Influence of Layer Configuration on the Morphology and Corrosion Resistance of CrAlN/TiSiN Multilayer Coatings Prepared via Cathodic Arc Deposition

Abstract

In this study, cathodic arc deposition was employed to synthesize CrAlN/TiSiN nanostructured multilayer coatings on silicon wafer substrates. The effects of the multilayer architecture on the microstructure and corrosion resistance of the coatings were systematically investigated. The structural characteristics and performance of the deposited films were analyzed using scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), X-ray diffraction (XRD), and electrochemical polarization measurements. The experimental results demonstrate that various CrAlN/TiSiN multilayer configurations were successfully deposited, forming dense multilayer coatings with a thickness of approximately 1–2 μm and a dominant FCC β₁-NaCl crystalline structure. The presence of nanostructured multilayer interfaces effectively inhibited columnar grain growth and contributed to microstructural refinement. XRD analysis revealed competitive growth between the (111) and (200) crystallographic orientations, indicating that the crystallization behavior is influenced by the interplay between surface energy minimization and strain energy accumulation. Contact angle measurements showed that all the coatings exhibited water contact angles exceeding 90°, indicating hydrophobic characteristics and potential anti-fouling capacity. In particular, the CrAlN outer layer structure presented lower surface free energy, which further enhances the coating system’s anti-fouling capacity. Electrochemical polarization results indicate that the corrosion current density of all the coatings remained in the order of 10⁻⁷ A/cm², demonstrating excellent chemical stability. Overall, the CrAlN/TiSiN nanostructured multilayer coatings exhibit pronounced interface strengthening and densification growth mechanisms, which effectively enhance the chemical stability of silicon-based material surfaces. These results could provide valuable insights for the structural design and optimization of high-performance protective coatings.

1. Introduction

Due to advances in technology, modern industry demands higher performance from tool applications, particularly in regard to enhancing mechanical and tribological properties to extend service life. In cutting and forming tool applications, single-layer hard coatings (such as CrN, TiN, and Ti(C,N)) have gradually evolved toward multilayer coating architectures, as these coatings are struggling to meet the increasing requirements for higher hardness and wear resistance. Multilayer designs are therefore developed to significantly improve durability and prolong service life [1,2,3,4]. Accordingly, to achieve superior performance, nanostructured multilayer thin-film technology has been proposed. Some studies have focused on fabricating coatings that simultaneously exhibit sufficient toughness and superhard characteristics [5,6]. Among these, CrAlN/TiSiN nanostructured multilayer hard coatings have emerged as a recent research direction, primarily emphasizing the structural design and modulation of nanostructured multilayer architectures. This approach effectively addresses the inherent limitation of conventional single-layer coatings, for which achieving both high hardness and adequate toughness in practical applications remains challenging [6,7]. For example, Cao Xueqian conducted an in-depth comparison of single-layer TiSiN, bilayer CrAlN/TiSiN, and multilayer structures [8] and found that multilayer architectures can effectively reduce shear stress and enhance interfacial adhesion strength. Moreover, they exhibit excellent wear resistance and a stable coefficient of friction at elevated temperatures of up to 500 °C [8,9]. Some recent major research findings are given as follows: (1) When the bilayer period is reduced to the nanoscale (typically in the range of 2–20 nm), the strain fields arising from lattice mismatch between adjacent layers can effectively impede dislocation motion, thereby inducing a superhardening effect [5,10,11]; (2) Leveraging the structural stability of TiSiN as the top layer on CrAlN can promote the formation of refined columnar grain structures, which not only enhances hardness but also significantly improves the coating’s resistance to crack propagation [10,12].

Over the past decade, these nanostructured multilayer structures have typically been created using two alternating layered materials, with individual layer thicknesses ranging from 1 to 100 nm and a total coating thickness of less than 5 μm [5,13]. Ternary and quaternary nanoscale multilayer coatings (such as CrN/AlN, TiN/CrN, CrN/NbN, CrN/a-CNx, CrAlN/AlSiN, and TiAlN/CrN) have attracted considerable attention due to their excellent mechanical properties, thermal stability, and oxidation resistance [14,15]. These advantages are attributed to the additional energy required for dislocations to propagate across interfaces between layers with different elastic moduli, along with the formation of dense oxide films composed of aluminum (Al), chromium (Cr), and silicon (Si) on the coating’s surface [5,9,16]. Furthermore, studies have shown that incorporating Si into nanostructured multilayer structures can enhance their mechanical properties, primarily because of the formation of an amorphous phase [17,18]. However, most of these nanostructured multilayer coatings are deposited using magnetron sputtering. Compared with magnetron sputtering, cathodic arc deposition—characterized by a high deposition rate and intense ionization—poses greater challenges in regard to fabricating well-defined nanostructured multilayer structures [19,20]. To date, only a few studies have investigated nanostructured multilayer coatings prepared via cathodic arc deposition [20,21]. For example, some progress has been achieved by varying deposition parameters such as arc current, substrate bias, and nitrogen flow rate [19,21,22]. Nevertheless, studies on CrAlN/TiSiN nanostructured multilayer coatings prepared via cathodic arc deposition remain scarce [7,21], and investigations into their corrosion resistance are particularly limited.

Accordingly, in this study, CrAlN/TiSiN nanoscale multilayer coatings with different layer configurations were fabricated using cathodic arc deposition (CAD). The effects of varying layer architectures on the microstructural and electrochemical properties were systematically investigated. The results provide important insights for the design of CAD-deposited coatings to enhance the surface performance of silicon substrates.

2. Experimental Section

In this study, silicon wafers were used as the primary substrates, as they possess excellent surface flatness and a stable crystal structure, which can effectively improve the deposition uniformity, interfacial adhesion, and thermal stress compatibility of coatings. For coating technology, a cathodic arc deposition (CAD) system was employed. The chamber was evacuated to a base pressure below 5 × 10⁻³ Pa. The working pressure during deposition was maintained at approximately 0.4 Pa. The substrate bias voltage was fixed at −80 V, the arc current was 70 A, and the substrate temperature remained below 250 °C during deposition. The target materials used included Cr (99.9%), Ti (99.9%), Cr₅₀Al₅₀ alloy, and Ti₈₀Si₂₀ alloy. High-purity argon (Ar) was used as the working gas, while nitrogen (N₂) served as the reactive gas. CrN and TiN interlayers, as well as CrAlN/TiSiN nanostructured multilayer coatings, were deposited onto silicon substrates to achieve controlled design of different interfacial layer configurations.

Table 1. Layer configurations and specimen codes used in this study.

Specimen	Layer Configuration (Deposition Time)
S1	CrN (10 min)/CrAlN-TiSiN (30 min)/CrAlN (20 min)
S2	CrN (10 min)/CrAlN-TiSiN (30 min)/TiSiN (20 min)
S3	CrN (10 min)/CrAlN-TiSiN (50 min)
S4	TiN (10 min)/CrAlN-TiSiN (30 min)/CrAlN (20 min)
S5	TiN (10 min)/CrAlN-TiSiN (30 min)/TiSiN (20 min)
S6	TiN (10 min)/CrAlN-TiSiN (50 min)

lists the coating configurations and corresponding sample designations. The specimens were categorized into two groups: TiN and CrN interlayers groups. First, TiN and CrN interlayers were deposited on the silicon substrates, and then CrAlN/TiSiN nanostructured multilayer structures were formed. Subsequently, a top layer of either CrAlN or TiSiN nanocoating was individually deposited on the outermost surface. TiN or CrN interlayers were introduced to mitigate the mismatch in the coefficients of thermal expansion between the coating and the silicon substrate, thereby enhancing interfacial adhesion stability. In addition, the multilayer structural design enables the formation of highly dense and stable coatings on the silicon substrate. Further studies using one-variable-controlled sample sets and monolayer reference coatings are required to establish fully decoupled structure–property relationships.

After deposition, the CrAlN/TiSiN nanostructured multilayer coatings were characterized using various materials analysis techniques to evaluate their microstructural features and performance, as described below: (1) A field-emission scanning electron microscope (FE-SEM) was employed to observe the surface morphology and cross-sectional structures of the coatings. Energy-dispersive spectroscopy (EDS) was used in conjunction to analyze elemental distributions and verify the compositional stability of the multilayer interfaces. EDS analysis was used to evaluate the overall elemental distribution of the coatings; however, it cannot directly resolve nanoscale periodic compositional variations. (2) The crystal structure was evaluated using X-ray diffraction (XRD) analysis to identify the coatings’ crystalline phases. (3) Surface roughness measurements and water contact angle analysis were conducted to evaluate surface flatness and wettability, and to furthermore, assess their influence on contamination adhesion and interfacial oxidation risk. A surface profilometer was used for surface morphology analysis, with measurement parameters including the average roughness (Ra) and the maximum height roughness (Rz), in order to quantify particle deposition and the coatings’surface topographical variations. (4) Water contact angle measurements were performed using a contact angle goniometer. A fixed volume of deionized water was deposited onto the coating surface, and the contact angle formed between the droplet and the film surface was determined using an image analysis system to evaluate its wettability behavior. The magnitude of the contact angle reflects the coating’s surface free energy and chemical stability. (5) In addition, electrochemical polarization tests were conducted to evaluate corrosion behavior. A 1 M sulfuric acid (H₂SO₄) solution was used as the corrosive medium to simulate the effects of an acidic environment on the interfacial stability of the coatings. By measuring the polarization curves and analyzing corrosion potential (Ecorr), corrosion current density (Icorr), and polarization resistance (Rp), we assessed the coatings’ corrosion resistance and electrochemical stability.

3. Results and Discussion

3.1. Analysis of Coating Morphology

The microstructures of the CrAlN/TiSiN nanostructured multilayer coatings formed on the silicon substrates were primarily influenced by the high-energy ion bombardment effect inherent to cathodic arc deposition and the modulation of nanostructured multilayer interfaces. The high-energy ion flux enhances the kinetic energy and surface mobility of deposited atoms, enabling the formation of dense nanocolumnar structures even at relatively low deposition temperatures. SEM observations of the coatings’ surface morphology indicate that, under all six layer configuration conditions, the nanostructured multilayer coatings formed on the silicon substrates exhibited uniform and dense surface structures, as shown in Figure 1. However, the images in Figure 1 reveal the presence of droplets, which are characteristic morphological features generated during cathodic arc deposition. In the higher-magnification images (3000×), the droplets can be clearly observed to be uniformly distributed, while the matrix regions remain relatively smooth and flat. The particle size is on the order of several micrometers, and the particle distribution is comparable across all deposition conditions. Regarding the coatings’ chemical compositions, energy-dispersive spectroscopy (EDS) was employed to analyze the elemental distribution. Since the films were constructed with nanoscale periodic layering and the penetration depth of EDS exceeds the micrometer scale, it was not possible to resolve the periodic variation of each element. However, the analysis confirms that all target elements are present within the coatings, as shown in

Table 2. Chemical composition of the coatings (at%).

Specimen	Cr	Al	Ti	Si	N
S1	22.47	16.34	10.67	2.13	48.39
S2	16.11	10.68	19.40	6.01	47.80
S3	18.58	12.11	15.04	4.16	50.11
S4	20.94	16.44	12.87	2.67	47.08
S5	9.82	7.51	23.63	5.33	53.71
S6	16.03	12.11	17.79	3.43	50.64

. The available data still reveal certain trends. For instance, the outermost layers of specimens S1 and S4 are CrAlN coatings, resulting in higher Cr and Al content. In contrast, the top layer for specimens S2 and S5 is TiSiN, leading to a relatively higher Si content, exceeding 5%. In addition, specimens with a N content of approximately 50% (S3, S5, and S6) indicate a higher fraction of nitride phases in the coatings. Under the high-energy conditions of cathodic arc deposition, Cr, Al, Ti, Si, and N₂ undergo highly ionized plasma reactions, forming nanocomposite nitride coatings, such as TiN, CrN, AlN, Si₃N₄, TiSiN, CrAlN, or TiCrAlSiN. The metal vapor generated by the arc contains numerous metal ions (Ti⁺, Al⁺, Cr⁺, and Si⁺). When the reactive gas N₂ enters the chamber, it is activated by the high-energy plasma and dissociates into nitrogen ions, which subsequently react on the substrate surface, forming nitride phases. Typically, silicon (Si) is not incorporated into the FCC lattice under high-energy plasma conditions; instead, it forms an amorphous Si₃N₄ phase. The amorphous Si₃N₄ phase encapsulates the (Ti, Al, Cr)N nanocrystals, inhibiting grain growth (i.e., refining the columnar structure), which leads to a significant increase in hardness (reaching the superhard regime) and corrosion resistance.

Figure 2 shows the coatings’ cross-sectional SEM morphology. The images reveal good interfacial adhesion between the coating and the silicon substrate, with no obvious voids or delamination. These results indicate that the interlayer design proposed in this study effectively alleviates interfacial stress induced by thermal expansion mismatch. The high-density interfacial regions formed by the multilayer structure can suppress columnar grain growth, maintaining the grain size at the nanoscale and thereby enhancing the structural stability of the coating. The results of this study confirm that these multilayer structures can effectively modulate grain size and strain distribution, providing a microstructural basis for designing protective coatings on silicon wafer surfaces. Furthermore, the SEM images reveal that the coatings exhibit dense nanocolumnar structures, with well-defined interfaces between the interlayers and top layers. There are no obvious voids, cracks, or instances of delamination, indicating that all six coating configurations effectively enhance the interfacial adhesion stability between the films and the silicon substrates. The total coating thickness of the different layer configurations on the silicon wafers ranges from approximately 1.073 to 1.900 μm (as shown in

Table 3. Total layered thickness of the coatings.

Specimen	Total Layered Thickness (μm)
S1	1.276
S2	1.073
S3	1.139
S4	1.781
S5	1.439
S6	1.900

). This result indicates that the coating thickness is uniform and the interfaces are smooth, implying that the alternating multilayer deposition process possesses good thickness control capacity and reproducibility. Therefore, in applications involving silicon substrates, the density of the cross-sectional structure and the integrity of the interfaces directly influence the distribution and stability of interfacial stresses under thermal cycling conditions. These factors contribute to resistance against crack propagation and suppression of crack growth, and they also potentially hinder the penetration pathways of corrosive media, thereby affecting the overall corrosion resistance of the coating structure.

In this study, a surface profilometer was employed to analyze surface morphology. The parameters obtained include the arithmetic average roughness (Ra) and the maximum height roughness (Rz), which were used to quantitatively characterize the droplet distribution and surface topographical variations of the coatings. Since the cathodic arc deposition process inherently leads to droplet formation, the size and distribution of these droplets directly influence the surface smoothness of the films. In silicon wafer applications, surface roughness affects interfacial electrical stability, compatibility with subsequent film deposition, and packaging reliability. Therefore, by comparing the Ra and Rz values, one can evaluate the ability of multilayer structures to regulate surface flatness and analyze the influence of different interlayer designs on surface nucleation behavior.

Table 4. Surface roughness values (Ra and Rz) of the coatings.

Specimen	S1	S2	S3	S4	S5	S6
Ra (μm)	0.162	0.183	0.192	0.215	0.267	0.244
Rz (μm)	1.633	2.234	2.291	2.014	2.737	2.407

presents the measured Ra and Rz surface roughness values of the coatings on silicon substrates. Figure 3 presents the measured Ra and Rz surface roughness values of the CrAlN/TiSiN multilayer coatings deposited on silicon substrates. Both the multilayer structure and interlayer design have a significant influence on surface roughness. Compared with specimens incorporating a TiN interlayer, those with a CrN interlayer exhibit lower Ra and Rz values, indicating better surface smoothness (Table 4). This finding suggests that the CrN interlayer can effectively suppress the growth of droplets generated during the cathodic arc deposition of the top layer, thereby improving surface flatness. In other words, in the cathodic arc evaporation process, relative to TiSi targets, CrAl targets are indeed more likely to produce smoother surfaces. This is presumed to be mainly related to the influence of target material properties on droplet formation. Chromium (Cr) has a relatively high melting point (approximately 1907 °C) and, under arc discharge, tends to sublimate directly or generate finer ion fluxes. Although aluminum (Al) has a relatively low melting point, CrAl alloy targets, when properly composited, do not. The observed thickness differences mainly arise from the different deposition rates of the CrAl and TiSi target materials, while the former results in great thickness of the film, which helps reduce localized overheating-induced splashing. Titanium (Ti) has a relatively low melting point (approximately 1668 °C), and arc spots tend to form deeper molten pools on the target surface. In addition, although the incorporation of silicon (Si) can refine grain size, insufficient target density under high current conditions may instead lead to the generation of larger splashed droplets due to thermal stress. In summary, the arc spot velocity on chromium-based targets (such as Cr and CrAl) is generally higher than that on titanium-based targets (such as Ti and TiSi). A higher arc spot velocity results in a lower degree of localized melting on the target surface, thereby reducing the number of ejected molten metal droplets and consequently producing smoother coating surfaces. In silicon substrate applications, surface roughness directly affects the interfacial compatibility and adhesion quality of subsequent film deposition, as well as the uniformity and stability of surface electrical properties. A lower Ra value indicates a reduced particle density and improved morphological uniformity, helping to decrease the probability of interfacial defect formation and enhancing long-term stability. The multilayer-structured specimens exhibit finer particles with more uniform distributions, indicating that multilayer interfaces can suppress nucleation behavior and droplet growth.

3.2. Analysis of Layer Structure and Water Contact Angle

Figure 4 shows the results of X-ray diffraction (XRD) analysis of six coated specimen configurations, enabling the identification of crystalline structural characteristics and phase compositions under varying coating combinations. In nanolayered structures, distinct CrAlN and TiSiN diffraction peaks are only rarely observed; instead, coherent growth occurs between the two, resulting in a single averaged diffraction peak. As shown in Figure 4, the films in this study exhibit a consistent face-centered cubic (FCC) structure. According to the literature, both CrAlN and TiSiN coatings typically display a NaCl-type FCC structure in XRD patterns. Based on the melting point of metal, Cr-based cathodes generally exhibit smaller arc spots than Ti-based cathodes, leading to reduced local overheating and fewer emitted droplets. The main diffraction peaks of the CrAlN and TiSiN films are located in the (111), (200), and (220) planes. When these two materials are stacked in various configurations to form nanostructured multilayer coatings, the growth orientation of the films manifests as a common “averaged peak.” The overlap of diffraction peaks from CrAlN and TiSiN results in the formation of a broadened characteristic (200) peak, which possesses the lowest surface energy in the FCC structure (e.g., S3 and S6). For CrAlN-dominant films, the (111), (200), and (220) peaks are located at approximately 37.6°, 44.4°, and 65.7°, respectively. In contrast, the (111) and (200) peaks of TiSiN films appear at approximately 36.8° and 42.6°. The observed orientation evolution may be associated with the interplay between surface energy minimization and strain energy accumulation. Due to the lattice mismatch between CrAlN and TiSiN, significant elastic strain is generated at the interfaces, leading to a shift in the main diffraction peaks relative to standard positions (such as S3 or S6). Figure 4 shows that the (200) plane orientation of the S1 and S4 films shifts to higher angles (right shift) due to the influence of the CrAlN top layer. In contrast, the (200) plane of S2 and S5 exhibits a slight shift to lower angles (left shift), governed by the TiSiN top layer. For silicon substrate applications, variations in diffraction intensity ratios among different film structures are observed. The preferred crystallographic orientation significantly influences the distribution and relaxation behavior of residual stress, governs the formation of grain boundary diffusion pathways, and affects thermal stability, along with the structural integrity and long-term reliability of the interface. A dominant (111) orientation is typically associated with strain energy–controlled growth mechanisms, whereas a (200) orientation is generally related to surface energy minimization.

The water contact angle (WCA) is the most direct indicator for evaluating the wettability of a solid surface. In other words, a larger contact angle corresponds to better hydrophobicity. When the contact angle exceeds 90°, a water droplet adopts a nearly spherical shape on the surface and does not readily wet it. Figure 5 shows that the water contact angles of all six film specimens exceed 90°. All coatings exhibited hydrophobic behavior, which may contribute to potential anti-contamination performance. Figure 6 further compares the differences, and the results suggest that the CrAlN/TiSiN multilayer structures (S3 and S6) may exhibit enhanced hydrophobic behavior due to changes in interfacial configuration and compositional distribution that reduce surface energy. The S1 and S4 multilayer specimens, with CrAlN serving as the top surface layer, exhibit slightly higher water contact angles than the other structures, indicating a lower surface free energy. In contrast, S2 and S5, which possess a TiSiN top surface, show relatively poor wettability. This behavior can be attributed to the tendency of chromium (Cr) and aluminum (Al) to readily form a dense passive oxide layer on the surface. Such oxides typically possess lower surface energy, which contributes to increases in the water contact angle. Therefore, hydrophobicity is closely related to both film composition and surface microstructure.

3.3. Corrosion Resistance of the Coatings

Figure 7 presents the polarization curves of six coated specimens obtained after polarization tests using 1 M H₂SO₄ solution as the corrosive medium.

Table 5. Polarization data obtained from polarization tests conducted on the coatings.

Specimen	Icorr (A)	Ecorr (V)	Rp
S1	1.293 × 10−7	0.0604	8.9 × 106
S2	1.431 × 10−7	−0.0723	6.8 × 106
S3	1.072 × 10−6	−0.269	1.11 × 106
S4	1.248 × 10−7	−0.1813	9.27 × 106
S5	1.545 × 10−7	−0.1197	7.5 × 106
S6	1.344 × 10−7	−0.1054	7.5 × 106

further summarizes the parameters calculated from the polarization measurements, including the corrosion potential (Ecorr), corrosion current density (Icorr), and polarization resistance (Rp). The polarization curves (Figure 7) show that all the coated specimens exhibit good corrosion resistance. Table 5 shows that the corrosion current densities of the six nanolayered specimens are on the order of 10⁻⁷ A/cm², and the corrosion potentials are close to 0 V, indicating excellent corrosion barrier performance. A lower Icorr corresponds to a lower corrosion rate, while a higher Ecorr indicates better electrochemical stability. The inferior corrosion resistance of S3 is due to the absence of an additional top sealing layer, which may facilitate electrolyte penetration through surface defects or interface pathways. In addition, a higher polarization resistance (Rp) reflects superior corrosion resistance. Since the differences in corrosion resistance among the six nanolayered specimens are relatively small, we infer that the multilayer interface structure effectively interrupts the continuous diffusion pathways formed along columnar grain boundaries, thereby reducing the rate of penetration of corrosive species. The presence of multiple interfaces forces corrosive media to diffuse along a more tortuous path, which significantly increases the corrosion potential and decreases corrosion current density. Furthermore, Cr, Al, and Ti tend to form stable oxide layers in corrosive environments. For example, the CrAlN layer readily forms dense Al₂O₃ and Cr₂O₃ films during corrosion, both of which exhibit excellent resistance to chemical attacks. This corrosion analysis is limited to short-term polarization measurements; future studies should include EIS and long-term exposure testing. Specimen S3 exhibited a relatively higher corrosion current density, indicating inferior corrosion resistance relative to the other coatings.

4. Conclusions

• In this study, dense nanolayered structures with thicknesses of approximately 1.073–1.900 μm were successfully deposited, exhibiting a typical FCC β₁-NaCl crystalline phase. The use of CrN and TiN as interlayers effectively improved interfacial adhesion and stability between the silicon substrate and the CrAlN/TiSiN coatings. We found that the preferred crystallographic orientation significantly influences the distribution and relaxation behavior of residual stress, governs the formation of grain boundary diffusion pathways, and affects thermal stability, as well as the structural integrity and long-term reliability of the interface. A dominant (111) orientation is typically associated with strain–energy–controlled growth mechanisms, whereas a (200) orientation is generally related to surface energy minimization. These strengthening effects are proposed to be plausible mechanisms based on structural observations and previous studies. Therefore, these measurements are required in future work.
• The nanolayered interfaces effectively suppress the growth of columnar grains. XRD analysis revealed a competitive growth behavior between the (111) and (200) preferred orientations, confirming that the crystallographic evolution is governed by the interplay between surface energy and strain energy, which in turn influences the distribution of residual stress and structural stability.
• Surface morphology analysis indicated that the multilayer structure refines droplet size and reduces the Ra value. Among the interlayers, the CrN interlayer exhibits a more pronounced effect on improving surface smoothness relative to the TiN interlayer.
• The water contact angles of all six coatings exceed 90°, indicating that the films exhibit hydrophobic characteristics. Among them, the structures with CrAlN as the outermost surface layer show lower surface free energy, a characteristic that has a positive effect on enhancing anti-contamination performance.
• Electrochemical polarization analysis showed that the corrosion current densities of the coatings were maintained at the level of 10⁻⁷ A/cm², while polarization resistance reached 10⁶–10⁷ Ω·cm². These results indicate that the multilayer interfaces effectively block the continuous diffusion pathways along columnar grain boundaries, creating a tortuous corrosion path. In addition, the formation of dense protective oxide layers via Cr, Al, and Ti significantly enhances corrosion resistance.