Investigation of the Corrosion Behavior of L245 Steel in 3.5 wt.% NaCl Solution with Varying Concentrations of Na₂S₂O₃

Abstract

In the extraction of shale gas, Cl⁻ and S₂O₃²⁻ are one of the important factors causing severe corrosion and failure of equipment and pipelines. Addressing the Cl⁻/S₂O₃²⁻ corrosion challenge in shale gas exploitation pipeline steels, this study evaluates the corrosion rates of L245 steels under diverse conditions, including S₂O₃²⁻ concentration and exposure time, utilizing the weight loss method. The microstructural, elemental, and phase compositions of the corrosion products were examined, and the electrochemical behavior of L245 steel was scrutinized under various conditions. Findings indicate that S₂O₃²⁻ addition intensifies localized corrosion on L245 steel, with the corrosion nature being contingent upon S₂O₃²⁻ concentration in the Cl⁻-containing solution. Concurrently, an escalation in S₂O₃²⁻ concentration correlates with a reduction in capacitive arc diameter and a significant decrease in film resistance, culminating in an accelerated corrosion rate.

1. Introduction

In recent years, many countries have been continuously exploring the extraction of natural gas from shale to ensure energy security, and the development of shale gas extraction has been rapid [1]. In the extraction of shale gas, corrosion is an important factor affecting safety, and failures caused by it account for about 25% of all equipment failures [2], among which Cl⁻ and S₂O₃²⁻ are one of the important factors causing severe corrosion and failure of equipment and pipelines. Pipelines are affected by chlorides, sulfides, and formation water, which can lead to severe corrosion [3,4]. Among these corrosive media, polysulfates, especially thiosulfate ions S₂O₃²⁻, have been identified as one of the most harmful substances in the oil and gas industry [5,6,7]. S₂O₃²⁻ alone does not cause corrosion, but when S₂O₃²⁻ and high concentrations of Cl⁻ coexist, it can lead to severe pitting or crevice corrosion [8,9,10,11]. However, when the concentration of S₂O₃²⁻ is high, it can mitigate pitting corrosion, as a large amount of S₂O₃²⁻ can neutralize the acidic solution in the pit [12,13,14].

Given the severe corrosion phenomena caused by the synergistic effect of S₂O₃²⁻ and Cl⁻, the corrosion mechanism of S₂O₃²⁻ and Cl⁻ on various metals or alloys has attracted widespread attention [14,15,16,17]. Fu et al. [18] studied the effect of thiosulfate ions on alloy 800 through electrochemical research and found that under the synergistic action of S₂O₃²⁻ and Cl⁻, Cl⁻ is first absorbed onto the alloy surface to destroy the passive film, while S₂O₃²⁻ adsorbs on the damaged passive film surface and reacts with the alloy matrix, hindering the passivation of alloy 800. Zakeri et al. [19] also found that the pit repassivation potential decreases with the addition of S₂O₃²⁻. In the presence of S₂O₃²⁻, a significant decrease in the critical concentration of cations within a single corrosion pit may indicate its effect on the repassivation potential by reducing the critical concentration of pit chemistry. Cui et al. [20] demonstrated through potentiostatic current and electrochemical impedance spectroscopy (EIS) that S₂O₃²⁻ promotes the dissolution process of the passive film, increases the passive current density, and reduces the polarization resistance. Choudhary et al. [21] discovered that pitting corrosion occurs in S₂O₃²⁻ solutions because the thin film on the alloy substrate and the S₂O₃²⁻ on the free surface are reduced to adsorbed sulfur during the reaction process, which accelerates the anodic dissolution and inhibits the repassivation process. Zhang et al. [22] demonstrated that under high anodic potentials, S₂O₃²⁻ is reduced to H₂S, and H₂S catalyzes the anodic dissolution of duplex stainless steel, with the reaction as follows:

$${\mathrm{S}}_2{\mathrm{O}}_3^{2-}+6{\mathrm{H}}^{+}+4{\mathrm{e}}^{-}\to 2\mathrm{S}+3{\mathrm{H}}_2\mathrm{O}$$

(1)

or disproportionate reaction:

$${\mathrm{S}}_2{\mathrm{O}}_3^{2-}+{\mathrm{H}}^{+}=\mathrm{S}+\mathrm{H}\mathrm{S}{\mathrm{O}}_3^{-}$$

(2)

The generated sulfur combines with H⁺:

$$\mathrm{S}+2{\mathrm{H}}^{+}+2{\mathrm{e}}^{-}={\mathrm{H}}_2\mathrm{S}$$

(3)

H₂S ultimately reacts with metal ion M:

$${\mathrm{M}}^{\mathrm{n}+}+{\displaystyle \frac{\mathrm{n}}{2}}{\mathrm{H}}_2\mathrm{S}=\mathrm{M}{\mathrm{S}}_{{\displaystyle \frac{\mathrm{n}}{2}}}+\mathrm{n}{\mathrm{H}}^{+}$$

(4)

Ning et al. [23] confirmed that thiosulfate (S₂O₃²⁻) can be reduced to aqueous hydrogen sulfide (H₂S) within pits at low potentials or under acidic pitting conditions, promoting the active dissolution of metal within the pits.

Although extensive research has been conducted on the synergistic corrosion of nickel-based alloys and stainless steels by S₂O₃²⁻ and Cl⁻, there is no mature research on the corrosion problems of L245 steel, which is commonly used in shale gas production and transportation pipelines, in environments containing S₂O₃²⁻ and Cl⁻. This article aims to explore the corrosion mechanism and patterns by studying the effects of different concentrations of S₂O₃²⁻ solution containing Cl⁻ and different immersion times on L245 steel, providing theoretical support and experimental basis for the corrosion and protection of L245 steel in the field of shale gas.

2. Materials and Methods

2.1. Sample Preparation

Specimens measuring 40 mm × 13 mm × 2 mm were sectioned from commercially procured L245 steel produced by Xinyou Instrument Factory, Gaoyou, China, with chemical compositions detailed in

Table 1. Chemical composition of L245 steel (wt.%).

Element	C	Si	Mn	P	S	Cr	Ni	Mo	V	Nb	Ti	Fe
L245	0.202	0.254	0.396	0.0189	0.0139	0.023	0.020	0.018	0.0014	0.003	0.0025	Bal.
API 5L	≤0.26	-	≤1.20	≤0.030	≤0.030	-	-	-	Nb + V ≤ 0.06; Nb + V+Ti ≤ 0.15

. Prior to the weight-loss experiment, specimens were meticulously polished with 150#, 600#, and 800# sandpaper to achieve a smooth, defect-free surface. Subsequent degreasing with acetone, cleaning with alcohol, and drying with cold air preceded final weighing and recording in a desiccated state within a drying oven.

2.2. Weight-Loss Experiment

A 3.5 wt.% NaCl solution was prepared using deionized water, to which Na₂S₂O₃ was added at concentrations of 0 mol/L, 0.5 mol/L, 1.0 mol/L, and 1.5 mol/L. To mitigate O₂ interference with Na₂S₂O₃, the deionized water was purged with high-purity N₂ for 4 h prior to Na₂S₂O₃ introduction. Experiments were conducted at a controlled temperature of 40 °C for immersion periods of 5, 10, and 15 days. And the corrosion solution was replaced every 7 days. The corrosion samples were retrieved, and the macroscopic morphology of the corrosion products was documented after drying. Surface corrosion products were removed in accordance with ISO 8407:2021 [24], followed by cleaning, drying, and reweighing by a scale with an accuracy of 0.1 mg to calculate the corrosion rate using Formula (5) [25].

$$CR={\displaystyle \frac{87600\Delta w}{\rho St}}$$

(5)

where CR represents the corrosion rate with the unit mm/a, 87,600 is a constant, Δw is the mass loss due to corrosion before and after the experiment with the unit g, ρ is the density of the steel (7.85 g/cm³) with the unit g/cm³, S is the surface area of the specimen (12.52 cm²) with the unit cm², and t is the immersion time with the unit h. To reduce accidental errors, three specimens were prepared for each experiment.

2.3. Corrosion Product Analysis

Corrosion products on the corroded L245 steel were analyzed using a DX-2700 X-ray diffractometer manufactured by Haoyuan Instrument Co., Ltd., Dandong, China to determine the phases. The test angle range was 10°~70°, with a step of 0.02° and a scanning speed of 2°/min. The corrosion surface morphology of L245 steel was observed using secondary electron by a ZEISS EV0 MA15 scanning electron microscope (SEM) manufactured by Carl Zeiss Co., Ltd., Oberkochen, Germany, with an accelerating voltage of 20 kV and 50 nm step size. The chemical composition of corrosion products was analyzed by energy-dispersive spectroscopy (EDS) integrated with the SEM. Finally, the product composition was ascertained through a synthesis of SEM-EDS data and X-ray diffraction results.

2.4. Electrochemical Testing

The post-immersion specimens were coated with epoxy resin, exposing only a 1 cm² area as the active surface. Electrochemical experiments were performed using a PGSTAT302N workstation manufactured by Metrohm AG Ltd., Herisau, Switzerland, employing a three-electrode configuration with the L245 steel as the working electrode, a platinum plate as the counter electrode, and a saturated calomel electrode as the reference. Open-circuit potential (OCP) was monitored for 2000s to ensure system stabilization. EIS was conducted within a frequency spectrum of 10⁵ to 10⁻² Hz at an amplitude of ±5 mV. Data were analyzed using ZSimpWin software 3.60, and potentiodynamic polarization curves were generated from −0.5 to 1.0 V (Vs. OCP) at a scan rate of 1 mV/s, with data fitting performed using OriginPro 2024b 10.1.5.132 to ascertain i_corr and E_cor values.

3. Results

3.1. Corrosion Weight Loss Test

Figure 1 delineates the corrosion rates of L245 steel in media of varying concentrations over different time intervals. It is observed that, at equivalent exposure durations, the average corrosion rate escalates with increasing S₂O₃²⁻ concentration. Conversely, at constant S₂O₃²⁻ concentrations, the corrosion rate diminishes with extended immersion periods, suggesting that surface-forming corrosion products may impede further corrosion [26].

3.2. Analysis of the Corrosion Product

3.2.1. Macroscopic Corrosion Morphology

Figure 2 shows the samples corroded for different durations at various concentrations of S₂O₃²⁻ in a 3.5 wt.% NaCl solution. As shown in Figure 2, L245 steel shows different morphology after corrosion in solution containing S₂O₃²⁻ and without S₂O₃²⁻. It is obvious that the steel surface corroded in the solution containing S₂O₃²⁻ is covered with a layer of uneven black corrosion products compared with the solution without S₂O₃²⁻. In addition, the L245 steels with 1.0 mol/L and 1.5 mol/L S₂O₃²⁻ had thick corrosion products on the surface and showed severe localized corrosion, which shows that the increase in S₂O₃²⁻ concentration can promote the localized corrosion and the formation of corrosion products. Moreover, localized corrosion occurred under the combined action of Cl⁻ and S₂O₃²⁻, which is consistent with the conclusions obtained by Ning et al. [23].

3.2.2. Phase Analysis by XRD

Figure 3 shows the XRD spectra of L245 steel after being corroded for 15 days under four different conditions. The XRD patterns reveal the presence of different phases on the corrosion film of L245 steel in NaCl solutions with varying concentrations of S₂O₃²⁻, including FeOOH, FeCl₃, FeS, and Fe₂O₃. In the solutions without S₂O₃²⁻, the corrosion films are composed of FeOOH, FeCl₃, and Fe₂O₃. Although the dissolution of steel in NaCl solution first produces Fe²⁺, it is more common for these products to be oxidized to Fe³⁺ when exposed to the air [27]. The presence of FeS peaks in the XRD patterns of the Cl⁻/S₂O₃²⁻ solution clearly describes the formation of an FeS layer on the surface of the carbon steel.

3.2.3. Corrosion Product and Elemental Composition Analysis by SEM-EDS

Figure 4 shows the surface morphology and energy spectrum of L245 steel samples after being corroded for 15 days in a 3.5 wt.% NaCl solution with different concentrations of S₂O₃²⁻. The corrosion morphologies of L245 steels at different S₂O₃²⁻ concentrations also showed differences. As can be seen from Figure 4a, a thick layer of corrosion products has formed on the surface of the corroded L245 steel, which is relatively dense and has a certain protective effect on the substrate, but it cannot effectively prevent the corrosive medium from entering the film/substrate interface to corrode the substrate. In the pure NaCl solution, the main elements of the corrosion products are C, O, Fe, and a small amount of Cl, which indicates that the corrosion film is mainly composed of iron oxides and chlorides, such as FeOOH and Fe₂O₃ [28,29].

From Figure 4b, it can be seen that a thick layer of corrosion products has formed on the surface of the corroded L245 steel with S₂O₃²⁻, which is loose and has many pores, providing poor protection to the substrate and failing to effectively prevent the corrosive medium from entering the film/substrate interface to corrode the substrate. Compared with Figure 4a, the corrosion is more severe. The main elements of corrosion products are C, O, Fe, S, and a small amount of Cl, indicating that the corrosion film is mainly composed of iron sulfides, oxides, and chlorides. The S content at the corroded site is 7.18 wt.%, indicating that iron sulfides dominate in the intact corrosion products. However, when the concentration of S₂O₃²⁻ is low, it is difficult to form a dense, protective corrosion product film, and no dense iron sulfide corrosion products are observed on the sample surface.

From Figure 4c, it can be seen that a thick layer of corrosion products has formed on the surface of L245 after corrosion, which is loose and has many evenly distributed larger pores, providing poor protection to the substrate. The main corrosion products are C, O, Fe, S, and a small amount of Cl, among which the S content reaches 10.41 wt.%, indicating that the corrosion products are mainly composed of iron sulfides, with some oxides and a small amount of chlorides.

Analyzing Figure 4d, it can be found that compared with Figure 4b,c, the surface of L245 steel has produced a denser corrosion product, and the diameter of the pores has also been relatively reduced, indicating that the increase in S₂O₃²⁻ concentration has intensified the corrosion process, resulting in a denser corrosion product film. The elements of the corrosion products are also mainly Fe, O, Cl, and S, with the S content in the elements reaching as high as 20.17 wt.%, which is the highest S content among the four states. This indicates that as the concentration of S₂O₃²⁻ increases, the proportion of S element in the corrosion products also increases. In addition, under this condition, the high concentration of S₂O₃²⁻ is conducive to the precipitation of Fe sulfides; hence, the surface is observed to have denser and thicker corrosion products. The research of Cao et al. [30] also proved this point.

L245 steel exhibits uniform corrosion in pure NaCl solution, while obvious local corrosion is observed when Cl⁻ and S₂O₃²⁻ ions coexist in the solution. EDS experiments showed the elemental analysis results of the products formed on the surface of L245 steel in 3.5 wt.% NaCl solution with different concentrations of S₂O₃²⁻: the main components of the corrosion product film in pure NaCl solution are iron oxides and chlorides. When Na₂S₂O₃ is added to the NaCl solution, a sulfur peak appears, indicating the formation of a sulfide layer on the surface. Therefore, the corrosion products are a mixture of iron oxides and chlorides in pure Cl⁻ solution and oxides, chlorides, and sulfides in Cl⁻/S₂O₃²⁻ solution. Combined with the relevant literature and XRD results, reactions that produce FeOOH, FeCl₃, Fe₂O₃, and FeS occur during the corrosion process, forming a pseudo-passivation layer [18], which then leads to pitting corrosion. The schematic diagram of Cl⁻/S₂O₃²⁻ corrosion is shown in Figure 5. S₂O₃²⁻ in the solution reacts with H⁺ to form elemental sulfur, and then reacts with free H⁺ to form H₂S. H₂S contacts with the metal matrix and then forms FeS corrosion products, which are covered on the surface of the matrix. Cl⁻ reacts directly with the metal matrix to form FeCl₂.

3.3. Corrosion Electrochemical Testing

3.3.1. Potentiodynamic Polarization Curve

Figure 6 shows the polarization curves of L245 steel after 5 days of corrosion in 3.5 wt.% NaCl solution with different concentrations of S₂O₃²⁻. It can be observed that both the anodic and cathodic processes of L245 steel are electrochemically controlled, and the polarization curves exhibit nonlinear behavior, which is more pronounced at a concentration of 1.5 mol/L S₂O₃²⁻. The solution with 1.5 mol/L S₂O₃²⁻ shows a constant current in the potential range of −0.62 V to −0.55 V, indicating the formation of a pseudo-passivation layer on the surface of carbon steel (with a current roughly maintained at 0.12 mA/cm²). From the increase in current, it can be seen that this layer is eventually pierced when the breakdown potential (Eb) is −0.55 V. The pseudo-passivation layer is composed of a mixture of sulfides and oxides, but its main component is likely to be sulfides, as the formation of sulfides is thermodynamically more feasible than the formation of oxides [21].

Figure 7 shows the polarization curves of L245 steel after 10 days of corrosion in 3.5 wt.% NaCl solution with varying concentrations of S₂O₃²⁻. It can be observed that the anodic process of L245 steel is more significantly inhibited under the corrosion conditions of 1.0 mol/L and 1.5 mol/L S₂O₃²⁻ concentrations. A region similar to the passivation zone appears during the anodic process. However, judging from the anodic current density in this “passivation zone”, it is not a true passivation zone, as the anodic current in a typical passivation process is generally in the range of a few to several tens of µA/cm². It is obvious that the anodic process is suppressed by the corrosion product film, leading to a decrease in anodic current density. Concurrently, as the concentration of S₂O₃²⁻ increases, there is a noticeable fluctuation in the current density near the corrosion potential.

Figure 8 shows the polarization curves of L245 steel after 15 days of corrosion in 3.5 wt.% NaCl solution with different concentrations of S₂O₃²⁻. It can be observed that the anodic process of L245 steel is also significantly inhibited under the corrosion conditions at a concentration of 0.5 mol/L S₂O₃²⁻.

Figure 9 illustrates the polarization curves of L245 steel in solutions with varying concentrations of S₂O₃²⁻ and 3.5 wt.% NaCl after being corroded for 5 days, 10 days, and 15 days, respectively. Regardless of the concentration of the corrosive medium, the general trend is that as the number of corrosion days increases, the corrosion product film gradually becomes more complete, significantly suppressing the anodic process. However, due to the composition of the corrosion product film, it fails to protect the substrate, resulting in an increase in corrosion current density and accelerated corrosion rate, which is most evident in solutions with added S₂O₃²⁻ and a certain shift in the corrosion potential.

To understand the behavior observed in the presence of Na₂S₂O₃, whether it is due to the sole presence of Cl⁻ ions or the combined effect of S₂O₃²⁻ and Cl⁻ ions, polarization experiments were conducted in pure NaCl solution, with results as shown in Figure 9a. With the increase in corrosion days, the polarization behavior does not exhibit the nonlinear behavior observed with the addition of Na₂S₂O₃. At the same time, the corrosion current density increases from 7.9 × 10⁻⁶ to 3.9 × 10⁻⁵ A/cm², but it is always lower than that in the presence of Na₂S₂O₃ for the same corrosion days. This indicates that the addition of S₂O₃²⁻ promotes corrosion on the surface of carbon steel. However, in the presence of S₂O₃²⁻, the curve exhibits nonlinear behavior, which may be due to the formation of a porous and non-protective sulfide layer on the surface. This suggests that the susceptibility to pitting corrosion is enhanced only when both Cl⁻ and S₂O₃²⁻ are present. Therefore, when the solution contains only NaCl, carbon steel undergoes uniform corrosion, and when the solution contains both substances, carbon steel may exhibit pitting corrosion, depending on the concentration of these two substances. Wu et al. [31] studied the effects of chloride-to-thiosulfate concentration ratio (CTCR) on the corrosion behavior of passive films in neutral solutions, and confirmed that S₂O₃²⁻ has a significant promoting effect on pitting corrosion at high concentration ratios.

The logarithm of current density increases linearly with the applied potential in the range of tens to hundreds of millivolts relative to E_corr. Therefore, the Tafel extrapolation method [32] is used to fit the polarization curve.

Table 2. Corrosion potential (E_corr) and corrosion current density (i_corr) values obtained from polarization curves.

Days	Corrosion Medium Concentration	Βa (mV/dec)	−βc (mV/dec)	Ecorr (V)	Icorr (A/cm2)
5 d	3.5 wt.%NaCl	257	146	−0.81	7.8 × 10−6
	3.5 wt.% NaCl + 0.5 mol/L S2O32−	213	186	−0.69	1.3 × 10−5
	3.5 wt.% NaCl + 1.0 mol/L S2O32−	73	29	−0.91	2.6 × 10−5
	3.5 wt.% NaCl + 1.5 mol/L S2O32−	192	139	−0.75	2.7 × 10−5
10 d	3.5 wt.% NaCl	359	170	−0.45	2.7 × 10−5
	3.5 wt.% NaCl + 0.5 mol/L S2O32−	291	104	−0.84	9.4 × 10−5
	3.5 wt.% NaCl + 1 mol/L S2O32−	205	126	−0.82	1.0 × 10−4
	3.5 wt.% NaCl + 1.5 mol/L S2O32−	196	30	−0.81	1.7 × 10−4
15 d	3.5 wt.% NaCl	213	127	−0.94	3.9 × 10−5
	3.5 wt.% NaCl + 0.5 mol/L S2O32−	250	120	−0.89	9.0 × 10−5
	3.5 wt.% NaCl + 1 mol/L S2O32−	250	69	−0.89	9.1 × 10−5
	3.5 wt.% NaCl + 1.5 mol/L S2O32−	335	81	−0.87	1.1 × 10−4

lists the electrochemical parameters such as corrosion current density (i_corr), corrosion potential (E_corr), and Tafel slopes obtained from the fitting of polarization curves under different corrosion conditions using the software built into the system. The polarization curves clearly describe the significant impact of S₂O₃²⁻ on the electrochemical behavior of carbon steel in the NaCl solution. A main observation from these polarization curves is that when the concentration of S₂O₃²⁻ increases from 0 mol/L to 1.5 mol/L, the icorr value for L245 steel samples corroded for 5 days increases from 7.9 × 10⁻⁶ A/cm² to 2.7 × 10⁻⁵ A/cm², and similar increasing trends are observed for samples corroded for 10 days and 15 days. This indicates that the addition of S₂O₃²⁻ in the NaCl solution accelerates the corrosion rate of carbon steel.

Due to the presence of S₂O₃²⁻ ions in the NaCl solution, the cathodic corrosion current density increases significantly, which is attributed to the reduction reactions of these substances. The typical cathodic reactions that occur on the metal surface are determined by the pH of the solution, the electrochemical potential, and the properties of the corroded surface (passivated/unpassivated). Additionally, the elemental sulfur produced may further react with Fe²⁺ to form an FeS film, thereby accelerating the corrosion process; that is, the addition of S₂O₃²⁻ affects both anodic and cathodic reactions in the NaCl solution [33].

The reduction of S₂O₃²⁻ to sulfur is more likely to occur on the unpassivated surface of L245 steel. In the NaCl solution, the surface of the L245 steel is not fully covered by an oxide film, which triggers the formation of sulfur, leading to the formation of an FeS layer on the surface of L245 steel. As the concentration of S₂O₃²⁻ increases, the formation rate of FeS may increase, resulting in the formation of a pseudo-passivation layer, which can be seen from the constant current region of the polarization curve. At higher anodic potentials, the pseudo-passivation layer breaks down, indicating that carbon steel is susceptible to localized corrosion [34], which is consistent with the surface morphology observations of the corroded samples mentioned earlier. Generally speaking, compared with the oxide film, the sulfide film formed in the early stage of corrosion is more porous and loose and does not provide protection [35]. Therefore, this pseudo-passivation layer cannot fully protect the metal substrate, and uniform corrosion can also occur simultaneously at higher concentrations of S₂O₃²⁻.

By plotting the data obtained from the fitting in Table 2, the trends of E_corr and i_corr with the change of S₂O₃²⁻ concentration can be obtained, as shown in Figure 10 and Figure 11. Comparing Figure 10 with Figure 4, it can be seen that although there are some deviations in the fitting results, the trend of change in corrosion current density of the polarization curve is consistent with the trend of average corrosion rate obtained by the weight loss method mentioned earlier. And the i_corr increased with the increase in S₂O₃²⁻ concentration, which is consistent with the research results of Al-mamun et al. [9] and Kappes et al. [7].

i_corr increases with the concentration of S₂O₃²⁻, which can be explained as follows [7,9,36,37]: In pure NaCl solutions, the passive film formed on the surface of L245 steel has a certain hindrance to Cl⁻, which makes the i_corr low. When S₂O₃²⁻ was added to the solutions containing Cl⁻, the synergistic effect of S₂O₃²⁻ and Cl⁻ could promote the degradation of the passive film on the surface of L245 steel, thereby increasing i_corr. In addition, the element S spontaneously generated by the reduction or disproportionation of S₂O₃²⁻ also reacts with Fe, resulting in an increase in i_corr. Hence, the addition of S₂O₃²⁻ has a significant impact on the corrosion of L245 steel in Cl⁻-containing solutions.

3.3.2. Electrochemical Impedance Spectroscopy Test

Figure 12 presents the EIS data obtained for L245 steel after corrosion under four different concentrations of S₂O₃²⁻ at condition 1 (40 °C, 3.5 wt.% NaCl solution, static, 5 days). From the Nyquist plot in Figure 12a, it is evident that upon the addition of S₂O₃²⁻, as the concentration of S₂O₃²⁻ increases, the diameter of the capacitive arc gradually decreases, and the corresponding film resistance significantly diminishes, leading to an enhanced corrosion rate. This indicates that the higher the concentration of S₂O₃²⁻ added, the more severe the corrosion of L245 steel. These findings are in agreement with the conclusions drawn from the previous weight loss tests and polarization measurements.

In order to gain a more in-depth understanding of the corrosion of L245 steel surfaces with added Na₂S₂O₃, the original impedance spectrum data are fitted to an equivalent electric circuit (EEC). Figure 12d represents the equivalent electric circuit model [38,39] corresponding to the EIS data, and taking into account the inhomogeneity of the corrosion product film and the surface of the steel, two constant phase angle elements are used instead of a capacitor, where R_s denotes the solution resistance, R_f and Q_f represent the resistance and capacitance of the passive film, R_ct and Q_dl are the charge transfer resistance and the double-layer capacitance, respectively. The fitting parameters are provided in

Table 3. Optimal EEC parameters obtained by dissolution of L245 in different corrosive medium (5 d).

EEC Parameters	3.5 wt.% NaCl	3.5 wt.% NaCl + 0.5 mol/L S2O32−	3.5 wt.% NaCl + 1 mol/L S2O32−	3.5 wt.% NaCl + 1.5 mol/L S2O32−
Rs (Ωcm−2)	33.00	7.15	2.76	33.26
Qf (Ω−1sncm−2)	3.141 × 10−3	9.683 × 10−3	7.092 × 10−3	1.021 × 10−3
nf	0.3400	0.8046	0.7649	0.4319
Rf (Ωcm−2)	48.87	29.26	27.33	24.04
Qdl (Ω−1sncm−2)	9.350 × 10−3	8.866 × 10−3	8.088 × 10−3	1.095 × 10−2
ndl	0.6047	0.8841	0.8574	0.8392
Rct (Ωcm−2)	1046	527	438	283

.

It can be seen that Q_f decreases with the increase in S₂O₃²⁻ concentration after the addition, indicating the thickening of the surface corrosion product film with the increase in S₂O₃²⁻ concentration, while the decrease in n_f indicates the increase in the inhomogeneity of the corrosion product film. In addition, R_ct decreasing with increasing S₂O₃²⁻ concentration shows that the charge transfer process is accelerated and the corrosion rate increased, which is consistent with the results of the polarization curves.

Figure 13 presents the EIS data obtained for L245 steel after corrosion under four different concentrations of S₂O₃²⁻ at condition 2 (40 °C, 3.5 wt.% NaCl solution, static, 10 days). From the Nyquist plot in Figure 13a, it can be observed that as the concentration of S₂O₃²⁻ increases, the diameter of the capacitive arc decreases, and the corresponding film resistance significantly decreases, leading to an increased corrosion rate. The trend in EIS analysis with a corrosion period of 10 days is consistent with that of 5 days, and the results are also in agreement with the corrosion rate variation laws determined by weight-loss experiments. At the same time, it can be seen from Figure 13a that in all cases, two time constants (capacitive loops) are observed in the impedance spectra after 10 days of corrosion.

From

Table 4. Optimal EEC parameters obtained from the dissolution of L245 in different corrosive media (10 d).

EEC Parameters	3.5 wt.% NaCl	3.5 wt.% NaCl + 0.5 mol/L S2O32−	3.5 wt.% NaCl + 1 mol/L S2O32−	3.5 wt.% NaCl + 1.5 mol/L S2O32−
RS (Ωcm−2)	31.77	27.21	25.64	24.31
Qf (Ω−1sncm−2)	2.060 × 10−3	1.847 × 10−2	2.587 × 10−2	1.180 × 10−4
nf	0.3691	0.8341	0.8190	0.4635
Rf (Ωcm−2)	37.37	5.15	3.48	1.71
Qdl (Ω−1sncm−2)	1.085 × 10−2	4.269 × 10−2	1.802 × 10−2	4.651 × 10−2
ndl	0.5156	0.7140	0.8545	0.8000
Rct (Ωcm−2)	217	539	472	457

, it is evident that as the concentration of S₂O₃²⁻ increases, R_f decreases. Consequently, the total impedance associated with the Faradaic reaction decreases with the increasing concentration of S₂O₃²⁻. Additionally, with the increase in S₂O₃²⁻ concentration, R_ct decreases. The results indicate that the rate of the cathodic reaction increases as the concentration of S₂O₃²⁻ increases. The total impedance caused by these parameters decreases with the increase in S₂O₃²⁻ concentration. This corresponds to the increased corrosion rate with the increase in S₂O₃²⁻ concentration observed in the polarization measurements mentioned earlier.

Figure 14 shows the impedance spectra obtained for L245 steel after corrosion under four different concentrations of S₂O₃²⁻ at condition 3 (40 °C, 3.5 wt.% NaCl solution, static, 15 days). It can be observed that two time constants (capacitive loops) are also seen in the impedance spectra after 15 days of corrosion in the presence of S₂O₃²⁻. Therefore, the equivalent electrical circuit shown in Figure 14d was used to quantitatively analyze the impedance data. From the Nyquist plot in Figure 14a, it can be seen that as the concentration of S₂O₃²⁻ increases, the diameter of the capacitive arc decreases, and the corresponding film resistance significantly decreases, leading to an increased corrosion rate. The results are consistent with the changes in corrosion rate obtained from weight loss and polarization tests.

Table 5. Optimal EEC parameters obtained from the dissolution of L245 in different corrosive media (15 d).

EEC Parameters	3.5 wt.% NaCl	3.5 wt.% NaCl + 0.5 mol/L S2O32−	3.5 wt.% NaCl + 1.0 mol/L S2O32−	3.5 wt.% NaCl + 1.5 mol/L S2O32−
RS (Ωcm−2)	6.240	0.690	0.549	2.599
Qf (Ω−1sncm−2)	1.420 × 10−3	1.966 × 10−3	7.457 × 10−3	2.022 × 10−2
nf	0.8490	0.8311	0.7952	0.7638
Rf (Ωcm−2)	41	110	61	22
Qdl (Ω−1sncm−2)	1.156 × 10−2	1.807 × 10−2	1.378 × 10−2	2.416 × 10−2
ndl	0.8289	0.5769	0.5503	0.7082
Rct (Ωcm−2)	201.4	627	525	487

provides the corresponding EIS fitting results.

It can be seen that after 15 days of immersion, Q_f increases with increasing S₂O₃²⁻ concentration, indicating a decrease in the thickness of the corrosion product film, while the decrease in n_f indicates an increase in the inhomogeneity of the corrosion product film. The thinness and inhomogeneity of the corrosion product film promote the reaction between the corrosion medium and the collective, and thus the corrosion rate is increased. Moreover, the decrease in R_f and R_ct also shows an increase in corrosion rate.

As expected from the polarization behavior, uniform corrosion was observed on the surface of L245 steel in the pure NaCl solution, while localized corrosion was observed in the presence of S₂O₃²⁻, and the phenomenon of localized corrosion became more pronounced with increasing concentration of S₂O₃²⁻. This phenomenon supports the findings from weight-loss experiments and polarization measurements, which suggest that when Cl⁻ and S₂O₃²⁻ ions are present in the solution simultaneously, a pseudo-passivation layer is formed, which is further eroded by Cl⁻ ions, leading to localized corrosion with a tendency towards pitting.

4. Conclusions

The corrosion of L245 steel commonly used in shale gas gathering and transmission pipelines in neutral environments containing S₂O₃²⁻ and Cl⁻ has been studied. The main conclusions are as follows:

The results of weight-loss experiments and potentiodynamic polarization tests confirmed that S₂O₃²⁻ had a significant effect on the corrosion of L245 steels in Cl⁻-containing solutions. The synergistic effect of S₂O₃²⁻ and Cl⁻ could promote the localized corrosion of L245 steels, and the corrosion rate of L245 steel increases with the increase in S₂O₃²⁻ concentration. Moreover, under the same corrosive medium conditions, the longer the corrosion time, the smaller the corrosion rate.

S₂O₃²⁻ has an obvious effect on the formation of surface corrosion products of L245 steel. The higher the concentration of S₂O₃²⁻, the thicker the films formed on the surface of L245 steels. However, the corrosion product films are porous and loose, offering very weak protection. EDS and XRD results indicate that the main corrosion products in this experiment are FeCl₃, Fe₂O₃, and FeOOH, and the addition of S₂O₃²⁻ leads to the formation of FeS in addition to Fe oxides and chlorides.

The concentration of S₂O₃²⁻ can affect the surface corrosion morphology of L245 steel. The addition of S₂O₃²⁻ causes localized corrosion on the surface of L245 steel, and the nature of corrosion (uniform or localized) depends on the CTCR. Uniform corrosion occurs when S₂O₃²⁻ is not added, while localized corrosion is observed when S₂O₃²⁻ is added. Therefore, referring to this study, the corrosion protection of L245 steel in shale gas exploitation could be possible by regulating CTCR.

This article only studied the ideal corrosion situation of S₂O₃²⁻ containing Cl⁻, while in reality, formation water may also contain other ions such as SO₄²⁻ and CO₃²⁻ that can affect corrosion. Therefore, the study of the effects of other ions or the combined action of multiple ions on S₂O₃²⁻ corrosion should have significant practical implications.