As an important functional material, titanium metal, with its advantages such as low density, high specific strength, and excellent corrosion resistance, is widely used in aerospace, energy, and medical applications. The development of medical titanium and titanium alloys can be roughly divided into three periods: the first period was represented by pure titanium and Ti-6Al-4V; the second period was α+β alloys, represented by Ti-5Al-2.5Fe and Ti-6Al-7Nb; and the third period focused on the development of β-type titanium alloys with improved bioperformance and lower elastic modulus. The application of new titanium alloy materials will be the current mainstream direction of medical device development.
Research on medical titanium alloys in my country began in the 1970s, with the development of Ti-2.5Al-2.5Mo-2.5Zr (TAMZ) by the Northwest Research Institute of Nonferrous Metals. In the 1990s, the Ti-6Al-4V, Ti-Al-2.5Fe, and Ti-6Al-7Nb materials, with independent intellectual property rights, were subsequently developed. The Chinese Academy of Sciences has also developed a new β-titanium alloy, Ti-24Nb-4Zr-7.6Sn. Current titanium alloy development in my country focuses on breakthrough new materials and the active application of titanium alloys.
Titanium Corrosion Properties
Titanium is a thermodynamically unstable metal with a relatively negative passivation potential, with a standard electrode potential of -1.63V. Therefore, it readily forms a passivating oxide film in the atmosphere and aqueous solutions, resulting in excellent corrosion resistance.
Titanium Corrosion Resistance in Different Media
Studying the corrosion resistance of medical materials is crucial. On the one hand, the penetration of some metal ions or corrosion products from implanted materials into biological tissues can trigger varying degrees of physiological reactions. On the other hand, the presence of body fluids can severely degrade the performance of certain materials, leading to rapid damage or even failure. The relatively complex human environment is prone to the dissolution of trace elements, altering the stability of the oxide layer. Slight friction can damage the passive film formed on the titanium surface to varying degrees. For example, in an oxygen-deficient environment, the oxide layer becomes less stable, and damaged oxide layers cannot be repaired or replaced, making it more susceptible to corrosion. This situation is almost unavoidable during repetitive human movement and the use of equipment. Plastic deformation alters the material's structural state, thereby affecting its corrosion resistance. Different degrees of plastic deformation have significantly different effects on the material's corrosion resistance. During plastic deformation, internal stress concentration creates defects at interfaces and within grains, thus weakening the material's corrosion resistance.
Titanium Corrosion Mechanism
Titanium is a Group IVB transition element with a relatively active chemical nature and a strong affinity for oxygen. In any oxygen-containing medium, a dense passive film readily forms on the titanium surface. This film is extremely thin, typically measuring a few to tens of nanometers thick. The presence of a titanium alloy passive film reduces the surface area available for active dissolution, slowing the dissolution rate and thus resisting damage caused by dissolution. Furthermore, the passive film is self-repairing; when damaged, it rapidly forms a new protective film. Therefore, titanium exhibits excellent corrosion resistance. Corrosion of titanium metal implanted in a living organism can be categorized as pitting, stress corrosion, crevice corrosion, galvanic corrosion, and wear corrosion.
Stress corrosion analysis
Stress corrosion refers to the phenomenon that metals crack 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 crack, 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 crack, forming cracks perpendicular to the tensile stress, and even leading to fracture.
1. Factors affecting stress corrosion of titanium alloys
The occurrence of SCC (stress corrosion cracking) in titanium alloys is the result of the combined action of three factors: environment, stress and material. SCC is highly selective. As long as any of the three factors mentioned above is changed, SCC will not occur.
(1) Environment
• Medium: Titanium alloys may undergo SCC under the action of many media such as aqueous solutions, distilled water, organic solutions and hot salts. The SCC mechanism is different in different media.
• pH value: There are still considerable differences in the effect of pH value on SCC of titanium alloys. 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, a strong corrosive environment with a pH value of 2-3 can even be formed in the front section of the local crack where SCC changes occur.
• Potential: The influence of potential on the degree of SCC is crucial. The corrosion system composed of the alloy and the medium is different, and its SCC sensitive potential is different.
• Temperature: Temperature is one of the important factors affecting the generation of SCC in titanium alloys. Generally speaking, the SCC sensitivity increases with increasing temperature. However, the temperature sensitivity of materials implanted in the human body is limited.
• Cl ion concentration: The higher the Cl- concentration in the solution, the greater its SCC sensitivity.
(2) Stress
SCC accidents caused by residual stress generated in the alloy during cold working, forging, welding, heat treatment or assembly account for 40% of the total SCC accidents. In addition, external stress generated during work or external stress caused by the volume effect of corrosion products may also cause the occurrence of SCC.
In summary, the corrosion performance of medical titanium is a key factor that must be considered when it is used as an implant material. By deeply understanding the corrosion mechanism of titanium and its performance in different environments, a scientific basis can be provided for the selection and design of medical titanium alloy materials, thereby ensuring its safety and reliability in practical applications.
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