Simplified Chemical Treatments for Improved Adhesive Bonding Durability and Corrosion Protection of High-Pressure Die-Cast Aluminum Alloy AlSi10MnMg

Abstract

The adhesive bonding of high-pressure die-cast (HPDC) aluminum alloy AlSi10MnMg is extensively applied in the aerospace and automotive sectors. Surface pretreatment of HPDC aluminum prior to bonding is crucial for enhancing bonding strength and durability, as it regulates surface roughness, and chemical properties. Traditional multi-step surface treatments including chromic acid anodizing for HPDC AlSi10MnMg are hazardous, complex, and often fail to balance adhesive bonding durability and corrosion protection, limiting their industrial applicability. This study examined the impact of various chemical treatments on the adhesive bonding performance of an AlSi10MnMg aluminum alloy. The treated surfaces were bonded using a structural adhesive, and bonding performance was evaluated via wedge tests under pristine conditions and after accelerated aging. A scanning electron microscope (SEM) was used to study the surface morphology, chemical composition, and corrosion characteristics of the treated surfaces. Energy dispersive spectroscopy (EDS), electrochemical impedance spectroscopy (EIS), and potentiodynamic polarization measurements were employed. Excellent adhesion characteristics, dominated by the cohesive failure of the adhesive, were observed in H₂O₂-treated samples. The H₂O₂-treated samples exhibited the shortest initial crack length, indicating a superior baseline bonding quality, and showed minimal crack propagation (only slight extension) after aging under extreme environmental conditions (70 °C and 100% relative humidity for 4 weeks). Electrochemical measurements revealed that the SG200-treated sample achieved the lowest corrosion current density (0.25 ± 0.03 μA/cm²) with an excellent corrosion resistance, while sol–gel-treated samples generally suffered from a poor adhesion, with interfacial failure. This study proposes a simplified, single-step chemical treatment using an H₂O₂ solution that effectively achieves both a strong adhesive bonding and an excellent corrosion resistance, without the drawbacks of conventional methods. It offers a viable alternative to conventional multi-step hazardous surface treatments.

1. Introduction

Adhesive bonding serves as a vital technical support for high-pressure die-cast (HPDC) aluminum component applications across critical industries [1,2,3]. Compared with traditional mechanical fasteners like bolts and rivets, adhesive bonding for HPDC components spreads stress across entire contact areas. This reduces peak loads by up to 70% compared to mechanical fasteners and avoids stress concentration points that can initiate failure under cyclic loading. Additionally, adhesives can also prevent galvanic corrosion between dissimilar metals in HPDC assemblies, a particularly valuable benefit in harsh environments like marine settings where saltwater accelerates electrochemical reactions. In aerospace, it enables the reliable assembly of HPDC aluminum brackets used in avionics support and structural load bearing. In automotive manufacturing, it is indispensable for the application of HPDC aluminum in chassis nodes. For marine applications, it supports the use of HPDC aluminum housings exposed to harsh saltwater and humidity environments [4].

High-pressure die casting produces complex, near-net-shape aluminum components [5,6,7,8]. AlSi10MnMg is a widely used HPDC aluminum alloy in automotive and aerospace sectors due to its high castability, good mechanical strength, and cost-effectiveness. However, its inherent porosity and surface segregation of Si/Mn phases increase the difficulty of achieving stable adhesive bonds and corrosion protection, making targeted surface treatments essential [4]. Traditional methods, particularly acid anodizing, have dominated for decades [9,10,11,12,13]. Shi et al. conducted surface treatment on HPDC using H₂TiF₆ and achieved favorable results; however, a further exploration of alternative methods is still needed from the perspectives of cost-effectiveness and environmental friendliness [4]. Emrich carried out a crack propagation study using AlMg3 as the substrate, but only applied a simple surface treatment without implementing a systematic chemical treatment [14]. Processes like chromic acid anodizing (CAA), sulfuric acid anodizing (SAA), and phosphoric acid anodizing (PAA) generate highly uniform, adherent oxide layers [9,15]. Crack lengths measured in wedge tests of chemically treated HPDC aluminum surfaces show performances comparable to those achieved with conventional anodizing processes. Zhang et al. reported an adhesion performance equivalent to minimal crack propagation for aluminum anodized with phosphoric and chromic acids, with slightly greater crack extensions observed for surfaces anodized with boric and sulfuric acids [16]. Anodization typically involves harsh acid treatments and multiple steps, including native oxide removal during pretreatment and pore sealing in post-processing. There is an urgent need for environmentally safe alternatives to HPDC AlSi10MnMg according to these drawbacks. It is a key alloy in high-demand industries where both bonding durability and corrosion resistance are critical. Laser-based surface modification has emerged as a leading sustainable alternative due to its high precision, zero pollutant emission, and controllable texture formation, addressing the limitations of chemical anodizing. Baldan reviewed that laser treatment enables the selective removal of weak surface layers and creates tailored micro-textures, significantly enhancing the interfacial wettability and chemical bonding between metals and composites. Unlike CAA, it avoids toxic chemicals and can be integrated into automated production lines for large-scale manufacturing. Recent studies have further advanced laser technology’s applicability [17]. Parodo et al. developed a stress field-guided laser texturing strategy for CFRP single-lap joints. By limiting treatment to high-stress regions (instead of full-area processing), they reduced laser processing time by ~60% while achieving a 63% increase in shear strength compared to untreated samples—resolving the efficiency bottleneck of conventional full-area laser treatment [18]. Xu et al. used an Nd:YVO4 UV nanosecond laser to pattern CFRP surfaces with annular, 2 × 2, 3 × 3, and 4 × 4 micro-pit arrays. They found that large-diameter, large-spacing annular patterns optimally balanced carbon fiber exposure and residual resin retention, maximizing mechanical interlocking and chemical bonding with the adhesive. This design improved the bonding strength of CFRP/Al6061 heterogeneous joints to 28.58 MPa, with cohesive failure as the dominant mode, revealing that laser pattern type directly regulates fiber exposure continuity and interface failure mechanisms [19]. Despite these advances, laser-based methods face practical limitations for HPDC AlSi10MnMg components: high equipment costs restrict small-to-medium enterprise adoption, and the alloy’s inherent porosity and complex near-net-shape geometries make uniform laser texturing challenging. Thus, there remains an urgent need for simplified, low-cost, and scalable surface treatments that achieve both a strong adhesive bonding and excellent corrosion resistance without the drawbacks of multi-step chemical anodizing or high-cost laser systems.

Standard phosphate coatings are less effective on aluminum alloys compared to steel. While significantly reducing sludge volume and toxicity compared to anodizing, some conversion coatings still generate waste streams requiring management. Rare-earth and sol–gel coatings show promise for corrosion protection. Their performance in the adhesive bonding of HPDC AlSi10MnMg remains understudied. This creates a gap between corrosion-focused research and industrial bonding requirements. Rare-earth metal-based conversion coatings, particularly those utilizing cerium or lanthanum salts, represent another promising avenue. These coatings form protective cerium oxide/hydroxide layers that actively inhibit cathodic corrosion reactions, mimicking the passivating effect of chromates without the toxicity [20,21,22]. Sol–gel technology, based on the controlled hydrolysis and condensation of organometallic precursors, enables the formation of dense hybrid organic–inorganic films at relatively low temperatures [23]. This low-temperature processing is particularly advantageous for heat-sensitive HPDC aluminum components, which are prone to microstructural changes or dimensional shifts under high heat. Incorporating rare-earth compounds into the sol–gel matrix creates a synergistic effect. It combines the inherent benefits of sol–gel films (superior moisture/ion barrier properties, strong adhesion to aluminum substrates, and precise thickness control) with the active corrosion inhibition of rare-earth cations. These cations are gradually released in response to corrosion-induced pH changes, forming insoluble hydroxide precipitates that block anodic sites and slow degradation, significantly enhancing long-term performance in aggressive environments [24].

However, rare-earth conversion coatings and sol–gel coating have received limited attention in adhesive bonding applications for HPDC aluminum. Most existing studies focus on their standalone corrosion resistance rather than their ability to promote and maintain strong adhesive bonds under service conditions. This gap is significant because surface chemistry and topography requirements for effective adhesion differ substantially from those for corrosion protection alone. Adhesive bonding demands not only a stable oxide layer but also sufficient surface roughness for mechanical interlocks, as well as chemical functionality to form covalent or hydrogen bonds with adhesive polymers.

In this study, a straightforward technique designed to simultaneously roughen the HPDC aluminum surface and form a cerium protective layer was introduced. This one-step treatment with sand blasting eliminates the need for separate cleaning and roughening steps, simplifying production workflows and reducing processing time. The adhesive bonding strength of treated surfaces was evaluated by wedge tests conducted both before and after accelerated aging. The aging test involved an exposure to 70 °C and 100% relative humidity for four weeks, a condition that accelerates moisture-induced degradation mechanisms common in automotive and marine environments. Additionally, the performance of various chemical treatments, focusing on how each one affects the surface morphology and chemical composition, were compared. Scanning electron microscopy (SEM) characterized changes in surface topography, while energy dispersive spectroscopy (EDS) analyzed the chemical composition. Corrosion properties were evaluated using electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization measurements, which provided quantitative data on corrosion rates and protective layer integrity. By combining these electrochemical results with adhesion test data, a comprehensive understanding of how each pretreatment balances corrosion resistance and adhesive performance can be developed.

2. Experimental Procedure

2.1. Materials and Chemical Treatment

Commercial AlSi10MnMg primary alloy ingots, with the nominal compositions provided in

Table 1. Chemical compositions of AlSi10MnMg primary alloys [4].

	Si	Fe	Cu	Mn	Mg	Zn	Ti	Al
wt.%	9.5–11.0	<0.25	<0.05	0.4–0.7	0.1–0.4	<0.07	0.03–0.12	Balance

(Trimet SE, Essen, Germany), were melted in a furnace at approximately 700 °C and subsequently cast into plates (260 × 150 × 4.0 mm³) using an HPDC machine (Buehler Evolution 530B, Holland, MI, USA) [4]. After being ejected from the die, the AlSi10MnMg plates underwent natural cooling in the air.

The chemical treatment, including sand blasting, ultrasonic cleaning, and chemical coating application, is depicted in Figure 1. The process starts with the sandblasting of the high-pressure die-cast aluminum AlSi10MnMg samples. This stage is followed by ultrasonic cleaning in acetone. Aluminum oxide grit serves as the abrasive medium for sandblasting. Sandblasting conditions were selected based on industry standards for HPDC aluminum surface roughening, where 8 bar pressure ensures sufficient roughness for mechanical interlocking without damaging the substrate [25,26]. Maintaining a 10 cm distance between the nozzle and the HPDC aluminum surface ensures consistent abrasion across the entire specimen. A 45° angle relative to the horizontal was selected for the nozzle. This orientation promotes even distribution of abrasive particles across the surface. Sandblasting duration was standardized at 60 s per sample. After sandblasting, the samples are rinsed and then subjected to ultrasonic cleaning in acetone. The ultrasonic cleaning parameters were established at a power of 400 W and a frequency of 40 kHz. Each sample receives 10 min of ultrasonic treatment. The temperature of the solution during cleaning is maintained below 40 °C. Cerium-based conversion coatings were chosen for their non-toxicity and cathodic inhibition effect. Three different chemical solutions are then used to coat the samples at room temperature. These include an APS solution (0.1 M Ce(NO₃)₃ + 0.1 M (NH₄)₂S₂O₈ + ethanol) applied for 5 min, an H₂O₂ solution (0.1 M Ce(NO₃)₃ + 0.1 M H₂O₂ + ethanol) also applied for 5 min, and a sol–gel solution (0.1 M Ce(NO₃)₃ + 0.1 M C₆H₈O₇ + ethanol) applied for 30 s [27,28,29]. Final processing involves drying the chemically treated samples. One group labeled S-SG80 is dried at 80 °C for 30 min. Another group consisting of sol–gel-coated samples and labeled S-SG200 undergoes drying at 200 °C for 30 min. The respective designations for samples treated with APS, H₂O₂, and sol–gel solutions are S-APS, S-H₂O₂, S-SG80, and S-SG200. Each schematic shows deoxidized samples immersed in their particular solutions. The figures provide a detailed overview of the chemical composition of each solution, as well as the expected surface reactions. The APS and H₂O₂ solutions are engineered to produce cerium-based chemical coatings.

The formation mechanism of cerium-based sol–gel coatings via the reaction system consisting of cerium nitrate hexahydrate (Ce(NO₃)₃·6H₂O), anhydrous ethanol (C₂H₅OH), and citric acid (C₆H₈O₇) involves a series of synergistic chemical processes encompassing complexation, hydrolysis, and polycondensation reactions. Cerium nitrate initially dissociates in anhydrous ethanol to release Ce³⁺ cations, which immediately interact with citric acid molecules upon their introduction into the system. Citric acid acts as a dual-function reagent: its carboxyl (-COOH) and hydroxyl (-OH) groups form stable multidentate chelates with Ce³⁺ through six-membered ring coordination structures, effectively preventing premature precipitation of cerium hydroxides and ensuring colloidal stability.

Simultaneously, controlled hydrolysis reactions occur where Ce³⁺ cations react with trace water in the system, generating Ce(OH)₃ intermediates via the following reversible reaction [30]:

$${\mathrm{C}\mathrm{e}}^{3+}+3{\mathrm{H}}_2\mathrm{O}\rightleftharpoons \mathrm{C}\mathrm{e}{\left(\mathrm{O}\mathrm{H}\right)}_3+{3\mathrm{H}}^{+}$$

(1)

The polycondensation stage follows, during which adjacent Ce(OH)₃ undergo dehydration to form Ce-O-Ce covalent bonds through the following reaction:

$$-\mathrm{C}\mathrm{e}-\mathrm{O}\mathrm{H}+\mathrm{H}\mathrm{O}-\mathrm{C}\mathrm{e}-\to -\mathrm{C}\mathrm{e}-\mathrm{O}-\mathrm{C}\mathrm{e}+{3\mathrm{H}}_2\mathrm{O}$$

(2)

The esterification reactions that occur between the carboxyl and hydroxyl groups of citric acid molecules, which create a hybrid organic–inorganic three-dimensional structure, work in concert to increase the formation of inorganic networks in a way that is synergistic. As the reaction proceeds, the homogeneous solution progressively changes into a sol state that is defined by uniformly distributed colloidal particles, which then changes into gel as the network crosslinking density increases when the aging process occurs. Subsequent drying removes entrapped ethanol and moisture.

2.2. Wedge Test

Wedge sample preparation began with cutting the high-pressure die-cast aluminum plates into two equal parts. Two half-plates were then attached to each other by means of adhesive Betamate 1480 according to DIN65448:1988 [31]. This standard specifies sample preparation, curing conditions, and crack length measurement methods, ensuring the wedge test results are comparable and reproducible. Figure 2 depicts the AlSi10MnMg sample with Betamate 1480 (Dow Corning, Wiesbaden, Germany) epoxy adhesive for wedge tests. For both wedge tests and electrochemical measurements, four independent replicates were conducted per sample group. Additionally, 0.3 mm thick spacers were placed along the edges of the half-plates to control the adhesive thickness. The bonded specimens were cured in an oven set to 180 °C, with the curing period lasting 30 min. Once cured, one hour of storage was spent in the lab environment for the samples. At this stage, the initial crack length was assessed using calibrated instruments (MarCal 16 FN, Goettingen, Lower Saxony, Germany). In accordance with this preliminary measurement, the wedge samples were put in a climate chamber. The chamber was set to 70 °C and 100% relative humidity, and the samples remained exposed to these conditions for 4 weeks. Upon completion of the aging time, the crack length was measured again. The difference between the post-aging and initial measurements gave the crack extension value for each sample. The samples were subsequently reconditioned by being stored in air at room temperature. This step enabled them to achieve stability prior to conducting additional tests. In the end, Instron 5567 testing equipment (INSTRON, Norwood, MA, USA) was used to open the wedge samples.

2.3. Electrochemical Measurement

Electrochemical measurement was carried out using a 3.5 wt.% NaCl solution as the electrolyte. All measurements were conducted using a VersaSTAT4 electrochemical workstation from Princeton Applied Research (Oak Ridge, TN, USA). The three electrodes were fitted with a 3M KCl solution filled with Ag/AgCl. The high-pressure die-cast aluminum samples worked as the working electrode, and a platinum mesh was used as the counter electrode. The working electrode was mounted in a flat cell, exposing a precise 1.0 cm² area to the electrolyte. Prior to initiating formal measurements, open circuit potential (OCP) was monitored for 900 s. This step allowed characterization of the sample’s electrochemical behavior and ensured the system reached a stable state. The scan rate was set at 1 mV/s, which was used to conduct dynamic polarization measurement. The potential range extended 0.1 V beyond the measured OCP in both directions. Electrochemical impedance spectroscopy was conducted at the steady OCP. The frequency range extended from 100 kHz to 10 mHz, with a consistent sinusoidal signal amplitude of 20 mV applied during the entire sweep. All impedance data were analyzed using ZView software (Version 3.1, Scribner Associates, Inc., Southern Pines, NC, USA). Fitting was performed using a predefined equivalent circuit to model the electrochemical interface and extract relevant parameters.

Model A, shown in Figure 3a, was developed to interpret electrochemical impedance spectroscopy recorded before the active electrochemical processes began [32]. In this model, R_s represents the solution resistance. For high-pressure die-cast aluminum, only one layer exists, which was defined by two parameters: oxide film resistance R_f and oxide film capacitance Q_f. Herein, constant phase elements (CPEs) are included to obtain more correct fitting results. These elements account for the system’s non-ideal capacitive behavior, represented here as Q_f and Q_dl. The mathematical expression for a CPE is as follows:

$$Z\left(j\omega \right)={\left(Y_0\right)}^{-1}{\left(j\omega \right)}^{-n}$$

(3)

Y₀ denotes the CPE constant with units of F·cm²·sⁿ⁻¹ in this expression. The imaginary unit is represented by j, the angular frequency is denoted by ω, and the degree of deviation between 0 and 1 from ideal capacitive behavior is shown by n. When n approaches 1, the capacitance approximates the Y₀ value of the CPE. In the case of HPDC aluminum samples with chemical coatings, Q_f represents the capacitance of the coating alone. The chemical coatings in the electrolyte gradually break down over time. This degradation increases pore formation, leading to a more uniform distribution of electrochemical reactions across the surface. R_p relates to the presence of pores and microcracks in the chemical coatings. Model B, illustrated in Figure 3b, was used to analyze these ongoing electrochemical reactions. It also illustrates that this reaction is modeled as a parallel combination of charge transfer resistance R_ct and double-layer capacitance Q_dl, representing the electrochemical component.

3. Results and Discussion

Figure 4 presents SEM micrographs showing the surface morphologies of the AlSi10MnMg substrates and treated samples (S-APS and S-H₂O₂). As shown in Figure 4a, the uncoated HPDC aluminum sample features a relatively rough surface due to the sandblasting treatment. This intentional surface roughness can, to a certain extent, facilitate subsequent bonding processes. In contrast, the SEM micrographs in Figure 4b (S-APS) and Figure 4c (S-H₂O₂) reveal the formation of protective films composed of cerium agglomerates. These coatings appear dense yet porous, with a nearly complete surface coverage across the HPDC aluminum substrate. The deposits exhibit a homogeneous distribution, forming a uniform layer that adheres well to the underlying metal.

Cerium deposition initiates at cathodic sites created by the intermetallic compounds within the aluminum matrix. The reduction of hydrogen peroxide during treatment induces localized pH increases, which facilitate the growth of cerium deposits. Additionally, the solution’s moderately acidic pH range promotes the generation of hydroxyl ions, further supporting deposit formation [33]. Cerium is accumulated in this way, which effectively blocks the cathodic active sites, decreases the system cathodic current, and improves the corrosion protection performance. The coating deposition procedure was assisted by either ammonium persulfate or hydrogen peroxide additives. Variations in the cerium oxide coverage across the treated surfaces can be attributed to differences in the surface reactivity. These morphological characteristics directly influence the performance in subsequent tests, including wedge tests evaluating the adhesive bonding properties.

Figure 5a,b display SEM micrographs that illustrate the surface morphology of cerium oxide films on the S-SG80 and S-SG200 samples, respectively. Both micrographs reveal that the films fully cover the surface of the AlSi10MnMg substrate, demonstrating a complete surface coverage. However, a notable feature of these sol–gel coatings is the presence of microcracks, a common issue associated with dense sol–gel film formation [34]. These microcracks result from shrinkage that occurs during the drying process of the sample. During thermal treatment, water evaporation from the coating induces shrinkage, which generates internal stress that ultimately leads to cracking. The parameters influencing sol–gel coating formation of temperature, curing conditions, and heat treatment play important roles in crack development [35] and will be further elucidated.

Comparing the two samples, the S-SG200 sample (treated at 200 °C for 30 min) exhibits a greater density of microcracks than the S-SG80 sample. As the coating dries, the shrinkage process creates mechanical stress that drives microcrack formation [36]. These microcracks compromise the coating integrity in multiple ways: they act as initiation points for further crack propagation, negatively impact adhesive bonding strength, and facilitate the penetration of corrosive media. When exposed to electrolytes over time, corrosive agents can travel through these microcracks to reach the underlying substrate, resulting in interfacial corrosion that further weakens the adhesive bonding strength evaluated by wedge tests. The corresponding energy dispersive spectroscopies from these surface regions are shown in Figure 5c,d, with distinct peaks confirming the presence of cerium in both samples. These spectra reveal that the sol–gel coatings consist primarily of cerium, aluminum, silicon, and carbon across the treated surfaces.

Figure 6 illustrates the potentiodynamic polarization curves that depict the corrosion behavior of HPDC aluminum samples treated with various surface treatments, assessed in a 3.5 wt.% NaCl solution at ambient temperature. The curves plot the logarithmic current density (y-axis) against the potential relative to the Ag/AgCl reference electrode (x-axis), providing a quantitative description to assess how each treatment modifies the corrosion activity. All tested samples exhibit similar polarization profiles because their corrosion mechanism is consistent: the anode reaction is dominated by Al dissolution, while the cathode reaction is oxygen reduction in 3.5 wt.% NaCl solution. For all samples, the corrosion potentials are all around −0.65 V vs. Ag/AgCl. Notable differences emerge in current density magnitudes. The treated samples (S-H₂O₂ and S-SG200) display a marked reduction in cathodic and anodic currents nearly two orders of magnitude lower than the typical baseline corrosion currents for the AlSi10MnMg substrate. Among these, the S-SG200 sample indicates the lowest corrosion current density, measuring 0.25 ± 0.03 μA/cm², which directly correlates to a superior corrosion resistance.

This enhanced performance stems from the formation of a protective cerium oxide layer, which functions as an effective corrosion inhibitor by actively suppressing cathodic reaction kinetics. The oxide layer significantly slows the anodic dissolution of the underlying metal by hindering these reduction reactions. The development of such protective cerium oxide/hydroxide films is promoted by synergistic effects: elevated temperatures, combined with the presence of oxidizing agents (hydrogen peroxide and ammonium persulfate), accelerate film growth through well-documented reaction mechanisms.

$${\mathrm{O}}_2+2{\mathrm{H}}_2\mathrm{O}+4{\mathrm{e}}^{-}\to 4{\mathrm{O}\mathrm{H}}^{-}$$

(4)

$${\left({\mathrm{N}\mathrm{H}}_4\right)}_2{\mathrm{S}}_2{\mathrm{O}}_8+2{\mathrm{H}}_2\mathrm{O}\to {2\mathrm{N}\mathrm{H}}_4\mathrm{H}{\mathrm{S}\mathrm{O}}_4+{\mathrm{H}}_2{\mathrm{O}}_2$$

(5)

$${\mathrm{H}}_2{\mathrm{O}}_2+2{\mathrm{e}}^{-}\to 2{\mathrm{O}\mathrm{H}}^{-}$$

(6)

$${\mathrm{C}\mathrm{e}}^{3+}+{\mathrm{O}\mathrm{H}}^{-}\to \mathrm{C}\mathrm{e}{\left(\mathrm{O}\mathrm{H}\right)}_3\to {\mathrm{C}\mathrm{e}}_2{\mathrm{O}}_3$$

(7)

Electrochemical impedance spectroscopy (EIS) was used to evaluate the corrosion behavior of both the untreated and surface-treated HPDC aluminum samples in a 3.5 wt.% NaCl solution, with corresponding Nyquist plots shown in Figure 7. In this figure, the x-axis denotes real impedance (Zre, in ohm·cm²) while the y-axis represents imaginary impedance (Zim, in ohm·cm²), establishing a direct reference for comparing electrochemical responses across different samples.

A notable aspect of Figure 7 is the Nyquist plot for the S-SG200 sample. It reveals an arc indicating a continuous protective sol–gel film acting as the main corrosion barrier. This stands in clear contrast to the plots of other samples (S-APS, S-H₂O₂, and S-SG80), which exhibit smaller, less distinct semicircles throughout the entire frequency range. These samples consist of two separate capacitive loops: a high-frequency loop associated with the properties of the oxide layer and a low-frequency loop corresponding to interfacial reactions between the coating/oxide layer and the underlying alloy substrate.

It is remarkable that the phase angles deviate from the ideal −90°. Constant phase elements were employed in EIS fitting to account for non-ideal capacitive behavior. Q_c represents the capacitance of the oxide layer, while Q_dl accounts for the electrochemical double-layer capacitance at the reaction interface.

The quantitative data extracted from Figure 7 align closely with the potentiodynamic polarization results shown in Figure 6. The S-SG200 sample exhibits the largest semicircle in the Nyquist plot, corresponding to the highest impedance value of 9.9 ± 0.7 kohm·cm², confirming its superior corrosion resistance. The S-H₂O₂ sample, with a moderately sized loop, shows a lower yet substantial impedance of 4.8 ± 0.5 kohm·cm². In contrast, the S-SG80 sample displays the smallest loop in Figure 7, with an impedance of only 3.4 ± 0.3 kohm·cm², indicating a comparatively weaker protective performance. These visual and numerical trends in Figure 7 directly reflect the effectiveness of surface treatments: the sol–gel treatment (particularly S-SG200) forms a protective layer that restricts electrolyte access to the alloy surface, as evidenced by the large, single capacitive loop. Samples with less effective coatings exhibit smaller arcs, indicating an increased corrosion activity at exposed interfaces.

Wedge tests were conducted to evaluate the adhesion performance of high-pressure die-cast aluminum samples subjected to various surface treatments. Figure 8 presents the measured crack lengths from these tests, enabling a direct comparison of treatment effectiveness between the substrate and modified samples. In the wedge test analysis, crack length serves as a critical metric. The shorter initial cracks and minimal post-aging propagation indicate superior adhesion and environmental durability.

Under pristine conditions, the untreated HPDC aluminum sample exhibited an initial crack length associated with mixed interfacial and cohesive failure modes, measuring approximately 37.88 ± 10.61 mm. As shown in Figure 8, distinct performance variations emerged among treated samples. The samples processed with APS, H₂O₂, SG80, and SG200 solutions displayed measurable crack lengths, reflecting their respective bonding qualities. Notably, the H₂O₂-treated sample showed the shortest initial crack length at approximately 21.33 ± 0.71 mm, indicating the strongest bond integrity, while the SG80-treated sample developed a longer initial crack (31.88 ± 1.99 mm) than the untreated sample.

Aging tests were performed at 70 °C and 100% relative humidity for four weeks in order to cause the crack to develop, which is shown in Figure 8. However, when compared to the untreated sample, treated samples generally kept shorter total crack lengths. Post-aging measurements revealed varying propagation degrees: the substrate sample showed a significant crack extension, with the total length increasing to approximately 82.44 ± 7.87 mm, while APS-treated and H₂O₂-treated samples retained relatively compact crack lengths at 38.63 ± 1.34 mm and 36.08 ± 3.75 mm, respectively. The H₂O₂-treated sample continued to exhibit the shortest total crack length after aging, with minimal propagation from its initial measurement, followed by the APS-treated sample. In contrast, SG80-treated and SG200-treated samples displayed more extensive post-aging crack growth, with SG80 showing the longest total crack length (79.13 ± 3.15 mm) among all treated groups. These results, supported by crack length comparisons in Figure 8, demonstrate that H₂O₂ treatment provides the most durable adhesion according to the minimal initial cracking and limited propagation under both pristine and aged conditions. This enhanced performance stems from hydrogen peroxide’s oxidative action on the HPDC aluminum surface, which forms a dense chemical coating. This layer acts as a barrier against moisture ingress and prevents bonding degradation over time. Intermediate aging measurements revealed key trends in crack propagation behavior. The untreated substrate showed rapid crack growth in the first 2 weeks and slowed thereafter, indicating early moisture penetration and bond degradation. S-H₂O₂ samples exhibited a stable crack growth, confirming that the H₂O₂-treated surface forms a durable moisture barrier that delays degradation. S-SG200 samples showed an abrupt crack extension between 2 and 3 weeks, likely due to microcrack propagation in the sol–gel coating allowing electrolyte ingress. These intermediate data confirm that S-H₂O₂ not only has the shortest final crack length but also maintains a stable bonding performance throughout aging. The H₂O₂ treatment can be a promising approach for improving adhesive joint durability in HPDC aluminum, particularly in harsh environments.

Figure 9 presents the fracture images of chemically treated samples (S-APS, S-H₂O₂, S-SG80, S-SG200) alongside a control sample subjected to sandblasting and acetone degreasing. These images, combined with the wedge test crack length data, reveal distinct failure modes: cohesive failure, interfacial failure, and mixed failure. The S-APS and S-H₂O₂ samples, which exhibited full cohesive failure in the wedge tests, were further evaluated for environmental durability by exposing the bonded specimens to extreme conditions (70 °C, 100% relative humidity) for four weeks. As shown in Figure 8, post-aging wedge test measurements confirmed minimal crack extension for these samples. It presents cohesive failure modes despite harsh exposure and indicates robust environmental stability.

This study emphasizes chemical treatments as a simplified alternative to complex anodization processes, which often rely on hazardous acids and extensive pre/post-treatment procedures. In contrast, the S-SG80 and S-SG200 samples demonstrate a strong corrosion resistance, with the electrochemical impedance values of 9.9 ± 0.7 kohm·cm² and 3.4 ± 0.3 kohm·cm², respectively. However, they exhibited significant crack extension in wedge tests, primarily associated with interfacial failure [37]. The cerium-based sol–gel coatings, while offering commendable corrosion protection, failed to balance corrosion resistance with adhesion performance. This stems from weak interfacial adhesion between the sol–gel coating and HPDC aluminum substrate, as shown by the crack propagation along the coating–substrate interface in Figure 9. Future work will focus on the parameters influencing sol–gel coating formation to achieve both minimal crack propagation in wedge tests and resistance to crack extension under environmental stress.

Figure 10 presents a schematic diagram illustrating the failure modes of cerium-containing coated HPDC aluminum samples, with its analysis complemented by Figure 9. As shown in the schematic, the S-APS- and S-H₂O₂-treated samples exhibit cohesive failure characteristics, consistent with the observations in Figure 9. These samples display failure signatures dominated by adhesive tearing, with no visible bare metal exposure and substantial adhesive residue remaining on the fracture surfaces, indicating strong bonding at the coating–adhesive interface. In contrast, the substrate and S-SG80-treated samples show distinct interfacial failure patterns. Their schematics feature smooth, bare metal regions where the coating has delaminated from the substrate, with minimal adhesive residue left—clear indicators of weak bonding at the coating–substrate interface. The S-SG200-treated sample demonstrates a mixed failure behavior, combining characteristics of both cohesive and interfacial failure. Its schematic reveals partial areas with adhesive tearing alongside regions of bare metal where coating delamination occurred. Figure 10 provides critical visual clarification of the failure modes identified in conjunction with Figure 9, directly illustrating how different surface treatments influence bond integrity: cohesive failure for S-APS and S-H₂O₂, interfacial failure for the substrate and S-SG80, and mixed failure for S-SG200. This schematic can enhance the understanding of the relationship between cerium-based coating treatments and bond failure mechanisms in HPDC aluminum joints.

It is well established that, in coating systems, cerium typically exists as Ce³⁺ or undergoes oxidation to Ce⁴⁺ when exposed to condensed moisture and atmospheric oxygen. Ce³⁺ ions can transform into Ce⁴⁺ through the formation of cerium hydroxides and oxides at cathodic sites, a process driven by local increases in pH. Bare metal revealed by coating defects creates an environment where Ce⁴⁺ can revert to Ce³⁺, which in turn reduces cerium solubility. This dynamic redox cycle enables the coating to release cerium ions that can reprecipitate, potentially self-healing minor coating defects.

According to the wedge test results shown in Figure 5, the samples treated with sol–gel coatings (S-SG80 and S-SG200) exhibited a poor adhesion to high-pressure die-cast aluminum substrates. The sol–gel coating was nearly completely removed both before and after aging, indicating a low adhesive strength for this coating on HPDC aluminum AlSi10MnMg. This poor performance probably results from the lack of strong chemical or mechanical attachments of the sol–gel coatings to some metallic surfaces. While sol–gel methods are commonly used to create superhydrophobic surfaces or enhance bond strength with various additives, the specific formulation employed here shows limited effectiveness in achieving a durable adhesion on HPDC aluminum.

An advantage of the reported method is its elimination of multiple surface treatment steps. This simplified approach significantly enhances the environmental durability of adhesively bonded joints through a straightforward chemical treatment procedure. Current research efforts are focused on investigating precursor concentrations and thermal treatment temperatures to identify the optimal combination that maximizes barrier properties against corrosive environments. These ongoing studies aim to systematically optimize processing conditions to enhance both the corrosion resistance and long-term durability of the protective coatings.

4. Conclusions

This study presents a simplified chemical treatment methodology developed to enhance adhesive bonding performance in high-pressure die-cast aluminum AlSi10MnMg. The approach focuses on improving bond integrity and long-term durability through environmentally benign procedures, with effectiveness quantitatively assessed using wedge tests where crack length serves as the primary performance indicator. The chemical treatment significantly mitigated initial crack formation in HPDC aluminum specimens. H₂O₂-treated samples exhibited the shortest initial crack length, indicating a superior baseline bonding quality. Following aging under extreme environmental conditions (70 °C and 100% relative humidity for 4 weeks), the H₂O₂-treated specimens showed minimal crack propagation, with only a 14.75 mm increase in total crack length compared to other treatment samples. Electrochemical impedance spectroscopy provided additional confirmation of the corrosion resistance advantages, revealing that samples with cerium chemical layers showed lowered corrosion current densities, with the SG200-treated group achieving a value of 0.25 ± 0.03 μA/cm². These combined results demonstrate that the treatment method simultaneously enhances both the adhesive performance and the long-term durability. Although sol–gel coatings exhibit an excellent corrosion resistance, the crack propagation is remarkable during wedge tests. Among the tested methods, the H₂O₂-based chemical treatment (S-H₂O₂) is the optimal surface preparation method for HPDC AlSi10MnMg, balancing adhesive bonding strength, environmental durability, and corrosion resistance. Future research should prioritize developing sol–gel formulations with improved substrate bonding capabilities. In contrast, chemical coatings prepared using APS or H₂O₂ solutions demonstrated balanced performance advantages, combining enhanced corrosion resistance with reduced crack formation and propagation. SEM surface morphology analysis revealed uniform, dense cerium oxide films on treated samples, which directly correlated with cohesive failure modes observed in wedge test fractures. This failure mode confirms robust adhesive bonding, as cracks propagated through the adhesive layer rather than at the critical substrate–coating interface. This simple, cost-effective, and environmentally safe treatment offers substantial industrial applicability. It can serve as an alternative to conventional multi-step surface treatments that rely on hazardous chemicals.