Effects of zirconium and nitrogen plasma immersion ion ... · PDF fileHowever, galvanic...

Effects of zirconium and nitrogen plasma immersion ion implantation onthe electrochemical corrosion behavior of Mg–Y–RE alloy in simulatedbody fluid and cell culture medium

Mohammed Ibrahim Jamesh a,b, Guosong Wu a, Ying Zhao a, Weihong Jin a, David R. McKenzie b,⇑,Marcela M.M. Bilek b, Paul K. Chu a,⇑a Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, Chinab Applied and Plasma Physics, School of Physics (A28), University of Sydney, Sydney, NSW 2006, Australia

a r t i c l e i n f o

Article history:Received 9 December 2013Accepted 29 May 2014Available online 7 June 2014

Keywords:A. MagnesiumB. EISB. XPSB. Ion implantation

a b s t r a c t

The effects of dual Zr & N plasma immersion ion implantation (PIII) on the corrosion behavior of WE43Mgalloy are evaluated in simulated body fluid (SBF) and cell culture medium (cDMEM). Zr & N PIII improvesthe corrosion resistance of WE43 which exhibits smaller icorr, larger R1 & R2, smaller CPE2, and largerphase angle maxima in SBF and cDMEM. The Zr & N PIII WE43 samples exhibit 12-folds decrease in icorr

in SBF and 71-folds decrease in icorr with near capacitive EIS in cDMEM. Analysis of the corrosion productsreveals calcium phosphate.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Stainless steels, titanium-based, and cobalt–chromium-basedalloys are common metallic biomaterials due to their combinationof high mechanical strength and fracture toughness. However,most metallic biomaterials are not naturally degradable. In thisrespect, biodegradable implants are attractive in some applicationsbecause a follow-up surgery to remove the biomedical implantscan be eliminated. Some commercial polymeric materials such aspolydioxanone (PDS), polyglycolic acid (PGA), and polylactic acid(PLA) are biodegradable [1,2], but many common polymericmaterials suffer from radiolucency and insufficient mechanicalproperties [3]. Mg is a biodegradable metal and also an essentialelement in the human body. Furthermore, the density of Mg alloysranges from 1.74 to 2.0 g/cm3 that is similar to that of humanbones (1.74–2.0 g/cm3) and their Young’s modulus (GPa) of41–45 is also closer to that of human bones (3–20 GPa) comparedto Co–Cr alloys (230 GPa), hydroxyapatite (73–117 GPa), stainlesssteels (189–205 GPa), and Ti alloys (110–117 GPa). The bettermechanical match reduces the stress at the implant/bone interfaceand can potentially induce rapid new bone growth and increasethe implant stability [4]. Mg also plays an important role in bone

growth, is an essential cofactor in different enzymes, and stabilizesthe structures of protein and DNA [5]. However one of the majorproblems associated with Mg biomedical implants is the poor cor-rosion resistance as Mg is highly reactive and especially vulnerablein a physiological environment with large chloride concentrations[4,6–8].

In order to improve the surface corrosion resistance while pre-serving the biodegradability, alloying and surface modification areviable options. Various Mg alloys such as Mg–RE [7,9,10], Mg–Zn[11], Mg–Ca [12], Mg–Sr [13], Mg–Al–Zn [7], Mg–Nd–Zn–Zr [14],and Mg–Zn–Mn [15,16] have been developed and studied. In par-ticular, Mg–Zn alloys have attracted much attention. Zn is anessential element in the human body and also plays an importantrole in the formation of bone [17]. The corrosion characteristics ofMg–Zn alloy have been compared to those of pure Mg [16]. Mg–Znalloy shows small corrosion resistance enhancement in the earlystage and the improvement is more noticeable in the later stage.However, galvanic corrosion can occur between the alloyingelement and Mg substrate as its standard electrode potential is�2.372 V which is comparatively more negative than otherelements. The galvanic effect can be explained by the followingrelationship [18]:

I ¼ ðEC � ESubÞ= RpðSubÞ þ RpðCÞ þ Rs þ RSub—C� �

; ð1Þ

where EC represents the corrosion potential of the cathode, ESub

represents the corrosion potential of the anode, Rp(Sub) and Rp(C)

http://dx.doi.org/10.1016/j.corsci.2014.05.0200010-938X/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding authors. Tel.: +852 34427724; fax: +852 34420542 (P.K. Chu).E-mail addresses: [email protected] (D.R. McKenzie), paul.chu@

cityu.edu.hk (P.K. Chu).

Corrosion Science 86 (2014) 239–251

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represent the polarization resistance of the anode and cathode,respectively, RS indicates the electrical resistance of the electrolyte,and RSub–C represents the electrical resistance between the anodeand cathode. To improve the corrosion resistance the galvanic cur-rent (I) should be smaller. There have been other studies on Mg–RE[7,9,10].

The other alloying element, Y, in Mg–RE is believed to play animportant role in the corrosion resistance because Y has a standardelectrode potential (�2.372 V) similar to that of Mg. Mg–Y–RE(WE43) which is commercially available has been used in biode-gradable stents. It has reduced restonosis rates compared to stain-less steel stents and also possesses suitable antiproliferative andmechanical properties [19,20]. Witte et al. [7] have observed thatWE43Mg alloy degrades entirely in 18 weeks in vivo. The corrosionbehavior of pure Mg, Mg alloys, and surface modified Mg and Mgalloys has been evaluated by measuring the hydrogen evolution,electrochemical impedance spectroscopy (EIS) and potentiody-namic polarization studies [8,16,21–24]. Hydrogen evolution is asimple and quantitative method to determine the corrosion rateof Mg and Mg alloys and EIS is a useful and non-destructive tech-nique to determine the corrosion resistance and mechanism.

Surface modification is a common method to improve theproperties of Mg and Mg alloys [18]. Plasma immersion ionimplantation (PIII) is especially promising because of the confor-mal nature and that selective surface properties can be optimizedwhile the favorable bulk attributes can be preserved [18,25,26]. Liuet al. [27] and Zhao et al. [10,28] plasma implanted Ti and Al intoMg alloys and observed improved corrosion resistance as a resultof the formation of TiO2 and Al2O3 layers on the surface. Wanget al. [29,30] plasma implanted Ta and Y into Mg alloy and alsoachieved better corrosion resistance because of the formation ofa surface oxide layer. Xu et al. [31] plasma implanted Cr into Mgalloy and the enhanced corrosion resistance stemmed from the for-mation of chromium oxide on the surface. Zn and Si which is alsoimportant to bone formation [17,32,33] have also been plasmaimplanted into Mg and its alloys [9,34]. Zn ion implantationinduces more rapid degradation because of the galvanic effectsbetween Zn and Mg, but Si ion implantation improves the corro-sion resistance because of the silicon oxide layer on the surface.Zr compounds are used in dental and orthopedic devices [35,36]as well as knee and hip replacements [37,38]. Our recent studiesreveal enhanced corrosion resistance, cytocompatibility, and anti-microbial properties after Zr and O plasma treatment on Mg alloys[39,40]. The biological effects of nitrogen plasma treatment havealso been studied. Nitrogen plasma treatment enhances osteoblastadhesion and early proliferation on poly(butylene succinate) [41],mesenchymal stem cell adhesion and their proliferation on poly-tetrafluoroethylene [42], and the corrosion resistance and in vitroand in vivo biocompatibility of Ti6Al4V alloy [43]. In addition,nitrogen plasma immersion ion implantation improves the corro-sion resistance of Ni–Ti alloy in simulated body fluid (SBF) [44]and nitrogen plasma treatment of stainless steel stents introducesa high degree of covalent immobilization of human tropoelastinprotein molecules consequently improving both endothelial cellproliferation and blood compatibility [45]. The high degree ofcovalent immobilization of bioactive proteins and extended reten-tion of bioactivity on the surface of the nitrogen plasma treatedsamples have been observed [46] and Zr with N compounds pos-sess good antibacterial activity as well [47]. Hence, the combinedzirconium and nitrogen plasma treatment improves the corrosionresistance and enhances the biocompatibility of biomaterials, butthe effects of dual Zr and N plasma treatment on the corrosionbehavior of WE43Mg alloy have not been studied systematically.Therefore, in this work, the synergetic effects rendered by dual Zrand N PIII on the corrosion resistance of WE43Mg alloy are inves-tigated in details in SBF and cell culture medium.

2. Experimental details

2.1. Plasma surface modification and characterization

As-cast WE43 plates (Mg with Y 4 wt.% and Nd 3 wt.%) havingdimensions of 10 mm � 10 mm � 5 mm were used substrates.The samples were ground with SiC papers of different grit size,mechanically polished to a mirror finish using 1 lm alumina paste,cleaned ultrasonically in ethanol, and dried by compressed air. Thesamples were subjected to two plasma treatments, namely Zrplasma ion implantation followed by N PIII. The schematic diagramof the PIII process is shown in Fig. 1 and more details about theprocess can be found from our previous paper [25]. Zr plasmaion implantation was performed on the HEMII-80 ion implanter(Plasma Technology Ltd.) equipped with a zirconium cathodic arcsource at 25 kV for 20 min and a base pressure of 8.0 � 10�4 Pa.N PIII was conducted on the GPI-100 ion implanter for 3 h at8 kV (RF power = 1000 W, pulse width = 20 ls, pulse frequency =80 Hz, nitrogen flow rate = 20 sccm, and base pressure = 10�5 torr).The elemental depth profiles and chemical states were determinedby X-ray photoelectron spectroscopy (XPS) with Al Ka irradiation.The sputtering rate was estimated to be about 26 nm/min based onthe analysis of a standard SiO2 film under the same conditions.

2.2. Electrochemical studies

The corrosion behavior of the untreated WE43Mg alloy(UT-WE43) and Zr & N plasma implanted WE43Mg alloy(ZrN-WE43) was assessed in simulated body fluid (SBF) andcomplete cell culture medium at 37 �C by potentiodynamicpolarization and electrochemical impedance spectroscopy (EIS)conducted on the Zahner Zennium electrochemical work station.More details about the electrochemical experiments can be foundfrom our previous papers [9,16,48,49]. The SBF used in this studyhad inorganic ion concentrations similar to those in human bodyfluids. The ionic concentrations (in mM) were: 142.0 Na+, 5.0 K+,1.5 Mg2+, 2.5 Ca2+, 147.8 Cl�, 4.2 HCO3

�, 1.0 HPO42�, and 0.5 SO4

2�.The SBF was prepared from the following reagent grade chemicals:8.035 g/l NaCl, 0.355 g/l NaHCO3, 0.225 g/l KCl, 0.231 g/l K2HPO4-

�3H2O, 0.311 g/l MgCl2�6H2O, 1.0 M HCl (39 ml), 0.292 g/l CaCl2,

SubstrateSubstrate

holder

Plasma sheath

Nitrogen gassupply

Vacuumsystem

High voltage pulser

Plasmachamber

Fig. 1. Schematic diagram of the plasma immersion ion implantation process.

240 M.I. Jamesh et al. / Corrosion Science 86 (2014) 239–251

and 0.072 g/l Na2SO4 in distilled water and buffered at pH 7.4 usingtrishydroxymethyl aminomethane (TRIS, 6.118 g/l) and 1.0 M HCl[50]. The complete cell culture medium used in this study con-sisted of a mixture of Dulbecco’s modified eagle medium (DMEM)and 10% fetal calf serum. The DMEM powder was bought from LifeTechnologies Corporation and its composition [49] (in mg/L) was:

2.2.1. Amino acids30 Glycine, 84 L-Arginine hydrochloride, 63 L-Cystine 2 HCl, 584

L-Glutamine, 42 L-Histidine hydrochloride-H2O, 105 L-Isoleucine,105 L-Leucine, 146 L-Lysine hydrochloride, 30 L-Methionine, 66L-Phenylalanine, 42 L-Serine, 95 L-Threonine, 16 L-Tryptophan,104 L-Tyrosine disodium salt dehydrate and 94 L-Valine.

2.2.2. Vitamins4 Choline chloride, 4 D-Calcium pantothenate, 4 Folic acid, 4

Niacinamide, 4 Pyridoxine hydrochloride, 0.4 Riboflavin, 4 Thia-mine hydrochloride and 7.2 i-Inositol.

2.2.3. Inorganic salts200 Anhydrous CaCl2, 0.1 Fe(NO3)3�9H2O, 97.67 anhydrous

MgSO4, 400 KCl, 6400 NaCl and 125 NaH2PO4�H2O.

2.2.4. Other components4500 D-Glucose (Dextrose) and 15 Phenol red.The DMEM was prepared with the powder using distilled water

and NaHCO3 (3.7 g/l). The completed cell culture medium(DMEM + 10% fetal calf serum) was designated as cDMEM in thisstudy. The electrochemical experiments were carried out using aconventional three electrode cell. The UT-WE43 and ZrN-WE43formed the working electrode with the platinum sheet andsaturated calomel electrode (SCE) serving as the counter andreference electrodes, respectively. Only 1 cm2 of the UT-WE43and ZrN-WE43 samples was exposed to the SBF and cDMEM.Potentiodynamic polarization was performed followed by EIS mea-surements at a scanning rate of 1 mV/s in the potential rangebetween �250 mV in the cathodic direction and +500 mV in theanodic direction from their respective open circuit potentials(OCPs). By means of Tafel extrapolation, the corrosion potential(Ecorr) and corrosion current density (icorr) were calculated. Theimpedance spectra were recorded at their respective OCPs afterstabilizing in SBF and cDMEM for 3 min by applying a sinusoidalperturbing signal of 10 mV (root mean square) in the frequencyrange between 10 kHz and 100 mHz. The EIS parameters weredetermined using ZSimpWin 3.21 software from the Nyquist plots.The Bode impedance and Bode phase angle plots were alsorecorded.

2.3. Corrosion product analysis

The surface morphology of the corrosion product after polariza-tion studies in SBF and cDMEM was examined by scanning electronmicroscopy (SEM, JEOL JSM-820) and the elemental composition ofthe corrosion product was determined by energy-dispersive X-rayspectroscopy (EDS).

3. Results and discussion

The XPS depth profile of Zr-N WE43 is depicted in Fig. 2. Thehigh-resolution Zr 3d, Mg 1s, Y 3d, O1s, and N 1s XPS spectra ofthe ZrN-WE43 are depicted in Fig. 3. The ZrN-WE43 sample(Fig. 2) does not show a Gaussian Zr distribution but instead a Zrrich layer about 53 nm thick. Fig. 2 discloses a Gaussian shape Npeak in the ZrN-WE43 with a thickness of about 66 nm. Thehigh-resolution XPS spectra of Zr 3d in Fig. 3(d) shows that the

binding energy of Zr 3d5/2 is 182.2 eV arising from Zr4+ and suggestthat ZrO2 is formed on the surface [51]. The high-resolution spectrashow a broad peak at a depth of about 13 nm due to the presenceof zirconium oxide and nitride [52]. The shift of the Zr 3dpeak towards smaller binding energy is observed from the high-resolution XPS spectra in Fig. 3(d) revealing that Zr4+ correspond-ing to nitride changes gradually to Zr0 with depth and only metallicZr is observed at a depth of about 79 nm. The high-resolution XPSspectra of Mg 1s in Fig. 3(a) of the ZrN-WE43 show that Mg existsin the possible form of MgO and/or Mg3N2 [53,54] and the shift ofthe Mg 1s peak towards smaller binding energy suggests that Mg2+

changes gradually to Mg0 with depth and at a depth of about53 nm, only the metallic state is observed.

The open circuit potential of the UT-WE43 and ZrN-WE43 sam-ples immersed for 180 s in SBF and cDMEM are depicted in Fig. 4.Compared to the UT-WE43 samples (�2028 mV vs SCE in SBF and�1997 mV vs SCE in cDMEM), the ZrN-WE43 samples (�2008 mVvs SCE in SBF and �1894 mV vs SCE in cDMEM) show shifts in thepotential towards the noble direction. In SBF and cDMEM, bothUT-WE43 and ZrN-WE43 show potential shifts towards the nobledirection with respect to time and the extent of the potential shiftis more pronounced for ZrN-WE43. After immersion for 180 s,UT-WE43 shows potential shifts of about 143 mV vs SCE and63 mV vs SCE in SBF and cDMEM, respectively, whereasZrN-WE43 exhibits potential shifts of about 285 mV vs SCE and312 mV vs SCE in SBF and cDMEM, respectively. The shifts towardsthe noble direction and more pronounced potentials shift overtime observed from ZrN-WE43 suggest that it is better protectedthan UT-WE43 in SBF and cDMEM.

The Nyquist plots of UT-WE43 and ZrN-WE43 after immersionin SBF and cDMEM for 180 s are depicted in Fig. 5. The Nyquistplots acquired in SBF (Fig. 5a) show three well defined loops sug-gesting three time constants, but the diameters of the capacitiveloops are different suggesting different corrosion rates. Xin et al.[55] observed three time constants from the untreated and coatedAZ91Mg alloy in SBF and Jamesh et al. [16,56] observed three timeconstants from pure Mg and Mg–Zn–Mn alloy immersed in Ring-er’s solution as well as for pure Mg immersed in SBF. Song et al.[57] determined three time constants for AZ31Mg alloy immersedin SBF and Srinivasan et al. [58] observed three time constantsfrom the untreated and polymer treated AZ31Mg alloy in SBF.The capacitive loop in the higher frequency region is associatedwith the charge transfer resistance. This is because of the resis-tance during electron transfer in the Faradic process in parallel tothe double layer capacitance at the interface between the surfaceof the metal and SBF. The medium frequency capacitive loop isattributed to mass transport in the solid phase arising from ion dif-fusion through the corrosion product layer. The inductive loop in

Fig. 2. XPS depth profiles of ZrN-WE43.

M.I. Jamesh et al. / Corrosion Science 86 (2014) 239–251 241

the lower frequency is attributed to the adsorption process. Theequivalent electrical circuit obtained by fitting the EIS spectra ofUT-WE43 and ZrN-WE43 in SBF are displayed in Fig. 6a. In theequivalent electrical circuit, RS represents the solution resistanceand R1 indicates the charge transfer resistance. The constant phaseelement (CPE1) is used instead of the double layer capacitance (Cdl)because the Nyquist plots of UT-WE43 and ZrN-WE43 in SBF showdepressed semicircles. R2 indicates the resistance and CPE2 repre-sents the capacitance of the corrosion product layer. L indicatesthe inductance and RL is the inductive resistance. However, theNyquist plots of UT-WE43 and ZrN-WE43 in cDMEM (Fig. 5b) showtwo well defined loops suggesting two time constants, but thediameters of the capacitive loops are different indicating differentcorrosion rates. Wu et al. [14,23] observed two time constants

from the Mg–Nd–Zn–Zr alloy immersed in SBF and 0.9% NaClsolution and Wu et al. [49] also determined two time constantsfor the untreated as well as aged Mg–Al–Zn–Mn alloy immersedin the complete cell culture medium. Zhao et al. [59] found twotime constants from the untreated and heat treated AZ91Mg alloyimmersed in the complete cell culture medium and Badawy et al.[60] observed two time constant from the Mg, Mg–Al–Zn andMg–Al–Zn–Mn alloy in aqueous acidic and neutral solutions. Sim-ilarly, Luo et al. [61] observed two time constant from the AZ91Mgalloys containing Y in 0.35% NaCl solution. The higher frequencycapacitive loop is associated with the charge transfer resistance.The lower frequency capacitive loop is attributed to mass transportin the solid phase arising from ion diffusion through the corrosionproduct layer. The EIS spectra of UT-WE43 and ZrN-WE43 in

Fig. 3. High resolution XPS spectra of (a) Mg 1s, (b) O 1s, (c) Y 3d, (d) Zr 3d and (e) N 1s obtained at various sputtering time from ZrN-WE43.


cDMEM are fitted using the equivalent electrical circuit in Fig. 6b inwhich RS represents the solution resistance, R1 and CPE1 representthe charge transfer resistance and capacitance, respectively, R2 isthe resistance of the corrosion product, and CPE2 indicates the

corresponding capacitance. The equivalent electrical circuit is cho-sen based on the non-linear least square fit of the experimentaldata with less than 5% error. Table 1 shows the EIS parametersobtained from the Nyquist plots of UT-WE43 and ZrN-WE43 inSBF and cDMEM after fitting the data. The experimental and fitteddata show good agreement with v2 of about 10�4. Compared toUT-WE43, the diameter of the Nyquist plots of ZrN-WE43 is obvi-ously large which also reflects the increased resistance decreasedcapacitance in both SBF and cDMEM. The rate of the electrochem-ical processes at the substrate/electrolyte interface is controlled bythe charge transfer resistance which is the key factor to determinethe corrosion resistance. ZrN-WE43 shows larger charge transferresistance than UT-WE43 in both SBF and cDMEM (Table 1). Thissuggests that the plasma process hinders the charge transfer pro-cess at the substrate/electrolyte interface. Improved corrosionresistance is observed from ZrN-WE43 in both media. The EIS datashowing that Zr & N plasma ion implantation leads to better corro-sion resistance in SBF and cDMEM.

The Bode impedance and Bode phase angle plots are displayedin Fig. 7. ZrN-WE43 shows larger impedance at higher and lowerfrequencies than UT-WE43 in both SBF and cDMEM (Fig. 7).UT-WE43 shows phase angle maxima of �28� and �61� in SBFand cDMEM, respectively) and ZrN-WE43 shows larger phase anglemaxima of �64� and �81� in SBF and cDMEM, respectively (Fig. 8).The larger impedance and phase angle maxima observed fromZrN-WE43 indicate improved corrosion resistance in both SBFand cDMEM after dual Zr & N plasma ion implantation comparedto UT-WE43. Zr-N-WE43 exhibits a larger phase angle maximumof �81� and larger phase angles over a wide range of frequency.Moreover, UT-WE43 and Zr-N-WE43 have two time constants inthe Bode impedance plot in which a steady increase in the imped-ance is observed from UT-WE43 up to 70 Hz and ZrN-WE43shows a similar increase up to 0.9 Hz. In addition, the Nyquist plotof ZrN-WE43 discloses a near capacitive behavior, suggestingimproved corrosion resistance of ZrN-WE43 in both mediaalthough the effect is more pronounced in cDMEM.

The potentiodynamic polarization curves obtained from theUT-WE43 and ZrN-WE43 in SBF and cDMEM are displayed inFig. 9 and Table 2 lists the corrosion potential (Ecorr) and corrosioncurrent density (icorr) derived from the polarization curves.

Fig. 4. Open circuit potential of the UT-WE43 and ZrN-WE43 samples immersed inSBF and cDMEM at 37 �C for 180 s.

Fig. 5. Nyquist plots of UT-WE43 and ZrN-WE43 samples recorded after immersionfor 180 s in (a) SBF and (b) cDMEM at 37 �C.

Solution(cDMEM)

Substrate

R1 R2

CPE1 CPE2

RE WE

RS

(b)

(a)

Solution(SBF)

Substrate

R1

R2

RL L

CPE1

CPE2

RE WE

RS

Fig. 6. Equivalent electrical circuit of UT-WE43 and ZrN-WE43 samples immersionin (a) SBF and (b) cDMEM.


Generally, the cathodic region of the polarization curve describesthe cathodic hydrogen evolution reaction associated with waterreduction and the anodic part of the polarization curve representsdissolution of the sample at an elevated potential. In comparison,the cathodic branch of ZrN-WE43 shifts towards a lower currentdensity than UT-WE43 in both SBF and cDMEM, indicating thatthe cathodic hydrogen evolution reaction is suppressed on ZrNWE43 in both media. In contrast to UT-WE43, ZrN-WE43 exhibitsthe characteristic of two-regime anodic polarization behavior inSBF (Fig. 9(a)). In the anodic region, the dissolution rate ofZrN-WE43 suddenly turns from slow to rapid when the potentialis raised to a certain value. This shows that ZrN-WE43 may bevulnerable to pitting corrosion at an elevated potential in SBF.The surface morphology of UT-WE43 and ZrN-WE43 after thepotentiodynamic polarization test in SBF is depicted in Fig. 10.UT-WE43 shows uniform corrosion whereas ZrN-WE43 showspitting corrosion. This is in agreement with the anodic branch of

the polarization curve (Fig. 9(a)) where UT-WE43 shows a slowincrease in the current density whereas ZrN-WE43 shows a slowto sudden increase. However, ZrN-WE43 (Fig. 10(d)) reveals lesssurface damage than UT-WE43 (Fig. 10(c)), suggesting thatZrN-WE43 offers better corrosion resistance than UT-WE43 inSBF. In cDMEM, UT-WE43 exhibits the two-regime anodic polariza-tion behavior whereas ZrN-WE43 shows a simple activationcontrolled anodic behavior (Fig. 9(b)). The exposed sample is sus-ceptible to pitting corrosion as the applied potential exceeds thecorrosion potential because the sample is in the active anodicregion with respect to its polarization curve. The surface morphol-ogy of UT-WE43 and ZrN-WE43 after potentiodynamic polariza-tion tests in cDMEM are depicted in Fig. 11 and both UT-WE43and ZrN-WE43 experiences pitting corrosion. This is in agreementwith the anodic branch of the polarization curve (Fig. 9(b)) whereUT-WE43 and ZrN-WE43 are in the active region at higher appliedpotential. However, ZrN-WE43 (Fig. 11(b)) shows less surface

Table 1Fitted data of UT-WE43 and ZrN-WE43 in SBF and cDMEM obtained from the electrochemical impedance spectra.

SBF cDMEM

UT-WE43 ZrN-WE43 UT-WE43 ZrN-WE43

RS (X cm2) 13.48 ± 1 13.81 ± 1.2 15.99 ± 5.7 15.78 ± 3.7CPE1 (X�2 cm�2 S�n) (1.168 ± 0.136) � 10�4 (6.167 ± 0.152) � 10�4 (3.208 ± 1.2) � 10�6 (3.223 ± 2.432) � 10�6

n1 0.74 0.47 0.87 0.92R1 (X cm2) 42.97 ± 4 828.7 ± 20 651.8 ± 73 3487 ± 848CPE2 (X�2 cm�2 S�n) (4.077 ± 0.126) � 10�5 (3.53 ± 3.91) � 10�6 (2.012 ± 1.4) � 10�4 (3.224 ± 2.433) � 10�6

n2 0.99 0.94 0.69 0.92R2 (X cm2) 8.04 ± 1 737 ± 19 564 ± 81 3486 ± 849v2 6.178 � 10�5 2.675 � 10�4 9.217 � 10�4 7.092 � 10�4

Fig. 7. (a and c) Bode impedance and (b and d) Bode phase angle plots of UT-WE43 and ZrN-WE43 recorded after immersion for 180 s in (a, b) SBF and (c, d) cDMEM at 37 �C.


damage than UT-WE43 (Fig. 11(a)), indicating that ZrN-WE43offers better corrosion resistance than UT-WE43 in cDMEM. Com-pared to UT-WE43, Ecorr of ZrN-WE43 shifts towards the nobledirection and icorr decreases in both SBF and cDMEM. In SBF,UT-WE43 has icorr of 368 lA/cm2 and ZrN-WE43 shows 29.8 lA/cm2 which is 12 times smaller. In cDMEM, UT-WE43 shows36.6 lA/cm2 and ZrN-WE43 shows 0.51 lA/cm2 which is 71 timeslesser. Wu et al. [14] observed decrease in icorr from the Mg–Nd–Zn–Zr alloy from 4.341 � 10�4 A/cm2 to 1.834 � 10�4 A/cm2 forthe 3 h O implanted sample, 1.671 � 10�4 A/cm2 for the 5 h Oimplanted sample, 1.527 � 10�4 A/cm2 for the Al & O implantedsample, and 3.832 � 10�5 A/cm2 for the Cr & O implanted samplein SBF. Xu et al. [62] observed decrease in icorr from pure Mg from2.58 � 10�4 A/cm2 and 6.52 � 10�5 A/cm2 to 2.63 � 10�5 A/cm2

and 1.88 � 10�6 A/cm2 for C implanted sample in SBF and DMEM,respectively. Jamesh et al. [39] observed decrease in icorr from theMg–Zn–Zr alloy from 4.09 � 10�4 A/cm2 to 7.0 � 10�5 A/cm2 forthe Zr implanted sample and 1.1 � 10�5 A/cm2 for the Zr & Oimplanted sample in SBF. Zhao et al. [40] observed decrease in icorr

from the Mg-Ca alloy from 2.3 � 10�4 A/cm2, 2.8 � 10�4 A/cm2,and 1.5 � 10�5 A/cm2 to 1.2 � 10�4 A/cm2, 4.9 � 10�5 A/cm2, and6.4 � 10�6 A/cm2 for the Zr implanted sample and 2.6 � 10�5 A/cm2, 4.1 � 10�5 A/cm2 & 4.6 � 10�6 A/cm2 for the Zr & O implantedsample in SBF, tryptic soy broth and cell culture medium, respec-tively. We also observed decrease in icorr from the Mg–Sr alloy from1.0 � 10�3 A/cm2, 4.3 � 10�4 A/cm2, and 2.3 � 10�5 A/cm2 to2.5 � 10�4 A/cm2, 4.2 � 10�5 A/cm2, and 5.0 � 10�6 A/cm2 for Zrimplanted sample and 1.7 � 10�4 A/cm2, 4.0 � 10�5 A/cm2, and4.0 � 10�6 A/cm2 for the Zr & O implanted sample in SBF, trypticsoy broth and cell culture medium, respectively. The present dataprovide evidence that Zr & N plasma ion implantation leads to bet-ter corrosion resistance on WE43Mg in both SBF and cDMEM and

the enhancement in the corrosion resistance is more pronouncedin cDMEM.

Smaller corrosion current density, larger resistance, biggerimpedance, and larger phase angle maxima are observed fromWE43Mg after Zr & N plasma ion implantation. Mg3N2, ZrN, ZrO2,Y2O3, and MgO are formed in the ZrN-WE43 sample. Huang et al.[63] observed decrease in the corrosion current density on theZrN coated steel in 5% NaCl and 1 M H2SO4 with 0.05 M KSCN solu-tions. Xin et al. observed improved corrosion resistance from theZrN coated AZ91Mg alloy in SBF [64]. The corrosion protectionbehavior of Mg(OH)2 will be discussed later in this paper andZrO2 provides better corrosion resistance. We observed that Zrion implantation followed by oxygen PIII produced a robust ZrO2

layer which enhanced the corrosion resistance of Mg–Zn–Zr,Mg–Ca and Mg–Sr alloys [39,40]. Zr & O plasma ion implantation

Fig. 8. Comparison of phase angle (�) maximum of UT-WE43 and ZrN-WE43recorded after immersions for 180 s in (a) SBF and (b) cDMEM at 37 �C.

Fig. 9. Potentiodynamic polarization curves acquired after the EIS measurement ofUT-WE43 and ZrN-WE43 samples immersed in (a) SBF and (b) cDMEM at 37 �C.

Table 2Corrosion potentials, Ecorr and corrosion current densities, icorr obtained frompotentiodynamic polarization data of UT-WE43 and ZrN-WE43 in SBF and cDMEM.

SBF cDMEM

UT-WE43 ZrN-WE43 UT-WE43 ZrN-WE43

Ecorr (mV vs SCE) �1997 ± 9 �1820 ± 57 �1780 ± 108 �1517 ± 112icorr (lA/cm2) 368 ± 124 29.8 ± 31 36.6 ± 34 0.51 ± 12


increases the corrosion resistance of ZK60Mg alloy in SBF as evi-denced by 37-folds decrease in icorr and 62-folds increase in RP

[39]. Zr & O plasma ion implantation enhances the corrosion resis-tance of Mg–Ca and Mg–Sr alloys in SBF, tryptic soy broth (TSB;Bacto), and cell culture medium [40]. Xin et al. [55] observed twoorders of magnitude decrease in the corrosion current density fromZrO2 coated AZ91Mg alloy in SBF. Ardelean et al. [65] observed sig-nificant improvement in the corrosion resistance from AM50 andAZ91Mg alloys coated with ZrO2 in Na2SO4 solution and aftercathodic and anodic polarization, the ZrO2 remained unaffectedin the Na2SO4 solution. Liang et al. [66] observed stable and noblepotentials from ZrO2 coated AM50Mg alloy after exposure to a0.1 M NaCl solution for 50 h. The presence of Y2O3 on the Mg alloyalso enhances the corrosion resistance [9,67]. Hence the presenceof ZrN, ZrO2 and Y2O3 in the ZrN layer of the Zr & N plasma ionimplanted WE43Mg alloy samples may be responsible for theimproved corrosion resistance. These also suggest that theimprovement in corrosion resistance results from the few nm thickimplanted layers forming a passive protective layer by reducing

galvanic effects. Wu et al. [23] observed second phases from thesurface of C2H2 plasma treated Mg–Nd alloy whereas the plasmamodified surface gave rise to significant improvement in the corro-sion resistance on Mg–Nd alloy in 0.9% NaCl solution. The secondphase in the plasma treated samples is also observed here (Figs. 10and 11). The galvanic effect can be explained by the following rela-tionship [18]:

I ¼ ðEC � ESubÞ= RpðSubÞ þ RpðCÞ þ Rs þ RSub—C� �

; ð1Þ

where EC represents the corrosion potential of the cathode, ESub rep-resents the corrosion potential of the anode, Rp(Sub) and Rp(C) repre-sent the polarization resistance of the anode and cathode,respectively, RS indicates the electrical resistance of the electrolyte,RSub–C represents the electrical resistance between the anode andcathode, RS depends on the geometry of the system and conductiv-ity of the electrolyte, and RSub–C is related to the conductivitybetween the implanted layer cathode and Mg alloy substrate anode.Obviously the implanted layer hiders the charge transfer process

100 µm 100 µm

1 µm 1 µm

40 µm 40 µm

(a) (b)

(c) (d)

(e) (f)

UT-WE43 in SBF (lower magnification) ZrN-WE43 in SBF (lower magnification)

UT-WE43 in SBF (medium magnification) ZrN-WE43 in SBF (medium magnification)

UT-WE43 in SBF (higher magnification) ZrN-WE43 in SBF (higher magnification)

Corroded region

Intact regionCorroded region

Intact region

Corroded region

Corroded region

Intact region

Fig. 10. Surface morphology of (a, c, e) UT-WE43 and (b, d, f) ZrN-WE43 samples (at different magnification) after the potentiodynamic polarization test in SBF at 37 �C.


(Table 1) and can increase the polarization resistance of the anodewhich may decrease the galvanic current (I).

Generally, Mg alloys offer better corrosion resistance in thepresence of a protein containing medium than without due toadsorption of protein molecules onto the surface [68]. However,the corrosion resistance of plasma modified ZrN-WE43 is morepronounced in cDMEM than SBF. Plasma modification can generatea covalent binding surface for protein molecules [69]. Bilek et al.

[70] have proposed that the covalent binding on the plasma treatedsurface may be due to the generation of a free radical density n0 ofabout 6 � 1025 m�3 on the surface. The plasma treated surface pro-duces layers with unpaired electrons yielding a platform to bindprotein and provide enough electron mobility inside the layer.The amino acids in the protein molecules can be covalently linkedby the unpaired electrons present in the free radicals. Ho et al. [71]and Nosworthy et al. [72] observed good covalent binding of

100 µm (a) (b)100 µm

40 µm 40 µm(c) (d)

10 µm 10 µm(e) (f)

UT-WE43 in cDMEM(X 100 magnification)

ZrN-WE43 in cDMEM(X 100 magnification)





Corroded region

Corroded region

(g) (h)

Intactregion

Intactregion

Intactregion

10 µm (g) (h)10 µm



Fig. 11. Surface morphology of (a, c, e, g) UT-WE43 and (b, d, f, h) ZrN-WE43 samples (at different magnification) after the potentiodynamic polarization test in cDMEM at37 �C.


protein molecules on plasma treated surfaces. Moreover, Yin et al.[46] observed better covalent binding of protein molecules on thenitrogen plasma treated surface than that without nitrogen plasmatreatment. Hence, the pronounced corrosion resistance observedfrom the plasma modified ZrN-WE43 in cDMEM may be due tothe covalent binding of proteins on the plasma modified surface.

The EDS results acquired from the corroded and intact regionsof UT-WE43 and ZrN-WE43 after polarization in SBF and cDMEMare depicted in Figs. 12 and 13 respectively. The corroded regionson UT-WE43 and ZrN-WE43 contain Mg, Ca, P, and O and the intact

areas contain Mg after immersion in SBF and cDMEM. The intensityof Ca and P is larger in UT-WE43 than ZrN-WE43 sample afterimmersion in SBF (Fig. 12) but it is larger on ZrN-WE43 thanUT-WE43 sample after immersion in cDMEM (Fig. 13). However,the atomic concentration excludes oxygen which cannot bedetected by EDS. Moreover, ZrN-WE43 (spectrum 3) shows stron-ger Mg peaks than UT-WE43 (spectrum 1) in both SBF (Fig. 12)and cDMEM (Fig. 13). This suggests that the thickness of the corro-sion product layer is larger on UT-WE43 than ZrN-WE43. Jameshet al. [39] has also observed thicker corrosion products on

(a)EDS analysis on corroded region

UT-WE43 in SBFSpectrum 1

CaMg PO

Spectrum 1

Spectrum 2

Spectrum 2(b) UT-WE43 in SBF

100 µm

100 µm

Mg

Spectrum 3

CaMg

PO

EDS analysis on intact region

EDS analysis on corroded region Spectrum 3

100 µm

ZrN-WE43 in SBF(c)

Element Atomic%

Mg 36.97

P 43.81

Ca 19.23

Element Atomic%

Mg 54.57

P 30.50

Ca 14.93

Mg

Spectrum 4

EDS analysis on intact regionSpectrum 4

ZrN-WE43 in SBF(d)

Fig. 12. Surface morphology of (a) UT-WE43 and corresponding EDS spot analysis of the corroded region, (b) UT-WE43 and corresponding EDS spot analysis of the intactregion, (c) ZrN-WE43 and corresponding EDS spot analysis of the corroded region, (d) ZrN-WE43 and corresponding EDS spot analysis of the intact area after thepotentiodynamic polarization test in SBF at 37 �C.


untreated ZK60 than Zr & O implanted ZK60 samples after immer-sion in SBF for 30 h. This may be due to the protective effectsrendered by the implanted layer which reduces surface damagecompared to the untreated Mg alloy.

The corrosion products are possibly Mg(OH)2 and calcium phos-phate as reported previously [16,39,49,57]. Song et al. [57] observedthe formation of protective calcium phosphate as a corrosion prod-uct on the AZ31Mg alloy after immersion in SBF for 1 h. Wu et al.[49] observed the formation of protective calcium phosphate as a

corrosion product on the Mg–Al–Zn–Mn alloy immersed in the cellculture medium for 24 h. Hence, the calcium phosphate here can beprotective in nature. When WE43Mg alloys are immersed in SBFand cDMEM, the intermetallic compound serve as the cathodeand Mg matrix serve as the anode. As a result of this galvanic cou-ple, the following electrochemical reaction occurs on the anode andcathode. Dissolution of Mg into Mg2+ occurs at the anode:

Mg!Mg2þ þ 2e�: ð2Þ

CaMg

100 µm

P

(a)

EDS analysis on corroded region

UT-WE43 in cDMEM

100 µm

MgEDS analysis on intact region

UT-WE43 in cDMEM(b)

100 µm

Ca

Mg

P

EDS analysis on corroded region

ZrN-WE43 in cDMEM(c)

Spectrum 1

Spectrum 2

Spectrum 3

Spectrum 1

Spectrum 2

Spectrum 3

O

O

Spectrum 4

Spectrum 4

100 µm

MgEDS analysis on intact region

ZrN-WE43 in cDMEM(d)

Fig. 13. Surface morphology of (a) UT-WE43 and corresponding EDS spot analysis of the corroded region, (b) UT-WE43 and corresponding EDS spot analysis of the intactregion, (c) ZrN-WE43 and corresponding EDS spot analysis of the corroded region, (d) ZrN-WE43 and corresponding EDS spot analysis of the intact area after thepotentiodynamic polarization test in cDMEM at 37 �C.


Hydrogen evolution occurs at the cathode.

2H2Oþ 2e� ! H2 " þ2OH�: ð3Þ

Mg(OH)2 precipitate is formed by the following overall reaction:

Mgþ 2H2O!MgðOHÞ2ðprecipitateÞ þH2 " : ð4Þ

The SBF solution contains Cl� ions at a concentration 147.8 mM andthe cDMEM contains Cl� ions (from inorganic salt) at a concentra-tion 118.5 mM and soluble MgCl2 is formed from insoluble Mg(OH)2

to raise of interfacial pH to above 10 because of the generation of alarge amount of OH�:

MgðOHÞ2 þ 2Cl� !MgCl2 þ 2OH�: ð5Þ

OH� reduces the phosphate ions in the SBF [6,49,57]:

H2PO�4 þ 2OH� ! PO3�4 þ 2H2O; ð6Þ

2HPO2�4 þ 2OH� ! 2PO3�

4 þ 2H2O: ð7Þ

The phosphate ions quickly react with Ca2+ in the SBF deposit-ing calcium phosphate as the corrosion product. Among thedifferent kinds of calcium phosphate thermodynamically stablewith the smallest solubility constant of Ksp of 1.6 � 10�58, calciumphosphate hydroxyapatite (Ca10(PO4)6(OH)2, HA) or metastablephase amorphous calcium phosphate (Ca3(PO4)2�3H2O, ACP) canbe formed in the corrosion product [39,49,73,74]. Calcium phos-phates such as Ca10(PO4)6(OH)2 or Ca3(PO4)2�3H2O are essentialmineral phases in bone formation. The present study reveals thatdual Zr & N plasma ion implantation increases the corrosion resis-tance of WE43Mg alloy in both SBF and cDMEM. The initial corro-sion rate can be reduced by Zr & N plasma ion implantation and thecorrosion rate can be tailored by varying the Zr and N ion implan-tation fluence and energy. The flexibility, simplicity, and controlla-bility of this method is attractive to clinical applications.

4. Conclusion

The corrosion behavior of UT-WE43 and ZrN-WE43 is evaluatedin both SBF and cDMEM by potentiodynamic polarization and EIS.The following conclusions can be drawn.

1. Dual Zr & N plasma ion implantation forms a 53 nm thick Zr richlayer and 66 nm thick N rich layer on the WE43Mg alloy.

2. In SBF and cDMEM, ZrN-WE43 shows smaller icorr, bigger R1 &R2, smaller CPE2, and larger phase angle maxima than UT-WE43, suggesting enhanced corrosion resistance possibly dueto the formation of ZrN, ZrO2 and Y2O3 in the ZrN implantedlayer.

3. ZrN-WE43 in SBF shows a 12-folds decrease in icorr whereasZrN-WE43 in cDMEM shows a 71-folds decrease in icorr togetherwith a near capacitive EIS behavior suggesting that the corro-sion resistance is more pronounced in cDMEM.

4. Calcium phosphate is detected from the corrosion product onUT-WE43 and ZrN-WE43 after immersion in SBF and cDMEM.

Acknowledgments

This study was financially supported by the Hong KongResearch Grants Council (RGC) General Research Funds (GRF) No.CityU 112212 and Australian Research Council. The authors thankMr. Sebastian Lau for discussion on SEM.

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