Role of Phase Composition of PEO Coatings on AA 2024 for In-Situ LDH Growth

Plasma electrolytic oxidation (PEO) is an environmentally friendly anodizing technique leading to the formation of a ceramic-like coatings under high-voltage discharges. Layered double hydroxides (LDHs) were grown directly on γ, α, and amorphous Al2O3 powders, respectively, in order to investigate the phase responsible for in-situ LDH growth on PEO coating. Furthermore, it is shown that LDH growth is limited by the high tortuosity of the PEO layer and the accessibility of Al(OH)− 4 anions from the substrate covered with thin amorphous aluminum oxide, through the pores.


Introduction
Plasma electrolytic oxidation (PEO) is an environmentally friendly anodizing process leading to the formation of ceramic-like coatings under high-voltage discharges [1].Hard and strongly-adherent PEO layers developed on metallic substrates improve both corrosion and wear properties of the material [2].Unfortunately, the electrical discharges and high stresses in the layer are also responsible for the formation of channels, pores from gas inclusions, and cracks in the coating.Different optimization trials (such as alternative current/voltage regimes [3,4], post-treatments [5,6], particle incorporation [7][8][9], etc.) have been performed in order to overcome this disadvantage.Currently all these treatments (e.g., immersion in phosphate, silicate or borate-containing electrolytes, as well as sol-gel, polymers, and other approaches [6,10,11]) aim to improve the long-term corrosion resistance of PEO coatings by sealing the porous structure that allowed fast penetration of corrosive species to the interface.However, the improvement of barrier properties does not always ensure a long service life of the material due to the formation of scratches, cracks, and other defects caused by exploitation.One way to avoid corrosion propagation in such defects is to use the concept of active inhibition on demand in the presence of specific triggers.
Recently, layered double hydroxides (LDHs) have been widely studied as possible green containers for the active corrosion protection of bare [12][13][14][15][16][17][18], pre-anodized [19,20], and PEO-coated [21,22] aluminum alloys.LDH films have been grown in-situ, intercalated with corrosion inhibitors, and used for "smart" controlled corrosion protection as a result of anion exchange between inhibiting species and the aggressive environment [16,17,23,24].In previous works, authors have already investigated the influence of PEO thickness and Al(OH) −  4 availability for the in-situ LDH growth [22].The advantage of the PEO layer is that it includes data from both the electrolyte and the treated substrate.In the framework of this study, we have synthesized Zn-Al LDH on the surface and in the pores of the PEO coating as a first step and then analyzed the role of its phase composition.The behavior of α, γ, and amorphous aluminum oxide was checked, and their impact on LDH growth is discussed.

Materials
The list of materials used for this work is: Zinc nitrate hexahydrate (Zn(NO 3 )

Methods
AA2024 aluminum alloy with a nominal composition of (wt %): 3.8-4.9Cu, 0.5 Fe, 0.1 Cr, 1.2-1.8Mg, 0.3-0.9Mn, 0.5 Si, 0.15 Ti, 0.25 Zn, 0.15 others, and Al balance-was used as substrate.The surface pre-treatment was performed according to the procedure previously described by Kuznetsov et al. [19].Briefly, the AA2024 samples were first cleaned in Metaclean T1200 solution for 25 min at 68 • C followed by an alkaline etching in Turco Liquid Aluminetech N2 for 45 s at 60 • C, and finally an acid etching in Turco Liquid Smutgo NC for 7 min at 30 • C.
The PEO processing was performed in aqueous electrolyte with parameters presented in Table 1 following the methodology described by different groups and adopted from [19,[25][26][27].Coatings 2017, 7,190 The advantage of the PEO layer is that it includes data from both the electrolyte and the treated substrate.In the framework of this study, we have synthesized Zn-Al LDH on the surface and in the pores of the PEO coating as a first step and then analyzed the role of its phase composition.The behavior of α, γ, and amorphous aluminum oxide was checked, and their impact on LDH growth is discussed.

Methods
AA2024 aluminum alloy with a nominal composition of (wt %): 3.8-4.9Cu, 0.5 Fe, 0.1 Cr, 1.2-1.8Mg, 0.3-0.9Mn, 0.5 Si, 0.15 Ti, 0.25 Zn, 0.15 others, and Al balance-was used as substrate.The surface pre-treatment was performed according to the procedure previously described by Kuznetsov et al. [19].Briefly, the AA2024 samples were first cleaned in Metaclean T1200 solution for 25 min at 68 °C followed by an alkaline etching in Turco Liquid Aluminetech N2 for 45 s at 60 °C, and finally an acid etching in Turco Liquid Smutgo NC for 7 min at 30 °C.
The synthesis of LDH-NO3 using three different aluminum oxide-containing powders (metallic aluminum covered with natural oxide film, γ-Al2O3 and α-Al2O3) as potential source of aluminum cations was carried out using the same method described above; 0.5 g of each powder was added to a solution of 0.1 M Zn(NO3)2 and 0.6 M NH4NO3 at pH 6.5, 95 °C for 30 min.The resulting powders were centrifuged, rinsed with deionized water, and dried under 60° for 4 h.
The synthesis of LDH-NO 3 using three different aluminum oxide-containing powders (metallic aluminum covered with natural oxide film, γ-Al 2 O 3 and α-Al 2 O 3 ) as potential source of aluminum cations was carried out using the same method described above; 0.5 g of each powder was added to Coatings 2017, 7, 190 3 of 7 a solution of 0.1 M Zn(NO 3 ) 2 and 0.6 M NH 4 NO 3 at pH 6.5, 95 • C for 30 min.The resulting powders were centrifuged, rinsed with deionized water, and dried under 60 • for 4 h.

Characterizations
Planar views and cross-sections of coatings were examined with a Tescan Vega3 SB scanning electron microscope (SEM, Brno, Czech Republic) equipped with an energy dispersive X-ray (EDS) spectrometer (eumeX Instrumentebau GmbH, Heidenrod, Germany).Phases observed on the PEO-coated samples were characterized by grazing incidence X-ray measurements using a PANalytical X'Pert Powder diffractometer (Karlsruhe, Germany) (Ni-filtered Cu Kα radiation, step size 0.02 • , dwell time ~1.5 s) at room temperature in θ-θ geometry.Incident angle was set to 3 • .

Results and Discussion
In order to analyze the LDH layer on the surface, grazing incidence angle XRD (GIXRD) was performed.The measured diffraction patterns of PEO without any LDH, PEO coated with LDH-NO 3 , and PEO with LDH-VO x are shown in Figure 1a.

Characterizations
Planar views and cross-sections of coatings were examined with a Tescan Vega3 SB scanning electron microscope (SEM, Brno, Czech Republic) equipped with an energy dispersive X-ray (EDS) spectrometer (eumeX Instrumentebau GmbH, Heidenrod, Germany).Phases observed on the PEO-coated samples were characterized by grazing incidence X-ray measurements using a PANalytical X'Pert Powder diffractometer (Karlsruhe, Germany) (Ni-filtered Cu Kα radiation, step size 0.02°, dwell time ~1.5 s) at room temperature in θ-θ geometry.Incident angle was set to 3°.

Results and Discussion
In order to analyze the LDH layer on the surface, grazing incidence angle XRD (GIXRD) was performed.The measured diffraction patterns of PEO without any LDH, PEO coated with LDH-NO3, and PEO with LDH-VOx are shown in Figure 1a.
The XRD pattern of PEO treated sample shows two peaks at 19.1° and 31.9°which correspond to γ-Al2O3 found in the PEO layer.There is also a broadened signal in the range of 17-30° due to the presence of Al2O3 in amorphous or nano-crystalline form.No peak of α-Al2O3 (normally at 25.6° [28]) was detected in the XRD pattern.
For PEO-LDH-NO3, the XRD pattern shows a peak at 9.5° which is due to the (003) reflection of LDH [29].These reflections correspond to a basal spacing of 9.1 Å, and given that the total thickness of Zn/Al hydroxide layer is about 4.7 Å [26], the space available for NO3 − corresponds to approximately 4.4 Å.The (006) peak of LDH-NO3 is overlapped with the reflection of γ-Al2O3.Thus, the signals were deconvoluted via fitting of two Pearson VII peaks, as shown in Figure 1b.After the anion exchange reaction between nitrate and vanadate anions, the peak positions in the XRD pattern shifted as indicated by an arrow in Figure 1b.Both (003) and (006) peaks of LDH-VOx are detected at 9.1° and 18.5° respectively.The shift in the peaks of LDH-VOx is due to an increase in the gallery height from 4.4 Å to 4.9 Å d(003) at 9.5 Å to d(003) at 9.1 Å, respectively.No clear assignable diffraction pattern corresponding to the LDH-NO3 was detected after the anion exchange reaction.
The morphology of original PEO-treated AA2024 surface as well as LDH flakes formed on PEO was investigated using SEM. Figure 2 shows a plan view SEM micrographs of as-prepared PEO and PEO-LDH-NO3 specimens.The PEO sample (Figure 2a) shows the typical surface morphology of plasma electrolytic oxidation layers with pores and cracks at the discharge channel sites, due to the thermal stresses and gas evolution through the molten oxide material during the PEO process, respectively [1].For PEO-LDH-NO 3 , the XRD pattern shows a peak at 9.5 • which is due to the (003) reflection of LDH [29].These reflections correspond to a basal spacing of 9.1 Å, and given that the total thickness of Zn/Al hydroxide layer is about 4.7 Å [26], the space available for NO 3 − corresponds to approximately 4.4 Å.The (006) peak of LDH-NO 3 is overlapped with the reflection of γ-Al 2 O 3 .Thus, the signals were deconvoluted via fitting of two Pearson VII peaks, as shown in Figure 1b.
After the anion exchange reaction between nitrate and vanadate anions, the peak positions in the XRD pattern shifted as indicated by an arrow in Figure 1b.Both (003) and (006) peaks of LDH-VO x are detected at 9.1 • and 18.5 • respectively.The shift in the peaks of LDH-VO x is due to an increase in the gallery height from 4.4 Å to 4.9 Å d(003) at 9.5 Å to d(003) at 9.1 Å, respectively.No clear assignable diffraction pattern corresponding to the LDH-NO 3 was detected after the anion exchange reaction.
The morphology of original PEO-treated AA2024 surface as well as LDH flakes formed on PEO was investigated using SEM. Figure 2 shows a plan view SEM micrographs of as-prepared PEO and PEO-LDH-NO 3 specimens.The PEO sample (Figure 2a) shows the typical surface morphology of plasma electrolytic oxidation layers with pores and cracks at the discharge channel sites, due to the thermal stresses and gas evolution through the molten oxide material during the PEO process, respectively [1].Synthesis of LDH-NO3 on the PEO-coated AA2024 specimen led to the coverage of the surface with a layer of flake-like microstructures (Figure 2b).These flakes are significantly smaller in comparison with those previously published for TSA (tartaric sulfuric acid anodization)-treated surface (200-400 nm vs. 1-5 μm for TSA treatment) [20].Thus, growth-limiting factors must determine the LDH formation process.Based on recent studies, the following explanation can be proposed.Zn-Al LDH synthesis is based on the following chemical reactions [30]: (1) The LDH growth strongly depends on the dissolution of Al2O3 both from the bulk of the PEO coating (γ-Al2O3 + amorphous Al2O3 in this case) and/or amorphous Al2O3 from inner PEO layer (Figure 3).The complex surface system, containing PEO pores and cracks, limits species (ion) transport; the direct dissolution of aluminum from the substrate is suppressed.The smaller size of the LDH flakes on the present PEO system can be explained by the lower accessibility of Al(OH) -4 .For the PEO system, the porosity ε and the tortuosity τ of the coating (which limit the diffusion for thicker PEO layer systems) should be considered.The tortuosity-"connectivity"-describes the accessibility of the electrolyte to the surface, and is a factor to quantify the interconnection of pores, channels, and cracks (τ = 1 for an infinitely small layer thickness).For typical pore structures, it is between 1.33 and 4, and is related to the porosity via the Bruggeman relation [31].From the presented results of LDH growth, one can conclude that either the tortuosity of the PEO layer is significantly higher than for TSA layer and/or the Al-containing phases of the PEO layer are not suitable for LDH growth.Synthesis of LDH-NO 3 on the PEO-coated AA2024 specimen led to the coverage of the surface with a layer of flake-like microstructures (Figure 2b).These flakes are significantly smaller in comparison with those previously published for TSA (tartaric sulfuric acid anodization)-treated surface (200-400 nm vs. 1-5 µm for TSA treatment) [20].Thus, growth-limiting factors must determine the LDH formation process.Based on recent studies, the following explanation can be proposed.Zn-Al LDH synthesis is based on the following chemical reactions [30]: The LDH growth strongly depends on the dissolution of Al 2 O 3 both from the bulk of the PEO coating (γ-Al 2 O 3 + amorphous Al 2 O 3 in this case) and/or amorphous Al 2 O 3 from inner PEO layer (Figure 3).The complex surface system, containing PEO pores and cracks, limits species (ion) transport; the direct dissolution of aluminum from the substrate is suppressed.The smaller size of the LDH flakes on the present PEO system can be explained by the lower accessibility of Al(OH) − 4 .For the PEO system, the porosity ε and the tortuosity τ of the coating (which limit the diffusion for thicker PEO layer systems) should be considered.The tortuosity-"connectivity"-describes the accessibility of the electrolyte to the surface, and is a factor to quantify the interconnection of pores, channels, and cracks (τ = 1 for an infinitely small layer thickness).For typical pore structures, it is between 1.33 and 4, and is related to the porosity via the Bruggeman relation [31].From the presented results of LDH growth, one can conclude that either the tortuosity of the PEO layer is significantly higher than for TSA layer and/or the Al-containing phases of the PEO layer are not suitable for LDH growth.
The second assumption mentioned above is further verified by XRD analysis of γ-Al 2 O 3 , α-Al 2 O 3 , and metallic aluminum (MP-Al) powders after attempting to grow LDH-NO 3 on them (Figure 4a).The characteristic peaks at 9.9 • and 19.8 • corresponding to the reflections (003) and (006), respectively, attesting the presence of LDH-NO 3 can be only seen on the metallic aluminum powder which was covered with natural amorphous aluminum oxide.These peaks are not present for both γ-Al 2 O 3 and α-Al 2 O 3 , which suggests that the growth of LDH-NO 3 could not be achieved on crystalline γ-Al 2 O 3 and α-Al 2 O 3 .
thicker PEO layer systems) should be considered.The tortuosity-"connectivity"-describes the accessibility of the electrolyte to the surface, and is a factor to quantify the interconnection of pores, channels, and cracks (τ = 1 for an infinitely small layer thickness).For typical pore structures, it is between 1.33 and 4, and is related to the porosity via the Bruggeman relation [31].From the presented results of LDH growth, one can conclude that either the tortuosity of the PEO layer is significantly higher than for TSA layer and/or the Al-containing phases of the PEO layer are not suitable for LDH growth.Coatings 2017, 7,190 The second assumption mentioned above is further verified by XRD analysis of γ-Al2O3, α-Al2O3, and metallic aluminum (MP-Al) powders after attempting to grow LDH-NO3 on them (Figure 4a).The characteristic peaks at 9.9° and 19.8° corresponding to the reflections (003) and (006), respectively, attesting the presence of LDH-NO3 can be only seen on the metallic aluminum powder which was covered with natural amorphous aluminum oxide.These peaks are not present for both γ-Al2O3 and α-Al2O3, which suggests that the growth of LDH-NO3 could not be achieved on crystalline γ-Al2O3 and α-Al2O3.In order to check whether all aluminum oxides in amorphous state are suitable for LDH synthesis, the PEO treatment of AA2024 under 250 V, 350 V, and 450 V was performed in a different electrolyte (see Table 1).The conditions were chosen in order to form amorphous layers with different thicknesses but not crystalline phases.The XRD patterns for the samples after growth of LDH are presented in Figure 4b.It can be seen that the LDH-NO3 only grew on the sample treated under 250 V (average thickness 1-2 μm), but not on the samples prepared at 350 V and 450 V (with average thicknesses of about 30 and 50 μm, respectively).These results, together with reduced size of LDH flakes shown in Figure 2, clearly confirm that for the investigated layered systems the predominant source of aluminum cation is the Al substrate through the PEO pores.The availability of aluminum cations is strictly limited by the access of electrolyte to the substrate interface and a possible reaction between Zn(OH) + and Al(OH) -4 .

Conclusions
The part played by γ, α, and amorphous Al2O3 composing the PEO coating on the in-situ growth of LDH is analyzed in the frame of the current work.It is shown that crystalline γ-Al2O3 and α-Al2O3 cannot be dissolved and used as source for LDH formation, while natural amorphous oxide layer and inner layer, obtained during PEO treatment, can be.The LDH growth is strictly limited by high tortuosity of PEO layer and accessibility of Al(OH) -4 anions dissolved from the substrate.A compromise between the barrier properties of the PEO layer (higher thickness) and the growth of LDH for active protection (thinner layer) should be reached.For future works, the accurate identification of phases (e.g., boehmite as main component of natural aluminum oxide layer) suitable for LDH growth should be performed.In order to check whether all aluminum oxides in amorphous state are suitable for LDH synthesis, the PEO treatment of AA2024 under 250 V, 350 V, and 450 V was performed in a different electrolyte (see Table 1).The conditions were chosen in order to form amorphous layers with different thicknesses but not crystalline phases.The XRD patterns for the samples after growth of LDH are presented in Figure 4b.It can be seen that the LDH-NO 3 only grew on the sample treated under 250 V (average thickness 1-2 µm), but not on the samples prepared at 350 V and 450 V (with average thicknesses of about 30 and 50 µm, respectively).These results, together with reduced size of LDH flakes shown in Figure 2, clearly confirm that for the investigated layered systems the predominant source of aluminum cation is the Al substrate through the PEO pores.The availability of aluminum cations is strictly limited by the access of electrolyte to the substrate interface and a possible reaction between Zn(OH) + and Al(OH) − 4 .

Conclusions
The part played by γ, α, and amorphous Al 2 O 3 composing the PEO coating on the in-situ growth of LDH is analyzed in the frame of the current work.It is shown that crystalline γ-Al 2 O 3 and α-Al 2 O 3 cannot be dissolved and used as source for LDH formation, while natural amorphous oxide layer and inner layer, obtained during PEO treatment, can be.The LDH growth is strictly limited by high tortuosity of PEO layer and accessibility of Al(OH) − 4 anions dissolved from the substrate.A compromise between the barrier properties of the PEO layer (higher thickness) and the growth of LDH for active protection (thinner layer) should be reached.For future works, the accurate identification of phases (e.g., boehmite as main component of natural aluminum oxide layer) suitable for LDH growth should be performed.

Figure 3 .
Figure 3.The schematic presentation of Al(OH) -4 availability for the LDH growth.

Figure 3 .
Figure 3.The schematic presentation of Al(OH) -4 availability for the LDH growth.

Figure 3 .
Figure 3.The schematic presentation of Al(OH) − 4 availability for the LDH growth.

Acknowledgments:
This work was partially supported by MULTISURF (Marie Skłodowska-Curie grant agreement No 645676) European project.Marta Mohedano is grateful to the MINECO (Spain) for financial support via Young Researchers Challenges Programme MAT2015-73355-JIN (Proyectos Retos Jovenes Investigadores).