The Synergistic Inhibitions of Tungstate and Molybdate Anions on Pitting Corrosion Initiation for Q235 Carbon Steel in Chloride Solution

In this work, the synergistic inhibitions of tungstate (WO42−) and molybdate (MoO42−) anions, including role and mechanism, on the initiation of pitting corrosion (PC) for Q235 carbon steel in chloride (Cl−) solution were investigated with electrochemical and surface techniques. The pitting potential (Ep) of the Q235 carbon steel in WO42− + MoO42- + Cl− solution was more positive than that in WO42− + Cl− or MoO42− + Cl− solution; at each Ep, both peak potential and affected region of active pitting sites in WO42− + MoO42− + Cl− solution were smaller than those in WO42− + Cl− or MoO42− + Cl− solution. WO42− and MoO42− showed a synergistic role to inhibit the PC initiation of the Q235 carbon steel in Cl− solution, whose mechanism was mainly attributed to the influences of two anions on passive film. Besides iron oxides and iron hydroxides, the passive film of the Q235 carbon steel formed in WO42− + Cl−, MoO42− + Cl−, or WO42− + MoO42− + Cl− solution was also composed of FeWO4 plus Fe2(WO4)3, Fe2(MoO4)3, or Fe2(WO4)3 plus Fe2(MoO4)3, respectively. The film resistance and the defect quantity for Fe2(WO4)3 plus Fe2(MoO4)3 film were larger and smaller than those for FeWO4 plus Fe2(WO4)3 film and Fe2(MoO4)3 film, respectively; for the inhibition of PC initiation, Fe2(WO4)3 plus Fe2(MoO4)3 film provided better corrosion resistance to Q235 carbon steel than FeWO4 plus Fe2(WO4)3 film and Fe2(MoO4)3 film did.


Introduction
Pitting corrosion (PC) is one of the most common and universal localized forms of corrosion for metals and alloys, and the remarkable characteristic of PC is latent, random and sudden, particularly during the process of PC initiation [1]. Therefore, it is generally accepted that the key point of PC inhibition is restraining the nucleation of the corrosion pit [2].
It is confirmed that for metals and alloys working in service environments, there are two essential factors for the initiation of PC: the establishment of surface passivation on metals and alloys and the presence of aggressive anions in service environments, particularly the chloride anion (Cl − ) [1]. Further, it is reported repeatedly that in the service environments of metals and alloys, the addition of an introduced substance, mainly some inorganic and organic species, can inhibit the initiation of PC effectively [3]. At present, reported inorganic species for the inhibition of PC initiation include borate (BO 4 3− ) [4], tetraborate (B 4 O 7 2− ) [5], chromate (CrO 4 2− ) [6], dichromate (Cr 2 O 7 2− ) [7], molybdate (MoO 4 2− ) [8], nitrite (NO 2 − ) [9], nitrate (NO 3 − ) [6], phosphate (PO 4 3− ) [6], tripolyphosphate (P 3 O 10 5− ) [10], tungstate (WO 4 2− ) [11] anions and so on; by contrast, reported organic species for inhibiting PC initiation are relatively more, such as benzotriazole (BTA) [12], calcium lignosulfonate (CLS) [13], imidazoline (IM) [14], octaphenylpolyoxyethyiene (OP) [15], sodiumdodecylbenzenesulfonate (SDBS) [16], sodium dodecyl sulfate (SDS) [17], sodium oleate (SO) [18], thioureido imidazoline (TAI) [19], tetrabethylenepentamine (TEPA) [20], vitamin B5 [21] and others. However, in recent years, the research of PC inhibition has begun to focus on the synergistic role and mechanism of two or more species, and the conclusion that the simultaneous additions of some species into environmental media exhibit a better role for PC inhibition than the single addition of corresponding one species do has been proposed [22][23][24][25][26][27][28][29][30][31]. Up to now, confirmed combinations owning a synergistic role for PC inhibition are as follows: B 2 O 7 2− and PO 4 3− [22], Cr 2 O 7 2− and MoO 4 2− [23], MoO 4 2− and NO 2 − [24], MoO 4 2− and BTA [25], MoO 4 2− and CLS [26], NO 2 − and TAI [27], NO 2 − and TEPA [28], WO 4 2− and CLS [29], CLS and SO [30], IM and OP [31], and so on. Carbon steel is a kind of metal engineering material with relatively weak surface passivation capability; therefore, in pure chloride solution, general corrosion is the main type of corrosion damage for carbon steel [2]. Further, for carbon steel in pure chloride solution, it is reported largely that the additions of organic species and their adsorption role as well as the additions of inorganic species and their film-forming role can inhibit the general corrosion of carbon steel [32]. At the same time, it is also confirmed that if an inorganic species, whose inhibition against general corrosion is derived from its role for the surface passivation of carbon steel, its addition into chloride solution can induce the initiation of PC for carbon steel. The related report of the above result has been mentioned in Na 3 BO 4 + NaCl solution [4], Na 3 PO 4 + NaCl solution [6], NaNO 2 + NaCl solution [9], Na 2 WO 4 + NaCl solution [11], Na 2 MoO 4 + NaCl solution [25] and so on. Further, from the above reports, it is concluded that for carbon steel in chloride solution containing inorganic species, particularly inorganic species with a certain degree of oxidation ability, the occurrence of PC for carbon steel results from the combined actions of Cl − and inorganic species, and their appropriate proportion plays a critical role in the initiation of PC.
In our previous works [33][34][35], for Q235 carbon steel in pure Na 2 WO 4 or/and Na 2 MoO 4 solutions free of Cl − , we investigated its electrochemical behavior and surface passivation and summarized some rules about the influences of WO 4 2− and MoO 4 2− on its corrosion and passivation behaviors. Both WO 4 2− and MoO 4 2− could promote the surface passivation of the Q235 carbon steel, which was due to their influence on the composition and microstructure of the passive film. However, due to the absence of an aggressive anion, particularly Cl − , in pure Na 2 WO 4 or/and Na 2 MoO 4 solutions, the occurrence of PC for Q235 carbon steel is avoided [36]. Further, in the presence of Cl − , for carbon steel, in Na 2 WO 4 + NaCl solution, Gao et al. [11] reported that the inhibition of WO 4 2− on the initiation of PC was attributed to its role in promoting the formation of γ-Fe 2 O 3 , which was the main composition of the passive film on the carbon steel surface. Jabeera et al. [37] reported that the presence of WO 4 2− resulted in the formation of FeWO 4 , which repaired the defect of the passive film because of the preferential deposition of FeWO 4 at the defect sites. Fujioka et al. [38] reported that the pitting potential and repassivation potential of carbon steel moved to the positive direction with the increase inWO 4 2− concentration, and the role of WO 4 2− resulted from repairing the defect of the passive film and from inhibiting the development of the corrosion pit. At the same time, for carbon steel in Na 2 MoO 4 + NaCl solution, Zhao et al. [23] reported that the presence of MoO 4 2− restrained the nucleation and development of the corrosion pit, which was attributed to the pH value of the pit interior raised with the increase inMoO 4 2− concentration. Zhou et al. [25] reported that the inhibition of MoO 4 2− on PC initiation was due to the role of MoO 4 2− to promote the transformation from FeOOH to Fe 2 O 3 in the passive film and to further enhance the stability of the passive film. Fujioka et al. [38] also reported the influence of MoO 4 2− concentration on the pitting potential and repassivation potential and its role in repairing the passive film defect. Saremi et al. [39] reported that the adsorption and reduction of MoO 4 2− was beneficial to the high resistance and low permeability of the passive film.
As stated above, in the single presence of WO 4 2− or MoO 4 2− in chloride solution, its role and mechanism on the PC inhibition of carbon steel has been greatly reported; however, the synergistic role and mechanism of WO 4 2− and MoO 4 2− on PC inhibition, particularly on the inhibition of PC initiation, of carbon steel are absent. In this work, Na 2 WO 4 , Na 2 MoO 4 and NaCl are introduced into de-ionized water to obtain three solutions: WO 4 2− + Cl − solution, MoO 4 2− + Cl − solution and WO 4 2− + MoO 4 2− + Cl − solution. In the above three solutions, the synergistic inhibitions of WO 4 2− and MoO 4 2− , including the inhibitive role and mechanism, on the initiation of PC for Q235 carbon steel are investigated with electrochemical and surface techniques.

Materials and Methods
The investigated material of this work was Q235 carbon steel with the following chemical composition (weight percent): C, 0.160; Mn, 0.530; P, 0.015; S, 0.045; Si, 0.300; and Fe, balance. Q235 carbon steel was processed into some samples with the three-dimensional size of 10 × 10 × 3 mm; after that, all samples were manually abraded up to 1000 grit with SiC abrasive paper, rinsed with de-ionized water and degreased in alcohol.
There were three solutions investigated in this work: besides Na + , Solution I comprising WO 4 2− and Cl − , Solution II comprising MoO 4 2− and Cl − , and Solution III comprising WO 4 2− , MoO 4 2− and Cl − . The detailed information of the three solutions, including component and pH value, are listed in Table 1. Solutions I, II and III were prepared with analytical grade agents and de-ionized water. The measurements of electrochemical techniques, including open circuit potential (OCP) evolution, potentiodynamic polarization, electrochemical impedance spectroscopy (EIS) and Mott-Schottky plot, were carried out by a Princeton 2273 electrochemical workstation (USA) at room temperature (RT). A typical three-electrode system was applied for electrochemical measurements: working electrode was a sample of the Q235 carbon steel, counter electrode was a platinum sheet, and reference electrode was a saturated calomel electrode (SCE). Before each electrochemical test, the surface area of the working electrode (a sample of the Q235 carbon steel) was restricted into a square with the two-dimensional size of 2 × 2 mm with room-temperature-cured silicone rubber. In OCP evolution tests, the record frequency of OCP was 5 Hz; in potentiodynamic polarization tests, the scanning rate of applied potential was 0.5 mV/s; in EIS tests, a perturbation potential of 10 mV amplitude was performed in the frequently range from 10 5 to10 −2 Hz; in Mott-Schottky plot tests, the scanning rate of applied potential was 5 mV/s, and the scanning range of applied potential was from −0.2 to 1.0 V SCE . However, all potentiodynamic polarization tests were terminated when corresponding current density increased suddenly and sharply, and thus, their potential scanning ranges were not proposed in this work.
The measurements of surface techniques included spatial potential distribution and surface chemical composition: a former test was performed by an XMU-BY electrochemical scanning tunneling microscope (ESTM) instrument (Xiamen Legang Materials Technology Co., Ltd., Xiamen, China), and a latter test was conducted by an ESCALAB-250 X-ray photoelectron spectroscopy (XPS) instrument (Waltham, MA, USA). Figure 1 shows the OCP evolutions of the Q235 samples in WO 4 2− + Cl − , MoO 4 2− + Cl − and WO 4 2− + MoO 4 2− + Cl − solutions. For Q235 samples in three solutions, with the extension of test time, OCP moves to the positive direction from 0 to 10 min; after that, from 10 to 20 min, the change of OCP is slight. Therefore, prior to the subsequent electrochemical measurements of potentiodynamic polarization, EIS and Mott-Schottky plot, Q235 samples were immersed in corresponding solutions for 20 min to ensure the stability of OCP.  Figure 1 shows the OCP evolutions of the Q235 samples in WO4 2− + Cl − , MoO4 2− + Cl − and WO4 2− + MoO4 2− + Cl − solutions. For Q235 samples in three solutions, with the extension of test time, OCP moves to the positive direction from 0 to 10 min; after that, from 10 to 20 min, the change of OCP is slight. Therefore, prior to the subsequent electrochemical measurements of potentiodynamic polarization, EIS and Mott-Schottky plot, Q235 samples were immersed in corresponding solutions for 20 min to ensure the stability of OCP. However, the stable OCP value of the Q235 carbon steel in WO4 2− + Cl − , MoO4 2− + Cl − or WO4 2− + MoO4 2− + Cl − solution increases in turn. For carbon steels in alkaline environments at open circuit condition, anodic half-reaction is the oxidation of an iron element from Fe to Fe 2+ with the standard potential (Es) of −0.684 VSCE [40]:

OCP Evolution
The equilibrium potential (Ee) of Fe oxidation is described as follows: Ee (Fe 2+ /Fe) = (−0.684 + 0.059/2 log αFe2+) VSCE At the same time, cathodic half-reaction is the reduction of an oxygen element from O2 to OH − with the Es of −0.157 VSCE [41]: The Ee of O2 reduction is described as follows: In three solutions, their same pH value suggests the approximate Ee of O2 reduction; therefore, according to Nernst theory, the difference in a stable OCP value is mainly attributed to the different Ee of Fe oxidation. In the subsequent result of potentiodynamic polarization, it will be seen that in WO4 2− + Cl − , MoO4 2− + Cl − or WO4 2− + MoO4 2− + Cl − solution, the corrosion current density of the Q235 carbon steel decreases in turn, indicating that Fe 2+ concentration near the solution/electrode interface decreases in turn. According to Equation (2), it can be calculated that in WO4 2− + Cl − , MoO4 2− + Cl − or WO4 2− + MoO4 2− + Cl − solution, the Ee of Fe oxidation moves to the positive direction in turn, resulting in the stable OCP value of the Q235 carbon steel that increases in turn.  However, the stable OCP value of the Q235 carbon steel in WO 4 2− + Cl − , MoO 4 2− + Cl − or WO 4 2− + MoO 4 2− + Cl − solution increases in turn. For carbon steels in alkaline environments at open circuit condition, anodic half-reaction is the oxidation of an iron element from Fe to Fe 2+ with the standard potential (E s ) of −0.684 V SCE [40]:

Potentiodynamic Polarization
The equilibrium potential (E e ) of Fe oxidation is described as follows: At the same time, cathodic half-reaction is the reduction of an oxygen element from O 2 to OH − with the E s of −0.157 V SCE [41]: The E e of O 2 reduction is described as follows: In three solutions, their same pH value suggests the approximate E e of O 2 reduction; therefore, according to Nernst theory, the difference in a stable OCP value is mainly attributed to the different E e of Fe oxidation. In the subsequent result of potentiodynamic polarization, it will be seen that in WO 4 2− + Cl − , MoO 4 2− + Cl − or WO 4 2− + MoO 4 2− + Cl − solution, the corrosion current density of the Q235 carbon steel decreases in turn, indicating that Fe 2+ concentration near the solution/electrode interface decreases in turn. According to Equation (2), it can be calculated that in WO 4 2− + Cl − , MoO 4 2− + Cl − or WO 4 2− + MoO 4 2− + Cl − solution, the E e of Fe oxidation moves to the positive direction in turn, resulting in the stable OCP value of the Q235 carbon steel that increases in turn. and WO 4 2− + MoO 4 2− + Cl − solutions. For the Q235 sample in 0.1 mM NaCl solution, anodic current density increases persistently with the positive shift of applied potential, indicating that Q235 carbon steel presents the electrochemical behavior of activation in 0.1 mM NaCl solution [42]. In contrast, for Q235 samples in WO 4 2− + Cl − , MoO 4 2− + Cl − and WO 4 2− + MoO 4 2− + Cl − solutions, with the positive shift of applied potential, anodic current density firstly increases slowly from the corrosion potential (E c ) to approximately −0.3 V SCE , then maintains steadily from −0.3 V SCE to the pitting potential (E p ), and finally increases rapidly when the applied potential is up to E p . Q235 carbon steel presents the electrochemical behavior of activation-passivation-pitting in WO 4 2− + Cl − , MoO 4 2− + Cl − and WO 4 2− + MoO 4 2− + Cl − solutions [43].

Potentiodynamic Polarization
For the Q235 sample in 0.1 mM NaCl solution, anodic current density increases persistently with the positive shift of applied potential, indicating that Q235 carbon steel presents the electrochemical behavior of activation in 0.1 mM NaCl solution [42]. In contrast, for Q235 samples in WO4 2− + Cl − , MoO4 2− + Cl − and WO4 2− + MoO4 2− + Cl − solutions, with the positive shift of applied potential, anodic current density firstly increases slowly from the corrosion potential (Ec) to approximately −0.3 VSCE, then maintains steadily from −0.3 VSCE to the pitting potential (Ep), and finally increases rapidly when the applied potential is up to Ep. Q235 carbon steel presents the electrochemical behavior of activation-passivation-pitting in WO4 2− + Cl − , MoO4 2− + Cl − and WO4 2− + MoO4 2− + Cl − solutions [43]. However, the influence of solution component on Ec and corrosion current density (ic) is negligible, but on Ep and repassivation potential (Er), it is very significant. Table 2 lists the calculated values of Ec, ic, Ep and Er, in which each Ec, ic, Ep or Er datum is the average value from ten parallel potentiodynamic polarization tests. For Q235 carbon steel in three solutions, the values of Ep and (Ep − Er) in WO4 2− + MoO4 2− + Cl − solution are respectively larger and smaller than those in WO4 2− + Cl − or MoO4 2− + Cl − solution, indicating that WO4 2− and MoO4 2− show a synergistic role for the inhibition of PC initiation [44]. Because the main aim of this work is to reveal the synergistic role and mechanism of WO4 2− and MoO4 2− on the inhibition of PC initiation, the following discussion of this work is mainly focused on physical and chemical properties about the surface of the Q235 carbon steel at the applied potential of Ep.  However, the influence of solution component on E c and corrosion current density (i c ) is negligible, but on E p and repassivation potential (E r ), it is very significant. Table 2 lists the calculated values of E c , i c , E p and E r , in which each E c , i c , E p or E r datum is the average value from ten parallel potentiodynamic polarization tests. For Q235 carbon steel in three solutions, the values of E p and (E p − E r ) in WO 4 2− + MoO 4 2− + Cl − solution are respectively larger and smaller than those in WO 4 2− + Cl − or MoO 4 2− + Cl − solution, indicating that WO 4 2− and MoO 4 2− show a synergistic role for the inhibition of PC initiation [44]. Because the main aim of this work is to reveal the synergistic role and mechanism of WO 4 2− and MoO 4 2− on the inhibition of PC initiation, the following discussion of this work is mainly focused on physical and chemical properties about the surface of the Q235 carbon steel at the applied potential of E p .

Spatial Potential Distribution (SPD)
In order to obtain spatial electrochemical information of PC initiation and propagation, the applied potentials of 0.01, 0.09 and 0.17 V SCE , respectively, being very close to the E p of the Q235 carbon steel in WO 4 2− + Cl − , MoO 4 2− + Cl − and WO 4 2− + MoO 4 2− + Cl − solutions, are exerted on the working electrode artificially.  Figure 3a, an active pitting site with the peak potential of 1.22 mV is detected at the bottom right corner of ESTM tip scanning region, suggesting the initiation of a corrosion pit [45]; by contrast, in the MoO 4 2− + Cl − solution shown in Figure 3b and in the WO 4 2− + MoO 4 2− + Cl − solution shown in Figure 3c, no obvious active pitting sites are observed in the ESTM tip scanning region.

Spatial Potential Distribution (SPD)
In order to obtain spatial electrochemical information of PC initiation and propagation, the applied potentials of 0.01, 0.09 and 0.17 VSCE, respectively, being very close to the Ep of the Q235 carbon steel in WO4 2− + Cl − , MoO4 2− + Cl − and WO4 2− + MoO4 2− + Cl − solutions, are exerted on the working electrode artificially. Figure 3 shows the SPD images of the Q235 samples in WO4 2− + Cl − , MoO4 2− + Cl − and WO4 2− + MoO4 2− + Cl − solutions at the applied potential of 0.01 VSCE. At 0.01 VSCE, in the WO4 2− + Cl − solution shown in Figure 3a, an active pitting site with the peak potential of 1.22 mV is detected at the bottom right corner of ESTM tip scanning region, suggesting the initiation of a corrosion pit [45]; by contrast, in the MoO4 2− + Cl − solution shown in Figure 3b and in the WO4 2− + MoO4 2− + Cl − solution shown in Figure 3c, no obvious active pitting sites are observed in the ESTM tip scanning region.         Figures 3a, 4a and 5a, suggesting the continuous propagation of PC [47]. In MoO 4 2− + Cl − solution, both the peak potential and affected region of the active pitting sites at 0.17 V SCE are larger than those at 0.09 V SCE , as shown in Figures 4b and 5b; at 0.17 V SCE , in the WO 4 2− + MoO 4 2− + Cl − solution shown Figure 5c, an active pitting site with the peak potential of 1.06 mV is detected at the middle position of the ESTM tip scanning region.
From the above results of polarization and SPD, on the one hand, the E p of the Q235 carbon steel in WO 4 2− + MoO 4 2− + Cl − solution is more positive than that in WO 4 2− + Cl − or MoO 4 2− + Cl − solution; on the other hand, at each E p , both the peak potential and affected region of the active pitting sites in WO 4 2− + MoO 4 2− + Cl − solution are smaller than those in WO 4 2− + Cl − or MoO 4 2− + Cl − solution. The above two aspects conclude that for Q235 carbon steel in Cl − solution, the inhibitive role of WO 4 2− and MoO 4 2− on PC initiation when both are used together is better than that when one is used solely.
both the peak potential and affected region of the active pitting sites at 0.17 VSCE further increase compared with those at 0.09 and 0.01 VSCE, as shown in Figures 3a, 4a and 5a, suggesting the continuous propagation of PC [47]. In MoO4 2− + Cl − solution, both the peak potential and affected region of the active pitting sites at 0.17 VSCE are larger than those at 0.09 VSCE, as shown in Figures 4b and 5b; at 0.17 VSCE, in the WO4 2− + MoO4 2− + Cl − solution shown Figure 5c, an active pitting site with the peak potential of 1.06 mV is detected at the middle position of the ESTM tip scanning region.   which is also reported regarding their oxidation for stainless steel [49] and cold rolling steel [50], similarly. Figure 6 shows the wide-scan XPS of the Q235 samples polarized to Ep in WO4 + Cl , MoO4 2− + Cl − and WO4 2− + MoO4 2− + Cl − solutions. For Q235 samples in three solutions, four peaks, Fe 2p at about 711 eV, O 1s at about 531 eV, C 1s at about 286 eV and Fe 3p at about 55 eV, are gathered by XPS analysis, which is independent of the solution component. Besides the above XPS peaks, W 4d peak at about 247 eV as well as W 4f peak at about 36 eV in WO4 2− + Cl − solution, Mo 3d peak at about 235 eV in MoO4 2− + Cl − solution, and W 4d peak at about 247 eV, Mo 3d peak at about 235 eV as well as W 4f peak at about 37 eV in WO4 2− + MoO4 2− + Cl − solution are also gathered by XPS analysis. The result of wide-scan XPS indicates that the passive film of the Q235 carbon steel formed in the three solutions has different chemical composition.    [48]; in MoO4 2− + Cl − solution shown in Figure 7b and in WO4 2− + MoO4 2− + Cl − solution shown in Figure 7c, each Fe 2p spectrum also reveals three peaks corresponding to Fe 3+ , Fe 2+ and Fe 0 . However, the intensity of the Fe 3+ peak in MoO4 2− + Cl − and WO4 2− + MoO4 2− + Cl − solutions is obviously stronger than that in WO4 2− + Cl − solution; conversely, the intensities of Fe 2+ and Fe 0 are weaker in MoO4 2− + Cl − and WO4 2− + MoO4 2− + Cl − solutions than in WO4 2− + Cl − solution. This result implies that the oxidation ability of MoO4 2− is stronger than that of WO4 2− , which is also reported regarding their oxidation for stainless steel [49] and cold rolling steel [50], similarly. In this work, the relatively strong oxidation ability of MoO4 2− plays an important role in the synergistic inhibitions of WO4 2− and MoO4 2− . In the subsequent results of EIS and the Mott-Schottky plot, it will be seen that the passive film of the Q235 carbon steel formed in WO4 2− + MoO4 2− + Cl − solution has larger passive film resistance and a smaller passive film defect than that formed in WO4 2− + Cl − or MoO4 2− + Cl − solution, which is very closely associated with the relatively high content of Fe 3+ in the passive film. On the other hand, according to bipolar model [51], more Fe 3+ and less Fe 2+ transfer across the passive film from an anion-selective type to a cation-selective type, which is beneficial to prevent passive film from the adsorption and attack of Cl − .    In this work, the relatively strong oxidation ability of MoO 4 2− plays an important role in the synergistic inhibitions of WO 4 2− and MoO 4 2− . In the subsequent results of EIS and the Mott-Schottky plot, it will be seen that the passive film of the Q235 carbon steel formed in WO 4 2− + MoO 4 2− + Cl − solution has larger passive film resistance and a smaller passive film defect than that formed in WO 4 2− + Cl − or MoO 4 2− + Cl − solution, which is very closely associated with the relatively high content of Fe 3+ in the passive film. On the other hand, according to bipolar model [51], more Fe 3+ and less Fe 2+ transfer across the passive film from an anion-selective type to a cation-selective type, which is beneficial to prevent passive film from the adsorption and attack of Cl − .  When carbon steels are exposed in an atmosphere environment, an air-formed passive film forms on their surface spontaneously [52]; further, when served in alkaline environments, previous passive film formed in air can rearrange a double-layer microstructure [53]. The inner dense layer of the passive film is composed of FeOOH and Fe2O3, and the outer loose layer of the passive film is composed of Fe(OH)2·nH2O and Fe(OH)3·nH2O [54]. The related mechanism of the above statement is as follows [55]:

XPS
FeOH − ads → FeOHads + e FeOHads + OH − → Fe(OH)2 + e (7) Although there are many defects in the outer loose layer of the passive film [56], some introduced substances, such as WO4 2− and MoO4 2− , can repair them [49,50]. According to the present XPS results and our previous works [33][34][35], it is concluded that besides iron   When carbon steels are exposed in an atmosphere environment, an air-formed passive film forms on their surface spontaneously [52]; further, when served in alkaline environments, previous passive film formed in air can rearrange a double-layer microstructure [53]. The inner dense layer of the passive film is composed of FeOOH and Fe2O3, and the outer loose layer of the passive film is composed of Fe(OH)2·nH2O and Fe(OH)3·nH2O [54]. The related mechanism of the above statement is as follows [55]: FeOH − ads → FeOHads + e FeOHads + OH − → Fe(OH)2 + e (7) Although there are many defects in the outer loose layer of the passive film [56], some introduced substances, such as WO4 2− and MoO4 2− , can repair them [49,50]. According to the present XPS results and our previous works [33][34][35], it is concluded that besides iron When carbon steels are exposed in an atmosphere environment, an air-formed passive film forms on their surface spontaneously [52]; further, when served in alkaline environments, previous passive film formed in air can rearrange a double-layer microstructure [53]. The inner dense layer of the passive film is composed of FeOOH and Fe 2 O 3 , and the outer loose layer of the passive film is composed of Fe(OH) 2 ·nH 2 O and Fe(OH) 3 ·nH 2 O [54]. The related mechanism of the above statement is as follows [55]: FeOH − ads → FeOH ads + e FeOH ads + OH − → Fe(OH) 2 + e (7) Although there are many defects in the outer loose layer of the passive film [56], some introduced substances, such as WO 4 2− and MoO 4 2− , can repair them [49,50]. According to the present XPS results and our previous works [33][34][35], it is concluded that besides iron oxides and iron hydroxides, the passive film of the Q235 carbon steel formed in WO 4 2− + Cl − , MoO 4 2− + Cl − , or WO 4 2− + MoO 4 2− + Cl − solution also comprises FeWO 4 plus Fe 2 (WO 4 ) 3 , Fe 2 (MoO 4 ) 3 , or Fe 2 (WO 4 ) 3 plus Fe 2 (MoO 4 ) 3 , respectively. The detailed composition of the passive film on the surface of the Q235 carbon steel formed in three solutions is listed in Table 3.  Figure 10 shows the EIS of the Q235 samples in 0.1 mM NaCl solution and in WO 4 2− + Cl − , MoO 4 2− + Cl − and WO 4 2− + MoO 4 2− + Cl − solutions. For the Q235 sample in 0.1 mM NaCl solution, the corresponding EIS is composed of only one capacitive semicircle in the entire frequency zone; in contrast, for Q235 samples in WO 4 2− + Cl − , MoO 4 2− + Cl − and WO 4 2− + MoO 4 2− + Cl − solutions, each EIS is composed of a relatively small capacitive semicircle in the high-frequency zone and another relatively large capacitive semicircle in the low-frequency zone. In the passivation system of the electrode/electrolyte, the appearance of the capacitive semicircle in EIS mainly results from the formation of the passive film on the interface between the electrode and electrolyte [57]. The present EIS results of the two capacitive semicircles confirm the double-layer microstructure of the passive film, and it is reasonable to infer that the small capacitive semicircle of EIS is attributed to the inner dense layer of the passive film, and another large one is due to the outer loose layer [35]. Fe2(MoO4)3, or Fe2(WO4)3 plus Fe2(MoO4)3, respectively. The detailed composition of the passive film on the surface of the Q235 carbon steel formed in three solutions is listed in Table 3. Table 3. Composition of the passive film on Q235 carbon steel surface formed in WO4 2− + Cl − , MoO4 2− + Cl − and WO4 2− + MoO4 2− + Cl − solutions. Figure 10 shows the EIS of the Q235 samples in 0.1 mM NaCl solution and in WO4 2− + Cl − , MoO4 2− + Cl − and WO4 2− + MoO4 2− + Cl − solutions. For the Q235 sample in 0.1 mM NaCl solution, the corresponding EIS is composed of only one capacitive semicircle in the entire frequency zone; in contrast, for Q235 samples in WO4 2− + Cl − , MoO4 2− + Cl − and WO4 2− + MoO4 2− + Cl − solutions, each EIS is composed of a relatively small capacitive semicircle in the high-frequency zone and another relatively large capacitive semicircle in the lowfrequency zone. In the passivation system of the electrode/electrolyte, the appearance of the capacitive semicircle in EIS mainly results from the formation of the passive film on the interface between the electrode and electrolyte [57]. The present EIS results of the two capacitive semicircles confirm the double-layer microstructure of the passive film, and it is reasonable to infer that the small capacitive semicircle of EIS is attributed to the inner dense layer of the passive film, and another large one is due to the outer loose layer [35].

EIS
However, the influence of the solution component on the radius of the capacitive semicircle in the high-frequency zone is slight, but on that in the low-frequency zone, it is severe. That is to say, for the Q235 carbon steel in three solutions, the inhibitive role and mechanism of WO4 2− and MoO4 2− on PC initiation are mainly derived from their influence on the outer loose layer of the passive film, rather than to the inner dense layer. The radius of the large capacitive semicircle in WO4 2− + Cl − , MoO4 2− + Cl − or WO4 2− + MoO4 2− + Cl − solutions enlarges in turn, indicating that the corrosion resistance of the outer loose layer for the Fe2(WO4)3 plus Fe2(MoO4)3 film is better than that for the FeWO4 plus Fe2(WO4)3 film or Fe2(MoO4)3 film. However, the influence of the solution component on the radius of the capacitive semicircle in the high-frequency zone is slight, but on that in the low-frequency zone, it is severe. That is to say, for the Q235 carbon steel in three solutions, the inhibitive role and mechanism of WO 4 2− and MoO 4 2− on PC initiation are mainly derived from their influence on the outer loose layer of the passive film, rather than to the inner dense layer. The radius of the large capacitive semicircle in WO 4 2− + Cl − , MoO 4 2− + Cl − or WO 4 2− + MoO 4 2− + Cl − solutions enlarges in turn, indicating that the corrosion resistance of the outer loose layer for the Fe 2 (WO 4 ) 3 plus Fe 2 (MoO 4 ) 3 film is better than that for the FeWO 4 plus Fe 2 (WO 4 ) 3 film or Fe 2 (MoO 4 ) 3 film.
Further, the method of equivalent electrical circuit (EEC) fitting is applied to interpret EIS. Combining the EIS results shown in Figure 10 and our predecessors' research studies [58], the model of EEC shown in Figure 11 is feasible to EIS interpretation. In Figure 11, R s represents solution resistance; CPE o and R o respectively represent the capacitance and resistance of the outer loose layer in the passive film; CPE i and R i represent the capacitance and resistance of the inner dense layer in the passive film, respectively.  [58], the model of EEC shown in Figure 11 is feasible to EIS interpretation. In Figure 11, Rs represents solution resistance; CPEo and Ro respectively represent the capacitance and resistance of the outer loose layer in the passive film; CPEi and Ri represent the capacitance and resistance of the inner dense layer in the passive film, respectively.  Table 4 lists the fitted values of CPEo, Ro, CPEi and Ri, in which each CPEo, Ro, CPEi or Ri datum is the average value from ten parallel EIS tests. It was reported that the value of the passive film resistance reflected the anti-corrosion protection of the passive film, and the larger the resistance, the better the anti-corrosion protection [32]; the value of the passive film capacitance indicated the damaged area of the passive film, and the larger the capacitance, the more severe the film damage [57]. For Q235 carbon steel in WO4 2− + Cl − , MoO4 2− + Cl − or WO4 2− + MoO4 2− + Cl − solution, the values of CPEo and Ro change obviously, but the values of CPEi and Ri change slightly, confirming that for the passive film, the influence of WO4 2− and MoO4 2− on its outer loose layer is greater than that on its inner dense layer; on the other hand, the value of CPEo decreases, and the value of Ro increases in turn, indicating that the Fe2(WO4)3 plus Fe2(MoO4)3 film shows better corrosion resistance against PC than the FeWO4 plus Fe2(WO4)3 film or Fe2(MoO4)3 film.  Figure 12 shows the Mott-Schottky plots of the Q235 samples in WO4 2− + Cl − , MoO4 2− + Cl − and WO4 2− + MoO4 2− + Cl − solutions. For the Q235 samples in three solutions, the slope of the straight line part for each Mott-Schottky plot exhibits a positive value, suggesting that the FeWO4 plus Fe2(WO4)3 film, Fe2(MoO4)3 film and Fe2(WO4)3 plus Fe2(MoO4)3 film satisfy the property of the n-type semiconductor [59]. It is generally accepted that for the passive film of n-type characteristic, its Mott-Schottky plot can be interpreted with the following equation [60]:

Mott-Schottky Plot
In Equation (9), C represents space charge layer capacitance, E represents applied potential, Uf represents flat band potential, k is Boltzmann constant, T is absolute temperature, e is electron charge, ε represents passive film permittivity, ε0 is free space permittivity, and ND represents donor density. Therein, the value of ND can reflect the defect quantity of the passive film: the larger the ND of the numerical value, the more defects in  or R i datum is the average value from ten parallel EIS tests. It was reported that the value of the passive film resistance reflected the anti-corrosion protection of the passive film, and the larger the resistance, the better the anti-corrosion protection [32]; the value of the passive film capacitance indicated the damaged area of the passive film, and the larger the capacitance, the more severe the film damage [57]. For Q235 carbon steel in WO 4 2− + Cl − , MoO 4 2− + Cl − or WO 4 2− + MoO 4 2− + Cl − solution, the values of CPE o and R o change obviously, but the values of CPE i and R i change slightly, confirming that for the passive film, the influence of WO 4 2− and MoO 4 2− on its outer loose layer is greater than that on its inner dense layer; on the other hand, the value of CPE o decreases, and the value of R o increases in turn, indicating that the Fe 2 (WO 4 ) 3 plus Fe 2 (MoO 4 ) 3 film shows better corrosion resistance against PC than the FeWO 4 plus Fe 2 (WO 4 ) 3 film or Fe 2 (MoO 4 ) 3 film. 3.6. Mott-Schottky Plot Figure 12 shows the Mott-Schottky plots of the Q235 samples in WO 4 2− + Cl − , MoO 4 2− + Cl − and WO 4 2− + MoO 4 2− + Cl − solutions. For the Q235 samples in three solutions, the slope of the straight line part for each Mott-Schottky plot exhibits a positive value, suggesting that the FeWO 4 plus Fe 2 (WO 4 ) 3 film, Fe 2 (MoO 4 ) 3 film and Fe 2 (WO 4 ) 3 plus Fe 2 (MoO 4 ) 3 film satisfy the property of the n-type semiconductor [59]. It is generally accepted that for the passive film of n-type characteristic, its Mott-Schottky plot can be interpreted with the following equation [60]: Materials 2022, 15, x FOR PEER REVIEW 13 of 16 the passive film; at the same time, the value of Uf can reflect the corrosion susceptibility of the working electrode in the electrolyte, similar to that of Ec [61]. The influence of the solution component on ND is greater than that on Uf, and Table  5 lists the fitted values of ND and Uf, in which each ND or Uf datum is the average valuefrom ten parallel Mott-Schottky plot tests. For Q235 carbon steel in WO4 2− + Cl − , MoO4 2− + Cl − or WO4 2− + MoO4 2− + Cl − solution, the value of ND increases in turn, indicating that the Fe2(WO4)3 plus Fe2(MoO4)3 film takes along smaller defects than the FeWO4 plus Fe2(WO4)3 film and Fe2(MoO4)3 film [61]; the value of Uf changes slightly, which is similar to the value of Ec, implying that Q235 carbon steel presents the approximate characteristic of uniform corrosion in three solutions. In our predecessors' research studies on PC, there are many theories and models proposed to reveal the initiation of a corrosion pit, such as acidification theory [62], chemical dissolution theory [63], depassivation-repassivation theory [64], anion penetration/migration model [65], chemical-mechanical model [66] and point defect model [67]. However, in the above theories and models [62][63][64][65][66][67], a critical step of PC initiation, the adsorption and the attack of aggressive anions on the surface of the passive film, was approved consistently by our predecessors. On the passive film, defective sites were more likely to be adsorbed and attacked by aggressive anions than other regions [1]. From the above results of EIS and Mott-Schottky plot, for the FeWO4 plus Fe2(WO4)3 film, Fe2(MoO4)3 film and Fe2(WO4)3 plus Fe2(MoO4)3 film, the film resistance (Ri+Ro) increases and the defect quantity decreases in turn. Therefore, the Fe2(WO4)3 plus Fe2(MoO4)3 film has a better capability to resist the adsorption and attack of Cl − than FeWO4 plus Fe2(WO4)3 film or Fe2(MoO4)3 film [68], and WO4 2− and MoO4 2− exhibit a better inhibitive role on PC initiation when both are used together than when one is used solely. In Equation (9), C represents space charge layer capacitance, E represents applied potential, U f represents flat band potential, k is Boltzmann constant, T is absolute temperature, e is electron charge, ε represents passive film permittivity, ε 0 is free space permittivity, and N D represents donor density. Therein, the value of N D can reflect the defect quantity of the passive film: the larger the N D of the numerical value, the more defects in the passive film; at the same time, the value of U f can reflect the corrosion susceptibility of the working electrode in the electrolyte, similar to that of E c [61].
The influence of the solution component on N D is greater than that on U f , and Table 5 lists the fitted values of N D and U f , in which each N D or U f datum is the average valuefrom ten parallel Mott-Schottky plot tests. For Q235 carbon steel in WO 4 2− + Cl − , MoO 4 2− + Cl − or WO 4 2− + MoO 4 2− + Cl − solution, the value of N D increases in turn, indicating that the Fe 2 (WO 4 ) 3 plus Fe 2 (MoO 4 ) 3 film takes along smaller defects than the FeWO 4 plus Fe 2 (WO 4 ) 3 film and Fe 2 (MoO 4 ) 3 film [61]; the value of U f changes slightly, which is similar to the value of E c , implying that Q235 carbon steel presents the approximate characteristic of uniform corrosion in three solutions. In our predecessors' research studies on PC, there are many theories and models proposed to reveal the initiation of a corrosion pit, such as acidification theory [62], chemical dissolution theory [63], depassivation-repassivation theory [64], anion penetration/migration model [65], chemical-mechanical model [66] and point defect model [67]. However, in the above theories and models [62][63][64][65][66][67], a critical step of PC initiation, the adsorption and the attack of aggressive anions on the surface of the passive film, was approved consistently by our predecessors. On the passive film, defective sites were more likely to be adsorbed and attacked by aggressive anions than other regions [1]. From the above results of EIS and Mott-Schottky plot, for the FeWO 4 plus Fe 2 (WO 4 ) 3 film, Fe 2 (MoO 4 ) 3 film and Fe 2 (WO 4 ) 3 plus Fe 2 (MoO 4 ) 3 film, the film resistance (R i +R o ) increases and the defect quantity decreases in turn. Therefore, the Fe 2 (WO 4 ) 3 plus Fe 2 (MoO 4 ) 3 film has a better capability to resist the adsorption and attack of Cl − than FeWO 4 plus Fe 2 (WO 4 ) 3 film or Fe 2 (MoO 4 ) 3 film [68], and WO 4 2− and MoO 4 2− exhibit a better inhibitive role on PC initiation when both are used together than when one is used solely.

Conclusions
(1) In Cl − solution, the simultaneous additions of WO 4 2− and MoO 4 2− showed better inhibition on PC initiation than the single addition of WO 4 2− or MoO 4 2− , and the synergistic role between WO 4 2− and MoO 4 2− for the inhibition of PC initiation was present.