Abstract
In order to improve the NiTi alloy biocompatibility, surface treatments become very important. Nevertheless, researchers use different solutions to simulate the body fluids in electrochemical assays, and the correlation between the obtained results is difficult and might not even be possible. The present paper evaluated the electrochemical behavior of polished NiTi surfaces exposed to different simulated body fluid solutions: Hanks solution, Hanks’ balanced salt (HBSS) solution, saline body fluid (SBF) solution, and Ringer solution. The electrochemical behavior of NiTi was evaluated by open circuit potential (OCP) and cyclic voltammetry tests. The surfaces of the samples were also characterized by scanning electron microscopy, which was performed after the electrochemical tests. The results demonstrated that the NiTi alloy shows the same corrosion mechanism (pitting) in all simulated body fluids that were studied. However, the corrosion potential changes for each electrolyte, being HBSS, SBF and Ringer the most corrosive solutions. Furthermore, the Hanks and HBSS solutions demonstrated good reproducibility of the electochemical results. Considering that the HBSS represents an extreme environment, this solution seems to be the most indicated to study the corrosion behavior of NiTi treated surfaces.
metallic biomaterial; electrochemical behavior; NiTi; simulated body fluids
1 Introduction
The NiTi alloy is extensively used in the biomedical field due to its notable
mechanical properties of superelasticity and shape memory11. Shabalovskaya SA, Rondelli GC, Undisz AL, Anderegg JW, Burleigh
TD and Rettenmayr ME. The electrochemical characteristics of native Nitinol
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http://dx.doi.org/10.1016/j.biomaterials.2009.03.034.
PMid:19345407
http://dx.doi.org/10.1016/j.biomaterials...
. Comparative studies of biocompatibility between
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show that the
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, plasma immersion ion implantation1313. Liu XM, Wu SL, Chu PK, Chung CY, Chu CL, Yeung KWK, et al.
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, hydroxiapatite/zirconia
composite coating1414. Qiu D, Wang A and Yin Y. Characterization and corrosion behavior
of hydroxyapatite/zirconia composite coating on NiTi fabricated by
electrochemical deposition. Applied Surface Science. 2010; 257(5):1774-1778.
http://dx.doi.org/10.1016/j.apsusc.2010.09.014.
http://dx.doi.org/10.1016/j.apsusc.2010....
, TiN
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al. The underlying biological mechanisms of biocompatibility differences between
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, sol-gel
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metal vapor
vaccum arc plasma source1717. Zhao T, Li Y, Xiang Y, Zhao X and Zhang T. Surface
characteristics, nano-indentation and corrosion behavior of Nb implanted NiTi
alloy. Surface and Coatings Technology. 2011; 205(19):4404-4410.
http://dx.doi.org/10.1016/j.surfcoat.2011.03.061.
http://dx.doi.org/10.1016/j.surfcoat.201...
. The
correlation of the results from these studies in regards to the corrosion resistance
is complex due to the different simulated body fluids that were used as
electrolytes. The American Society for Testing and Materials (ASTM) standard to
determine the corrosion susceptibility of small implant devices does not specify the
electrolyte formulation – only an ion concentration range is mentioned1818. American Society for Testing and Materials – ASTM. F2129-08:
Standard Test method for conducting cyclic potentiodynamic polarization
measurements to determine the corrosion susceptibility of small implant devices.
West Conshohocken; 2008. http://dx.doi.org/10.1520/F2129-08.
http://dx.doi.org/10.1520/F2129-08...
. However, the different
electrolyte compositions may result in distinct aggressiveness and corrosion
mechanisms. Some of the most used solutions to simulate body fluids include:
Ringer’s solution1919. Liu XM, Wu SL, Chu PK, Chung CY, Chu CL, Chan YL, et al. In
vitro corrosion behavior of TiN layer produced on orthopedic nickel–titanium
shape memory alloy by nitrogen plasma immersion ion implantation using different
frequencies. Surface and Coatings Technology. 2008; 202(11):2463-2466.
http://dx.doi.org/10.1016/j.surfcoat.2007.08.017.
http://dx.doi.org/10.1016/j.surfcoat.200...
, Hanks
solution2020. Bayat N, Sanjabi S and Barber ZH. Improvement of corrosion
resistance of NiTi sputtered thin films by anodization. Applied Surface Science.
2011; 257(20):8493-8499.
http://dx.doi.org/10.1016/j.apsusc.2011.05.001.
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,2121. Shi P, Cheng FT and Man HC. Improvement in corrosion resistance
of NiTi by anodization in acetic acid. Materials Letters. 2007;
61(11-12):2385-2388.
http://dx.doi.org/10.1016/j.matlet.2006.09.020.
http://dx.doi.org/10.1016/j.matlet.2006....
, Hanks balanced salt solution
(HBSS)2222. Chrzanowski W, Neel EAA, Armitage DA and Knowles JC. Effect of
surface treatment on the bioactivity of nickel-titanium. Acta Biomaterialia.
2008; 4(6):1969-1984. http://dx.doi.org/10.1016/j.actbio.2008.05.010.
PMid:18565807
http://dx.doi.org/10.1016/j.actbio.2008....
, saline body fluid
solution (SBF)2323. Park HH, Park IS, Kim KS, Jeon WY, Park BK, Kim HS, et al.
Bioactive and electrochemical characterization of TiO2 nanotubes on titanium via
anodic oxidation. Electrochimica Acta. 2010; 55(20):6109-6114.
http://dx.doi.org/10.1016/j.electacta.2010.05.082.
http://dx.doi.org/10.1016/j.electacta.20...
.
A study of Qiu et al.1414. Qiu D, Wang A and Yin Y. Characterization and corrosion behavior
of hydroxyapatite/zirconia composite coating on NiTi fabricated by
electrochemical deposition. Applied Surface Science. 2010; 257(5):1774-1778.
http://dx.doi.org/10.1016/j.apsusc.2010.09.014.
http://dx.doi.org/10.1016/j.apsusc.2010....
used
mechanically polished and hydroxiapatite/zirconia (HAP/ZrO2)
electrodeposited NiTi. The electrochemical corrosion test was performed in an SBF
solution. These authors observed a corrosion current density
(icorr) of 3.98 × 10–7 A/cm2
for the NiTi uncoated sample, and of 7.00 × 10–9 A/cm2 for the
HAP/ZrO2 coated sample, which is almost 60 times lower1414. Qiu D, Wang A and Yin Y. Characterization and corrosion behavior
of hydroxyapatite/zirconia composite coating on NiTi fabricated by
electrochemical deposition. Applied Surface Science. 2010; 257(5):1774-1778.
http://dx.doi.org/10.1016/j.apsusc.2010.09.014.
http://dx.doi.org/10.1016/j.apsusc.2010....
. They also found that the
breakdown potential (Ebr) of the bare NiTi sample appeared at nearly 600
mV, while coated samples did not break at studied potentials1414. Qiu D, Wang A and Yin Y. Characterization and corrosion behavior
of hydroxyapatite/zirconia composite coating on NiTi fabricated by
electrochemical deposition. Applied Surface Science. 2010; 257(5):1774-1778.
http://dx.doi.org/10.1016/j.apsusc.2010.09.014.
http://dx.doi.org/10.1016/j.apsusc.2010....
. Zhao et al.1717. Zhao T, Li Y, Xiang Y, Zhao X and Zhang T. Surface
characteristics, nano-indentation and corrosion behavior of Nb implanted NiTi
alloy. Surface and Coatings Technology. 2011; 205(19):4404-4410.
http://dx.doi.org/10.1016/j.surfcoat.2011.03.061.
http://dx.doi.org/10.1016/j.surfcoat.201...
, on the other hand, used a commercial NiTi polished
alloy samples, as well as samples polished by metal vapor vaccum arc plasma (MEVVA
100) with different parameters. The electrochemical tests were conducted in Hank’s
solution. Nb-NiTi samples exhibit much higher Ecorr (-396 mV) and
Ebr (1094 mV) than the untreated NiTi samples (Ecorr = -
478 mV, Ebr = 420 mV). The authors state that Ebr is an
indication to evaluate the susceptibility to pitting corrosion, thus, higher
Ebr values means higher resistance to pitting corrosion1717. Zhao T, Li Y, Xiang Y, Zhao X and Zhang T. Surface
characteristics, nano-indentation and corrosion behavior of Nb implanted NiTi
alloy. Surface and Coatings Technology. 2011; 205(19):4404-4410.
http://dx.doi.org/10.1016/j.surfcoat.2011.03.061.
http://dx.doi.org/10.1016/j.surfcoat.201...
. They also found that the
icorr values of the Nb-NiTi samples (5.89 ×
10–10 A/cm2) were much lower than that of the untreated
NiTi (1.07 × 10–8 A/cm2), suggesting a much slower eroding
rate. In both studies, the developed surface treatment improved the corrosion
resistance of the NiTi. Even though the Nb alloy treatments developed lower current
densities, it is important to notice that the corrosion densities found by Zhao et
al.1717. Zhao T, Li Y, Xiang Y, Zhao X and Zhang T. Surface
characteristics, nano-indentation and corrosion behavior of Nb implanted NiTi
alloy. Surface and Coatings Technology. 2011; 205(19):4404-4410.
http://dx.doi.org/10.1016/j.surfcoat.2011.03.061.
http://dx.doi.org/10.1016/j.surfcoat.201...
were also lower than
the values obtained by Qiu et al.1414. Qiu D, Wang A and Yin Y. Characterization and corrosion behavior
of hydroxyapatite/zirconia composite coating on NiTi fabricated by
electrochemical deposition. Applied Surface Science. 2010; 257(5):1774-1778.
http://dx.doi.org/10.1016/j.apsusc.2010.09.014.
http://dx.doi.org/10.1016/j.apsusc.2010....
for the NiTi without surface treatment. Therefore, when
the same substrate (NiTi) is analyzed in different electrolytes that simulate body
fluids, there may be problems in interpreting the results obtained by different
authors.
Shahrabi et al.1111. Shahrabi T, Sanjabi S, Saebnoori E and Barber ZH. Extremely high
pitting resistance of NiTi shape memory alloy thin film in simulated body
fluids. Materials Letters. 2008; 62(17–18):2791-2794.
http://dx.doi.org/10.1016/j.matlet.2008.01.052.
http://dx.doi.org/10.1016/j.matlet.2008....
studied bulk
NiTi, polished to 1500 grit, in both Ringer’s and Hank’s solutions. The authors
observed a higher Ebr in Ringer’s solution (747 mV). Furthermore, the
passive region of the bulk in Ringer’s solution has shifted to the right (higher
current density) in comparison with Hank’s solution1111. Shahrabi T, Sanjabi S, Saebnoori E and Barber ZH. Extremely high
pitting resistance of NiTi shape memory alloy thin film in simulated body
fluids. Materials Letters. 2008; 62(17–18):2791-2794.
http://dx.doi.org/10.1016/j.matlet.2008.01.052.
http://dx.doi.org/10.1016/j.matlet.2008....
. Another work, by Liang and Mou2424. Liang C-h, Mou Z. Effects of different simulated fluids on
anticorrosion biometallic materials. Transactions of Nonferrous Metals Society
of China. 2001;11(4):579-582., studied the effects of different electrolytes
(Ringer’s, PBS and Hank’s solution) on anticorrosion biometallic materials (SUS316L
stainless steel, Co-Cr alloy and Ti-6Al-4V). The results indicate that the corrosion
caused by the Ringer’s solution is the strongest, followed by the PBS and Hank’s
solutions. In addition, the decrease of the pH of the solution significantly
increased the corrosion rate and the susceptibility to localizated corrosion of
SUS316L and Co-Cr. On the other hand, Ti-6Al-4V alloys exhibited stability and only
a slight increase of corrosion rate with decreasing pH.
In general, as mentioned before, studies envolving biomedical alloys already showed
different electrochemical results when distincts electrolytes were used to simulated
the body fluids, which indicates the important influence of the electrolyte
composition. Besides, different alloys can present distint behavior in the same
electrolyte. In this context, the aim of this work is to evaluate the influence of
the different electrolytes used to simulate the body fluids on assessing the NiTi
corrosion resistance. For this purpose, the Ringer, Hanks, HBSS and SBF solutions
were chosen. Additionally a 0.9% NaCl solution was used following some authors
suggestions that its corrosion potential is similar to the body fluid due to its
high chloride concentration11. Shabalovskaya SA, Rondelli GC, Undisz AL, Anderegg JW, Burleigh
TD and Rettenmayr ME. The electrochemical characteristics of native Nitinol
surfaces. Biomaterials. 2009; 30(22):3662-3671.
http://dx.doi.org/10.1016/j.biomaterials.2009.03.034.
PMid:19345407
http://dx.doi.org/10.1016/j.biomaterials...
,2525. Flamini DO, Saugo M and Saidman SB. Electrodeposition of
polypyrrole on Nitinol alloy in the presence of inhibitor ions for corrosion
protection. Corrosion Science. 2014; 81:36-44.
http://dx.doi.org/10.1016/j.corsci.2013.11.063.
http://dx.doi.org/10.1016/j.corsci.2013....
,2626. Maho A, Delhalle J and Mekhalif Z. Study of the formation
process and the characteristics of tantalum layers electrodeposited on Nitinol
plates in the 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide
ionic liquid. Electrochimica Acta. 2013; 89:346-358.
http://dx.doi.org/10.1016/j.electacta.2012.11.026.
http://dx.doi.org/10.1016/j.electacta.20...
.
2 Experimental
2.1 Sample preparation
The near-equiatomic superelastic NiTi (Ni 55.8 wt%) alloy was used for this study. NiTi samples were cut using the Electric Discharge Machining (EDM). The test specimen dimensions were 15 mm x 15 mm x 1 mm. All samples were polished with silicon carbide sandpaper up to the 600 grade and subsequently isolated with Araldite®, defining an area of 0.91 cm² for electrochemical tests.
2.2 Preparation of electrolytes
The following electrolytes were prepared according to published articles:
Ringer2727. Khalil-Allafi J, Amin-Ahmadi B and Zare M. Biocompatibility and
corrosion behavior of the shape memory NiTi alloy in the physiological
environments simulated with body fluids for medical applications. Materials
Science and Engineering C. 2010; 30(8):1112-1117.
http://dx.doi.org/10.1016/j.msec.2010.06.007.
http://dx.doi.org/10.1016/j.msec.2010.06...
, Hanks2828. Siu HT and Man HC. Fabrication of bioactive titania coating on
nitinol by plasma electrolytic oxidation. Applied Surface Science. 2013;
274:181-187. http://dx.doi.org/10.1016/j.apsusc.2013.03.008.
http://dx.doi.org/10.1016/j.apsusc.2013....
, HBSS2222. Chrzanowski W, Neel EAA, Armitage DA and Knowles JC. Effect of
surface treatment on the bioactivity of nickel-titanium. Acta Biomaterialia.
2008; 4(6):1969-1984. http://dx.doi.org/10.1016/j.actbio.2008.05.010.
PMid:18565807
http://dx.doi.org/10.1016/j.actbio.2008....
and SBF2929. Kokubo T and Takadama H. How useful is SBF in predicting in vivo
bone bioactivity? Biomaterials. 2006; 27(15):2907-2915.
http://dx.doi.org/10.1016/j.biomaterials.2006.01.017.
PMid:16448693
http://dx.doi.org/10.1016/j.biomaterials...
. A 9 g.L–1 (153. mmol/L) NaCl (0.9%
NaCl) distilled water solution was prepared. The ion concentration of each
prepared solution is described on Table
1. For comparison purposes, the human blood plasma ion concentrations
is also demonstrated on Table 12828. Siu HT and Man HC. Fabrication of bioactive titania coating on
nitinol by plasma electrolytic oxidation. Applied Surface Science. 2013;
274:181-187. http://dx.doi.org/10.1016/j.apsusc.2013.03.008.
http://dx.doi.org/10.1016/j.apsusc.2013....
.
The pH measurements were perfomed in a Sanxin PHS-3D pHmeter at 37 ºC for Hanks, HBSS, SBF and Ringer solutions, and 25 °C for NaCl 0.9% (Table 2).
pH measurements of studied solutions at 37 °C (Hanks, HBSS, SBF and Ringer) and 25 °C (NaCl 0.9%).
2.3 Electrochemical characterization
Open circuit potential (OCP) measurements and potentiodynamic polarization curves
were performed threefold, under an inert atmosphere, in an Autolab PGSTAT302N
potenciostat/galvanostat, according to the ASTM F2129-08 standard1818. American Society for Testing and Materials – ASTM. F2129-08:
Standard Test method for conducting cyclic potentiodynamic polarization
measurements to determine the corrosion susceptibility of small implant devices.
West Conshohocken; 2008. http://dx.doi.org/10.1520/F2129-08.
http://dx.doi.org/10.1520/F2129-08...
. The test temperature was of
25°C (0.9% NaCl), with a maximum variation of 1°C, and of 37°C – for the other
solutions, with the same variation. The 0.9% NaCl solution was prepared at room
temperature to reproduce the methodology described by several authors11. Shabalovskaya SA, Rondelli GC, Undisz AL, Anderegg JW, Burleigh
TD and Rettenmayr ME. The electrochemical characteristics of native Nitinol
surfaces. Biomaterials. 2009; 30(22):3662-3671.
http://dx.doi.org/10.1016/j.biomaterials.2009.03.034.
PMid:19345407
http://dx.doi.org/10.1016/j.biomaterials...
,2525. Flamini DO, Saugo M and Saidman SB. Electrodeposition of
polypyrrole on Nitinol alloy in the presence of inhibitor ions for corrosion
protection. Corrosion Science. 2014; 81:36-44.
http://dx.doi.org/10.1016/j.corsci.2013.11.063.
http://dx.doi.org/10.1016/j.corsci.2013....
,2626. Maho A, Delhalle J and Mekhalif Z. Study of the formation
process and the characteristics of tantalum layers electrodeposited on Nitinol
plates in the 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide
ionic liquid. Electrochimica Acta. 2013; 89:346-358.
http://dx.doi.org/10.1016/j.electacta.2012.11.026.
http://dx.doi.org/10.1016/j.electacta.20...
. The OCP and potentiodynamic
polarization were performed in a conventional three-electrode electrochemical
cell. The working electrode was the NiTi samples, the counter electrode was a
platinum wire, and the reference electrode was a satured calomel electrode
(SCE). The OCP was monitored for 1 hour, the period that was necessary to
stabilize the electrochemical system. The cyclic voltammetry was performed from
the OCP potentials until 2 V in reference to the SCE with a scan rate of 1
mV.s–1.
2.4 Superficial characterization
The superficial characterization was performed in polished NiTi after potenciodynamic polarization in 0.9% NaCl, Hanks, HBSS, Ringer and SBF solution using an scanning electron microscopy (SEM), JEOL 6060. Characterization was also made in polished NiTi without the electrochemical test for comparison purposes.
3 Results and Discussions
3.1 Electrochemical characterization
The OCP values that were obtained are shown in Figure 1. The lowest OCP value, measured at 3600 seconds (- 291 mV),
which is the most active potential, was developed in HBSS solution. Considering
the electrolytes studied at 37 °C, HBSS, SBF and Ringer demonstrated the highest
corrosion potential, which may be associated to a higher Cl-
concentration in these electrolytes (Table
1). NiTi alloy is susceptible to pitting corrosion in chloride
containing solutions3030. Li X, Wang J, Han EH and Ke W. Influence of fluoride and
chloride on corrosion behavior of NiTi orthodontic wires. Acta Biomaterialia.
2007; 3(5):807-815. http://dx.doi.org/10.1016/j.actbio.2007.02.002.
PMid:17467350
http://dx.doi.org/10.1016/j.actbio.2007....
.
Attack by Cl- in NiTi result in Ni being released into the solution
and decrease in the local Ni concentration at the pitting sites3131. Hu T, Chu C, Xin Y, Wu S, Yeung KWK and Chu PK. Corrosion
products and mechanism on NiTi shape memory alloy in physiological environment.
Journal of Materials Research. 2010; 25(02):350-358.
http://dx.doi.org/10.1557/JMR.2010.0051.
http://dx.doi.org/10.1557/JMR.2010.0051...
. The remaining Ti reacts
with dissolved oxygen from the solution to form titanium oxides in the corroded
area3131. Hu T, Chu C, Xin Y, Wu S, Yeung KWK and Chu PK. Corrosion
products and mechanism on NiTi shape memory alloy in physiological environment.
Journal of Materials Research. 2010; 25(02):350-358.
http://dx.doi.org/10.1557/JMR.2010.0051.
http://dx.doi.org/10.1557/JMR.2010.0051...
. The corrosion
product layer expands over the entire surface and is composed of
TiO2, Ti2O3, and TiO with depleted Ni3131. Hu T, Chu C, Xin Y, Wu S, Yeung KWK and Chu PK. Corrosion
products and mechanism on NiTi shape memory alloy in physiological environment.
Journal of Materials Research. 2010; 25(02):350-358.
http://dx.doi.org/10.1557/JMR.2010.0051.
http://dx.doi.org/10.1557/JMR.2010.0051...
. The change in the Ni
release rate is related to the corrosion defects such as pitting pores on the
NiTi specimens3131. Hu T, Chu C, Xin Y, Wu S, Yeung KWK and Chu PK. Corrosion
products and mechanism on NiTi shape memory alloy in physiological environment.
Journal of Materials Research. 2010; 25(02):350-358.
http://dx.doi.org/10.1557/JMR.2010.0051.
http://dx.doi.org/10.1557/JMR.2010.0051...
. In the
early immersion period, nickel ions area released gradually into the surrounding
solution3131. Hu T, Chu C, Xin Y, Wu S, Yeung KWK and Chu PK. Corrosion
products and mechanism on NiTi shape memory alloy in physiological environment.
Journal of Materials Research. 2010; 25(02):350-358.
http://dx.doi.org/10.1557/JMR.2010.0051.
http://dx.doi.org/10.1557/JMR.2010.0051...
. As the
immersion continues, corrosion results in the formation of pitting pores that
promote nickel release, leading to a high Ni release rate3131. Hu T, Chu C, Xin Y, Wu S, Yeung KWK and Chu PK. Corrosion
products and mechanism on NiTi shape memory alloy in physiological environment.
Journal of Materials Research. 2010; 25(02):350-358.
http://dx.doi.org/10.1557/JMR.2010.0051.
http://dx.doi.org/10.1557/JMR.2010.0051...
. However, if the immersion time is long
enough, the corrosion-induced titanium oxide layer seals the pitting pores on
the NiTi surface, thereby reducing the Ni release rate3131. Hu T, Chu C, Xin Y, Wu S, Yeung KWK and Chu PK. Corrosion
products and mechanism on NiTi shape memory alloy in physiological environment.
Journal of Materials Research. 2010; 25(02):350-358.
http://dx.doi.org/10.1557/JMR.2010.0051.
http://dx.doi.org/10.1557/JMR.2010.0051...
. Early pitting corrosion takes place on many
sites on the surface. Early corrosion process take place on surface defects
initially and involves the breakdown of original thin oxide layer and Ni-Ti bond
after attack by Cl-, and this process releases Ni ions3131. Hu T, Chu C, Xin Y, Wu S, Yeung KWK and Chu PK. Corrosion
products and mechanism on NiTi shape memory alloy in physiological environment.
Journal of Materials Research. 2010; 25(02):350-358.
http://dx.doi.org/10.1557/JMR.2010.0051.
http://dx.doi.org/10.1557/JMR.2010.0051...
. As time elapses, the
pitting sites propagate and noticeable pores form on the surface3131. Hu T, Chu C, Xin Y, Wu S, Yeung KWK and Chu PK. Corrosion
products and mechanism on NiTi shape memory alloy in physiological environment.
Journal of Materials Research. 2010; 25(02):350-358.
http://dx.doi.org/10.1557/JMR.2010.0051.
http://dx.doi.org/10.1557/JMR.2010.0051...
. Ni released from the bulk
materials into the solution reduces the local concentration of Ni in the
materials3131. Hu T, Chu C, Xin Y, Wu S, Yeung KWK and Chu PK. Corrosion
products and mechanism on NiTi shape memory alloy in physiological environment.
Journal of Materials Research. 2010; 25(02):350-358.
http://dx.doi.org/10.1557/JMR.2010.0051.
http://dx.doi.org/10.1557/JMR.2010.0051...
. Meanwhile,
the remaining Ti reacts with dissolved oxygen in the solution to form titanium
oxides around the pitting sites and pores3131. Hu T, Chu C, Xin Y, Wu S, Yeung KWK and Chu PK. Corrosion
products and mechanism on NiTi shape memory alloy in physiological environment.
Journal of Materials Research. 2010; 25(02):350-358.
http://dx.doi.org/10.1557/JMR.2010.0051.
http://dx.doi.org/10.1557/JMR.2010.0051...
. As the corrosion process proceeds, the titanium
oxide layer grows and propagates over the entire surface of the NiTi
specimen3131. Hu T, Chu C, Xin Y, Wu S, Yeung KWK and Chu PK. Corrosion
products and mechanism on NiTi shape memory alloy in physiological environment.
Journal of Materials Research. 2010; 25(02):350-358.
http://dx.doi.org/10.1557/JMR.2010.0051.
http://dx.doi.org/10.1557/JMR.2010.0051...
. The pores
formed during corrosion are possibly blocked by the titanium oxides formed
inside the pores. As a result, a uniform and dense oxide layer is formed3131. Hu T, Chu C, Xin Y, Wu S, Yeung KWK and Chu PK. Corrosion
products and mechanism on NiTi shape memory alloy in physiological environment.
Journal of Materials Research. 2010; 25(02):350-358.
http://dx.doi.org/10.1557/JMR.2010.0051.
http://dx.doi.org/10.1557/JMR.2010.0051...
. After the titanium oxide
layer has formed on the entire surface of the NiTi specimen, it will serve as a
passivation film to retard the corrosion process3131. Hu T, Chu C, Xin Y, Wu S, Yeung KWK and Chu PK. Corrosion
products and mechanism on NiTi shape memory alloy in physiological environment.
Journal of Materials Research. 2010; 25(02):350-358.
http://dx.doi.org/10.1557/JMR.2010.0051.
http://dx.doi.org/10.1557/JMR.2010.0051...
. This susceptibility increases when the chloride
concentration is higher3030. Li X, Wang J, Han EH and Ke W. Influence of fluoride and
chloride on corrosion behavior of NiTi orthodontic wires. Acta Biomaterialia.
2007; 3(5):807-815. http://dx.doi.org/10.1016/j.actbio.2007.02.002.
PMid:17467350
http://dx.doi.org/10.1016/j.actbio.2007....
. In
the Hanks solution, the NiTi alloy developed a less active potential (- 214 mV).
Figure 2 presents the voltammograms of
NiTi in different studied solutions. A slight increase in the current density
occurred in the anodic potential scan, indicating a superficial oxide disruption
allowing the current flow. This phenomenon of anodic dissolution in the surface
indicates the beginning of the oxide layer breakdown. When the applied potential
reaches 2 V, it is reversed, and for all systems the current density of the
inverse scan is higher (Figure 2). This
behavior is associated to the pitting corrosion of the NiTi alloy in the studied
solutions2727. Khalil-Allafi J, Amin-Ahmadi B and Zare M. Biocompatibility and
corrosion behavior of the shape memory NiTi alloy in the physiological
environments simulated with body fluids for medical applications. Materials
Science and Engineering C. 2010; 30(8):1112-1117.
http://dx.doi.org/10.1016/j.msec.2010.06.007.
http://dx.doi.org/10.1016/j.msec.2010.06...
.
Open circuit potentials in different simulated body fluids at 37 °C and in 0.9% NaCl at 25 °C.
Potentiodynamic polarization in different simulated body fluids at 37 °C and in 0.9% NaCl at 25 °C. Voltammogram detail showing the passive region of NiTi in (a) HBSS solution, (b) SBF solution and (c) Ringer solution.
In the 0.9% NaCl solution, the NiTi samples developed current densities that were
one order of magnitude lower than the others studied solutions (Figure 2) indicating that a lower
temperature (25 °C) and the absence of other ions, such as
HCO3-1, can make the medium less aggressive to this
alloy. However the difference in temperature, 25 °C for the test in 0.9% NaCl
solution compared to 37 ºC for the other electrolytes can contribute for the
differences observed between NaCl solution and the other solutions. Few studies
have investigated the effects of temperature on the corrosion resistance of
nitinol, but a study in Hank's physiological solution at temperatures ranging
from 10 to 80 °C show that the ability of NiTi to repassivate is significantly
reduced by an increase in temperature3232. Trépanier C and Pelton AR. Effect of temperature and pH on the
corrosion resistance of Nitinol. In: Medical Device Materials II: Proceedings
from the Materials & Processes for Medical Devices Conference 2004; 2004.
St. Paul Minnesota: ASM International; 2005. p. 392.. Studies using other alloys proved that a
bicarbonate ion can induce pitting corrosion3333. Han J, Zhang J and Carey JW. Effect of bicarbonate on corrosion
of carbon steel in CO2 saturated brines. International Journal of Greenhouse Gas
Control. 2011; 5(6):1680-1683.
http://dx.doi.org/10.1016/j.ijggc.2011.08.003.
http://dx.doi.org/10.1016/j.ijggc.2011.0...
. Furthermore, in a 0.9% NaCl solution it is
possible to observe the disruption of the passive layer at 757 mV (Figure 2). Considering the solutions that
were analyzed at 37 °C, the Hanks solution demonstrated the lowest current
densities in the passive zone of the polarization curve (Figure 2). This is probably associated to
HCO3- ion concentration in the Hanks solution (Table 1). The oxide layer was firstly
ruptured, in Hanks solution, when the applied potential reached 99 mV (current
density increased from 1.55 × 10–7 A/cm2 to 8.72 ×
10–7 A/cm²) while this value was – 78 mV (current density
increased from 3.20 × 10–6 A/cm² to 1.17 × 10–5
A/cm2) for Ringer, - 119 mV (current density increased from 2.80
× 10–6 A/cm² to 1.17 × 10–5 A/cm2) for HBSS and
-148 mV (current density increased from 2.80 × 10–6 A/cm2
to 1.39 × 10–5 A/cm²) for SBF. In the Hanks solution the NiTi shows a
lower tendency to corrosion due to a minor passive current and a higher pitting
potential, when compared to other solutions studied at 37°C. This results
demonstrated a lower corrosive tendency of NiTi in Hanks solution in comparison
with other studied solutions. The variation in the passive zone of the
polarization curve of different alloys due to the concentration of bicarbonate
in the electrolyte was described by other authors3434. Torres-Islas A, Gonzalez-Rodriguez JG, Uruchurtu J and Serna S.
Stress corrosion cracking study of microalloyed pipeline steels in dilute NaHCO3
solutions. Corrosion Science. 2008; 50(10):2831-2839.
http://dx.doi.org/10.1016/j.corsci.2008.07.007.
http://dx.doi.org/10.1016/j.corsci.2008....
. Ions bicarbonate accelerated the cathodic
reactions, promoting an oxide growth on the surface, and resulting in lower
anodic current densities. Despite the curves in the other solutions having
similar behavior (Figure 2), the
potentials in which sudden increases of current density occurred varied. This
fact suggests that it is not possible to compare the results of studies
performed in different electrolytes, even if they have all been performed in
simulated body fluids. A standardization of all of the parameters for corrosion
tests is important to compare the different studies about the protection of NiTi
surfaces. It should be mentioned that, according to some authors, it is
difficult to choose an ideal solution to simulate the body fluids due to the
complexity of the human physiological enviroment2424. Liang C-h, Mou Z. Effects of different simulated fluids on
anticorrosion biometallic materials. Transactions of Nonferrous Metals Society
of China. 2001;11(4):579-582.. Other authors suggest that organic acids found in
the blood should also be considered in the preparation of simulated body
fluids2727. Khalil-Allafi J, Amin-Ahmadi B and Zare M. Biocompatibility and
corrosion behavior of the shape memory NiTi alloy in the physiological
environments simulated with body fluids for medical applications. Materials
Science and Engineering C. 2010; 30(8):1112-1117.
http://dx.doi.org/10.1016/j.msec.2010.06.007.
http://dx.doi.org/10.1016/j.msec.2010.06...
. In the present
paper, the Hanks and the HBSS solutions demonstrated a good reproducibility of
the results.
Considering that all samples were made of the same alloy and were submitted to
the same superficial treatment, and the electrochemical tests were performed
under the same parameters (temperature, sealing and scan rate), the corrosion
potential variance is probably due to the different aggressiveness levels of the
studied electrolytes, despite the slight difference in the pH among the
solutions, which can influence the cathodic curves, thus affecting the corrosion
potential. Nevertheless, this influence is probably not relevant, since all of
the solutions are in neutral pH range. A study developed in Hank's physiological
solution with pH varying from 1 to 9 showed that corrosion resistance of NiTi is
not significantly affected by pH values3232. Trépanier C and Pelton AR. Effect of temperature and pH on the
corrosion resistance of Nitinol. In: Medical Device Materials II: Proceedings
from the Materials & Processes for Medical Devices Conference 2004; 2004.
St. Paul Minnesota: ASM International; 2005. p. 392.. The Hanks solution showed the lowest
aggressiveness in comparison to the other simulated body fluid solutions, which
is evidenced by the less active corrosion potential and by the evolution of
minor current densities. Other authors also demonstrated that the Ringer
solution is stronger than the Hanks. This variation was associated to the
presence of Na2PO4 and KH2PO4, which
benefits the biomaterials against corrosion, in Hanks solution1111. Shahrabi T, Sanjabi S, Saebnoori E and Barber ZH. Extremely high
pitting resistance of NiTi shape memory alloy thin film in simulated body
fluids. Materials Letters. 2008; 62(17–18):2791-2794.
http://dx.doi.org/10.1016/j.matlet.2008.01.052.
http://dx.doi.org/10.1016/j.matlet.2008....
,2424. Liang C-h, Mou Z. Effects of different simulated fluids on
anticorrosion biometallic materials. Transactions of Nonferrous Metals Society
of China. 2001;11(4):579-582.. It is important to notice
that the Cl- ions concentration is lower in the Hanks solution, in
comparison to the others studied solutions. Studies indicate that the NiTi alloy
is suscetible to pitting corrosion in chloride solutions, and in increasing
chloride concentration, the pitting potential decreases3030. Li X, Wang J, Han EH and Ke W. Influence of fluoride and
chloride on corrosion behavior of NiTi orthodontic wires. Acta Biomaterialia.
2007; 3(5):807-815. http://dx.doi.org/10.1016/j.actbio.2007.02.002.
PMid:17467350
http://dx.doi.org/10.1016/j.actbio.2007....
. In the HBSS solution the NiTi alloy
developed a higher current density and more active potentials (Figure 2), which was expected due to the
results obtained by the OCP (Figure
1).
3.2 Superficial characterization
After the electrochemical tests, the NiTi samples were analyzed by scanning
electron microscopy in order to determine the relation to the corrosion
mechanism found in the electrochemical tests (Figures 3, 4, 5, 6,
7 and 8). Although the electrochemical results demonstrated that the NiTi
alloy shows the same corrosion mechanism (pitting) in all studied simulated body
fluids, only with the HBSS solution it was possible to notice the presence of
oxidation products in the surface (Figure
6), resulting from more active potentials (Figure 2) and a higher current density; such products have
been described by other authors3535. Milošv I, Strehblow H-H, Navinšek B and Metikoš-Huković M.
Electrochemical and thermal oxidation of TiN coatings studied by XPS. Surface
and Interface Analysis. 1995; 23(7-8):529-539.
http://dx.doi.org/10.1002/sia.740230713.
http://dx.doi.org/10.1002/sia.740230713...
,3636. Tomashov ND, Chukalovskaya TV, Myedova LL and Yegorov FF.
Corrosion and anodic behaviour of carbide, nitride and boride of titanium in
sulphuric and phosphoric acids. Zaschyta Myetallov.
1985;5:682-688. as titanium hydroxides, which are more soluble,
less protective than the oxides, and are known by an increase in the current
between ~1,0 and 1,5 V. Other studies3737. Rondelli G. Corrosion resistance tests on NiTi shape memory
alloy. Biomaterials. 1996; 17(20):2003-2008.
http://dx.doi.org/10.1016/0142-9612(95)00352-5. PMid:8894095
http://dx.doi.org/10.1016/0142-9612(95)0...
have showing pitting corrosion on NiTi, in
physiological solution, at potentials between +190 and +280 mV versus SCE.
Oshida et al.3838. Oshida Y, Sachdeva R, Miyazaki S and Fukuyo S. Biological and
Chemical Evaluation of TiNi Alloys. Materials Science Forum. 1990;
56-58:705-710.
http://dx.doi.org/10.4028/www.scientific.net/MSF.56-58.705.
http://dx.doi.org/10.4028/www.scientific...
, evidenced
NiTi low pitting potential values, and presence of numerous oscillations of
anodic passive current for potentials lower than 500 mV.
NiTi samples after electrochemical tests in Hanks solution, by scanning electron microscope.
NiTi samples after electrochemical tests in Ringer solution, by scanning electron microscope.
NiTi samples after electrochemical tests in 0.9% NaCl solution, by scanning electron microscope.
4 Conclusions
Evaluating coatings through an appropriated method is as important as developing surface treatments for NiTi alloys. Considering that the corrosion resistance is a major issue in these alloys, it is relevant to standardize the electrochemical procedure parameters, as the electrolytes, allowing a comparison between the various coatings developed by several researchers.
This study compared the most commonly used electrolytes to simulated body fluids: Hanks, HBSS, SBF, Ringer and 0.9% NaCl. Electrochemical and morphological results, show that the NiTi presents the same corrosion mechanism (pitting induced by Cl- and HCO3- ions) in all studied simulated body fluids. However, the Hanks solution demonstrated less aggressiveness in comparison to other simulated body fluids, which is evidenced by its less active corrosion potential and by the development of lowest current densitie values, probably due to a lower chloride concentration. In the HBSS, SBF and Ringer solutions, the NiTi showed higher current densities and a more active potential. The Hanks and HBSS solutions presented the best reproducibility of results. Considering that the HBSS represents an extreme environment, this solution seems to be the most indicated to study the corrosion behavior of NiTi treated surfaces.
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Publication Dates
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Publication in this collection
Jan-Feb 2015
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Received
26 June 2014 -
Reviewed
03 Feb 2015