Research on Corrosion Behavior of 20 Steel in Simulated High Chloride Desulfurization Wastewater

Highlights

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High Cl⁻ concentrations reduce the uniform corrosion rate of 20 steel in desulfurization wastewater.

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Chloride promotes formation of corrosion product films with partial protective properties.

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Increasing Cl⁻ concentration shifts corrosion from uniform attack to localized pitting.

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Maximum pitting depth increases dramatically, reaching ~153 μm at 100,000 mg·L⁻¹ Cl⁻.

Abstract

Corrosion of pipelines by flue gas desulfurization (FGD) wastewater compromises the normal operation of the desulfurization tower, and corrosion under high-chloride conditions in particular severely damages the tower’s internal structure. To further elucidate the corrosion mechanism at elevated Cl⁻ concentrations, the corrosion behavior of 20 steel exposed to high-chloride FGD wastewater at different Cl⁻ concentrations was investigated through weight-loss measurements, electrochemical tests, immersion corrosion experiments, composition analysis, and microscopic morphology characterization. The results revealed that higher Cl⁻ concentrations corresponded to lower corrosion rates: the corrosion rate reached 0.1964 mm/y in the absence of Cl⁻, but decreased to 0.1537 mm/y at a Cl⁻ concentration of 100,000 mg/L. XPS analysis showed that as the Cl⁻ concentration increased, the corrosion film gradually transformed from porous FeOOH into dense Fe₃O₄. Localized pitting analysis indicated a positive correlation between Cl⁻ concentration and pitting susceptibility. At Cl⁻ concentrations of 0 and 100,000 mg/L, the corrosion current density decreased from 32.44 μA/cm² to 6.43 μA/cm² after 72 h, decreasing by a factor of approximately 5.05. This behavior is attributed to the fact that Cl⁻ increases solution conductivity in high-chloride environments, thereby promoting the formation rate of the corrosion film. Additionally, high Cl⁻ levels reduce dissolved oxygen in the solution, causing the corrosion film to progressively react and form denser Fe₃O₄. Nevertheless, the high penetrability of Cl⁻ continues to aggravate pitting corrosion of 20 steel.

1. Introduction

Coal-fired power generation remains one of the mainstream power generation methods worldwide [1,2]. Abundant coal resources have been proven in the TuSan–Hami (hereinafter referred to as Turpan–Hami) region of Xinjiang, China, with predicted reserves reaching the trillion-ton level. However, the coal species in this region are characterized by high chlorine content, with an average chlorine content of approximately 0.5% and a maximum of 1% [3,4,5]. Large amounts of chlorine-containing and sulfur-containing flue gases released during the combustion of high-chlorine coal cause severe corrosion problems to key internal components of boilers and downstream desulfurization systems [6,7]. Among existing desulfurization technologies, wet flue gas desulfurization (WFGD) is one of the most widely used methods [8]. The desulfurization wastewater generated by this method is usually weakly acidic with a temperature of about 55 °C, and 68.88%–77.31% of the chlorine in coal needs to be discharged through desulfurization wastewater [9]. Therefore, the Cl⁻ concentration in general desulfurization wastewater ranges from 1000 to 10,000 mg/L [10,11,12,13], while the Cl⁻ concentration in desulfurization wastewater generated by the combustion of high-chlorine coal in the Turpan–Hami region can be as high as 80,000–100,000 mg/L. Currently, 20 steel is widely used for desulfurization wastewater pipelines. On-site investigations at power plants have found that high-chloride desulfurization wastewater causes severe corrosion to 20 steel, leading to frequent pipeline replacements.

Researchers have conducted corresponding studies on the corrosion behavior of pipelines caused by desulfurization wastewater. Cl⁻ has a small ionic radius, excellent diffusion performance, is an acidic anion with erosiveness [14,15,16,17], and is easy to adsorb on metal surfaces, inducing pitting corrosion [18,19]. Dastgheib et al. [20] tested the corrosion resistance of carbon steel and stainless steel under different conditions in lime sludge flue gas desulfurization. The results showed that the corrosion resistance of stainless steel in limestone slurry is significantly better than that of carbon steel. Adding 5000 ppm of Cl⁻ to the slurry significantly increased the corrosion rates of both carbon steel and stainless steel. This study only examined the effect of a single Cl⁻ concentration. Fan et al. [21] conducted a study on the effect of different Cl⁻ concentrations on metal corrosion and found that when the chloride concentration increased from 20,000 mg·L⁻¹ to 50,000 mg·L⁻¹, the mass loss rates of 316L (AISI 316L/UNS S31603) and 2205 stainless steels (UNS S32205) increased by 32% and 93% respectively. Smith et al. [22] studied the effect of high concentrations of Cl⁻ on metal corrosion and found that the effect of Cl⁻ on the corrosion rate of carbon steel varies in different concentration ranges: when the Cl⁻ content increases from 0.05% to 3.5%, the corrosion rate increases significantly; while in the range of 3.5% to 10%, the effect of Cl⁻ on the corrosion rate is not obvious.

Regarding corrosion in a mixed SO₄²⁻ and Cl⁻ environment, Deng et al. [23] conducted immersion tests on 316 stainless steel (AISI 316/UNS S31600). In a solution containing 0.5% Cl⁻, the SO₄²⁻ concentration was adjusted by adding Na₂SO₄ within the range of 0 to 0.8%. When the SO₄²⁻ concentration was lower than 0.42%, the Critical Pitting Temperature (CPT) of 316 stainless steel (AISI 316/UNS S31600) was lower than that without SO₄²⁻, indicating that SO₄²⁻ accelerates the initiation of pitting corrosion. When the SO₄²⁻ concentration exceeded 0.42%, its CPT was higher than that in the SO₄²⁻-free system, demonstrating the inhibitory effect of SO₄²⁻ on pitting initiation. Huttunen-Saarivirta et al. [24] carried out experiments on various austenitic stainless steels including 1.4539 (EN 1.4539/AISI 904L/UNS N08904). The results showed that the corrosion rate was 0.04 mm/y or lower in sulfuric acid solutions with mass fractions. This reduction arises from the salting-out effect of high NaCl concentrations, with significant decreases of 5%, 10%, 20%, and 50% at 50 °C. When 3000 ppm of Cl⁻ was added to the 20% mass fraction sulfuric acid solution, the corrosion rate reached a maximum of 0.17 mm/y. Zhao et al. [25] analyzed the effect of varying Cl⁻ content on the corrosion behavior of 316L (AISI 316L/UNS S31603) stainless steel. Their study revealed that with a constant SO₄²⁻ content, the increase in Cl⁻ concentration led to decreases in the radius of the capacitive arc, polarization resistance (R_p), and pitting potential (E_b). When the Cl⁻ content increased from 0 to 50 g/L, significant corrosion occurred on the surface of 316L (AISI 316L/UNS S31603) stainless steel. Many scholars have focused on the corrosion behavior of stainless steel, mostly under conditions of low chloride ion concentration [26,27,28,29]. There are relatively few studies on the corrosion of 20 steel, which is more commonly used in desulfurization wastewater pipelines, especially the corrosion mechanism in high-chloride environments remains unclear.

Based on this, this study aims to investigate the effect of high-chloride desulfurization wastewater on the corrosion behavior of 20 steel: the corrosion rate under different corrosion environments was calculated by the weight loss method; electrochemical methods were used to analyze the effect of the components of desulfurization wastewater on the corrosion process and calculate the corresponding film thickness; combined with characterization techniques such as XRD, SEM, and white light interferometry, the corrosion products and surface morphology of 20 steel in high-chloride environments were clarified, thereby revealing the corrosion mechanism of high-chloride desulfurization wastewater on 20 steel under certain temperature and low oxygen conditions.

2. Samples and Methods

2.1. Preparation of Samples and Solutions

All experimental samples were made of 20 steel (GB/T 699, China; equivalent to AISI 1020/SAE 1020/UNS G10200/ASTM A29 1020/DIN 1.1151 (C22E)/EN 10083-2 C22E), a commonly used low-carbon steel in desulfurization towers. Its specific chemical composition is detailed in

Table 1. The chemical composition of 20 steel (wt.%).

C	Si	Mn	P	S	Cr	Ni	Cu	Fe
0.19	0.22	0.55	0.012	0.002	0.03	0.02	0.01	Bal.

. Specimens with dimensions of 10 mm × 10 mm × 3 mm were welded to wires, then embedded and encapsulated with epoxy resin, with only a single metal surface exposed as the working surface. Subsequently, the encapsulated specimens were ground sequentially with 240-mesh, 400-mesh, 600-mesh, 800-mesh, 1000-mesh, 1500-mesh, and 2000-mesh silicon carbide sandpapers, followed by mechanical polishing using diamond polishing paste with a particle size of 2.5 μm. After polishing, the samples were subjected to ultrasonic degreasing with acetone and rinsing with ethanol in sequence to remove surface grease. After thorough drying, the samples were stored in a desiccator for later use.

The solution prepared for the experiment was a simplified simulated wastewater of high-chloride desulfurization wastewater, in which the SO₄²⁻ and Cl⁻ concentrations were controlled by adding MgSO₄ (Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) and NaCl (Tianjin Zhiyuan Chemical Reagent Co., Ltd., Tianjin, China), respectively. For the Cl⁻-containing corrosive solution, the pH was adjusted to 4 using HCl (Sinopharm Chemical Reagent Co., Ltd., Shanghai, China), while for the Cl⁻-free corrosive solution, the pH was adjusted to 4 using H₂SO₄ (Sinopharm Chemical Reagent Co., Ltd., Shanghai, China). A total of four solution systems with different ion concentrations were set up in this experiment. The SO₄^2- concentration in all corrosive media was fixed at 10,000 mg/L, while the Cl⁻ concentrations were set to 0, 60,000, 80,000 and 100,000 mg/L respectively. The specific compositions are shown in

Table 2. Ion concentrations in different solutions.

	C1	C2	C3	C4
SO42− (mg/L)	10,000	10,000	10,000	10,000
Cl− (mg/L)	0	60,000	80,000	100,000
Mg2+ (mg/L)	2500	2500	2500	2500

. The immersion corrosion tests were conducted in strict accordance with ASTM G31-21 “Standard Guide for Laboratory Immersion Corrosion Testing of Metals” and NACE TM0169-2016 “Laboratory Corrosion Testing of Metals”, with appropriate adaptations made for the simulated high-chloride desulfurization wastewater environment. Three parallel specimens were tested in each solution system. The test duration was 100 h, the temperature was precisely maintained at 55.0 ± 0.5 °C, and the solution volume-to-specimen surface area ratio was 300 mL/cm², which fully meets the requirements specified in the ASTM G31 standard. The flow chart is shown in Figure 1.

2.2. Electrochemical Measurements

The tests were conducted using a CORRTEST CS350M (Wuhan Corrtest Instruments Corp., Ltd., Wuhan, China) electrochemical workstation, with the samples immersed in four different solution systems for 100 h. The electrochemical test specimens had an exposed working surface area of 1.0 cm². A standard three-electrode system was adopted: 20 steel served as the working electrode (WE), a saturated calomel electrode (SCE) as the reference electrode (RE), and a platinum (Pt) electrode as the counter electrode (CE). Thirty minutes after connecting the electrodes, the open circuit potential (OCP) tended to stabilize. For electrochemical impedance spectroscopy (EIS), a sinusoidal excitation signal of 10 mV was applied within the frequency range of 10⁵ Hz to 10⁻² Hz. Potentiodynamic polarization measurements were conducted across a potential window of −1.25 V to −0.15 V (vs. SCE) with a constant scan rate of 0.5 mV/s.

2.3. Weight Loss and Morphology Characterization

Prior to the experiment, the initial mass of all specimens was measured using an analytical balance (Sartorius BSA224S-CW, Sartorius AG, Goettingen, Germany). This balance features a resolution of 0.1 mg and a maximum weighing capacity of 220 g. Calibration was performed with standard weights before each measurement to ensure measurement accuracy. Three parallel specimens, each with an effective exposed area of 1.0 cm², were tested in each solution system. After 100 h of corrosion exposure, the specimens were taken out, rinsed with deionized water, dehydrated with ethanol, dried with cold air, and then stored in a desiccator for subsequent characterization.

Three of the samples were ultrasonically cleaned in a mixed solution of 5 wt.% HCl and 0.5% C₆H₁₂N₄ to remove corrosion products, then rinsed with deionized water and ethanol, dried, and reweighed. After being rinsed with deionized water and ethanol, and then dried, it was reweighed. The average corrosion rate of 20 steel was calculated using the following formula:

$$v_{coor}=8.76\times {10}^4\times {\displaystyle \frac{\left(W_0-W_1\right)}{\rho \times A\times t}}$$

(1)

where v_corr is the average corrosion rate of the sample (mm/y), W₀ is the initial mass of the sample (g), W₁ is the mass of the sample after removing corrosion products (g), ρ is the density of the sample (g·cm⁻³), A is the area of the sample (cm²), and t is the immersion time (h).

After the immersion test, the samples with retained corrosion products were gently rinsed with deionized water and dried under a nitrogen stream. These samples were directly characterized using the following techniques. X-ray diffraction (XRD) was performed on a Rigaku Smart Lab 9 kW diffractometer (Rigaku Corporation, Tokyo, Japan) with Cu Kα radiation (λ = 1.5406 Å) at an operating voltage of 45 kV and a current of 200 mA. Diffractograms were recorded over a 2θ range of 5–90° with a step size of 0.02° and a scan rate of 2°/min. Surface and cross-sectional morphologies were examined using scanning electron microscopy (SEM) coupled with energy-dispersive X-ray spectroscopy (EDS) on a Phenom Pure Plus integrated system (Thermo Fisher Scientific, Eindhoven, The Netherlands) at an accelerating voltage of 15 kV and a working distance of approximately 10 mm. Cross-sectional samples were prepared by cold-mounting in epoxy resin, followed by sequential grinding with SiC papers and polishing. X-ray photoelectron spectroscopy (XPS) analysis was conducted on a Thermo Scientific K-Alpha spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) using monochromatic Al Kα radiation with an X-ray spot size of 400 μm. Pass energies of 50 eV for survey spectra and 20 eV for high-resolution core-level spectra were employed, with a step size of 0.1 eV. For samples requiring removal of corrosion products, they were first ultrasonically cleaned in ethanol and then immersed in an acid pickling solution containing 5 wt.% HCl and 0.5% C₆H₁₂N₄ to remove the corrosion products, followed by rinsing with deionized water and drying. The distribution and depth of corrosion pits were measured using a white light interferometer (MicroXAM-3D, KLA Corporation, Milpitas, CA, USA) with a vertical scanning resolution of 0.01 nm, a lateral resolution of 0.11–8.8 μm, and an adjustable field of view.

3. Results and Discussion

3.1. Corrosion Rate and Corrosion Products Analysis

The corrosion rates of 20 steel in solutions with different ion concentrations were calculated using the average corrosion rate formula, where the density of 20 steel is ρ = 7.85 g·cm⁻³, the exposed area A = 1 × 10⁻⁴ m², and the immersion time t = 100 h. The corrosion rate results are shown in Figure 2. In the four different solutions, C1, C2, C3, and C4, the corrosion rates were 0.1964 mm/y, 0.1875 mm/y, 0.1792 mm/y, and 0.1537 mm/y, respectively. As can be seen from Figure 2, adding Cl⁻ to the solution containing 10,000 mg/L SO₄²⁻ inhibits the corrosion rate of 20 steel, and the corrosion rate decreases with increasing Cl⁻ concentration. The corrosion rate of 0.1964 mm/a measured under Cl⁻-free conditions is close to the value of 0.22 mm/y reported by Gong et al. [30] for Q235B steel after 7 days of immersion in desulfurization wastewater (pH 6.95, SO₄²⁻ 15,300 mg/L). The corrosion rates obtained in this study also fall within the range of 0.13–0.89 mm/y reported by Smith et al. [22] for carbon steel in aqueous salt solutions containing 0.05%–10% Cl⁻. Furthermore, the trend of decreasing corrosion rate with increasing Cl⁻ concentration is consistent with the “threshold effect” reported by Cáceres et al. [31] and Ahmed et al. [32].

3.2. Macro-Image Analysis

The macroscopic morphological evolution of the four specimen groups C1, C2, C3, and C4 over different immersion times is presented in Figure 3.

The C1 group exhibited a pronounced color change after 6 h of immersion, with the surface turning light gray accompanied by the progressive accumulation and spreading of corrosion products. By 24 h, a large portion of the surface was uniformly covered with yellowish-brown corrosion products, and after 100 h, the surface was almost entirely enveloped by a compact, dark brown corrosion product layer. The C2 group remained relatively stable within the first 24 h, presenting a light gray product layer on the surface; after 24 h, localized yellowish-brown corrosion stains began to nucleate and spread rapidly. In contrast, the C3 group maintained outstanding surface integrity throughout the entire immersion period. From 2 h to 72 h, the surface sustained an overall grayish-green tone, with only a small number of discrete dark pitting spots distributed across it; even after 100 h of immersion, no obvious large-scale color change was observed macroscopically. The C4 group likewise displayed good surface stability during the first 72 h, exhibiting a light grayish-green appearance, and slight yellowing began to emerge locally at 100 h.

3.3. Analysis of pH Value and Dissolved Oxygen Content

To elucidate the role of dissolved oxygen in the corrosion mechanism, the DO concentration and bulk solution pH were monitored at 0, 24, 48, 72, and 100 h using a portable dissolved oxygen meter (Hach HQ40d, accuracy ±0.1 mg/L) and a calibrated glass-electrode pH meter (Mettler Toledo FE28, accuracy ±0.01), respectively. Both instruments were calibrated before each measurement session. All reported values are the mean of three parallel specimens ± standard deviation.

The evolution of dissolved oxygen (DO) is presented in Figure 4a. At 55 °C, the initial DO concentration decreased progressively with increasing Cl⁻ concentration: 5.22 mg/L for C1, 3.98 mg/L for C2, 3.38 mg/L for C3, and 2.72 mg/L for C4. This reduction arises from the salting-out effect of high NaCl concentrations, which significantly decreases the solubility of molecular oxygen in aqueous solutions [18]. Over the 100 h immersion period, the DO in all four systems decreased as oxygen was consumed by the cathodic oxygen reduction reaction (ORR). Throughout the entire immersion period, the DO in the C4 system remained consistently at the lowest level, reaching 1.22 mg/L at 100 h, compared to 2.41 mg/L for C1. The decline rate was most rapid during the first 24 h, after which the DO approached a quasi-steady state as the corrosion product film formed and partially suppressed further oxygen diffusion toward the metal surface. The overall trend is unambiguous: higher Cl⁻ concentrations maintain the DO at consistently lower levels, directly weakening the cathodic driving force of ORR.

The evolution of the bulk solution pH is shown in Figure 4b. The initial pH of all four systems was 4.00. After 100 h of immersion, the pH of all systems exhibited a slight increase, but the magnitude decreased markedly with increasing Cl⁻ concentration: C1 rose to 4.35, C2 to 4.18, C3 to 4.18, and C4 to 4.03. The pH increase in the weakly acidic environment is primarily attributed to H⁺ consumption via the cathodic ORR. The progressively smaller pH shift at higher Cl⁻ concentrations reflects two synergistic effects: (i) the lower overall corrosion rate reduces the rate of H⁺ consumption, and (ii) the autocatalytic acidification inside active corrosion pits (HCl generation via FeCl₂ hydrolysis) produces a diffusive flux of H⁺ toward the bulk solution, which partially counteracts the pH-raising effect of ORR. The near-constant bulk pH in the C4 system suggests that these opposing processes approach a dynamic equilibrium in high-chloride environments. Several non-monotonic fluctuations were observed, which are attributed to minor temperature fluctuations, localized film detachment, and transient outward diffusion of HCl from active pits during pitting outbreaks—all physically reasonable in this corrosion system.

These results quantitatively confirm that high Cl⁻ concentrations substantially reduce the available dissolved oxygen via the salting-out effect, which in turn weakens the cathodic ORR rate and contributes to the observed decrease in the macroscopic uniform corrosion rate. Meanwhile, the minimal bulk pH change in high-Cl⁻ systems does not contradict the locally aggressive environment within active pits, where the autocatalytic mechanism sustains highly acidic conditions independent of the bulk solution chemistry.

3.4. Corrosion Product Analysis

As aggressive ions, Cl⁻ increase the effective reactive area between the steel substrate and the corrosive medium, which leads to the formation of more corrosion products on the surface of 20 carbon steel and accelerates the corrosion process. The integrity and compactness of these corrosion product films directly determine the corrosion susceptibility of 20 carbon steel.

The high-resolution XPS spectra of Fe 2p, O 1s, S 2p, Mg 1s and Cl 2p for the corrosion products on 20 carbon steel are shown in Figure 5. Iron in the corrosion products exists in two valence states, i.e., Fe²⁺ and Fe³⁺, with Fe³⁺ as the dominant valence state. As the Cl⁻ concentration increases, the Fe²⁺/Fe³⁺ ratio rises, indicating an increased formation of low-valence iron compounds. The O 1s spectra confirm that the increase in Cl⁻ concentration leads to an elevated proportion of M-O and a reduced proportion of -OH in the corrosion products, which demonstrates that the formation of dense crystalline oxides is promoted while the generation of porous iron oxyhydroxides is inhibited, this is because the decrease in oxygen content gradually reduces FeOOH to lower-valence Fe₃O₄. The S 2p spectra reveal that Cl⁻ facilitates the reduction process of sulfate, and the contents of S⁰ and S²⁻ increase significantly with the elevation of Cl⁻ concentration. Meanwhile, Cl⁻ can adsorb on and penetrate through the corrosion product film, while Mg²⁺ in the solution can participate in the formation of magnesium–iron spinel phase, which improves the chemical stability of the corrosion product film.

XRD results show that the main crystalline phases of the corrosion products are spinel-type Fe₃O₄ (PDF#19-0629, ICDD) and MgFe₂O₄ (PDF#73-1960, ICDD), accompanied by a small amount of FeOOH (goethite, PDF#29-0713, ICDD) oxyhydroxide. With the increase in Cl⁻ concentration, the intensity of the characteristic peaks of FeOOH decreases, and the relative content of the spinel phases increases significantly.

3.5. Electrochemical Measurement Results

EIS results measured on 20 steel after immersion in four different solutions for varying durations are presented in Figure 6. Among them, Figure 6a,d,g,j are Nyquist plots, Figure 6b,e,h,k are Bode phase angle plots, and Figure 6c,f,i,l are Bode impedance modulus plots.

In the four different media, the diameter of the Nyquist semicircles increases progressively from C1 to C4 in tandem with rising Cl⁻ concentration; concurrently, the low-frequency impedance modulus |Z| exhibits a similar upward trend. These apparent changes in the electrochemical response reflect the increased coverage of corrosion products on the metal surface and the altered interfacial state under high-Cl⁻ conditions. At high Cl⁻ concentrations, Cl⁻ induces pitting that disrupts the original oxide film; simultaneously, however, it promotes the deposition and accumulation of corrosion products in the extra-pit regions, forming a thicker product layer with greater surface coverage [33]. This layer presents a physical barrier to charge transfer, which macroscopically manifests as increased R_ct and |Z| in the EIS measurements. The Bode phase angle plots reveal that the addition of Cl⁻ yields a higher phase angle peak and a broader peak width. This indicates the formation of a relatively compact surface film and an increase in interfacial heterogeneity. The enhanced interfacial non-uniformity is closely associated with the Cl⁻-induced localized corrosion process; the underlying mechanism likely involves promoting pitting initiation and propagation, thereby causing the substrate surface microstructure to become increasingly porous and rough.

In the C1 solution, as shown in Figure 6b,c, the impedance modulus at high frequencies remains approximately 20 Ω·cm². At 2 h, the |Z| value is about 558.81 Ω·cm², with a peak value of approximately 49.80°. By 6 h, the |Z| value decreases to 467.33 Ω·cm², and the peak value drops to 45.67°—this decrease indicates that active anions in the solution attacked the initial oxide film on the metal during the early corrosion stage. Subsequently, due to the formation of new corrosion products, the |Z| value rises to 799.15 Ω·cm², with the corresponding phase angle peak shifting to lower frequencies; in addition, the Nyquist plot exhibits a larger capacitive loop. At 24 h, the |Z| value decreases to 487.14 Ω·cm², which can be attributed to the initiation of dissolution and detachment of the film layer. From 48 h to 100 h, the |Z| value increases from 773.87 Ω·cm² to 817.06 Ω·cm², the phase angle peak decreases slightly from 59.12° to 57.31° and shifts marginally toward lower frequencies, and the corresponding capacitive loop in the Nyquist plot becomes larger. This indicates an increase in R_p and suggests that the corrosion rate of the steel decreases with prolonged immersion.

In the other Cl⁻-containing solutions, the metal specimens exhibit the same variation trend as those in the C1 solution within the first 48 h. At 72 h, the impedance values of C2 and C3 show a decreasing trend, with |Z| values of 636.64 Ω·cm² and 901.05 Ω·cm², respectively. This is attributed to the instability of the corrosion film layer, which readily dissolves in the solution, leading to a decrease in impedance. For C4, the surface roughness is significantly higher than that of the other two groups; despite partial dissolution of the film layer, the impedance value increases to 4000.25 Ω·cm² due to the influence of surface corrosion pits.

To further clarify the interface reaction mechanism of 20 steel in the solutions, an equivalent circuit was used to fit the EIS data. As shown in Figure 7a,b, two equivalent circuit models, R(QR) and R(Q(R(QR))), were employed to fit the EIS data acquired at various times during the 100 h immersion. The R(QR) model was used for C1 during the early corrosion stage (2–6 h) and for all measured time points of C2, C3, and C4, whereas the R(Q(R(QR))) model was applied to C1 for the period of 12–100 h. Herein, R_s is the solution resistance, R_ct is the charge transfer resistance, Q_f is the film capacitance, and Q_dl is the double-layer capacitance. Q_dl is a constant phase element (CPE) related to the double-layer capacitance. Considering the roughness and inhomogeneity of the material surface, the constant phase element Q_dl was used instead of the ideal capacitance C. The impedance of the CPE (Z_Q) can be expressed as follows:

$$Z_Q={Y_0}^{-1}j{\omega }^{-n}$$

(2)

where Y₀ and n are characteristic parameters of Q_dl: Y₀ represents the magnitude parameter of Q_dl, with dimensions of Ω⁻¹ cm⁻² sⁿ; n is the dispersion exponent; j denotes the imaginary unit; ω is the angular frequency of the sinusoidal perturbation, which is equivalent to 2πf, where f is the frequency of the voltage signal.

Table 3. Fitting parameters of impedance spectroscopy plots for 20 steel in different solutions.

Samples	Time (h)	Rs (Ω cm2)	Qf		Rf (Ω cm2)	Qdl		Rct (Ω cm2)	χ2 (×10−4)
			Y0 (Ω−1 cm−2 sn)	nf		Y0 (Ω−1 cm−2 sn)	ndl
C1	2	26.05	-	-	-	3.85 × 10−4	0.7906	533.4	4.5
	6	21.5	-	-	-	5.33 × 10−4	0.7311	438.4	3.8
	12	22.7	4.62 × 10−4	0.8006	26.92	1.97 × 10−3	0.8346	772.3	4.1
	24	20.54	2.58 × 10−4	0.9854	20.74	8.17 × 10−4	0.6906	468.2	3.5
	48	19.87	8.46 × 10−4	0.8780	195.4	3.31 × 10−4	0.7987	581.8	3.7
	72	19.61	8.45 × 10−4	0.8746	255.8	4.86 × 10−4	0.8036	545.6	3.2
	100	20.15	9.66 × 10−4	0.8664	288.4	5.85 × 10−4	0.8265	549.7	3.6
C2	2	1.75	-	-	-	1.31 × 10−4	0.8685	1560	3.9
	6	1.82	-	-	-	3.41 × 10−4	0.8084	845.9	3.1
	12	1.79	-	-	-	2.14 × 10−4	0.8679	1038	4.2
	24	1.71	-	-	-	4.23 × 10−3	0.6228	695.8	2.9
	48	1.80	-	-	-	4.38 × 10−4	0.8960	1422	3.3
	72	1.68	-	-	-	1.36 × 10−3	0.8288	668.5	2.7
	100	1.84	-	-	-	7.06 × 10−4	0.8742	1400	3
C3	2	1.53	-	-	-	2.11 × 10−4	0.8475	1288	3.4
	6	1.48	-	-	-	2.57 × 10−4	0.8890	929.9	4.5
	12	1.41	-	-	-	1.57 × 10−4	0.9101	2429	8.6
	24	1.50	-	-	-	4.34 × 10−4	0.8765	977.3	6.4
	48	1.49	-	-	-	5.39 × 10−4	0.8593	1138	2.9
	72	1.41	-	-	-	7.68 × 10−4	0.8292	949.5	5.4
	100	1.40	-	-	-	9.33 × 10−4	0.8262	1115	5.6
C4	2	1.27	-	-	-	1.05 × 10−4	0.7782	1858	2.8
	6	1.48	-	-	-	2.57 × 10−4	0.8537	929.9	6.7
	12	1.27	-	-	-	1.29 × 10−4	0.9107	2572	2.6
	24	1.33	-	-	-	4.04 × 10−4	0.8923	1670	5.8
	48	1.27	-	-	-	5.01 × 10−4	0.8834	1934	4.6
	72	1.30	-	-	-	3.85 × 10−4	0.8702	4042	6.9
	100	1.32	-	-	-	5.63 × 10−4	0.8618	1863	4.5

summarizes the corresponding electrochemical parameters derived from fitting the EIS data. EIS measurements indicate that the introduction of Cl⁻ decreases the R_s. Analysis of Table 3 reveals that the charge R_ct values in the four solutions exhibit a negative correlation with the corrosion rate: specifically, a higher R_ct value corresponds to a lower corrosion rate. Notably, the introduction of Cl⁻ results in an increase in R_ct and a concomitant reduction in corrosion rate, with this effect becoming more significant as the Cl⁻ concentration increases. EIS data fitting was performed using ZSimpWin software 3.21. Two equivalent-circuit models, R(QR) and R(Q(R(QR))), were employed according to the number of time constants resolved in the Bode phase-angle plots. The quality of each fit was assessed by the chi-squared value and the relative error of each fitted parameter. All spectra yielded chi-squared values below 10⁻³.

The increase in R_ct with rising Cl⁻ concentration can be physically attributed to two factors: (1) enhanced ion-transport resistance caused by the thickening of the corrosion-product film, and (2) a reduction in the effective cathodic area as active sites become covered by corrosion products, leading to an overall decrease in the charge-transfer rate. However, the increase in R_ct represents only an apparent enhancement of electrochemical inertness at the macroscopic scale; it does not counteract the locally accelerated corrosion within pits driven by the Cl⁻ enrichment effect and the HCl autocatalytic mechanism.

Figure 8a presents the potentiodynamic polarization curves of 20 steel after 100 h of immersion in the four corrosion media (C1–C4). Compared with the Cl⁻-free C1 medium, increasing Cl⁻ concentration shifts the E_corr in the positive direction and reduces the overall polarization current density. This trend indicates a decreased thermodynamic tendency for corrosion that becomes more pronounced at higher Cl⁻ concentrations.

The corrosion current density was derived from the polarization resistance using the Stern–Geary equation [34]:

$$i_{corr}={\displaystyle \frac{B}{R_p}}$$

(3)

where B is the Stern–Geary constant. For carbon steel under active corrosion conditions, a value of B = 26 mV was adopted [35], and R_p is the polarization resistance in Ω·cm². The obtained results are shown in Figure 8c. The presence of Cl⁻ reduces the i_corr of 20 steel. When the Cl⁻ concentration reaches 100,000 mg/L, the i_corr is significantly lower than that of the groups with other Cl⁻ concentrations. In Cl⁻-containing systems, the cathodic region of the potentiodynamic polarization curve exhibits the characteristic of oxygen-limited diffusion. In the high Cl⁻ concentration range, the solubility of dissolved oxygen in the medium decreases significantly with the increase in Cl⁻ concentration. Whereas the corrosion process of carbon steel in weak acidic desulfurization wastewater is controlled by the cathodic oxygen depolarization reaction, the reduction in dissolved oxygen content greatly weakens the cathodic driving force of the corrosion reaction. This inhibition effect outweighs the corrosive attack effect of Cl⁻ itself on the metal matrix, ultimately resulting in a continuous decrease in corrosion rate with increasing Cl⁻ concentration.

Figure 8b shows the OCP of 20 steel as a function of immersion time. The OCP shifts in the positive direction over time for all media, and the OCP values for Cl⁻-containing samples are consistently higher than those for the Cl⁻-free C1 sample. This trend is consistent with the noble shift in E_corr observed in the potentiodynamic polarization curves. Pseudo-passivation behavior—characterized by a reduced rate of current increase in the anodic region—is observed in the Cl⁻-containing media. The pseudo-passivation region widens with increasing Cl⁻ concentration, reaching its maximum extent at the highest Cl⁻ concentration. This phenomenon arises from the geometric coverage of active dissolution sites and the diffusion barrier effect of the porous corrosion product film rather than from true thermodynamic passivation.

The electrochemical parameters obtained in this study are consistent with literature values reported for carbon steels in similar environments. For instance, the Rct of 20 steel measured in this work ranges from 549.7 to 1863 Ω·cm², which falls within the range reported by Xue et al. [36] for low-carbon steel in bicarbonate solutions containing chloride ions (500–3000 Ω·cm²). The i_corr decreased from 32.44 μA/cm² to 6.43 μA/cm², representing a reduction by a factor of approximately 5.05. This trend is analogous to the observations of You et al. [37], who reported that the introduction of chloride ions into pore solutions of alkali-activated slag pastes leads to an increase in Rct and a decrease in the overall corrosion rate of low-carbon steel due to the formation of a surface corrosion product layer. The positive shift in E_corr and the broadening of the pseudo-passive region observed in this study further corroborate the general electrochemical behavior of carbon steel in high-chloride systems, where chloride-induced film formation kinetically inhibits anodic dissolution despite promoting localized pitting.

In summary, high concentrations of Cl⁻ suppress the uniform corrosion rate of 20 steel by weakening cathodic oxygen reduction and promoting the formation of a corrosion product film, as evidenced by the decrease in i_corr and the noble shift in E_corr. However, Cl⁻ can penetrate the product film locally, increasing the risk of pitting. Consequently, the reduction in i_corr does not necessarily indicate an overall improvement in corrosion resistance, and localized corrosion must be evaluated in conjunction with surface morphology analysis.

3.6. Electron Microscopy Cross-Sectional Analysis

Figure 9 presents the cross-sectional morphologies of 20 steel under different Cl⁻ concentrations after the immersion test, where Figure 9a–d correspond to cross-sectional images with Cl⁻ concentrations of 0, 60,000 mg/L, 80,000 mg/L, and 100,000 mg/L, respectively, and Figure 9e is the EDS elemental composition map obtained by point analysis on the cross-section.

In the solution with 0 Cl⁻ concentration, the cross-sectional morphology exhibits characteristics of general corrosion, as shown in Figure 9a. For the sample immersed in the 60,000 mg/L solution (Figure 9b), some shallow pits appear on the steel surface, and the corrosion film shows distinct stratification: the outer layer is a film formed by uniform corrosion, while the inner layer consists of corrosion products accumulated from pitting corrosion of the metal matrix induced by Cl⁻. With increasing Cl⁻ concentration, the corrosion film is dominated by products from pitting corrosion, and the outer corrosion products peel off as the film thickens. In Figure 9c,d, the corrosion pits are significantly enlarged, accompanied by a thicker corrosion product film.

Figure 9e demonstrates a distinct variation in iron content between the inner and outer regions of the corrosion layer, with a higher iron content in the inner layer adjacent to the base metal. This phenomenon arises because, under low-oxygen conditions, cathodic reduction reactions involve not only the reduction of hydrogen and oxygen but also the reduction of iron corrosion products. Within the corrosion layer, the oxide film imposes a certain barrier to oxygen diffusion, resulting in the predominance of iron corrosion product reduction—with the primary reaction being the reduction of FeOOH to Fe₃O₄. This observation is consistent with the findings of Xue et al. [36] on corrosion behavior in low-oxygen environments, thereby validating the reduction in corrosion products.

When the Cl⁻ adsorbed on the specimen surface reaches a certain concentration threshold, a corrosion cell with a small anode and a large cathode is formed. Under the catalytic action of chloride ions, the metallic bonding forces between the iron atoms in the surface layer are weakened, accelerating the dissolution of iron and the formation of vacancy defects [38]. The dissolved Fe²⁺ combines with Cl⁻ to form FeCl₂, which rapidly diffuses away from the reaction interface, thereby preventing the inhibition of the anodic reaction by product accumulation. Since the affinity of OH⁻ for iron atoms is higher than that of Cl⁻ and thus favors the formation of stable hydroxides, FeCl₂ is subsequently converted into Fe(OH)₂, accompanied by the generation of HCl that acidifies the local solution. In this process, H⁺ participates in the reduction reaction while Cl⁻ is simultaneously released again, allowing chloride ions to cyclically participate in the corrosion process. The depolarization reaction equations are as follows:

$${\mathrm{F}\mathrm{e}}^{\mathrm{2}\mathrm{+}}\mathrm{+}\mathrm{2}{\mathrm{C}\mathrm{l}}^{\mathrm{-}}\mathrm{+}\mathrm{2}{\mathrm{H}}_{\mathrm{2}}\mathrm{O}\mathrm{\to }\mathrm{F}\mathrm{e}\mathrm{C}{\mathrm{l}}_{\mathrm{2}}\mathrm{·}\mathrm{2}{\mathrm{H}}_{\mathrm{2}}\mathrm{O}$$

(4)

$$\mathrm{F}\mathrm{e}{\mathrm{C}\mathrm{l}}_{\mathrm{2}}\mathrm{·}\mathrm{2}{\mathrm{H}}_{\mathrm{2}}\mathrm{O}\mathrm{\to }\mathrm{F}\mathrm{e}{\left(\mathrm{O}\mathrm{H}\right)}_{\mathrm{2}}\mathrm{+}\mathrm{2}\mathrm{H}\mathrm{C}\mathrm{l}$$

(5)

$$\mathrm{2}\mathrm{H}\mathrm{C}\mathrm{l}\mathrm{+}\mathrm{2}{\mathrm{e}}^{\mathrm{-}}\mathrm{\to }{\mathrm{H}}_{\mathrm{2}}\mathrm{+}{\mathrm{2}\mathrm{C}\mathrm{l}}^{\mathrm{-}}$$

(6)

3.7. Corrosion Pit Analysis

The results of corrosion of 20 steel after 100 h in solutions without Cl⁻ and with different concentrations of Cl⁻ are shown in Figure 10. Among them, Figure 10a corresponds to the solution system containing only SO₄²⁻ without Cl⁻, revealing that 20 steel undergoes only uniform corrosion. Figure 10b–d are SEM images of metal corrosion in solutions with Cl⁻ concentrations increasing in sequence. The introduction of Cl⁻ into the solution induces pitting corrosion; as the Cl⁻ concentration increases, the corrosion pits on the surface become denser and deeper. When the concentration reaches 100,000 mg/L, the corrosion pits are the deepest. Even at high Cl⁻ concentrations, Cl⁻ still exhibits a strong pitting ability.

The equilibrium microstructure of 20 steel consists of a ferrite (α-Fe) matrix with a small amount of dispersed pearlite (lamellar α-Fe + Fe₃C eutectoid). Given its carbon content of 0.19 wt.%, the pearlite volume fraction is approximately 20%–25% after annealing. A significant electrochemical potential difference exists between ferrite and cementite (Fe₃C): cementite is more noble (approximately −0.4 V vs. SHE) and serves as the cathodic phase, whereas ferrite is more active (approximately −0.6 V vs. SHE) and acts as the anodic phase. These two phases form numerous microscopic galvanic couples in the corrosive medium, which theoretically accelerate the preferential dissolution of ferrite and render the pearlite/ferrite interfaces susceptible sites for pit initiation.

Figure 11 presents the white light interferometry images and corresponding depth profiles of 20 steel after 100 h immersion. For quantitative evaluation, all discernible pits within a scanned area of 3.5 mm × 2.5 mm per specimen were analyzed using a depth threshold of 1 μm. The C1 specimen (0 mg/L Cl⁻) exhibited uniform corrosion without identifiable pits, with a maximum surface roughness below 1 μm. In contrast, 22, 20, and 18 pits were detected on the C2, C3, and C4 specimens, respectively, corresponding to pit densities of 2.51, 2.29, and 2.06 pits/mm². The gradual decrease in pit density with increasing Cl⁻ concentration indicates that high chloride levels do not nucleate more pits but instead intensify the propagation of individual pits. The maximum pitting depth increased from 3.78 μm (C2) to 17.74 μm (C3) and 152.77 μm (C4), far exceeding the average uniform corrosion penetration of 2.14, 2.05, and 1.76 μm calculated from weight-loss data. Consequently, the pitting factor—defined as the ratio of maximum pit depth to average uniform corrosion depth—increased sharply from 1.8 (C2) to 8.7 (C3) and 87.1 (C4). The depth distribution histograms (Figure 12) reveal that 86% of C2 pits were shallower than 3.5 μm, 65% of C3 pits fell within 6–14 μm, and 78% of C4 pits exceeded 60 μm.

Figure 12 further presents the relationship between pit mouth diameter and pit depth. The C2 pits (red circles) cluster in the lower-left region with diameters of 10–42 μm and depths below 4.5 μm, exhibiting a broad, dish-like morphology. The C3 pits (green triangles) occupy an intermediate zone with diameters of 26–58 μm and depths of 4.8–17.74 μm, corresponding to bowl-shaped geometry. The C4 pits (blue diamonds) populate the upper-right region with diameters of 42–105 μm and depths of 42–152.77 μm; notably, all C4 pits exceed a depth-to-diameter ratio of 1.0, indicating a tunnel-like morphology that is deeper than it is wide. The progressive transition from dish-like (C2) to bowl-like (C3) to tunnel-like (C4) pits reflects the intensification of the Cl⁻ autocatalytic mechanism, wherein occluded-cell acidification preferentially drives downward dissolution. The tunnel-like C4 pits achieve depths nearly 90 times greater than the uniform corrosion depth, underscoring that the localized penetration risk posed by high-chloride desulfurization wastewater far outweighs the modest reduction in average wall thickness.

3.8. High-Chloride Corrosion Mechanism

During the corrosion experiment, the cathodic reaction mainly involves the reduction of oxygen, with the side reaction being the reduction of H⁺. The cathodic reactions can be expressed by the following equations:

$${\mathrm{O}}_{\mathrm{2}}\mathrm{+}\mathrm{4}{\mathrm{H}}^{\mathrm{+}}\mathrm{+}\mathrm{4}{\mathrm{e}}^{\mathrm{-}}\mathrm{\to }{\mathrm{2}\mathrm{H}}_{\mathrm{2}}\mathrm{O}$$

(7)

$$\mathrm{2}{\mathrm{H}}^{\mathrm{+}}\mathrm{+}\mathrm{2}{\mathrm{e}}^{\mathrm{-}}\mathrm{\to }{\mathrm{H}}_{\mathrm{2}}$$

(8)

Anodic reactions are as follows:

$$\mathrm{F}\mathrm{e}\mathrm{\to }{\mathrm{F}\mathrm{e}}^{\mathrm{2}\mathrm{+}}\mathrm{+}\mathrm{2}{\mathrm{e}}^{\mathrm{-}}$$

(9)

The main reactions of 20 steel in high-chloride desulfurization wastewater are as follows:

$${\mathrm{F}\mathrm{e}}^{\mathrm{2}\mathrm{+}}\mathrm{+}{\mathrm{2}\mathrm{H}}_{\mathrm{2}}\mathrm{O}\mathrm{\to }\mathrm{F}\mathrm{e}{\mathrm{\left(}\mathrm{O}\mathrm{H}\mathrm{\right)}}_{\mathrm{2}}\mathrm{+}\mathrm{2}{\mathrm{H}}^{\mathrm{+}}$$

(10)

$${\mathrm{F}\mathrm{e}}^{\mathrm{2}\mathrm{+}}\mathrm{+}{\mathrm{S}\mathrm{O}}_{\mathrm{4}}^{\mathrm{2}\mathrm{-}}\mathrm{\to }\mathrm{F}\mathrm{e}{\mathrm{S}\mathrm{O}}_{\mathrm{4}}$$

(11)

$$\mathrm{4}\mathrm{F}\mathrm{e}{\mathrm{\left(}\mathrm{O}\mathrm{H}\mathrm{\right)}}_{\mathrm{2}}\mathrm{+}{\mathrm{O}}_{\mathrm{2}}\mathrm{+}{\mathrm{2}\mathrm{H}}_{\mathrm{2}}\mathrm{O}\mathrm{\to }\mathrm{4}\mathrm{F}\mathrm{e}{\mathrm{\left(}\mathrm{O}\mathrm{H}\mathrm{\right)}}_{\mathrm{3}}$$

(12)

$$\mathrm{F}\mathrm{e}{\mathrm{\left(}\mathrm{O}\mathrm{H}\mathrm{\right)}}_{\mathrm{2}}\mathrm{+}\mathrm{2}\mathrm{F}\mathrm{e}{\mathrm{\left(}\mathrm{O}\mathrm{H}\mathrm{\right)}}_{\mathrm{3}}\mathrm{\to }\mathrm{F}{\mathrm{e}}_{\mathrm{3}}{\mathrm{O}}_{\mathrm{4}}\mathrm{+}\mathrm{4}{\mathrm{H}}_{\mathrm{2}}\mathrm{O}$$

(13)

$$\mathrm{F}\mathrm{e}{\mathrm{\left(}\mathrm{O}\mathrm{H}\mathrm{\right)}}_{\mathrm{3}}\mathrm{\to }\mathrm{F}\mathrm{e}\mathrm{O}\mathrm{O}\mathrm{H}\mathrm{+}{\mathrm{H}}_{\mathrm{2}}\mathrm{O}$$

(14)

The main corrosion products of 20 steel in high-chloride desulfurization wastewater are Fe₃O₄, with a small amount of FeSO₄. Under low-oxygen conditions, FeOOH is slowly formed, but the resulting FeOOH is unstable and is reduced back to Fe₃O₄ in subsequent reactions, as shown in Figure 13. Due to the instability of the film layer, some products dissolve and detach during the corrosion process. The addition of Cl⁻ increases the charge transfer resistance of 20 steel and reduces the self-corrosion current density by up to a factor of 11.36. The corrosion product film formed on the outer surface outside the pits (mainly composed of Fe₃O₄, FeOOH, and MgFe₂O₄) provides a physical barrier to the substrate, increasing the diffusion resistance of H⁺ and O₂ toward the substrate surface, thereby lowering the overall uniform corrosion rate.

Specifically, the Cl⁻-induced pitting corrosion follows a classical autocatalytic mechanism. In the regions at the pit mouth and outside the pit, a corrosion product layer is deposited. Under high Cl⁻ concentrations, loose FeOOH is reduced to dense Fe₃O₄. On the one hand, this film suppresses uniform corrosion outside the pit; on the other hand, it precisely indicates the presence of an active dissolution process inside the pit. The higher the Cl⁻ concentration, the more severe the pitting corrosion, the greater the outward diffusion of corrosion products, and the thicker the deposited film outside the pit. In the absence of Cl⁻, 20 steel undergoes only uniform corrosion, with a maximum corrosion depth not exceeding 1 μm. In the presence of Cl⁻, both the pit density and pit depth increase with rising Cl⁻ concentration. At a Cl⁻ concentration of 100,000 mg/L, the maximum pitting depth reaches 152.77 μm, and the corrosion pits become progressively denser.

It is worth noting that in addition to Fe, 20 steel also contains trace alloying elements, including C (0.19%), Si (0.22%), Mn (0.55%), Cr (0.03%) and Ni (0.02%). Although these elements are present in low concentrations, they exert certain potential effects on the corrosion behavior. Hyun and Kim [39] investigated the effects of Cr and Mn on the corrosion performance of carbon steel in simulated seawater. The results showed that Cr can effectively improve the corrosion resistance by forming a stable oxide film, while Mn accelerates corrosion due to the formation of galvanic coupling with the iron matrix. Given that the overall concentrations of these alloying elements are low, the dominant corrosion processes—iron dissolution, oxide/hydroxide formation, and Cl⁻-induced pitting corrosion—are still governed by the electrochemical behavior of Fe, as illustrated in Figure 11.

4. Conclusions

This study investigates the corrosive effect of high Cl⁻ concentrations on 20 steel and clearly demonstrates that a high Cl⁻ concentration reduces dissolved oxygen via the salting-out effect, weakens the cathodic oxygen reduction reaction, and causes film transformation, thereby suppressing the macroscopic uniform corrosion rate; meanwhile, it induces deep, narrow, tubular pitting through an autocatalytic mechanism, resulting in a localized penetration depth that reaches nearly 90 times the uniform corrosion thinning. The following specific conclusions are drawn:

(1) High-concentration Cl⁻ significantly reduces the uniform corrosion rate of 20 steel in simulated desulfurization wastewater containing 10,000 mg/L SO₄²⁻. As Cl⁻ concentration increases from 0 to 100,000 mg/L, the corrosion rate decreases from 0.1964 mm/y to 0.1537 mm/y, due to lower dissolved oxygen and enhanced formation of a protective corrosion product film. While high Cl⁻ suppresses general corrosion, it drastically aggravates localized pitting. Without Cl⁻, only uniform corrosion occurs with a maximum depth below 1 μm; at 100,000 mg/L Cl⁻, the maximum pitting depth reaches 152.77 μm.

(2) The corrosion product film undergoes a distinct compositional and structural transformation with increasing Cl⁻ concentration. XPS and XRD analyses reveal a phase evolution from porous FeOOH to dense spinel-type Fe₃O₄ and MgFe₂O₄, accompanied by an elevated Fe²⁺/Fe³⁺ ratio. This transition enhances the physical barrier effect against charge transfer, as evidenced by the increased charge transfer resistance and low-frequency impedance modulus.

(3) Electrochemical measurements demonstrate that Cl⁻ reduces the free corrosion current density of 20 steel and induces pseudo-passivation behavior. The anodic polarization curves exhibit a broadened pseudo-passive region with increasing Cl⁻ concentration, indicating that the corrosion product film imposes a kinetic inhibition on the anodic dissolution process, even though it does not provide true thermodynamic passivation.

(4) The dual role of Cl⁻ in high-chloride desulfurization wastewater is elucidated: while it promotes the formation of a thicker, denser corrosion product film that suppresses uniform corrosion macroscopically, its high penetrability simultaneously initiates and propagates localized pitting through the classical autocatalytic mechanism. Consequently, the apparent reduction in corrosion current density does not indicate an overall improvement in corrosion resistance, and localized corrosion must be critically evaluated in the integrity assessment of 20 steel pipelines.