Mixed Zn/Li/Al-LDH based coating grown on the surface AA7075 to improve its corrosion resistance

Mixed Zn/Li/Al-LDH based coating grown on the surface AA7075-T6 to improve its corrosion resistance Valeryia Kasneryk a , * , Tatsiana Shulha a , Michal Mazur b , Maria Serdechnova a , Nico Scharnagl a , Carsten Blawert a , Mikhail L. Zheludkevich a , c , d a Institute of Surface Science, Helmholtz-Zentrum Hereon, Geesthacht 121502, Germany b Department of Physical and Macromolecular Chemistry, Faculty of Science, Charles University, Prague 212800, Czech Republic c Faculty of Engineering, CAU Kiel University, Kiel 224143, Germany d Kiel Nano, Surface and Interface Science (KiNSIS), Kiel University, Kiel D-24118, Germany ARTICLE INFO Keywords: AA7075-T6 alloy Layered double hydroxide Corrosion protection Conversion coating Adhesion Functional coatings ABSTRACT Alternative chromium-free conversion coatings are still in focus of interest. In this work, the formation of mixed Zn/Li/Al LDH-CO 32- /OH − (layered double hydroxide)-based multilayered CC (conversion coatings) was demonstrated for the first time. It was grown in-situ on the surface of AA7075-T6 aluminium alloy under mild conditions (30 ◦C) in a treatment bath containing 0.1 M Li 2 CO 3 at pH =11.5 and in the presence of NH 4 OH. The Li + ions required for the LDH structure formation came from the treatment bath, while the Zn 2+ and Al 3+ were provided by the alloy dissolution. The formed Zn/Li/Al LDH-CO 32- /OH - CC significantly enhanced the corrosion resistance of the AA7075-T6 alloy. Based on the results of electrochemical impedance spectroscopy (EIS), the total impedance modulus |Z|at 0.01 Hz was increased by one order of magnitude compared to the bare substrate, while the salt spray test (SST) revealed no pitting after 816 h of exposure. These excellent protective properties were associated with the changes of the AA7075-T6 alloy surface during the LDH growth process, formation of a passive oxide layer with good barrier protection ability and LDH's smart nanocontainer function. 1. Introduction Nowadays, aluminium alloys are widely used in the aerospace, automotive and marine industries [1–4]. Such high interest is related to their good strength-to-weight ratio and high damage tolerance. However, operating conditions often involve metallic surfaces coming into contact with aggressive media particularly in marine and aerospace applications, which include corrosive salts, fuels, de-icing fluids, and hydraulic fluids etc [5]. This can cause corrosion processes, degradation, loss of integrity, and consequently a decreased service life of aluminium-based products. Therefore, protecting aluminium alloys from corrosion has become a highly important industrial task. One of the most effective approaches to improve the anticorrosion properties of metallic materials is to form surface conversion coatings (CC) [6] in combination with paint application. In recent decades, conversion coatings based on layered double hydroxides (LDHs) have been the focus of numerous investigations. LDHs are functional materials from the class of anionic clays [7,8]. Their structures are made of positively-charged mixed metal M I /M III or M II /M III hydroxide layers, which are balanced by anions (A y− ) intercalated in the gallery and separated by water molecules [9,10]. As cheap and environmentally friendly materials, they can act as physical barriers to corrosive media [8,11]. Even more, LDHs are smart “nanocontainers”, i.e. they are capable of intercalating, storing and releasing functional species on demand, e.g. in the presence of corrosive species or in response to mechanical damage [8,12,13]. In the context of corrosion protection, the formation of Zn/Al LDH CC on the surface of AA2024 alloy have been the subject of extensive study [14–19]. These investigations reported growing Zn/Al LDH-NO 3 - coatings the AA2024 aluminium alloy surfaces and subsequently intercalating them with corrosion inhibitors, such as vanadate [14–17], molybdate, 2-mercaptobenzothiazole (MBT) [18–20] and 8-hydroxyquinoline [21]. The obtained coatings demonstrated a high level of corrosion resistance, which was associated with the capture of chlorides and the release of inhibitive species, and the self-healing ability. As an alternative to Zn/Al LDH CC, Li/Al LDH-CO 32- /OH - based CC were also * Corresponding author. E-mail address: [email protected] (V. Kasneryk). Peer review under the responsibility of Chongqing University Contents lists available at ScienceDirect Nano Materials Science journal homepage: www.keaipublishing.com/cn/journals/nano-materials-science/ https://doi.org/10.1016/j.nanoms.2025.07.016 Received 24 July 2024; Received in revised form 27 May 2025; Accepted 21 July 2025 Available online xxxx 2589-9651/© 2025 The Authors. Publishing services provided by Elsevier B.V. on behalf of KeAi Communications Co. Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Nano Materials Science xxx (xxxx) xxx Please cite this article as: V. Kasneryk et al., Mixed Zn/Li/Al-LDH based coating grown on the surface AA7075-T6 to improve its corrosion resistance, Nano Materials Science , https://doi.org/10.1016/j.nanoms.2025.07.016 developed for the AA2024 alloy [22,23]. These CC also possessed high corrosion resistance, primary due to their barrier protection ability. Moreover, in the context of Li/Al LDH-CO 32- /OH - formation, the investigations of P. Visser should be mentioned, who studied the formation of a lithium-based protective layer on the surface of the AA2024 alloy coated with polymer coatings containing Li 2 C 2 O 4 or Li 2 CO 3 fillers after immersion in a NaCl medium [24–29]. Under these conditions, multilayered coatings were formed consisting of dense amorphous Li-containing pseudoboehmite and crystalline Li/Al LDH-CO 32- /OH - phases. Even more, recent investigations by this group have demonstrated that this lithium-based protective layer preformed on the AA2024 alloy is capable to continuing its grow heterogeneously under corrosive conditions [30]. In contrast to the AA2024 alloy, the LDH treatments of the AA7075 aluminium alloy are significantly less in research focus. It was firstly introduced by Buchheit, who demonstrated possible formation of Li 2 [Al 2 (OH) 6 ] 2 CO 3 ⋅nH 2 O on the surface of AA7075-T6 alloy [22]. However, the protective ability of the formed hydrotalcite-like coatings was still limited. Later on, Petica et al. reported an improvement of the protective ability of the AA7075 alloy via in-situ growth of Li/Al LDH, followed by subsequent electrodeposition of cerium-based CC [31]. Cao et al. [32] recently showed the possible formation of an environmentally friendly protective coating on the surface of AA7075 alloy based on a combination of Zn/Al LDH CC and lignin obtained from biomass. Moreover, the hydrothermal synthesis of Co/Al LDH followed by the intercalation of sodium pyrithione, has been shown to improve the anticorrosion and antifouling properties of the AA7075 alloy. Regardless of the enhanced corrosion protection provided by the LDH formation, the main disadvantage of these LDH coatings is the synthesis strategy. The conversion treatment involves the application of elevated temperatures (85 ◦C to 95 ◦C) [31,32] or even autoclave conditions [33], which can significantly limit their further application, especially at an industrial level. In this work, the formation of mixed Zn/Li/Al LDH-CO 32- /OH - CC under mild conditions was reported for the first time. It included treatment in a reaction bath containing 0.1 M Li 2 CO 3 at pH =11.5 in the presence of NH 4 OH at 30 ◦C to 50 ◦C. The current study clarifies the processes taking place on the alloy surface during the process of Zn/Li/Al LDH-CO 32- /OH - formation and develops how the obtained Zn/Li/Al LDH-CO 32- /OH - based coatings further affect corrosion resistance and adhesive properties of the AA7075-T6 alloy. 2. Experimental 2.1. Substrate As a substrate, AA7075-T6 aluminium alloy was used with the nominal composition (analysed by spark analysis, in wt. %) of 90.42 % Al, 1.53 % Cu, 0.095 % Fe, 0.16 % Cr, 2.43 % Mg, 0.026 % Mn, 0.058 % Si, 0.036 % Ti, 5.17 % Zn. The samples were cut into plates of 30 mm × 40 mm ×1.6 mm or 50 mm ×50 mm ×1.6 mm (for adhesion tests). Prior to treatment they were ground with #1 200 and #2 500 grades of silicon carbide paper, washed with deionised water and dried at 25 ◦C in air. Upscaling tests were performed on the specimens with a size of 80 mm ×150 mm ×1.6 mm. Prior to Zn/Li/Al LDH in-situ growth, the AA7075-T6 specimens were subjected to a cleaning pre-treatment procedure, which includes consecutive treatment with Metaclean T2001/4 VP 2 (Chemie-Vertrieb Gruppe), Bonderite C-AK ALUM ETCH 2 (Henkel), and finally with Bonderite C-IC Smutgo NC AERO (Henkel). 2.2. Synthesis of LDH-CO 32- /OH - The Zn/Li/Al LDH-CO 32- /OH - conversion coatings were synthesised via the procedure recently reported by Stephan et al. [23]. Briefly, the Zn/Li/Al LDH-CO 32- /OH - was grown on the surface of the AA7075-T6 aluminium alloy from 0.1 M Li 2 CO 3 (≥99.0 %, Sigma Aldrich) solution preheated to 30 ◦C at pH =11.5 for 24 h under continuous stirring. The pH of the treatment bath was adjusted by adding NH 4 OH solution (28 wt % to 30 wt %, Thermo scientific). Zn and Al for LDH formation were provided by a dissolution of the AA7075-T6 substrate. In order to follow the impact of synthesis conditions on the protective ability of LDH CC, specimens were also prepared under the following conditions: 30 ◦C for 12 h and 50 ◦C for 12 h. After the treatment, the LDH-coated specimens were rinsed three times with deionised water and dried at room temperature in air. The specimens were named as LDH-N-m, where N is the temperature of the treatment bath in C and m is the treatment time in hours. 2.3. Synthesis chromate-based conversion coatings Reference chromate-based conversion coatings were prepared from an Alodine 1 200 solution based on the standard procedure described in Refs. [34–36]. After the treatment, the specimens were rinsed with deionised water and dried in air. 2.4. Evaluation of AA7075-T6 alloy dissolution during Zn/Li/Al LDH formation by in-situ atomic emission spectroelectrochemistry The process of the AA7075-T6 alloy transformation during Zn/Li/Al LDH formation was followed by in-situ atomic emission spectroelectrochemistry (AESEC), which was performed using an AMETEK ICP spectrometer (AMETEK, USA) connected to a Gamry Interface 1 000 potentiostat (Gamry, USA) similarly to the procedure described previously [37,38]. This set-up includes an in-situ cell allowing an electrochemical evaluation of the processes on the metallic surface and direct analysis of the elements released into the treatment solution. A Spectra/Por® 4 RC dialysis membrane splits up the cell to prevent the loss of dissolved species from the specimen during the electrochemical measurements. The solution containing released ions is continuously fed into the plasma using a peristaltic pump. The emission lines of the released ions are further analysed and compared with the electrochemical data. Prior to test, the AA7075-T6 specimens were ground with #1 200 and #2 500 grades of silicon carbide paper and rinsed with deionised water. The exposed area of the metallic plate in the AESEC cell was 0.5 cm 2 (⌀ =8 mm). The AESEC analysis was performed at 30 ◦C in an electrolyte with a composition needed for LDH growth, i.e. 0.1 M Li 2 CO 3 , pH =11.5 adjusted by NH 4 OH. The electrolyte volume was around 0.5 cm 3 , flow rate 2 mll⋅min −1 . The selectivity of Zn dissolution from the AA7075-T6 alloy was calculated using Eq. (1) and further plotted as a function of time: SZn =(nZn=nAl ) (AESEC) (nZn=nAl ) (EDS)(1) where n are the amounts of Zn and Al (in moles) measured by AESEC during AA7075-T6 alloy dissolution, or by EDS for initial alloy. The AESEC analysis demonstrated the presence of four stages during AA7075-T6 alloy dissolution. To clarify the processes taking place on the surface of AA7075-T6 during each of the stage, dissolution was tested by open circuit potential (OCP) similarly to one performed by AESEC, and was interrupted at particular times, namely 422 s, 765 s, 897 s, 9 690 s, which correspond to 1st, 2nd, 3rd, 4th stages respectively. The experiment was carried out in the cell with exposed area of 0.5 cm 2 (⌀ =8 mm). OCP was performed using a Gamry 1000 potentiostat (USA) with a three-electrode cell. A platinum wire was the counter electrode, a saturated Ag/AgCl electrode was the reference electrode and the AA7075-T6 plates were the working electrodes. After that, the obtained specimens during 1 st -4 th stage of the AA7075-T6 dissolution were analysed by SEM, EDS and Raman spectroscopy (see below). V. Kasneryk et al. Nano Materials Science xxx (xxxx) xxx 2 2.5. Microstructural analyses Surface morphology was analysed using a Vega3 SB scanning electron microscope (SEM, Tescan, Czechia) equipped with an energy dispersive X-ray spectrometer (EDS, Ultim Max 40 Oxford, UK). The morphology of the samples was analysed in secondary electron SE mode, acquisition was done with at a voltage of 8.0 kV to 10.0 kV and beam intensity of 6.0 to 12.0. Cross sections were analysed in back scattered electrons (BSE) mode. Prior to test, the LDH specimens were embedded in resin, ground through successive grades of silicon carbide paper (#1 200, #2 500, #4 000), washed with deionised water, dried in air, and sputtered with carbon. The size of the LDH flakes and the layer thicknesses were estimated using ImageJ software (National Institutes of Health) [39]. Time-of-Flight secondary ion mass spectrometry (TOF-SIMS) was performed using SEM Tescan Lyra 3 (TESCAN GmbH, Dortmund, Germany) microscope equipped with equipped with a ToF-SIMS (Tofwerk C-ToF) system. Scanning transmission electron microscopy (STEM) measurements were performed using a JEOL JEM NEOARM-200F microscope equipped with Schottky type field-emission gun operating at 200 kV. The lamella of the sample was prepared in Tescan LYRA dual-beam FIBSEM microscope using the lift-out technique, utilizing Ga + focused ion beam. The lamella was attached to a copper TEM grid (EMS, USA) and transferred to the STEM microscope. Images were collected in scanning mode using a JEOL annular dark-field (ADF) detector. Energy dispersive X-ray spectroscopy (EDS) element distribution maps were acquired using a JEOL JED-2300 energy dispersive X-ray analyser. 2.6. Structural characterisation A crystallographic study of the LDH phase (X-ray diffraction, XRD) was performed using a D8 Advanced Powder diffractometer (Bruker, Germany). Analyses were carried out with Cu Kα radiation in the range of 2θ from 3◦to 60◦with glancing angles of 0.5◦to 5◦, acquisition time of 10 s/step with a step size of 0.02◦. Further structural analysis of the specimens was performed by Raman spectroscopy with a confocal Raman microscope (Senterra, Bruker, Ettlinger, Germany). All data acquisition was performed at 532 nm laser wavelength, 20x objective lens, 25 mW of laser power, 50 μm aperture size, and 128 scans with an integration time of 4 s. Evaluation of the obtained Raman spectra was done using Bruker OPUS 7.5.18 software. X-ray photoelectron spectroscopy (XPS) analysis was performed using a KRATOS AXIS Ultra DLD (Kratos Analytical, Manchester, United Kingdom) equipped with a monochromatic Al Kα anode working at 15 kV (225 W). For the survey spectra a pass energy of 160 eV was used, while for the region spectra the pass energy was 20 eV. The investigated area was 700 μm ×300 μm. For the samples, charge neutralization was necessary. The evaluation and validation of data were carried out with the software CASA-XPS version 2.3.18. Calibration of the spectra was done by adjusting the C1s signal to 284.5 eV. For deconvolution of the region files, background subtraction (linear or Shirley) was performed before calculation. 2.7. Corrosion tests Salt spray tests (SST) were used for the initial characterisation of the specimen corrosion resistance. The test of small size specimens (30 mm ×40 mm ×1.6 mm) was carried out using 5 wt % NaCl aqueous solution at 35 ◦C in a salt spray test chamber ClimaCORR 1000-TL FR (VLM GmbH, Germany) according to ASTM B117 standard. The samples of bare AA7075-T6 alloy, LDH-30-12 and LDH-50-12 were subjected to SST for 48 h, while LDH-30-24 for 816 h. The LDH-30-24 sample was regularly visually inspected for corrosion damages, namely after 48 h, 96 h, 144 h, 240 h, 336 h, 504 h, 672 h and 816 h. Images of the corroded AA7075-T6 specimens (both bare and coated with Zn/Li/Al LDH) were recorded using a Canon EOS 760D camera. The estimation of the corrosion-affected surface (%) was made from the obtained photos using ImageJ software by colour-based thresholding approach [39]. Where necessary, the surface areas of the light and dark corrosion products were analysed separately and the final % of corroded surfaces represents the sum of both. The study of the upscaled specimens (80 ×150 mm ×1.6 mm) was performed according to ISO EN 9227 test in a test chamber VLM SAL 1000-FL RS FC TA ATU CC (VLM GmbH, Germany) for 168 h of exposure. All three upscaled specimens were treated parallelly and were characterised by XRD and SEM after SST analysis. The further corrosion performance of the AA7075-T6 specimens was evaluated via electrochemical impedance spectroscopy (EIS) in a 3.5 wt % NaCl solution at 22 ◦C for 7 days. Measurements were carried out using a Gamry 1000 potentiostat (USA) with a three-electrode cell comprising a platinum wire counter electrode, a saturated Ag/AgCl electrode and the testing metallic plates as working electrode with an exposed area of 0.5 cm 2 . Analysis was carried out in the frequency range from 100 kHz to 0.01 Hz at OCP, with 10 mV RMS sinusoidal potential perturbations using 9 points per frequency decade. The impedance data were fitted using ZView software, version 3.3c (Scribner, North Carolina, USA). 2.8. Adhesion tests Th adhesion properties (both dry and wet) were tested via pull-off adhesion and cross-cut adhesion tests. Comparison of the adhesion properties was performed for Zn/Li/Al LDH and chromate CC to evaluate the feasibility of replacing highly toxic chromate CC as well as the performance of both was compared to the bare AA7075-T6 alloy. Prior to testing, a two components SEEVENAX primer was applied to the 50 mm ×50 mm AA7075-T6 plates coated with Zn/Li/Al LDH or chromatebased CC. For that purpose, a water-based primer SEEVENAX Primer 313–02 (Mankiewicz) was mixed with a hardener SEEVENAX Hardener 315–00 (Mankiewicz) and deionised water. The obtained coatings were dried at room temperature and labelled as the “dry” test. Additionally, “wet” adhesion testing was carried out, which included immersion of the primed specimens in deionised water at 40 ◦C for a particular time. Dry and wet pull-off adhesion tests were performed in agreement with ISO 4624 using a PosiTest AT-M manual pull-off adhesion tester (DeFelsko, USA). Aluminium dollies (20 mm in diameter) were glued on top of dried primer with Araldite 2011 glue and dried for 20 h. The pulloff adhesion tests for “wet” and “dry” specimens were repeated three times for reproducibility reasons. Cross-cut adhesion tests were performed based on ISO 2409 using a TQC CC1000 test kit (TQC GmbH, Germany). On the top of the plates, 6 perpendicular cuts were made. Along of the diagonal lines, the cross-cut areas were scrubbed with the supplied brush. Adhesive tape was applied to one set of cuts and pulled off at a 60◦angle. In agreement to ISO standards, the adhesion properties were visually classified from 0 to 5, where 0 corresponds to full integrity of the coating, and 5 corresponds to the removal of more than 65 % of the cut area from the coating surface. 2.9. Thermodynamic calculations Thermodynamic calculations were performed using HydraMedusa software version of 18 Aug. 2009 to determine the possibility of Zn/Li/Al LDH formation, similarly to one reported in Refs. [23,40, 41]. The stability constants of all complexes were taken from the Hydra database of Jun. 2015 [42]. The following parameters were applied for the calculations: ionic strength I =2 mol/(kg H 2 O), temperature t = 25 ◦C, and concentrations 100 mM of CO 3 2- , 200 mM of Li + , 15 mM of NH 4 OH, 0.1 mM of Al 3+ and 0.01 mM of Zn 2+ . V. Kasneryk et al. Nano Materials Science xxx (xxxx) xxx 3 3. Results and discussions 3.1. Study of AA7075-T6 dissolution in 0.1 M Li 2 CO 3 at pH =11.5 in the presence of ammonia Prior to LDH growth, the process of the AA7075-T6 alloy dissolution in 0.1 M Li 2 CO 3 at pH =11.5 adjusted by NH 4 OH was studied by in-situ AESEC. This analysis allows to simultaneously evaluate the dissolution rate for selected elements and follow electrochemical changes occurring at the metallic interface. Furthermore, it is possible to observe how the composition of the treatment solution affects the various alloying elements or phases present in the alloy. [43–46]. The AESEC dissolution profiles for Al, Zn and the results of OCP values are presented in Fig. 1, for the dissolution profiles for Cu and Mg in Fig. SI–1. As it can be seen from the evolution of Zn, Al (Fig. 1), Cu and Mg dissolution (Fig. SI–1) and the OCP, the presence of four stages is characteristic for the dissolution of the AA7075-T6 aluminium alloy during the coating formation. The first period, which took place for the first 545 s of the AA7075-T6 plate immersion in the basic solution of Li 2 CO 3 , was characterised by highly intensive dissolution of Al. During this stage, Al was mainly dissolved from the surface, which could have been related to the disruption of the passive film with fast dissolution of the Al matrix. This period also included the spontaneous dissolution of Cu and Mg, taking part during the first 230 s and changing to a stable release from the substrate during all other stages (Fig. SI–1). Moreover, during the first stage, presence of distinctive spikes at 319 s (Fig. 1, green asterisk) was detected in the profiles of both Al and Zn elements. Such behaviour can be explained by the release of small Al-Zn-based intermetallic particles (IMP) from the alloy. The second period running from 545 s to 865 s was characterised by a high level of Zn dissolution, which was also accompanied by the rise of OCP. In turn, the Al concentration in the solution dropped during this time. Then, a sharp increase of Al concentration to a value around 10 −1 ppm accompanied by drops of both OCP value and zinc concentration to 10 −2 ppm was observed in the narrow third period located in the region from 865 to 1 000 s. During the second and third stages, preferential dissolution of Zn from the alloy matrix and intermetallics occurred, which can be especially seen from Fig. SI–2demonstrating the selectivity of Zn dissolution. The fourth period started from 1 000 s demonstrated the stoichiometric dissolution of the AA7075-T6 aluminium alloy (Fig. SI–2). Moreover, the amounts of both Zn and Al detected gradually decreased with the prolongation of dissolution till 8 000 s. The decrease of Al and Zn concentrations can be possibly related to the started formation of a coating on the surface and its barrier effect preventing further fast dissolution of the alloy. Moreover, the observed noncongruent changes in the Al and Zn profiles might be related to local pH changes at the interface defining the solubility of the respective species. To follow the processes taking place on the surface of the AA7075-T6 alloy during the four stages of AA7075-T6 alloy dissolution, four new specimens were prepared. The metallic coupons were put in a cell containing the treatment solution needed for LDH formation and studied by OCP test similarly to one during AESEC analysis. Once stage I-IV was reached, the experiments were interrupted, then the obtained samples were characterised by SEM, EDS and Raman spectroscopy. Fig. SI–3 represents the results of OCP for each specimen. As it can be followed from Fig. SI–3, stage I was interrupted after 422 s, stage II after 765 s, stage III after 897 s and stage IV after 9 690 s. Fig. 2demonstrates the EDS elemental distribution for Al, Zn, Cu, Fe, Mg, O and C in the initial bare AA7075-T6 alloy and the specimens obtained at stages I-IV of the dissolution process. The bare AA7075-T6 alloy was characterised by a homogenous distribution of Al on the surface, as the main component of the alloy. Moreover, the presence of Cu and Fe containing intermetallics can be seen on the surface. Few spots of O can also be detected, which were located in the areas of the intermetallics. After starting of the treatment, the specimen was characterised by less homogeneous distribution of aluminium (stage 1). Moreover, areas with an enhanced concentration of O can be also followed. Samples obtained during the periods II and III demonstrated an even less uniform distribution of both Al and O on the surface. Interestingly, the circles with a decreased concentration of Al and an increased amount of O were simultaneously formed around the intermetallics. It can be proposed that the formation of such circles with low Al concentration can be related to the more extensive formation of oxidised products in the areas close to the intermetallics. The amount of these circles decreased during the stage IV, which was related to elements’ dissolution from the entire surface of the alloy as well as the formation of LDH and amorphous hydroxides on the surface. It should also be mentioned, that all 5 specimens were characterised by a random distribution of the Cu-, Feand Mg-rich intermetallics on the surface. The evolution of the chemical composition of the AA7075-T6 Al alloy surface can be followed in Table 1 and SI-4 using EDS analysis at each stage (I-IV) of LDH growth. As previously discussed, the stage I was characterised by the selective dissolution of Al by AESEC analysis. However, the chemical composition of the specimen after the stage I was comparable to that of the bare AA7075-T6 alloy (Table 1). In contrast, the specimen from the stage II was characterised by a significantly decreased concentration of Al (viz. 62.0 ±0.2 v. 85.0 ±0.3 after the stage I) and an increased concentration of Zn (7.3 ±0.1 v. 5.8 ±0.1). At the same time, the specimen exhibited increased concentrations of both O and C. Such observations could be related to the predominant dissolution of Al taking place on the surface during the stage I, and consequently enrichment of the surface with Zn as well as with the low solubility of oxidised Zn products and the starting formation of hydroxides on the surface. The specimen obtained during the interruption of the stage III showed a slightly increased concentration of Al, and a decreased amount of Zn, which was associated with the selective dissolution of Zn from the surface, as it was demonstrated by AESEC. The long-term treatment of the AA7075-T6 alloy with ammonium solution of 0.1 M Li 2 CO 3 (stage IV) resulted in a further diminution of Al and Zn concentrations on the surface accompanied with a rise of O amount, which can be explained by the formation of a conversion coating on the surface. Additionally, the surface transformation during the first four stages was followed by Raman spectroscopy (Fig. 3). The spectra obtained during stages I and II of the AA7075-T6 dissolution were characterised mainly by the presence of bands in the region of 285 cm −1 to 650 cm −1 associated with the Me-O vibration [37,47]. In turn, the presence of Fig. 1. AESEC dissolution profiles (left y-axis) for Zn and Al plotted together with the OCP changes (right y-axis) during the LDH growth on the surface of AA7075-T6 for the first 8 000 s. The insert represents the region till 1 500 s. The green asterisks demonstrate spikes in the profiles, which is associated with the release of intermetallic particles. V. Kasneryk et al. Nano Materials Science xxx (xxxx) xxx 4 bands responsible for the CO 3 2vibration was detected after longer treatment (samples from stages III and IV). Thus, the bands at 1 040 cm −1 and 1 063 cm −1 can clearly be seen in these two spectra, which are typical for LDH structures intercalated with carbonate ions [48–50] and assigned to the CO 3 2symmetric stretching mode. Additionally to that, the spectrum of the specimen from stage IV demonstrates the presence of a shoulder at 1 090 cm −1 , which is attributed to the symmetric stretching mode of carbonate [51]. Moreover, bands in the region of 1 350 cm −1 to 1 440 cm −1 were present in the spectra, which belong to CO 3 2antisymmetric stretching modes [52]. It should also be noted that Fig. 2. EDS elemental mappings for the bare AA7075-T6 alloy and the specimens obtained during I-IV stages of the treatment with 0.1 M Li 2 CO 3 at pH = 11.5 (NH 4 OH). Table 1 Chemical compositions of the specimens by EDS, wt. %. Element/ Stage Bare AA7075-T6 Stage I Stage II Stage III Stage IV Al 85.5 ±0.6 85.0 ±0.3 62.0 ±0.2 64.8 ±0.4 53.9 ±0.2 Zn 6.1 ±0.1 5.8 ±0.1 7.3 ±0.1 5.6 ±0.2 4.9 ±0.1 O0.9 ±0.1 1.1 ±0.1 16.6 ±0.1 14.1 ±0.2 26.1 ±0.1 C3.4 ±0.1 4.0 ±0.3 8.4 ±0.2 10.0 ±0.4 9.5 ±0.2 Cu 1.5 ±0.1 1.6 ±0.1 2.3 ±0.1 2.1 ±0.2 1.3 ±0.1 Fe 0.3 ±0.1 0.2 ±0.5 0.3 ±0.9 0.3 ±0.9 0.3 ±0.2 Mg 2.2 ±0.1 2.4 ±0.1 3.0 ±0.1 3.1 ±0.1 3.9 ±0.1 Fig. 3. Raman spectra of the AA7075-T6 specimens obtained during I-IV stages of treatment with 0.1 M Li 2 CO 3 (pH =11.5, NH 4 OH). V. Kasneryk et al. Nano Materials Science xxx (xxxx) xxx 5 bands at 923 cm −1 (stage IV) and at 1 003 cm −1 (stages III and IV) were detected. Identification of these bands was tricky. A literature search showed that these bands could be attributed to C-O [53] and C-C [54] vibrations, which could be present on the surface after the long-term contact of the specimen with polymeric cell used for the experiment. 3.2. Zn/Li/Al LDH-CO 32- /OH - CC structure and surface morphology As it was previously discussed, the in-situ AESEC analysis demonstrated, that Al and Zn ions are permanently released from the AA7075T6 substrate into the LDH treatment bath. Consequently, the formation of a mixed Zn/Li/Al LDH can be assumed from a 0.1 M Li 2 CO 3 solution under the conditions applied. To confirm this assumption, detailed characterisation of the obtained coatings was performed. Fig. 4a and Fig. SI–5represent the XRD patterns of the LDH coatings grown on the surface of AA7075-T6. All three XRD patterns were characterised by the presence of two broad signals located at 11.5◦and 23.2◦2θ. The signals at current positions are typical for the (002) and (004) reflections of Li 2 Al 4 (CO 3 )(OH) 12 •3H 2 O [55,56] (PDF: 00-037-0728) as well as (003) and (006) reflections of Zn 6 Al 2 (CO 3 ) 16 (OH) 12 •4H 2 O [57] (PDF: 00-038-0486). As both types of LDHs have main characteristic signals in the same positions, the identification of the particular phase was limited by the facilities of the laboratory XRD. Concerning the impact of the treatment conditions, no differences were observed in the XRD patterns of the specimens obtained at 30 ◦C after 12 h and 24 h. In contrast, the XRD pattern of the specimen prepared at 50 ◦C showed the reflection peaks of LDH phase with significantly enhanced intensity, which can be associated to higher flakes size and the increased layer thickness. Such observation agrees with the SEM results presented in Figs. 4b to d demonstrating the highest LDH crystals size for the specimen prepared at 50 ◦C for 12 h, which reached values up to 0.5 μm. In the case of the LDH flakes grown on the surface of AA7075-T6 aluminium alloy at 30 ◦C for 12 h, their size was 2 times smaller with a maximum value of 0.2 μm. An increase of the treatment time to 24 h resulted in further growth of the LDH crystals, whose size was around 0.3 μm. Moreover, regardless of the conditions applied to form the coating, all specimens were characterised by homogenous surface coverage with LDH flakes. Moreover, for all three LDH-N-m coatings, there were no differences in the size or shape of the LDH flakes formed on the aluminium matrix or the intermetallic particles of the AA7075-T6 alloy. Based on the cross-section analysis (Fig. SI–6), all three coatings were dense, but LDH-50-12 contained cracks crossing the entire coating. The thickness of the coatings increased in the following order: LDH-30-12 <LDH-30-24 <LDH-50-12 demonstrating the values of 0.8–1 μm, 1–1.4 μm and up to 2 μm to 2.4 μm, respectively. It should also be mentioned that as it was observed in the case of LDH-30-12, the coating thickness was slightly lower in the areas close to the intermetallic particles (Fig. SI–6). Moreover, Fig. SI–6 exhibits EDS mapping of Al, O, Zn, C and Cu for all three coatings confirming the inhomogeneous distribution of elements within the LDH coatings. Moreover, EDS mapping of the LDH-30-24 surface (Fig. SI–7) also demonstrated that Al, Zn, O, C, Fe, Mg and Cu were inhomogeneously distributed on the surface. Besides, Al and Zn exhibited the areas of higher concentration located close to the Feand Cu-based intermetallics, similarly to the one discussed previously for the specimen obtained during stages II-IV of AA7075-T6 dissolution. Further structural characterisation was done by XRD at glancing incidence and TEM analyses. Fig. 5a represents a series of the XRD patterns for the LDH-30-24 specimen obtained by varying the glancing angle from 0.5 ◦to 5◦. At the lowest glancing angle (0.5◦), the LDH diffraction peak located at 11.5 2θ◦was significantly more intensive comparing to the substrate signals, which indicates that the LDH phase was located close to the coating surface. With a change of glancing angle to 1◦, the intensity of the LDH signals further increased, reaching its maximum value. Moreover, the presence of an amorphous phase was detected in the XRD pattern. This indicates that both phases were located closer to the substrate surface. Further increase of the glancing angle to 1.5◦and 2◦resulted in a decrease of the intensities corresponding to the LDH reflections, while the presence of the amorphous Fig. 4. a) XRD patterns of bare AA7075-T6 aluminium alloy and the specimens coated with the LDH CCs: LDH-30-12, LDH-30-24 and LDH-50-12 (the full range XRD is presented in SI). # – LDH, * – AlCuMg, •– MgZn 2, both originated from AA7075-T6 substrate. SEM micrographs of b) LDH-30-12, c) LDH-30-24 and LDH-50-12. V. Kasneryk et al. Nano Materials Science xxx (xxxx) xxx 6 phase was still clearly visible in the XRD patterns. It should be also mentioned that evident diffraction peaks corresponding to the substrate were presented in the patterns at a glancing angles of 0.5◦to 2◦, indicating that X-rays penetrated through the coating and reached the surface of the AA7075-T6 substrate. This is associated with intercrystalline surface porosity also demonstrated by SEM (Fig. 4), due to which X-rays can easily penetrate to the substrate. And the XRD spectra obtained at glancing angles of 2.5◦to 5◦were characterised by comparable intensities of diffraction peaks for the LDH and the substrate. The results of the TEM and TOF-SIMS analyses for the LDH-30-24 presented in Figs. 5b to dagree with the XRD data obtained at glancing incidence and display the multilayer structure of the Zn/Li/Al LDH coating on the AA7075-T6 aluminium alloy. As it can be seen from the TEM images (Figs. 5b and c) the structure was made of three distinct layers: a dense inner one with a thickness of 330–340 nm; a porous intermediate layer with 300 nm to 310 nm thick; and a 250 nm to 280 nm pillar layer on the top, which was characterised by a predominantly vertical orientation of the LDH. The thickness of the entire coating was around 1 μm, which is consistent with the results of SEM cross section analysis (Fig. SI–6). Figs. 5c and d reveal EDS and TOF-SIMS element distribution maps allowing to follow the elemental composition of each layer of the coating. Thus, O, Al, Zn and Mg were homogenously distributed in the dense inner layer based on the TEM analysis. The results obtained by TOF-SIMS analysis are consistent with the TEM analysis (Fig. 5d), which shows a homogenous distribution of Mg, Al and Li in this layer. However, this layer was mainly composed of Al, as the concentrations of both Li and Mg were deteriorated comparing to Al. Both TEM and TOF-SIMS showed, that all elements demonstrated inhomogeneous distribution within the middle and upper porous layers. Surprisingly, magnesium was mainly located in the middle layer, as can be followed from Fig. 5c exhibiting the superposition of Zn and Mg. Even more, Mg was nonuniformly spread throughout the layer, with a higher Fig. 5. (a) XRD patterns of the AA7075-T6 alloy covered with the LDH-30-12 obtained at glancing angles 0.5◦to 5◦; (b)–(c) cross section TEM micrographs of the LDH-30-24 coating at different magnification and the corresponding EDS mappings for the elemental distribution of O, Al, Zn and Mg; (d) TOF-SIMS maps (side view) for Li + , Mg + , Al + . V. Kasneryk et al. Nano Materials Science xxx (xxxx) xxx 7 concentration close observed close to the pillar-porous layers interface. The TOF-SIMS profile for Mg, presented in Fig. SI–8, also demonstrated a similar distribution of Mg throughout the coating. In turn, TOF-SIMS mapping and profiles for Li and Al (Fig. 5d Figs. SI-8a and c), revealed that the top layer was made of Li and Al, whose concentrations were enhanced in this layer of the coating. Based on the results of the both analyses, it can be proposed that that the dense layer was made of amorphous hydroxides, like amorphous Li-pseudoboehmite and Fig. 6. XPS spectra of the AA7075-T6 aluminium alloy coated with LDH CC at 30 ◦C for 24 h. (a) full survey spectrum, (b) Al 2p, (c) C 1s, (d) Li 1s, (e) O 1s, (f) Zn 2p. V. Kasneryk et al. Nano Materials Science xxx (xxxx) xxx 8 amorphous Al(OH) 3, in a manner similar to the conversion layer formed on the surface of the AA2024 alloy by lithium-leaching from a polyurethane coating [26,58]. In turn, the middle layer apparently represents a combination of amorphous phases, mainly amorphous Mg(OH) 2 , and crystalline LDH. And the top pillar layer was made by Zn/Li/Al LDH flakes composed of Zn, Al, Li and O, respectively. TEM analysis proved the presence of Zn and Al in the LDH flakes grown on the surface of the AA7075-T6 aluminium alloy. To understand whether mixed Zn/Li/Al LDH was obtained under the treatment conditions, XPS analysis was performed. XPS allows to investigate the presence of the elements in depth of the coating. Fig. 6 represents the XPS spectrum of LDH-30-24: (a) shows the full survey spectrum, while (b) to (f) represent the selected regions for the elements. The region spectrum of O 1s presented two signals located at 531.2 eV and 531.6 eV, which are attributed to the CO 3 2group [59] and Al 2 O 3 . The C 1s region spectrum contains three peaks at approximately 284.6 eV, 286.1 eV and 288.3 eV ascribed to C-C, C-O, and O-C=O, respectively. The first two signals are attributed to adventitious carbon contamination from the environment, while the last one was characteristic of CO 3 2- . Two peaks at 73.7 eV (Al 2p 3/2) and 74.2 eV (Al 2p 1/2) were present in the region spectrum of Al 2p and correspond to native Al 2 O 3 . The Zn 2p spectrum contains signals of Zn 2p1/2 at 1 044.7 eV and Zn 2p3/2 at 1 021.6 eV attributed to Zn 2+ species originated from Zn/Al LDH [19, 60]. The Li 1s region spectrum is characterised by a broad signal of low intensity at 54.9 eV, which can be associated to Li being surrounded of Zn and Al. Consequently, based on the results of the XPS and TEM analyses, a mixed Zn/Li/Al LDH-CO 32- /OH - was crystallised on the surface of the AA7075-T6 alloy. 3.3. Corrosion performance The corrosion protective ability of the obtained Zn/Li/Al LDH coatings was firstly evaluated by salt spray test (SST). Fig. 7 represents the photos of the bare AA7075-T6 aluminium alloy and the specimens coated under different conditions after 48 h of SST. Moreover, the evolution of the corrosion propagation for the LDH-30-24 system for 816 h is also demonstrated. The bare AA7075-T6 specimen was highly corroded already after 48 h of exposure, around 68 % of the surface was covered by the corrosion products. In turn, the specimens coated with Zn/Li/Al LDH were significantly less corroded. Moreover, the protective ability of the conversion coating strongly depended on the conditions of the in-situ growth of the LDH layer. Among all three specimens, the least protective was the one prepared under an elevated temperature (LDH-50-12), demonstrating 40 % of the damaged surface after 48 h of SST. Such deteriorated behaviour can be related to the presence of the cracks in the coating demonstrated by the cross-section analysis (Fig. SI–6). Only 27 % of the surface was corroded in the case of the coating in-situ grown at a lower temperature for the same treatment time (LDH-30-12). In turn, a significant improvement in protective ability was detected for the Zn/Li/Al LDH coating synthesised at 30 ◦C for 24 h, which showed almost no presence of visible corrosion products after 48 h of SST. Further prolongation of the exposure time demonstrated that the appearance of the first visible corrosion products was seen only after 96 h (4 % of corroded surface). Besides, the presence of dark points was also observed on the surface with further prolongation of the SST up to 144 h (highlighted by yellow in Fig. 7). Further prolongation of the SST resulted in the slow degradation of the AA7075-T6 specimen coated with Zn-Li/Al LDHFig. 7. Photos of bare AA7075-T6 aluminium alloy, LDH-30-12 h and LDH-50-12 subjected to SST for 48 h, and the evolution of SST photos for the LDH-30-24 h specimen (48 h to 816 h). The numbers in the photos represent the amount of corroded surface (in %) estimated using ImageJ software. V. Kasneryk et al. Nano Materials Science xxx (xxxx) xxx 9