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Article

Effect of Cr, Mo, and W Contents on the Semiconductive Properties of Passive Film of Ferritic Stainless Steels

by
Seung-Heon Choi
1,
Young-Ran Yoo
2,*,
Young-Cheon Kim
1,3,* and
Young-Sik Kim
1,3
1
Department of Materials Science and Engineering, Gyeongkuk National University, 1375 Gyeongdong-ro, Andong 36729, Republic of Korea
2
Department of Semiconductor Facilities, Gumi Campus of Korea Polytechnics, 84, Suchul-daero 3-gil, Gumi 39257, Republic of Korea
3
Materials Research Centre for Energy and Clean Technology, Gyeongkuk National University, 1375 Gyeongdong-ro, Andong 36729, Republic of Korea
*
Authors to whom correspondence should be addressed.
Crystals 2025, 15(8), 723; https://doi.org/10.3390/cryst15080723
Submission received: 4 July 2025 / Revised: 7 August 2025 / Accepted: 12 August 2025 / Published: 14 August 2025
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Abstract

This study investigated the influence of Cr, Mo, and W alloying elements incorporated into ferritic stainless steel on the characteristics of passive films formed under acidic chloride conditions. Electrochemical assessments demonstrated that increasing the amounts of Cr, Mo, and W reduces passive current density and enhances polarization resistance. Through XPS analysis, it was determined that the passive film exhibits a double-layer structure, consisting of an inner layer rich in metal oxides and an outer layer containing metal oxy-anions. Mott–Schottky analysis indicated the presence of both p-type and n-type semiconducting properties. To clarify the effect of these alloying elements on the passive films at the surface of stainless steel, this work introduces a new parameter termed the “Bipolar Index,” defined as |p-type slope| + |n-type slope|. With higher Cr, Mo, and W contents, the bipolar index increases, reflecting modifications in the semiconductive behavior. Consequently, the point defect concentration within the passive film decreases, causing a reduction in passive current density and a rise in polarization resistance.

1. Introduction

Stainless steel is an alloy mainly composed of iron and at least 13 wt. % chromium, and it offers outstanding corrosion resistance attributed to a thin, stable oxide film that forms spontaneously in air or oxidizing environments [1,2,3,4]. As a consequence, stainless steel finds extensive application in industries such as chemical processing, power generation, water treatment, and marine engineering, and is highly regarded as a construction material in environments with high corrosion risk. The superior corrosion resistance arises from a passive film, a few nanometers thick [5,6], which serves as a barrier between the metal substrate and the corrosive medium, thereby inhibiting metal ion dissolution and the ingress of aggressive ions (e.g., Cl−).
The passive film refers to a phenomenon in which a metal remains stable within a specific potential range, accompanied by a sharply reduced corrosion rate. This behavior has been addressed by multiple theories developed since the early 20th century. Notably, the “oxide film theory” posits that a stable oxide layer forms on the metal surface, serving as a protective barrier for the metal [7]. In contrast, the “adsorption theory” suggests that certain ions or molecules selectively adhere to the metal surface, thereby imparting corrosion resistance [8]. Building upon these passivation theories, various models of passive film formation mechanisms have been proposed, including the “Electronic Configuration-Induced Adsorption Passivity” theory and the “Ionic Space Charge-Induced Passivation” theory [9,10,11].
Based on this formation mechanism, XPS characterization has been employed to investigate the thickness and composition of passive films. Passive films on stainless steel are generally considered to be composed of double layers. The inner layer typically consists of a Cr-enriched dense oxide film (such as Cr2O3 or Cr(OH)3, etc.), which adheres directly to the metal substrate, while the outer layer is composed of Fe-based oxides (such as Fe2O3 and FeOOH, etc.) [12,13,14,15,16].
Several investigations, utilizing this double-layer model, have reported the presence of an electric field within the passive film. Consequently, analyses involving photocurrent measurements and capacitance-based Mott–Schottky methods have indicated that passive films exhibit semiconducting properties. From these experimental observations, two prominent theoretical models have been developed to explain the characteristics of passive films on stainless steel. The first is the Point Defect Model (PDM) introduced by Hoar, which describes the growth and breakdown behavior of passive films through the dynamics of oxygen and metal vacancies and is chiefly applied to interpret corrosion resistance [17,18]. The second model is the bipolar fixed charge-induced passivity model (Bipolar Model), which examines how alloying elements such as Cr and Mo influence the selectivity for cations or anions within the film [19,20,21].
Based on this theory, Mott–Schottky analysis has been used to study the semiconducting properties of each layer in a passive film. Notably, a number of studies have demonstrated that the semiconducting type—whether p-type, n-type, or a mixed p–n-type—can vary according to the applied potential [20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42], underscoring the complex nature of charge transport mechanisms within the passive film rather than a simple model.
Cr, Mo, and W are notable alloying elements that exert a significant impact on the electrochemical properties and corrosion resistance of passive films. Cr is present as Cr2O3 and CrO42− in the passive film, influencing both the electronic structure and the ion barrier characteristics of the film [43,44,45,46]. Although Mo does not independently form a passive film, it aids in stabilizing the passive film when incorporated into alloys [47,48,49]; MoO42− ions function as electron acceptors, enhancing metal–oxygen bonding and creating a cation-selective layer within the film, which results in improved corrosion resistance [19,20,45,50,51,52,53]. Furthermore, Mo has been found to participate in the formation of intermetallic compounds with Cr oxides or to support the stabilization of Cr2O3 in the passive film [50,53]. W, analogous to Mo, occurs as WO3 or WO42− and contributes to the stability and corrosion resistance of the passive film by reinforcing the oxide layer and restricting metal ion diffusion [54,55,56]. Specifically, WO42− ions decrease the positive charge density within the passive film, thereby suppressing the migration of ions like Fe3+ and Cr3+ and blocking chloride ion (Cl−) ingress [44,51]. Collectively, Cr, Mo, and W are considered to be integral in promoting stainless steel corrosion resistance by modulating the electrochemical behavior of the passive film.
It is well established that incorporating alloying elements enhances the corrosion resistance of stainless steel, a phenomenon primarily attributed to the influence of specific elements or chemical species within the passive film. In practical terms, Cr predominantly impacts p-type characteristics [28,57,58], Mo is known to affect n-type behavior [59,60,61,62], and W influences both p-type and n-type properties [36,63]. Nevertheless, previous research has often described these semiconductor characteristics only in the context of defect concentration variation, which restricts a deeper understanding of how the full electronic structure of the passive film relates to corrosion resistance. Some investigations into semiconductor properties have employed different environmental parameters, such as potential, temperature, and pH, as experimental variables.
This study evaluates the effects of Cr, Mo, and W alloying elements on the formation and semiconductive properties of passive films on ferritic stainless steel in an acidic chloride environment. Considering the double-layered structure of the passive film, XPS and Mott–Schottky analyses were used to distinguish the electrical properties of each layer and to analyze the relationship between the semiconductive characteristics of the film and its corrosion properties.

2. Materials and Methods

2.1. Materials

The alloys used in this study were ferritic stainless steels with varying compositions of Cr, Mo, and W. Specimen preparation involved the use of high-purity commercial-grade electrolytic iron (99.9%), Cr (99.9%), Ni (99.9%), Mo (99.9%), W (99.8%), Si (99.2%), and Mn (99.8%), and the alloys were produced as follows. A vacuum induction melting furnace (medium-low frequency type) was preheated to 400 °C for 15 min at 40 kW, after which electrolytic iron, Cr, Ni, Mo, and W were charged. Following complete melting, Mn and Si were introduced, and upon dissolution of all alloying additions, Al, Ca–Si, and Ti were wrapped in Al foil and immersed deeply into the molten metal for desulfurization. Once all elements had been fully dissolved, the power was decreased and the melt was allowed to settle for 2–3 min to facilitate slag removal. The melt was then cast into a ceramic mold (150 mm × 150 mm × 300 mm) at 1630–1650 °C to solidify individual ingots. The ingots were subsequently hot forged to a thickness of 30 mm, after which they were cut to appropriate sizes for different tests. The resulting alloys underwent an annealing heat treatment at 1075 °C for 10 min to eliminate impurities and relieve residual stresses introduced during casting and forging. An aliquot of the alloys was sectioned for compositional analysis using an ICP-MS (Inductively Coupled Plasma mass spectrometer, Agilent-7800, Agilent, Santa Clara, CA, USA). Analytical results are summarized in Table 1.

2.2. Anodic Polarization Test

To evaluate passive layer behavior, an anodic polarization test was conducted. The specimens were cut into 1.5 cm × 1.5 cm pieces, and a copper-coated wire was spot welded to one face for electrical connectivity. The specimens were then fixed using epoxy resin. Surfaces exposed for testing were finished using SiC paper, progressing from #80 to #2000 grit, with the exposed region fixed at precisely 1 cm2 through epoxy sealing of all other areas. Until the experiment, all specimens were kept in a desiccator to avoid contact with moisture and air. The anodic polarization measurements were conducted using a potentiostat (Interface 1000, Gamry Instruments, Warminster, PA, USA) with a saturated calomel electrode (SCE) as the reference and a high-density graphite rod as the counter electrode. The electrolyte (1 N NaCl + 1 N HCl) was kept at 30 °C and thoroughly deaerated by introducing N2 gas at 100 mL/min for 30 min prior to testing, and a linear potential scan rate of 0.33 mV/s was applied [64].

2.3. Potentiostatic EIS Test

To assess the resistance properties of the passive film, AC impedance measurements were carried out. The specimens were prepared using the same procedure as for the anodic polarization test, and the experimental solution (1 N NaCl + 1 N HCl) was kept at 30 °C, with deaeration achieved by introducing N2 gas at a flow rate of 100 mL/min for 30 min. After immersing the specimens in the solution for 20 min, a potentiostat (Interface 1000, Gamry Instruments, Warminster, PA, USA) was used to apply a potential of +500 mV (SCE) along the anodic polarization curve for 30 min, allowing a passive film to form. Following passive film formation, impedance was measured while maintaining the same potential, setting a frequency range from 10 kHz to 0.1 Hz. The polarization resistance (Rp) of the passive film was determined using the Randles model to extract the Rp value [65].

2.4. XPS Analysis

Specimens for passive surface analysis were ground with SiC paper from #80 to #2000 grit and subsequently mirror polished using 3 μm diamond paste. After ultrasonic cleaning with alcohol and drying, the samples were immersed for 20 min in a deaerated solution purged by N2 gas at 100 mL/min for 30 min. Subsequently, passive films were generated by applying an anodic potential of +500 mV (SCE) for 1 h with a potentiostat, based on the passive potential identified from anodic polarization tests. To avoid surface contamination, the specimens were then kept in a nitrogen atmosphere. Surface composition and chemical state analyses were conducted using X-ray Photoelectron Spectroscopy (XPS) with a K-alpha instrument (Thermo UK, Altrincham, UK). The X-ray source utilized was Al Kα radiation (1486.6 eV, 12 kV, 3 mA). Depth profiling was performed using Ar ion sputtering with a voltage of 1 kV and a current of 2 μA. The sputtering energy was adjusted to approximately 500 eV, and an etching rate of 0.05 nm/s based on SiO2 standard was applied to analyze the depth of the passive film. Pre-sputtering under these conditions was also performed to eliminate organic contaminants from the surface before analysis. Acquired spectra were analyzed with Avantage software (Version 6.8.1.4, Thermo Fisher Scientific, Waltham, MA, USA). The C 1s peak at 284.6 eV observed in all specimens was used as an internal reference for binding energy calibration. Oxidation states and distribution of chemical species for each element were determined by deconvoluting each elemental peak.

2.5. Mott–Schottky Analysis

Mott–Schottky analysis was employed to assess the semiconductive characteristics of the passive film. The test solution was kept at 30 °C and deaerated by purging N2 gas at a flow rate of 100 mL/min for 30 min. Following deaeration, the specimens were submerged in the deaerated solution for 20 min, then subjected to passive film formation at a potential of +500 mV (SCE) for 30 min using a potentiostat (Interface 1000, Gamry Instruments, Warminster, PA, USA), based on the anodic polarization curve. After passive film formation, Mott–Schottky analysis was performed to determine the capacitance of the passive film. The AC amplitude was maintained at 10 mV (peak to peak), and the frequency was held constant at 1580 Hz. The applied potential was scanned from +1.0 V (SCE) to −1.0 V (SCE) at a rate of 50 mV/s.

3. Results

3.1. Effect of Cr, Mo, and W Contents on the Electrochemical Properties

Figure 1 presents the electrochemical polarization behavior (a–c) along with the passive current density (a′–c′) at +500 mV (SCE) of ferritic stainless steel specimens containing varying Cr, Mo, and W contents in a deaerated 1 N NaCl + 1 N HCl solution at 30 °C. All electrochemical parameters (corrosion potential, corrosion current density, critical passive current density, passive current density) are presented as the average values of repeated measurements and rounded to an appropriate number of significant digits according to the experimental precision. Figure 1a,a′ demonstrate changes in electrochemical behavior according to Cr content (F-23Cr, F-26Cr, F-29Cr). In the anodic polarization profiles of Figure 1a, the corrosion potential (ER) ranged from −0.44 to −0.45 V (SCE), showing minimal variation among specimens, whereas the corrosion current density (iR) remained consistent, ranging from 85.5 to 98.9 μA/cm2. Notably, the critical passive current density (iC) decreased with increasing Cr content, displaying a trend of 8.3 × 103, 7.4 × 103, and 6.3 × 103 μA/cm2 μA/cm2, respectively. Figure 1a′ displays the passive current density (ip) measured at +500 mV (SCE), revealing that ip decreases markedly as Cr content rises, with values of 10.8, 2.0, and 1.7 μA/cm2. These results confirm that Cr functions as a key alloying element that concurrently lowers both the critical passive current density (iC) and passive current density (ip), thereby improving the electrochemical stability of the passive film.
Figure 1b,b′ present the changes in electrochemical behavior as the Mo content varies (F-0Mo, F-2Mo, F-3.5Mo). All electrochemical parameters (corrosion potential, corrosion current density, critical passive current density, passive current density) are presented as the average values of repeated measurements and rounded to an appropriate number of significant digits according to the experimental precision. In Figure 1b, the corrosion potential (ER) shifts gradually in a positive direction between −0.48 and −0.45 V (SCE) as the Mo content increases. The corrosion current density (iR) markedly declines with higher Mo content, with corresponding values of 1.0 × 103, 5.6 × 102, and 2.4 × 102 μA/cm2. Furthermore, the critical passive current density (iC) substantially decreases to levels of 6.5 × 104, 1.6 × 104, and 6.3 × 103 μA/cm2. In Figure 1b′, the passive current density (ip) is highest for F-0Mo at approximately 1.0 × 103 μA/cm2, then drops significantly to 1.7 μA/cm2 for both F-2Mo and F-3.5Mo. These findings indicate that Mo serves as a crucial alloying element that not only lowers the critical passive current density (iC) and passive current density (ip) but also enhances the electrochemical stability of passive films.
Figure 1c,c′ depict how passive film properties change with different W content (F-0W, F-1.5W, F-3W). All electrochemical parameters (corrosion potential, corrosion current density, critical passive current density, passive current density) are presented as the average values of repeated measurements and rounded to an appropriate number of significant digits according to the experimental precision. In Figure 1c, the corrosion potential (ER) remains nearly constant around −0.45 V (SCE), but the corrosion current density (iR) shows a slight increase with greater W content, with values of 91.6, 99.1, and 118.9 μA/cm2. The critical passive current density (iC) decreases to 7.9 × 103, 6.3 × 103, and 5.0 × 103 μA/cm2 as W content rises. In Figure 1c′, the passive current density (ip) is highest at 2.3 μA/cm2 for F-0W, then drops to 1.7 and 1.6 μA/cm2 for F-1.5W and F-3W, respectively. Thus, W not only reduces the critical passive current density (iC) and passive current density (ip), but also serves as an effective alloying element to enhance the electrochemical stability of passive films.
Figure 2 presents the AC impedance analysis of passive films formed at +500 mV (SCE) on ferritic stainless steel samples with varying Cr, Mo, and W contents in a deaerated 1 N NaCl + 1 N HCl solution at 30 °C. This figure comprises (a–c) Nyquist plots, (a′–c′) Bode plots, and (a″–c″) polarization resistance (Rp) data.
Figure 2a displays the Nyquist plot as a function of Cr content, where the diameter of the semicircular curve gradually increases with higher Cr content. This indicates an increase in interfacial resistance, suggesting the formation of a more stable passive film. In the Bode plot shown in Figure 2a′, the impedance magnitude (|Z|) rises progressively over the entire frequency range with increasing Cr content, which corroborates the Nyquist plot findings. Figure 2a″ illustrates the change in polarization resistance (Rp) with Cr content. The Rp values increase from approximately 186 Ω·cm2 at 23 wt. % Cr to 226 Ω·cm2 at 26 wt. % and 261 Ω·cm2 at 29 wt. %.
Figure 2b presents the Nyquist plot results as a function of Mo content, clearly demonstrating that the semicircle diameter expands as Mo content increases. Notably, the F-0Mo specimen exhibits very low film resistance and an almost absent semicircular profile, implying that the passive film is inadequately formed. The Bode plot in Figure 2b′ also demonstrates a general rise in impedance magnitude with increasing Mo content. In Figure 2b″, the Rp value of F-0Mo is nearly zero, while it increases to about 161 Ω·cm2 and 261 Ω·cm2 for the F-2Mo and F-3.5Mo specimens, respectively.
Figure 2c provides the Nyquist plot for varying W content, revealing that the semicircle diameter increases with higher W content. The Bode plot in Figure 2c′ shows a consistent increase in impedance magnitude across the entire frequency range. Figure 2c″ demonstrates that the Rp values for the F-0W, F-1.5W, and F-3W specimens are 159 Ω·cm2, 261 Ω·cm2, and 299 Ω·cm2, respectively, confirming that polarization resistance is enhanced with increased W content.
Hence, the rise in Cr, Mo, and W contents results in larger semicircle diameters in the Nyquist plots, higher impedance magnitudes in the Bode plots, and enhanced polarization resistance (Rp). These findings confirm that all three elements play a critical role in the development and stability of the passive film.

3.2. Effect of Cr, Mo, and W Contents on the XPS Analysis

Figure 3 presents the typical XPS peak analysis results for the chemical states of chromium, molybdenum, and tungsten. Specifically, Figure 3a shows the spectrum of chromium, Figure 3b corresponds to molybdenum, and Figure 3c illustrates tungsten. These spectra were interpreted based on the binding energy references listed in Table 2.
Figure 4 presents the XPS depth profile analysis of passive films developed at +500 mV (SCE) for 1 h on ferritic stainless steel specimens with varying Cr contents (F-23Cr, F-29Cr). The analysis included Fe 2p3/2, Cr 2p3/2, Mo (3d5/2 + 3d3/2), W (4f7/2 + 4f5/2), and O 1s species. The O concentration decreased from the exterior toward the interior of the passive film, while Fe, Cr, Mo, and W contents exhibited an increasing trend from the outer to inner layer. In particular, the concentration of Cr in the passive film increased with higher Cr content.
Figure 5 demonstrates how Cr content influences the distribution of Cr chemical species within the passive film. Figure 5a–e display the depth profiles of Cr-Metal, Cr2O3, CrO3, Cr(OH)3, and CrO42−, respectively. The Cr-Metal concentration was elevated in the inner film layer, particularly in the F-29Cr specimen. In contrast, Cr2O3, CrO3, and Cr(OH)3 were found to accumulate in the outer layer, with overall concentrations declining as Cr content increased. CrO42− was detected primarily in the outer layer, and its enrichment in this region became more pronounced with higher Cr content.
Figure 6 provides a summary of quantitative analyses regarding the distribution of oxides and metal oxyanions in passive films relative to Cr content. Figure 6a presents the overall oxide (Fe, Cr, Mo, W) proportions, where the F-29Cr specimens exhibited a consistent decline in oxide ratios with increasing passive film depth, whereas the F-23Cr samples displayed a less uniform distribution. Figure 6b depicts the variation in the Cr2O3/Cr(OH)3 ratio, showing an upward trend towards the inner layer for F-29Cr specimens. This result suggests that Cr oxide predominantly resides in the inner film layer, corroborating findings from related studies [12,13,14,15,16]. Figure 6c shows the distribution of metal oxyanions (e.g., CrO42−), highlighting greater concentration in the outer layers at higher Cr content. These observations align with the bipolar model, which proposes that a cation-selective outer layer promotes improved corrosion resistance [38,39,40,41,48,51].
Figure 7 presents the XPS depth profile analysis results for F-0Mo and F-2Mo specimens, which were passivated at +500 mV (SCE) for 1 h in deaerated 1 N NaCl + 1 N HCl solution at 30 °C. The analysis targeted five elements: Fe 2p3/2, Cr 2p3/2, Mo (3d5/2 + 3d3/2), W (4f7/2 + 4f5/2), and O 1s. The O concentration declined with increasing depth, whereas Fe, Cr, Mo, and W concentrations increased toward the interior of the passive film. Notably, F-2Mo exhibited a pronounced rise in Mo content from the outer surface to the inner region of the passive film.
Figure 8 illustrates the effect of Mo content on the distribution of Mo chemical species within the passive film. In the Mo-containing F-2Mo specimen, MoO3, MoO(OH)2, and MoO42− were predominantly located in the outer layer, and their concentrations declined with increased etch time. In contrast, these Mo species were absent in the Mo-free F-0Mo specimen. Since Mo was not intentionally added to the F-0Mo alloy and no Mo signal was detected in the XPS spectra, the Mo content was recorded as zero for comparison purposes.
Figure 9 provides a quantitative assessment of the variation in total oxide content, Cr oxide fraction, and metal oxyanion levels with respect to Mo content. Figure 9a compares the fraction of total oxides—which include Cr, Fe, Mo, and W oxides—and demonstrates that oxide concentrations are markedly higher in the outer region, regardless of Mo presence. The CrO3/Cr(OH)3 ratios displayed in Figure 9b exhibit a similar overall distribution regardless of Mo, but the F-2Mo specimens show a relatively greater ratio in the outer layer than F-0Mo. Figure 9c represents the distribution of metal oxyanions (such as MoO42−), indicating that the F-2Mo films feature slightly elevated concentrations in the outer layers. This result aligns with earlier studies; according to the bipolar model, a cation-selective layer forms in the outer region of the passive film, thereby enhancing corrosion resistance [19,20,66,67,68].
Figure 10 displays the XPS depth profile analysis of passive films formed on F-0W, F-1.5W, and F-3W specimens through passivation at +500 mV (SCE) for 1 h in a deaerated 1 N NaCl + 1 N HCl solution at 30 °C. The analysis focused on five elements: Fe 2p3/2, Cr 2p3/2, Mo (3d5/2 + 3d3/2), W (4f7/2 + 4f5/2), and O 1s. For all specimens, the O 1s decreased from the outer to the inner layers of the passive film, while Fe, Cr, Mo, and W concentrations increased from the outer to the inner regions. In the W-alloyed specimens, W concentration showed an increase towards the inner layer of the passive film, whereas W was not detected in the F-0W specimen.
Figure 11 illustrates the distribution of chemical states of W within the passive films of F-0W, F-1.5W, and F-3W specimens. Figure 11a–d depict the depth profiles of W-metal, WO2, WO3, and WO42−, respectively. The content of W-metal demonstrates an increasing trend in the depth direction as the W content increases. Oxidized species (WO2, WO3) and oxyanions (WO42−) are primarily located in the outer layer of the film and decrease gradually as etch time progresses. Since W was not intentionally added to the F-0W alloy and no W signal was detected in the XPS spectra, its content was marked as zero for comparison purposes.
Figure 12 presents the percentages of total oxide (Figure 12a), Cr oxide (Figure 12b), and metal oxyanion (Figure 12c) as a function of W content. The total oxide content exhibits minimal variation among the specimens. The proportion of Cr oxide increases slightly with higher W content, displaying a general upward trend toward the inner layers of the passive film. Metal oxyanions are more concentrated in the outer layers, particularly in the F-3W specimen, and decrease toward the interior. These findings support the bipolar model and align with previous reports indicating that cation-selective regions developing in the outer layer can enhance corrosion resistance [19,52,53,54,68,69].

4. Discussion

The alloying elements Cr, Mo, and W used in this study were found to have a significant impact on the corrosion resistance of the passive film due to changes in the oxide and metal oxygen-anion composition within the passive film. In particular, a double-layer structure with metal oxyanions enriched in the outer layer of the passive film and metal oxides in the inner layer was commonly identified. These structural properties are formed differently for each alloying element and are closely related to the semiconductor behavior of passive films. Therefore, Mott–Schottky tests were performed to analyze the semiconductor properties of passive films with different compositions.
Figure 13 shows the Mott–Schottky analysis results of passive films formed on ferritic stainless steel specimens with different Cr, Mo, and W contents by applying +500 mV (SCE) for 1 h in deaerated 1 N NaCl + 1 N HCl solution at 30 °C. Figure 13a shows the results for specimens with different Cr content, Figure 13b for Mo content, and Figure 13c for W content. For all specimens, a negative slope was observed between −0.6 V and −0.1 V (SCE) and a positive slope between +0.1 V and +0.6 V (SCE), indicating that passive films formed in acidic environments commonly exhibit p-type and n-type semiconductor behavior. This trend is consistent with previous studies [70,71]. F-0Mo was excluded from this analysis due to the absence of a stable passive film, as indicated by its near-zero polarization resistance (Rp).
Table 3 summarizes the results obtained from the Mott–Schottky analysis in Figure 13, giving the flat band potential (Efb), donor density (ND), and acceptor density (NA) of the passive film for each specimen. The Efb in the p-type semiconductor section shows similar values for all specimens, suggesting that the alloying element addition does not have a significant effect on the Efb. On the other hand, Efb in the n-type semiconductor segment tends to decrease gradually with increasing Cr, Mo, and W content. The donor density (ND) showed an overall decreasing trend with increasing Cr, Mo, and W contents. This is in line with previous studies reporting that Mo and W contribute to n-type semiconductor behavior [36,59,63]. Although Cr has been mainly associated with p-type behavior, the observed decrease in ND with increasing Cr content in this study suggests that Cr may also partially influence the n-type properties of the passive film. For the acceptor density (NA), a slight decrease was observed with increasing Cr and Mo content, which is consistent with previous studies [28,57,58,59,63]. However, in contrast to earlier reports that suggested a decreasing trend in NA with W addition, no significant change was observed in the present study.
Figure 14 presents the variation in semiconductive slopes and the bipolar index derived from the Mott–Schottky analysis of ferritic stainless steel specimens passivated at +500 mV (SCE) for 1 h in deaerated 1 N NaCl + 1 N HCl solution at 30 °C. Figure 14a–c illustrate the changes in p-type and n-type slopes with respect to Cr, Mo, and W content, respectively. Increasing Cr and Mo content results in higher p-type and n-type slopes, indicating these elements directly influence the semiconductive properties of the passive films. In contrast, for W, while the p-type slope increases in the F-1.5W specimen, it remains stable for F-3W; meanwhile, the n-type slope demonstrates no substantial change with varying W content. These results imply that Cr and Mo influence both p-type and n-type behavior, whereas W mainly affects the p-type response within a limited content range.
Figure 14a′–c′ display the changes in bipolar index as a function of individual alloying element content. The bipolar index is an introduced parameter designed to comprehensively assess the combined p-type and n-type semiconductive characteristics of passive films, and its definition is given in Equation (1).
Bipolar Index = |p-type slope| + |n-type slope|
This assessment is based on the premise that the passive film consists of two distinct space charge layers, corresponding to p-type and n-type semiconductive characteristics. Figure 14a′–c′ compare bipolar index values as Cr, Mo, and W contents vary. As Cr content increases, the bipolar index rises incrementally to 12.96, 13.70, and 16.51, respectively, which shows that Cr is an element that affects semiconductor properties. For Mo, the bipolar index exhibits an increasing trend from 6.23 to 7.98, confirming Mo’s role in affecting semiconducting characteristics. When W is examined, bipolar index values are 15.35, 16.71, and 17.02 for F-0W, F-1.5W, and F-3W specimens, respectively, indicating an initial increase with W content. However, after F-1.5W, the increase is not significant, suggesting that W has an effect up to a certain content, but beyond that, the effect on semiconductor properties is limited.
As shown above, the bipolar index varied with each alloy component. To further explore the connection between the bipolar index and passive film properties, the relationship between corrosion current density (ip) and polarization resistance (Rp) was examined.
Figure 15 displays the correlation between the proposed bipolar index and both passive current density (ip) and polarization resistance (Rp), which represent the characteristics of the passive film. Figure 15a–c depict the relationship between the bipolar index and ip for different Cr, Mo, and W contents, while Figure 15a′–c′ illustrate the relationship between the bipolar index and Rp under the same compositional conditions. In Figure 15a,a′, ip clearly decreases and Rp shows an increasing trend as the bipolar index increases. This result indicates that Cr enhances the bipolar index of passive films, which substantially reduces ip and increases Rp, thereby improving corrosion resistance. In Figure 15b,b′, ip shows little change regardless of the bipolar index, whereas Rp increases as the bipolar index increases. This result indicates that Mo enhances the bipolar index of passive films, which mainly contributes to increasing Rp and improving resistance characteristics. In Figure 15c,c′, ip decreases as the bipolar index increases, while Rp shows an increasing trend. This result indicates that W enhances the bipolar index of passive films, thereby lowering ip and improving resistance characteristics.
As discussed above, the bipolar index characterizes the semiconducting properties of passive films, and this work demonstrates its association with the film’s corrosion resistance. To interpret this from the point of view of the Point Defect Model, the correlation between the bipolar index and the concentration of defects in the passive film is shown in Figure 16.
Figure 16 demonstrates a distinct trend of decreasing defect concentration with increasing bipolar index. Specifically, as the bipolar index rises from approximately 12 to 17, the defect concentration decreases linearly from about 3.05 × 1028 cm−3 to 2.12 × 1028 cm−3. These findings indicate that a higher bipolar index corresponds to a lower concentration of point defects in the passive film, which aligns with enhancements in the film’s semiconducting properties. Consequently, the bipolar index shows that it can be used as a quantitative indicator of corrosion resistance.
Figure 17 presents a conceptual model that depicts the impact of Cr, Mo, and W alloying elements on the semiconductor characteristics and electrochemical stability of the passive film, as derived from this study’s results. The incorporation of Cr, Mo, and W enhances the concentration of Cr2O3/Cr(OH)3 oxides in the inner layer of the passive film, thereby strengthening p-type semiconductor behavior. Simultaneously, in the outer layer, the increased presence of metal oxyanions (e.g., MeO42−) improves n-type semiconductor properties. The schematic Mott–Schottky plot at the bottom visually conveys that increasing Cr, Mo, and W content raises the p-type and n-type slopes, respectively, leading to an increase in the proposed indicator, the bipolar index (|p-type slope| + |n-type slope|). A higher bipolar index reduces the total defect density within the passive film, thereby decreasing the passive current density (ip) and increasing the polarization resistance (Rp), which results in greater stabilization of the passive film.

5. Conclusions

This study presents the following main conclusions regarding the electrochemical and semiconductor behavior of passive films on ferritic stainless steels containing varying Cr, Mo, and W levels, assessed in an acidic chloride environment (1 N NaCl + 1 N HCl, 30 °C, deaerated):
(1)
Increasing Cr, Mo, and W content in stainless steel leads to a decrease in the passive current density (ip) and an enhancement in polarization resistance (Rp). In addition, XPS depth profiling identified that the passive film possesses a double-layer structure, comprising an inner layer with p-type semiconductor properties near the metal and an outer layer exhibiting n-type semiconductor characteristics at the film–solution interface.
(2)
Using Mott–Schottky analysis, the slopes corresponding to the p-type and n-type semiconductor characteristics of the passive film were determined, leading to the introduction of a new metric termed the “Bipolar Index,” defined as |p-type slope| + |n-type slope|. As the concentrations of Cr, Mo, and W increase, the bipolar index also rises, thereby enhancing the semiconductor characteristics of the passive film. This leads to a decrease in the concentration of point defects within the film, resulting in a reduction in the passive current density (ip) and an increase in the polarization resistance (Rp).

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.-C.K. and Y.-S.K.; supervision, Y.-C.K. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by the “Research Fund of Gyeongkuk National University” and “Grant number 2025–2026”.

Data Availability Statement

All relevant data are presented within this article.

Conflicts of Interest

The authors have stated that there are no conflicts of interest.

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Crystals 15 00723 g001
Figure 1. Electrochemical polarization behavior (a–c) and passive current density (a′–c′) at +500 mV (SCE) of ferritic stainless steels in 1 N NaCl + 1 N HCl (30 °C, deaerated): (a,a′) Cr content, (b,b′) Mo content, (c,c′) W content.
Figure 1. Electrochemical polarization behavior (a–c) and passive current density (a′–c′) at +500 mV (SCE) of ferritic stainless steels in 1 N NaCl + 1 N HCl (30 °C, deaerated): (a,a′) Cr content, (b,b′) Mo content, (c,c′) W content.
Crystals 15 00723 g001
Crystals 15 00723 g002
Figure 2. Impedance behavior of passive films formed on ferritic stainless steels in 1 N NaCl + 1 N HCl (30 °C, deaerated): (a–c) Nyquist plots, (a′–c′) Bode plots, (a″–c″) polarization resistance (Rp); (a,a′,a″) Cr content, (b,b′,b″) Mo content, (c,c′,c″) W content.
Figure 2. Impedance behavior of passive films formed on ferritic stainless steels in 1 N NaCl + 1 N HCl (30 °C, deaerated): (a–c) Nyquist plots, (a′–c′) Bode plots, (a″–c″) polarization resistance (Rp); (a,a′,a″) Cr content, (b,b′,b″) Mo content, (c,c′,c″) W content.
Crystals 15 00723 g002
Crystals 15 00723 g003
Figure 3. Typical deconvolution of chemical states of chromium, molybdenum and tungsten by XPS in the passive film in 1 N NaCl + 1 N HCl (30 °C, deaerated) at an applied potential of +500 mV (SCE); (a) Cr 2p3/2, (b) Mo 3d (3d5/2 + 3d3/2), (c) W 4f (4f7/2 + 4f5/2).
Figure 3. Typical deconvolution of chemical states of chromium, molybdenum and tungsten by XPS in the passive film in 1 N NaCl + 1 N HCl (30 °C, deaerated) at an applied potential of +500 mV (SCE); (a) Cr 2p3/2, (b) Mo 3d (3d5/2 + 3d3/2), (c) W 4f (4f7/2 + 4f5/2).
Crystals 15 00723 g003
Crystals 15 00723 g004
Figure 4. Depth profile by XPS on the passive film formed for 1 h at +500 mV (SCE) in 1 N NaCl + 1 N HCl (30 °C, deaerated); (a) F-23Cr, (b) F-29Cr.
Figure 4. Depth profile by XPS on the passive film formed for 1 h at +500 mV (SCE) in 1 N NaCl + 1 N HCl (30 °C, deaerated); (a) F-23Cr, (b) F-29Cr.
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Crystals 15 00723 g005
Figure 5. Effect of Cr contents on the depth distribution of Cr 2p3/2 species in the passive film formed for 1 h at +500 mV (SCE) in 1 N NaCl + 1 N HCl (30 °C, deaerated); (a) CrM, (b) Cr2O3, (c) CrO3, (d) Cr(OH)3, (e) CrO42−.
Figure 5. Effect of Cr contents on the depth distribution of Cr 2p3/2 species in the passive film formed for 1 h at +500 mV (SCE) in 1 N NaCl + 1 N HCl (30 °C, deaerated); (a) CrM, (b) Cr2O3, (c) CrO3, (d) Cr(OH)3, (e) CrO42−.
Crystals 15 00723 g005
Crystals 15 00723 g006
Figure 6. Effect of Cr on the depth profile of passive films formed for 1 h at +500 mV (SCE) in 1 N NaCl + 1 N HCl (30 °C, deaerated); (a) Metal oxide/[Metal + Metal oxide], (b) Cr2O3/Cr(OH)3, (c) Metal oxyanion/[Metal + Metal oxide].
Figure 6. Effect of Cr on the depth profile of passive films formed for 1 h at +500 mV (SCE) in 1 N NaCl + 1 N HCl (30 °C, deaerated); (a) Metal oxide/[Metal + Metal oxide], (b) Cr2O3/Cr(OH)3, (c) Metal oxyanion/[Metal + Metal oxide].
Crystals 15 00723 g006
Crystals 15 00723 g007
Figure 7. Depth profile by XPS on the passive film formed for 1 h at +500 mV (SCE) 1 N NaCl + 1 N HCl (30 °C, deaerated); (a) F-0Mo, (b) F-2Mo.
Figure 7. Depth profile by XPS on the passive film formed for 1 h at +500 mV (SCE) 1 N NaCl + 1 N HCl (30 °C, deaerated); (a) F-0Mo, (b) F-2Mo.
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Crystals 15 00723 g008
Figure 8. Effect of Mo contents on the depth distribution of Mo (3d5/2 + 3d3/2) species in the passive film formed for 1 h at +500 mV (SCE) in 1 N NaCl + 1 N HCl (30 °C, deaerated); (a) MoM, (b) MoO2, (c) MoO3, (d) MoO(OH)2, (e) MoO42−.
Figure 8. Effect of Mo contents on the depth distribution of Mo (3d5/2 + 3d3/2) species in the passive film formed for 1 h at +500 mV (SCE) in 1 N NaCl + 1 N HCl (30 °C, deaerated); (a) MoM, (b) MoO2, (c) MoO3, (d) MoO(OH)2, (e) MoO42−.
Crystals 15 00723 g008
Crystals 15 00723 g009
Figure 9. Effect of Mo on the depth profile of passive films formed for 1 h at +500 mV (SCE) in 1 N NaCl + 1 N HCl (30 °C, deaerated); (a) Metal oxide/[Metal + Metal oxide], (b) Cr2O3/Cr(OH)3, (c) Metal oxyanion/[Metal + Metal oxide].
Figure 9. Effect of Mo on the depth profile of passive films formed for 1 h at +500 mV (SCE) in 1 N NaCl + 1 N HCl (30 °C, deaerated); (a) Metal oxide/[Metal + Metal oxide], (b) Cr2O3/Cr(OH)3, (c) Metal oxyanion/[Metal + Metal oxide].
Crystals 15 00723 g009
Crystals 15 00723 g010
Figure 10. Depth profile by XPS on the passive film formed for 1 h at +500 mV (SCE) in 1 N NaCl + 1 N HCl (30 °C, deaerated); (a) F-0W, (b) F-1.5W, (c) F-3W.
Figure 10. Depth profile by XPS on the passive film formed for 1 h at +500 mV (SCE) in 1 N NaCl + 1 N HCl (30 °C, deaerated); (a) F-0W, (b) F-1.5W, (c) F-3W.
Crystals 15 00723 g010
Crystals 15 00723 g011
Figure 11. Effect of W contents on the depth distribution of W (4f7/2 + 4f5/2) species in the passive film formed for 1 h at +500 mV (SCE) in 1 N NaCl + 1 N HCl (30 °C, deaerated); (a) WM, (b) WO2, (c) WO3, (d) WO42−.
Figure 11. Effect of W contents on the depth distribution of W (4f7/2 + 4f5/2) species in the passive film formed for 1 h at +500 mV (SCE) in 1 N NaCl + 1 N HCl (30 °C, deaerated); (a) WM, (b) WO2, (c) WO3, (d) WO42−.
Crystals 15 00723 g011
Crystals 15 00723 g012
Figure 12. Effect of W on the depth profile of passive films formed for 1 h at +500 mV (SCE) in 1 N NaCl + 1 N HCl (30 °C, deaerated); (a) Metal oxide/[Metal + Metal oxide], (b) Cr2O3/Cr(OH)3, (c) Metal oxyanion/[Metal + Metal oxide].
Figure 12. Effect of W on the depth profile of passive films formed for 1 h at +500 mV (SCE) in 1 N NaCl + 1 N HCl (30 °C, deaerated); (a) Metal oxide/[Metal + Metal oxide], (b) Cr2O3/Cr(OH)3, (c) Metal oxyanion/[Metal + Metal oxide].
Crystals 15 00723 g012
Crystals 15 00723 g013
Figure 13. Effect of Cr, Mo and W content on Mott–Schottky plot of passive film formed in 1 N NaCl + 1 N HCl (30 °C, deaerated); (a) Cr content, (b) Mo content, (c) W content.
Figure 13. Effect of Cr, Mo and W content on Mott–Schottky plot of passive film formed in 1 N NaCl + 1 N HCl (30 °C, deaerated); (a) Cr content, (b) Mo content, (c) W content.
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Crystals 15 00723 g014
Figure 14. Variation in semiconductive properties of passive films formed on ferritic stainless steels in 1 N NaCl + 1 N HCl (30 °C, deaerated): (a–c) p-type and n-type slopes, (a′–c′) bipolar index; (a,a′) Cr content, (b,b′) Mo content, (c,c′) W content.
Figure 14. Variation in semiconductive properties of passive films formed on ferritic stainless steels in 1 N NaCl + 1 N HCl (30 °C, deaerated): (a–c) p-type and n-type slopes, (a′–c′) bipolar index; (a,a′) Cr content, (b,b′) Mo content, (c,c′) W content.
Crystals 15 00723 g014
Crystals 15 00723 g015
Figure 15. Relationship between bipolar index and passive film properties of passive films formed on ferritic stainless steels in 1 N NaCl + 1 N HCl (30 °C, deaerated): (a–c) bipolar index vs. passive current density (ip), (a′–c′) bipolar index vs. polarization resistance (Rp); (a,a′) Cr content, (b,b′) Mo content, (c,c′) W content.
Figure 15. Relationship between bipolar index and passive film properties of passive films formed on ferritic stainless steels in 1 N NaCl + 1 N HCl (30 °C, deaerated): (a–c) bipolar index vs. passive current density (ip), (a′–c′) bipolar index vs. polarization resistance (Rp); (a,a′) Cr content, (b,b′) Mo content, (c,c′) W content.
Crystals 15 00723 g015
Crystals 15 00723 g016
Figure 16. Correlation between bipolar index and total defect density of passive films formed in 1 N NaCl + 1 N HCl (30 °C, deaerated).
Figure 16. Correlation between bipolar index and total defect density of passive films formed in 1 N NaCl + 1 N HCl (30 °C, deaerated).
Crystals 15 00723 g016
Crystals 15 00723 g017
Figure 17. Model for the effect of Cr, Mo, and W additions on the bipolar index of the passive film.
Figure 17. Model for the effect of Cr, Mo, and W additions on the bipolar index of the passive film.
Crystals 15 00723 g017
Table 1. Chemical compositions of ferritic stainless steels.
Table 1. Chemical compositions of ferritic stainless steels.
AlloyChemical Composition, wt. %
CrMoWSiNiMnCSPFe
F-23Cr23.03.541.320.51.120.760.0060.0060.004Bal.
F-26Cr26.03.531.330.531.120.760.0070.0070.005Bal.
F-29Cr28.83.541.340.541.120.740.0090.0060.003Bal.
F-0Mo28.90.061.330.521.120.750.0110.0070.005Bal.
F-2Mo28.82.001.340.521.130.740.0110.0070.004Bal.
F-3.5Mo28.83.541.340.541.120.740.0090.0060.003Bal.
F-0W28.73.550.020.521.200.750.0070.0080.005Bal.
F-1.5W28.83.541.340.541.120.740.0090.0060.003Bal.
F-3W28.73.582.930.541.160.740.0130.0060.004Bal.
Table 2. Typical binding energy of some chemical species for the XPS analysis [40,66].
Table 2. Typical binding energy of some chemical species for the XPS analysis [40,66].
SpeciesCrMCr2O3Cr(OH)3CrO3CrO42−
Binding Energy, eV574.1576.3577.3578.1579.3
SpeciesMoMMoO2MoO(OH)2MoO3MoO42−
Binding Energy, eV227.9
230.9		228.1
231.9		230.2
233.5		232.4
235.4		232.2
234.8
SpeciesWMWO2WO3WO42−
Binding Energy, eV30.8
32.95		32.5
34.55		34.9
35.5
37.65
37.95		36.3
Table 3. Donor density, acceptor density, and flat-band potential of passive films formed on ferritic stainless steels with different Cr, Mo, and W contents in 1 N NaCl + 1 N HCl (30 °C, deaerated).
Table 3. Donor density, acceptor density, and flat-band potential of passive films formed on ferritic stainless steels with different Cr, Mo, and W contents in 1 N NaCl + 1 N HCl (30 °C, deaerated).
AlloyF-23CrF-26CrF-29CrF-2MoF-3.5MoF-0WF-1.5WF-3W
Efb by P slope,
V (SCE)		0.130.0980.100.100.100.0930.100.10
Efb by N slope, V (SCE)0.0780.0500.0370.0580.0370.0720.0440.033
NA (1028 cm–3)1.36 1.311.061.601.061.301.061.06
ND (1028 cm–3)1.431.331.131.451.131.081.111.07
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Choi, S.-H.; Yoo, Y.-R.; Kim, Y.-C.; Kim, Y.-S. Effect of Cr, Mo, and W Contents on the Semiconductive Properties of Passive Film of Ferritic Stainless Steels. Crystals 2025, 15, 723. https://doi.org/10.3390/cryst15080723

AMA Style

Choi S-H, Yoo Y-R, Kim Y-C, Kim Y-S. Effect of Cr, Mo, and W Contents on the Semiconductive Properties of Passive Film of Ferritic Stainless Steels. Crystals. 2025; 15(8):723. https://doi.org/10.3390/cryst15080723

Chicago/Turabian Style

Choi, Seung-Heon, Young-Ran Yoo, Young-Cheon Kim, and Young-Sik Kim. 2025. "Effect of Cr, Mo, and W Contents on the Semiconductive Properties of Passive Film of Ferritic Stainless Steels" Crystals 15, no. 8: 723. https://doi.org/10.3390/cryst15080723

APA Style

Choi, S.-H., Yoo, Y.-R., Kim, Y.-C., & Kim, Y.-S. (2025). Effect of Cr, Mo, and W Contents on the Semiconductive Properties of Passive Film of Ferritic Stainless Steels. Crystals, 15(8), 723. https://doi.org/10.3390/cryst15080723

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