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 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.

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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

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