Effect of Molybdate on Corrosion Performance of Oxide Coating Produced on 7075 Al Alloy Using PEO

Abstract

In this research, plasma electrolytic oxidation (PEO) coatings were prepared on 7075 Al alloy in a silicate-based solution with Na₂MoO₄ additive using a unipolar waveform at constant current density. The coatings displayed micro-pores, micro-cracks, pancake-like and crater-like features, and also solidified molten oxide particles on the surface. The coatings were majorly composed of Al₂O₃ (γ, δ, and α), SiO₂ (amorphous), and MoO₃ phases, which confirms the incorporation of molybdenum in the case of additive-containing coatings. Molybdenum species were transported through cracks, channels, and micropores, as the ready access pathways into the coating and partly sealed the coating pores. The EIS technique was used to evaluate the long-term corrosion performance of the coatings up to 168 h of immersion in 3.5 wt.% NaCl solution. The results showed that the barrier action of the PEO coatings was highly enhanced by adding Na₂MoO₄ due to the higher resistance that alumina achieved to chlorine absorption and also its higher stability by the incorporation of MoO₃. The coating formed in the presence of 5 g L⁻¹ Na₂MoO₄ showed the highest thickness and the lowest porosity percent (15.15%), which provided the highest corrosion performance at long immersion times.

1. Introduction

7075 aluminum alloy is widely used for aerospace and automotive industries due to a combination of properties, such as high strength-to-weight ratio, high fracture toughness, and good shape-ability. However, the weak corrosion resistance, particularly intergranular and pitting corrosion, represents a significant obstacle for its applications and has largely confined the broad use of 7075 aluminum alloy in some industries [1,2].

A naturally occurring passive oxide layer can be imparted corrosion resistance exposed to the atmosphere, but it cannot protect the Al alloy in harsh environments, such as marine or acidic environments [3]. Plasma electrolytic oxidation (PEO) has been considered as one of the most suitable surface treatments for generating a conformal oxide layer on the surface of various valve metals to improve their mechanical and electrochemical properties for industrial applications. PEO process is an anodic oxidation treatment applied above the breakdown voltage, with the consequent onset of short-lived discharges on the coating surface melting the substrate and the oxides simultaneously, which provide thick, uniform, and very adhesive coatings. The most attractive advantage of the PEO process is it is environmentally friendly, providing hard oxide ceramic layers with excellent corrosion and wear resistance and good electrical and thermal properties.

PEO coating properties are affected by processing parameters, such as the electrolyte composition, voltage, current density, waveform, and process time. Among them, the composition of the electrolytic baths plays a key role and affects the structure, phase composition, and element distribution in a PEO coating [4]. The use of rare earth salts and metal oxides, especially cerium, lanthanum, and molybdenum, has positive effects on corrosion resistance [5]. Arunnellaiappan et al. [6] reported that the corrosion performance of 7075 Al alloy would be improved remarkably by CeO₂ particles incorporation.

Molybdate is well known as a green corrosion inhibitor when present in the electrolyte as MoO₄²⁻ and Mo (VI) ions [7,8,9]. Because of its strong oxidation character, its reduction product is stable and can form a passive film on metals and shift the corrosion potential to nobler values. Molybdate also acts as an oxidant, such as chromate, in the chemical conversion processes, and its adsorbed products can inhibit the penetration of Cl⁻ ions to protect the substrate [10,11]. The inhibitive effect of molybdate on the corrosion of Al alloys has been the subject of several papers [11,12,13]. Wiggle et al. [7] studied the effect of molybdate on the pitting corrosion of two Al alloys, AA3003 and AA7072, and found that the corrosion potential becomes nobler, increasing monotonically with the concentration of molybdate ions.

Recently, molybdate salt as an additive has been used for magnesium and aluminum alloys to improve corrosion resistance during PEO treatment. Wang et al. [14] studied the structure and corrosion resistance of the coatings formed on ZL108 Al alloy in the silicate-Na₂MoO₄ electrolyte using the PEO process at different current densities. The results showed that the coatings were mainly composed of mullite, γ–Al₂O₃, SiO₂, MoO₂, and MoO₃. The microstructure of the coatings and corrosion performance was significantly affected by increasing current density. Kaseem et al. [15] also investigated the effect of current frequency on the structural features and electrochemical response of MoO₂–Al₂O₃ hybrid oxide films formed on Al-Mg-Si alloy. Numerous studies have investigated the synergistic effect of molybdate with tungstate [16], ZrO₂ [17], and TiO₂ [18] on tribological [3] and corrosion properties. Usually, the effect of molybdate on PEO coatings was found at a constant molybdate concentration and the effect of molybdate concentration in the bath solution and also the distribution of Mo in the coating layers were not found. Moreover, the effect of molybdate on the long-term corrosion behavior of the PEO coatings was not reported.

This paper studies the PEO process of 7075 Al alloy in a silicate-based electrolyte containing Na₂MoO₄ to evaluate the change in microstructure, composition, and long-term corrosion performance of the prepared coatings. The incorporation mechanism is considered to explain how the different species inside the electrolytic bath are incorporated in the coating and what their role is in coating microstructure and corrosion barrier properties.

2. Materials and Methods

2.1. Specimen Preparation

Disc-shape specimens of 7075 aluminum alloy with chemical composition (wt.%) of 2.45 Mg, 5.55 Zn, 0.16 Mn, 1.20 Cu, 0.37 Si, 0.33 Fe, 0.25 Cr, and balance Al, with a diameter of 20 mm and thickness of 5 mm, were prepared from T6 heat-treated bars and used as the substrate for PEO treatment. Both flat faces of the specimens were mechanical polished using silicon carbide (SiC) from 320 to 1200 grit. Then, they washed in deionized water and ultrasonically degreased in pure ethanol, and finally dried in cold airflow. Each specimen is connected to a copper wire used as the anode in the PEO treatment.

2.2. Plasma Electrolytic Oxidation Coating Procedure

PEO treatment was performed in a cylindrical cell (with 7 L volume) made from 316 L stainless steel, while the internal cell walls were used as the cathode. The electrolytic bath contained 3 g L⁻¹ sodium metasilicates (Na₂SiO₃·5H₂O), 2 g L⁻¹ potassium hydroxide (KOH), and 0, 3, and 5 g L⁻¹ sodium molybdate (Na₂MoO₄) as an additive. The electrolyte was stirred with a submersible centrifugal pump and cooled using a water bath equipped with a chiller to keep the cell temperature at 24 ± 1 °C. Furthermore, the pH and conductivity of the bath solutions were measured using a pH-meter (AZ instrument model: 827 digital pH lab) and a digital conductometer (SCHOTT GERATE CG 857, Labexchange, Burladingen, Germany), respectively.

The specimens were coated using a full-switching power supply equipped with a double isolated source (IGBT-based pulser). The coating process was performed in 60 min using a unipolar waveform at a frequency of 2 kHz, 20% duty cycle, with the average anodic current density of 6 A dm⁻². During the PEO treatment, the potential-time responses were recorded. The pH and conductivity of the electrolytic baths, the codes of the coated specimens, and the coating visual appearance are shown in

Table 1. Sample codes, pH, conductivity, and coating visual appearance of the coatings produced in various baths with different amounts of Na₂MoO₄.

Sample Code	Na2MoO4 (g L−1)	pH	Conductivity (mS·cm−1)	Visual Appearance
Mo-0	0	12.5	7.08
Mo-3	3	12.42	8.96
Mo-5	5	12.4	9.4

.

2.3. Coatings Characterization

A field emission scanning electron microscope (FESEM, FEI model Quanta FEG 450, FEI, Hillsboro, OR, USA) was applied to observe the surface and cross-section of the coatings. The chemical composition and elemental maps of the coatings were evaluated by an energy dispersive spectrometer (EDS, Octane Elite, Ametek, Mahwah, NJ, USA). The cross-section of the coatings was prepared through a grinding procedure using abrasive SiC papers up to 2400 grit. Then, the samples were polished using 0.5 microns Al₂O₃ particles. Finally, the samples were washed in deionized water, cleaned ultrasonically in ethanol, and dried in cold airflow. The average surface roughness (Ra) was measured using a stylus profilometer instrument (model Mitutoyo SJ210, Mitutoyo, Kawasaki, Japan). The thickness and porosity of the coatings were evaluated from the FESEM cross-sectional images by Image J 1.44p and MIP student software, respectively. The coating phase composition was detected using a glancing incident beam X-ray diffraction instrument (GIXRD, model ASENWARE AW-XDM300, Asenware, Shenzhen, China) by Cu Kα radiation generated at 40 kV and 30 mA. The X-ray diffraction patterns were obtained over the 2θ range of 10–80° with a step size of 0.05° and time per step of 5 s. The beam incident angle was 10°. X’pert Highscore software (Version 3.0.0) with powder diffraction file for inorganic materials (PDF2) database was used for data analysis.

2.4. Evaluation of Corrosion Performance of the Coatings

The electrochemical impedance spectroscopy (EIS) tests at long-term immersions in 3.5 wt.% NaCl, adjusted at pH 4 by adding dilute HCl solution, were performed to evaluate the barrier performance of the coatings by a potentiostat/galvanostat (model PARSTAT 2273, AMETEK, Oak Ridge, TN, USA). On the one face of the coated samples with 1.77 cm² area, a platinum plate and an Ag/AgCl electrode were applied as the working, counter, and reference, respectively, in a three-electrode cell. The frequency range from 100 kHz to 100 mHz and ±10 mV peak-to-peak voltage amplitude versus OCP were used for the measurements. For evaluating the performance of the coatings in aggressive solution, the EIS measurements were continued for 168 h using duplicate samples. The Power Suite and Zview software were applied to extract and fit the EIS data, respectively.

3. Results and Discussion

3.1. Voltage–Time Responses

Figure 1 shows the voltage vs. time response read during the PEO process in the baths containing 0, 3, and 5 g L⁻¹ Na₂MoO₄ at 6 A dm⁻². In the voltage–time transients (Figure 1), four different stages can be detected.

In the first stage (0–60 s), by applying the current density of 6 A dm⁻², the voltage increases with a very steep slope (stage I), which mainly involves the rapid electrochemical formation of a first insulating oxide film. In stage II (60–420 s), dielectric breakdown starts to occur, accompanied by numerous fine white sparks moving rapidly and uniformly over the entire sample surface. The addition of Na₂MoO₄ in the solution increased the conductivity. The values of the breakdown voltage depend upon the solution conductivity and decrease with increasing the value of conductivity [19,20]. Thus, the lowest breakdown voltage is achieved in the solution with 5 g L⁻¹ Na₂MoO₄ showing the highest conductivity of 9.40 mS cm⁻¹. In this stage, voltage increases nonlinearly with time, with a gentle slope, by forming a more compact and thicker oxide layer. In stage III, intensive discharges, such as orange glow, are observed, which are randomly distributed on the sample surface via slow-moving [21]. In this stage, the second inflection is defined as the critical voltage occurs. Then, the cell voltage (stage IV) reaches a relatively stable value, and gradually the spark size and intensity increase. This voltage–time transient is recorded for both PEO processes with and without molybdate.

3.2. Surface Morphology Observation and Roughness of the Coatings

The FESEM micrographs in Figure 2 show the surface morphology and roughness of the coatings. The micro-pores, micro-cracks, pancake-like, crater features, and solidified molten oxide particles were observed on the surface of all coatings. The oxide granules and micro-pores are mainly originated from ejected materials, random discharging, and gas entrapping [22]. In addition, the pancake and crater features are formed due to the ejection of molten aluminum from the substrate/coating interface along with the molten alumina from the inner/outer layer interface through the B-type and E-type discharge channels, respectively. As soon as the molten Al reaches the coating/electrolyte interface, it is oxidized and solidified, forming the pancakes [23,24]. The rapid solidification of molten oxide when it is surrounded by the solution develops the thermal stresses leading to micro-cracks. The difference between thermal coefficients of oxide layer and substrate and also the high pressure governed during the PEO process enhance the number of micro-cracks [25,26].

As the Na₂MoO₄ concentration in electrolyte increases, the density of oxide granules is increased, and the pancake features are decreased due to the reduction of strong discharges and the increment of A and C-type discharges, confirmed by decreasing breakdown voltage. Therefore, the surface morphology of the coatings seems rougher with increasing Na₂MoO₄ concentration (as seen in Figure 2c). Moreover, it seems that the diameter and number of the micropores on the coating surface decreased with increasing Na₂MoO₄ concentration [3].

As seen in Figure 2, the Ra increases with incrementing concentration from 0 to 5 g L⁻¹, which is also in agreement with the FESEM images. The higher final voltage results in a coarse microstructure [27,28]. Furthermore, the high tendency formation of crater features and granules of oxide compounds increases the surface roughness. The Mo-5 has the highest Ra with the highest final voltage.

3.3. Cross-Sectional Observation, Structure, and Composition of the Coatings

Figure 3 displays cross-sectional images of the coatings. Typically, PEO coating comprises up to three sub-layers, including a dense inner layer, a region with accumulated pores in the form of a pore band, and a more compact outer layer penetrated by discharge channels [29]. The inner layer adjacent to the substrate is very thin and displays a wavy-jagged appearance, which makes them an integral part of the substrate as the result of substrate dissolution at the early stage of the PEO process when discontinuous oxidation of the aluminum substrate occurs [30,31]. The middle layer involving porosity is orientated parallel to the substrate surface and originated due to the occurrence of D-type discharge. Moreover, the cross-sections show that the coated samples are composed of both micro-cracks and micro-pores, originating from various discharge types and thermal stresses.

The average thickness values of the PEO coatings were estimated from cross-sectional images using Image J software and are shown in Figure 3. According to Figure 3, the average thickness of the coating grown in the bath with no Na₂MoO₄ content (Mo-0) is 34.3 ± 3.6 μm, while with the addition of Na₂MoO₄, the average thickness increases. It is 38.5 ± 5.9 μm for the coating produced in 5 g L⁻¹ Na₂MoO₄ solution (Mo-5). As the concentration of sodium molybdate increases, the formation of Mo⁶⁺ ions increases, which increases the coating thickness and encourages a more passivation process [7]. The non-uniform distribution of discharges and higher aggressiveness of solution with the increasing of the Na₂MoO₄ concentration supports the higher coating thickness by the continually local thickening of the coating followed by breakdown events and substrate dissolution [7,32]. The increase of the coating thickness raises the surface roughness, as confirmed by the profilometry results (Figure 2). This is due to the increase in required energy for the current to pass from the thicker coating. Consequently, the diameter of the discharge channels and molten oxide eruption increases, resulting in a higher surface roughness [33].

The porosity percent of the PEO coatings, measured from FESEM cross-sectional images using MIP software, is inserted in Figure 3. The porosity percent decreases with increasing the Na₂MoO₄ concentration due to the higher Pilling–Bedworth ratio (RPB) of MoO₂ and MoO₃ (2.94 and 3.38, respectively) than Al₂O₃ (1.28) [14,34]. On the other hand, the lower breakdown voltage of the Mo-5 specimen reduces the destructive micro-discharges and decreases the porosity percent. Therefore, the presence and increasing the concentration of Na₂MoO₄ salt in electrolytic bath decreases the porosity of PEO coatings.

Figure 4 shows the distribution maps of elements in the coating cross-sections. As seen, the Al from the substrate is distributed uniformly throughout the coatings. The Si from the electrolyte is found mainly on the top surface of the outer layer due to the occurrence of A and C-type discharges [23]. Moreover, Si exists mostly inside the pore walls located in the middle layer. In the coatings containing Na₂MoO₄, Mo shows an even distribution across the coating thickness, while the accumulation of Mo is observed in the inner layer and the micro-pores.

Atomic percentages (at.%) of the elements taken from selected areas on the cross-section of the coatings and also at micro-pore walls (shown in Figure 5) using EDS analysis are listed in

Table 2. EDS analysis results of the coatings.

Sample	Element (at.%)
	Selected Area	Al	Mg	O	Mo	Si	Na	K
Mo-0	1	41.83	0.72	45.92	-	8.77	1.74	1.03
Mo-3	1	41.22	0.51	53.27	0.43	4.25	0.27	0.04
	2	23.42	0.12	64.71	3.93	7.05	0.60	0.18
Mo-5	1	43.44	0.61	52.91	0.56	2.09	0.30	0.09
	2	35.72	0.40	55.92	4.54	3.03	0.34	0.04

. The Si content of the coating is decreased by adding Na₂MoO₄ to the bath and increases with an increasing Na₂MoO₄ concentration. As the Na₂MoO₄ concentration increases, the value of Mo also increases in the coatings. It is worth noting that Mo tends to be accumulated in the vicinity and inside of the micro-pores of the coatings (as seen in EDS analysis for selected area 2 in Figure 5b,c), due to high surface energy, which reduces the ionic mobility of Mo ions [15].

According to previous papers [17], Na₂MoO₄ salt added in the bath dissociates based on the following reaction:

Na₂MoO₄ → 2Na⁺ + MoO₄²⁻

(1)

The incorporation of MoO₄²⁻ ions mainly occurs by migration during the anodic cycles under the electrical field and form the MoO₂ and MoO₃ phases by the chemical anodic reaction. Monfort et al. [35] suggested that the species of electrolytes are distributed and incorporated with different separate routes. Molybdenum species, such as phosphorus species, may be transported along short-circuit paths, such as grain boundaries, cracks, channels, and micropores, which provide ready access to the inner part of the coating and make the pores semi-sealed [35]. Figure 6 shows a schematic image explaining the incorporation mechanism of MoO₄²⁻ and SiO₃²⁻ anions during the PEO process. As seen, up-taking the MoO₄²⁻ and SiO₃²⁻ anions into coating mainly occurs through adhering on the melting pools obtained by the discharging events during the PEO process and electrostatic adsorption due to the applied negative charge. During discharging events, MoO₄²⁻ ions can enter the channel by the electric field, and the local melting promoted by the heat of sparking is responsible for sintering [36]. The lower melting point of MoO₂ and MoO₃ (1100 and 795 °C, respectively) compared to the melting point of Al₂O₃ (about 2000 °C) and other possible phases, such as Al₆O₁₃Si₂ and SiO₂ (1880 and 1710 °C, respectively), facilitates the sintering process. MoO₂ and MoO₃ phases are inertly incorporated into the coating due to their high thermodynamical stability [37].

3.4. The Phase Composition of the Coatings

Figure 7 shows XRD patterns of the coatings obtained in the solutions containing various Na₂MoO₄ concentrations. The coatings formed are mainly composed of alumina with different allotropy of γ-gamma, δ-delta, and α-alpha (γ–Al₂O₃–ICDD PDF no. 10–0425, δ–Al₂O₃–ICDD PDF no. 04–0877, and α–Al₂O₃–ICDD PDF no. 46–1212). The nuclei of γ–Al₂O₃ could directly originate from molten oxide under 1473 °C and supercooling during the PEO process [38]. The transformation of the γ–Al₂O₃ to the α–Al₂O₃ phase needs tremendous energy provided by micro-discharging to overcome the energy barrier [39]. The transforming drive force increases with the temperature rise. The presence of diffraction peaks of α–Al₂O₃ in the coatings indicates that the magnitude of the applied current density and reaction times are sufficient to support the phase transformation. The coatings formed in the bath containing Na₂MoO₄ show the obvious peaks with low diffraction intensity corresponding to molybdenum oxide (MoO₃–ICDD PDF no. 21–0569) in addition to the above-mentioned phases, which reconfirms the incorporation of molybdenum into the coatings. The diffractions relating to Al (ICDD PDF no. 85–1327) from the substrate are also evident but decrease with the increase of thickness and decrease of porosity of the coating by the increasing Na₂MoO₄ concentration. The patterns also show a halo in the 2θ range of 20–45°, denoting an amorphous phase, which should be mostly SiO₂. It was reported that the silicon may be incorporated in the coating by transport, respectively, in lattice or network sites of the crystalline and amorphous regions of the coating [35,40].

3.5. Long-Term Electrochemical Impedance Spectroscopy (EIS) Measurements

To evaluate and compare the long-term corrosion performance of the coatings, the EIS tests were performed in 3.5 wt.% NaCl solution (pH 4 ± 0.1) after 24, 48, and 168 h of immersion. The Nyquist and Bode-phase diagrams of the samples after different immersion times are shown in Figure 8. After 24 h of immersion, a two-time constant response with two humps can be seen in Bode-phase plots (Figure 8a’), which confirms the dual structure of the coatings [29]. The humps at low frequencies are related to the dense inner layers of the coatings, and those at high frequencies result from the porous outer layers. Correspondingly, in the Nyquist diagrams (Figure 8a), large and small capacitive loops indicate that the coatings have barrier functions against the aggressive solution to penetrate toward the middle layer (pore band-like) and substrate, respectively. With an increase in the immersion time to 48 h, the values of phase angle at the high frequencies decrease (Figure 8b’), which is related to the decrease of barrier resistance of the outer layer due to the penetration of aggressive solution inside the pores of the coatings [41,42]. After 168 h of immersion, the humps are merged to be flattened, as seen in Figure 8c′. In the Nyquist diagrams (Figure 8c), the Mo-5 sample shows the largest capacitive loop.

The best appropriate equivalent circuit (EC) model for fitting the EIS data is shown in Figure 9. In this EC model, R_s correspond to the resistance of solution, R_out and CPE_out represent the resistance and constant phase element of the outer layer, and R_in and CPE_in explain the resistance and constant phase element of the inner layer of the coatings. The constant phase element is introduced due to the non-ideal capacity behavior of the outer and inner layers. The black lines in Figure 8 represent the fitting curves, and the best-fitting parameters (χ² < 0.003) are summarized in

Table 3. Electrical elements extracted from EIS plots using Zview software for Mo-0, Mo-3, and Mo-5 specimens.

Specimens	Immersion Time	Outer Layer			Inner Layer
		CPEout (μF cm−2 Sn−1)	nout	Rout (kΩ cm2)	CPEin (μF cm−2 Sn−1)	nin	Rin (kΩ cm2)
Mo-0	24 h	0.22	0.79	1.82	1.87	0.67	266.05
	48 h	1.33	0.76	0.27	4.23	0.74	139.97
	168 h	16.35	0.76	0.07	11.23	0.77	27.18
Mo-3	24 h	3.53	0.59	19.79	11.16	0.56	57.69
	48 h	9.47	0.51	1.66	2.17	0.6	77.5
	168 h	27.55	0.56	0.49	1.27	0.91	56.13
Mo-5	24 h	0.51	0.74	1.41	11.75	0.52	56.42
	48 h	9.98	0.5	2.21	1.63	0.67	49.62
	168 h	14.25	0.5	0.38	1.56	0.79	94.55

.

According to the fitting data (Table 3), after 24 h, the maximum R_out value is for Mo-3 coating, which is probably caused by the sufficient sealing provided by MoO₃ particles in the outer layers of the coatings. With an increase in the immersion time, the R_out values are decreased, and their values become negligible for all coatings. The obtained results from the fitting data show that the R_in values are significantly higher than the R_out ones. Thus, the corrosion performance of the PEO coatings is mostly determined by their inner layer resistances. The dependence of R_in versus immersion time is drawn for the coatings in Figure 10. As seen, the Mo-0 coating shows the highest R_in value at the beginning of immersion. After 48 h, R_in values of the coatings fall, especially for the Mo-0 specimen. For this specimen, it is assumed that the R_in is strongly reduced due to its lower thickness and higher porosity. The decay in R_in is due to the up-taking of the corrosive solution through the pores of the middle layer, which touches the inner layer of the coating. For Mo-containing coatings, R_in is less affected by immersion time, and, notably, it even increases for Mo-5 at long immersion times. It is reported that aluminum oxides containing molybdenum are not stable at low pH (pH 4), and their preferential dissolution substantially changes the composition, making them richer in Mo(IV) compound (MoO₃ and MoO(OH)₂) at enough exposure time, which are more stable in acidic solutions [8]. Therefore, the increase of Mo content on the coating surface is responsible for better barrier properties. On the other hand, the aluminum oxide film, which contains molybdenum oxide, resists chlorine absorption because molybdenum oxide reduces the pH of aluminum oxide at the point of zero charges (isoelectric point) (from 9–9.5 to 3–6) [43]. This oxide film provides corrosion inhibition within the pH range 3–6 [7]. It was reported that MoO₄²⁻ ions are classified as anodic inhibitors and affect both general and pitting corrosion [44]. As a result, the Mo-5 coating has the highest barrier performance in long-term immersion, not only due to its higher thickness and lower porosity but also due to the higher molybdenum content.

Figure 11 shows the surface morphologies and the elemental distribution maps of the coatings after 168 h of immersion in the corrosive solution. All the coating surfaces show holes of different sizes due to localized attacks initiating at the weakest points of the coatings. Fewer damages appear for the sample coated in the solution containing 5 g L⁻¹ Na₂MoO₄. However, the absence of inductive loops in Nyquist curves indicates that these holes have no connection to the substrate and have not caused corrosive electrolyte contact with the substrate surface [45,46]. The elemental distribution maps show that the Cl⁻ ions’ adsorption decreases with increasing Na₂MoO₄ concentration. Molybdate can inhibit the penetration of Cl⁻ ions to protect the coatings and substrate, as described by refs. [10,11].

4. Conclusions

Plasma electrolytic oxidation of 7075 Al alloy in an alkaline silicate-based solution with different amounts of Na₂MoO₄ was performed using a unipolar waveform. The following results were obtained:

• The incorporation of Mo into the coating lowered the voltage-time response thanks to increasing the electrolyte conductivity and hence decreasing the breakdown voltage and intensity of micro-discharging.
• The coating’s average thickness was increased by raising the Na₂MoO₄ content. The highest porosity percentage (i.e., 29.64%) was calculated for the coating with no Mo content, while the lowest (i.e., 15.15%) was achieved for the specimen with the highest Mo content.
• Based on EDS analysis and elemental map, Mo tends to be accumulated in the vicinity and inside of the micro-porosity. The finer discharging channels were sealed due to the formation of molybdenum oxides. Further, Mo is transported to the inner parts of the coating, which further improves its barrier property.
• The coating produced in 5 g L⁻¹ Na₂MoO₄ solution displayed the highest barrier performance at long-term immersion due to the higher stability of aluminum oxide rich in molybdenum oxide, higher resistance to chlorine absorption, the highest thickness, and lowest porosity. The semi-sealed pores have also hindered the aggressive solution from touching the substrate surface, preventing the local attack on the substrate.