Potential and morphological transitions during bipolar plasma electrolytic oxidation of tantalum in silicate electrolyte

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Potential and morphological transitions during bipolar plasma electrolytic oxidation of tantalum in silicate electrolyte Yingliang Chenga,∗, Qinghe Zhanga, Zhunda Zhua, Wenbin Tua, Yuling Chenga, Peter Skeldonb a b

College of Materials Science and Engineering, Hunan University, Changsha, 410082, China Corrosion and Protection Centre, Department of Materials, The University of Manchester, Manchester, M13 9PL, UK

A R T I C LE I N FO

A B S T R A C T

Keywords: Plasma electrolytic oxidation Tantalum Cathodic process Hydrogen evolution

Surges in the cell potential, due to an increased overpotential for hydrogen evolution, and transitions in ceramic oxide coating morphology during plasma electrolytic oxidation (PEO) of tantalum under a pulsed bipolar current regime at 1000 Hz in a silicate electrolyte are investigated using real-time imaging of gas evolution, analytical scanning electron microscopy, X-ray photoelectron spectroscopy and supplementary potential-controlled electrochemical measurements. The coatings, which contained Ta2O5, TaO and incorporated silicon species, revealed a nodular morphology that transformed with treatment time to a “pancake” type and then a “coral reef” type. The first potential surge occurred only in the cathodic potential, coinciding with an increased spark intensity, more vigorous gas evolution, emergence of “pancake” structures and a reduction in the coating porosity. The later increases in both the anodic and cathodic potential, coincided with intensification of the sparking, the establishment of silicon-rich “coral reef” structures, and formation of a comparatively thick coating. The kinetics of coating growth differed significantly between the three morphological stages. Electrochemical measurements showed that anodic discharges increased the overpotential for hydrogen evolution in the subsequent cathodic pulse, which is proposed to be due to gas impeding the coating and at and near the coating surface increasing the resistance to ionic transport.

1. Introduction Plasma electrolytic oxidation (PEO) is an electrochemically-based technique that can generate wear and corrosion resistant, biocompatible, catalytic, and decorative ceramic coatings, commonly on Al [1], Mg [2,3], Ti [4,5] and Zr [6] and their alloys. Microdischarges on the workpiece lead to coating growth, and local melting and quenching of the coating material. The precise mechanisms of plasma and coating formation are still under study [7]. AC or pulsed regimes with the introduction of negative biasing are usually preferred to DC, e.g. due to faster growth and denser coatings with improved performance [8–12]. However, the role of the negative potential is unclear. Rogov et al. [13] summarized the main hypotheses, including electrostatic charge accumulation [14,15], hydration [16], intercalation [17,18], coating microstructural or plasma discharge modifications [19,20] and coating heating [21]. Others [22] proposed that solution agitation by hydrogen bubbles assisted transport of electrolyte species. Discharges are usually absent under cathodic polarization. However, Nomine et al. [23] reported light emission during cathodic polarization of magnesium alloy in NH4F electrolyte, and associated it

∗

with electric charge accumulation on switching from anodic to cathodic bias; the cathodic bias also influenced the subsequent anodic behavior [23]. Cathodic discharges have also been observed on thick coatings formed at high frequency (≈2 kHz) [24,25]. Nominé et al. [26] suggested that the applied electric field was then unshielded by the electrical double layer [15]. Negative pulses have also been associated with chemical or morphological changes in the coating [13]. Fatkullin et al. [27] studied the PEO of an Al alloy under bipolar conditions using a combination of positive and negative potential pulses of high or low amplitudes. They found that the coating morphologies depended on the selected combination of the pulses and different equivalent circuits were required to simulate the PEO process. Recent years have witnessed increased study of PEO of tantalum, focused on biomedical uses [28–31]. Stojadinovic et al. [32,33] investigated the microdischarge properties and morphology, chemical and phase composition of coatings formed in 12-tungstosilicic acid. Sowa et al. [30] reported incorporation of silicon species from a silicate electrolyte and improved corrosion resistance in simulated body fluid, and also studied solutions containing calcium, phosphorus and magnesium species [34]. Antonio et al. [28] produced hydroxyapatite

Corresponding author. E-mail addresses: [email protected], [email protected] (Y. Cheng).

https://doi.org/10.1016/j.ceramint.2020.02.120 Received 15 December 2019; Received in revised form 22 January 2020; Accepted 12 February 2020 0272-8842/ © 2020 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Please cite this article as: Yingliang Cheng, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2020.02.120

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coatings by a pulsed method. Other works have examined calcium- and phosphorus-incorporated coatings [35], phosphoric acid electrolyte with calcium nitrate and copper (II) nitrate [36], coating growth processes [37] and coating structure, bond strength, apatite-inducing ability and response to annealing [38]. Despite the above mentioned works, the mechanism of coating formation and especially the role of the negative biasing during bipolar PEO of tantalum are still not fully understood. The present work investigates the potential transients that occur during pulsed bipolar PEO of tantalum in a silicate electrolyte and their relation to the sparking behavior, gas generation and coating morphology. Real-time imaging, scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS) were used to examine the plasma discharges and the morphology and composition of the coatings, respectively. Conventional potentiodynamic cathodic polarization and controlled potential negative and bipolar pulses were also employed to investigate hydrogen evolution on prior PEO-treated specimens and the influence of anodic discharges on the hydrogen overpotential. The results in this study provide deeper insights into the influence of PEO processing parameters on the coating formation mechanism and its relationship with the coating morphology, gas evolution and discharging behavior. The findings derived here for PEO of tantalum are expected to be applicable to PEO of other valve metals.

2. Experimental Specimens were cut from a tantalum plate (99.95%, ~0.5 mm thick), then masked with acid- and alkali-resistant tape, leaving a 10 × 10 mm working area on both sides. The working surface was polished to a 2000 grit SiC finish, degreased in ethanol, rinsed in distilled water and dried in warm air. PEO was carried out in a 1 L glass cell, equipped with magnetic stirring and water cooling. Two 60 × 80 mm stainless steel plates, placed opposite the specimen, were used as the counter electrodes. The electrolyte contained 10 g L−1 Na2SiO3·9H2O + 1 g L−1 KOH, and was prepared from high purity reagents (Sinopharm Chemical Reagent Co., Ltd, Shanghai, China) and distilled water. A 5 kW power source (MAO-5D, Pulsetech Electrical Co., Ltd., Chengdu, China) provided a pulsed bipolar current (frequency 1000 Hz; duty cycle 20%) that was monitored by an oscilloscope (Tektronix TDS 1002C-SC). The average positive and negative current densities from the integrated waveforms were ~0.22 and ~0.11 A cm−2, respectively. The conditions were similar to those of previous studies of Al [39,40] or Zr [22,41] alloys. The gas emission during the PEO process was recorded by real time imaging under strong illumination using a Nikon D300 digital camera (exposure time 1/500 s), with a piece of tantalum (20 × 10 mm) mounted in resin that had been polished down to a 1000 grit SiC finish. The PEO-treated tantalum samples were also subjected to potentiodynamic polarization and controlled potential tests in the electrolyte used for PEO. The potentiodynamic polarization tests were carried out from 0 to −20 V at a scan rate of 1.22 V s−1 on a Keysight B2912A Precision Source/Measure unit in a two-electrode configuration with a platinum counter electrode. Polarization under potential control using pulsed cathodic potentials only, and also using a cathodic pulse preceded by a high anodic pulse to trigger plasma discharges were also employed. Coatings were examined by SEM (QUANTA FEG 250 or QUANTA 200, FEI, USA) equipped with energy-dispersive X-ray spectroscopy (EDS). EDS was carried out at an electron energy of 20 keV. The valence states of coating constituents were analyzed by XPS, using a K-Alpha ESCALAB 250Xi instrument (ThermoFisher-VG Scientific, USA), with Al Kα radiation as the excitation source. Data were charge corrected to a C 1s binding energy of 285.0 e V. The area of analysis was ~0.5 mm2.

Fig. 1. (a) Cell potential-time responses (absolute values of the negative potential) during PEO of tantalum in 10 g l−1 Na2SiO3·9H2O + 1 g l−1 KOH and (b) potential and (c) current density waveforms at 85, 166 and 220 s.

3. Results and discussion 3.1. Cell potential –time responses, electric waveforms, and real-time imaging Fig. 1 (a) shows typical positive and negative cell potential-time responses during PEO for 1500 s. The potentials are peak values, with absolute values given for the negative potential. The positive potential increases during the growth of the barrier film at a rate of 26 V s−1 to 2

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reach 395 V at 15 s, when there is an inflection as fine sparks initiate and PEO commences. The potential then increases at a slower rate of 0.7 V s−1 to 494 V at 166 s, when there is a second inflection, later shown to be associated with a change in spark intensity. The voltage then is almost constant up to ~1000 s, before rising with increasing fluctuations to the final potential of 542 V. The negative potential initially rises during the growth of the barrier film to a reach a plateau of ~55 V, followed by a surge to 150 V between ~150 and 235 s, coincident with the second inflexion in the anodic potential. The potential then increases slowly, similarly to the anodic potential, and a second potential escalation occurs that coincides with the rise in anodic potential, with the potential reaching a final plateau potential after 1500 s of −197 V. These anodic and cathodic potential escalations occurred over a longer period than the first escalation, starting slowly from about 700 s and accelerating after about 1100 s. Fig. 1 (b) and (c) show the cell potential and current waveforms at 85, 166 and 220 s, viz. before, during and after a negative potential escalation of −88 V. The plateau positive current density was in the range ≈1.0–1.05 A cm−2. However, the negative current density varied from ≈ −0.7, −0.6 to −0.5 A cm−2 at 85, 166 and 220 s, respectively. Unlike the anodic pulse, which causes multiple electrode processes such as coating formation and oxygen evolution, it is known that the cathodic pulse in PEO waveforms only leads to hydrogen evolution. Hence, the surge in the negative potential is indicative of a significantly enhanced overpotential for hydrogen evolution. The hydrogen evolution overpotential may be affected by factors such as the electrode material, surface condition, electrolyte composition, temperature and surface roughness [42]. Real-time images recorded during the PEO are presented in Fig. 2(a–g), with corresponding positive and negative potentials indicated. The image at 6 s displays the purple interference color of a thin anodic film [43]. The gas in the pre-sparking stage consists of electrochemically generated hydrogen produced during cathodic polarization and possibly oxygen generated during anodic polarization, which has been reported in some studies [44]. By 46 s, the whole electrode surface exhibits numerous fine white sparks. After 174 and 186 s (at the start of the first negative potential escalation), the number of the sparks is reduced, but the intensity of individual spark has increased and sparks are of an orange hue. By 206 and 308 s, the negative potential escalation

was complete and the spark intensity has further increased. After 1046 s, during the second potential escalation, sparking intensifies, sparks are less uniformly distributed, and regions of spark-free white coating are evident. Photographs of the specimens at different stages of growth (Fig. 2(h–j)) show a black coating at 300 s, with a few white islands, mainly at the edges, that then spread to other areas by 600 s and covered all the surface at 1500 s. Fig. 3(a–d) show images of the gas evolution on the electrode from a side view. Prior to the first escalation in the negative potential, numerous gas bubbles are present on the electrode. Bubbles also rise vertically into the electrolyte above the specimen (Fig. 3 (a, b)). Due to the external illumination, the fine sparks present at this stage (see Fig. 2 (b)) cannot be seen. A much greater amount of gas was evolved into the electrolyte as the negative cell potential and sparking intensity increased (Fig. 3(c and d)). Front views comparing the specimen prior to sparking, before the potential escalation and when the escalation was complete (Fig. 3(e–g), respectively) show an increase in gas evolution in each stage. The gas comprises hydrogen and oxygen, generated both electrochemically and by dissociation of water molecules [24]. Bubbles of water vapour may also be present, although condensation usually causes their collapse on the coating surface [45]. Clearly, more vigorous gas evolution and wider dispersal of bubbles into the electrolyte occurs in the presence of the stronger discharges.

3.2. Scanning electron microscopy of coating morphologies Fig. 4 shows the SEM images of the surfaces of coatings formed for 120, 140 and 180 s, representing before, during and after the first negative potential surge. The 120 s coating comprises numerous nodules (Fig. 4 (a)), similar to those of a previous study [46]. After 140 s, (Fig. 4(b–d)), comparatively smooth “pancakes” are present surrounded by nodules (Fig. 4 (c, d)); the former appear bright in the backscattered mode suggesting regions of higher atomic number (Fig. 4 (c)). “Pancakes” are common in other type of PEO coatings, for example on Al [11,12,47] and Zr [22,41,48] alloys. At 180 s, “pancakes” were the dominant surface feature (Fig. 4 (e, f)). By 600 s, which is at the start of the slow potential rise towards the second potential escalation (that occurs in both the anodic and cathodic potential), numerous “coral reef” features, reaching several tens of

Fig. 2. (a–g) Real-time images during PEO of tantalum with corresponding positive and negative potentials indicated. (h–j) Optical images for the coatings formed for 300, 600 and 1500 s, respectively. Sample size (area for imaging): 10 mm × 10 mm; Camera exposure time: 1/100 s. 3

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Fig. 3. Side and front views of the gas evolution on tantalum during PEO at 1000 Hz before (a, b, e) and after (c, d, g) the occurrence of strong sparks in the first negative potential surge. The treatment time and corresponding positive and negative cell potentials are indicated in each image. The arrows in (c) indicate sites of strong discharges. Sample size: 20 mm × 10 mm; Camera exposure time: 1/500 s.

they were silicon-rich. Sowa et al. showed that a coating formed on tantalum in K2SiO3 contained silica and silicate species, with the former dominating [30]. Furthermore, synthetic quartz can be grown by hydrothermal treatment of sodium carbonate or sodium hydroxide solutions supersaturated by silica [49,50]. The formation of PEO coatings in the “coral reef” features may be expressed by the following reactions [51]:

microns in size, had formed on parts of the coating surface (Fig. 5 (a). “Pancakes” remained evident between the “coral reef” features. The latter appear dark under backscattered imaging (Fig. 5 (b)) and, as shown by EDX analysis below, are silicon-rich. At this stage of coating growth, the “coral-reef” features are non-uniformly distributed on the coating surface (see the white materials in Fig. 2 (i)) and much of the surface is covered mainly by “pancake” material. The individual “coral reef” features appear to have a dendritic morphology with nodular branches (Fig. 5 (c)). Adjacent to them, pores of ≈ 2–3 μm in diameter, which are significantly larger than those observed in the “pancake” regions, were often present, suggesting sites of stronger discharges (Fig. 5 (c)). After 1200 s, “coral reef” features are more dominant, and regions containing still larger pores, of the order 20 μm diameter, surrounded by “coral reef” material are present (Fig. 5 (d)). Later crosssections show that these pores are confined to the “coral-reef” material, which is located above an inner tantalum oxide layer. Backscattered electron imaging revealed isolated bright regions of tantalum oxide up to several tens of microns in size (Fig. 5 (e). The example shown at high magnification (Fig. 5 (f)) appears to show a solidified globule of previously molten oxide emerging from the surface. Two dark spots on the surface of the globule are possible gas filled cavities. Semi-quantitative EDS analyses at the locations indicated in Fig. 4 are listed in Table 1. All the nodular features presented on the coatings before, during and after the escalation of negative potential reveal the presence of Ta, O and Si (Points “A”, “C” and “E”). At points “B” and “D” located on the “pancakes”, only Ta and O were detected, with an average O:Ta atomic ratio of 2.1; later XPS confirms the presence of mainly Ta2O5. However, some pancakes with slightly darker appearances in the backscattered electron micrograph (Fig. 4 (f)) reveal a Si content similar to that of the nodules, as shown by the composition at point “F”. The composition of the “coral reef” structure formed at later stage of PEO has also been analyzed. A “coral reef” feature yielded 72.9% O, 3.7% Na, 21.5% Si, 1.3% K, 0.6 Ta% (at. %), indicating that

SiO32 − (l) +  H2 O(l)

Plasma → SiO2 (s ) + 2OH− (l)

SiO32− → SiO2 +1/2 O2 +2 e−

(1)

(2)

The cross-section of the coating formed before the first escalation of negative potential (Fig. 6 (a)) reveals numerous nodules above a barrier layer approximately 1 μm thick, which is consistent with the surface morphology in Fig. 4 (a). Determination of precise thickness values is hindered by the non-planar coating/substrate interface. The thickness of the coating, measured from the substrate to the top of the nodules, is ~5 μm. Between the nodules, only the barrier layer appears to be present, often with a scalloped morphology due to increased oxidation of the tantalum substrate. The coating during the potential escalation shows regions similar to those of Fig. 6 (a) and an isolated region of compact oxide, about 3 μm thick, corresponding to a “pancake” ((Fig. 6 (b), see dashed lines and inset). The “pancake” width is ~15 μm, which agrees with that of “pancakes” at the coating surface in Fig. 4 (d). After the potential escalation of negative potential, “pancakes” cover most of the surface (Fig. 4 (e) and (f)) and the coating in cross-section (Fig. 6 (c)) comprises mainly compact oxide between 2 and 4 μm thick, with internal porosity being mostly located above a barrier layer roughly 0.5 μm thick. A sub-micron layer of loosely attached material appears to have been deposited at the coating surface, which may correspond with the very fine textured material on the surface of the “pancakes” (Fig. 4 (e)). 4

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Fig. 4. Scanning electron micrographs of the tantalum surface after PEO for (a) 120, (b–d) 140, and (e, f) 180 s. The times are before, during and after the first potential surge, respectively. (Secondary electron mode, except (c, f) which are backscattered electron mode). Locations of EDS analyses are lettered.

in morphology, as shown by the example of a coating formed for 600 s (Fig. 7 (a)), with the thickness lying between 2 and 5 μm. The exceptions were occasional locations where the thickness had increased in association with formation of overlying silicon-rich “coral-reef” material (Fig. 7 (b)). In the presented example (Fig. 7 (b)), the compact regions and the “coral reef” structures are both up to ≈ 20 μm thick, and significant local retreat of the underlying substrate/coating interface is evident. After 1200 s, the silicon-rich “coral reef” features are more numerous and the barrier region appears thicker, ranging from 4 to 9 μm (Fig. 7 (c, d)). The scalloping of the coating/substrate interface is generally more pronounced than at earlier times. The total thickness of the coating is 75 μm at the largest of the “coral-reef” features. Within the “coral reef” structures a mixture of porous tantalum oxide and porous silicon-rich material is evident by their respective light and dark appearances in backscattered electron imaging, with mainly silicon-rich material in the outer parts (Fig. 7 (d)). The scalloping of the substrate/

Any recession of the substrate/coating interface caused by oxidation of the tantalum, during formation of the individual “pancakes” was similar to or below the depth resolution possible from measurements on SEM cross-sections (≈1 μm, Fig. 6 (b)). The nodules appear to have been melted by the strong discharges (melting points of Ta2O5 and SiO2 are 1872 and 1710 °C, respectively), with most of the material of the original nodules being incorporated into the “pancakes”. The incorporation of the nodular material into the “pancakes” is consistent with EDX point analysis that detected silicon in some “pancakes” (Point “F” in Fig. 4 (f)). However, it cannot be ruled out that some loss of the nodular material may have occurred by mechanical removal, vaporization or dissolution into the electrolyte. The discharges appear not to have caused significant melting of the substrate, which has high melting point (3017 °C). In the period between the first and second potential surges, most regions of the coating showed no significant thickening or major change

5

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Fig. 5. Scanning electron micrographs of the tantalum surface after PEO for (a–c) 600 and (d–f) 1200 s. The times are near the start and after the second potential transitions, respectively. ((a, c, d) Secondary electron mode; (b, e, f) backscattered electron mode).

stronger discharges occur at this stage of coating growth. The temperatures and pressures in the discharges appear to have melted and redistributed the tantalum oxide, the latter possibly assisted by gas generation. Silica-rich material fills the interstices and overlays the tantalum oxide and gives rise to the white appearance of the coatings seen in the photographs of Fig. 2. The tantalum oxide regions reveal numerous cracks, which are probably formed under the stresses generated by rapid solidification and cooling of the coating once discharges are extinguished. The measured thickness of tantalum oxide is at all times low relative to that which might have formed if the anodic current were used only to generate Ta5+ ions contained in the coating. Tantala layers of ≈15, 30, 75 and 150 μm would be expected after PEO for 120, 240, 600 and 1200 s, compared with measured estimates of ≈3, 3, 4 and 20 μm, respectively (assuming a Pilling-Bedworth ratio of Ta2O5/Ta of 2.52 [52]).

Table 1 Elemental composition (at. %) at locations in Fig. 4. Location

A B C D E F

Elemental composition (at.%) Ta

O

Si

13.8 29.9 22.8 36.0 5.5 13.0

74.8 70.1 64.3 64.0 78.7 77.0

11.4 n.d. 12.9 n.d. 15.8 10.0

n.d. = not detected.

coating interface beneath individual “coral reef” features (see arrows in Fig. 7 (b)), the formation of thick overlying “coral reef” material, and the integration of tantalum oxide in the silicon-rich oxide suggest that 6

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Fig. 6. Scanning electron micrographs (backscattered electron mode) of cross-sections of tantalum after PEO for (a) 120, (b) 157 and (c) 240 s. The times are before, during and after the first potential surge, respectively.

Fig. 7. Scanning electron micrographs of cross-sections of tantalum after PEO for (a, b) 600 s and (c, d) 1200 s. The times are near the start and after the second potential transition, respectively. ((a–c) Secondary electron mode, (d) backscattered electron mode.).

are close to those for Ta2O5 (26.6 eV (Ta 4f 7/2) and 28.5 eV (Ta 4f 5/2) [53]). Ta 4f at 180 s was deconvoluted into Ta 4f 7/2 at 26.62 eV and Ta 4f 5/2 at 28.57 eV, belonging to Ta2O5, and two minor doublets (Ta 4f 7/2 at 23.91 eV and Ta 4f 5/2 at 25.86 eV) that can be ascribed to TaO [54]) and one (Ta 4f 7/2 at 28.05 eV and Ta 4f 5/2 at 30.15 eV that could not be identified from the literature.

3.3. XPS Fig. 8 presents high resolution spectra of Ta 4f, O 1s and Si 2p for coatings formed for 120 and 180 s, namely before and after the first surge in negative potential. Ta 4f at 120 s reveals a doublet with peak binding energies of 26.37 eV (Ta 4f 7/2) and 28.32 eV (Ta 4f 5/2), that 7

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Fig. 8. XPS spectra of Ta 4f, O 1s and Si 2P for coatings formed for 120 and 180 s. The times are before and after the first potential surge, respectively.

3.4. Evaluation of hydrogen evolution under different polarization regimes

The O 1s spectrum for the 120 s coating was deconvoluted into two peaks at 532.24 eV and 530.69 eV, respectively, due to SiO2 (532.39 eV in Ref. [47] and Ta2O5 (530.9 eV in Ref. [55]). The O 1s spectrum for the 180 s coating is similar, with deconvoluted peaks at 532.28 eV and 530.28 eV. Si 2p spectra for both coatings were similar and deconvoluted into a doublet with two peaks of Si 2p 3/2 and Si 2p 1/2, which are close to Si 2p 3/2 at 102.51 eV and Si 2p 1/2 at 103.11 eV reported for SiO2 [56]. The silicon detected after the potential escalation may be present in the residual areas of nodular coating (Fig. 4(e)), where it was detected in earlier EDX analysis; in the thin deposit above the “pancakes”; or within the outer region of “pancakes” following melting of the nodular material.

Cathodic potentiodynamic polarization curves of the tantalum prior to PEO and also following PEO for 120 and 220 s, representing times before and after the first escalation in the negative potential, revealed transitions to ohmic behavior at ~ −2.5, ~ −14.0 and ~ −14.2 V, respectively and slopes of 33, 47 and 47 Ω cm2 (Fig. 9). The current density of the untreated tantalum specimen is higher than that of the coated specimens at a given potential, which is attributable to the thin air-formed oxide that facilitates electron tunneling. It is surprising to observe that the two coatings showing different hydrogen overpotentials during PEO display nearly identical cathodic current 8

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Fig. 11. Photographs of specimens under a pulse cathodic potential of −54 V, with prior PEO of the tantalum for (a) 120 and (b) 190 s. The times are before and after the first potential surge, respectively. Fig. 9. Cathodic potentiodynamic polarization curves for untreated tantalum and following PEO for 120 and 220 s. The times are before and after the first potential surge, respectively.

time, while those of the 170 s specimen were not significantly affected. Both specimens exhibited similar waveforms after 90 s. Thus, when preceding anodic pulses were applied the overpotential for hydrogen evolution in the subsequent cathodic pulse was increased, which is more significant for the specimen formed for 170 s. The anodic pulses result in an increased ohmic resistance to ionic transport during the cathodic pulses. This might arise from several sources, for example a modified electrolyte composition within the coating porosity, deposition of material, and generation of gas by oxidation of OH− ions and H2O [57] or by boiling and dissociation of water [24,48,57,58]. As seen in Figs. 2–5, the first surge in the overpotential for hydrogen evolution coincides with stronger discharges, development of “pancake” structures and a more compact coating, and significantly increased gas generation. The coating morphology, discharge behavior and gas emission before and after the first escalation of the negative potential are schematically illustrated in Fig. 13. Before the escalation of negative potential, the surface of the PEO-treated specimen features nodules and fine sparks, and hydrogen evolution may occur across the whole surface. However, strong discharges, which generate “pancakes”, occur after the escalation of negative potential. The “pancakes” consist of solidified material produced by quenching of the discharges. Bubbles of water vapour and oxygen are expected to form above these active discharge channels during PEO. Some “pancakes” appear relatively pore-free, possibly due to flow of molten oxide into pores during the cathodic pulse, as light emission from an anodic pulse, suggesting a high coating temperature, has been observed to be maintained for up to ≈2 ms after an anodic pulse has ended [25]. The less porous coating would provide fewer routes for ionic transport and species exchange between the coating and the bulk electrolyte. Furthermore, the

densities in Fig. 9. To investigate the cathodic behavior under pulsed conditions, pulsed potential polarization at −54 and −150 V was applied to specimens coated for 120 and 190 s (before and after the negative potential escalation, respectively), using the same frequency and duty cycle as for PEO. Fig. 10 shows the results recorded at ~40 s after the application of the pulsed negative potentials. It was also observed that the waveforms changed little during the application of negative potentials. For both specimens, the peak cathodic current densities were similar ≈ 1.1 A cm−2 at −54 V and ≈ −3.5 A cm−2 at −150 V, indicating an approximately ohmic behaviour, with a resistance of 40 Ω cm2, of a similar order as measured from the potentiodynamic polarization curves. The same as the conventional polarization tests, this contrasts with an increased overpotential with voltage during PEO (Fig. 1). Photographs for both times of coating growth show numerous hydrogen bubbles evolving from across the electrode surfaces during pulsed polarization at −54 V (Fig. 11). Pulsed positive and negative potentials of 480 and -144 V, the former initiating discharges, were applied to specimens PEO-treated for 90 and 170 s, corresponding to before and after the negative potential transition. The resultant current waveform varied with the time of polarization (Fig. 12). After 10 s, the former specimen displayed higher positive and negative current densities e.g. a cathodic current density of −1.35 compared with −0.34 A cm−2 for the specimen treated for 170 s, and the anodic and cathodic current densities reduced with the

Fig. 10. Cell potential and current density waveforms using pulsed cathodic potentials of −54 and −150 V with prior PEO of the tantalum for (a) 120 and (b) 190 s. The waveforms were recorded at ~40 s after the application of negative potentials. 9

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Fig. 12. Cell potential and current density waveforms for pulsed anodic and cathodic potentials (480 V/-144 V), with prior PEO for (a) 90 and (b) 170 s. The times are before and after the first negative potential surge, respectively.

more uniform distribution across the coating during hydrogen evolution. Hence, the anodic current flow is not impeded by any residual gas that may remain elsewhere in the coating or coating surface following the cathodic pulses, and gas can readily escape from the discharge channels, pores and surface of the molten coating. Additionally, the oxide remains thin in the period immediately following the first cathodic potential surge, such that the dielectric breakdown potential required for continuation of sparking may not increase significantly. Later, however, the anodic potential rises gradually as “coral reef” features develop, especially from about 600 s onwards, under further intensified sparking. The intensification is possibly the result of a gradual thickening of the “pancake” dominated coating and healing of coating defects that results in current concentrating in fewer favoured

enhanced gas generation might further impede the ionic transport through the coating and near-surface electrolyte. It is also possible that the oxygen is generated by electrochemical oxidation and/or thermolysis of water directly on the molten “pancake” surface. In this regard, gas emission at the sites of strong discharges has been observed to persist during the cathodic polarization, since the discharge channels may not be immediately quenched to low temperatures after the end of anodic pulse. The gas bubbles may block access of the electrolyte to sites of hydrogen evolution during subsequent cathodic polarization, as seen in the image at the lower right hand corner in Fig. 13. The lack of an accompanying anodic voltage surge with the first cathodic voltage surge (see Fig. 1(a)) might be explained partly by the concentration of the anodic current in the discharges compared with a

Fig. 13. Schematic illustration showing the coating morphology, discharge behavior and gas emission on tantalum before and after the first escalation of the negative potential. 10

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Acknowledgement

regions for dielectric breakdown, for example, thinner oxide, defective oxide containing cracks, gas-filled cavities or impurities, and sites of field enhancement owing to the local oxide/substrate geometry. The more intense sparks may also reinitiate over a number of cycles at the same location, which has been observed to occur in previous work [59]. The “coral-reef” features appear to develop at sites of increased temperature that enables of thickening of the comparatively compact barrier layer (Fig. 7 (b)), possibly involving anodic and thermal oxidation of the substrate. The barrier layer growth is accompanied by deposition of porous SiO2, formed by thermolysis of SiO32− ions, that may be infiltrated by molten tantalum oxide. The later emergence of globular molten oxide might be a consequence of further temperature rises through current concentration in fewer, persistent discharges that cause melting of the substrate, which leads to more rapid localized oxidation of the tantalum and a less viscous oxide.

The authors thank the National Natural Science Foundation of China (Grant Number: 51671084) for support of this work. References [1] W. Xue, Z. Deng, Y. Lai, R. Chen, Analysis of phase distribution for ceramic coatings formed by microarc oxidation on aluminum alloy, J. Am. Ceram. Soc. 81 (1998) 1365–1368. [2] X. Lu, C. Blawert, Y. Huang, H. Ovri, M.L. Zheludkevich, K.U. Kainer, Plasma electrolytic oxidation coatings on Mg alloy with addition of SiO2 particles, Electrochim. Acta 187 (2016) 20–33. [3] Y. Chen, X. Lu, S.V. Lamaka, P. Ju, C. Blawert, T. Zhang, F. Wang, M.L. Zheludkevich, Active protection of Mg alloy by composite PEO coating loaded with corrosion inhibitors, Appl. Surf. Sci. 504 (2020) 144462. [4] F. Gao, L. Hao, G. Li, Y. Xia, The plasma electrolytic oxidation micro-discharge channel model and its microstructure characteristic based on Ti tracer, Appl. Surf. Sci. 431 (2018) 13–16. [5] M. Kaseem, H. 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4. Conclusions Ceramic coatings have been developed on the surface of tantalum by PEO treatment under pulsed bipolar conditions in a silicate electrolyte. The following conclusions were drawn: 1. PEO of tantalum under the selected constant pulsed current density regime results in transition between three different coating morphologies dependent upon the treatment time: an initial nodular coating, a subsequent “pancake” featured coating and a final “coralreef” featured coating. 2. The coatings contain Ta2O5, TaO and SiO2, with tantalum oxide contained mainly in the inner coating and silicon species near the surface. The coatings thicken at different rates in the various stages of growth. In particular, development of “coral-reef” features, which contain large amounts of SiO2, significantly accelerates the thickening of the coating. 3. The morphological transitions coincide with transients in the cell potential. The first transition leads to an escalation in the overpotential for hydrogen evolution (negative cell potential) that occurs 2–3 min after the start of the treatment. The transition is coincident with increase in sparking intensity and change of spark colour, formation of “pancake” coating features and increased gas generation. The second transition, and further in the increase of the overpotential, occurs more slowly and coincides with formation of “coral reef” features. Only in the latter instance is there an accompanying increase in the anodic potential, which may be related to significant growth of the barrier layer occurring only in the second transition. 4. The “pancake” structures result mainly from melting and re-distribution of the previous nodular material. Formation of silica-rich “coral reef” material is associated with the stronger discharges and higher temperature of the later stages of coating growth, which result in larger pores, locally increased oxidation of the substrate and ejection of molten tantalum oxide. 5. Electrochemical measurements in the absence of anodic sparks indicate that the overpotential for hydrogen evolution has an ohmic dependence on the current density due to the resistance of the coating and electrolyte. The overpotential is enhanced by anodic discharges, which is proposed to be due to the oxygen gas generated during anodic discharges that impede ionic migration during subsequent cathodic polarization.

Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. 11

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