A Comparative Evaluation of Microbiologically Induced Corrosion Behaviors of 316L Austenitic and 2205 Duplex Stainless Steels Inoculated in Desulfovibrio vulgaris

Abstract

Selecting appropriate materials is crucial for mitigating the severe economic and safety challenges posed by microbiologically induced corrosion (MIC) in marine and industrial settings. This study focuses on the MIC behavior of 316L austenitic stainless steel and 2205 duplex stainless steel that is caused by the metabolic activities of D. vulgaris during a life span of 7 days. Cell counts, weight loss, electrochemical measurements, and surface characterization were employed to evaluate the materials’ resistance to MIC. Specifically, 2205 DSS exhibited a 60% lower weight loss (0.02 vs. 0.05 mg/cm²), a 42% lower maximum pit depth (2.11 vs. 3.64 μm), and an orders-of-magnitude lower corrosion current density (0.094 vs. 2.0 μA cm⁻²) compared to 316L SS, demonstrating its superior resistance to D. vulgaris MIC. XRD and XPS analyses revealed that although FeS formed on both materials, FeS₂—a thermodynamically stable deep-sulfidation product—was only present on 316L, indicating a more advanced corrosion stage. The absence of FeS₂ on 2205 suggests limited sulfide corrosion progression. These findings confirm the advantage of duplex stainless steel in mitigating D. vulgaris-induced corrosion and provide insights into the selection of materials for MIC-prone environments.

1. Introduction

MIC originates from the metabolic activities of microorganisms that form biofilms on metal surfaces. MIC is a corrosion behavior that is induced by corrosive microbes [1,2,3,4]. It occurs in the marine industry, the oil and gas industry, water utility systems, etc. [5,6,7,8]. Corrosion is one of the key factors limiting the service life and structural reliability of metallic materials, especially in complex service environments such as marine, energy, chemical, and underground facilities. According to relevant statistics, the global economic losses caused by corrosion account for 3–4% of GDP annually, with a significant proportion related to invisible and uncontrollable MIC [9]. As a form of corrosion primarily driven by biological activity, MIC has gained widespread attention from researchers in the interdisciplinary fields of materials, environment, and microbiology due to its characteristics such as strong concealment, rapid development, and high remediation costs.

The essence of MIC is an electrochemical process triggered by the metabolic products of microorganisms in the biofilm formed on the metal surface, with corrosion mechanisms that are highly diversified and systemically coupled. The known microorganisms that induce MIC include sulfate-reducing bacteria (SRB) [10], iron-oxidizing bacteria (IOB) [11], nitrate-reducing bacteria (NRB) [12], and acid-producing bacteria (APB) [13], among others. Among them, SRB is widely considered one of the most corrosion-active microbial groups. In anaerobic environments, SRB uses sulfate as an electron acceptor, reducing it to H₂S, which reacts with metal ions to form corrosion products such as FeS, leading to severe localized corrosion and even perforation failure [14].

SRB are anaerobic organisms capable of utilizing sulfate as the final electron acceptor during respiration. In contrast to soluble organic carbon, elemental iron discharges electrons extracellularly, which are subsequently employed in sulfate reduction within the SRB cytoplasm through bio-catalysis, necessitating extracellular electron transfer (EET). Consequently, this form of MIC is referred to as “EET-MIC”, arising from the energy requirements of sessile cells in biofilms that are capable of conducting EET.

In recent years, with the advancement of electrochemical technologies and microbial detection methods, there has been a deeper understanding of the mechanisms of SRB-induced corrosion. Among these, cathodic depolarization theory, the extracellular electron transfer (EET) mechanism, and the localized acidic environment formed by microbial biofilms are considered key factors driving the MIC process [15,16,17]. Furthermore, the biofilm formed by SRB can induce electrochemical microenvironments and oxygen concentration cell effects, exacerbating localized corrosion. Existing studies have shown that SRB-related MIC has become one of the main causes of stainless steel failure in subsea cables, offshore platforms, crude oil transportation systems, and wastewater treatment facilities.

To address the material degradation caused by SRB, researchers have proposed various protective strategies, including the use of efficient biocides, controlling environmental pH and redox potential, and developing antimicrobial coatings [18,19,20,21]. However, traditional biocides are often inefficient against biofilms, have high operational costs, and cause adverse environmental impacts due to the release of disinfectant by-products. Additionally, after treatment, high-speed cleaning agents are required for cleaning and rinsing to remove organic debris [22]. Coatings can slow down the corrosion rate to some extent, but the corrosion problem still persists. Moreover, if the protective layer has defects such as peeling, the corrosion rate of underground pipelines will accelerate further [23]. Therefore, improving the resistance of metal materials to SRB-induced corrosion by focusing on the alloy composition and microstructure of the materials themselves is still considered a solution with higher intrinsic safety and longer service life [24]. In this context, stainless steel materials have attracted attention in the field of MIC protection due to the presence of their passive film, high chromium/nickel content, and good resistance to pitting corrosion.

The 316L austenitic stainless steel, as a traditional general-purpose stainless steel, is widely used in industries such as chemical, marine, and medical equipment due to its excellent machinability and weldability. However, in microbiologically enriched environments, the surface of 316L is prone to being covered by SRB biofilms, which promote localized pitting corrosion and even stress corrosion cracking, revealing its performance limitations in MIC environments [25]. In contrast, 2205 duplex stainless steel, with both austenitic and ferritic structures, not only outperforms 316L in mechanical properties but also exhibits stronger resistance to localized corrosion due to its higher content of Cr, Mo, and N. It has become an important candidate material for various extreme service conditions [26]. A direct, systematic comparison of their MIC resistance is therefore of significant practical importance. It provides engineers and asset managers with a clear, data-driven basis for material selection, balancing performance requirements with economic considerations in projects where failure due to MIC poses serious risks.

Even though previous studies have investigated the corrosion characteristics of these two stainless steels in specific microbial circumstances, there is a lack of systematic comparisons to better understand the MIC behaviors and mechanisms, biofilm characteristics, and surface morphology changes of 316L and 2205 stainless steel under SRB exposure conditions. Based on the above background, this study designed a standard anaerobic circumstance to cultivate the D. vulgaris, weight loss, electrochemical testing (open circuit potential, linear polarization, electrochemical impedance spectroscopy, potential polarization), cell count, confocal laser scanning microscope (CLSM), scanning electron microscope (SEM), and X-ray diffraction (XRD) were used to analyze the corrosion behavior of 316L stainless steel and 2205 duplex stainless steel. This research presents the relationship between alloy content and MIC resistance properties, providing theoretical support for MIC resistance. A key innovation of this work is the systematic comparison of MIC behavior between 316L and 2205 stainless steels through several characterization methods, which enables comprehensive elucidation differences in MIC behaviors. Furthermore, this study offers theoretical support for optimizing material selection in MIC environments, such as marine engineering and oil and gas transportation infrastructures.

2. Materials and Methods

2.1. Materials Preparation

In this study, 316L austenitic stainless steel and 2205 duplex stainless steel (Nanjing Iron and Steel Co., Ltd., Nanjing, China) were utilized for all experiments. The two types of stainless steels were processed into square coupons of 1 cm × 1 cm × 0.3 cm.

Table 1. Chemical composition of 316L stainless steel and 2205 duplex stainless steel (wt%). (PRE_N* = 1%Cr + 3.3%Mo + 16%N).

Material	Cl	Si	Mn	S	P	Cr	Ni	Mo	N	Fe	PREN*
316L	0.02	0.5	1.4	0.003	0.035	17.45	12.11	2.3	0.03	Bal.	25.52
2205	0.03	0.6	1.5	0.001	0.026	22.30	5.80	3.1	0.16	Bal.	35.09

lists the compositions of 316L austenitic stainless steel and 2205 duplex stainless steel.

The square coupons were sequentially polished using 180#, 400#, and 800# sandpapers to achieve consistent surfaces. The deionized water was used for cleaning and then air-dried in a clean environment or oil-free cold air. The epoxy resin was applied to seal the edges and back of the square coupons and electrochemical coupons, which were required to be dried and conserved in a clean environment. After cleaning with 100% isopropanol, the coupons were exposed to ultraviolet light for 20 min to eliminate contamination and ensure sterile conditions.

2.2. Cultural Medium

The ATCC 1249 culture medium shown in

Table 2. Composition of ATCC 1249 culture medium.

Component	Chemical	Amount
Component I	MgSO4∙7H2O	4.1 g/L
	Sodium citrate	5.0 g/L
	CaSO4∙2H2O	1.0 g/L
	NH4Cl	1.0 g/L
	Distilled water	400 mL/L
Component II	K2HPO4	0.5 g/L
	Distilled water	200 mL/L
Component III	Sodium lactate	4.5 mL/L
	Yeast extract	1.0 g/L
	Distilled water	400 mL/L
Component IV	Fe(NH4)2(SO4)2	1.0 g/L

was used to cultivate the D. vulgaris. The pH of the medium was adjusted to 7.0 ± 0.1, and 100 ppm of L-cysteine was added as a reducing agent. After sterilizing the medium at 121 °C for 20 min in an autoclave (Panasonic MLS-3751L, Kadoma, Japan), it was deoxygenated under continuous nitrogen gas flow (N₂) for 45 min to ensure an anaerobic broth.

The broth was inoculated into 200 mL of culture medium (with a total bottle volume of 450 mL), leaving 250 mL of headspace with 2 mL of seeds to maintain a ratio of 1:100. The broth was incubated at 37 °C for 7 days. The incubation time was set to 7 days, a common duration in SRB-MIC studies that is sufficient for stable biofilm formation and initial corrosion product development [3]. All inoculation and transfer procedures were conducted in an anaerobic chamber.

2.3. Cell Counts

After 7 days of immersion in ATCC 1249 broth, 1 mL of broth was extracted, diluted 10 times, and shaken for planktonic cell count. At the same time, the attached cells were scraped from the material surface, diluted 100 times with phosphate-buffered saline (PBS), mixed with a Kylin-Bell VORTEX-5 vortex mixer (Haimen Qilin Bell Instrument Manufacturing Co., Ltd., Nantong, China), and then counted by a hemocytometer, the ZEISS optical microscope (Oberkochen, Germany), at 50 × 20 magnification for sessile cell count.

2.4. Biofilm Conditions

Extensive research has established that the MIC behavior of SRB is strongly related to biofilm development, which acts as a critical determinant factor of the MIC process. Therefore, the biofilms of 316L austenitic stainless steel and 2205 duplex stainless steel were observed by CLSM (Zeiss LSM780, Carl Zeiss, Oberkochen, Germany) and SEM (FEI Quanta 250, Hillsboro, OR, USA).

For CLSM observation, the coupons were rinsed with PBS at pH 7.4 to remove the medium and loosely attached planktonic cells. The cells were stained using the LIVE/DEAD^TM BacLight^TM bacterial viability stain kit (Life Technologies, Grand Island, NY, USA) after the 7-day incubation period. The SYTO-9 dye in the kit stains live cells and dead cells green under fluorescence at excitation wavelengths of 488 nm and 559 nm. After approximately 1 h of incubation, the samples were placed under CLSM to observe the number of cells on the biofilm formed on the sample.

For SEM observation, the coupons were rinsed three times with PBS (15 s each) and then fixed with 2.5% (w/w) glutaraldehyde solution at 10 °C for 8 h. The samples were dehydrated using ethanol at concentrations of 50%, 70%, 80%, 90%, and 95% for 10 min each, and finally, 100% ethanol was used to dehydrate for 30 min after a 7-day immersion. The coupons were then gold-coated for conductivity and observed under SEM.

2.5. Weight Loss

The coupons were accurately weighed before and after the immersion period. After 7 days, MIC products were removed by a Clark solution and dried at 27 °C for 24 h. The dried coupons were measured for weight loss.

The corrosion rate $\left(V_{corr}\right)$ was calculated by Equation (1):

$$V_{corr}={\displaystyle \frac{\mathrm{\Delta }m}{\rho \ast A \ast t}}$$

(1)

where Δm is the weight loss, ρ is the density (7.98 g/cm³ for 316L austenitic stainless steel and 7.80 g/cm³ for 2205 duplex stainless steel), A is the exposed surface area (1 cm²), and t is the corrosion time. This method quantitatively assesses the corrosion performance of different stainless steels [27].

2.6. Electrochemical Testing

Electrochemical tests were conducted using a three-electrode system: the working electrode was 316L austenitic stainless steel and 2205 duplex stainless steel. The reference electrode was a saturated calomel electrode (SCE, filled with saturated KCl solution). The counter electrode was a platinum plate with a surface area of 1 cm². The experiments were conducted in 450 mL anaerobic bottles, with each bottle containing 200 mL of deoxygenated D. vulgaris culture medium with 2 mL of seeds.

The electrochemical tests were performed by the electrochemical station (Reference 600 Plus, Gamry, Warminster, PA, USA). Open circuit potential (OCP) was measured until the OCP value became stable. Linear polarization resistance (LPR) was scanned at a rate of 0.1667 mV s⁻¹ in the range of −10 mV to +10 mV vs. OCP each day. EIS was performed at OCP by applying a sinusoidal signal of 10 mV (amplitude) at a frequency ranging from 10⁴ to 10⁻² Hz each day. And we used ZView2 software to fit and analyze the obtained data. Potentiodynamic polarization curves were scanned from OCP to −200 mV vs. OCP and from OCP to +200 mV vs. OCP at a rate of 0.1667 mV s⁻¹ once at the end of the 7 d incubation. The corrosion potential (E_corr), corrosion current density (i_corr), and anodic and (absolute) cathodic Tafel slopes (β_a and β_c) were determined from a Tafel analysis of the polarization curves [8].

2.7. Corrosion Pits Observation

After 7 days of D. vulgaris incubation, the 316L austenitic stainless steel and 2205 duplex stainless steel coupons were retrieved. The SEM was used to observe the corrosion pits on the square coupon surfaces. The coupons were cleaned using a fresh Clarke’s solution to remove biofilms and corrosion products before pit image analysis under SEM. The chemical composition of Clarke’s solution consisted of 50 g of stannous chloride and 20 g of antimony chloride in 1 L of hydrochloric acid solution (specific gravity: 1.19).

The MIC pit depth was measured by CLSM. The profile with the deepest pit was chosen as the representative profile to indicate the severity of the MIC pitting corrosion on 316L austenitic stainless steel and 2205 duplex stainless steel.

2.8. Corrosion Product Analysis

After the 7-day culture experiment, the samples were removed from the anaerobic bottles, dried under nitrogen for about 10 min, and then subjected to X-ray diffraction (XRD; Bruker D8 Discovery model, Bruker AXS GmbH, Karlsruhe, Germany) and X-ray photoelectron spectroscopy (XPS; AXIS Supra, Shimadzu, Kyoto, Japan) to analyze the chemical composition of the corrosion products, with the latter providing more information. The top biofilm of 316L austenitic stainless steel and 2205 duplex stainless steel coupons was removed by a cotton swab to reveal corrosion products underneath. It was then dried under N₂ for approximately 10 min before being placed under XRD and XPS.

3. Results and Discussion

3.1. Cell Counting

The planktonic cell in the culture medium also showed slight differences between the two types of stainless steel. In Figure 1, the planktonic cell of the 2205 duplex stainless steel group was lower than that in the 316L stainless steel, with counts of 3.2 × 10⁷ ± 1.2 × 10⁶ cells/mL and 4 × 10⁷ ± 2.3 × 10⁶ cells/mL, respectively (error bars stand for the standard deviations from three independent coupons). A lower cell count typically leads to less corrosion damage, while a higher cell count may indicate more severe corrosion issues for the material.

The sessile cell count (Figure 2) on the 2205 duplex stainless steel was relatively lower (2.5 × 10⁶ ± 5.4 × 10⁵ cells/cm²) than the 316L stainless steel, which showed a higher attachment of D. vulgaris cells (4.0 × 10⁶ ± 8.9 × 10⁵ cells/cm²), which corresponds to the weight loss trends. Error bars stand for the standard deviations from three independent samples. Higher sessile cell count requires more free electrons as nutrients for D. vulgaris metabolism, thereby promoting the rapid growth of D. vulgaris on the 316L surface.

3.2. CLSM

CLSM was applied to evaluate the distribution of live cells on the coupons. From the fluorescence images (Figure 3), it can be observed that the green fluorescent spots on the surface of 2205 stainless steel are sparse, with a discrete distribution and low brightness, indicating limited attachment of D. vulgaris cells on its surface and weak biofilm formation capability. In contrast, the surface of 316L stainless steel exhibits dense fluorescent spots with enhanced brightness, suggesting that the surface of 316L is more prone to D. vulgaris adhesion and proliferation, and the biofilm development is more extensive. Thus, it can be inferred that 2205 stainless steel has an advantage in inhibiting D. vulgaris biofilm formation.

3.3. Weight Loss Analysis

The weight loss results of 2205 duplex stainless steel and 316L stainless steel after being immersed in D. vulgaris standard culture medium for 7 days are shown in Figure 4. It can be observed that the corrosion loss of 2205 duplex stainless steel is relatively low, which is related to its higher content of Cr and Mo. These two compositions typically enhance the traditional corrosion resistance property of iron [28,29], which also enhances the MIC resistance property. The results show that the weight loss of 2205 duplex stainless steel is 0.02 mg/cm², while that of 316L stainless steel is 0.05 ± 0.01 mg/cm². This indicates that the MIC induced by D. vulgaris on 316L stainless steel is more severe than that on 2205 duplex stainless steel. The trend of weight loss is consistent with sessile cell counts, which is followed by EET-MIC.

3.4. Pit Depths

Morphologies of MIC pits on the 316L stainless steel and 2205 duplex stainless steel after 7 days of incubation with biofilms and corrosion products removed were examined under CLSM, with pit depths of 3.635 μm and 2.113 μm (in Figure 5), respectively. The pit depth 2205 duplex stainless steel was relatively smooth with fewer MIC products, showing better MIC resistance than 316L austenitic stainless steel. This was attributed to higher Cr and Mo content, which forms a relatively stable passive layer to protect the rare material. Furthermore, both austenite and ferrite phases also contribute to the corrosion resistance of the 2205 duplex [30,31].

3.5. Electrochemical Performance Testing

In Figure 6, the OCP of both stainless steels fluctuates over time (the error bars represent the average values calculated from two parallel working electrodes). The overall OCP value of 2205 duplex stainless steel is lower than that of 316L stainless steel, indirectly proving that 2205 duplex stainless steel has a weaker thermodynamic tendency on the MIC process [21].

For MIC, an activated system, open circuit potential (OCP) as a thermodynamic tendency parameter often lacks high regularity and reliability. The kinetic parameters such as linear polarization resistance (LPR) offer higher reliability. In Figure 7, the R_p value of 2205 duplex stainless steel is higher than that of 316L stainless steel (the error bars represent the average values calculated from two parallel working electrodes). The R_p results are completely consistent with other experimental results, establishing a consistent mechanism to illustrate the outstanding MIC resistance of 2205 duplex stainless steel. The lower sessile cell count on 2205 duplex stainless steel (Figure 2) resulted in lower electron consumption to maintain the metabolism of the D. vulgaris requirement (EET-MIC), which directly induced a lower weight loss (Figure 4), higher R_p, and higher charge transfer (R_ct) (Table 3). and lower MIC current density (i_corr, Table 4).

Electrochemical impedance spectroscopy (EIS) is generally a non-destructive testing method for samples and provides more electrochemical information on the MIC rate [32]. The Nyquist and Bode plots for 2205 duplex stainless steel and 316L stainless steel for 1 day, 3 days, and 7 days are shown in Figure 8.

The equivalent circuit model R_s(Q_f(R_f(Q_dlR_ct))) was selected based on the physical characteristics of the system. It is a well-established model for MIC studies where a biofilm/corrosion product layer forms on the metal surface. In Table 3, R_s represents the solution resistance; R_f represents the biofilm resistance; R_ct represents the charge transfer resistance. The sum of biofilm resistance and charge transfer resistance is used to evaluate the corrosion rate in MIC. In Table 3, CPE represents the constant phase element, which simulates a non-ideal double-layer capacitance. A non-ideal capacitor differs from an ideal capacitor in that it has a time constant with a dispersion effect due to surface inhomogeneity. The impedance of the constant phase element is related to frequency as follows:

$$Z = {\displaystyle \frac{1}{Y_0}}{\left(j\omega \right)}^{-n}$$

(2)

where Y₀ is the modulus, j is the imaginary unit, ω is the angular frequency, and n is the element exponent.

The EIS fitting circuit model requires inclusion of the biofilm/corrosion product film resistance and charge transfer resistance, as shown in Figure 9. The charge transfer resistance (R_ct) for both steels is high (>10⁵ Ω cm²), indicative of their inherent passivity. R_ct values in the range of 10⁵–10⁶ Ω·cm² are well-documented in the literature for stainless steels in various near-neutral environments, even under MIC conditions. For example, Ebeagwu et al. reported R_ct values of 2 × 10⁵ Ω·cm² for 2205 DSS in a simulated marine environment [33]. Li et al. found that the Rct value of 316 SS in SRB medium was 10⁵ Ω·cm² [34]. However, the R_ct value for 2205 DSS is consistently higher than that for 316L SS at each time point, signifying a more effective barrier against charge transfer and thus a lower corrosion rate. Concurrently, the biofilm resistance (R_f) for 2205 increases more significantly over time (from 33 to 173 Ω cm²) compared to 316L (from 34 to 162 Ω cm²). The constant phase element exponents (n₁, n₂) are less than 1, reflecting the non-ideal capacitive behavior due to surface inhomogeneity caused by biofilm coverage and initial corrosion. The trends are consistent with weight loss, cell counts, open circuit potential, and linear polarization resistance.

Figure 8. Variation in EIS for 2205 duplex stainless steel and 316L stainless steel in 1, 3, and 7 days of incubation in 450 mL anaerobic bottles.

Figure 8. Variation in EIS for 2205 duplex stainless steel and 316L stainless steel in 1, 3, and 7 days of incubation in 450 mL anaerobic bottles.

Figure 9. Equivalent circuit diagram for Figure 8.

Figure 9. Equivalent circuit diagram for Figure 8.

Table 3. Equivalent circuit fitting parameters for Figure 9.

	$\mathbf{T} \left(\mathbf{d}\right)$	${\mathit{R}}_{\mathit{s}}$ $(\mathsf{\Omega } {\mathbf{c}\mathbf{m}}^2)$	${\mathit{Q}}_{\mathit{f}}$ ${(\mathsf{\Omega }}^{-1} {\mathbf{c}\mathbf{m}}^{-2} {\mathbf{s}}^{\mathbf{n}})$	${\mathit{n}}_1$	${\mathit{R}}_{\mathit{f}}$ $(\mathsf{\Omega } {\mathbf{c}\mathbf{m}}^2)$	${\mathit{Q}}_{\mathit{d}\mathit{l}}$ $({\mathsf{\Omega }}^{-1} {\mathbf{c}\mathbf{m}}^{-2} {\mathbf{s}}^{\mathbf{n}})$	${\mathit{n}}_2$	${\mathit{R}}_{\mathit{c}\mathit{t}}$ $(\mathsf{\Omega } {\mathbf{c}\mathbf{m}}^2)$
2205	1	25.36	5.39 × 10−5	0.87	33	6.21 × 10−5	0.85	4.23 × 105
	3	22.4	6.96 × 10−5	0.83	82	8.35 × 10−5	0.83	3.67 × 105
	7	24.66	7.19 × 10−5	0.81	173	6.10 × 10−5	0.82	4.04 × 105
316L	1	25.81	5.72 × 10−5	0.86	34	7.02 × 10−5	0.83	3.76 × 105
	3	23.32	7.63 × 10−5	0.81	107	6.83 × 10−5	0.81	3.16 × 105
	7	23.73	7.6 × 10−5	0.80	162	5.69 × 10−5	0.80	3.50 × 105

Table 3. Equivalent circuit fitting parameters for Figure 9.

	$\mathbf{T} \left(\mathbf{d}\right)$	${\mathit{R}}_{\mathit{s}}$ $(\mathsf{\Omega } {\mathbf{c}\mathbf{m}}^2)$	${\mathit{Q}}_{\mathit{f}}$ ${(\mathsf{\Omega }}^{-1} {\mathbf{c}\mathbf{m}}^{-2} {\mathbf{s}}^{\mathbf{n}})$	${\mathit{n}}_1$	${\mathit{R}}_{\mathit{f}}$ $(\mathsf{\Omega } {\mathbf{c}\mathbf{m}}^2)$	${\mathit{Q}}_{\mathit{d}\mathit{l}}$ $({\mathsf{\Omega }}^{-1} {\mathbf{c}\mathbf{m}}^{-2} {\mathbf{s}}^{\mathbf{n}})$	${\mathit{n}}_2$	${\mathit{R}}_{\mathit{c}\mathit{t}}$ $(\mathsf{\Omega } {\mathbf{c}\mathbf{m}}^2)$
2205	1	25.36	5.39 × 10−5	0.87	33	6.21 × 10−5	0.85	4.23 × 105
	3	22.4	6.96 × 10−5	0.83	82	8.35 × 10−5	0.83	3.67 × 105
	7	24.66	7.19 × 10−5	0.81	173	6.10 × 10−5	0.82	4.04 × 105
316L	1	25.81	5.72 × 10−5	0.86	34	7.02 × 10−5	0.83	3.76 × 105
	3	23.32	7.63 × 10−5	0.81	107	6.83 × 10−5	0.81	3.16 × 105
	7	23.73	7.6 × 10−5	0.80	162	5.69 × 10−5	0.80	3.50 × 105

Figure 10 shows the PDP curves at the end of the 7-day incubation. Table 4 lists the electrochemical parameters obtained from Tafel curve fitting, including the corrosion current density (i_corr), corrosion potential (Ecorr), and the cathodic (β_c) and anodic (β_a) Tafel slopes. The i_corr values of 2205 duplex stainless steel and 316L stainless steel were 0.094 μA cm⁻² and 2.0 μA cm⁻², respectively, indicating that the 316L stainless steel had a higher MIC rate, corroborating the weight loss, cell counts, pit depth, OCP, and LPR data.

Table 4. Tafel curve fitting results.

	icorr (μA cm−2)	Ecorr (V) vs. SCE	βa (mV/dec)	βc (mV/dec)
2205 SS	9.4 × 10−2	–0.37	121	–241
316L SS	2.0	–0.31	96	–709

Table 4. Tafel curve fitting results.

	icorr (μA cm−2)	Ecorr (V) vs. SCE	βa (mV/dec)	βc (mV/dec)
2205 SS	9.4 × 10−2	–0.37	121	–241
316L SS	2.0	–0.31	96	–709

Figure 10. Potentiodynamic polarization curves of 2205 duplex stainless steel and 316L stainless steel after being immersed in D. vulgaris standard culture medium for 7 days.

Figure 10. Potentiodynamic polarization curves of 2205 duplex stainless steel and 316L stainless steel after being immersed in D. vulgaris standard culture medium for 7 days.

3.6. Surface and Cross-Sectional Morphology

SEM observed the biofilm formation on the surface and cross-section of the D. vulgaris specimens, as shown in Figure 11. The surfaces of both samples are covered with loosely structured corrosion products, and their structures are similar, indicating that the two types of steel undergo the same corrosion mechanism.

The 2205 stainless steel shows only a thin layer of surface corrosion, with a clear interface between the corrosion layer and the substrate. No significant delamination or cracking is observed, suggesting that the corrosion has not progressed deeply, and the overall material structure remains intact. In contrast, 316L stainless steel shows significant thickening of the corrosion layer, with some areas exhibiting corrosion cracks and delamination. This proves that 2205 duplex stainless steel has higher reliability in engineering applications under microbiologically corrosive environments.

3.7. Corrosion Products

According to the XRD analysis (Figure 12), the results show that after being immersed in the D. vulgaris environment for 7 days, both 2205 and 316L stainless steels underwent sulfide reactions, generating FeS corrosion products. The 316L stainless steel (red curve) exhibits significant FeS diffraction peaks, indicating more severe sulfide corrosion in the D. vulgaris environment. In contrast, the FeS peaks of 2205 stainless steel (black curve) are weaker, suggesting stronger corrosion resistance and no significant sulfide corrosion. These results demonstrate that the XRD spectra of both stainless steels are similar, indicating that the chemical nature of MIC and the corrosion products generated are not affected by the different steel compositions.

Figure 13 and

Table 5. XPS analysis results of the relative composition of key corrosion products in 2205 and 316L after 7 days of immersion in the D. vulgaris environment.

Corrosion Product	2205		316L
	Binding Energy (eV)	Atom %	Binding Energy (eV)	Atom %
FeS	713.6	19.88	713.6	10.65
Fe2O3	711.6	42.82	711.6	46.60
Fe3O4	708.1	5.59	710.2	38.47
FeO	710.7	31.71	~	~
FeS2	~	~	707.3	4.28

present the high-resolution Fe 2p₃/₂ XPS spectra and the relative composition distribution of corrosion products of 2205 duplex stainless steel and 316L austenitic stainless steel after 7 days of immersion in the D. vulgaris environment. The fitting results show that the surface of the samples primarily formed corrosion products such as FeS, FeO, Fe₃O₄, and Fe₂O₃, with FeS₂ further detected on the 316L surface.

FeS in 2205 accounted for 19.88%, higher than 10.65% in 316L, indicating that both stainless steels were affected by D. vulgaris and underwent sulfidation. Li et al. observed a similar shift (713.6 eV) in iron sulfide produced by MIC [34]. Notably, FeS₂ was only detected on the 316L surface, with an atomic percentage of 4.28%. FeS₂ is a thermodynamically more stable deep-sulfidation product [35,36], typically formed from FeS under sustained anaerobic conditions, and its presence indicates that the corrosion process has entered a later, irreversible stage.

Regarding oxide products, both materials showed the presence of FeO, Fe₃O₄, and Fe₂O₃. These oxides represent different stages of the corrosion process: FeO is the primary oxidation product of divalent iron, with a loose, unstable structure, commonly seen in the early stages of corrosion; Fe₃O₄ is a mixed-valence oxide, formed in the intermediate corrosion stage, with certain electrochemical activity that may accelerate corrosion; Fe₂O₃, the final oxidation product of trivalent iron, is the most thermodynamically stable [37,38], and typically appears after the complete failure of the passive film, signaling the deepening of corrosion and the failure of surface protection mechanisms. Therefore, an increase in Fe₂O₃ can be considered an indicator of intensified oxidative corrosion.

Combining the quantitative results, the 2205 sample primarily contains Fe₂O₃ (42.82%), with relatively low amounts of FeO and Fe₃O₄. The corrosion reaction mainly remains in the early-to-intermediate stages. In contrast, 316L shows a significant increase in Fe₃O₄ (38.47%) alongside Fe₂O₃ (46.60%), indicating its inferior corrosion resistance.

4. Conclusions

This study investigates the corrosion behavior of 2205 duplex stainless steel and 316L austenitic stainless steel in the D. vulgaris environment over a 7-day immersion period. The results demonstrate that 2205 duplex stainless steel exhibits superior resistance to MIC compared to 316L austenitic stainless steel. Specifically, 2205 DSS showed a 60% lower weight loss (0.02 mg/cm² vs. 0.05 mg/cm²), 42% shallower maximum pit depth (2.11 μm vs. 3.64 μm), and an orders-of-magnitude lower corrosion current density (0.094 μA cm⁻² vs. 2.0 μA cm⁻²) than 316L SS.

The electrochemical and morphological analysis revealed that 2205 stainless steel has not only a lower corrosion rate but also significantly fewer D. vulgaris cells attached to its surface (2.5 × 10⁶ cells/cm² vs. 4.0 × 10⁶ cells/cm² for 316L SS) and less significant corrosion product accumulation. In contrast, 316L stainless steel showed higher D. vulgaris cell attachment, more pronounced pitting corrosion, and a higher corrosion rate, indicating its higher susceptibility to MIC.

XRD analysis confirmed the formation of FeS on both samples, with 316L showing more pronounced sulfide corrosion. In addition, XPS results revealed the presence of FeS₂ on 316L but not on 2205. As FeS₂ is a thermodynamically stable, deep-sulfidation product, its detection on 316L suggests that the corrosion process progressed to a more advanced and irreversible stage, whereas 2205 remained in an early-to-intermediate corrosion phase. This result reinforces that 2205 not only limits microbial adhesion and maintains structural integrity but also suppresses the formation of deep corrosion products under D. vulgaris influence.

Future work will involve long-term immersion experiments (e.g., 30 days or longer) to monitor corrosion evolution. Techniques such as Localized Electrochemical Impedance Spectroscopy (LEIS) and critical pitting temperature (CPT)/potential (CPP) measurements will be employed to gain deeper insights into the localized corrosion initiation and propagation mechanisms under MIC conditions.