1) Environment
Medium: SCC of titanium alloy may occur under the action of many media such as aqueous solution, distilled water, organic solution and hot salt. The SCC mechanism is different in different media.
PH value: there are still considerable differences in the influence of pH value on SCC of titanium alloy. Generally, with the increase of pH value, the SCC sensitivity of titanium alloy decreases. When the pH value is 13-14, SCC can often be inhibited. However, strong corrosive environment with pH value of 2-3 can even be formed in the front section of local crack with SCC change.
Potential: the effect of potential on the degree of SCC is very important. The SCC sensitive potential is different when the corrosion system composed of alloy and medium is different. For example, when the potential of b-titanium alloy in aqueous solution containing halide is near - 600mV, SCC will aggravate; Under the over passivation potential, cracks will also occur; However, when the potential is lower than - 1000mV, there is no crack. In the aqueous solution containing Cl - and Br -, the SCC sensitive potential of ti8al1mo1v is - 500mv - 600mV. In the aqueous solution containing I -, the sensitive potential region is above 0mV.
Temperature: temperature is one of the important factors affecting SCC of titanium alloy. Generally speaking, SCC sensitivity increases with the increase of temperature. Ti6al3mo2zr0.5% in hot salt air environment at 300 - 500 ℃ The stress corrosion of 5sn alloy is more sensitive to SCC above 450 ℃. The SCC sensitivity of Ti6Al4V alloy with a certain amount of PD or Mo in H2S + CO2 + NaCl + s solution at 200 ℃ is lower than that at 250 ℃. However, the materials implanted in human body have limited sensitivity to temperature.
Cl ion concentration: the higher the Cl - concentration in the solution, the greater the SCC sensitivity.
2) Stress
The residual stress caused by welding, forging or heat treatment in the whole process of SCC accounts for 40%. In addition, the external stress generated during operation or the external stress caused by the volume effect of corrosion products or the uneven stress caused by the volume effect of corrosion products are the stress sources of SCC. The higher the stress level, the shorter the time of SCC.
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 alloy can reduce its stress corrosion sensitivity. The SCC sensitivity of Ti6Al4V and ti15v3cr3al3sn alloys treated by peak aging is higher than that of annealed alloys. When the oxygen content in Ti6Al4V alloy is less than 0.13%, the SCC sensitivity can be greatly reduced.
2.1.2 common solutions
The following methods can be used to eliminate or reduce the SCC sensitivity of titanium alloy in a certain medium:
Elimination of residual stress: the local residual stress generated after parts manufacturing can be eliminated by overall annealing or local annealing. At this time, the negative effect of heat treatment on the strength, plasticity or toughness of the material should be considered.
Alloying: for traditional alloys, an appropriate amount of PD, Mo or Ru can be added to the alloy according to the situation to improve its SCC resistance.
Surface treatment: improve the biocompatibility and wear resistance of materials by improving the surface quality of titanium alloy, and reduce and delay the time and speed of crack generation.
2.2 crevice corrosion
When the medium is in the gap formed between metal parts and metal or non-metal, it can accelerate the corrosion of metal in the gap, which is called gap corrosion. Crevice corrosion is a kind of local corrosion. When there is a gap in titanium and titanium alloy, due to the lack of oxidizing substances in the gap, it becomes an anode and corrodes and destroys the passive film. Generally, crevice corrosion goes through three stages: ① consuming oxygen in the crevice; ② Form a macro battery and the pH value decreases; ③ The passive film is activated and dissolved until it is completely destroyed. It is found that in Hanks' solution at 37 ℃, the crevice corrosion degree of the material is as follows: NiTi > NiTiCu > 316L > Ti6Al4V ≈ Ti; Ti and Ti6AI4V have strong crevice corrosion resistance in Hanks' solution.
2.3 wear and corrosion
Wear corrosion is that when the metal and medium contact each other, the relative movement speed is large, resulting in the wear of the metal surface, which leads to the accelerated corrosion of the metal. When titanium is implanted as an implant, there will be a certain degree of wear with the operating instruments, resulting in the destruction of the oxide film on the surface. If this oxide film cannot be repaired in time, the implanted metal will further corrode or even fail.
Biomedical materials are the important material basis for the rapid development of modern clinical medicine and the main subject of material research in the 21st century. As a new type of corrosion-resistant material, titanium has made great progress. Because of its good biocompatibility and corrosion resistance, it is widely used in biomedical field. However, there are still many problems to be solved in the application of titanium in human environment. Therefore, the performance of titanium materials in all aspects should be deeply studied to design and start the faster development of biomedical materials.
