pore solution. J. Mater. Eng. Perform.29, 3141–3151 (2020). [Google Scholar]
16.Jin, Z. Q., Xiong, C. S. & Zhao, T. J. Passivation and depassivation properties of Cr-Mo alloyed corrosion-resistant steel in simulated concrete pore solution. Cem. Concr Compos.126, 104375 (2022). [Google Scholar]
17.Shi, J. J., Jing, M. & Wang, D. Q. Improved corrosion resistance of a new 6% cr steel in simulated concrete pore solution contaminated by chlorides. Corros. Sci.174, 108851 (2020). [Google Scholar]
18.Ha, H. Y. et al. Effects of Cr on pitting corrosion resistance and passive film properties of austenitic Fe–19Mn–12Al–1.5 C lightweight steel. Corros. Sci.206, 110529 (2022). [Google Scholar]
19.Tian, Y. W. et al. Cr-modified low alloy steel reinforcement embedded in mortar for two years: corrosion result of marine field test. Cem. Concr Compos.97, 190–201 (2019). [Google Scholar]
20.Shi, J. J., Wang, D. Q. & Jing, M. Long-term electrochemical behavior of low-alloy steel in simulated concrete pore solution with chlorides. J. Mater. Civ. Eng.30, 04018042 (2018). [Google Scholar]
21.Geng, Z., Yao, N., Zhou, X. & Shi, J. Understanding the intrinsic effect of fly ash on passivity and chloride-induced corrosion of carbon steel and stainless steel in cement extract solutions. Cem. Concr Compos.143, 105236 (2023). [Google Scholar]
22.Yao, N., Zhou, X., Liu, Y. & Shi, J. Synergistic effect of red mud and fly ash on passivation and corrosion resistance of 304 stainless steel in alkaline concrete pore solutions. Cem. Concr Compos.132, 104637 (2022). [Google Scholar]
23.Cui, L., Gao, X., Hang, M. & Chen, T. Comparative studies on steel corrosion resistance of different inhibitors in chloride environment: the effects of multi-functional protective film. Appl. Sci.13, 4446 (2023). [Google Scholar]
24.Zhang, Z. H. et al. Passivation behavior of Cr-modified rebar in simulated concrete pore solutions with different pH. J. Mater. Res. Technol.26, 246–259 (2023). [Google Scholar]
25.Shi, J. J., Ming, J. & Wu, M. Electrochemical behavior and corrosion products of Cr-modified reinforcing steels in saturated ca(OH)₂ solution with chlorides. Cem. Concr Compos.110, 103587 (2020). [Google Scholar]
26.Zou, G. N. et al. Revealing excellent passivation performance of a novel Cr-alloyed steel rebar in carbonized concrete environment. J. Mater. Res. Technol.23, 1848–1861 (2023). [Google Scholar]
27.Liu, T. et al. Focusing on the corrosion resistance enhancement of HRB400 rebar by Cr addition in the marine environment. Case Stud. Constr. Mater.20, e03236 (2024). [Google Scholar]
28.Wang, Z. et al. Effect of pH on the electrochemical behaviour and passive film composition of 316L stainless steel. Acta Metall. Sin-Engl. 32, 585–598 (2019). [Google Scholar]
29.Wang, L. et al. Electrochemical and XPS analytical investigation of the accelerative effect of bicarbonate/carbonate ions on AISI 304 in alkaline environment. Appl. Surf. Sci.492, 792–807 (2019). [Google Scholar]
30.Ai, Z. Y. et al. Enhanced passivation of alloy corrosion-resistant steel Cr10Mo1 under Carbonation — Passive film formation, the kinetics and mechanism analysis. Cem. Concr Compos.92, 178–187 (2018). [Google Scholar]
31.Ai, Z. Y. et al. Passive behaviour of alloy corrosion-resistant steel Cr10Mo1 in simulating concrete pore solutions with different pH. Appl. Surf. Sci.389, 1126–1136 (2016). [DOI] [PMC free article] [PubMed] [Google Scholar]
32.Liu, G. J., Zhang, P. P., Liu, C. & Zhang, Y. S. Influence of pH on passivation of mild steel in simulated concrete pore solution: Electrochemical, ToF-SIMS and ReaxFF study. Develop Built Environ.19, 100502 (2024). [Google Scholar]
33.Zhang, Z. H. et al. Corrosion resistance behavior of enhanced passivation Cr-modified rebars and their service life prediction based on Monte Carlo simulation. Constr. Build. Mater.438, 137130 (2024). [Google Scholar]
34.Li, H. Z., Zhao, X. & Xu, J. MRI-compatible Nb-60Ta-2Zr alloy for vascular stents: electrochemical corrosion behavior in simulated plasma solution. Mater. Sci. Eng. C. 56, 205–214 (2015). [DOI] [PubMed] [Google Scholar]
35.Li, Y. & Xu, J. Is niobium more corrosion-resistant than commercially pure ti-tanium in fluoride-containing artificial saliva? Electrochim. Acta. 233, 151–166 (2017). [Google Scholar]
36.Wang, W., Mohammadi, F. & Alfantazi, A. Corrosion behaviour of niobium in phosphate buffered saline solutions with different concentrations of bovine serum albumin. Corros. Sci.57, 11–21 (2012). [Google Scholar]
37.Liu, H. Z., Wang, L. J., Sun, J. K., Liu, W. J. & Wang, Z. W. Corrosion resistance of niobium microalloyed HRB500 mild steel rebar in the alkaline concrete pore solution. Int. J. Electrochem. Sci.15, 6596–6604 (2020). [Google Scholar]
38.OrjuelaG, A., Rincón, R. & Olaya, J. J. Corrosion resistance of niobium carbide coatings produced on AISI 1045 steel via thermo-reactive diffusion deposition. Surf. Coat. Technol.259, 667–675 (2014). [Google Scholar]
39.Barroux, A. et al. Combined XPS/TEM study of the chemical composition and structure of the passive film formed on additive manufactured 17-4PH stainless steel. Surf. Interfaces. 22, 100874 (2021). [Google Scholar]
40.Ji, P. F. et al. Effect of Nb addition on the stability and biological corrosion resistance of Ti-Zr alloy passivation films. Corros. Sci.170, 108696 (2020). [Google Scholar]
41.Guan, L., Li, Y., Wang, G., Zhang, Y. K. & Zhang, L. C. pH dependent passivation behavior of niobium in acid fluoride-containing solutions. Electrochim. Acta. 285, 172–184 (2018). [Google Scholar]
42.You, N. Q., Shi, J. J. & Zhang, Y. M. Electrochemical performance of low-alloy steel and low-carbon steel immersed in the simulated pore solutions of alkali-activated slag/steel slag pastes in the presence of chlorides. Corros. Sci.205, 110438 (2022). [Google Scholar]
43.Shi, J. J., Ming, J. & Sun, W. Electrochemical performance of reinforcing steel in alkali-activated slag extract in the presence of chlorides. Corros. Sci.133, 288–299 (2018). [Google Scholar]
44.Mundra, S. S., Criado, M., Bernal, S. A. & Provis, J. L. Chloride-induced corrosion of steel rebars in simulated pore solutions of alkali-activated concretes. Cem. Concr Res.100, 385–397 (2017). [Google Scholar]
45.Sliem, M. H., Radwan, A. B., Mohamed, F. S., Alnuaimi, N. A. & Abdullah, A. M. An efficient green ionic liquid for the corrosion inhibition of reinforcement steel in neutral and alkaline highly saline simulated concrete pore solutions. Sci. Rep.10, 14565 (2020). [DOI] [PMC free article] [PubMed] [Google Scholar]
46.Hang, M. Y., Zhou, X. B., Wang, J. C. & Sun, M. J. Corrosion behavior and mechanism analysis of triethanolamine and imidazoline in the simulated pore solution of concrete. Case Stud. Constr. Mater.18, e01907 (2023). [Google Scholar]
47.Wang, F. J. et al. Inhibition mechanisms of HAIB on Q235 rebar in the simulated concrete pore solution. J. Build. Eng.72, 106565 (2023). [Google Scholar]
48.Volpi, E., Olietti, A., Stefanoni, M. & Trasatti, S. P. Electrochemical characterization of mild steel in alkaline solutions simulating concrete environment. J. Electroanal. Chem.736, 38–46 (2014). [Google Scholar]
49.Cabrini, M. & Lorenzi, S. Cyclic voltammetry evaluation of inhibitors for localized corrosion in alkaline solutions. Electrochim. Acta. 124, 156–164 (2014). [Google Scholar]
50.Montes, F., Frontini, A., Vázquez, M. & Valcarce, M. B. Silicate ions as corrosion inhibitors for carbon steel in chloride-contaminated concrete pore simulating solutions. Corrosion. 76, 1147–1154 (2020). [Google Scholar]
51.Andrade, C. et al. Electrochemical behavior of steel rebars in concrete: influence of environmental factors and cement chemistry. Electrochim. Acta. 46, 3905–3912 (2001). [Google Scholar]
52.Freire, L. et al. Electrochemical and analytical investigation of passive films formed on stainless steels in alkaline media. Cem. Concr Compos.34, 1075–1081 (2012). [Google Scholar]
53.Hamadou, L., Kadri, A. & Benbrahim, N. Characterisation of passive films formed on low carbon steel in borate buffer solution (pH 9.2) by electrochemical impedance spectroscopy. Appl. Surf. Sci.252, 1510–1519 (2005). [Google Scholar]
54.Ming, H. B. et al. Photocatalytic activation of peroxymonosulfate by carbon quantum dots functionalized carbon nitride for efficient degradation of bisphenol A under visible-light irradiation. Chem. Eng. J.424, 130296 (2021). [Google Scholar]
55.Liu, T. Y. et al. Temperature-Dependent Water Oxidation kinetics: implications and insights. ChemRxiv. 10.26434/chemrxiv-2024-j5q6g (2024). [DOI] [PMC free article] [PubMed] [Google Scholar]
56.Macdonald, D. D. The history of the point defect model for the passive state: a brief review of film growth aspects. Electrochim. Acta. 56, 1761–1772 (2011). [Google Scholar]
57.Han, S. X., Zhang, J., Lei, X. W., Yang, R. N. & Wang, N. Insight into the anisotropic electrochemical corrosion behaviors of laser metal deposited Ni-based single crystal superalloy. Corros. Sci.217, 111111 (2023). [Google Scholar]
58.Liu, Q. et al. Influence of Ru-Ni-Nb combined cathode modification on corrosion behavior and passive film characteristics of Ti-6Al-4V Alloy used for oil country tubular goods. Corros. Sci.207, 110569 (2022). [Google Scholar]
59.Zhang, J. B. et al. Picosecond laser remelting of electrodeposited Ni–P coating: parameters optimization and electrochemical corrosion behavior. Surf. Coat. Technol.471, 129877 (2023). [Google Scholar]
60.Xu, H., Sun, D. & Yu, H. Repassivation behavior of 316L stainless steel in borate buffer solution: kinetics analysis of anodic dissolution and film formation. Appl. Surf. Sci.357, 204–213 (2015). [Google Scholar]
61.Mu, Y. et al. Highly selective Electrochemical Baeyer – Villiger Oxidation through Oxygen Atom transfer from Water. J. Am. Chem. Soc.146, 13438–13444 (2024). [DOI] [PubMed] [Google Scholar]
62.Fei, M. C. et al. Highly selective photocatalytic methane coupling by Au-Modified Bi₂WO₆. ACS Catal.14, 1855–1861 (2024). [Google Scholar]
63.Jacob, K. T., Shekhar, C. & Vinay, M. Thermodynamic properties of Niobium Oxides. J. Chem. Eng. Data. 55, 4854–4863 (2010). [Google Scholar]
64.Gaboreau, S. & Vieillard, P. Prediction of Gibbs free energies of formation of minerals of the alunite supergroup. Geochim. Cosmochim. Ac. 68, 3307–3316 (2004). [Google Scholar]
65.Brewer, L. The Thermodynamic properties of the Oxides. California: Lawrence Berkeley National Laboratory, University of California, UCRL-104, 1–46 (1948).

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Data Availability Statement

All data generated or analyzed during this study are included in this published article.

[CR1] 1.Zeng, Z. Y. et al. Effect of nb content and thermal deformation on the microstructure and mechanical properties of high-strength anti-seismic rebar. Mater. Sci. Eng. A. 840, 142929 (2022). [Google Scholar]

[CR2] 2.James, A. et al. Rebar corrosion detection, protection, and rehabilitation of reinforced concrete structures in coastal environments: a review. Constr. Build. Mater.224, 1026–1039 (2019). [Google Scholar]

[CR3] 3.Lundgren, K., Robuschi, S. & Zandi, K. Methodology for testing rebar-concrete bond in specimens from decommissioned structures. Int. J. Concr Struct. Mater.13, 38 (2019). [Google Scholar]

[CR4] 4.Tian, Y. et al. Corrosion of steel rebar in concrete induced by chloride ions under natural environments. Constr. Build. Mater.369, 130504 (2023). [Google Scholar]

[CR5] 5.Tang, Y. B., Wang, S. N., Xu, Y. P. & Ni, J. X. Influence of calcium nitrite on the passive films of rebar in simulated concrete pore solution. Anti-corros Method Mater.64, 265–272 (2017). [Google Scholar]

[CR6] 6.Lv, S. L. & Li, K. F. Semiconducting behaviour and corrosion resistance of passive film on corrosion-resistant steel rebars. Materials. 15, 7644 (2022). [DOI] [PMC free article] [PubMed] [Google Scholar]

[CR7] 7.Macdonald, D. D. et al. Corrosion of rebar in concrete. Part I: calculation of the corrosion potential in the passive state. Corros. Sci.177, 109018 (2020). [Google Scholar]

[CR8] 8.Diaz, B., Guitian, B. & Novoa, X. R. The effect of chlorides on the corrosion behaviour of weathered reinforcing bars. Electrochim. Acta. 336, 135737 (2020). [Google Scholar]

[CR9] 9.Poyet, S., Dridi, W., L’Hostis, V. & Meinel, D. Microstructure and diffusion coefficient of an old corrosion product layer and impact on steel rebar corrosion in carbonated concrete. Corros. Sci.125, 48–58 (2017). [Google Scholar]

[CR10] 10.Shao, W., Nie, Y. H. & Liang, F. Y. A novel comprehensive evaluation method for the corrosion initiation life of RC hollow piles in chloride environments. Constr. Build. Mater.249, 118801 (2020). [Google Scholar]

[CR11] 11.Li, K. Q. et al. Influence of partial rust layer on the passivation and chloride-induced corrosion of Q235b steel in the carbonated simulated concrete pore solution. Metals. 12, 1064 (2022). [Google Scholar]

[CR12] 12.Yoon, H. N., Seo, J., Kim, S., Lee, K. H. & Park, S. Hydration of calcium sulfoaluminate cement blended with blast-furnace slag. Construct Build. Mater.268, 121214 (2021). [Google Scholar]

[CR13] 13.Lv, D. Y. et al. Quantitative characterization of passivation process of steel reinforcement in concrete towards durability against anticorrosion based on electrochemical methods. Appl. Sci.14, 1297 (2024). [Google Scholar]

[CR14] 14.Beddoe, R. E., Mullauer, W. & Heinz, D. On leaching mechanisms of major and trace elements from concrete - carbonation, exposure to deicing salt and external sulphates. J. Build. Eng.45, 103435 (2022). [Google Scholar]

[CR15] 15.Zhu, M., Zhang, Q., Yuan, F. Y., Guo, S. Y. & Pan, J. Passivation behavior of 2507 super duplex stainless steel in simulated concrete pore solution. J. Mater. Eng. Perform.29, 3141–3151 (2020). [Google Scholar]

[CR16] 16.Jin, Z. Q., Xiong, C. S. & Zhao, T. J. Passivation and depassivation properties of Cr-Mo alloyed corrosion-resistant steel in simulated concrete pore solution. Cem. Concr Compos.126, 104375 (2022). [Google Scholar]

[CR17] 17.Shi, J. J., Jing, M. & Wang, D. Q. Improved corrosion resistance of a new 6% cr steel in simulated concrete pore solution contaminated by chlorides. Corros. Sci.174, 108851 (2020). [Google Scholar]

[CR18] 18.Ha, H. Y. et al. Effects of Cr on pitting corrosion resistance and passive film properties of austenitic Fe–19Mn–12Al–1.5 C lightweight steel. Corros. Sci.206, 110529 (2022). [Google Scholar]

[CR19] 19.Tian, Y. W. et al. Cr-modified low alloy steel reinforcement embedded in mortar for two years: corrosion result of marine field test. Cem. Concr Compos.97, 190–201 (2019). [Google Scholar]

[CR20] 20.Shi, J. J., Wang, D. Q. & Jing, M. Long-term electrochemical behavior of low-alloy steel in simulated concrete pore solution with chlorides. J. Mater. Civ. Eng.30, 04018042 (2018). [Google Scholar]

[CR21] 21.Geng, Z., Yao, N., Zhou, X. & Shi, J. Understanding the intrinsic effect of fly ash on passivity and chloride-induced corrosion of carbon steel and stainless steel in cement extract solutions. Cem. Concr Compos.143, 105236 (2023). [Google Scholar]

[CR22] 22.Yao, N., Zhou, X., Liu, Y. & Shi, J. Synergistic effect of red mud and fly ash on passivation and corrosion resistance of 304 stainless steel in alkaline concrete pore solutions. Cem. Concr Compos.132, 104637 (2022). [Google Scholar]

[CR23] 23.Cui, L., Gao, X., Hang, M. & Chen, T. Comparative studies on steel corrosion resistance of different inhibitors in chloride environment: the effects of multi-functional protective film. Appl. Sci.13, 4446 (2023). [Google Scholar]

[CR24] 24.Zhang, Z. H. et al. Passivation behavior of Cr-modified rebar in simulated concrete pore solutions with different pH. J. Mater. Res. Technol.26, 246–259 (2023). [Google Scholar]

[CR25] 25.Shi, J. J., Ming, J. & Wu, M. Electrochemical behavior and corrosion products of Cr-modified reinforcing steels in saturated ca(OH)₂ solution with chlorides. Cem. Concr Compos.110, 103587 (2020). [Google Scholar]

[CR26] 26.Zou, G. N. et al. Revealing excellent passivation performance of a novel Cr-alloyed steel rebar in carbonized concrete environment. J. Mater. Res. Technol.23, 1848–1861 (2023). [Google Scholar]

[CR27] 27.Liu, T. et al. Focusing on the corrosion resistance enhancement of HRB400 rebar by Cr addition in the marine environment. Case Stud. Constr. Mater.20, e03236 (2024). [Google Scholar]

[CR28] 28.Wang, Z. et al. Effect of pH on the electrochemical behaviour and passive film composition of 316L stainless steel. Acta Metall. Sin-Engl. 32, 585–598 (2019). [Google Scholar]

[CR29] 29.Wang, L. et al. Electrochemical and XPS analytical investigation of the accelerative effect of bicarbonate/carbonate ions on AISI 304 in alkaline environment. Appl. Surf. Sci.492, 792–807 (2019). [Google Scholar]

[CR30] 30.Ai, Z. Y. et al. Enhanced passivation of alloy corrosion-resistant steel Cr10Mo1 under Carbonation — Passive film formation, the kinetics and mechanism analysis. Cem. Concr Compos.92, 178–187 (2018). [Google Scholar]

[CR31] 31.Ai, Z. Y. et al. Passive behaviour of alloy corrosion-resistant steel Cr10Mo1 in simulating concrete pore solutions with different pH. Appl. Surf. Sci.389, 1126–1136 (2016). [DOI] [PMC free article] [PubMed] [Google Scholar]

[CR32] 32.Liu, G. J., Zhang, P. P., Liu, C. & Zhang, Y. S. Influence of pH on passivation of mild steel in simulated concrete pore solution: Electrochemical, ToF-SIMS and ReaxFF study. Develop Built Environ.19, 100502 (2024). [Google Scholar]

[CR33] 33.Zhang, Z. H. et al. Corrosion resistance behavior of enhanced passivation Cr-modified rebars and their service life prediction based on Monte Carlo simulation. Constr. Build. Mater.438, 137130 (2024). [Google Scholar]

[CR34] 34.Li, H. Z., Zhao, X. & Xu, J. MRI-compatible Nb-60Ta-2Zr alloy for vascular stents: electrochemical corrosion behavior in simulated plasma solution. Mater. Sci. Eng. C. 56, 205–214 (2015). [DOI] [PubMed] [Google Scholar]

[CR35] 35.Li, Y. & Xu, J. Is niobium more corrosion-resistant than commercially pure ti-tanium in fluoride-containing artificial saliva? Electrochim. Acta. 233, 151–166 (2017). [Google Scholar]

[CR36] 36.Wang, W., Mohammadi, F. & Alfantazi, A. Corrosion behaviour of niobium in phosphate buffered saline solutions with different concentrations of bovine serum albumin. Corros. Sci.57, 11–21 (2012). [Google Scholar]

[CR37] 37.Liu, H. Z., Wang, L. J., Sun, J. K., Liu, W. J. & Wang, Z. W. Corrosion resistance of niobium microalloyed HRB500 mild steel rebar in the alkaline concrete pore solution. Int. J. Electrochem. Sci.15, 6596–6604 (2020). [Google Scholar]

[CR38] 38.OrjuelaG, A., Rincón, R. & Olaya, J. J. Corrosion resistance of niobium carbide coatings produced on AISI 1045 steel via thermo-reactive diffusion deposition. Surf. Coat. Technol.259, 667–675 (2014). [Google Scholar]

[CR39] 39.Barroux, A. et al. Combined XPS/TEM study of the chemical composition and structure of the passive film formed on additive manufactured 17-4PH stainless steel. Surf. Interfaces. 22, 100874 (2021). [Google Scholar]

[CR40] 40.Ji, P. F. et al. Effect of Nb addition on the stability and biological corrosion resistance of Ti-Zr alloy passivation films. Corros. Sci.170, 108696 (2020). [Google Scholar]

[CR41] 41.Guan, L., Li, Y., Wang, G., Zhang, Y. K. & Zhang, L. C. pH dependent passivation behavior of niobium in acid fluoride-containing solutions. Electrochim. Acta. 285, 172–184 (2018). [Google Scholar]

[CR42] 42.You, N. Q., Shi, J. J. & Zhang, Y. M. Electrochemical performance of low-alloy steel and low-carbon steel immersed in the simulated pore solutions of alkali-activated slag/steel slag pastes in the presence of chlorides. Corros. Sci.205, 110438 (2022). [Google Scholar]

[CR43] 43.Shi, J. J., Ming, J. & Sun, W. Electrochemical performance of reinforcing steel in alkali-activated slag extract in the presence of chlorides. Corros. Sci.133, 288–299 (2018). [Google Scholar]

[CR44] 44.Mundra, S. S., Criado, M., Bernal, S. A. & Provis, J. L. Chloride-induced corrosion of steel rebars in simulated pore solutions of alkali-activated concretes. Cem. Concr Res.100, 385–397 (2017). [Google Scholar]

[CR45] 45.Sliem, M. H., Radwan, A. B., Mohamed, F. S., Alnuaimi, N. A. & Abdullah, A. M. An efficient green ionic liquid for the corrosion inhibition of reinforcement steel in neutral and alkaline highly saline simulated concrete pore solutions. Sci. Rep.10, 14565 (2020). [DOI] [PMC free article] [PubMed] [Google Scholar]

[CR46] 46.Hang, M. Y., Zhou, X. B., Wang, J. C. & Sun, M. J. Corrosion behavior and mechanism analysis of triethanolamine and imidazoline in the simulated pore solution of concrete. Case Stud. Constr. Mater.18, e01907 (2023). [Google Scholar]

[CR47] 47.Wang, F. J. et al. Inhibition mechanisms of HAIB on Q235 rebar in the simulated concrete pore solution. J. Build. Eng.72, 106565 (2023). [Google Scholar]

[CR48] 48.Volpi, E., Olietti, A., Stefanoni, M. & Trasatti, S. P. Electrochemical characterization of mild steel in alkaline solutions simulating concrete environment. J. Electroanal. Chem.736, 38–46 (2014). [Google Scholar]

[CR49] 49.Cabrini, M. & Lorenzi, S. Cyclic voltammetry evaluation of inhibitors for localized corrosion in alkaline solutions. Electrochim. Acta. 124, 156–164 (2014). [Google Scholar]

[CR50] 50.Montes, F., Frontini, A., Vázquez, M. & Valcarce, M. B. Silicate ions as corrosion inhibitors for carbon steel in chloride-contaminated concrete pore simulating solutions. Corrosion. 76, 1147–1154 (2020). [Google Scholar]

[CR51] 51.Andrade, C. et al. Electrochemical behavior of steel rebars in concrete: influence of environmental factors and cement chemistry. Electrochim. Acta. 46, 3905–3912 (2001). [Google Scholar]

[CR52] 52.Freire, L. et al. Electrochemical and analytical investigation of passive films formed on stainless steels in alkaline media. Cem. Concr Compos.34, 1075–1081 (2012). [Google Scholar]

[CR53] 53.Hamadou, L., Kadri, A. & Benbrahim, N. Characterisation of passive films formed on low carbon steel in borate buffer solution (pH 9.2) by electrochemical impedance spectroscopy. Appl. Surf. Sci.252, 1510–1519 (2005). [Google Scholar]

[CR54] 54.Ming, H. B. et al. Photocatalytic activation of peroxymonosulfate by carbon quantum dots functionalized carbon nitride for efficient degradation of bisphenol A under visible-light irradiation. Chem. Eng. J.424, 130296 (2021). [Google Scholar]

[CR55] 55.Liu, T. Y. et al. Temperature-Dependent Water Oxidation kinetics: implications and insights. ChemRxiv. 10.26434/chemrxiv-2024-j5q6g (2024). [DOI] [PMC free article] [PubMed] [Google Scholar]

[CR56] 56.Macdonald, D. D. The history of the point defect model for the passive state: a brief review of film growth aspects. Electrochim. Acta. 56, 1761–1772 (2011). [Google Scholar]

[CR57] 57.Han, S. X., Zhang, J., Lei, X. W., Yang, R. N. & Wang, N. Insight into the anisotropic electrochemical corrosion behaviors of laser metal deposited Ni-based single crystal superalloy. Corros. Sci.217, 111111 (2023). [Google Scholar]

[CR58] 58.Liu, Q. et al. Influence of Ru-Ni-Nb combined cathode modification on corrosion behavior and passive film characteristics of Ti-6Al-4V Alloy used for oil country tubular goods. Corros. Sci.207, 110569 (2022). [Google Scholar]

[CR59] 59.Zhang, J. B. et al. Picosecond laser remelting of electrodeposited Ni–P coating: parameters optimization and electrochemical corrosion behavior. Surf. Coat. Technol.471, 129877 (2023). [Google Scholar]

[CR60] 60.Xu, H., Sun, D. & Yu, H. Repassivation behavior of 316L stainless steel in borate buffer solution: kinetics analysis of anodic dissolution and film formation. Appl. Surf. Sci.357, 204–213 (2015). [Google Scholar]

[CR61] 61.Mu, Y. et al. Highly selective Electrochemical Baeyer – Villiger Oxidation through Oxygen Atom transfer from Water. J. Am. Chem. Soc.146, 13438–13444 (2024). [DOI] [PubMed] [Google Scholar]

[CR62] 62.Fei, M. C. et al. Highly selective photocatalytic methane coupling by Au-Modified Bi₂WO₆. ACS Catal.14, 1855–1861 (2024). [Google Scholar]

[CR63] 63.Jacob, K. T., Shekhar, C. & Vinay, M. Thermodynamic properties of Niobium Oxides. J. Chem. Eng. Data. 55, 4854–4863 (2010). [Google Scholar]

[CR64] 64.Gaboreau, S. & Vieillard, P. Prediction of Gibbs free energies of formation of minerals of the alunite supergroup. Geochim. Cosmochim. Ac. 68, 3307–3316 (2004). [Google Scholar]

[CR65] 65.Brewer, L. The Thermodynamic properties of the Oxides. California: Lawrence Berkeley National Laboratory, University of California, UCRL-104, 1–46 (1948).

PERMALINK

Passivation performance of Nb microalloyed rebar in concrete carbonation environments with different pH

Zeyun Zeng

Shangjun Gu

Jie Wang

Fulong Wei

Xiang Xie

Zhiying Li

Hui Yang

Changrong Li

Abstract

Introduction

Experimental procedures

Material preparations

Table 1.

Fig. 1.

Table 2.

Simulated solutions

Table 3.

Electrochemical tests

Characterization methods

Results and discussions

Analysis of passivation reaction peak

Fig. 2.

Semiconductor properties of passive film

Fig. 3.

Corrosion resistance of passive film

Fig. 4.

Fig. 5.

Table 4.

Fig. 6.

Formation process of passive film

Fig. 7.

Fig. 8.

Fig. 9.

Fig. 10.

Surface analysis of passive film

Fig. 11.

Fig. 12.

Table 5.

Fig. 13.

Passivation mechanism

Fig. 14.

Conclusions

Acknowledgements

Author contributions

Data availability

Declarations

Competing interests

Footnotes

References

Associated Data

Data Availability Statement

ACTIONS

PERMALINK

RESOURCES

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