The Effect of Chromium Contents on the Corrosion Performance of Fe-22Mn-0.6C TWIP Steels in Sulfate-Containing Environments

Abstract

This study evaluates the corrosion behavior of Fe-22Mn-0.6C TWIP steels containing 0%, 5%, and 10% chromium after 28 days of exposure to a neutral sulfate solution. By combining electrochemical testing with a surface and spectroscopic analysis, we explored how Cr influences the formation and stability of oxide layers. The results reveal a clear trend: as the chromium content increases, the corrosion resistance improves significantly. The 10% Cr alloy stood out for its high impedance and stable electrochemical response, pointing to the development of a dense, protective oxide layer that limits the corrosive attack. The SEM/EDS and Raman spectroscopy revealed that chromium not only enhances the oxide’s compactness but also alters its composition, transitioning from iron-rich, porous oxides to Cr-containing spinels and oxyhydroxides with superior barrier properties. These structural and chemical improvements were confirmed by electrochemical parameters, which showed a reduced capacitance and increased film homogeneity. To tie these findings together, we propose a schematic model describing how chromium shapes the passivation process in these steels. Altogether, this study highlights the essential role of Cr in enhancing long-term corrosion protection in high-Mn TWIP steels under sulfate-rich conditions.

1. Introduction

The development of advanced steels with exceptional mechanical properties and corrosion resistance properties is increasingly important to meet the demands of safety, durability, and performance in industries such as mining, petrochemicals, and construction. In this context, Twinning-Induced Plasticity (TWIP) steels have gained considerable attention due to their outstanding strength–ductility balance, high work-hardening rate, and excellent formability [1].

TWIP steels typically contain 18–22 wt.% Mn and 0.1–0.8 wt.% C, with alloying additions such as Al, Si, V, Ti, and Cr [2,3,4,5,6,7]. Their mechanical behavior is governed by deformation mechanisms—mainly mechanical twinning and dislocation glide—that are closely related to the stacking fault energy (SFE) [1,4,8]. Depending on the SFE, other mechanisms—such as ε-martensite transformations, dynamic strain aging (DSA), or α′-martensite formation—may also contribute [9,10].

Manganese (Mn), a key component in TWIP steels, plays a dual role: it stabilizes the austenitic phase and enhances the mechanical performance, but it also reduces the corrosion resistance due to the formation of unstable oxides, especially under acidic or chloride-rich conditions [11]. On the other hand, chromium (Cr) has been shown to promote corrosion resistance by favoring the formation of stable Cr₂O₃ and Fe-Cr spinel oxides that improve the compactness and adherence of protective layers [12,13]. Nevertheless, excessive Cr additions may lead to carbide precipitation (e.g., (Cr,Fe)₇C₃ or (Cr,Fe)₂₃C₆), which can deplete Cr locally and compromise both mechanical and electrochemical properties [14,15].

Although chloride-induced corrosion in TWIP steels has been widely investigated, corrosion in sulfate environments has received less attention despite its relevance to real operating conditions in mining and industrial wastewater systems [16,17,18,19]. Sulfate ions are known to modify the composition and mechanical integrity of oxide layers, with some studies suggesting more severe damage than chlorides under an alkaline or neutral pH [18,20,21,22,23]. Furthermore, many studies have relied solely on electrochemical data without incorporating surface characterization techniques, such as SEM/EDS or Raman spectroscopy, which are essential for identifying corrosion products and understanding anticorrosion mechanisms [24,25]. This lack of complementary evidence has led to contradictory findings regarding the influence of Cr on the stability and composition of oxide films in TWIP steels.

To address these knowledge gaps, the present study investigates the effect of Cr additions (0, 5, and 10 wt.%) on the microstructure and corrosion behavior of Fe-22Mn-0.6C TWIP steels in a neutral 0.1 M Na₂SO₄ solution. This work combines electrochemical techniques (open-circuit potential, OCP, and electrochemical impedance spectroscopy, EIS) with a post-corrosion surface analysis (scanning electron microscopy, SEM, and energy-dispersive spectroscopy, EDS, and Raman spectroscopy) to elucidate the role of Cr in the oxide layer formation and durability in sulfate-containing environments. The results aim to support the design of corrosion-resistant TWIP steels for industrial applications.

2. Materials and Methods

2.1. TWIP Samples Preparation

TWIP steels were produced via induction melting to obtain ingots with dimensions of 100 × 100 × 300 mm. These ingots were hot-forged at 1200 °C into plates of 25 × 200 × 950 mm, followed by water quenching. A homogenization heat treatment was conducted at 1100 °C for 24 h, and the plates were then hot-rolled to a final thickness of 20 mm. Corrosion coupons were sectioned from these plates into 20 × 40 × 2 mm specimens according to the ASTM G31 standard [26]. The chemical composition was verified by optical emission spectroscopy (OES), with special attention to carbon and chromium content for alloy validation.

2.2. Microstructural Characterization

Standard metallographic preparation was employed, including grinding, polishing, and etching with Vilella’s reagent (100 mL ethanol, 5 mL HCl, and 1 g picric acid). Optical microscopy (Olympus, Tokyo, Japan) was used to evaluate grain structure, and grain size was determined according to ASTM E112 [27].

Chemical composition variations were analyzed using a HITACHI SU 3500 scanning electron microscope (SEM) (Tokyo, Japan) equipped with a Bruker XFlash 410 M energy-dispersive X-ray spectroscopy (EDS) detector (Billerica, MA, USA).

Room-temperature (25 °C) X-ray diffraction (XRD) patterns were collected from the free-standing alloy films with a LabX XRD-6000 diffractometer (Bruker, Kyoto, Japan) operating with Cr Kα radiation (λ = 0.229 nm). Instrumental broadening was established in advance using the LaB₆ NIST SRM 660c reference (a = 0.415704 ± 0.000008 nm). The measured peak full widths at half maximum were fitted to the Caglioti equation, and the resulting coefficients were applied to correct the experimental profiles across the full 2θ range. Phase identification and structural refinement were then performed by Rietveld analysis in MAUD (version 2.9995). Popa’s anisotropic microstructure model—capable of simultaneously describing crystallite-size and microstrain broadening, preferred orientation (texture), and Warren-type planar defects—was adopted to obtain a reliable fit to the diffraction line shapes in the present samples [28,29].

2.3. Electrochemical Testing

Prior to electrochemical measurements, specimens were ground with SiC papers up to 2400 grit, rinsed with distilled water, degreased with acetone and ethanol, and dried with hot air. All tests were performed immediately after surface preparation, and each condition was tested in triplicate to ensure reproducibility.

Electrochemical experiments were conducted in a three-electrode cell, utilizing the TWIP sample as the working electrode, a saturated calomel electrode (SCE) as the reference, and a platinum wire as the counter electrode. The electrolyte was a 0.1 M Na₂SO₄ solution at 25 ± 1 °C. This neutral sulfate medium, chosen for its industrial relevance, facilitates the formation of oxide layers while maintaining low solution resistance.

The electrochemical techniques employed included open-circuit potential (OCP) monitoring and electrochemical impedance spectroscopy (EIS). OCP was recorded until stabilization (approximately 1 h), followed by EIS measurements at the stabilized potential (E_oc) using a 10 mV sinusoidal perturbation over the frequency range of 100 kHz to 3 mHz. Immersion periods of 1, 7, 14, 21, and 28 days were studied.

2.4. Characterization of Corrosion Products

After electrochemical testing, the morphology and chemical composition of corrosion products were analyzed using a TESCAN VEGA scanning electron microscope (SEM) (Brno, Czech Republic) equipped with an energy-dispersive X-ray spectroscopy (EDS) detector. Raman spectroscopy was conducted using a custom-built setup with an Andor Shamrock spectrometer, iDus CCD detector, and a 488 nm laser (2.54 eV) in a backscattering configuration. Laser power was maintained below 400 μW to prevent thermal effects, and the spot size was less than 1 μm. These techniques were employed to identify corrosion products and assess the composition of the oxide films.

3. Results and Discussion

3.1. Microstructure and Phase Analysis

Table 1. Chemical composition (wt.%) of TWIP steels with varying Cr content, determined by optical emission spectroscopy.

Steel	Elements (wt.%)
	C	Mn	Si	Cr	P	S	Fe
TWIP 0%Cr	0.57	21.20	0.12	0.14	0.030	0.005	Balance
TWIP 5%Cr	0.63	22.90	0.10	5.35	0.022	0.002	Balance
TWIP 10%Cr	0.60	21.23	0.06	10.20	0.067	0.006	Balance

summarizes the nominal and measured chemical compositions of the TWIP steel samples. All alloys present Mn and C contents within the target range for TWIP steels, ensuring a fully austenitic matrix stabilized by high Mn levels [19,30]. The Cr content was accurately adjusted to 0.14, 5.35, and 10.20 wt.%, confirming that the desired alloying levels were achieved through careful melt processing and homogenization. The minor variations observed are within acceptable tolerances for laboratory-scale alloy designs. These baseline compositions serve as the foundation for interpreting the microstructural and corrosion behavior discussed in the following sections.

Figure 1 shows the optical micrographs of the Fe-22Mn-0.6C TWIP steels with 0%, 5%, and 10% wt.% Cr after hot rolling and homogenization. All samples exhibit a fully austenitic structure, characterized by equiaxed grains ranging in size from 170 to 200 μm and no evidence of ferrite or martensitic phases. This confirms the stability of the γ-phase due to the high Mn content, which acts as a strong stabilizer of the austenite [1,4].

Figure 2 presents the X-ray diffraction patterns of the Fe-22Mn-0.6C TWIP steels with 0%, 5%, and 10% Cr. All samples display the characteristic diffraction peaks of face-centered cubic (FCC) austenite, corresponding to the (111), (200), and (220) planes, confirming the fully austenitic nature of the alloys. No ferrite or martensite was detected, which is consistent with the stabilizing effect of the high Mn content and the processing conditions employed [31].

Low-intensity XRD peaks were detected in the Cr-alloyed samples and were attributed to secondary phases. Two carbide species were unambiguously identified in the 5 wt % and 10 wt % Cr alloys: primary (Cr,Fe)₂₃C₇ carbides that nucleated during solidification and secondary (Cr,Fe)₇C₃ carbides that precipitated during the heat treatment. The quantitative XRD shows that the 5 wt % Cr alloy contains only 1–2 vol % carbides, whereas the 10 wt % Cr alloy contains 4–5 vol %. Spot EDS–SEM analyses further confirm the chromium-rich segregation along grain boundaries in both alloys. In the 10 wt % Cr, the matrix contains 8.36 ± 0.40 wt % Cr and 21.29 ± 0.20 wt % Mn, while the grain boundary precipitates contain 15.06 ± 2.9 wt % Cr and 19.93 ± 2.84 wt % Mn. For the 5 wt % Cr alloy, the corresponding values are 3.96 ± 0.30 wt % Cr and 19.18 ± 0.80 wt % Mn in the matrix and 8.4 ± 0.50 wt % Cr and 17.52 ± 0.10 wt % Mn within the grain boundary. These results are consistent with the authors’ previous observations on the same alloy after a different homogenization treatment [32].

Such localized variations in the chemical composition may result in chromium-depleted regions adjacent to the carbides, potentially reducing the corrosion resistance of the material [14,17]. However, the amount of chromium carbide precipitates in alloys with 5% Cr and 10% Cr is not significant enough to cause a noticeable reduction in the alloy’s corrosion potential (Ecorr) in Na₂SO₄ environments. Nonetheless, further research is needed to explore this phenomenon in greater detail.

3.2. Electrochemical Response (OCP and EIS)

Figure 3 shows the evolution of the open-circuit potential (OCP) for Fe-22Mn-0.6C TWIP steels with 0%, 5%, and 10% wt.% Cr immersed in the 0.1 M Na₂SO₄ solution over 28 days.

All samples exhibited an initial potential shift during the first hour of immersion, followed by a gradual stabilization, indicating the formation of a protective oxide layer on the surface. The OCP values were more negative for the 0% Cr alloy throughout the test, with values ranging from −0.43 V to −0.39 V vs. the SCE. In contrast, the 5% Cr alloy exhibited more nobles potentials, ranging from −0.38 V to −0.31 V, while the 10% Cr alloy maintained the most positives values, stabilizing near −0.27 V vs. the SCE after 28 days.

The progressive ennoblement of the OCP with the increasing Cr content suggests a greater tendency toward the spontaneous passivation and improved thermodynamic stability of the protective oxide layer [19]. This trend is consistent with the known role of chromium in promoting the formation of compact and adherent Cr-rich oxide layers [33].

While the 10% Cr alloy demonstrated the most positive potential over time, a slight decrease in the OCP was observed after 21 days, potentially indicating the minor degradation or local instability of the oxide layer under extended exposure. This behavior aligns with SEM observations discussed in Section 3.3, which revealed the partial delamination of the oxide layer in the same alloy.

Overall, the OCP trends indicate that chromium additions enhance the passivation behavior in TWIP steels exposed to sulfate-rich environments, providing a stable electrochemical foundation for interpreting the subsequent EIS and polarization results.

Figure 4 presents the Nyquist and Bode plots for TWIP steels with 0%, 5%, and 10% wt.% Cr after 1, 7, 14, 21, and 28 days of immersion in the 0.1 M Na₂SO₄ solution at the open-circuit potential. In this condition, the impedance behavior has a contribution of the anodic reaction related to the metal oxidation and cathodic reaction attributed to the oxygen reduction reaction. Therefore, the electrolyte resistance, double-layer capacitance, charge transfer resistance of both reactions, and possibly a Warburg impedance is involved. Nyquist plots reveal capacitive behavior characterized by depressed semicircles, indicating surface heterogeneity and non-ideal double-layer behavior. The diameter of the semicircle increased with the chromium content and immersion time, reflecting the enhanced corrosion resistance and stabilization of the oxide layer [24,25,34]. In particular, the 0% Cr alloy showed a small and poorly defined semicircle at early exposure times (1–7 days), indicating a low charge transfer resistance and an unstable oxide film that could be prone to localized corrosion. As the immersion time increased to 14 and 21 days, only slight improvements were observed, suggesting a limited self-passivation capability in the absence of Cr. In contrast, the 5% Cr alloy exhibited a significantly larger semicircle from day 7 onward, indicating an early and sustained passivation response. The 10% Cr alloy presented the most substantial impedance values throughout the entire test period, with semicircle diameters exceeding those of the other samples by more than twofold at day 28, which is consistent with the formation of a more compact and protective oxide layer. These findings agree with prior reports demonstrating that Cr enhances the stability of oxide films by promoting the formation of Cr₂O₃ and FeCr₂O₄ spinel-type oxides [30,33].

In the Bode plots, the impedance modulus (|Z|) versus the frequency showed clear differences among the samples with varying Cr contents. The 0% Cr alloy exhibited the lowest |Z| values throughout the entire immersion period, indicating a limited barrier effect of the oxide layer formed in this alloy, likely due to poor adhesion, porosity, or the formation of less protective iron oxides, such as Fe₃O₄ and γ-Fe₂O₃. The low-frequency region (<1 Hz) reflected the ease of the ionic transfer across the interface, suggesting inadequate corrosion protection.

The 5% Cr alloy demonstrated intermediate |Z| values, showing a steady increase over immersion time, especially in the low-frequency domain. This behavior indicates the gradual evolution of a more resistive and protective oxide layer. The improvement can be attributed to the formation of Cr-containing oxides such as FeCr₂O₄, which enhance the compactness and stability of the oxide layer. The two distinct frequency regions observed—one at a high frequency (solution resistance and outer layer) and another at a low frequency (inner barrier layer)—are consistent with a duplex oxide layer structure, as also reported in the literature for similar Fe-Cr systems [19,30,35].

In contrast, the 10% Cr alloy exhibited the highest |Z| values across the entire frequency spectrum, particularly after 14 and 28 days of immersion. The increase in the impedance modulus at low frequencies reflects a highly resistive oxide layer that effectively suppresses the charge transfer. This response is indicative of a dense and chemically stable oxide film enriched in Cr oxides such as FeCr₂O₄ and α-FeOOH. The high |Z| values in this alloy indicate a significant improvement in corrosion protection, which is consistent with observations made in other high-Cr austenitic steels exposed to neutral sulfate environments [11,23].

Figure 5 shows the variation in the impedance modulus at a low frequency (|Z|_4mHz) of TWIP steels as a function of the immersion time. At low frequencies, the impedance modulus provides a sensitive measure of the barrier properties of the oxide layer; as expected, this parameter exhibits a marked increase with the increasing Cr content. The 0% Cr alloy showed values below 10³ Ω·cm² throughout the exposure period, which is indicative of a porous or poorly adherent oxide layer that offers limited protection against corrosion. In contrast, the 5% Cr alloy exceeded 10⁴ Ω·cm² by day 28, and the 10% Cr alloy reached values above 10⁵ Ω·cm². This gradual increase in impedance overtime may reflect the development of more compact and resistive surface layers. Although further analysis is required to confirm the composition of these layers. The 10% Cr alloy exhibited the highest impedance, exceeding 10⁵ Ω·cm² after 28 days. This response may indicate the development of a more stable and resistive oxide layer over time. Although the exact nature of this layer cannot be confirmed from impedance data alone, we believe this can be linked to the presence of Cr-containing compounds such as FeCr₂O₄ and α-FeOOH. These results reinforce the notion that Cr additions significantly enhance the integrity and long-term stability of the oxide layer in sulfate-containing environments, in agreement with prior studies on Cr-alloyed high-Mn steels [30,33].

Bode plots of the TWIP steels reveal a broad peak centered between 10 and 100 Hz, which is associated with the resistance and capacitance previously mentioned. However, significant differences were observed among the samples. The 0% Cr alloy exhibited a progressive decrease in the phase angle magnitude over time, accompanied by a shift toward lower frequencies, indicating the degradation of the oxide film and reduced electrochemical stability. In contrast, the 5% and 10% Cr alloys exhibited higher and more stable phase angles, consistent with the development of robust and adherent oxide layers that enhance corrosion resistance.

Interestingly, the 10% Cr alloy exhibited a temporary drop in the phase angle on day 21 (from ~70° to ~45°), which may be attributed to local oxide dissolution followed by re-passivation, as previously reported for Fe–Mn–Cr alloys under sulfate exposure. These findings align with the more noble and stable OCP trends observed in Cr-containing steels and are consistent with reduced corrosion current densities reported for high-Mn TWIP steels with Cr additions.

The shape of the phase angle curves for all compositions exhibited non-homogeneous frequency dispersion, indicating surface inhomogeneity and the presence of different relaxation processes at the metal–electrode interface. Therefore, a constant phase element (CPE) was employed to adjust the impedance response more accurately. The CPE parameters—Q (pseudo-capacitance) and α (dispersion coefficient)—were extracted from Bode plots to evaluate deviations from the ideal capacitive behavior. The ideal capacitive response is defined by α = 1; values below this threshold are indicative of surface roughness, compositional heterogeneity, or porous oxide films.

In our study, α values remained below 0.9 for all samples, confirming non-ideal behavior. However, the α parameter increased with the Cr content, reaching ~0.88 for the 10% Cr alloy on day 28. This trend indicates the formation of a more homogeneous and compact oxide layer with an increasing Cr concentration. Similarly, the Q parameter showed a marked decrease with an increasing Cr content, suggesting the reduced dielectric leakage and enhanced barrier properties of the oxide layer, in line with earlier findings for Cr-rich protective layers.

Table 2. Electrochemical parameters (Q, α, and C_dl) estimated from the graphical analysis of Bode plots and the Brug formula for TWIP steel with 0%, 5%, and 10% Cr after immersion in 0.1 M Na₂SO₄. R_e: electrolyte resistance, Q and α: constant phase element parameters, and C_dl: double-layer capacitance.

Exposure Time (Days)	TWIP Steel	Re (Ωcm−2)	αHF	αLF	QLF × 10−5 (Fcm−2s−(1−a))	Cdl × 10−6 (μF cm−2)
1	0% Cr	17.2	0.59	0.81	594.4	1244.1
7		19.3	0.57	0.52	564.3	1095.4
14		18.1	0.43	1.07	916.9	893.2
21		13.7	0.46	1.06	1114.5	1264.9
28		12.9	0.50	1.09	999.1	1281.9
1	5% Cr	19.2	0.74	0.98	53.8	112.5
7		26.1	0.70	0.95	110.7	244.2
14		21.9	0.69	0.53	125.5	253.1
21		18.9	0.75	0.51	108.8	306.76
28		21.3	0.71	0.42	153.8	382.9
1	10% Cr	19.7	0.74	0.52	29.2	47.6
7		20.5	0.70	0.64	37.8	47.3
14		22.5	0.82	0.45	17.3	49.3
21		21.4	0.33	0.56	8.3	1.7 × 10−4
28		19.8	0.77	1.00	17.1	32.1

summarizes the Q and α values obtained graphically, along with the electrolyte resistance (R_e), which remained relatively stable (19.7–22.5 Ω·cm²) throughout the immersion period. This stability suggests consistent electrode–electrolyte contact and negligible changes in the solution’s conductivity, reinforcing the integrity of the oxide layers. Additionally, the double-layer capacitance (C_dl) was estimated using the Brug formula. The observed decline in C_dl values with the increasing Cr content is associated with either an increase in the oxide layer thickness or a reduction in its dielectric constant. Both scenarios align with the Cr-induced passivation and densification of the oxide film.

The EIS analysis further supports the beneficial effect of Cr on the corrosion resistance of TWIP steels. As observed in the Nyquist and Bode plots (Figure 4 and Figure 5), Cr additions lead to the formation of a more resistive and stable oxide layer. To rationalize these results, the equivalent electrical circuit presented in Figure 6a was employed to model the experimental impedance spectra. This circuit comprises two R–CPE (resistor–constant phase element) pairs, each associated with distinct electrochemical contributions. The high-frequency loop is commonly attributed to the properties of the outer oxide layer (e.g., oxide resistance and pseudo-capacitance), while the low-frequency loop reflects the charge transfer resistance and ionic diffusion through the inner barrier layer and oxide/electrolyte interface [36].

Importantly, the assignment of these time constants is not necessarily unique and may vary depending on the system complexity, including variations in the surface morphology, porosity, or localized corrosion processes. Nevertheless, the use of a dual time-constant circuit provides a reasonable approximation of the dominant electrochemical phenomena observed in Cr-containing alloys. In particular, the 10% Cr alloy exhibited the highest impedance values and the lowest pseudo-capacitance (Q), which is consistent with the development of a dense, compact, and chemically stable oxide film enriched in spinel-type phases such as FeCr₂O₄.

Although not shown here, the fitted curves closely matched the experimental data, particularly in the low-frequency region, which supports the validity of the proposed model in describing the corrosion response of Cr-containing alloys. A comparative visualization of fitted vs. experimental spectra for each alloy will be considered in future supplementary analyses.

It is worth noting that while the selected equivalent circuit assumes a serial configuration of processes, this model may not fully capture localized effects, especially in the 0% Cr alloy, where heterogeneous surface features and poorly protective oxide films likely give rise to lateral inhomogeneities and parallel transport pathways. Consequently, future studies may benefit from exploring more advanced circuit topologies that account for the distributed elements or spatial variability across the electrode surface.

3.3. Surface and Corrosion Product Analysis

The SEM analyzed the corroded surfaces’ morphology, as shown in Figure 7, revealing distinct morphologies. After 28 days of exposure, the 0% Cr alloy exhibited a voluminous spherical rust layer with widespread cracking and poor adhesion to the substrate. At a higher magnification (a₁), extensive delamination, cracking, and an uneven surface topography were observed, indicating early stages of localized corrosion. These features are likely attributed to the instability of the oxide layer and inhomogeneous dissolution processes [37,38]. In contrast, the 5% Cr alloy showed a more compact oxide layer, although some cracks and localized detachment were still present (b). Secondary corrosion products were found surrounding delaminated regions, suggesting the initiation of localized re-passivation (b₁). The 10% Cr sample exhibited a significantly more uniform oxide layer with a characteristic flake-like structure (c and c₁) [39,40]. In both samples (5 and 10% Cr), the characteristics of the corrosion products formed are associated with the composition and phase distribution of the TWIP steel studied [41].

The morphology in 0% Cr is consistent with the electrochemical data, which shows low impedance values and decreasing phase angles, indicating a non-protective and porous film [42]. In contrast, the improvement of the film integrity with 5% Cr, with respect to the alloy without Cr, correlates with the intermediate EIS parameters and phase angle stability observed for this composition [43].

The 10% Cr alloy exhibited a dense and chemically stable oxide film. The cracking and detachment were minimal, supporting the notion of a dense, chemically stable oxide film. These observations are consistent with the high impedance modulus, low Q values, and near ideal capacitive behavior measured electrochemically, which reinforces the role of Cr in promoting the formation of continuous, protective oxide structures [44]. As shown in Figure S1 of the Supplementary Material, the alloys exhibit a smooth surface morphology prior to corrosion testing, in contrast to the images in Figure 7, which display corrosion products for all three alloy compositions. Furthermore, Figure S2 in the Supplementary Material displays the EDS analysis results for the three alloy compositions following 28 days of immersion in a Na₂SO₄ solution.

To determine the composition of the corrosion products, Raman spectroscopy was used to analyze the exposed surfaces. Figure 8 shows the corresponding Raman spectra for the three alloys. The 0% Cr sample (Figure 8a) revealed four bands corresponding to a mixture of hematite (α-Fe₂O₃) e maghemite (γ-Fe₂O₃) at 339, 498, 612, and 1421 cm⁻¹ and a strong signal for magnetite (Fe₃O₄) at 688 cm⁻¹. These phases are typical of loosely bound, less protective iron oxides formed in mildly oxidizing environments [38].

In the 5% Cr alloy (Figure 8b), the Raman spectra showed bands corresponding to lepidocrocite (γ-FeOOH) at 244, 373, 522, 1048, and 1299 cm⁻¹ and a broad band corresponding to chromite (FeCr₂O₄) at 640 cm⁻¹, indicating the development of a more protective film enriched in spinel-type oxides. In the 10% Cr sample (Figure 8c), an additional six intense bands assigned to the mixture of goethite (α-FeOOH) and lepidocrocite were observed at 211, 269, 382, 510, 1057, and 1311 cm⁻¹, alongside the persistent chromite signal at 640 cm⁻¹ [42].

The formation of α-FeOOH and γ-FeOOH is particularly significant, as both are associated with enhanced corrosion resistance due to their chemical stability and low solubility in sulfate environments. The presence of Cr-rich spinels also suggests an inward Cr diffusion during passivation, which favors film compaction, electrochemical stability, passivity, and resistance to localized corrosion in sulfate and chloride environments [36,40].

These findings are consistent with previous reports on Cr-alloyed high-Mn steels, where the emergence of α-FeOOH, γ-FeOOH, and FeCr₂O₄ has been linked to an increased passivity and resistance to localized corrosion in sulfate and chloride environments [15,35].

Although cross-sectional EDS or depth-profiling techniques (e.g., XPS, GDOES) were not employed in this study, the Raman and SEM analyses strongly support the hypothesis that Cr promotes the formation of stable, adherent oxide films with enhanced barrier properties. No Mn-rich phases were detected on the surface by the Raman characterization, although the presence of this element was observed in the EDS analysis (S1), with higher amounts in the 5% Cr and 10% Cr samples. In Fe–Mn alloys, a manganese enrichment at the metal–oxide interface has been reported due to the preferential dissolution of Fe and Cr, particularly in sulfate-containing environments. This enrichment can influence the electronic and structural properties of the oxide film, potentially stabilizing oxyhydroxide phases or forming mixed Mn–Fe oxides. While this phenomenon was not directly observed here, it may contribute to the overall corrosion behavior and warrants further investigation using depth-sensitive analytical techniques [15,18,43]. This aspect warrants further investigation, particularly given the relevance of Mn enrichment in selective dissolution scenarios.

While the 10% Cr alloy exhibited the most stable passive behavior overall, a minor localized oxide detachment was observed after the extended immersion, suggesting that even highly protective films may be susceptible to long-term degradation under static exposure. Future studies should include cross-sectional SEM/EDS or depth-profiling methods to validate the internal morphology, composition gradients, and film thicknesses. These analyses would provide direct insight into the stratification of oxide layers and help confirm the proposed corrosion mechanism. Additionally, evaluating the electrochemical behavior under a dynamic flow or thermal cycling would better simulate service conditions and further assess the durability of Cr-induced oxide films [44].

3.4. Schematic Representation of Corrosion Evolution in Cr-Alloyed TWIP Steels

Based on the electrochemical measurements, surface morphology, and phase composition data obtained in this study, a conceptual model is proposed to illustrate the corrosion mechanism of Fe-22Mn-0.6C TWIP steels as a function of the Cr content (Figure 9). This model integrates the observed trends in the open-circuit potential, impedance response, SEM morphology, and Raman spectroscopy to describe the evolution of the oxide film structure and stability under the sulfate exposure.

For the 0% Cr alloy, the oxide film formed upon immersion is thin, porous, and chemically unstable. This insufficient barrier permits the continuous ingress of aggressive species, leading to localized attacks and extensive oxide detachment. The SEM micrographs and Raman spectra revealed the presence of non-protective iron oxides such as Fe₃O₄ and γ-Fe₂O₃, while EIS measurements indicated a low charge transfer resistance and high pseudo-capacitance, confirming the formation of a defective oxide layer with a poor electrochemical performance.

In contrast, the 5% Cr alloy develops a more continuous and chemically stable oxide film. The presence of FeCr₂O₄ and γ-FeOOH, as detected by Raman spectroscopy, suggests a partial Cr incorporation into the oxide layer. This addition enhances its protective character, as reflected by the increased impedance modulus and more stable phase angles over time. However, SEM images revealed localized detachment and cracking, suggesting that the film, although improved, may still exhibit structural or compositional inhomogeneities that limit its long-term stability.

The 10% Cr alloy exhibits the most robust corrosion resistance. The oxide film is characterized by a dense, flake-like morphology, enriched in FeCr₂O₄ and α-FeOOH. The electrochemical response is consistent with a compact, highly resistive oxide layer, as indicated by the lowest Q and C_dl values, high α coefficients, and the most noble and stable OCP. Although some minor oxide delamination was observed at 28 days, these features may reflect dynamic reorganization or localized re-passivation processes, which are common in Cr-rich systems.

The proposed model in Figure 9 represents a conceptual summary of these findings. It highlights the progressive improvement in the oxide film integrity and corrosion protection as the Cr content increases, from a highly defective and reactive layer in the 0% Cr alloy to a continuous, compact, and chemically stable oxide film in the 10% Cr alloy. This schematic is grounded in the current experimental observations but should be validated through a future cross-sectional characterization (e.g., SEM-EDS, GDOES, or XPS depth profiling) to confirm the oxide layer stratification and elemental distribution.

4. Conclusions

This work presents a comprehensive evaluation of the corrosion behavior of Fe-22Mn-0.6C TWIP steels with varying chromium (Cr) contents when exposed to a neutral sulfate solution. The results demonstrate that chromium plays a central role in enhancing passivation by promoting the formation of compact, stable, and chemically protective oxide layers. As the Cr content increases, the electrochemical performance improves significantly, as evidenced by the higher impedance modulus values, lower pseudo-capacitance, and more stable phase angle behavior. These trends are particularly evident in the 10% Cr alloy, which exhibited the most effective corrosion resistance throughout the 28-day immersion period.

Surface and spectroscopic analyses supported these findings by revealing a shift in the morphology and composition of the corrosion product. While the Cr-free alloy formed porous and unstable iron oxides, Cr-containing alloys developed adherent films enriched in spinel-type oxides and iron oxyhydroxides. These structural and compositional changes contributed to enhanced barrier properties and a reduced film degradation over time.

Taken together, the electrochemical and surface data allowed for the development of a conceptual model that illustrates the Cr-dependent evolution of oxide layers in TWIP steels. The findings highlight the potential of chromium as a key alloying element to enhance long-term corrosion protection in high-Mn steels exposed to sulfate-rich environments, providing valuable insights relevant to alloy design in demanding service conditions.