Microstructure and Corrosion Behavior of PEO-Coated AA7075 Under Pulsed Unipolar Potential Control Mode

Abstract

Plasma electrolytic oxidation (PEO) has emerged as a promising surface coating technique producing high-quality ceramic coating for light metals like Al, Mg, Ti, and their alloys. AA7075 is one of the commonly used Al alloys for aircraft structures, gears and shafts, and automotives as it provides high yield and tensile strength. However, Al and its alloys have drawbacks that limit their further application. Thus, surface treatments are proposed to improve the metal and its alloy’s properties. In this study, the PEO of AA7075 was carried out with an AC power source under a pulsed unipolar potentiostatic mode at varying voltages of 425 and 450 V in 1000 Hz and at 80% duty cycles of 30 m. The effect of varying voltages on the morphology, coating thickness, and corrosion resistance of the PEO-coated samples was investigated. Surface morphology, elemental distribution, and phase composition were characterized using SEM, EDX, and XRD. A porous structure with a pancake-like shape, a crater, and nodular structures were observed with coating thickness ranges from 12.1 to 55.3 ± 4.67 µm. Al, α-alumina, and γ-alumina were detected in all surface coatings. The PEO-coated sample at 450 V exhibited higher corrosion resistance evaluated via potentiodynamic polarization and EIS.

1. Introduction

Aluminum (Al) and its alloys are the most widely used metals in different applications across the marine, aerospace, automotive, metal packaging, and transportation industries due to their unique properties owing to low density (2.7 g/cm³), high specific strength, and thermal and electrical conductivities [1,2,3]. Al 7075 (AA7075) is an Al alloy with zinc, magnesium, and copper as its main alloying elements and is commonly used for aircraft structures, gears and shafts, and automotive components. Despite the alloy’s extensive use in various applications, it is limited by its low hardness, high adhesion tendency, and poor corrosion and wear resistance. Thus, surface modification treatments are proposed to improve the alloy’s properties.

Plasma electrolytic oxidation (PEO), also known as micro-arc oxidation (MAO), is a promising surface coating technique that produces high-quality ceramic coating for light metals like Al, Mg, Ti, and their alloys. PEO is a surface modification treatment, similar to electroplating, PVD/CVD, and anodizing, that forms a protective coating to improve the metal’s surface properties [4]. PEO originates from the conventional anodic oxidation of light metals and their alloys in aqueous solutions but operates above the surface oxide’s breakdown voltage [5,6,7]. The formed coatings predominantly comprise of the substrate oxide but can also contain more complex oxides depending on the electrolyte [5,6,7]. They also possess high hardness, good wear resistance, and improved corrosion resistance. Although PEO originates from anodizing, it is simpler, more environmentally friendly, and more cost-effective. Unlike conventional anodizing, which typically uses acidic electrolytes, such as concentrated sulfuric acid, phosphoric acid, and other salts that can cause significant environmental harm, PEO uses environmentally friendly, water-based alkaline electrolytes without heavy metals [8,9]. This choice of electrolytes reduces the environmental impact of chemical waste disposal and minimizes risks to human health.

Currently, PEO is considered an emerging coating technology, valued for its superior surface coating performance. Compared to anodizing and commercial chromate conversion coatings, PEO offers higher hardness, better wear resistance, and improved corrosion resistance [10]. These enhanced coating properties lead to increased durability and a longer service life for coated components, which reduces the need for frequent replacements or maintenance. Additionally, PEO coatings are associated with lower emissions and improved energy efficiency. They are also compatible with lightweight alloys and offer good recyclability, contributing further to their sustainability and environmental benefits [11,12].

PEO is done through plasma micro-discharge events formed at potentials beyond the breakdown voltage. Micro-discharges generated at breakdown voltage contribute significantly to the enhancement of coating growth characteristics [13]. These micro-discharges are produced in different modes by applying constant voltage and current.

Electrical parameters play an important role in phenomena like voltage breakdown and local melting, and oxidation of substrates will be achieved during the process. Ultimately, these processes influence the final quality of the PEO layer. PEO can be accomplished in different modalities with respect to the way voltage or current is controlled during the process [13]. For example, the galvanostatic mode is a PEO process that applies a constant current. In this mode, the voltage rapidly increases as the coating growth increases over time. This mode offers greater control over thickness, with thicker coating associated, but less control over coating properties and a greater risk of uneven coating formation due to possible fluctuations. The potentiostatic mode, on the other hand, is PEO performed at a constant voltage, thereby preventing voltage fluctuations. In this mode, the current is limited by the conductivity of the oxide layer. A large current spike in the beginning is followed by a rapid fall in amplitude due to oxide layer growth. A maximum number and size of micro-discharge is exhibited at the beginning, when the current is at its maximum. As the current decreases, the individual micro-discharge size rises to a certain level and then decreases. In this manner, the potentiostatic mode limits the maximum micro-discharge energy and reduces high-intensity discharges detrimental to the oxide layer [14]. This mode offers better control over the coating properties by reducing the risk of uneven coating thickness. However, the potentiostatic mode for PEO is a slower coating process and requires a precise and optimized control of settings to have uniform coatings compared to the galvanostatic mode. Aside from a unidirectional electrical mode, the PEO process can be done with pulsed electrical regimes, such as the pulsed bipolar or pulsed unipolar regimes, as shown in Figure 1. These approaches are designed to create varying micro-discharge formation electrical regimes to prevent arcing during PEO as this limits the formation of more energetic and larger simultaneous micro-discharges [13]. A pulsed bipolar regime is set to have positive and negative half cycles, as shown in Figure 1a. A so-called “soft-sparking” is established for this mode as a result of the positive and negative charge ratio. The negative half cycle is responsible for the de-charging of the oxide layer and changing the dynamic behavior of the micro-discharges [13,15,16,17]. Yerokhin et al. claim that PEO under a pulsed bipolar regime provides better control over discharges, and an optimal combination of coating growth rate and energy consumption can be achieved at a pulse frequency between 1 and 3 kHz [18]. In a study by Zhu et al., an increase of negative current was observed to gradually improve the compactness of the coating and corrosion resistance, though excessive negative current will cause ablation on the sample surface [15]. For the pulsed unipolar mode, soft-sparking is not applicable. In this mode, only one polarity (positive or negative) is applied, as seen in Figure 1b, and the proportionality of duty cycle is considered to compensate for the pulse-on time (T_on) [13]. The pulse-on time (T_on) is the period when micro-discharges appear beyond breakdown voltage, while pulse-off time (T_off) is the period when there is no electrical pulse applied to the substrate. Shahzamani et al. reported that a pulsed unipolar DC current allows for control over the discharge duration by means of a duty cycle. A lower duty cycle and lower current density decrease the localized temperature discharge, lowering the probability of α-alumina and mullite formation [19]. Dehnavi et al. studied the effect of electrical parameters in a pulsed unipolar current regime at varying pulse frequencies, duty cycles and current densities on the phase distribution and composition of PEO-coated AA6061. They found that lower duty cycles may result in a slightly higher number of micro-discharges and lower intensity micro-discharges [20]. Azghandi et al. reported that an increase in both duty cycles and frequency led to a lower corrosion rate due to changes in the microstructure [21]. Similarly, a study by T. Arunnaellaiappan et al. found that coatings produced at a high frequency (1000 Hz) significantly improved corrosion resistance, albeit at a lower duty cycle. Thus, higher duty cycles and frequencies can enhance coating thickness and uniformity, further improving the corrosion resistance of coated samples [22]. Mengesha et al. reported that the breakdown voltage for the PEO coating decreased with an increasing duty cycle; however, the addition of borax to the electrolyte increased the breakdown voltage [23]. In another study, by Chen et al., PEO coatings produced at higher duty cycles resulted in a thinner coating with larger pore sizes, but this coating had better corrosion resistance due to a more compact microstructure [24].

The majority of research tends to conduct PEO under the galvanostatic mode due to the greater level of control it offers over uniform coating and oxide thickness. This mode ensures steady oxide growth, even heat distribution, and controlled reaction kinetics. However, between the two electrical modes, the potentiostatic mode offers greater control over micro-discharges and can prevent excessive high electrical discharges that cause localized damage, cracking, or uneven coating formation, which affects the coating properties. Currently, there are limited studies that have explored the properties of PEO coatings on Al alloys created under the potentiostatic mode in a pulsed unipolar regime. The use of this oxidation mode could provide unique surface PEO microstructures on the Al alloy that may lead to interesting properties. This study investigates the microstructure and corrosion behavior of PEO coating on AA7075 created via a pulsed unipolar mode under potential control (potentiostatic). The PEO treatment was conducted in a silicate-based electrolyte under constant frequency, duty cycle, and treatment time. The applied potentials were determined from the breakdown voltage of the AA7075 with respect to the electrolyte’s properties. The duty cycle and frequency were chosen based on studies examining their effects on corrosion behavior in PEO coatings. Research indicates that a high frequency (1000 Hz) improves coating uniformity, results in a finer microstructure, and reduces heat generation, which minimizes thermal stress on the substrate. Conversely, a high duty cycle (80%) enhances coating growth and thickness but can lead to higher porosity. Nonetheless, both a high frequency and high duty cycles can improve corrosion resistance when optimized. The surface morphology and the elemental and phase composition of the PEO AA7075 samples were characterized using SEM, EDX, and XRD. The corrosion behavior was characterized using potentiodynamic polarization and electrochemical impedance spectroscopy in a 3.5 wt% NaCl solution.

2. Materials and Methods

2.1. Materials and PEO Process

An aluminum alloy 7075 (0.4% Si, 0.5% Fe, 1.2%–2.0% Cu, 0.3% Mn, 2.1%–2.9% Mg, 0.18%–0.28% Cr, 5.1%–6.1% Zn, 0.2% Ti) sheet with dimensions of 3 cm × 3 cm × 0.4 cm was used as a substrate. A corner side of each substrate was mechanically punched to create a hole for the connection of the AA7075 to the anode terminal of the PEO equipment using Al wire. Prior to the PEO treatment, substrates were ground and polished using silicon carbide papers at 240–1200 grit size, then ultrasonically washed in acetone, ethanol, and deionized water for 10 m. Substrates were subsequently acid pickled in nitric acid dip (500 mL 67% HNO₃ in 1 L distilled water) for 3 m, then rinsed with DI water and air dried. The samples were subjected to a water break test after acid pickling to check the surface cleanliness. This is to ensure that the surface is free from oil, fingerprints, and other hydrophobic contaminants by splashing running water. All samples after the acid pickling showed a single well-defined and continuous sheet of water indicating a clean surface.

The electrolyte used for this study consisted of 3 g/L sodium metasilicate [Na₂SiO₃, Sigma–Aldrich, St. Louis, MO, USA] and 2 g/L sodium hydroxide [KOH, Sigma–Aldrich]. The conductivity, pH, and viscosity of the electrolyte were measured over a range of temperatures (25–40 °C) using a handheld conductivity meter (Horiba Scientific, Kyoto, Japan, LAQUAact-EC110), pH meter (Horiba Scientific, LAQUAact-PH110), and viscometer (Visco, Atago, Bellevue, WA, USA). Results from this test are shown in

Table 1. Measured conductivity, pH, and viscosity of the PEO electrolyte.

Na2SiO3:KOH (g/L)	Conductivity (mS/cm)		pH		Viscosity (cP)
	25 °C	40 °C	25 °C	40 °C	25 °C	40 °C
3:2	13.26	14.21	12.63	12.23	5.39	4.90

. The PEO was carried out in a pulsed unipolar mode at 1000 Hz, 80% duty for 30 m, as summarized in

Table 2. PEO electrical parameters used in the study.

Sample Code	Voltage (V)	Frequency (Hz)	Duty Cycle (%)	Ton (ms)	Toff (ms)
HC425	425	1000	80	0.8	0.2
HC450	450

. A low and high voltage of 425 V and 450 V were set as the oxidation potentials for the process while maintaining an electrolyte temperature of 25–40 °C. After the coating process, substrates were washed with DI water and then air-dried.

2.2. Characterization of PEO Coatings

The morphological features, elemental composition, and coating thickness of PEO-coated samples were characterized by evaluating their surface and cross-section coating using a scanning electron microscope (SEM, Hitachi SU3800, Tokyo, Japan) and energy dispersive X-ray spectroscopy (EDX, Oxford Ultim Max 40, Oxford, UK). Average coating thickness and pore size diameters were measured using ImageJ with at least 10–50 points in each substrate. The phase composition of the surface coatings was evaluated via an X-ray diffractometer (XRD, Shimadzu XRD-1700 PharmaSpec, Kyoto, Japan) using Cu Kα radiation at the 2θ range of 10° to 90° with a 0.02° step size.

The sample’s corrosion performance was evaluated via cyclic potentiodynamic polarization (CPP) and electrochemical impedance spectroscopy (EIS) in a 3.5 wt% NaCl aqueous solution using SP150 Biologic potentiostat (Orlando, FL, USA). A three-electrode system was employed for the corrosion test, consisting of Ag/AgCl as the reference electrode; platinum as the counter electrode; and test the specimen, with an exposed surface area of 0.7854 cm², as the working electrode. The CPP test was performed by polarizing the sample over the potential range of −1.0 to 1.5 V vs. OCP at a 10 mV/s scan rate. The EIS test was conducted using ± 10 mV amplitude in the frequency range of 10 mHz to 100 kHz. To ensure the reliability and consistency of the experimental results, each tested sample was subjected to three replicate trials and the mean values were reported for the electrochemical studies.

3. Results and Discussion

3.1. Surface Morphology of PEO Coatings

Figure 2 shows the surface morphology of PEO-coated AA7075 at different potentials: 425 and 450 V in a pulsed unipolar potentiostatic mode. The typical PEO porous structure was observed for both samples. The porous structure represents the plasma discharge channels generated during the process, which is consistent for all PEO coating [25]. Both PEO-coated samples at low and high voltage had the same uniform surface morphologies of a pancake or crater-like and nodular structure, as shown in Figure 2a–d. The average pore diameter of the pancake structure was 2.25 ± 0.83 µm for the 425 V sample and 4.05 ± 1.26 µm for the 450 V sample. The cratered and pancake-like structures are discharge channels where molten material has been ejected from the coating–substrate interface due to high temperatures and a strong electric field. Upon ejection of molten alumina, the material rapidly solidifies when it comes into contact with the electrolyte. Most studies have reported that nodular structures are rich in Si and are typically observed near the surroundings of the cratered region. It is anticipated from a silicate-based electrolyte that the SiO₃²⁻ will generate SiO₂ or SiO₂·nH₂O on interface between layer and electrolyte. These nodular structures are formed when a molten alumina is ejected from the discharging channels at high temperature and combines with the SiO₂ compound, forming the mullite phases (Al₂O₃·nSiO₂). As the mullite is punched around the discharging channel by the force of the plasma sparks, it then suddenly solidifies when it comes into contact with the electrolyte [20,26,27].

Some microcracks were observed in both samples due to thermal stress and high pressure created by the high reaction temperature at the higher deposition potential observed in Figure 2b,c. Pore size appears to be directly related to the applied potential. The larger pore size obtained at the higher potential (450 V) was likely due to the larger and more intense discharge channels generated at such a high input energy. In contrast, the pore density was inversely proportional to the applied potential, and, by extension, to the generated pore diameter, as seen in Figure 3. The decrease in pore density can be related to fewer large discharges present within the surface area at a higher voltage, which is consistent with findings from Wang et al. and Nadaraia et al. [28,29]. The decrease in pore density at the higher voltage is due to high energy discharges that can promote intense localized melting and re-solidification of the coating material, reducing the pore size and density as the molten material can fill the gaps and voids. However, a few large and intense discharges are still generated and often occur at the weak sites of the substrates, resulting in large pore diameters. Figure 4 shows the histogram of pores size distribution on the PEO-coated samples. The histogram reveals that the 425 V sample predominantly featured smaller pores, ranging from 0–2 µm, whereas the 450 V sample showed a more prominent pore size of 2–3 µm. This observation is consistent with the average pore sizes measured.

The coating thickness of the PEO-coated AA7075 samples was observed from a cross-section of the specimens and measured using ImageJ, as shown in Figure 5. The measured average coating thickness of the 425 V and 450 V samples was 12.1 ± 0.68 µm and 55.3 ± 4.67 µm, respectively. The Al specimen oxidized at 450 V exhibited a higher coating thickness (greater than four times higher) and higher compactness as compared to Al oxidized at 425 V, as shown in Figure 5b. This was expected, as the higher applied potential enhanced the rate of oxide generation. This is because during the PEO process, electrochemical reactions are intensified at higher voltages, increasing the rate at which metal ions are oxidized, which accelerates the formation of the oxide layer. The frequency and intensity of micro-discharges are also enhanced and increased at higher voltages. These micro-discharges are critical in the formation of the oxide layer as it provides the high temperature and the pressure necessary for localized melting and re-solidification in both substrates, which grow the oxide layer. Moreover, ion transportation is enhanced as a strong electric field is generated at higher voltages, promoting a faster diffusion of ions through the oxide layer, which leads to rapid oxide layer formation. The growth of the coating is similarly promoted at a high voltage as the intense discharges are also capable of penetrating the oxide layer and inducing oxidation of the underlying metal substrate. At an elevated voltage, the activation energy is reduced thus allowing the process to occur at a faster and more efficient rate, resulting in the formation of a thicker and denser coating. Furthermore, it is also good to take note that agitation is crucial in the PEO process, especially at high voltages, which generates significant heat due to high energy micro-discharges. This will dissipate the heat away from substrate’s surface and maintain the temperature of the electrolyte bath, preventing localized overheating that can cause thermal stress and damage to the coating. In addition, agitation will aid in maintaining a homogeneous concentration of electrolyte components throughout the bath, ensuring a constant supply of reactive ions to the reaction sites, which prevent local depletion and maintain the rate of oxide formation and uniform coating. However, despite the enhanced growth rate of the oxide layer through a high voltage, an excessive voltage can cause uneven coating thickness, excessive surface roughness, cracks, porosity, and other structural defects. Thus, optimizing the control of applied voltages is essential in preventing coating deterioration.

3.2. Elemental and Phase Composition of the PEO Coatings

Figure 6 shows the results from SEM–EDX analysis of the PEO-coated samples at (a) 425 V and (b) 450 V. Based on the EDX analysis, the main elemental composition formed on the surface coating is represented by Al, O, and Si.

Table 3. EDX elemental content of PEO-coated AA7075 at 425 V and 450 V.

wt%	Al	O	Si	Mg	Na
HC425	46.55	44.74	6.2	1.5	1.1
HC450	41.15	45.31	10.9	1.56	1.04

shows the elemental wt% content of the PEO-coated samples. Other elements, like Mg and Na, were attributed to the substrate’s alloying elements and the electrolyte’s composition. Element composition mapping of the PEO samples showed that Al and O were evenly distributed throughout the surface coating. A higher O wt% content at 450 V indicates that more surface oxide was present and can correlate to a higher coating growth rate at a higher voltage.

More Si was also detected in the coating generated at 450 V. The high local temperatures likely promoted the transformation and subsequent incorporation of Si ions and other constituents from the electrolyte into the generated oxide layer. During oxidation, molten products are ejected along the discharge channels, forming nodular structures at the coating–electrolyte interface. This nodular structure present in the surroundings of the crater-like structure was confirmed to be Si-rich based on the EDX analysis. The XRD pattern of PEO-coated AA7075 at 425 and 450 V is presented in Figure 7. The strong diffraction peaks (38.79, 44.91, 65.3, and 78.44) of Al detected in all specimens are from the substrate [30]. Moreover, peaks of aluminum oxide (Al₂O₃) corresponding to both γ- and α-alumina were present. Aluminum, γ-alumina, and α-alumina were identified using ICDD 01-085-1327, 00-050-0741, and 00-010-0173, respectively. α-alumina is a stable oxide phase with a trigonal crystal structure and a high melting point of 2050 °C, while γ-alumina is metastable with a lower melting point compared to α-alumina of around 80–1200 °C [31,32]. It is believed that during the preliminary stages of oxidation, γ-alumina is initially formed and transforms to α-alumina at higher temperatures [29,32]. Although nodular structures were identified to be Si-rich in the SEM–EDX analysis, no mullite phase was detected from the XRD pattern for both samples. This was unexpected as most of the other studies reported by Tavares et al. and Denhavi et al. claimed to observe the presence of mullite phase alongside the nodular structures [20,33]. The absence of mullite phase in both samples indicates that the nodular structures are purely Si-rich. These formed nodular structures are possibly structures that do not diffuse or combine with alumina to form mullite under rapid processing conditions. The formation of Si nodular structures instead of mullite phase could be primarily associated with rapid, localized, and non-uniform heating during the process. Typically, mullite is formed at temperatures above 1200 °C; although the PEO process can generate high temperatures, it is possible that mullite formation did not occur due to a rapid cooling of the oxide layer where the required temperature and reaction time necessary for alumina (Al₂O₃) and silica (SiO₂) to form were not sustained sufficiently. The formation of Si-rich nodular structures in both samples indicates that the setting conditions in the study did not favor the reaction of alumina to form mullite. The absence of Si peaks in the XRD could be due to its lower content and possibly detection limitations of the XRD machine.

3.3. Corrosion Behavior of PEO-Coated AA7075

Figure 8 shows the potentiodynamic polarization curves of the bare AA7075 and PEO-coated samples carried out in 3.5 wt% NaCl solution and tested through a potential range of −1.75 to 1.5 V. The corrosion potential (E_corr) and corrosion current density (i_corr), were evaluated from potentiodynamic polarization curves via the Tafel fit method. The corresponding corrosion relevant values are presented in

Table 4. Corrosion parameters derived from potentiodynamic polarization curves.

Sample	Ecorr (V)	icorr (µA/cm2)
Bare AA7075	−1.121 ± 0.015	30.99 ± 8.69
HC425	−0.826 ± 0.026	0.82 ± 0.59
HC450	−0.744 ± 0.004	0.19 ± 0.16 s

. The E_corr represents the thermodynamic tendency of the sample to corrode, while i_corr is proportional to the corrosion rate. From Figure 8 and Table 4, the bare AA7075 sample exhibited a significantly higher i_corr in its potentiodynamic polarization curve when compared to the PEO-coated samples. Initially, the AA7075 sample formed a passive film, which provided a protective barrier against corrosion. However, as the potential exceeded a threshold, a rapid onset of localized corrosion occurred, specifically pitting corrosion. This phenomenon was anticipated, as bare AA7075 is known to be susceptible to pitting corrosion when the applied potential surpasses its pitting potential. The experiment was terminated before reaching the set final potential due to the rapid onset of pitting corrosion. On the other hand, the PEO-coated samples shifted to more positive corrosion potentials accompanied by a lesser corrosion current density at increasing voltage compared to the bare AA7075, with the sample created at 450 V exhibiting the highest E_corr and lowest i_corr values. These indicate that the PEO-coated samples exhibited better corrosion resistance behavior than the uncoated Al, likely due to the coatings’ ability to prevent interaction between the corrosive environment and the substrate surface. This confirms the PEO coating’s ability to enhance the quality of Al. The pitting potential (Ep) is another parameter that can be determined from the potentiodynamic polarization curve. The pitting potential describes the potential at which the surface oxide breaks down and pitting occurs. A more positive E_p indicates better resistance of the metal to pitting corrosion. From Figure 8, pitting potential can be found in the anodic branch of the curve. The bare AA7075 and the PEO-coated sample at 425 V had E_p values of −0.56 V and −0.578 V vs. Ag/AgCl, respectively, while the PEO-coated sample at 450 V did not show an E_p value at the tested potential range. This clearly indicates that the 450 V sample had much more positive Ep values (i.e., occurring at potentials above 1.5 V).

Electrochemical impedance spectroscopy (EIS) measurements of the samples were carried out to analyze further the corrosion behavior of the PEO coatings. This test evaluated the properties of the coating in terms of the relevant phenomenon occurring (e.g., charge-transfer, diffusion) at the electrolyte–electrode interface and the corrosion resistance via impedance assessment [30]. Figure 9 shows the Nyquist and Bode plots of the bare AA7075 and PEO-coated samples in 3.5 wt% NaCl. The plotted symbols correspond to the experimental EIS data of the samples and the solid lines are for theoretical fitting results. The equivalent circuit (EC) models used for the mathematical fitting of the EIS data are seen in Figure 10, while the corresponding circuit element values of the bare AA7075 and PEO-coated samples are listed in

Table 5. Summary of corresponding EIS equivalent circuit values after mathematical fitting.

Specimen	Rs/Area (Ω/cm2)	Q1 (F.s(a − 1)/ cm2)	a1	R1/Area (Ω/cm2)	Q2 (F.s(a − 1)/ cm2)	a2	R2/Area (Ω/cm2)	Q3 (F.s(a − 1)/cm2)	a3	R3/Area (Ω/cm2)	X2/|Z|
Bare 7075	18.93 ± 0.24	1.52 × 10−5 ± 1.6 × 10−8	0.955	5.76 × 103 ± 4.9 × 10−1	-	-	-	-	-	-	0.405
425 V	33.96 ± 0.54	8.80 × 10−7 ± 1.27 × 10−7	0.4136	6.06 × 104 ± 4.38 × 103	2.25 × 10−8 ± 1.15 × 10−9	0.9652	1.25 × 102 ± 4.38 × 100	-	-	-	0.030
450 V	26.33 ± 0.27	2.79 × 10−8 ± 2.42 × 10−10	0.9581	1.57 × 104 ± 7.19 × 101	2.26 × 10−6 ± 1.37 × 10−9	0.9505	1.26 × 105 ± 5.65 × 101	9.18 × 10−7 ± 4.02 × 10−9	0.6782	4.66 × 104 ± 1.23 × 102	0.320

.

The Nyquist plot in Figure 9a shows that the uncoated AA7075 had the smallest impedance among the tested specimens, consistent with its poor corrosion behavior. Conversely, the high impedance values in the PEO-coated samples confirm their improved corrosion resistance, particularly the 450 V sample, which exhibited the highest impedance among the tested specimens. These observations are consistent with the results from the polarization tests. Analysis of the Bode plot shows that the bare AA7075 possesses a prominent time constant at the middle frequency, with the high phase angle (~80°) suggesting a strongly capacitive element. This is consistent with the EC model to describe the bare alloy’s EIS behavior (Figure 10a), consisting of a resistor-capacitor pair. In this model, Rs corresponds to the solution resistance, and Q1 and R1 are the surface film capacitance, represented by a constant phase element (CPE) and film resistance, respectively. This model typically describes an intact surface, which reflects the expected passive layer formed at the aluminum surface. CPEs are used to describe the behavior of a non-ideal capacitor. The unit of a CPE is F⋅s^{(a − 1)}, indicating Farads per second. The dimensionless parameter “a” ranges from 0 to 1. When a = 1, the CPE behaves like an ideal capacitor, while for a < 1, it represents a system that behaves more like a resistor or a non-ideal capacitive element [30,34].

For the PEO-coated specimens, the 425 V sample had two time constants of a smaller diameter at high frequency and a larger diameter at mid-frequency, while the 450 V sample showed two time constants at high frequency and a large diameter at low frequency, which is equivalent to two time constants, respectively. These two time constants correspond well with the EC models describing the coated specimens’ EIS behavior (Figure 10b,c, respectively). The EC model in Figure 10b describes a porous film layer on the coated sample at 425 V, where Rs is the solution resistance, Q1 refers to the PEO coating’s capacitance (CPE), R1 is the film’s pore resistance, and Q2 and R2 refers to the double-layer capacitance (CPE) and the charge transfer–film resistance, respectively. Although, the coated sample at 450 V shows two recognizable time constants observed from the bode plot in Figure 9b with a larger diameter at low frequency, its equivalent circuit fits to the EC model in Figure 10c with three RC pairs. The EC model in Figure 10c corresponds to layer coatings where Rs is the solution resistance, Q1 refers to the PEO coating’s capacitance (CPE), R1 is the film’s pore resistance, Q2 and R2 refers to the dense inner layer capacitance (CPE) and dense inner resistance, and Q3 and R3 refers to the double-layer capacitance (CPE) and the charge transfer–film resistance, respectively.

The porous film model is consistent with the porous structure of the PEO coating, as seen from the SEM images. The film’s strong impedance, particularly exhibited by the 450 V specimen, can be traced from the dense inner layer and very small value of CPE and high charge transfer–film resistance (~10⁴ Ω), in agreement with those reported for PEO-coated Al [35]. This EIS data simply confirms the good corrosion resistance of the PEO-coated samples (in particular, the 450 V specimen) and agrees well with the results from the potentiodynamic polarization.

Overall, the 450 V sample offered the best corrosion resistance among the tested alloys. The excellent corrosion performance of the 450 V sample can be associated with its higher coating thickness and a more stable oxide phase on the surface coating. Although the 450 V sample has a larger pore diameter than the 425 V specimen, its pore density is lower. This may have reduced the opportunity for extensive contact between Cl-ions and the substrate. It is known that surface coating bearing less porosity/defects, greater thickness, and a stable phase composition would be beneficial as it can provide suitable corrosion protection to the metal substrate in an aggressive environment [28].

3.4. Oxide Growth Mechanism

The PEO process under the potentiostatic mode follows the same mechanism stages that occur in the galvanostatic mode, such as anodic oxidation, spark discharge, and micro-arc discharge. Unlike the galvanostatic mode, where voltage is increased to maintain the current density, for the potentiostatic mode, a constant voltage is set at or beyond the breakdown voltage range with respect to the metal/metal alloy substrate and the electrolyte’s conductivity and pH. During the process, a passive film is expected to form on the metal substrate, followed by avalanches of micro-discharges when dielectric breakdown is reached, forming a porous coating layer [36,37]. The dielectric breakdown/resistance of the oxide layer in a potentiostatic mode tends to increase as the coating layer thickens. Thus, the set constant voltage will not be able to maintain the discharge intensity for long processing times. This is confirmed from a visual observation of the process, as shown in Figure 11. During the first few minutes of oxidation, intense micro-discharges were observed on the entire substrate, as seen in Figure 11a. However, over time, the number of micro-discharges eventually decreased as the micro-sparks gradually converted to micro-arcs, as seen in Figure 11b.

In the pulsed unipolar mode, t_on and t_off are defined as periods where voltage/current is ‘on’ or ‘off’ in a single cycle and the duty cycle is defined by Equation (1):

$${\mathrm{D}}_{\mathrm{t}}={[\mathrm{t}}_{\mathrm{o}\mathrm{n}}/{(\mathrm{t}}_{\mathrm{o}\mathrm{n}}+{\mathrm{t}}_{\mathrm{o}\mathrm{f}\mathrm{f}}\left)\right]\times 100$$

(1)

During PEO coating, different processes such as oxide formation, dissolution, and gas evolution occur in the alkaline solution. The formation mechanism ${\mathrm{A}\mathrm{l}}_2{\mathrm{O}}_3$ in the PEO process is described by the following equations below. At the t_off period where there is no voltage/current applied, chemical reactions etch the Al substrate, leading to the formation of aluminate ion $(2{\mathrm{A}\mathrm{l}\mathrm{O}}_2^{-}$) and $\mathrm{A}\mathrm{l}{\left(\mathrm{O}\mathrm{H}\right)}_4^{-}$ in the electrolyte, as shown in Equations (2) and (3) [19,38]:

$$2\mathrm{A}\mathrm{l}+2{\mathrm{H}}_2\mathrm{O}+2{\mathrm{O}\mathrm{H}}^{-}\to 2{\mathrm{A}\mathrm{l}\mathrm{O}}_2^{-}\left(\mathrm{a}\mathrm{q}\right)+{3\mathrm{H}}_2$$

(2)

$$\mathrm{A}\mathrm{l}+4{\mathrm{O}\mathrm{H}}^{-}\to \mathrm{A}\mathrm{l}{\left(\mathrm{O}\mathrm{H}\right)}_4^{-}\left(\mathrm{g}\mathrm{e}\mathrm{l}\right)$$

(3)

The aluminum hydroxide could be re-dissolved by ${\mathrm{O}\mathrm{H}}^{-}$

$$\mathrm{A}\mathrm{l}(\mathrm{O}{\mathrm{H})}_3+{\mathrm{O}\mathrm{H}}^{-}\to \mathrm{A}\mathrm{l}{\left(\mathrm{O}\mathrm{H}\right)}_4^{-}$$

(4)

and aluminum oxidation may also occur;

$$2\mathrm{A}\mathrm{l}+3{\mathrm{H}}_2\mathrm{O}\to {\mathrm{A}\mathrm{l}}_2{\mathrm{O}}_3+{3\mathrm{H}}_2\uparrow$$

(5)

The surface the of Al substrate is chemically dissolved in ${\mathrm{O}\mathrm{H}}^{-}$, originating from the KOH in the electrolyte (6). The ${\mathrm{O}\mathrm{H}}^{-}$ anions diffuse due to the electrode surface and chemical processes.

$$\mathrm{K}\mathrm{O}\mathrm{H}\to {\mathrm{K}}^{+}+{\mathrm{O}\mathrm{H}}^{-}$$

(6)

At the ${\mathrm{t}}_{\mathrm{o}\mathrm{n}}$ period, when the voltage is above the breakdown voltage, micro-discharges start to appear. The electrolyte components KOH (6), water (7), and Na₂SiO₃ (8) start to decompose and ionize during this period, due to the high electric field generated during the PEO process:

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

(7)

$${\mathrm{N}\mathrm{a}}_2{\mathrm{S}\mathrm{i}\mathrm{O}}_3{\to 2\mathrm{N}\mathrm{a}}^{+}+\mathrm{S}\mathrm{i}{\mathrm{O}}_3^{2-}$$

(8)

Anodic dissolution of aluminum releases cations into the electrolyte (9). Under the high electric field, oxygen anions (${\mathrm{O}}^{2-})$ migrate towards the anode and react with aluminum cation ${(\mathrm{A}\mathrm{l}}^{3+}$) to form aluminum oxide (10). Some aluminum cations ejected from the electrolyte might react with hydroxide (11) or silicate (12):

$${\mathrm{A}\mathrm{l}\to \mathrm{A}\mathrm{l}}^{3+}+{3\mathrm{e}}^{-}$$

(9)

$${2\mathrm{A}\mathrm{l}}^{3+}+{3\mathrm{O}}^{2-}\to {\mathrm{A}\mathrm{l}}_2{\mathrm{O}}_3$$

(10)

$${\mathrm{A}\mathrm{l}}^{3+}+{3\mathrm{O}\mathrm{H}}^{-}\to {\mathrm{A}\mathrm{l}\left(\mathrm{O}\mathrm{H}\right)}_3$$

(11)

$${2\mathrm{A}\mathrm{l}}^{3+}+3\mathrm{S}\mathrm{i}{\mathrm{O}}_2^{2-}\to {\mathrm{A}\mathrm{l}}_2({{\mathrm{S}\mathrm{i}\mathrm{O}}_3)}_3$$

(12)

In addition, the thickness of alumina coating can decrease due to chemical dissolution:

$${\mathrm{A}\mathrm{l}}_2{\mathrm{O}}_3+{2\mathrm{O}\mathrm{H}}^{-}+3{\mathrm{H}}_2\mathrm{O}\to \mathrm{A}\mathrm{l}{\left(\mathrm{O}\mathrm{H}\right)}_4^{-}\left(\mathrm{g}\mathrm{e}\mathrm{l}\right)\to 2\mathrm{A}\mathrm{l}({\mathrm{O}\mathrm{H})}_3\downarrow +{\mathrm{O}\mathrm{H}}^{-}$$

(13)

$$\mathrm{A}\mathrm{l}{\left(\mathrm{O}\mathrm{H}\right)}_4^{-}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{A}\mathrm{l}\mathrm{O}}_2\mathrm{H}\downarrow +{\mathrm{H}}_2\mathrm{O}+{\mathrm{O}\mathrm{H}}^{-}$$

(14)

While metal deposition and hydrogen evolution occur in the cathodic cycle:

$${\mathrm{A}\mathrm{l}}^{3+}+{3\mathrm{e}}^{-}\to \mathrm{A}\mathrm{l}$$

(15)

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

(16)

4. Conclusions

The AA7075 was successfully coated using PEO under the pulsed unipolar potentiostatic mode at varying potentials. The porous structure observed in the PEO-coated Al was due to the discharge channels generated at the surface during the process. Both oxidized Al samples obtained the uniform surface morphology of a pancake or crater-like and nodular structure. Larger pore size diameter and higher coating thickness resulted from the high-intensity discharge produced at the 450 V deposition potential. The oxide coating of the samples was composed of Al, γ-alumina, and α-alumina. The higher O wt% on the sample confirms that more oxide coating was formed at 450 V. Although the EDX confirms that the nodular structures are Si-rich, peaks of mullite phase were not detected from XRD. The absence of mullite phase in both samples indicates that the nodular structures are purely Si-rich nodular structures. These formed nodular structures are possibly structures that do not diffuse or combine with alumina to form mullite under rapid processing conditions. The formation of Si-rich nodular structures in both samples indicates that the setting conditions in the study do not favor the reaction of alumina to form mullite. The absence of Si peaks could be due to its lower content and possibly detection limitations of the XRD machine.

Potentiodynamic polarization and EIS analyses show the improved corrosion resistance of the PEO-coated samples compared to the bare AA7075. The 450 V sample exhibited the highest E_corr and lowest i_corr values, indicating a low tendency to undergo electrochemical corrosion and a slow corrosion rate. The EIS results agree with the better corrosion performance achieved at higher potential applied at 450 V. The better corrosion performance of the 450 V sample is associated with its surface coating having less porosity/defects, a higher coating thickness, an inner dense layer coating, and a more stable oxide phase composition, which are beneficial in providing suitable corrosion protection to the metal in an aggressive environment.