The Effect of Oxidation Time on the Organization and Corrosion Performance of 6061 Aluminum Alloy Micro-Arc Oxidation Coatings

Abstract

This study investigates the influence of micro-arc oxidation (MAO) duration on the formation and characteristics of ceramic coatings on 6061 aluminum alloys. MAO treatments were conducted in a silicate-based electrolyte for varying durations. The effects of oxidation time on film thickness, surface roughness, morphology, and phase composition were meticulously assessed using a film thickness gauge, surface roughness meter, scanning electron microscopy (SEM), and X-ray diffraction (XRD) analysis. The corrosion behavior of the coatings was evaluated in a 3.5% NaCl solution using an electrochemical workstation. The experimental results demonstrate that both the film thickness and surface roughness exhibit a positive correlation with oxidation time. Notably, the film growth rate initially increased before plateauing, suggesting an optimal oxidation duration. The coatings primarily comprise γ-Al₂O₃, α-Al₂O₃, and Al phases, with the proportions of γ-Al₂O₃ and α-Al₂O₃ increasing progressively with extended oxidation times. Comparative electrochemical tests revealed that the treated specimens exhibited significantly higher corrosion potential and reduced corrosion current densities compared to the untreated aluminum alloy substrate. Specifically, a coating produced with a 20 min oxidation time demonstrated an exceptional corrosion current density (J_corr) of 1.545 × 10⁻⁶ A·cm⁻² and a corrosion resistance (R_p) of 2.716 × 10⁴ Ω·cm², signifying the best corrosion resistance achieved in this study. These results underscore the pivotal role of oxidation time in tailoring the properties of micro-arc-oxidized layers on 6061 aluminum alloys, offering valuable insights for enhancing corrosion protection in practical applications.

1. Introduction

Aluminum alloys are renowned for their advantageous properties such as wear resistance, thermodynamic stability, high electrical conductivity, and ease of machining and forming. These characteristics have positioned them as primary materials in lightweight manufacturing, and they have been extensively used in aerospace structural components and automotive manufacturing [1,2,3]. Despite these benefits, the application of aluminum alloys is significantly hampered by their poor corrosion resistance. To address this limitation, numerous protective techniques have been developed for industrial applications, aimed at enhancing the performance of aluminum alloys [4]. These techniques encompass surface mechanical treatments [5], ion implantation [6], physical vapor deposition [7], chemical vapor deposition [8], laser surface treatments [9], and micro-arc oxidation [10]. Many of the methods described above, such as physical vapor deposition (PVD) techniques, demonstrate commendable corrosion resistance for specific functional films, although their effectiveness in enhancing the key properties of aluminum alloys, such as their hardness and corrosion resistance, remains relatively limited. Furthermore, PVD processes often involve the use of toxic or hazardous gases, raising environmental and safety concerns [11]. Chemical vapor deposition (CVD), although capable of producing coatings with high hardness and excellent corrosion resistance, presents significant challenges in terms of process complexity and economic feasibility.

Micro-arc oxidation (MAO), also referred to as plasma electrolytic oxidation, is an advanced surface engineering technique for generating stable oxide layers on valve metals such as Al, Mg, and Ti. Compared to conventional anodizing methods that rely on acidic electrolytes, MAO offers significant advantages. It employs environmentally friendly alkaline electrolytes, which not only enhance the process’s sustainability but also result in coatings with superior corrosion resistance, exceptional wear resistance, and dense oxide structures that exhibit excellent adhesion to substrates. Additionally, the simultaneous application of a high voltage and current during MAO enables rapid coating growth, making it a highly efficient method for surface modification [12,13,14]. This comparative analysis highlights MAO as a promising alternative to conventional PVD and CVD methods, particularly for 6061 aluminum alloy treatment, offering a balanced combination of high performance, cost-effectiveness, and environmental sustainability [15].

However, many studies have shown that there are many defects and cracks in MAO coatings that lead to corrosion and poor wear resistance. These defects and cracks in MAO coatings are controlled by a variety of factors, such as electrical parameters, additives, electrolyte composition, and oxidation time [16]. Among these variables, oxidation time has been reported to have a significant effect on the corrosion and wear resistance of MAO coatings. Li et al. [17] examined the impact of oxidation time on the properties of micro-arc-oxidized coatings on aluminum alloys. Their study revealed that the coating’s thickness increased with oxidation time, and correspondingly, the film’s frictional wear decreased. Dos et al. [18] also investigated the effects of varying oxidation times on micro-arc-oxidized coatings prepared on 5052 aluminum alloys. They observed that prolonging the oxidation time resulted in increased oxide thickness and roughness while concurrently reducing material wear. However, they analyzed fewer cases of film corrosion. Several studies have reported the effect of MAO process parameters and electrolytes on the friction and corrosion properties of coatings. One study reported that the order of electrical parameters affecting the thickness and hardness of coatings is forward voltage, forward duty cycle, and pulse frequency and the order of electrical parameters affecting the corrosion resistance of coatings is pulse frequency, forward voltage, and forward duty cycle [19]. Zheng et al. [20] prepared ceramic coatings with different pulse frequencies on an LY12 aluminum alloy substrate using the micro-arc oxidation technique; the results show that film thickness, roughness, microporous size, and abrasion resistance increase with decreasing pulse frequency. Abbas et al. [21] explored the influence of electrical parameters and electrolyte composition on the microstructure of micro-arc-oxidized coatings on 7075 aluminum alloy substrates. Their research revealed a direct correlation between cathodic current and duty cycle with the coating’s thickness. The above studies show that micro-arc oxidation technology can improve the surface morphology of aluminum alloys, their friction resistance, and other properties, but the results for the final micro-arc oxidation application requirements are not enough, such as corrosion resistance. In addition, there are limited studies reporting the effect of oxidation time on the film wear performance and corrosion resistance of MAO coatings simultaneously.

This research endeavors to address the existing shortfall by leveraging micro-arc oxidation technology to develop ceramic-like oxide coatings on the surface of 6061 aluminum alloys, utilizing a silicate electrolyte system. Our research focuses on elucidating the influence of oxidation time on critical film attributes, including thickness, surface roughness, microstructure, phase composition, and element distribution. Furthermore, we utilized an electrochemical workstation to assess how oxidation time impacts the corrosion resistance of the coatings. The insights gained from this study are intended to provide robust data support for the practical application of ceramic coatings on 6061 aluminum alloys treated with micro-arc oxidation, thereby advancing the field of surface engineering for aluminum alloy materials.

2. Test Materials and Methods

2.1. Test Materials

The test material utilized in this study was a 6061 extruded aluminum alloy in T6 temper condition, which was sectioned into specimens measuring 20 mm × 15 mm × 3 mm using a wire cutting method. The T6 temper treatment involved a solution heat treatment at 530 °C for 2 h, followed by water quenching and subsequent artificial aging at 160 °C for 8 h. The chemical composition of the 6061 aluminum alloy is presented in

Table 1. Chemical composition of 6061 aluminum alloy (mass fraction/%).

Si	Mg	Fe	Cu	Mn	Cr	Zn	Ti	Al
0.4~0.8	0.8~1.2	0.7	0.15~0.4	0.15	0.25	0.25	0.15	Bal

. To ensure consistent and accurate results, the specimens underwent a meticulous surface preparation process prior to testing. The specimens were prepared using a standardized metallographic polishing procedure on a P-1 polishing machine. The polishing process consisted of three sequential stages: (1) initial rough polishing with 400-grit SiC abrasive paper for 2 min at a wheel speed of 500 rpm, (2) intermediate polishing with 800-grit SiC abrasive paper for 2 min under identical conditions to remove surface contaminants and oxidized layers, and (3) final precision polishing with 1200-grit SiC abrasive paper for 2 min to achieve a mirror-like surface finish. Between each polishing stage, the specimens were ultrasonically cleaned in ethanol for 5 min to prevent cross-contamination of abrasives. The final surface roughness (Ra) was measured to be less than 0.05 μm using a surface profilometer, ensuring consistent surface conditions for subsequent micro-arc oxidation treatment. To eliminate any remaining particulates and contaminants, the specimens were then ultrasonically cleaned with deionized water for a duration of 5 min. After cleaning, the specimens were air-dried to remove excess moisture before proceeding with the micro-arc oxidation treatments.

2.2. Micro-Arc Oxidation Preparation

The micro-arc oxidation setup comprised a 720V/30A power supply, an electrolysis tank, U-shaped stainless steel electrodes, and a water circulation cooling system (experimental micro-arc oxidizing equipment, Dongguan Micro-Arc Environmental Protection Technology Co., Ltd., Dongguan, China). The electrolyte was prepared with the following composition: 16 g/L Na₂SiO₃·9H₂O, 3 g/L NaOH, 2 g/L KF·2H₂O, and 3 g/L C₃H₈O₃ (AR, Xilong Science Co., Ltd., Shantou, China). The temperature of the electrolyte was maintained below 20 °C using a water circulation cooling system. The pH and conductivity of the electrolyte were measured using a PHS-3C pH meter and a DDS-307A conductivity meter, with readings of 12.25 and 23.4 mS/cm, respectively. During the process, the 6061 aluminum alloy specimen served as the anode, while the U-shaped stainless steel acted as the cathode. The process parameters were set as follows: a current density of 6 A/dm², a duty cycle of 30%, a frequency of 1000 Hz, and oxidation times of 15, 20, 25, and 30 min.

2.3. Performance Tests

The thickness of the micro-arc oxide layer was determined using a Minitest 2500 Digital Eddy Current Thickness Gauge (ElektroPhysik, Cologne, Germany), with thickness measurements taken at 25 random points on the specimen’s surface. Anomalies were excluded, and the average thickness value was calculated. A point was randomly selected on the surface of the specimen, with this point as the center, and four points were randomly selected in the four directions above, below, left, and right of this point. Measurements were made with a JITAI 820 surface roughness tester manufactured by Beijing JITAI Inspection Equipment Co., Ltd. (Beijing, China). Measurements were taken at five distinct locations on each sample, and the average roughness values were calculated to assess the surface roughness of the oxide coatings.

An elemental analysis of the oxide surfaces was performed using an Oxford (Xplore30. Aztec one, Oxford, UK) energy-dispersive spectrometer (EDS). The porosity, pore size, and number of pores on the surface of the oxide layer were calculated and counted using Image J software (version 1.54m). The phase composition of the coatings was analyzed using a Bruker (D8 ADVANCE, Billerica, MA, USA) X-ray diffractometer (XRD), with a scanning speed of 5 °/min and a scanning range of 20° to 90°. For electrochemical performance testing, a Shanghai Chenhua CHI660E electrochemical workstation (Shanghai, China) was employed. The test utilized a standard three-electrode system, with the micro-arc-oxidized sample serving as the working electrode, a saturated calomel electrode serving as the reference electrode, and a platinum electrode serving as the auxiliary electrode. The electrolyte used was a 3.5% NaCl solution. The scanning speed for the electrochemical tests was set at 5 mV/s, with a scanning interval from −2 V to 0 V. Prior to testing, the specimens were immersed in 3.5% NaCl solution for 30 min to ensure stable conditions. The open-circuit potential (OCP) was measured prior to both potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) measurements. The OCP was monitored for 15 min in the electrolyte until a stable value was achieved (potential fluctuation < 2 mV/min), ensuring the establishment of steady-state conditions before conducting subsequent electrochemical tests. The collected test data were analyzed using Zview 2 software to interpret the corrosion resistance characteristics of the oxide layers.

3. Results and Discussion

3.1. Process of Anode Voltage Change

Figure 1 illustrates the relationship between oxidation time and anode voltage during the micro-arc oxidation process. The voltage curve can be segmented into three distinct stages. During the initial rapid growth stage, the anode voltage spikes sharply, reaching 429 V within the first minute of oxidation. This corresponds to a growth rate of 429 V/min, indicating a vigorous onset of the oxidation process. The subsequent slow growth stage is characterized by a more gradual increase in anode voltage. By the 10 min mark, the voltage has risen to 501 V, showcasing a moderate growth rate during this intermediate phase. Finally, the stable growth phase ensues from the 10 min mark to the 30 min mark, where the voltage exhibits minimal fluctuations, stabilizing at a growth rate of 2.75 V/min. This phase suggests that the oxidation process has entered a more consistent and controlled state. These stages in the anode voltage curve provide insight into the dynamics of the micro-arc oxidation process and are crucial for understanding the formation and growth of the oxide layer on the 6061 aluminum alloy surface.

3.2. Effect of Oxidation Time on Film Thickness and Surface Roughness

3.2.1. Film Thickness

Figure 2 presents the impact of varying oxidation times on the film thickness of the micro-arc-oxidized layer. The data reveal a progressive increase in film thickness as the oxidation time is extended. The rate of thickness increase is more pronounced in the earlier stages of oxidation, slowing down as the process continues. At 15 min of oxidation, the film thickness reached approximately 11.0 μm, with an initial growth rate of 0.73 μm/min from the start to 15 min. When the oxidation time was increased to 20 min, the coating’s thickness grew to about 15.4 μm, with a growth rate of 0.88 μm/min between 15 and 20 min. Further oxidation to 25 min resulted in a rapid increase in film thickness to approximately 20.6 μm, with a growth rate of 1.04 μm/min from 20 to 25 min. Finally, at 30 min of oxidation, the film thickness was about 23.1 μm, but the growth rate diminished to 0.50 μm/min between 25 and 30 min. The analysis suggests that in the initial 15 min following the breakdown voltage, the coatings experienced numerous discharges, marking the beginning of growth despite the low discharge intensity. As oxidation time increased, the process entered a high-intensity discharge stage, leading to rapid film growth. The growth rate peaked between 20 and 25 min of oxidation. Between 25 and 30 min, the film thickness continued to increase, but the growth rate slowed due to the increased resistance of the coatings, which impeded the penetration of voltage and reduced the generation of molten oxide, thereby decreasing the film growth rate.

3.2.2. Surface Roughness

Figure 3 illustrates the influence of different oxidation times on the surface roughness of the micro-arc-oxidized layer. The data indicate that surface roughness increases progressively with oxidation time, although the rate of increase diminishes over time. Figure 3a shows the arithmetic mean roughness, Ra. Initially, during the 25 min prior to oxidation, Ra increased more rapidly, reaching an Ra value of 1.65 μm. Subsequently from 25 to 30 min, the increase in Ra slowed down, culminating in an Ra value of 1.75 μm. Figure 3b shows Rz, which represents the vertical distance between the highest peak and the lowest valley in the surface profile, and Rz rises faster in the time period of 25 min before oxidation to 10.81 μm, rises relatively slowly in the time period of 25~30 min, and the final Rz is 12.43 μm. The analysis suggests that during the early stages of micro-arc oxidation, the surface of the coatings transitions from a stage of weak micro-arc discharges to one of strong micro-arc discharges, intensifying the oxidation process and leading to an initial rapid increase in surface roughness. As oxidation time increases, the coating thickens, which in turn reduces its conductivity. This results in a partial discharge phenomenon occurring on the coating’s surface, filling the micropores and causing a gradual increase in roughness.

3.3. The Effect of the Oxidization Time on the Surface Morphology of the Coatings

Figure 4 depicts the SEM surface morphology of micro-arc-oxidized coatings subjected to different oxidation times. The analysis of these images provides insights into the structural changes that occur as the oxidation process proceeds. In Figure 4a,b, which represent the coatings at 15 min of oxidation, the surface is characterized by a prevalence of large pores and numerous small pores, with only a few small cracks visible. The accumulation of ceramic particles on the surface is relatively modest at this stage. Figure 4c,d illustrate the surface morphology after 20 min of oxidation. Here, the large pores have nearly vanished, and there is a significant increase in the accumulation of ceramic particles on the surface. However, fine cracks are still present. The SEM images in Figure 4e,f show the coatings at 25 min of oxidation. At this point, there is a slight increase in large pores, while the number of small pores decreases compared to the previous stages. The buildup of ceramic particles on the surface continues to grow, and the microcracks in the coatings are coarser than those observed at 20 min of oxidation. Finally, Figure 4g,h present the surface morphology after 30 min of oxidation. The number of macropores has increased, and the microcracks in the coatings are significantly rougher compared to the 25 min stage. The surface is uneven, with a proliferation of micropores of varying sizes and a substantial accumulation of ceramic particles. The observed surface morphology is a result of the micro-arc oxidation process, where the high temperature causes the surface of the 6061 aluminum alloy substrate to melt and oxidize. The molten material, which is ejected from the coatings due to the plasma discharge, quickly cools upon coming into contact with the electrolyte, forming a “volcanic rock”-like bulge on the surface, accompanied by the appearance of micropores [22]. As the oxidation time increases, more molten oxides gather on the surface, which increases the resistance to ejection from the interior, leading to larger bumps. The microcracks around the “volcanic rock”-like protrusions are attributed to the differential expansion coefficients and expansion directions of the constituent phases in the coatings. The transformation of amorphous phases into γ-Al₂O₃ and the subsequent conversion of γ-Al₂O₃ into α-Al₂O₃, due to volume changes and thermal stresses, result in the formation of microcracks.

To further analyze the surface morphology of the coatings, the surface porosity, pore diameter, and pore number were measured. The relationship between the surface porosity and pore size of the micro-arc-oxidized coatings and oxidation time is depicted in Figure 5. The data reveal that the porosity, maximum pore size, and pore size distribution exhibit a trend of decreasing and then increasing with increasing oxidation time. Figure 5a illustrates the porosity and maximum pore size as a function oxidation time. The porosity and maximum pore size initially decrease before increasing again as oxidation time progresses. For instance, at 15 min of oxidation, the coating’s surface had a porosity of 3.01% and a maximum pore size of 7.5 μm and was characterized by numerous “volcano-like” discharge pores. As the oxidation time increased to 20 min, the “volcanic rock”-like pores became smaller, with the coating’s surface porosity dropping to 1.79% and the maximum pore diameter dropping to 4.2 μm. After 25 min of oxidation, the coating’s surface exhibited the least number of discharge pores, with a porosity of 1.76% and a maximum pore size of 4.4 μm, representing the lowest porosity at that time. After 30 min of oxidation, while the porosity of the ceramic coatings increased, the maximum pore size decreased slightly. Figure 5b shows the pore size distribution as a function of oxidation time, which also tends to decrease and then increase with increasing oxidation time. The statistics on pore size distribution indicate that most discharge holes are within a range of 0 to 3 μm, with larger holes predominantly distributed between 1 and 2 μm and smaller holes between 0 and 1 μm. The maximum range of pore size distribution was observed after 15 min of oxidation. After 20 min, the number of discharge holes across all sizes was significantly reduced. The coatings after 25 min had the lowest number of surface discharge holes, but there were more holes with diameters between 1 and 3 μm compared to the coatings after 20 min. After 30 min, the number of discharge holes of various sizes increased.

These findings suggest that in the early stage of micro-arc oxidation, the rapid growth in voltage leads to an intense oxidation degree, resulting in the formation of a thin oxide film with a high number of conductive micropores and porosity reaching a maximum value. As oxidation time increases, the coating thickens, requiring more energy for discharge breakdown and leading to the gradual filling of small-sized channels with molten oxide. In the late stage of oxidation, the coating reaches its maximum thickness, its conductivity becomes poor, and discharge becomes difficult, resulting in a reduction in the number of discharge sparks. However, the volume of individual sparks increases, leading to the formation of larger ion discharge channels. Simultaneously, the reduction in the number of ion discharges decreases the film overlap, causing an increase in the number of discharge holes left on the surface of the layer.

Aluminum (Al) and oxygen (O) are predominantly found within the disk-like structures that are scattered across the surface (Figure 6). These disk structures are likely to represent the γ-Al₂O₃ phase, which is a common outcome of applying the micro-arc oxidation process to aluminum alloys. The concentration of Al and O within these structures suggests a high degree of oxidation and strong bond formation at these sites. Conversely, silicon (Si) is more prominently distributed in the regions surrounding the disk structures and within the holes of the coatings. This distribution pattern indicates that SiO₂, which is formed by the reaction of silicon in the alloy with the oxygen in the electrolyte, is associated with the less dense areas of the film. The SiO₂ phase contributes to the film’s porosity and may influence its mechanical properties and wear resistance. The variation in elemental concentrations across the coating’s surface highlights the dynamic nature of the oxidation process and the potential for tailoring the film’s properties by controlling the oxidation time and electrolyte composition.

Table 2. Surface composition of coatings prepared with different oxidation times.

Oxidation Time (min)	Element Content (wt.%)
	Al	O	Si	F
15	42.43	40.38	16.49	0.70
20	43.16	41.77	14.49	0.68
25	42.95	41.29	15.26	0.50
30	40.51	42.23	16.68	0.58

displays the surface composition of the ceramic coatings resulting from micro-arc oxidation for varying durations: 15, 20, 25, and 30 min. The coatings contain Al, O, Si, and F. While Al and Si are inherent elements of the 6061 aluminum alloy, the Si content is elevated relative to the alloy due to the incorporation of silicates from the electrolyte into the coating’s discharge channels. O element is primarily sourced from the electrolyte, F element is derived from the electrolyte KF. The primary constituents of the coatings are Al, O, and Si. The relative content of Al exhibits a slight increase followed by a decrease with increasing oxidation time. The O content remains relatively stable, while the Si content shows a trend of decreasing, then slightly increasing, and finally stabilizing. Despite some variations in elemental content with oxidation time, the overall composition of the oxide film is consistent, indicating that the extension of the oxidation time does not significantly affect the coating’s composition.

3.4. The Effect of the Oxidation Time on the Cross-Sectional Morphology of the Coatings

Figure 7 presents the SEM cross-sectional morphology of the coatings after oxidation for 15, 20, 25, and 30 min. The coatings are characterized by two distinct regions: a loose layer (Layer I) and a dense layer (Layer II). The loose layer is permeated with numerous micropores and has a less compact structure, whereas the dense layer is tightly adhered to the substrate. The coating’s thickness is observed to increase progressively with oxidation time, which is in agreement with the findings in Figure 2 and the conclusions drawn by Li et al. [23]. In Figure 7a, the cross-section after 15 min of oxidation reveals a thin film with a high number of micropores and an indistinct boundary between the loose and dense layers. Figure 7b shows after 20 min of oxidation, the coating has thickened, and the dense layer becomes more prominent, forming a strong metallurgical bond with the substrate without any visible defects. Figure 7c depicts the cross-section at 25 min of oxidation, where the coating continues to grow but begins to exhibit visible micropores and discharge channels within the dense layer. Finally, Figure 7d displays the cross-section at 30 min of oxidation, which shows minimal change from the 25 min layer, indicating that while the film thickness has increased, the overall morphology and the presence of micropores and discharging channels in the dense layer remain consistent.

3.5. The Effect of the Oxidation Time on the Phase Composition of the Coatings

Figure 8 displays the XRD patterns of the micro-arc-oxidized layers at various oxidation times. The patterns reveal prominent diffraction peaks corresponding to γ-Al₂O₃, α-Al₂O₃, and Al. The Al peaks are attributed to the 6061 aluminum alloy substrate, confirming that the ceramic coating has formed in situ on the aluminum surface. As the oxidation time increases, the relative intensity of the Al diffraction peak decreases, whereas the peaks for γ-Al₂O₃ and α-Al₂O₃ intensify. This trend suggests a decrease in the relative volume fraction of aluminum in the coatings and a corresponding increase in the volume fractions of γ-Al₂O₃ and α-Al₂O₃. The higher content of γ-Al₂O₃ relative to α-Al₂O₃ can be explained by the lower nucleation free energy of γ-Al₂O₃ compared to α-Al₂O₃. The rapid cooling of the high-temperature melt ejected from the discharge channels during micro-arc oxidation promotes the formation of the metastable γ-Al₂O₃ phase. With prolonged oxidation, the surface oxidation of the 6061 aluminum alloy becomes more complete, and there is a slight enhancement in the α-Al₂O₃ peak. This is likely due to the transformation of a small fraction of γ-Al₂O₃ into α-Al₂O₃ when the plasma discharge temperature reaches approximately 3000~10,000 K during the oxidation process, as reported in the literature [22,24].

3.6. The Effect of the Oxidation Time on the Electrochemical Corrosion Properties

Figure 9 presents the polarization curves for five specimens with varying oxidation times, measured in a 3.5% NaCl solution. The electrochemical parameters derived from these curves are summarized in

Table 3. Polarization curve parameters of coatings with different oxidation times.

Oxidation Time (min)	Ecorr (V)	Jcorr (A·cm−2)	Rp (Ω·cm2)
0	−1.374	1.077 × 10−4	4.314 × 102
15	−0.911	2.025 × 10−6	2.116 × 104
20	−0.831	1.545 × 10−6	2.716 × 104
25	−0.751	7.767 × 10−6	5.784 × 103
30	−0.732	4.982 × 10−6	8.448 × 103

. The data in Figure 9 and Table 3 reveal that the kinetic potential polarization curves of the coatings differ significantly with increasing oxidation time. Compared to the 6061 aluminum alloy substrate, the corrosion potential (E_corr) of the micro-arc-oxidized coating increases, while the corrosion current density (J_corr) decreases by two orders of magnitude, indicating a substantial reduction in corrosion rate. As the oxidation time increases, the ceramic coating thickens, enhancing its ability to block the corrosive medium and improving its corrosion resistance. After 20 min of oxidation, the corrosion current density reaches a minimum of 1.545 × 10⁻⁶ A·cm⁻², and the corrosion resistance (R_p) attains a maximum of 2.716 × 10⁴ Ω·cm⁻². However, when the oxidation time reaches 25 min, an increase in J_corr suggests a decline in the corrosion resistance. This is due to the appearance of coarse microcracks and discharge holes on the surface of the coatings. In the process of the electrochemical reaction, the radius of erosion ions is small, and they are preferentially adsorbed into the surface pores of the micro-arc oxidation coatings and microcracks and so on [25]. When the membrane layer surface microporous diameter is larger, the microcracks become coarser and erosive ions will pass through these defects quickly through the loose layer to reach the dense layer, thus reaching the aluminum substrate and then causing a corrosion reaction, which increases the sensitivity of the substrate to corrosion [26]. The corrosion resistance of the coating is also closely related to its density; a denser layer exhibits a stronger ability to isolate the external environment, resulting in improved corrosion resistance [27]. The optimal corrosion resistance is achieved after 20 min of oxidation, which correlates with the results observed in Figure 4 and Figure 7, indicating a uniform and dense coating at this time point. Consequently, the micro-arc-oxidized specimens exhibit significantly improved corrosion resistance, with the best performance reached after 20 min of oxidation.

To delve deeper into the influence of oxidation duration on the corrosion resistance of the specimens, Figure 10 exhibits the AC impedance spectra for five specimens that underwent different micro-arc oxidation periods. The equivalent circuit derived from the impedance spectrum is represented in Figure 11, and the fitted parameters are summarized in

Table 4. Fitting data for equivalent circuits.

Oxidation Time/(min)	Rs/(Ω·cm2)	CPE1/(Ω−1·sn·cm2)	n1	R1/(Ω·cm2)	CPE2/(Ω−1·sn·cm2)	n2	R2/(Ω·cm2)	Equivalence Model
0	38.54	8.182 × 10−6	0.877	3.208 × 103	----	----	----	R(QR)
15	25.61	3.870 × 10−7	0.766	7.853 × 103	2.197 × 10−6	0.649	1.584 × 104	R{Q[R(QR)]}
20	36.54	2.084 × 10−7	0.856	2.238 × 103	5.080 × 10−6	0.509	2.261 × 104	R{Q[R(QR)]}
25	40.39	6.813 × 10−7	0.779	4.047 × 103	3.146 × 10−5	0.899	1.127 × 103	R{Q[R(QR)]}
30	44.44	1.053 × 10−6	0.751	5.076 × 103	4.519 × 10−6	0.773	4.893 × 103	R{Q[R(QR)]}

. In this context, Rs stands for the resistance of the 3.5% NaCl solution, CPE₁ and CPE₂ represent the capacitance of the sparse and dense layers of the film, R1 is the resistance of the microporous layer of the micro-arc oxidation film, and R₂ is the polarization resistance of the aluminum alloy substrate that responds to corrosion. The Nyquist plot in Figure 10a shows that a larger capacitance arc radius correlates with a lower corrosion rate in the solution, which in turn signifies better corrosion resistance [28]. The specimen oxidized for 20 min demonstrates the best corrosion resistance, as evidenced by the largest arc radius. In the case of the micro-arc-oxidized ceramic coatings, a higher R₂ value indicates a stronger resistance to corrosive fluids, thereby reflecting improved corrosion resistance. The data in Table 4 reveal that the R₂ value for the 20 min-oxidized specimen is higher than those of the specimens oxidized for other durations, confirming that this coating has a reduced microporosity, greater density, and enhanced resistance to the penetration of aggressive ions, leading to superior corrosion resistance. The Bode plot in Figure 10b illustrates that the impedance modulus of the micro-arc-oxidized specimens is significantly greater than that of the 6061 aluminum alloy substrate. The impedance modulus, |Z|, is maximized at an oxidation time of 20 min, indicating that specimens that underwent this oxidation duration possess the highest corrosion resistance to the 3.5% NaCl solution. This outcome is in agreement with the polarization curves presented in Figure 9, providing a consistent assessment of the corrosion resistance attributes of the micro-arc-oxidized layers.

While the MAO process demonstrated in this study shows promising results for enhancing the corrosion performance of 6061 aluminum alloys, it is important to acknowledge its potential limitations. First, the scalability of the MAO process may pose challenges in industrial applications. The current experimental setup is optimized for small-scale laboratory conditions, and scaling up to larger substrates or continuous production lines may require significant adjustments in parameters such as the voltage, current density, and electrolyte flow rate. Future studies should investigate the feasibility of scaling up the process while maintaining the desired coating properties. Second, variations in electrolyte composition could significantly influence the MAO process and the resulting coating characteristics. In this study, we used a specific electrolyte formulation to achieve optimal results. However, in practical applications, the electrolyte composition may vary due to factors such as raw material availability or cost constraints. Such variations could affect the coating’s microstructure, adhesion, and corrosion resistance. Therefore, further research is needed to evaluate the robustness of the MAO process under different electrolyte conditions and to develop guidelines for adapting the process to varying compositions.

4. Conclusions

• As the micro-arc oxidation time increases, the thickness and surface roughness of the coatings also increase, with the “volcanic rock”-like morphology becoming more pronounced. The growth rate of the coatings exhibits an initial rise followed by a decline with extended oxidation time. Conversely, the porosity of the coatings, along with the maximum pore diameter and pore distribution, tends to decrease initially before increasing again as the oxidation time is prolonged.
• The oxidation time does not alter the physical phase composition of the coatings, which predominantly consists of γ-Al₂O₃, α-Al₂O₃, and Al. Notably, the relative content of γ-Al₂O₃ and α-Al₂O₃ within the coatings progressively increases with the extension of oxidation time.
• The micro-arc oxidation process markedly enhances the corrosion resistance of the specimens. Compared to the aluminum alloy substrate, the corrosion potential of the coatings after varying oxidation durations gradually increases, while the corrosion current density significantly decreases by two orders of magnitude. Specifically, the coating resulting from 20 min of oxidation exhibits a corrosion current of 1.545 × 10⁻⁶ A·cm⁻² and a polarization resistance of 2.716 × 10⁴ Ω·cm², showcasing the best corrosion resistance among the tested specimens.