| Titanium is one of the biological materials with highly-safety performance. With the development of cast and process technology, titanium and its alloy has been widely used in stomatology fields. Pure titanium is characterized by such merits as good biocompatibility, little mass, low cost and so on, becoming one of ideal dental materials. However, in order to enlarge the application of titanium and its alloy in prosthodontics fields and to make up the deficiency of commercial pure titanium in its strength, hardness and corrosion resistance, the author studied Ti-6Al-7Nb alloy, which is superior to pure titanium in strength and hardness. In the mid 1980s, a new titanium alloy named Ti-6Al-7Nb was developed by Switzerland. The microstructure and properties of Ti-6Al-7Nb was similar to Ti-6Al-4V, however, Ti-6Al-7Nb's composition does not have potential biological toxicity. In addition, Ti-6Al-7Nb was up to the ISO standard so that it could be rapidly used into clinical treatment. Since the application of metal as prosthodontics materials, the corrosion problem has become been becoming the long-standing problem of people. By comparing with commercial-type titanium TA2 prosthodontics and other commonly used alloys in the aspects of galvanic corrosion, stress corrosion cracking and static corrosion immersion experiments, this paper evaluates the corrosion resistance properties of Ti-6Al-7Nb comprehensively and analyzed the impact of its corrosion resistance factors, providing experimental basis for Ti-6Al-7Nb alloy as casting alloy used in the dental restoration fields.Contents and Methods:1. The free corrosion potential (Ecorr) and Tafel curve of Ti-6Al-7Nb, TA2, Co-Cr and Ni-Cr in 37℃artificial saliva and the galvanic current density of three galvanic couples between Ti-6Al-7Nb alloy and TA2, Co-Cr alloy are detected by using electrochemical method, and the anticorrosive property of three galvanic couples is analyzed statistically.2. To evaluate the corrosion resistance of Ti-6Al-7Nb alloy with pure titanium under loading in corrosion environment, the Ti-6Al-7Nb alloy and pure titanium (TA2 type) for five levels are loaded in acid fluoride solutions by using sustained tensile-loaded fixture and immerge, and the microstructure, chemical composition and crystal structure of corrosion products are observed with scanning electron microscopy and X-ray diffraction instruments.3. the ion precipitation situation of Ti-6Al-7Nb alloy and pure titanium in artificial saliva, 1% lactic acid solution, 2% NaF artificial saliva (pH = 7) and 2% NaF artificial saliva (pH = 3) is studied to analyze the influence of pH value and fluoride ions on metal ion precipitation. Meanwhile, the microstructure, element composition, crystalline structure and ion precipitation concentration of corrosion products are observed and detected by using scanning electron microscopy, energy dispersive spectrometer, X-ray diffraction device and inductively coupled plasma mass spectrometer.Results:1. The Ecorr values (mV) of Ti-6Al-7Nb, TA2, Co-Cr and Ni-Cr are -111.8±0.02, -156.6±0.03, -246.2±0.07 and -298±0.08 respectively (P<0.01). The Rp values (M? cm2) of Ti-6Al-7Nb, TA2, Co-Cr and Ni-Cr are 0.96±2.02,0.74±0.97,0.13±0.6 and 0.10±0.88 respectively (P<0.01). The Icorr values (μA/cm2) of Ti-6Al-7Nb, TA2, Co-Cr and Ni-Cr are 0.77±1.25, 2.67±0.71, 21.36±1.04 and 23.22±0.33 respectively (P<0.01). The galvanic current densities(μA/cm2) of three galvanic couples between Ti-6Al-7Nb alloy and TA2, Co-Cr alloy, Ni-Cr alloy are 0.46±0.21, 1.37±0.90 and 2.25±1.52 respectively (P<0.01).2. In the stress and the combined effect of fluoride ion, the surface of pure titanium and Ti-6Al-7Nb alloy in each group generated corrosion products. However, at the same level of stress load, the product of Titanium Group corrosion is more than that of Ti-6Al-7Nb alloy group, and the pitting pit volume titanium group exceeds that of Ti-6Al-7Nb alloy group. the outcome under Scanning electron microscopy shows that the pitting pit and the diameter of pit of titanium group is deeper and larger than that of Ti-6Al-7Nb group. The corrosion products of pure titanium are mainly Na3TiF6 and Na2TiF6 while Ti-6Al-7Nb alloy corrosion products are mainly Na3TiF6, Na3AlF6 and Na2NbF6 under X-ray diffraction's analysis.3. The surface color of each group immersed in artificial saliva and lactic acid solution does not change significantly while color of the specimen surface darkens and grays out in neutral fluoride solution and fluoride acid solution. The color of titanium group changes most obviously. X-ray diffraction analysis In the neutral fluoride solution group, Ti-6Al-7Nb alloy corrosion products are Na3TiF6 and Na3AlF6 while pure titanium Group corrosion products are Na3TiF6. In the Acidic fluoride solution group, Ti-6Al-7Nb alloy corrosion products are Na3TiF6,Na3AlF6 and Na2NbF6 while pure titanium Group corrosion products are Na3TiF6, Na2TiF6 and Na3TiOF5 respectively under X-ray diffraction's analysis. In the neutral and acidic fluoride solution group, Ti-6Al-7Nb alloy corrosion products are the main elements Ti, Al, Nb, Na, F, K and Si whlie pure titanium elemental composition of corrosion products are mainly Ti, Na, F, K and Si under Spectroscopy analysis. Ti-6Al-7Nb titanium alloy ion precipitates respectively vary from 16.1ng/ml, 170.5ng/ml, 20.7μg/ml and 218.3μg/ml, lower than the pure titanium 62.8ng/ml, 371ng/ml, 32.5μg/ml and 301.2μg/ml in artificial saliva, lactic acid, neutral and acidic solution of fluorine-containing. Titanium ion is the largest amount of precipitation in acid fluoride solution.Conclusions:1. Ti-6Al-7Nb alloy have the best corrosion resistance properties in four kinds of alloys. The best materials that coordinate with Ti-6Al-7Nb alloy is pure titanium, Co-Cr alloy and Ni-Cr alloy in turn. The galvanic corrosion between Ti-6Al-7Nb alloy and common dental alloys in artificial saliva is not taken into account.2. Under the joint effect of stress load and corrosion environment, both Ti-6Al-7Nb alloy and pure titanium generate stress corrosion, and the more the loading stress the severer the corrosion, but the corrosion of Ti-6Al-7Nb alloy is lighter than that of Titanium. Hence, Ti-6Al-7Nb alloy is more excellent in the stress corrosion resistance than that of pure titanium. 3. Solution to lower pH values and the existence of fluoride ion can accelerate the release of Ti ions. Soaking solution at all, Ti ion concentration in precipitation of Ti-6Al-7Nb alloy was lower than that of pure titanium. Ti-6Al-7Nb alloy performs well in acid and fluoride environment than pure titanium. Therefore, the corrosion-resistance of Ti-6Al-7Nb alloy is higher than that of pure titanium.
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