Corrosion Resistance of Ti Coatings, Cr Coatings, and Ti/Cr Multilayer Coatings Prepared on 7050 Aluminum Alloy by Magnetron Sputtering

Abstract

Al7050 aluminum alloy substrates were coated with Cr, Ti, and Ti/Cr multilayer films via direct current magnetron sputtering to enhance their corrosion resistance. A comprehensive analysis, employing SEM and XRD, characterized the coatings’ morphology and composition, while electrochemical experiments assessed their corrosion performance. The Cr coating, despite exhibiting the largest thickness (588 nm), revealed a porous microstructure with inherent structural weaknesses. Conversely, the Ti coating, while possessing a dense structure, presented a significantly reduced thickness (96 nm). The Ti/Cr multilayer coating, with a thickness of 242 nm, achieved an optimal balance between structural density and overall thickness. Critically, the layered architecture of the Ti/Cr multilayer coating effectively impeded crack propagation and facilitated the formation of tortuous corrosion pathways. This intricate pathway morphology significantly hindered the diffusion of the corrosive medium, resulting in a notably low corrosion current density of 3.83 × 10⁻⁷ A·cm⁻². Comparative analysis revealed that the corrosion current density of the Ti/Cr multilayer coating was substantially lower than that of both the Cr and Ti coatings, demonstrating improvements of 2386% and 222%, respectively. These findings underscore the pivotal role of the multilayer structure in augmenting the corrosion resistance of aluminum alloys by providing a superior barrier to corrosive medium.

1. Introduction

Aluminum and aluminum alloys have garnered increasing attention due to their low density, high strength-to-weight ratio, and excellent thermal conductivity. With their superior mechanical properties, relatively low production and processing costs, and good recyclability, they have taken a leading position in the production and manufacturing of various products [1,2]. However, corrosion of aluminum alloys is the primary mode of equipment failure, with the economic costs resulting from material corrosion damage estimated at approximately 3–5% of annual gross domestic product [3]. Therefore, there is a need to find a solution that provides excellent corrosion protection for aluminum alloys under harsh working conditions. Many researchers have explored various methods, such as adding rare earth elements to aluminum alloys, multi-stage aging treatment, and severe plastic deformation to enhance corrosion resistance [4,5,6]. Coating technology has been proven to be one of the most effective methods for extending the service life and reliability of metal alloys parts. It achieves this by depositing corrosion-resistant coatings that isolate the substrate from corrosive media without altering the substrate’s properties [7,8]. As a result, various protective coatings are widely used in industries such as automotive manufacturing, marine engineering, aerospace, and other.

Coatings prepared by physical vapor deposition (PVD) are widely used due to their compact structure, uniform composition, and low roughness [9]. Among these, Cr coatings have been proven to effectively enhance the service life and reliability of metal parts made of steel and various alloys by protecting them from chemical corrosion and increasing their wear resistance [10,11,12]; Ti coatings, with their excellent mechanical properties and superior corrosion resistance (forming a robust protective oxide layer on the metal surface), are widely applied for protecting critical components in corrosive environments [13,14,15]. However, despite the many superior properties of Ti coatings and Cr coatings, structural defects (such as pores and cracks) caused by the continuous growth of columnar crystals during coating process often allow corrosive media (such as Cl⁻ and SO₄²⁻) to penetrate the substrate, thereby reducing the corrosion resistance of the protected parts [16,17]. To overcome these defects, researchers have attempted to develop multilayer coatings to achieve reliable corrosion resistance and reduce interfacial contact resistance. Multilayer coatings can exhibit layered interfaces parallel to the coating surface within the coating structure, overcoming most coating defects and providing the coating with high corrosion resistance [18]. Liu et al. [19] compared the structure and corrosion behavior of VN coatings and multi-layer VN/C coatings prepared on 316L steel via PVD. They found that the C nanolayer disrupted the growth of large grains in the VN layer, thereby enhancing coating toughness and suppressing crack initiation, while the nano-multi-layer structure hindered the penetration of corrosive media. Onur [20] prepared TiN, TiAlN coatings, and TiAlN/TiN multilayer coatings on the surface of Ti45Nb and analyzed their corrosion resistance. The results showed that compared to single-layer coatings, multilayer coatings have finer grain sizes and more stable coating structures, thereby improving corrosion resistance. Additionally, the wetting characteristics of different coating surfaces also influence corrosion resistance. Xie et al. [21] deposited AlN coatings and (Al/AlN)₅ multilayer coatings on AZ31 magnesium alloy and analyzed their corrosion resistance. The superior corrosion resistance of the multilayer coatings was attributed to their denser layered structure, better crack propagation resistance, and deformation coordination between the coating and substrate during corrosion.

While multilayer coatings have demonstrated effectiveness in enhancing the corrosion resistance of materials like steel, titanium, and magnesium alloys, a comprehensive comparison of Cr, Ti, and Ti/Cr multilayer coatings applied directly to aluminum alloy substrates remains limited. Addressing this gap, the present study employs direct current magnetron sputtering to deposit Ti, Cr, and Ti/Cr multilayer coatings onto 7050 aluminum alloy. This research investigates the structural characteristics and corrosion protective behavior of these coatings in 3.5% NaCl solution environment. Furthermore, by analyzing electrochemical mechanisms, the study aims to elucidate the corrosion evolution process and detailed corrosion mechanisms associated with each coating type—Ti, Cr, and the Ti/Cr multilayer configuration—providing a deeper understanding of their respective strengths and weaknesses in protecting aluminum alloys from corrosion. This in-depth analysis will contribute to the development of optimized coating strategies for aluminum alloys in harsh marine environments.

2. Materials and Methods

2.1. Substrate Preparation

A sample made of 7050 aluminum alloy (Southwest Aluminium (Group) Co., Ltd., Chongqing, China) with dimensions of Ø28 mm × 3 mm was used as the substrate. The substrate was progressively ground using SiC sandpaper with grit sizes ranging from #400 to #2000, followed by mechanical polishing with diamond slurry. Subsequently, the substrate samples were ultrasonically cleaned with 99.5% acetone for 10 min. It is worth noting that, to characterize the microstructure of the coating more efficiently, silicon wafers (P-type, crystal orientation (100), resistivity <0.02 ohm) were also used for coating preparation, with the same deposition conditions as the aluminum substrate.

2.2. Coating Deposition

A direct current magnetron sputtering system (MSP-300C, Beijing Chuangshi Weina Technology Co., Ltd., Beijing, China) was used to deposit the coating on silicon wafers and on aluminum substrates that have been ground and polished. The system’s base vacuum level is set to 8 × 10⁻⁴ Pa, followed by the introduction of high-purity Ar gas to maintain a pressure of 0.3 Pa. The deposition is performed at room temperature by controlling the system to sputter a high-purity Ti target (99.995%, ZhongNuo Advanced Material, Beijing, China) and Cr target (99.995%, ZhongNuo Advanced Material, Beijing, China). Both the Ti target and Cr target operate at a power of 200 W. Prior to coating deposition, each target is cleaned via argon ion sputtering for 10 min. Due to the differing sputtering rates of the Cr target and Ti target, the deposition times of the two targets are controlled to prepare Cr coatings, Ti coatings, and Ti/Cr multilayer coatings. The total deposition time for all three coatings was 80 min. Previous experiments have demonstrated that under similar sputtering conditions, the sputtering rates of Ti and Cr targets are approximately one-third of each other [22]. To avoid the influence of sublayer thickness in multilayer coatings, the Ti/Cr multilayer coating was prepared by alternately sputtering the Ti target (15 min) and Cr target (5 min) four times using a shutter system, with the outermost layer being the Cr coating.

2.3. Microstructural Characterization

The phase composition of the coated silicon wafer samples was examined using GIXRD analysis with a D/max-2500/PC diffractometer (DMAX-RB, Rigaku Corporation, Tokyo, Japan) equipped with Cu Kα radiation. The incident angle was 0.25°, the step size was 0.02°, the scanning speed was 4°/min, and the 2θ range was 20–90°. The surface morphology of coated aluminum alloy samples before and after corrosion, as well as the morphology and thickness of the cross-section of coated silicon wafer samples, were observed using a scanning electron microscope (SEM) equipped with energy-dispersive spectroscopy (EDS) (Hitachi SU5000, Tokyo, Japan).

2.4. Corrosion Resistance Study

At room temperature, the electrochemical performance of uncoated, Cr-coated, Ti-coated, and Ti/Cr multilayer-coated aluminum alloys was tested using a three-electrode electrochemical system (CHI 760D, Shanghai Chenhua Instrument Co., Ltd., Shanghai, China). A standard calomel electrode was used as the reference electrode, a platinum electrode as the counter electrode, and the coated samples as the working electrode, with an exposed area of 1 cm². First, the coated samples were immersed in a 3.5 wt.% NaCl solution for 30 min to ensure the test system was in equilibrium. Then, the impedance spectra of the samples were measured using a sinusoidal alternating signal with an amplitude of 10 mV as the excitation signal. The test frequency range is 10⁻² to 10⁵ Hz. After the impedance test, a dynamic potential polarization test is performed at a scan rate of 1.0 mV/s within the voltage range of −1.2 V to +1.2 V. For aluminum alloy, the dynamic potential polarization test is conducted at a scan rate of 1.0 mV/s within the voltage range of −2 V to 0 V. Each sample underwent three separate experiments to ensure experimental reproducibility.

3. Results

3.1. Coating Phase Structure

Figure 1 shows the XRD patterns of the coatings deposited on the Si substrate. The XRD analysis results indicate that the Cr coating exhibits (110) crystal plane preferential orientation, belonging to the body-centered cubic (BCC) structure; while the Ti coating exhibits (101) crystal plane preferential orientation, belonging to the hexagonal close-packed (HCP) structure. In the Ti/Cr multilayer coating, both Ti(101) and Cr(110) crystal plane-corresponding diffraction peaks were observed, indicating the coexistence of two-phase crystal structures in the multilayer structure [23]. Notably, all coating samples exhibit a shift in diffraction peaks toward higher angles, a phenomenon primarily attributed to internal stresses introduced during high-power magnetron sputtering, which cause lattice compression in the coating, reducing interplanar spacing and resulting in the shift in diffraction peaks toward higher angles [24]. Additionally, the width of the diffraction peaks observed in the 20–30° range in all patterns originates from the diffraction response of the Si substrate, a common feature in XRD patterns of silicon substrates [25].

3.2. Morphology of the Coatings

Figure 2 shows the microstructural organization of cross-sections of different coatings on silicon wafers. The thicknesses of the Cr coating, Ti coating, and Ti/Cr multilayer coating are approximately 588 nm, 94 nm, and 242 nm, respectively. This difference primarily stems from the varying deposition rates of the different target materials during the magnetron sputtering process, with the Ti target exhibiting a significantly lower deposition rate than the Cr target. Microscopic structural observations reveal that the Cr coating has a relatively loose structure with numerous pore defects visible within the coating; the Ti coating exhibits a dense columnar crystal structure. This difference primarily stems from the lower surface diffusion coefficient and higher sputtering yield of Cr atoms, leading to rapid deposition but insufficient atomic rearrangement, resulting in a loosely packed structure. In contrast, Ti atoms have higher surface mobility and stronger interatomic bonding, enabling the formation of a dense and uniform film despite the slower deposition rate [26]. The cross-sectional morphology of Ti/Cr multilayer coatings is regulated by the multilayer structure, which relatively suppresses the large-scale growth of columnar crystals while effectively bridging the pores in the Cr coating, resulting in a relatively dense and uniform columnar crystal structure.

Figure 3 shows the morphological characteristics of different coatings on the aluminum substrate surface. Due to the characteristics of the direct current magnetron sputtering process, the coating surface is generally smooth, with almost no obvious particle or droplet defects [27]. The Cr coating shown in Figure 3a exhibits a typical pyramid-shaped morphology, with visible through cracks on the surface. This defect is attributed to the accumulation of internal stress caused by high-power magnetron sputtering bombardment of the Cr target material, leading to coating cracking [28]; the Ti coating shown in Figure 3b exhibits a granular structure and is the most dense; The Ti/Cr multilayer coating shown in Figure 3c also exhibits a granular morphology. Although the outermost layer is a Cr sublayer, due to the template effect [29], its growth morphology on the Ti sublayer is similar to that of the Ti coating shown in Figure 3b, both exhibiting a granular structure. However, due to the inherent properties of the Cr layer, cracks defects can still be observed on the surface of this multilayer coating.

3.3. Electrochemical Properties

Figure 4 shows the open-circuit potential changes in the coated and uncoated aluminum alloy samples tested in a 3.5 wt.% NaCl solution for 1800 s. The open-circuit potential values of the coated samples were all higher than those of the uncoated aluminum alloy, indicating that the aluminum alloy had undergone surface modification through coating deposition. The open-circuit potential of the Cr coating samples decreases gradually with increasing test time and exhibits fluctuating characteristics, attributed to the penetration of the corrosion medium through defects into the substrate; the open-circuit potential of the Ti coating samples decreases gradually with increasing test time, as the Ti coating samples is too thin, shortening the corrosion medium’s path [15]. In the early stages of testing, the Ti/Cr multilayer coating samples experienced only minor corrosion due to the barrier effect at the multilayer interfaces, and this resistance lasted longer than that of the Cr coating and Ti coating samples. However, when the test time reached approximately 1200 s, the open-circuit potential began to decrease, as the corrosive medium penetrated the tortuous corrosion pathways and multilayer barriers to diffuse into the substrate, causing pitting corrosion [30].

Figure 5 shows the Potentiodynamic polarization curves of uncoated, Cr-coated, Ti-coated, and Ti/Cr multi-layer coated samples. The corrosion parameters obtained by Tafel extrapolation fitting of the polarization curves are listed in

Table 1. Tafel fitting parameters of Potentiodynamic polarization curves.

Samples	Ecorr (V)	Icorr (A·cm−2)	${\mathit{\beta }}_{\mathit{a}}$ (V/dec)	${\mathit{\beta }}_{\mathit{c}}$ (V/dec)	Rp, calculated values (Ω·cm2)
Cr	−0.737	9.14 × 10−6	0.06	2.48	1123
Ti	−0.773	8.50 × 10−7	0.04	0.88	8939
Ti/Cr	−0.760	3.83 × 10−7	0.04	0.47	12,940
Al7050	−1.216	3.46 × 10−5	3.30	0.15	61.8

. The corrosion current density of the aluminum alloy is one to two orders of magnitude higher than that of the coated samples. Theoretically, a lower corrosion current density typically indicates higher thermodynamic stability and superior corrosion resistance of the material, which helps delay the destruction and dissolution of the passivation film [31,32]. The bare aluminum alloy exhibits pitting corrosion behavior at approximately –0.875 V, indicating that the in situ-generated Al₂O₃ passivation film on its surface has been penetrated, losing its protective function for the aluminum alloy interior [33]. Cr and Ti coatings reduced the corrosion current density of the aluminum alloy (3.46 × 10⁻⁵ A·cm⁻²) to 9.14 × 10⁻⁶ and 8.50 × 10⁻⁷ A·cm⁻², respectively. This is attributed to the thermodynamically stable TiO₂ and Cr₂O₃ passivation films formed on the surface, which delay the corrosion medium’s erosion of the substrate [34,35]. It is worth noting that the Cr coating exhibited significant current fluctuations within the passivation zone, which is related to its porous structure. This can cause localized delamination of the passivation film on the surface, exposing pores and forming corrosion pits, thereby leading to sudden fluctuations in corrosion current density [36]. In contrast, the corrosion current density of the Ti/Cr multilayer coating samples further decreased to 3.83 × 10⁻⁷ A·cm⁻², demonstrating the best corrosion resistance performance. In addition, the polarization resistance was calculated using the Stern–Geary equation:

$$R_{p,\mathrm{c}\mathrm{a}\mathrm{l}\mathrm{c}\mathrm{u}\mathrm{l}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{d}\mathrm{v}\mathrm{a}\mathrm{l}\mathrm{u}\mathrm{e}\mathrm{s}}={\displaystyle \frac{{\beta }_a{\beta }_c}{2.303I_{corr}\left({\beta }_a+{\beta }_c\right)A}}$$

where β_a and β_c represent the anodic and cathodic Tafel slopes, respectively; I_corr denotes the corrosion current density; and A is the exposed surface area of the tested sample. This parameter reflects the difficulty of the corrosion reaction: in general, a larger R_{p, calculated value} corresponds to superior corrosion resistance of the coating [37]. As shown in Table 1, the Ti/Cr multilayer coating exhibits the highest calculated polarization resistance, reaching 12,940 Ω·cm².

The corrosion resistance of 7050 aluminum alloy substrates and coated samples was evaluated using electrochemical impedance spectroscopy, with the test results shown in Figure 6. The aluminum alloy substrate exhibited a capacitive loop in the high-frequency region and Warburg impedance characteristics in the mid- and high-frequency regions, indicating that corrosion products had formed on the aluminum substrate surface, diffusion processes had occurred, and the substrate had been severely corroded. Therefore, subsequent corrosion resistance rankings were performed solely on the coatings [38]. The impedance spectra of each coating sample exhibited similar shapes, with a capacitive loop in the high- and medium-frequency ranges and an inductive loop in the low-frequency range. This indicates that the corrosive medium can diffuse through defects in the coating to the coating/substrate interface, triggering electrochemical reactions [39]. Generally, the larger the capacitive arc radius, the better the material’s corrosion resistance [40]. The corrosion resistance performance follows the following order: Cr coating < Ti coating < Ti/Cr multilayer coating.

The electrochemical Bode plots of the coated samples in a 3.5 wt.% NaCl solution are shown in Figure 7. In the impedance-frequency curve, the impedance modulus values in the high-frequency and low-frequency regions, respectively, reflect the resistance of the solution and coating in the electrochemical system. The greater the difference between the impedance modulus values in the low-frequency and high-frequency regions, the higher the polarization resistance of the representative system [41]. As shown in Figure 7a, it is evident that the polarization resistance of the Ti/Cr multilayer coating is the highest, with the Ti coating exhibiting higher polarization resistance than the Cr coating. In the phase angle-frequency curve, a phase angle of −90° corresponds to an insulating layer with high resistance and low capacitance, indicating better protective performance of the coating [42]. The peak of the Ti/Cr multilayer coating is closest to −90°, followed by the Ti coating, while the Cr coating performs the worst. Additionally, phase angle peaks are observed in both the high-frequency and low-frequency regions for both Cr and Ti coatings, indicating dual-layer electrochemical behavior [43]. The time constants in the high-frequency region correspond to the capacitance characteristics of formed Cr₂O₃ and TiO₂, while the low-frequency region represents the charge transfer process between the coating and the aluminum alloy, indicating that the corrosive medium has reached the coating/substrate interface [44,45].

In Figure 8a, R_s denotes the solution resistance, while R_ct and CPE_dl are connected in parallel to form the main capacitance loop, corresponding to the non-ideal capacitance of the interfacial electric double layer and the charge transfer pathways induced by defect sites in the oxide film, respectively. This path is closely related to the enrichment of intermetallic compounds, which can easily lead to localized breakdown of the passivation film and exposure of the substrate. W₁ represents the Warburg impedance, indicating the corrosion process where the corrosive medium invades the aluminum alloy substrate at low frequencies, marking an important transition in the corrosion kinetics from the diffusion-controlled stage to the diffusion-controlled stage [46]. In Figure 8b, R_s denotes the solution resistance. R_f denotes the coating resistance, CPE_f denotes the coating capacitance, and the mid-frequency capacitive loop originates from the diffusion process of the coating on the alloy surface. R_ct is the charge transfer resistance associated with metal oxidation reactions, while CPE_Qdl represents the double-layer capacitance, describing the low-frequency capacitive loop. R_L and L describe the low-frequency inductive loop, reflecting the adsorption–desorption behavior of the coating during Faraday reactions and the current oscillations triggered in the initial stage of localized corrosion, attributed to corrosion nucleation in the initial stage of localized corrosion [47,48]. The chi-squared fitting coefficients for all circuits are in the 10⁻³ order of magnitude, indicating that the equivalent circuits have good matching. The electrochemical impedance spectroscopy (EIS) fitting results for the coating samples are summarized in

Table 2. EIS fitting results for coating samples in the equivalent circuit.

Coating Samples	Rs	CPEf		Rf	CPEQdl		Rct	RL	L
	(Ω·cm2)	Y0 (Ω−1·cm−2·sn)	n	(Ω·cm2)	Y0 (Ω−1·cm−2·sn)	n	(Ω·cm2)	(Ω·cm2)	-
Cr	6.816	2.11 × 10−3	0.614	83	9.56 × 10−5	0.791	325	1367	1345
Ti	7.513	1.99 × 10−5	0.759	116	1.77 × 10−3	0.717	5571	1086	16,191
Ti/Cr	10.58	1.96 × 10−5	0.858	10,708	1.59 × 10−4	0.749	3100	3674	31,182

.

The impedance of an electrochemical system is composed of the coating resistance R_f and the charge transfer resistance R_ct, i.e., the polarization resistance R_p can be calculated using the formula R_p = R_f + R_ct. Additionally, the polarization resistance R_p reflects the penetration resistance of the coating or oxide film and can be used to evaluate the protective performance of the coating or oxide film; the larger the R_p value, the stronger the corrosion resistance [49]. The R_p values for Cr coatings, Ti coatings, and Ti/Cr multilayer coatings are 408, 5687 and 13,808 Ω·cm², respectively. The corrosion resistance performance, from highest to lowest, is consistent with the results of the polarization curves.

3.4. Corrosion Morphology Analysis

Figure 9 and

Table 3. EDS Surface Scanning Analysis Results of the Coating and Substrate Surface After Electrochemical Testing.

Samples	Al (wt.%)	O (wt.%)	Cl (wt.%)	Cr (wt.%)	Ti (wt.%)
Cr	21.71	40.30	0.42	37.55	0.01
Ti	58.31	34.23	0.34	0.38	6.74
Ti/Cr	44.26	44.56	0.90	5.12	5.17
Al7050	36.54	60.10	3.18	0.05	0.14

show the EDS surface scan results of each sample after electrochemical corrosion testing. The Ti/Cr multilayer coating maintained its overall integrity after corrosion, with only minor corrosion traces appearing in localized areas. During the corrosion process, part of the surface Cr sublayer was consumed, exposing the Ti sublayer in localized areas. The surface element distribution was uniform, and Cl element signals were barely detected, indicating that the coating effectively blocked the penetration of the corrosive medium. In contrast, although the Ti coating has a dense structure, its thickness is relatively thin, resulting in complete exposure of the substrate during corrosion; the Cr coating, due to the presence of numerous pores, allows corrosion media to penetrate internally along defect pathways, leading to substrate exposure and corrosion. The exposed Al7050 alloy surface underwent severe corrosion, covered with extensive corrosion products, and showed significantly elevated Cl and O element contents, reflecting intense chloride ion erosion. As such, multi-layer structures demonstrate a clear advantage in enhancing coating density and corrosion resistance.

4. Discussion

The corrosion resistance of coatings is influenced by a combination of factors, primarily the number of defects penetrating the thickness, the length of the corrosion path, and the resistance to crack propagation [22]. This study systematically compared the structural characteristics and corrosion behavior of three coatings (Cr coating, Ti coating, and Ti/Cr multilayer coating), formed by a direct current magnetron sputtering process. The results indicate that corrosion resistance not only depends on the coating thickness but is also closely related to the coating’s structural density, interface structure, galvanic corrosion behavior, and defect control capability. First, although the Cr coating has the greatest thickness (approximately 588 nm), it tends to form columnar crystals and a loose structure with high porosity during deposition. These structural defects provide direct pathways for corrosion media penetration. In 3.5% NaCl solution environment, Cl⁻ ions easily diffuse into the coating along the pores, penetrate the coating to reach the substrate, and trigger pitting corrosion. In addition, due to the potential difference between chromium (Cr) and aluminum (Al), the potential difference drives the initial corrosion process, and under the action of microelectrochemical corrosion, the aluminum substrate is more susceptible to corrosion [15]. The significant enrichment of Cl elements in the Cr coating sample shown in Figure 9a confirms this corrosion mechanism. In contrast, although the Ti coating is the thinnest (only 94 nm), it has high atomic mobility, a slow and uniform film formation process, and forms a structurally dense columnar crystal structure, thereby possessing a certain corrosion barrier capability. Although Cl signals appear after corrosion, no large-area corrosion products are observed in Figure 9b, indicating that the Ti coating can effectively delay the diffusion of the corrosion medium in the short term. However, due to the limited coating thickness, once local surface damage occurs, corrosion pathways rapidly penetrate to the substrate, thereby limiting its long-term protective performance [50]. Notably, the Ti/Cr multilayer coating demonstrates the best corrosion resistance performance. The multilayer design effectively interrupts the continuity of defects (such as pores and cracks) in the Cr sublayer, resulting in a more compact coating structure. SEM cross-sectional analysis also confirms its uniform columnar crystal structure. Each Ti/Cr interface can be regarded as a “refraction surface” for corrosion pathways, altering the direction of corrosion medium transmission and forming a “meandering diffusion mechanism,” thereby significantly increasing the diffusion distance and time of corrosion pathways [51]. This mechanism is validated in the EDS of Figure 9c, where almost no Cl elements are detected penetrating into the substrate. The introduction of the Ti layer partially reduces the potential difference between the Cr layer and the Al substrate, effectively mitigating the localized anodic reaction caused by galvanic corrosion; simultaneously, since the outermost layer is Cr, the Ti layer acts as a “barrier intermediate layer” to further delay the attack process of Cl⁻. Additionally, the multilayer structure facilitates crack passivation and deflection. When cracks traverse the hard-soft layer interfaces, they are often deflected or terminated due to differences in elastic modulus and crystal orientation, thereby preventing the formation of through cracks in the coating during service and enhancing the overall structural stability [52].

5. Conclusions

In this study, Cr coatings, Ti coatings, and Ti/Cr multilayer coatings were prepared on the surface of Al7050 aluminum alloy using direct current magnetron sputtering technology. The structural characteristics and corrosion resistance of these coatings were systematically investigated. The results showed that although the Cr coating had the greatest thickness, its structure was loose and contained a large number of pore defects. The Ti coating exhibited a dense structure but had the smallest thickness and was susceptible to localized corrosion penetration. In contrast, the layered structure of the Ti/Cr multilayer coating disrupts the continuity of pores in the Cr coating, effectively blocking corrosion pathways, while also increasing the overall thickness of the Ti layer, enhancing its barrier performance. Electrochemical test results showed that the corrosion current density of the uncoated Al7050 aluminum alloy reached 3.46 × 10⁻⁵ A·cm⁻², with significant corrosion dissolution observed on the surface; after protection by Cr and Ti coatings, the corrosion current densities were reduced to 9.14 × 10⁻⁶ and 8.50 × 10⁻⁷ A·cm⁻², respectively, while the corrosion current density of the Ti/Cr multilayer coating further decreased to 3.83 × 10⁻⁷ A·cm⁻², demonstrating the most outstanding corrosion resistance. This performance improvement is not only attributed to the blocking effect of pore continuity in the multilayer structure but also to the Ti/Cr multilayer coating weakening the galvanic corrosion driving force between Cr and the Al7050 substrate, while extending the corrosion medium’s transmission path within the Ti layer, thereby enhancing its corrosion resistance.

Further research will focus on optimizing the individual sublayer thicknesses and the number of Ti/Cr bilayers to maximize long-term corrosion protection. Specifically, investigations will explore the impact of varying sputtering parameters, such as substrate temperature and gas pressure, on the multilayer coating’s microstructure, adhesion strength, and mechanical properties. The potential of incorporating corrosion inhibitors or using reactive sputtering to introduce alloying elements into the layers will also be explored to further enhance the corrosion resistance and tailor the coating properties for specific environmental conditions. Long-term exposure testing in simulated and real-world corrosive environments is crucial to validate the performance of optimized multilayer coatings and assess their suitability for industrial applications.