Investigation Biomedical Corrosion of Implant Alloys in Physiological Environment

Nabaa S. Radhi, PhD, Lecturer
University of Babylon-Hilla-Iraq
E-mail: dr.nabbaa@gmail.com.
Abstract:
Biomaterials have been out of commonly used materials in biomedical applications in recent years. Today among the mostly used biocompatible metals are especially Ni-Ti alloys.
Thus, the biomaterials used in these implants have been limited to bioinert titanium based alloys, stainless steel as well as to alumina and zirconia ceramics.
In This research studied the corrosion properties for both samples titanium and stainless steel in different corrosive media i.e in NaCl, H2SO4 and NaF, and tested the microstructure before and after corrosive media. Then the measurement surface roughness with and without corrosive media. Finally, tested XRD for both titanium and stainless steel samples.
Keywords: Biomaterials, biomedical, biocompatible, implants, bioinert, XRD.
1. Introduction:
Some metals are used as passive substitutes for hard tissue replacement such as total hip and knee joints, for fracture healing aids as bone plates and screws, spinal fixation devices, and dental implants because of their excellent mechanical properties and corrosion resistance. Some metallic alloys are used for more active roles in devices such as vascular stents, catheter guide wires, orthodontic archwires, and cochlea implants. The biocompatibility of the metallic implant is of considerable concern because these implants can corrode in an in vivo environment [Williams, 1982]. The consequences of corrosion are the disintegration of the implant material per se, which will weaken the implant, and the harmful effect of corrosion products on the surrounding tissues and organs.
1.1 Stainless Steels
The first stainless steel utilized for implant fabrication was the 18-8 (type 302 in modern classification), which is stronger and more resistant to corrosion than the vanadium steel. Vanadium steel is no longer used in implants since its corrosion resistance is inadequate in vivo. Later 18-8sMo stainless steel was introduced which contains a small percentage of molybdenum to improve the corrosion resistance in chloride solution (salt water). This alloy became known as type 316 stainless steel. In the 1950s the carbon content of 316 stainless steel was reduced from0.08 to a maximum amount of 0.03% (all are weight percent unless specified) for better corrosion resistance to chloride solution and to minimize the sensitization and hence, became known as type 316L stainless steel. The minimum effective concentration of chromium is 11% to impart corrosion resistance in stainless steels.
1.2 Pure Ti and Ti Alloys
Attempts to use titanium for implant fabrication dates to the late 1930s. It was found that titanium was tolerated in cat femurs, as was stainless steel and Vitallium® (CoCrMo alloy). Titanium’s lightness (4.5 g/cm3, see Table 1) and good mechanochemical properties are salient features for implant application.

TABLE 1 Specific Gravities of Some Metallic Implant Alloys.
Alloys Ti and its alloys 316 Stainless steel CoCrMo CoNiCrMo NiTi
Density (g/cm3) 4.5 7.9 8.3 9.2 6.7

Osseointegration is defined as direct contact without intervening soft tissue between viable remodeled bone and an implant. Surface roughness of titanium alloys have a significant effect on the bone apposition to the implant and on the bone implant interfacial pull out strength. The average roughness increased from 0.5 to 5.9 ?m and the interfacial shear strength increased from 0.48 to 3.5 MPa [Feighan et al., 1995]. Highest levels of osteoblast cell attachment are obtained with rough sand blast surfaces where cells differentiated more than those on the smooth surfaces [Keller et al., 1994]. Chemical changes of the titanium surface following heat treatment is thought to forma TiO2 hydrogel layer on top of the TiO2 layer as shown in (Figure .2). The TiO2 hydrogel layer may induce the apatite crystal formation [Kim et al., 1996]. In general, on the rougher surfaces there are lower cell numbers, decreased rate of cellular proliferation, and increased matrix production compared to smooth surface. Bone formation appears to be strongly related to the presence of transforming growth factor ?1 in the bone matrix [Kieswetter et al., 1996].

Figure .2 Chemical change of titanium implant surface of alkali following heat treatment [Kim et al., 1996].
2. Experimental work and procedure
It is involve the materials and equipment that are used in this work, and the sequence of operations and tests that have been accomplished.
The base alloy which used in this work are stainless steel and titanium alloy, all the above alloys should be of high purity (99.9 wt %) because they are for human use, and any undesirable elements may cause side effects on the human health.
The materials and alloys that are used in this work are:
1. High purity titanium alloy of 99.9 wt%, which was analyzed by XRD.
2. Stainless steel has the purity of 99.9 wt%, which was analyzed by XRD.
3. NaCl has purity of 99.8wt%
4. NaF of purity 99.99 wt% purchased from (Fluka Company).
5. H2SO4 high purity of 99.99 wt%.
2.1 Preparation of samples
The square samples of titanium and stainless steel cutting with dimension (10*10) mm, and grinding to prepare for testing which include:
2.1.1 X-Ray Diffraction Analysis
The X-ray diffraction analyses have been performed on stainless steel and titanium alloy to determine the existing phases in the samples. The X-ray diffraction device used is Lab, XRD-6000,Shimadzu supplied with single wave length Cu–K? is 1.54 A?, with nickel filter.
2.1.2 Microstructure and Observation
The microstructure of samples of stainless steel and titanium alloy have been observed and studied using optical microscope, the specimens were taken from each alloy and cold mounted, then wet ground using different grades of emery papers (180,220, 400, 800, 1000, 1200), the grinding operations were achieved using a rotary disk machine.
The microstructure took for specimens before and after corrosion test in different electrolyte solution was observed using optical microscope type (Nikon , Eclipse , ME 600 L , made in Japan).
2.1.3 Corrosion Test
In this work, there are three types of corrosion solution which have been imbedded the samples of stainless steel and titanium alloys. The electrolyte solutions used in corrosion tests, whose composition is shown in Table (2).
Before imbedded the samples in each electrolyte solution we take the original weights and original surface areas then after each 24 hour will go out these samples from electrolyte solution and washing with distilled water and alcohol then drying by hot air finally taken the weight by using sensitive balance, and put each sample in the same electrolyte solution which taken from it. After 15 days calculate the corrosion rate for each sample from this equation:
C.R=?w/A*t
Where:
C.R: Corrosion Rate.
?w: Weight Change (g).
A: Original Area (mm2).
t: Time (days).
Table (2) The Composition Of Electrolyte Solution.
No. Constituent Concentration
1 NaCl 5%
2 NaF 5%
3 H2SO4 10%
2.1.4 Surface roughness test
The surface roughness of samples of stainless steel and titanium alloy have been observed and studied by using surface roughness tester, the surface roughness taken for specimens before and after corrosion test in different electrolyte solution.
3. Result and dissection
3.1 X-Ray Diffraction
X-ray diffraction test was carried out on the specimens of stainless steel and titanium alloys to determine the existing phases in each specimen. The range of the diffraction angle (2??) was (20?-70?), the resulting phases are explained below.
3.1.1 X – Ray Diffraction
X-ray diffraction analyses of stainless steel has been studied in order to identify metal used in this study as illustrated in figure (3) and table (3).

Figure (3) illustrates X-ray diffraction results for stainless steel sample.

Table (3): illustrate (2??), d-spacing, phases, and (hkl) of stainless steel samples.
2?? d (measured) A? Phase hkl
37.6192 2.07336 ? (111)
44.4793 2.03524 ?? (110)
83.8239 1.22635 ? (200)
From figure (3) and table (3) can be shown the phases that appear in this samples which ? and ??.
While X-ray diffraction analyses of titanium has been studied in order to identify metal used in this study as illustrated in figure (4) and table (4).


Figure (4) Illustrates X-ray diffraction results for titanium sample.
Table (4): illustrates (2??), d-spacing, phases, and (hkl) of Titanium alloys.
2?? d (measured) A? Phase Hkl
35.000 2.5570 Ti (100)
36.750 2.3420 Ti (002)
40.255 2.2440 Ti (101)
55.870 1.7260 Ti (102)
65.755 1.4750 Ti (110)

X-ray diffraction technique is one of analytical techniques that reveal information about the crystallographic structure, chemical composition, and physical properties of materials. This technique is based on observing the scattered intensity of an X-ray beam hitting a sample. An important parameter is the wavelengths; X-rays have wavelengths in the order of angstroms, in the range of typical interatomic distances in crystalline solids. For this reason, X-ray diffraction (XRD) has provided information regarding the crystallographic structures in material, [Shuichi Miyazaki]. But also, it can be used to determine molecular structures. X-ray diffraction provided important evidence of atoms. XRD directed at the solid provides is the simplest way to determine interring atomic spacing. The intensity of the diffracted beams depends on the arrangement and atomic number, but also on the other parameters.
4.2 Topographic Observation
4.2.1 Topographic Observation of Stainless steel
From the below figures (5(a, b, c and d)) observed the surface of stainless steel samples can seen cracks from corrosion and results from corrosion which causes high value for roughness measurement tests than titanium alloys in same media.
Figure (5(a)) shows the topographic of stainless steel sample after grinding without any electrolyte solution.
Figure (5(b)) illustrates the topographic of stainless steel sample after imbedded 15 days in 5% NaCl electrolyte solution.
Figure (5(c)) shows the topographic of stainless steel sample after imbedded 15 days in 5% NaF electrolyte solution.
Figure (5m (d)) illustrates the topographic of stainless steel sample after imbedded 15 days in 10% H2SO4 electrolyte solution.

(a) (b)

(c) (d)
Figure (5) shows the topographic of stainless steel sample after grinding without any electrolyte solution and after imbedded 15 days in 5% NaCl, 5% NaF and 10% H2SO4 electrolyte solutions.

4.2.2 Topographic Observation of titanium alloy
From the below figures (6(a, b, c and d) observed the surface of titanium samples can seen mini cracks from corrosion and results from corrosion which causes low value for roughness measurement tests than stainless steel in same media.
Figure (6(a)) shows the topographic of titanium sample after grinding without any electrolyte solution.
Figure (6(b)) illustrates the topographic of titanium sample after imbedded 15 days in 5% NaCl electrolyte solution.
Figure (6(c)) shows the topographic of titanium sample after imbedded 15 days in 5% NaF electrolyte solution.
Figure (6(d)) illustrates the topographic of titanium sample after imbedded 15 days in 10% H2SO4 electrolyte solution.

(a) (b)

(c) (d)
Figure (6) shows the topographic of titanium samples after grinding without any electrolyte solution and after imbedded 15 days in 5% NaCl, 5% NaF and 10% H2SO4 electrolyte solutions.
4.3 Corrosion test:
Internal fluids have chloride ion concentrations about seven times higher than that of oral fluids. Figure (7) illustrates the corrosion behavior of the stainless steel and titanium samples after imbedded 15 days in 5% NaCl solution. Which the results illustrated titanium samples best corrosion resistance than stainless steel after imbedded 15 days in 5% NaCl solution because demonstrated the presence of a thick oxide film mainly composed of TiO2. It was mentioned that the oxide layer contributed to the higher resistance of localized corrosion in 5% NaCl solution, compared with that of the stainless steel that made conclude precepts salts on the surface of sample after imbedded 5% NaCl, that causes increasing in weight.
From above figures (5b and 6b) can be observed pits on the surface of stainless steel samples because attack of these electrolyte solution to the surface which effect to values of surface roughness test which display in table (5).

Figure (7) illustrates the corrosion behavior of the stainless steel and titanium samples after imbedded 15 days in 5% NaCl, solutions.
Due to its non-corrosive properties titanium has excellent biocompatibility. The material passivity itself in vivo by the formation of an adhesive oxide layer. Samples tested were covered mainly with the rutile type of TiO2,
Figure (8) illustrates the corrosion behavior of stainless steel and titanium samples after imbedded 15 days in 5% NaF solutions
From above figures (5c and 6c) can be observed pits on the surface of stainless steel and titanium samples because attack of these electrolyte solution to the surface which effect to values of surface roughness test which display in table (5).

Figure (8) illustrates the corrosion behavior of stainless steel and titanium samples after imbedded 15 days in c solutions.
The fluoride ions could cause the breakdown of the protective passivation layer that normally exists on the titanium and its alloys, leading to pit corrosion.[Schiff N, Grosgogeat B, Lissac M, Dalard F, 2002]. Fluorine was detected on the titanium surface immersed in the solution containing fluoride, and dissolution of the titanium was confirmed. Due to its high strength, low weight and non-corrosive properties, titanium and its alloys are used in a wide range of medical applications.
Figure (9) illustrates the corrosion behavior of stainless steel and titanium samples after imbedded 15 days in 10% H2SO4 solution.

Figure (9) illustrates the corrosion behavior of stainless steel and titanium samples after imbedded 15 days in 10% H2SO4 solutions.
Internal fluids have chloride ion concentrations about seven times higher than that of oral fluids. A diet rich in sodium chloride, added to large volumes of acidulated beverages (phosphoric acid), provides a continuous source of corrosive agents despite the relatively short exposure. Both sulfur dioxide and hydrogen sulfide have been found to accelerate the tarnishing and corrosion of metal implants. In addition to the above listed intraoral chemistry and electrochemistry, it is important to know how to simulate the intraoral environments when the in vitro chemical or electrochemical corrosion test is prepared and conducted. Figure (10) illustrates the corrosion behavior of stainless steel and titanium samples after imbedded 15 days in 5% NaCl, 5% NaF and 10% H2SO4 solutions.

Figure (10) illustrates the corrosion behavior of stainless steel and titanium samples after imbedded 15 days in 5% NaCl, 5% NaF and 10% H2SO4 solutions.
4.4 Surface roughness test
Table (5) illustrates the surface roughness test before and after corrosion test for stainless steel and titanium samples.
Table (5) illustrate the surface roughness of samples.
Samples Roughness value (µm)
Stainless steel 0.053
St.st. in NaCl 0.049
St.st. in NaF 0.123
St.st. in H2SO4 0.125
Titanium 0.066
Ti. in NaCl 0.120
Ti. in NaF 0.091
Ti. in H2SO4 0.072

The biocompatibility of Ti- derives from the formation of an oxide layer (TiO2) on the substrate surface. This is similar to the TiO2 on CpTi (commercially pure titanium), which enhances its biocompatibility as an implant material,[ Oshida Y, Miyazaki S., 1991]. The passivation layer can range in thickness from 2 nm to ~1 ?m [Trépanier C, Tabrizian M, Yahia L’H, Bilodeau L, Piron D L, 1998]. Resistance of this layer to damage correlates with the corrosion resistance and, hence, biocompatibility of the implants. Overall thickness of the passivation layer is less germane to biocompatibility than its uniformity,[ Filip P, Lausmaa J, Musialek J, Mazanec K, 2001]. Because the oxide layer is a brittle ceramic, the superelasticity of the Ti substrate can induce stresses in the passivation layer as the implant deforms, causing cracking and resulting in a pitting attack of the Ti substrate,[Villermaux F, Tabrizian M, Yahia L’H, Czeremuszkin G, Piron D L, 1996]. Obtaining the integrity of the passivation layer is paramount with nitinol to prevent the potential release of metallic Ni into the body. It has been established that Ni in vivo is highly toxic, producing severe inflammatory responses, along with being a potential carcinogen,[Ayers R A, Bateman T A, Simke S J, 2000]. The surface-preparation techniques have been shown to have a significant effect on the biocompatibility of the alloy,[Assad M, Lomardi S, Bernèche S, DesRosiers E A, Yahia L.H, Rivard C H,1994]. Ion release measurements on Ti–Ni alloy have shown that the initial rate of Ni ion release is high, but falls rapidly within 2 days, which is similar to that of Ni released from 316L stainless steel. Although 316L stainless steel contains only 8 wt%. Armitage et al.97 prepared Ti–Ni which was mechanically polished, followed by buff-polishing with the diamond paste, in order to study the influences of surface modifications on the biocompatibility, [Oshida Y, Miyazaki S., 1991].
5. Conclusion:
This work included the compartment of Titanium and stainless steel by surface roughness, Topography and corrosion test in different media such as NaCl, NaF and H2SO4, which the conclusions are as follows:-
1. The results of corrosion illustrated titanium alloy best of stainless steel in different corrosion media.
2. The surface roughness and topographic tests show the titanium is best when compared with stainless steel.


Acknowledgments
The Ministry of Higher Education, and Babylon University in Iraq is gratefully acknowledged. The experimental part was performed in the laboratory of Babylon University.

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