Influence of Additives on the Macroscopic Color and Corrosion Resistance of 6061 Aluminum Alloy Micro-Arc Oxidation Coatings

In this study, we successfully employed the plasma electrolytic oxidation (PEO) technique to create a uniform white ceramic layer on the surface of the 6061 aluminum alloy using K2ZrF6 and Na2WO4 as colorants. A scanning electron microscope (SEM) equipped with an energy-dispersive X-ray spectrometer (EDS) and X-ray diffraction (XRD) were used to characterize the coatings, and we used an electrochemical workstation to test their corrosion protection properties. The corrosion resistance of the coatings was analyzed using potentiodynamic polarization curves. The results showed that K2ZrF6 addition whitened the coating with ZrO2 as the main phase composition, inhibiting Al substrate depletion and enhancing coating corrosion resistance. A small amount of Na2WO4 decreased the coating’s L* value, successfully constructing ceramic coatings with L* (coating brightness) values ranging from 70 to 86, offering broad application prospects for decorative coatings.


Introduction
In recent years, aluminum and its alloys have been extensively utilized in various fields, such as aerospace and 3C electronics due to their high specific strength, excellent corrosion resistance, and good thermal conductivity [1][2][3][4].However, certain products, particularly electronic devices, necessitate surface coloring treatment to meet decorative requirements.Common coloring techniques for aluminum alloys, including dye coloring and anodic oxidation coloring, are limited by factors such as low hardness, poor bonding, and complex processes [5][6][7].
PEO technology, a novel surface treatment approach evolved from traditional anodic oxidation technology, has significantly enhanced processing efficiency by increasing the anode voltage from tens to hundreds of volts.In this process, valve metals (e.g., Mg, Al, and Ti) and their alloys are placed in an alkaline electrolyte as the anode, while stainless steel and other inert electrode plates serve as the cathode.When the voltage surpasses the metal anode's capacitance threshold, plasma discharge occurs, and a ceramic layer grows on the metal substrate surface.This coating exhibits properties such as high bonding strength and good corrosion resistance [8,9].Moreover, the alkaline electrolyte is easier to dispose of, reducing environmental pollution.By introducing different additives to the electrolyte, coatings of various colors can be prepared on the metal surface.Indeed, black, white, green, blue, and other colored coatings have been successfully produced on aluminum alloys [10,11].The commonly used colorants include Na 2 WO 4 , K 2 ZrF 6 , NH 4 VO 3 , K 2 TiF 6 , etc. [12][13][14].Wang et al. [15] prepared white, brown, grey, and black coatings on the surface of the AZ91D magnesium alloy using copper acetate, sodium tungstate, and sodium orthovanadate.It was not difficult to find that the presentation of different colors was caused by the formation of oxides on the coating surface by different metal cations.When the AZ31 magnesium alloy is micro-arc-oxidized in a cerium nitrate electrolyte, the coating shows a yellow color due to the combined effect of cerium dioxide and cerium oxide, and prolonging the time of micro-arc oxidation can make the yellow color deepen [16].Jiang [17] varied the amount of sodium hexametaphosphate to investigate its effect on the color of the coating.However, these coating colors were only qualitative, lacking quantitative data expression, such as color interval ranges.Although numerous studies have investigated colored coatings, few have focused on different shades of the same coating color.This study aimed to adjust the coating color using different colorants, establish a library of white color gradients for the coating, and investigate the coating's corrosion resistance, thereby extending the application field of PEO technology.
In this study, Na 2 WO 4 and K 2 ZrF 6 were selected as colorants to prepare white coatings on aluminum alloy surfaces using PEO.By varying the colorant content, the coatings' chromaticity value, L* [18] (brightness), was made to fall within the range of 70-86, making them adaptable for decorative purposes in different fields.The corrosion resistance and coloring mechanism of the coatings were also analyzed and discussed.

Experimental Procedures 2.1. Coating Deposition Conditions
The substrate used was a 6061 aluminum alloy with dimensions of Φ35 × 4 mm (wt%: Cu 0.28, Mg 0.88, Fe 0.34, Si 0.5, Mn 0.14, Zn 0.09, Cr 0.12, Ti 0.4, and Al).Each sample's surface was polished with 2000 mesh silicon carbide paper and ultrasonically cleaned in ethanol.All chemicals used in the coating preparation process were analytically pure (AR), with specific details listed in Table 1.Table 2 presents the electrolyte compositions and denotes the samples as P-Zr0, P-Zr2, P-Zr4, P-Zr15, P 6 -W0, P 6 -W0.4,and P 6 -W0.8.PEO was performed using a pulsed unipolar power supply (10 W) in the constant current mode, with the solution temperature maintained below 20 • C through cooling and stirring.The micro-arc oxidation process parameters were set to a current density of 2 A/dm 2 , a duty cycle of 20%, a frequency of 500 HZ, and a treatment time of 15 min.

Microscopic Characterization
The PEO coatings' morphology and surface elemental composition were observed using scanning electron microscopy (SEM, JEOL, JSM-7900F, Tokyo, Japan) and energy dispersive X-ray spectroscopy (EDS, JEOL, JSM-IT500A, Japan).In the SEM technique, the accelerating voltage was 15 KV, with the backscattering mode used for the coating cross-section scans, and the secondary electron mode was used for the coating surface scans.X-ray diffraction (XRD, D/Max-2400, Akishima, Japan) was employed to determine the coatings' phase composition using a Cu target (Kα1 = 0.15406 nm) as the anode target in the parallel light mode with a grazing angle of 2 • .The samples were scanned in the 2θ range from 20 to 90 • with a step size of 0.02 • and a scanning speed of 10 • /min.An electrochemical workstation (Metrohm Autolab PGSTAT302 N, Herissau, Switzerland) was used to test the coatings' corrosion protection properties in a 3.5 wt.% NaCl solution at room temperature, employing a conventional three-electrode battery system with a Ag/AgCl electrode as the reference electrode and a platinum electrode as the counter electrode.Potentiodynamic polarization curves were tested at a scan rate of 10 mV/s after 1 h of immersion.The coating thickness was measured using an eddy current thickness gauge (FMP20, Fisher, Schwerte, Germany), while optical emission spectroscopy (OES, Ideaoptics PG2000-Pro, Shanghai, China) was used to study the discharge sparks' emission spectra during the PEO process.The surface roughness was measured using a surface roughness tester (Ra200, Jingmere Technology Co., Ltd., Beijing, China).The coatings' chromaticity was analyzed using the LS171 colorimeter to obtain the chromaticity values L* (luminance), a* (red to green range), and b* (yellow to blue range).

Voltage-Time Response
The time-voltage curve for the micro-arc oxidation process after adding K 2 ZrF 6 is shown in Figure 1a.The three stages of micro-arc oxidation are delineated by dashed lines.When the content of K 2 ZrF 6 is between 0 g/L and 2 g/L, the electrolyte remains stable.However, when the content of K 2 ZrF 6 is increased to 4 g/L and 15 g/L, a significant voltage drop occurs during the first stage of micro-arc oxidation, known as the anodic oxidation stage.This is due to the generation of a large number of bubbles and the formation of the oxide layer.As the working voltage reaches the breakdown voltage, the process enters the second stage of micro-arc oxidation.At this point, the acidic solution partially dissolves the newly formed oxide layer, creating pores and resulting in a decrease in the coating resistance, leading to a voltage drop.As the reaction progresses and the coating growth rate surpasses the dissolution rate, the working voltage gradually rises and stabilizes.
dispersive X-ray spectroscopy (EDS, JEOL, JSM-IT500A, Japan).In the SEM technique, the accelerating voltage was 15 KV, with the backscattering mode used for the coating crosssection scans, and the secondary electron mode was used for the coating surface scans.Xray diffraction (XRD, D/Max-2400, Akishima, Japan) was employed to determine the coatings' phase composition using a Cu target (Kα1 = 0.15406 nm) as the anode target in the parallel light mode with a grazing angle of 2°.The samples were scanned in the 2θ range from 20 to 90° with a step size of 0.02° and a scanning speed of 10°/min.An electrochemical workstation (Metrohm Autolab PGSTAT302 N, Herissau, Switzerland) was used to test the coatings' corrosion protection properties in a 3.5 wt.% NaCl solution at room temperature, employing a conventional three-electrode battery system with a Ag/AgCl electrode as the reference electrode and a platinum electrode as the counter electrode.Potentiodynamic polarization curves were tested at a scan rate of 10 mV/s after 1 h of immersion.The coating thickness was measured using an eddy current thickness gauge (FMP20, Fisher, Schwerte, Germany), while optical emission spectroscopy (OES, Ideaoptics PG2000-Pro, Shanghai, China) was used to study the discharge sparks' emission spectra during the PEO process.The surface roughness was measured using a surface roughness tester (Ra200, Jingmere Technology Co., Ltd., Beijing, China).The coatings' chromaticity was analyzed using the LS171 colorimeter to obtain the chromaticity values L* (luminance), a* (red to green range), and b* (yellow to blue range).

Voltage-Time Response
The time-voltage curve for the micro-arc oxidation process after adding K2ZrF6 is shown in Figure 1a.The three stages of micro-arc oxidation are delineated by dashed lines.When the content of K2ZrF6 is between 0 g/L and 2 g/L, the electrolyte remains stable.However, when the content of K2ZrF6 is increased to 4 g/L and 15 g/L, a significant voltage drop occurs during the first stage of micro-arc oxidation, known as the anodic oxidation stage.This is due to the generation of a large number of bubbles and the formation of the oxide layer.As the working voltage reaches the breakdown voltage, the process enters the second stage of micro-arc oxidation.At this point, the acidic solution partially dissolves the newly formed oxide layer, creating pores and resulting in a decrease in the coating resistance, leading to a voltage drop.As the reaction progresses and the coating growth rate surpasses the dissolution rate, the working voltage gradually rises and stabilizes.
Figure 1b illustrates the time-voltage curve after adding a small amount of Na2WO4 solution.Due to the low Na2WO4 content, the solution stabilizes, and the time-voltage curves of the three electrolytes are consistent.When the voltage reaches the breakdown voltage, entering the second stage of micro-arc oxidation, the rate of voltage rise is lower than in the first stage, which is also attributed to the acidic nature of the solution.Figure 1b illustrates the time-voltage curve after adding a small amount of Na 2 WO 4 solution.Due to the low Na 2 WO 4 content, the solution stabilizes, and the time-voltage curves of the three electrolytes are consistent.When the voltage reaches the breakdown voltage, entering the second stage of micro-arc oxidation, the rate of voltage rise is lower than in the first stage, which is also attributed to the acidic nature of the solution.

Effect of Different Additives on the Surface Structure and Micro-Structure of the Coatings
Based on the micro-arc oxidation time-voltage curve, a point was selected at the third stage of micro-arc oxidation (300th s of the large spark stage).The discharged sparks generated during the process were captured, and the discharged ions were analyzed using optical emission spectroscopy (OES) between 200 and 1000 nm. Figure 2a shows the OES spectra of the four solutions after adding K 2 ZrF 6 .The discharge intensity of the electrolyte solution first increases, then decreases, and finally increases with the addition of different K 2 ZrF 6 content, consistent with the trend of its operating voltage.Comparing the solutions with and without K 2 ZrF 6 reveals that during discharging, not only the strong discharge of Na 589.46 nm is present, but also K 767.11 nm is involved in the discharge reaction.This indicates that the electrolyte is primarily consumed during the discharge process.The absence of an Al peak in the spectra further suggests that the substrate is seldom consumed in the energy supply discharge.Additionally, Hα 656.41 nm in the electrolyte also participates in the reaction, which is related to the acidic nature of the solution after the hydrolysis of K 2 ZrF 6 .Figure 2b shows the OES spectra after adding a small amount of Na 2 WO 4 .The spectra stabilize due to the low Na 2 WO 4 content and the lack of obvious voltage changes during the discharge process.The presence of strong Na peaks further indicates that the discharge process mainly consumes the electrolyte.

Effect of Different Additives on the Surface Structure and Micro-Structure of the Coatings
Based on the micro-arc oxidation time-voltage curve, a point was selected at the third stage of micro-arc oxidation (300th s of the large spark stage).The discharged sparks generated during the process were captured, and the discharged ions were analyzed using optical emission spectroscopy (OES) between 200 and 1000 nm. Figure 2a shows the OES spectra of the four solutions after adding K2ZrF6.The discharge intensity of the electrolyte solution first increases, then decreases, and finally increases with the addition of different K2ZrF6 content, consistent with the trend of its operating voltage.Comparing the solutions with and without K2ZrF6 reveals that during discharging, not only the strong discharge of Na 589.46 nm is present, but also K 767.11 nm is involved in the discharge reaction.This indicates that the electrolyte is primarily consumed during the discharge process.The absence of an Al peak in the spectra further suggests that the substrate is seldom consumed in the energy supply discharge.Additionally, Hα 656.41 nm in the electrolyte also participates in the reaction, which is related to the acidic nature of the solution after the hydrolysis of K2ZrF6.Figure 2b shows the OES spectra after adding a small amount of Na2WO4.The spectra stabilize due to the low Na2WO4 content and the lack of obvious voltage changes during the discharge process.The presence of strong Na peaks further indicates that the discharge process mainly consumes the electrolyte.Figure 3 illustrates the macroscopic color of the micro-arc oxidation coating after adding additives to the two electrolyte systems.The coating color becomes darker with the addition of a small amount of Na2WO4, while the addition of K2ZrF6 results in a whiter coating due to the presence of the corresponding tungsten and zirconium compounds.Tungstate is commonly used in the preparation of black coatings for micro-arc oxidation, so even a small amount of Na2WO4 can significantly decrease the whiteness.ZrO2, being a white compound, further demonstrates that the formation of micro-arc oxide coatings involves the deposition of compounds from the electrolyte, as evidenced by the change in coating color.Figure 3 illustrates the macroscopic color of the micro-arc oxidation coating after adding additives to the two electrolyte systems.The coating color becomes darker with the addition of a small amount of Na 2 WO 4 , while the addition of K 2 ZrF 6 results in a whiter coating due to the presence of the corresponding tungsten and zirconium compounds.Tungstate is commonly used in the preparation of black coatings for micro-arc oxidation, so even a small amount of Na 2 WO 4 can significantly decrease the whiteness.ZrO 2 , being a white compound, further demonstrates that the formation of micro-arc oxide coatings involves the deposition of compounds from the electrolyte, as evidenced by the change in coating color.

Effect of Different Additives on the Surface Structure and Micro-Structure of the Coatings
Based on the micro-arc oxidation time-voltage curve, a point was selected at the third stage of micro-arc oxidation (300th s of the large spark stage).The discharged sparks generated during the process were captured, and the discharged ions were analyzed using optical emission spectroscopy (OES) between 200 and 1000 nm. Figure 2a shows the OES spectra of the four solutions after adding K2ZrF6.The discharge intensity of the electrolyte solution first increases, then decreases, and finally increases with the addition of different K2ZrF6 content, consistent with the trend of its operating voltage.Comparing the solutions with and without K2ZrF6 reveals that during discharging, not only the strong discharge of Na 589.46 nm is present, but also K 767.11 nm is involved in the discharge reaction.This indicates that the electrolyte is primarily consumed during the discharge process.The absence of an Al peak in the spectra further suggests that the substrate is seldom consumed in the energy supply discharge.Additionally, Hα 656.41 nm in the electrolyte also participates in the reaction, which is related to the acidic nature of the solution after the hydrolysis of K2ZrF6.Figure 2b shows the OES spectra after adding a small amount of Na2WO4.The spectra stabilize due to the low Na2WO4 content and the lack of obvious voltage changes during the discharge process.The presence of strong Na peaks further indicates that the discharge process mainly consumes the electrolyte.Figure 3 illustrates the macroscopic color of the micro-arc oxidation coating after adding additives to the two electrolyte systems.The coating color becomes darker with the addition of a small amount of Na2WO4, while the addition of K2ZrF6 results in a whiter coating due to the presence of the corresponding tungsten and zirconium compounds.Tungstate is commonly used in the preparation of black coatings for micro-arc oxidation, so even a small amount of Na2WO4 can significantly decrease the whiteness.ZrO2, being a white compound, further demonstrates that the formation of micro-arc oxide coatings involves the deposition of compounds from the electrolyte, as evidenced by the change in coating color.To express the difference in the coating color visually and accurately, a color difference analyzer was employed to demonstrate the coating color change through CIE L* a* b*.As shown in Table 3, with an increasing K 2 ZrF 6 concentration, the coating color gradually becomes lighter and tends to be pure.Tu et al. found that K 2 ZrF 6 does not continuously enhance the whiteness of the coating, and when the concentration reaches a certain level, the whiteness rises slowly [18].Simultaneously, the addition of a small amount of Na 2 WO 4 darkened the coating color, with the L* value dropping as low as 70.Table 4 presents the surface element contents of the aluminum alloy micro-arc oxidation coatings with different additives.Before adding the additives, the main elements of the coating were Al and O, and the coating color was influenced by the substrate composition.After adding K 2 ZrF 6 , the coating elements primarily consisted of Al, O, and Zr.The Zr in the coating originated from the electrolyte and increased with the rise in K 2 ZrF 6 content, indicating that the electrolyte composition plays a major role in the coating growth process.The decrease in the Al content also suggests that the addition of K 2 ZrF 6 suppresses the influence of the Al substrate on the coating color, ultimately leading to an increase in the coating L* value.Similarly, W was also present in the dark coating, but the small amount of Na 2 WO 4 content resulted in a low W content in the coating.However, even a small amount of W led to a deeper coating color, as Na 2 WO 4 is commonly used in the preparation of black coatings.Figure 4 illustrates the relationship between the thickness and roughness of the aluminum alloy micro-arc oxidation coatings with different additives.It was observed that with an increasing K 2 ZrF 6 concentration, the thickness of the micro-arc oxidation coating under the same electric parameters increased, indicating that zirconium ions participated in the reaction and were incorporated into the coating.K 2 ZrF 6 fully dissolved, further suggesting that K 2 ZrF 6 promotes coating growth.The increase in thickness also led to an increase in coating roughness, and the changing trend of roughness was consistent with the changing trend of thickness, as evident in the figure.This demonstrates that within the same electrolyte, the coating thickness is a direct factor affecting the surface roughness of the coating.Simultaneously, due to the low Na 2 WO 4 content and the same working voltage, no obvious changes in coating thickness or roughness were observed.
gesting that K2ZrF6 promotes coating growth.The increase in thickness also led to an increase in coating roughness, and the changing trend of roughness was consistent with the changing trend of thickness, as evident in the figure.This demonstrates that within the same electrolyte, the coating thickness is a direct factor affecting the surface roughness of the coating.Simultaneously, due to the low Na2WO4 content and the same working voltage, no obvious changes in coating thickness or roughness were observed.Figure 5 shows the results of the physical phase analysis of the micro-arc oxidation coatings prepared with different additives.From Figure 5a, it can be seen that the main phase compositions of the coatings without the addition of K2ZrF6 are α-Al2O3 and γ-Al2O3, with a small amount of the Al substrate diffraction peaks also present.This is because the micro-arc oxidation coating has a loose, porous structure, and the X-rays pass through the pores on the coating surface, revealing the diffraction peaks of the Al substrate.With the addition of K2ZrF6, a gradual decrease in α-Al2O3 and γ-Al2O3 is clearly observed, indicating that the addition of K2ZrF6 reduces the consumption of the Al substrate.When the K2ZrF6 content reached 15 g/L, α-Al2O3 and γ-Al2O3 completely disappeared, as the increase in the K2ZrF6 content completely suppressed the depletion of the Al substrate.All the Zr in the coatings came from the electrolyte, and tetragonal zirconia (t-ZrO2) appeared, which was the reason for the increase in the L* values of the coatings.This further demonstrates that the presence of additives in the micro-arc oxidation process inhibits substrate depletion, and the coating growth is mainly derived from the deposition of electrolyte compounds, with the coating growth mode dominated by deposition.Moreover, the thickening of the coating has a shielding effect, which reduces the Al diffraction peaks.Figure 5b shows the change in the coating phase composition after adding Na2WO4.Due to the small content of the additive, Na2WO4 could not affect the coating phase composition, and the coating maintained α-Al2O3 and γ-Al2O3 as the main crystalline phases.gesting that K2ZrF6 promotes coating growth.The increase in thickness also led to an increase in coating roughness, and the changing trend of roughness was consistent with the changing trend of thickness, as evident in the figure.This demonstrates that within the same electrolyte, the coating thickness is a direct factor affecting the surface roughness of the coating.Simultaneously, due to the low Na2WO4 content and the same working voltage, no obvious changes in coating thickness or roughness were observed.Figure 5 shows the results of the physical phase analysis of the micro-arc oxidation coatings prepared with different additives.From Figure 5a, it can be seen that the main phase compositions of the coatings without the addition of K2ZrF6 are α-Al2O3 and γ-Al2O3, with a small amount of the Al substrate diffraction peaks also present.This is because the micro-arc oxidation coating has a loose, porous structure, and the X-rays pass through the pores on the coating surface, revealing the diffraction peaks of the Al substrate.With the addition of K2ZrF6, a gradual decrease in α-Al2O3 and γ-Al2O3 is clearly observed, indicating that the addition of K2ZrF6 reduces the consumption of the Al substrate.When the K2ZrF6 content reached 15 g/L, α-Al2O3 and γ-Al2O3 completely disappeared, as the increase in the K2ZrF6 content completely suppressed the depletion of the Al substrate.All the Zr in the coatings came from the electrolyte, and tetragonal zirconia (t-ZrO2) appeared, which was the reason for the increase in the L* values of the coatings.This further demonstrates that the presence of additives in the micro-arc oxidation process inhibits substrate depletion, and the coating growth is mainly derived from the deposition of electrolyte compounds, with the coating growth mode dominated by deposition.Moreover, the thickening of the coating has a shielding effect, which reduces the Al diffraction peaks.Figure 5b shows the change in the coating phase composition after adding Na2WO4.Due to the small content of the additive, Na2WO4 could not affect the coating phase composition, and the coating maintained α-Al2O3 and γ-Al2O3 as the main crystalline phases.Figures 6 and 7 illustrate the surface morphology of the coatings with the two additives.The images reveal that the coating surfaces generated after the micro-arc oxidation treatment exhibited more discharge channels and cracks, which were caused by excessive cooling contraction, leading to brittle fractures within the internal texture.Figure 6d shows the surface morphology of the coatings without the addition of K 2 ZrF 6 , characterized by numerous small holes and the absence of the "pancake" structure typically observed after strong B-type discharge in the base electrolyte [19].This phenomenon is attributed to the acidic nature of the electrolyte, where the coating growth rate is lower than the dissolution rate, and both rates remain consistent, thereby explaining the lack of a decrease in the operating voltage.However, with the addition of K 2 ZrF 6 , as depicted in Figure 6a, a granular structure emerged in the coating due to the involvement of K 2 ZrF 6 in the reaction, with discharges occurring in the form of A and C-type discharges.Previous studies have reported that A and C-type discharges tend to incorporate electrolyte substances into the coating, while B-type discharges dope the coating through the melting of the substrate components.The growth of the coating during micro-arc oxidation is attributed to the oxidation of molten aluminum as it exits through the discharge channel created by the breakdown of the oxide layer [20].Consequently, the formed aluminum oxide is ejected from the channel, encountering the rapidly cooled electrolyte on the coating surface, forming a pancake structure and indicating that the main component of the film originates from the substrate metal, highlighting the significance of B-type discharges.In contrast, both A and C-type discharges occur in the upper layer of the coating without contacting the substrate, involving only the interaction between the electrolyte and the oxide in the coating.As the K 2 ZrF 6 content increased, the alumina content in the coating decreased until it disappeared, explaining the absence of the granular structure in Figure 6c.It was replaced by fewer and larger pores, which provided the foundation for the thermal control properties of the coating.Figure 7b displays a distinct "pancake" structure after B-type discharge.The surface morphology in Figure 7a resembles that in Figure 6d, which is attributed to the acidity of the solution.Although the additives induced A and C-type discharges, the discharges still occurred in the form of B-type discharges due to the low content of Na 2 WO 4 .Figures 6 and 7 illustrate the surface morphology of the coatings with the two additives.The images reveal that the coating surfaces generated after the micro-arc oxidation treatment exhibited more discharge channels and cracks, which were caused by excessive cooling contraction, leading to brittle fractures within the internal texture.Figure 6d shows the surface morphology of the coatings without the addition of K2ZrF6, characterized by numerous small holes and the absence of the "pancake" structure typically observed after strong B-type discharge in the base electrolyte [19].This phenomenon is attributed to the acidic nature of the electrolyte, where the coating growth rate is lower than the dissolution rate, and both rates remain consistent, thereby explaining the lack of a decrease in the operating voltage.However, with the addition of K2ZrF6, as depicted in Figure 6a, a granular structure emerged in the coating due to the involvement of K2ZrF6 in the reaction, with discharges occurring in the form of A and C-type discharges.Previous studies have reported that A and C-type discharges tend to incorporate electrolyte substances into the coating, while B-type discharges dope the coating through the melting of the substrate components.The growth of the coating during micro-arc oxidation is attributed to the oxidation of molten aluminum as it exits through the discharge channel created by the breakdown of the oxide layer [20].Consequently, the formed aluminum oxide is ejected from the channel, encountering the rapidly cooled electrolyte on the coating surface, forming a pancake structure and indicating that the main component of the film originates from the substrate metal, highlighting the significance of B-type discharges.In contrast, both A and C-type discharges occur in the upper layer of the coating without contacting the substrate, involving only the interaction between the electrolyte and the oxide in the coating.As the K2ZrF6 content increased, the alumina content in the coating decreased until it disappeared, explaining the absence of the granular structure in Figure 6c.It was replaced by fewer and larger pores, which provided the foundation for the thermal control properties of the coating.Figure 7b displays a distinct "pancake" structure after B-type discharge.The surface morphology in Figure 7a resembles that in Figure 6d, which is attributed to the acidity of the solution.Although the additives induced A and C-type discharges, the discharges still occurred in the form of B-type discharges due to the low content of Na2WO4.The cross-sectional morphology of the six coatings is presented in Figures 8 and 9.It is observed that the cross-sections of the micro-arc oxidation coatings after the addition of K2ZrF6 exhibit similar characteristics, all featuring the presence of macropores.The varying contents of K2ZrF6 resulted in different coating thicknesses, which is consistent with the findings in Figure 4.This further confirms that the addition of K2ZrF6 promotes coating growth, and the decrease in the working voltage does not impact the coating thickness.Moreover, as the K2ZrF6 concentration increased, the zirconium-containing oxides filled the coating pores more uniformly, attributable to the fact that the product of K2ZrF6 after electrolysis facilitated the formation and buildup of the coating during the micro-arc oxidation process.The coating with the addition of Na2WO4 did not exhibit significant changes, except for the thickness due to its small amount.Table 4 reveals that in the absence of K2ZrF6, the coating is predominantly composed of Al and O, indicating that alumina is the primary constituent.Figures 10 and 11 illustrate the elemental distribution of the coatings with the addition of K2ZrF6 and Na2WO4, respectively.It is observed that after the addition of K2ZrF6, the coating is primarily composed of Al, O, and Zr, with a significant increase in the Zr content as the K2ZrF6 content increases.The XRD pattern suggests that the coating is dominated by ZrO2 at this point, which increases with the rise in K2ZrF6 content.Furthermore, the shallow distribution of Al elements in the coating indicates that the addition of K2ZrF6 inhibits the consumption of the Al substrate, aligning with the XRD results.The F element did not exhibit a significant presence, potentially due to the short oxidation time.According to Tu's study, F was The cross-sectional morphology of the six coatings is presented in Figures 8 and 9.It is observed that the cross-sections of the micro-arc oxidation coatings after the addition of K 2 ZrF 6 exhibit similar characteristics, all featuring the presence of macropores.The varying contents of K 2 ZrF 6 resulted in different coating thicknesses, which is consistent with the findings in Figure 4.This further confirms that the addition of K 2 ZrF 6 promotes coating growth, and the decrease in the working voltage does not impact the coating thickness.Moreover, as the K 2 ZrF 6 concentration increased, the zirconium-containing oxides filled the coating pores more uniformly, attributable to the fact that the product of K 2 ZrF 6 after electrolysis facilitated the formation and buildup of the coating during the micro-arc oxidation process.The coating with the addition of Na 2 WO 4 did not exhibit significant changes, except for the thickness due to its small amount.The cross-sectional morphology of the six coatings is presented in Figures 8 and 9.It is observed that the cross-sections of the micro-arc oxidation coatings after the addition of K2ZrF6 exhibit similar characteristics, all featuring the presence of macropores.The varying contents of K2ZrF6 resulted in different coating thicknesses, which is consistent with the findings in Figure 4.This further confirms that the addition of K2ZrF6 promotes coating growth, and the decrease in the working voltage does not impact the coating thickness.Moreover, as the K2ZrF6 concentration increased, the zirconium-containing oxides filled the coating pores more uniformly, attributable to the fact that the product of K2ZrF6 after electrolysis facilitated the formation and buildup of the coating during the micro-arc oxidation process.The coating with the addition of Na2WO4 did not exhibit significant changes, except for the thickness due to its small amount.Table 4 reveals that in the absence of K2ZrF6, the coating is predominantly composed of Al and O, indicating that alumina is the primary constituent.Figures 10 and 11 illustrate the elemental distribution of the coatings with the addition of K2ZrF6 and Na2WO4, respectively.It is observed that after the addition of K2ZrF6, the coating is primarily composed of Al, O, and Zr, with a significant increase in the Zr content as the K2ZrF6 content increases.The XRD pattern suggests that the coating is dominated by ZrO2 at this point, which increases with the rise in K2ZrF6 content.Furthermore, the shallow distribution of Al elements in the coating indicates that the addition of K2ZrF6 inhibits the consumption of the Al substrate, aligning with the XRD results.The F element did not exhibit a significant presence, potentially due to the short oxidation time.According to Tu's study, F was  The cross-sectional morphology of the six coatings is presented in Figures 8 and 9.It is observed that the cross-sections of the micro-arc oxidation coatings after the addition of K2ZrF6 exhibit similar characteristics, all featuring the presence of macropores.The varying contents of K2ZrF6 resulted in different coating thicknesses, which is consistent with the findings in Figure 4.This further confirms that the addition of K2ZrF6 promotes coating growth, and the decrease in the working voltage does not impact the coating thickness.Moreover, as the K2ZrF6 concentration increased, the zirconium-containing oxides filled the coating pores more uniformly, attributable to the fact that the product of K2ZrF6 after electrolysis facilitated the formation and buildup of the coating during the micro-arc oxidation process.The coating with the addition of Na2WO4 did not exhibit significant changes, except for the thickness due to its small amount.Table 4 reveals that in the absence of K2ZrF6, the coating is predominantly composed of Al and O, indicating that alumina is the primary constituent.Figures 10 and 11 illustrate the elemental distribution of the coatings with the addition of K2ZrF6 and Na2WO4, respectively.It is observed that after the addition of K2ZrF6, the coating is primarily composed of Al, O, and Zr, with a significant increase in the Zr content as the K2ZrF6 content increases.The XRD pattern suggests that the coating is dominated by ZrO2 at this point, which increases with the rise in K2ZrF6 content.Furthermore, the shallow distribution of Al elements in the coating indicates that the addition of K2ZrF6 inhibits the consumption of the Al substrate, aligning with the XRD results.The F element did not exhibit a significant presence, potentially due to the short oxidation time.According to Tu's study, F was Table 4 reveals that in the absence of K 2 ZrF 6 , the coating is predominantly composed of Al and O, indicating that alumina is the primary constituent.Figures 10 and 11 illustrate the elemental distribution of the coatings with the addition of K 2 ZrF 6 and Na 2 WO 4 , respectively.It is observed that after the addition of K 2 ZrF 6 , the coating is primarily composed of Al, O, and Zr, with a significant increase in the Zr content as the K 2 ZrF 6 content increases.The XRD pattern suggests that the coating is dominated by ZrO 2 at this point, which increases with the rise in K 2 ZrF 6 content.Furthermore, the shallow distribution of Al elements in the coating indicates that the addition of K 2 ZrF 6 inhibits the consumption of the Al substrate, aligning with the XRD results.The F element did not exhibit a significant presence, potentially due to the short oxidation time.According to Tu's study, F was found to enter the coating at the late stage of micro-arc oxidation [21].The coating without Na 2 WO 4 and the coating without K 2 ZrF 6 share the same main elemental composition of alumina.After the addition of Na 2 WO 4 , W is uniformly distributed in the coating, confirming that a small amount of Na 2 WO 4 participates in the micro-arc oxidation process and enters the coating, deepening its color.However, due to the small quantity of Na 2 WO 4 , no crystalline phase is observed.
Materials 2024, 17, x FOR PEER REVIEW 9 of 12 found to enter the coating at the late stage of micro-arc oxidation [21].The coating without Na2WO4 and the coating without K2ZrF6 share the same main elemental composition of alumina.After the addition of Na2WO4, W is uniformly distributed in the coating, confirming that a small amount of Na2WO4 participates in the micro-arc oxidation process and enters the coating, deepening its color.However, due to the small quantity of Na2WO4, no crystalline phase is observed.

Effect of Various Additives on the Corrosion Resistance of Aluminum Alloy Coatings
Figure 12 presents the kinetic potential polarization curves measured in a 3.5% NaCl solution at room temperature for samples with and without additives after micro-arc oxidation.Table 5 lists the fitting results of the corrosion potential (Ecorr) and corrosion current density (icorr).In general, the corrosion potential reflects the coating's stability; a more positive corrosion potential indicates a more stable coating.The corrosion current density reflects the coating's corrosion rate; a lower corrosion current density corresponds to a more corrosion-resistant coating.The Ecorr is typically related to the material's thermodynamic stability, and a lower Ecorr value suggests a higher susceptibility to localized corrosion [22].The addition of Na2WO4 resulted in a 53 mV increase in the Ecorr compared to P6-W0, which may be attributed to the increased crystallinity of the coating.Furthermore, the icorr value decreased after the addition of Na2WO4, indicating the improved corrosion resistance of the coatings.The addition of K2ZrF6 also led to a one-order-of-magnitude decrease in the corrosion current density (icorr), which is due to the increased coating thickness and the consequent slowing of the corrosion rate, resulting in better corrosion protection performance.However, the corrosion potential (Ecorr) decreased by 268 found to enter the coating at the late stage of micro-arc oxidation [21].The coating without Na2WO4 and the coating without K2ZrF6 share the same main elemental composition of alumina.After the addition of Na2WO4, W is uniformly distributed in the coating, confirming that a small amount of Na2WO4 participates in the micro-arc oxidation process and enters the coating, deepening its color.However, due to the small quantity of Na2WO4, no crystalline phase is observed.

Effect of Various Additives on the Corrosion Resistance of Aluminum Alloy Coatings
Figure 12 presents the kinetic potential polarization curves measured in a 3.5% NaCl solution at room temperature for samples with and without additives after micro-arc oxidation.Table 5 lists the fitting results of the corrosion potential (Ecorr) and corrosion current density (icorr).In general, the corrosion potential reflects the coating's stability; a more positive corrosion potential indicates a more stable coating.The corrosion current density reflects the coating's corrosion rate; a lower corrosion current density corresponds to a more corrosion-resistant coating.The Ecorr is typically related to the material's thermodynamic stability, and a lower Ecorr value suggests a higher susceptibility to localized corrosion [22].The addition of Na2WO4 resulted in a 53 mV increase in the Ecorr compared to P6-W0, which may be attributed to the increased crystallinity of the coating.Furthermore, the icorr value decreased after the addition of Na2WO4, indicating the improved corrosion resistance of the coatings.The addition of K2ZrF6 also led to a one-order-of-magnitude decrease in the corrosion current density (icorr), which is due to the increased coating thickness and the consequent slowing of the corrosion rate, resulting in better corrosion protection performance.However, the corrosion potential (Ecorr) decreased by 268

Effect of Various Additives on the Corrosion Resistance of Aluminum Alloy Coatings
Figure 12 presents the kinetic potential polarization curves measured in a 3.5% NaCl solution at room temperature for samples with and without additives after micro-arc oxidation.Table 5 lists the fitting results of the corrosion potential (Ecorr) and corrosion current density (icorr).In general, the corrosion potential reflects the coating's stability; a more positive corrosion potential indicates a more stable coating.The corrosion current density reflects the coating's corrosion rate; a lower corrosion current density corresponds to a more corrosion-resistant coating.The Ecorr is typically related to the material's thermodynamic stability, and a lower Ecorr value suggests a higher susceptibility to localized corrosion [22].The addition of Na 2 WO 4 resulted in a 53 mV increase in the Ecorr compared to P6-W0, which may be attributed to the increased crystallinity of the coating.Furthermore, the icorr value decreased after the addition of Na 2 WO 4 , indicating the improved corrosion resistance of the coatings.The addition of K 2 ZrF 6 also led to a one-order-of-magnitude decrease in the corrosion current density (icorr), which is due to the increased coating thickness and the consequent slowing of the corrosion rate, resulting in better corrosion protection performance.However, the corrosion potential (Ecorr) decreased by 268 mV, and the cross-sectional morphology of the coatings revealed a loose and porous structure after the addition of K 2 ZrF 6 , making them more susceptible to corrosion.The increased thickness contributed to the enhanced corrosion resistance of the coating in the same environment [23].mV, and the cross-sectional morphology of the coatings revealed a loose and porous structure after the addition of K2ZrF6, making them more susceptible to corrosion.The increased thickness contributed to the enhanced corrosion resistance of the coating in the same environment [23].

Coloring Analysis
The presence of ZrO2 on the surface is the primary reason for the coating's white color.The additives in the electrolyte during the oxidation process led to the occurrence of A and C-type discharges on the coating surface.These additives are doped into the porous layer and inhibit the generation of Al2O3.The elemental distribution of the coating cross-section further confirms the deposition of colorants within the layer.Although a small amount of Na2WO4 may not produce the crystalline phase of WO2, the presence of the W element contributes to the darkening of the coating color.This explains the observed change in the coating color from darker to lighter colors.

Limitations and Future Research
Despite the classification of coating chromaticity under the same color system using the additives, the color range is relatively limited.Therefore, this system can be expanded and enriched.Moreover, the content of additives depends on the final rendering of the coating, requiring more pre-experiments for verification.
Cost reduction can be achieved not only by adjusting the additives but also by modifying the electrical parameters, such as the oxidation time and current density.To achieve coating color diversification, substrates with trace color elements can be employed in experiments.
Furthermore, the addition of K2ZrF6 results in a coating that not only meets decorative requirements but also exhibits a low absorption rate.The presence of zirconium oxide in the coating improves its emissivity, making it suitable for aerospace applications.

Coloring Analysis
The presence of ZrO 2 on the surface is the primary reason for the coating's white color.The additives in the electrolyte during the oxidation process led to the occurrence of A and C-type discharges on the coating surface.These additives are doped into the porous layer and inhibit the generation of Al 2 O 3 .The elemental distribution of the coating cross-section further confirms the deposition of colorants within the layer.Although a small amount of Na 2 WO 4 may not produce the crystalline phase of WO 2 , the presence of the W element contributes to the darkening of the coating color.This explains the observed change in the coating color from darker to lighter colors.

Limitations and Future Research
Despite the classification of coating chromaticity under the same color system using the additives, the color range is relatively limited.Therefore, this system can be expanded and enriched.Moreover, the content of additives depends on the final rendering of the coating, requiring more pre-experiments for verification.
Cost reduction can be achieved not only by adjusting the additives but also by modifying the electrical parameters, such as the oxidation time and current density.To achieve coating color diversification, substrates with trace color elements can be employed in experiments.
Furthermore, the addition of K 2 ZrF 6 results in a coating that not only meets decorative requirements but also exhibits a low absorption rate.The presence of zirconium oxide in the coating improves its emissivity, making it suitable for aerospace applications.

Conclusions
Ceramic layers with L* values between 70 and 86 were successfully prepared using PEO technology.The following conclusions can be drawn from the characterization and corrosion resistance studies of the coatings prepared under different conditions: 1.
K 2 ZrF 6 and Na 2 WO 4 were fully dissolved and involved in the reaction, resulting in micro-arc oxidation coatings with L* values between 70 and 86 after adding the additives.A crystalline phase of ZrO 2 appeared in the coating, which lightened the coating color, while a small amount of Na 2 WO 4 did not form a crystalline phase, but the presence of elemental W in the coating darkened the coating color; 2.
The addition of additives to the coating during the reaction decreased its thermal stability due to the reduction of the crystalline phase compared to the coatings without the additives.However, the corrosion current decreased by one order of magnitude, indicating improved corrosion resistance.The addition of the additives not only enhanced the corrosion resistance of the coating but also increased its wear resistance, extending the service life of the aluminum alloy; 3.
The addition of additives inhibited the depletion of the Al substrate, and the crystalline phases α-Al 2 O 3 and γ-Al 2 O 3 in the coatings decreased with an increasing additive concentration.Moreover, the addition of the additives altered the discharge type, leading to changes in the coating surface morphology.The increased coating thickness and the presence of macropores in the K 2 ZrF 6 solution laid the foundation for the thermal control performance of the coatings.

Figure 1 .
Figure 1.Time-voltage response curve of the aluminum alloy micro-arc oxidation process: (a) after adding K2ZrF6; (b) after adding Na2WO4.

Figure 2 .
Figure 2. OES spectra of aluminum alloy micro-arc oxidation at the same time: (a) in the K2ZrF6 solution with different contents; (b) in the Na2WO4 solution with different contents.

Figure 2 .
Figure 2. OES spectra of aluminum alloy micro-arc oxidation at the same time: (a) in the K 2 ZrF 6 solution with different contents; (b) in the Na 2 WO 4 solution with different contents.

Figure 2 .
Figure 2. OES spectra of aluminum alloy micro-arc oxidation at the same time: (a) in the K2ZrF6 solution with different contents; (b) in the Na2WO4 solution with different contents.

Figure 4 .
Figure 4. Relationship between the thickness and surface roughness of the aluminum alloy microarc oxidation coatings with different additives.

Figure 4 .
Figure 4. Relationship between the thickness and surface roughness of the aluminum alloy micro-arc oxidation coatings with different additives.

Figure 5
Figure5shows the results of the physical phase analysis of the micro-arc oxidation coatings prepared with different additives.From Figure5a, it can be seen that the main phase compositions of the coatings without the addition of K 2 ZrF 6 are α-Al 2 O 3 and γ-Al 2 O 3 , with a small amount of the Al substrate diffraction peaks also present.This is because the micro-arc oxidation coating has a loose, porous structure, and the X-rays pass through the pores on the coating surface, revealing the diffraction peaks of the Al substrate.With the addition of K 2 ZrF 6 , a gradual decrease in α-Al 2 O 3 and γ-Al 2 O 3 is clearly observed, indicating that the addition of K 2 ZrF 6 reduces the consumption of the Al substrate.When the K 2 ZrF 6 content reached 15 g/L, α-Al 2 O 3 and γ-Al 2 O 3 completely disappeared, as the increase in the K 2 ZrF 6 content completely suppressed the depletion of the Al substrate.All the Zr in the coatings came from the electrolyte, and tetragonal zirconia (t-ZrO 2 ) appeared, which was the reason for the increase in the L* values of the coatings.This further demonstrates that the presence of additives in the micro-arc oxidation process inhibits substrate depletion, and the coating growth is mainly derived from the deposition of electrolyte compounds, with the coating growth mode dominated by deposition.Moreover, the thickening of the coating has a shielding effect, which reduces the Al diffraction peaks.Figure5bshows the change in the coating phase composition after adding Na 2 WO 4 .Due to the small content of the additive, Na 2 WO 4 could not affect the coating phase composition, and the coating maintained α-Al 2 O 3 and γ-Al 2 O 3 as the main crystalline phases.

Figure 4 .
Figure 4. Relationship between the thickness and surface roughness of the aluminum alloy microarc oxidation coatings with different additives.

Figure 5 .
Figure 5. XRD patterns of micro-arc oxidation coatings on aluminum alloys (a) with different contents of K 2 ZrF 6 and (b) with different contents of Na 2 WO 4 .

Figure 5 .
Figure 5. XRD patterns of micro-arc oxidation coatings on aluminum alloys (a) with different contents of K2ZrF6 and (b) with different contents of Na2WO4.

Figure 12 .
Figure 12.Kinetic potential polarization curves of the aluminum alloy micro-arc oxidation coatings with different additives immersed in a 3.5 wt.% NaCl solution for 1 h.

Table 1 .
List of the chemical reagents used in the experiment.

Table 2 .
Composition of the seven electrolytes.

Table 3 .
L* a* b* CIE results of the aluminum alloy micro-arc oxidation coatings with different additives.

Table 4 .
Surface element content of the aluminum alloy micro-arc oxidation coatings with different additives.

Table 5 .
Fitting results of the dynamic potential polarization curves.