Plasma electrolytic oxidation of AZ31 magnesium alloy in aluminate-tungstate electrolytes and the coating formation mechanism

Accepted Manuscript Plasma electrolytic oxidation of AZ31 magnesium alloy in aluminate-tungstate electrolytes and the coating formation mechanism Wenbin Tu, Yulin Cheng, Xinyao Wang, Tingyan Zhan, Junxiang Han, Yingliang Cheng PII:

S0925-8388(17)32482-9

DOI:

10.1016/j.jallcom.2017.07.117

Reference:

JALCOM 42533

To appear in:

Journal of Alloys and Compounds

Received Date: 12 April 2017 Revised Date:

2 July 2017

Accepted Date: 9 July 2017

Please cite this article as: W. Tu, Y. Cheng, X. Wang, T. Zhan, J. Han, Y. Cheng, Plasma electrolytic oxidation of AZ31 magnesium alloy in aluminate-tungstate electrolytes and the coating formation mechanism, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.07.117. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Graphical abstract

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(~ 104 A cm-2)

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Plasma electrolytic oxidation of AZ31 magnesium alloy in aluminate-tungstate electrolytes and the coating formation mechanism

Cheng *

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Wenbin Tu, Yulin Cheng, Xinyao Wang, Tingyan Zhan, Junxiang Han，Yingliang

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College of Materials Science and Engineering, Hunan University, Changsha, 410082, China

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Abstract: Plasma electrolytic oxidation (PEO) of AZ31 magnesium alloy under pulsed bipolar regimes has been carried out in an aluminate electrolyte with the addition of 0-25 g l-1 Na2WO4·2H2O. Black coatings are formed with the addition of tungstate. Sequential anodizing has also been adopted to investigate the coating

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formation mechanisms by tracing the elemental distribution of W and Al in the coatings. The coatings develop an outer layer, inner layer and a barrier layer after a certain period of PEO. At the later stage of the PEO, the coating grows inwardly,

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which was accompanied by the strong penetrating discharges. The penetrating

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discharges have caused significant anion deposition, and the electrolyte species, such as W and Al, can be transported to the coating/substrate interface instantly. The anodic current density within the penetrating discharge channels is estimated to be ~104 A cm-2, which is high enough to melt the coating materials beneath the pancake structure and cause the direct thermal decomposition of water and hence the anomalous gas evolution reported for PEO. X-ray photoelectron spectroscopy (XPS) denies that free state W exists in PEO coatings. *Corresponding authors. Tel.: +86 731 88821727 ; Fax: +86 731 88823554 E-mail addresses: [email protected], [email protected](Y. Cheng).

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Keywords: Plasma electrolytic oxidation, Magnesium alloy, Tungstate, aluminate,

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formation mechanism.

1. Introduction

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Owing to their lightness, high strength to weight ratio, abundant raw material

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resources, good electrical conductivity and high thermal conductivity, magnesium and its alloys have been widely used in automobile, aerospace, electronic communication and biomedical industries [1-6]. However, magnesium alloys also have a few notable shortcomings such as high chemical reaction activity, low corrosion resistance, poor

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creep and wear resistance, which greatly restrict their extensive applications in technical and biomedical industries [4-9]. In order to avoid such disadvantages, surface treatment is indispensable for the practical application of magnesium alloys

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[10,11]. A various methods can be used for surface modification of magnesium and its

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alloys, which include anodizing [12], PVD or CVD depositing [13,14], electroplating [15] and plasma electrolytic oxidation (PEO) [16-19]. Among these methods, PEO, also called micro-arc oxidation (MAO), is viewed to be the most effective to improve the surface properties of magnesium alloys[4, 17, 20-23].

PEO is developed from conventional anodizing but works under higher voltages, which causes the dielectric breakdown of the oxide films, manifested by moving

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ACCEPTED MANUSCRIPT plasma discharges at the surface of the treated workpieces. Due to high temperatures of the plasma, several physical-chemical processes, typically electrochemical, plasma chemical and thermal diffusion, occur simultaneously during PEO, which result in

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rather complex coating formation mechanisms [24,25]. A recent review has gathered the most important advancements on the mechanisms of PEO coating formation, however, a complete explanation for the coating formation mechanisms has not yet

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been proposed [4]. PEO is normally viewed to be significantly related with the plasma

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discharges and coatings grow discontinuously, with cycles of coating formation, dielectric breakdown, and material deposition in the discharge channel after its termination [23, 26, 27]. However, a recent study of the PEO on Al by Zhu et al., suggested that the thin amorphous alumina layer at the coating base was grown by an

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ionic migration mechanism and hence, “PEO was not an abrupt ejection of a molten material but a gentle growth process”[28]. Various attempts have been made to investigate the coating growth mechanism and associated species transportation

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process, for example, the using of 18O [26] and oxide particles as tracers [20].

The microstructures of PEO coatings are determined by many factors, such as the substrate metal, the electronic properties of the formed oxides, electrolyte concentration and the forming electrical regimes [25,29,30]. In a recent study, the high insulating and semiconductor properties of ZrO2 and TiO2, respectively, were attributed to the formation of “coral reef” and pancake structures on the coatings on the respective titanium and zirconium alloys [29]. The electrolyte concentration, and

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ACCEPTED MANUSCRIPT hence the anion deposition process, have much effect on plasma discharge bahavior and coating morphologies, too [30]: Energetic penetrating discharges were found for PEO with less anion deposition, favoring the formation of pancake structures,

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however, the PEO in concentrated electrolytes with heavy anion deposition was accompanied by weak discharges and an absence of pancake features. There are also other special sturctures on the PEO coatings, such as “the characteristic solidification

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structure” on the PEO coatings on zirconium alloys [31-35]. The characteristic the low thermal

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solidification structure, which is thought to be related with

conductivity of zirconia [31,33], is featured by a cluster of equiaxed grain in the central region, surrounded by a ring of radially orientated, elongated grains. Up to the

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present, this structure is only found with the PEO of zirconium alloys.

The PEO of magnesium and its alloys is normally carried out in electrolytes based on silicate, phosphate and aluminate [36-42]. The coatings from these electrolytes are

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normally white in color, however, coatings with other colors are attractive for

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decorative and also other functional purposes [43-46]. As an example, black coatings with high emittance and high absorptance are highly desired for the internal components of the spacecraft [44]. In recent years, tungsten-containing oxide layers on Al and its alloys have been explored due to their catalytic, semiconducting and corrosion resistant properties [47-53], and the incorporation of W is also known to result in black coatings on Al alloys [44,49]. However, as compared with Al alloys, there are less works on the PEO of magnesium alloys using sodium tungstate as a

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ACCEPTED MANUSCRIPT component of the electrolyte [54-57]. Furthermore, Zhao et al. [57] reported that sodium tungstate can lighten the coating colour on AZ91 magnesium alloy, which

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seems to contradict to the role of tungsten for PEO coating on Al alloys.

In the present study, PEO of an AZ31 magnesium alloy has been carried out under

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pulsed bipolar regime in aluminate based electrolytes with the addition of 0-25 g l-1

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Na2WO4·2H2O. The coating morphology, microstructure and phase compositions before and after the addition of Na2WO4·2H2O have been characterized in detail. Sequential anodizing has also been performed to explore the coating formation mechanism. By tracing the distribution of the tungsten species in the coatings, the

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2. Experimental

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mechanism for the coating formation has been discussed.

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A rolled AZ31 magnesium alloy plate, ~ 6 mm in thickness, was cut and mounted in epoxy resin to provide specimens with working areas of 10 x 20 mm. The nominal composition of the alloy (wt%) is: Al 3.0, Zn 1.0, Mn 0.2, Mg Balance. The specimens were successively polished to a 2000 grit SiC finish, degreased in ethanol, rinsed in distilled water and, finally, dried in a stream of warm air. Aqueous electrolytes of 10 g l-1 NaAlO2 + 3 g l-1 C6H8O7·H2O(citric acid) + 2 g l-1 KOH, with the addition of 0-25 g l-1 Na2WO4·2H2O, were used for the PEO treatments.

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The PEO power supply and experimental arrangement was the same as that in our previous paper [58]. PEO was carried out in a glass vessel, equipped with magnetic

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stirring and a water cooling system to cool the electrolyte during PEO. Pulsed bipolar constant current regimes, using a frequency of 1000 Hz and a duty cycle of 20% were employed for the PEO treatments. An oscilloscope (Tektronix TDS 1002C-SC) was

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used to monitor the current waveforms. Average positive and negative current

integration of the waveforms.

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densities of ~0.22 and 0.09 A cm-2 were determined for the PEO processes from the

The surface and cross-sectional morphology of the coatings were examined by a field

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emission gun scanning electron microscopy (SEM, QUANTA 250, FEI, USA) assisted by energy dispersive spectroscopy (EDS). Phase compositions of the coatings were determined by X-ray diffraction (XRD) with a Rigaku D/MAX 2500

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diffractometer (Cu-Kα radiation). X-ray Photoelectron Spectroscopy (XPS), using a

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K-Alpha 1063 instrument (Thermo Fisher Scientific, UK), with Al Kα radiation as the excitation source (12 kV, 6 mA), was used to examine the chemical states of W species in the coating. The charging effect was corrected using the binding energy of C1 s at 285.0 eV. An eddy current thickness gauge (TT260, Time group, Beijing) was used to measure the thickness of the coatings.

A commercial digital camera (Canon EOSD300) was used to record the appearance of plasma discharges with a fast exposure time of 0.125 ms. The OES spectra of the 6

ACCEPTED MANUSCRIPT plasma during PEO were recorded using an Ocean Optics Spectrometer (HR4000), in the wavelength range from 250 to 700 nm. The distance between the detector of the spectra recording system (a 74 UV collimating lens) and the surface of the specimen was fixed at ~10 cm and the integration time for the spectra acquiring was 2 s. All

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data collected by the spectrometer was sent to the computer. Atomic and ionic lines in

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the collected spectra were identified using the NIST online spectral database [59].

The determination of the coating growth with reference to the original metal surface,

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i.e., the outward growth and inward growth, respectively, is useful for the understanding of the coating growth mechanism. The outward and inward growth can be differentiated by inert marker for conventional anodic films [60]. However, in PEO study, the respective coating growth can be known by measuring the variation in

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geometrical dimensions before and after treatment [24, 61], or by directly observing the cross section of the coating with reference to the original metal surface [23, 62]. In

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the present paper, an AZ31 sample was first ground to 5000 grit SiC paper followed by polishing to 1 µm diamond paste. A small fraction of the polished surface was then

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masked and protected by epoxy resin. The as-prepared sample was PEO-treated in the aluminate based electrolyte with the addition of 10 g l-1 Na2WO4·2H2O for ~ 240 s. The cross-section at the coating/masked area boundary was then observed to determine the outward and inward growth.

It is found in the present study that the W species in the coatings can be easily traced by SEM in backscattered electrons due to the atomic number contrast between W and 7

ACCEPTED MANUSCRIPT other coating elements (Mg, Al and O), hence, sequential anodizing has also been employed to trace the coating formation mechanism. Two different sequential anodizing procedures have been adopted. In the first procedure, the AZ31 magnesium

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alloy was first treated in the aluminate based electrolyte to form a W-free precursor coating, then the precursor coating was treated in W-containing electrolyte for different times. The distribution of W-species in the sequential anodized coatings was

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then traced by SEM. The second procedure used a reversed sequence, i.e., a

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W-containing precursor coating was first formed and then PEO-treated in the W-free aluminate electrolyte (10 g l-1 NaAlO2 + 3 g l-1 C6H8O7·H2O + 2 g l-1 KOH).

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3. Results and discussion

3.1 Cell potential--time responses, current waveforms, discharge and coating

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appearance

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Fig.1(a) and (b) show the cell potential-time responses, current waveforms during PEO of the AZ31 alloy in electrolytes with different concentrations of Na2WO4·2H2O. It has been found in this study that the addition of Na2WO4·2H2O leads to faster coating growth. In order to obtain coatings with similar thicknesses, a shorter treatment time (480 s) was employed for PEO in the electrolytes with higher tungstate concentrations. The positive cell potential-time responses of different samples show an initial rapid rise and a subsequent stage with much reduced rate of potential rise.

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ACCEPTED MANUSCRIPT The inflexion point in the positive cell potential-time responses is designated as breakdown potential in literature [63]. The negative potentials also show an initial rapid rise to ~ 43-72 V (absolute values), after which the potentials drop a little and

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then rise again in a trend similar to that of the positive potentials. The values of negative potentials are much lower than those of the positive potentials. It is observed that the addition of the Na2WO4·2H2O has not significantly altered potential-time

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response, however, a reduced value in breakdown potential was noticed when the

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concentration of Na2WO4·2H2O was increased from 0 to 20 g l-1 and the coating formed with the addition of 20 g l-1 Na2WO4·2H2O shows a higher cell potential at prolonged treatment times. According to Fig.1(b), the waveforms of current densities were similar for the PEO treatment in different electrolytes. Average positive and

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negative current densities of ~ 0.22 and 0.09 A cm-2 respectively were determined by integration of the waveforms.

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The plasma discharges during PEO are associated with coating formation mechnism

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[30]. Plasma discharges are normally numerous at the early stage of PEO, however, the discharges will be more powerful and less numerous when the coating thickens. Real time imaging has been used to record the appearance of discharges [30,33,63]. In order to learn the discharge behaviour at the later stage of PEO of the present AZ31 magnesium alloy, the discharges after ~ 600 s PEO treatment in the aluminate based electrolyte with the addition of 10 g l-1 Na2WO4·2H2O were recorded in a sequential order within a duration of ~ 2 s (Fig.1(c-g)). The results show that there are only a few

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ACCEPTED MANUSCRIPT powerful discharges on the electrode and the discharges are movable. In Fig.1(d) and (f), most of the discharges are extinguished, which may imply that the images were recorded during the negative pulse or the pulse-off duration since the exposure time

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for the images is 0.125 ms, which is shorter than a waveform cycle.

The appearances of the coatings formed in electrolytes with different concentrations

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of Na2WO4·2H2O are displayed in Fig.1(h-l). It is evident that the addition of

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Na2WO4·2H2O endows a black color to the coatings, with the darkest appearance for 10 and 15 g l-1 Na2WO4·2H2O. A further increase to 20 g l-1 Na2WO4·2H2O leads to a rougher surface and some small points of white materials on the coating. The coating thickness increases with the increase of the concentration of Na2WO4·2H2O. The

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coating formed in the W-free electrolyte for 600 s is 46.0 ± 1.2 µm, while the coating formed for 480 s with the addition of 20 g l-1 Na2WO4·2H2O reaches a thickness of

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64.7 ± 4.4 µm.

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3.2 OES spectra

Fig. 2 (a-d) shows the typical OES spectra for the PEO of AZ31 magnesium alloy in the aluminate based electrolytes with the addition of 0-20 g l-1 Na2WO4·2H2O. The strongest signal in all the spectra is the lines of Na I (“I” and “II” denote the neutral and singly ionized atoms, respectively) duplets, at 588.99 and 589.59 nm, respectively, which come from the electrolyte. The lines of other species from the electrolyte, such

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ACCEPTED MANUSCRIPT as O, Hα and Hβ, H2O+ are also detected. The signal of Al I line at 396.1 nm, which is an abundant electrolyte species, could be found in all the magnified spectra, but in Fig.2(a-d), its intensity is very low and hardly is seen. In Fig.2(a), there is a very weak

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peak at 568.82 nm, which is another line for Na I [30]. However, we assigned the lines at the same position for spectra recorded in the W-containing electrolytes to W I lines at 567.96 and 568.96 nm. In order to verify that the lines were correctly assigned,

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Fig.1(e) and (f) have compared the magnified spectra between 500 and 600 nm

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recorded in a W-free electrolyte with 15 g l-1 NaAlO2 and the present aluminate based electrolyte (10 g l-1) with the addition of 10 g l-1 Na2WO4·2H2O. We can see that the signal at ~568 nm is significantly higher for the latter electrolyte despite the same Na+ concentration in the two electrolytes. Hence, it is reasonable that the lines at ~ 568 nm

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in the latter electrolyte come from W. However, it can also been seen from Fig.2 that the intensity of W lines seems not to be dependent on the concentration of Na2WO4 in

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the electrolytes.

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Besides the lines from the electrolyte species, the lines of Mg, which come from the substrate, are relatively strong in all the cases. According to our previous study [30], the presence of substrate elements in the OES spectra indicates that the discharges during the coating formation are mainly the high intensity, penetrating discharges, which usually form the pancake structures and were previously termed as “B” type discharges in a model proposed by Hussein et al.[64]. The W lines and Al lines indicate that the anions from the electrolyte also have taken part in the coating

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ACCEPTED MANUSCRIPT formation, although previous study shows that for the heavy anion deposition in concentrated electrolyte, the spectral lines of most species, except the Na I lines, cannot be detected [30]. The W and Al atoms were possibly excited by the high

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temperature and pressure in the penetrating discharge channels, giving off light when they fall back to ground state.

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3.3 Phase compositions

The phase compositions of the coatings formed in electrolytes with the addition of 0-20 g l-1 Na2WO4·2H2O and a coating after the removal of its outer layer have also been examined by XRD. Fig.3(a) shows that the coating obtained in the W-free

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electrolyte is composed of the phases of MgAl2O4 (Spinel) and MgO. The peaks of Mg in the XRD pattern come from the substrate. As the concentration of Na2WO4·2H2O in the PEO electrolytes increases, the additional peaks of WO3 and

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W18O49 and a suspected peak of W (2θ = ~40.3 degree) become more evident in XRD

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patterns ( see Fig.3(b-e)). The peak at 2θ = ~40.3 degree is also presented for the PEO coatings on Al alloys in tungstate containing electrolytes and was attributed to free state W [47,48,53, 65]. These authors ascribed the formation of W by the reaction of WO3 with the substrate metal(Al) in the discharge channels: WO3 + 2Al→Al2O3 + W

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It seems that the same mechanism can be applied to the present coatings, i.e., the free

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ACCEPTED MANUSCRIPT state W is produced by the reaction of WO3 with Mg. However, we used the term “suspected” in the above discussion and a “?” is used for the W peak in XRD patterns, since common sense tells us that free metal is difficult to exist in the high temperature

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discharge channels during PEO. Furthermore, there is also a small bump at the same position in 2θ for the coating formed in the electrolyte without Na2WO4·2H2O (Fig.3(a)), which supports our suspicion that the peak is not from W. Fig.3(d) is the

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XRD of the inner part of a coating formed in 10 g l-1 Na2WO4·2H2O, obtained by

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polishing the sample down to ~20 µm (the original coating is ~59.3 µm ). Judging from the intensity of the suspected W peak, there is possibly no enrichment of that phase at the lower part of the coating. Whether there is free state W in the coatings

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will be further explored by XPS in this study.

3.4 Effect of Na2WO4 on the morphologies of the coatings

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Fig.4 shows the morphologies of a coating formed for 600 s in the aluminate based

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electrolyte, without the addition of Na2WO4·2H2O. The lower magnification image of the cross section displays a bi-layered structure, showing big or lateral pores between the outer and inner layer. However, a magnification of the interfacial region(the inset at the left hand corner of Fig.4(a)) shows that there is still a barrier layer at the base of the coating, which has also been found in magnesium PEO coatings by others [37]. The detail of a large pore is shown by inset in Fig.4(a), which has a dimension more than 50 µm in width. The features as revealed in Fig.4(a) are also found in our

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ACCEPTED MANUSCRIPT previous work for the PEO of aluminium and zirconium alloys [25,30,66]. The reason for the formation of the big pores and hence the bi-layered structure is due to the gas evolution beneath the pancake structures which were formed with the strong

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penetrating discharges during PEO [25,30]. The surface morphology of the coating is shown in Fig.4(b). As expected, numerous pancake features, ~ 20 µm in diameter, dominate the coating surface. The presence of the pancakes supports the previous

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hypothesis on the coating formation mechanism. Furthermore, it has been mentioned

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that the PEO coating structure is strongly related with the electronic property of the PEO coatings, i.e., the highly insulating dielectric oxide coating and semiconducting coating will favor the formation of pancake and coral reef structures, respectively [29]. The main phase composition of the present coating, MgAl2O4, is known to be an

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electrical dielectric, with a wide band gap of 7.8 eV [67], which is in accordance with the observed coating morphology.

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Fig.5 (a) shows the cross section of a coating formed for 600 s in the aluminate

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electrolyte with the addition of 5 g l-1 Na2WO4·2H2O. The cross section is largely similar to the coating formed without the addition of Na2WO4·2H2O, however, a band of light materials was found at the coating/substrate interface. The light materials in the backscattered electron image imply the enrichment of higher atomic number elements, since heavier elements will backscatter more electrons. The point EDS analyses of the three different positions in the cross section, A, B and C as shown in the inset of Fig.5(a), are listed in Table 1. The result shows that W element enriches at

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ACCEPTED MANUSCRIPT the coating/substrate interface, reaching 19.87 wt% (point A), the outer layer also has a relatively high W content (11.66 wt% at point C), however, the intermediate part of the coating (the inner layer) has the lowest W content (4.11 wt% at point B). Fig. 5(c)

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shows an EDS line scan across the cross section of the same coating. It is evident that a high peak is present at the coating/substrate interface for the W scan. The W species in the coating come from the electrolyte, however, the other element, Al, which is also

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from the electrolyte, distributes in a different way. The Al shows a decreasing trend

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from the outer layer to the inner layer. The different distribution patterns of W and Al in the coating may reflect their different transportation behaviours during PEO.

Fig.5(b) shows the surface morphology of the coating formed in 5 g l-1 Na2WO4·2H2O.

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The morphology is somewhat similar to that of the coating formed without the addition of Na2WO4·2H2O, however, a special feature, as indicated by the arrow D in the figure, has been found on the coating surface. This feature is more evident when

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the concentration of Na2WO4·2H2O increases and will be discussed later. The inset in

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Fig.5(b) is a backscattered electron image of the coating surface, showing the presence of micro-cracks. The cracks may be formed due to the releasing of thermal stress during the solidification process of the PEO coating. Fig.5(d) is the EDS spectrum made on the feature D, which reveals a composition in wt% of O 27.23, Mg 32.06, Al 28.21, W 12.50. The peak of Au in Fig.5(d) is due to the sputtering of the coating surface before SEM examination.

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ACCEPTED MANUSCRIPT The morphologies of the coatings formed in the electrolytes with higher concentrations of Na2WO4·2H2O are displayed in Fig.6. The cross section of the coating formed for 600 s in the electrolyte with the addition of 10 g l-1 Na2WO4·2H2O

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is shown in Fig.6(a), which is similar to the coating formed in 5 g l-1 Na2WO4·2H2O, exhibiting a band of light materials at the base of the coating. The cross section of the coating formed in the electrolyte with a higher concentration of Na2WO4·2H2O, 15 g

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l-1, shows a more uniform distribution of bright materials along the coating depth (see

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Fig.6(c)). However, the EDS line scan of the coating still displays an enrichment of W at the region close to the base of the coating. The distribution of Al along the coating depth is similar to that revealed in Fig.5(c).The cross sections of the coatings formed in 20 and 25 g l-1 Na2WO4·2H2O are similar to that in Fig.6(c) and only the cross

the inset in Fig.6(e).

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section of the coating formed for 480 s in 25 g l-1 Na2WO4·2H2O has been provided in

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Fig.6(b) and (d) are the corresponding surface morphologies for the respective cross

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sections at left hand side, and Fig.6(e) and (f) are the surface morphology of a coating formed in 25 g l-1 Na2WO4·2H2O. From these images, the special feature as indicated by the arrow “D” in Fig.5(b) is more evident. Fig.6(f) shows that the feature looks like a cluster of lotus seeds in the seedpod. Actually, similar features have been found in the PEO study of zirconium alloys, which was termed “the characteristic solidification structure” as discussed in the Introduction. The formation of such features was believed to be related with the solidification process of a micro melting pool on the

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ACCEPTED MANUSCRIPT coating surface and was assisted by the low thermal conductivity of zirconia [31,33]. The presence of the characteristic solidification structure on the coatings formed by PEO of the AZ31 magnesium alloy in W-containing electrolytes may imply that the

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tungsten oxides also have low thermal conductivities. There seems to be a lack of data for the thermal conductivity of the oxides of tungsten. However, Wang et al. [68] have measured the thermal conductivity of the thin films of tungsten Oxide, showing values

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of 1.63 Wm-1 K-1 for WO3 thin films and 1.28 W m-1 K-1 for WO2/WO3 films, which

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are really low values compared to those of the bulk periclase (MgO) or spinel (MgAl2O4) (46.2 Wm-1 K-1 and 11.8 Wm-1 K-1, respectively [69]).

For the coatings formed in 20 g l-1 Na2WO4·2H2O, it has been previously mentioned

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that some small white points will appear on the coating surface at later stage of PEO. This phenomenon has also been found in PEO of a zirconium alloy in concentrated aluminate electrolytes and it was attributed to the heavy anion deposition process at

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some particular positions [30]. The white points on a coating formed for 600 s in 20 g

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l-1 Na2WO4·2H2O have been examined by SEM and EDS. The circled region in Fig.7(a) shows such a white point on the coating surface, however it is not “white” in SEM. The feature is slightly dark in the backscattered electrons compared to its surroundings. Fig.7(b) is a magnification of the central part of the circled region in Fig.7(a), showing some irregular patches of white materials. The EDS of the white materials shows higher W content at ~50 wt% ( Fig.7(c)). The EDS made at point B, a location somewhat remote from the central white materials, shows a high Al content

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ACCEPTED MANUSCRIPT at ~47 wt%. The Mg content at both points shows a low value of ~12 wt%. The results presented here support the view that these white points are mainly formed by the

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deposition of electrolyte anions.

3.5 Coating growth with reference to the original alloy surface

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In order to clarify the outward and inward growth of the coating, the boundary region

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including both coating and the masked alloy has been examined in cross section by SEM. As described in the Experimental section, the coating was formed for ~ 240 s in the aluminate electrolyte with the addition of 10 g l-1 Na2WO4·2H2O. Figure 8 shows that the coating grows in directions both inward and outward to the original plane of

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the untreated alloy. According to Fig.8(b), the inner layer and the W-enriched barrier layer are well below the original plane of the alloy. The outer layer, which is loose

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and cracked, grows above the original alloy surface.

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3.6 The distribution of W, Al along coating depth at different PEO time

The cross sections of the coatings formed for 600 s in 5 and 10 g l-1 Na2WO4·2H2O show a W-rich light band along the coating/substrate interface. The W species originates from the electrolyte, which was possibly decomposed by the high temperature of the PEO plasma. In the first look, the decomposition likely occurs at the coating surface, since sparks are observed on the electrode surface during PEO.

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ACCEPTED MANUSCRIPT Hence W species might access to the coating outer layer first. In order to know the element distribution at different stages of PEO, EDS line scans for the coatings formed at 30, 60, 120, 180 and 300 s in the aluminate electrolyte with the addition of

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10 g l-1 Na2WO4·2H2O are made and compared in Fig.9. According to the line scan results, W element distributes relatively uniformly at initial stages of 30 and 60 s, however, for treatment time longer than 120 s, the W species begin to enrich at the

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lower part of the coating and a thin W-enriched layer has been formed for the coating

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formed at 300 s. In contrast to the W element, the distribution of Al always decreases from the outer layer to the inner layer.

3.7 Sequential anodizing

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3.7.1 PEO in W-free and then W-containing electrolytes

In this section, the AZ31 magnesium alloy was first treated in the aluminate based

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electrolyte (10 g l-1 NaAlO2 + 3 g l-1 C6H8O7·H2O + 2 g l-1 KOH) for 300 s to form a

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precursor coating without W species. Then the precursor coating was treated for different times in the aluminate electrolyte with the addition of 10 g l-1 Na2WO4·2H2O.

Fig.10 shows the cross sections of the coatings after being treated in the second W-containing electrolyte for 60, 300 and 600 s, respectively. It was found that even for a short time of 60 s, W species have reached the innermost of the coating at some

19

ACCEPTED MANUSCRIPT particular locations, forming a thin layer of white materials along the coating/substrate interface (Fig.10(a)). However, there are also parts of intact coating as shown by the inset at the left lower corner of the image. It is also shown in Fig.10(a) that the coating

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has been thickened at the locations with the incorporation of W species (see the insets). The white band at the coating/alloy interface is more evident when the treatment times of the second step were increased to 300 and 600 s (see Fig.10(b) and (c),

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respectively).

Fig.11 shows the surfaces of the sequential anodized coatings after being treated in the W-containing electrolyte for 60, 120, 300 and 600 s, respectively. The low magnification image of the coating treated for the shortest time of 60 s (Fig.11(a))

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displays two morphologies: the dark areas, which are the original surface of the precursor coating, and the white areas, which were modified by the incorporation of W due to the PEO in the second electrolytes. The inset in Fig.11(a) is a magnification

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at the white area, showing pancakes with lighter or darker colors. When treatment

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time is further prolonged, the sample surfaces are covered by more white areas and become a more or less homogenous distribution of white and dark coating materials. The insets in Fig.11 show that the morphologies of the different coatings with the incorporation of W species are similar at higher magnification.

Figs.10 and 11 show clearly that the coating morphologies have been modified by the treatment in the second electrolyte. The cross sections in Fig.10 imply that new

20

ACCEPTED MANUSCRIPT coating materials are formed at the base of the coating. Evidently, the new coating materials are grown discontinuously and should be related with the discharges in the second electrolyte. The clusters of white areas on the coating surface in Fig.11(a)

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imply that the treatment in the W-containing for 60 s has experienced too many discharging events. In order to have a better insight into the effect of the discharge on the coating formation, the precursor coating was treated for only 3 s in the

SC

W-containing electrolyte. The visual observation of the PEO shows that the sample

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only has undergone several sparks at some particular locations. Fig.12(a) shows the backscattered electron micrograph of the surface of the treated sample, there are only a few white spots, which were caused by PEO in the second electrolyte. The inset in Fig.12(a) shows a pancake structure with a ring of white coating material, which was

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possibly caused by one or several discharges. Fig.12(b), which is a higher magnification image, shows some irregularly patches of white materials on the coating surface, which may result from the directly decomposition of the

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W-containing electrolyte species. A cluster of white equiaxed grains is also present in

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the middle of the micrograph and was further shown in secondary electrons by the inset. Fig.12(c) shows a feature that was found by chance on the coating surface. A close examination shows that the feature is actually the inner coating, which was exposed by the loss of its outer coating materials. The damage of the outer coating may be caused by the violent gas evolution during the discharges and can be inferred from the residue of a broken pancake structure. The white colour of the feature implies the incorporation of W species. Fig.12(d) shows the inner layer of the coating

21

ACCEPTED MANUSCRIPT after being polished down to a thickness of ~10 µm. White island-like features, which extend to a length of ~100 µm, are present after the removal of the outer coating. These findings show that large amount of W-containing materials from the electrolyte

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have accessed to the interior of the coating. The pancake with the white ring (point A in Fig.12(a)) and the W-enriched coating materials in the inner layer (point B in Fig.12(c)) have been analyzed by EDS (Fig.12(e) and (f), respectively). The result

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shows that the inner layer has a W-content of 16.50 Wt%, which is higher than that of

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the pancake on the surface ( 6.35 wt%).

3.7.2 PEO of the W-containing precursor coating in W-free electrolytes

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PEO of the magnesium alloy in a reversed sequence has also been carried out. The alloy was first treated in the W-containing electrolyte (10 g l-1 NaAlO2 + 3 g l-1 C6H8O7·H2O + 2 g l-1 KOH + 10 g l-1 NaWO4) for 300 s, then in the W-free aluminate

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electrolyte for different times.

Fig.13(a) shows the cross section of the sequential anodized coating after being treated in the W-free electrolyte for 120 s. Most parts of the coating exhibits a lighter color, which is the original W-containing coating materials formed at the first step. However, at some particular locations, darker coating materials are present. The inset shows that the coating has been thickened at the location with the darker materials. Fig.13(b) shows the surface morphology of the same coating. Many relatively big

22

ACCEPTED MANUSCRIPT sized pancake structures with dark color are seen, and the inset shows that the dark pancake structure has a diameter ~50 µm, with a hole in the center. The area fraction of the dark pancakes has been roughly estimated, being ~23%. Fig.13(c) shows the

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cross section of the coating with a longer treatment time of 600 s in the W-free electrolyte. The outer layer of the coating is similar to that of the coating in Fig.13(a), showing the W-rich coating materials with a lighter color. However, the inner layer is

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much thickened and displays a non-homogenous nature. The inner layer also shows a

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darker color, implying the depletion of W-element, however, an EDS analysis made at the coating/alloy interface still shows the enrichment of W in the most inner layer, being 12.47 wt%. The surface of the coating is displayed in Fig.13(d), which is similar to the coating with a shorter treatment time of 120 s, however, the numbers of

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the dark pancakes are slightly increased after longer treatment and the area fraction of the dark pancakes has been increased to ~45%. The darker pancakes are modified by the discharges in the second electrolyte, and apparently, not all the surface areas have

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experienced plasma discharging in the PEO treatment in the second electrolyte.

3.8 SEM and XPS analysis of the backside of a stripped coating

This section is devoted to verify if there is free state W in the coatings, although previous XRD study shows that a suspected peak of free state tungsten. The previous SEM examination indicates that W-species are prone to enrich at the coating/substrate interface, hence it will be of interest to know the chemical state of the W element at

23

ACCEPTED MANUSCRIPT the lowest part of the coating. In order to do this, the coating formed in the aluminate electrolyte with the addition of 10 g l-1 Na2WO4·2H2O for 600 s was immersed in a dilute phosphoric acid to dissolve the magnesium alloy substrate and the backside of

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the free standing coating was examined by SEM and XPS.

Fig.14 shows the morphologies of the free standing coating. Fig.14(a) is a low

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magnification micrograph (secondary electrons) of the backside of the stripped

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coating. The backside is dominated by many mound-like features, some of which have a size of ~ 100 µm, and a fine porous nature. The higher magnification micrograph in backscatter electrons in Fig.14(b) shows many light colored features on the surface of the backside. Fig.14 (c) is the cross section of the stripped coating,

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which is obtained by polishing the epoxy-mounted free coating. Although the previous white band at the coating/substrate interface for the non-stripped coating is not so apparent, we can still see some enrichment of light materials at the lowest part

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of the coating. The EDS made on a large area of the backside of the coating shows a

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high W content of 22.18 wt%, the value is close to that of the W content in the barrier layer (see Table 1).

Fig.15 is the W 4f scan of the XPS of the backside of the stripped coating. The spectrum can be deconvoluted into two doublets. One of the doublet shows the binding energies of 36.01 eV (W 4f7/2) and 38.11 eV (W 4f5/2), respectively, which corresponds to WO3 [70,71]. The other doublet has W 4f7/2 and W 4f5/2 peaks at 34.3

24

ACCEPTED MANUSCRIPT and 36.5 eV, which may correspond to W18O49 [72]. According to Fig.15, there is no free state W in the coating since the free state W has a 4f7/2 peak at 31.0 eV [73].

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The area examined by XPS has a diameter of ~ 400 µm, hence the result can be viewed as representative. The backside of the coating is also the place where the free state W is most likely to be found since it was most close to the substrate which can

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provide Mg metal for the reduction reaction ( if it is so). Furthermore, the SEM shows

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that there is an enrichment of W species at the coating/substrate interface, whereas the present XRD shows that the suspected W phase is not enriched at the lower part of the coating. As a result, it can be safely inferred that free state W does not exist in the present PEO coatings. The peak in XRD patterns, which was assigned to W in the

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present study and in literature, is largely incorrect. It should be pointed out that some polymorphs of alumina also have a diffraction line at 2θ = ~40.3 degree, for example,

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PDF# 12-0539, #11-0517 and #10-0414.

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3.9 Coating formation mechanism

The present study shows that following a certain period of PEO treatment, a structure with an outer layer and an inner layer and a barrier layer in contact with the substrate will be developed for the coatings. Big pores are also present between the outer and inner layers. According to the observation in this study, the coating grows both outward and inward, and the outer layer is above the original alloy surface, while the

25

ACCEPTED MANUSCRIPT inner layer is below the original metal surface.

Although outward growth and inward growth for the coatings have been discerned in

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this study, it has been found that coatings are mainly grown by the inward growth at the later stage of PEO. This is revealed by the sequential anodizing results in Fig.13. The thickness of the outer layer derived from the W-containing precusor coating has

SC

changed little after being treated in the W-free electrolyte for different times, only the

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inner layer has been significantly thickened after the prolonged treatment time. The present study shows that the outer layer not having experienced discharges will be kept intact during PEO. However, plasma discharges have caused some enlarged pancake structure on the coating surface. Those pancake structures acted as channels

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for mass transportation during PEO, through which the electrolyte anions entered the coating interior and gases were released into the electrolyte through the central hole in the pancakes. The pancakes are only confined within the outer layer since the inner

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layers below as exposed in Fig.12(c) and also in our previous study of the PEO an

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Al-Cu-Li alloy [66] display different morphologies. In [66], the growth of inner layer was attributed to an additional “D” type discharge which happens in the pores, however, it seems to be impossible to verify the presence of such discharges. The plasma discharges are actually ionized gases and may be only phenomena associated with certain coating formation mechanisms. Hence, it may be better to say “coatings are formed “with” instead of “by” certain types of discharge”.

26

ACCEPTED MANUSCRIPT The OES spectra for the present study show the strong lines from the substrate element. Hence, according to [30], strong penetrating discharges dominated the PEO processes, which can also be supported by the visual observation of the discharging

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behaviour in this study. The penetrating discharges, i.e., the B type discharges in Hussein’s model, are known to result in the pancakes, which were previously thought to be a structure involving mainly substrate elements [29, 64, 74]. However, the EDS

SC

of a pancake structure in Fig.12 shows that the Al content is comparable to that of Mg.

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Further, the observation of the cross sections of coatings formed by sequential anodizing shows that electrolyte species, for example, W, can easily get access to the base of the coating, possibly following only one single discharge. These findings help us have a better understanding of the strong penetrating discharges and hence, coating

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formation mechanism. In the past, the three types of discharges proposed by Hussein et al. [64] have been widely adopted to explain the PEO coating formation mechanism. Apart from the B type discharge, the A and C type discharges, which was thought to

EP

happen at the upper or top coating, were used to account for the features derived for

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electrolyte species [25, 63, 74]. However, our recent study shows that it needs not to adopt those hypothetical discharge types to explain the coating formation mechanism [30]. The PEO process is a combination of the oxidation of the substrate and the anion deposition process, and it has been shown that the anion deposition process requires less energy, that is why weak discharges accompany the PEO process in concentrated electrolytes [30]. Hence, the features previously assigned to the A and C type discharges are merely a result from the anion deposition. It is evident that the

27

ACCEPTED MANUSCRIPT energetic penetrating discharges (the B type discharges) are by no means less able to cause the decomposition of electrolyte component and anion deposition. That is why large amount of W and Al species from the electrolyte can be found on the pancake

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feature and also at the coating/substrate interface following a single penetrating discharge. The previous thinking that the B type discharge was mainly associated with the oxidation of substrate may be due to the dilute nature of the electrolyte which

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cannot afford sufficient electrolyte anions for coating formation.

The coating formation mechanism at the later stage of PEO for the present magnesium alloy is schematically illustrated in Fig.16. As shown in the figure, an outer layer and inner layer have already been developed. Two discharging locations are shown in the

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figure. At the later stage of PEO, the number of the sparks will be significantly reduced, which can be seen in Fig.1(c-g), and in some cases, only one moving spark was observed on the electrode at the later stage of PEO [75]. Most of the anodic

EP

current will be concentrated at the discharging locations since the main coating

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materials (such as MgO and MgAl2O4) are dielectric and can hardly convey charge carriers. The diameter of the pancake structure, such as those shown in Fig.13, is approximately 50 µm. Assuming that each strong discharging spark in Fig.1 corresponds to one discharging event and one pancake structure, it will be reasonable to think that there are approximately 5 discharging events on 1 cm2 coating surface at an instant at the later PEO stage. Then the anodic current density passing through the discharge channels can be evaluated under the present PEO condition with a peak

28

ACCEPTED MANUSCRIPT anodic current density of 1 A cm-2 (see Fig.1(b)). A value of ~ 104 A cm-2 is obtained, which is significantly higher than the microdischarge current density of 1.8 to 5 A cm-2 in a previous study [76]. The difference may be due to that earlier PEO stage was

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considered in [76] and the area of the sparks instead of the pancakes was used for the calculation. The visible sparks are ionized gases and certainly have larger dimensions than the discharge channels beneath them. Evidently, such a high anodic current

SC

density has great power to melt the oxides within the discharge channels. Hence, a

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large amount of coating material is melted and extends to greater areas beneath the pancake structures in Fig.16, which is supported by SEM observation in the present study. The pancake structures, which were displayed with a maroon red color in the illustration, might be partially melted during the PEO since the pancakes contact the

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cold electrolyte, especially at the negative pulse and pulse-off duration. Large number of electrolyte anion species will be drawn into the discharge channels by the electric field [27] and contribute to the coating formation. The electrolyte with the anions may

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have already presented in the inner pores of the coating before the occurrence of a

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certain discharge. Great amounts of gases, possibly in ionized state, i.e., plasma, are generated simultaneouly within the discharge channels and were eventually released into the electrolyte. After the extinguishment of a strong discharge, the volumes taken by the plasma underneath the pancakes as depicted in Fig.16 will form the big pores as shown in this study and also in previous studies [25, 30,66].

The present findings support the view that PEO coating grows discontinuously, and

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ACCEPTED MANUSCRIPT new coating materials are formed at the location of plasma discharges. Previously, the discharge channel is viewed as a circular column leading to the substrate, forming a pancake after discharge termination [77,76]. However, the discharge channels in the

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present study consists of two parts as depicted in Fig.16. The upper part of the discharge channel is the pancake structure in the outer layer, however the lower part of the discharge channel expands to a greater area below the pancake. The present

SC

study may also disapprove a previous view that PEO is an ejection of molten oxides

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onto the top surface of the discharge channels and a thickening of coating at that location [28,77]. The inner layer and outer layer(the pancake) usually show different morphologies and they are seperated by big pores, it is reasonable that they are grown independently. It is shown in Fig.16 that high tempearture ionized gases are present

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between the inner melted coating materials and the pancake at the outer layer during a discharging event. Hence, the molten materials at the inner coating cannot easily reach the outer layer. As a result, the inward coating growth plays an important role

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and the thickness of the outer layer changes little during the sequential anodizing as

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shown in Fig.13.

The PEO of valve metals has been found to be associated with anomalous gas evolution, i.e., the amount of the liberated oxygen far exceeds the Faradaic yield [29,78,79]. The anomalous gas evolution was found to be more evident when the strong discharges occur [29]. Snizhko et al. first think that the major reason for the anomalous gas evolution is due to the radiolytic effect of plasma discharge on the

30

ACCEPTED MANUSCRIPT adjacent electrolyte volume [78], however, the phenomenon was later attributed by the same authors to “primarily peroxide decomposition due to the interaction with both hydroxyl ions and radicals at the discharge–electrolyte interface” [79]. Our

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previous observation showed that the big gas bubbles appeared to be ejected out of the discharge channels [29], which implies that the anomalous gases are generated within the discharge channels. Due to the extremely high anodic current density (~ 104 A

SC

cm-2) in the discharge channels, which is evaluated in this study, we proposed here

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that the direct thermal decomposition of water in the discharge channels is the main reason for the anomalous gas evolution. Although Snizhko et al. have also mentioned the effect of thermal decomposition, but they thought that this effect was unimportant due to the extremely strong bonds of water molecule [78]. The temperature inside the

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discharge channel is hard to evaluate, however, it is reasonable to assume that the value might be higher than that of the plasma above the pancake structures. Even the temperature of the plasma in electrolyte, which is reported to be ~ 6000 K [30], is

EP

much higher than the temperatures for direct thermal decomposition of water. It is

AC C

reported that the thermal decomposition of water begins at 2000 K [80,81] and the process is nearly completed at ~4000 K [82].

It is interesting to have observed in the present study that the W species are prone to be enriched at the coating/substrate interface while the Al species distribute in a decreasing trend for the outer layer to the inner layer. One reason for this phenomenon may lie in the fact that the electrolyte species can be transported to innermost of the

31

ACCEPTED MANUSCRIPT coating following a plasma discharge as shown by the sequential anodizing in this study. The other reason may be due to the faster inward migration rate of W species in the molten oxide compared to Al, however, future investigations are required for this

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assumption.

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4. Conclusions

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PEO of AZ31 magnesium alloys was carried out under pulsed bipolar regimes in an aluminate electrolyte with the addition of 0-25 g l-1 Na2WO4·2H2O. The addition of tungstate endows a black colour to the obtained coatings. Sequential anodizing has also been adopted to investigate the PEO mechanism. The following conclusions can

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be drawn:

1. Periclase (MgO) and spinel (MgAl2O4) are the main phases for the coatings formed

EP

in the W-free aluminate electrolyte. The addition of tungstate to the aluminate

AC C

electrolyte leads to the formation of additional WO3 and W18O49 phases. A suspected W peak has been found in the XRD pattern for the PEO coatings with the incorporation of W species, however, X-ray Photoelectron Spectroscopy (XPS) has not detected any free state W in the coatings.

2. The surfaces of the coatings are featured by pancake structures, which are related to the dielectric properties of the spinel (MgAl2O4) phase. The incorporation of W

32

ACCEPTED MANUSCRIPT species has caused “the characteristic solidification structures” in the coatings, due to the low thermal conductivity of WO3.

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3. The cross section of the coatings is featured by an outer layer, inner layer and an additional barrier layer at the coating/substrate interface. W-species are prone to be enriched at the innermost of the coating for the coatings formed for longer times in the

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W-containing electrolyte.

4. Coatings are formed with the energetic penetrating discharges, however, there is possibly no ejection of molten oxide onto the coating surface. The outer layer is formed at the earlier stage of PEO and is a result of outward growth. At the later stage

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of PEO, the coating is dominated by inward growth.

5. The penetrating discharge can cause significant anion deposition. Species from the

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electrolyte anions such as W and Al have been transported to the innermost of the

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coating following the penetrating discharges. Extremely high anodic current density, ~ 104 A cm-2, exists in the discharge channels, which have caused the melting of large amount of inner coating materials beneath the pancakes and may be the main reason for the direct thermal decomposition of water and hence the anomalous gas evolution for PEO.

Acknowledgement

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ACCEPTED MANUSCRIPT

The authors thank the National Natural Science Foundation of China (Grant Numbers:

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51671084 and 51071066) for support of this work.

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formed by microarc oxidation on Al-Cu-Mg alloy, Thin Solid Films 372（2000）

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processes of coatings formed by AC plasma electrolytic oxidation of aluminium，

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Electrochim. Acta. 54 (2009) 6767–6778. [63] Y. Cheng, M. Mao, J. Cao, Z. Peng，Plasma electrolytic oxidation of an Al-Cu-Li alloy in alkalinealuminate electrolytes: A competition between growth and dissolution for the initial ultra-thin films，Electrochim. Acta. 138 (2014) 417-429. [64] R.O. Hussein, X. Nie, D.O. Northwood, A. Yerokhin, A. Matthews, Spectroscopic study of electrolytic plasma and discharging behaviour during the plasma electrolytic oxidation(PEO) process, J. Phys. D: Appl. Phys. 43 (2010)

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Sens. Actuators, B: Chem. 219(2015) 346-353. [71] A.P. Shpak, A.M. Korduban, M.M. Medvedskij, V.O. Kandyba, XPS studies of active elements surface of gas sensors based on WO3−x nanoparticles, J. Electron Spectrosc. Relat. Phenom. 156-158 (2007) 172-175. [72] A.V. Naumkin, A. Kraut-Vass, S.W. Gaarenstroom, C.J. Powell, NIST X-ray Photoelectron Spectroscopy Database (Version 4.1), National Institute of Standards and Technology, 2012

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aluminium, J. Phys. D: Appl. Phys. 36 (2003) 2110-2120.

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ACCEPTED MANUSCRIPT water, possibilities for improvement of process efficiency, Int. J. Hydrogen Energ. 29 (2004) 1451-1458. [81] J. Lede, F. Lapicque, J. Villermaux, Production of hydrogen by direct thermal

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decomposition of water, Int. J. Hydrogen Energ. 8 (1983) 675-679. [82] X. Zhai, K. Liu, Q. Han, New Energy Technology 2nd ed, Chemical Industry Press, Beijing, 2010 In Chinese.].

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Figure captions:

Figure 1. Positive and negative (absolute value) peak cell potential-time responses (a) and current waveforms (b) for the PEO of AZ31 magnesium alloy in 10 g l-1 NaAlO2 + 3 g l-1 C6H8O7·H2O + 2 g l-1 KOH with the addition of different concentration of

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Na2WO4·2H2O. (c-g) are the discharging behaviours recorded in sequential order within a duration of ~ 2 s after ~ 600 s PEO treatment in the aluminate electrolyte with the addition of 10 Na2WO4·2H2O. (h-l) are the appearance of coatings formed in

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electrolytes with the addition of 0, 5, 10, 15 and 20g l-1 Na2WO4·2H2O, respectively.

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The coatings in (h-j) are formed with a duration of 600 s, and the last two coatings are 480 s. The size of specimens is 10×20 mm.

Figure 2. (a-d) The typical OES spectra recorded at different stages of PEO in the aluminate electrolyte (10 g l-1 NaAlO2) with the addition of different concentrations of Na2WO4·2H2O: (a) 0 g l-1; (b) 5 g l-1; (c) 10 g l-1; (d) 20 g l-1. (e, f) A comparison of the OES spectra between 500- 600 nm for PEO in (e) 15 g l-1 NaAlO2 and (f) 10 g l-1

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ACCEPTED MANUSCRIPT NaAlO2 + 10 g l-1 Na2WO4·2H2O, the other additives ( citric acid and KOH) are also added in the two electrolytes.

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Figure 3. XRD patterns of the PEO coatings formed in the aluminate based electrolytes with the addition of different concentrations of Na2WO4·2H2O: (a) 0 g l-1; (b) 5 g l-1; (c,d) 10 g l-1; (e) 20 g l-1. (d) is the inner part of the coating after being

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polished down to ~20 µm. All the samples are formed with a duration of 600 s, except

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the sample formed in 20 g l-1 Na2WO4·2H2O, which is 480 s.

Figure 4. Morphology of the cross section and surface of a coating formed for 600 s in the aluminate based electrolyte (10 g l-1 NaAlO2) without the addition of

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Na2WO4·2H2O. The cross section and surface are shown in backscattered and secondary electrons, respectively.

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Figure 5. Morphology and EDS analyses of the cross section and surface of a coating

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formed for 600 s in the aluminate based electrolyte with the addition of 5 g l-1 Na2WO4·2H2O. (a) cross section in backscattered electrons; (b) surface in secondary and backscattered electrons(the inset), respectively; (c) An EDS line scan across the substrate and coating; (d) The EDS spectrum for the feature “D” in (b).

Figure 6. Morphology of the cross sections and surfaces of the coatings formed for 600 or 480 s in the aluminate based electrolyte with the addition of different Na2WO4

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ACCEPTED MANUSCRIPT concentrations. (a, b) 10 g l-1 Na2WO4·2H2O, 600 s ; (c,d) 15 g l-1 Na2WO4·2H2O 4, 480 s, the inset in (c) is an EDS line scan ; (e,f) 25 g l-1 Na2WO4·2H2O, 480 s. The

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cross sections are shown in backscattered electrons.

Figure 7. Morphology shown in backscattered electrons for a white point feature on

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the coating formed for 600 s in the aluminate based electroltye with the addition of 20

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g l-1 Na2WO4·2H2O. (a) low magnification of the feature, which is the circled region. (b) the central part of the circled region in (a). (c) and (d) are the EDS analyses of point A and B, respectively.

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Figure 8. (a) The cross section including the coating and the masked alloy, and (b) is a magnification of boxed area in (a). The coating was formed for ~240 s in the

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aluminate electrolyte with the addition of 10 g l-1 Na2WO4·2H2O.

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Figure 9. The EDS line scans of elements W and Al from the substrate to the outer layer. The coatings were formed for 30-300 s in the aluminate electrolyte with the addition of 10 g l-1 Na2WO4·2H2O.

Figure 10. Backscattered electron micrographs of the cross section of coatings formed by PEO of the W-free precursor coating in W-containing electrolyte (the aluminate based electrolyte with the addition of 10 g l-1 Na2WO4·2H2O) for different durations:

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ACCEPTED MANUSCRIPT (a) 60 s; (b)300 s; (c) 600 s. The precursor coating was formed in the aluminate based electrolyte for 300 s.

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Figure 11. Backscattered electron micrographs of the surface of the coatings formed by PEO of the W-free precursor coating in W-containing electrolyte (the aluminate based electrolyte with the addition of 10 g l-1 Na2WO4·2H2O) for different durations:

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(a) 60 s; (b)120 s; (c) 300 s; (d) 600 s. The precursor coating was formed in the

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aluminate based electrolyte for 300 s.

Figure 12. Backscattered electron micrographs of the W-free precursor coating after being treated in W-containing electrolyte (the aluminate based electrolyte with the

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addition of 10 g l-1 Na2WO4·2H2O) for 3 s. (a-c) surface morphologies at different locations and magnifications; (d) the inner coating exposed by polishing down to ~10 µm; (e) and (f) are the EDS analyses at points A and B, respectively. The precursor

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coating was formed in the aluminate based electrolyte for 300 s.

Figure 13. Backscattered electron micrographs of the cross section and surface of the coatings formed by PEO of the W-containing precursor coating in W-free electrolyte (the aluminate based electrolyte) for different times: (a,b) 120 s; (c,d) 600 s; (e) EDS analysis at the coating/substrate interface (point A in (c)). The precursor coating was formed in the aluminate based electrolyte with the addition of 10 g l-1 Na2WO4·2H2O for 300 s.

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Figure 14. Scanning electron micrographs of a stripped coating, the coating was formed in aluminate based electrolyte with the addition of 10 g l-1 Na2WO4·2H2O for

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600 s. (a) the backside of the coating in secondary electrons, (b) a higher magnification micrograph of the coating backside in backscattered electrons, (c) cross

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(430×570 µm) on the backside of the coating.

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section of the stripped coating (backscattered electrons), (d) EDS of a boxed area

Figure 15. XPS of W 4f scan on the backside of the stripped coating in Fig.14.

Figure 16. An illustration of the coating formation mechanism at the later stage of

Table captions:

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PEO under the penetrating discharges.

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Table 1. EDS analyses of the positions as indicated in the inset in Fig.5(a)

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ACCEPTED MANUSCRIPT Table 1. EDS anlyses of the positions as indicated in the inset in Fig.5(a) Position

Element composition in wt.% and at.% ( the values in brackets), respectively. O Mg Al W 21.14 (34.37)

56.32 (60.25)

02.67 (02.57)

19.87 (02.81)

B

27.29 (38.00)

53.45 (48.99)

15.15 (12.51)

04.11 (00.50)

C

27.48 (41.30)

30.12 (29.79)

30.74 (27.39)

11.66 (01.52)

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A

ACCEPTED MANUSCRIPT Figure 1 3

Positive potentials

500

-1

400

10 g l Na2WO4

300 -1

20 g l Na2WO4

200

Negative potentials

-1

0 g l Na2WO4

100

0

100

200

300

400

500

0 g l Na2WO4 -1

10 g l Na2WO4

2

-1

20 g l Na2WO4 1

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-2

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10 g l Na2WO4

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600

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(b)

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(k)

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25 µm

25 µm

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(c)

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(b)

(a)

100 µm

10 µm

10 µm

(d)

＋ B

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The residual part of a pancake

10 µm Element

Mg

Al

33.19

46.22

Mg

33.88

31.05

Al

26.59

21.96

W

06.35

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W

At%

00.77

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O

O

Wt%

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(e)

W

(f)

Element

Mg

O

Al

25 µm Wt%

At%

O

29.81

45.90

Mg

28.47

28.85

Al

25.22

23.03

W

16.50

02.21

W W

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(a) 50 µm

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50 µm

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＋ A

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(d)

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(e)

Element

Wt%

At%

O

35.99

51.57

Mg

33.09

31.20

Al

18.45

15.67

W

12.47

01.56

TE D

Mg

O

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Al

200 µm

W W

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(b)

(a)

100 µm

(c)

10 µm

(d)

Element

Wt%

At%

O

26.74

43.54

Mg

37.59

40.29

Al

13.49

13.02

22.18

03.14

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Resin

W

Backside

Resin

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25 µm

ACCEPTED MANUSCRIPT Figure 15

3500

A,B: W 18O19

3000

C,D: WO3

W 4f scan

C 2000

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Intensity/a.u.

fitted 2500

D

1500

A

B 1000

0 42

40

38

36

34

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500

32

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Figure 16

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0 600

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Wavelength/nm 10000

W I duplets 567.96&568.96

600

650

700

-1

20 g l

480 s

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+

Hβ

O II

4000

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Na I

+

1000

H2O

Na I

530

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520

Mg I

Intensity/a.u.

Na I +

H2O

510

-1

480 s

1500

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Mg I

500

-1

10 g l NaAlO2 + 10 g l Na2WO4 2 H2O

W I duplets

360 s 1500

1000

700

·

2000

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15 g l NaAlO2

(e)

650

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Mg I

10 g l

Na I

Mg I

(c)

500

Mg I

10000 -1

Mg I

Intensity/a.u.

8000

450

Wavelength/nm

Hα

550

Na I

500

W I duplets

450

+

400

H2O

350

H2O

Hβ

OII

2000

+

4000

5gl 240 s

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Mg I duplets

Intensity/a.u.

Hα

6000

0 300

-1

8000

Hα

240 s

NaI

0gl

Mg I

(b)

-1

Na I 568.82

Mg I duplets 517.26&518.36 Hβ

2000

O II duplets 447.79&446.93

Mg I duplets 382.93&383.83

4000

Al I 396.1

Intensity/a.u.

8000

6000

Na I duplets588.99&589.59

10000

(a) (a)

500

0 500

510

520

530

540

550

560

570

Wavelength/nm

580

590

600

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Figure 3

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(b) 25 µm

Barrier layer

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50 µm

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5 µm

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(b)

C B A

D

10 µm

100 µm

(d)

(c)

Mg

OK Mg K

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Al

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25 µm

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(a)

O

Al K

W

20 µm

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WM

Au

50 µm

ACCEPTED MANUSCRIPT Fiure 6

(a)

50 µm

25 µm

(d)

(c)

OK Mg K Al K

(e)

WM

(e)

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(f)

(f)

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ACCEPTED MANUSCRIPT Figure 7

+ A

+ B 200 µm Wt%

At%

O

22.23

50.52

Mg

11.87

Al

Element

Wt%

At%

O

24.57

39.88

17.75

Mg

12.12

12.94

16.27

21.92

Al

46.56

44.81

49.62

09.81

16.75

2.37

AC C

EP

TE D

W

(d)

SC

Element

20 µm

M AN U

(c)

RI PT

(b)

(a)

W

ACCEPTED MANUSCRIPT Figure 8 (a)

(b) Original alloy surface

25 µm

AC C

EP

TE D

M AN U

SC

250 µm

RI PT

Original alloy surface

ACCEPTED MANUSCRIPT Figure 9 40

10

W

20

4 2

10

10 0

0 40

8

0

5

10

15

20

25

30

35

6

40

45

180 s

2

10 40 0 0

2

4

6

8

10

12

14

16

18

120 s

30

2

10

10 0

0 40

8

0

2

4

6

8

10

12

14

16

18

15

0

10

10 0

40 0 2

4

6

8

30

2

4

6

8

10

2

4

6

8

10

10

30 s

30

0

2

4

35

12

14

12

14

6

M AN U

0

25

40

16

18

16

8

60 s 10

2 µm

0

EP

TE D

Distance/ m

6

1

2

3

4

Distance/ m

5

6

Resin

5

30 s

30 s

Coating

4

60 s

5 µm

Mg alloy

3

0

µ

2

µ

1

120 s

10 µm

18

10

2 0

10 µm

20

4 0

180 s

45

120 s

0

2

6

20

180 s

20

4

8

10

30

60 s

6

5

20

4

AC C

At%

0 30

10 0 6

20 µm

20

4

8

300 s

300 s

Al

30

300 s

RI PT

6

SC

8

ACCEPTED MANUSCRIPT ★ PEO of AZ31 magnesium in aluminate-tungstate electrolytes. ★ PEO coating is not formed by an ejection of molten oxide. ★ Penetrating discharges caused the inward coating growth and anion deposition.

RI PT

★ Anodic current density is estimated to be ~104 A cm-2 within discharge channels.

AC C

EP

TE D

M AN U

SC

★ Thermal decomposition of water causes anomalous gas emission.