• <tr id="yyy80"></tr>
  • <sup id="yyy80"></sup>
  • <tfoot id="yyy80"><noscript id="yyy80"></noscript></tfoot>
  • 99热精品在线国产_美女午夜性视频免费_国产精品国产高清国产av_av欧美777_自拍偷自拍亚洲精品老妇_亚洲熟女精品中文字幕_www日本黄色视频网_国产精品野战在线观看 ?

    LDH conversion film for active protection of AZ31 Mg alloy

    2023-03-24 09:24:22PilloMingoelOlmoMtykinKooijmnGonzlezGrciArrlMoheno
    Journal of Magnesium and Alloys 2023年1期

    B.Pillo ,B.Mingo ,R.el Olmo,c ,E.Mtykin ,A.M.Kooijmn ,Y.Gonzlez?Grci ,R.Arrl,M.Moheno,?

    a Departamento de Ingeniería Química y de Materiales,Facultad de Ciencias Químicas,Universidad Complutense,Madrid 28040,Spain

    b Department of Materials,University of Manchester,Oxford Road,Manchester M13 9PL,UK

    c Institute of Optoelectronics,Military University of Technology,2 Kaliskiego Str.,Warsaw 00-908,Poland

    dMaterials Science and Engineering Department,Delft University of Technology,Mekelweg 2,Delft 2628 CD,the Netherlands

    Abstract Zinc aluminium (Zn-Al) and lithium aluminium (Li-Al)– layered double hydroxides (LDH) coatings with incorporated inhibitors (Li?,Mo?and W?based) were successfully synthesized on AZ31 Mg alloy.Zn?Al LDH W and Li?Al LDH Li showed the highest corrosion resistance and were selected for further evaluation.SEM cross?section examination revealed a bi?layer structure composed of an outer part with loose fla es and a denser inner layer.XRD,FTIR,and XPS analysis confirme the incorporation of the inhibitors.Post?treatments with corrosion inhibitors containing solutions resulted in the selective dissolution of the most external layer of the LDH coating,reducing the surface roughness,hydrophilicity and paint adhesion of the layers.Active corrosion properties were confirme by SVET evaluation for the Zn?Al LDH W coating.The proposed active corrosion mechanism involves the ion?exchange of aggressive Cl?ions,deposition of hydroxides and competitive adsorption of W?rich corrosion inhibitors.

    Keywords: AZ31;Magnesium;Layered double hydroxides;Corrosion inhibitors;Active corrosion protection.

    1.Introduction

    Conversion coatings are one of the most cost?effective approaches for preventing the degradation of metallic alloys[1].In the last decade,layered double hydroxides (LDHs)have shown promising results on Al alloys [2,3] and more recently,there has been growing interest of these layers on Mg alloys [4–9].

    LDH or hydrotalcite?like systems can be described as positively?charged mixed metal (M2+,M3+) hydroxide layers and interlayers occupied by anions (Am?:NO3?,Cl?,CO32?,etc.) and water molecules.The general formula of the most common LDHs can be represented as [M2+(1?x)M3+x(OH)2]x+[(Am?)x/n·nH2O]x?[10].The M(II)/M(III) ratio may vary according to the synthesis conditions and initial salts concentrations.In most cases,this ratio lies between 0.10 and 0.33.The stability of LDH increases in the order Mg2+

    In-situgrowth and co?precipitation methods are commonly used to synthesize LDH coatings.In-situprocesses can be subdivided into several categories (one?step,two?step,hydrothermal,urea hydrolysis,steam coating,etc.).These are simple,versatile and use the substrate as a source for cations.Co?precipitation methods allow for a greater variety of LDH chemical compositions and,in the case of Mg alloys,are usually carried out in solutions with M2+and M3+ions under high temperature and pressure conditions (i.e.,hydrothermal synthesis).

    From a corrosion point of view,the most interesting property of LDHs is the anion exchange capacity due to the lack of crosslinking between the hydrotalcite?like layers [13,14].This capacity can be fin ?tuned by controlling the size,charge/ratio of metal cations and anions and amount of water[10].As a result,LDH can serve as a nanotrap for corrosive anions or as a container for corrosion inhibitors.

    Table 1 [15–26] summarizes corrosion results reported for alloy AZ31 with LDH coatings.In general,LDH coatings improve the corrosion performance by up to 4 orders of magnitude in terms of corrosion current density and impedance modulus.Most studies focus on Mg?Al LDH.Other systems include Mg?Fe,Zn?Al and Li?Al.The most common synthesis method is the hydrothermal route,although steam coating,two?step,and urea hydrolysis have also been used.

    Table 1 Synthesis method and corrosion performance of different LDH coatings on AZ31.

    Only a few studies [22,24–26] addressed the intercalation of inhibitors.Zeng et al.[22] synthesized a molybdate intercalated hydrotalcite coating with nanosized lamellar structures that,according to FTIR measurements,released MoO42?which acted as anodic inhibitor.Chen et al.[24]found that Mg?Al?ASP?LDHs had better corrosion resistance than Mg?Al?NO3?LDHs owing to the corrosion inhibition of aspartic acid (ASP) ions and the larger specifi surface area to capture Cl?.Anjum et al.[25] studied the effect of intercalation of 8?hydroxyquinoline (8HQ)corrosion inhibitor into Mg?Al based LDH coating.The enhancement of corrosion resistance was attributed to Cl?and HQ?ion exchange and the redeposition of Mg(HQ)2.Tang et al.[26] intercalated Cl?,VO43?,PO43?,and MoO42?.The results showed that the corrosion resistance decreased in the following order: Zn?Al?VO43?>Zn?Al?MoO42?>Zn?Al?PO43?>Zn?Al?Cl?>Zn?Al?NO3?.The better corrosion behaviour of Zn?Al?VO43?LDH was attributed to its greater ion?exchange ability.

    In this study,new Zn?Al and Li?Al LDH coatings with incorporated inhibitors (W-,Mo-and Li-based species) are produced on AZ31 Mg alloy.After screening and ranking the coatings by electrochemical impedance spectroscopy (EIS),the best LDH coatings are investigated byX?ray photoelectron spectroscopy (XPS),X?ray diffraction (XRD),contact angle measurements and Fourier?transform infrared spectroscopy (FTIR).The corrosion performance is further evaluated by Scanning Vibrating Electrode Technique (SVET).

    2.Material and methods

    2.1.Material

    AZ31B Mg alloy specimens (composition in wt.%: Al 3.1,Zn 0.73,Mn 0.25,Si 0.02,Ca<0.01,Fe 0.005,Cu<0.001,Ni<0.001,Zr<0.001,others<0.30,and Mg bal.) with dimensions of 25×40×3 mm3were pre?treated in a two?step commercial process: (1) alkaline cleaning in 90 g L?1Bonderite C?AK 4181 L for 12 min at 80?90 °C;and(2) acid etching in sulphuric?based solution (10 g L?1Bonderite C?IC 3610,3 min at room temperature).Finally,the specimens were rinsed in deionized water,cleaned with isopropyl alcohol and dried in warm air.Cleaning solutions were replaced every 20 specimens.

    2.2.LDHs synthesis

    LDH coatings were synthesized using 500 mL aqueous solutions (pH ~10) consisting of 0.25 M Zn(NO3)2·6H2O+0.125 M Al(NO3)3·6H2O and 0.125 M LiNO3+0.125 M Al(NO3)3·6H2O,for the so called Zn?Al and Li?Al systems (the naming refers to the main cations used in the solutions).0.0625 M Na2CO3was added as a precipitating agent and the pH was adjusted with 1 M NaOH.The stirred solutions were transferred to a PTFE?lined stainless steel autoclave in which the samples were vertically immersed for 24 h at 125 °C.

    Inhibitor?containing systems were obtained via immersion of LDH?coated specimens in aqueous solutions containing Na2WO4·2H2O,Na2MoO4·4H2O or LiNO3(0.1 M,pH ~10)during 2 h at 45 °C,followed by rinsing in deionized water and drying in warm air.The specimens without inhibitors were identifie as Zn?Al LDH and Li?Al LDH.The coatings with inhibitor follow the same naming with the addition of the main element at the end,e.g.,Zn?Al LDH W.

    The inhibitors used in this work were selected based on the positive results of previous studies with WO4?[27,28],MoO42?[29]and Li+[30].WO4?and MoO42?behave as anodic inhibitors that adsorb on the surface at fl wed areas [31].The role of Li+is more complex,but it seems to be related to the modificatio of the hydroxide layer that forms on the surface [32,33].These inhibitors can be considered “green”alternatives to chromates,which are known to be toxic and carcinogenic [34].It is worth mentioning that the mechanism of inhibitor incorporation may differ depending on the charge of the active species.Anions are expected to incorporate into the intergallery spaces,whereas cations may interact with the hydroxide layer [35].

    2.3.Surface characterisation

    Surfaces and cross?sections of studied specimens were examined by scanning electron microscopy (SEM,JEOL JSM?6400) and fiel emission scanning electron microscope(FESEM,JEOL JSM 6335F).Coatings cross-sections were prepared by grinding through successive grades of silicon carbide paper with fina polishing to 1 μm diamond fin ish.Both microscopes are equipped with an Oxford Link Energy Dispersive X?ray (EDS) microanalysis spectrometer.The phase composition was analysed by X?ray diffraction(XRD,Philips X’Pert diffractometer,Cu Kα=0.154056 nm,from 10° to 90°,0.05° step size,6 s per step,0.5° grazing angle).Measurements of features such as dimensions of fla es and coatings thicknesses were carried out by image analysis using AxioVision 4.8 software.

    Fourier transform infrared (FTIR) analysis of the LDH coatings was performed with a Nicolet iS50 spectrophotometer equipped with a KBr beam?splitter,a DTSG?KBr detector and a SpectraTech Performer ATR accessory equipped with a diamond glass.Reported measurements are the average of 128 scans with 4 cm?1resolution.

    XPS analysis of the samples was carried out using a Thermo Scientifi K?Alpha ESCA instrument equipped with aluminium Kαmonochromatized radiation at 1486.6 eV X?ray source.Due to the non-conducting nature of the samples,it was necessary to use an electron floo gun to minimize surface charging.Neutralization of the surface charge was performed by using both a low?energy floo gun (electrons in the range of 0 to 14 eV) and a low?energy Argon ions gun.

    The roughness analysis was conducted with a focus?variation optical profilomete (Infinite ocusSL,Alicona) witha×50 magnificatio lens and the IF?Measure Suite software.Cited roughness parameters (Sa;arithmetical mean height and S10z;ten?point height) were determined from the primary surface area and are the average of three measurements.

    Water contact angle (?) measurements were performed by means of a drop shape analysis system (FTA 1000) with an incorporated high?speed camera (Edmund Optics 5582,Navitar lens) and FTA32 software.Cited values are the average of duplicated specimens.For each specimen,three drops were measured in 20 frames acquired during 15 s from the release of the drop.

    Paint adhesion tests were carried out in accordance with EN ISO 2409 [36] by 5 line cross?cut with 1 mm spacing obtained using certifie tools (Zehntner Testing Instruments).A three-component epoxy layer applied with a drawbar (~25 μm-thick layer) and cured for 1 h at 80 °C was used.

    2.4.Corrosion tests

    Electrochemical impedance spectroscopy (EIS) measurements were conducted using a GillAC (ACM Instruments)computer?controlled potentiostat.The exposed area was limited to 1 cm2.Samples were measured by triplicate in 3.5 wt.% NaCl solution at room temperature,(22 ± 2)°C.A three?electrode cell was used.The counter electrode was a graphite electrode and the reference electrode was a silver?silver chloride electrode in 3 M KCl solution(Ag/AgCl KCl).The specimen was connected as the working electrode.A sinusoidal disturbance of 10 mV amplitude was applied in the frequency range of 10 kHz?0.01 Hz.

    The polarization curves were measured in 0.5 wt.% NaCl solution within ?200 mV to +1500 mV with respect to the OCP using a potential scan rate of 0.3 mV/s.Samples were measured by triplicate.Values of corrosion potential (Ecorr)and corrosion current density(icorr)were measured to evaluate the corrosion properties of the materials.Corrosion current density was calculated using the cathodic Tafel slope.

    The scanning vibrating electrode technique (SVET) was used to measure the local current density at the site of the artificia defects.Defects were produced with a scratcher instrument (CSM Revetest) with a 200 μm Rockwell C diamond indenter by applying a constant load of 4 N,which gives the possibility to prepare reproducible scratches with a controlled depth of ~5 μm and 1 mm length.A commercial SVET manufactured by Applicable ElectronicsTMand controlled with the software provided by Science WaresTMwas used.The assembly uses an insulated microelectrode of Platinum?iridium manufactured by MicroprobeTMwith a Platinum black deposited on its tip of?=~20 μm as a vibrating electrode.The microelectrode was placed at 150 μm above the surface sample.The probe vibration frequency normal to the sample was 67 Hz and the peak-to-peak vibration amplitude was approximately 40 μm.Before the experiments,the microelectrode was calibrated in the working electrolyte following a common procedure described in detail elsewhere [37].All the experiments were carried out in 0.05 M NaCl solution.The area of interest surrounding the defect was masked using a thin layer of sealing lacquer (Electrolube Bloc Lube Red).SVET maps of,on average 2×2.5 mm were recorded on grid of 31×31 points.

    Scribed samples were exposed to 0.05 M NaCl solution for 48 h.Specimens were manually scribed with a standard zirconia tip across the sample surface (a cross-shaped scribe,with a width of 0.1 mm and a length of 1 cm;the depth of the scribe was larger than the coating thickness and reached the underlying substrate).Scribed specimens were evaluated by SEM/EDS analysis.

    3.Results and discussion

    3.1.Inhibitor screening

    Eight different LDH systems,with and without inhibitors,were screened by EIS testing following 1 h of immersion in 3.5 wt.% NaCl solution (Fig.1).Examples of the obtained Nyquist diagrams and Bode plots are depicted in supplementary material (Supplementary Fig.S1).In general,coatings show a capacitive arc at high frequencies and an inductive loop at low frequencies.However,those coatings with a higher impedance modulus show a diffusion tail at low frequencies.It is important to note that the comparison of the impedance modulus (|Z|) at low frequency response (0.01 Hz)is a common tool for ranking the corrosion performance of coatings [27],although it has some limitations as it does not always match the corrosion performance obtained by other methods such as salt spray testing.

    Fig.1.Scatter diagram of impedance modulus at 0.01 Hz for the AZ31 alloy with and without LDH coatings.Filled symbols correspond to the specimens loaded with corrosion inhibitors.Note that each system is measured by triplicate,although some points are overlapped so only one or two are observed.

    Compared with the bare alloy,Zn?Al and Li?Al LDH coatings increase the impedance modulus by one and two orders of magnitude,respectively.Loading of Li?,Mo?and W?based corrosion inhibitors improved the corrosion resistance of Zn?Al LDH,with sodium tungstate yielding the highest impedance values (|Z|0.01Hz~105Ωcm2).As for the Li?Al LDH system,only the Li?based inhibitor increased the impedance response (|Z|0.01Hz~6×104Ωcm2).

    Zn?Al LDH W and Li?Al LDH?Li systems were selected for further evaluation based on this initial corrosion screening.A quick comparison with the values included in Table 1 reveals that the selected systems are among the best in terms of low frequency impedance response.In the following sections,inhibitor?free LDH systems are also included for comparison.

    3.2.Characterization

    Fig.2 shows the SEM plan?view micrographs of Zn?Al and Li?Al LDHs,where their fla e?like morphology is clearly visible.Both coatings cover the entire surface and show LDH islands that are larger in the Zn?Al system.The agglomeration of fla es that form the islands is attributed here to the increased amounts of available cations(i.e.,Mg2+,Al3+),preferentially at the location of Al?Mn inclusions.Enhanced dissolution is expected to occur in these regions due to two phenomena: i) micro?galvanic corrosion between inclusions and the surrounding matrix;and ii) selective Al dissolution or de-alloying in the Al?Mn inclusions due to the highly alkaline conditions(pH 10)during treatment.Note that chemical dissolution of the Al?Mn inclusions is still possible even when they should be acting as cathodes [38].Fig.2d shows an example of partially dissolved Al?Mn inclusions surrounded by the thicker coating material.

    Fig.2.Planar view micrographs of Zn?Al LDH (a,c,e) and Li?Al LDH (b,d,f) coatings.The EDS analysis results are collected on Table 2.

    Fig.2d shows an example of partially dissolved Al?Mn inclusions surrounded by the thicker coating material.

    EDS area and point analysis labelled in Fig.2 are shown in Table 2,along with the EDS results obtained for the bare material (not shown in Fig.2).Both coatings show increased Al content (6 ?8 at.%) in comparison with the bare material (~2.4 at.%).This is consistent with the incorporation of this element into the LDH structure.Similarly,the Zn?Al LDH shows a higher amount of Zn on its surface (~2.9 at.%)compared with the as?received alloy (~0.4 at.%).Li was not observed in the Li?Al system due to the difficultie in detecting this element by EDS.Note the high amount of Mn and Fe and the low Al/Mn ratio in point 2 in Fig.2d,which evidences the preferential dissolution of Al in the Al?Mn inclusion.It is worth mentioning that not all the islands show the presence of Al?Mn inclusions (e.g.,point 2,Fig.2a).

    Table 2 EDS quantificatio in at.% on AZ31,Zn–Al LDH (Fig.2a),Li–Al LDH(Fig.2b and 2d),Zn–Al LDH W and Li–Al LDH Li (Fig.4) in the specifie areas.

    Micrographs at higher magnificatio revealed that the fla es are mostly oriented perpendicularly to the surface,indicating a faster growth rate in the direction of the bulk solution(Fig.2e and 2f).This is typically observed in LDHs systems and is the result of their anisotropy (i.e.fla es growth in the ab?direction faster than in the c?direction)and the hampered growth of fla es oriented horizontally to the surface [39].

    Fig.3.This figur presents the cross?sectional view (a) of the Zn?Al LDH and (b) Li?Al LDH coatings.The interface between the coating and the bulk material is indicated here by white arrows.Blue arrows mark the overall thickness of the LDH coatings (For interpretation of the references to color in this figure the reader is referred to the web version of this article.).

    Flakes in the Zn?Al LDH are thicker and larger than those in the Li?Al system.Image analysis measurements (obtained from planar views micrographs) yielded thickness and surface area values of (300±50) nm/(31.6 0.5) μm2and(5±3) nm/(560±50) nm2,for the coating fla es,respectively.Considering that similar conditions were used for both LDH treatments,it is evident that the differences in size are related to the characteristics of Zn2+and Li+cations.

    Cross-section examination of the LDH coatings shows a bi?layer structure composed of an outer part with loose fla es(~30% of the layer) and a denser inner layer (Fig.3).The overall LDH conversion layer is thicker when Zn cations are used (5.2±0.5 μm) in comparison with the Li?based solution (3.8±0.4 μm).This bi?layer structure is often seen in LDH conversion coatings [19,22,40] and is related to differences in fla e?size and crystallinity of the coating material.For instance,Lin et al.[41] reported that the inner layer was less crystalline than the outer one in a Mg?Fe?LDH coating formed on a 99.9% Mg.It is suggested here that the interface between the two layers roughly corresponds to the original surface as it shows a very fla profil (further studies are needed to confir this).

    Detailed mechanistic studies of LDH fil formation can be found elsewhere [42,43].Differences in structure and morphology of LDH film are mainly related to the source and availability of cations.The high supply of cations coming from dissolution of the substrate results in high a nucleation rate due to faster achievement of the solubility limit of Mg2+compounds.This leads to small but numerous LDH fla es in the inner layer (next to the substrate/solution interface).Further away from the substrate,the concentration of cations is lower and,consequently,the nucleation rate decreases,resulting in fewer but larger LDH fla es in the outer layer [43].

    Fig.4 shows the SEM characterization of the studied LDH systems after post?treatment with W?and Li?based inhibitors.The Zn?Al LDH?W coating shows a smoother morphology than the Zn?Al LDH coating,although some small islands are still scattered over the surface (Fig.4a).The EDS analysis shows that these islands contain slightly more W (~0.5 at.%) than the surrounding coating material(~0.1 at.%).Some Na contamination is also present in the coating (~0.6 at.%) (Table 2).According to the Pourbaix diagram of W(298 K,[WO42?]=10?6molL?1)[44],tungstates are soluble in alkaline aqueous solutions.Therefore,W?rich precipitates such as WO3are not expected to form during post?treatment at pH 10.The precipitation of Al2(WO4)3can also be ruled out as the amount of Al in the deposits is quite small (0.2 at.%).Therefore,the presence of W in the coating is most likely due to the incorporation of WO42?ion into the LDH structure or to the precipitation of MgWO4(pKα=6.46).Li?rich deposits or precipitates,if any,were not detected in the Li?Al LDH Li coating.The surface also appeared smoother than the inhibitor?free coating (Fig.4b,d compared to Fig.2b,d,f) .

    High magnificatio plan views reveal that,after post?treatment with inhibitors,LDH fla es are smaller and are not very well?define (Fig.4c and 4d).This is attributed to the partial dissolution of the outer layer,as evidenced by the cross sections (Fig.4e and 4f).During post?treatment at pH 10 and under the non-saturated conditions,LDH fla es gradually dissolve,particularly those in the outer layer as they are loosely bonded to the surface.Note that some of the dense inner layers were also lost during post?treatment,but the thickness loss can be considered negligible (<0.5 μm).After post?treatment,the coating thicknesses were ~2.7 and ~2.5 μm for the Zn?Al LDH W and Li?Al LDH Li,respectively.

    Fig.4.Plan view and cross section SEM micrographs after post?treatment.Zn?Al LDH planar?view (a and c) and cross?view (e).Li?Al LDH plan?view(b and d) and cross?view (f).The EDS analysis results are collected in Table 2.

    Fig.5 presents the grazing angle X?ray diffraction(XRD) pattern for the studied coatings before and after post?treatment.All coatings show the characteristic peaks of hydrotalcite?like LDH structure with a rhombohedral unit cell and R?3 m space group [45].Mg(OH)2was also identified which is a common subproduct formed during the synthesis of LDH in alkaline conditions (pH>10.8) [46].Despite using glancing angle for the measurement and due to the low thickness of the studied coatings,peaks from theα?Mg phase of the substrate are also identifie at 34,36 and 47°.

    Table 3 shows the basal plane spacingdcalculated using Bragg’s equation and the unit cell parametersaandc(a=2d110;c=3d003[47]) of LDH structures calculated from (003),(006) and (110) reflection at ~11°,~18.5° and 62°,respectively.The inhibitor-free structures present a d003value of 0.8098 and 0.8058 nm,for Zn?Al LDH and Li?Al LDH,respectively,which are consistent with hydrotalcite?like systems intercalated with NO3?anions [47].The shoulder peaks identifie for (003) and (006)refl xions at slightly higher 2?theta values suggest the partial intercalation of CO32?/OH?ions between the LDH layers[48].The d003values correspond to the basal spacing of two consecutive hydrotalcite?like layers,therefore,it is possible to calculate the intergallery height by subtracting the basal spacing of the cationic layer (brucite?like,4.8) (Table 3).The intergallery height of both systems is comparable,being 3.298 and 3.258for Zn?Al LDH and Li?Al LDH,respectively.

    Table 3 XRD peak indexing results for the (003) and (006) planes of the LDH structure.

    Fig.5.(a) XRD patterns for the coatings.(b) Amplificatio of selected region from 10 to 28°.

    After the post?treatment,the characteristic LDH reflec tions (003) and (006) of both LDH systems show a lower intensity and remain at a relatively invariable 2q (Fig.5b).The lower intensity is related to the removal of the external loose layer of the coating during the post?treatment.It is worth mentioning that the shoulder peak identifie for (003)refl xion at slightly higher 2?theta values became clearer in the case of Zn?Al LDH-W (labelled?in Fig.5b) which is probably related to an intense intercalation of partial CO32?/OH?ions with a smaller size compared to NO3?between the LDH layers during the post?treatment.

    The invariable reflectio 2θsuggests that the corrosion inhibitors were not incorporated within the intergallery space,although they may be incorporated in edge positions,as suggested by Sels [49].Therefore,the basal plane spacing,d,and the unit cell parameter c remained constant after the treatment.Theab?axis values also remained constant,which indicates that the inhibitors did not modify the cationic hydroxide layers.In Zn?Al LDH-W,W?rich particles in the form of MgWO4were detected at 12,16.5 and 29° and observed on the SEM micrographs (Fig.4).This confirm their incorporation into the LDH system.Considering the low solubility of MgWO4,these are likely to be physically adsorbed on the LDH’s most external layers.The inhibitor Li+in the Li?Al LDH-Li system is not likely to be incorporated between the LDH layers due to its positive charge.Li+is most probably located at the most external layers (top and bottom) of the LDH systems creating an electric double layer with the NO3?ions which remain attracted by electrostatic interactions to the LDH layers.

    Fig.6.FTIR spectra of Zn?Al LDH,Zn?Al LDH W,Li?Al LDH and Li?Al LDH Li coatings on AZ31 Mg alloy.

    The FTIR spectra of the different LDH coatings with and without intercalated inhibitors are shown in Fig.6.The intense band located at 3683 cm?1corresponds to O?H stretching mode of hydroxyl groups of the LDH layers (Zn?OH,Mg?OH,and Al?OH) [50].The bands at 3650?3170 cm?1and 1726?1505 cm?1are assigned to tension and bending vibrations,respectively,of the O?H bonds of water molecules intercalated between the LDH layers.The bands at 1690?1480,760 and 578 cm?1correspond to the asymmetric stretching,out?of?plane symmetric and antisymmetric deformation modes of NO3?ions intercalated between the LDH layers,respectively [48].The band at 1690?1480 cm?1could also be correlated to the symmetric stretching vibrations of O?C?O bond of CO32?anions.The bands at lower wavenumbers (761?546 cm?1) correspond to the stretching vibrations of M-O (M: Al,Zn) of the LDH [51].The bands between 3000 and 2775 cm?1correspond to C?H vibrations associated with the presence of superficia contamination in the form of hydrocarbons.

    Fig.7.XPS spectra of the studied specimens a) before and b) after 10 min of argon sputtering.c) High?resolution Zn,W and Li spectra obtained after 10 min of argon sputtering of the LDH coatings on AZ31.

    XPS analysis(Fig.7)was carried out to obtain quantitative compositional information of the studied materials (Table 4).Fig.7a and 7b show the XPS spectra of the LDH systems before and after 10 min of argon sputtering,respectively.Fig.7c shows the high?resolution XPS spectra of elements Zn,W and Li after sputtering.

    Table 4 XPS elemental composition (at.%) of studied coatings.

    The most superficia layer of all the studied materials in the as?received condition shows varying amounts of adventitious C.The C 1s signals at 285 and 286 eV correspond to long chain hydrocarbons (C–C,C–H) which were also evident in the FTIR analysis.After sputtering,the signals at 285 and 286 eV diminished and a small peak at ~290 eV,corresponding to carbonate ions,appeared(Fig.7b).This suggests the intercalation of CO32?between the LDH galleries,as evidenced by the shift of the peak (003) in XRD.The intensity of this signal increases on the deeper layers of the coating as the superficia contamination is sputtered away.In the as?received condition,only one O 1s signal is identifie at ~532 eV which is assigned to O atom in metal?hydroxide species (or hydroxyl groups -OH) [52,53].Another confirma tion of the presence of magnesium hydroxides is the Auger parameter values of 997.23 eV to 997.47 eV [54].The latter is calculated by the difference between Mg 1s and Mg KLL peaks[54].After sputtering,its intensity decreases and a new contribution appears at lower binding energy (~530 eV),which can be assigned to metal carbonates.It is worth mentioning that the peak related to MgO was not observed (typically at ~1.5 eV of Mg 1s and Mg 2s binding energies[55,56]).The Al 2p peak at ~74 eV is possibly related to the bonding energy of Al(OH)3.Zn is present in Zn?Al LDH samples;in the Li?Al LDH system,small amounts of Zn were also detected but disappeared after sputtering,suggesting superficia contamination.The twin peaks at ~1021 eV and ~1044 eV are assigned to Zn 2p3/2and Zn 2p1/2,respectively (Fig.7c),suggesting that Zn is present in the LDH in the divalent oxidation state [57].In the case of Zn?Al LDHW system,a peak associated with tungsten is observed at 50 eV.In the as-received condition,the W 4f high resolution spectra show a split peak at 35.48 and 37.58 eV,while after sputtering (Fig.7c),two doublets 4f7/2 ?4f5/2 are fitte at 34.08?35.94 eV and 35.99?38.04 eV,respectively,which are associated with a tungsten oxidation state +6,probably in the form of WO42?[58,59].Li was identifie in Li?Al LDH and Li?Al LDH-Li systems at 55.33 eV (Li 1s).

    Table 4 shows the quantitative chemical analysis before and after sputtering of the studied materials.In the as received condition,in both Zn?Al LDH and Li?Al LDH systems,N is identified from the intercalation of NO3?anions between the LDH layers.It is worth mentioning that Zn?Al LDH-W did not present N,suggesting the partial intercalation of CO32?/OH?ions between the LDH layers.This is in agreement with the shoulder peaks identifie for (003) and (006) refl xions at slightly higher 2?theta values.In the case of Li?Al LDH system,the N content increased after the post?treatment in LiNO3,where further NO3?are incorporated into the structure.Although a characteristic peak of Li was identified the detected amount was below the limit of quantification

    In the as received condition,the (Mg+Zn)/Al ratios are~2.3 and ~1.0 for Zn?Al and Li?Al LDH,respectively.After sputtering,the ratios increase as the surface contamination is removed(~3.9 and ~1.5)for Zn?Al and Li?Al LDH,respectively).In both systems,the specimens containing corrosion inhibitors show a slightly lower ratio which could be associated with the selective dissolution of Mg during the immersion post?treatment,where the most superficia layer of the coating is completely removed.

    3.3.Potentiodynamic polarization measurements

    Fig.8 shows the polarization curves of the studied coatings after 1,24 and 48 h immersion in 0.5 wt.% NaCl.The electrochemical data obtained from the curves are gathered in Table 5,including the standard deviation estimated from three separate measurements.The results acquired for the uncoated AZ31 alloy after 1 h of immersion are shown for comparison.

    After 1 h of immersion,all the coatings revealed nobler corrosion potential,Ecorr,and lower corrosion current density,icorr(1100–1200 mV,~10?4mA/cm2).This indicates improved corrosion protection performance compared to the substrate (~1240 mV,10?3mA/cm2,Table 5).It is also evident that the coatings increased the pitting potential,Epit,with values up to ~700 mV.Note that the bare substrate showed an anodic branch with a very low slope,indicating spontaneous pitting under non-polarized conditions.

    A quick comparison with the literature data included in Table 1 reveals that the relatively thin coatings developed in this work are amongst the best performers.In general,the icorrvalues reported by other studies are close to 10?3mA/cm2,which is one of order of magnitude higher than the values obtained in this study.

    Table 5 Polarization data of tested materials as a function of immersion time in 0.5 wt.% NaCl aerated solution.

    Comparison between the polarization curves does not reveal significan differences between the studied coatings with the exception of the Li-Al LDH coating which,after 24 h and 48 h of immersion,shows a very steep anodic branch without pitting potential.Therefore,the applied post-treatment reduced the resistance to localized corrosion,possibly due to the dissolution of the outer LDH layer.

    Fig.8.DC polarization curves of (a) Zn-Al LDH,(b) Zn-Al LDH W,(c) Li-Al LDH Li and (d) Li-Al LDH Li after 1 h,24 h and 48 h of immersion in 0.5 wt.% NaCl.The polarization curve of AZ31 uncoated material after 1 h of immersion is shown for comparison.

    3.4.SVET measurements

    To evaluate the corrosion protection efficien y offered by the encapsulated corrosion inhibitors,the coatings were artifi cially scratched and their electrochemical response was analysed by SVET up to 6 days of immersion in 0.05 M NaCl.

    Fig.9 shows the optical images of the artificia defect and the SVET maps at the same location at different immersion times.

    For Zn?Al LDH,right after the immersion,two features were observed: the discolouration of the surface and blurring of the defect,making it indistinguishable from the intact coating.At this stage,the current density values remained relatively low (±10 mA/cm2) and no H2bubbles were formed.This behaviour continued after 2 and 6 days of immersion and the corrosion response at the location of the defect remained relatively unchanged.This behaviour may be explained by the partial dissolution of the LDH fla es and redeposition of the coating material at the location of the artificia defect.The specimen containing the corrosion inhibitor (Zn?Al LDH W)showed a similar trend,although the loss of brightness was less severe,indicating an improved corrosion resistance.The defect became indistinguishable in the optical image and low currents in the range of ±10 mA/cm2were registered.Two cathodic spots are formed after 2 days of immersion,indicating the electrochemical activity associated with corrosion initiation.

    Fig.9.Optical images and SVET 2D current density maps of 1 mm scratch defect and the surrounding area up to 6 days of immersion in 0.05 M NaCl solution.

    Interestingly,after 6 days of immersion,these two spots disappeared,which could be related to an active protection mechanism.Fig.10 and Table 6 show the SEM/EDS results of Zn?Al LDH and Zn?Al LDH W during the firs 2 days of immersion in 0.05 M NaCl.

    Before immersion,the scratches are clean and with depth values greater than 7 μm (measurements not shown).EDS results at the location of the scratches show high and low amounts of Mg and O,respectively,in comparison to nonscribed regions (Tables 6 and 2),showing that the defect has reached the substrate.After 2 days of immersion,there is an increase in the O content in the scribe,which is indicative of corrosion,however,there is also W and Zn enrichment(Table 6).The presence of W inside the scribe,where there was none before the immersion,suggests that WO42?is released from the intact coating zones and then precipitates at the defect.Note that the non-scribed areas in the Zn-Al LDH specimen show islands of corrosion products,which explains the loss of brightness that was previously mentioned.

    Table 6 EDS quantificatio in at.% inside the scratch for Zn–Al LDH (Fig.9a,c),and Zn–Al LDH W (Fig.9b,d).

    Table 7 Roughness parameters (Sa;arithmetical mean height and S10z;ten?point height).

    Fig.10.SEM micrographs of the scratched regions for up to 2 days of immersion in 0.05 M NaCl.Zn?Al LDH (a,c,e) and Zn?Al LDH W (b,d,f).

    The general protective mechanism could be associated with two phenomena : (i) the dissolution?redeposition of the LDH coating material (some liberation of Zn2+ions from the LDH structure may also occur,which are likely to precipitate in the form of Zn(OH)2) and,(ii) the dissolution/redeposition of MgWO4.In both cases,these insoluble deposits would isolate the exposed magnesium substrate from the aggressive media,thus reinstating partially the passive properties of the coating while preventing corrosion propagation.The protective corrosion mechanism is represented in Fig.11 which is based on the combination of three phenomena: ion?exchange,deposition of hydroxides and competitive adsorption.

    In the case of the Li?Al LDH system with and without corrosion inhibitors,a high anodic activity (50 mA/cm2) is identifie at the location of the defect from the beginning of the immersion test.This is accompanied by an intense generation of H2bubbles.Over time,corrosion progresses catastrophically through the exposed substrate and several initiation points are also developed outside the scribed area.This indicates that the barrier properties of this system are not that good when there is a defect on the surface [60].

    3.5.Contact angle and surface analysis

    Fig.11.Schematic representation of Zn?Al LDH W protective corrosion mechanism.

    The contact angle measurements were performed to evaluate the hydrophilicity/hydrophobicity of the developed coatings (Fig.12).All coatings show a hydrophilic behaviour.Zn?Al LDH shows a 0° contact angle and the water drop was completely spread on the coating’s surface.Li?Al LDH presents a slightly higher contact angle,13°,which is also considered highly hydrophilic.In both systems,this behaviour can be explained by the presence of hydroxyl groups within the LDH structures that can easily interact with water molecules through hydrogen bonding [61].This extreme hydrophilicity may cause the aggregation of the LDH fla es and significan water adsorption,which may promote corrosion initiation if the aqueous environment reaches the substrate.Both specimens containing corrosion inhibitors show higher contact angles,58nd 43°for Zn?Al LDH and Li?Al LDH,respectively.This increase can be related to the loss of the outer layer that modifie the roughness of the coating and to the presence of inhibitor-containing species at the LDH surface that prevents the formation of hydrogen bonding.In case of Zn?Al LDH W,these could be W?rich solid precipitates,while in case of Li?Al LDH-Li it could be associated with the adsorption of Li+cations.

    The roughness values of the studied coatings are presented in Table 7.The analyses of Zn?Al LDH and Li?Al LDH reveal fundamental differences between them.The Zn?Al LDH shows the highest values among studied coatings.This may be related with the presence of agglomerated fla es on top of the intermetallic particles,as observed in the SEM micrographs(Fig.2).This promotes the water adsorption,which is in accordance with the contact angle values.The post?treatment for that system results in the dissolution of the most external layer of the coating,including the above?mentioned agglomerations,leaving the dense and uniform inner layer exposed.As expected,this leads to a reduction of the superfi cial roughness (Saand S10z) and,consequently a decrease in the hydrophilicity as observed in the increased contact angle values after the post?treatment.The differences in the surface roughness for the Li?Al LDH system before and after post?treatments were not significant probably due to the lack of agglomerate fla es and their smaller size in comparison to Zn?Al LDH.

    3.6.Paint adhesion

    Fig.12.Water contact angle measurements: a) Zn?Al LDH,b) Zn?Al LDH W,c) Li?Al LDH and d) Li?Al LDH Li.

    Fig.13.Paint adhesion of LDH coatings on AZ31:a)Zn?Al LDH.b)Li?Al LDH.c) Zn?Al LDH W.d) Li?Al LDH Li.

    The paint adhesion property of the coatings was evaluated according to the EN ISO 2409 standard using a water-based paint,where scores are allocated to quantify the area affected by paint delamination.The scale ranges from 0 to 5,where 0 corresponds to a 0% area delaminated,1 to 〈5%,2 to 5 -15%,3 to 15-35%,4 to 35-65% and 5 to an area 〉65%.Fig.13 shows the cross?cut test results and the allocated scores.Some differences are observed between the inhibitor?free and the inhibitor?containing LDH coatings.The Zn?Al and Li?Al LDH specimens show a paint adhesion score of 1,since some detachment of small fakes of the coatings was observed at the intersection of the cuts.This indicates a relatively good paint adhesion property,which can be explained by the high hydrophilicity and roughness of the most superficia layer of the LDH coating to prior the immersion post?treatment that provides a high surface area for an optimal paint anchorage.

    The coatings containing corrosion inhibitors,Zn?Al LDH W and Li?Al LDH Li,obtained after the post?treatment,exhibit a slightly higher level of delamination (score 2).However,it remains in the lower range (~5%).The coatings fla e along the edges and at the intersections of the cuts.The slight decrease in paint adhesion is due to the physical and chemical changes suffered by the most external layer of the coating after the post?treatment resulting in a smoother surface with slightly higher hydrophobicity.Consequently,the contact area available for the pain anchorage is reduced,resulting in a decrease of the paint adhesion.

    4.Conclusions

    The main conclusions of this work are summarised as follows:

    -Different Zn?Al and Li?Al LDH systems containing Li?,Mo?and W?based corrosion inhibitors were successfully synthesized and optimized in terms of corrosion resistance.The coatings with the highest corrosion resistance Zn?Al LDH W and Li?Al LDH Li were selected for further evaluation.

    -XRD,FTIR and XPS analysis confirme the incorporation of the inhibitors into the LDH structure.W is incorporated in the form of WO42?and it is likely to be physically adsorbed on the LDH’s most external layers.In contrast,Li is incorporated in the cationic form Li+and remains attached to the top and bottom layers of the LDH system attracted by electrostatic interactions.

    -The applied post?treatments results in the selective dissolution of the outermost layer of the LDH coating,reducing hydrophilicity and paint adhesion of the coatings.

    -Potentiodynamic polarization tests revealed that all the studied coatings reduced the corrosion current density and increased the pitting resistance of the AZ31 alloy.

    -The active corrosion properties of the Zn?Al?LDH W coating were confirme by SVET and SEM/EDS results on scribed specimens.The Li?Al LDH system did not show active protection.

    -The inhibition corrosion mechanism is attributed to the combination of three phenomena:ion?exchange of aggressive Cl?ions,dissolution?redeposition of LDH coating material,including W?rich precipitates.

    Declaration of Competing InterestThe authors declare that they have no known competing financia interests or personal relationships that could have appeared to influenc the work reported in this paper.

    Acknowledgments

    The authors gratefully acknowledge the support of the RTI2018-096391-B-C33 FEDER/ Ministerio de Ciencia e Innovación?Agencia Estatal de Investigación,S2018/NMT?4411 Regional Government of Madrid and EU Structural and Social Funds and PID2021-124341OBC22 (MCIU/AEI/FEDER,UE).M.Mohedano is grateful for the support of RYC-2017 21843,Ministerio de Ciencia e Innovación.B.Mingo is supported by the Royal Academy of Engineering through the RAEng Research Fellowship and by EPSRC (EP/V026097/1)

    Supplementary materials

    Supplementary material associated with this article can be found,in the online version,at doi:10.1016/j.jma.2022.09.014.

    国产成年人精品一区二区| 在线观看免费日韩欧美大片| 欧美+亚洲+日韩+国产| 久久精品夜夜夜夜夜久久蜜豆 | 又黄又爽又免费观看的视频| 中亚洲国语对白在线视频| 国产亚洲精品一区二区www| 中国美女看黄片| 黄色 视频免费看| 免费在线观看视频国产中文字幕亚洲| 久久性视频一级片| 久久中文字幕人妻熟女| 三级国产精品欧美在线观看 | 嫩草影院精品99| 日本一二三区视频观看| 法律面前人人平等表现在哪些方面| 女人爽到高潮嗷嗷叫在线视频| 色综合婷婷激情| 不卡一级毛片| 亚洲精品美女久久久久99蜜臀| 日韩av在线大香蕉| 欧美3d第一页| 亚洲欧美精品综合久久99| 国产av麻豆久久久久久久| 熟女电影av网| 午夜福利高清视频| 国产黄a三级三级三级人| 久久精品91无色码中文字幕| 国产久久久一区二区三区| 国产伦在线观看视频一区| 久久久久久九九精品二区国产 | 啦啦啦韩国在线观看视频| 麻豆成人av在线观看| 久久久国产欧美日韩av| 久久中文字幕一级| 欧美色欧美亚洲另类二区| 免费观看精品视频网站| 日韩欧美 国产精品| 香蕉av资源在线| 亚洲一码二码三码区别大吗| 亚洲精品国产一区二区精华液| av中文乱码字幕在线| 国产精品久久久人人做人人爽| 91av网站免费观看| 国产精品亚洲av一区麻豆| 精品高清国产在线一区| 18禁观看日本| 色综合站精品国产| 午夜老司机福利片| 黑人操中国人逼视频| 国产精品综合久久久久久久免费| 十八禁网站免费在线| 色精品久久人妻99蜜桃| 丰满人妻熟妇乱又伦精品不卡| 欧美日韩瑟瑟在线播放| 亚洲成人中文字幕在线播放| 最近最新中文字幕大全免费视频| 国产97色在线日韩免费| 一本大道久久a久久精品| 又粗又爽又猛毛片免费看| 欧美黑人欧美精品刺激| 99riav亚洲国产免费| 亚洲国产精品久久男人天堂| 国产亚洲av高清不卡| 午夜a级毛片| 一夜夜www| 夜夜夜夜夜久久久久| 99国产精品99久久久久| 久久久久久九九精品二区国产 | 国产免费男女视频| 99re在线观看精品视频| 国产精品 国内视频| 精品欧美一区二区三区在线| 欧美激情久久久久久爽电影| 黑人操中国人逼视频| 亚洲av美国av| 亚洲av中文字字幕乱码综合| 国内精品久久久久久久电影| av视频在线观看入口| 黄色视频不卡| 男女做爰动态图高潮gif福利片| 超碰成人久久| 久久天躁狠狠躁夜夜2o2o| 变态另类丝袜制服| 国产欧美日韩一区二区三| 久久久国产欧美日韩av| 黄色 视频免费看| 成人特级黄色片久久久久久久| 在线观看舔阴道视频| 国产1区2区3区精品| 50天的宝宝边吃奶边哭怎么回事| 欧美成人免费av一区二区三区| 久久这里只有精品19| 后天国语完整版免费观看| 国产又色又爽无遮挡免费看| 亚洲人与动物交配视频| 搡老妇女老女人老熟妇| 亚洲精品一区av在线观看| 日韩欧美 国产精品| 日本免费一区二区三区高清不卡| 国产一级毛片七仙女欲春2| 两性午夜刺激爽爽歪歪视频在线观看 | 琪琪午夜伦伦电影理论片6080| 哪里可以看免费的av片| 成人特级黄色片久久久久久久| 色综合欧美亚洲国产小说| 欧美成人一区二区免费高清观看 | 他把我摸到了高潮在线观看| 18禁黄网站禁片免费观看直播| 每晚都被弄得嗷嗷叫到高潮| 一区二区三区激情视频| 久热爱精品视频在线9| 老熟妇乱子伦视频在线观看| 欧美精品啪啪一区二区三区| 午夜日韩欧美国产| 亚洲午夜理论影院| 超碰成人久久| 成人永久免费在线观看视频| 少妇熟女aⅴ在线视频| 欧美av亚洲av综合av国产av| 亚洲五月天丁香| 高清在线国产一区| 12—13女人毛片做爰片一| videosex国产| 成人三级做爰电影| 色哟哟哟哟哟哟| 琪琪午夜伦伦电影理论片6080| 99久久精品国产亚洲精品| 超碰成人久久| 天堂动漫精品| 成年女人毛片免费观看观看9| 亚洲色图av天堂| 美女黄网站色视频| 亚洲欧美激情综合另类| 亚洲va日本ⅴa欧美va伊人久久| 日韩 欧美 亚洲 中文字幕| 1024视频免费在线观看| 亚洲午夜理论影院| 他把我摸到了高潮在线观看| 99热6这里只有精品| 国产伦在线观看视频一区| 日韩精品免费视频一区二区三区| 亚洲欧美一区二区三区黑人| 免费在线观看亚洲国产| 国产精品野战在线观看| 白带黄色成豆腐渣| 国产真人三级小视频在线观看| 俄罗斯特黄特色一大片| 在线观看日韩欧美| 999精品在线视频| 变态另类成人亚洲欧美熟女| 久久人人精品亚洲av| 一二三四社区在线视频社区8| 国产成人精品久久二区二区91| 午夜老司机福利片| 久久久久免费精品人妻一区二区| 999久久久精品免费观看国产| 久久亚洲精品不卡| 黄色视频,在线免费观看| 日韩高清综合在线| 中文字幕最新亚洲高清| 婷婷精品国产亚洲av在线| 国产成人精品无人区| 中文在线观看免费www的网站 | 欧美成人一区二区免费高清观看 | 免费在线观看影片大全网站| 亚洲精品中文字幕一二三四区| 精品久久久久久久久久久久久| 一区二区三区激情视频| 51午夜福利影视在线观看| 正在播放国产对白刺激| 人人妻人人澡欧美一区二区| 久久精品91无色码中文字幕| 看片在线看免费视频| 国产成人系列免费观看| 1024视频免费在线观看| 国产成+人综合+亚洲专区| 男女下面进入的视频免费午夜| 91国产中文字幕| 日韩欧美在线乱码| 99热只有精品国产| 免费在线观看成人毛片| 老熟妇乱子伦视频在线观看| 亚洲狠狠婷婷综合久久图片| 九色国产91popny在线| 嫩草影视91久久| 亚洲av熟女| 亚洲天堂国产精品一区在线| 在线视频色国产色| 99在线视频只有这里精品首页| 人人妻,人人澡人人爽秒播| 久久人妻福利社区极品人妻图片| 欧美极品一区二区三区四区| 一本大道久久a久久精品| 正在播放国产对白刺激| 操出白浆在线播放| 黄色丝袜av网址大全| 成年版毛片免费区| 午夜福利欧美成人| 黄频高清免费视频| 可以免费在线观看a视频的电影网站| 窝窝影院91人妻| 在线观看66精品国产| 久久亚洲精品不卡| а√天堂www在线а√下载| x7x7x7水蜜桃| 啦啦啦韩国在线观看视频| 国产一级毛片七仙女欲春2| 国产私拍福利视频在线观看| 麻豆av在线久日| svipshipincom国产片| 国语自产精品视频在线第100页| 搡老岳熟女国产| 嫩草影院精品99| 99精品久久久久人妻精品| 搞女人的毛片| 国产精品爽爽va在线观看网站| 精品久久久久久成人av| av中文乱码字幕在线| 18禁裸乳无遮挡免费网站照片| 99久久99久久久精品蜜桃| 香蕉av资源在线| www.www免费av| 黄色毛片三级朝国网站| 欧美黄色片欧美黄色片| 欧美中文综合在线视频| 国产精品1区2区在线观看.| 亚洲精品久久成人aⅴ小说| 亚洲国产欧美一区二区综合| 在线观看免费午夜福利视频| 久久热在线av| 日本黄色视频三级网站网址| 欧美午夜高清在线| 亚洲精品久久国产高清桃花| 国内久久婷婷六月综合欲色啪| 在线观看免费视频日本深夜| 香蕉av资源在线| 青草久久国产| 高潮久久久久久久久久久不卡| 99久久久亚洲精品蜜臀av| 久久精品综合一区二区三区| 免费在线观看亚洲国产| 国产伦一二天堂av在线观看| 巨乳人妻的诱惑在线观看| 亚洲黑人精品在线| 亚洲国产精品久久男人天堂| 亚洲国产高清在线一区二区三| 国产蜜桃级精品一区二区三区| 国产日本99.免费观看| 国产亚洲精品久久久久久毛片| 黄色视频,在线免费观看| 男女那种视频在线观看| 亚洲精品在线观看二区| 91成年电影在线观看| 真人一进一出gif抽搐免费| 欧洲精品卡2卡3卡4卡5卡区| 亚洲欧美精品综合一区二区三区| 一区福利在线观看| 中亚洲国语对白在线视频| 法律面前人人平等表现在哪些方面| 婷婷精品国产亚洲av| 亚洲成人国产一区在线观看| 给我免费播放毛片高清在线观看| 夜夜躁狠狠躁天天躁| 亚洲专区中文字幕在线| 久久久精品欧美日韩精品| 免费看日本二区| 免费无遮挡裸体视频| 免费电影在线观看免费观看| 99在线视频只有这里精品首页| 成人精品一区二区免费| 亚洲自拍偷在线| 亚洲欧美日韩高清专用| 亚洲午夜精品一区,二区,三区| 欧美一区二区精品小视频在线| 久久久水蜜桃国产精品网| 欧美色欧美亚洲另类二区| 国产午夜福利久久久久久| 亚洲黑人精品在线| 久久久久国产精品人妻aⅴ院| 九九热线精品视视频播放| 99热这里只有是精品50| 久久精品国产清高在天天线| 亚洲男人的天堂狠狠| 国产男靠女视频免费网站| tocl精华| 日韩中文字幕欧美一区二区| 久久精品91无色码中文字幕| 少妇熟女aⅴ在线视频| a级毛片a级免费在线| 观看免费一级毛片| 亚洲激情在线av| 可以在线观看的亚洲视频| 精品久久久久久久人妻蜜臀av| 精品人妻1区二区| 欧美日韩亚洲综合一区二区三区_| 亚洲成a人片在线一区二区| 免费在线观看视频国产中文字幕亚洲| 国产不卡一卡二| 久久久久九九精品影院| 一级毛片精品| 午夜影院日韩av| 男女视频在线观看网站免费 | 999精品在线视频| 免费观看精品视频网站| 免费在线观看影片大全网站| 日韩欧美免费精品| 日本三级黄在线观看| 三级国产精品欧美在线观看 | 欧美性猛交╳xxx乱大交人| 少妇的丰满在线观看| 欧美激情久久久久久爽电影| 午夜免费观看网址| 久久香蕉激情| 欧美绝顶高潮抽搐喷水| 国产高清激情床上av| 真人一进一出gif抽搐免费| 好看av亚洲va欧美ⅴa在| 国产蜜桃级精品一区二区三区| 国产真实乱freesex| 99国产精品99久久久久| 五月伊人婷婷丁香| 成年人黄色毛片网站| 真人做人爱边吃奶动态| 国产精品久久久av美女十八| 99riav亚洲国产免费| 人人妻人人看人人澡| 国产一区二区在线观看日韩 | 三级男女做爰猛烈吃奶摸视频| 99热只有精品国产| 级片在线观看| 亚洲成人国产一区在线观看| 精品日产1卡2卡| 神马国产精品三级电影在线观看 | 好男人电影高清在线观看| 日韩高清综合在线| 一区二区三区激情视频| 黄片大片在线免费观看| 一区二区三区激情视频| 欧美成狂野欧美在线观看| 一二三四在线观看免费中文在| 日日干狠狠操夜夜爽| 亚洲一区二区三区不卡视频| 丰满人妻一区二区三区视频av | 岛国在线观看网站| 深夜精品福利| 老司机午夜福利在线观看视频| 精品国产亚洲在线| 丝袜美腿诱惑在线| 中文在线观看免费www的网站 | 非洲黑人性xxxx精品又粗又长| 国产乱人伦免费视频| 亚洲中文av在线| 美女扒开内裤让男人捅视频| 日韩欧美 国产精品| 欧美zozozo另类| 99精品在免费线老司机午夜| www日本黄色视频网| 1024手机看黄色片| 国产乱人伦免费视频| 一个人免费在线观看的高清视频| 亚洲天堂国产精品一区在线| 亚洲美女黄片视频| 777久久人妻少妇嫩草av网站| 18禁黄网站禁片免费观看直播| 一本久久中文字幕| 免费在线观看视频国产中文字幕亚洲| 精品乱码久久久久久99久播| 久久久国产成人精品二区| 亚洲国产高清在线一区二区三| 叶爱在线成人免费视频播放| 日本三级黄在线观看| 神马国产精品三级电影在线观看 | 变态另类成人亚洲欧美熟女| 亚洲人成77777在线视频| 久久久久久久久免费视频了| 欧美av亚洲av综合av国产av| 久久香蕉激情| 亚洲专区国产一区二区| 中文字幕熟女人妻在线| xxx96com| 男人舔女人下体高潮全视频| 中文字幕精品亚洲无线码一区| 婷婷精品国产亚洲av| 久久中文字幕人妻熟女| 免费观看精品视频网站| 欧美又色又爽又黄视频| 亚洲av第一区精品v没综合| 日日夜夜操网爽| 国产av一区二区精品久久| 国内精品久久久久久久电影| 国产精品亚洲一级av第二区| 黄色视频,在线免费观看| 久久久久久久久免费视频了| 国产成人系列免费观看| 在线观看日韩欧美| 又紧又爽又黄一区二区| 18禁美女被吸乳视频| 波多野结衣巨乳人妻| 色综合亚洲欧美另类图片| 两人在一起打扑克的视频| 国产99白浆流出| 免费人成视频x8x8入口观看| 欧美日本视频| 五月玫瑰六月丁香| 亚洲avbb在线观看| 午夜影院日韩av| 亚洲欧美日韩高清专用| 手机成人av网站| 嫩草影院精品99| 日日夜夜操网爽| 免费av毛片视频| 别揉我奶头~嗯~啊~动态视频| 国产精品99久久99久久久不卡| 琪琪午夜伦伦电影理论片6080| 日韩欧美国产在线观看| 久久国产乱子伦精品免费另类| tocl精华| 777久久人妻少妇嫩草av网站| 嫩草影院精品99| 精品久久久久久久久久久久久| 色在线成人网| 制服诱惑二区| 国产精品99久久99久久久不卡| 香蕉丝袜av| 国产精品电影一区二区三区| 他把我摸到了高潮在线观看| 色综合欧美亚洲国产小说| 又紧又爽又黄一区二区| 我要搜黄色片| 免费一级毛片在线播放高清视频| 久久精品亚洲精品国产色婷小说| www.999成人在线观看| 成人一区二区视频在线观看| 亚洲九九香蕉| 免费在线观看视频国产中文字幕亚洲| 91成年电影在线观看| 脱女人内裤的视频| 日韩欧美三级三区| 美女 人体艺术 gogo| 成人国语在线视频| 国产精品久久久久久久电影 | 国产又黄又爽又无遮挡在线| 人人妻,人人澡人人爽秒播| 精品欧美国产一区二区三| 中文资源天堂在线| 在线免费观看的www视频| 欧洲精品卡2卡3卡4卡5卡区| 亚洲国产精品999在线| 国产日本99.免费观看| 久久久久久久久免费视频了| 在线观看www视频免费| 搡老妇女老女人老熟妇| 中文字幕av在线有码专区| av欧美777| x7x7x7水蜜桃| 亚洲熟女毛片儿| 日韩av在线大香蕉| 搡老熟女国产l中国老女人| 久久精品综合一区二区三区| 国内精品久久久久久久电影| www国产在线视频色| 久久婷婷成人综合色麻豆| 一边摸一边抽搐一进一小说| 99国产精品一区二区三区| 露出奶头的视频| 淫妇啪啪啪对白视频| 欧美在线黄色| 亚洲人与动物交配视频| 国产精品久久久久久久电影 | 亚洲欧美精品综合一区二区三区| 1024手机看黄色片| 男女午夜视频在线观看| 黄色丝袜av网址大全| 变态另类丝袜制服| 日韩国内少妇激情av| 精品久久久久久久末码| 天天躁夜夜躁狠狠躁躁| 精品日产1卡2卡| 黄片小视频在线播放| 人人妻人人看人人澡| 亚洲av熟女| 国产午夜福利久久久久久| 免费在线观看亚洲国产| 两个人免费观看高清视频| 欧美最黄视频在线播放免费| 日本免费a在线| 国语自产精品视频在线第100页| 后天国语完整版免费观看| 午夜影院日韩av| 国产片内射在线| 亚洲国产欧美人成| 欧美不卡视频在线免费观看 | 色在线成人网| 国产高清videossex| 久久久久久国产a免费观看| 少妇熟女aⅴ在线视频| 久久性视频一级片| 国产野战对白在线观看| 国产aⅴ精品一区二区三区波| 久久久国产精品麻豆| 国产午夜精品论理片| 久久精品91无色码中文字幕| 成人国产一区最新在线观看| 亚洲av电影不卡..在线观看| 亚洲精品美女久久久久99蜜臀| 亚洲专区字幕在线| 日韩大码丰满熟妇| 亚洲精品一区av在线观看| 国产片内射在线| 久久久久久大精品| 变态另类成人亚洲欧美熟女| 非洲黑人性xxxx精品又粗又长| 午夜a级毛片| 成人一区二区视频在线观看| 欧美成人性av电影在线观看| 免费在线观看影片大全网站| 色综合欧美亚洲国产小说| 色在线成人网| 在线播放国产精品三级| 亚洲av日韩精品久久久久久密| 成年版毛片免费区| 一进一出抽搐动态| 国产精品av视频在线免费观看| 99精品在免费线老司机午夜| 亚洲aⅴ乱码一区二区在线播放 | 婷婷丁香在线五月| 国产高清视频在线观看网站| 亚洲专区字幕在线| 香蕉国产在线看| 亚洲国产精品999在线| 99热这里只有是精品50| 国产黄色小视频在线观看| 久久久久久免费高清国产稀缺| 最新在线观看一区二区三区| 亚洲精品色激情综合| 一本大道久久a久久精品| 91国产中文字幕| 日韩欧美精品v在线| 两个人看的免费小视频| 国产精品av久久久久免费| 日本成人三级电影网站| 国产视频内射| 88av欧美| 两性午夜刺激爽爽歪歪视频在线观看 | 成人欧美大片| 日本免费a在线| 亚洲电影在线观看av| 久久久国产成人精品二区| 五月玫瑰六月丁香| 成人高潮视频无遮挡免费网站| 欧美人与性动交α欧美精品济南到| 少妇人妻一区二区三区视频| 国产成人av激情在线播放| 少妇裸体淫交视频免费看高清 | av超薄肉色丝袜交足视频| 亚洲全国av大片| 丰满人妻熟妇乱又伦精品不卡| 精品久久久久久久毛片微露脸| 国产亚洲av高清不卡| 欧美黄色淫秽网站| 国产欧美日韩精品亚洲av| 亚洲午夜精品一区,二区,三区| 国产成人aa在线观看| 国产精品久久久人人做人人爽| 色综合婷婷激情| 高清毛片免费观看视频网站| 亚洲国产精品sss在线观看| 一级毛片女人18水好多| 啪啪无遮挡十八禁网站| 午夜福利在线在线| 男女那种视频在线观看| 两个人视频免费观看高清| 18禁观看日本| 日韩欧美国产在线观看| 一个人免费在线观看电影 | 国产午夜精品久久久久久| 曰老女人黄片| 很黄的视频免费| 美女高潮喷水抽搐中文字幕| 岛国在线观看网站| 午夜激情福利司机影院| 香蕉av资源在线| 老司机深夜福利视频在线观看| 99精品久久久久人妻精品| 国产精品久久久人人做人人爽| 亚洲一区二区三区不卡视频| 99在线视频只有这里精品首页| 国产高清激情床上av| 欧美日本视频| 好男人在线观看高清免费视频| 在线观看一区二区三区| 中文字幕av在线有码专区| 黄片大片在线免费观看| 日韩 欧美 亚洲 中文字幕| 人人妻人人看人人澡| 嫁个100分男人电影在线观看| 精华霜和精华液先用哪个| 男男h啪啪无遮挡| 日本一二三区视频观看| 一a级毛片在线观看| netflix在线观看网站| 亚洲欧美日韩高清专用| 99久久精品热视频| 又黄又粗又硬又大视频| 精品熟女少妇八av免费久了| 欧美午夜高清在线| 少妇熟女aⅴ在线视频| 非洲黑人性xxxx精品又粗又长| 国产成年人精品一区二区| 亚洲色图av天堂| 国产69精品久久久久777片 | 免费在线观看黄色视频的| 国产欧美日韩一区二区三| 亚洲最大成人中文|