Impact of plastic deformation on the corrosion behavior of S32304 duplex steel in a simulated industrial solution

2.1 Specimens and surface preparation

The study employed S32304 duplex stainless steel with a 1 mm thickness as the base material. The mass fractions of the components in stainless steel are as follows: 23.36 % chromium (Cr), 3.64 % nickel (Ni), 1.52 % manganese (Mn), 0.57 % silicon (Si), 0.33 % copper (Cu), 0.157 % nitrogen (N), 0.15 % molybdenum (Mo), 0.026 % phosphorus (P), 0.017 % carbon (C), 0.001 % sulfur (S), with the remainder being iron (Fe). Before conducting the experiments, the stainless steel plates underwent an annealing process at 1,100 °C for 4 h, followed by quenching in water.

The duplex stainless steel plates underwent cold deformation in a single direction by passing them between working rolls to gradually reduce their thickness through successive passes. The plates were cold rolled to achieve reductions of 10 %, 30 %, 50 %, 70 %, and 90 % in thickness. Subsequently, the samples were cut into dimensions of 10 mm × 12 mm, ensuring that the surfaces were parallel to the rolling direction. Surface preparation involved sequential grinding using SiC paper with grit sizes ranging from 400 to 2000, followed by polishing with 0.1 μm diamond powders. Afterward, the samples were thoroughly rinsed with deionized water and alcohol, and finally dried before further analysis.

2.2 Material characterization

The electron backscatter diffraction (EBSD) samples were prepared through a series of steps including mechanical grinding, polishing, and subsequent electropolishing. Electropolishing was carried out using an electrolyte mixture consisting of 15 % perchloric acid and 85 % ethanol by volume. This process helped to achieve a smooth and polished surface suitable for EBSD analysis. The raw EBSD data obtained from the samples were processed using Aztec Crystal software, enabling detailed analysis and interpretation of the crystallographic information contained within the microstructure of the material.

For X-ray photoelectron spectroscopy (XPS) analysis, the sample underwent electrochemical polishing to obtain the passive film. XPS analysis was conducted using the Thermo Escalab 250XI instrument, which features a beam diameter of 650 μm and operates at an operating voltage of 1.486 kV. Charge correction was achieved using the carbon contamination peak (C1s = 284.8 eV). High-resolution spectra were obtained for oxygen (O), iron (Fe), and chromium (Cr). The XPS data were analyzed using commercial XPSpeak software with Shirley background subtraction. The C1s peak from adventitious carbon at 284.8 eV was utilized as the calibration peak to correct for charging shifts, and all other peaks were calibrated using the C1s peak.

2.3 Electrochemical test

Electrochemical measurements were conducted using a VersaSTAT 3F with a conventional three-electrode cell system. A saturated calomel electrode (SCE) served as the reference electrode, while the sample and a platinum sheet were chosen as the working and counter electrodes, respectively. Initially, the specimens underwent a constant potential of −1.1 V_SCE for 60 s to eliminate the oxide layer formed in air. Subsequently, the open circuit potential (OCP) was monitored for 60 min to ensure the stability of the test system. Finally, dynamic polarization curves were generated at a scanning rate of 0.5 mV/s, with the scanning range spanning from −0.8 to −1.3 V_SCE. Corrosion current density values (i _corr ) were determined using the Tafel region extrapolation method.

For capacitance measurements, Mott–Schottky plots were scanned at a frequency of 1,000 Hz within a potential range from −1,000 to 1,000 mV SCE, employing a perturbation voltage of 10 mV. The scan rate was set at 36 mV/steps to prevent any alterations in the passive film. The test solutions utilized throughout all experiments were simulated industrial environment solutions, consisting of 0.01 mol/L NaHSO₃ with 3.5 wt% NaCl. Following the protocol outlined by Burstein et al. (Burstein and Ilevbare 1996; Burstein and Vines 2001), experiments were repeated at least three times to ensure the reproducibility of the obtained results. All the above experiments were carried out at room temperature.

2.4 Immersion test

The initiation sites of pitting corrosion were pinpointed through a combination of immersion testing and EBSD analysis, following the methodology outlined in previous studies (Fu et al. 2020; Liu et al. 2024b). Immersion experiments were carried out using a simulated industrial environment solution. The immersion temperature was room temperature. After immersion for 24 h, the sample underwent thorough cleaning. Specific areas were then selected for detailed analysis. Finally, EBSD was utilized to examine the corrosion behavior within various microstructural regions, providing insights into the localized corrosion mechanisms at play.

3.1 Characterization of microstructure

Figure 1 illustrates the phase diagrams of S32304 stainless steel under various deformation conditions (0 %, 10 %, 30 %, and 50 %), revealing a relatively uniform distribution of phases in the samples. In duplex stainless steels, the austenite phase acts as the secondary phase in the microstructure, and its deformation is constrained by the surrounding ferrite phase. Under moderate deformation conditions, the ferrite phase can effectively accommodate the deformed austenite grains.

Figure 1:

The EBSD phase maps of S32304 duplex stainless steel depict varying degrees of cold deformation: (a) without deformation, (b) 10 % deformation, (c) 30 % deformation, and (d) 50 % deformation.

Following a 24-h immersion of the raw materials in a simulated industrial environment, an EDS surface scan image was acquired. Figure 2 displays both the SEM image and the EDS surface scan image of unaltered S32304 following a 24-h immersion in a simulated industrial environment. In the SEM image, raised areas appear gray, showcasing a concentration of nickel elements, indicative of the austenite phase. Conversely, recessed areas exhibit a grayish-white hue, highlighting a concentration of chromium elements, representing the ferrite phase. The austenite phase is distributed in an elongated, blocky, and irregular dendritic shape within the ferrite matrix, aligning with findings from previous literature (Saha et al. 2020).

Figure 2:

The EDS surface scan image of undeformed S32304 duplex stainless steel after immersion in a simulated industrial environment solution for 24 h.

Figure 3 illustrates SEM images of S32304 duplex stainless steel subjected to various deformations after immersion in a simulated industrial environment solution for 24 h. Under the condition of 0 % deformation, as seen from Figure 3 (a1), austenite is regularly distributed in the ferrite matrix. In the middle corrosion area, it can be seen that ferrite is corroded while austenite shows a structure distributed in a pattern similar to a rhombus and is not corroded. In the lower right corner, slight pitting pits can also be observed in the austenite area. In Figure 3 (a2), the distribution of austenite is relatively disordered. In the middle corrosion area, it can also be observed that ferrite is preferentially corroded. And it is observed that the uncorroded austenite has a disordered orientation and intersects with each other. The orientations and morphologies of the two areas are different, but in both cases, the ferritic phase is preferentially corroded. At deformation rates of 10 %, 30 %, and 50 %, it can be seen that the corrosion morphologies of the same sample in different areas are different. This may be related to the orientation of the material. However, in the corrosion areas, it can be observed that ferrite is corroded away and austenite stands out. Some local phase boundary corrosion occurs, and the ferritic phase is also preferentially corroded. Of course, some slight pitting pits are observed to occur in the austenite area. Upon reaching a deformation of 70 %, significant corrosion pits emerge, with diameters ranging from 60 to 80 μm, spreading into the austenite region. Irregular prism-shaped pits also appear. This may be crystallographic pits, which have been reported in some papers (Shahryari et al. 2009; Tang and Aday 2024). The primary ferrite phase undergoes corrosion, with some observed on the austenite as well. When the deformation reaches 90 %, severe local corrosion appears on the surface. Austenite becomes the center of pit formation, indicating that the austenite phase is preferentially corroded. The surface morphology observed via laser confocal microscopy using OLYMPUS LEXT OLS 4100 yielded the following results, as summarized in Table 1. Under the condition of 0 % deformation, corrosion pits primarily formed in the ferrite phase, with a depth of 202.835 ± 12.122 μm. Under the deformation condition of 10 %, the observed maximum pitting pit further increases to 245.573 ± 13.222 μm. It may be that the ferrite is deformed and its Volta potential is increased, resulting in an increased potential difference between ferrite and austenite and enhanced galvanic corrosion. Under 30 % deformation, the maximum observed pit depth was 206.986 ± 11.114 μm, displaying a regular circular shape. Under the deformation condition of 50 %, the maximum pitting depth of 121.797 ± 8.154 μm is observed. Pitting develops into local corrosion, mainly developing along the ferrite area and presenting an irregular dendritic shape. With 70 % deformation, corrosion depth reduced to 90.754 ± 7.126 μm. Pitting pits can be seen to germinate from ferrite and spread in a circular shape. While ferrite is corroded, local austenite is also corroded. Under the deformation condition of 90 %, the maximum corrosion pit depth of 28.565 ± 6.142 μm can be observed. At the same time, it is observed that pitting has changed from germinating at ferrite to germinating at austenite, and austenite is preferentially corroded. The findings presented in Figure 4 demonstrate a notable trend in the variation of maximum pit depth concerning the deformation. Initially, there is an increase in the maximum pit depth, followed by a gradual decrease. This observation suggests a nuanced behavior in the pitting corrosion resistance of the material, characterized by an initial decrease followed by an increase. One potential explanation for this behavior lies in the distinct responses of the corrosion resistance exhibited by the two phases of the material in response to stress variation. Specifically, the corrosion resistance of ferrite exhibits a steady decrease with increasing stress, while the corrosion resistance of austenite shows an initial increase under low deformation conditions, followed by a sharp decline with higher degrees of deformation.

Figure 3:

SEM image of S32304 with different cold deformation levels after immersion in a simulated industrial environment solution for 24 h: (a_1–2) 0 %, (b_1–2) 10 %, (c_1–2) 30 %, (d_1–2) 50 %, (e_1–2) 70 %, and (f_1–2) 90 % (α: ferrite phase, γ: austenite phase).

Table 1:

The laser confocal results of S32304 duplex stainless steel with different deformation levels after immersion in a simulated industrial environment solution for 24 h.

Deformation	3D topography	3D height distribution map	Maximum pitting depth (μm)
0 %			202.835 ± 12.122
10 %			245.573 ± 13.222
30 %			206.986 ± 11.114
50 %			121.797 ± 8.154
70 %			90.754 ± 7.126
90 %			28.565 ± 6.142

Figure 4:

Maximum pitting depth changes with different deformation after immersion in a simulated industrial environment solution for 24 h: (a) 10 %, (b) 30 %, (c) 50 %, (d) 70 %, and (e) 90 %.

The specific potential difference between the ferrite and austenite phases, typically ranging from 50–100 mV, plays a crucial role in this corrosion behavior (Mondal et al. 2019; Örnek and Engelberg 2015; Örnek et al. 2019). Electrochemical data indicate that in a 1 M H₂SO₄ environment, the dissolution potentials of austenite and ferrite are −281 mV_SCE and −323 mV_SCE, respectively (Mondal et al. 2019). This potential difference theoretically induces galvanic corrosion, accelerating the corrosion process in the ferritic phase (Tsai and Chen 2007). This dynamic interplay between stress, corrosion resistance, and potential difference contributes to the complex corrosion behavior observed.

Figure 5 illustrates the EBSD results of S32304 following immersion in a simulated industrial environment solution for 24 h. Table 2 provides the proportion of low-angle grain boundaries under varying deformations. In the phase diagram, green denotes ferrite, while red signifies austenite. At 10 % deformation, 97.2 % of grain boundaries are low angle. As deformation increases to 30 %, ferrite exhibits preferential corrosion, reducing the proportion of low-angle grain boundaries to 87.0 %. At 50 % deformation, though ferrite remains preferentially corroded, austenite shows increased corrosion severity, with low-angle grain boundaries declining to 60.3 %. At 70 % deformation, minimal austenite is observed, with the majority being ferrite, and low-angle grain boundaries decrease to 56.6 %. At 90 % deformation, austenite is scarcely visible, indicating a corrosion transition from ferrite to austenite. Further deformation beyond 70 % doesn’t significantly alter the proportion of low-angle grain boundaries. This highlights a corrosion preference reversal from ferrite to austenite around 70 % deformation.

Figure 5:

EBSD of S32304 with various deformation levels after immersion in a simulated industrial environment solution for 24 h.

3.2 Electrochemical behavior analysis

In Figure 6, potentiodynamic polarization results of S32304 duplex stainless steel in a solution of 0.01 mol/L NaHSO₃ containing 3.5 wt% NaCl at room temperature are depicted. Deformation appears to influence the corrosion process reactions concurrently. The calculated values of corrosion potential (E _corr), corrosion current density (i _p), and critical pitting potential (E _pit) are presented in Table 3. Notably, The E _pit represents the potential at which the current density reaches 100 μA cm⁻². The E_pit is chosen as the point on the anodic polarization curve where the current density rises sharply and remains elevated without passivating again, which reflects the corrosion tendency of the material and the ease of destruction of the surface passivation film. As deformation increases to 10 %, the E _corr shifts in a positive direction, while i _p and E _pit decrease. At this deformation level, general corrosion resistance improves, but pitting corrosion resistance is reduced. The deformation causes a rise in potential difference between the two phases, reducing the material’s resistance to pitting corrosion.

Figure 6:

Potentiodynamic polarization curves of S32304 duplex stainless steel with different deformation level in 0.01 mol/L NaHSO₃ solution containing 3.5 wt% NaCl at room temperature.

As deformation increases to 30 %, the E _corr shifts toward the negative direction. At the same time, the i _p increases, while the E _pit still shows a downward trend. This observation may stem from the enhanced resistance of austenite to general corrosion, as documented in previous literature (Luo et al. 2017). However, the decrease in ferrite’s corrosion resistance exacerbates the corrosion current. Despite this, the potential difference between the two phases continues.

With a further increase in deformation to 50 %, i _p continues to rise, and the corrosion resistance of austenite begins to decline. Consequently, there is an increase in E _pit, accompanied by an increase in the potential difference between the two phases. At a deformation level of 70 %, the i _p rapidly escalates to 2.165 ± 0.15 μA/cm². Surprisingly, the E _pit increases to 280.636 ± 9 mV_SCE, indicating that although the corrosion intensity increases, the resistance to pitting corrosion is improved. This increase in pitting potential corresponds to a reduction in the potential difference between the two phases.

Upon further deformation to 90 %, the i _p continues to climb to 2.255 ± 0.16 μA/cm², resulting in further deterioration in resistance to general corrosion. However, the E _pit also experiences a further increase to 295.051 ± 10 mV_SCE. This suggests that despite the heightened general corrosion, the material’s resistance to pitting corrosion improves.

The Mott–Schottky approach was utilized to analyze the semiconducting properties of the material in a 0.01 mol/L NaHSO₃ solution containing 3.5 wt% NaCl under various levels of cold deformation. In Figure 7, the Mott–Schottky plots illustrate the relationship between the semiconductor’s capacitance and the applied potential. Each curve on the plot demonstrates two distinct regions with positive and negative slopes, separated by a narrow potential plateau region. In this plateau region lies the flat band potential (E _FB), which is a crucial parameter indicating the absence of charge accumulation or depletion. The flat band potential (E _FB) was observed to be approximately 0 mV_SCE, indicating that the material was neither accumulating nor depleting charges at this potential. This suggests that the material’s surface was in a stable, nonpolarized state in the tested electrolyte solution. As shown in Figure 7, the passive film of S32304 duplex stainless steel presents a p-n heterojunction, independently of cold deformation. The positive slope (E > 0 V_SCE) suggests the electronic behavior of an n-type semiconductor, indicating that oxygen vacancies and/or cation interstitials are the major defect. The negative slope (E < 0 V_SCE) suggests the electronic behavior of a p-type semiconductor, indicating that cation vacancies are the major defect in the passive film (Taveira et al. 2010).

Figure 7:

The Mott–Schottky curve of passive films of S32304 duplex stainless steel with different deformation level in 0.01 mol/L NaHSO₃ solution containing 3.5 wt% NaCl at room temperature.

The values of donor density (N _D), acceptor density (N _A), and E _FB were determined from the Mott–Schottky curves. In Table 4, the N _D, N _A, and E _FB values of the passive film formed in 0.01 mol/L NaHSO₃ solution containing 3.5 wt% NaCl under various cold deformation levels are presented. It’s noted that there were no significant differences observed in EFB with changes in the cold deformation level. The N _D and N _A values ranged from 10²⁰ to 10²¹ cm⁻³, indicating a consistent order of magnitude compared to previous reports on stainless steel in different environments (Hakiki et al. 2000; Luo et al. 2012; Taveira et al. 2010).

The trend observed in the results indicates that N _D initially decreases, then slightly increases, and eventually stabilizes. This trend is mirrored in the changes observed for N _A. The presence of a high donor density is found to be unfavorable for the stability of the passivation film, aligning with findings from electrochemistry studies.

3.3 Characterization of surface passive film

The stability, protective character, and semiconducting properties of S32304 duplex stainless steel are associated with the composition of the passive film. The full spectrum result of XPS, as depicted in Figure 8, reveals the predominant elements in the passive film. These include Mo, O, Cr, Fe, and Ni, with respective atomic percentages of 6.54 %, 70.66 %, 7.25 %, 13.11 %, and 2.23 %.

Figure 8:

Spectra of X-ray photoelectron spectroscopy (XPS): (a) full spectrum of XPS. The percentage is atomic concentration (b) Fe 2p, (c) Cr 2p, and (d) O 1s.

The XPS spectra depicted in Figure 8 provide detailed insight into the chemical states of individual element within the passive film. The fine spectrum of Fe 2p reveals iron existing in four distinct chemical states: metallic iron (Fe_met), iron (II) oxide (FeO), iron (III) oxide (Fe₂O₃), and iron oxyhydroxide (FeOOH), with FeO and Fe₂O₃ representing the most prominent peaks [43]. The Cr 2p spectrum demonstrates chromium present in three different forms within the passive film: metallic chromium (Cr_met), chromium (III) oxide (Cr₂O₃), and chromium hydroxide (Cr(OH)₃), with Cr₂O₃ and Cr(OH)₃ being the predominant constituents(Wei et al. 2022). The fine spectrum of O 1s primarily manifests two significant chemical states: hydroxide (OH⁻) and superoxide (O₂ ⁻) (Zhao et al. 2022b).

From the XPS analysis, it is evident that the passive film is primarily composed of iron oxides (FeO and Fe₂O₃) and chromium oxides (Cr₂O₃ and Cr(OH)₃). These chemical states contribute to the stability and protective properties of the passive film on UNS S32304 duplex stainless steel.