Influence of Nb/V on the corrosion behavior of high-strength anti-seismic rebar in marine environments | npj Materials Degradation

npj Materials Degradation volume 8, Article number: 76 (2024) Cite this article

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In this study, the immersion test, surface analysis, cross-section analysis, quantitative analysis and electrochemical analysis were used to study the influence of Nb/V on the corrosion behavior of high-strength anti-seismic rebar in marine environments. The corrosion results clarified that the addition of Nb/V improved the corrosion resistance of the rebar, thereby reducing the corrosion rate of the rebar and improving the stability of corrosion layers. Firstly, the addition of Nb/V promoted the transformation of unstable Fe oxyhydroxides to stable Fe oxyhydroxides in the surface corrosion layers of the rebar, thus increasing the α/(β + γ) ratio, corrosion potential and total impedance value. Secondly, the addition of Nb/V induced the formation of Nb oxides and V oxides in the surface corrosion layers of the rebar, and the existence of these oxides repaired the surface defects of corrosion layers, thus enhancing the corrosion resistance performance of surface corrosion layers of the rebar.

The micro-alloyed high-strength anti-seismic rebar, also known as hot-rolled ribbed rebar, are widely used in the major projects such as bridge engineering and construction engineering1,2,3. When the rebar is exposed to the marine environment, the aggression of corrosive ions (such as Cl−, SO42−, etc.) will cause the corrosion of the rebar4,5,6. The corrosion of the rebar will reduce the bearing area of the rebar and greatly increase the corrosion rate of the rebar, and the volume expansion of surface corrosion products will lead to the destruction of concrete structures, which directly affects the durability and service life of reinforced concrete structures7,8. Therefore, from the perspective of safety and service life of reinforced concrete structures, the development of high-performance and high-corrosion-resistant rebar has important strategic significance for marine engineering structures and reinforced concrete structures.

As is well known, compared with the corrosion of ordinary carbon rebar, the improvement of corrosion performance of micro-alloyed high-strength anti-seismic rebar depended on two factors, the one was the structure and properties of surface corrosion layers of the rebar, and the other was the effect of trace alloying elements on the corrosion layers9,10,11. As the corrosive ions penetrated the corrosion layers, the structure of corrosion layers was broken, thus the corrosion rate of the rebar gradually increased12,13,14. Previous studies4,14,15 have considered that the corrosion layers on the surface of alloy steel were mainly composed of Fe oxyhydroxides (α-FeOOH (Goethite), β-FeOOH (Akaganeite), γ-FeOOH (Lepidocrocite)), Ferrihydrite (Fe5HO8·H2O), Fe oxides (Fe3O4 (Magnetite) and Fe2O3 (Maghemite)), green rust ([FeII3FeIII(OH)8]+[Cl·H2O]-) in the marine environment, in which, Fe2O3 (Maghemite) mainly included γ-Fe2O3 (maghemite) and α-Fe2O3 (hematite). According to the formation free energy of corrosion products16,17, the stability comparison of corrosion products were as follows: α-FeOOH (ΔG = −755.76 kJ/mol)＞ α-Fe2O3 (ΔG = −742.74 kJ/mol)＞ γ-Fe2O3 (ΔG = −709.52 kJ/mol)＞ γ-FeOOH (ΔG = −704.68 kJ/mol). For the characteristics of corrosion layers, various evaluation methods have been proposed to obtain the corrosion properties of alloy steel, such as content of α-FeOOH, α/γ ratio, α/(β + γ) ratio, electrochemical methods18,19,20. Zhang et al.21, Li et al.22, and Fan et al.23 reported that the corrosion layers of weathering steel were mainly composed of Fe oxyhydroxides and Fe oxides, and the Fe oxyhydroxides/Fe oxides ratio was used to evaluate the corrosion resistance and stability of corrosion layers. Xu et al.24 and Jia et al.25 considered that the α/γ ratio of corrosion layers in nickel-based alloy steel was one of the most effective methods to evaluate the corrosion resistance and stability of corrosion layers. Fan et al.26 pointed out that with the increases of Cl− ion contents, the inner corrosion films of nickel-based weathering steel transformed from α-FeOOH to γ-FeOOH, while the outer corrosion films transformed from γ-FeOOH to β-FeOOH. Li et al.27 pointed out that with the increases of corrosion time, the content of α-FeOOH in HSLA-100 steel gradually increased, which promoted the uniform distribution and densification of corrosion layers.

About the role of microalloying elements in the corrosion layers, previous studies9,10,11,25,26,27 predominantly emphasized that the enrichment of microalloying elements (such as Cr, Cu, Ni, Mo, Co, Nb, Ti, etc.) in the corrosion layers significantly influenced the corrosion resistance of alloy steel, thereby directly affecting the chemical and physical properties of corrosion layers. Firstly, the addition of microalloying elements significantly affected the composition and structure of corrosion layers, which directly promoted the transformation of stable phase α-FeOOH, thus reducing the conductivity of corrosion layers and promoting the stability of corrosion layers25,26,27. Secondly, the addition of microalloying elements promoted the uniform corrosion distribution and inhibited the propagation of pitting pit, thus repairing the defect of corrosion layers28,29,30. Thirdly, the addition of microalloying elements in alloy steel mainly formed the corresponding oxides in the corrosion layers, and these oxides promoted the formation of stable oxyhydroxides, so that the corrosion layers of alloy steel had good corrosion resistance27,28,29,31,32,33. Previous studies21,22,23,24,34,35 also showed that after experiencing the long-term corrosion, the addition of Cr, Cu, Ni gradually enriched in the corrosion layers of alloy steel, which promoted the stability of corrosion layers. Zhou et al.33 and Zhang et al.29 pointed out that after experiencing a long period of dry-wet cycle corrosion test, the Cr element in Cr-Ni-Cu steel was mainly enriched in the inner corrosion layers, the Ni element was enriched in the whole corrosion layers, and the enrichment degree of Cu was small, indicating that the addition of Cr and Ni was beneficial to promote the formation of corrosion layers and improving the corrosion resistance of corrosion layers. Numerous researchers36,37,38 reported that during the long-term corrosion process, the compounds of microalloying elements were effectively enriched in the whole corrosion layers, which was beneficial to promote the transformation of β-FeOOH and γ-FeOOH to the stable α-FeOOH, thus reducing the corrosion potential and corrosion rate of weathering steel.

However, the role of microalloying elements (such as Nb, V, Ti, etc.) on the corrosion behavior of alloy steel has rarely been reported. Li et al.39 studied the effect of Nb on the localized corrosion behavior of hydrogen-containing steel, the results found that the precipitation of nano-NbC was beneficial to inhibit the activation of hydrogen and reduce the generation of hydrogen-induced cracks. Qiao et al.40 studied the effect of Nb on the stress corrosion cracking in low-alloy steel, indicating that the precipitation of nano-NbC inhibited the initiation and propagation of stress corrosion cracks. Barroux et al.41 studied the structure and composition of surface passive film of 17-4PH stainless steel, the results found that the nano-NbC in 17-4PH stainless steel was beneficial to the formation of Nb oxides, thus promoting the stability and compactness of film. Therefore, based on the above research results, the corrosion mechanism of microalloying elements (such as Nb, V, Ti, etc.) in the surface corrosion layers of alloy steel was not clear, thus the effect of microalloying elements on the structure and composition of corrosion layers in low-alloy steel was also the worthy study and discussion.

This study was necessary to discuss and study the roles and existent forms of microalloying elements (such as Nb, V, Ti, etc.) in the corrosion layers of micro-alloyed high-strength anti-seismic rebar. Therefore, the immersion tests, surface analysis, cross-section analysis, quantitative analysis and electrochemical analysis were used to study the effects of microalloying elements (such as Nb, V, Ti, etc.) on the structure and composition of surface corrosion layers of the rebar in the simulated marine environment, and the corrosion mechanism was proposed, which provided a theoretical basis for expanding the application of microalloying elements (such as Nb, V, Ti, etc.) in the corrosion layers and the development of a new type of micro-alloyed corrosion-resistant high-strength anti-seismic rebar.

The changes of corrosion weight loss and corrosion rate of two rebars at different corrosion times were given in Fig. 1.

a Corrosion weight loss of two rebars (Standard deviation of error bar is ± 0.0424). b Corrosion rate of two rebars (Standard deviation of error bar is ± 0.00707).

Figure 1a shows that with the increases of corrosion time, the corrosion weight loss of two rebars gradually increased, and the corrosion weight loss of CS rebar was higher than that of NB rebar, which was more obvious at 20 d – 30 d. The corrosion weight loss curves of two rebars were fitted by the Allometric1 function (\(y=a\times {x}^{b}\)), and the significance level R2 value (R2 ≥ 99) indicated that the fitting value was consistent with the experimental value. During the 5 d – 20 d corrosion test, the corrosion weight loss curves of two rebars increased steeply, indicating that the surface corrosion layers of two rebars were unstable. During the 20 d – 30 d corrosion test, the corrosion weight loss curves of two rebars increased steadily, indicating that the surface corrosion layers of two rebars were stable, which effectively promoted the protective ability of rebar substrate. However, the corrosion weight loss of NB rebar was less than that of CS rebar at 30 d, which meant that the surface corrosion layers of NB rebar were denser than that of CS rebar.

It can be seen from Fig. 1b that with the increases of corrosion time, the corrosion rates of two rebars decreased gradually, which considered that the stable corrosion layers were formed on the surface of two rebars. The corrosion rate curves of two rebars was fitted by the Allometric1 function (\(y=a\times {x}^{b}\)), and the significance level R2 value (R2 ≥ 99) indicated that the fitting value was consistent with the experimental value. During the 5 d – 10 d corrosion test, the corrosion rate curves of two rebars decreased steeply, indicating that the corrosion rates of two rebars was larger. During the 15 d – 30 d corrosion test, the corrosion rate curves of two rebars decreased more smoothly, indicating that the corrosion rates of two rebars were small. Notably, with the increases of corrosion time, the corrosion rate of NB rebar was lower than that of CS rebar, this was attributed to the compactness of corrosion layers of NB rebar.

The XRD results of surface corrosion layers of two rebars at different corrosion times were given in Fig. 2a, b.

a, b XRD images. c, d Quantitative analysis. a, c CS rebar. b, d NB rebar.

Figure 2a and b illustrates that at different corrosion times, the surface corrosion layers of two rebars were mainly composed of goethite (α-FeOOH), akaganeite (β-FeOOH), lepidocrocite (γ-FeOOH), magnetite (Fe3O4) and maghemite (γ-Fe2O3). With the increases of corrosion time, in the surface corrosion layers of two rebars, the peak intensity of goethite, magnetite and maghemite gradually increased, the peak intensity of lepidocrocite gradually weakened, and the peak intensity of akageneite did not change significantly. When the corrosion time reached 30 d, compared with CS rebar, the peak intensity of goethite in the surface corrosion layers of NB rebar significantly enhanced, indicating that the surface corrosion layers of NB rebar were composed of a large amount of goethite. Previous studies37,38,42 reported that the goethite was an important phase, which promoted the compactness of surface corrosion layers of alloy steel and played a protective role in alloy steel. Therefore, after suffering the long-term corrosion, compared with CS rebar, the formation of goethite in the surface corrosion layers of NB rebar can promote the stable and compactness of corrosion layers.

On the basis of the relative strength ratio method (RIR), X ‘pert Highscore Plus was used to perform the quantitative analysis of surface corrosion layers of two rebars (the relative error is about 10%)37,38,42,43,44, and the quantitative analysis results were displayed in Fig. 2c, d. Figure 2c, d shows that with the increases of corrosion time, the content of α-FeOOH in the surface corrosion layers of two rebars increased gradually, and the content of α-FeOOH in the surface corrosion layers of NB rebar was greater than that of CS rebar, which indicated that the surface corrosion layers of NB rebar presented more protective effect than CS rebar. Notably, in the surface corrosion layers of NB rebar, the content of γ-FeOOH increased, the content of β-FeOOH not changed, and the content of Fe3O4/γ-Fe2O3 decreased, which indicated that the addition of microalloying elements (such as Nb, V, Ti, etc.) promoted the formation of γ-FeOOH and the transformation of γ-FeOOH to α-FeOOH. Several studies38,45,46,47,48 have reported that the unstable phase (β-FeOOH and γ-FeOOH) in the surface corrosion layers of alloy steel were more easily converted into the stable phase (α-FeOOH), thus making the stability and protection of surface corrosion layers.

Some researchers37,38,43,44 pointed out that the mass ratio of α-FeOOH/γ-FeOOH (α/γ) or α-FeOOH/(γ-FeOOH+β-FeOOH) (α/(γ + β)) in the surface corrosion layers of alloy steel can describe the stability and protection of corrosion layers. Figure 2c, d shows that with the increases of corrosion time, the mass ratio of α/(γ + β) in the surface corrosion products of two rebars increased gradually, indicating that the surface corrosion products of two rebars presented the strong corrosion resistance. When the corrosion time reached 30 d, the mass ratio of α/(γ + β) in the surface corrosion layers of CS rebar and NB rebar reached 0.52 and 0.95, respectively, indicating that the surface corrosion layers of NB rebar had higher α/(γ + β) than that of CS rebar, which was attributed to the fact that the γ-FeOOH and β-FeOOH in NB rebar were easily converted into α-FeOOH, which was consistent with previous reports37,43,44,45,46.

In fact, the XRD test only analyzed the phase composition of surface corrosion layers of micro-alloyed steel. When the amorphous phase and the crystalline phase were mixed in the corrosion layers, these phase compositions cannot be accurately identified by XRD. Therefore, the phase composition and distribution of inner and outer corrosion layers of two rebars were tested by micro-Raman spectroscopy, and the peak intensity positions of inner and outer corrosion layers of two rebars was determined by Table 5, and the Raman analysis results were displayed in Fig. 3.

a 5 d. b 10 d. c 20 d. d 30 d. (Left (a–d): CS Rebar. Right (a–d): NB Rebar).

It can be seen from Fig. 3a–d that in the inner corrosion layers of CS rebar, with the increases of corrosion time, the peak intensity of α-FeOOH slowly increased, the peak intensity of γ-FeOOH, Fe3O4 and γ-Fe2O3 gradually decreased, while in the outer corrosion layers of CS rebar, the peak intensity of α-FeOOH slowly decreased, the peak intensity of γ-FeOOH, Fe3O4 and γ-Fe2O3 gradually increased (see the left in Fig. 3a–d). In the inner corrosion layers of NB rebar, with the increases of corrosion time, the peak intensity of α-FeOOH gradually increased, the peak intensity of γ-FeOOH greatly decreased, the peak intensity of Fe3O4 and γ-Fe2O3 slowly increased, while in the outer corrosion layers of NB rebar, the peak intensity of α-FeOOH gradually decreased, the peak intensity of γ-FeOOH greatly increased, the peak intensity of Fe3O4 and γ-Fe2O3 slowly decreased (see the right Fig. 3a–d).

Obviously, as can be seen in Fig. 3, the addition of microalloying elements has a great influence on the phase composition and distribution of inner and outer corrosion layers of the rebar. Compared with CS rebar, a large amount of α-FeOOH, Fe3O4 and γ-Fe2O3 were detected in the inner corrosion layers of NB rebar, and a large amount of γ-FeOOH and a small amount of Fe3O4 and γ-Fe2O3 were detected in the outer corrosion layers. The results showed that the inner corrosion layers of NB rebar were mainly composed of a large number of stable Fe hydroxyl oxides and Fe oxides, which promoted the stable and dense of inner corrosion layers, while the outer corrosion layers were mainly composed of unstable Fe hydroxyl oxides, resulting in the unstable existence of outer corrosion layers, which was consistent with the reported research results43,46.

The full spectrum of all elements in the surface corrosion layers of two rebars at different corrosion times were given in Fig. 4. The XPS fine spectrum of Fe2p and O1s in the surface corrosion layers of two rebars at different corrosion times were given in Figs. 5, 6. The XPS fine spectrum of Nb3d, V2p and Ti2p in the surface corrosion layers of NB rebar at different corrosion times were given in Fig. 7. The semi-quantitative analysis of all elements in the surface corrosion layers of two rebars at different corrosion times were given in Fig. 8.

a CS Rebar. b NB Rebar.

a, e 5 d. b, f 10 d. c, g 20 d. d, h 30 d. (a–d Fe2p. e–h O1s).

a, e 5 d. b, f 10 d. c, g 20 d. d, h 30 d. (a–d Fe2p. e–h O1s).

a, e, i 5 d. b, f, j 10 d. c, g, k 20 d. d, h, m 30 d. (a–d Nb3d. e–h V2p. i–m Ti2p).

a CS Rebar (Standard deviation of error bar is ± 0.0354). b NB Rebar (Standard deviation of error bar is ± 0.0354).

Figure 4a, b shows that the all elements of two rebars at different corrosion times were mainly composed of Fe2p and O1s, while the microalloying elements (such as Nb, V, Ti, etc.) of NB rebar were mainly composed of Nb3d, V2p and Ti2p. In both CS rebar or NB rebar, the peak intensity of Fe2p and O1s was the strongest, indicating that the surface corrosion layers of two rebars were mainly composed of various compounds of Fe. The existence forms of microalloying elements in the surface corrosion layers of two rebars will be further discussed in the fine spectrum.

Figure 5a–d and Fig. 6a–d shows that the binding energy peaks of Fe2p in the surface corrosion layers of two rebars at different corrosion times can be divided into Fe0 (Fe metal) (binding energy 717.5 eV and 727.4 eV)47, Fe2+ (FeO) (binding energy 709.8 eV and 725.6 eV)36,47, Fe3+ (Fe2O3 and Fe3O4) (binding energy 710.5 eV and 732.3 eV)24,26,36,46,47, FeOOH (α-FeOOH(large), β-FeOOH, γ-FeOOH) (binding energy 711.9 eV and 723.8 eV)24,26,36,46,47. Notably, the satellite peaks of Fe were detected at 715.65 eV and 720.71 eV in the surface corrosion layers of two rebars, indicating that the Fe2p spectrum of two rebars presented a satellite peak, which contained the complex multiple splitting peaks of FeO36,47.

Figure 5e–h and Fig. 6e–h shows that the binding energy peaks of O1s can be divided into O2− (such as FeO, Fe2O3, Fe3O4, etc.) (binding energy 529.3 eV)24,26,36,47, OH- (such as α-FeOOH, γ-FeOOH, etc.) (binding energy 530.5 eV)24,26,36,47 and H2O (531.1 eV)24,26,36,47. It can be seen from Fig. 8a, b that with the increases of corrosion time, compared with CS rebar, the mass ratio of Fe3+/Fe2+, FeOOH/(Fe2++Fe3+) and O2−/OH− in the surface corrosion layers of NB rebar gradually increased, indicating that the surface corrosion layers of NB rebar were composed of a large number of Fe oxyhydroxides and Fe oxides.

According to the Gibbs free energy formation theory of metal oxide49,50,51, at 298.15 K, the standard Gibbs free energies of NbO, NbO2, Nb2O5 were −391.490 kJ·mol−1, −740.923 kJ·mol−1, −1764.585 kJ·mol−1, respectively, the standard Gibbs free energies of V2O3, VO2, V2O5 were −274.80 kJ·mol−1, −320.10 kJ·mol−1, −351.40 kJ·mol−1, respectively, and the standard Gibbs free energies of TiO and TiO2 were −101.1 kJ·mol−1 and −181.4 kJ·mol-1, respectively, this indicated that the Nb oxides were easier to form than V oxides and Ti oxides, and the higher the valence state of these oxides, the more stable the formation of these oxides, which explained that the Nb oxides, V oxides and Ti oxides were easy to form at room temperature.

Figure 7a–d shows that the binding energy peaks of Nb3d in the surface corrosion layers of NB rebar can be divided into Nb2+ (NbO) (binding energy 204.5 eV)52, Nb4+ (NbO2) (binding energy 205.8 eV)41,52,53 and Nb5+ (Nb2O5) (binding energy 206.5 eV)52,53, respectively, while the Nb5+ can only be detected in the corrosion layers after 30 d. Figure 7e–h shows that the binding energy peaks of V2p in the surface corrosion layers of NB rebar can be divided into V3+ (V2O3) (binding energy 515.1 eV)54, V5+ (V2O5) (binding energy 516.3 eV)54 and V4+ (VO2) (binding energy 517.8 eV)54, respectively. The above results indicated that compared with CS rebar, the oxides of Nb and V existed in the surface corrosion layers of NB rebar55,56,57. Figure 7i–m shows that at different corrosion times, the binding energy peak of Ti2p in the corrosion layers of NB rebar can only detect the impurity peak. When the corrosion time reached 10 d, two kinds of Ti oxides (TiO and TiO2) can be only detected in the binding energy peak of Ti2p, and the corresponding binding energies were 455.5 eV and 458.1 eV, respectively. The peak intensity of these Ti oxides was relatively weak, and the relative content of TiO and TiO2 in the corrosion layers of NB rebar was relatively small. This indicated that a small amount of Ti oxides had little effect on the stability and compactness of the surface corrosion layers of NB rebar.

It can be seen from Fig. 8b that with the increases of corrosion times, the mass ratios of Nb4+ (NbO2)/Nb2+ (NbO) and V5+ (V2O5)/(V3+ (V2O3) + V4+ (VO2)) in the surface corrosion layers of NB rebar gradually increased, resulting in the increases of Fe3+/Fe2+ and FeOOH/(Fe2++Fe3+), indicating that the addition of Nb and V promoted the formation of Fe3O4 and the transformation of Fe3O4 to stable α-FeOOH in the surface corrosion layers, which was beneficial to the compactness and corrosion resistance of surface corrosion layers of NB rebar. Therefore, the above XPS results showed that the addition of Nb and V in NB rebar were contributed to promote the formation of stable corrosion layers, and the evolution mechanism of Nb and V in the surface corrosion layers will be explained in the discussion section.

The EIS results of surface corrosion layers of two rebars at different corrosion times were given in Fig. 9. Figure 9a, d shows that at different corrosion times, the surface corrosion layers of two rebars presented two typical semi-circular resistance capacitance circuits. Firstly, in the 10−2 Hz–101 Hz region, the surface corrosion layers of two rebars produced a large semi-circular capacitance circuit. Secondly, in the 102 Hz–105 Hz region, the interface circuit between the surface corrosion layers of two rebars and the solution was composed of a small charge transfer resistance. The capacitance loop direction of surface corrosion layers of two rebars was consistent at different corrosion times, indicating that the corrosion process of surface corrosion layers of two rebars was mainly controlled by the electrochemical reaction process. With the increases of corrosion time, the radius of the electrochemical capacitance loop of surface corrosion layers of two rebars gradually increased, and the radius of the capacitance loop of surface corrosion layers of NB rebar was larger than that of CS rebar, indicating that the surface corrosion layers of NB rebar was more protective than that of CS rebar.

a, d Nyquist. b, e Bode. c and f Phase angle. (a–c CS Rebar. d–f NB Rebar).

Figure 9b, e shows that in the 10−2 Hz–101 Hz region, with the increases of corrosion time, the impedance value of surface corrosion layers of two rebars gradually increased, and the impedance value of surface corrosion layers of NB rebar was greater than that of CS rebar. When the corrosion time reached 30 d, the impedance values of surface corrosion layers of CS rebar reached 1021.5 Ω·cm2, and the impedance values of surface corrosion layers of NB rebar reached 1032.7 Ω·cm2, this showed that the corrosion resistance of surface corrosion layers of NB rebar was stronger than that of CS rebar. Notably, in the 102 Hz–105 Hz region, with the increases of corrosion time, the impedance values of surface corrosion layers of two rebars were relatively close.

Figure 9c, f shows that in the 10−2 Hz – 101 Hz region, the phase angles of surface corrosion layers of two rebars gradually increased with the increases of corrosion time, and the phase angle of surface corrosion layers of NB rebar was larger than that of CS rebar. Notably, when the phase angles of surface corrosion layers of two rebars reached the maximum value, the phase angle curve showed a certain width of smoothness, which indicated that the surface corrosion layers of two rebars presented a double-layer capacitance loop characteristic. When the corrosion time reached 30 d, the phase angles of surface corrosion layers of NB rebar and CS rebar were 64 degrees and 60 degrees, respectively, which indicated that the surface corrosion layers of NB rebar were more stable than that of CS rebar26,27,29,35,37.

The EIS results indicates that the surface corrosion layers of two rebars exhibited the double-layer capacitance, thus the fitting circuit diagram in Fig. 10 was used to fit the EIS data in Fig. 9.

Fitting circuit diagram.

In Fig. 10, RS represents the resistance of simulated solution, RC represents the resistance of corrosion layers, Rct represents the resistance of charge transfer, CPEC and CPEdl represents the constant phase elements of corrosion layers and charge transfer, respectively, which reflect the capacitance behavior of surface corrosion layers of two rebars42,44,46,47. The CPE is defined as24,27,35,48:

In the Equation, Z0 and n are the admittance value and fitting index of the constant phase element CPE, respectively, the value range of n is 0 – 1, w is the angular frequency, i2 = 1. The fitting index n is used to define CPE. When n = 1, CPE is considered to be an ideal double-layer circuit capacitor; when n = 0, CPE is considered to be a non-ideal double-layer circuit capacitor; when n = 0.5, CPE is considered to be a capacitance circuit with Warburg impedance24,27,35.

Relevant references58,59,60 reported that the Brug formula and the Hsu formula and the Mansfeld formula were used to evaluate the effective capacitance (C) of CPE, and the effective capacitance (C) was obtained by the equation \(C={Q}^{\frac{1}{n}}{R}^{\frac{1-n}{n}}\), where the R value in the Brug formula was the resistance of the simulated solution, and the R value in the Hsu formula and the Mansfeld formula was the film resistance on the surface of alloy. In order to evaluate the effective capacitance (CC) of the film resistance between the simulated solution and the corrosion layer and the effective capacitance (Cdl) of the charge transfer resistance between the corrosion layer and the steel matrix, the formulas of the effective capacitance (CC and Cdl) were defined by Brug formula, Hsu formula and Mansfeld formula as follows61,62,63:

The effective capacitance values (CC and Cdl) in the surface corrosion layers of two rebars were calculated using Eqs. (2) and (3), and the EIS results in Fig. 9 were fitted by the fitting circuit in Fig. 10, and the best fitting value were given in Table 1.

It can be seen from Table 1 that when the Chi-square value (χ2) ≤ 10−3, the EIS fitting data of two rebars were good, finding that the fitting index n of surface corrosion layers of two rebars was the range of 0.80 – 0.90, and considering that the CPE presented the intermediate capacitance loop properties between the ideal capacitor (n = 1) and the warburg impedance capacitor (n = 0.5)27,35,46,47. The effective capacitance of the surface corrosion layers of two rebars showed a double-layer capacitance characteristic, which was consistent with the reported research results61,62,63.

Table 1 also shows that with the increases of corrosion time, the RC and Rct of surface corrosion layers of two rebars gradually increased, and the CC and Cdl gradually decreased. The RC and Rct of surface corrosion layers of NB rebar were higher than that of CS rebar, and the CC and Cdl were lower than that of CS rebar. Previous researchers61,62,63 reported that the charge transfer resistance can predict the electrochemical corrosion behavior. When the corrosion time reached 30 d, compared with CS rebar, the Rct value and Cdl value of surface corrosion layers of NB rebar reached 2077.2 ± 44 Ω·cm2 and (1.87 ± 0.3) × 10−5 F·cm−2, respectively, indicating that the Rct value of surface corrosion layers of NB rebar was the largest and the effective capacitance Cdl value was the lowest, which indicated that the corrosion resistance of NB rebar was the best. These results indicated that the compactness of surface corrosion layers of NB rebar was higher than that of CS rebar, the main reason was that the surface corrosion layers of NB rebar obtained the superiority of RC and Rct, thereby improving the corrosion resistance of NB rebar.

The dynamic potential polarization curves of surface corrosion layers of two rebars were displayed in Fig. 11a, d. The corrosion potential (Ecorr) and corrosion current density (Icorr) were obtained by solving the tangent methods in the linear polarization region of the dynamic potential polarization curves, and the best fitting results were given in Table 2.

a–c CS Rebar. d–f NB Rebar. (Fig. 11b, e are the enlarged drawing of the red dotted lines in Fig. 11a, d, and the green dotted lines in Fig. 11c, f represents the experimental simulated solution).

Figure 11a, d and Table 2 shows that with the increases of corrosion time, the Ecorr value of surface corrosion layers of two rebars gradually presented the positive shifts, the icorr value gradually decreased, and the Ecorr value of surface corrosion layers of NB rebar was more positive than that of CS rebar, indicating that the corrosion resistance of surface corrosion layers of NB rebar gradually enhanced. The change of Ecorr and icorr values was due to the fact that the cathodic and anodic reaction processes of surface corrosion layer of the rebar were mainly affected by the structure, properties and thickness of corrosion layers. Previous studies23,26,45,47 showed that the cathodic reaction process of alloy rebar was mainly controlled by the reduction reaction of active substances (γ-FeOOH, β-FeOOH, etc.) in the corrosion layers, while the anodic reaction process was mainly controlled by the dissolution of rebar substrate. The difficulty degree of cathodic reaction process mainly depended on the compactness degree of corrosion layers and the content of active materials, while the difficulty degree of anodic reaction process mainly depended on the inhibiting effect of corrosion layer thickness on the aggressive ions26,47.

In order to better understand the reaction system of surface corrosion layer of two rebars and the change process of corrosion products, the pourbaix diagram calculated by the Ecorr value of surface corrosion layers of two rebars relative to the SHE electrode were showed in Fig. 11c, f. Figure 11c, f shows that in the experimental simulated solution (green dotted area), the surface corrosion layers of two rebars were a complex reaction process. In the initial corrosion stage, the anodic reaction process on the surface of the rebar was mainly dominated by the dissolution of rebar matrix, while the cathodic reaction process was mainly dominated by the diffusion of dissolved oxygen22,23,26,27,45. Due to the dissolution of Fe formed a large amount of Fe2+, the dissolutions of rebar matrix and the decreases of dissolved oxygen strengthened the anodic dissolution process and the cathodic reaction process, thus a layer of loose and porous initial products (Fe hydroxides and hydroxyl oxides) covered on the surface of the rebar23,26,27. In the later corrosion stage, with the progress of corrosion reaction process, the initial products on the surface of the rebar gradually transformed into the stable products (Fe hydroxyl oxides and Fe oxides), which reduced the dissolution of rebar matrix, thus the anodic dissolution process and cathodic reaction process on the surface of the rebar were inhibited during the whole corrosion process, resulting in the positive shift of Ecorr value and the decrease of icorr value, which greatly improved the stability and corrosion resistance of surface corrosion layers of the rebar22,26,27. Therefore, compared with CS rebar, the Ecorr value of surface corrosion layers of NB rebar presented more positive shifts, and the icorr value greatly decreased, indicating that the density degrees of corrosion layers increased, which inhibited the cathodic and anodic reaction process of surface corrosion layers of NB rebar and enhanced the protective ability of surface corrosion layers of NB rebar.

Relevant research22,23,26,27 reported that the corrosion current density can predict the corrosion tendency of micro-alloyed steel, and the corrosion rates of two rebars were calculated by Faraday’s law formula, which evaluated the corrosion tendency of two rebars. The Faraday’s law formula was defined as follows:

Where, CR is the corrosion rate (mm/y); n is the valence of iron ions; F is the Faraday constant (96485 C/mol); K is a constant (3.27 × 10−3 for mm/y unit); icorr is the corrosion current density (A·cm-2); EW is the equivalent weight (g/eq). The corrosion rates of two rebars calculated by Faraday’s law in Table 2 were very close to the experimental value of corrosion rate in Fig. 1b, and the error was 0.015% – 0.045%, which showed that the corrosion rate calculated by Faraday’s law was in good agreement with the experimental value. Table 2 shows that compared with CS rebar, with the increases of corrosion time, the decrease of corrosion current density in the surface corrosion layers of NB rebar led to a significant decrease of corrosion rate, indicating that the corrosion tendency of NB rebar was small, which was consistent with the experimental value in Fig. 1b.

The surface morphology of CS rebar and NB rebar at different corrosion times were depicted in Figs. 12, 13, respectively.

a–c 5 d. d–f 10 d. g–i 20 d. j–m 30 d. ((b, e, h, k) is the enlarged drawing of the white box in (a, d, g, j). (c, f, i, m) is the enlarged drawing of the white box in (b, e, h, k)).

a–c 5 d. d–f 10 d. g–i 20 d. j–m 30 d. ((b, e, h, k) is the enlarged drawing of the white box in (a, d, g, j). (c, f, i, m) is the enlarged drawing of the white box in (b, e, h, k)).

It can be seen from Fig. 12a, d, g, j that after experiencing the different corrosion times, the overall morphology of surface corrosion layers of CS rebar presented the unevenness and inhomogeneity, and the surface corrosion layers existed the large holes and some cracks. With the increases of corrosion time, the large holes and apparent cracks of surface corrosion layers of CS rebar gradually decreased. Figure 12b, c, e, f shows that when the corrosion times reached 5 d – 10 d, the surface corrosion layers of CS rebar were mainly presented the fish scale-like structure and a small amount of branch rod-like structure (see Fig. 12c–f), the fish scale-like structure was γ-FeOOH22,27,29,38,44, and the branch rod-like structure was β-FeOOH27,29,44. Figure 12h, i, k, m shows that when the corrosion times reached 20 d – 30 d, the surface corrosion layers of CS rebar were mainly presented the fish scale-like structure, branch rod-like structure and a small amount of needle-like structure (see Fig. 12i–m), the needle-like structure was α-FeOOH27,29,38,44,45.

Fig. 13a, d, g, j indicates that after experiencing the different corrosion times, the overall morphology of surface corrosion layers of NB rebar presented the smooth and uniform distribution, and the surface corrosion layers existed the small number of holes and cracks. As the corrosion time increased, the cracks and pores of surface corrosion layers of NB rebar greatly reduced, and the surface corrosion layers presented the compactness and stable. Figure 13b, c, e, f shows that when the corrosion times reached 5 d – 10 d, the surface corrosion layers of NB rebar were mainly composed of a large numbers of fish scale-like structure and branch rod-like structure and a small amount of needle-like structure (see Fig. 13c, f), indicating that the surface corrosion layers of NB rebar were unstable at the initial stage of corrosion. Figure 13h, i, k, m shows that when the corrosion times reached 20 d – 30 d, the surface corrosion layers of NB rebar presented the substantial needle-like structure, a small amounts of fish scale-like structure and branch rod-like structure (see Fig. 13i, m), indicating that the surface corrosion layers of NB rebar were more stable at the later stage of corrosion. The results of XRD and Raman shows that due to the weak detection intensity of Fe3O4 and γ-Fe2O3 in the surface corrosion layers of two rebars, the morphology of two corrosion products were difficult to be detected by SEM. The above results considered that compared with CS rebar, the loose and porous corrosion layers of NB rebar after the corrosion times 30 d greatly reduced, and the notable enrichment of α-FeOOH was beneficial to increase the density and uniformity of corrosion structure, thus improving the stability and protection of corrosion layers. Therefore, compared with CS rebar, the addition of Nb and V can promote the compactness of surface corrosion layers of NB rebar and reduce the holes and cracks of surface corrosion layers, thus improving the corrosion resistance and decreasing the corrosion rate of NB rebar.

The distribution of all elements on the cross-section corrosion layers of CS rebar and NB rebar at different corrosion times were described in Figs. 14, 15, respectively.

a 5 d. b 10 d. c 20 d. d 30 d.

a 5 d. b 10 d. c 20 d. d 30 d.

It can be seen from Fig. 14 that during the corrosion times 5 d – 10 d, Fe and O were distributed in the whole cross-section corrosion layers of CS rebar, Mn and Si was not distributed in the whole cross-section corrosion layers of CS rebar (see Fig. 14a, b), and the thickness of whole corrosion layers increased from 110 μm to 136 μm, the thickness of inner corrosion layers increased from 46 μm to 68 μm, indicating that the whole cross-section corrosion layers of CS rebar was mainly composed of Fe oxides and Fe oxyhydroxides. During the corrosion times 20 d – 30 d, Fe and O were also distributed in the whole cross-section corrosion layers of CS rebar, Mn and Si were less enriched in the inner cross-section corrosion layers of CS rebar (see Fig. 14c, d), and the thickness of whole corrosion layers increased from 157 μm to 166 μm, the thickness of inner corrosion layers increased from 96 μm to 109 μm, indicating that the compounds of Mn and Si were not involved in the densification of inner cross-section corrosion layers, while the outer cross-section corrosion layers was formed by a large number of loose and porous FeOOH, thereby the protective ability and corrosion resistance of corrosion layers was poor.

It can be seen from Fig. 15 that during the corrosion times 5 d – 10 d, Nb and V were gradually enriched in the inner cross-section corrosion layers of NB rebar, and Nb and V were less enriched in the outer cross-section corrosion layers of NB rebar (see Fig. 15a, b), and the thickness of whole corrosion layers increased from 121 μm to 150 μm, the thickness of inner corrosion layers increased from 71 μm to 89 μm. During the corrosion times 20 d – 30 d, Nb and V were obviously enriched in the inner cross-section corrosion layers of NB rebar (see Fig. 15c, d), Mn and Si were not distributed in the whole corrosion layers, and the thickness of whole corrosion layers increased from 168 μm to 186 μm, the thickness of inner corrosion layers increased from 104 μm to 125 μm, indicating that the compounds of Nb and V in the surface corrosion layers of NB rebar were effective participated in the formation of density and stability of inner corrosion layers, and the thickness of corrosion layers gradually increased, thus hindering the contact between the corrosive ions (such as Cl−, SO42−, etc.) and the substrate, and promoting the formation of more protective ability of corrosion layers. Since the Ti content in NB rebar was only 0.015 wt.%, combined with XPS results, the formation of Ti oxides was more difficult than that of Nb oxides and V oxides at 298.15 K. At the initial stage of corrosion, the Ti element was not significantly enriched in the whole corrosion layers of NB rebar. When the corrosion time reached 30 d, a small amount of Ti element was enriched in the inner corrosion layers of NB rebar. This showed that the Ti element was less distributed in the surface corrosion layers of NB rebar, and a small amount of Ti oxides was formed in the surface corrosion layers of NB rebar, thus indicating that the addition of a small amount of Ti had a weak effect on the stability and compactness of surface corrosion layers of NB rebar, which was consistent with the XPS results.

Figure 15a–d shows that compare with the CS rebar, the addition of Nb and V promoted the stability change of surface corrosion layers of NB rebar mainly reflected two points. Firstly, with the increases of corrosion time, Nb and V obviously distributed in the corrosion layers of NB rebar, which repaired the microcracks and holes of corrosion layers. Secondly, with the increases of corrosion time, Nb and V formed the corresponding oxides, and the formation of these oxides not only can promote the transformation of unstable Fe hydroxy oxides into stable Fe hydroxy oxides, but also can induce the formation of stable Fe oxides, thus more stable corrosion products formed on the surface of NB rebar, which played a role in inhibiting corrosive ions and protecting the rebar matrix. Some studies9,10,11,25,27,46,47 also showed that in the marine environment, the enrichment of microalloying elements (such as Cu, Cr, Ni, etc.) in the corrosion layers of micro-alloyed steel promoted the formation of protective α-FeOOH, which was conducive to strengthen the protective ability of steel matrix. The above results confirmed that the addition of Nb and V can be enriched in the surface corrosion layers of NB rebar, and the formation of Nb oxides and V oxides repaired the surface defects of corrosion layers and promoted the formation of stable Fe oxyhydroxides, thus enhancing the formation of more stable corrosion layers on the surface of NB rebar, which was consistent with the experimental results of XRD, Raman, XPS, etc.

In this study, the simulated marine environment was a corrosive environment with high concentration of Cl−, SO42−, as well as room temperature and high humidity. The micro-alloyed high-strength anti-seismic rebar permanently worked in the tropical marine environment, and the short-term corrosion of the rebar was inevitable. Thus, the stable corrosion layers will be produced on the surface of the rebar, which will further hinder the corrosion electrochemical reaction between the corrosive ions (such as Cl−, SO42−, etc.) and the rebar12,13,14,64,65.

The properties of corrosion layers have an important influence on the density and stability of surface corrosion layers of the rebar. The compactness of surface corrosion layers of the rebar depended on the structure and composition of corrosion layers. In terms of spatial structure27,66,67, the α-FeOOH and γ-FeOOH belonged the orthogonal crystal structure, the β-FeOOH belonged the tetragonal space structure, the Fe3O4 belonged the inverse spinel face-centered cubic structure, and the Fe2O3 belonged the α-type unit cell structure. From the thermodynamic view27,46,47,66,67, the α-FeOOH was a thermodynamically stable hydroxyl iron, which was stable in the marine environment, the β-FeOOH was a thermodynamically unstable hydroxyl iron, which was easily oxidized and converted into more stable α-FeOOH in the marine environment, and the γ-FeOOH was slightly more thermodynamically stable than β-FeOOH, which was prone to occur the reduction reaction in the marine environment. Previous studies15,22,23,26,31 also considered that in the containing Cl− ions marine environment, the γ-FeOOH and β-FeOOH on the surface corrosion layers of steel presented the unstable structure, which was conducive to becoming a channel for Cl− ions to penetrate the corrosion layers, resulting in the formation of tiny cracks in the corrosion layers and accelerating the corrosion of steel.

In this study, a series of corrosion experimental results found that the surface corrosion layers of two rebars were mainly composed of Fe oxyhydroxides (α-FeOOH, β-FeOOH, γ-FeOOH, etc.) and Fe oxides (Fe3O4, γ-Fe2O3, etc.). With the increases of corrosion time, compared with CS rebar, the surface of NB rebar gradually formed the more stable corrosion layers, which farther increased the protective ability of corrosion layers. Zhang et al.21,29,45,48 and Fan et al.23,26 reported that the surface corrosion layers of CS steel in the marine atmosphere mainly presented the combination of β-FeOOH and γ-FeOOH, and the surface corrosion layers of WS steel mainly presented the substantial formation of α-FeOOH. Previous studies23,24,37,38,43,44 considered that the quantitative analysis of α/γ and α/(β + γ) was the main index of corrosion resistance on the surface corrosion layers of the rebar. In this study, the results of α/(β + γ) showed that the surface corrosion layers of NB rebar presented the more stable corrosion layers than that of CS rebar, which effectively increased the corrosion resistance and electrochemical performance of NB rebar. Wu et al.43 and Fan et al.23,26 considered that compared with CS steel, the inner corrosion layers of WS steel mainly presented the combination of α-FeOOH and γ-FeOOH, and the outer corrosion layers mainly presented the combination of β-FeOOH and γ-FeOOH. In this study, compared with CS rebar, the inner corrosion layers of NB rebar mainly presented the combination of stable α-FeOOH (large) and Fe oxides (γ-Fe2O3 and Fe3O4), the outer corrosion layers mainly presented the combination of stable α-FeOOH (little), β-FeOOH, γ-FeOOH and γ-Fe2O3 (large), which indicated that the inner corrosion layers of NB rebar was denser and considered that the NB rebar had better corrosion resistance than CS rebar, which was consistent with the results of XRD, Raman, EPMA, etc.

The addition of Nb and V have an important influence on the evolution of surface corrosion layers of two rebars, and the evolution mechanism diagram of surface corrosion layers of two rebars in the simulated marine environment were given in Fig. 16.

a–d CS Rebar. e–h NB Rebar.

In the initial stage of corrosion, due to the accumulation and invasion of a large amount of water vapor on the surface of two rebars, the oxygen and chloride ions contacted with the substrate through the water vapor film and occurred the corrosion electrochemical reaction. Fe was oxidized to Fe2+ in the anode area, and the oxygen absorption reaction occurred in the cathode area22,24,26,33,36,38,42,68,69,70. These reactions were given in Eqs. (5) and (6).

With the progress of corrosion electrochemical reaction, the Fe2+ in the anode area further moved to the cathode area, while OH- and Cl− in the cathode area moved to the anode area, thus forming the Fe(OH)2 and FeCl222,26,28,29,33,36,46 in the anode area, and the Fe(OH)2 was further hydrolyzed into Fe(OH)322,26,28,29,33,36,46, and these reactions were given in Eqs. (7), (8), (9).

During the corrosion electrochemical reaction process, the Fe hydroxides (Fe(OH)2 and Fe(OH)3) were further oxidized to form an intermediate unstable products (the mechanical mixtures of Fe2+ and Fe3+), which was further oxidized to form a loose, porous and unstable Fe hydroxyloxides (β-FeOOH and γ-FeOOH)26,29,33,36,38,42,46 (see Fig. 16a, e). These reactions were given in Eqs. (10) and (11).

In the initial corrosion stage of two rebars, the formed cathode and anode areas were unevenly distributed on the surface of the rebar, and the formation of surface corrosion layers were loose and porous, thus the corrosion rates of two rebars gradually increased (see Fig. 1b). As the corrosion time increased, the Cl− ions and O2 penetrated the corrosion layers, resulting in the higher electrochemical activity of surface corrosion layers, forming more and more anode and cathode areas. Meanwhile, the surface of corrosion layers appeared the inner and outer corrosion layers, the unstable Fe oxyhydroxides (β-FeOOH and γ-FeOOH) and some Fe hydroxides (Fe(OH)3) were oxidized to form the stable Fe oxyhydroxides (α-FeOOH) (see Eqs. (12) and (13)), these stable Fe oxyhydroxides were distributed in the corrosion layers24,33,36,38,42,46, while some unstable Fe oxyhydroxides (β-FeOOH and γ-FeOOH) were reduced to Fe3O4 (see Eq. (14)), and Fe3O4 was further transformed into Fe2O3 (see Eq. (15)), thus the corrosion layers presented more and more stable and compact26,29,33,36,38,42,46 (see Fig. 16b and f).

Previous studies24,29,33,36,38,42,46,66,67 showed that the unstable Fe oxyhydroxides (β-FeOOH and γ-FeOOH) in the corrosion layers was the effective cathode phase, and the unstable Fe oxyhydroxides largely participated in the cathodic reduction process. However, the cathodic reduction process of unstable Fe oxyhydroxides (β-FeOOH and γ-FeOOH) in the corrosion layers was mainly controlled by the electron transfer, and when the corrosion layers were a wetting state, the Fe2+ participated in the reduction of unstable Fe oxyhydroxides (β-FeOOH and γ-FeOOH) in the corrosion cathode area to form Fe3O424,29,36,38,42,46. These reactions were given in Eq. (16).

During the long-term corrosion process, the anode and cathode areas of surface corrosion layers closely distributed, and the dense and stable corrosion layers gradually presented on the surface of two rebars (see Fig. 16c, g). With the increases of corrosion time, compared with CS rebar, the density of surface corrosion layers of NB rebar gradually increased, the permeability of Cl− ions and O2 greatly decreased, and the corrosion resistance and stability of NB rebar gradually increased26,33,36,46, which confirmed that the addition of Nb and V participated the densification process of surface corrosion layers of NB rebar.

According to the Gibbs free energy formation theory of metal oxide49,50,51, the Nb oxides and V oxides can be easily formed at 298.15 K, and the higher the valence state of oxides, the more stable the formation of oxides. The interesting experimental results of XPS and EPMA found that when the corrosion time reached 30 d, compared with CS rebar, the addition of Nb and V in NB rebar were strongly enriched in the inner corrosion layers and formed the Nb oxides (NbO, NbO2, Nb2O5) (see Eqs. (17)–(19)) and V oxides (VO2, V2O3, V2O5) (see Eqs. (20)–(22)), and seeing Fig. 16d, h.

Nam et al.52 also reported that in corrosive environment, with the increases of Nb content, the resistance of surface corrosion products of low-alloy steel increased, indicating that the low-alloy steel showed the higher corrosion resistance, which was attributed to form the protective Nb oxide layers (Nb2O5) on the surface of low-alloy steel, thus inhibiting the dissolution of steel matrix. Previous studies55,56,57 also showed that in corrosive environment, the addition of Nb will form a layer of Nb oxides on the surface of alloy steel, mainly including NbO, NbO2, Nb2O5, and the NbO and NbO2 were oxidized to more stable Nb oxides (Nb2O5), which enhanced the corrosion resistance of alloy steel. Chukwuike et al.53 studied the corrosion mechanism of Nb oxides in each corrosive medium, which found that the stable Nb2O5 film was mainly formed in each corrosive medium, and with the increases of Nb content, the Nb2O5 film thickened to protect the matrix. Surnev et al.54 studied the formation mechanism of V oxides in different fields of corrosion electrochemistry, and the results found that the addition of V mainly formed three oxides, namely VO2, V2O3 and V2O5, in which the V2O5 was the most stable and protective oxides.

Interestingly, the existence of Nb oxides and V oxides improved the cation selectivity of corrosion layers and accelerated the transformation of unstable Fe oxyhydroxides (β-FeOOH and γ-FeOOH) to stable Fe oxyhydroxides (α-FeOOH) (see Eqs. (23)–(26)), thus repairing the defects (such as microcracks, pores, etc.) of corrosion layers, enhancing the impedance of corrosion layers, improving the corrosion resistance and stability of corrosion layers, and reducing the corrosion rates of NB rebar.

Therefore, the addition of Nb and V have a special role in the surface corrosion layers of NB rebar, and the key points of this study can be summarized as follows. Firstly, the addition of Nb and V in the simulated marine environment was beneficial to improve the stability of surface corrosion layers of micro-alloyed high-strength anti-seismic rebar, resulting in a significant reduction in the corrosion rate of the rebar and an increase in the corrosion resistance of surface corrosion layers of the rebar. Secondly, the enrichment of Nb and V in the inner corrosion layers of micro-alloyed high-strength anti-seismic rebar promoted the formation of a dense structure within these layers, which inhibited the anodic dissolution and induced the formation of Nb oxides and V oxides, and the existence of these oxides repaired the surface defects of inner corrosion layers, thereby enhancing the compactness and protection of inner corrosion layers. Thirdly, the addition of Nb and V facilitated the transformation of unstable phase Fe oxyhydroxides to stable phase Fe oxyhydroxides, leading to increases of α/(β + γ) ratio, corrosion potential and total impedance.

During the long-term corrosion process, the addition of Nb and V have a strong enrichment effect in the surface corrosion layers of micro-alloyed high-strength anti-seismic rebar, which greatly improves the corrosion resistance of the rebar. This work may provide a theoretical reference for the addition and application of microalloying elements (such as Nb, V, etc.) in the corrosion field of micro-alloyed high-strength anti-seismic rebar.

The raw materials in this study were ordinary carbon steel rebar (Named the CS rebar), and the micro-alloyed high-strength anti-seismic rebar (Named the NB rebar) were prepared on the basis of ordinary carbon steel rebar. According to the new national standard GBT 1499.2-2018, the chemical composition control of microalloying elements (such as Nb, V, Ti, etc.) in NB rebar were designed, and the NB rebar was melted by vacuum induction furnace, and the alloy materials were mainly composed of ferroniobium alloy, ferrovanadium alloy, ferrotitanium alloy. Finally, the CS rebar and NB rebar were forged into a diameter (100 mm) × height (50 mm) steel ingots. The chemical composition of CS rebar and NB rebar were tested by the inductively coupled plasma emission spectroscopy (ICP-OES/MS), carbon sulfur analyzer (CS844), nitrogen hydrogen oxygen analyzer (ONH836), and the chemical composition results of two rebars were given in Table 3.

The microstructure and mechanical properties of CS rebar and NB rebar were given in Table 4 and Fig. 17.

a CS rebar. b NB rebar. (Yellow box represents the enlarged drawing of the pearlite).

Table 4 and Fig. 17 illustrates that the microstructures of two rebars were mainly composed of ferrite and pearlite, and the ferrite was polygonal and the pearlite was lamellar. Compared with CS rebar, the ferrite grain size (dF) and pearlite lamellar spacing (S0) of NB rebar refined to 10.15 μm and 0.142 μm, respectively, thus the tensile strength and yield strength of NB rebar reached 678 MPa and 541 MPa, respectively, and the elongation after fracture reached 28.08%, this showed that the addition of microalloying elements played a role in refining the grain and improving the mechanical properties of NB rebar.

The samples of two rebars were prepared with dimensions of length (10 mm) × width (10 mm) × thickness (5 mm). The samples of two rebars were coarsely abraded and finely polished using different types of metallographic sandpaper (100# – 3000#), and these samples were sealed completely using the epoxy resin (excepting for the observation surface of the samples). Finally, the samples surface of two rebars were polished using the metallographic sample polishing machine (PA-I1) and diamond polishing agent (the particle size was 0.5 μm, 1 μm, 2 μm), and making the surface smooth and flat of the samples. The electrochemical samples were welded together with the nickel sheet using a spot welder (GLITTER801D) to make the nickel sheet act as a connecting line for the electrode, and then the electrochemical samples were sealed completely using the epoxy resin (excepting for the observation surface of the samples). The sealed all samples were rinsed and vibrated in ultrasonic + ethanol solution, and the surface of all samples was dried and sealed using the silica gel. Finally, the dried samples were placed in a vacuum drying oven for immersion tests and characterization tests.

The plateau humid marine environment in southwest China has the following characteristics. Firstly, the average annual humidity in this area reached 66.44%. Secondly, the average annual temperature in this area reached 22 °C. Thirdly, the seawater in this area contained a large number of aggressive ions (such as Cl−, SO42−, etc.), and the Cl− ions was the most important component. In this study, the composition of 3.5 wt.% NaCl + 0.1 wt.% CaCl2 + 0.05 wt.% Na2SO4 + 0.05 wt.% CaSO4 was used to simulate the plateau humid marine environment in Southwest China22,24,25, and the pH value was 6.8. The simulated solution was prepared by the NaCl analysis pure powder, CaCl2 analysis pure powder, Na2SO4 analysis pure powder, CaSO4 analysis pure powder, and deionized water (0.3 us/cm), and then the simulated solution uniformly stirred with a glass rod for 24 h. Finally, the pH value of the simulated solution was measured using a PHS-3E pH meter and the solution was sealed with a preservative film.

The corrosion tests of all rebar samples were carried out in the immersion tests. The testing time was set to 120 h (Named the 5 d), 240 h (Named the 10 d), 480 h (Named the 20 d) and 720 h (Named the 30 d), respectively. After completing the corrosion tests, the samples of two rebars were taken out and dried naturally in the drying chamber for the subsequent characterization. Three parallel samples were prepared for each test to ensure the accuracy of corrosion process of two rebars under the condition of each test time.

Before the samples were corroded, the weight of two rebar samples was weighed and recorded using a balance (accuracy of 0.001 g). After completing the corrosion of two rebar samples at different times in the immersion tests, according to the ASTM G1 standard, the surface corrosion products of the corroded samples were removed in a rust removal solution of 500 ml HCl + 3.5 g hexamethylene tetramine + 500 ml distilled water. When the surface corrosion products of the samples were removed, the surface of the samples was cleaned and vibrated with ultrasonic and ethanol solution. Finally, the surface of the samples was washed and dried with alcohol, and the weight of the samples was measured using the balance and recorded. In order to ensure the accuracy of the experimental results, the three parallel samples were tested for the samples that tested the corrosion rate. The corrosion weight loss and corrosion rate of two rebar samples were calculated using the Eqs. (27) and (28)71,72,73.

In the Equation, mg (g/cm2) is the mass loss of surface per unit area of the rebar; w0 (g) is the surface mass of the rebar before corrosion; w1 (g) is the surface mass of the rebar after corrosion; S (cm2) is the corrosion surface area of the rebar; vcorr (mm/y) is the corrosion rate of the rebar; △W (W0-W1) (g) is the weight loss of the rebar; ρ (7.85 g/cm3) is the density of the rebar; t (h) is the corrosion time.

The electrochemical properties of two rebars were tested after completing the natural corrosion for 5 d, 10 d, 20 d and 30 d in simulated solution (not inflation). The electrochemical tests were performed on a CHI760E electrochemical workstation using a three-electrode system, in which the saturated calomel electrode (SCE) was the reference electrode, the platinum sheet was the auxiliary electrode, and the two rebar samples were the working electrodes. Three parallel samples were tested at each time to ensure the stability of the experimental results.

The electrochemical experiments were conducted at room temperature. The test solution was the simulated solution of marine environment. The open circuit potential (OCP) was tested for 30 min and the final OCP data were recorded. The voltage range of the polarization curve was −0.8 V – 0.6 V (vs. SCE), and the scanning rate was 0.5 mV/s. The electrochemical impedance spectroscopy (EIS) was performed on the open circuit potential, the amplitude of AC potential was 10 mV, and the frequency range was 10−2 Hz – 105 Hz. The EIS data were fitted by ZView software.

The corrosion morphology of two rebars at different corrosion times were characterized by field emission scanning electron microscopy (SEM, TESCAN MIRA LMS). The accelerating voltage was 200 eV – 30 keV, the resolution was 1.2 nm, and the probe current was 3 PA – 20 nA.

The corrosion products of two rebars were scraped off by the blade and ground into the powder. The X-ray diffraction (XRD, Bruker AXS D8 Advance) were used to analyze the powder phase composition of surface corrosion products of two rebars. The target was Cu target, the tube current was 40 mA, the tube voltage was 40 kV, the scanning rate was 50/min, the 2θ range was 100–850. The XRD datas were processed by HighScore Plus software.

The phase composition and morphology of cross-section corrosion layers of two rebars were characterized by the micro laser Raman spectrometer (Raman, Thermo DXR). The laser emission source was 532 nm, the Raman shift range was 0 – 1800 cm−1, and the acquisition time was 30 s. The surface corrosion layers of two rebars were tested at three parallel points of each time. The Raman shift peak of surface corrosion layers of two rebars was calibrated according to the literatures, and the Raman shift peak of corrosion layers were given in Table 5.

The existent forms of microalloying elements (such as Nb, Ti, V, etc.) in the corrosion layers of two rebars at different corrosion times were characterized by X-ray photoelectron spectroscopy (XPS, Thermo Scientific K-Alpha), and the distribution of Fe, O, Nb, V, Ti in the corrosion layers of the rebar at different corrosion times was determined. The excitation source was Al Kα ray (hv = 1486.6 eV), the operating voltage was 12 kV, and the filament current was 6 mA. The binding energy correction of each element took the binding energy of C1s = 284.80 eV as the energy standard to avoid the interference of other charges. The peaks of each element were fitted by Avantage software.

The cross-section element composition of surface corrosion layers of two rebars at different corrosion times were characterized by electron probe microanalyzer (EPMA, JXA-8530F PLUS), which determined the distribution of elements (such as Fe, O, Nb, Ti, V, etc.) in the surface corrosion layers of two rebars.

Relevant data supporting this study can be obtained from corresponding author according to reasonable requirements.

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This work was supported by the National Natural Science Foundation of China [Grant No. 52074095]; Supported by Guizhou Provincial Basic Research Program (Natural Science) (Grant No. QKHJC-ZK [2023] YB072); Supported by Guizhou Provincial Key Technology R&D Program (Grant No. QKHZC [2023] YB404); Supported by Guizhou Provincial Key Technology R&D Program (Grant No. QKHZC [2022] YB053).

College of Materials and Metallurgy, Guizhou University, Guiyang, 550025, Guizhou, China

Zeyun Zeng, Zhiying Li, Hui Yang & Changrong Li

Guizhou Provincial Key Laboratory of Metallurgical Engineering and Process Energy Saving, Guiyang, 550025, Guizhou, China

Zeyun Zeng, Zhiying Li, Hui Yang & Changrong Li

Shougang Shuicheng Iron and Steel (Group) Co., Ltd., Liupanshui, 553000, Guizhou, China

Shangjun Gu, Jie Wang & Fulong Wei

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Zeyun Zeng and Shangjun Gu conceived and planned the experiments. Jie Wang and Fulong Wei carried out the experiments. Zhiying Li and Hui Yang contributed to the interpretation of the results. Changrong Li took the lead in writing the manuscript. All authors provided critical feedback and helped shape the research, analysis, and manuscript.

Correspondence to Changrong Li.

The authors declare no competing interests.

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Zeng, Z., Gu, S., Wang, J. et al. Influence of Nb/V on the corrosion behavior of high-strength anti-seismic rebar in marine environments. npj Mater Degrad 8, 76 (2024). https://doi.org/10.1038/s41529-024-00493-3

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Received: 21 February 2024

Accepted: 07 July 2024

Published: 22 July 2024

DOI: https://doi.org/10.1038/s41529-024-00493-3

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