50 Innovations in Corrosion and Materials Science many fieldsfortheirgoodbiocompatibility[1-2]andcorro- many Puretantalum(Ta)anditsalloyshavebeenappliedin 1. INTRODUCTION E-mail: [email protected] P.R.China; Tel:+86-731-88876179; Fax:+86-731-88879692; and EngineeringCentralSouth University HunanChangsha410083 School ofMaterialsScience *Address correspondencetothisauthor atthe Keywords: 2 1 Likun Hu

Chengguan town, Yongxing County, Hunan Chenzhou,423301,P.R.China. Chengguan town,YongxingCounty,Hunan Lithium tantalate(LiTaO Lithium solutions. behavior oftantaluminalkaline little [9,13-14]wasreportedonthecorrosion knowledge, fort beingfocusedonsurfacereinforcement [11-12],toour thisneedwithanextensiveef- Despite of greatimportance. toharden thesurfaceofpuretantalumis Therefore, method sion ofalkalinesolutionscont issue. Furthermore,puretantalum cannotwithstandtheero- applications wherecompactionanddeformationmaybe an hardness ofpuretantalumissomehowabitlowforsome [7-8].However,thesurface of densesurfaceoxidefilm lent anticorrosionperformanceisattributedtothegeneration knownthattheirexcel- and neutralsolutions[3-6].It’swell especiallyinacids sion resistanceinawidevarietyofmedia, Changsha South Tantalum Niobium Co.,Ltd,HunanChangsha,410083,P.R.China; Changsha SouthTantalumNiobium ScienceandEngineering,CentralSouthUniversity,HunanChangsha,410083,P.R.China; School ofMaterials  10.2174/2352094909666190211125527 DOI: Accepted: January30,2019 Revised: December3,2018 Received: June28,2018 A RTICLEHSOY RESEARCH ARTICLE 1 LiTaO , SichengYuan Formation ofCorrosionResistantHardCoatingLiTaO in MoltenLiNO

 3 thin film, anodic oxidation, anticorrosion, NaOH solution, tantalum substrate, moltenLiNO thinfilm,anodicoxidation,anticorrosion,NaOHsolution,tantalum 

3 ), ofexceptionalhighhardness display higherpolarizationresistancewithtocorrosion. analysis ofOCPcurvesandpotentiodynamicpolarizationcurves.Uponanodicoxidation,samples lower corrosioncurrentdensitiesthanthoseofpuretantalumsubstrate,accordingtotheresultsand Conclusion: polarization curvesandelectrochemicalimpedancespectra(EIS). ther examinedbyelectrochemicalmethodsincludingopen-circuitpotential(OCP),potentiodynamic to thoseofpureTasubstrate.PuretantalumsampleandcoatedbyLiTaO results indicatedthatLiTaO NaOHsolutionat50 into 10wt% voltage andreactiontimeofthecoatingprocess.Selected specimens,aftercoating,wereimmersed with thatofpuretantalum.Thevaluehardnesswasfoundtograduallyincreasetemperature, Results ers hardness,respectively. microscopy(SEM)andVick- film wereinvestigatedbyX-raydiffraction(XRD),scanningelectron Methods tantalum substrate Abstract: aining NaOH[9]orKOH[10]. 1 , PanpingXie 3 :

Ourresultsshowedthatsurfacehardnessofallcoatedsampleshasbeenincreasedcompared : Theeffectsoftemperature,voltageandtimeoncomposition,morphologyhardness Background We havefoundthatsamplescoatedwithLiTaO We 1 5.0.0 ©2019Bentham Science Publishers 2352-0957/19 $58.00+.00 via 1 Innovations in Corrosion and Materials Science, Innovations inCorrosionandMaterials , DengfengXu anodicoxidation. : Lithiumtantalate(LiTaO 3 coatedsampleshaveasmallermasslossandcorrosionratecompared o C for96htoexploretheiranti-corrosion performance.Immersing

sputtering havebeentriedtosynthesizeLiTaO method, chemicalvapordeposition(CVD)andmagnetron linear opticalproperties[18-20].Laserdeposition,sol-gel lectric, ferroelectric,electro-optic,acousto-opticandnon- toitsuniquepiezoelectric,pyroe- owing (RAM), memories optical waveguides,switchesandrandomaccess and piezoelectricdetectors,surfaceacousticwavefilters, 6.6×10 lhiefroteetoi eie 1-7 i pyroelectric in alchoice foroptoelectronicdevices[15-17] from LiNO from ble to search for an effective method todealwiththisther- ble tosearchforaneffectivemethod between thelayer ofLiTaO between Note here,LiTaO (5.5-6 Mohs), hasreceivedconsiderableattentionasmateri- (5.5-6 Mohs), LiTaO may provideapossibilitytogetgoodprotectivelayer may thermal expansioncoefficients(16.1×10 thesubstrate due totheirlargedifferencein off easilyfrom worth trying.Phasediagram ofTa Anodicoxidation[26]seems tobeapromisingway mal expansionproblem. be usedinan environment against solutionerosion.As some Ta-basedproductstendto 2 , ZhiPeng 3 -6

vs /K of tantalum). Similar casesoccurredforinstance Similar /K oftantalum). 3 ) thinfilmwassynthesizedandinsitucoatedon 3×10 3 ) alsoshowstheappearance of 2 , AxiXie -6 /K ofsilicon)[25]. Itisthenhighlydesira- /K 3 3 Send Orders for Reprints [email protected] Send OrdersforReprints film prepared by these means maybe flake maybe preparedbythesemeans film thinfilmexhibithigherpotentialsand 2019, 9,50-59 3 3 No.75 EastRenminRoad, andFengZheng 

containing 10 wt% NaOH,the containing 10 wt% 3 andsilicon(16.1×10 3 byAnodizing 2 3 O thin film werefur- thinfilm 5 withLiOH(obtained -6  /K ofLiTaO LiTaO 3.

1,2,* 3 film [21-24]. film 3 [27],this

-6 /K of /K 3

vs

eISSN: 2352-0957 ISSN: 2352-0949 Formation of Corrosion Resistant Hard Coating of LiTaO3 Innovations in Corrosion and Materials Science, 2019, Vol. 9, No. 1 51 objective of present work is then to synthesize and coat LiTaO3 thin film on tantalum substrate in situ by anodic oxi- dation. After coating, we will investigate the change of sur- face hardness and corrosion behavior of samples in NaOH solution. In comparison, composition, morphology, surface hardness and anticorrosion performance of pure tantalum and those coated by LiTaO3 film will be examined and discussed in detail. Both traditional immersion corrosion tests and elec- trochemical measurements will be employed to study the performance of anticorrosion.

2. EXPERIMENTAL PROCEDURE

2.1. Materials and Synthesis

Tantalum foil, 19 mm in diameter × 0.5 mm in thickness, was provided by Changsha South Tantalum Niobium Co., Fig. (1). Experimental setup. Ltd, China, with nominal chemical composition (mass frac- tion, %): H≤0.0015, C≤0.01, N≤0.01, O≤0.015, Si≤0.005, Fe≤0.005, Ni≤0.002, W≤0.01, Mo≤0.01, Ti≤0.002, Nb≤0.03, Table 1. Synthesis conditions for tested samples. with Ta be balance. The apparent exposed area was 2 2.84 cm . Samples Time/h Voltage/V Temperature/oC All chemicals used here are of analytical grade purchased     from Sinopharm Chemical Reagent Co. Ltd. (China) without S0    further purification. Acetone and ethanol were employed to S1 1 10 490 degrease Ta foils. LiNO3 and NaOH were used and acted as the electrolyte of anodic oxidation and solute of corrosion S2 3 10 490 solution, respectively. Note here, LiNO3 (melting point of S3 5 10 490 255°C) is a strong oxidant, can absorb water and produce derivatives such as LiOH. Water was therefore carefully S4 3 5 490 removed during our processing of melting LiNO3 (400°C to     600°C) to prevent such things from happening. S5 3 15 490 S6 3 10 450 Setup of equipment for synthesizing LiTaO3 thin film was shown in Fig. (1) schematically. Steps to synthesize and S7 3 10 530 coat LiTaO3 thin film were according to the followings: 1) Heating nickel crucible to 490°C and holding at this 2.2. Immersion Corrosion Tests temperature until LiNO3 electrolyte to be melted completely. 2) Rinsing Ta foils one after another with acetone, etha- Preliminary immersion corrosion tests were conducted to nol, distilled water, followed by drying them at 80°C for 1h. investigate anticorrosion of Ta substrate and LiTaO3 thin film coated samples. First, accurately weighed flaky NaOH 3) Bolting cleaned Ta foils onto anode and immersing was dissolved in distilled water to prepare 10wt% NaOH them completely in molten LiNO3. Then apply voltage be- solution. Then, tantalum substrate and samples coated by tween anode and cathode by DC power for electrochemical LiTaO3 thin film were immersed into as-prepared 10 wt% reactions. NaOH solution at 50°C for 96h to monitor their mass chang- 4) Pulling out samples at the end of coating (after going es. During immersion testing, all samples went through re- through required reactions) and cleaning them ultrasonically peated pulling-out, ultrasonically cleaning in distilled water, in distilled water, drying at 80°C in air for further tests and drying and weighing in air at interval of 12h. Finally, a mi- characterizations. crostructure of surface was characterized by SEM for each sample at the end of immersion test set at 96h. Besides, the In addition, the influences of temperature, voltage and corrosion rate and corrosion inhibition efficiency were calcu- reaction time on composition, morphology and hardness of lated through Eqs. 1 and 2 [28, 29]: reacted thin film were investigated. Composition and surface    morphology of coated samples were measured using X-ray   (1) diffraction (XRD, D/Max 2500, Rigaku, Japan) and scanning   electron microscopy (SEM, Quanta-200, FEI, USA), respec-   (2)  tively. The surface hardness of Ta substrate and LiTaO3 thin film coated samples was measured by Vickers hardness test- Where and are initial and final mass of tested sample, is er (DHV, 1000) under the loading of 4.9N at dwelling time apparent exposed area of the sample, is immersion time, and of 15s. Pure tantalum (S0) and coated samples (S1-S7) were denote the mass losses of Ta substrate and coated samples nominated according to their synthesis parameters and listed in tested solution. in Table 1. 52 Innovations in Corrosion and Materials Science, 2019, Vol. 9, No. 1 Hu et al.

2.3. Electrochemical Measurements for Ta2O5 and Ta2O (even Ta) still visible (S3-5h). That is to say, prolonged reaction will increase the content of LiTaO Electrochemical tests were carried out in 10 wt% NaOH 3 while consuming tantalum oxides. In summary, crystalline solution at 50°C by the workstation (ZAHNER IM6ex), in phase of LiTaO grows with coating time at consumption of order to further study the anticorrosion performance of tanta- 3 those of Ta2O5 and Ta2O. In another word, the formation of lum substrate and samples coated by LiTaO3 thin film. Plati- LiTaO3 was somehow the result of a series of reactions num foil and saturated calomel electrode (SCE) were used as + among Li ions in solution with surface oxides Ta O and auxiliary and reference electrodes, respectively. To avoid 2 5 Ta O. These results are in agreement with those of early re- contamination, the SCE was connected via a bridge filled by 2 port [30]. The phase evolution observed here could be de- 10 wt% NaOH solution with tip being pressed against the scribed as such, surface of working electrode to minimize IR drop. All poten- tials obtained in this work were referred to this reference Oxidation of pure Ta substrate occurred initially and sponta- electrode. Prior to polarization, the working electrode was neously: immersed in testing solution for 1h to stable open-circuit 4Ta + O
 2Ta O







(3)
potential (OCP). Upon equilibrium, potentiodynamic polari- 2 2 zation curve was measured at scanning rate of 1 mV/s, cov- and ering from -1 to 0.5 V. Electrochemical impedance spectra 2Ta2O + 4O2

2Ta2O5 (4) (EIS) was performed using AC signal of amplitude 10 mV at pre-set potential covering frequency range of 100 kHz to 0.01 or

Hz. Each testing was conducted with freshly prepared solution. 4Ta + 5O2

2Ta2O5 with ∆rGm(490°C) = -407 .7 kJ/mol (5) 3. RESULTS AND DISCUSSION Followed by reactions between Ta2O5 with molten LiNO3 3.1. Composition Analysis solution: XRD spectra of samples prepared in different conditions 2Ta2O5 + 4LiNO3 
4LiTaO3 + 4NO
+ 3O2
, are presented in Figs. (2) and (3). All crystalline phases ob- ∆rGm(490°C) =
19.3 kJ/mol






(6)
servable here are lithium tantalate (LiTaO3, JCPDS No. 29- or 0836), tantalum pentoxide (Ta2O5, JCPDS No. 18-1304), 2Ta2O5 + 4LiNO3 
4LiTaO3 + 4NO2
+ O2
, tantalum oxide (Ta2O, JCPDS No. 74-2305) and tantalum (Ta, JCPDS No. 04-0778). ∆rGm(490°C) = -19.6 kJ/mol






(7)
or Ta2O5 + 2LiNO3 
2LiTaO3 + NO2
+ NO
+ O2
, ∆rGm(490°C) = -19.4 kJ/mol






(8)
In addition, following reaction may also occur at the same time:

Ta2O5 + 2LiOH 
2LiTaO3 + H2O
, ∆rGm(490°C) =-30.1 kJ/mol
















Fig. (2). XRD spectra of samples synthesized as function of coating time.

Fig. (2) shows the influence of coating time on XRD spectra for the growth of LiTaO3 thin film [XRD spectra of Ta substrate is shown in the inset of Fig. (2)]. Note, strong diffraction peaks for sample S1-1h are corresponding to Ta2O5, Ta2O and Ta (being substrate) with one weak peak of
LiTaO3. Up to 3h, both peak number and intensity of LiTaO3 increased, while those of Ta2O5 and Ta2O declined (S2-3h). Fig. (3). XRD spectra of coated samples synthesized at different These become even more profoundly at 5h, with weak peaks voltage and temperature. Formation of Corrosion Resistant Hard Coating of LiTaO3 Innovations in Corrosion and Materials Science, 2019, Vol. 9, No. 1 53

Note here, values of ∆rGm (Gibbs free energy change) strate. That is why the XRD spectra of LiTaO3 coated sam- were calculated by HSC 6.0. As ∆rGm of Eqs. (5-9) is nega- ples also contained tantalum oxides. Therefore, it is very tive, meaning these reactions can take place spontaneously. important to select reasonable experimental parameters for There are no ∆rGm values assigned for Eqs. (3-4) due to lack actual production. In this paper, the optimal preparation of data of Ta2O in HSC database. However, we were able to condition is S2 (synthesis condition is 490°C-5V-3h) and detect trace of Ta2O phase in XRD spectra, which allows us more details will be discussed in Section 3.4. to propose such reactions for oxidation of Ta substrate to 3.2. Morphology Analysis first form Ta2O, followed by formation of Ta2O5. The differ- ence of ∆rGm value for Eqs. (6-8) is negligible which mean- Surface morphologies of LiTaO3 thin film synthesized at ing that these reactions can all take place independently to different coating time are presented in Fig. (4a-c) associated form LiTaO3. If in case there is indeed some LiOH (melting with 1, 3 and 5h, respectively. In general, all samples were point of 462°C, one possible intermediate product in our composed of similar coarse grains on surface with some lam- system) presented, it will also form LiTaO3 by reacting with inar asperities. Specifically, laminar asperities were different Ta2O5 (seeing Eqn. 9). from each other and all gowning with coating time, being small at 1 (Fig. 4a) and 3 h (Fig. 4b), and got larger at 5 h The influence of coating voltage and temperature on XRD (Fig. 4c). This might due to the existence of various defects patterns of LiTaO thin film is presented in Fig. (3) (XRD 3 in Ta substrate, which provide different driving force (ac- spectra of Ta substrate is shown in the inset of Fig. 3). All diffraction peaks for sample S4-5V matched well with those of cording to Yang’s equation: σLS = σαS + σLαcosθ) for nuclea- tion and growth. Defects reduce energy barrier of heteroge- Ta O , Ta O and Ta (being substrate). Once coating voltage 2 5 2 neous nucleation which are beneficial to diffusion, aggrega- increased to 15V, we began to see the peaks for LiTaO 3 tion, clusters coalescence and growing of coating [31]. This which is co-existed with those of Ta O and Ta O (S5- 2 5 2 could explain why the size of laminar asperities coated of 5h 15V). There must be a threshold of voltage during coating, + (Fig. 4c) is much larger than those of 1 (Fig. 4a) and 3h (Fig. above which tantalum oxides can react with Li to form 4b). LiTaO3. That is to say, improving voltage will first acceler- ate the formation of tantalum oxides as an intermediate and Typical surface morphologies of LiTaO3 thin film synthe- then transformed into LiTaO3 film. For sample S6-450°C, sized as function of coating voltage (Figs. 5a and 5b) and strong and sharp peaks were assigned to Ta2O5, Ta2O and temperature (Figs. 5c and 5d) are depicted in Fig. (5). Coated Ta (being substrate), with one feeble peak belonging to sample synthesized at 5V has smooth surface accompanying LiTaO3. Nevertheless, peak number and intensity of with very small laminar asperities (Fig. 5a), this becomes LiTaO3 and Ta2O5 changed constantly with coating temper- rough with many larger laminar asperities as voltage in- ature up to 530°C, while those of Ta2O and Ta declined creased to 15V (Fig. 5b). Similar morphology change has (S7-530°C). Comparing S6-450°C, S2-490°C, S7-530°C, been occurred for samples coated at 450 (Fig. 5c) and 530°C we found the fact that the content of Ta2O5 was increased (Fig. 5d).According to the classical theory of thermally in- rapidly, accompanying with slow increase of LiTaO3. The- duced crystallization and electrical field induced crystalliza- se changes indicated that the effect of temperature on the tion, energy exerted from external forces is proportional to formation of Ta2O5 is greater than that of LiTaO3. These temperature or voltage which affects rate of diffusion or ion- results may suggest that there is an optimal coating temper- ic electromigration [32]. Increasing voltage or temperature ature range for the formation of LiTaO3. Giving enough will for sure provide more energy for nucleation, diffusion, time, intermediate tantalum oxides can produce pure merge of atoms and even worse abnormal grain growth. This LiTaO3 thin film theoretically. However, we found that is why surface morphology gets rough and coarsen for sam- continuously formed films retard the reaction of Li+ ions ples synthesized at higher coating voltage and temperature. and even worse, some surface began to peel off as so many Surface morphology of coatings formed at different voltage hard and brittle LiTaO3 particles presented there. Using our and temperature may not distinguishable, attributed to their current technique, we could not get pure LiTaO3 on Ta sub- similar influence on nucleation and growth.

Fig. (4). SEM images of LiTaO3 film synthesized as function of coating time: (a) 1 h, (b) 3 h, (c) 5 h. 54 Innovations in Corrosion and Materials Science, 2019, Vol. 9, No. 1 Hu et al.

Fig. (5). SEM images of LiTaO3 film synthesized at different coating voltage and temperature:(a) 5 V, (b) 15 V, (c) 450°C, (d) 530°C.

Fig. (6). Cross-sectional morphology (a) and EDS (b) of LiTaO3 coated sample S2. To further determine coating thickness, Back Scattered ples is similar to the thickness proportional to voltage (S4- Electron (BSE) images of cross-sections of coated samples S5) or temperature (S6-S7). were measured. Cross-section morphology (Fig. 6a) and EDS 3.3. Hardness Analysis (Fig. 6b) of LiTaO3 coated sample S2 are shown in Fig. (6). There is no clear boundary between substrate and coating Vickers hardness measurement was used to study the (Fig. 6a) due to low contrast. We could not get the thickness mechanical properties of samples coated by LiTaO3 thin film directly from BSE images of cross-sections. Instead, the film (Fig. 7). Note here, the hardness of Ta substrate is about 140 thickness was taken from the distance of elemental distribu- HV. As can be seen from Fig. (7), surface hardness of all tions of tantalum and oxygen along the cross-sections (Fig. coated samples are much higher (≥ 300 HV) and increased 6b). Thickness of S2 was estimated to be around 3μm. Note with coating time (Fig. 7a), voltage (Fig. 7b) and tempera- here, this is total thickness of mixed film containing LiTaO3 ture (Fig. 7c) as well, due to in situ formation of increased and tantalum oxides, as we could not get pure LiTaO3. Sam- amount hard phase of LiTaO3 and tantalum oxides (see Fig. 2). ples (S1-S7) were also treated in the same way with corre- For example, the content of LiTaO3 in sample S7 (530°C) is sponding thickness data listed in Table 2. Generally speak- slightly higher than that of S6 (450°C), while its surface ing, the cross-section of coating morphology of these sam- hardness is much higher than the later. As our coating was a Formation of Corrosion Resistant Hard Coating of LiTaO3 Innovations in Corrosion and Materials Science, 2019, Vol. 9, No. 1 55 mixed film containing LiTaO3 and tantalum oxides, we could 3.4. Immersion Corrosion Tests not attribute it to the contribution of pure product with regard Ta substrate and samples coated by LiTaO thin film to hardness. In fact, it was their combined effect affecting the 3 were immersed into 10 wt% NaOH solution for 96h to study change of surface hardness. their anti-corrosion performance. Mass change of each spec- imen was registered with results exhibited in Fig. (8a) while Table 2. Film thickness of LiTaO coated samples. corresponded corrosion rate being calculated according to 3 Eqn. (1) and displayed in Fig. (8b). Note, all samples coated by LiTaO thin film showed lower mass loss ( 4 mg),         3  Samples S1 S2 S3 S4 S5 S6 S7 comparing to that of Ta substrate (up to 23 mg). This is a clear indication such that Ta substrate becomes almost 6x d/μm 1.5 3.0 4.1 1.7 3.7 2.3 3.3 more resistive to corrosion upon anodic oxidation. Corrosion rate of 8×10-6 g.cm2.h-1 was obtained for coated samples in- stead of 47×10-6 g.cm2.h-1 for Ta substrate. Corresponding corrosion inhibition efficiency was calculated according to Eqn. 2 and listed in Table 3. It is clear that all coated samples showed high corrosion inhibition efficiency of over 83%, suggesting that formed LiTaO3 thin film as well as tantalum oxides inhibit the corrosion of NaOH alkali solution. Samples of S2, S3 and S7 had the minimum corrosion rate (Fig. 8b) and maximum corrosion inhibition efficiency (Table 3) with high resistance for corrosion. Accounting for energy saving and production efficiency in practical application, however, lowering temperature or shortening reaction time will reduce power consumption and cost. Thus, S2 (synthesis condition is 490°C-5V-3h) was regarded as optimal among all samples.

Fig. (7). Hardness of coated samples synthesized as function of (a) Fig. (8). Mass change (a) and corrosion rate (b) of samples after time, (b) voltage and (c) temperature. immersion test. 56 Innovations in Corrosion and Materials Science, 2019, Vol. 9, No. 1 Hu et al.

Table 3. Corrosion inhibition efficiency of coated samples.

Samples S0 S1 S2 S3 S4 S5 S6 S7

∆ m/g 0.023 0.004 0.001 0.001 0.003 0.002 0.002 0.001

IEcoor - 0.83 0.96 0.96 0.87 0.91 0.91 0.96

Fig. (9). Surface morphologies of samples after immersion test: (a) pure Ta, (b) and (c) LiTaO3 coated.

Fig. (10). OCPs (a) and potentiodynamic polarization curves (b) of samples immersed in 10 wt% NaOH solution at 50°C as function of time.

SEM images of pure Ta and sample coated by LiTaO3 loss of Fig. (8a) to demonstrate the effect of LiTaO3 thin thin film (S2) are depicted in Fig. (9) (Fig. 9a from pure Ta, film on anticorrosion. Figs. 9b and 9c from S2). We can see clearly that pure Ta was indeed eroded by NaOH solution in almost all range 3.5. Electrochemical Measurements leading to the uneven surface with many holes (Fig. 9a), corresponding to the maximum mass loss showed in Fig. OCP and potentiodynamic polarization curves of Ta substrate (8a). Sample S2, however, was corroded slightly with only and LiTaO3 thin film coated sample are presented in Fig. (10). a few small pits (Fig. 9b) and appeared on a nearly smooth Compared to continuously OCP drop of pure Ta (started at - surface. One magnified image (see Fig. 9c, the insert of 0.754V), that of coated sample remains almost stable (re- Fig. 9b) may demonstrate a pit eroded slightly by NaOH mained at -0.096V) (Fig. 10a). This drop of OCP of pure Ta solution. These contrastive experiments showed that with time is indicative of dissolution of protective oxide film LiTaO3 thin film can prevent Ta substrate from ionic attack formed naturally in air. Similar behavior has been reported for in NaOH solution. We can also relate those results to mass Niobium (Nb) in NaOH solution [33]. In addition, the OCP Formation of Corrosion Resistant Hard Coating of LiTaO3 Innovations in Corrosion and Materials Science, 2019, Vol. 9, No. 1 57

Fig. (11). Surface morphologies of samples after potentiodynamic polarization test: (a) pure Ta and (b) LiTaO3 coated. value of LiTaO3 coated sample is higher than that of Ta sub- with one slope of -0.63 in low frequency range, another -0. strate with less chance to be eroded in NaOH solution. 59 in high frequency range, accompanying with phase angle of -48°. Besides the difference in OCPs, shape of their poten- tiodynamic polarization curves are alike (Fig. 10b), indi-  cated that possible erosion reactions occurred at metal- solution interface should be similar. Of Tafel behavior, corrosion current density of these samples was obtained by extrapolation of linear fitting. Tafel values of Ta substrate and LiTaO3 coated sample are listed in Table 4, calculation of corrosion current density of pure Ta and LiTaO3 coated sample to be 6.4×10-7 and 4.7×10-9 A.cm-2. Note here, of higher corrosion potential and about two orders lower cor- rosion current density, corrosion reactions between LiTaO3 thin film and NaOH must be very weak. Fig. (11) shows surface morphologies of samples after potentiodynamic polarization test (Fig. 11, a for pure Ta and b for LiTaO3 coated sample).As can be seen from Fig. (11a), there are many stripes and a few holes appeared in the surface of Ta substrate. It is likely that electrochemical corrosion oc- curred on the surface during the polarization test. However, the surface morphology of LiTaO3 coated sample (Fig. 11b) does not change after polarization test. These results showed that the corrosion resistance has been improved after coating (by LiTaO3).

Table 4. Corrosion potential and corrosion current density of tested samples.

2 Samples Ecoor (mV/SCE) Icoor (A/cm )

S0: Pure Ta -754 6.4×10-7

S2: with LiTaO 3 -96 4.7×10-9 coating

Nyquist and Bode plots of tested samples are depicted in Figs. (12) and (13). Nyquist plot of Ta displays one semicir- cle (Fig. 12a) while Bode plot of Ta shows a linear relation- Fig. (12). Nyquist plots of tested samples in 10 wt% NaOH solution ship between and with slope of -0.97 and phase angle of -87° at 50°C: (a) pure Ta, (b) LiTaO3 coated. for high frequency range (Fig. 13a). These are characteristics of predominantly capacitive behavior. In contrast, Nyquist To illustrate the details of Nyquist plots, a commonly plot of LiTaO3 coated sample shows two capacitance circles accepted equivalent circuit is employed by considering the (Fig. 12b) with impedance higher than that of Ta. Bode plot properties of corrosive solution and samples, as shown in of Fig. (13b) depicts two linear relationships between and Fig. (14). Where R0 is the resistance of electrolyte, R1 is the 58 Innovations in Corrosion and Materials Science, 2019, Vol. 9, No. 1 Hu et al.

Table 5. Parameters for equivalent circuits.

  Samples R0 R1 C R2 CPE 1 CPE 2

 2  2  2  2  -1 -2 -n -1 -2 -n (Ω·cm ) (Ω·cm ) (μF·cm ) (Ω·cm ) Y1 (Ω ·cm ·S ) n1 Y2 (Ω ·cm ·S ) n2

Pure Ta   -3      4.319 11425 9.048×10     

With 11.1 40548  7.949×105 5.015×10-8 0.60 3.461×10-6 0.63 LiTaO3

resistance of charge transfer, R2 is the resistance of LiTaO3 coating, C is the capacitor resistance, CPE 1 and CPE 2 are the constant phase elements related to capacitance of double layer and LiTaO3 film, respectively. The impedance of CPE is defined as      (10) Fig. (14). Equivalent circuits of our experiments: (a) pure Ta, (b) LiTaO3 coated.

Values of fitted equivalent circuit parameters are listed in Table 5. Note here, LiTaO3 coated sample showed larger values of capacitor resistance (for CPE 1 and CPE 2) and charge transfer resistance (R1) than those of pure Ta sub- strate. The former had a very high resistance of passive film 5 2 with value of 7.949×10 Ω.cm (R2). Notice that the re- sistance of R2 referred to the total value of passive film, as our coating being mixed film containing LiTaO3 and tanta- lum oxides.Therefore, we could not attribute it to the contri- bution of pure product with regard to corrosion resistance. Generally speaking, the larger value of the impedance, the better corrosion resistance of the sample. According to these analysis, anticorrosion performance of LiTaO3 coated sample must be superior to that of pure Ta substrate. This is in ac- cordance with our findings of immersion tests. The protec- tive LiTaO3 thin film coated on Ta surface can effectively block reactants of NaOH solution from attacking the surface underneath.

CONCLUSION

1) LiTaO3 thin film has been prepared and coated on Ta substrate by anodic oxidation in molten LiNO3. Both content and hardness of LiTaO3 coating increase gradually with synthesized time, voltage and tempera- ture.

2) Morphology of LiTaO3 coating changed with synthe- sized voltage and temperature. 3) Coated samples have smaller mass loss ( 4 mg) and corrosion rate (8×10-6 g.cm2.h-1) in immersion corro- sion tests compared to those of pure Ta substrate (23 mg and 47×10-6 g.cm2.h-1). 4) Coated samples have demonstrated lower corrosion

current density (4.7×10-9 A.cm-2), higher impedance Fig. (13). Bode plots of tested samples in 10 wt% NaOH solution at (7.949×105 Ω.cm2) and 6-times better corrosion re- 50°C: (a) pure Ta, (b) LiTaO3 coated. sistance in NaOH solution.

Formation of Corrosion Resistant Hard Coating of LiTaO3 Innovations in Corrosion and Materials Science, 2019, Vol. 9, No. 1 59

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