Le Sun,Ying M,?,Binfeng Fn,Sheng Wng,Zhnying Wng

a State Key Laboratory of Advanced Processing and Recycling of Non-ferrous Metals,Lanzhou University of Technology,Lanzhou 730050,China

Abstract Novel hybrid coatings on pure magnesium were prepared by combining plasma electrolytic carburizing(PEC)with micro-arc oxidation(MAO)to further enhance the anti-corrosion property in this paper.Scanning electron microscopy(SEM)was used to observe the microstructure of the coatings,meanwhile,energy dispersive spectrometry(EDS)and X-ray diffraction(XRD)were separately used to investigate the elemental as well as phase compositions of the coatings.The anti-corrosion property of the coatings was evaluated by potentiodynamic polarization curves as well as electrochemical impedance spectroscopy(EIS).The results show that PEC process is closely related with the effects of adsorption as well as diffusion of the activated carbon atoms,and it can provide a favorable pretreatment surface with predesigned chemical composition to obtain a new kind of phase,namely SiC with superior corrosion resistance and chemical stability,in the following PEC+MAO hybrid coatings.Meanwhile,PEC preprocessing also can afford an excellent micro-structure to increase the coating thickness as well as to improve the compactness of the PEC+MAO hybrid coatings.During the fabrication process of the PEC+MAO hybrid coatings,an overlapping phenomenon in regard to coating thickness can be observed instead of heaping up layer by layer.Compared with both single PEC surface modification layers as well as single MAO coatings,the PEC+MAO hybrid coatings exhibit more superior anti-corrosion property.Especially,the EIS data reveal that the PEC+MAO hybrid coatings can act as an effective protection system to provide relatively excellent long-range anti-corrosion protection.Note also that employing same MAO technique for both single MAO treatment as well as PEC+MAO combining procedure is the key to this research.

Keywords:Pure magnesium;Plasma electrolytic carburizing;Micro-arc oxidation;Surface modification layers;Hybrid coatings;Anti-corrosion property.?Corresponding author.

1.Introduction

Magnesium and its alloys,owing to their superior properties such as low density,high strength to weight ratio,good damping performance,biocompatibility,good electromagnetic shielding characteristics,as well as recyclability,are widely used in the aerospace,transportation,electronic 3C,biomedical and energy sectors[1,2].However,high activity of magnesium weakens the anti-corrosion ability of this metal and its alloys especially in corrosive media,which tremendously hinders their large-scale industrial applications[3,4].Hence,some surface treatment techniques are necessary to be adopted to enhance the corrosion resistance of magnesium and its alloys before they are used.

At present,plasma electrolytic saturation(PES)has been acted as an efficient surface modification approach to improve the comprehensive properties(e.g.hardness,anti-corrosion performance and wear resistance)of the metals.According to different kinds of elements,PES can be divided into plasma electrolytic carburizing(PEC),nitriding(PEN),boronizing(PEB),nitrocarburizing(PEN/C)and borocarburizing(PEB/C),etc.,among which PEC is the most common[5].During PEC process,a continuous gaseous envelope is broken down at the critical voltage,resulting in the plasma discharge adjacent to the cathode and an enhanced interstitial diffusion of active carbon atoms.PEC treatment,owing to its excellent advantages such as environmentally-friendly,high efficiency,low cost,simple process and directly quenching in the electrolyte,becomes a popular surface modification technique.In recent years,a considerable amount of researchers have paid more attention to PEC of ferrous metals(e.g.pure iron[6,7],carbon steel[8–11]as well as stainless steel[12,13]).

In the case of the non-ferrous metals,the investigations associated with PEC technique are primarily focused on pure titanium and titanium alloys surface[14–16].Aliofkhazraei et al.[14]produced a TiC/WC ultra hard nanocomposite layers on pure titanium by PEC technique.The maximum hardness of the nanocomposite layers was about 2580 HV and the corrosion resistance of pure titanium was significantly improved by the nanocomposite layers.Rouhaghdam et al.[15]have analyzed the effect of frequency on PEC of pure titanium.The results indicated that the carburized layers obtained at moderate frequency(400 Hz)exhibit superior anticorrosion property.Aliofkhazraee et al.[16]have confirmed that frequency and duty cycle can affect PEC of titanium alloy.It is revealed that the corrosion current density decreases from 0.68 μA/cm2for substrate to 0.10 μA/cm2for the samples carburized at 1000 Hz and duty cycle of 10%.But the researches associated with PEC of magnesium as well as its alloys had been infrequently reported.

Micro-arc oxidation is a kind of prevalent surface treatment approach due to its low cost,simple operation as well as environmentally-friendly.This kind of surface modification method is widely used to fabricate protective coatings for the non-ferrous metals(e.g.Mg,Al,Ti)under the condition of high-voltage environment by generating plasma discharges[17–20].The obtained MAO coatings exhibit strong adhesion in the coating/substrate interface and reveal superior anti-corrosion property,which can provide effective corrosion protection.

Currently,in order to further enhance the anti-corrosion property of magnesium and its alloys,more and more researchers have tried to prepare films with multilayers,composite coatings or hybrid coatings.And the specific processing methods could be ranked as following:two-step MAO treatments[21–23],combination of inorganic silane,sol-gel sealing,laser surface melting,electrochemical deposition,ENplating,electroless plating with MAO technology[24–30].Meanwhile,Nezamdoust et al.[31]prepared composite coatings by combining sol-gel with chemistry conversion technology on AM60B Mg alloys.Particularly,Toorani et al.[32]successfully fabricated a MAO/Silane/Epoxy three-layer coating as an effective protection system for the preservation of AZ31B Mg alloys.Nevertheless,the researches of simultaneously combining PES with MAO processes to prepare hybrid coatings on pure magnesium had been infrequently reported.

In this investigation,hybrid coatings were prepared by combining PEC with MAO on pure magnesium.After that,the characteristics of the anti-corrosion property between single PEC surface modification layers,single MAO coatings as well as the PEC+MAO hybrid coatings were contrastively investigated.In addition,the formation processes and mechanism of single PEC surface modification layers as well as the PEC+MAO hybrid coatings were discussed,respectively.It should be noted that selecting pure magnesium as the research object in this paper was to make an attempt in the first stage,since the disturbance from other alloy elements could be kept out during the coating formation.Once the hybrid coatings were fabricated on pure magnesium successfully,and then this novel surface modification technique would be applied to magnesium alloys and also improving the corrosion resistance of the coatings would be carried out further in the next stage.

2.Experimental

2.1.Fabrication of the PEC+MAO hybrid coatings

Cuboid specimens(30 mm×20 mm×12 mm)of pure magnesium(99.89%)were used in this study.These specimens were mechanically ground with water-resistant silicon carbide(SiC)papers progressively from 150 to 2000 grit before surface treatment.After that,they were rinsed with distilled water,ultrasonically cleaned with absolute ethanol,dried with cool air and stored in a sealed desiccator.Subsequently,the pure magnesium samples were subdivided into three parts,the first part was processed merely with PEC approach,the second part was processed merely with MAO technique,whereas the third part was treated by combining PEC with MAO to prepare hybrid coatings.

Both PEC and MAO processes were conducted by a homemade bipolar pulsed power supply.During the PEC pretreatment,the cathode was the pure magnesium substrate,while the stainless steel sheet was applied as the anode.The aqueous solution of PEC pretreatment consisted of glycerol(C3H8O3325 ml/L)and NaOH(5 g/L).The preparation of the PEC surface modification layers were carried out at a constant voltage of 180 V for 30 min under DC mode.In the case of MAO process,the pure magnesium substrate as well as the stainless steel sheet were connected as the anode and cathode,respectively.Meanwhile,the MAO coatings were prepared at a constant voltage of 400 V for 10 min under AC mode with frequency 700 Hz as well as duty ration 20% in silicate-based aqueous solution(Na2SiO315 g/L,KF 13 g/L,NaOH 2 g/L).During the fabrication of the PEC+MAO hybrid coatings,the pure magnesium samples were pretreated firstly with PEC technique,and then the PEC coated samples were treated by MAO process.Fig.1 exhibits the fabrication process of the PEC+MAO hybrid coatings.It is worth noting that the MAO technique was kept constant for both single MAO treatment as well as the preparation of the PEC+MAO hybrid coatings.

2.2.Characterization of the coatings

The surface roughness of the coatings was measured on the front as well as back of the sample by applying a surface roughness tester(2206 made in China),and the examination was carried out at least 5 times for each side of the sample.Scanning electron microscopy(SEM,QUANTA-450FEGand JSM-6700F)were used to observe the surface and crosssectional morphologies of the coatings.Image J software was employed to analyze the distribution characteristics of the micro-pore as well as the surface porosity and the thickness of the coatings.The elemental compositions of the coatings were characterized by energy dispersive spectrometry(EDS).The phase compositions of the coatings were examined using an X-ray diffraction(XRD,Ricoh,D/MAX2500PC)with a Cu target at 6°/min scanning rate over a 2θrange from 10°to 80°

Fig.1.Preparation process of the PEC+MAO hybrid coatings.

2.3.Characterization of anti-corrosion property

Potentiodynamic polarization curves as well as electrochemical impedance spectroscopy(EIS)were applied to characterize the anti-corrosion property of the coatings using an electrochemical workstation(CHI660C).A conventional three-electrode cell system was used in the 3.5 wt.% neutral NaCl solution,among which the coated samples(1 cm2exposed area),the platinum sheet,and the saturated calomel electrode(SCE)were served as the working electrode,counter electrode as well as reference electrode,respectively.Potentiodynamic polarization curves were performed from-1900 to-1400 mV at 1 mV/s sweeping rate on the coated specimens after 30 min of immersion.While the EIS diagrams were gathered from 10 mHz to 100 kHz with an interference potential of 10 mV on the coated samples after immersion of 0.5,2,5,10,20,44 and 68 h,respectively.The Zsimp-Win3.20 software was employed to fit the EIS results as well as establish the equivalent circuits.In addition,the potentiodynamic polarization curves and EIS measurements for every specimen were repeated no less than four times to ensure the reliability as well as stability.

3.Results

3.1.Surface morphologies and the distribution characteristics of the micro-pores

The surface morphologies of the coatings prepared by different processes are presented in Fig.2.The surface roughness(Ra)of the coatings prepared by different processes are exhibited in Table 1.As shown in Fig.2a and Table 1,PEC surface modification layers exhibit“dry cement”features and relatively rough surface characteristics(Ra≈2.01 μm),which can be ascribed to the violent plasma etching effect and the local high-temperature effect[5].While the surface roughness of both MAO coatings(Ra≈1.09 μm)and the PEC+MAO hybrid coatings(Ra≈1.25 μm)are lower than that of PEC surface modification layers.Meanwhile,it is obvious that the difference of the surface roughness between MAO coatings and the PEC+MAO hybrid coatings is relatively little,and it is worth to note that the thickness of the PEC+MAO hybrid coatings(Fig.5c)was approximately 5 μm thicker than that of the single MAO coatings(Fig.5b).

Table 1Surface roughness(Ra)of the coatings prepared by different processes.

Additionally,as exhibited in Fig.2b,MAO coatings reveal porous structural characteristics,resulting from the mechanism of MAO technique.Although the PEC+MAO hybrid coatings also show porous microstructure(Figs.2c-d),two types of typical characteristics could be observed on the surface of the PEC+MAO hybrid coatings,that is less micropore areas labeled by some ellipses(Fig.2c)as well as a sign of sintering seal pores of the coating materials shown in Fig.2d,which could reduce the pore number as well as the pore size,respectively.

Fig.3 displays the distribution characteristics of the micropores.Fig.4 illustrates the micro-pore size as well as surface porosity of the coatings.As shown in Figs.3 and 4,the per-centage of the micro-pore(＜1 μm)of the PEC+MAO hybrid coatings(51.83%)is approximately 7.76% more than MAO coatings(44.07%).Particularly,the percentage of the micro-pore(＞3 μm)of the PEC+MAO hybrid coatings is only 0.53%,which is 1/15 of MAO coatings(7.69%).Meanwhile,the average micro-pore size of MAO coatings is approximately 2.7 μm,which is 1.6 μm larger than that of the PEC+MAO hybrid coatings(1.1 μm).Additionally,the surface porosity of the PEC+MAO hybrid coatings is approximately 4.3%,which is merely half of MAO coatings(9.5%).These results confirm that the PEC+MAO hybrid coatings are more compact than that of MAO coatings.

Fig.2.Surface morphologies of the(a)PEC,(b)MAO,as well as(c)PEC+MAO coatings;(d)magnification image of the rectangle region in Fig.2(c).(For interpretation of the references to colour in this figure legend,the reader is referred to the web version of this article.)

Fig.3.Distribution characteristics of the micro-pores of the(a)MAO corresponding to Fig.2(b),as well as(b)PEC+MAO corresponding to Fig.2(c)coatings.

Fig.4.Micro-pore size as well as the surface porosity of the coatings prepared by different processes.(For interpretation of the references to colour in this figure legend,the reader is referred to the web version of this article.)

3.2.Cross-sectional morphologies and elemental compositions

Fig.5 illustrates the cross-sectional morphologies as well as the corresponding EDS line scan of the coatings.Fig.5a exhibits that there is an obvious diffusion region in the crosssection of PEC surface modification layers as well as the contrast is clear.As shown in Fig.5c,some defects(e.g.small-scale pits together with micro-pores with different size)can be observed in the cross-section of MAO coatings,implying that MAO coatings are not dense enough.The crosssection morphology of the PEC+MAO hybrid coatings exhibited in Fig.5e reveals obviously that the PEC+MAO hybrid coatings are more compact than MAO coatings,even though some tiny micro-pores are distributed in the outer regions of the PEC+MAO hybrid coatings.In addition,as exhibited in Fig.5e,the PEC+MAO hybrid coatings show relatively higher thickness in comparison with MAO coatings.

As can be observed from Fig.5b that Mg,O and C are detected in PEC surface modification layers,which indicates that the C element really had diffused into pure magnesium substrate.Meanwhile,according to the distribution state of C as well as O,PEC surface modification layers can be divided into an oxide layer as well as a diffusion layer.It is evident that the O element predominantly focuses on in the oxide layer,whereas the C element predominantly concentrates on the diffusion layer.Obviously,there is a carbon peak in the outer region of the diffusion layer,but the content of C gradually reduces in the inner zone of the diffusion layer,which is primarily attributed to the temperature of the inner part of the coated sample is relatively low resulting in the C diffusion is gradually inhibited.

Fig.5d shows that MAO coatings consist of Mg,O,Si and F,demonstrating that both sodium silicate and potassium fluoride in the electrolyte had participated in the coating formation reactions during MAO process.From the EDS line scan it appears that the concentrations of O and Si in the outer part of MAO coatings are higher than those in the inner part.Conversely,the F concentration increases in the inner part of MAO coatings,namely the coating/substrate interface is enriched with F.The reason is that the radius of F-,O2-,OH-as well as SiO32-ions are 0.126,0.141,0.152 and 0.195 nm,respectively[33].Generally,the smaller the radius is,the easier the movement is,and the higher the migration rate is.Compared with O2-,OH-and SiO32-ions,the Fions exhibit the smallest radius.Consequently,the F-ions show more competitive advantage and can quickly migrate into the discharge channel and preferentially combine with Mg2+ions to form MgF2deposited in the inner part of MAO coatings.In other words,F will take up more regions in the inner part of MAO coatings resulting in more O and Si have to distribute in the outer part of MAO coatings.

As for the PEC+MAO hybrid coatings,the elements of Mg,C,O,Si as well as F are detected in the cross-section of the coatings,as can be confirmed from Fig.5f.Evidently,the distribution state of O,Si as well as F in the PEC+MAO hybrid coatings is similar to those exhibited in MAO coatings,as verified by Fig.5d.However,the distribution characteristics of element C in the PEC+MAO hybrid coatings are slightly different from those in PEC surface modification layers.As can be seen from Fig.5b,the content of C is relatively higher in the outer region of the diffusion layer,while the content of C in the inner part of the diffusion layer shows a downward trend,and the decrement is relatively large.As exhibited in Fig.5f,it is obvious that the element of C distributes more evenly in the whole PEC+MAO hybrid coatings than that in Fig.5b,but an upward trend of C element distribution can be observed in the inner part of the hybrid coatings as well.The distribution differences of the C content between PEC surface modification layers and PEC+MAO hybrid coatings can be attributed to the PEC+MAO hybrid coatings are fabricated by MAO technique based on PEC surface modification layers,namely the C diffusion can be further enhanced under the condition of local high temperature in plasma discharge regions and the treatment duration during MAO process,which is helpful to generate more excellent SiC phase(Fig.6c)to improve the anti-corrosion property of the PEC+MAO hybrid coatings.

3.3.Phase compositions

The XRD detection results of the coatings prepared by different processes are shown in Fig.6.It can be seen from Fig.6a that MgO,MgC2and Mg2C3phases are detected in PEC surface modification layers.The appearance of MgC2and Mg2C3phases once again confirms that C element certainly had penetrated into pure magnesium.Moreover,it should be noted that MgO is also formed on the surface of pure magnesium(cathode).In general,it is scarce that oxide compounds or oxide film are generated on the surface of cathode in the process of PES,however,this type of special situation is an objective fact.Taheri et al.[34]prepared modified layers on pure titanium surface by cathode PEC/N technique.The XRD result demonstrates that titanium oxide(TiO2)is formed in the PEC/N surface modification layers.Inour previous work,Sun et al.[35]had researched the cathode PEN/C of AZ91D magnesium alloys.It is revealed that MgO as well as Al2O3are generated in the PEN/C surface modification layers,meanwhile,an oxide film was formed in the outer part of PEN/C surface modification layers.In the case of the ferrous metals,a considerable amount of researchers[9–13]had reported that some oxide compounds(e.g.FeO,Fe2O3,Fe3O4)can be detected in the surface modification layers after the specimens were processed by cathode PEN/C treatment,particularly,an oxide film composed of Fe3O4was formed on 45# steel surface[11].

Fig.5.Cross-section morphologies as well as the corresponding EDS line scan of the(a,b)PEC,(c,d)MAO,and(e,f)PEC+MAO coatings.

Hence,according to the analysis of the above literatures,the formation mechanism of MgO on the surface of cathode in this investigation can be interpreted as follows.During PEC pretreatment,Mg could react with water vapor to generate Mg(OH)2(Eq.(1))under the effect of local high-temperature in plasma vapor.Subsequently,Mg(OH)2will quickly dehydrate to obtain MgO(Eq.(2)).In this case,the oxide layer will be gradually generated on the surface of cathode(pure magnesium)with increasing plasma discharge time.

As shown in Figs.6b and c,MgO,MgF2and Mg2SiO4phases are formed both in MAO coatings as well as the PEC+MAO hybrid coatings.In the case of MAO treatment,the dissolution of anode(pure magnesium)will generate Mg2+ions shown in Eq.(3).And then under the effect of the strong electric field,Mg2+ions will fast migrate into the discharge channel.Subsequently,these Mg2+ions will react with F-,O2-,OH-and SiO32-ions to generate MgO,MgF2and Mg2SiO4phases depicted in Eqs.(5)-(10)[22].

Fig.6.XRD detection results of the(a)PEC,(b)MAO,as well as(c)PEC+MAO coatings.

Moreover,some compounds such as SiC,MgC2as well as Mg2C3are detected in the PEC+MAO hybrid coatings,too.After PEC preprocessing,Mg2C3as well as MgC2are generated in the surface modification layers.While during MAO treatment,a part of Mg2C3and MgC2phases with chemical instability would disintegrate into activated carbon atoms under the effect of violent plasma discharge.Subsequently,these activated carbon atoms would combine with SiO32-ions to form SiC phase shown in Eqs.(11)and(12).

3.4.Anti-corrosion property

3.4.1.Potentiodynamic polarization curves

Fig.7 presents the potentiodynamic polarization curves of the coatings prepared by different processes in 3.5 wt.% neutral NaCl solution.The fitting results of the potentiodynamic polarization curves are shown in Table 2.Generally,the corrosion potential(Ecorr)and the corrosion current density(Jcorr)are used to evaluate the anti-corrosion properties of the coatings.Meanwhile,the higher corrosion potential and the lower corrosion current density demonstrates superior corrosion resistance.

As can be observed from Fig.7 and Table 2,theEcorrof pure magnesium substrate and the coatings fabricated by different processes are obtained to be in the following order:the PEC+MAO hybrid coatings(-1.60 V)＞MAO coatings(-1.65 V)＞PEC surface modification layers(-1.67 V)＞pure magnesium substrate(-1.69 V).Meanwhile,theJcorrcan be ranked as following:the PEC+MAO hybrid coatings(0.207 μA cm-2)＜MAO coatings(1.77 μA cm-2)＜PEC surface modification layers(85.4 μA cm-2)＜pure magnesium substrate(128 μA cm-2).These results reveal that the three types of protective films can enhance the anti-corrosion property of pure magnesium substrate.Particularly,the PEC+MAO hybrid coatings show more excellent anticorrosion performance as compared to single PEC surface modification layers as well as single MAO coatings.

Table 2Fitting results of potentiodynamic polarization curves exhibited in Fig.7.

Table 3Fitted data of EIS plots of pure magnesium substrate after 0.5 h of immersion.

Fig.7.Potentiodynamic polarization curves of substrate as well as the coatings prepared by different processes in 3.5 wt.% neutral NaCl solution.(For interpretation of the references to colour in this figure legend,the reader is referred to the web version of this article.)

3.4.2.Electrochemical impedance spectroscopy

Fig.8 shows the EIS plots of PEC surface modification layers after immersion with different duration.The fitted data are presented in Table 4.Fig.9 exhibits the corresponding equivalent circuits,among whichRsrepresents the resistance of solution,Rosignifies the oxide film resistance in parallel with constant phase elementQo,Rdrepresents the resistance of diffusion layer paralleled withQd,RLis the inductance resistance in series with the inductanceL,Rfis the resistance of the passive film generated on pure magnesium substrate exposed to the corrosive media paralleled withQf,Cdlis the double-layer capacitance,as well asRtis the resistance of the charge transfer.Additionally,for comparison,the EIS plots of pure magnesium substrate after 0.5 h of exposure are exhibited in Figs.8,10 and 12,too.And the corresponding fitted results of them are listed in Table 3.

As for the EIS,the capacitive loop radius as well as the impedance modulus are usually used to evaluate the anticorrosion property of the coatings.Meanwhile,the larger capacitive loop radius and the higher impedance modulus indicates excellent anti-corrosion property.

According to Fig.8 and Table 4,it is obvious that both the capacitive loop radius and the impedance modulus of PEC surface modification layers are relatively high after 0.5 h of exposure.This indicates that PEC surface modification layers exhibit superior protective behavior during the initial immersion process,which is consistent with the results of polarization curves.From 2 to 5 h,the capacitive loop radius and the impedance modulus decrease gradually,demonstrating that PEC surface modification layers deteriorate gradually with the penetration of corrosive media.In the subsequent immersion duration(10-20 h),PEC surface modification layers are found to deteriorate drastically and the substrate is gradually corroded with the invasion of aggressive ions,which can be verified by the low-frequency phase angle shows positive value(Fig.8c)and the low-frequency inductive loop could be observed from the corresponding Nyquist diagram(Fig.8a).And with the increase of testing duration up to 44 h,PEC surface modification layers have absolutely failed due to the capacitive loop radius as well as the impedance modulus of PEC surface modification layers decrease fast and are lower in comparison with the substrate during this immersion stage.

Fig.10 illustrates the EIS diagrams of MAO coatings after immersion with different duration.Analyses of these EIS diagrams indicate that two types of different equivalent circuit models should be required(Fig.11).Meanwhile,the fitted values are given in Table 5.Note also thatRmsignifies the resistance of MAO coatings in parallel with constant phase elementQm,Rirepresents the resistance of coating/substrate interface paralleled withQi.

As seen in Fig.10 and Table 5,MAO coatings show a large capacitive loop and high impedance modulus after 0.5 h of immersion,indicating that MAO coatings exhibit excellent protective function at the initial stage of immersion.From 0.5 to 2 h,both the capacitive loop radius and the impedance modulus of MAO coatings decrease gradually,which suggests that the corrosive medium(e.g.Cl-)has attacked MAO coatings through the microscopic defects,resulting in the corrosion resistance of MAO coatings has gradually got worse.With the increase of testing duration from 2 to 20 h,the capacitive loop radius and the impedance modulus decrease still further,demonstrating that the deterioration of MAO coatingsis also enhanced during this immersion stage.After 44 h of exposure,the low-frequency inductive loop could be found from the Nyquist diagram(Fig.10a),which implies that the corrosive medium have already infiltrated into the coating/substrate interface and gradually corroded the substrate.But it appears that after 68 h of immersion,the capacitive loop radius and the impedance modulus of MAO coatings are still larger as compared to the substrate,which means that MAO coatings have not absolutely failed at this instance of time.

Fig.8.Experimental and fitted(a)Nyquist together with(b and c)Bode plots of substrate immersed for 0.5 h as well as PEC surface modification layers after immersion with different duration.

Fig.9.Equivalent circuits of PEC surface modification layers after immersion with different time:(a)0.5?5 h,(b)10?20 h,as well as(c)44?68 h.

Fig.12 shows the EIS plots of the PEC+MAO hybrid coatings after immersion with different duration.The corresponding equivalent circuit as well as the fitted data are presented in Fig.13 and Table 6,respectively.Notably,Rhis the PEC+MAO hybrid coatings resistance in parallel with constant phase elementQh,Rirepresents the resistance of theinterface between the PEC+MAO hybrid coatings and substrate paralleled withQi.

Table 4Fitted data of EIS plots of PEC surface modification layers after immersion with different duration.

Table 5Fitted data of EIS plots of MAO coatings after immersion with different duration.

Table 6Fitted data of EIS plots of the PEC+MAO hybrid coatings after immersion with different duration.

Fig.10.Experimental and fitted(a)Nyquist together with(b and c)Bode plots of substrate immersed for 0.5 h as well as MAO coatings after immersion with different duration.

Fig.11.Equivalent circuits of MAO coatings after immersion with different duration:(a)0.5?20 h,as well as(b)44–68 h.

Fig.12.Experimental and fitted(a)Nyquist together with(b and c)Bode plots of substrate immersed for 0.5 h as well as the PEC+MAO hybrid coatings after immersion with different time.

Fig.13.Equivalent circuit of the PEC+MAO hybrid coatings after immersion with different time.

Although the capacitive loop radius and the impedance modulus of the PEC+MAO hybrid coatings decrease gradually with the increase of immersion time from 0.5 to 68 h,the fluctuation of the data is quite slight(Fig.12 and Table 6),demonstrating that the PEC+MAO hybrid coatings are more stable than both single PEC surface modification layers as well as single MAO coatings.In particular,the PEC+MAO hybrid coatings exhibit weak corrosion tendency along all the immersion duration,which can be confirmed by the lowfrequency inductive loop is not be found from the Nyquist diagram even after 68 h of exposure(Fig.12a).

Analyses of the above EIS results indicate that PEC surface modification layers could offer short-term corrosion resistance protection during the whole testing duration.MAO coatings exhibit relatively higher protective behavior as compared to PEC surface modification layers.Whereas the PEC+MAO hybrid coatings show weak corrosion tendency and could offer relatively superior long-range anti-corrosion inhibiting effect.Consequently,compared with both single PEC surface modification layers as well as single MAO coatings,the PEC+MAO hybrid coatings possess higher protective function,which is also consistent with the conclusion drawn from the potentiodynamic polarization curves.

4.Discussion

4.1.Formation mechanisms of PEC surface modification layers as well as the PEC+MAO hybrid coatings

According to the adsorption mode of the activated carbon atoms,the plasma electrolytic carburizing(PEC)process on pure magnesium mainly experience two stages:physical adsorption as well as chemical adsorption.During PEC preprocessing,the treatment temperature adjacent to the cathode gradually rise with the increase of the applied voltage.This scenario the local-boiling glycerol(C3H8O3)at near-cathode region will fast decompose to generate a continuous vapor envelope which contains CH4as well as CO depicted in Eq.(13)[5].With the increase of the voltage up to the critical breakdown voltage,the vapor envelope will be broken down,resulting in the plasma discharge and the liberation of the activated carbon atoms appears on the surface of cathode Eqs.(14)-((15)).After that,these activated atoms will agglomerate on the surface of pure magnesium(cathode)by physical adsorption and then they will penetrate into the magnesium lattice via interstitial diffusion under the influence of the local high temperature condition.Subsequently,under the effect of chemical adsorption,MgC2and Mg2C3are gradually generated shown in Eqs.(16)-(17),meanwhile,two kinds of supersaturated solid solution as well as compounds can possibly exist.In this case,the diffusion layer will be gradually generated with increasing plasma discharge time,which is presented in Fig.14.

Fig.14.Formation mechanism of PEC surface modification layers.

Fig.15 illustrates the formation process of the PEC+MAO hybrid coatings.Evidently,after PEC pretreatment,the surface modification layers are generated on pure magnesium and the PEC surface modification layers consist mainly of an oxide layer as well as a diffusion layer.After that,the PEC coated samples would be processed by MAO treatment to fabricate hybrid coatings.And literature[36]has reported that MAO coatings could grow inward as well as outward concurrently.Under this circumstances,the MAO coatings grown inward would overlap with the diffusion layer,whereas the MAO coatings grown outward would absolutely replaced the very thin outer oxide layer generated during PEC pretreatment process.Hence,in the preparation process of the PEC+MAO hybrid coatings,an overlapping phenomenon in regard to coating thickness instead of heaping up layer by layer could be observed due to the PEC+MAO hybrid coatings were fabricated by MAO approach based on PEC surface modification layers.

Fig.15.Formation process of the PEC+MAO hybrid coatings.

4.2.Effects of PEC pretreatment on the PEC+MAO hybrid coatings

Based on the above analysis,PEC preprocessing could offer an excellent microstructure to increase the coating thickness for the PEC+MAO hybrid coatings followed.Meanwhile,during PEC preprocessing,owing to the radius of the activated carbon atoms is relatively smaller in comparison with magnesium atoms,the activated carbon atoms would infiltrate into the magnesium lattice to generate supersaturated solid solution as well as compounds of MgC2together with Mg2C3exhibited in Fig.6a.And the lattice deformation of pure magnesium induced from the formation process of supersaturated solid solution,which could improve the compactness of the following PEC+MAO hybrid coatings,as verified by(Figs.2–5).Additionally,PEC process also could provide a favorable pretreatment surface with predesigned chemical composition to obtain a new kind of phase,namely SiC with superior anti-corrosion property and chemical stability,in the following PEC+MAO hybrid coatings.

Consequently,PEC preprocessing would exert a significant effect on the comprehensive characteristics of the PEC+MAO hybrid coatings,such as thickness,compactness as well as composition.

4.3.Analyses of the anti-corrosion property

According to the EIS results,it is evident that the corrosion processes of the three kinds of protective films are different.The deterioration of PEC surface modification layers predominantly experience three stages:the corrosive medium gradually penetrates into PEC surface modification layers,then corrodes the substrate,and finally PEC surface modification layers absolutely fail.As for MAO coatings,the corrosion process can be subdivided into two stages:the corrosive medium gradually penetrates into MAO coatings,and then down to the coating/substrate interface,eroding the substrate,but the MAO coatings have not completely failed in the second stage.Whereas the PEC+MAO hybrid coatings exhibit weak corrosion tendency as well as superior anti-corrosion performance along all the immersion duration.

Literature[37]has indicated that the anti-corrosion property of the coatings is influenced by a great many of microcosmic characteristic variables including the thickness,compactness(surface porosity,micro-pore size and morphology),chemical composition as well as phase composition,and the defects(large-sized micro-pores and micro-cracks)of the coatings.

In the case of this study,as exhibited in Fig.5,the PEC+MAO hybrid coatings posses higher thickness as compared to both single PEC surface modification layers as well as single MAO coatings.Apparently,the higher coating thickness can act as an excellent physical barrier to inhibit the penetration of the corrosive medium.And as shown in(Figs.2–5),the PEC+MAO hybrid coatings are more compact than single MAO coatings.The dense coatings can effectively hinder the aggressive Cl-ions from attacking the substrate due to the corrosion paths are hard to form in the compact coatings.At the meantime,a new SiC phase with superior anti-corrosion performance and chemical stability is formed in the PEC+MAO hybrid coatings,which could be confirmed from Fig.6(c).Furthermore,after PEC pretreatment,the anti-corrosion property of the surface modification layers have been improved,whereas the PEC+MAO hybrid coatings were fabricated by MAO approach based on PEC surface modification layers.That is to say,the appearance of PEC surface modification layers will be beneficial to further enhance the anti-corrosion property of the PEC+MAO hybrid coatings.Consequently,the anti-corrosion performance of the PEC+MAO hybrid coatings is the most excellent among the three types of protective films under the synergetic effect among the above microcosmic characteristic variables.

5.Conclusions

1)Novel hybrid coatings are fabricated successfully by combining PEC with MAO on pure magnesium to further enhance the anti-corrosion property.

2)PEC pretreatment can afford an excellent microstructure to increase the coating thickness and to improve the compactness of the PEC+MAO hybrid coatings.Meanwhile,it also can provide a favorable pretreatment surface with predesigned chemical composition in order to obtain a new kind of phase,namely SiC with superior performances for the PEC+MAO hybrid coatings followed.

3)Compared to both single PEC surface modification layers as well as single MAO coatings,the PEC+MAO hybrid coatings show more excellent anti-corrosion property.In particular,the PEC+MAO hybrid coatings can act as an effective protection system to offer relatively superior long-term anti-corrosion inhibiting effect.

Prime novelty statement

?A novel surface modification technique was developed to enhance corrosion resistance of pure magnesium,namely hybrid coatings were fabricated on pure magnesium by combining plasma electrolytic carburizing(PEC)with micro-arc oxidation(MAO).

?PEC pretreatment can offer an excellent micro-structure to increase the thickness and compactness of PEC+MAO hybrid coatings.Meanwhile,it also can present a favorable pretreatment surface with predesigned chemical composition to obtain a newly born phase,such as SiC with superior performances,in the following PEC+MAO hybrid coatings.

?PEC+MAO hybrid coatings can further improve anticorrosion performance of pure magnesium.Particularly,it can act as an effective protection system to provide relatively superior long-term anti-corrosion inhibiting effect.

Acknowledgements

This work was financially supported by the Creative Research Group Fund Grant of Gansu Province,China(1111RJDA011).