Medical titanium analysis
As an important functional material, titanium metal is widely used in aerospace, energy industry, medical supplies and other fields due to its advantages such as low density, high specific strength and good corrosion resistance. The development process of medical titanium and titanium alloys can be roughly divided into three periods:
The first period is represented by pure titanium and Ti-6AI-4V; the second period is αβ-type alloys, represented by Ti-5A1-2.5Fe and Ti-6Al-7Nb; the third period is better biological properties and elasticity development of. Beta titanium alloys with lower modulus are the main line of defense. The application of new titanium alloy materials will be the development direction of current mainstream medical devices.
Research on medical titanium alloy materials in my country began in the 1970s. Northwest Nonferrous Metal Research Institute developed Ti-2.5Al-2.5Mo-2.5Zr (TAMZ). In the 1990s, Ti-2.5Al-2.5Mo-2.5Zr (TAMZ) with independent intellectual property rights was developed. -6Al-4V, Ti-Al-2.5Fe and Ti-6Al-7Nb materials. The Chinese Academy of Sciences has also developed a new beta titanium alloy Ti-24Nb-4Zr-7.6Sn. At present, the development of titanium alloys in my country is mainly aimed at breakthroughs in new materials and active applications of titanium alloy materials.
Corrosion of titanium
Titanium is a thermodynamically unstable metal with a relatively negative passivation potential, with a standard electrode potential of -1.63V. Therefore, it is easy to form an oxide film with passivation properties in the atmosphere and aqueous solution, and has good corrosion resistance.
Corrosion resistance of titanium in different media
It is very important to study the corrosion resistance of medical materials. On the one hand, some metal ions or corrosion products of implanted materials penetrate into biological tissues, which can trigger physiological reactions of varying degrees; on the other hand, due to the presence of body fluids, the performance of some materials may be severely degraded, causing rapid damage or even failure. The human body environment is relatively complex, which is more likely to cause the dissolution of trace elements and change the stability of the oxide layer. Slight friction will cause varying degrees of damage to the passivation film formed on the titanium surface. For example, in an oxygen-deficient environment, the stability of the oxide layer is weakened. When damaged, it cannot be repaired immediately or a new oxide layer can be formed, making it easier to cause corrosion. This situation is almost inevitable during repeated movements of the human body and use of equipment. Plastic deformation will change the structural state of the material, thereby affecting the corrosion performance of the material. Different degrees of plastic deformation have very different effects on the corrosion properties of materials. During the plastic deformation process, internal stress is concentrated, causing defects in the interface and grains. Therefore, plastic deformation weakens the corrosion resistance of the material.
Titanium corrosion mechanism
Titanium is a transition element of Group IVB. The chemical properties are relatively active and have a great affinity with oxygen. In any oxygen-containing medium, a dense passivation film is easily formed on the titanium surface. This passivation film is extremely thin, with a thickness usually ranging from a few nanometers to tens of nanometers. The existence of titanium alloy passivation film reduces the area of surface active dissolution and slows down the dissolution rate, thus resisting the damage caused by dissolution. In addition, the passivation film can be automatically repaired and a new protective film can be quickly formed when damaged. Therefore, titanium has good corrosion resistance. The corrosion forms of titanium metal implanted in living organisms can be divided into pitting corrosion, stress corrosion, crevice corrosion, galvanic corrosion and wear corrosion.
Stress corrosion refers to the phenomenon of metal cracks when tensile stress and corrosion act simultaneously. The general process is: the action of tensile stress causes the protective film formed on the metal surface to begin to rupture, forming a crack source of pitting corrosion or crevice corrosion, which develops deeper. At the same time, the action of tensile stress will cause the protective film to crack repeatedly, forming cracks perpendicular to the direction of the tensile stress. Cracks or even breakage.
Factors affecting stress corrosion of titanium alloys
The occurrence of titanium alloy SCC is the result of the interaction of three factors: environment, stress and material. SCC is highly selective. As long as any one of the above three factors is changed, SCC will not occur.
Titanium alloys may undergo stress corrosion cracking under the action of various media such as aqueous solution, distilled water, organic solution, hot salt, etc. The SCC mechanisms in different media are different.
There are still large differences in the influence of pH value on titanium alloy SCC. Generally speaking, as the pH value increases, the SCC sensitivity of titanium alloys decreases. When the pH value is 13-14, SCC can often be inhibited. However, in the front section of local cracks where SCC changes, a strong corrosive environment with a pH value of 2 to 3 may even form.
The influence of electrical potential on the degree of SCC is crucial. The corrosion systems composed of alloys and media are different, and their SCC sensitivity potentials are also different. For example, in B-titanium alloys in aqueous solutions containing halides, the SCC increases when the potential is around -600mV; cracks will also occur at over-passivation potentials; but at potentials lower than -1000mV, no cracks will occur. crack. In aqueous solutions containing Cl- and Br-, the SCC sensitivity potential of Ti8Al1Mo1V is -500mV--600mV. In an aqueous solution containing I-, the sensitive potential area is above 0mV.
Temperature is one of the important factors affecting the occurrence of SCC in titanium alloys. In general, as temperature increases, the susceptibility of SCC increases. In a hot salt air environment of 300-500°C, the stress corrosion of Ti6Al3Mo2Zr0.5Sn alloy is more sensitive to SCC above 450°C. The SCC sensitivity of Ti6Al4V alloy with a certain amount of Pd or Mo added in H2S CO2 NaCl S solution at 200°C is less than that at 250°C. However, materials implanted in the human body have limited sensitivity to temperature.
When the medium is in the gap formed between the metal component and the metal or non-metal, it can accelerate the corrosion of the metal in the gap, which is called crevice corrosion. Crevice corrosion is a type of localized corrosion. When there are gaps in titanium and titanium alloys, due to the lack of oxidized substances in the gaps, they become anodes and corrode, destroying the passivation film. Generally, crevice corrosion goes through three stages: ① The consumption of oxygen in the crevice; ② The formation of macro cells and the reduction of pH value; ③ The passivation film is activated and dissolved until it is completely destroyed. The study found that in Hanks solution at 37°C, the crevice corrosion degree of the materials in descending order is: NiTi>NiTiCu>316L>Ti6Al4V≈Ti; Ti and Ti6AI4V have strong crevice corrosion resistance in Hanks solution. .
Wear corrosion is when metal comes into contact with the medium, the relative movement speed is large, causing the metal surface to be worn, which in turn causes accelerated corrosion of the metal. When titanium is implanted as an implant, it will wear to a certain extent with surgical instruments, causing the oxide film existing on the surface to be destroyed. If this oxide film cannot be repaired in time, the implanted metal will further corrode or even fail.
Biomedical materials are an important material basis for the rapid development of modern clinical medicine and are the main topic of materials research in the 21st century. Titanium has made great progress as a new corrosion-resistant material and is widely used in the biomedical field due to its good biocompatibility and corrosion resistance. However, there are still many problems that need to be solved for the application of titanium in the human environment. Therefore, it is necessary to conduct in-depth research on various aspects of the properties of titanium materials to design and initiate faster development of biomedical materials.
refer to:
[1] Qin Ying, Wang Shaoan. Research on the corrosion properties of titanium and titanium alloy dental implants [J]. International Journal of Oral Medicine, 2008, 35(4): 255-258.
[2] Huang Yongguang. Titanium alloy materials for surgical implantation and their standardization [J]. Progress in Titanium Industry, 2002, (1): 36-39
