Gen Zhang ,Liang Wu ,Maria Serdehnova ,Aitao Tang ,Cheng Wang ,Carsten Blawert ,Fusheng Pan,Mikhail L.Zheludkevih,e

aNuclear Power Institute of China,Chengdu,Sichuan 610213,China

bCollege of Materials Science and Engineering,Chongqing University,Chongqing 400044,China

c Institute of Surface Science,Helmholtz-Zentrum Hereon,Max-Planck Str.1,Geesthacht 21502,Germany

d National Engineering Research Center for Magnesium Alloys,Chongqing University,Chongqing 400044,China

e Institute for Materials Science,Faculty of Engineering,University of Kiel,Kaiserstra?e 2,Kiel 24143,Germany

Abstract In this work,plasma electrolytic oxidation (PEO) coatings were produced on magnesium alloy AZ31 in aluminate,silicate and phosphatebased electrolytes,and followed by hydrothermal treatments in order to synthesis layered double hydroxides (LDHs) based nanocontainers.LDHs synthesis was done in three different growth solutions (deionized water,sodium nitrate and aluminum nitrate containing solution).In frame of this work it was shown,that it was difficult to form LDHs on Si-based PEO coating,due to more stable silicate phases in comparison with aluminate and phosphate phases in respective PEO coatings.The obtained hybrid LDH/PEO coatings were characterized using SEM,EDS and GDOES,and then the corrosion protection was further investigated by EIS.Based on the obtained results,it was confirmed that,the hydrothermal treatments in Al3+ containing solution played an important role on overall corrosion resistance for phosphate and silicate-based PEO coatings,but not for Al-based PEO coatings.

Keywords: Layered double hydroxides;Plasma electrolytic oxidation;Corrosion resistance;Magnesium alloys.

1.Introduction

Magnesium (Mg) alloys as the lightest metallic structural materials have been widely used in many engineering applications.However,they are highly reactive and have relatively poor corrosion resistance [1–3].Although this characteristic makes Mg alloys used as degradable biomaterial,it limits their applications in fact at a larger scale in engineering [4].Therefore,various methods have been proposed to prevent Mg alloys from corrosion so far.Examples,including,purification[5,6],homogenization of microstructure [7–10]and alloying of Mg (e.g.Al,Mn,Ca and rare earth) [11–13],are metallurgical methods to improve the corrosion resistance of Mgbased alloys.The other strategy is the formation of protective coatings on them,which is a widespread and cost-effective approach to enhance protection for Mg substrate against general and localized corrosion [14–16].

In recent years,active protection functionality,together with traditional conventional barrier coatings,has been one of the most interesting research topics in the field of corrosion protection.Such “smart” coatings are capable to heal minor damage or defects during service,without any external intervention[17,18].For examples,sol-gel-based CeO2/phytic acid composite coating [19],halloysite nanotubes doped plasma electrolytic oxidation(PEO)coating[20,21],Ca/P doped PEO coating [22],etc.In addition,preparation of layered double hydroxides (LDHs) on traditional anodic coatings or PEO coatings has been widely investigated to enhance active corrosion protection of aluminum (Al) [16–28]or magnesium(Mg) alloys [29–32].The combination has several advantages over other methods:

(i) The pores and micro-cracks of anodic or PEO coatings are sealed,resulting in an barrier enhancement of corrosion behavior of underlying substrate.

(ii) Preparation of LDHs-based nanocontainers on anodic or PEO coatings is beneficial for further encapsulation of corrosion inhibitors.

(iii) In comparison with directly loading of corrosion inhibitors into pores of anodic or PEO coatings,triggered release can be achieved using LDHs encapsulation strategy.Thus,it may provide a long-term self-healing effect and corrosion protection.

(iv) One can expect better adhesion to the substrate,as a result of anodic/PEO coating use as the internal sources of cations for LDHs preparation.

In our previous works,we had proposed a novel and easy way to prepare LDHs on anodic oxide film of Mg alloys via conversion treatment [33,34],and further discussed the mechanism of LDHs coating growth behaviors via two-dimensional layer (2D) to three-dimensional (3D)growth model [35].As a result,the effect of PEO coatings,e.g.,composition,structure and morphology,on the LDHs growth was clarified [36].It was found that LDHs growth was strongly determined by the dissolution of the PEO coatings.In particular,due to higher stability of spinel phase(MgAl2O4),the number of LDHs flakes on the surface decreased significantly.After that,we had prepared a composite coating consisting of LDHs/Mg(OH)2/CeO2/Ce(OH)3with a non-uniform Ce distribution using PEO treatments,via hydrothermal post-treatments.We obtained an enhanced corrosion protection,and finally confirmed the synergistic effect between Ce species and phosphate loaded LDHs for active corrosion protection [37].However,all these studies were done using anodic oxide films or PEO coatings formed in aluminate electrolyte since Mg2+and Al3+were supposed to be more accessible and allowed to prepare MgAl-LDHs on them more easily.And,in case of phosphate or silicate-based PEO coatings,LDHs growth behaviors are still completely unclear.Toulabifard et al.[38]systematically studied preparation PEO coatings on AZ31 in aluminate,phosphate,and silicate-based electrolytes,but without further post treatment for PEO coatings in their study.

The most reasonable approach in such a case would be a systematic study of the three main different PEO coatings classes followed by further LDHs based modification.In addition,it is worth to mention,that suitable solutions containing M2+cation (such as,Zn2+,Mg2+or Ni2+) as post-treatment growth solution to synthesize LDHs on PEO layers for Al alloys [23–28]are required.For Mg as substrate,M3+cations(such as,Al3+,Cr3+or Fe3+) containing solutions were required for LDHs preparation on Mg [29,30,39].In our work we have found that MgAl-LDHs could simply form using water as post-treatment growth solution [40].Therefore,the effect of growth solutions on LDHs formation on different PEO layers also needs to be considered.

Thus,in this work the growth behavior of MgAl-LDHs on aluminate-,silicate-and phosphate-based PEO coatings under consideration of the effect of the growth solution (deionized water,sodium nitrate and aluminum nitrate solution)was studied respectively by means of SEM,EDS,XRD and GDOES.The corrosion performance of resulting coatings was determined by EIS and discussed.

2.Experimental methods

2.1.Materials and reagents

Mg alloy AZ31 with a nominal composition of Al 2.5–3.5%,Zn 0.6–1.3%,Mn 0.2–1%,Ca 0.04%,Si 0.1%,Cu 0.05% (all in wt.%),and balance Mg was used.The chemicals used in this study were: sodium aluminate(NaAlO2,>99%,Sigma-Aldrich,Germany),sodium phosphate(Na3PO4,>96%,Sigma-Aldrich,Germany),sodium silicate (Na2SiO3,25.5–28.5% SiO2,Sigma-Aldrich,Germany),sodium hydroxide (NaOH,>98%,Carl Roth GmbH,Germany),sodium nitrate (NaNO3,>99%,Honeywell GmbH,Germany) and aluminum nitrate (Al(NO3)3·9H2O,>98%,ThermoFisher GmbH,Germany).Deionized water was used as the solvent.

2.2.Specimens preparation

Prior to PEO treatment,specimens of AZ31 alloy with dimension of 20 mm × 20 mm × 5 mm were ground on all surfaces up to 1200 grit using SiC papers,and then dried with warm air.After that,three different classes of PEO specimens were prepared in aluminate-,phosphate-and silicate-based electrolytes using a pulsed DC power supply.In detail,the compositions of the three different electrolytes were:

All PEO treatments were conducted at a constant voltage of 350 V for 600 s withton:toff=2 ms: 8 ms,the electrolyte was continuously stirred during the PEO treatment and kept at 20±5°C by a water cooling system.The prepared specimens were rinsed in deionized water and dried in warm air.The obtained PEO specimens in the three different electrolytes were designated as Al/PEO,P/PEO and Si/PEO,respectively.

Subsequently,Al/PEO,P/PEO and Si/PEO specimens were hydrothermally treated in three different solutions in a Teflonlined stainless steel autoclave at 398 K for 16 h in order to investigate the influence of conversion solution on LDHs formation.These three solutions were: (i) deionized water;(ii) 0.1 M NaNO3aqueous solution and (iii) 0.1 M Al(NO3)3aqueous solution with pH=10.5 (adjusted by NaOH).For clarity of discussion,the specimens obtained at this stage are called as:

(a) Al/PEO-W,P/PEO-W and Si/PEO-W (in water),

(b) Al/PEO-N,P/PEO-N and Si/PEO-N (in NaNO3) and

(c) Al/PEO-Al,P/PEO-Al and Si/PEO-Al (in Al(NO3)3),respectively.

The whole preparation process is illustrated in Scheme 1.

2.3.Characterization

Scanning electron microscopy (SEM;Tescan Vega3,Czech) equipped with an EDS detector was used to study the surface and cross-sectional morphologies as well as the chemical compositions of the specimens.The crystal structure and phase composition were characterized using X-ray diffraction measurement (XRD;Bruker AXS D8 ADVANCE,Germany)using a Cu Kαradiation (40 kV,40 mA) at a glancing angle of 3°,in the range of 2θfrom 5 to 80° with 0.02° step and at a scanning rate of 1 s/ step.Depth profile analysis was carried out using glow discharge optical emission spectroscopy(GDOES;HORIBA GD-Profiler 2,France) with an anode of 4 mm in diameter.

Electrochemical impedance spectra (EIS) were measured using Gamry Interface 1000 potentiostat with a conventional three-electrode set-up,consisting of a saturated Ag/AgCl reference electrode,a platinum counter electrode and the coated Mg substrate as the working electrode with an exposed area of 1 cm2.The EIS measurements were performed in 3.5 wt.%NaCl solution after different immersion times with a sine signal with an amplitude of 10 mV RMS.The range of measured frequencies extended from 100 kHz to 10 mHz,with a logarithmic sweep of 7 points per decade.The experimental impedance plots were fitted by using equivalent circuits by means of the Zview software.

3.Results

3.1.Morphology,composition and structure of PEO coatings

Fig.2 shows the differences in surface morphology of the three different PEO coatings.As for Si/PEO,it revealed a smaller number but larger-size pores in its surface,while Al/PEO and P/PEO revealed a higher number of porosity but smaller size pores.These differences in surface morphology were related to the different discharge characteristic during PEO processing,which are strongly influenced by the composition of electrolyte [41].Apart from electrolyte composition,temperature of electrolyte [42],its concentration [43],applied energy (voltage and current density) [44,45],additives (such as TiO2,CeO2) [46,47]and secondary phases of the substrate alloy[26,48]also play important roles for the morphologies of PEO coating (dimensions and distribution of pores),and thus contributing to different corrosion resistance.In the case of P/PEO one can observe some micro-cracks,which are likely attributed to the thermal stresses as a result of local melting and solidification of the hard ceramic compounds during formation of PEO coating [36].

XRD patterns of Al/PEO,P/PEO and Si/PEO are shown in Fig.3.In addition to the peaks corresponding Mg and MgO,a broad hump peak marked by blue rectangle in Fig.3 was observed,which indicated that amorphous phases (or nanocrystalline),likely silicate-based sodium or/and magnesium,existed in Si/PEO.For P/PEO specimen,no peak associated with crystalline phosphorus appeared in the XRD pattern as well and MgO was the main phase.Similar to Si/PEO,a broad hump peak,which was also marked (yellow rectangle) in Fig.3,is visible.As reported by previous studies [49,50],this amorphous(or nano-crystal)phase was likely Mg3(PO4)2.For Al/PEO specimen,the only detected crystalline phase was associated with MgO.It’s remarkable that the intensity of MgO peaks in the phosphate solution was significantly stronger than in the others.This observation is in a good agreement with previously published results [51].

Fig.3.XRD patterns of different PEO specimens formed in different electrolytes.

Fig.4 shows the cross-sectional SEM images of Al/PEO,P/PEO and Si/PEO specimens.It can be seen that the thickness of all of the PEO layers was relatively uniform,however,pores in PEO layer could be observed clearly for Al/PEO and P/PEO specimens from Fig.4a and b.In contrast,Si/PEO had a relatively compact and uniform PEO layer.In addition,the thickness of all three PEO coatings was very similar(5.0 ± 0.4 μm for Al/PEO,5.3 ± 0.6 μm for P/PEO and 4.5 ± 0.5 μm for Si/PEO).Yoichi et al.[41]suggested that the growth rate of PEO coatings was remarkably higher in phosphate electrolytes compared to silicate electrolytes.However,there is no larger differences in thickness for P/PEO and Si/PEO in our study.A wavy interface with low defect levels and no fracture sites in the interface of coating and substrate indicated a good adhesion between PEO coating and Mg substrate.

Fig.4.Cross-sectional SEM micrographs of (a) Al/PEO;(b) P/PEO and (c) Si/PEO.

In order to estimate the elemental distribution in the PEO layer,all the coatings were divided into three zones (layers):outer layer (OL),middle layer (ML) and inner layer (IL).Four different points in each layer were analyzed via point EDS analysis and results are shown in Table 1.It can be seen,that Al for Al/PEO,P for P/PEO and Si for Si/PEO are present throughout the oxide thickness almost down to the interface between substrate and oxide.This observation can be explained via coating growth,which occurs mostly in the interface between oxide and solution.The content of Al,P and Si decreased from outer layer to middle layer and to inner layer.As suggested Khaselev and Yahalom [52],such a high concentration of Al,P and Si in the outermost part of the coating were due to adsorption of respective anions on the film surface.Moreover,our previous works [36]and Khaselev’s works [53]found that Al content in the coating increased with an applied voltage,applied current density and Al content in the electrolyte,likely same as phosphate electrolyte and silicate electrolyte.It is important to mention,that Al is also present in P/PEO and Si/PEO specimens,confirming,that not only the electrolyte but also AZ31 alloy substrate works as a source of aluminum.However,the Al content in P/PEO and Si/PEO was significantly lower as expected.

Table 1Elemental compositions and Al/Mg ratio from different positions in cross section of different PEO specimens.

Table 2Fitted parameters for EIS spectra after 3 h immersion.

Table 3Fitted parameters for EIS spectra after 168 h immersion.

3.2.Corrosion resistance of PEO coatings

The EIS was employed for the evaluation of corrosion resistance of PEO coatings formed in the three different electrolytes.EIS plots of Al/PEO,P/PEO and Si/PEO specimens,shown in Fig.5,were recorded after 3 h,6 h,24 h and 48 h immersion in 3.5 wt.% NaCl solution.In contrast P/PEO and Si/PEO systems,the initial impedance of Al/PEO was significantly lower at lower frequency part and drops further with longer immersion time.However,for P/PEO,the impedance obviously increased at initial immersion stage (3 h to 6 h)and reached a maximum (|Z|0.01 Hz=8.11 × 104Ωcm2) after ca.6 h immersion,followed by the decrease.This increase in impedance is likely due to the amorphous phosphorus components (most likely Mg3(PO4)2),which dissolve during the immersion leading to spontaneous integration of dissolved Mg2+into newly formed stable coating.Si/PEO specimen has shown highest low frequency impedance and the better stability,when the immersion time was less than 6 h Nevertheless,a rapid decrease in the low frequency impedance was evident after prolonging to 24 h of immersion.Furthermore,impedances modules of Si/PEO after 24 h and 48 h immersion were lower in comparison with P/PEO system after 48 h of immersion.

Fig.5.Bode and Nyquist plots of (a) Al/PEO;(b) P/PEO and (c) Si/PEO after 3 h,6 h,24 h and 48 h immersion in 3.5 wt.% NaCl solution.

3.3.Composition,structure and morphology of PEO coatings modified with LDHs

XRD patterns of (a) Al/PEO-W;(b) Al/PEO-N;(c)Al/PEO-Al;(d) P/PEO-W;(e) P/PEO-N;(f) P/PEO-Al;(g)Si/PEO-W;(h) Si/PEO-N and (i) Si/PEO-Al are shown in Fig.6.The characteristic diffraction peaks of LDHs located at about 11.3° and 22.2°,indicating that their interlayers are loaded with hydroxides [30].In most of the cases,it is beneficial to get nitrate pillared LDHs,because nitrate exchange is easier,and thus nitrate pillared LDHs are usually used as precursors for ion exchange [54,55].Unexpectedly,for all PEO specimens regardless in which solution (NaNO3,Al(NO3)3or water) only hydroxide containing LDHs were formed.After hydrothermal treatment,peaks corresponding to Mg(OH)2appeared for all specimens.As for Al-based PEO coatings,the hydrothermal treatment in deionized water has resulted in not only high content of MgAl-LDHs,but also high content of Mg(OH)2(see Fig.6).Moreover,the addition of Al3+cations into growth solution did not lead to an increase of MgAl-LDHs content as expected (even a slight decrease of peak intensity for Si/PEO specimens was observed,compared with that in NaNO3).As for P/PEO coating,MgAl-LDHs content of growing in NaNO3or Al(NO3)3solutions apparently higher than that of growing in deionized water.In contrast to the other two coatings,it is evident that the growth of MgAl-LDHs on Si-based PEO coating was more difficult and only the small XRD peaks of LDHs were observed.

Fig.6.XRD patterns of (a) Al/PEO-W;(b) Al/PEO-N;(c) Al/PEO-Al;(d)P/PEO-W;(e) P/PEO-N;(f) P/PEO-Al;(g) Si/PEO-W;(h) Si/PEO-N and (i)Si/PEO-Al.

Fig.7 shows the surface morphologies of the prepared LDHs flakes on the different PEO coatings in the different growth solutions.The hydrothermal treatment in deionized water reveal typical flake-like LDHs structure on the surface of Al/PEO-W and the size of the hexagonal flakes was relatively large (ca.4 μm).In addition,its surface morphology shows a dramatic change and the irregular pores of the PEO coating are completely disappeared.For P/PEO-W and Si/PEO-W systems,the typical PEO pores remain and are still visible in the respective coatings.In P/PEO-W system,even original micro-cracks are still visible.These results are in a good agreement with XRD results,where the diffraction peaks associated with LDHs were very low.The hydrothermal treatment in NaNO3solution,there were flakelike LDHs in the surface of both Al/PEO-N and P/PEO-N systems.However,only tiny nanoflakes formed in the original PEO pores on Si/PEO-N.When hydrothermal treatment was performed in Al(NO3)3solution,Al/PEO-Al and P/PEOAl systems have similar surface morphologies.The original PEO layer completely fragmented leading to the formation of island-like structure,covered by tiny nanoflakes.For Si/PEOAl system,the original PEO morphology remained intact,but PEO pores are sealed well by tiny nanoflakes.It can be explained by relatively low concentrations of metal ions and low basicity (or pH) near the interface,which leads to a low degree of supersaturation,which normally is beneficial for LDHs growth in a big flakes as a result of a slow nucleation rate [56].

Fig.7.SEM micrographs of PEO specimens after hydrothermal treatment in different growth solutions.

Fig.8.Cross-sectional SEM micrographs and EDS mappings of PEO specimens after hydrothermal treatment in deionized water.

The cross-sectional SEM images and corresponding EDS mappings of PEO specimens treated hydrothermally in deionized water,NaNO3solution and Al(NO3)3solution are shown in Figs.8,9 and 10,respectively.Whether treated in deionized water,NaNO3or Al(NO3)3solutions,all obtained specimens revealed a typical two-layer structure.Based on EDS mapping results,it can be seen that the coating mainly grow toward inwards,which were formed as a result of the dissolution of Mg substrate.This result also demonstrated that PEO layer contains pores and could not provide an effective barrier protection for underlying Mg substrate.When the hydrothermal treatment of PEO specimens is performed in Al-free electrolyte,it can be seen that the outer layer became relatively Al-rich based on EDS mapping results(marked with arrows in Fig.9).The enrichment of Al in outer layer may be explained via outwards diffusion of Al alloying element from Mg substrate.The evolution of average thickness of the coating obtained by the cross-sectional SEM micrograph is plotted in Fig.11.The thickness of outer layer of hydrothermally treated PEO specimens is almost the same as respective thickness of original PEO layers.Moreover,for Al/PEO series of hybrid coatings,the total thickness of the resulting layer depends strongly on the conversion solution used,while a constant thickness is found in the case of hydrothermal treatment of P-based and Si-based PEO coatings.That may be explained via better PEO protection performance against Mg substrate dissolution of P-based and Si-based PEO coatings compared to Al-based PEO coating (see Fig.5).The addition of either NaNO3or Al(NO3)3into the LDHs conversion treatment solution always shows an inhibitive effect on the inward growing of coating in the case of Al/PEO coating.

Fig.9.Cross-sectional SEM micrographs and EDS mappings of PEO specimens after hydrothermal treatment in NaNO3 solution.

Fig.10.Cross-sectional SEM micrographs and EDS mappings of PEO specimens after hydrothermal treatment in Al(NO3)3 solution.

Fig.11.Evolution of total coating thickness.

In order to clarify the component of two layers,the GDOES depth profiles for (a) Al/PEO-W;(b) P/PEO-W;(c)Si/PEO-W;(d) Al/PEO-N;(e) P/PEO-N;(f) Si/PEO-N;(g)Al/PEO-Al;(h) P/PEO-Al and (i) Si/PEO-Al were done.The results are shown in Fig.12.For Al-based PEO coatings hydrothermally treated in the three different growth solutions,Al signal in zone I is higher than in zone II.H,O and Mg signals can be detected both in zone I and zone II.Combined with above analyses,that indicated that the outer layer can consist of mixed original PEO layer/Mg(OH)2/ and LDHs,while inner layer consist of mixed Mg(OH)2/LDHs only.However,after hydrothermal treatment for P-based and Si-based PEO coatings,the Al signal in zone I is almost the same as in zone II and the signal is lower than the Al-based PEO (especially Al signal is very low in the Si-based PEO).This might be explained that Si/PEO has lowest porosity and it is difficult for aqueous electrolyte to reach the interface between PEO layer and substrate,preventing its dissolution.Accordingly,not enough Al ions was available for MgAl-LDHs formation.The obtained GDOES results are in nice agreement with XRD result presented before (Fig.6) and surface morphology investigations (Fig.7).Additionally,we speculated that the component of outer and inner layer of P-based and Si-based are the same as for Al/PEO,i.e.,outer layer may be consisted of PEO layer/Mg(OH)2/ LDHs mixture,while inner layer may contain only Mg(OH)2/ LDHs.

Fig.12.GDOES depth elemental profiles of (a) Al/PEO-W;(b) P/PEO-W;(c) Si/PEO-W;(d) Al/PEO-N;(e) P/PEO-N;(f) Si/PEO-N;(g) Al/PEO-Al;(h)P/PEO-Al and (i) Si/PEO-Al.

Fig.13.Bode and Nyquist plots of (a) Al/PEO-W;(b) P/PEO-W and (c) Si/PEO-W after 3 h,24 h,72 h and 168 h immersion in 3.5 wt.% NaCl solution.

Fig.14.Bode and Nyquist plots of (a) Al/PEO-N;(b) P/PEO-N and (c) Si/PEO-N after 3 h,24 h,72 h and 168 h immersion in 3.5 wt.% NaCl solution.

3.4.Corrosion resistance of PEO coatings modified with LDHs

In order to evaluate the corrosion resistance and analyze the corrosion behavior,EIS spectra of hydrothermally treated PEO specimens in deionized water,NaNO3solution and Al(NO3)3solution were measured.The results are shown in Figs.13,14 and 15,respectively.The suggested equivalent circuit,employed to fit the electrical parameters,is shown in Fig.16.For the equivalent circuit,Rsol,RoutandRinnrepresented the resistance of solution,resistance of outer layer and inner compact layer respectively.Due to the nonhomogeneity and roughness of the surface,constant phase elements (CPEs)were used herein to demonstrate the non-ideal capacitive behavior of the coated specimens [57,58].Accordingly,CPEoutandCPEinnin Fig.16 were used to describe the constant phase elements of outer layer and inner layer.Corrosion of Mg substrate occurs only when the aggressive media (such as Cl-in our case) diffuses through PEO layer and reaches to the interface between coating and substrate.This corrosion process can be characterize by the double layer capacitance of the electrolyte/substrate interface (CPEdl) and charge transfer resistance(Rct)[37].The chi-squareχ2for all fitting lines was reached less than 10-2and the fitting result is displayed in Fig.17.Tables 2 and 3 lists fitted parameters for EIS spectra after 3 h and 168 h immersion.At initial immersion stage in 3.5 wt.% NaCl solution (～3 h),the results of EIS measurements of hydrothermally treated Al-based PEO specimens in all three solutions show that the impendence value of Al/PEOW in low frequency obviously increase,in comparison with original Al/PEO specimen (Fig.5).The impendence values of Al/PEO specimens after treatment in three electrolytes can be ranked at initial immersion in increasing order as follows:in Al(NO3)3solution

Fig.16.Equivalent circuits used to fit the EIS spectra.

Fig.17.The evolution of (a) Rout;(b) Rinn and (c) Rct as a function of immersion time.

At middle and later immersion stages (24 h to 168 h),RoutandRinnvalues dramatically decreased for all three Al-based specimens after hydrothermal treatment (Fig.17),suggesting that all Al-based coatings,including both PEO coating and post-treated PEO coating,could work in corrosion protection in a short term in this case,but not for longer term.In contrast,P-based and Si-based PEO specimens hydrothermally treated in deionized water and NaNO3solution had a similar stability against corrosion (see Figs.13,14 and 17).Nevertheless,for hydrothermal treatment in Al(NO3)3solution,Si/PEO-Al shows higher stability than P/PEO-Al.In summary,in terms of overall corrosion resistance,original PEO layer plays more significant role than newly formed LDHs/Mg(OH)2layer.

4.Discussion

4.1.Corrosion behavior of PEO specimens

In the case of Al/PEO,the coating contains a small size pores,but in a large amount.From cross sectional image,the whole coating has a uniform thickness with defects.When Al/PEO specimen was immersed in Cl-containing solution,aggressive Cl-ions could penetrate into the pores.After the concentration of Cl-increased above a critical threshold,the inner compact layer of PEO coating breaks down and then the Mg substrate is rapidly dissolved,further leading to the formation of Mg(OH)2[59].Although it was previously suggested [60]that such Mg(OH)2layer could inhibit the expansion and spreading of pitting corrosion,it is generally confirmed that the protection effect of single Mg(OH)2is very limited.Hence,the corrosion resistance of Al/PEO is very poor in comparison with P/PEO and Si/PEO.In addition,it is important to mention that more stable phase MgAl2O4(spinel)can be formed also in aluminate electrolytes under higher applied voltage [36].As suggested by Oscar et al.[53],the content of MgAl2O4can be controlled by the aluminate concentration.Based on the literature data,MgAl2O4is more difficult to dissolve than MgO and Al2O3,and therefore it leads to a better corrosion resistance in comparison with PEO coatings containing only MgO phases.In other words,in order to get specimens with effective corrosion resistance in aluminate electrolyte,it is necessary to prepare the PEO coating consisted of MgAl2O4,rather than PEO coating consisted of MgO.

Concerning the P/PEO system,a certain level of selfhealing effect was observed,which can be attributed to the dissolution of amorphous phosphorus compounds from PEO layer (e.g.Mg3(PO4)2),followed by the formation of stable renewed phases.As suggest by Koji Murakami et al.[61],this renewed phases could be insoluble or poorly soluble salt as magnesium phosphate(Mg3(PO4)2·4H2O,Mg3(PO4)2·8H2O) or magnesium hydrophosphate (MgHPO4·3H2O,MgHPO4·7H2O).Moriet al.[41]thought it was Mg3(PO4)2·22H2O,and the reaction can be expressed by Eqs.(1) and (2):

Although researchers debate on the composition of this renewed phase,they all agreed on a fact that PEO coating formed in phosphate electrolytes had a self-healing effect for Mg based substrate.In our current XRD results,phosphorus containing components in P/PEO specimen exist in the amorphous form.It has been proved that the amorphous phosphates in aqueous solutions are not stable and can transform spontaneously into crystalline ones.Therefore,amorphous phosphorus in P/PEO is likely to further strengthen this self-healing ability (see Fig.5b).Moreover,the highest MgO content in P/PEO (see Fig.3) meant a large number of Mg(OH)2formation during immersion.Together with the above-mentioned factors,this determines a good self-healing ability for P/PEO.

In contrast,initial Si/PEO system has the highest corrosion resistance between the three different PEO coatings at initial moments of immersion.That may be due to the less pores and defects,what we have observed during a cross section analysis of the coatings (see Fig.4c) and due to the higher stability of silicate-based phase than Mg(OH)2[62].

It was previously shown,that the corrosion resistance of the PEO coating formed in a silicate containing electrolyte is higher than the one formed in a phosphate electrolyte [50,51].Although another data reports [49],that there was a similar self-healing phenomenon in Si-based PEO coating,our EIS results showed that the impedance dramatically decrease after 24 h immersion.

4.2.Growth behavior and corrosion behavior of LDHs on different PEO specimens and the effect of different growth solutions

Differences in preparation of LDHs on traditional anodic layer,LDHs/Mg(OH)2coating herein mainly grows inwards to the substrate,rather than inwards growth to the substrate and outwards growth simultaneously [35].Hence,inwards growth is more favorable from point of providing additional Mg2+and Al3+for MgAl-LDHs formation.It had been proved that there were two primary sources to provide Mg2+and Al3+,which are necessary for nucleation and growth of MgAl-LDHs: (i) dissolution of alloy from the substrate and(ii) degradation of PEO coatings [26,35].Based on XRD results(Fig.3),MgO existed in all three kinds of PEO coatings.Mg2+is easily available during hydrothermal treatment due to the dissolution of MgO and giving rise to Mg2+as a result.In such a case,MgAl-LDHs growth on PEO layer is mainly controlled by the accessibility of Al(OH)-4,which is in agreement with previous reports in literature [27].In the case of Al-based PEO layers,due to the existence of Al oxide in PEO layer,all the reactants for LDHs formation are the most available.As a result,the diffraction peaks corresponding to LDHs are the most evident even though Al-free growth solutions were used (see Fig.6).However,for hydrothermally treated Al/PEO in Al(NO3)3solution,the LDHs content did not follow a concentration effect (more Al ions,more LDHs formation was expected).That might be ascribed to a high alkalinity of growth solution (pH 10.5).This suggestion can be also supposed by literature data,where e.g.Liu et al.[56]have reported that it’s not beneficial for LDHs nucleation and growth if the release and diffusion of Al3+is too fast.

Concerning P/PEO system,it consist of MgO and amorphous phosphates,while there is more stable amorphous silicate beside MgO in Si/PEO.As it was shown in Fig.5,Si/PEO possessed the best corrosion resistance at initial stage,which was more than twice higher than P/PEO.Furthermore,the cations available from the substrate are determined by the porosity of the coating,which explains that when a part of PEO coating dissolve,the access to the required compound becomes easier through the defects.Lü et al.[63]reported that the LDHs nucleation sites are normally located on the surface of the anodized layer at initial stages,rather than in the area of un-anodized substrate and precipitation from the growth solution.The stable amorphous silicate in Si/PEO is more stable than MgO phase.As a result,it is hard to nucleate on this PEO surface.This may be a reason why it was difficult to form LDHs on Si/PEO specimens in frame of this work.Moreover,this may explain why the addition of Al3+cations into the LDHs growth solution does not result in an obvious increase in the LDHs content on Si/PEO surface.Serdechnova et al.[27]have reported similar finding that an amorphous oxide layer was in favor of LDHs formation,but crystallineγ-Al2O3andα-Al2O3not.Important is that,hydrothermal treatment of P/PEO and Si/PEO systems in Al-free growth solution led to a sharp decrease in corrosion resistance,compared with initial respective PEO specimens.This can be explained that the original PEO layer is damaged in case of there is no Al ions in growth solution.Another possibility is that such a destruction was a result of a low alkalinity of growth solutions.Moreover,in both NaNO3and Al(NO3)3solutions,MgAl-LDHs intercalated with nitrate cannot be formed during hydrothermal treatment and MgAl-LDHs intercalated with hydroxide forms preferentially.Overall,LDHs coating formation on PEO treated Mg alloys can be explained via following chemical reactions [25,64]:

As for the chemical reactions of formation of MgAl-LDHs on P/PEO and Si/PEO,it is similar with that of on Al/PEO.The difference is that the acquisition of Al ions is more difficult than Al-based PEO coating (see Table 1).Eqs.(3)–(8)

5.Conclusion

As a summary,based on our current experimental results and available literature data,some key “rules” for LDHs formation on PEO treated magnesium alloys can be formulated:

(1) In order to obtain a high amount of MgAl-LDHs,it is better to focus on Al-based PEO coatings.However,with the increase of MgAl2O4content in the coating,the formed LDHs amount will decrease.

(2) From point of corrosion resistance,the work with Sibased PEO coatings would be a good choice,but the growth of LDHs on Si-based PEO coating was more difficult.

(3) For preparing MgAl-LDHs on phosphate-and silicatebased PEO coatings,addition of Al3+cations into the growth solution as the source of elemental aluminum for LDHs formation is very important,from point of improvement of corrosion resistance;

(4) For further improvement of corrosion resistance and better self-healing ability,the best way is to perform hydrothermal treatment of phosphate-based PEO coatings.An ion-exchange with suitable corrosion inhibitors can be considered as a next step.

Acknowledgments

This work was supported by the International Cooperation in Science and Technology Innovation between Governments,National Key Research and Development Program of China (No.2018YFE0116200),the National Natural Science Foundation of China (51971040,51701029),the Fundamental Research Funds for the Central Universities (2020CDJQY-A007),China Postdoctoral Science Foundation Funded Project (2017M620410,2018T110942) and the Chongqing Postdoctoral Scientific Research Foundation(Xm2017010).G.Z.thanks also China Scholarship Council for the award of fellowship and funding (No.201806050047).M.S.and C.B.additionally thank the ACTICOAT project(Era.Net RUS Plus Call 2017,Project 477) for the financial support of this work.Special thanks the support from Mr.Volker Heitmann and Mr.Ulrich Burmester.