Wei Wu, Lili Zhu, Peilin Chai, Niyun Liu, Longfei Song, Zhiyong Liu, and Xiaogang Li, Atmospheric corrosion behavior of Nb- and Sb-added weathering steels exposed to the South China Sea, Int. J. Miner. Metall. Mater., 29(2022), No. 11, pp.2041-2052. https://dx.doi.org/10.1007/s12613-021-2383-2
Cite this article as:
Wei Wu, Lili Zhu, Peilin Chai, Niyun Liu, Longfei Song, Zhiyong Liu, and Xiaogang Li, Atmospheric corrosion behavior of Nb- and Sb-added weathering steels exposed to the South China Sea, Int. J. Miner. Metall. Mater., 29(2022), No. 11, pp.2041-2052. https://dx.doi.org/10.1007/s12613-021-2383-2
Wei Wu, Lili Zhu, Peilin Chai, Niyun Liu, Longfei Song, Zhiyong Liu, and Xiaogang Li, Atmospheric corrosion behavior of Nb- and Sb-added weathering steels exposed to the South China Sea, Int. J. Miner. Metall. Mater., 29(2022), No. 11, pp.2041-2052. https://dx.doi.org/10.1007/s12613-021-2383-2
Cite this article as:
Wei Wu, Lili Zhu, Peilin Chai, Niyun Liu, Longfei Song, Zhiyong Liu, and Xiaogang Li, Atmospheric corrosion behavior of Nb- and Sb-added weathering steels exposed to the South China Sea, Int. J. Miner. Metall. Mater., 29(2022), No. 11, pp.2041-2052. https://dx.doi.org/10.1007/s12613-021-2383-2
The atmospheric corrosion behavior of new-type weathering steels (WSs) was comparatively studied, and the effects of Nb and Sb during corrosion were clarified in detail through field exposure and characterization. The results showed that the addition of Nb and Sb played positive roles in corrosion resistance, but there was a clear difference between these two elements. Nb addition slightly improved the rust property of conventional WS but could not inhibit the electrochemical process. In contrast, Sb addition significantly improved the corrosion resistance from the aspects of electrochemistry and rust layer. Compared with only 0.06wt% Nb, the combination of 0.05wt% Sb and 0.06wt% Nb could better optimize the rust structure, accelerate the formation of a high proportion of dense and protective α-FeOOH, repel the invasion of Cl−, and retard the localized acidification at the bottom of the pit.
Weathering steels (WSs) are widely used in bridges, rails, transmission towers, and infrastructures owing to their wonderful weldability, mechanical properties, and corrosion resistance [1]. Generally, this excellent corrosion resistance is due to the addition of a small number of alloying elements to form a dense and well-adhered corrosion product film (CPF), which is primarily composed of α-FeOOH, β-FeOOH, γ-FeOOH, Fe3O4, and amorphous phases [2–6]. However, the CPF may be destroyed in marine atmospheres, which are characterized by corrosive ions and high humidity, high temperature, and heavy rainfall, resulting in a shortening of the service life of WS [7–10]. Thus, many studies have been developed to clarify the effect of alloying elements on CPFs to optimize the corrosion resistance of WSs in marine environments.
Conventional WSs are designed according to different combinations of Cr, Ni, and Cu elements [11–14]. The role of Cr and Ni in WSs is to form spinel Fe2CrO4 or Fe2NiO4 [15–20], which increases the proportion of α-FeOOH and effectively improves the protection of CPF. Currently, the corrosion resistance mechanism of Cr, Ni, and Cu has been basically understood, and more attention has gradually been paid to the role of microalloy elements, such as Sb, Sn, Ca, and Nb. It is generally believed that Nb and Sb are acid corrosion resistant elements [21–22]. Nam and Kim [23] found that WS containing Nb exhibited good passivation behavior in a 10wt% H2SO4 environment. As the content of Nb increased, the passivation current density decreased significantly, and the surface showed uniform corrosion characteristic with very few pits. Similarly, Sb can enhance the corrosion resistance of WS in sulfuric acid or hydrochloric acid environments. Studies have shown that adding a small amount of Sb, usually less than 0.1wt%, can significantly improve the resistance of WSs to sulfuric acid dew point corrosion by restraining electrochemical reactions [24–25]. Owing to the synergistic effect of Sb and Cu, this influence can be enhanced in the presence of Cl− [26]. Yang et al. [27] also found that 0.1wt% Sb could promote the transformation of γ-FeOOH to Fe3O4 in the CPF formed in acidic atmosphere. It is widely accepted that localized acidification occurring underneath the CPF is the main reason for the corrosion of WSs in marine atmospheres. The addition of Nb and Sb is very likely to alleviate acidification and further reduce the corrosion rate of WSs. However, relevant research has not been carried out so far. Therefore, it is necessary to compare the atmospheric corrosion behavior of Nb- and Sb-added WSs in marine environment.
In this paper, some new WSs were designed to reveal the roles of Nb and Sb on the corrosion behavior of WSs in a real tropical marine atmosphere. The electrochemical and corrosion behaviors were investigated by electrochemical impedance spectroscopy (EIS), potentiodynamic polarization, and field exposure tests. The CPFs were analyzed in detail by various characterization, and the difference between Nb addition and Sb addition was clarified.
2.
Experimental
2.1
Materials
The steels used in the experiment were numbered as 6Nb and 6Nb5Sb, respectively, whose chemical compositions were listed in Table 1. These steels were first prepared into round ingots by vacuum melting, heated at 1200°C for 2 h, and then rolled into 12-mm-thick strips. After rolling, the strips were quenched in water to 440°C and air-cooled to room temperature.
Table
1.
Chemical compositions of WSs used in this study wt%
Samples were first ground to 2000#, polished with 1.5 μm diamond paste, and then etched by a 4vol% nitric acid solution for approximately 8 s. Subsequently, the microstructure was observed with an FEI Quanta 250 scanning electron microscopy (SEM) at an accelerating voltage of 20 kV. Electron back-scattered diffraction (EBSD) samples were electropolished in a solution consisting of 10vol% perchloric acid and 90vol% ethanol at 30 V for about 5 s. A TEAM data acquisition software integrated with SEM was used to finish EBSD tests. An OIM Analysis 7.3 software was used to acquire the inverse pole figure (IPF) and grain boundary rotation angle (RA) data.
2.2
Electrochemical measurements
Electrochemical measurements were performed on a Versa STAT-3 workstation with a traditional three-electrode cell. The working electrode was the steel with a geometrical exposed area of 1 cm2, the reference electrode was a saturated calomel electrode, and foil was used as a counter electrode. The steel sample was ground to 2000# with silicon carbide sandpaper and then washed using deionized water and alcohol. The solution for test was 0.1wt% NaCl + 0.05wt% CaCl2 + 0.05wt% Na2SO4, which was widely accepted as a simulated medium for atmospheric condition in the South China Sea [28–29]. The open circuit potential (OCP) was monitored for 30 min until it reached stabilization. EIS was measured from 100 kHz to 10 mHz with a potential amplitude of 10 mV. Then, the potentiodynamic polarization test was performed from −1 to 0 V vs. SCE with a scan rate of 0.5 mV/s. All tests were performed at least three times at room temperature (~25°C) to ensure repeatability.
2.3
Field exposure test
The exposure site was the Wenchang atmospheric corrosion test station in the South China Sea. The climate condition was a typical tropical marine environment, with an annual average temperature of 27°C, an annual average humidity of 77.4%, an annual rainfall of more than 1500 mm and Cl− deposition rate of 112.68 mg/(m2·d). The detailed changes of temperature and rainfall with time were displayed in Fig. 1. The monthly average maximum temperature was in the range of 26 and 32°C, and the minimum temperature was in the range of 21 and 28°C. In addition, rainfall was abundant from June to October and was relatively low in other months.
Fig.
1.
Average monthly temperature and monthly rainfall of the South China Sea.
Prior to the test, five duplicate samples with a size of 100 mm × 50 mm × 5 mm were prepared for each steel. All samples were ground to 1200# with silicon carbide sandpaper in turn, cleaned by alcohol, and weighted to obtain the original mass w0. Then, the samples were installed on the specimen holder in the South China Sea. The exposure period was from April 2018 to April 2019.
2.4
Corrosion rate measurements
The samples retrieved from the South China Sea were immersed in solution (500 mL HCl + 500 mL H2O + 3.5 g C6H12N4) for 15 min to remove corrosion products by an ultrasonic cleaning machine [8,30–31]. Then, the final mass (wt) was obtained after washing and drying the sample. Meanwhile, three blank samples went through the same processes to calibrate the mass loss during the removal of corrosion products. The corrosion rate was calculated using the weight method based on the following equation:
R=(w0−wt)×104Sρt
(1)
where R represents the corrosion rate, μm/a; w0 − wt is the mass loss, g; S is the surface area, cm2; ρ is the steel density (7.8 g/cm3); t is the exposure time, a.
2.5
Corrosion products analysis
The surface and cross-sectional morphology of the CPFs were observed by using a VHX-2000 stereomicroscope and FEI Quanta 250 SEM. The chemical composition was analyzed by energy dispersive spectroscopy (EDS). X-Ray diffraction (XRD) tests were performed on a MAC Science-M21X analyzer using a Cu target to identify the main crystal phase of the CPF. The scanning angle was 10°–90°, and the scanning rate was 4°/min. After the removal of rust, the surface morphology of the steel was observed with SEM, and the pits on the surface were analyzed by a Keyence VKX250 laser confocal microscope. Specifically, the diameter and depth of the pits were statistically analyzed by randomly selecting sixteen fields of view.
3.
Results and discussion
3.1
Microstructure
The microstructure of the prepared steels is shown in Fig. 2. It can be seen that the original steel is dominated by lath and granular bainite, and the width of the lath is close to 1 μm. As shown in Fig. 2(a), a large number of island and block carbon-rich phases are distributed at the lath bainite boundary (LBB) and prior austenite grain boundary (PAGB). Many reports believe that these island-like carbon-rich phases are martensite/austenite (M/A) structure [32–33]. Adding Nb has an obvious effect on the microstructure, as shown in Fig. 2(b). The size of the carbon-rich phase and the width of lath bainite decrease, indicating the grain refinement effect of Nb, which is consistent with the previous reports [34–36]. As shown in Fig. 2(c), adding Sb on the basis of Nb addition does not change the microstructure, which suggests that Sb addition has little effect on the microstructure.
Fig.
2.
Microstructure of the steels: (a, d) BM, (b, e) 6Nb, and (c, f) 6Nb5Sb. The red arrows represent PAGB, the yellow arrows represent LBB, the blue arrows represent carbon-rich phases distributed along PAGB, and the green arrows represent carbon-rich phases distributed along LBB.
The IPF and RA data of these steels are shown in Fig. 3. The IPF diagrams show that three steels mainly have bcc structure with uniform crystal orientation. Generally, there are two types of grain boundaries: high-angle grain boundary (HAGB, 15°–180°) and low-angle grain boundary (LAGB, 2°–15°). The corrosion resistance of these two kinds of grain boundaries is different [37]. Due to the irregular atomic arrangement and high energy, the resistance of HAGBs to corrosion is weak. And intergranular corrosion is prone to occur. In contrast, LAGBs can resist the intergranular corrosion to a certain extent. From the RA image, we can find that the PAGBs and most LBBs in the prepared steels are HAGBs, while some subgrain boundaries and a small amount of LBBs belong to LAGBs. This phenomenon is consistent with the grain boundary characteristic of E690 steel reported in the literature [26].
Fig.
3.
EBSD results of the steels: (a1, a2) BM, (b1, b2) 6Nb, and (c1, c2) 6Nb5Sb. 1 is the IPF data, and 2 is the RA data. Blue, green, and red lines represent the boundaries with the misorientations of 15°–180°, 5°–15°, and 2°–5°, respectively.
Fig. 4(a) shows the polarization curves. It can be seen from the curve shape that the electrochemical mechanism does not change after microalloying. It mainly includes the active dissolution process at the anode and the oxygen reduction process at the cathode. Despite that the addition of Nb and Sb does not change the electrochemical mechanism, it does affect the anodic and cathodic reactions. There is no obvious difference between the BM steel and 6Nb steel. This suggests that single Nb addition has little effect on the electrochemical process. However, when 0.05wt% Sb is added on the basis of 0.06wt% Nb, the anodic and cathodic processes are clearly inhibited. Fig. 4(b) shows the fitted results after Tafel method [38]. It is found that the corrosion current density (icorr) decreases with the addition of Nb, but the reduction is slight. In contrast, the simultaneous addition of Nb and Sb greatly reduces the icorr value. Besides, the corrosion potential (Ecorr) also increases from −364.8 to −336.9 mV vs. SCE. This indicates that Sb addition works better than Nb addition in improving corrosion resistance from an electrochemical point of view.
Fig.
4.
(a) Polarization curves and (b) the fitted parameters of the steels.
The EIS curves are shown in Fig. 5, in which symbols represent the experimental data and solid lines represent the fitted data. The Nyquist diagram of the steels exhibits a large compressed semicircle, as shown in Fig. 5(a). This indicates that they have similar electrochemical mechanism. Moreover, both the arc diameter and impedance modulus increase after adding Nb and Sb. Generally, the difference in corrosion resistance of the steels can be reflected by comparing the radius of the arc and the impedance modulus. Thus, the addition of Nb and Sb improves the corrosion resistance. This result has been proved in similar studies [22–23,39].
Fig.
5.
EIS curves of the steels: (a) Nyquist; (b) Bode; (c) EEC for EIS fitting.
In addition, the Nyquist diagram includes only an arc, and the Bode diagram shows only a peak with a symmetrical phase angle distribution, as shown in Fig. 5(b). This characteristic means that there is only one time constant in the electrochemical system. In other words, the CPF generated on the surface of the sample is very loose during OCP monitoring. Thus, it is very hard to distinguish the resistance produced by the CPF from the charge transfer resistance [40–42]. Based on the literature [43–44], the spectra in our study are fitted using an equivalent electronic circuit (EEC) shown in Fig. 5(c), where Rs is the solution resistance, Rp is the polarization resistance, and Qdl is the constant phase element of the double electrode layer. Qdl can reflect the deviation from the ideal capacitor due to the unevenness of the electrode [45–46]. The fitted parameter values are listed in Table 2. Compared with previous studies, the fitted values are in a similar order of magnitude [43–44,47]. Small chi-square (χ2) values indicate that the fitted quality is high [48]. From Table 2, we can see that Rp increases whether Nb or Sb is added. In particular, the Rp value of the 6Nb5Sb steel increases obviously, that is, corrosion is more difficult to occur after the combined addition of Sb and Nb.
Table
2.
Fitted results of the EIS curves via the proposed EEC
Sample
Rs / (Ω·cm²)
Qdl / (Ω−1·cm−2·sN)
N
Rp / (Ω·cm²)
χ²
BM
98.41
1.184 × 10−3
0.7446
1578
3.16 × 10−3
6Nb
154.4
1.539 × 10−3
0.7919
1632
2.77 × 10−3
6Nb5Sb
122.9
9.028 × 10−3
0.7782
1853
3.25 × 10−3
Note: N is the exponent of the constant phase element Qdl.
The corrosion rate of the three steels after exposure is shown in Fig. 6. Obviously, BM steel shows the highest corrosion rate, approximately 70 μm/a. With the addition of Nb or Sb, the corrosion rate of WS decreases gradually. The corrosion rates of the 6Nb steel and the 6Nb5Sb steel are 65 and 55 μm/a, respectively. The change of corrosion rate is consistent with the above electrochemical results, both confirming the improvement of corrosion resistance after microalloying. Generally, the CPF formed under the atmosphere determines the corrosion resistance. Thus, the following tries to clarify the effect of Nb and Sb on the properties of the CPF during exposure.
Fig.
6.
Corrosion rate of the steels after exposure for one year.
Fig. 7 shows the surface morphologies of the steels after exposure for one year in the South China Sea. For all steels, there is almost no difference in the macroscopic morphologies, as shown in Fig. 7(a), (d), and (g). All surfaces look compact and smooth. But the microscopic morphologies are completely different. The CPF surface of BM steel has many particles, and the adhesion of each particle seems to be weak, as shown in Fig. 7(b). This morphology indicates that the corrosive electrolyte cannot be prevented from reaching the CPF/metal interface since CPF is composed of porous needle-like particles.
Fig.
7.
Surface morphologies of the steels after exposure for one year: (a, b, c) BM; (d, e, f) 6Nb; (g, h, i) 6Nb5Sb.
For the 6Nb and 6Nb5Sb steels, the CPF is relatively smooth and dense, as shown in Fig. 7(e) and (h). Specifically, the CPF surface of the 6Nb steel is composed of many particles with needle-like attachments and voids. According to previous results [20,49–50], the corrosive electrolyte composed of acid radical ions (such as C1−, SO2−4, and H+) can accumulate on the surface and penetrate into the CPF through cracks and pores. For the 6Nb5Sb steel, the CPF on the surface is obviously complete. From the microscopic image, we can find that the needle-like corrosion products are denser, and only a very narrow crack appears, as shown in Fig. 7(i). Thus, it is difficult for the electrolyte to reach the interface of CPF and steel matrix to accelerate corrosion process. Obviously, the physical structure of the CPF formed on 6Nb5Sb steel has a better corrosion resistance property.
3.4.2
Cross section morphologies and chemical compositions
Fig. 8 shows the cross-sectional morphologies and elemental map distribution of the CPFs formed on the steels after exposure for one year. The thicknesses of the CPFs are 300, 200.6, and 130.5 μm for the BM, 6Nb, and 6Nb5Sb steel, respectively. The decrease in CPF thickness shows the inhibition of Nb and Sb on atmospheric corrosion. For the BM, some wide cracks appear in the inner part of the CPF, and Cl− is enriched in these cracks as well as the outer layer of the CPF. Clearly, the loose structure of the CPF on the BM provides a fast channel for Cl− permeation. Compared with the BM steel, 6Nb steel has a compact CPF structure without obvious pores and cracks, as shown in Fig. 8(b). However, the enrichment of Cl− is still found at the interface between CPF and matrix. This implies that the dense CPF formed on the 6Nb steel has a poor capacity to resist the aggression of Cl−. For the 6Nb5Sb steel, the inner part is very dense, while the outer part has some cracks. EDS result shows that the distribution of Cl− is relatively uniform, and only the cracked area has a certain aggregation of Cl−, as shown in Fig. 8(c). Wu [51] reported that the incorporated Sb had a synergy with Cu to produce an electronegative inner layer so that Cl− is hard to enter the CPF. It can be summarized that the addition of Sb has a significant effect on the elemental distribution of the CPF, which improves the CPF property and enhances the corrosion resistance of WS.
Fig.
8.
Cross-sectional morphologies, EDS semi-quantitative analysis, and elemental mapping distributions of the CPFs on the steels: (a) BM; (b) 6Nb; (c) 6Nb5Sb.
The phase compositions of the CPFs are analyzed by XRD, and the results are shown in Fig. 9. It is obvious that the main phases of the CPFs are composed of α-FeOOH, β-FeOOH, γ-FeOOH, Fe3O4, and γ-Fe2O3. These crystal phases are common components of the corrosion products generated in marine [52]. In addition, XRD spectra further show that the intensity of the peaks related to iron oxyhydroxides changes, while the intensity of the peaks corresponding to iron oxides does not change, owing to the addition of Nb and Sb. This shows the percentage difference of each crystal phase in the CPF. It is well-known that α-FeOOH has a compact structure and relatively stable electrochemical state in CPF, which can prevent the intrusion of chloride ions and delay the corrosion process effectively. Thus, the relative intensity ratio method (RIR) [53–56] is used to determine the proportion of α-FeOOH, and the results are shown in Fig. 9(b). The proportions of α-FeOOH in 6Nb and 6Nb5Sb steel are 5.89% and 8.71%, respectively, which are clearly higher than that in BM steel (2.38%). This shows that with the addition of Nb and Sb, the proportion of α-FeOOH in the CPF gradually increases, and the protective ability of the CPF gradually increases as well. Besides, adding 0.06wt% Nb on the basis of 0.05wt% Sb is much better than adding it alone.
Fig.
9.
(a) Phase compositions and (b) the proportion of α-FeOOH in the CPF after exposure.
Fig. 10 shows the surface morphologies of the steels after removing the CPFs. It can be seen that the surface of the steels contains many pits of different sizes. For BM steel, the depth and width of the pits are large, and the surface is rough. After adding 0.06wt% Nb, the depth and diameter of the pits are slightly reduced. By comparing Fig. 10(b) and (c), the steel with 0.06wt% Nb and 0.05wt% Sb has a relatively smoother surface than 6Nb steel. The depth and width of the pits are clearly reduced. This phenomenon means that the corrosion mode of WSs in a tropical marine atmosphere changes from localized corrosion into uniform corrosion after Sb addition. This transformation is closely related to the localized acidification behavior at the pit bottom underneath the CPF, and the details will be analyzed in the following.
Fig.
10.
Surface morphologies of the steels after the removal of CPF: (a, d) BM; (b, e) 6Nb; (c, f) 6Nb5Sb.
Meanwhile, the 3D surface morphologies of the steels after rust removal are shown in Fig. 11. Compared with Fig. 11(a), the proportion of uniformly corroded area in Fig. 11(b) and (c) is larger, and the depth of corrosion pits is smaller. This confirms that Nb and Sb additions inhibit the uneven corrosion process and the development of corrosion pits. The maximum pit depths are approximately 132.01, 107.09, and 100.44 μm for the BM steel, 6Nb steel, and 6Nb5Sb steel, respectively. Clearly, Nb and Sb additions both obviously reduce the pit depth. To better describe the characteristic of the corrosion pit, a statistical analysis is performed, and the results are shown in Fig. 12. It can be seen that with the addition of Nb and Sb, the slope of the fitted line, i.e., the value of b, gradually increases. This change implies that the pits gradually expand in the width direction instead of the depth direction. It can be confirmed from the pits statistics that adding Nb and Sb can transform the corrosion mode of WSs in a tropical marine atmosphere from localized corrosion into uniform corrosion.
Fig.
11.
3D morphologies of the steels after rust removal: (a) BM; (b) 6Nb; (c) 6Nb5Sb. The left image shows the 3D morphology, and the right image presents the profile curve along the white line in the left image.
The pH value in the local pit can be calculated based on the model of Galvele [57]. Generally, the pit shape is associated with the thermodynamics of the local condition within the pits [58]. Dissolution can occur when the metal in the pits is in equilibrium. Meanwhile, hydrolysis appears as the metallic ions are produced. These reactions are shown in Eqs. (2) and (3).
Me→Men++ne−
(2)
Men++nH2O→Me(OH)n+nH+
(3)
where Me represents metal, n is the number of electrons transferred during the reaction.
There are also various material transfer processes when the reactions occur. The fluxion of iron atoms, oxygen- and hydrogen-containing substances can be described in Eqs. (4), (5), and (6), respectively. And there is a certain equilibrium process, as shown in Eqs. (7) and (8).
D1dC1dx+D4dC4dx=inF
(4)
D2dC2dx+D3dC3dx+D4dC4dx=0
(5)
2D2dC2dx+D3dC3dx+D4dC4dx+D5dC5dx=0
(6)
∗K1=C4⋅C5C1
(7)
Kω=C3⋅C5
(8)
where Cj is the concentration of substance j, mol/cm3; Dj is the diffusion coefficient of substance j, cm2/s; F is Faraday’s constant; ∗K1 is the hydrolysis parameter of Fe2+; Kω is the hydrolysis parameter of H+ and OH−; x is the pit depth of the three-dimensional morphology after removing the CPF; i is the corrosion current density. Table 3 lists the values of some constants.
Table
3.
Values of constants needed for the above equations
Based on the model of Galvele [57–58], the value of pH within the pit is calculated. Taking the ionization equilibrium of Fe(OH)2 as an example, the calculated process is briefly shown as follows. The detailed process has been described in the previous study [27].
n⋅F⋅D1∗K1C4⋅C5+n⋅F⋅C4⋅D4=x⋅i+k
(9)
C5⋅D5⋅Kω1C5−C4⋅D4=k
(10)
Supposing that the concentration of Fe2+ in the outermost CPF is saturated, the ion concentration of various substances can be calculated based on the ionization reaction process of Fe(OH)2, as follows:
Fe(OH)2→Fe2++2OH−
(11)
Then, the relationship between x·i and pH can be drawn as follows:
It should be noted that the above calculation results are the most conservative pH values. When there are pollutants or salt particles on the outermost surface, the actual pH value will be lower than the calculated value.
Based on the above, the pH values of the corrosion pits of these steels are obtained and listed in Table 4. The pH values are 4.38, 4.41, and 4.53 for BM, 6Nb, and 6Nb5Sb, respectively. Clearly, both Nb and Sb additions can inhibit the localized acidification process at the pits. In contrast, Nb addition has a much weaker effect than Sb addition. This is consistent with our previous results. The good effect of Sb may result from the formation of acid-resisting oxides, like Sb2O3 or Sb2O5, during atmospheric corrosion [27,51]. Thus, it can be concluded that Sb addition not only improves the CPF structure and promotes the formation of α-FeOOH, but also retards localized acidification at the pits, thus leading to a significant increase in the corrosion resistance of WS.
The above analysis shows that Nb and Sb addition has a significant influence on the electrochemical process and corrosion resistance of the WS in a marine atmosphere. The microstructure indicates that Nb addition in steel reduces the sizes of the carbides and grains (Fig. 2(b)). But the microstructural change does not significantly inhibit the electrochemical process (Figs. 4(a) and 5(a)) because the electrochemical activity is usually determined by the chemical composition of WS. Although the corrosion rate of WS is reduced after adding 0.06wt% Nb, the reduction is very limited (Fig. 6). This is attributed to the slight effect of Nb on atmospheric corrosion process. Nb usually exists in steel in the form of carbide, i.e., NbC, which is difficult to participate in the formation and stabilization of CPF. Only a very small amount of Nb element is dissolved in the matrix [12]. These Nb atoms serve as nucleation sites for the formation of products at the initial stage of corrosion, and promote the subsequent transformation of each phase. Thus, the surface of the CPF of the Nb-added steel is dense and flat (Fig. 7(e)). However, the number of Nb atoms is small, and its effect is very limited, too. In addition, Nb has no obvious effect on improving the element distribution in CPF, particularly the distribution of Cl. Thus, the localized acidification induced by Cl− is barely inhibited when only 0.06wt% Nb is incorporated in WS.
Sb addition on the basic of Nb addition has a very obvious effect on atmospheric corrosion process. The addition of Sb has almost no effect on the microstructure of the steel, but the electrochemical process is inhibited (Figs. 4(a) and 5(a)). Previous study [24] showed that a protective film could be formed when low alloy steel with 0.1wt% Sb was polarized. This conclusion is also verified in our study. Sb atoms are dissolved in the matrix, thus generating significant influences on corrosion behavior. First, the presence of Sb atoms in CPF provides the nucleation sites for the formation of crystal phase, and this effect is basically the same with that of Nb. Second, the addition of Sb prevents the invasion of Cl− into the inner part of CPF, which further inhibits the localized corrosion caused by Cl− aggregation. It is accepted that this effect results from the synergy of Sb with Cu in steel [12,24]. The literature [51] reported that a layer enriched by Sb and Cu was produced in the inner CPF. Unfortunately, this phenomenon is not observed in our study, probably due to the low Sb content in the steel. Third, Sb addition has a good effect in alleviating local acidification process within the pits, as calculated in Table 4. This is highly related to the distribution of Cl− in CPF. These effects contribute largely to the improvement of corrosion resistance. Combined with the benefits of the addition of Nb described above, it is reasonable to believe that the steel with 0.06wt% Nb and 0.05wt% Sb has a high corrosion resistance in a real tropical marine atmosphere.
4.
Conclusions
Based on the experimental results and analysis, there are some conclusions drawn as follows.
(1) Adding 0.06wt% Nb into WS has a weak effect on corrosion resistance in a tropical marine atmosphere. Although Nb addition optimizes the physical structure and enhances the proportion of α-FeOOH phase, it has very little effect on the distribution of Cl− in CPF, the electrochemical reactions, and the localized acidification process.
(2) Adding 0.05wt% Sb on the basis of 0.06wt% Nb has a significant effect on corrosion resistance. This can be attributed to the beneficial role of Sb addition, which inhibits the anodic and cathodic reactions, repels the invasion of Cl−, and restrains the localized acidification within the pits. As a result, the steel with 0.06wt% Nb and 0.05wt% Sb shows the highest corrosion potential (−336.9 mV vs. SCE), the lowest corrosion current density (0.758 μA/cm2), the lowest corrosion rate (55.76 μm/a), and the strongest protection of CPF.
Acknowledgements
This work was financially supported by the National Natural Science Foundation of China (No. 52101068) and the China Postdoctoral Science Foundation (No. 2022M710348).
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