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Article

Semiconductive Tendency of the Passive Film Formed on Super Austenitic Stainless Steel SR-50A in Acidic or Alkaline Chloride Solutions

by
Seung-Heon Choi
1,
Young-Ran Yoo
2 and
Young-Sik Kim
1,2,*
1
Department of Materials Science and Engineering, Andong National University, 1375 Gyeongdong-ro, Andong 36729, Republic of Korea
2
Materials Research Centre for Energy and Clean Technology, Andong National University, 1375 Gyeongdong-ro, Andong 36729, Republic of Korea
*
Author to whom correspondence should be addressed.
Crystals 2024, 14(9), 766; https://doi.org/10.3390/cryst14090766
Submission received: 13 August 2024 / Revised: 23 August 2024 / Accepted: 26 August 2024 / Published: 28 August 2024
(This article belongs to the Section Crystalline Metals and Alloys)
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Abstract

:
Stainless steel is widely used in various industrial fields due to its excellent corrosion resistance and mechanical properties. The key to this corrosion resistance is the thin passive film that naturally forms on the metal surface. Passive films are characterized by oxide film theory and adsorption theory, each uniquely explaining the structure and mechanism of the protective film on the metal surface. Research on the semiconductive properties of passive films on stainless steel offers diverse viewpoints, classifying theories into the point defect model and the bipolar fixed charge-induced passivity. Specific changes in passive film attributes that lead to degradation, however, are not fully understood. In this study, we analyzed the inner and outer layers of the passive film on super austenitic stainless steel SR-50A under various conditions in acidic and alkaline chloride environments. The interpretations of these results were based on the point defect model and the bipolar model for the passivation mechanism, and correlations between p-type and n-type semiconductor properties and passivation behavior were examined. The surface of the stainless steel forms a passive film comprising two layers with p-type and n-type semiconductive properties, independent of the pH of the solutions. The corrosion resistance increases as the p-type and n-type semiconductive tendencies become more balanced, consequently enhancing the properties of the passive film.

1. Introduction

Stainless steel is widely used in various industrial fields for its excellent corrosion resistance and mechanical properties. The key to this corrosion resistance is the thin passive film naturally formed on the metal surface. The passive film on stainless steel consists mainly of oxides or hydroxides of Fe and Cr [1,2,3], and its stability is enhanced by the addition of more than 12% of Cr [4]. Therefore, the structure and composition of the passive film are crucial in determining the corrosion resistance of stainless steel.
These passive films can be categorized into two perspectives. The first proposes that the passive film acts as a diffusion barrier formed by reaction products. This layer, composed of metal oxides or other compounds, isolates the metal from its environment and reduces the reaction rate. This concept is known as the oxide film theory [5]. The second perspective suggests that the metal’s passive state is maintained by a chemisorbed oxygen film, which displaces the adsorbed water molecules and lowers the anodic dissolution rate through hydration of the metal ions. This means the adsorbed oxygen decreases the exchange current density of the dissolution reaction M = Mz+ + ze and heightens the anodic overvoltage. It indicates that even a single layer of adsorbed film can provide a passivation effect [6], despite not serving as a diffusion barrier. This theory is referred to as the adsorption theory [6].
Passivity theories are grouped under the two aforementioned perspectives (oxide film theory and adsorption theory), with each theory offering distinct explanations based on the structure and function of the protective film on the metal surface. Notable theories include the “Electronic Configuration-Induced Adsorption Passivity” [7] and the “Ionic Space Charge-Induced Passivation” [8]. The former theory posits that the metal surface’s electronic structure prompts the chemical adsorption of oxygen or other elements, consequently forming a protective film that shields the metal from corrosion. However, this model focuses exclusively on oxygen adsorption onto the metal and neglects to consider the kinetic stability of the passive film [9]. Conversely, the latter theory by Fromhold et al. [8] suggests that the ionic current in a barrier layer can be minimized by the presence of an ionic space charge, compared to what would occur without such a charge, relying on field-assisted ionic conduction. Yet, this theory, which concentrates solely on ionic currents within the film, does not address the dissolution rate of the barrier layer that determines the passive metal’s overall dissolution rate [9].
Research on passive films has focused on analyzing their thickness and composition using X-ray Photoelectron Spectroscopy (XPS). Studies on passive films can be summarized as follows: A.M.P. Simoes et al. [10] proposed that the passive film on Fe-18Cr in a solution of 0.075 M Na2B4O7∙10 H2O + 0.05 M H3BO3 (pH 9.2) consists of a single layer of Cr and Fe oxides. V. Maurice et al. [11] suggested that the passive film on Fe-Cr-Ni alloy in a 0.5 M H2SO4 environment consists of internal Cr and Fe oxides, and external Cr and Ni hydroxides. G. Lorang et al. [12] stated that the passive film on Fe-18Cr-8Ni alloy in a 0.075 M Na2B4O7∙10 H2O + 0.05 M H3BO3 (pH 9.2) environment consists of an inner layer of Cr oxides and an outer layer of Fe oxides and Fe hydroxides. P. Marcus et al. [13] proposed that the passive film on Fe-17Cr-8Ni alloy in a 0.5 M H2SO4 environment contains internal Cr oxides and chromium nitrides, and external Cr hydroxides. E.D. Vito et al. [14] suggested that the passive film on Fe-18Cr-8Ni-1.5Mo alloy in a 0.5 M H2SO4 environment comprises internal Cr oxides and external Cr hydroxides and Mo6+ states.
These findings reflect diverse views on the specific structure and composition of passive films. This diversity is attributed to the extremely thin thickness of the film, typically only a few nanometers (nm), and to the fact that ex-situ analysis techniques like XPS can alter the film’s properties when exposed to air, complicating the revelation of its true properties. Nevertheless, the discovery of an electric field within the passive film has established techniques such as photocurrent measurement and capacitance measurement through Mott–Schottky analysis as powerful tools for the in-situ analysis of passive films on metals and alloys.
The research on the semiconductor characteristics of passive films was summarized using photocurrent measurement and Mott–Schottky analysis: H. Tsuchiya et al. [15] found that the passive film on Fe-18Cr alloy in a 0.6 M 3H3BO3 + 0.15 M Na2B4O7 (pH = 8.4) solution consists of internal Cr oxides and external Cr hydroxides, both displaying n-type semiconductor characteristics. S. Fujimoto et al. [16] reported that the passive film on Fe-18Cr alloy in both 0.15 M H2BO3 + 0.0375 M Na2B4O7 (pH = 8.4) solution and 0.1 M H2SO4 solution comprises Cr oxides internally and Cr hydroxides externally. The former solution exhibits n-type characteristics for both layers, while the latter shows p-type characteristics internally and n-type externally. R. Babic et al. [17] found that the passive film on STS 304 and STS 316 alloys in a 0.5 M NaCl environment includes an internal layer of p-type Cr oxide and an external layer of n-type Fe oxide and hydroxide. N. E. Hakiki et al. [18] suggested that the internal part of the passive film in a 0.075 M Na2B4O7∙10H2O + 0.05 M H3BO3 (pH 9.2) solution consists of p-type Cr2O3, while the external part consists of n-type γ-Fe2O3. Z. Feng et al. [19] reported that the passive film on STS 316L alloy in a 0.05 M H3BO3 + 0.075 M Na2B4O7∙10H2O solution transitions between n-type (Fe2O3, MoO3, FeO(OH)) and p-type (Cr2O3, MoO2, FeCr2O4, NiO) based on the specific potential range. H. Luo et al. [20] suggested that the passive film on 2205 duplex stainless steel in a 0.03 M Na2CrO4 + 1 M NaCl solution consists of an internal Cr-rich p-type oxide and an external Fe-rich n-type oxide, with these characteristics varying by specific potentials.
As reviewed above, the research on the semiconducting properties of passive films on stainless steel presents diverse viewpoints. The theories concerning the semiconducting properties of the passive film are broadly classified into two categories: the point defect model and the bipolar fixed charge-induced passivity. The point defect model, proposed by Hoar [21], employs mathematical analysis to describe several phenomena occurring in passive films. This model is based on the presence of highly defective oxide films characterized by a significant number of oxygen and metal vacancies (1020~1021 cm−3). It elucidates the growth and breakdown of the film by modeling the generation and annihilation reactions of defects at the metal/passive film interface and the passive film/solution interface [22]. Research based on the point defect model primarily explains the corrosion resistance of stainless steel in terms of the concentrations of donors and acceptors within the passive film, as determined through Mott–Schottky analysis. However, while specific elemental additions can be discussed in relation to defect generation and annihilation at the metal–passive film and passive film–solution interfaces [23,24,25,26,27,28,29], explanations and research on the formation mechanisms remain incomplete. The second theory, proposed by Sakashita and Sato, is the bipolar fixed charge-induced passivity. This theory highlights the pivotal role of ion selectivity due to the fixed charges in the hydrated oxide film on the metal in passive formation [30,31]. Numerous studies have shown that the hydrated oxide film on the metal forms an anion-selective layer of fixed cations on the metal side and a cation-selective layer of fixed anions on the solution side [32,33,34]. Drawing on this theory, C.R. Clayton et al. and Y.S. Kim et al. interpreted the corrosion resistance of stainless steel from the perspective of bipolar fixed charge-induced passivity. C.R. Clayton’s research suggests that the positive and negative charges in the passive film form two layers, explaining the effects of Cr and Mo through the ion selectivity of chromate and molybdate ions formed in the outer layer of the film via a solid-state reaction [35,36,37,38]. Y.S. Kim et al. identified Mo’s influence in increasing the passive current density of stainless steel in sulfuric acid solution through the ion selectivity of molybdate ions formed in the outer layer of the passive film [39]. Furthermore, they analyzed the effect of nitrogen on the corrosion resistance of stainless steel based on the formation of nitro-oxyanion in the outer layer of the film and the bipolar model [40].
In this study, the super austenitic stainless steel SR-50A served as the experimental alloy, and its electrochemical properties were assessed through anodic polarization tests and Electrochemical Impedance Spectroscopy (EIS) measurements in acidic or alkaline chloride environments. Passive films were subsequently formed in test solutions, and the chemical states of the inner and outer layers of these films were examined using XPS. The findings were evaluated through Mott–Schottky analysis and interpreted based on the point defect model and the bipolar model of passivation. The correlations between p-type and n-type semiconductor properties and passivation behavior were analyzed, leading to a newly proposed semiconductive tendency of the passivation.

2. Materials and Methods

2.1. Experimental Alloy and Test Solution

To study the effect of the corrosion environment on the semiconductor properties of passivation on super corrosion-resistant stainless steels, UNS S32050 (super austenitic stainless steel, SR-50A (KOMETECH Research, Seoul, Korea)) was employed as the experimental alloy. This material is highly corrosion-resistant, with a PREN30 value of 52.1 and an austenitic single phase structure. The chemical composition of the experimental alloy is detailed in Table 1.
Electrochemical tests were conducted using chloride solutions at 30 °C with various pH levels. Acidic chloride solutions were prepared by adding HCl to a 1 N NaCl neutral chloride solution to adjust the pH from 1.98 to −0.38. Alkaline chloride solutions were created by adding NaOH to a 1 N NaCl neutral solution to adjust the pH from 10.29 to 11.53.

2.2. Anodic Polarization Test

Anodic polarization tests were conducted to explore passivation behavior. Specimens were cut into 1.5 cm × 1.5 cm pieces, with a rubber-coated copper wire spot-welded to one side for electrical connection and secured with epoxy resin. The specimen surfaces were polished with SiC paper #2000, then mirror-polished using 1 µm diamond paste. The exposure area was limited to 1 cm2, with the rest covered by epoxy resin. Specimens were stored in a desiccator until testing. The anodic polarization tests utilized a potentiostat (ZIVE MP1, Won A Tech, Seoul, South Korea), with a saturated calomel electrode (SCE) as the reference electrode and a high-density graphite rod as the counter electrode. The test solution was deaerated using N2 gas at 100 mL/min for 30 min, and the experiment proceeded at a scan rate of 0.33 mV/s. Corrosion current density was calculated by using the Tafel extrapolation method.

2.3. Potentiostatic EIS Test

The potentiostatic EIS test was conducted to determine the resistance of the passive film. Specimens were prepared similarly to those in the anodic polarization test. After deaerating the test solution with N2 gas at 100 mL/min for 30 min, the specimens were immersed in the solution. Using the potentiostat (Interface 1000, Gamry Instruments, Warminster, PA, USA), a passive film was formed at the anodic polarization curve potential of Ep (+400 mV (SCE)) for 30 min, and the impedance was measured under potentiostatic conditions. Measurements extended over a frequency range of 10 kHz to 0.01 Hz. The polarization resistance of the passive film was calculated by applying the resulting data to an equivalent circuit, where the Randles model was used to calculate the Rp value [41].

2.4. Mott–Schottky Anlysis

To explore the semiconducting properties of passive films, Mott–Schottky plots were conducted using specimens prepared identically to those in the anodic polarization test. The specimens were immersed in the experimental solution and deaerated with N2 gas at 100 mL/min for 30 min before forming the passive film at a potential of +400 mV (SCE) on the anodic polarization curve for 30 min using a potentiostat (Interface 1000, Gamry Instruments, Warminster, PA, USA). Subsequently, the capacitance was measured by reducing the applied potential at a rate of 50 mV/s from +1 V (SCE) to −1.5 V (SCE), with the AC amplitude fixed at 10 mV (peak-to-peak) and the frequency set at 1580 Hz.

2.5. XPS

The specimens for surface analysis were polished using SiC paper #2000 and 1 µm diamond paste to achieve a mirror finish, followed by cleaning with alcohol. The experimental solutions consisted of acidic chloride (1 N NaCl + 0.5 N HCl) and alkaline chloride (1 N NaCl + 0.005 N NaOH), and they were deaerated with N₂ gas at 100 mL/min for 30 min before immersing the specimens. A potentiostat (ZIVE MP1, Won A Tech, Seoul, South Korea) was used to perform cathodic reduction for 10 min, followed by passivation at a passive potential of +400 mV (SCE) for 3 h, in accordance with the anodic polarization behavior. The passivated specimens were stored under a nitrogen atmosphere until analysis.
Surface analysis was conducted using X-ray Photoelectron Spectroscopy (XPS) with a Thermo Fisher Scientific K-alpha instrument (Multilab-2000, Thermo Fisher Scientific, Waltham, MA, USA). Spectra for each element were acquired using an Al-Kα (1486.6 eV, 12 kV, 3 mA) X-ray source. Before the depth profile was obtained, the surface was sputter-cleaned with a 1 kV Ar ion, and the depth profile Ar-Sputtering (1 kV, 2 µA) was performed to time the sputtering from 0 to 50 s in 2 s increments for each element. After the XPS measurements, the chemical states of each element were analyzed using the Avantage software 4.78 (Thermo Fisher Scientific, Waltham, MA, USA). The binding energy of the measured C1s spectrum was corrected relative to the standard C1s binding energy (284.6 eV), followed by deconvolution for each chemical species.

3. Results

3.1. Effect of pH on Corrosion Behavior and Semiconductive Properties

Figure 1 illustrates the impact of pH on the polarization behavior of super austenitic stainless steel (SR-50A) in a deaerated 1 N NaCl + x N HCl solution at 30 °C. Figure 1a displays the influence of the pH of the acidic chloride solution on the passivation behavior of the experimental alloy. The values for corrosion potential (ER), corrosion current density (iR), critical passive current density (iC), and passive current density (iP) derived from the polarization curves are summarized in Table 2. As the pH diminished (i.e., the concentration of hydrogen ions increased), the ER for the experimental alloys generally decreased to −0.26 V (SCE), −0.27 V (SCE), −0.32 V (SCE), and −0.29 V (SCE), respectively. For iR, the observed values were 3.548 × 10−7 A/cm2, 5.248 × 10−7 A/cm2, 6.457 × 10−6 A/cm2, and 4.266 × 10−6 A/cm2, and for iC, the values were 1.349 × 10−6 A/cm2, 3.388 × 10−6 A/cm2, 1.288 × 10−4 A/cm2, and 2.630 × 10−4 A/cm2, respectively. Particularly, the change in passive current density corresponding to the pH shift (hydrogen ion concentration) is presented in Figure 1b, where iP at +400 mV (SCE) was measured. Specifically, as the hydrogen ion concentration (H+) increased, the passive current density escalated from 1.175 × 10−6 A/cm2 to 1.288 × 10−6 A/cm2, 1.413 × 10−6 A/cm2, and 2.692 × 10−6 A/cm2, a trend consistent with multiple studies [42,43,44].
Figure 2 shows the influence of pH on the polarization behavior of super austenitic stainless steel (SR-50A) in deaerated 1 N NaCl + x N NaOH solutions at 30 °C. Figure 2a shows the polarization curves of the experimental alloy at various pH levels in alkaline chloride solutions, from which ER, iR, transpassive potential (Etr), and iP were determined and summarized in Table 3. As the pH increases (i.e., as the concentration of hydroxyl ions increases), the ER tends to decrease to −0.069 V (SCE), −0.14 V (SCE), and −0.48 V (SCE), respectively. Correspondingly, the iR tends to increase, measuring 8.318 × 10−9 A/cm2, 6.166 × 10−8 A/cm2, and 1.318 × 10−7 A/cm, respectively. Notably, the Etr decreases rapidly with increasing hydroxyl ion concentrations to +0.74 V (SCE), +0.64 V (SCE), and +0.54 V (SCE), respectively. Moreover, Figure 2b shows how iP at +400 mV (SCE) and hydroxyl ion concentration (OH−) are related. As the hydroxyl ion concentration rises, the passive current density increases to 3.548 × 10−6 A/cm2, 5.129 × 10−6 A/cm2, and 1.549 × 10−5 A/cm2, respectively, consistent with trends observed in other studies [45,46,47].
Figure 3 presents the EIS measurement results for the passive film formed on the experimental alloy, which was held at a passive potential (+400 mV (SCE)) for 30 min in deaerated acidic chloride (1 N NaCl + x N HCl) at 30 °C. Figure 3a displays the Nyquist plots of the passive film on SR-50A across various pH levels of acidic chloride solutions. As the pH decreases (i.e., the hydrogen ion concentration increases), the magnitude of the semicircle in the Nyquist plot decreases, which indicates a reduction in the polarization resistance (Rp) of the passive film with decreasing pH. Figure 3b presents the Bode plots of the passive film on SR-50A at different pH levels of acidic chloride solutions, showing that the impedance values in both low- and high-frequency regions decrease as the pH decreases. Figure 3c shows the results derived from calculating the polarization resistance (Rp) value using the Randles model for the passive film formed in this manner as a function of hydrogen ion concentration. In this case, the polarization resistance value refers to the charge transfer resistance between the metal and the environment after the passive film is formed on the metal surface. The polarization resistance diminishes significantly with increasing hydrogen ion concentration, suggesting that higher hydrogen ion concentration leads to a decrease in the protective properties of the passive film. It should be noted that although an increased concentration of hydrogen or hydroxyl ions augments the solution’s activity, leading to the thermodynamic stabilization of ions and the degradation of passive film properties, the specific changes in passive film characteristics responsible for this degradation are not fully understood.
Figure 4 shows the EIS measurement results for the passive film formed on the surface of the experimental alloy after maintaining the passive potential (+400 mV (SCE)) for 30 min, as obtained from polarization curves in deaerated alkaline chloride solutions (1 N NaCl + x N NaOH) at 30 °C with increasing pH. Figure 4a shows the Nyquist plots, revealing that the magnitude of the semicircle in the Nyquist plot decreases as the pH increases. This suggests that the resistance of the passive film declines as the pH of the solution increases. Figure 4b depicts the Bode plots for the passive film formed in alkaline chloride solutions, indicating that impedance decreases in both the low- and high-frequency regions as the pH increases. Figure 4c shows the calculated polarization resistance (Rp) of the passive film using the Randles model, demonstrating that polarization resistance decreases with increasing hydroxyl ion concentration (OH−), implying a diminution in the protective properties of the passive film as the hydroxyl ion concentration in the solution rises.
It is crucial to note that higher concentrations of hydrogen ions or hydroxyl ions in the solution increase the activity of the solution, leading to the thermodynamic stabilization of ions and the subsequent degradation of passive film properties. However, the specific changes in the passive film characteristics leading to this degradation are not thoroughly explained. Specifically, the specimen of SR-50A used in this study is super austenitic stainless steel with a PREN of 52.1. In acidic or alkaline chloride solutions test environments, no localized corrosion such as pitting was observed, although there were significant changes in the passive current density.
Figure 5 shows the influence of pH on the Mott–Schottky behavior of the passive films formed on the experimental alloy by applying a potential of +400 mV (SCE) for 30 min in deaerated acidic and alkaline chloride solutions at 30 °C. Figure 5a shows the semiconductor properties of the passive film formed in acidic chloride solutions. Between −0.5 V and 0 V (SCE), a straight line with a negative slope indicates p-type semiconductor behavior, whereas a straight line with a positive slope from 0 V to +0.5 V (SCE) indicates n-type semiconductor behavior [28,48,49]. As the pH decreases in the acidic chloride solution, both slopes tend to decrease. Conversely, Figure 5b displays the semiconductor properties of the passive film formed in alkaline chloride solutions. A negative slope from −1.3 V to −1.0 V (SCE) suggests p-type semiconductor behavior, and a positive slope from −0.5 V to −0.25 V (SCE) suggests n-type semiconductor behavior. As the pH increases in the alkaline solution, both slopes tend to decrease.
Table 4 summarizes the flat band potential (Efb) based on the Mott–Schottky plots of the passive films formed by applying +400 mV (SCE) in acidic and alkaline chloride solutions. The flat band potential is defined as the potential at which the C−2 value reaches zero (C is capacitance), determined from the slopes of tangent lines for p-type and n-type semiconductors in the Mott–Schottky plot [50]. As the pH decreases in the acidic solution, the Efb generally increases. However, as the pH increases in the alkaline solution, the Efb generally decreases, although this trend is not sharply defined.

3.2. Surface Analysis on the Passive Film Formed in Acidic or Alkaline Chloride Solutions

To analyze the structure and composition of passive films on super austenitic stainless steel in various corrosive environments, XPS analysis was utilized.
Figure 6 presents the depth profiles of key elements in the passive films formed on the experimental alloy after applying +400 mV (SCE) for 3 h in deaerated acidic chloride (1 N NaCl + 0.5 N HCl) and alkaline chloride (1 N NaCl + 0.005 N NaOH) solutions at 30 °C. Figure 6a illustrates the depth profile of Fe 2p3/2 in the passive film and the influence of pH. The passive film formed in acidic chloride solution shows a depletion of Fe in the outer layer, while in the alkaline chloride solution, depletion occurs in both the outer and inner layers. Figure 6b shows the depth profile of Cr 2p3/2 in the passive film and its pH influence. In acidic chloride solution, an enriched Cr layer is observed in the outer layer, whereas in alkaline chloride solution, Cr depletion occurs in this layer. Figure 6c presents the depth profile of Mo (3d5/2 + 3d3/2) in the passive film, along with pH influence. It reveals that the Mo concentration in the passive film from the acidic chloride solution exceeds that from the alkaline chloride solution. Figure 6d shows the depth profile of O 1s in the passive film and its pH effect. The distribution of oxygen suggests the potential to estimate the passive film’s approximate thickness. The oxygen content in the passive film from acidic chloride solution decreases significantly with sputtering time, while in the alkaline solution, it remains high even at extended sputtering durations. This suggests that the passive film in the alkaline chloride solution is thicker than in the acidic solution. Figure 6e showcases the depth profile of Ni 2p3/2 in the passive film and its pH influence. Here, Ni is depleted up to 20 s of sputtering in acidic solution and up to 40 s in alkaline chloride solution. Figure 6f illustrates the depth profile of Cl− 2p in the passive film and the influence of pH. The Cl− distribution reveals a wide presence in the passive film made from the acidic chloride solution, but it is undetected in the film made from the alkaline chloride solution. There remains no consensus regarding the presence of chloride in passive films [51,52].
When summarizing the depth profiles of the passive films in Figure 6, it can be observed that the elemental distributions of passive films formed in acidic and alkaline chloride solutions differ, as do the distributions between the outer and inner layers of these films. Moreover, the elemental distributions vary between the outer and inner layers. Based on the oxygen distribution, the chemical states of each element in both layers were deconvoluted, with the outer and inner layers defined by the sputtering time up to 25 s for acidic chloride and 50 s for alkaline chloride solutions, respectively.
Figure 7a,b present the depth profiles of the chemical states of Fe 2p3/2 in the passive films formed at +400 mV (SCE) in acidic and alkaline chloride solutions, respectively. In the outer layer of the passive film formed in the acidic solution, FeO and Fe2O3 are enriched, while in the alkaline solution, FeO, Fe2O3, and FeOOH are present in higher concentrations. In both environments, metallic Fe increases with depth.
Figure 8a,b show the depth profiles of the chemical states of Cr 2p3/2 in the passive films formed at +400 mV (SCE) in acidic and alkaline chloride solutions, respectively. In the outer layer of the passive film formed in acidic chloride solution, Cr2O3, Cr(OH)3, CrO3, and CrO42− are enriched. Conversely, in the passive film formed in alkaline chloride solution, these compounds show slight enrichment near the middle region of the film.
Figure 9a,c depict the depth profiles of the chemical states of Mo (3p5/2 + 3p3/2) in the passive films formed at +400 mV (SCE) in acidic and alkaline chloride solutions, respectively. The associated Figure 9b,d, provide enlarged views. In the outer layer of the passive film formed in acidic chloride solution, the enrichment includes MoO2, MoO3, MoO(OH)2, MoO42−, and notably, MoCl3. In contrast, the passive film formed in the alkaline chloride solution exhibits high concentrations of MoO2 throughout, with MoO3, MoO(OH)2, and MoO42− enriched in the outer layer, but no MoCl3 detected, consistent with the Cl2p depth profile in Figure 6f.
Figure 10a,b illustrate the depth profiles of the chemical states of O 1s in the passive films formed at +400 mV (SCE) in acidic and alkaline chloride solutions, respectively. In both solutions, the outer layers of the passive films are enriched with O2−, OH−, and H2O, while the inner layers primarily contain O2−. This indicates that the outer layer of the passive film is predominantly composed of hydroxides, whereas the inner layer consists mainly of oxides. Additionally, as demonstrated by the chemical states of Cr and Mo, the outer layer forms a concentrated layer of metal oxyanions such as CrO42− and MoO42− [39,40,53,54,55,56,57].
Figure 11a,b present the depth profiles of the chemical states of Ni 2p3/2 in the passive films formed at +400 mV (SCE) in acidic and alkaline chloride solutions, respectively. In the acidic chloride solution, Ni predominantly exists as NiO. Conversely, in the alkaline chloride solution’s passive film, NiO is concentrated in the middle region, while Ni(OH)2 and Ni2O3 are detected in the outer layer.

4. Discussion

As described above, the impact of pH on the passivation behavior of SR-50A, a super austenitic stainless steel, in acidic and alkaline chloride solutions is summarized as follows: an increase in the concentration of hydrogen ions or hydroxyl ions in the corrosive environment leads to an increase in the passive current density and a reduction in polarization resistance. Analyses of the semiconductor characteristics of the passive film showed that it comprises both p-type and n-type layers, regardless of the corrosive environment. Chemical state analysis reveals that the outer layer of the passive film predominantly consists of hydroxides and metal oxyanions, whereas the inner layer primarily contains oxides.
It is essential to elucidate why the passive characteristics of stainless steel vary with the corrosion environment. The point defect model [21], a widely accepted theory used to explain passive film formation, explains these variations. Figure 12 displays the total defect density in the passive films formed on SR-50A at +400 mV (SCE) in acidic and alkaline chloride solutions. The total defect density was determined using the Mott–Schottky equation, which estimates donor and acceptor concentrations from the slopes of the p-type and n-type regions [50]. Figure 12a shows the correlation between hydrogen ion concentration (H⁺) and total defect density (circle dot), highlighting a rising trend (dashed line) in total defect density with an increase in hydrogen ion concentration (i.e., decreasing pH). Conversely, the total defect density (square dot) in alkaline chloride solutions, as depicted by the function of hydroxyl ion concentration, exhibits a lesser degree of proportionality (Figure 12b). Table 5 enumerates the donor density (ND), acceptor density (NA), and total defect density as functions of pH.
Figure 13 analyzes the relationship (dashed line) among passive current density, polarization resistance, and total defect density in acidic and alkaline chloride environments. Figure 13a displays a proportional relationship between passive current density and total defect density in acidic environments, whereas this proportionality is much less pronounced in alkaline environments, as shown in Figure 13b. Figure 13c illustrates a linear decline in polarization resistance with increasing total defect density in acidic environments, while the correlation in alkaline environments appears considerably weaker, as indicated by Figure 13d. Generally, the point defect model effectively elucidates the passive behavior in neutral or acidic solutions [27,28]. According to this model, the rise in passive current density (or the fall in polarization resistance) accompanying an increase in hydrogen ion concentration in acidic chloride solutions correlates with an increase in total defect density, aligning well with predictions of the model [23,24,25,26,27,28,29]. This observation is crucial for understanding the corrosion resistance of passive films, since a high defect density within the passive film suggests a greater number of structural defects, which could impair the electrochemical properties.
However, while the total defect density results derived from the point defect model can explain the creation and annihilation of vacancies at the metal–passive film and passive film–solution interfaces, there is a lack of mechanistic explanation for these reactions. Notably, in alkaline chloride solutions, although an increase in hydroxyl ion concentration leads to an increased passive current density (or decreased polarization resistance), the proportionality with total defect density is insufficient. Therefore, while the point defect model effectively characterizes the properties of the passive film, it falls short in explaining why defect density varies with fluctuations in the corrosion environment or the alloy’s corrosion resistance (e.g., PRE).
The passivation formation theory that we will apply to resolve these questions is the bipolar model. Initially proposed by Sakashita and Sato [30,31] as a theory of bipolar fixed-charge passive films, the bipolar model was later employed by C.R. Clayton et al. [35,36,37,38] to interpret the effects of Cr and Mo on the passivation behavior of stainless steel. Y.S. Kim et al. have also utilized the bipolar model to clarify the influences of Mo and nitrogen on the corrosion resistance of stainless steel [39,40]. However, previous studies on the bipolar model have largely relied on XPS results to describe the bipolar nature of passive films, without providing direct measurement data on these characteristics.
Figure 14a shows the p-type and n-type slopes (dashed line) obtained from the Mott–Schottky plot (Figure 5a) under various acidic chloride conditions, plotted against hydrogen ion concentration. As the hydrogen ion concentration increases, both p-type and n-type slopes decrease linearly. Conversely, Figure 14b displays the p-type and n-type slopes (dashed line) from the Mott–Schottky plot (Figure 5b) under various alkaline chloride conditions, plotted against hydroxyl ion concentration. Here, the correlation between the slopes and hydroxyl ion concentration is less clear, initially increasing before decreasing.
Figure 15 presents the ratios of the slopes derived in Figure 14 as a function of the solution’s pH. As previously discussed, the passive film comprises two layers, each with different chemical states of the constituent elements. The Mott–Schottky plot also confirms that the film is composed of two layers with p-type and n-type characteristics—the inner layer primarily consists of oxides (p-type) and the outer layer is composed of hydroxides and metal oxyanions (n-type). Therefore, the passive film formed under various conditions may show a more pronounced p-type or n-type tendency. By taking the absolute values of the slopes from the Mott–Schottky plot and calculating their ratios, the tendencies can be derived. The semiconductive tendency is calculated as follows:
(1)
p-type semiconductive tendency, % = |p|/(|p| + |n|) × 100;
(2)
n-type semiconductive tendency, % = |n|/(|p| + |n|) × 100;
(3)
Difference in semiconductive tendencies = (1) − (2).
As depicted in Figure 15a, the difference between p-type and n-type tendencies increases with the decreasing pH in acidic chloride solutions. This trend corresponds to the rise in passive current density shown in Figure 1 and the reduction in polarization resistance displayed in Figure 3. Similarly, Figure 15b shows that the difference between p-type and n-type properties grows with the increase in pH in alkaline chloride solutions, consistent with the increased passive current density seen in Figure 2 and the decreased polarization resistance in Figure 4.
Figure 16 shows the difference in semiconductive tendency between p-type and n-type and plots the relationship (dashed line) between passive current density (circle dot) and polarization resistance (square dot). The effect of the difference in semiconductive tendency on the passive current density and polarization resistance is shown in Figure 16. It can be seen that regardless of the acidity or alkalinity of the solution, as the difference in semiconductive tendency becomes smaller, the passive current density decreases and the polarization resistance increases.
Therefore, a novel bipolar model for passive films on stainless steel in acidic and alkaline chloride solutions can be proposed, as shown in Figure 17. This model postulates that a passive film comprising two layers with p-type and n-type characteristics forms on the stainless steel surface. The dashed line in the figure represents the interface between the p-type and n-type layers. Variations in hydrogen or hydroxyl ion concentration in the environment modify the slopes of the Mott–Schottky plot, consequently weakening or strengthening the properties of the passive film. However, the properties and corrosion resistance of the passive film improve when the ratio of the bipolar semiconductor tendency is balanced.

5. Conclusions

In this study, the following conclusions were drawn regarding the super austenitic stainless steel SR-50A (PREN 52.1), based on corrosion characteristics, passive properties, and surface analysis in acidic and alkaline chloride solutions:
(1)
As the concentration of hydrogen ions or hydroxyl ions increases in either acidic or alkaline chloride solutions, the passive current density escalates and the polarization resistance diminishes. The passive film formed at a constant potential consists of an outer layer primarily containing metal hydroxides and metal oxyanions, and an inner layer mainly composed of metallic oxides, exhibiting semiconductive properties;
(2)
The surface of the stainless steel develops a passive film comprising two layers with p-type and n-type semiconductive properties regardless of the solution’s pH. When the concentration of hydrogen ions or hydroxyl ions in the environment fluctuates, the semiconductive tendencies of the passive film change, leading to either a weakening or strengthening of the passive film’s properties. Notably, the corrosion resistance improves as the p-type and n-type semiconductive tendencies become more balanced, enhancing the properties of the passive film.

Author Contributions

Conceptualization, S.-H.C.; methodology, Y.-R.Y. and S.-H.C.; investigation, S.-H.C.; data curation and analysis, Y.-R.Y.; writing—original draft preparation, Y.-R.Y. and S.-H.C.; writing—review and editing, Y.-R.Y. and Y.-S.K.; supervision, Y.-S.K. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by “Research Fund pf Andong National University” and “Grant number 2023–2024”.

Data Availability Statement

Data are contained within the article.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Effect of pH on the polarization behavior of SR-50A in a deaerated 1 N NaCl + x N HCl at 30 °C; (a) polarization curves; (b) passive current density at +400 mV (SCE).
Figure 1. Effect of pH on the polarization behavior of SR-50A in a deaerated 1 N NaCl + x N HCl at 30 °C; (a) polarization curves; (b) passive current density at +400 mV (SCE).
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Figure 2. Effect of pH on the polarization behavior of SR-50A in deaerated 1 N NaCl + x N NaOH at 30 °C; (a) polarization curves, (b) passive current density at +400 mV (SCE).
Figure 2. Effect of pH on the polarization behavior of SR-50A in deaerated 1 N NaCl + x N NaOH at 30 °C; (a) polarization curves, (b) passive current density at +400 mV (SCE).
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Figure 3. EIS results on the passive film of super austenitic stainless steel (SR-50A) formed at +400 mV (SCE) in 1 N NaCl + x N HCl at 30 °C; (a) Nyquist plot, (b) Bode plot, (c) polarization resistance obtained by Randle’s model.
Figure 3. EIS results on the passive film of super austenitic stainless steel (SR-50A) formed at +400 mV (SCE) in 1 N NaCl + x N HCl at 30 °C; (a) Nyquist plot, (b) Bode plot, (c) polarization resistance obtained by Randle’s model.
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Figure 4. EIS results on passive film of super austenitic stainless steel (SR-50A) formed at +400 mV (SCE) in 1 N NaCl + x N NaOH at 30 °C; (a) Nyquist plot, (b) Bode plot, (c) polarization resistance obtained by Randle’s model.
Figure 4. EIS results on passive film of super austenitic stainless steel (SR-50A) formed at +400 mV (SCE) in 1 N NaCl + x N NaOH at 30 °C; (a) Nyquist plot, (b) Bode plot, (c) polarization resistance obtained by Randle’s model.
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Figure 5. Effect of pH on the Mott–Schottky behavior of passive film on super austenitic stainless steel (SR-50A) formed at +400 mV (SCE) in deaerated solution at 30 °C; (a) 1 N NaCl + x N HCl, (b) 1 N NaCl + x N NaOH.
Figure 5. Effect of pH on the Mott–Schottky behavior of passive film on super austenitic stainless steel (SR-50A) formed at +400 mV (SCE) in deaerated solution at 30 °C; (a) 1 N NaCl + x N HCl, (b) 1 N NaCl + x N NaOH.
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Figure 6. Depth profile of (a) Fe 2p3/2, (b) Cr 2p3/2, (c) Mo (3d5/2 + 3d3/2), (d) O 1s, (e) Ni 2p3/2, (f) Cl− 2p on the passive film of super stainless steel (SR-50A) formed at +400 mV (SCE) in acidic or alkaline solutions.
Figure 6. Depth profile of (a) Fe 2p3/2, (b) Cr 2p3/2, (c) Mo (3d5/2 + 3d3/2), (d) O 1s, (e) Ni 2p3/2, (f) Cl− 2p on the passive film of super stainless steel (SR-50A) formed at +400 mV (SCE) in acidic or alkaline solutions.
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Figure 7. Chemical states of Fe 2p3/2 and depth profiles in the passive film on super stainless steel (SR-50A) formed at +400 mV (SCE) in (a) 1 N NaCl + 0.5 N HCl and (b) 1 N NaCl + 0.05 N NaOH.
Figure 7. Chemical states of Fe 2p3/2 and depth profiles in the passive film on super stainless steel (SR-50A) formed at +400 mV (SCE) in (a) 1 N NaCl + 0.5 N HCl and (b) 1 N NaCl + 0.05 N NaOH.
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Figure 8. Chemical states of Cr 2p3/2 and depth profiles in the passive film on super stainless steel (SR-50A) formed at +400 mV (SCE) in (a) 1 N NaCl + 0.5 N HCl and (b) 1 N NaCl + 0.05 N NaOH.
Figure 8. Chemical states of Cr 2p3/2 and depth profiles in the passive film on super stainless steel (SR-50A) formed at +400 mV (SCE) in (a) 1 N NaCl + 0.5 N HCl and (b) 1 N NaCl + 0.05 N NaOH.
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Figure 9. Chemical states of Mo (3d5/2 + 3d3/2) and depth profiles in the passive film on super stainless steel (SR-50A) formed at +400 mV (SCE) in (a,b) 1 N NaCl + 0.5 N HCl (c,d) 1 N NaCl + 0.05 N NaOH.
Figure 9. Chemical states of Mo (3d5/2 + 3d3/2) and depth profiles in the passive film on super stainless steel (SR-50A) formed at +400 mV (SCE) in (a,b) 1 N NaCl + 0.5 N HCl (c,d) 1 N NaCl + 0.05 N NaOH.
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Figure 10. Chemical states of O 1s and depth profiles in the passive film on super stainless steel (SR-50A) formed at +400 mV (SCE) in (a) 1 N NaCl + 0.5 N HCl and (b) 1 N NaCl + 0.05 N NaOH.
Figure 10. Chemical states of O 1s and depth profiles in the passive film on super stainless steel (SR-50A) formed at +400 mV (SCE) in (a) 1 N NaCl + 0.5 N HCl and (b) 1 N NaCl + 0.05 N NaOH.
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Figure 11. Chemical states of Ni 2p3/2 and depth profiles in the passive film on super stainless steel (SR-50A) formed at +400 mV (SCE) in (a) 1 N NaCl + 0.5 N HCl and (b) 1 N NaCl + 0.05 N NaOH.
Figure 11. Chemical states of Ni 2p3/2 and depth profiles in the passive film on super stainless steel (SR-50A) formed at +400 mV (SCE) in (a) 1 N NaCl + 0.5 N HCl and (b) 1 N NaCl + 0.05 N NaOH.
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Figure 12. Total defect density of the passive film formed at +0.4 V (SCE) in acidic and alkaline solutions on super stainless steel (SR-50A); (a) [H+] concentration vs. total defect density, (b) [OH−] concentration vs. total defect density.
Figure 12. Total defect density of the passive film formed at +0.4 V (SCE) in acidic and alkaline solutions on super stainless steel (SR-50A); (a) [H+] concentration vs. total defect density, (b) [OH−] concentration vs. total defect density.
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Figure 13. Effect of total defect density on passive current density and anodic resistance in acidic and alkaline chloride solutions; (a) total defect density vs. passive current density at +400 mV (SCE) in acidic solutions, (b) total defect density vs. passive current density at +400 mV (SCE) in alkaline solutions, (c) total defect density vs. polarization resistance in acidic solutions, (d) total defect density vs. polarization resistance in alkaline solutions.
Figure 13. Effect of total defect density on passive current density and anodic resistance in acidic and alkaline chloride solutions; (a) total defect density vs. passive current density at +400 mV (SCE) in acidic solutions, (b) total defect density vs. passive current density at +400 mV (SCE) in alkaline solutions, (c) total defect density vs. polarization resistance in acidic solutions, (d) total defect density vs. polarization resistance in alkaline solutions.
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Figure 14. Comparison of p-type and n-type slopes (C−/V) demonstrating semiconductor properties in the Mott–Schottky plot under acidic and alkaline chloride solutions: (a) acidic, (b) alkaline.
Figure 14. Comparison of p-type and n-type slopes (C−/V) demonstrating semiconductor properties in the Mott–Schottky plot under acidic and alkaline chloride solutions: (a) acidic, (b) alkaline.
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Figure 15. Effect of pH on the proportion of semiconductive properties of passive films formed on super austenitic stainless steel (SR-50A) in (a) acidic and (b) alkaline chloride solutions.
Figure 15. Effect of pH on the proportion of semiconductive properties of passive films formed on super austenitic stainless steel (SR-50A) in (a) acidic and (b) alkaline chloride solutions.
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Figure 16. Relationship between difference in semiconductive tendencies and passive current density or polarization resistance; (a) acidic chloride solution and (b) alkaline chloride solution.
Figure 16. Relationship between difference in semiconductive tendencies and passive current density or polarization resistance; (a) acidic chloride solution and (b) alkaline chloride solution.
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Figure 17. Newly proposed bipolar passivation model formed on stainless steel in acidic or alkaline chloride solutions.
Figure 17. Newly proposed bipolar passivation model formed on stainless steel in acidic or alkaline chloride solutions.
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Table 1. Chemical composition of SR-50A stainless steel.
Table 1. Chemical composition of SR-50A stainless steel.
MaterialChemical Compositions, wt. %* PREN30
UNS S32050
(SR-50A)		CrNiMoMnSiCuNCPFe52.1
23.5222.016.240.280.090.080.320.0160.023bal.
* PREN30 (Pitting Resistance Equivalent Number) = %Cr + 3.3(%Mo + 0.5%W) + 30%N.
Table 2. Polarization factors obtained in deaerated acidic chloride solutions at 30 °C.
Table 2. Polarization factors obtained in deaerated acidic chloride solutions at 30 °C.
Hydrogen Ion
Concentration, mol/L		ER, V (SCE)iR, A/cm2iC, A/cm2iP at +400 mV (SCE), A/cm2
0.0105 (pH −0.38)−0.263.548 × 10−71.349 × 10−61.175 × 10−6
0.7413 (pH −0.2)−0.275.248 × 10−73.388 × 10−61.288 × 10−6
1.5849 (pH 0.13)−0.326.457 × 10−61.288 × 10−41.413 × 10−6
2.3988 (pH 1.98)−0.294.266 × 10−62.630 × 10−42.692 × 10−6
Table 3. Polarization factors obtained in deaerated alkaline chloride solutions at 30 °C.
Table 3. Polarization factors obtained in deaerated alkaline chloride solutions at 30 °C.
Hydroxyl Ion
Concentration, mol/L		ER, V (SCE)iR, A/cm2Etr, V (SCE)iP at +400 mV (SCE), A/cm2
1.95 × 10−4 (pH 10.29)−0.0698.318 × 10−90.743.548 × 10−6
1.318 × 10−3 (pH 11.12)−0.146.166 × 10−80.645.129 × 10−6
3.388 × 10−3 (pH 11.53)−0.481.318 × 10−70.541.549 × 10−5
Table 4. Flat band potential of passive films on super austenitic stainless steel (SR-50A) formed at +400 mV (SCE) in acidic and alkaline solutions.
Table 4. Flat band potential of passive films on super austenitic stainless steel (SR-50A) formed at +400 mV (SCE) in acidic and alkaline solutions.
Hydrogen/Hydroxyl Ion Concentration, mol/L2.39881.58490.74131.05 × 10−21.9 × 10−41.32 × 10−33.39 × 10−3
Efb by N slope, V (SCE)0.0450.0460.047−0.1−0.55−0.63−0.65
Efb by P slope, V (SCE)−0.05−0.140.03−0.51−0.97−0.99−0.1
Table 5. Donor density, acceptor density and total defect density of passive film on super austenitic stainless steel (SR-50A) formed in acidic and alkaline solutions.
Table 5. Donor density, acceptor density and total defect density of passive film on super austenitic stainless steel (SR-50A) formed in acidic and alkaline solutions.
Hydrogen Ion Concentration, mol/LHydroxyl Ion Concentration, mol/L
2.39881.58490.74130.01051.95 × 10−41.318 × 10−33.388 × 10−3
ND (cm−3)2.88 × 10281.71 × 10281.26 × 10281.05 × 10280.076 × 10280.061 × 10280.084 × 1028
NA (cm−3)8.97 × 10284.01 × 10281.75 × 10281.75 × 10280.049 × 10280.038 × 10280.048 × 1028
Total defect density (cm−3)11.8 × 10285.72 × 10283.87 × 10+2.80 × 10280.12 × 10280.099 × 10280.13 × 1028
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Choi, S.-H.; Yoo, Y.-R.; Kim, Y.-S. Semiconductive Tendency of the Passive Film Formed on Super Austenitic Stainless Steel SR-50A in Acidic or Alkaline Chloride Solutions. Crystals 2024, 14, 766. https://doi.org/10.3390/cryst14090766

AMA Style

Choi S-H, Yoo Y-R, Kim Y-S. Semiconductive Tendency of the Passive Film Formed on Super Austenitic Stainless Steel SR-50A in Acidic or Alkaline Chloride Solutions. Crystals. 2024; 14(9):766. https://doi.org/10.3390/cryst14090766

Chicago/Turabian Style

Choi, Seung-Heon, Young-Ran Yoo, and Young-Sik Kim. 2024. "Semiconductive Tendency of the Passive Film Formed on Super Austenitic Stainless Steel SR-50A in Acidic or Alkaline Chloride Solutions" Crystals 14, no. 9: 766. https://doi.org/10.3390/cryst14090766

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