As an important functional material, titanium metal is widely used in aerospace, the energy industry, medical supplies, and other fields due to its advantages such as low density, high specific strength, and good corrosion resistance. The development history of the corrosion performance 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 α+β type alloys, represented by Ti-5A1-2.5Fe and Ti-6Al-7Nb; the third period is the development of β-type titanium alloys with better biological properties and lower elastic modulus as the main line of defense. The application of new titanium alloy materials will be the direction of the development of mainstream medical devices.
Corrosion performance 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 a passivating oxide film 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 performance of medical titanium. On the one hand, some metal ions or corrosion products of implanted materials penetrate biological tissues, which may cause physiological reactions of varying degrees; on the other hand, due to the presence of body fluids, the performance of some materials may be seriously reduced, causing them to be rapidly damaged or even fail. The human body environment is relatively complex, which makes it easier for trace elements to dissolve and change the stability of the oxide layer. Slight friction can cause the passivation film formed on the titanium surface to suffer varying degrees of damage. For example, in an oxygen-poor environment, the stability of the oxide layer is weakened. When damaged, it cannot be immediately repaired or a new oxide layer is formed, which is more likely to cause corrosion. This situation is almost unavoidable in the repeated movement of the human body and the use of instruments. Plastic deformation will change the tissue state of the material, thereby affecting the corrosion performance of medical titanium. Different degrees of plastic deformation have different effects on the corrosion performance of the material. During the plastic deformation process, due to the concentration of internal stress, defects are generated in the interface and grains. Therefore, plastic deformation will weaken the corrosion resistance of the material.
Corrosion mechanism of titanium
Titanium is a transition element of group IVB, with relatively active chemical properties and a great affinity with oxygen. In any oxygen-containing medium, a dense passivation film is easily formed on the surface of titanium. This passivation film is extremely thin, and its thickness is usually a few nanometers to tens of nanometers. The existence of the titanium alloy passivation film reduces the surface active dissolution area and slows down the dissolution rate, thereby resisting the damage caused by dissolution. In addition, the passivation film can also repair itself. When it is damaged, it can quickly form a new protective film. Therefore, the corrosion performance of medical titanium. The corrosion forms of metal titanium implanted in the body can be divided into pitting corrosion, stress corrosion, crevice corrosion, galvanic corrosion, and wear corrosion.
Stress corrosion analysis
Stress corrosion refers to the phenomenon that metals break when tensile stress and corrosion act at the same time. The general process is: that the action of tensile stress causes the protective film generated on the metal surface to begin to break, forming a crack source of pitting or crevice corrosion, and developing in depth. At the same time, the action of tensile stress can cause the protective film to break repeatedly, forming cracks perpendicular to the tensile stress, and even causing fractures.
Factors affecting stress corrosion of titanium alloys
The occurrence of SCC (stress corrosion cracking) in titanium alloys is the result of the combined effects of three factors: environment, stress, and material. SCC is highly selective. As long as any of the three factors is changed, SCC will not occur.
(1) Environment
• Medium: Titanium alloys may undergo SCC in 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 the 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, in the front section of the local crack where SCC changes occur, a strong corrosion environment with a pH value of 2-3 can even be formed.
• Potential: The effect 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 occurrence 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 during cold working, forging, welding, heat treatment, or assembly of alloys 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 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.
