Abstract: With the progress of society and economic development, especially the advancement of technology, the medical industry has achieved significant development achievements. Among them, the medical product materials developed in recent years have an irreplaceable role in treating and repairing human tissues, enhancing the function of human tissues and organs, and the application of metal titanium and titanium alloys is extremely extensive. Below, this article briefly analyzes the corrosion performance of medical titanium.
Keywords: medical titanium; corrosive; SCC； Hydrogen embrittlement of titanium alloys
Preface
Titanium metal, as an important functional material, is widely used in fields such as aerospace, energy industry, medical supplies, etc. due to its advantages of 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 characterized by α+β type alloys, represented by Ti-5A1-2.5Fe and Ti-6Al-7Nb; The third phase focuses on developing beta titanium alloys with better biological properties and lower elastic modulus as the main defense line. The application of new titanium alloy materials will be the current direction of mainstream medical device development.
The research on medical titanium alloy materials began in China in the 1970s. The Northwest Nonferrous Metals Research Institute developed Ti-2.5Al-2.5Mo-2.5Zr (TAMZ), and in the 1990s, they successively developed Ti-6Al-4V, Ti-Al-2.5Fe, and Ti-6Al-7Nb materials with independent intellectual property rights. The Chinese Academy of Sciences has also developed a new β titanium alloy Ti-24Nb-4Zr-7.6Sn. The current development of titanium alloys in China is mainly focused on breakthrough new materials and the active application of titanium alloy materials.
1、 Corrosion of titanium
Titanium is a thermodynamically unstable metal with a negative passivation potential and 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.
1. Corrosion resistance of titanium in different media
It is quite important to study the corrosion resistance of medical materials. On the one hand, the infiltration of some metal ions or corrosion products from implanted materials into biological tissues can trigger varying degrees of physiological reactions; On the other hand, due to the presence of bodily fluids, the performance of certain materials may significantly decrease, leading to rapid damage or even failure. The human environment is relatively complex, making it easier for trace elements to dissolve and alter the stability of the oxide layer. Slight friction can cause varying degrees of damage to the passivation film formed on the surface of titanium. For example, in an oxygen poor environment, the stability of the oxide layer is weakened, and when it is damaged, it cannot be immediately repaired or a new oxide layer is formed, which is more likely to cause corrosion. And this situation is almost unavoidable in the repeated movement of the human body and the use of equipment. Plastic deformation can alter the microstructure of materials, thereby affecting their corrosion resistance. The influence of different degrees of plastic deformation on the corrosion performance of materials varies greatly. During plastic deformation, defects are generated at the interface and in the grains due to the concentration of internal stress, which weakens the corrosion resistance of the material.
2. Corrosion mechanism of titanium
Titanium is a transition element in the IVB group, with relatively active chemical properties and strong affinity for oxygen. In any oxygen-containing medium, a dense passivation film is easily formed on the surface of titanium, which is extremely thin and usually has a thickness of several nanometers to tens of nanometers. The presence of passivation film on titanium alloy reduces the area of surface active dissolution, slows down the dissolution rate, and thus resists the damage caused by dissolution. In addition, the passivation film can also automatically repair and quickly form a new protective film when it is 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.
2.1 Stress Corrosion
Stress corrosion refers to the phenomenon of metal fracture caused by the simultaneous action of tensile stress and corrosion. The general process is as follows: the action of tensile stress causes the protective film generated on the metal surface to begin to rupture, forming a crack source for pitting or crevice corrosion, which develops in depth. At the same time, the action of tensile stress can cause the protective film to repeatedly rupture, forming cracks perpendicular to the tensile stress direction, and even leading to fracture.
2.1.1 Factors affecting stress corrosion of titanium alloys
The occurrence of SCC in titanium alloys is the result of the combined effects of environmental, stress, and material factors. SCC has a high degree of selectivity, and as long as any one of the three factors mentioned above is changed, SCC will not occur.
1) Environment
(1) Medium
Titanium alloys may undergo SCC in various media such as aqueous solutions, distilled water, organic solutions, and hot salts. The SCC mechanism varies in different media.
(2) PH value
There is still considerable disagreement on the effect of pH value on SCC of titanium alloys. In general, as the pH value increases, the sensitivity of titanium alloys to SCC decreases. When the pH value is 13-14, SCC can often be inhibited. But in the early stage of local cracks where SCC changes occur, a strong corrosive environment with a pH value of 2-3 can even be formed.
(3) Potential
The influence of potential on the degree of SCC is crucial. The SCC sensitivity potential varies depending on the corrosion system composed of alloys and media. When the potential of B-titanium alloy is around -600mV in an aqueous solution containing halides, SCC intensifies; Under the passivation potential, cracks will also occur; But no cracks appeared at potentials below -1000mV. In an aqueous solution containing Cl - and Br -, the SCC sensitive potential of Ti8Al1Mo1V is -500mV to -600mV. In aqueous solutions containing I -, the sensitive potential region is above 0mV.
(4) Temperature
Temperature is one of the important factors affecting the occurrence of SCC in titanium alloys. Generally speaking, as the temperature increases, the sensitivity to SCC increases. In a hot salt air environment of 300-500 ℃, the stress corrosion of Ti6Al3Mo2Zr0.5Sn alloy is more sensitive to SCC above 450 ℃. The SCC sensitivity of Ti6Al4V alloy with a certain amount of Pd or Mo added in a solution of H2S+CO2+NaCl+S is lower at 200 ℃ than at 250 ℃. But the sensitivity of materials implanted in the human body to temperature is limited.
(5) Cl ion concentration
The higher the concentration of Cl - in the solution, the greater its sensitivity to SCC.
2) Stress
Residual stresses generated during cold working, forging, welding, heat treatment, or assembly of alloys account for 40% of all SCC accidents. In addition, external stress generated during work or uneven stress caused by the volume effect of corrosion products or the volume effect of corrosion products are all sources of stress that cause SCC. The higher the stress level, the shorter the time for SCC to occur.
3) Materials
In the same environmental medium, if the chemical composition, segregation, microstructure, grain size, crystal defects, properties, heat treatment, and surface state of materials are different, their stress corrosion behavior and degree are also different. Adding a small amount of Pd, Mo, or Ru to titanium alloys can alleviate their stress corrosion sensitivity. The SCC sensitivity of Ti6Al4V and Ti15V3Cr3Al3Sn alloys subjected to peak aging treatment is higher than that of annealed state. When the oxygen content in Ti6Al4V alloy is below 0.13%, it can greatly reduce the sensitivity to SCC.
2.1.2 Common Solutions
The following methods can be used to eliminate or reduce the SCC sensitivity of titanium alloys in a certain medium:
1) Eliminate residual stress
Local residual stresses generated after component manufacturing can be eliminated through overall annealing or local annealing. At this point, the negative impact of heat treatment on material strength, plasticity, or toughness should be considered.
2) Alloying
For traditional alloys, appropriate amounts of Pd, Mo, or Ru can be added to the alloy according to the situation to improve its SCC resistance.
3) Surface treatment
By improving the surface quality of titanium alloys, the biocompatibility and wear resistance of the material can be enhanced, reducing and delaying the time and speed of crack formation.
2.2 Gap Corrosion
When the medium is in the gap formed between a metal component and a metal or non-metal, it can accelerate the corrosion of the metal inside the gap, which is called crevice corrosion. Gap corrosion is a type of localized corrosion. When there are gaps in titanium and titanium alloys, due to the lack of oxidizing substances in the gaps, they become anodes and corrode, damaging the passivation film. In general, crevice corrosion goes through three stages: ① consuming oxygen within the crevice; ② Forming a macroscopic battery with a decrease in pH value; ③ Activate and dissolve the passivation film until it is completely destroyed. Research has found that in Hanks' solution at 37 ℃, the degree of crevice corrosion of materials is ranked in descending order: NiTi>NiTiCu>316L>Ti6Al4V ≈ Ti; Ti and Ti6AI4V have strong resistance to crevice corrosion in Hanks' solution.
2.3 Wear and corrosion
Wear and corrosion refers to the accelerated corrosion of metals caused by the wear and tear of their surfaces due to the relatively high relative motion speed when they come into contact with a medium. When titanium is implanted as an implant, it will experience a certain degree of wear and tear with the operating instruments, causing the existing oxide film on the surface to be destroyed. If this oxide film cannot be repaired in time, the implanted metal will further corrode or even fail.
2、 Conclusion
Biomedical materials are an important material foundation for the rapid development of modern clinical medicine and a major topic in materials research in the 21st century. Titanium, as a new type of corrosion-resistant material, has made great progress and is widely used in the biomedical field due to its good biocompatibility and corrosion resistance. However, there are still many issues that need to be addressed in the application of titanium in the human environment. Therefore, in-depth research should be conducted on the various properties of titanium materials to design and accelerate the development of biomedical materials.
Reference:
[1] Qin Ying, Wang Shao'an. Study on the Corrosion Performance of Titanium and Titanium Alloy Oral Implants [J]. International Journal of Stomatology, 2008, 35 (4): 255-258
[2] Huang Yongguang. Titanium Alloy Materials for Surgical Implants and Their Standardization [J]. Advances in Titanium Industry, 2002, (1): 36-39
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