Breakdown of Passive Films and Repassivation of Ti-6Al-4 V Alloy with Rapid Elongation in Simulated Body Fluid including Osteoblast-like Cells

Abstract

Breakdown of passive films and repassivation of Ti-6Al-4 V alloy in a simulated body environment were examined using a rapid straining electrode test. Samples previously immersed in cell culturing medium with or without cells were elongated rapidly with a straining rate of 0.67 s−1. The transients of stress, strain and current were recorded during and after elongation. Larger charge density after elongation was observed on samples immersed longer in the simulated body fluid and covered with cells. It is supposed that the proteins and cells that adhered on the sample surface hinder the repassivation. An occluded space is formed between proteins and cells and substrate alloy. Since the diffusion is blocked in the occluded space, the pH becomes lower in this space as the result of hydrolysis of dissolved cations. Therefore, the repassivation is hindered and a localized corrosion occasionally occurred along slip step under cells and proteins. Localized corrosion occurred after elongation when applied potential is more noble than some critical potential, which could not be determined with conventional electrochemical tests.

Synergistic Effects of Plastic Deformation and Chloride Ions on Corrosion Initiation of Steel Rebars in Simulated Concrete Pore Solution

“…In our previous studies, the rapid increase in current density in the plastic region was attributed to slip step formation on metal substrates. 14,22,23,27 Figure 7 shows the SEM image of the surface of the tensile specimen, which was observed as prepared (Fig. 7a), after elastic deformation up to the strain of 1% (Fig.…”

“…Our research group has previously reported that mechanical stress breaks down the passive film and induces localized corrosion of metallic materials. [14][15][16] For example, Ti alloy and stainless steel, which are used as biomedical implants, form a passive film and show high corrosion resistance in the environment of the human body; however, the passive film is broken when it is subjected to fatigue stresses due to the movements of the body such as walking. 16 The implants that are exposed to mechanical stress are expected to have a shorter lifetime when compared to those without mechanical stress.…”

“…It is well known that Cl − breaks down the passive film and inhibits repassivation which is a repair process of the passive film. 14,[17][18][19] However, the effect of mechanical stress and Cl − on the repassivation of steel rebars in concrete has not yet been investigated.…”

Electrochemical Behavior of Type 316L Stainless Steel during Cyclic Deformation under Cell Culturing

“…Furthermore, proteins present in the body fluid may also adsorb on the newly created surface and suppress the repassivation rate. 21) …”

“…19) The dissolution and repassivation of Ti-6Al-4V alloy was previously investigated by the rapid straining electrode test, which showed that proteins and cells adhered on the sample retard repassivation and increase the charge density generated after elongation. 20,21) This means that, in the event of passivity breakdown, proteins and cells will accelerate metal dissolution. Cells and proteins may also hinder repassivation under cyclic stress, thus accelerating crack initiation and propagation, and eventually resulting in fracture of the material by corrosion fatigue.…”