Kun Qian,Weizhou Li,?,Xiaopeng Lu,Xinxin Han,Yong Jin,Tao Zhang,Fuhui Wang

aSchool of Resources,Environment and Materials,Guangxi University,Nanning 530004,China

b Guangxi Key Laboratory of Processing Non-ferrous Metals and Features Materials,Guangxi University,Nanning 530004,China

c Shenyang National Laboratory for Materials Science,Northeastern University,3-11 Wenhua Road,Shenyang 110819,China

d School of Materials Science and Engineering,Northeastern University,Shenyang 110819,China

Received 6 August 2019;received in revised form 20 December 2019;accepted 4 May 2020 Available online 8 August 2020

Abstract This study investigated the effect of sealing treatment on the corrosion performance of plasma electrolytic oxidation(PEO)coated AZ91D Mg alloy with and without addition of corrosion inhibitor.The microstructure,phase composition and corrosion property of the sealed and unsealed coatings were evaluated by using scanning electron microscopy(SEM),energy dispersion spectroscopy(EDS),x-ray diffraction(XRD),x-ray photoelectron spectroscopy(XPS),polarization,and electrochemical impedance spectroscopy(EIS)tests.Electrochemical experiments and salt spray tests showed that,after sealing in phosphate solution containing corrosion inhibitor,the corrosion current density of PEO-coated AZ91D decreased more than 10-fold and the anti-corrosion time in a salt spray environment increased more than three-fold.The corrosion rate of the PEO coating slowed down due to the releasing and adsorbing of the corrosion inhibitors in the pores and cracks of the coating during the corrosion process.

Keywords:AZ91D magnesium alloy;Plasma electrolyte oxidation;Sealing treatment;Corrosion inhibitor.

1.Introduction

Due to their good performances in things such as a high strength to weight ratio,dimensional stability,damping capacity,castability,machinability and weldability,Mg alloys are widely used in the aerospace,automotive,communication and biomedicine industries,among others[1–5].However,Mg alloys have poor corrosion resistance due to their high chemical activity and an inability to form a stable,protective,natural oxide layer on their surface,which shortens their service life[6].

Multiple methods of improving the service life of Mg alloys in corrosion environments have been investigated according to previous studies,with the use of alloying[7,8],adding corrosion inhibitors into the application environment[9],surface treatment such as gas-phase deposition,laser melting,surface coatings like chemical conversion coatings and plasma electrolytic oxidation(PEO)[10–16].Until now,a variety of high-efficiency corrosion inhibitors acting on Mg alloys have been discovered,and their corrosion inhibition mechanisms have been completely explained[9,17–23].Hu et al.[23]has successfully synthesized 5,10,15,20-Tetraphenylporphyrin(TPP)and studied its inhibition effect on AZ91D magnesium alloy in NaCl solution.The research results suggests that TPP molecules can chelate with Mg via their N atoms to form a TPP-Mg complex,which can precipitate as a protective layer on AZ91D alloy.Research of Huang et al.[21]showed that the combination of sodium phosphate and dodecylbenzenesulfonate(SDBS)as an inorganic-organic inhibitor package has significantly improved the inhibition efficiency on GW103 at ambient and high temperatures.However,corrosion inhibitors take effect only in closed environments,which limits their practical use.Among all the surface treatment techniques,PEO is a relatively popular and advanced method for improving the corrosion resistance of magnesium alloys that feature high productivity,ecological friendliness,economic efficiency,as well as it coating with excellent bonding onto the substrate[24].PEO coating has many advantages,like good adhesion,high hardness,high stability,and low conductivity[25–27].However,a PEO coating cannot provide long-term protection due to its looseness and porous structure because corrosive media can penetrate into the pores and accumulate,then severely erode the coating,finally reaching and corroding the substrate[28,29].To overcome the shortcomings of PEO coating,sealing post-treatments,which feature easy implementation and high efficiency,have been tried to improve the corrosion resistance of PEO coating using inorganic substances and organic materials according to previous studies.Yun-Il Choi et al.[30]have observed an improvement of corrosion resistance of PEO coating on AZ31 alloys after post-treatment by using Na2SnO3solution under water bath at 77°C for 2–60min.They found that the polarization resistance value increased by more than 100 times in the initial stage after post-treatment and decreased to only three times after 24h of immersion..Nguyen et al.[31]have studied the combination of post-treatments with Ce(NO3)3solution under a water bath at 25°C and NaH2PO4solution under a water bath at 90°C on AZ31 alloys with PEO coating.The results showed that the PEO coating after a combined treatment with Ce(NO3)3and NaH2PO4,each for 20min,maintained the best corrosion resistance in 20 d of immersion in 0.5M NaCl solution.Castellanos[32]has researched the difference of performance for AZ91D alloys with PEO coatings after post-treatments with different substances.The impedance values indicated that post-treatment with polymeric matrix,water-based polytetrafluoroethylene and hybrid organic-inorganic silica sol increased the corrosion resistance of PEO coatings by five and four times respectively.Yu et al.[33]prepared the sealing solution by mixing 1.0%acetic acid and different concentrations of Chitosan,then the PEO treated Mg–4Li–1Ca alloy was sealed by a withdrawing method in sealing solution.The results of polarization curves showed that when the concentration of chitosan was 10g/L,the corrosion resistance of sealed coating increased for about 1.5 times.According to the results of previous studies[30-37],sealing post treatments can effectively improve the corrosion resistance of PEO coating on Mg alloys,but they cannot provide a long-term corrosion protection since the pores on the coatings are still permeable to the container for corrosive medium such as Cl?after the deposition coating is penetrated.

According to the advantages and disadvantages of corrosion inhibitors,it can be assumed that loading corrosion inhibitor into PEO coating can weaken the effects of corrosive media.Until now,some reports have focused on research involving the addition of corrosion inhibitors into PEO coatings.Gnedenkov et al.[38]immersed the PEO-coated MA8 Mg alloy in alkaline 8-hydroxyquinoline solution for 120min and treated the coating under 140°C for 20min to crystallize the 8-hydroxyquinoline.The corrosion resistance of corrosion-inhibitor-containing PEO coating was 5.7-times higher than pure PEO coating.However,the loading of corrosion inhibitor onto the surface of PEO coating is difficult to maintain at high levels during the corrosion process.Lamka et al.[39]added 8-hydroxyquinoline during the preparation of sol-gel solution using zirconium(IV)propoxide mixed with(3-glycidyloxypropyl)trimethox-ysilane.Then,the PEO coated AZ31 Mg alloy was immersed in the sol-gel solution at a rate of 18cm/min,and the coating was solidified at 130°C for 60min to prepare the inhibitor-doped sol-gel coatings.After treatment,the corrosion resistance of the coating increased for 3 times according to the EIS results.Chen et al.[40]prepared sol-gel solution by mixing two different sols using controllable hydrolysis of(3-glycidoxypropyl)trimethoxysilane(GPTMS)and titanium(IV)propoxide(TPOT).PEO-coated AZ91 specimens were immersed in the hybrid sol-gel solution for 60s at a constant dipping rate of 3mm/s before curing at 120°C for 80min.During preparation process,three kinds of corrosion inhibitors sodium glycolate,sodium 4-aminosalicylate,and sodium 2,6-pyridinedicarboxylate were incorporated and immobilized between the organic coating and the inorganic coating by impregnation.336h of EIS tests showed that the impedance value of sol-gel sealed coating without corrosion inhibitor decreased by up to four orders of magnitude,and the impedance value of sol-gel sealed coatings with the addition of the corrosion inhibitor decreased by only one order of magnitude,showing more excellent corrosion resistance.However,the poor adhesion between the sol-gel coating and PEO coating restricts its service life.In this study,we tried to seal the pores and also load the corrosion inhibitor into the coating during sealing post-treatment,without any other complicated process.Previous research on corrosion inhibitors has important guiding significance for the selection of corrosion inhibitors in this study.Among these inhibitors,sodium fumarate(C4H2Na2O4)was found to be a kind of most efficient corrosion inhibitor for Mg alloy[9].Therefore,sodium fumarate was used in the present study to enhance the corrosion resistance of PEO coating.

The main substance for sealing post-treatment,NaH2PO4,is a kind of buffer under acidic conditions with a stable pH of around 4.2,which can undoubtedly cause the dissolution of PEO coating.Nguyen et al.found that sealing time has an obvious influence on the morphology and corrosion resistance of sealed PEO coating[31].Since sealing post-treatment is the process of dissolution and deposition,controlling the degree of dissolution and deposition of the coating by adjusting the sealing time is especially critical.When the sealing time is too short,not enough dissolution takes place.However,the amount of sodium fumarate sealed in the pores of the coating will decrease at the same time.When the sealing time is too long,the acidic solution will damage the integrity of coating.Therefore,we chose 15min,30min and 60min to research the properties of sealed coatings in this study.

2.Materials and experiment

2.1.Experimental materials

AZ91D magnesium alloy specimens of size 30mm×15mm×5mm with a mass fraction of 8.5–9.5%Al,0.45–0.9% Zn,0.17–0.40% Mn,max 0.08% Si,and Mg balance were used as the substrate.The specimens were ground with different grit(up to 1000)emery sheet and cleaned with ethanol before the experiment.

2.2.Coating preparation process

The PEO coating was formed on the surface of AZ91D Mg alloy with an alkaline solution containing 10–20g/L Na2SiO3,1–4g/L KF,and 1–5g/L NaOH.A pulse reverse power was used for the PEO process,which provides 500Hz of frequency and a duty ratio of 30.The current was constant at 2 A/dm2for 15min.The samples were rinsed in distilled water and dried in warm air after PEO.PEO-coated AZ91D Mg alloys were sealed in a NaH2PO4solution with the addition of sodium fumarate for 15min,30min and 60min.PEO-coated AZ91D Mg alloys sealed in a NaH2PO4solution without sodium fumarate for 60min were used as control group to observe the effect of inhibitor on performance of sealed coatings.After sealing post-treatment,the samples were rinsed in distilled water and dried in warm air.

2.3.Characterization

The surface and section morphologies of coatings were obtained via a scanning electron microscope(SEM,PHILIPS,XL-30FEG,Holland)equipped with an X-ray energydispersive spectrometer(EDS,Oxford,Holland),which was used to analyze the element content of the coatings.In order to examine the phase composition of the coating,an X-ray diffractometer(XRD,PHILIPS,PW1700,Cu Kαradiation,andλ=0.154059nm,Holland)and X-ray photoelectron spectroscopy(XPS,ESCALAB250,Al-Ka radiation(1253.6eV)at 300V,Germany)spectrometer were used.Data was collected after 30s of ion etching of samples.Then XPSPEAK4.1 software was used to fit the XPS curves.

2.4.Corrosion tests

The change of corrosion rate in 3.5wt% NaCl solution to PEO-coated alloy with different sealing post-treatment was studied by preparing the collection volume of H2evolution.The equipment used to collect the evolution of H2which was invented by Song et al.[41],is shown in Fig.1.Considering that the corrosion solution cannot be replaced during the experiment process to maintain the numerical accuracy,only 48h of H2evolution measurement were performed to prevent the environment change of NaCl solution.

Fig.1.Equipment for collecting the evolution of H2 during the corrosion progress on unsealed and sealed PEO coatings:(a)acid burette,(b)funnel,(c)PEO coating sample,(d)solution.

Electrochemical tests were performed in a 3.5wt% NaCl solution at 30°C±0.5°C by using a Zahner Zennium electrochemical workstation(Zahner,Germany).The sample was connected as the working electrode,a saturated calomel electrode(SCE,saturated KCl)was used as the reference electrode and a platinum slice served as the counter electrode.The potentiodynamic polarization experiments and impedance spectroscopy(EIS)measurements were carried out after immersion for 30min under the open-circuit potential(OCP)tests to reach a stable potential.All tests were carried out using an electrode area of 3 cm2and were repeated least three times.

Potentiodynamic polarization experiments were carried out on the surface of the coatings under a scan rate of 0.333mV/s.EIS experiments were carried out by applying an AC disturbance signal.Eight points were taken per decade between a frequency range from 100,000Hz to 0.01Hz.The results of the EIS experiments were analyzed by Zsimpwin software,and the equivalent circuits were fitted.

The salt spray tests of each sample were carried over 30 d by using a salt spray cabinet(CCI/CCM-MX).According to the standard ASTM B117-02,a salt spray atmosphere was created using an aqueous solution of 5wt% NaCl at neutral pH with a temperature of 30°C.A Canon S95 digital camera was used to record the macroscopic appearance of samples.

3.Results

3.1.Microstructure of coatings

Fig.2 shows the surface microstructure of the PEO coating and coatings after post-treatment for different times.The surface of the PEO coating(Fig.2(a))presented a porous structure with volcanic projections,and the size of the craterlike pores was between 0.5μm and 10μm.These pores occurred at the sites of the discharge channels of the evolution gas due to the rapid solidification of molten oxide caused by electrolyte during the PEO process[42–44].After sealing treatment,the nodular protrusion of the volcanic structure disappeared(Fig.2(b)–(e)),which can be explained by the dissolution of the PEO coating when in contact with acidic sealing solution.The number and size of pores on the coating decreased accompanied by the appearance of cracks.These were due to the deposition after dissolution and the increase in internal stress resulting from the volume expansion of PEO coating with deposition during sealing treatment[31,45].Coatings treated for different times exhibited similar surface morphology.Thus,it can be concluded that short treatment times can also provide an excellent sealing effect for the PEO coating.

Fig.2.Surface morphology of PEO coating without sealing(a),sealed coating without inhibitor for 60min(b),sealed coating with inhibitor for 15min(c),sealed coating with inhibitor for 30min(d),sealed coating with inhibitor for 60min(e).

Fig.3.Cross-sectional morphology of PEO coating without sealing(a),sealed coating without inhibitor for 60min(b),sealed coating with inhibitor for 15min(c),sealed coating with inhibitor for 30min(d),sealed coating with inhibitor for 60min(e).

Fig.3 shows the cross-section microstructure of the coatings.According to the cross-section of PEO coating(Fig.3(a)),it can be observed that the PEO coating retains a relatively uniform thickness of 18±2μm.The coating can be divided into two parts,a loose,porous outer layer and a dense inner layer[43,45].Many open pores that connect to the outside can be found in the PEO coating.After sealing treatment,the thickness of the coating did not change significantly,staying at about 20–23μm(Fig.3(b)–(e)).It can be seen that the surface of sealed PEO coating produced a deposition layer,so the sealed coating can be divided into three parts,a deposition layer,a loose porous outer layer and a dense inner layer.The results of EDS showed that the deposition layer contains 10.7–16.1 at% of P element,indicating that this deposition layer is derived from the deposition of NaH2PO4.The content of P increased with increasing sealing time due to the increased deposition.The deposition layer sealed open pores in the PEO coating,so the size and number of pores on its surface were reduced.However,parts of the red circle in Fig.3(b)and Fig.3(e)indicated that when sealed for 60min,open pores in PEO coating can be overly dissolved by the acidic sealing solution so the inner cavity of open pores was enlarged.Therefore,the density of coatings sealed for 15min,30min,and 60min is quite different,although their surface morphology is similar.Among them the coating sealed for 15min presented the densest structure.

Fig.4.X-ray diffraction patterns of unsealed PEO coating,PEO coating sealed without inhibitor,and coating sealed with inhibitor for different times.

3.2.Composition of coatings

X-ray diffraction(XRD)analysis indicated that PEO coating formed in silicate-based electrolyte on AZ91D alloy was mainly composed of two kinds of crystal phases,MgO and Mg2SiO4(Fig.4).Pattern peaks corresponding to the Mg phase can also be observed,which is attributed to the porous structure of PEO coating resulting in the penetration of X-rays arriving to the substrate[36].The MgHPO4was discovered as the new crystal phase after sealing post-treatment.According to the previous EDS results,it was found that content of P element increased with the increasing of sealing time.Since XRD showed that MgHPO4was the deposition product,it is known that more MgHPO4deposited on the surface of PEO coating accompanied with the increasing of sealing time.The H2PO4?from sealing solution ionized into H+and HPO42?,then the acidic H+led to the dissolution of MgO from PEO coating.After such a reaction,the pH in the space between the surface of coating and solution increased due to the consumption of H+,which further promoted the ionization of H2PO4?in turn.Finally,the dissolved Mg2+combined with the ionized HPO42?and deposited on the surface of PEO coating to seal the pores.Reaction formulas for this overall process are as follows:

However,no sodium fumarate can be found according to the previous analysis.To further investigate any traces of inhibitor,X-ray photoelectron spectroscopy(XPS)was used to analyze the chemical compositions of the coatings.To decrease any interference from the absorption of external substances,sample surfaces were sputtered for 30s.The existence of P peaks could be found on the survey spectra after sealing post-treatment which proved the production of the phosphorus-related compounds.High-resolution XPS spectra of C 1s,Si 2p and P 2p of three kinds of coatings were analyzed.The C 1s spectrum(Fig.5(b))on PEO coating sealed with corrosion inhibitor for different times revealed four peaks at 284.5eV,286.3eV,287.8eV and 289.2eV,corresponding respectively to elemental C in C=C,C–C/C–H,C=O and COO?[46–49],which proved the existence of sodium fumarate.Thus,it can be concluded that sodium fumarate is effectively sealed in coatings even at treatment times of only 15min.For the Si 2p binding energy(Fig.5(c)),two peaks at 102.5eV and 103.6eV existed,which respectively represented SiO2and Mg2SiO4[50,51].The P 2p spectrum(Fig.5(d))of coatings sealed for 30min and 60min revealed two peaks at 133.76eV and 132.33eV,which respectively corresponded to PO43?and HPO42?.This indicates that during the longtime sealing post-treatment,in addition to the ionization of HPO42?,there is also a certain amount of PO43?ionized by HPO42?[52,53].However,in coating sealed for 15min,only a peak corresponding to HPO42?can be found.This means that less H+and PO43?are ionized by HPO42?because the short sealing time limits the progress of pH increase in the region between solution and coating surface by the dissolution of coating.We can speculate that this relates to the denser structure of coating sealed for 15min since less H+damaging the coating is produced.To investigate the distribution of sodium fumarate in the coating,the coating was subjected to a high-resolution single peak of C 1s with a sputtering time of 1000s,2000s,and 3000s(Fig.5(e)).This sputtering time can detect a depth of 0.1–0.3μm in the coating,which is about half the thickness of the deposited layer.The results showed that at a thickness of 0.3μm,peaks of C=C,C–C/C–H,C=O,and COO?,which could prove the presence of sodium fumarate still existed.It could be concluded that sodium fumarate was not only absorbed onto the surface of the deposition layer but participated in co-deposition during the formation process of deposition layer to be loaded inside the deposition layer.

According to previous characterization of the microstructure and composition of coatings,the formation mechanism of the inhibitor-containing composite coating can be explained.A schematic diagram of the post-treatment progress with NaH2PO4after adding corrosion inhibitor is shown in Fig.6.A deposition layer consisting of MgHPO4is formed on surface of PEO coating by the dissolution and redeposition reaction in NaH2PO4solution.Inhibitor in the solution is loaded in the deposition layer along with the deposition reaction.Fig.6(a)describes the whole progress.When the sealing time is too long,the situation will be as shown in Fig.6(b).Acidic solution will overly dissolved the coating,causing the increase of depth and cavity volume of open pores in PEO coating.

Fig.5.X-ray photoelectron spectroscopy for unsealed PEO coating,PEO coating sealed without inhibitor,coating sealed with inhibitor for different times,(a)survey spectra,(b)high resolution single peak of C 1s,(c)high resolution single peak of Si 2p,(d)high resolution single peak of P 2p,(e)high resolution single peak of C 1s under different sputtering time.

3.3.H2 evolution tests

Fig.7 shows the volume of H2evolution by PEO-coated AZ91D Mg alloy and PEO-coated alloy after sealing treatment both with and without inhibitor.The results show that the H2volume of PEO-coated Mg alloy was 0.11mL/cm2and after sealing treatment without inhibitor,the H2volume decreased to 0.08mL/cm2.However,the rate of H2evolution recovered to the same as PEO-coated Mg alloy during the later stage of testing,which meant the failure of the deposition layer by sealing-treatment after immersion.For a PEOcoated alloy sealed with inhibitor,the volume of H2evolution dropped to an extremely low value,and the alloy sealing for 15min kept the lowest H2volume(0.01mL/cm2).This indicated that the corrosion tendency of coatings sealed with inhibitor reduced significantly compared with unsealed PEO coating during 48h corrosion time,and coatings sealed for 15min had the lowest corrosion degree.

Table 1The results on electrochemical parameters of potentiodynamic polarization curves from Fig 8.

Fig.6.Schematic diagram of the sealing progress with NaH2PO4 after adding corrosion inhibitor:(a)suitable sealing time,(b)long sealing time.

Fig.7.H2 evolution volume of PEO-coated AZ91D Mg alloy both with and without sealing treatment after immersing for 48h.

3.4.Electrochemical measurements

To study the effect of the sealing post-treatment on the short-term and long-term corrosion resistance of PEO coatings both with and without inhibitor,electrochemical tests have been used to study the corrosion performance of sealed PEO coatings.

Fig.8 shows potentiodynamic polarization curves of PEOcoated AZ91D alloy without sealing post-treatment,sealed with NaH2PO4for 60min,and sealed with NaH2PO4and inhibitor for 15,30,and 60min.The coating that was only sealed with NaH2PO4showed lower corrosion current density(icorr)than the coating without sealing post-treatment[54].For the coating sealed with inhibitor,the corrosion current density further decreased.These results indicated that in addition to the effect of post-treatment on the reduction of corrosion rate,adding inhibitor during the sealing process further improved the corrosion resistance of PEO coating on AZ91D alloy.It can be seen that when sealed with inhibitor,the corrosion current density increased sequentially as the sealing time increased,so the treatment showed a minimum corrosion current density after being sealed for 15min,which represented the best corrosion resistance.Table 1 lists the electrochemical parameters.

Fig.8.Polarization curves of unsealed PEO coating,PEO coating sealed with NaH2PO4,Coating sealed with inhibitor for different times.

To better understand the effects of PEO coatings with and without sealing post-treatment as barrier systems during the corrosion process,EIS was used to gain complementary information for morphology and composition.Different frequency segments on impedance permits the effective evaluation of the different components of the coating system,such as capacitance and resistance of the outer and inner layers,charge transfer resistance,and double layer capacitance associated with electrochemical activities on the surface[34].The impedance of coatings was continuously measured for 120h at 24h intervals.Bode and Nyquist diagrams responding to the EIS of PEO coatings with and without sealing post-treatment in 3.5% NaCl solution are shown in Fig.9 for 0.5h,Fig.10 for 48h and Fig.11 for 120h.

With 0.5h immersion before testing,the PEO coating processed good corrosion protection.Three overlapped time constants are present corresponding to three relaxation processes,which are found to be an equivalent circuit in Fig.12(a)and are chosen to simulate such progress.PEO coating is considered to have a two-layer structure during the initial corrosion stage,wherein the CPEoutandRoutin the high frequency regime correspond to the capacitance and resistance behavior of the loose,porous,outer layers in coatings.CPEinandRinbetween the middle and low frequency regimes relate to the capacitance and resistance behavior of the dense inner layers.CPEdlandRctat low frequencies can be used to describe the double layer capacitance on the electrolyte and metal interface and the charge transfer resistance of corrosion processes[55].After 24h of immersion,the corrosive solution which entered the pores of the outer layer,corroded the outer layer and further corroded the inner layer of the PEO coating,resulting in the disappearing of corrosion protection by the outer layer.At the same time,some parts of the inner layer were penetrated by the corrosive medium,which caused pitting corrosion on the interface of the coating and substrate.So RLand L,which relate to the diffusion resistance of pitting corrosion appeared[56].The equivalent circuit in Fig.12(c)reflects such a situation.After 120h of corrosion,most of the area of the inner layer was penetrated and completely lost any protection of the substrate.Under such circumstances,the equivalent circuit in Fig.12(c)is chosen and the fitted values are listed in Table 2.

Fig.9.Impedance spectra of unsealed PEO coating,PEO coating sealed with NaH2PO4,and coating sealed with inhibitor for different times after 0.5h immersing.

However,for coatings after sealing without inhibitor,totally different behavior of EIS during 120h immersion is observed,as seen in Figs.9,10 and 11.After 0.5h of corrosion,the coating exhibited relatively highRoutandRin,which showed that the coating sealed with NaH2PO4provides more effective corrosion protection compared with PEO coating.Here,Routand CPEoutassociated with the outer layer corresponds to the combination of outer porous layer and deposition layer,as shown in Fig.13(a).In this stage,three relaxation processes corresponding to three overlapped time constants are found.Then,the protection of the outer layer disappeared after experiencing 24h immersion accompanying with the undetectability of high-frequency time constants corresponding to the outer layer.During the corrosion from 24h to 120h,the inner layer provided excellent protection to the substrate,wherein two overlapped time constants corresponding to relaxation processes of inner layer and double layer capacitance were present.The equivalent circuit in Fig.13(b)showed this state of the coating,and the fitted values are listed in Table 3.

Coating sealed with inhibitor had similar corrosion process behavior to coating only sealed with NaH2PO4,yet it exhibited higherRoutandRin.This can be explained by the fact that after the infiltration of corrosive medium,the sodium fumarate loaded in the deposition layer was released and acted as the corrosion inhibitor to be adsorbed on the surface of inner layer where corrosion started,which slowed down the corrosion rate.Therefore,the increase ofRinwas relatively smoother and slower.The equivalent circuit and schematic diagram in Fig.14 described this corrosion process.Afterwards,the corrosion resistance of PEO coatings sealed with inhibitor for different times were analyzed.After 0.5h of immersion before testing,PEO coatings with different sealing times showed good corrosion protection.Three overlapped time constants corresponding to three relaxation processes can be found according to the Nyquist and Bode plots,which speak to the integrity of outer layer and inner layer of coatings.Among them,the coating sealed for 15min presented the most excellent resistance ofRoutandRindue to it having the densest coating structure.After 24h of immersion,the outer layer of sealed coatings was penetrated by corrosive solution as the solution entered the pores of the coatings.Therefore,coatings mainly provided protection relying on the dense inner layer and the released corrosion inhibitor.It can be seen that unlike the impedance value of the outer layer,the impedance value of the inner layer of coating sealed for 15min did not have a big advantage compared with other coatings,but still maintained the maximum value during 5 days.It can be observed that the coating sealed for 15min possessed the best corrosion protection when exposed to a long-period corrosion environment.The fitted values are listed in Table 4.

Fig.10.Impedance spectra of unsealed PEO coating,PEO coating sealed with NaH2PO4,and coating sealed with inhibitor for different times after 48h immersing.

Table 2Fitting results of EIS plots of the PEO coating in 3.5% NaCl solution before 120h of immersion.

Table 3Fitting results of EIS plots of the PEO coating sealed with NaH2PO4 for 60min in 3.5% NaCl solution before 120h of immersion.

Fig.11.Impedance spectra of unsealed PEO coating,PEO coating sealed with NaH2PO4,and coating sealed with inhibitor for different times after 120h immersing.

Fig.12.Equivalent circuits for fitting the impedance data of PEO coating.

Fig.13.Equivalent circuits for fitting the impedance data of PEO coating sealed with NaH2PO4.

Fig.14.Equivalent circuits for fitting the impedance data of PEO Coating sealed with inhibitor.

Table 4Fitting results of EIS plots of the PEO Coating sealed with inhibitor for different times in 3.5% NaCl solution before 120h of immersion.

Here,we useRtotalto represent the total amount of resistance associated with corrosion,and the formula for calculatingRtotalis as follows:

Fig.15 shows the comparison ofRtotalto coatings in 120h of immersion.TheRtotalof all the coatings decreased as the corrosive time increased,whileRtotalof coating sealed with inhibitor for 15min was always better than other coatings,which proved its better corrosion resistance.

Fig.15.Change in Rtotal of unsealed PEO coating,PEO coating sealed with NaH2PO4,and coating sealed with inhibitor for different times.

3.5.Salt spray tests of coatings

The coating with or without sealing post-treatment were also examined by a salt spray test.Fig.16 shows the change on the surface of coatings after 30 d of corrosion.The PEO coating showed the fastest initiation,with the appearance of corroded areas after 10 days.When it reached the 30th day,the surface of the coating was completely filled with corrosion products.To coating sealed with NaH2PO4for 60min,the situation was slightly more optimistic since there were fewer corroded areas on the 10th day compared with PEO coating.However,on the 30th day,the surface of the coating showed similarly severe corrosion.Different from both of these,the surface of coating sealed with inhibitor for 60min maintained great integrity after 30 days of salt spray.This result is consistent with the electrochemical results.It can be seen from the result that the post-treated coating can maintain better corrosion resistance due to its denser outer layer obtained by sealing in the first ten days of salt spray,but it cannot provide a long-term protection once the corrosive medium enters the pores after an extended corrosion time.Only this coating can resist the infiltration of the corrosive medium in the long-period corrosion process relying on the corrosion inhibition to them by corrosion inhibitor added to the pores of coating during sealing post-treatment.

The unsealed and sealed coatings after salt spray tests were tested via SEM.Fig.17 shows the morphology of coatings after 30 days of corrosion.The lower magnification micrograph shown in Fig.17(a)indicates that the PEO coating had been severely damaged due to corrosion.Deep pores created by corrosion in the corroded area indicates that corrosion continued to increase in the corroded area in PEO coating due to its poorer ability of resisting corrosive media compared with the uncorroded area.The higher magnification micrograph shown in Fig.17(b)shows a large amount of accumulated corrosion products appearing on the surface of coating.It can be found from Fig.17(c)and(d)that the corrosion area of coating sealed with NaH2PO4solution was in the same condition as unsealed PEO coating.For coating sealed with NaH2PO4solution containing inhibitor,the corroded area showed completely different topography.Corrosion products in the corrosion area were not accumulated,but formed a thin and dense layer on the corrosion pore.This layer effectively prevented corrosion from proceeding deep into the coating,thus protecting the substrate.Therefore,no corrosion pits were formed in the corrosion area of the coating.This corrosion product layer also appeared on the surface of AZ91 Mg alloy after corrosion in 3.5wt% NaCl solution with added inhibitor(Fig.17(h)).This proves that this layer produced by the adsorption of inhibitor,which released from the corrode area of the coating.

Fig.16.Surface appearance of the PEO coatings with and without sealing post-treatment during 30 days of salt spray test.

Fig.17.Surface morphologies of coatings after salt spray tests:unsealed PEO coating(a)and(b),PEO coating sealed without inhibitor(c)and(d),coating sealed with inhibitor(e)and(f).Morphologies of AZ91 Mg alloys immersed in 3.5wt% NaCl solution for 48h without(g)and with inhibitor(h).

4.Conclusions

–The sealing post-treatment was successfully developed on PEO-coated AZ91D magnesium alloy with corrosion inhibitor.The coating became smoother and the size of pores on the surface of the coating decreased after sealing treatment.The sealed coating obtained the densest structure when the sealing time was 15min.

–Composition characterization implied that the main ingredient of the deposition layer was MgHPO4.Inhibitor was effectively loaded in PEO coating regardless of whether the sealing time was 15min,30min,or 60min.

–Electrochemical tests and salt spray tests revealed that corrosion properties of the coating improved after applying the sealing post-treatments with corrosion inhibitor.This could be attributable to the barrier effect to corrosive media of dense coating by sealing and inhibition of the released corrosion inhibitor.After being sealed for 15min,theRtotalof the coating increased by 58.7-fold,and theicorrof the coating decreased by 52.3-fold,which proved its best corrosion resistance.

Declaration of Competing Interest

None.

Acknowledgments

The authors acknowledge the financial support of the National Natural Science Foundation of China(No.U1737102,51531007,51371059),Young Elite Scientists Sponsorship Program by CAST(2017QNRC001),Guangxi Natural Science Foundation of China(Nos.2016GXNSFDA380022),Major Science and Technology Projects in Guangxi(No.AA18118030 and AA17204100),Project of Development of Science and Technology of Nanning(No.20181191-2),and the Fundamental Research Funds for the Central Universities(N170203006).