Optimization of Corrosion Resistance in Magnetron-Sputtered CrAlN Coatings for Alkaline Seawater Electrolysis via Nitrogen Flow Ratio Control: Microstructural Evolution and Corrosion Mechanism

Abstract

Designing materials with superior corrosion resistance is critical for seawater electrolysis systems to achieve efficient and long-term stable hydrogen production. In the current study, CrAlN coatings were deposited on TA1 titanium substrates by reactive magnetron sputtering with nitrogen flow ratios ranging from 40%–70% to investigate the effect of nitrogen stoichiometry on corrosion behavior in simulated alkaline seawater (pH ≈ 14, chloride-containing). Microstructural characterization (Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Spectroscopy (EDS), Grazing Incidence X-Ray Diffraction (GIXRD), Transmission Electron Microscopy (TEM), X-Ray Photoelectron Spectroscopy (XPS), Atomic Force Microscopy (AFM)) reveals that a 60% nitrogen ratio promotes grain refinement, improved CrN/AlN phase stoichiometry, and reduced oxygen-related defects, resulting in a dense columnar structure with minimized diffusion pathways. Electrochemical measurements show that this condition yields the lowest corrosion current density (0.297 μA·cm⁻²) and the highest polarization resistance (123.9 kΩ·cm²). Electrochemical impedance spectroscopy confirms enhanced charge transfer resistance and suppressed ionic transport at the coating/electrolyte interface. The results establish a clear correlation between nitrogen-controlled phase evolution, defect density, and passivation kinetics in highly alkaline chloride environments relevant to seawater electrolysis. This study targets the fabrication of protective coatings for alkaline seawater electrolysis via nitrogen flow ratio optimization. The optimized CrAlN coating achieves remarkably improved corrosion resistance compared with existing coatings, showing promising practical value for long-term stable seawater electrolysis.

1. Introduction

Renewable energy is expected to become the mainstream in future energy systems. Developing renewable energy is also crucial to align with global energy trends, to reduce carbon dioxide emissions, and to accelerate the transition towards carbon neutrality. Hydrogen is recognized as one of the most sustainable energy carriers as its only byproduct during electricity generation is water. Furthermore, hydrogen possesses a high energy density (140 MJ/kg) [1,2,3]. Seawater electrolysis for hydrogen production has attracted extensive attention due to its abundant resources, environmental friendliness, and sustainability [4,5]. However, selecting reliable structural materials for hydrogen production equipment and platforms poses significant challenges. It is well known that seawater contains high levels of chlorine, with chloride ions concentrations up to about 0.6 M, which imposes a significant corrosion threat to most metallic materials [6,7,8]. Developing highly corrosion-resistant materials is essential for seawater electrolysis systems, as it ensures efficient and durable hydrogen production.

Metal nitrides (MNs) exhibit high structural stability and chemical inertness owing to the strong metal-nitrogen bonds [9,10,11,12,13]. Several MNs, such as TiN, CrN, ZrN, and NbN, exhibit high performance in challenging service conditions (corrosion, high temperatures, and frictional environments, etc.) [14,15,16,17,18]. Therefore, MNs represent promising coating candidates for service in harsh environments. In contrast to single-metal nitride coatings such as CrN and TiN, the incorporation of multiple metallic elements into nitrides has been widely reported to yield a denser microstructure and significantly improved corrosion resistance [19,20,21,22]. Deng et al. [23] fabricated CrN, TiN, CrAlN, and TiAlN coatings via Cathodic Arc Evaporation, and evaluated their microstructure and erosion behavior. Their results showed that the multi-metallic elements nitride coatings (CrAlN and TiAlN) exhibited higher erosion wear resistance than those without Al (CrN and TiN). Chim et al. [24] evaluated the oxidation behavior of TiN, CrN, TiAlN, and CrAlN coatings, and their results demonstrated that the ternary nitride coatings (TiAlN, CrAlN) exhibited evidently better oxidation resistance than their binary counterparts. Barshilia et al. [25] prepared and characterized CrN and CrAlN coatings. The results showed the CrAlN coating sample exhibited much reduced corrosion current (1.03 μA·cm⁻² ) than that of CrN coating sample (6.95 μA·cm⁻²). All the results demonstrate that the ternary nitride coatings provide better performance than binary nitride coating.

The preparation conditions also greatly impact the structure and properties of coatings. Khamseh et al. [26] investigated the fabrication of thin CrAlN films under different pulse frequencies, demonstrating that the prepared CrAlN films could achieve higher hardness, increased internal stress, and finer grain size when the pulse frequency exceeds 160 kHz. Furthermore, Li et al. [27] prepared CrAlN coatings with different Ar/N₂ gas ratios. Their results indicated that the CrAlN coating deposited at N₂ pressure of 3.0 Pa exhibited the highest corrosion resistance. In addition, Su et al. [28] investigated CrAlN coatings prepared by unbalanced magnetron sputtering, and further synthesized MAX-phase Cr₂AlN coatings via annealing at 1000 °C in an inert atmosphere. Their results indicated that the formation of Cr₂O₃ and Al₂O₃ on the CrAlN coating surface is favorable for improving the corrosion resistance of the coating, which exhibited a corro-sion current as low as 0.535 μA/cm² in a simulated PEMFC environment. Despite advances in depositing CrAlN coatings to enhance corrosion resistance on various substrates, their behavior on titanium alloys in seawater electrolysis environments remains underexplored.

Alkaline seawater electrolysis serves as a core route for large-scale green hydrogen production, but the highly alkaline and chloride-rich harsh environment causes severe corrosion failure of key components, which has become a key bottleneck restricting the stable operation and service life of electrolyzers.

The present study aims to elucidate the role of nitrogen flow ratio in governing phase constitution, defect structure, and corrosion passivation behavior of magnetron-sputtered CrAlN coatings deposited on TA1 titanium alloy substrates. By systematically varying the nitrogen ratio (40%–70%) during reactive sputtering, we investigate microstructural evolution using SEM, AFM, GIXRD, TEM, and XPS, and correlate these structural features with electrochemical performance in simulated alkaline seawater. Particular emphasis is placed on establishing the relationship between nitrogen-controlled stoichiometry, ionic transport resistance, and passive film formation mechanisms. Through combined electrochemical and surface chemical analyses, this work seeks to clarify how defect engineering via nitrogen regulation can optimize corrosion resistance in highly alkaline chloride environments relevant to seawater electrolysis systems. In comparison with prior studies, our study focused on the corrosion performance in the complex environment of alkaline and simulated seawater, and the sample still shows good corrosion resistance relative to the single seawater environment.

2. Materials and Methods

These CrAlN films were deposited on TA1 substrates (20 mm × 20 mm × 1 mm) using PVD magnetron sputtering equipment. Prior to coating deposition, the substrate was polished using sandpaper up to 2000# grit and subsequently ultrasonically cleaned in acetone and anhydrous ethanol for 30 min. They were then rinsed with deionized water, dried in a vacuum drying oven, and mounted onto the sample rotation tray in the magnetron sputtering chamber. The experiment was conducted using the DZD600 composite coating system manufactured by Shenyang Keyou Vacuum Technology Co., Ltd. (Shenyang, China) This system comprises four sputtering target modules, a vacuum assembly, and a circulating cooling water system. The substrate was maintained at a floating potential during deposition.

Before deposition, the targets were subjected to glow discharge pre-sputtering in a high-purity argon atmosphere (99.999%) to eliminate surface contaminants and native oxide layers. The vacuum level in the sputtering chamber was maintained at 5.0 × 10⁻⁴ Pa. A Cr buffer layer was first deposited for 30 min to serve as a transition zone. The CrAlN functional layer was then synthesized atop the Cr layer with a total deposition time of 120 min. The deposition process utilized Cr and Al targets at a constant power of 150 W each, with a sample rotation speed of 5 rpm and a target-to-substrate distance of 150 mm. Argon was introduced at a flow rate of 10 sccm to maintain the plasma. To assess the effect of nitrogen content on the properties of the Cr/CrAlN coatings, the nitrogen gas flow ratio was adjusted to 40%, 50%, 60%, and 70%, with a total gas flow rate of 20 sccm. The corresponding N₂: (Ar + N₂) flow rates for the four samples were 8:12, 10:10, 12:8, and 14:6 sccm, respectively. The detailed deposition parameters are summarized in Table 1.

Table 1. The PVD parameters for the deposition of Cr/CrAlN films.

Parameters	Cr Layer	CrAlN Working Layer
Temperature (°C)	60	60
Base Pressure (Pa)	5.0 × 10−4	5.0 × 10−4
Nitrogen ratios	N/A	40%~70%
Gas Flow Rate (sccm)	10	20
Working Pressure (Pa)	0.8	1.0
Coating Time (min)	30	120
Cr Target Power (W)	150	150
Al Target Power (W)	N/A	150
Substrate Bias (W)	−50	−50

Surface microstructure and cross-sectional morphology were characterized using a scanning electron microscope (SEM, Quanta FEG 250, FEI Company, Hillsboro, OR, USA) and an atomic force microscope (AFM, Dimension Icon, Bruker Nano GmbH, Karlsruhe, Germany). The chemical composition of the coating surface was identified via energy-dispersive X-ray spectroscopy (EDS, with the matching detector of the above SEM system). In the EDS results, the elemental contents of N and O are for reference only and mutually corroborate the XRD and XPS results. For elemental quantification: The relative atomic percentages (at.%) of elements were calculated from the integrated peak areas of high-resolution spectra, normalized by the atomic sensitivity factors (ASF) provided by the instrument manufacturer. An X’Pert Pro X-ray diffractometer (PANalytical, Almelo, The Netherlands) was employed, utilizing a Cu-Kα radiation source with a step size of 0.02°. The sampling range (2θ) was set from 20 to 80°. The phase composition and preferred crystallographic orientation of the coatings were determined via X-ray diffraction (XRD) analysis.

X-ray photoelectron spectroscopy (XPS, Thermo Scientific K-Alpha, Thermo Fisher Scientific, Waltham, MA, USA) was utilized to characterize and analyze the surface elemental constituents and chemical compound types of the as-deposited coatings. The instrumental parameters for XPS were set as follows: an accelerating voltage of 12 kV, a filament current of 6 mA, and an Al-Kα radiation source with a characteristic photon energy of 1486.6 eV.

To simulate marine corrosion, electrochemical measurements were performed using a Corrtest-CS350 workstation (Wuhan Corrtest Instruments Co., Ltd., Wuhan, China). An artificial seawater electrolyte (composition detailed in Table 2) served as the corrosive medium. For electrochemical impedance spectroscopy (EIS), measurements were conducted at an initial potential of 0 V vs. OCP, covering a frequency range from 10⁵ to 10⁻² Hz with a 10 mV sinusoidal amplitude after an initial 3600 s stabilization period. Potentiodynamic polarization (Tafel) tests were performed at a scan rate of 1.0 mV/s, ranging from −1.2 V to 0.5 V relative to the open circuit potential. All corrosion tests were conducted in a static mixed solution of simulated seawater and NaOH.

Table 2. Chemicals in Simulated Seawater Electrolysis Solution.

Chemical Composition	Content (g/L)
NaOH	240.0
NaCl	24.53
MgCl2	5.20
Na2SO4	4.09
CaCl2	1.16
KCl	0.695
NaHCO3	0.201
KBr	0.101

3. Results

3.1. Microstructure and Phase Composition of CrAlN Coatings

Figure 1 displays the surface and cross-sectional SEM morphologies of the coatings synthesized at various nitrogen flow ratios. After a detailed examination, it was observed that the CrAlN grains became finer with the increase in N₂/N₂ + Ar ratio up to 60%. However, when the ratio increased to 70%, the CrAlN grains were coarsened. The appearance of “macro-particles” on the coating surfaces is common in magnetron sputtering, which occurs as ionized argon atoms bombard the target surface, causing material clusters to detach and deposit onto the substrate. As shown in Figure 1b, the total coating thickness was approximately 310 nm, featuring a distinct interface between the Cr transition layer and the CrAlN functional layer. It can be seen from Table 3 that with the increase in the ratio, the thickness of the CrAlN coating sputtered on silicon wafers gradually decreases from 331 nm to 297 nm. This is because the reduction in Ar content decreases the flux of ions bombarding the target, thereby reducing the sputtering and deposition rate of the coating. The cross-sectional analysis confirms a dense coating microstructure with high interfacial adhesion to the titanium alloy substrate. Subsequent performance tests indicate that coating thickness exerts an insignificant influence on the corrosion resistance within the thickness range investigated in this study. The surface morphology of the coatings was characterized using AFM and the results are shown in Figure 2. As the N₂/(N₂ + Ar) ratio increased from 40% to 60%, the average surface roughness (Ra) decreased significantly from 20.3 nm to 13.5 nm, indicating progressive surface densification and grain refinement. However, further increasing the nitrogen ratio to 70% led to a pronounced rise in roughness (20.4 nm), which could imply the onset of microstructural instability or defect formation under excessive nitrogen conditions. Reduced surface roughness is generally associated with lower defect density and fewer open diffusion pathways for aggressive species. Therefore, the coating deposited at a 60% N₂/(N₂ + Ar) ratio exhibits the most compact surface morphology, which is expected to minimize electrolyte penetration and enhance corrosion resistance.

Figure 1. Surface morphology (a) and cross-sectional morphology (b) of CrAlN coatings deposited at different N₂ ratios.

Table 3. Coating thickness of CrAlN coatings prepared with different N₂ ratios.

Nitrogen Ratios (%)	Coating Thickness (nm)	Standard Deviation (nm)
40%	331	±7.21
50%	307	±5.12
60%	305	±3.17
70%	297	±4.24

Figure 2. AFM height image of the CrAlN coating surface: (a) 40%, (b) 50%, (c) 60%, (d) 70%.

Elemental compositions and spatial distributions of the coatings were analyzed by EDS, as presented in Figure 3a–d. The elemental maps demonstrate a homogeneous distribution of Cr, Al, and N across the coating surface, indicating uniform co-deposition without apparent elemental segregation. With increasing N₂/(N₂ + Ar) flow ratio, the nitrogen content in the films gradually increased, accompanied by a corresponding decrease in oxygen concentration, reflecting improved nitridation and reduced oxide incorporation during deposition. Notably, EDS semi-quantitative results (for light elements N and O, for reference only) show that the coating prepared at a 60% nitrogen ratio exhibits the lowest relative oxygen content, suggesting enhanced structural compactness and reduced defect-mediated oxygen uptake. In contrast, at a 70% nitrogen ratio, a slight reduction in Cr and Al contents is observed together with an unexpected increase in relative oxygen signal. This relative trend, consistent with subsequent XPS quantitative analysis, implies that excessive nitrogen flow may induce microstructural defects or stress-related porosity, thereby facilitating oxygen incorporation and increasing susceptibility to surface oxidation.

Figure 3. EDS elemental maps (a–d) and corresponding elemental composition percentage curves (e) of CrAlN coatings deposited at different N₂ ratios: (a) 40%, (b) 50%, (c) 60%, and (d) 70%.

Figure 4 presents the GIXRD patterns of the CrAlN coatings deposited at different nitrogen flow ratios. Diffraction results demonstrate that the coatings exhibit a dominant face-centered cubic (fcc) CrN-type structure, with Al atoms dissolved into the lattice to form a CrAlN solid solution. All samples exhibit characteristic reflections corresponding to fcc-CrN, confirming that the dominant phase remains structurally consistent across the investigated nitrogen ratios. A preferred orientation evolution is observed, with pronounced diffraction intensity along the (200) and (111) planes depending on the nitrogen content. With increasing the N₂/(N₂ + Ar) ratio up to 60%, the diffraction peaks become sharper and slightly shift toward the standard positions of CrN(110) (≈37.5°) and CrN(200) (≈43.7°), suggesting improved crystallinity and enhanced phase stoichiometry. The peak sharpening implies grain refinement and reduced structural disorder, while the peak position adjustment indicates a lattice parameter closer to that of stoichiometric CrN. These structural changes confirm that optimizing the nitrogen ratio promotes the formation of a more crystalline and chemically stable CrAlN phase. No obvious characteristic diffraction peaks of oxide phases were observed in the GIXRD analysis. Combined with the issue of oxygen content obtained from EDS analysis, a detailed investigation will be conducted in subsequent XPS measurements.

Figure 4. GIXRD patterns with the corresponding enlarged regions (a) and corresponding grain sizes (b) calculated by the Debye-Scherrer formula of the CrAlN films deposited with different N₂ ratios.

The average crystallite size of the coatings was estimated using the Debye-Scherrer equation (Equation (1)) [29], where K = 0.9, λ = 0.15406 nm (Cu Kα radiation), β represents the corrected full width at half maximum (FWHM) in radians, and θ is the Bragg diffraction angle. As shown in Figure 4b, the calculated crystallite size decreases progressively with increasing nitrogen ratio up to 60%, indicating pronounced grain refinement. This refinement is associated with enhanced nitridation and improved phase formation of CrN and AlN, which promote nucleation over grain growth during deposition. The reduced crystallite size suggests a denser microstructure with minimized intercolumnar spacing, which is expected to improve barrier properties against corrosive species penetration.

$$D={\displaystyle \frac{K\lambda }{\beta cos \theta }}$$

(1)

Figure 5 displays the high-resolution XPS core-level spectra and corresponding peak deconvolution results for CrAlN coatings deposited at different nitrogen ratios. XPS measurements were directly conducted on the coating surface to characterize its surface properties. The Cr 2p spectra (Figure 5a) exhibit the characteristic spin–orbit doublet located at binding energies of approximately 576.2 ± 0.4 eV (Cr 2p₃/₂) and 585.8 ± 0.4 eV (Cr 2p₁/₂). Deconvolution of the Cr 2p₃/₂ envelope reveals multiple chemical states: the component centered at ~575.5 eV is assigned to Cr-N bonds in CrN, while peaks located at higher binding energies are attributed to oxidized chromium species, including Cr₂O₃ and non-stoichiometric CrO_x species [30]. A clear compositional trend is observed with an increasing N₂/(N₂ + Ar) ratio. Samples prepared with nitrogen ratios ≥ 60% exhibit a higher relative intensity of the Cr-N component (74.3%) and a reduced contribution from Cr-O species (25.7%) compared to those deposited at ≤50% nitrogen. This shift in chemical state distribution indicates enhanced nitridation and improved phase stoichiometry under optimized nitrogen supply, resulting in reduced oxygen incorporation and suppressed surface oxidation during deposition.

Figure 5. XPS spectra of CrAlN coatings (a) Cr 2p, (b) Al 2p, (c) N 1s, (d) O 1s at different N₂ ratios.

The high-resolution Al 2p spectra of the coatings prepared at different nitrogen ratios are presented in Figure 5b. The spectra can be deconvoluted into two primary components: a dominant peak centered at ~74.2 eV corresponding to Al-N bonding in AlN, and a secondary peak near ~74.9 eV attributed to Al-O species in non-stoichiometric AlO_x [31,32]. As the N₂/(N₂ + Ar) ratio increases, the relative intensity of the Al-N component progressively rises and reaches a maximum proportion of 89.3%, while the contribution from Al-O species gradually decreases to a minimum value of 10.7%. This evolution indicates enhanced nitridation efficiency and improved chemical stoichiometry under higher nitrogen supply, leading to preferential formation of the AlN phase. The compositional trend observed in XPS is consistent with the GIXRD results (Figure 4), where enhanced crystallinity and strengthened diffraction features associated with nitride phases were detected at optimized nitrogen ratios. Together, these findings confirm that nitrogen flow regulation plays a critical role in controlling phase constitution and suppressing oxide incorporation during film growth.

Figure 5c presents the high-resolution N 1s spectra of the CrAlN coatings deposited at different nitrogen ratios. At a 40% nitrogen ratio, the dominant peak located at ~396.8 eV is attributed to N-Al bonding in AlN, while a broader component centered near ~399.7 eV can be assigned to N-O-Cr(Al) configurations or chemisorbed nitrogen-containing species, possibly originating from surface oxidation or post-deposition exposure [33]. With increasing N₂/(N₂ + Ar) ratio, the component near ~396.6 eV, corresponding to N-Cr bonding in CrN, becomes progressively more pronounced, indicating enhanced formation and increased fraction of the CrN phase. This evolution is consistent with the GIXRD results, which show improved crystallinity and strengthened diffraction features of CrN at optimized nitrogen ratios. Concurrently, subtle shifts in peak position and relative intensity suggest a redistribution of nitrogen between Cr-N and Al-N bonds, reflecting compositional and structural adjustment within the CrAlN solid solution.

The deconvoluted O 1s spectra (Figure 5d) exhibit two principal components at approximately 530.5 eV and 531.8 eV, which are attributed to lattice oxygen in Cr₂O₃/Al₂O₃ and non-stoichiometric surface oxide species, respectively. The relative decrease in these oxygen-related contributions at intermediate nitrogen ratios further supports the suppression of oxide incorporation and the enhancement of nitride phase stability under optimized nitrogen flow conditions.

Figure 6 presents the cross-sectional transmission electron microscopy (TEM) image and corresponding selected-area electron diffraction (SAED) pattern of the CrAlN coating deposited at the optimal 60% nitrogen ratio. The sample were obtained via FIB milling. The cross-sectional TEM image in Figure 6a reveals a well-defined bilayer architecture consisting of a Cr interlayer (~180 nm) and the CrAlN functional layer above it. The Cr interlayer exhibits a typical columnar morphology growing perpendicular to the TA1 substrate surface, providing a transitional bonding layer that enhances adhesion and accommodates lattice mismatch. The overlying CrAlN layer shows dense columnar growth with limited intercolumnar voids, supporting the conclusion that the 60% nitrogen ratio promotes a compact microstructure favorable for corrosion protection. As shown in Figure 6b, the SAED pattern exhibits a series of concentric diffraction rings that can be indexed to the (110)/(200)/(100)/(220) family planes of the fcc CrN-based structure with AlN incorporation. The ring-like morphology confirms the polycrystalline nature of the coating and indicates the absence of strong epitaxial texture, consistent with the GIXRD results (Figure 4). The relatively sharp and continuous diffraction rings further suggest improved crystallinity under the optimized nitrogen condition.

Figure 6. The TEM results of CrAlN Coating prepared at 60% N₂ ratio: (a) cross-sectional image and (b) SAED pattern.

3.2. Properties and Corrosion Behavior of CrAlN Coatings

Figure 7 presents the water contact angle measurements of the CrAlN coatings deposited at different nitrogen ratios. The contact angles marked in the figure were measured in five replicate tests, with the standard deviation within 3°. The contact angle increases progressively from 40% to 60% N₂/(N₂ + Ar), followed by a decrease at 70%, indicating a non-monotonic dependence on nitrogen content. The maximum contact angle is observed at 60%, reflecting enhanced surface hydrophobicity under the optimized deposition condition. The increased contact angle suggests reduced surface wettability and lower effective surface energy, which can limit electrolyte spreading and decrease the residence time of corrosive species on the coating surface. Such hydrophobic characteristics are beneficial for corrosion mitigation, as they reduce the probability of localized electrolyte accumulation and restrict ionic penetration into surface defects. Therefore, the improved wettability behavior at 60% nitrogen ratio contributes synergistically to the enhanced corrosion resistance observed in electrochemical measurements.

Figure 7. Contact Angle images of CrAlN coatings with N₂ ratios of 40% (a), 50% (b), 60% (c) and 70% (d).

Regarding mechanical integrity, adhesion between the coating and substrate is primarily facilitated by mechanical interlocking and interfacial cohesion provided by the Cr interlayer. Scratch testing was conducted to quantitatively assess the adhesion strength of the coatings deposited on the TA1 substrate. As illustrated in Figure 8 and summarized in Table 4, the coating prepared at a 60% N₂/(N₂ + Ar) ratio exhibits the highest critical failure load (Lc₃), reaching 6283 mN at a scratch distance of 1176 μm. Compared to the 40% ratio specimen (4840 mN), the 60% ratio specimen shows better film-substrate adhesion. The enhanced adhesion at this nitrogen ratio can be attributed to the optimized microstructure and improved phase stoichiometry, which reduce internal defects and promote load transfer across the coating-substrate interface. In contrast, coatings deposited at lower or higher nitrogen ratios exhibit comparatively reduced critical loads, likely due to increased structural defects or stress-induced microstructural instability. The superior adhesion at 60% nitrogen is consistent with its dense morphology and contributes to improved mechanical stability under corrosive service conditions.

Figure 8. (a) Scratch morphology and (b) relationship between critical load and force/frictional load of CrAlN coatings prepared under different nitrogen ratios.

Table 4. Mechanical properties of CrAlN coatings prepared with different N₂ ratios.

Nitrogen Ratios (%)	Adhesion Strength Lc3 (mN)	Standard Deviation (mN)
40%	4840	±41.12
50%	6169	±31.58
60%	6283	±29.43
70%	6227	±27.31

The potentiodynamic polarization curves are shown in Figure 9a, and the extracted electrochemical parameters are summarized in Table 5. The corrosion current density (i_corr) serves as a quantitative indicator of corrosion kinetics, with lower values reflecting suppressed anodic dissolution and improved barrier performance. As the N₂/(N₂ + Ar) ratio increases from 40% to 60%, i_corr decreases markedly from 8.139 μA·cm⁻² to 0.297 μA·cm⁻², indicating a substantial reduction in corrosion rate. A slight increase to 0.303 μA·cm⁻² is observed at 70%, suggesting the onset of microstructural deterioration.

Figure 9. Electrochemical tests of CrAlN coatings prepared with different N₂ ratios: (a) Potentiodynamic polarization curves (Tafel plots), (b) Nyquist plots of impedance and (c) Bode plots of impedance.

Table 5. Electrochemical corrosion properties of CrAlN coatings prepared with different N₂ ratios.

Nitrogen Ratios (%)	βa (V/dec)	βc (V/dec)	Ecorr (V)	icorr (μA·cm−2)	Rp (kΩ·cm2)
40%	0.257	0.137	−0.819	8.139	4.774
50%	0.250	0.087	−1.144	1.06	26.473
60%	0.163	0.176	−0.843	0.297	123.884
70%	0.146	0.149	−0.927	0.303	105.815

The polarization resistance (R_p), calculated using the Stern-Geary equation (Equation (2)) [31], further confirms this trend. The coating deposited at 60% nitrogen exhibits the highest R_p (123.884 kΩ·cm²), nearly 26 times higher than that of the 40% sample, demonstrating optimal electrochemical stability.

$$R_p={\displaystyle \frac{{\beta }_c\times {\beta }_a}{2.3\times i_{corr}\times \left({\beta }_c+{\beta }_a\right)}}$$

(2)

where β_c and β_a represent the slopes of the cathodic and anodic curves, respectively, and i_corr denotes the corrosion current density.

Electrochemical Impedance Spectroscopy (EIS) results (Figure 9b,c) corroborate this trend. The Nyquist plots display depressed capacitive semicircles characteristic of non-ideal capacitive behavior. The largest semicircle diameter is observed at 60% nitrogen, indicating maximum polarization resistance (R_po). Bode plots reveal higher impedance modulus at low frequencies for this condition, reflecting enhanced barrier performance. Equivalent circuit fitting (Figure 10) shows that the constant phase element exponent (ncoat) reaches its highest value (0.9206) at 60% nitrogen (Table 6), approaching ideal capacitive behavior and suggesting a more homogeneous and defect-free coating surface.

Figure 10. Equivalent circuit diagram for EIS fitting.

Table 6. Equivalent circuit fitting parameters of EIS spectra for CrAlN coatings prepared with different N₂ ratios.

Nitrogen Ratios (%)	Rs (Ω·cm2)	CPEcoat (Ω−1·cm2·sn)	ncoat	Rpo (kΩ·cm2)
40%	4.0596	0.000257	0.7581	5.339
50%	4.2238	0.000137	0.8949	28.754
60%	4.0631	0.000119	0.9206	122.164
70%	4.3555	0.000314	0.8724	100.857

The CPEcoat (Q) value also reaches its minimum at 60% nitrogen, indicating that the coating exhibits the highest compactness and the smallest effective contact area with the electrolyte, thus demonstrating the optimal barrier performance against corrosive species. In line with the EIS fitting results, the CPEcoat value of the 70% nitrogen ratio sample is notably higher than that of the 60% sample, while the ncoat value decreases to 0.8724, deviating further from ideal capacitive behavior. This quantitative variation directly indicates an increased defect density and reduced structural homogeneity in the 70% N₂ coating, which creates more available diffusion pathways for corrosive ions and consequently leads to slightly inferior corrosion resistance compared with the optimal 60% nitrogen condition.

The optimized CrAlN coating (60% N₂) delivers an ultra-low corrosion current density of 0.297 μA·cm⁻² and a high polarization resistance of 123.9 kΩ·cm² in the target highly alkaline chloride-containing environment (pH ≈ 14), outperforming the corrosion data of CrAlN-based coatings reported in previous literature (e.g., 0.535 μA·cm⁻² in Ref. [26]). This marked enhancement in corrosion performance, particularly under the harsh alkaline conditions mimicking industrial seawater electrolysis, underscores the practical application value of the optimized coating (Table 7).

Table 7. Comparison of corrosion parameters obtained from potentiodynamic polarization measurements for various nitride coatings.

Coating System	Preparation Method	Corrosion Environment	icorr (μA·cm−2)	Ref.
CrN	Inductively Coupled Plasma Magnetron Sputtering	3.5 wt.% NaCl	4.6	[34]
CrN	Reactive DC Magnetron Sputtering	3.5 wt.% NaCl	1.73	[35]
ZrN	Reactive High-power Impulse Magnetron Sputtering	3.5 wt.% NaCl	0.49	[36]
TiN	Multi-arc Ion Plating	3.5 wt.% NaCl	0.51	[37]
Ti/Al2O3	Reactive Mag-netron Sputtering	3.5 wt.% NaCl	6.3	[38]
CrAlN	Reactive DC Mag-netron Sputtering	3.5 wt.% NaCl	1.03	[25]
Max-Cr2AlN	Unbalanced Magnetron Sputtering + Anneal	Simulated PEMFC Environment	0.535	[28]

Notably, the Rs values exhibit negligible variation, indicating that the test conditions were essentially consistent across all samples and that measurement errors were minimal.

To evaluate long-term stability, the optimal coating (60% nitrogen) was subjected to 96 h immersion in simulated alkaline seawater. The open circuit potential evolution (Figure 11a) shows an initial negative shift followed by gradual stabilization at approximately −0.3 V, indicating the formation of a protective surface film. The transient potential drop corresponds to initial surface activation, whereas subsequent stabilization suggests passivation. Post-immersion polarization curves (Figure 11b) show a positive shift in Ecorr (from −0.75 V to −0.63 V) with minimal variation in i_corr, indicating stable electrochemical performance after prolonged exposure. This behavior confirms the formation of a chemically stable passive layer.

Figure 11. Open circuit potential (a) and potentiodynamic polarization tests (b) before and after corrosion for the CrAlN coating prepared under a 60% nitrogen ratio in a corrosive environment.

EDS and XPS analyses (Figure 12) reveal significant surface chemistry evolution after immersion. EDS indicates decreased Al content and increased oxygen concentration. High-resolution Al 2p spectra display a dominant peak at ~74.9 eV corresponding to Al-O bonding, confirming the formation of Al₂O₃-rich species. The Cr 2p spectra exhibit oxidized chromium states, including CrO₃ and CrO_x, and a peak at ~580.5 eV associated with Na-Cr-O species (Na₂CrO₄). These findings suggest that alkaline exposure promotes partial oxidation of CrN and AlN phases, resulting in a composite passive film composed of aluminum oxides and chromium-based oxides/chromates. This in situ formed composite layer acts as a chemically stable and ion-impermeable barrier, consistent with the electrochemical observations.

Figure 12. EDS results of the CrAlN coating (a,b) and XPS analysis showing the full spectrum (c), high-resolution Al 2p spectrum (d), and high-resolution Cr 2p spectrum (e), before and after corrosion testing.

To further evaluate the service life of the coating in a seawater electrolysis environment, the optimal coating (60% N₂) was subjected to potentiostatic polarization at 2 V for 6 h in simulated alkaline seawater. As shown in Figure 13a, the polarization current decreased rapidly within the initial half hour and then stabilized at approximately 80 μA·cm⁻², indicating that the surface reactions of the coating tended to be steady. The Tafel plots recorded after potentiostatic polarization (Figure 13b) exhibited a slight positive shift in E_corr (about 0.05 V) and a negligible change in i_corr. These results are consistent with those of the immersion tests, further verifying that this sample possesses excellent long-term corrosion resistance and durability.

Figure 13. Potentiostatic polarization curves (a) and Potentiodynamic polarization curves (b) before and after potentiostatic polarization for the CrAlN coating prepared under 60% nitrogen ratio in a corrosive environment.

Figure 14 schematically illustrates the corrosion mechanisms under different nitrogen ratios. In nitrogen-deficient coatings, under-stoichiometric Cr_xN/Al_xN phases contain higher vacancy concentrations and intercolumnar defects. These structural discontinuities serve as rapid diffusion pathways for Cl⁻ and OH⁻ ions, leading to localized degradation and eventual substrate exposure. At the optimal 60% nitrogen ratio, near-stoichiometric CrN and AlN phases form with refined grains and reduced defect density. The dense microstructure limits ionic transport through tortuous grain boundary pathways. Simultaneously, a dual-passivation mechanism operates:

Figure 14. Schematic diagrams of corrosion mechanism of (a) Ti substrate; (b) CrAlN coating prepared with low nitrogen-to-argon ratio; and (c) CrAlN coating prepared with high nitrogen-to-argon ratio.

• The CrN matrix provides electronic stability and forms chromium oxides/chromates under alkaline conditions.
• The AlN phase preferentially oxidizes to produce a dense Al₂O₃-rich layer that seals nanoscale defects and grain boundaries.

This synergistic interaction between structural densification and chemically stable oxide formation significantly enhances corrosion resistance.

As evidenced by the impedance fitting data, excessive nitrogen (e.g., 70%) likely introduces elevated residual stress or microcracking, increasing defect density and slightly reducing barrier performance. Therefore, the superior corrosion resistance at 60% nitrogen arises from the optimized balance between phase stoichiometry, microstructural compactness, and formation of a stable dual-oxide passivation layer, ensuring sustained protection of the titanium substrate in highly alkaline chloride environments.

The superior corrosion resistance achieved at the optimal 60% nitrogen ratio originates from the synergistic coupling of the CrN and AlN phases and the resulting microstructural densification. As schematically illustrated in Figure 13, the nitrogen flow ratio governs phase stoichiometry and defect evolution, thereby dictating barrier performance. At low nitrogen ratios, nitrogen-deficient Cr_xN and Al_xN phases form, accompanied by a high density of vacancies, columnar gaps, and intergranular defects. These structural discontinuities serve as rapid diffusion channels for aggressive Cl⁻ and OH⁻ ions, accelerating electrolyte penetration and substrate attack.

In contrast, deposition at 60% nitrogen approaches near-stoichiometric fcc-CrN and AlN formation, enhancing thermodynamic stability and promoting grain refinement. The resulting compact, fine-grained architecture increases diffusion tortuosity and suppresses through-thickness transport pathways. The protective mechanism operates through a dual-barrier effect: the CrN matrix ensures mechanical robustness and electronic passivation, while AlN preferentially oxidizes to form a dense Al₂O₃-rich layer in alkaline media. This in situ oxide film seals nanoscale defects and grain-boundary grooves, producing a chemically stable composite surface that effectively impedes electrolyte ingress. When the nitrogen ratio exceeds the optimum (e.g., 70%), excessive residual stress and potential microcracking disrupt this synergy, leading to reduced protection. Therefore, the 60% nitrogen ratio provides the optimal balance between stoichiometry, structural compactness, and interfacial stability, ensuring sustained protection of the titanium substrate under simulated alkaline seawater electrolysis conditions.

4. Conclusions

CrAlN coatings were deposited on TA1 titanium alloy by magnetron sputtering with nitrogen flow ratios of 40%–70%. The nitrogen ratio was identified as the critical parameter governing phase constitution, microstructure, adhesion, and corrosion performance. It was found that the coating prepared at 60% nitrogen exhibited a dense, fine-grained structure with well-developed fcc-CrN and AlN phases. Electrochemical tests demonstrated that this sample achieved the lowest corrosion current density (0.297 μA·cm⁻²) and the highest polarization resistance (123.884 kΩ·cm²), indicating superior barrier efficiency and long-term stability. Compared with previously reported CrAlN coatings evaluated in neutral NaCl solution or standard artificial seawater, the optimized coating in this work presents more superior corrosion resistance in the highly alkaline chloride-containing environment that is highly consistent with the actual service conditions of alkaline seawater electrolysis.

This study reveals the regulation mechanism of nitrogen flow ratio on the microstructure and corrosion passivation of CrAlN coatings, providing a feasible design strategy and experimental basis for developing high-performance protective coatings for seawater electrolysis equipment.