Tantalum Nitride Films for Corrosion Protection of Biomedical Mg-Y-RE Alloy

Journal of Alloys and Compounds 764 (2018) 947e958

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Journal of Alloys and Compounds

journal homepage: http://www.elsevier.com/locate/jalcom

Tantalum nitride ﬁlms for corrosion protection of biomedical Mg-Y-RE alloy

Weihong Jin a, b, Guomin Wang a, Xiang Peng a, Wan Li a, Abdul Mateen Qasim a, * Paul K. Chu a, a Department of Physics and Department of Materials Science and Engineering, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China b Institute of Advanced Wear & Corrosion Resistant and Functional Materials, Jinan University, Guangzhou, 510632, China article info abstract

Article history: Based on their natural degradation in the human body, magnesium-based alloys are attractive for Received 3 April 2018 temporary implants such as cardiovascular stents and bone fixation devices. Unfortunately, rapid Received in revised form degradation of magnesium-based alloys in the physiological environment is a major obstacle limiting 5 June 2018 wider application. Herein, the WE43 Mg alloy deposited with tantalum nitride films by reactive Accepted 14 June 2018 magnetron sputtering shows enhanced corrosion resistance. The modified WE43 exhibits a 95-fold Available online 18 June 2018 decrease in the corrosion current density and 100-fold initial increase in the charge transfer resistance in simulated body fluids (SBF). The ion concentration and pH of the soaking solution show that corrosion Keywords: fi Corrosion of the modi ed WE43 is remarkably retarded during a long-term immersion in SBF for 30 days. The fi Electrochemical impedance spectroscopy corrosion propagation mechanism of the untreated and modi ed WE43 during long-term immersion in Scanning electron microscopy SBF is investigated. Thin films © 2018 Elsevier B.V. All rights reserved. Vapor deposition

1. Introduction Tantalum (Ta) is a refractory transition metal commonly used in capacitors in electronics. Owing to the good biocompatibility, Ta Magnesium (Mg) and its alloys have been extensively investi- has also been used as components in primary and revision total hip gated for potential applications to temporary implants such as arthroplasty and clinical studies support further investigation of cardiovascular stents, bone fixation plates, screws, pins, and nails porous Ta as an alternative to traditional implant materials [6]. The [1,2]. The main driving force to develop Mg-based implants is their excellent bioactivity has also been confirmed [7e9]. Ta has good natural degradation under physiological conditions thereby elimi- inertness in many aggressive environments due to the formation of nating follow-up surgeries to remove the implants after sufficient a stable, compact, and adherent Ta2O5 film on the surface [10,11]. tissue healing [3]. Among the various Mg-based alloys, the ones Xu et al. [12] and Díaz et al. [13] observed that Ti-6Al-4V alloy and alloyed with rare-earth elements such as WE43 do not contain steel with a Ta2O5 nanocoating barrier prepared by double glow aluminum with neurotoxicity and also possess high strength and discharge plasma technique and low-temperature atomic layer ductility, thereby making them attractive to biomedical applica- deposition as well as filtered cathodic arc deposition, respectively tions such as temporary implants [4]. Unfortunately, a major had enhanced anticorrosion ability. In comparison, modification of concern of Mg-based implants is rapid degradation in the chloride- Mg alloys with Ta has seldom been reported. In our recent study, Ta containing physiological system. The accompanying drawbacks oxide fabricated by reactive magnetron sputtering enhanced both include premature loss of the mechanical integrity during tissue the early-stage corrosion resistance and in vitro biocompatibility of healing, excessive hydrogen evolution, and local alkalinity which Mg alloy [14]. Wang et al. revealed that Ta ion implantation may cause implant failure [5]. As a result, degradation of Mg and its improved the corrosion resistance of AZ31 Mg alloys and suggested alloys must be controlled prior to clinical acceptance of the tem- that formation of a dense MgO layer and protective Ta2Al barrier porary implants. rather than Ta2O5 formation was responsible for the improvement [15,16]. Similar to other transition metals such as Ti, Zr, and Nb, Ta has a high affinity to nitrogen, which can react with Ta to form inert Ta * Corresponding author. E-mail address: [email protected] (P.K. Chu). nitride to improve the bacterial and corrosion resistance. Ag- and https://doi.org/10.1016/j.jallcom.2018.06.151 0925-8388/© 2018 Elsevier B.V. All rights reserved. 948 W. Jin et al. / Journal of Alloys and Compounds 764 (2018) 947e958

Cu-incorporated Ta nitride nanocomposite thin films have been 0.292 g CaCl2, and 0.072 g Na2SO4) in 1000 mL of deionized water. observed to have anti-bacteria and anti-wear properties [17]. Choe The SBF solution was buffered at pH 7.4 with 6.118 g L 1 trishy- et al. [18] and Alishahi et al. [19] found that Ta nitride coatings droxymethyl aminomethane and 1.0 M HCl. The ion concentrations þ þ þ þ produced by different methods retarded corrosion of 316L stainless in the SBF solution were 142.0 Na , 5.0 K , 1.5 Mg2 , 2.5 Ca2 ,147.8 2 steels in a 0.05 M H2SO4 þ 0.2 ppm HF solution and the materials Cl , 4.2 HCO3 , 1.0 HPO4 , and 0.5 SO4 mM, which are close to those were good bipolar plates in polymer electrolyte membrane fuel of human blood plasma [22]. The bottom and sides of the speci- cells. Ta nitride films were also used to protect biomedical stainless mens were sealed with silica gel and the area of the working steels in the artificial physiological solution [20,21]. However, electrode exposed to the electrolyte was 1 cm2 during the elec- enhancement of the surface corrosion resistance of Mg and its al- trochemical measurements. EIS was conducted with a 10 mV loys by Ta nitride has not been reported. In this work, the potential alternating signal from 100 kHz to 100 mHz at the open circuit and effectiveness of Ta nitride films fabricated by reactive magne- potential (OCP) after immersion in SBF for 1, 24, 48, 72, 96, and tron sputtering to enhance the corrosion resistance of the WE43 Mg 120 h. The samples were immersed in the solution for 15 min and alloy are investigated. Electrochemical and long-term immersion the polarization curves were collected at a scanning rate of studies are performed systematically to evaluate the effects and 1mVs 1 from 300 mV and 500 mV with respect to OCP. mechanisms. 2.3. Long-term immersion tests 2. Experimental details To monitor degradation of the WE43 Mg alloy before and after 2.1. Sample preparation and surface characterization surface modification, long-term immersion tests were carried out in SBF. According to ASTM-G31-72 [23], the specimens were The 10 mm 10 mm 5 mm specimens were prepared from immersed in SBF and maintained at 37 C for 30 days. At different the Mg-Y-RE alloy (Mge4.03 wt% Ye2.03 wt% Nde1.15 wt% immersion time of 1, 3, 7, 15, and 30 days, the concentrations of Gde0.53 wt% Zr, WE43). Prior to deposition, the specimens were leached Mg from the samples were determined by inductively- mechanically ground with SiC abrasive paper up to 1200 grit, ul- coupled plasma optical emission spectroscopy (ICP-OES, PE Op- trasonically cleaned with ethanol in an ultrasonic bath for 15 min, tima 2100DV, Perkin Elmer, Waltham, MA, USA). The pH of the and finally dried by nitrogen. The Ta nitride films were deposited in corresponding SBF was recorded. The cross sectional and surface a direct-current reactive magnetron sputtering system (ATC-Orion morphologies as well as elemental information were acquired by 5 UHV with Load-Lock, AJA International, Inc., Scituate, Massa- SEM and EDS (INCAx-sight, Oxford Instruments, Abingdon, chusetts, USA). After loading the specimens, the system was pum- Oxfordshire, UK) from the uncoated and coated WE43 samples ped down to a base pressure of 6.0 10 4 Pa. A high purity (99.99%) after immersion in SBF for 7, 15, and 21 days. Ta target was used. Both the reactive N2 (99.999%) and inert Ar (99.999%) flow rates were adjusted to 6 sccm during deposition, 3. Results and discussion while the working pressure was maintained at 0.37 Pa. The power was 100 W and deposition was conducted for 3 h without substrate 3.1. Surface characterization heating. X-ray photoelectron spectroscopy (XPS, PHI 5802, Physical Fig. S1 shows the optical microstructure of the WE43 Mg alloy. Electronics, Inc., Eden Prairie, Minnesota, USA) with Al Ka irradia- The alloy is composed of the Mg matrix and a distribution of second tion was performed to determine the chemical states of the film phases which seem to be crystallized along the grain boundaries as and the binding energies were calibrated referenced to the C 1s line well as in the matrix. The XPS full spectrum of the coated WE43 at 284.8 eV. All the XPS spectra were collected after sputtering by Ar after sputtering for 50 nm are depicted in Fig. 1a. The peaks of Ta, N, for 50 nm, which was based on a SiO2 reference. The phase and and O are observed from the XPS full spectrum in Fig. 1a. Oxygen structure were determined by grazing-incidence X-ray diffraction arises from natural oxidation/adsorption and/or residual H2OorO2 (GIXRD, SmartLab X-ray Diffractometer, Rigaku Corporation, Tokyo, in the deposition chamber. The high-resolution XPS spectra of Ta 4f, Japan) with Cu Ka radiation. The diffractometer was operated at N 1s, and O 1s are presented in Fig. 1b and Fig. S2. As shown in 40 kV and 30 mA with an incident angle of 1. The XRD spectra were Fig. 1b, the high-resolution XPS Ta 4f spectrum is resolved into 1 recorded over a wide range from 15 to 80 at a rate of 0.02 s . The three sets of 4f7/2 and 4f5/2 doublets with a spin-orbit splitting of film thickness and elemental maps of the cross-sectioned sample 1.9 eV and intensity ratio of 4:3. The peaks at 22.8 eV for Ta 4f7/2 and were determined by scanning electron microscopy (SEM, JSM-820, 24.7 eV for Ta 4f5/2 are in good agreement with the chemical state of JEOL Ltd., Tokyo, Japan) equipped with energy-dispersive X-ray Ta in TaN [24]. Two more sets of Ta 4f peaks appear at higher spectroscopy (EDS, INCAx-sight, Oxford Instruments, Abingdon, binding energies of 23.3 eV/25.2 eV and 24.6 eV/26.5 eV attributed Oxfordshire, UK). The surface morphology was observed by SEM to Ta suboxide in the film [24e26]. Fig. S2a shows that the N 1s peak with a field emission source (JSM-6335F, JEOL Ltd., Tokyo, Japan). overlaps the peak of Ta 4p3/2 and the strong peak at 397.5 eV for N 1s corresponds to the formation of TaN. With regard to the O 1s in 2.2. Electrochemical tests Fig. 2b, the peaks at 530.4 eV and 531.0 eV are associated with Ta suboxides and that at 531.9 V arises from -OH due to air exposure Potentiodynamic polarization and electrochemical impedance [27]. Therefore, a surface layer composed of TaN and Ta suboxide spectroscopy (EIS) were performed in simulated body fluids (SBF) (TAN) is formed on the WE43 Mg alloy by reactive magnetron at 37 C on an electrochemical workstation (Zennium, Zahner, sputtering. Fig. 1c presents the GLXRD spectrum of the deposited Kansas City, Missouri, USA) using a three-electrode cell system. The film on silicon. The peak at 34 is assigned to the (110) reflection of saturated calomel electrode (SCE), platinum sheet, and specimens TaN (JCPDS No. 71-0253). A broad peak in the range of 42e60 also were the reference electrode, counter electrode, and working emerges from the deposited film in accordance with those electrode, respectively. Unless specifically specified, the potentials observed previously from amorphous oxidized Ta films [14] and by were referred to SCE. The SBF solution was prepared by dissolving Kulisch et al. [28]. Consequently, the sputtered film has a mixed reagent grade chemicals (8.035 g NaCl, 0.355 g NaHCO3, 0.225 g KCl, structure of crystalline TaN and amorphous Ta suboxide. Fig. 2a and 0.231 g K2HPO4$3H2O, 0.311 g MgCl2$6H2O, 1.0 M HCl (39 mL), b shows the cross-sectional SEM image of the TAN-coated WE43 W. Jin et al. / Journal of Alloys and Compounds 764 (2018) 947e958 949

Fig. 1. (a) XPS full spectrum of the TaN film on WE43 and (b) High-resolution XPS spectrum of Ta 4f acquired from the TaN film on WE43 after sputtering for 50 nm; (c) GIXRD spectrum of the TaN film deposited on silicon.

(TAN-WE43) sample and corresponding EDS elemental maps of Ta and a smaller corrosion current density corresponds to a lower and Mg. The thickness of the deposited TAN film is 621 ± 4 nm. corrosion rate. The corrosion current density of TAN-WE43 de- Fig. 2c and d presents two representative regions of surface creases to 6.2 ± 1.4 mAcm 2, which is approximately two orders morphology of the TAN-WE43 sample observed by SEM. Fig. 2c smaller than that of the bare WE43 sample, thus providing evi- depicts a continuous and dense TAN film formed on the WE43 Mg dence that the TaN film notably retards the corrosion of WE43 Mg alloy. However, as shown in Fig. 2d there are some defects such as alloy in SBF. Fig. 4 shows the OCP of WE43 and TAN-WE43 during large particles (indicated by arrows), which are not closely con- immersion in SBF for 120 h. The OCP of the untreated WE43 in- nected with the uniform parts of the film forming some large gaps. creases rapidly during the initial 24 h, rises slowly in the next 24 h, This may be caused by low substrate temperature during film and stabilizes thereafter. The initial OCP increase is caused by the deposition or incomplete removal of contaminants from the sub- formation of corrosion products and the surface is covered uni- strate surface during sample preparation. formly by the corrosion products after immersion for 48 h. The OCP of the TAN-WE43 rises gradually in the first 48 h and then stabilizes. This phenomenon is common due to continuous plugging of 3.2. Electrochemical studies the microspores of the TaN film by the corrosion products in the initial stage. Fig. 3 presents the electrochemical potentiodynamic polariza- As one of the useful techniques to investigate the corrosion tion curves of WE43 and TAN-WE43 after immersion in SBF for processes at the electrode/electrolyte interface and evaluate the 15 min. The corrosion potential Ecorr, corrosion current density Icorr, corrosion resistance, EIS is conducted after immersion in SBF for and cathodic Tafel slope bc are calculated from the cathodic polar- different periods of time up to 120 h. Fig. 5 shows the Nyquist plots ization curves according to the method described previously [29] and fitted curves of the WE43 and TAN-WE43 samples and the and the values are listed in Table 1. The corrosion potential of the corresponding experimental and fitted Bode impedance and phase modified WE43 in SBF shifts to 1767.4 ± 18.2 mV, which is slightly angle plots are given in Figs. 6 and 7. As shown in Fig. 5a and b, the more positive than that of the bare WE43. No clear positive shift Nyquist plot of the untreated WE43 during the entire immersion was also reported by Jia et al. from SiP-coated Mge1Ca alloy [30]. time shows two arcs corresponding to two time constants. The two The corrosion rate depends on the corrosion current density, which loops of the bare WE43 show a larger radius with soaking time. is an important parameter in corrosion resistance characterization 950 W. Jin et al. / Journal of Alloys and Compounds 764 (2018) 947e958

Fig. 2. (a) EDS map of Ta and (b) SEM image of the cross-section of TAN-WE43; (c, d) Two representative regions of surface morphology of the TAN-WE43 sample observed by SEM using the secondary electron mode.

Fig. 4. OCP of WE43 and TAN-WE43 during the 120-h immersion in SBF.

Fig. 3. Polarization curves of WE43 and TAN-WE43 after 15 min immersion in SBF.

separated and integrated after immersion for 72 h. The initial Table 1 separation is also indicated by the reduced slope in the middle b Ecorr, Icorr, and c of WE43 and TAN-WE43 in SBF calculated from the polarization frequency range of the Bode impedance plots (Fig. 6a) and the two curves. split phase angle maxima (Fig. 7a) [32]. When the immersion time 2 1 Ecorr (mV) Icorr (mAcm ) bc (mV decade ) is increased to 72 h, the change in the slope in the middle frequency WE43 1953.3 ± 18.6 597.8 ± 38.2 346.5 ± 9.1 becomes blurred and the two phase angle maxima also merge into TAN-WE43 1767.4 ± 18.2 6.2 ± 1.4 366.7 ± 9.1 each other (Figs. 6b and 7b) suggesting two integrated arcs. An equivalent circuit model with two time constants, Rs(CPEdlRct)(CPEdiffRdiff), as shown in Fig. 8a is adopted to ﬁt the EIS Ascencio et al. reported a similar behavior of WE43 Mg alloy in data of the WE43 Mg alloy. Rs is the resistance of the electrolyte modiﬁed-SBF [31]. In the beginning, the two arcs are clearly between the working electrode and reference electrode, CPEdl, W. Jin et al. / Journal of Alloys and Compounds 764 (2018) 947e958 951

Fig. 5. Nyquist plots of (a, b) WE43 and (c, d) TAN-WE43 in SBF at 1, 24, 48, 72, 96, and 120 h. represents the capacitance of the double layer at the electrolyte/ Fig. 5c and d, as the immersion time is increased, two apparent metal interface, and Rct is the relevant charge transfer resistance. loops in the medium and low frequency ranges of the TAN-WE43 CPEdiff and Rdiff are related to the capacitance and relevant resis- sample become smaller and a similar phenomenon has been re- tance induced by the diffusion process in a finite region. The same ported by Ishizaki et al. for the AZ31 Mg alloy coated with super- equivalent circuit has been implemented by Wu et al. [33]to hydrophobic films [35]. This is also confirmed by the fitted EIS re- simulate the EIS data of the Mg-Nd-Zn-Zr alloy in the 0.9 wt% NaCl sults and the reasons will be discussed in that part. A parallel solution. As shown in Fig. 8a, the high-frequency arc in the combination of CPEfRf is introduced to construct an equivalent cir- impedance diagram is assigned to the effect of the charge transfer cuit model with three time constants, Rs(CPEfRf)(CPEdlRct) resistance and double layer capacitance, whereas the low- (CPEdiffRdiff) in Fig. 8btofit the EIS data. CPEf is ascribed to the frequency arc is related to the diffusion process, which is also capacitance of the deposited film and Rf is the pore resistance implied by the upward curve rather than horizontal line at low resulting from the formation of ionically conductive paths across frequencies in the Bode impedance plot (Fig. 6a and b) [32]. Even the film. The three loops of TAN-WE43 at high, middle, and low though corrosion products cover the substrate surface during im- frequencies are associated with the surface film, charge transfer mersion, only one arc emerges at high frequencies. The resistance process, and diffusion process, respectively. The equivalent circuit of the corrosion products layer with a loose structure (confirmed by for TAN-WE43 is different but similar to that proposed for the following SEM images of the samples after immersed in SBF) is polypropylene-coated mild steel [36] and polymer-coated Mg-Gd-Y so small in comparison with the charge transfer resistance that the alloy [37]. As shown in Figs. 5e7, the Nyquist and Bode plots of TAN- corresponding arc of the corrosion layer is virtually undetectable in WE43 are fitted well, so the equivalent circuit in Fig. 8b can be used the impedance diagram [34]. to analyzed the EIS data of TAN-WE43. With respect to TAN-WE43, the Bode phase diagrams in Figs. 6c, The fitted values of Rct and Rdiff of WE43 and Rf, Rct, and Rdiff of d and 7c, d shows three arcs as it is a very sensitive method to TAN-WE43 are shown in Fig. 9. For the untreated WE43 Mg alloy, identify the number of time constants. During the 120-h immersion prolonged exposure to SBF increases both Rct and Rdiff. The charge period, the second loop and third loop are clearly separated, while transfer resistance, Rct, is considered the most appropriate param- the first two loops are also separated according to the phase angle eter to monitor the protective properties of the film because it re- vs frequency diagram, even though the first loop is too small and flects the corrosion rate of the metal substrate [38]. Rct of WE43 cannot be clearly observed from the Nyquist diagram. As shown in increases from 87 U cm2 at 1 h to 432 U cm2 at 120 h and so the 952 W. Jin et al. / Journal of Alloys and Compounds 764 (2018) 947e958

Fig. 6. Bode impedance plots of (a, b) WE43 and (c, d) TAN-WE43 in SBF at 1, 24, 48, 72, 96, and 120 h. corrosion rate of the uncoated WE43 decreases with immersion penetration of the electrolyte through the existing and newly time. This is ascribed to the continuous formation of the corrosion formed defects in the film. A small Rf has been also observed by Wu products layer on the WE43 Mg alloy and the corrosion products et al. from the Mge3.0Nde0.2Zne0.4Zr alloy modified by a can serve as a barrier layer for the WE43 substrate against further diamond-like carbon film fabricated by plasma immersion ion corrosion to some extent. The decreased dissolution rate comes implantation and deposition after immersion in 0.9% NaCl for from the increased thickness of the corrosion layer with better 30 min arising from random defects in the diamond-like carbon protective ability. However, very limited protection is provided if film [33]. Therefore, it is important to design effective strategies to the corrosion products layer has a loose structure (confirmed by reduce defects in the film, for example, by optimizing the deposi- SEM). Hence, Rct of the untreated WE43 Mg alloy does not reach a tion conditions and pretreatment. Rct of TAN-WE43 Mg decreases very large value. Meanwhile, when diffusion occurs, it cannot be rapidly during immersion for 24 h and then decreases gradually ignored because the corrosion process can be affected by the afterwards. This means that the metal substrate underneath the movement of chemical species. When the immersion time is film continues to dissolve and the corrosion rate of the TAN-WE43 2 increased to 120 h, Rdiff of the WE43 sample increases to 659 U cm . Mg alloy in SBF is quite large in early immersion but decreases after For the uncoated WE43, Rdiff represents the resistance against 24 h. Compared to the untreated WE43, the modified WE43 shows 2þ diffusion of the produced Mg through the corrosion layer away a larger Rct at each time point during the immersion period of 120 h, 2 from the Mg substrate. Transportation of the produced species is especially in the initial stage. Rct of TAN-WE43 reaches 8850 U cm , more difficult when the corrosion products layer becomes thicker. which is 101 times that of the WE43 substrate after immersion for 2 With regard to the WE43 Mg alloy with the TaN film, the trends 1 h. Even after 120 h, Rct of TAN-WE43 is 1281 U cm , which is much 2 of Rct and Rdiff are different and another parameter Rf is discussed. larger than 432 U cm of WE43. These results prove that the As shown in the inset image in Fig. 9, after immersion for 1 h, TAN- dissolution rate of the WE43 Mg alloy underneath the TaN film is WE43 shows a small Rf suggesting that after a short time, the notably smaller over the 120 h immersion period. Meanwhile, as electrolyte absorbs and reaches the metallic substrate through the degradation of the Mg substrate under the TaN film only occurs in þ latent defects or discontinuities, which provide the pathways for defective areas, diffusion of the produced Mg2 species though the the solution to reach the substrate/film interface and have a accumulated corrosion products away from the metal surface is detrimental effect on the corrosion performance [39]. The increase much more difficult under the TaN film. In the beginning, the in Rf at 24 h may be due to plugging of pores by the corrosion deposited film has limited imperfections and diffusion of the pro- products [40]. Rf decreases subsequently due to increased duced species is strongly impeded, resulting in a large Rdiff.As W. Jin et al. / Journal of Alloys and Compounds 764 (2018) 947e958 953

Fig. 7. Bode phase angle plots of (a, b) WE43 and (c, d) TAN-WE43 in SBF at 1, 24, 48, 72, 96, and 120 h.

Fig. 8. Equivalent circuits of (a) WE43 and (b) TAN-WE43 after immersion in SBF. immersion proceeds, the corrosion process at the film/metal WE43 Mg alloy in SBF. interface produces more defects in the film making it easier for the produced species to diffuse resulting in Rdiff reduction. After immersion in SBF for 1e120 h, the diffusion resistance of TAN-WE43 3.3. Long-term degradation behavior reaches 8578e1748 U cm2, whereas that of the bare WE43 is only 23e659 U cm2. Consequently, the EIS results shown above show To monitor the long-term degradation behavior, release of Mg that the TaN film provides protection and retards dissolution of the ions and pH value change of the immersed solution are monitored for up to 30 days. Fig. 10 depicts the concentrations of released Mg 954 W. Jin et al. / Journal of Alloys and Compounds 764 (2018) 947e958

To further investigate corrosion propagation along the longitudinal and lateral directions, SEM images and EDS line scans are acquired from WE43 and TAN-WE43 after immersion for 7, 15, and 21 days. Since the longitudinal corrosion propagation of the bare and coated Mg alloy is not uniform, it is better to monitor different regions to monitor the actual corrosion conditions. As shown in Figs. 11e13 and Figs. S3eS8, two representative regions including a slightly corroded region and severely corroded region are present. Fig.11a and b shows that all the regions on WE43 are covered by the corrosion products and the maximum thickness of the corrosion layer on the untreated WE43 is 91.5 mm after 7 days. Fig. 11a and b also reveal that the corrosion layer has a loose structure with many cracks providing channels for SBF and limited protection. As shown in Fig. S3, Mg, P, Ca, and O are detected from the corrosion layer on the untreated WE43 due to the formation of MgO, Mg(OH)2, phosphate, and carbonate during immersion [31]. The former three heavy elements are determined by EDS [29] and the line scan is Fig. 9. R and R of WE43 and R , R and R TAN-WE43 obtained from the fitted EIS ct diff f ct diff acquired from up to down along the red line across the resin, results in SBF at 1, 24, 48, 72, 96, and 120 h. corrosion layer, and Mg substrate. The top layer of the corrosion products on the uncoated WE43 shows stronger P and Ca signals ions from WE43 and TAN-WE43 after immersion in SBF for 1, 3, 7, and weaker Mg signal and the P and Ca signals weaken and Mg 15, and 30 days and the corresponding pH values of the immersed signal strengthens towards the Mg substrate. The Mg, P, and Ca solution. As shown in Fig. 10a, WE43 shows fast release of Mg ions signals cannot be observed at some cracks. in the first 7 days. Compared to the untreated WE43 sample, TAN- After immersion for 7 days, TAN-WE43 shows less corrosion as WE43 shows much less leaching of Mg ions during the whole im- shown in Fig. 11cef. It can be seen that the maximum thickness of mersion process. The concentrations of released Mg from the un- the corrosion products on the TaN film is 44.4 mm, which is much treated and treated WE43 are 206.1e941.7 mgmL 1 and smaller than that observed from the bare WE43. The slightly- and 32.7e536.1 mgmL 1, respectively, during the 30-day immersion severely-corroded surfaces show three different structures: period. With increasing soaking time in SBF, OH is released from A intact region, B e slightly corroded region with blisters, and C e both WE43 and TAN-WE43 to raise the pH. Release of OH in the severely corroded region with ruptured film. The corrosion layer second phase with a relatively notable potential as a cathode is beneath the TaN film has a loose structure with some cracks proportional to the cathodic reaction and reflects the overall resulting in regions B and C. Fig. S4 show Ta at the top of the corrosion rate. As shown in Fig. 10b, the pH of the solutions of the corrosion layer although the signal is weak. Mg, P, and Ca are also untreated and coated WE43 Mg alloy increases rapidly in the initial detected from the film region because the loose corrosion products 7 days and gradually afterwards. After 1 day, the pH of the solution may slip to the film side during sample preparation. The P and Ca of the bare Mg alloy rises to 8.25 ± 0.19 and that of TAN-WE43 is concentrations decrease from the top to inner layers of the corro- only 7.51 ± 0.01, which is only slightly larger than that of the SBF of sion products of TAN-WE43 and the Mg content exhibits a reverse 7.4. The solution of TAN-WE43 exhibits lower alkalinity than that of tendency. WE43 at each immersion time point. After 30 days, the solution of After exposure to SBF for 15 and 21 days, corrosion of the un- TAN-WE43 is 8.39 ± 0.14, which is much smaller than that of the treated WE43 extends to deeper locations and the corrosion untreated WE43 of 9.3 ± 0.13. The trend of Mg leaching is similar to penetration depths are 136.2 mm and 163.4 mm as shown in Fig. 12a that of the pH and the results confirm that degradation of the WE43 and b as well as Fig. 13a and b. The untreated WE43 also shows Mg alloy is effectively mitigated by the TaN film. rapid penetration in the first 7 days and it slows after 15 days. The

Fig. 10. (a) Concentrations of Mg ions leached from WE43 and TAN-WE43 after immersion in SBF for 1, 3, 7, 15, and 30 days and (b) corresponding pH of the immersion solutions. W. Jin et al. / Journal of Alloys and Compounds 764 (2018) 947e958 955

Fig. 11. SEM cross-sectional images of the slightly corroded region and severely corroded region of (a, b) WE43 and (deg) TAN-WE43 after immersion in SBF for 7 days.

Fig. 12. SEM cross-sectional images of the slightly corroded region and severely corroded region of (a, b) WE43 and (deg) TAN-WE43 after immersion in SBF for 15 days. thickness of TAN-WE43 Mg alloy reaches 99.2 mm and 119.9 mm the initial stage. With respect to TAN-WE43 after immersion for 15 after immersion in SBF for 7 and 15 days (Figs. 12cef and 13cef), days, regions without corrosion are still found as shown in Fig. 12c. which are obviously smaller than those of the untreated substrate. Region C exhibits more blistering and delamination allows easier The increasing rate of the maximum thickness of the corrosion transportation of the produced species to accelerate the reaction products on TAN-WE43 is smaller than the coated one especially in with Mg. As shown in Fig. 13d and f, after 21 days, parts of the TaN 956 W. Jin et al. / Journal of Alloys and Compounds 764 (2018) 947e958

Fig. 13. SEM cross-sectional images of the slightly corroded region and severely corroded region of (a, b) WE43 and (deg) TAN-WE43 after immersion in SBF for 21 days.

film detach from region D (severely corroded region with film intact or just blisters. When the immersion time is increased to 15 delamination). The condition in this region is similar to that of the days, the area of region C increases and those of regions A and B corroded WE43 without treatment. Dissolution of the Mg substrate decrease. After 21 days, the TaN film delaminates as shown in re- in this region is easier than that the other three regions and the TaN gion D, which may result from the hydrogen gas and corrosion film in regions A and B provides a relatively good barrier against products produced during corrosion [41,42]. corrosion. The corresponding EDS line scans in Figs. S5eS8 Corrosion of Mg-based alloys in aqueous environments often demonstrate that the P and Ca contents tend to decrease except involves microgalvanic coupling between the cathodic regions such at the crack and fall-off regions from the top to inner layers of the as the second phase and anodic regions namely the Mg matrix since corrosion products of WE43 and TAN-WE43, whereas the Mg signal the second phase has a nobler potential and is less reactive than the exhibits a reverse phenomenon similar to that observed after im- Mg matrix. The overall reaction for the corrosion of Mg is expressed mersion in SBF for 7 days. The results indicate with penetration of as: SBF, Mg in the outer layer continues to react with Ca and P species and Mg in the inner layer is gradually consumed to begin to Mg þ 2H2O / Mg(OH)2 þ H2 (1) combine with P and Ca species to form products. Some different phenomena are also observed during the late immersion stage. The The overall reaction may be the sum of several partial reactions þ EDS line scans in Fig. S5b do not show Mg, P, and Ca in some regions including the anodic production of Mg2 (Equation (2)), cathodic due to the large cracks or fall-off of the looser corrosion products hydrogen production (Equation (3)), and product formation during sample preparation. The volcano-like precipitates in (Equation (4)) as follows: Figs. 12b and 13a are observed from the corrosion layer on the þ e untreated WE43 after immersion for 15 days and 21 days. The EDS Mg / Mg2 þ 2e (2) results in Fig. S7a reveal that the volcano-like precipitates have a e larger Ca ratio consistent with the observation by Ascencio et al. 2H2O þ 2e / H2 þ 2OH (3) from corroded WE43 Mg alloy immersed in modified SBF [31]. 2þ Fig. 14 depicts the surface morphology of WE43 and TAN-WE43 Mg þ 2OH / Mg(OH)2 (4) after immersion in SBF for 7, 15, and 21 days. Even though the samples are coated with carbon, some areas shown in Fig. 14aec For the untreated WE43 Mg alloy, the Mg(OH)2 layer forms still appear white due to charging of the thick insulating corrosion gradually on the surface during immersion in SBF. The fitted EIS products. As shown in Fig. 14aec, a severely corroded surface with results (Fig. 9) show that this corrosion products layer can provide many large cracks is observed from the bare WE43 Mg alloy after protection to some extent to the WE43 substrate against further the 7, 15, and 21 days. The volcano-like precipitates also emerge corrosion and better protective ability is achieved by a thicker from the corroded WE43 surface after 7 days. Fig. 14def shows that corrosion layer up to an exposure time of 120 h. The cross-sectional the modified WE43 Mg alloy has less local corrosion and corrosion and surface SEM views of the corroded WE43 at 7, 15, and 21 days propagation after 21 days than the untreated WE43. The SEM im- (Figs. 11a, b and 12a, b, as well as Fig. 13a and b) disclose that the ages further confirm regions A, B, C, and D. Parts of the TaN film corrosion layer has a loose structure with many cracks offering rupture after immersion for 7 days, while most of the surface is limited protection. For the WE43 Mg alloy with the TaN film, after W. Jin et al. / Journal of Alloys and Compounds 764 (2018) 947e958 957

Fig. 14. Surface morphology of (a, b, c) WE43 and (d, e, f) TAN-WE43 after immersion in SBF for 7, 15, and 21 days.

immersion for 1 h, a small Rf value is obtained (Fig. 9), implying that anticorrosion properties are investigated by electrochemical tests the electrolyte has traversed through the imperfections onto the and immersion tests for up to 30 days. The corrosion current Mg substrate after a short time. Compared to the untreated WE43, density of TAN-WE43 in SBF is approximately two orders less than TAN-WE43 has a larger charge transfer resistance Rct at each time that of the bare WE43 substrate translating into a smaller disso- point during the 120 h immersion period and Rct of TAN-WE43 is lution rate. During extended exposure to SBF, the charge transfer 101 times that of the WE43 substrate after 1 h (Fig. 9). The larger Rct resistance Rct and diffusion resistance Rdiff of WE43 tends to in- leads to a smaller dissolution rate of the WE43 Mg alloy under the crease whereas those of TAN-WE43 increase. The modified WE43 TaN film. After immersion in SBF for 1e120 h, the diffusion resis- exhibits larger Rct and Rdiff during 120-h immersion in SBF, espe- 2 tance Rdiff of TAN-WE43 reaches 8578e1748 U cm , which is much cially in the initial stage, suggesting the effective corrosion pro- larger than 23e659 U cm2 of the bare WE43 (Fig. 9). Diffusion of the tection by the TaN film. The untreated WE43 Mg alloy shows rapid produced species though the accumulated corrosion products away corrosion propagation in both the longitudinal and lateral di- from the Mg surface becomes more difficult under the TaN film and rections during immersion because the loose corrosion layer the large charge transfer resistance and diffusion resistance miti- formed provides limited protection. The corrosion layer on TAN- gate corrosion in SBF. The surface SEM images of the immersed WE43 is thicker larger than that on the untreated WE43 after samples (Fig. 14) reveal that the WE43 surface is fully covered by exposure for 7 days because the TaN film acts as an effective barrier corrosion products after 7 days, whereas the coated WE43 Mg alloy to protect the WE43 substrate from the SBF. As degradation of the shows less local corrosion even after immersion for 21 days. The Mg substrate under the TaN film occurs finally, more and larger cross-sectional SEM images of the immersed samples (Fig. 1113) imperfect areas are produced to expedite dissolution of Mg. When show that TAN-WE43 experiences less corrosion penetration and the exposure time is long enough, film rupturing and delamination lateral propagation than the untreated WE43 during 21-day im- occur. In summary, the TaN film deposited on the WE43 Mg miti- mersion. The excellent corrosion resistance of the TaN film is also gates the corrosion of the WE43 Mg alloy. demonstrated by the smaller corrosion current density (Fig. 3), Mg leaching (Equation (2)), as well as pH variation (Equation (3)) in the Acknowledgments 30-day study (Fig. 10). Both the cross section and surface SEM images of the corroded TAN-WE43 immersed for 7 and 15 days This work was supported by City University of Hong Kong (Figs. 11cef, 12cef, and 14d and e) confirm the three regions: Research Grants Council (RGC) General Research Funds (GRF) Nos. A intact region, B e slightly corroded region with blisters, and C e CityU 11301215 and 11205617. severely corroded region with ruptured film. As the immersion time is increased, the area of regions A and B decreases and those of Appendix A. Supplementary data regions C and D increase (Fig. 13cef, and Fig. 14f). As degradation of fi the Mg substrate under the TaN lm occurs, more and larger Supplementary data related to this article can be found at imperfect areas are produced to further accelerate dissolution of https://doi.org/10.1016/j.jallcom.2018.06.151. Mg and when the exposure time is long enough, film rupture and delamination occur. References

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[23] ASTM G31-72: standard practice for laboratory immersion corrosion testing of Mater. Interfaces 8 (2016) 10014e10028. Supporting Information

Tantalum nitride films for corrosion protection of biomedical Mg-Y-RE alloy

Weihong Jin1,2, Guomin Wang1, Xiang Peng1, Wan Li 1, Abdul Mateen Qasim1, Paul K. Chu1, *

1 Department of Physics and Department of Materials Science and Engineering, City University

of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China

2 Institute of Advanced Wear & Corrosion Resistant and Functional Materials, Jinan University,

Guangzhou 510632, China

* Corresponding Author:

Prof. Paul K. Chu

Tel.: +852 34427724; Fax: +852 34420542

E-mail: [email protected]

Fig. S1: Optical mircrostructure of the WE43 Mg alloy.

Fig. S2: High-resolution XPS spectra of (a) N 1s and (b) O 1s acquired from the TaN film on

WE43 after sputtering for 50 nm.

Fig. S3: EDS line scans of the (a) slightly corroded region and (b) severely corroded region of

WE43 after immersion in SBF for 7 days. The scans start from the top of the red line drawn in

the corresponding SEM cross-sectional images.

Fig. S4: EDS line scans of the (a) slightly corroded region and (b) severely corroded region of

TAN-WE43 after immersion in SBF for 7 days. The scans start from the top of the red line

drawn in the corresponding SEM cross-sectional images.

Fig. S5: EDS line scans of the (a) slightly corroded region and (b) severely corroded region of

WE43 after immersion in SBF for 15 days. The scans start from the top of the red line drawn in

the corresponding SEM cross-sectional images.

Fig. S6: EDS line scans of the (a) slightly corroded region and (b) severely corroded region of

TAN-WE43 after immersion in SBF for 15 days. The scans start from the top of the red line

drawn in the corresponding SEM cross-sectional images.

Fig. S7: EDS line scans of the (a) slightly corroded region and (b) severely corroded region of

WE43 after immersion in SBF for 21 days. The scans start from the top of the red line drawn in

the corresponding SEM cross-sectional images.

Fig. S8: EDS line scans of the (a) slightly corroded region and (b) severely corroded region of

TAN-WE43 after immersion in SBF for 21 days. The scans start from the top of the red line

drawn in the corresponding SEM cross-sectional images.