Biomaterials 22 (2001) 1853}1859

Corrosion behavior of titanium nitride coated Ni}Ti shape

memorysurgical alloy

D. Starosvetsky, I. Gotman*

Department of Materials Engineering, Technion City, Haifa 32000, Israel

Received 1 February2000; accepted 27 October 2000

Abstract

Nickel}titanium (NiTi, nitinol) shape memoryalloywas nitrided using an original powder immersion reaction assisted coating

(PIRAC) method in order to modifyits surface properties. PIRAC nitriding method is based on annealing the samples in the
atmosphere of highlyreactive nitrogen supplied bydecomposition of unstable nitride powders or, alternatively, byselective di!usion
of the atmospheric nitrogen to the sample surface. Being a non-line-of-sight process, PIRAC nitriding allows uniform treatment of
complex shape surgical implants. Hard two-layer titanium nitride (TiN)/TiNi coatings were obtained on NiTi surface after PIRAC

anneals at 900 and 10003C. PIRAC coating procedure was found to considerablyimprove the corrosion behavior of NiTi alloyin
Ringer's solution. In contrast to untreated nitinol, no pitting was observed in the samples PIRAC nitrided at 10003C, 1 h up to 1.1 V.
The coated samples were also characterized byverylow anodic currents in the passive region and byan exceedinglylow metal ion
release rate. The research results suggest that PIRAC nitriding procedure could improve the in vivo performance of NiTi alloys
implanted into the human body.

2001 Elsevier Science Ltd. All rights reserved.

Keywords: NiTi surgical alloy; Titanium nitride coating; Pitting corrosion; Corrosion resistance; Ringer's solution; Metal ion release

1. Introduction

Since the discoveryof the shape memoryproperties of

nickel}titanium intermetallic (NiTi) in the early1960s,
the alloyhas been proposed for various practical applica-
tions, in most of which corrosion problems were of no
concern. Over the last decade, however, NiTi alloys have
been increasinglyconsidered for use in external and
internal biomedical devices, e.g. orthodontic wires, self-
expanding cardiovascular and urological stents, bone
fracture "xation plates and nails, etc. [1}5]. For applica-
tions in the human body, the corrosion resistance of NiTi
becomes extremelyimportant, as the amount and toxic-
ityof corrosion products control the alloybiocompatibil-
ity. Several studies of the corrosion resistance of NiTi in
simulated human body #uids have been reported [6}9].
It has been found that the behavior of NiTi in dilute
chloride solutions is markedlybetter than that of 316L
stainless steel. At the same time, NiTi exhibits poor

* Corresponding author. Tel.: #[phone]; fax: #[phone]-

1978.

E-mail address: [e-mail] (I. Gotman).

resistance to localized corrosion in chloride-containing
environments, with arguablylow pitting potential values.
In addition, the healing of the passive "lm on NiTi has
been reported to be a slow and di$cult process.

On the whole, the corrosion behavior of NiTi (an alloy

of Ni and Ti) in the body #uids is much closer to that of
corrosion resistant Ti than to that of non-biocompatible
Ni. However, even if NiTi takes on the excellent cor-
rosion resistance of Ti, passive ion release to the environ-
ment will inevitablyoccur, as it happens for the most
passive Ti alloys. If a NiTi implant is placed in the
physiological environment, Ti and Ni ions will be re-
leased into the adjacent tissues, with potentiallyharmful
local and systemic e!ects. The release of nickel, whose
toxicity, carcinogenicity and allergic hazards are well
documented, arouses special concern [10}12].

It is clear that anyimprovement in corrosion resist-

ance achieved via surface modi"cation will be bene"cial
for the patient and for the long-term implant stability.
It should be remembered, however, that corrosion
kinetics of implant materials is frequentlyenhanced by
mechanical processes such as wear. It would be advant-
ageous, therefore, if surface modi"cation simultaneously
achieved the goal of wear and corrosion resistance

0142-9612/01/$ - see front matter

2001 Elsevier Science Ltd. All rights reserved.

PII: S 0 1 4 2 - 9 6 1 2 ( 0 0 ) 0 0 3 6 8 - 9

improvement. Coating with TiN seems attractive, as it
was previouslyreported to reduce wear damage and to
improve corrosion resistance of Ti alloys [13}16]. Tita-
nium nitride coating has been accepted bythe Federal
Food and Drug Administration for use with titanium
alloyparts because of its biocompatibility[16], and it
can also be considered as a suitable blood-contacting
material [17]. Reports on corrosion behavior of TiN-
coated NiTi alloys are very scarce. Ion nitrided NiTi
samples hardened bya TiN surface layer were reported
to exhibit excellent wear resistance, as well as markedly
improved corrosion behavior in HCl and HSO solu-

tions [18,19]. TiN coating prepared byarc ion plating
improved the corrosion resistance of Ni}Ti alloyin 0.9%
NaCl solution in the low-potential region, but increased
pitting corrosion susceptibilityat potentials above
500 mV [20].

In this work, the resistance of TiN-coated NiTi alloyto

a localized corrosion attack occurring in chloride con-
taining neutral media has been studied. An original PI-
RAC nitriding method was used for the coating of NiTi
with a hard TiN layer [21]. PIRAC TiN-coatings have
been alreadyshown to improve the corrosion behavior of
Ti}6Al}4 V surgical alloyunder static and fretting condi-
tions, as well as to reduce fretting wear damage [22,23].
Good biocompatibilityand bone bonding properties of
PIRAC-coated Ti}6Al}4 V samples, comparable to those
of the uncoated implants, have also been reported [24].

2. Materials and methods

1.5 mm thick plates of a superelastic NiTi alloySE-508

(Nickel: 55.6 wt%, Titanium: balance, trace elements:

(

0.1 wt%) was supplied byPerformance Alloys & Ma-

terials, Ltd. Rectangular samples &5

;10 mm were pre-

pared bycutting and polishing with a 3

m diamond

paste.

Part of the samples were PIRAC nitrided byannealing

at 900 and 10003C in sealed containers made of a stain-
less steel foil. The steel foil contained large percent of Cr
that reacted with the atmospheric oxygen preventing it
from reaching the sample's surface. At the same time,
N atoms could easilydi!use through the container walls
due to the rather low chemical a$nityof Cr for nitrogen,
therebycreating a low nitrogen pressure inside the con-
tainer. Reactive di!usion of this veryactive monatomic
nitrogen resulted in the formation of a nitrogen-rich layer
on the samples' surfaces. More details on PIRAC nitrid-
ing procedure can be found in [21]. An important point
to mention is that, as the substrate is directlyinvolved in
the process of coating formation, PIRAC coatings are
characterized byan excellent conformityand strong ad-
hesion [23]. Being a non-line-of-sight process, PIRAC
nitriding allows uniform treatment of complex shape
surgical implants. In spite of the relativelyhigh process-

ing temperature, PIRAC nitriding is not expected to
compromise the superelastic properties of SE-508 whose
transformation temperature, A"153C, lies below room

temperature.

Phase identi"cation and microstructure characteriza-

tion of PIRAC nitrided samples were performed employ-
ing X-raydi!raction (XRD) and scanning electron
microscopy(SEM) with energydispersive spectrometry
(EDS).

The corrosion and electrochemical behavior of un-

coated and PIRAC coated (9003C, 1.5 h and 10003C, 1 h)
NiTi samples was studied in Ringer's solution: 9.00 g/l
NaCl, 0.20 g/l NaHCO, 0.25g/l CaCl ) 6H0, 0.4 g/l

KCl. The solution was prepared with analytically pure
reagents and doublydistilled water. Tests were conduc-
ted in a 0.5 l well-capped electrochemical cell under nitro-
gen atmosphere. Electrodes and gas bubblers were tightly
inserted into the cell through glass grinds sealed with
a silicon rubber sealant. Prior to testing, the medium was
purged for 1 h with a pure nitrogen gas in the electro-
chemical cell. Subsequently, the working electrode that
had been positioned in the cell head space during deaer-
ation was immersed. The solution with the immersed
sample was purged for 2 h, after which the experiment
was started. Potentials were measured using Luggin cap-
illary. The resulting potential values were referred to
a saturated calomel electrode (SCE). All the experiments
were conducted at the bodytemperature (37$0.53C).

The experiments were performed with a potentiostat

M273 EG & G. The electrochemical behavior of PIRAC
nitrided NiTi samples was compared with that of un-
treated NiTi. The corroded surfaces of the coated and
uncoated NiTi samples were studied in SEM.

To studythe e!ect of PIRAC nitriding on metal ion

release, the coated (10003C, 1 h) and untreated NiTi sam-
ples were placed in 100 ml Ringer's solution and
maintained at 37$0.53C. The surface area of each
sample group was approximately10 cm

. Evaporation of

the test solution was compensated bythe addition of
doublydistilled water. After 400 h, the Ti and Ni concen-
trations in the saline solution were measured via atomic
absorption spectrometry(AAS).

3. Results

PIRAC nitriding of NiTi alloyat 900}10003C yielded

a characteristic golden-colored TiN coating on the sam-
ples' surface. According to the XRD results, Fig. 1, the
nitrided NiTi surface contains titanium nitride, TiN, and
a Ti-rich intermetallic, TiNi. SEM/EDS analysis re-

vealed that the coatings consist of two layers, the thin
surface layer being TiN, and the thicker layer beneath
TiNi. Examples of PIRAC coatings grown on NiTi

samples byannealing at 10003C for 1 and 4 h, respective-
ly, are shown in Fig. 2. The thicknesses of TiN and TiNi

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D. Starosvetsky, I. Gotman / Biomaterials 22 (2001) 1853}1859

Fig. 1. XRD pattern taken from the surface of a NiTi sample PIRAC
nitrided at 9003C, 1.5 h.

Fig. 2. Cross-sections of NiTi samples PIRAC nitrided at 10003C:
(a) for 1 h; (b) for 4 h SEM.

Table 1
Thickness of TiN and TiNi layers on the NiTi surface after PIRAC

nitriding

PIRAC treatment

Layer thickness,

m

TiNi

TiN

9003C, 1.5 h

0.6

0.1

10003C, 1 h

1

0.4

Fig. 3. Corrosion potential (E) transients for untreated and PIRAC

nitrided NiTi samples in deaerated Ringer's solution.

Table 2
Summaryof corrosion characteristics of untreated and PIRAC nitrided NiTi alloyin deaerated Ringer's solution at 373C

NiTi

E, V

E, V

E, V

i at 0.25 V,

A/cm

p.d.

p.s.

p.d.

p.s.

Untreated

!

0.420

0.25}0.31

0.35}0.42

!

0.02}!0.05

3

0.07}0.08

PIRAC 9003C, 1.5 h

!

0.130

0.52}0.57

0.6

!

0.25}!0.30

0.2

)

0.01

PIRAC 10003C, 1 h

!

0.04

*

*

*

&

0.1

)

0.01

Potentiodynamic test.

Potentiostatic test.

layers on nitinol samples that were used for corrosion
studies (see below) are given in Table 1. The microhard-
ness &3.5 GPa was measured on the surface of the sam-
ples PIRAC nitrided at both 900 and 10003C, compared
to&2 GPa for the untreated NiTi. Due to the verysmall
TiN and TiNi layer thickness, Table 1, the obtained

microhardness values must be stronglyin#uenced bythe

soft underlying material. Still, the hardening e!ect of
PIRAC coatings on NiTi alloyis obvious, and it should
lead to an improved wear resistance.

Fig. 3 shows corrosion potential (E) transients for

untreated and

PIRAC nitrided NiTi

samples in

deaerated Ringer's solution. It can be seen that through-
out all the immersion period, the corrosion potentials of
the coated samples are much more positive than that of
untreated NiTi alloy. E values after 2 h exposure are

given in Table 2.

D. Starosvetsky, I. Gotman / Biomaterials 22 (2001) 1853}1859

1855

Fig. 4. Potentiodynamic curves of untreated and PIRAC nitrided NiTi
samples in deaerated Ringer's solution.

Fig. 5. Potentiostatic curves of NiTi alloyin deaerated Ringer's solu-
tion: (a) untreated sample at initiallyapplied potential below E mea-

sured in the potentiodynamic test (E

); (b) sample PIRAC nitrided at

9003C, 1.5 h at initiallyapplied potential below E

; (inset) sample

PIRAC nitrided at 9003C, 1.5 h at initial potential above E

.

In Fig. 4, representative cyclic polarization curves of

untreated and

PIRAC nitrided NiTi

samples in

deaerated Ringer's solution obtained at the potential
sweep of 1 mV/s are presented. Five separate polarization
tests were conducted for each type of samples, with the
results summarized in Table 2.

In addition to cyclic polarization test, breakdown po-

tentials of NiTi samples were measured bymeans of
a stepwise potentiostatic test. The test was started at
the initiallyapplied potential of 0.2 and 0.5 V for the
untreated and PIRAC coated samples, respectively, i.e.
below the corresponding E measured bycyclic polariza-

tion. After a 10 min exposure, the applied potential was
increased up to breakdown with a 20}50 mV step and
10 min exposure at each applied potential. The results
of the potentiostatic test are presented in Fig. 5 and in
Table 2 (summaryof 5 separate tests). Fig. 6 shows
representative surface structures of untreated and
PIRAC nitrided NiTi samples before and after potentios-
tatic polarization.

The release of titanium and nickel was measured for

untreated NiTi and for NiTi PIRAC nitrided at 10003C
for 1 h. The results of dissolution test are given in Table 3.

4. Discussion

Passivityin body#uids is the major characteristic of

surgical alloys. The stability of passive condition of
PIRAC nitrided and untreated NiTi is illustrated bythe
corrosion potential time curve in Fig. 3. It can be seen
that E of both uncoated and PIRAC coated at 9003C,

1.5 h samples decreases with time, the decrease being
much more pronounced for the uncoated material. In

contrast to this, E of NiTi PIRAC nitrided at 10003C

slightlyincreases with time indicating strong surface pas-
sivation.

Untreated NiTi alloyis passive in a wide potential

region up to the breakdown occurrence at E+

0.25}0.31 V, as can be seen in the positive scan part of
the anodic polarization curve, Fig. 4. Numerous spikes of
anodic current, I , are observed in the region of passivity
at potentials close to E indicating initiation and repas-

sivation of metastable pits. At potentials above E,

anodic current graduallyincreases indicating nucleation
and development of stable pits. As can be seen in the
backscan part of the curve, the pits initiated in untreated
NiTi at potentials above 0.25}0.31 V undergo repassiva-
tion at E+!0.02 to !0.05V (Table 1), i.e. a few

hundreds mV above E.

The positive scan part of the curve obtained for sam-

ples PIRAC nitrided at 9003C, 1.5 h, Fig. 4, shows the
wide potential region of passivityand the region of pit-
ting attack. The region of passivityfor such samples is
much wider than that of the uncoated NiTi, and is
characterized bysigni"cantlylower current densities.
Breakdown of passivityof NiTi samples PIRAC nitrided
at 9003C, 1.5 h occurred at E+0.52}0.57V (Table 1), i.e.

signi"cantlyabove the values measured on the uncoated
NiTi. After breakdown, a sharp current increase is ob-
served indicating the development of active pits. The
backscan part of the cyclic curves reveals that, in spite of

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D. Starosvetsky, I. Gotman / Biomaterials 22 (2001) 1853}1859

Fig. 6. SEM micrographs of an uncoated NiTi sample (a) and samples PIRAC nitrided at 10003C, 1 h (b) and at 9003C, 1.5 h (c) after potentiostatic test;
(d) PIRAC nitrided NiTi sample before the test.

Table 3
Concentration of Ti and Ni ions in aerated Ringer's solution after 400 h
exposure at 373C

Material

Metal ion concentration, mg/cm

Ti

Ni

Untreated NiTi

0.012

0.011

PIRAC nitrided NiTi
(10003C, 1 h)

not detectable

not detectable

the fact that the pitting of the coated (9003C, 1.5 h)
samples occurs at a more positive potential, the pits
undergo repassivation at a potential signi"cantlylower
than that measured on the uncoated NiTi (!0.25}!0.30
versus !0.02}!0.05 V, Table 1), as well as below the
initial corrosion potential of the coated samples mea-
sured before the potential sweep application (&!0.1 V).

NiTi samples PIRAC coated at 10003C, 1 h were pass-

ive in the whole tested potential range, and no break-
down of passivitywas observed up to 1.1 V (see Fig. 4).
Apparently, the 0.4

m thick surface layer of TiN was

su$cient to e!ectivelyprotect NiTi from localized cor-
rosion attack.

The results of the potentiostatic test, Fig. 5, show that

at applied potentials below the breakdown, anodic cur-
rents of both untreated and PIRAC coated (9003C, 1.5 h)

NiTi reached maximum immediatelyafter potential ap-
plication and then decreased graduallywith time indicat-
ing electrode passivation. The spikes observed in the
curve of untreated NiTi during exposure are attributed to
the initiation and repassivation of metastable pits.
A sharp current increase indicating nucleation and devel-
opment of active pits was observed in the curve of un-
treated NiTi, Fig. 5(a), at the potential of 0.39 V after
a few minutes of incubation period. As seen in Table 2,
the activation of untreated NiTi in all the potentiostatic
tests occurred at potentials several tens of mV higher
than those measured bypotentiodynamic polarization.
The higher resistance of NiTi to pitting attack in the
potentiostatic versus potentiodynamic test is, probably,
due to variations in the nature of passive "lm and the
kinetics of its formation under di!erent conditions of
potential application. SEM observation of the surface of
untreated NiTi after exposure at E (0.39V), Fig. 6(a),

reveals irregular cavities suggesting that localized cor-
rosion proceeds bythe formation and coalescence of very
small (submicron) pits.

As can be seen from the anodic current transient of

NiTi PIRAC coated at 9003C, 1.5 h, Fig. 5(b), no break-
down of passivitywas observed in the whole tested po-
tential region when the initiallyapplied pressure was
below E measured in the potentiodynamic test. How-

ever, when the initiallyapplied potential was 0.6 V, i.e.
above E measured in the potentiodynamic test, the

D. Starosvetsky, I. Gotman / Biomaterials 22 (2001) 1853}1859

1857

anodic current stronglyincreased after a short incuba-
tion period indicating surface activation, Fig. 5(b), inset.
Corrosion damage can be clearlyseen in such sample,
Fig. 6(b), with local cracking of the coating presumably
caused bypitting attack on the alloyunderneath.

For NiTi samples PIRAC nitrided at 10003C, 1 h, no

breakdown of passivitywas observed in the potentio-
static test, with the coating remaining intact in the whole
potential range (up to 0.8 V) at anyinitiallyapplied
potential, Table 2 and Fig. 6(c). This result clearlyillus-
trates the favorable e!ect of PIRAC nitriding procedure
on the corrosion resistance of NiTi alloy.

Verylow anodic currents were measured in the region

of passivityduring potentiostatic test for both untreated
and PIRAC nitrided NiTi, see Table 2. Current density
values for the samples PIRAC coated at 900 and 10003C
are veryclose and are &7 to 10 times smaller than those
of the untreated alloy. Therefore, lower passive dissolu-
tion rates can be expected in PIRAC-coated NiTi com-
pared to its untreated counterpart. As can be seen in
Table 3, small but measurable amounts (&0.01 mg/cm

)

of Ti and Ni ions have been released from the untreated
NiTi sample after 400 h immersion in Ringer's solution.
For PIRAC coated samples (10003C, 1 h) no Ti or Ni was
detected in the solution byAAS con"rming our earlier
assumption that PIRAC nitriding reduces metal ion re-
lease from NiTi alloy. The absence of Ni in the solution is
noteworthygiven its known toxic e!ects in the human
body.

5. Summary

The surface of NiTi shape memoryalloywas modi"ed

bythe original PIRAC nitriding treatment based on
selective di!usion of nitrogen from the atmosphere. After
short PIRAC anneals at 9003C (1.5 h) or 10003C (1 h), the
modi"ed NiTi surface consisted of a thin outer layer of
titanium nitride, TiN, and a thicker TiNi layer under-

neath. In spite of the small coating thickness, the surface
hardness of PIRAC nitrided NiTi was noticeablyhigher
than that of the untreated alloyimplying a better wear
resistance.

Untreated NiTi was found susceptible to pitting cor-

rosion in the saline (Ringer's) solution under conditions
typically encountered in the human body. PIRAC nitrid-
ing treatment allowed to signi"cantlyimprove the cor-
rosion behavior of NiTi. Thus, NiTi samples PIRAC
nitrided at 10003C for 1 h were passive in a wide range of
potentials with no signs of activation up to 1.1 V. PIRAC
coated NiTi was characterized byverylow anodic cur-
rents in the passive region and byan exceedinglylow
metal ion release rate. The absence of pitting coupled
with negligible in vitro release of harmful Ni ions, as well
as increased surface hardness of the coated samples sug-
gest that PIRAC nitriding procedure can improve the

in vivo performance of NiTi alloyimplanted into the
human body.

An important point to be noted is that NiTi surgical

devices can undergo signi"cant deformations associated
with the shape memorye!ect or superelasticity. In order
to provide an appropriate corrosion protection, the
PIRAC coating should be able to sustain these deforma-
tions without cracking. Although there is a dearth of
literature data on the strength of thin TiN layers, there
are certain indications that several micron thick TiN
coatings can accommodate *1% elastic strain without
cracking [25]. In our earlyfour-point bending experi-
ments, no cracks were observed in the PIRAC coating
(10003C, 1 h) on the tensile side of the bent NiTi sample
up to the roughlyestimated 2}4% strain. More accurate
mechanical tests of PIRAC nitrided NiTi samples are
currentlyunderway.

Acknowledgements

This work was supported bythe Israel Ministryof

Science through research Grant No.1094-1-98.

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