The Role of Surface Adhering Organic Substances on Metal Dissolution and Repassivation during Cyclic Deformation of Ti-6Al-4V Alloy

In the body environment, passive film is broken by cyclic stress induced by the motion of body, then newly created surface is exposed to the body fluid which includes Cl^- ions and organic substances. Consequently, corrosion fatigue occurs. However, the role of the organic substances such as proteins and cells on the passivity breakdown and repassivation has been little clarified. In this work, therefore, we examined the passivity breakdown and repassivation behavior of Ti-6Al-4V alloy with cyclic deformation in simulated body fluid including proteins and cells. The samples of Ti-6Al-4V alloy for tensile test with a gauge size of 8×4×1 mm³ were mirror-finished and sterilized in an autoclave at 121 °C for 900 s. Then samples were immersed in a-MEM+10 % FBS which is commonly used for cell culturing. The immersion period was 1day or 1 week ((1) 1 day immersed, (2) 1 week immersed). In order to examine the role of cells adhering on the samples, osteoblast-like cells (MC3T3-E1) were cultured on some samples for 1 week ((3) 1week immersed with cells). Samples without immersion ((4) in the air) were examined for comparison. Then a cyclic stress which was modulated as sinusoidal wave was applied to the samples. The stress ratio was 0.1 and the maximum stress was 400~800 MPa, which did not exceed the elastic region of Ti-6Al-4V alloy. The stress frequency was 10 Hz. During the test, the samples were accommodated in the electrochemical cell filled with a-MEM + 10 % FBS kept at 37 °C under atmosphere controlled as 5 % CO₂, 20 % O₂ and 75 % N₂ (1 day immersed, 1 week immersed and 1 week immersed with cells) or in the air kept at 28 °C (in the air) . The transients of strain, stress and corrosion potential were recorded with time.  Figure shows the fatigue life cycles of samples under the four conditions. The fatigue life cycles of the samples deformed in the solution were shorter than the samples deformed in the air, because passivity breakdown and metal dissolution enhanced the growth of fatigue crack. Therefore, the fatigue lives of 1 week immersed and 1 week immersed with cells were shorter than that of 1 day immersed. This result indicates that the proteins and cells decrease the fatigue life of Ti-6Al-4V alloy. When the passive films are broken by cyclic deformation, the corrosion potential shifts less noble and when the area of newly created surface decreases by reapassivation, the corrosion potential shifts noble. The corrosion potential of 1 day immersed shifted less noble at only first cycle and then shifted monotonously noble direction. On the other hand, for 1 week immersed and 1 week immersed with cells, the corrosion potential continued to shift less noble until about 1000th cycle and then shifted noble. These results indicate that proteins and cells adhered on the sample surface hinder the repassivation. It is considered that the hindrance to repassivation by protein and cells led the less fatigue life of Ti-6Al-4V alloy.