High Corrosion Resistance of Ti₃C₂T_x/Al6061 Composites Achieved via Equal Channel Angular Pressing

Abstract

This study systematically investigates the synergistic corrosion resistance enhancement mechanisms in aluminum matrix composites (AMCs) through the combined implementation of equal channel angular pressing (ECAP) and Ti₃C₂T_x MXene reinforcement. The results demonstrate that ECAP treatment significantly refines the microstructure, reducing grain sizes to an average of 8.7 µm after three passes, while improving mechanical properties such as hardness by 40.6–45.1%. Additionally, the incorporation of Ti₃C₂T_x enhances corrosion resistance by establishing a physical barrier that impedes the diffusion of corrosive mediators and prevents localized corrosion. Electrochemical tests reveal that the composite subjected to three ECAP passes exhibits the lowest corrosion current density (I_corr) and a remarkable 3.4-fold increase in charge transfer resistance (R_ct) compared to untreated material. These findings highlight the potential of synergistically integrating ECAP and Ti₃C₂T_x to develop high-performance AMCs with enhanced mechanical strength and corrosion resistance, offering significant implications for applications in marine equipment, aerospace, and new energy vehicles.

1. Introduction

Al alloys have long been recognized as critical materials in various industrial applications due to their unique combination of properties, including low density, high strength, and excellent corrosion resistance [1,2,3,4]. In the domain of alloy research, Al6061 has been identified as a material of particular significance due to its versatility. The substance has gained a notable presence in a variety of industries, including aerospace, automotive, and construction, owing to its unique set of properties [5,6]. However, Al and its alloys are susceptible to localized corrosion (e.g., pitting, intergranular corrosion) in humid, salty, or strongly acidic and alkaline environments, which severely limits their long-term service performance under harsh operating conditions [7,8]. While the corrosion resistance of Al matrix can be enhanced to a certain extent through alloying, surface coating, or the addition of traditional reinforcing phases (e.g., carbon fibers, silicon carbide particles), these methods frequently encounter challenges such as uneven dispersion of reinforcing phases, inadequate interfacial bonding, or a solitary corrosion protection mechanism. This results in a constrained protective effect and complicates the consideration of enhanced mechanical properties [9,10,11]. Consequently, the exploration of novel reinforcing phases to achieve synergistic optimization of the mechanical and corrosion resistance properties of AMCs has become a pivotal research direction in this field.

In recent years, two-dimensional transition metal carbon-nitrides (MXenes) have demonstrated considerable potential as an emerging reinforcing phase in the field of composites. This is due to their distinctive layered structure, high specific surface area, excellent chemical stability, and tunable surface functional groups. Among them, MXene is of particular significance, with the chemical formula Ti₃C₂T_x (T_x stands for surface functional groups, e.g., -O, -OH, -F), which has attracted much attention due to its unique lamellar structure, excellent mechanical properties, and good metal affinity [12,13,14,15]. Ti₃C₂T_x nanosheets have been shown to effectively prolong the corrosion resistance of corrosive mediators’ (e.g., H₂O, Cl⁻) diffusion path in the matrix, and the lamellae themselves are chemically stable. The integration of these elements into metal matrices, particularly AMCs, is anticipated to establish a substantial physical barrier, thereby impeding the onset of corrosion and preventing expansion [16,17]. This approach offers a novel concept for the development of a new generation of high-performance AMCs with enhanced corrosion resistance. ECAP, a typical severe plastic deformation (SPD) technique, has proven highly effective in addressing the aforementioned challenges. The principal advantage of ECAP lies in its unique capability to impose significant pure shear strains via specially designed angular channels within the die while maintaining the original cross-sectional dimensions of the workpiece. Through multi-pass shear deformation, this process achieves substantial grain refinement, enables precise microstructure regulation, and facilitates uniform dispersion of reinforcements within the matrix [18,19,20,21]. Experimental studies confirm that ECAP not only enhances material strength and toughness but also improves corrosion resistance by eliminating porosity, optimizing grain boundary architectures, and strengthening interfacial bonding between reinforcing phases and the matrix [22,23]. For instance, ECAP-processed Al alloys exhibit densified surface passivation films and reduced corrosion rates, primarily attributed to grain size refinement and increased dislocation density. Nevertheless, synergistic integration of the ECAP process with Ti₃C₂T_x reinforcement to modulate microstructural evolution in AMCs—thereby achieving breakthroughs in corrosion resistance—remains scarcely explored in the existing literature [24,25].

In light of this background, this study synergistically combines the ECAP process with Ti₃C₂T_x reinforcement in AMCs to investigate their combined effect on corrosion resistance. Utilizing scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) characterization, we elucidate the effects of grain refinement, Ti₃C₂T_x dispersion, and second-phase particle size on corrosion behavior. Furthermore, pitting corrosion behavior was investigated in a simulated corrosive environment (3.5 wt.% NaCl solution) using electrochemical impedance spectroscopy (EIS), potentiodynamic polarization curves, and surface morphology analysis after corrosion. This work presents a novel approach for developing lightweight materials with enhanced corrosion resistance, holding significant potential for applications in marine equipment and new energy vehicles.

2. Materials and Methods

2.1. Experimental Materials

In this experiment, Al6061 alloy (~0.99 wt.% Mg, ~0.55 wt.% Si, ~0.33 wt.% Fe, ~0.20 wt.% Cu, and ~0.12 wt.% Cr for the main alloying elements) was chosen as the base material. The Al powder (purity > 99%, particle size: 100–200 mesh) used for the preparation of the intermediate alloy was obtained from Changsha Tianju Metal Materials Co., Ltd. (Changsha, China). The MAX phase Ti₃AlC₂ precursor for the preparation of Ti₃C₂T_x was purchased from Forsman Technology Co., Ltd. (Beijing, China). The as-received Ti₃AlC₂ particles displayed a lamellar morphology with lateral dimensions of 2–7 µm. Hydrofluoric acid (HF, 49 wt.%) and dimethyl sulfoxide (DMSO) for Ti₃C₂T_x synthesis—required for selective etching and delamination—were procured from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China), and used as received.

2.2. Preparation of Ti₃C₂T_x

The Ti₃C₂T_x was synthesized through selective etching of Al atoms from the Ti₃AlC₂ phase using an HF-based protocol [26]. As schematically demonstrated in Figure 1a, (1) first, 4–5 g of weighed Ti₃AlC₂ powder was loaded into a PTFE reactor. (2) Subsequently, 50–80 mL of 49% HF solution was measured and dripped into the reactor under magnetic stirring. (3) The reaction system was maintained at 70 °C with continuous stirring for 12 h. (4) The etched mixture was centrifuged for 5 min, followed by repeated washing with deionized water until the supernatant reached near-neutral pH. (5) The resulting precipitate was vacuum-filtered through a 0.45 µm PTFE membrane and vacuum-dried at 80 °C for 12 h. (6) For delamination, 2 g of the dried powder was dispersed in 40 mL DMSO, followed by ultrasonic intercalation (20 kHz, 1 h) to induce layer separation. (7) The mixture was washed sequentially with ethanol and deionized water to eliminate residual DMSO. (8) The purified Ti₃C₂T_x suspension was filtered and vacuum-dried at 60 °C for 24 h. (9) The final Ti₃C₂T_x powder was milled and then sealed and set aside.

2.3. Preparation of Ti₃C₂T_x/Al6061 Composites

The fabrication process of Ti₃C₂T_x/Al6061 composites is illustrated in Figure 1b. Ti powder was dispersed in ethanol through ultrasonic agitation (2 h, 40 kHz) to achieve uniform dispersion. A stoichiometrically controlled amount of aluminum powder was subsequently introduced into the suspension, followed by homogenization via magnetic stirring (1300 rpm, 1 h). The mixed slurry was vacuum-filtered, with the collected powder vacuum-dried (60 °C, 12 h) to eliminate residual solvents. The dried composite powder was mechanically alloyed using planetary ball milling (150 rpm, 5 h) with a ball-to-powder ratio of 10:1, where stearic acid (0.3 wt.%) served as a process control agent to inhibit cold welding. The milled powder was uniaxially cold-pressed under 120 MPa (10 min dwell time), and it was sectioned into 10 mm × 10 mm × 5 mm specimens for melt processing. The Ti₃C₂T_x/Al6061 composites were synthesized via a magnetic levitation vacuum melt-casting technique. Predetermined amounts of Al6061 alloy were placed in a melting crucible, and the system was evacuated using a precision control unit. The alloy was then completely melted in a magnetically levitated vacuum induction furnace for 30 min to ensure thermal homogeneity. Subsequently, the Al-Ti₃C₂T_x preforms were introduced into the molten matrix, and electromagnetic stirring was applied via controlled coil currents to promote dispersion of the reinforcement phase. After 10 min of stirring, the 2 wt.% Ti₃C₂T_x/Al6061 composites were fabricated by casting the molten mixture into a steel mold featuring a cavity dimension of ø11.8 mm × 80 mm.

An ECAP die with a channel intersection angle (Φ) of 100°, an outer corner angle (Ψ) of 50°, and a mold channel of ø12 mm was employed for processing. Before ECAP deformation, molybdenum disulfide (MoS₂) lubricant was applied to the plunger surfaces, die channels, and pre-deformed specimens to minimize frictional forces. The deformation was conducted at 523 K with an extrusion rate of 5 mm/s. Following extrusion, the specimens were subjected to aging treatment at 175 °C for 12 h. In the experimental protocol, the composites underwent multi-pass extrusion of 1, 2, and 3 passes. All subsequent descriptions and characterizations will be designated as ECAP 1-pass, ECAP 2-pass, and ECAP 3-pass.

2.4. Electrochemical Measurements and Immersion Corrosion Tests

All specimens were sequentially ground to metallographic finish, polished using diamond suspensions, and ultrasonically cleaned in anhydrous ethanol to achieve contaminant-free surfaces. Potentiodynamic polarization measurements were conducted via a Princeton P4000 electrochemical workstation employing a conventional three-electrode configuration (working electrode: 1 cm² exposed specimen area; counter electrode: platinum mesh; reference electrode: saturated calomel electrode (SCE)) in 3.5 wt.% NaCl solution (to simulate marine and coastal atmospheric environments) maintained at 25 ± 0.5 °C under atmospheric conditions. To stabilize the open-circuit potential (OCP), specimens were immersed in electrolyte for 20 min pre-test. Scanning parameters included polarization curves at 1 mV/s sweep rate within ±300 mV vs. OCP and EIS over 0.01 Hz-100 kHz frequency range with 10 mV AC amplitude. Five independent replicates per group were performed under thermostatic control. Pre-immersion protocols involved 15 min ultrasonic cleaning in 95 wt.% ethanol followed by nitrogen-purge drying before immersion in 3.5 wt.% NaCl solution for controlled durations (10/20/30 days). Mass loss quantification utilized a precision balance (0.0001 g resolution) through differential weighing pre/post-immersion. Post-corrosion treatment comprised sequential immersion in 20 g/L CrO₃ + 1.69 g/mL H₃PO₄ solution for corrosion product removal, secondary ethanol ultrasonic cleaning (15 min), and SEM morphological characterization. All experimental procedures, including electrochemical analyses, were repeated in quintuplicate to ensure statistical significance. The corrosion rate is calculated as [27]:

$${\mathrm{V}}_{\mathrm{M}}={\displaystyle \frac{{\mathrm{m}}_0-{\mathrm{m}}_1}{\mathrm{A}\times \mathrm{t}}}$$

(1)

In the corrosion rate formulation, ${\mathrm{V}}_{\mathrm{M}}$ (mg·cm⁻²·d⁻¹) quantifies the mass loss rate under corrosive degradation, where ${\mathrm{m}}_0$ (mg) and ${\mathrm{m}}_1$ (mg) denote the mass values prior to and post-immersion, respectively. A (cm²) corresponds to the electrochemically active surface area exposed to the corrosive medium, while t (d) represents the cumulative corrosion exposure duration.

2.5. Density, Porosity, and Hardness Tests

The true density ($\rho$) of the composites was determined via Archimedes’ principle, utilizing deionized water as the immersion medium. The density calculation followed the established relationship [28]:

$$\rho ={\displaystyle \frac{M_1}{V}}={\displaystyle \frac{M_1}{\frac{M_1-M_2}{{\rho }_1}}}$$

(2)

where $M_1$ is the mass of the specimen weighed on an electronic scale, $M_2$ is the mass obtained from the specimen in water, and ${\rho }_1$ is the density of water. The theoretical density ${\rho }_2$ and porosity $P$ of the specimen were calculated using the following equations [29]:

$${\rho }_2={\sum }_{a=1 }^n{\rho }_a{V}_a$$

(3)

$$P={\displaystyle \frac{{\rho }_2-\rho }{{\rho }_2}}\times 100\%$$

(4)

In the above equation, ${\rho }_a$ is the theoretical density of each component of the composite specimen and $V_a$ is the volume fraction of each component of the specimen.

Vickers hardness was measured using a microhardness tester (HVX-1000A, Jinan Huayin Testing Instrument Co., Ltd., Jinan, China) with an applied load of 200 gf and dwell time of 15 s. An equidistant grid comprising 100 measurement points was established across the specimen surface. The Vickers hardness values were statistically averaged to obtain the representative bulk hardness value.

2.6. Microstructural Characterization

Field emission SEM (Apreo 2S HiVac, Thermo Scientific, Waltham, MA, USA) was used to characterize the microstructure and corrosion morphology of Ti₃AlC₂, Ti₃C₂T_x powders and composites. EBSD (Sigma 560, ZEISS, Oberkochen, Germany) was used to characterize the grain size of the composites after ECAP deformation. The powder phase composition was measured by X-ray diffraction (XRD, D8ADVANCE, Bruker Corporation, Berea, KY, USA) using Cu Ka radiation (wavelength 0.154 nm).

3. Results and Discussion

3.1. Microstructure and Physical Phase Analysis of Ti₃C₂T_x

Ti₃C₂T_x, a two-dimensional layered material, is synthesized by selectively etching the Al layers from Ti₃AlC₂ using a HF solution, as schematically illustrated in Figure 2. EDS mapping analysis reveals distinct structural differences between the precursor and etched phases. The unetched Ti₃AlC₂ phase exhibits a dense lamellar structure (Figure 2a), whereas the etched phase displays an expanded multilayer architecture (Figure 2b), demonstrating the successful removal of the Al layers.

XRD characterization was systematically performed on both the pristine Ti₃AlC₂ phase and its derived Ti₃C₂T_x product. As illustrated in Figure 3, the XRD patterns reveal a significant reduction in primary characteristic peak intensity after the etching process, indicating structural evolution from the Ti₃AlC₂ phase to Ti₃C₂T_x. Notably, both materials exhibit well-defined (002) diffraction peaks at 2θ angles of 10.2° and 8.9°, respectively. This angular shift demonstrates interlayer spacing modification during the phase transformation process. The interplanar distances were calculated using Bragg’s law [30]:

$$2dsin\theta =n\lambda$$

(5)

where d represents the interplanar spacing, θ denotes the Bragg angle between incident X-rays and crystallographic planes, λ is the Cu-Kα radiation wavelength (0.154 nm), and n is the diffraction order (n = 1). The calculated (001) d-spacing of Ti₃C₂T_x (1.03 nm) exhibits a 20% expansion compared to the Ti₃AlC₂ phase (0.86 nm) via Equation (5). This pronounced interlayer dilation conclusively demonstrates the successful selective removal of Al atomic layers through HF etching, followed by effective intercalation-induced structural expansion. Furthermore, the preservation of a sharp (110) diffraction peak confirms the structural integrity of the Ti₃C₂T_x framework post-etching.

3.2. Microstructural Analysis of T_i3C₂T_x/Al6061 Composites

The SEM of Ti₃C₂T_x/Al6061 composites treated with different passes is shown in Figure 4. In Figure 4a, the white phases correspond to Ti₃C₂T_x particles and the iron-rich phase, respectively. Both phases show no significant difference in grayscale contrast but exhibit distinguishable morphological characteristics. Following ECAP treatment, both the matrix grain size and the secondary phase dimensions in the composite demonstrate substantial refinement. As shown in Figure 4b, the microstructure evolves into a homogeneous fine-grained structure where the secondary phase undergoes fragmentation into uniformly distributed fine particles, while the iron-rich phase is fractured into smaller fragments. This microstructural evolution can be attributed to the shear stresses generated during extrusion-induced deformation, which progressively disintegrate the secondary phases. The accumulated stress magnitude increases with successive extrusion passes, leading to continued comminution of secondary phases into submicron particles that undergo coordinated flow and redistribution within the matrix [25]. Concurrently, ECAP processing reduces the aspect ratio of Ti₃C₂T_x particles through mechanical fragmentation. The resultant reinforcement particles become more homogenously dispersed in the matrix with enhanced interfacial bonding characteristics. These observations align with prior investigations on SPD processing of particle-reinforced AMCs [31].

The EBSD analysis of Ti₃C₂T_x/Al6061 composites under as-cast and ECAP three-pass processing conditions is presented in Figure 5. The inverse pole figure (IPF) maps are shown in Figure 5a. High-angle grain boundaries (HAGBs ≥ 15°) and low-angle grain boundaries (LAGBs 2–15°) are depicted by black and pink lines, respectively. Quantitative analysis of Figure 5a–c reveals grain coarsening (average size: 28.6 µm) with partial recrystallization in the as-cast specimen, attributable to thermally activated grain growth during homogenization and aging treatments. The boundary misorientation distribution demonstrates 21.4% LAGBs and 78.4% HAGBs. After ECAP three-pass processing (Figure 5d–f), significant grain refinement is achieved with an average grain size of 8.7 µm, representing a 69.6% reduction compared to the initial state. The microstructure exhibits shear-oriented grain elongation with intensified LAGB fraction (72.2%). This substantial increase in LAGBs predominantly originates from grain rotation and fragmentation under accumulated shear stress, generating dense dislocation networks that evolve into subgrain boundaries through dislocation entanglement and accumulation. Concurrently, limited HAGB formation correlates with dynamic recovery and localized recrystallization during ECAP deformation [32].

3.3. Densification and Hardness Analysis

The effects of ECAP deformation on the density and porosity of Ti₃C₂T_x/Al6061 composites are illustrated in Figure 6. The results demonstrate progressive density enhancement (2.736 g/cm³ theoretical density at ECAP three-pass) with concomitant porosity reduction (0.72% after three passes) under multi-pass ECAP conditions. This densification mechanism primarily originates from the synergistic effects of compressive stress and shear-induced plastic flow during SPD, which effectively alleviate intrinsic casting defects in the matrix. The shear-dominated deformation mode facilitates inter-pore connectivity reduction through coordinated material flow along the shear plane. As shown in Figure 6, the incremental density gain per pass decreases from 0.014 g/cm³ (first pass) to 0.003 g/cm³ (third pass), while porosity reduction efficiency declines from 1.071% to 0.11% between successive passes. This attenuation phenomenon arises from progressive accumulation of strain-hardening effects (Vickers hardness increase 38 HV after three passes) and geometric work hardening, which exponentially increases the energy barrier for void closure. The findings conclusively establish the pass-dependent nature of ECAP’s densification efficacy in composites.

Figure 7 illustrates the Vickers hardness evolution of Ti₃C₂T_x/Al6061 composites in both as-cast and ECAP-processed states. As depicted in Figure 7a, the as-cast composite exhibits a hardness distribution ranging from 80.2 to 89.1 HV with an average value of 84.3 HV. ECAP processing induces substantial hardening, evidenced by a microhardness increase to 107.6 HV after the first pass. Progressive enhancement is observed with successive processing, reaching 118.5 HV and 122.3 HV after the second and third ECAP passes, respectively, corresponding to 40.6% and 45.1% improvements relative to the initial condition. The strengthening mechanism primarily originates from grain refinement and improved particle dispersion, as evidenced by SEM characterization. The average grain size decreases from 28.6 µm in the as-cast condition to 8.7 µm after ECAP three-pass processing (Figure 4), consistent with Hall–Petch strengthening behavior. Concurrently, SEM micrographs (Figure 4) demonstrate enhanced Ti₃C₂T_x particle distribution with reduced agglomeration, where micron-scale reinforcement particles (1–3 µm) promote load transfer efficiency and constrain localized deformation.

3.4. Surface Morphology Analysis After Corrosion

Pitting corrosion, recognized as one of the most detrimental corrosion mechanisms, predominantly initiates at localized regions such as defects, pits, or coarse secondary phases, substantially degrading the comprehensive corrosion resistance of composite materials. Following a 20-day immersion in a 3.5 wt.% NaCl solution, surface corrosion products were meticulously removed from the fabricated composite to characterize its corrosion morphology, as illustrated in Figure 8. Figure 8a reveals pronounced localized surface deterioration accompanied by extensive and deep corrosion pits. This phenomenon arises from micro-galvanic interactions between coarse secondary-phase particles and the adjacent aluminum matrix, exacerbating corrosion propagation. Comparatively, Figure 8b–d delineate the corrosion morphologies of specimens subjected to one-pass, two-pass, and three-pass ECAP treatments, respectively. A progressive reduction in both pit dimensions and density is discernible with increasing ECAP passes, correlating with enhanced corrosion resistance. Notably, the three-pass ECAP-treated specimen demonstrates minimal pit formation, attributed to SPD-induced grain refinement and secondary-phase fragmentation during ECAP processing. These microstructural modifications effectively suppress corrosion kinetics, as evidenced by reduced corrosion rates.

Figure 9 compares the longitudinal corrosion profiles of Ti₃C₂T_x/Al6061 composites before and after ECAP following 20 days of immersion. In the untreated composite (Figure 9a), corrosion-induced intergranular cracking reaches a depth of 37.58 µm, driven by stress localization at interfaces between secondary phases and the aluminum matrix, where mismatched strain accommodation under corrosive conditions exacerbates crack nucleation and growth. Post-ECAP treatment (Figure 9b) reduces the crack depth to 16.73 µm, accompanied by diminished groove formation. This improvement stems from ECAP-generated homogeneous strain distribution, which eliminates interfacial strain mismatches between the matrix and secondary phases. The imposed severe shear deformation disrupts clustered secondary phases, minimizes interfacial defects, and redistributes internal stresses [33]. Consequently, corrosive agents encounter fewer localized energy gradients, suppressing adsorption-driven dissolution at grain boundaries.

Mass loss analysis is a fundamental and reliable method for evaluating average corrosion rates; Figure 10 illustrates the mass loss data of specimens immersed in 3.5 wt.% NaCl solution for 10, 20, and 30 days. A rapid initial corrosion rate is observed across all specimens, with minimal differentiation in mass loss during the first 10 days. Subsequently, the corrosion rate decelerates due to the progressive formation of protective surface layers that impede further corrosive attack. After 20 days of immersion, the mass losses for the as-cast, one-pass, two-pass, and three-pass ECAP-treated specimens are 0.028, 0.019, 0.015, and 0.012 g, respectively. This progressive reduction in mass loss with increasing ECAP deformation demonstrates enhanced barrier effectiveness of the corrosion product layers.

Figure 11 presents the OCP and potentiodynamic polarization curves of Ti₃C₂T_x/Al6061 composites under as-cast and ECAP-processed conditions. As shown in Figure 11a, a relatively stable state was achieved by all specimens after 1200 s of immersion. The OCP of the as-cast Ti₃C₂T_x/Al6061 composite stabilized at approximately −0.74 V, whereas the corrosion potential (E_corr) of ECAP-processed composites shifted to more noble values compared to the as-cast material. A comparative analysis of the potentiodynamic polarization curves (Figure 11b) revealed pronounced electrochemical differences: the ECAP three-pass-processed composite exhibited the most positive E_corr value. Critically, the variations in E_corr (quantified via Tafel extrapolation in Figure 11c) followed a hierarchical order: ECAP three-pass > ECAP two-pass > ECAP one-pass > as-cast, though E_corr alone cannot definitively predict corrosion resistance.

Instead, the corrosion current density (I_corr)—directly reflecting corrosion kinetics—showed a significant reduction in ECAP-processed composites. The I_corr values decreased progressively with ECAP passes, reaching a minimum for the ECAP three-pass composite (from 4.09 to 2.27 µA/cm²). The absence of a classical passivation region in Figure 11b aligns with the known behavior of Al-Mg-Si alloys in chloride-containing environments, where metastable pitting dominates over stable passivation [34]. Notably, ECAP-processed composites exhibit a coupled trend: samples with nobler E_corr (e.g., ECAP three-pass) consistently demonstrate lower I_corr values (Figure 11c). This inverse E_corr-I_corr relationship indicates that ECAP processing modulates anodic oxidation kinetics rather than promoting passivity [35].

Figure 12 illustrates the Nyquist and Bode plots of Ti₃C₂T_x/Al6061 composites under different states, obtained through equivalent circuit simulation using ZView 4.0 software (as shown in the equivalent circuit model in Figure 12c). As observed from Figure 12a, both the as-cast and ECAP-processed specimens exhibit a single capacitive loop across the entire frequency range. The diameter of the capacitive arc is directly related to the charge transfer resistance (R_ct), which is a key parameter indicative of the corrosion resistance of the material. The radius of the impedance arc for the ECAP-processed specimen is larger than that of the as-cast Ti₃C₂T_xAl6061 composite. With increasing ECAP passes, the impedance radius further enlarges, indicating a progressive increase in charge transfer resistance and thus enhanced corrosion resistance. In the Bode plot (Figure 12b), all specimens display a single time-constant behavior. As the frequency varies, broader peak widths and higher phase angle peak magnitudes are associated with superior corrosion resistance. Notably, the low-frequency impedance modulus (|Z|) of the ECAP-processed specimens exceeds that of the as-cast counterpart. Among them, the ECAP three-pass-processed composite exhibits the highest impedance modulus, suggesting optimal corrosion resistance.

The EIS results were analyzed using an equivalent circuit model, with the corresponding fitted electrochemical parameters presented in

Table 1. Fitting data extracted from the EIS data.

Specimen	Rs (Ω·cm2)	Rct (Ω·cm2)	CPE-T (F·cm−2)	n
As-cast composites	26.07	7274	1.75 × 10−5	0.87
ECAP 1-pass	18.84	14,253	4.91 × 10−6	0.83
ECAP 2-pass	22.23	20,199	8.24 × 10−6	0.86
ECAP 3-pass	21.86	24,584	8.85 × 10−6	0.85

. In the model, R_s, R_ct, and CPE_dl represent solution resistance, charge transfer resistance, and the double-layer capacitance between solution and electrode, respectively. The R_s value is determined by the electrolyte conductivity, while the magnitude of R_ct depends on the corrosion resistance of the composite material. The CPE_dl element is defined by two parameters (CPE-T and n); when the n value approaches 1, the CPE-T component behaves more like a pure capacitor; conversely, when n approaches 0, it resembles a pure resistor. As shown in Table 1, the R_ct value of ECAP-processed specimens significantly exceeds that of as-cast samples. After multi-pass ECAP treatment, the composite material exhibited increased R_ct values, indicating improved corrosion resistance. Particularly notable was the sample after three passes of ECAP processing, which demonstrated a 3.4-fold increase in the R_ct value compared to the unprocessed material. These experimental results confirm that ECAP processing effectively enhances the corrosion resistance of the material.

3.5. Corrosion Mechanism

In chloride-containing corrosive environments, localized corrosion may occur in specific regions of the composite material due to anodic dissolution reactions. The presence of secondary phase particles (e.g., Mg₂Si and Al₈Fe₂Si) within the composite creates potential differences with the α-Al matrix. These secondary phases act as cathodic sites [9,36], forming micro-galvanic couples with the anodic α-Al matrix, thereby initiating preferential dissolution and subsequent pitting corrosion at their peripheries. Concurrently, Cl⁻ preferentially accumulates near these active pits, accelerating localized corrosion through autocatalytic processes. ECAP processing fundamentally alters this corrosion behavior by modifying the microstructure: EBSD and SEM analysis confirmed ECAP-induced grain refinement (reducing average grain size to 8.7 µm, Figure 5) and homogeneous dispersion of secondary phases (Figure 4). This refinement and dispersion directly reduce electrochemical heterogeneity and diminish micro-galvanic coupling effects, as validated by the 3.4-fold increase in R_ct for ECAP three-pass samples (Table 1) and the shift to nobler E_corr/lower I_corr (Figure 11c). Cross-sectional analysis (Figure 9) further shows significantly shallower corrosion grooves (16.73 µm depth vs. 37.58 µm in as-cast) due to mitigated interfacial strain mismatch and reduced cathodic site continuity. Porosity reduction (0.72% after three passes, Figure 6) impedes corrosive media infiltration pathways.

As shown in Figure 13b, the substantial dispersion of secondary phases suppresses infiltration along grain boundaries and interrupts corrosion propagation. Consequently, under prolonged corrosion, the ECAP three-pass sample exhibits superior resistance compared to untreated composites. Furthermore, micro-corrosion pits resulting from matrix detachment (Figure 8d) are associated with localized corrosion at dispersed secondary-phase particles. Furthermore, corrosion rate measurements via the weight loss method (Figure 10) demonstrate progressively lower corrosion rates with increasing ECAP passes, reaching a minimum for the ECAP three-pass sample. This enhancement is directly attributed to the microstructural modifications achieved by ECAP: substantial grain refinement (Figure 5) and homogeneous phase dispersion (Figure 4) reduce electrochemical driving forces for corrosion, while reduced porosity (Figure 6) hinders electrolyte penetration.

4. Conclusions

In summary, this study systematically investigates the synergistic corrosion resistance enhancement mechanisms in AMCs through the combined implementation of ECAP processing and Ti₃C₂T_x reinforcement. The main conclusions are as follows:

(1)

Following ECAP treatment, SPD induces fragmentation and redistribution of secondary/reinforcing phases. Increasing ECAP passes enhances grain refinement and orientation texture, with EBSD confirming an average grain size of 8.7 µm after three passes.

(2)

ECAP significantly improves hardness: microhardness increases to 107.6 HV (one-pass), 118.5 HV (two-pass), and 122.3 HV (three-pass)—representing 40.6% and 45.1% increases versus as-cast for two-pass and three-pass, respectively. Density increases while porosity decreases (0.72% after three passes).

(3)

ECAP processing reduces corrosion susceptibility by (i) diminishing I_corr and achieving a 3.4-fold R_ct increase through grain refinement and homogeneous phase dispersion, which suppress electrochemical heterogeneity and micro-galvanic coupling; (ii) reducing corrosion groove depth from 37.58 µm (as-cast) to 16.73 µm (three-pass) by mitigating interfacial strain mismatch; and (iii) impeding electrolyte infiltration via porosity reduction.