• <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 ?

    覆有微弧氧化涂層的AZ91D鎂合金在氯化鈉溶液中的極化行為

    2012-12-21 06:33:32常林榮曹發(fā)和蔡景順劉文娟鄭俊軍張鑒清曹楚南
    物理化學(xué)學(xué)報 2012年1期
    關(guān)鍵詞:微弧極化曲線鎂合金

    常林榮 曹發(fā)和,* 蔡景順 劉文娟 鄭俊軍 張鑒清,2 曹楚南,2

    (1浙江大學(xué)化學(xué)系,杭州310027;2中國科學(xué)院金屬研究所,金屬腐蝕與防護(hù)國家重點實驗室,沈陽110016)

    覆有微弧氧化涂層的AZ91D鎂合金在氯化鈉溶液中的極化行為

    常林榮1曹發(fā)和1,*蔡景順1劉文娟1鄭俊軍1張鑒清1,2曹楚南1,2

    (1浙江大學(xué)化學(xué)系,杭州310027;2中國科學(xué)院金屬研究所,金屬腐蝕與防護(hù)國家重點實驗室,沈陽110016)

    由于結(jié)構(gòu)和成分的影響,覆有微弧氧化涂層的AZ91D鎂合金的極化曲線有多種不同的表現(xiàn)形式.事實上,覆有微弧氧化涂層的AZ91D鎂合金在NaCl溶液中的極化曲線行為不僅與涂層的主要組成和微觀結(jié)構(gòu)有關(guān),也與極化曲線測試條件,如氯離子濃度、溶液pH值、陰極極化程度和樣品的暴露面積有關(guān).由于微弧氧化涂層的不穩(wěn)定性,這些因素通過改變氧化涂層的組成和微觀結(jié)構(gòu),繼而影響極化曲線的形狀.本文用傅里葉變換顯微紅外成像和對應(yīng)的光學(xué)照片研究了氧化涂層的成分和結(jié)構(gòu)的變化.結(jié)合物理表征,我們提出一個模型,用以闡明微弧氧化涂層組成和結(jié)構(gòu)在NaCl溶液中的變化.對于浸泡在NaCl溶液中的AZ91D微弧氧化涂層,陽極溶解和陰極還原反應(yīng)的速控步驟分別是傳質(zhì)過程和電荷轉(zhuǎn)移過程.所以從極化曲線上擬合出來的腐蝕電流密度不能準(zhǔn)確反映腐蝕速率,而且誤差也難以避免.

    AZ91D;微弧氧化膜;極化曲線;傅里葉變換顯微紅外成像;電化學(xué)阻抗譜

    1 Introduction

    Polarization test is one of the most conventional methods in measuring the polarization behavior of metals with or without coating,such as steel,copper,aluminum,titanium,and so on.1-4In a certain corrosion system,polarization curve makes it possible to obtain information of the corrosion reaction kinetics,protection of the passive film,corrosion rate of the metal from the pattern of the experimental curve.5In fact,it is not easy to extract any of the above information because the experimental curve is the sum of the real anodic and cathodic polarization curves.6

    Magnesium and its alloys are much more reactive than any other metals mentioned above;therefore,they are easier to be corroded in air or solution without suitable surface treatment. Micro-arc oxidation(MAO),a relatively new surface treatment for magnesium and its alloys,has been widely used to enhance the corrosion resistance of magnesium alloys by producing a MAO coating.Many attempts had been performed to make various MAO coatings on different magnesium alloy substrates, such as AZ91D,7-10AZ31,11AM60,12AM50,13ZK60,14and so on.The results indicated that the properties of MAO coating are related to its primary composition and structure which depend on several factors,such as the compositions of the substrate,15,16the electric parameters,17-19the concentration and chemical composition of electrolyte.9,10,20In these investigations above,polarization curve is also an important electrochemical method to evaluate corrosion behavior of magnesium alloy with MAO coating.The polarization curves obtained in above papers showed various patterns which were considered to only depend on the composition and structure of the MAO coating.

    Unfortunately,those electrochemical measurements were not often carried out in the same experimental conditions.In fact,the MAO coating,whose main compositions are MgO and Mg(OH)2,is porous and unstable in aqueous solution.21The experimental conditions,including the concentration and chemical composition of electrolyte,the exposed area of specimen,and the cathodic polarization degree,may also have influence on the electrochemical behavior of magnesium alloy with MAO coating.

    The present work is aimed at discussing the influence of the measurement conditions on the polarization patterns of magnesium alloyAZ91D with MAO coating.

    2 Experimental

    The composition of AZ91D is same as shown in our previous work.8AZ91D specimens of 30 mm×20 mm×5 mm size, cutting from ingot,were used as substrates for the anodization process.Their two faces and four sides were ground with SiC abrasive paper(grit 320 to 1000),degreased by acetone and then washed with distilled water before anodizing by a constant pulsed voltage power source for 3 min in the electrolyte containing 50.0 g·L-1NaOH(analytical reagent),10.0 g·L-1H3BO3(analytical reagent),20.0 g·L-1Na2B4O7·10H2O(analytical reagent),and 10.0 g·L-1C6H5Na3O7·2H2O(analytical reagent).The initial temperature of the electrolyte was(30±1)°C.

    The morphology and composition of the MAO coating were observed using SIRION-100 field scanning electron microscope(FEI,Netherland)and GENESI 4000 Energy Dispersive Spectrometer.The optical photographs were taken by XTL-3400 stereoscopic microscope made by Cany Precision Instruments Co.,Ltd.

    The electrochemical tests were carried out in aqueous solution and before each electrochemical test the working electrode was immersed in the solution for 30 min to obtain the stable open circuit potential(OCP).A three-electrode cell containing AZ91D with micro-arc oxidation coating as working electrode, saturated calomel electrode(SCE)as reference electrode,and platinum sheet with large area as counter electrode was employed in the test.Polarization curves were made using CHI630C Potentiostat(CH Instruments Inc.,USA).The ratio of aqueous solution volume to sample area was 50 mL·cm-2. After immersion for 30 min,scanning was conducted at a rate of 0.5 mV·s-1.Electrochemical impedance measurements were performed in the 10 kHz-10 mHz frequency range using a 20 mV peak to peak alternative current excitation with a VMP2 multichannel potentiostat produced by PARC Corporation of USA.The experimental impedance spectra were interpreted on the basis of equivalent circuit using fitting software(ZView).

    Fourier transform infrared spectroscopy microscopic mapping(FTIR microscopic mapping)was carried out by a FTIR 6700 spectrometer(Nicolet,USA)with Continuμm microscope.Spectrum acquisition and evaluation were performed with OMNIC 7.3 software using the 3D and MAP packages.

    3 Results

    3.1 Patterns of polarization curves of AZ91D with

    MAO coating

    Based on the results of previous investigations,the polarization curves of AZ91D with micro-arc oxidation coating showed several typical patterns due to the difference of the composition of structure of the MAO coating.Usually,a wide linear region can be observed from the cathodic branches of these polarization curves,while the shapes of the anodic branches are relatively complicated.8,10,15,16,22,23Occasionally,the anodic branch of the polarization curve is much steeper than its corresponding cathodic branch and there is no Tafel region in its anodic branch.8,10In contrast,a linear region or an inflexion can be observed in the anodic branch of the polarization curve,15,22,23while the anodic branch of the polarization curve sometimes shows a pseudo passivation plateau,and then a current eruption can be observed when the electrode potential is high enough.16Actually,the polarization curve of the specimen prepared under the same condition also shows different patterns in difference measurement conditions,which implies that the composition and structure of the MAO coating is not the only reason determining the pattern of polarization curve of AZ91D with MAO coating.In the next sections,the influence factors of the pattern of polarization curve besides the composition and structure of the MAO coating will be further discussed.

    Fig.1 Cross-sections of micro-arc oxidation(MAO)coating with different structures(a,thick and compact;b,thin and porous)and their corresponding polarization curves(a?,b?)in 3.5%(w)NaCl solution

    3.2 Effect of structure of MAO coating

    Many investigations have been done to prove that the properties of MAO coating can be improved by modifying the composition of MAO coating.24-26Our previous work27indicates that the primary compositions of the MAO coating obtained in our experimental condition are similar to each other;therefore,the effect of the minor composition will not be further discussed in this work.

    Undoubtedly,the pattern of polarization curve of AZ91D with MAO coating is related to the structure of the MAO coating.The cross-section images of MAO coatings with different structures and their corresponding polarization curves obtained in 3.5%(w)NaCl solution are shown in Fig.1.The two specimens were prepared at different applied voltages(a,200 V;b, 120 V)while the other experimental conditions were fixed.It is obvious that the structure of anodic coating has a great influence on the pattern of the polarization curve.As shown in Fig.1a,the MAO coating is about 20 μm thick and even with two layers(outer porous layer and inner barrier layer),whose corresponding polarization curve(Fig.1a?)has linear regions in both cathodic branch and anodic branch.On the contrary,the MAO coating in Fig.1b is much thinner than that in Fig.1a and uneven and no barrier layer can be observed from its cross-section morphology.Obviously,the shape of its corresponding polarization curve(Fig.1b?)is different from that of Fig.1a?,especially in anodic branch.

    3.3 Effect of chloride ion and pH value

    Fig.2 Polarization curves of magnesium alloy coated with MAO coatings immersed in different aqueous solutions

    As shown in Fig.2,the pattern of polarization curve of AZ91D with MAO coating which was prepared under the same condition is strongly influenced by the composition and concentration of aqueous solution.Linear region can be observed in both cathodic and anodic branches of the polarization curves measured in Na2SO4and H2SO4solutions.Although cathodic current densities are different,the configurations of cathodic branches of these polarization curves are similar to each other.But the anodic branches of polarization curves in NaCl solution are much different from those obtained in Na2SO4and H2SO4solutions.Linear region can hardly observed in the anodic branches of polarization curves measured in NaCl solution, and their shapes change with the concentration of NaCl solution.It is worth to point out that the difference between open circuit potential and corrosion potential(Ecorr)of the polarization curve obtained in 3.5%NaCl solution is larger than others, more than 50 mV.It is apparent that the effect of chloride ion on the electrochemical behavior of AZ91D with MAO coating is much different from that of other ions.Various patterns of polarization curves measured in NaCl solution are related to the concentration of chloride ion.

    Based on the results above,the chloride ion and hydrogen ion remarkably affect the anodic and cathodic reactions of AZ91D with MAO coating,which is related to the pattern of the polarization curve ofAZ91D with MAO coating.

    3.4 Effect of cathodic polarization degree

    Polarization curves of magnesium alloy with MAO coating show various patterns when measurements are carried out from different initial potentials,which means that the cathodic polarization degree can influence the pattern of polarization curve. As shown in Fig.3,the cathodic polarization is relatively weak when the initial potentials are OCP-10 mV and OCP-50 mV. The OCP(about-1.6 V(vs SCE))approximates to the Ecorrobtained from polarization curves at this time,while there is an apparent inflexion in the anodic branch of each polarization curve.In contrast,when the cathodic polarization is strong enough(initial potential is OCP-200 mV or OCP-300 mV), the anodic branch shows a current burst pattern and there is no inflexion before the current density quickly increases to the limitation with a very steep slope.Moreover,the Ecorrshifts positively and the difference between OCP and Ecorris up to about 80 mV.

    Fig.3 Polarization curves of magnesium alloy with MAO coatings under different cathodic polarizations in 3.5%(w)NaCl solution

    3.5 Effect of exposed area of specimen

    All of the MAO coatings used in this section were prepared under the same condition and measured with different exposed areas in 3.5%NaCl solution.As shown in Fig.4a,a linear region and a current burst are observed in the cathodic branch and anodic branch,respectively,when the exposed area of the specimen is relatively larger(more than 2 cm2).After measurement,a number of pits scatter on the surface of exposed area of specimen,which is a common phenomenon when the exposed area of the specimen is relatively large(more than 2 cm2).

    Fig.4 Polarization curves and their corresponding photographs (22×)after polarization of magnesium alloy with MAO coatings under different exposed areas in 3.5%(w)NaCl solution (a)2.66 cm2;(b,c)0.38 cm2

    Polarization curves were measured on a relatively small surface area(0.38 cm2)and two different patterns of the polarization curves are presented in Fig.4b and Fig.4c.As shown in Fig.4b,the pattern of polarization curve is similar to that in Fig.4a.Several pits disperse on the exposed area of the specimen after polarization as shown in the inset of Fig.4b.In contrast,a pseudo passivation region can be observed in the anodic branch of the polarization curve in Fig.4c.Only one pit appears on the exposed area after polarization,as observed in the inset of Fig.4c.

    When the exposed area of the specimen is relatively larger (more than 2 cm2),the pits are not distributed uniformly on the exposed area,but concentrated on part of the area,just like the combination of the insets in Fig.4b and Fig.4c.So the optical photograph of the exposed area after polarization is not presented in Fig.4a.

    The results above indicate that many vulnerable regions are randomly distributed in the MAO coating and the pattern of the polarization curve is related to the number and distribution of vulnerable regions in the exposed area.

    4 Discussion

    4.1 Transformation of AZ91D with MAO coating

    OCP curves of AZ91D with MAO coating measured in 0.62 mol·L-1NaCl and Na2SO4aqueous solutions are shown in Fig.5.The OCP of AZ91D with MAO coating in Na2SO4solution is always higher than that in NaCl solution during 30 min immersion period.The OCP in Na2SO4solution increases with immersion time,up to a plateau at the end of immersion.In NaCl solution,the OCP quickly increases to a peak value in 3 min,and then dramatically decreases and fluctuates within a lower value range.

    Fig.5 OCPcurves ofAZ91D with MAO coatings immersed in different aqueous solutions

    The IR map of O―H in Mg(OH)2of MAO coatings can be used to illustrate the distribution and amount of Mg(OH)2on the surface of the anodic coating.21Fig.6 shows the IR maps (1375 cm-1)of the MAO coating before and after immersion in 3.5%NaCl solution for 2 min and corresponding optical micrographs(150×).The blue color indicates the lowest infrared absorbance,whereas the red color indicates the highest infrared absorbance.These maps were analyzed by Envi 4.7 software which is used to process and analyze geospatial imagery and the results are shown in Fig.7.Ratios of red,yellow,and green to the whole area of the specimen after immersion are higher than that of blank specimen,while ratios of light blue area and blue area are lower,which indicates that amount of Mg(OH)2on the surface of the MAO coating increases after immersion in the electrolyte only for 2 min.Only these big pores(about 10 μm)can be observed in the optical photograph.As shown in black circles of Fig.6(b,d),some of these pores disappear and some appear on the surface of MAO coating in the 2 min immersion period.The above experimental results illustrate that the distribution and amount of Mg(OH)2on the surface of MAO coating are quickly changed in a short immersion time, while the surface morphology of anodic coating is also slightly changed,with formation and disappearance of micro-pores.

    It is well known that the main compositions of MAO coating are MgO and Mg(OH)2.28-30Thermodynamically,Mg(OH)2is more stable than MgO,and MgO will transform into Mg(OH)2in aqueous solution under standard condition because ΔG=-26.989 kJ·mol-1for reaction MgO+H2O=Mg(OH)2. Mg(OH)2could only stably exist when the pH value is higher than 11.46,while it will be dissolved in neutral and acidic aqueous solutions according to the following reaction:Mg(OH)2+ 2H+=Mg2++2H2O.31Additionally,the Cl-can be adsorbed on AZ91D magnesium and transform Mg(OH)2to more soluble magnesium salt MgCl2via the following reaction:Mg(OH)2+ 2Cl-=MgCl2+2OH-.32

    It is apparent that the transformation of MgO and dissolution of Mg(OH)2simultaneously occur on the surface of anodic coating during the initial immersion stage,but the transformation of MgO is the primary process at this time because of the increment of the amount of Mg(OH)2on the surface of anodic coating.The same reactions also occur in micro-pores of the anodic coating,leading to the positive shift of OCP in the initial immersion stage as shown in Fig.5.

    In neutral Na2SO4solution,the dissolution of Mg(OH)2,according to the equation Mg(OH)2+2H+=Mg2++2H2O,is slow enough to be ignored during the initial immersion stage.Meanwhile the transformation from MgO to Mg(OH)2will make micro-pores in the anodic coating be sealed because the molar volume of Mg(OH)2is larger than that of MgO.In contrast,in NaCl solution the dissolution rate of Mg(OH)2is accelerated by chloride ion and can not be ignored.Therefore,the OCP of magnesium alloy with anodic coating in NaCl solution is very unstable,which can be attributed to the competition between the transformation of MgO and the dissolution of Mg(OH)2.

    Based on the discussion above,the MAO coating is not stable during the initial immersion stage due to the transformation of MgO and dissolution of Mg(OH)2which are accelerated by chloride ion.

    In fact,the cathodic polarization also has a strong influence on the composition and structure of MAO coating.Fig.8 shows IR maps(1375 cm-1)and optical photographs(150×)of the MAO coating with or without cathodic polarization(from-200 to 0 mV)in 3.5%NaCl solution.It is clear that the dissolution of Mg(OH)2is much faster than the transformation of MgO in the cathodic polarization process,which results in the decrease of Mg(OH)2on the surface of anodic coating.At the same time,more big micro-pores appear on its surface,despite there are still some micro-pores sealed in the cathodic process.All results above demonstrate that the deterioration of MAO coating can be accelerated by cathodic polarization.

    Fig.6 Infrared(IR)maps(1375 cm-1)(a,c)and optical photographs(150×)(b,d)of the MAO coating before(a,b)and after(c,d)immersion in 3.5%(w)NaCl solution for 2 min

    Fig.7 Proportion of color areas in Fig.6(a,c)

    4.2 Transformation model

    As shown in Fig.9,a transformation model is summarized based on the results of Section 4.1.It is confirmed that the MAO coating of magnesium alloy,which is porous,can not protect the substrate perfectly.Many kinds of pores and holes can be observed in the cross-section of the anodic coating.As shown in Fig.9a,there are two kinds of holes in the MAO coating,through-holes and non-through-holes.The non-throughholes provide protection against corrosion because they keep corrosive media from contacting to the substrate.In the through-hole structure,the corrosive media penetrate and contact with the substrate directly and result in corrosion of the substrate alloys.33

    Before each test,the specimen must be immersed in the electrolyte for 30 min to a stable OCP.Based on the results above, the MAO coating,which is immersed in the electrolyte,is not stable and its composition and structure could be changed in a short immersion period.Therefore,as shown in Fig.9(b,c),the non-through-holes may be broken through and transformed into through-holes due to the transformation of MgO and the dissolution of Mg(OH)2during the immersion process.The new deterioration products,such as Mg(OH)2and MgCl2,are not compact enough to prevent the contact between the corrosive media and the substrate.The more through-holes appear in the anodic coating,the more corrosive media would contact with the substrate directly.The region where the corrosive media contact with the substrate directly is so called vulnerable region.31

    Fig.9 Schematic diagram of the microstructure of an MAO coating

    Generally speaking,a polarization test is conducted from the cathodic polarization region because the specimen is destroyed irreversibly by the anodic polarization.In the cathodic scanning region,the reaction of electrode could be expressed as 2H2O+2e→2OH-+H2.For magnesium alloy with MAO coating,the reduction reaction occurs in the interface between substrate and MAO coating where the ohm resistance is the smallest and the porous structure is beneficial to the formation of hydrogen bubble.The local pH value increases in these regions where the reduction of hydrogen occurs,which will accelerate the formation of Mg(OH)2.A mass of deterioration products (Mg(OH)2)could strongly retard the anodic reaction but not efficiently inhibit the cathodic reaction,

    At the beginning of potentiodynamic scan,intense hydrogen gas evolution occurs immediately at the vulnerable regions because of the reduction reaction of H+and the structure of the anodic coating which is benefit for the evolution of gas bubbles. Part of the deterioration products formed in the immersion process is also extruded out of the through-holes whose diameters become larger.The deterioration products,which are compact enough to act like a passive film,are formed at the vulnerable regions(as shown in Fig.9d)due to the increase of local pH value caused by reduction of H+,while anions,especially Cl-, are expelled from the interface between the MAO coating and the substrate by the electric field and concentrate in the outside layer of the MAO coating.The concentrated chloride ion will accelerate the deterioration of the outside layer of the MAO coating,which is proved in Section 4.1.The deterioration prod-uct film,which is stable in a wide potential range because of the absence of Cl-,has a great influence on the anodic and cathodic reactions.The property of the deterioration products film is related to all these factors discussed above,such as the concentration of chloride ion,the pH value,the cathodic polarization degree,the exposed area of specimen,the structure of MAO coating and so on.As shown in Fig.9e,some of deterioration product films will be broken down when the potential imposed on the specimen is positive enough.

    Fig.10 Nyquist plots for magnesium alloy with MAO coating immersed in 3.5%NaCl solution for 30 min at different applied potentialsThe inset is the equivalent circuit for fitting these impedance data.

    The difference between the OCP and the Ecorrof the polarization curve in NaCl solution,as mentioned in Section 3.3, should be caused by the deterioration product film formed in the cathodic polarization process.The difference can not be observed in Na2SO4and H2SO4(pH=2)solutions,because the main composition of the deterioration product is Mg(OH)2which is always unstable in H2SO4(pH=2)solution and stable in neutral Na2SO4solution.Therefore,the fresh deterioration products can not influence the anodic and cathodic reactions.But in NaCl solution,only the fresh Mg(OH)2formed in the cathodic polarization process is stable to form a deterioration product film which significantly retards the anodic reaction and make the Ecorrof the polarization curve more positive than OCP.

    4.3 Rate determining step

    Nyquist plots for magnesium alloy with anodic coating at different applied potentials in 3.5%NaCl solution for 30 min are shown in Fig.10.The Nyquist plots are similar to each other except for the difference in the diameter of loops.In all cases,there are two capacitive loops in the high and medium frequency domains,respectively,and an inductive loop in the low frequency domain.The high frequency capacitive loop originates from the anodic coating formed on the AZ91D and themedium frequency one can be attributed to the reduction of hydrogen ion,while the cause of the inductive component of the lower frequency domain is very complicated and out of the aim in this research,which will not be further discussed.The two capacitive loops in the high and medium frequencies were fitted using the equivalent circuit shown in the inset of Fig.10, where Rsis the electrolyte resistance,Rsurfis the coating resistance(determined by solution concentration,thickness,and porosity of the coating),Rtis the charge transfer resistance of hydrogen ion reduction,Csurfand Cdlare the coating capacitance and the solution/metal interface capacitance,respectively.As shown in Table 1,the data of Rsurfare random,which may be caused by the complicated structure of the anodic coating.But the Rtdecreases with cathodic polarization degree.It is clear that Rtis much larger than Rsurf,which indicates that the electrochemistry step of the cathodic process is the rate determining step,while the influence of the anodic coating on the reduction of hydrogen is slight.

    Table 1 Fitting data of Nyquist plots showed in Fig.10

    Fig.11 Typical polarization curve of bareAZ91D in 3.5%NaCl solution

    A typical polarization curve of bare AZ91D in 3.5%NaCl solution is shown in Fig.11.It is obvious that there is a Tafel region in the cathodic branch of the polarization curve.For magnesium alloy in NaCl solution,the hydrogen reduction is the sole cathodic reaction.34The exchange current density of hydrogen reduction was calculated using the linear fit of the cathodic branch of the polarization curve.3The calculated reversible potential of the hydrogen reduction reaction is about-0.41 V(vs SHE)or-0.65 V(vs SCE)at pH 7 in 3.5%NaCl solution.The exchange current density of hydrogen reduction(I0(H+/H2))and the slope of the cathodic branch of the polarization curve(βc) are 1.18×10-11A·cm-2and-141 mV·dec-1,respectively,which mean that the hydrogen reduction on AZ91D is very slow and under activation control.

    Based on the transformation model,the substrate is isolated from the electrolyte in the vulnerable regions after the deterioration product films are formed in these regions.Therefore,the anodic reaction and the cathodic reaction may occur on the different areas of these vulnerable regions.The anodic reaction occurs at the interface of the substrate and the passive film, while the cathodic reaction occurs at the interface of the deterioration product film and the electrolyte.In that case,the rate determining steps of the anodic reaction and the cathodic reaction should be different from each other.For the anodic reaction,Mg+2OH--2e→Mg(OH)2,the charge transfer step is fast due to very reactive magnesium alloy(the potential of Mg/ Mg(OH)2(vs SHE)is-2.690 V),but the product diffusion which is retarded by the deterioration product film should be very slow.Therefore,the rate determining step of the anodic reaction is the diffusion step.According to the results of electrochemical impedance spectroscopy(EIS)and polarization curve of bare AZ91D,the charge transfer step,which is also retarded by the deterioration product film,should be the rate determining step for the cathodic reaction.

    4.4 Deconstruction of polarization curve

    Fig.12a is a schematic of the deconstruction of the polarization curve for AZ91D with MAO coating.5Based on the polarization model above,the anodic reaction is controlled by its diffusion step in a wide potential range,which is represented by a current plateau,but the height of the current plateau is different under different measurement conditions.The height of the current plateau depends on many factors,such as cathodic polarization degree and the distribution of the vulnerable regions in specimen.When the applied potential is high enough to break down the deterioration product film,the anodic current will suddenly erupt to another plateau.In fact,the anodic current does not always show a plateau shape because the deterioration product film will change with the applied potential.In neutral NaCl aqueous solution,the corrosion reaction of magnesium could be expressed as Mg+2OH--2e→Mg(OH)2,which is still intensely carrying out in the cathodic region according to the standard electrode potential of Mg/Mg(OH)2(-2.690 V).Even when the potential is sufficient negative,the anodic current is still not small enough to be omitted even in the cathodic region.As a result,the current recorded in the intense cathodic region is still the sum of the anodic and cathodic currents.

    It is presumed that the two cathodic reactions,which are much similar to each other in two polarization tests,are replaced by one process in Fig.12a.In this case,the patterns of the two polarization curves are only dependent on the anodic reactions.In case 1,the intersection of the anodic line and the cathodic line is at potential Ecorr1,then a current eruption can be observed in the anodic branch of the polarization curve shown in Fig.12b.In case 2,a pseudo passivation plateau appears in the anodic branch of the curve shown in Fig.12c,when the intersection is at potential Ecorr2.In fact,the cathodic reaction also has an effect on the pattern of polarization curve in actual measurements(in Fig.3).The magnitude of the cathodic current density is related to several factors,such as the number and area of the vulnerable region in the MAO coating,the cathodic polarization and the property of deterioration product film.But the slope of the cathodic curve should be similar to each other due to the fact that the cathodic reaction is controlled by charge transfer step.

    Based on the conclusions above,current density(I)and potential(E)can be written as: I=ILa

    Fig.12 Anodic and cathodic polarization processes of magnesium alloy coated with MAO coating in 3.5%NaCl solution

    -I0cexp[(E-Ee,c)/bc] (1) where ILais the limiting diffusion current density of anodic reaction,I0cis the exchange current density of hydrogen reduction,Ee,cis the equilibrium potential of cathodic reaction,and bcis the cathodic Tafel slope.At the beginning of each test,the deterioration product film is formed at the vulnerable regions (in Fig.9d)due to the increase of local pH value caused by reduction of H+.The property of this deterioration product film is related to the concentration of chloride ion,the pH value,the cathodic polarization degree,exposed area of specimen,and the structure of MAO coating.The diffusion step is the rate determining step of the anodic reaction which is significantly retarded by the deterioration product film.

    It is obvious that the pattern of polarization curve of AZ91D with MAO coating depends on the active site of the MAO coating and the diffusion of corrosion products,which can not completely meet the assumptions of Tafel extrapolation.Therefore, the Icorrfitted from polarization curve is not accurate corrosion rate and the error is inevitable.Nevertheless,as the results shown in Section 3.2,the pattern of polarization curve AZ91D with MAO coating is related to the property of the MAO coating.The Icorrfitted from polarization curve can still be used to evaluate the corrosion resistance of AZ91D with MAO coating as a relative parameter not an absolute corrosion rate.

    5 Conclusions

    For AZ91D with MAO coating immersed in NaCl solution, the rate determining steps of anodic reaction and cathodic reaction are the mass diffusion step and the charge transfer step,respectively.Excepting the primary composition and structure of the MAO coating,the pattern of polarization curve AZ91D with MAO coating is also affected by several measurement conditions,such as concentration of chloride ion,pH value of electrolyte,cathodic polarization degree,and exposed area of specimen which can change the composition and structure of the MAO coating in the measurement process.Although the Icorrfitted from polarization curve is not accurate corrosion rate and the error is inevitable,it can still be used to evaluate the corrosion resistance of AZ91D with MAO coating as a relative parameter not an absolute corrosion rate.

    (1) Khaled,K.F.;Amin,M.A.Corrosion Sci.2009,51,1964.

    (2)Chidambaram,D.;Clayton,C.R.;Halada,G.P.;Kendig,M.W. J.Electrochem.Soc.2004,151,B605.

    (3) Sandenbergh,R.F.;Van der Lingen,E.Corrosion Sci.2005,47, 3300.

    (4) Otmacic,H.;Stupnisek-Lisac,E.Electrochim.Acta 2003,48, 985.

    (5) Flitt,H.J.;Schweinsberg,D.P.Corrosion Sci.2005,47,2125.

    (6) Mansfeld,F.Advances in Corrosion Science and Technology 1976,6,163.

    (7) Zhang,L.J.;Zhang,Z.;Zhang,J.Q.Acta Phys.-Chim.Sin. 2008,24,1831.[張麗君,張 昭,張鑒清.物理化學(xué)學(xué)報, 2008,24,1831.]

    (8) Cao,F.H.;Cao,J.L.;Zhang,Z.;Zhang,J.Q.;Cao,C.N. Mater.Corros.2007,58,676.

    (9) Ardelean,H.;Frateur,I.;Zanna,S.;Atrens,A.;Marcus,P. Corrosion Sci.2009,51,3030.

    (10) Laleh,M.;Rouhaghdam,A.S.;Shahrabi,T.;Shanghi,A. J.Alloy.Compd.2010,496,548.

    (11) Wu,C.S.;Zhang,Z.;Cao,F.H.;Zhang,L.J.;Zhang,J.Q.; Cao,C.N.Appl.Surf.Sci.2007,253,3893.

    (12) Cao,F.H.;Lin,L.Y.;Zhang,Z.;Zhang,J.Q.;Cao,C.N. Trans.Nonferrous Met.Soc.China 2008,18,240.

    (13)Ghasemi,A.;Raja,V.S.;Blawert,C.;Dietzel,W.;Kainer,K.U. Surf.Coat.Technol.2008,202,3513.

    (14)Wu,H.L.;Cheng,Y.L.;Li,L.L.;Chen,Z.H.;Wang,H.M.; Zhang,Z.Appl.Surf.Sci.2007,253,9387.

    (15)Arrabal,R.;Matykina,E.;Viejo,F.;Skeldon,P.;Thompson,G. E.Corrosion Sci.2008,50,1744.

    (16) Cakmak,E.;Tekin,K.C.;Malayoglu,U.;Shrestha,S.Surf. Coat.Technol.2010,204,1305.

    (17) Park,I.S.;Jang,Y.S.;Kim,Y.K.;Lee,M.H.;Yoon,J.M.;Bae, T.S.Surf.Interface Anal.2008,40,1270.

    (18)Lv,G.H.;Chen,H.;Gu,W.C.;Li,L.;Niu,E.W.;Zhang,X. H.;Yang,S.Z.J.Mater.Process Tech.2008,208,9.

    (19) Srinivasan,P.B.;Liang,J.;Blawert,C.;Stormer,M.;Dietzel, W.Appl.Surf.Sci.2009,255,4212.

    (20) Ryu,H.S.;Hong,S.H.J.Electrochem.Soc.2009,156,C298.

    (21) Chang,L.R.;Cao,F.H.;Cai,J.S.;Liu,W.J.;Zhang,J.Q.; Cao,C.N.Electrochem.Commun.2009,11,2245.

    (22) Luo,H.H.;Cai,Q.Z.;Wei,B.K.;Yu,B.;He,J.;Li,D.J. J.Alloy.Compd.2009,474,551.

    (23)Arrabal,R.;Matykina,E.;Hashimoto,T.;Skeldon,P.; Thompson,G.E.Surf.Coat.Technol.2009,203,2207.

    (24) Hsiao,H.Y.;Tsung,H.C.;Tsai,W.T.Surf.Coat.Technol. 2005,199,127.

    (25) Liang,J.;Guo,B.G.;Tian,J.;Liu,H.W.;Zhou,J.F.;Liu,W. M.;Xu,T.Surf.Coat.Technol.2005,199,121.

    (26) Chen,F.;Zhou,H.;Yao,B.;Qin,Z.;Zhang,Q.F.Surf.Coat. Technol.2007,201,4905.

    (27) Chang,L.R.;Cao,F.H.;Cai,J.S.;Liu,W.J.;Zhang,Z.; Zhang,J.Q.Trans.Nonferrous Met.Soc.China 2011,21,307.

    (28) Hsiao,H.Y.;Chung,P.;Tsai,W.T.Corrosion Sci.2007,49,781.

    (29) Duan,H.;Yan,C.;Wang,F.Electrochim.Acta 2007,52,3785.

    (30) Birss,V.;Xia,S.;Yue,R.;Rateick,R.G.J.Electrochem.Soc. 2004,151,B1.

    (31) Zhang,Y.J.;Yan,C.W.;Wang,F.H.;Li,W.F.Corrosion Sci. 2005,47,2816.

    (32) El-Taib Heakal,F.;Fekry,A.M.;Fatayerji,M.Z.Electrochim. Acta 2009,54,1545.

    (33) Shi,Z.M.;Song,G.L.;Atrens,A.Corrosion Sci.2005,47, 2760.

    (34) Song,G.L.Corrosion and Protection of Magnesium Alloy; Chemical Industry Press:Beijing,2006;pp 75-84. [宋光鈴.鎂合金腐蝕與防護(hù).北京:化學(xué)工業(yè)出版社,2006:75-84.]

    September 21,2011;Revised:November 9,2011;Published on Web:November 11,2011.

    Polarization Behavior of Magnesium Alloy AZ91D with Micro-Arc Oxidation Coating in NaCl Solution

    CHANG Lin-Rong1CAO Fa-He1,*CAI Jing-Shun1LIU Wen-Juan1ZHENG Jun-Jun1ZHANG Jian-Qing1,2CAO Chu-Nan1,2
    (1Department of Chemistry,Zhejiang University,Hangzhou 310027,P.R.China;2State Key Laboratory for Corrosion and Protection,Institute of Metal Research,Chinese Academy of Sciences,Shenyang 110016,P.R.China)

    The polarization curves of magnesium alloy AZ91D with a micro-arc oxidation(MAO)coating showed several typical patterns caused by differences in the composition and structure of the coating.The pattern of the polarization curve of magnesium alloy AZ91D with a MAO coating depends on the primary composition and structure of the MAO coating and many experimental factors,such as the concentration of chloride ions,pH of the electrolyte,degree of cathodic polarization,and the exposed area of the specimen. These factors change the pattern of polarization curve of magnesium alloy AZ91D with MAO coating by affecting the main composition and structure of the MAO coating because of its instability in aqueous solution.Compositional and structural changes in the MAO coating on magnesium alloy AZ91D were investigated by Fouriertransform infrared microscopic mapping and the corresponding optical photographs,respectively.A model was proposed to describe the transformation of the MAO coating in aqueous NaCl solution.For magnesium alloy AZ91D with a MAO coating immersed in NaCl solution,the rate determining steps of the anodic and cathodic reactions are the mass diffusion and charge transfer steps,respectively.As a result,the corrosion current density fitted from the polarization curve is not an accurate corrosion rate.

    AZ91D;Micro-arc oxidaton coating;Polarization curve;Fourier transform infrared spectroscopy microscopic mapping;Electrochemical impedance spectroscopy

    10.3866/PKU.WHXB201111112

    *Corresponding author.Email:nelson_cao@zju.edu.cn;Tel:+86-571-87952318;Fax:+86-571-87951895.

    The project was supported by the National Natural Science Foundation of China(51131005,51171172,50801056)and Natural Science Foundation of Zhejiang Province,China(Y4110074).

    國家自然科學(xué)基金(51131005,51171172,50801056)及浙江省自然科學(xué)基金(Y4110074)資助項目

    O646

    猜你喜歡
    微弧極化曲線鎂合金
    路譜激勵對于燃料電池極化曲線特征參數(shù)影響研究
    汽車科技(2018年3期)2018-09-04 09:57:50
    鎂合金微弧氧化膜的制備及其性能研究
    先進(jìn)鎂合金技術(shù)與應(yīng)用高峰論壇在滬召開
    鎂合金微弧氧化膜的制備及性能研究
    物理化學(xué)中極化曲線的測定實驗改革
    電化學(xué)實驗教學(xué)中極化曲線的測量與應(yīng)用
    鎂合金微弧氧化工藝的研究
    AZ91D鎂合金磷酸鹽轉(zhuǎn)化膜的制備與表征
    鎂合金的化學(xué)表面處理探討
    鋁合金微弧氧化制備含二硫化鉬的減磨膜層
    国产视频首页在线观看| 色视频在线一区二区三区| 亚洲精品日韩在线中文字幕| 午夜福利一区二区在线看| 每晚都被弄得嗷嗷叫到高潮| 2021少妇久久久久久久久久久| 免费av中文字幕在线| 国产成人免费观看mmmm| 纵有疾风起免费观看全集完整版| 男女床上黄色一级片免费看| 人妻 亚洲 视频| 久久久精品区二区三区| 国产精品熟女久久久久浪| 国产亚洲欧美精品永久| 伊人亚洲综合成人网| 天天添夜夜摸| 亚洲第一青青草原| av在线app专区| 亚洲国产精品成人久久小说| 亚洲中文日韩欧美视频| 亚洲欧洲日产国产| e午夜精品久久久久久久| 久久精品亚洲av国产电影网| 天天操日日干夜夜撸| 欧美xxⅹ黑人| 黄片小视频在线播放| 亚洲欧美一区二区三区黑人| 丝瓜视频免费看黄片| 性色av一级| 一级黄片播放器| 高清不卡的av网站| 中文字幕av电影在线播放| 可以免费在线观看a视频的电影网站| 成人亚洲欧美一区二区av| 亚洲成人免费av在线播放| 国产一级毛片在线| 亚洲欧美精品综合一区二区三区| 美女扒开内裤让男人捅视频| 伊人亚洲综合成人网| 国产免费福利视频在线观看| 夫妻性生交免费视频一级片| 在线亚洲精品国产二区图片欧美| 国产精品一区二区在线不卡| 成年女人毛片免费观看观看9 | 真人做人爱边吃奶动态| 国产成人91sexporn| 黑丝袜美女国产一区| 亚洲 国产 在线| 免费日韩欧美在线观看| 热re99久久国产66热| 老司机午夜十八禁免费视频| 精品免费久久久久久久清纯 | av在线app专区| 欧美成人午夜精品| www日本在线高清视频| av一本久久久久| 亚洲av男天堂| 国产一区二区三区av在线| 大话2 男鬼变身卡| 国产在线免费精品| 免费看不卡的av| 日本欧美视频一区| 91九色精品人成在线观看| 极品人妻少妇av视频| 欧美xxⅹ黑人| 90打野战视频偷拍视频| 一本一本久久a久久精品综合妖精| 午夜精品国产一区二区电影| 秋霞在线观看毛片| 一级毛片女人18水好多 | 丝袜脚勾引网站| 纵有疾风起免费观看全集完整版| 欧美日韩综合久久久久久| 免费少妇av软件| 人人妻,人人澡人人爽秒播 | 亚洲中文字幕日韩| 亚洲精品成人av观看孕妇| 亚洲精品久久午夜乱码| 观看av在线不卡| a级毛片在线看网站| 欧美黑人欧美精品刺激| 国产又色又爽无遮挡免| 亚洲精品一区蜜桃| 2021少妇久久久久久久久久久| av不卡在线播放| 丰满迷人的少妇在线观看| svipshipincom国产片| 国产精品亚洲av一区麻豆| 桃花免费在线播放| 母亲3免费完整高清在线观看| 久久毛片免费看一区二区三区| 丁香六月天网| 成年av动漫网址| 亚洲欧美日韩另类电影网站| 国产1区2区3区精品| 久久久久久久精品精品| av天堂久久9| 久久久久久久久免费视频了| 一区二区三区四区激情视频| 亚洲精品久久午夜乱码| 久久ye,这里只有精品| 大片电影免费在线观看免费| 色综合欧美亚洲国产小说| 国产国语露脸激情在线看| 精品熟女少妇八av免费久了| 国产成人免费观看mmmm| 国产麻豆69| 首页视频小说图片口味搜索 | 波多野结衣av一区二区av| 国产亚洲精品第一综合不卡| 999精品在线视频| 久久久久久人人人人人| 亚洲国产欧美日韩在线播放| 亚洲国产毛片av蜜桃av| 18禁国产床啪视频网站| 男女床上黄色一级片免费看| 日本黄色日本黄色录像| 最近中文字幕2019免费版| 丝袜美腿诱惑在线| 久久久亚洲精品成人影院| 国产精品成人在线| 亚洲av男天堂| 大片电影免费在线观看免费| 久久99热这里只频精品6学生| 9191精品国产免费久久| 精品福利观看| 亚洲精品国产区一区二| 纯流量卡能插随身wifi吗| 涩涩av久久男人的天堂| 人妻 亚洲 视频| 看免费成人av毛片| 国产成人精品久久二区二区91| 黄频高清免费视频| 天天操日日干夜夜撸| 1024视频免费在线观看| 一区二区av电影网| 少妇的丰满在线观看| 国产视频首页在线观看| 亚洲精品国产色婷婷电影| 色综合欧美亚洲国产小说| 国产亚洲午夜精品一区二区久久| 国产女主播在线喷水免费视频网站| 亚洲一区二区三区欧美精品| 国产国语露脸激情在线看| 成人国语在线视频| 国产精品一区二区免费欧美 | 人人妻人人添人人爽欧美一区卜| 大片免费播放器 马上看| 精品少妇一区二区三区视频日本电影| 性色av一级| 天天躁狠狠躁夜夜躁狠狠躁| 国产午夜精品一二区理论片| 深夜精品福利| 性色av乱码一区二区三区2| 色精品久久人妻99蜜桃| 最黄视频免费看| 制服诱惑二区| 麻豆av在线久日| 精品国产一区二区久久| 女警被强在线播放| 国产精品免费大片| 亚洲精品久久午夜乱码| 久久精品久久精品一区二区三区| 亚洲一区中文字幕在线| 久久久久网色| 久久ye,这里只有精品| 中国美女看黄片| 少妇被粗大的猛进出69影院| 美女午夜性视频免费| 日韩欧美一区视频在线观看| 91麻豆精品激情在线观看国产 | 男女高潮啪啪啪动态图| 久久青草综合色| 欧美乱码精品一区二区三区| 九色亚洲精品在线播放| 老司机靠b影院| 日本欧美视频一区| 国产一卡二卡三卡精品| 国产高清不卡午夜福利| 最近最新中文字幕大全免费视频 | 中文字幕高清在线视频| 美女国产高潮福利片在线看| 亚洲专区国产一区二区| 免费在线观看影片大全网站 | 男女床上黄色一级片免费看| 天天添夜夜摸| 欧美精品一区二区大全| 只有这里有精品99| 首页视频小说图片口味搜索 | videos熟女内射| 天天躁狠狠躁夜夜躁狠狠躁| 黄片小视频在线播放| 精品国产一区二区三区久久久樱花| 亚洲七黄色美女视频| 丝袜人妻中文字幕| 亚洲国产av影院在线观看| 欧美少妇被猛烈插入视频| 一级毛片电影观看| 国产精品一区二区免费欧美 | 爱豆传媒免费全集在线观看| 日韩一卡2卡3卡4卡2021年| 国产男女内射视频| 欧美日韩亚洲国产一区二区在线观看 | 别揉我奶头~嗯~啊~动态视频 | 亚洲人成电影观看| 日韩制服骚丝袜av| 黑人猛操日本美女一级片| 校园人妻丝袜中文字幕| 色婷婷久久久亚洲欧美| 韩国高清视频一区二区三区| 国产精品一区二区精品视频观看| 欧美成人精品欧美一级黄| 亚洲第一av免费看| 在线天堂中文资源库| 亚洲男人天堂网一区| 国产熟女欧美一区二区| 久久久久久久久久久久大奶| 国产成人一区二区三区免费视频网站 | 久久鲁丝午夜福利片| 亚洲成色77777| 在线观看免费日韩欧美大片| xxx大片免费视频| 99九九在线精品视频| 老司机靠b影院| 国产又色又爽无遮挡免| 亚洲av电影在线进入| 老鸭窝网址在线观看| 国产精品亚洲av一区麻豆| 久久青草综合色| 亚洲国产精品成人久久小说| 国产日韩欧美亚洲二区| 国产在线免费精品| 嫁个100分男人电影在线观看 | 妹子高潮喷水视频| 国产又色又爽无遮挡免| 久久人妻熟女aⅴ| 国产麻豆69| 少妇猛男粗大的猛烈进出视频| 成年人午夜在线观看视频| 亚洲av成人不卡在线观看播放网 | 精品少妇黑人巨大在线播放| 亚洲av欧美aⅴ国产| 国产欧美日韩综合在线一区二区| 国产激情久久老熟女| 91九色精品人成在线观看| 免费在线观看影片大全网站 | 美女高潮到喷水免费观看| 国产精品 国内视频| 老司机午夜十八禁免费视频| 99国产精品99久久久久| 韩国高清视频一区二区三区| 午夜福利乱码中文字幕| 狠狠婷婷综合久久久久久88av| 黄色片一级片一级黄色片| 亚洲精品久久午夜乱码| 国产日韩欧美视频二区| 精品一区二区三卡| 免费观看av网站的网址| 日韩大片免费观看网站| 在线观看免费高清a一片| 亚洲精品国产区一区二| 亚洲人成网站在线观看播放| 欧美精品一区二区大全| 精品久久蜜臀av无| 天天躁日日躁夜夜躁夜夜| 国产精品久久久久久精品古装| 国产av一区二区精品久久| 人成视频在线观看免费观看| 国产精品人妻久久久影院| 日日夜夜操网爽| 热re99久久精品国产66热6| 黄片小视频在线播放| 婷婷色麻豆天堂久久| 夫妻午夜视频| 美女国产高潮福利片在线看| 国产xxxxx性猛交| 中文字幕人妻丝袜制服| 国产成人91sexporn| 中文字幕高清在线视频| 久久久久久久国产电影| 男女床上黄色一级片免费看| 亚洲激情五月婷婷啪啪| 高清av免费在线| 欧美日韩精品网址| 精品国产一区二区三区四区第35| 黑丝袜美女国产一区| 亚洲国产av新网站| 精品久久久精品久久久| 久久久久久人人人人人| 欧美人与善性xxx| 国产黄频视频在线观看| 国产激情久久老熟女| 女性被躁到高潮视频| tube8黄色片| 99国产综合亚洲精品| 精品久久久久久电影网| 日韩免费高清中文字幕av| 国产爽快片一区二区三区| 欧美激情极品国产一区二区三区| 麻豆国产av国片精品| 人人妻人人添人人爽欧美一区卜| 黄网站色视频无遮挡免费观看| 人妻 亚洲 视频| 欧美在线一区亚洲| 自拍欧美九色日韩亚洲蝌蚪91| 最近手机中文字幕大全| 国产真人三级小视频在线观看| 免费观看人在逋| 在线av久久热| 欧美国产精品va在线观看不卡| 亚洲国产看品久久| 国产成人精品久久久久久| 首页视频小说图片口味搜索 | 岛国毛片在线播放| 日本一区二区免费在线视频| 国产xxxxx性猛交| 国产有黄有色有爽视频| 首页视频小说图片口味搜索 | 精品国产一区二区三区久久久樱花| 18禁裸乳无遮挡动漫免费视频| 热99国产精品久久久久久7| 国产淫语在线视频| 这个男人来自地球电影免费观看| 欧美久久黑人一区二区| 啦啦啦啦在线视频资源| www.av在线官网国产| 日韩伦理黄色片| 男男h啪啪无遮挡| 免费黄频网站在线观看国产| 亚洲国产欧美一区二区综合| 久久国产精品人妻蜜桃| 婷婷色av中文字幕| 国产亚洲欧美在线一区二区| 国产麻豆69| 午夜91福利影院| 精品亚洲成a人片在线观看| 一级毛片黄色毛片免费观看视频| 十分钟在线观看高清视频www| netflix在线观看网站| 少妇精品久久久久久久| 在线看a的网站| 91麻豆av在线| 亚洲五月色婷婷综合| 18禁观看日本| 国产一卡二卡三卡精品| 亚洲熟女精品中文字幕| 中文字幕高清在线视频| 国产精品麻豆人妻色哟哟久久| 亚洲av欧美aⅴ国产| 伦理电影免费视频| 悠悠久久av| 99国产综合亚洲精品| 国产男女内射视频| 婷婷成人精品国产| 黄色一级大片看看| 亚洲免费av在线视频| 下体分泌物呈黄色| 亚洲国产精品999| 视频区图区小说| 国产精品久久久久成人av| 男女无遮挡免费网站观看| 国产成人影院久久av| 两个人看的免费小视频| 中文字幕制服av| 叶爱在线成人免费视频播放| 日韩中文字幕视频在线看片| 久久久久网色| 18禁国产床啪视频网站| 久久精品久久久久久噜噜老黄| 国产1区2区3区精品| 巨乳人妻的诱惑在线观看| 老鸭窝网址在线观看| 青青草视频在线视频观看| av天堂在线播放| 亚洲国产精品一区二区三区在线| 夫妻午夜视频| 日韩中文字幕欧美一区二区 | 9色porny在线观看| 亚洲图色成人| 交换朋友夫妻互换小说| 久久综合国产亚洲精品| 天天躁夜夜躁狠狠躁躁| 中文字幕另类日韩欧美亚洲嫩草| svipshipincom国产片| 啦啦啦啦在线视频资源| 成人亚洲精品一区在线观看| 一边亲一边摸免费视频| 国产精品一二三区在线看| 性高湖久久久久久久久免费观看| av又黄又爽大尺度在线免费看| 久久久久久免费高清国产稀缺| 叶爱在线成人免费视频播放| 欧美中文综合在线视频| 飞空精品影院首页| 黄频高清免费视频| 青春草亚洲视频在线观看| 欧美+亚洲+日韩+国产| 亚洲黑人精品在线| 真人做人爱边吃奶动态| 精品国产一区二区三区四区第35| 男人操女人黄网站| 中文字幕精品免费在线观看视频| 热99久久久久精品小说推荐| 精品国产一区二区久久| 曰老女人黄片| 成年人免费黄色播放视频| 777米奇影视久久| 精品国产超薄肉色丝袜足j| 久久国产精品大桥未久av| 亚洲精品国产一区二区精华液| 国产成人欧美| 大型av网站在线播放| 人人妻人人添人人爽欧美一区卜| 免费黄频网站在线观看国产| 大话2 男鬼变身卡| 国精品久久久久久国模美| 天天影视国产精品| 男人操女人黄网站| 欧美日韩精品网址| 91精品三级在线观看| 亚洲精品乱久久久久久| 韩国高清视频一区二区三区| 国产免费现黄频在线看| 国产精品免费大片| 丝袜在线中文字幕| 中文字幕人妻丝袜制服| 天堂8中文在线网| 欧美av亚洲av综合av国产av| 久久久久久久久久久久大奶| 真人做人爱边吃奶动态| h视频一区二区三区| 国产熟女午夜一区二区三区| 欧美国产精品va在线观看不卡| 在线天堂中文资源库| 大片电影免费在线观看免费| 中文字幕亚洲精品专区| 亚洲黑人精品在线| 9191精品国产免费久久| av视频免费观看在线观看| 午夜老司机福利片| 在线观看www视频免费| 国产成人精品久久二区二区91| 一级毛片女人18水好多 | 国产麻豆69| 韩国高清视频一区二区三区| 精品国产乱码久久久久久男人| 欧美日韩视频高清一区二区三区二| 日韩 欧美 亚洲 中文字幕| 亚洲人成电影免费在线| 啦啦啦 在线观看视频| 91麻豆精品激情在线观看国产 | 日韩视频在线欧美| 亚洲人成电影免费在线| 97精品久久久久久久久久精品| 岛国毛片在线播放| svipshipincom国产片| 人人妻,人人澡人人爽秒播 | 久久ye,这里只有精品| 久久久久久久大尺度免费视频| 国产精品 国内视频| 午夜福利影视在线免费观看| 亚洲精品美女久久久久99蜜臀 | 欧美成人午夜精品| 大话2 男鬼变身卡| 真人做人爱边吃奶动态| 国产精品国产av在线观看| 国产97色在线日韩免费| 免费看av在线观看网站| 伊人亚洲综合成人网| 天天躁日日躁夜夜躁夜夜| 国产精品九九99| av天堂久久9| 日本欧美视频一区| 国产国语露脸激情在线看| 亚洲五月色婷婷综合| 美女脱内裤让男人舔精品视频| 成人18禁高潮啪啪吃奶动态图| 免费少妇av软件| 午夜老司机福利片| 热re99久久精品国产66热6| 欧美精品人与动牲交sv欧美| 久久精品国产综合久久久| 亚洲久久久国产精品| 爱豆传媒免费全集在线观看| 自线自在国产av| 久久精品亚洲av国产电影网| 国产片特级美女逼逼视频| 色94色欧美一区二区| 精品一区二区三区av网在线观看 | 青草久久国产| 亚洲欧美精品综合一区二区三区| 国产高清不卡午夜福利| 脱女人内裤的视频| 亚洲视频免费观看视频| 久久久久视频综合| 人人妻,人人澡人人爽秒播 | 日本av手机在线免费观看| 赤兔流量卡办理| 国产亚洲欧美精品永久| 超碰97精品在线观看| 亚洲欧美一区二区三区黑人| 久久 成人 亚洲| 人人妻人人爽人人添夜夜欢视频| 777米奇影视久久| 精品人妻1区二区| 亚洲欧美成人综合另类久久久| 日韩一区二区三区影片| 亚洲一码二码三码区别大吗| 欧美日韩国产mv在线观看视频| av有码第一页| 高潮久久久久久久久久久不卡| 午夜免费男女啪啪视频观看| 国产成人免费观看mmmm| 妹子高潮喷水视频| 日韩大码丰满熟妇| 九色亚洲精品在线播放| 51午夜福利影视在线观看| 一本大道久久a久久精品| 女人久久www免费人成看片| 高清不卡的av网站| 一二三四社区在线视频社区8| 黑人欧美特级aaaaaa片| 日韩精品免费视频一区二区三区| 久久久久视频综合| 成年av动漫网址| 人人妻人人澡人人看| 女人被躁到高潮嗷嗷叫费观| 国产精品成人在线| 一本久久精品| 亚洲男人天堂网一区| 久久久久久久久免费视频了| 国产伦理片在线播放av一区| 制服人妻中文乱码| 99re6热这里在线精品视频| 人人澡人人妻人| 久久久久精品人妻al黑| 丰满迷人的少妇在线观看| 一本一本久久a久久精品综合妖精| 丰满迷人的少妇在线观看| 高清视频免费观看一区二区| 精品一区二区三区av网在线观看 | 少妇人妻久久综合中文| 亚洲图色成人| 99久久精品国产亚洲精品| 亚洲成人免费av在线播放| 两人在一起打扑克的视频| 无遮挡黄片免费观看| 久久久久国产一级毛片高清牌| 午夜影院在线不卡| 自拍欧美九色日韩亚洲蝌蚪91| 精品视频人人做人人爽| svipshipincom国产片| 午夜免费男女啪啪视频观看| av天堂久久9| 国产成人一区二区在线| 久久99热这里只频精品6学生| 女人久久www免费人成看片| 精品高清国产在线一区| 亚洲中文av在线| 欧美日韩国产mv在线观看视频| 人体艺术视频欧美日本| 高清欧美精品videossex| 久久精品国产亚洲av高清一级| 9热在线视频观看99| 亚洲av国产av综合av卡| 日韩一区二区三区影片| 成年人黄色毛片网站| 中文字幕亚洲精品专区| 国产精品秋霞免费鲁丝片| 自线自在国产av| 菩萨蛮人人尽说江南好唐韦庄| av在线app专区| 国产一区亚洲一区在线观看| 国产精品香港三级国产av潘金莲 | 精品一品国产午夜福利视频| 欧美少妇被猛烈插入视频| 亚洲国产精品999| av国产精品久久久久影院| 国产精品一区二区在线观看99| 亚洲国产最新在线播放| 欧美xxⅹ黑人| 一二三四在线观看免费中文在| 久久九九热精品免费| 亚洲 国产 在线| 一级毛片 在线播放| 国产xxxxx性猛交| 国产片内射在线| www.999成人在线观看| 99国产精品99久久久久| 成在线人永久免费视频| 超碰成人久久| 又大又爽又粗| 亚洲精品中文字幕在线视频| 一区二区av电影网| 巨乳人妻的诱惑在线观看| 欧美少妇被猛烈插入视频| 欧美黑人欧美精品刺激| 美女福利国产在线| 中文字幕另类日韩欧美亚洲嫩草| 手机成人av网站| 免费人妻精品一区二区三区视频| 晚上一个人看的免费电影| 性少妇av在线| 男人添女人高潮全过程视频| 国产精品免费视频内射| 亚洲一区二区三区欧美精品| 亚洲三区欧美一区| 视频区图区小说| 日韩视频在线欧美| 久久国产精品人妻蜜桃| 九草在线视频观看| 国语对白做爰xxxⅹ性视频网站| 国产女主播在线喷水免费视频网站| 一级黄片播放器| 在线观看免费午夜福利视频|