LDH Post-Treatment of Flash PEO Coatings

: This work investigates environmentally friendly alternatives to toxic and carcinogenic Cr (VI)-based surface treatments for aluminium alloys. It is focused on multifunctional thin or ﬂash plasma electrolytic oxidation (PEO)-layered double hydroxides (LDH) coatings. Three PEO coatings developed under a current-controlled mode based on aluminate, silicate and phosphate were selected from 31 processes (with di ﬀ erent combinations of electrolytes, electrical conditions and time) according to corrosive behavior and energy consumption. In situ Zn-Al LDH was optimized in terms of chemical composition and exposure time on the bulk material, then applied to the selected PEO coatings. The structure, morphology and composition of PEO coatings with and without Zn-Al-LDH were characterized using XRD, SEM and EDS. Thicker and more porous PEO coatings revealed higher amounts of LDH ﬂakes on their surfaces. The corrosive behavior of the coatings was studied by electrochemical impedance spectroscopy (EIS). The corrosion resistance was enhanced considerably after the PEO coatings formation in comparison with bulk material. Corrosion resistance was not a ﬀ ected after the LDH treatment, which can be considered as a ﬁrst step in achieving active protection systems by posterior incorporation of green corrosion inhibitors.


Introduction
Plasma electrolytic oxidation (PEO) is a plasma-assisted electrochemical surface treatment characterized by the utilization of high voltages (100-600 V) in alkaline electrolytes to produce ceramic-like coatings on light alloys such as aluminium [1], magnesium [2] and titanium [3]. This technology may become a potential alternative to conventional toxic and highly carcinogenic chromic acid anodizing (CAA) for niche applications [4][5][6].
The PEO process can be conducted under direct current (DC) [7], alternate current (AC) [8], unipolar [9] or bipolar pulsed regimes [10] involving polarization of the alloy to voltages above the dielectric breakdown of the oxide. This results in the formation of multiple short-lived microdischarges on the metallic surface that trigger the formation of highly stable ceramic phases [1,11,12]. The resulting PEO coatings present high hardness, thermal stability and adherence to the substrate, which lead to enhanced corrosion and tribological properties. Moreover, the coatings' microstructures and compositions can be tailored as required by controlling electrical parameters during the coating synthesis by electrolyte selection and by the application of pre-and post-treatments [13,14].
However, the costs associated with PEO technology are relatively high, which is mainly due to high energy consumption. General current densities and voltages for the PEO process are in the range of 1.5-15 A·dm −2 and 400-500 V for Mg [15], 6-20 A·dm −2 and 200-500 V for Ti [16] and 10-60 A·dm −2 and 200-400 V for Al [17]. Typical PEO treatment times are within a 15-60 min range.

Synthesis of Zn-Al-LDH Growth
Zn-Al LDH-nitrate (LDH-NO 3 ) was synthesized on AA1050-H18 aluminium alloy. The specimens were immersed in the solution for different times under continuous stirring in order to form LDH (Table 2), then rinsed in deionized water and dried in air at room temperature. All treatments were developed in 100 mL of aqueous solution at 95 • C. pH values were adjusted to 6.5 using 1 vol.% ammonia.

Characterization
Planar and cross-sectional views of the specimens were examined using a JEOL JSM 6335F (Tokyo, Japan) field emission scanning electron microscope (FESEM) working at 20 kV and equipped with an energy dispersive (EDS) spectrometer (OXFORD X-MAX, Oxford, UK). Coating cross-sections were ground through successive grades of SiC paper and polished to a 1 µm diamond finish. Phase composition was examined by X-ray diffraction (XRD), with a Philips X'Pert MRD (Amsterdam, The Netherlands, Cu Kα = 1.54056 Å). The XRD patterns were taken using grazing incidence with a step size of (0.01 • -1 • ) and a dwell time of 6 s per step at room temperature.
The specific energy consumption was calculated by integration of the voltage-time and current-time transients acquired by the power supply (SM 400AR-8 Systems electronic) during the PEO treatment (Equation (1)). With the obtained results, the energy consumption was calculated in terms of kW·h·m −2 according to Equation (1), and the specific energy consumption in kW·h·m −2 ·µm −1 was obtained dividing P tot by the coating thickness.

Electrochemical Behavior
Electrochemical impedance spectroscopy (EIS) was used to evaluate the corrosion resistance of the different coatings in an aqueous saline solution (NaCl 3.5 wt.%) at 25 • C. For that, a GillAC (ACM Instruments, Cumbria, UK) computer-controlled potentiostat and a three-electrode cell were used. The specimen was connected as a working electrode with an exposed area of 1 cm 2 . A graphite electrode and a silver-silver chloride (Ag/AgCl) electrode used as the counter and the reference electrode, respectively. The solution inside the reference electrode was KCl 3 M, which provided a potential of 0.210 V with respect to the standard hydrogen electrode. The tests applying a sinusoidal perturbation of 10 mV RMS amplitude in the frequency range of 30 kHz-0.01 Hz were carried out after 1 h of immersion. All measurements were duplicated to ensure reproducibility.

PEO Coating Screening
The first screening process to select one PEO coating per electrolyte type was conducted in accordance with three factors: (i) presence of microdischarges during coating formation, (ii) visually uniform coating morphology and (iii) coating thickness ≥1 µm.
The last step of the screening process consisted of a corrosion evaluation based on the value of the modulus of the impedance obtained by electrochemical impedance spectroscopy (EIS). Figure 1 shows the |Z| × 10 −2 Hz values of the studied materials, providing an estimation of the corrosion resistance, where higher values of |Z| indicate a lower corrosion rate [41].
The last step of the screening process consisted of a corrosion evaluation based on the value of the modulus of the impedance obtained by electrochemical impedance spectroscopy (EIS). Figure 1 shows the |Z| × 10 −2 Hz values of the studied materials, providing an estimation of the corrosion resistance, where higher values of |Z| indicate a lower corrosion rate [41]. With the aim of studying the influence of electrolytes on LDH growth, three PEO coatings were selected (one per electrolyte composition: aluminate A3.1, phosphate 2.1 and silicate S4). The specific energy consumption of selected coatings was calculated ( Table 3) by integration of the voltage-time and current-time transients ( Figure 2) recorded during the PEO process to verify that the developed coatings were energy efficient. The obtained values reveal the effect of electrolyte composition on energy consumption, coating growth rate and breakdown voltage values ( Table 3).
As can be seen in Figure 2, the current drop was observed only in aluminate and silicate electrolyte cases (Figure 2a,c) when 400 and 350 V limitations (Table 1) were achieved after 40 and 75 s of treatment, respectively, and the power supply switched to a constant voltage-control mode. Typically, when the current density was below 20 mA·cm −2 , the microdischarges extinguished; the anodizing, however, was carried on till set time in order to repair microdefects in the oxide material [42].
The fact that the limiting voltage was reached (hence the current drop) could be explained by high coating density and, consequently, higher resistance of the oxide to charge and mass transfer that were achieved at early stages [18]. High value of energy consumption in the case of aluminate is mainly due to the fact that aluminate species in the electrolyte gave rise to the formation of coating composed of nearly pure alumina, which has very low electron conductivity (i.e., the current flows mainly by ion and not by electron transfer) [12]. Further, the high breakdown voltage (320 V) in the aluminate electrolyte compelled the use of a higher voltage limit (400 V) in order to ensure a long enough period of sparking in order to achieve a uniform coating of a significant thickness; this yielded a higher specific energy consumption value. With the aim of studying the influence of electrolytes on LDH growth, three PEO coatings were selected (one per electrolyte composition: aluminate A3.1, phosphate 2.1 and silicate S4). The specific energy consumption of selected coatings was calculated ( Table 3) by integration of the voltage-time and current-time transients ( Figure 2) recorded during the PEO process to verify that the developed coatings were energy efficient. The obtained values reveal the effect of electrolyte composition on energy consumption, coating growth rate and breakdown voltage values (Table 3).  As a result, the dielectric breakdown voltage was high, the limiting set voltage was achieved quickly and, as a consequence of the current drop, the sparking period was short, hence the low coating growth rate [22,43]. Similarly, high value of energy consumption in the case of phosphate electrolyte, where sparking was observed until the end of the treatment and coating growth rate was relatively high, was due to the absence of current drop, because the limiting 350 V were never reached. In this case, higher resistance of the oxide to charge and mass transfer were achieved at 60 s, giving rise to intense microdischarges and voltage variations, and therefore the treatment was stopped at 115 s in order to maintain coating uniformity.
The lowest energy consumption of 2.2 kW·h·m −2 ·μm −1 was achieved in case of silicate electrolyte. This was mainly the result of its high electrical conductivity and, therefore, low Ubd. The onset of microdischarges early in the treatment and the relatively long sparking period before current decay As can be seen in Figure 2, the current drop was observed only in aluminate and silicate electrolyte cases (Figure 2a,c) when 400 and 350 V limitations (Table 1) were achieved after 40 and 75 s of treatment, respectively, and the power supply switched to a constant voltage-control mode. Typically, when the current density was below 20 mA·cm −2 , the microdischarges extinguished; the anodizing, however, was carried on till set time in order to repair microdefects in the oxide material [42].
The fact that the limiting voltage was reached (hence the current drop) could be explained by high coating density and, consequently, higher resistance of the oxide to charge and mass transfer that were achieved at early stages [18]. High value of energy consumption in the case of aluminate is mainly due to the fact that aluminate species in the electrolyte gave rise to the formation of coating composed of nearly pure alumina, which has very low electron conductivity (i.e., the current flows mainly by ion and not by electron transfer) [12]. Further, the high breakdown voltage (320 V) in the aluminate electrolyte compelled the use of a higher voltage limit (400 V) in order to ensure a long enough period of sparking in order to achieve a uniform coating of a significant thickness; this yielded a higher specific energy consumption value.
As a result, the dielectric breakdown voltage was high, the limiting set voltage was achieved quickly and, as a consequence of the current drop, the sparking period was short, hence the low coating growth rate [22,43]. Similarly, high value of energy consumption in the case of phosphate electrolyte, where sparking was observed until the end of the treatment and coating growth rate was relatively high, was due to the absence of current drop, because the limiting 350 V were never reached. In this case, higher resistance of the oxide to charge and mass transfer were achieved at 60 s, giving rise to intense microdischarges and voltage variations, and therefore the treatment was stopped at 115 s in order to maintain coating uniformity.
The lowest energy consumption of 2.2 kW·h·m −2 ·µm −1 was achieved in case of silicate electrolyte. This was mainly the result of its high electrical conductivity and, therefore, low U bd . The onset of microdischarges early in the treatment and the relatively long sparking period before current decay resulted in the intermediate coating growth rate value ( Table 3) [44]. It is well known that DC conditions promote low growth rates in comparison with AC conditions [18,22]; however, in this study the values obtained were considerably lower compared with the available data for different PEO treatments on commercial Al alloys, which can be as high as 26.7 kW·h·m −2 ·µm −1 [18]. The present findings demonstrate that in order to reduce specific energy consumption under DC conditions it is necessary to (i) limit the final forming voltage that ensures a current drop, and (ii) use electrolytes with conductivity, which ensures low microdischarges onset voltages and extended sparking periods, as in the case of the S4 electrolyte. Figure 3 shows the planar view and cross-section scanning electron micrographs of AA1050 coated by selected PEO coatings. All selected treatments show a thin oxide layer of 1-2.5 µm (Table 3). This is particularly evident in aluminate electrolyte-based PEO coating (A3.1), where the Al-Fe intermetallic compounds from the substrate are still visible in the coating (Figure 3a, inset) due to its low thickness. This coating is also more heterogeneous (Figure 3b) than the rest, which is attributable to its high breakdown voltage values ( Figure 2) [37,45,46] and, as mentioned before, its low coating growth rate. Phosphate electrolyte-based PEO coating (P2.1) showed a homogeneous (Figure 3c) surface appearance (Figure 3c), and the highest thickness value (Figure 3d). This was mainly due to the presence of polyphosphate species that participated in PEO coating formation and favored its high coating growth rate [47]. Silicate electrolyte-based PEO coating (S4) (Figure 3e), with the lowest breakdown voltage, showed a homogeneous surface morphology with very sparse submicrometric pores. The latter may be attributable to the formation of a thin superficial glassy layer of SiO 2 , which can be surmised from the EDS analysis where the presence of 1.5 at.% Si in the coatings and a greater content of oxygen than in the other two coatings was confirmed. the presence of polyphosphate species that participated in PEO coating formation and favored its high coating growth rate [47]. Silicate electrolyte-based PEO coating (S4) (Figure 3e), with the lowest breakdown voltage, showed a homogeneous surface morphology with very sparse submicrometric pores. The latter may be attributable to the formation of a thin superficial glassy layer of SiO2, which can be surmised from the EDS analysis where the presence of 1.5 at.% Si in the coatings and a greater content of oxygen than in the other two coatings was confirmed.

LDH Screening
The effect of reactant composition and treatment time during the growth of LDH coatings were investigated. Figure 4 depicts the XRD patterns of the different LDH treatments (Table 2)  The effect of reactant composition and treatment time during the growth of LDH coatings were investigated. Figure 4 depicts the XRD patterns of the different LDH treatments (Table 2) grown on the bulk material. The presence of peaks at 9.6° and 19.9° corresponding to the characteristic (003) and (006) reflections of LDHs intercalated with NO 3− [32,48,49] indicates the formation of LDH under the different conditions. It was revealed that treatments containing NaNO3 in the solution led to more defined and intense peaks, probably due to the presence of sodium ions in the LDH gallery [50]. On the contrary, the presence of NH4NO3 drove the formation of broadened peaks [51]. In fact, just in the case of LDH grown in NH4NO3, a small peak was revealed at 9.9° that could be associated with an LDH phase intercalated with carbonate, due to the formation of LDH layers under atmospheric conditions [52].
LDHs formed under long treatment times (LDH 2 and 4) showed very strong peaks in comparison with LDHs formed under short treatment times (LDH 1 and 3), mainly because an increment in the LDH degree of crystallinity took place [51].
The correlation between XRD patterns of studied LDH coatings and planar view scanning electron micrographs were investigated. Figure 5 shows secondary electron images of the LDH treatments (Table 2) grown on the pure aluminium.
The typical flake-like LDH structure could be clearly observed for LDH carried out in NH4NO3 at short treatment times, whereas LDH developed in NaNO3 formed this structure at long exposure times. According to Figure 5a, the LDH structure carried out in the presence of NH4NO3 is in good agreement with the XRD patterns that showed broadened peaks and, consequently, a highly open LDH structure. Additionally, Figure 5b clearly shows (also consistent with the XRD pattern) a nondefined LDH structure, which is usually attributed to the incorporation of carbonate ions into the LDH gallery [48]. However, LDH developed in NaNO3 showed typical curved plate-like LDH microcrystals at long exposure times, and a non-defined structure at short exposure times ( Figure  5c,d). This was mainly due to an increment in the degree of LDH crystallinity that was observed in XRD patterns (Figure 4).
In order to evaluate the correlation between the corrosion protection and the structure of studied LDHs, a screening process based on corrosion performance (EIS) was carried out. Figure 6 depicts the Bode and Nyquist plots for AA1050 alloy with studied LDH coatings. It was revealed that treatments containing NaNO 3 in the solution led to more defined and intense peaks, probably due to the presence of sodium ions in the LDH gallery [50]. On the contrary, the presence of NH 4 NO 3 drove the formation of broadened peaks [51]. In fact, just in the case of LDH grown in NH 4 NO 3 , a small peak was revealed at 9.9 • that could be associated with an LDH phase intercalated with carbonate, due to the formation of LDH layers under atmospheric conditions [52].
LDHs formed under long treatment times (LDH 2 and 4) showed very strong peaks in comparison with LDHs formed under short treatment times (LDH 1 and 3), mainly because an increment in the LDH degree of crystallinity took place [51].
The correlation between XRD patterns of studied LDH coatings and planar view scanning electron micrographs were investigated. Figure 5 shows secondary electron images of the LDH treatments (Table 2) grown on the pure aluminium.
The typical flake-like LDH structure could be clearly observed for LDH carried out in NH 4 NO 3 at short treatment times, whereas LDH developed in NaNO 3 formed this structure at long exposure times. According to Figure 5a, the LDH structure carried out in the presence of NH 4 NO 3 is in good agreement with the XRD patterns that showed broadened peaks and, consequently, a highly open LDH structure. Additionally, Figure 5b clearly shows (also consistent with the XRD pattern) a non-defined LDH structure, which is usually attributed to the incorporation of carbonate ions into the LDH gallery [48]. However, LDH developed in NaNO 3 showed typical curved plate-like LDH microcrystals at long exposure times, and a non-defined structure at short exposure times (Figure 5c,d). This was mainly due to an increment in the degree of LDH crystallinity that was observed in XRD patterns (Figure 4).
In order to evaluate the correlation between the corrosion protection and the structure of studied LDHs, a screening process based on corrosion performance (EIS) was carried out. Figure 6 depicts the Bode and Nyquist plots for AA1050 alloy with studied LDH coatings.   From the point of view of coating structure, the presence of spheroidal particles in flake-like LDH 1 and LDH 4 ( Figure 5) [14] was associated with the presence of secondary phases [35][36][37] that favored aluminium cation dissolution due to their highly cathodic behavior [48]. In addition, the porous structure of these spheroidal particles also favored Cl − anion penetration into the LDH gallery, and for this reason these coatings showed the lowest corrosion protection among the studied LDH coatings ( Figure 6) [53][54][55]. From the point of view of corrosion protection of non-defined LDH coatings, it should be noted that LDH 2 showed similar corrosion behavior in comparison with LDH 4, which can be attributed to its intermediate non-porous LDH structure ( Figure 5). On the contrary, LDH 3 provided a beneficial effect to corrosion protection in comparison with all the studied LDH coatings. This may be due to the presence of sodium ions in the LDH gallery, which facilitate the formation of non-porous corrosion-protective LDH coating ( Figure 6). For this reason, LDH 3 was the selected treatment to use for the selected PEO coatings and study their anti-corrosion properties ( Figure 6). coatings. This may be due to the presence of sodium ions in the LDH gallery, which facilitate the formation of non-porous corrosion-protective LDH coating ( Figure 6). For this reason, LDH 3 was the selected treatment to use for the selected PEO coatings and study their anti-corrosion properties ( Figure 6). PEO coatings based on aluminate and silicate electrolytes showed the typical peaks at 9.6° and 19.9°, which corresponded to the characteristic (003) and (006) reflections of LDH intercalated with NO3 − [32,48,49]. Additionally, the presence of ZnO and Al2Si2O5(OH)4 characteristic peaks in the S4-LDH XRD pattern came from LDH chemical composition (Table 2) and silicate electrolytes, respectively (Table 1).

PEO-LDH Coating Characterization
In the particular case of PEO coating developed in phosphate electrolyte, there were no peaks detected in that range. This could be attributed to the formation of non-crystalline phases, or to only a small amount that could not be detected at the selected scan rate (Figure 7). Figure 8 highlights a detailed morphology of the selected PEO-LDH coatings, which reveals the importance of PEO coating composition. The characteristic flake-like LDH structure can be clearly observed for the A3.1-LDH coating, in which LDH flakes are covering the whole PEO coating (Figure  8a,b). This is also observable in the S4-LDH coating, but in this case is more heterogenous (Figure  8e,f). These results are in accordance with XRD patterns that showed the presence of these characteristic reflections (Figure 7). However, for the P2.1-LDH coating there is a drastic decrease of the density of LDH-like flakes (Figure 8c,d), which is in accordance with the XRD patterns ( Figure 7).   (Table 2) and silicate electrolytes, respectively (Table 1).
In the particular case of PEO coating developed in phosphate electrolyte, there were no peaks detected in that range. This could be attributed to the formation of non-crystalline phases, or to only a small amount that could not be detected at the selected scan rate (Figure 7). Figure 8 highlights a detailed morphology of the selected PEO-LDH coatings, which reveals the importance of PEO coating composition. The characteristic flake-like LDH structure can be clearly observed for the A3.1-LDH coating, in which LDH flakes are covering the whole PEO coating (Figure 8a,b). This is also observable in the S4-LDH coating, but in this case is more heterogenous (Figure 8e,f). These results are in accordance with XRD patterns that showed the presence of these characteristic reflections (Figure 7). However, for the P2.1-LDH coating there is a drastic decrease of the density of LDH-like flakes (Figure 8c,d), which is in accordance with the XRD patterns (Figure 7). This fact is in strong agreement with a high dependence on the availability of Al(OH)2 + cations necessary to form Zn-Al-LDH. According to previous studies [40,56], Zn-Al LDH synthesis can be explained via the following chemical reactions: Zn OH Al OH 2NO → LDH NO As mentioned before, the in situ growth method was used in the present work and, consequently, Al(OH)2 + cations were an essential requirement to form the LDH layers. Due to the porous structure of PEO coatings and their compositions, two sources can provide Al(OH)2 + cations: (i) the aluminium metal matrix (due to the electrochemical interactions with LDH solutions), and PEO coating thickness [57].
Firstly, in consideration of the PEO coating cross-section ( Figure 3) and planar view micrographs after LDH treatment (Figure 8), it can be concluded that the amount of LDH flakes on the selected PEO coatings was highest for A3.1, and lowest for the P2.1 and S4 coatings. This could firstly be explained by thickness, and secondly by the chemical composition of the PEO coating surfaces. Due to the low thickness of the A3.1 PEO coating (~1 μm) (Figure 3a,b), the migration capacity of Al(OH)2 + cations from the aluminium metal matrix towards the coating surface was ensured. However, the P2.1 (Figure 3c  This fact is in strong agreement with a high dependence on the availability of Al(OH) 2 + cations necessary to form Zn-Al-LDH. According to previous studies [40,56], Zn-Al LDH synthesis can be explained via the following chemical reactions: As mentioned before, the in situ growth method was used in the present work and, consequently, Al(OH) 2 + cations were an essential requirement to form the LDH layers. Due to the porous structure of PEO coatings and their compositions, two sources can provide Al(OH) 2 + cations: (i) the aluminium metal matrix (due to the electrochemical interactions with LDH solutions), and PEO coating thickness [57]. Firstly, in consideration of the PEO coating cross-section ( Figure 3) and planar view micrographs after LDH treatment (Figure 8), it can be concluded that the amount of LDH flakes on the selected PEO coatings was highest for A3.1, and lowest for the P2.1 and S4 coatings. This could firstly be explained by thickness, and secondly by the chemical composition of the PEO coating surfaces. Due to the low thickness of the A3.1 PEO coating (~1 µm) (Figure 3a,b), the migration capacity of Al(OH) 2 + cations from the aluminium metal matrix towards the coating surface was ensured.

Corrosion Resistance of PEO + LDH Coatings
In order to evaluate the effect of LDH formation on selected PEO coatings, corrosion resistance was measured by electrochemical impedance spectroscopy (EIS) for 1 h of immersion in 3.5 wt.% NaCl solution at room temperature (Figure 9).

Corrosion Resistance of PEO + LDH Coatings
In order to evaluate the effect of LDH formation on selected PEO coatings, corrosion resistance was measured by electrochemical impedance spectroscopy (EIS) for 1 h of immersion in 3.5 wt.% NaCl solution at room temperature (Figure 9). As can be seen in Figure 9, PEO coatings with and without LDH treatment showed high similitude of the impedance modulus.
It is important to note that, in this work, the corrosion resistance of different PEO coatings was strongly connected with PEO coating porosity, because the lack of pores restricts the Cl − penetration and prevents its detrimental effects [58]. For this reason, A3.1 showed the lowest corrosion resistance due to the combination of pores and heterogeneities in comparison with the P2.1 and S4 coatings. The highest corrosion protection exhibited by the S4 PEO coating (∼5 × 10 6 Ω·cm 2 ) may be attributed to its sparse surface porosity due to the formation of a glassy silica-rich layer.
In this work, the results showed similar behavior of PEO-LDH compared with PEO coatings without any clear improvement after the post-treatment. This could be attributed to several factors, for instance: (i) LDH flake resistance was negligible compared with that of the PEO coating, and (ii) PEO coating lost some of its barrier properties during the formation of LDH, somewhat retracting from the possible small beneficial effect of the LDH layer.
It should be noted that no studies of LDH formation on flash PEO coatings (at <5 min anodizing time) have been reported so far. However, based on this work and few works carried out with nonflash PEO of aluminium, some conclusions can be drawn regarding the effects of LDH posttreatments. For instance, when an LDH layer is not loaded with inhibitors, the corrosion resistance remains unchanged or degrades slightly, as has been shown in [37]. On the other hand, when LDH is intercalated with an inhibitor (e.g., vanadate ions), an improvement in corrosion resistance is observed with immersion time due to an active protection effect [37,38].
The present findings highlight that the development of LDH-container layers did not deteriorate the corrosion resistance of flash PEO coatings, which has a potential for added active protection functionality. Therefore, the first stage of active protection system building can be considered successful. The second stage would consist of loading the LDH scaffold with corrosion inhibitors that would ensure enhanced corrosion protection.
In conclusion, these results are highly relevant for understanding the relation between coherent and uniform LDH layer formations on flash PEO coatings, which is the first step in achieving active protection systems through the incorporation of green corrosion inhibitors into the LDH layer. As can be seen in Figure 9, PEO coatings with and without LDH treatment showed high similitude of the impedance modulus.
It is important to note that, in this work, the corrosion resistance of different PEO coatings was strongly connected with PEO coating porosity, because the lack of pores restricts the Cl − penetration and prevents its detrimental effects [58]. For this reason, A3.1 showed the lowest corrosion resistance due to the combination of pores and heterogeneities in comparison with the P2.1 and S4 coatings. The highest corrosion protection exhibited by the S4 PEO coating (~5 × 10 6 Ω·cm 2 ) may be attributed to its sparse surface porosity due to the formation of a glassy silica-rich layer.
In this work, the results showed similar behavior of PEO-LDH compared with PEO coatings without any clear improvement after the post-treatment. This could be attributed to several factors, for instance: (i) LDH flake resistance was negligible compared with that of the PEO coating, and (ii) PEO coating lost some of its barrier properties during the formation of LDH, somewhat retracting from the possible small beneficial effect of the LDH layer.
It should be noted that no studies of LDH formation on flash PEO coatings (at <5 min anodizing time) have been reported so far. However, based on this work and few works carried out with non-flash PEO of aluminium, some conclusions can be drawn regarding the effects of LDH post-treatments. For instance, when an LDH layer is not loaded with inhibitors, the corrosion resistance remains unchanged or degrades slightly, as has been shown in [37]. On the other hand, when LDH is intercalated with an inhibitor (e.g., vanadate ions), an improvement in corrosion resistance is observed with immersion time due to an active protection effect [37,38].
The present findings highlight that the development of LDH-container layers did not deteriorate the corrosion resistance of flash PEO coatings, which has a potential for added active protection functionality. Therefore, the first stage of active protection system building can be considered successful. The second stage would consist of loading the LDH scaffold with corrosion inhibitors that would ensure enhanced corrosion protection.
In conclusion, these results are highly relevant for understanding the relation between coherent and uniform LDH layer formations on flash PEO coatings, which is the first step in achieving active protection systems through the incorporation of green corrosion inhibitors into the LDH layer.

Conclusions
The following can be summarized from this preliminary study of LDH growth on flash PEO coatings: • Flash PEO coatings with~1-2 µm thickness and~2-5 KW·h·m −2 ·µm −1 energy consumption were genereated on a commercially pure aluminum alloy. Low energy consumption was ensured through relatively high electrolyte conductivity and a transition of the anodizing regime from constant current to constant voltage control. • The first stage of the active protection system was successfully completed on flash PEO coatings via the development of an LDH layer. LDH coating is continuous and well defined when the PEO layer is thin (~1 µm), and the LDH formation is further facilitated when additional Al(OH) 2 + cations are lixiviated from the coating.

•
Corrosion resistance of inhibitor-free flash PEO/LDH coatings is mainly determined by the low porosity of the PEO layer. Formation of the LDH layer does not compromise the corrosion resistance of flash PEO coatings. Loading of the LDH scaffold with corrosion inhibitors is necessary in order to achieve an enhanced corrosion protection.