Protective Cerium-Based Layered Double Hydroxides Thin Films Developed on Anodized AA6082

In this work, CeMgAl-LDHs protective thin films were developed directly on the anodized aluminum surface, and on the “hot water-sealed” anodized aluminum specimens. (e synthesized coatings were investigated by SEM-EDS and XRD and through long-term electrochemical impedance spectroscopy (EIS) spectra.(e growth of CeMgAl-LDHs into/onto the micropores/defects of anodized film was found to significantly improve the LDH barrier properties with delaying coating degradation compared to LDHs developed on the “hot water-sealed” surface. (e unmodified LDHs “without cerium addition” were also developed to compare the influence of cerium on the structural and electrochemical properties of LDHs. It is also noteworthy that LDHs grown on the anodized surface provided dense and finer growth, while the addition of cerium ions was found to exhibit influential higher long-term corrosion resistance properties after the 1200 h immersion period.


Introduction
Layered double hydroxides (LDHs) based coatings are a new class of smart coating systems, where enough corrosion resistance properties can be achieved due to (i) LDHs anion exchange mechanism/self-healing capability to inhibit the damaging effect of corrosion reactions [1][2][3] and (ii) LDH barrier properties which resist the interaction of corrosive species to the substrate [4][5][6]. Considering the first aspect, various corrosion inhibitors are introduced to provide efficient anticorrosion properties [7][8][9][10], where entrapment of corrosive species and release of inhibitors result in delaying the degradation process of coatings. To promote the barrier properties, the synthesis of LDHs on the anodized aluminum substrate can provide significant active and passive corrosion protection to obtain a compact coating system [11]. Conceptually, the LDH grown on the anodized surface may seal the micropores of the anodized surface (improved barrier properties), while LDH itself provides active protection via entrapment of aggressive species and through self-healing properties [12]. Moreover, LDH preferentially grows on the entire surface of the anodic film including the bumpy areas and on the complicated surface providing a suitable approach to protect the aluminum alloys [13].
Recently, rare earth elements were found to have a significant inhibiting effect and are the object of considerable scientific interest [14], exhibiting nontoxic nature [15]. Among rare earth elements, cerium-based coatings attracted significant attention, specifically also in case of incorporation inside the LDH network [16]. However, due to the larger ionic radius of cerium, it is difficult to introduce it in layered double hydroxide structure [17].
ere are few reports, where the introduction of cerium through the urea hydrolysis method inside the LDH framework [18,19] or solgel approach is reported [20]. However, a detailed study is needed to understand the influence of cerium addition on the LDHs long-term protective properties and possible interaction with the anodized film. e complex evaluation of the inhibition efficiency showed that solutions of Ce(III) ammonium nitrate salt reveal better inhibitive ability compared with those of Ce(IV) ammonium nitrate one, at relatively similar conditions of concentration and exposure time [21,22]. Herein, we synthesize the Ce-doped MgAl LDHs directly on the anodized aluminum surface, and longterm corrosion resistance properties are systematically discussed. e paper is composed of two main sections, the synthesis of cerium-based LDH on the "hot water-sealed" anodized surface and, secondly, the development of CeM-gAl-LDH directly on the microporous anodized surface. e effect of both approaches on corrosion performance is compared with unmodified LDHs and the possible mechanism of cerium interaction with LDH is described.

Synthesis of the Anodized
Layer. An AA6082 (wt.%; 0.50 iron, 0.6-1.2 magnesium, 0.70-1.30 silicon, 0.10 copper, 0.40-1.0 manganese, and balance aluminum) round extruded bar (3.14 cm 2 surface area) was cut in 1 cm thick small cylinders. e circular surface was mechanically ground with SiC grit papers, i.e., 500, 1000, 2400, 4000, and immersed in 0.1 M NaOH solution for 1 minute, followed by 0.1 M HNO 3 solution for five minutes, rinsed with distilled water after each step. e anodization process was carried out in 0.1 M H 2 SO 4 electrolyte at a constant voltage of 14 V for 30 minutes at room temperature. e stainless-steel plate was used as a cathode, while pretreated AA6082 specimen acted as the anode. e surface area of the cathode was about 50 cm 2 and the distance between the electrodes about 5 cm. After anodization, the specimens were divided into two groups: the first group was treated with "hot boiling water" for 20 minutes to seal the porous anodized structure and further was used to developed LDHs on it, and the other was immersed in the mixture solution of cerium and magnesium nitrates, which act as a building block to fabricate LDH film on/into the anodic surface.  3 on the anodized surface at high pH is also a wellexplained phenomenon [19]. e pH of the solution was maintained at 10 with a dropwise addition of NH 4 OH solution. All the chemicals were purchased from Sigma Aldrich. (b) For reference, without cerium, MgAl-LDHs were also developed on the above-mentioned anodized surfaces, at the same synthetic conditions (80°C-24 hr-10 pH) with similar magnesium salt concentration (0.01 M). e general schematic representation of LDH synthesis on the anodized surface is shown in Figure 1. For simplicity, Table 1 describes the general terms to use further for the developed specimens.

Characterization
e structure and cross-section analysis of LDHs were characterized by SEM (JEOL JSM-IT300 equipped with an energy dispersive spectrometer (EDS)) including crosssectional imaging. XRD (X'Pert High Score diffractometer (Rigaku, Japan)) was performed using copper K α emission source at 10 mA and 30 kV conditions. e corrosion resistance properties were analyzed by electrochemical impedance spectroscopy (EIS) by using Parstat-2273 equipment with 0.1 M NaCl aqueous solution at open circuit potential (E OCP ) at different exposure times, maximum 1200 hrs. A conventional three-electrode cell was used consisting of a Ag/AgCl reference electrode (+210 mV vs SHE), a platinum ring as a counter electrode, and the investigated samples as a working electrode with an exposed area of 3.14 cm 2 . e electrochemical impedance spectroscopy (EIS) measurements were collected with an amplitude of 10 mV at the frequency range from 0.01 Hz to 100 kHz. "Zsimpwin" software was used for EIS dataset fitting to further evaluate the coating film parameters and their effect on long-term stability.

Results and Discussion
XRD patterns of the developed LDHs films, with and without cerium addition, are presented in Figure 2. In all the specimens, well-resolved reflections of (003), (006), and (009) are observed clearly at around 11.8°, 23°, and 34°, respectively, that are typical reflection peaks of LDHs. e "003" reflection peak corresponds to 0.75 nm basal spacing, which confirms presence of carbonates inside LDHs [23]. It is found that the incorporation of cerium ions in LDH structure did not make any dominant difference in LDH characteristics peaks with similar basal spacings, and thus we can assume that cerium ions are incorporated in the film in the amorphous form [9]. However, slight variation in peak intensities is observed on cerium addition which indicates a little variation of crystallinity on cerium addition into LDH lamellar layers. No dominant characteristics peaks of CeO 2 / Ce 2 O 3 are observed, thus suggesting the incorporation of cerium inside LDHs layers in the amorphous form [19]. e top view of SEM images ( Figure 3) shows the anodized film before and after the LDH treatments. It is found that LDHs microcrystals grown on the anodized surface covered the entire substrates with a dense network of curvy  (Figure 4(c)). It may be due to the precipitation of more cerium content on open micropores porous aluminum oxide layer than the "hot watersealed" one. e elements Mg, Ce, Al, C, and O were found mainly to take part in the film formation. Moreover, the Mg/Ce molar ratio was found to be 7.16 and 3.75 for the hot water sealed (CeLDH-H) and LDH precursors sealed films (CeLDH), respectively. e significant decrease of Mg/Ce contents proposed the incorporation of effective cerium ions inside the CeLDH structure. e addition of cerium leads to decrease in the number of other elements in LDH surface as shown in EDS quantitatively analysis, which confirmed the presence of cerium ions in the LDHs network, either in amorphous form or/ and in the form of intermediate species Ce (OH) 2 or Ce (OH) 3 that can lead to the formation of CeO 2 [24]. However, XRD analysis supports the incorporation of cerium in the amorphous form in the LDH network and no impure cerium ions phase is observed.
It is found that LDH film grows simultaneously "inward" and "outward" on the substrate, growth inward dense layer formed at LDH/substrate interface and outward porous network of LDHs formed at LDH/solution interface [25]. e dissolution of the aluminum oxide layer and interaction with the LDH precursors start LDH inward growth, while during that aluminum is oxidized to Al 3+ . us, thanks to the simultaneous presence of Ce 3+ and Mg 2+ ions, on interaction with OH − groups Al(OH) 3 , Ce(OH) 3 , and Mg(OH) 2 are formed. In addition, cerium ions can diffuse inside LDH layers, resulting in the formation of MgAlCe-LDH. In cross-sectional analysis ( Figure 5), a two-layer structure is formed on the substrate, and anodic film starts partial dissolving on contact with the LDH precursors and results in the formation of LDH film on the anodic film and forms a compact structure [19]. It is reported that higher pH causes the hydrolysis of Ce 3+ , and this can promote the precipitation of Ce(OH) 3 on the anodized surface. So, cerium participates in the sealing of micropores and in the building block of LDHs in the amorphous form [19]. e presence of Ce 3+ ions may lead to partial oxidation to Ce 4+ and can form mixed-species containing CeO 2 , Ce(OH) 4 depending on pH of the surrounding solution [26]. e measured film thickness (anodization + LDH layer) of the developed specimens is shown in Figure 5(b), where "hot water-sealed" LDHs have shown comparatively a little higher film thickness than LDHs grown on microporous anodic film.
To understand the corrosion resistance behavior of modified and unmodified LDHs, electrochemical impedance spectroscopy spectra were collected in 0.1 M NaCl solution. e electrolyte has been selected in order to promote degradation of the LDH layer which allows us to monitor the potential inhibitive effect of Ce in the time scale investigated. e EIS spectra acquired after 1 h of immersion are reported in Figure 6. impedance modulus and phase plots are shown in order to better highlight the presence of the relaxation processes in the high-frequency range.
For general comparison among different developed LDH films, the impedance modulus in the low-frequency range can be employed for a rough estimation of the stability and corrosion resistance of the investigated samples [27]. Accordingly, as far as the impedance modulus at 0.01 Hz is concerned (|Z| 0.01 ), the introduction of cerium in both forms (CeLDH, CeLDH-H) has shown initially an improvement compared to the reference samples. In particular, an increase of about 1.5 orders of magnitude of |Z| 0.01 is observed for cerium modified LDHs, compared to the unmodified LDHs ( Figure 6(a)). e phase plots ( Figure 6(b)) of cerium modified and cerium-free LDHs both suggest the presence of two   Advances in Materials Science and Engineering 3 relaxation processes. However, significant differences are observed: in the case of the Ce containing coatings, one time constant is located in the high-frequency range (around 10 4 Hz) and the other in the low-frequency range (around 10 −1 Hz), while for the LDH and LDH-H, the first time constant is shifted towards lower frequency range (around 10 3 Hz) and the second one towards the middle frequency range (around 10 0 -10 1 Hz). To a first approximation, the high-medium frequency relaxation process is attributed to the outer porous surface layer and the other in medium-low frequency relaxation process is associated with the inner barrier layer. e time constant in the low-frequency range is believed to account also for the faradic process occurring at the substrate/film interface. A deeper discussion of the reasons underneath this attribution is provided at a later stage in the manuscript when the electrical equivalent circuits (e.e.c.) employed for the fitting are discussed. Considering the impedance plot in Figure 6(a), the CeMgAl-LDH depicted higher impedance values in the middle-high frequency range, suggesting the better resistance of the upper layer as compared to unmodified LDH. As previously anticipated, higher impedance modulus values also in the low-frequency range have been observed for the Ce based LDHs, thus suggesting a reduced corrosion rate. e significantly enhanced corrosion resistance of Ce based LDHs can be explained by the following; (i) the passive protection of compact LDH layer and (ii) the presence of cerium in the LDH frame, which was found to provide protection by its inhibition action by dissolving from the LDHs structure, entering the solution and precipitating at the cathodic sites (mechanism is explained later in the manuscript). To better investigate the role of Ce in the LDH layers, the long-term stability of the developed coatings has been investigated upon exposure to the chloride containing media. e impedance plots of the unmodified LDH after immersion for 1 h to 168 h are shown in Figures 7(a) and 7(b). Modified Ce containing LDHs after various time intervals of immersion (from 1 h to 1200 h) is shown in Figure 8 (including an enlarged version of the low-frequency impedance modulus in Figures 8(b) and 8(d)   Advances in Materials Science and Engineering almost the same |Z| 0.01 value∼10 6.5 Ω·cm 2 after 1 h of immersion. It is observed that |Z| 0.01 value reduced more rapidly from 1 to 72 h probably due to upper porous LDH layer, but on a later stage, a slower reduction rate is observed. On later immersion stages, CeLDH shows a lower decreasing rate of the low-frequency impedance compared to the CeLDH-H sample. e |Z| 0.01 value of CeLDHs reduced from 10 6.5 to about 10 5 Ω·cm 2 after 1200 hrs, while in case of CeLDH-H, it reduced from 10 6.5 to about 10 4.5 Ω·cm 2 . is finding suggests higher stability of the CeLDH structure against the corrosive solution compared to the CeLDH-H.
To better compare the investigated samples, the |Z| 0.01 values of modified and unmodified LDHs as a function of immersion time are shown in Figure 9. e |Z| 0.01 values of CeLDH are much higher than other developed LDH films throughout the whole immersion period. Moreover, a decrease in the declining rate is observed on exposure with long-term corrosive solutions. At the early stage of the corrosion mechanism, a relatively rapid decrease from 10 6.5 to about 10 5.5 Ω·cm 2 was observed for CeLDH, while the decline rate was more severe for CeLDH-H, from 10 6.5 to about 10 5.2 Ω·cm 2 . On the second stage of the exposure (after about 200 h), a slower degradation process was observed for the CeLDH and CeLDH-H samples. ese findings suggest that, regardless of the difference between the Ce containing LDH coatings, the presence of Ce in the coatings improves the initial protection properties as well as the durability upon prolonged immersion in the electrolyte. is effect is believed to be related to the LDH development inside the anodic film layers and active role of cerium in providing a sort of healing effect thanks to its recognized effect as a corrosion inhibitor.
To further analyze the properties of the developed thin films containing Ce, the EIS data were fitting by using the "ZSimpwin" software [28]. Two different circuits were employed to investigate the EIS experimental dataset. At an early stage of immersion (1-120 h), two time constants were observed: one located in the high-frequency range (around 10 4 Hz) and the other in the low-frequency range (around 10 −1 Hz). According to the literature, it seems quite well established that the former can be associated with the response of the porous anodic oxide/LDH layer covering the metal [29][30][31][32][33][34][35][36][37]. Conversely, the attribution of the latter is not univocal and different interpretations can be found in the literature. Hoar and Wood [35] and, later, Mansfeld [36] ascribed the middle-low frequency time constant to the properties of the barrier layer between the bottom of the pores and the metal oxide. Similarly, Huang et al. [31,32] interpreted the middle-low frequency relaxation process as the contribution of the inner barrier layer of the anodic oxide. On the other hand, Zhu et al. [30] described the lowfrequency loop as the superimposition of the contribution of the inner barrier oxide and the charge transfer process: a polarization resistance and a constant phase element (CPE) to account for the electrical double layer and barrier layer were used to fit the relaxation process. Similarly, Usman et al. [29] associated the low-frequency time constant with the charge transfer processes related to the barrier layer and, for prolonged immersion time, with the corrosion of the substrate. For the sample under investigation, at the beginning of the immersion the |Z| 0.01 value is relatively high, thus suggesting that the low-frequency response can be attributed to the superimposition of the contribution of the inner barrier oxide and the charge transfer process. A contribution associated with the corrosion process will be introduced for longer immersion time. A similar approach has been employed by other authors [37]. e circuit depicted in Figure 10(a) was employed to fit the EIS dataset at an early stage of immersion (1-120 h): R s stands for the electrolyte resistance; R LDH and CPE LDH refer to the conductive paths and the dielectric properties of the porous anodization layer + LDH coating, respectively; R b accounts for the resistance of the barrier oxide, while CPE b refers to the dielectric response of the electrical double layer and barrier layer [30]. Constant phase element (CPE) was employed instead of pure capacitances, according to the general expression, i.e., Z CPE = 1/(Q(ωj)) α . CPEs are recognized to be a flexible fitting parameter to account for capacitance distributions that originated from surface roughness and heterogeneities, electrode porosity, variation of coating composition, nonuniform potential, and current distribution [38].
In the second stage of immersion (168-1200 h), the phase peak in the low-frequency range is no more symmetrical and cannot be properly fitted by means of the e. e. c. depicted in Figure 10(a). Furthermore, a significant decrease of the impedance modulus in the low-frequency domain is observed (see Figure 9). According to other authors [37] from 168 h of immersion in the saline solution, a contribution of the corrosion process is introduced in the e. e. c. employed to fit the experimental EIS dataset (Figure 10(b)). Notice that an additional time constant to account for the Faradic process is introduced. In particular, R s stands for the electrolyte resistance; R LDH and CPE LDH refer to the conductive paths and the dielectric properties of the porous anodization layer + LDH coating, respectively; R b accounts for the resistance of the barrier oxide, while CPE b refers to its dielectric response; R ct stands for the charge transfer resistance while CPE dl refers to the dielectric response of the doublelayer capacitance. e parameter obtained by means of the fitting of the raw EIS data is reported in Tables 2 and 3.
As far as the fitting parameters related to the porous oxide/LDH layer are concerned (Q LDH , R LDH , α LDH ), notice that the resistances of porous layer are lower than the associated barrier layer resistance. is is in accordance with previous studies which explained this finding with the higher conduction in the sealed pores compared to the barrier layers [37]. e Q LDH values are quite scattered, probably due to the permeable structure of the porous anodic layer as well as the LDH layer. e corresponding values of the exponent of the CPE, α LDH , are in the 0.58-0.84 range during immersion time. is parameter is recognized to be dependent on the fractal nature of the surface (affected by roughness and porosity) [38], on the roughness of the interphase [39], and on heterogeneities of the electrode surface [40]. Within this framework, the obtained data suggest a quite significant evolution of the surface roughness and porosity of the porous anodic oxide/LDH layer during immersion time. Partial dissolution of the porous anodic oxide and precipitation of insoluble compound over the surface of the electrodes are likely to occur and would explain the α LDH values (which are quite far from unity). e surface conditions of the electrodes after the immersion will be investigated at the end of this section to further support this hypothesis. On the other hand, given such values of α LDH , Q LDH cannot be clearly ascribed to a specific physical property. Q LDH is believed to account for the dielectric properties of the porous anodic oxide/LDH layer but a more defined description is not possible.
On the other hand, R b values, suggesting the barrier properties of the layer, have shown significant resistance values and demonstrate improved barrier properties for CeLDH compared to the CeLDH-H. It can also be seen that R b values for CeLDH show a decrease from 2.2·10 7 Ω·cm 2 and 1.5·10 6 Ω·cm 2 to 3.8·10 5 Ω·cm 2 and 4.7·10 3 Ω·cm 2 for CeLDH and CeLDH-H, respectively, with the immersion of 1 hr to 1200 hr in NaCl solution. ese findings suggest that the hot water sealing treatment seems not beneficial to further improve the barrier properties of the CeLDH coatings. e Q b values, although slightly scattering in the first days of immersion, are almost stable during immersion time. e numerical values of the CeLDH and CeLDH-H sample are comparable: this suggests that the thickness of the barrier layer for the two different samples is similar. As far as the exponent of the CPE, α b is concerned, it ranges from 0.66 to 0.89 for both sealed and unsealed samples. ese values suggest that the barrier oxide is heterogenous [40]. e sealing was supposed to be able to "plug" the flaws present in barrier layers, thus increasing its homogeneity [37]. However, no relevant differences are observed between sealed and unsealed samples in our case.
is finding further supports the reduced beneficial effect of the hot water sealing for the investigated samples.
Similarly, R ct values indicate that the extent of the corrosion process occurring on CeLDH-H is more pronounced compared to CeLDH. In fact, the R ct , which is inversely proportional to corrosion rate, is a parameter representing the resistance of the electron transfer across the metal surface [41]. It is recognized that the higher R ct is, the more difficult the corrosion reaction is and hence the lower the corrosion rate is [28].
XRD pattern of the investigated samples after 1200 h of immersion in the electrolyte has been collected in order to investigate the different species developed on the surface upon immersion in the saline solution. XRD diffraction parameters of CeLDH and CeLDH-H after 1200 hrs of immersions are shown in Figure 11 e X-ray diffraction   3 . e low intense cerium oxide/hydroxide peaks support the presence of crystalline cerium oxide in the LDH layer. Considering that these compounds were not found before the exposure in the electrolyte, it is likely that a release of Ce occurred from the amorphous domains in the anodic oxide/LDH layer. Ce entered the solution from the LDH structure and the consequent precipitated over cathodic sites upon reaction with OH − species. is mechanism is believed to be responsible for the anticorrosion properties of LDHs due to the inhibition of the cathodic reaction provided by the precipitated compounds (mixed cerium oxide/hydroxides). Matter et al. [21] previously investigated the cerium inhibition effect and reported that Ce(III) attributes better inhibitive ability than Ce(IV) and thus Ce(III) addition on the LDHs is likely to be responsible for a sort of self-healing effect by forming Ce(OH) 3 /CeO 2 on protective film degradation. e corrosion protection ability of cerium salts is already well reported in various works, where the inhibiting mechanism is recognized to involve the impediment of the cathodic oxygen reduction reaction due to the formation of cerium hydroxides/oxides [42][43][44].
e SEM images of CeLDH after treatment with 0.1 M NaCl solution after 360, 720, and 1200 hrs are shown in Figure 12, where results indicated the good intactness and stability of the structure after 360 hrs of immersion and    similarly after 720 hrs of immersion. However, corrosion products seem to precipitate over the surface after 720 h of immersion. After 1200 h of immersion, the partial destruction of regular platelet structure can be seen and results in an expansion of platelet structure with the accumulation of corrosion products on the surface. However, XRD analysis confirms the intactness of LDH geometry after 1200 h immersion in 0.1 M NaCl solution. e observation of LDH geometry is well correlated with the EIS findings which suggest a limited degradation extent of the protective layers.

Conclusion
Ce based MgAl-LDH conversion films were developed by a single-step in situ growth method on the anodized AA6082 surface. e sealing effect of CeMgAl-LDHs precursors on the anodized porous network and corresponding barrier effect in terms of long-term corrosion resistance behavior and structural evaluation is discussed. e introduction of cerium directly on the anodized porous network (CeLDH) has shown improved corrosion resistance properties, compared to the CeLDH-H which were grown on "hot water-sealed" anodized surface. e findings demonstrated that CeLDH provided comparatively better active and passive corrosion protection than other developed specimens. e evaluation of coating parameters defined the corrosion mechanism for LDHs and their behaviour on long-term stability against corrosive species. e CeLDH was found intact after 360 hrs of immersion and slightly corroded after 760 hrs of immersion and provided stable longer period protection (1200 hrs) for the underlying AA6082 substrate. Ce oxides/hydroxides were found on the LDH surface after prolonged exposure to the chloride solution. is finding suggested the capability of the film to release cerium cations to the environment and their capability to precipitate in correspondence of the cathodic sites upon reaction with OH − species. e CeLDH developed directly on the porous anodic film can be potentially used for further functionalization for the enhanced protection of aluminum alloys.

Data Availability
Data are available on request from Muhammad Ahsan Iqbal (University of Trento, Department of Industrial Engineering, via Sommarive n. 9, 38123 Trento, Italy, muhammadahsan.iqbal@unitn.it).