(Invited) Evaluation of Long-Term Corrosion Behavior of Metallic Biomaterials in Simulated Body Fluid By Electrochemical Impedance Spectroscopy
Authors: Tsutsumi Yusuke 
Published: December 2024

One of the major problems associated with the use of metallic biomaterials is their corrosion. Corrosion reactions, which are unavoidable in metals, cause not only deterioration or fracture of the products but also serious harm to living tissue. Nevertheless, metallic biomaterials are commonly used in the fields of medicine and dentistry because of their superior mechanical properties: an excellent combination of strength and ductility, resulting in high fracture toughness can not be substituted by other materials. Therefore, highly corrosion-resistant metals and alloys such as Ti, Ti alloys, Co-Cr alloys, and stainless steels are used for implant devices. All of them show superior corrosion resistance results from the formation of a passive film on the surface of the metal. However, very small quantities of the metallic ions actually dissolve through the passive film on these metallic biomaterials. There are already many literatures about the corrosion resistance of metallic biomaterials about the breakdown of the passive film. However, only limited information is available concerning the corrosion with extremely small rate accompanying metal ion release via stable passive film. Therefore, in this study, long-term measurement of in-situ electrochemical impedance spectroscopy, dissolution test, and surface analysis were performed to clear the difference in the corrosion behaviors of various metallic biomaterials.
Disk shaped specimens of pure Ti (Ti), Ti-33.5Nb-5.7Ta beta-type Ti alloy (Ti-Nb-Ta), Ti-Ti-50.8mol%Ni super elastic alloy (Ti-Ni), Co-20Cr-15W-10Ni alloy (L605, ASTM F90) (Co-Cr), type 316L stainless steel (316 SS), and pure Zr (Zr) were prepared. The specimens for corrosion tests were mechanically ground to #800 grit SiC abrasive paper. The specimens for XPS were additionally polished and mirror-finished with 9 μm diamond paste and 0.04 μm colloidal silica. The specimens were then ultrasonically cleaned in acetone and ethanol. In this study, the simulated body fluid, 5.85 gL-1 NaCl and 10.0 gL-1 lactic acid, as a corrosion test solution was prepared. The pH of the solution was checked in advance and was confirmed to be within the range of 2.30 ± 0.05 just after preparation. This solution was intended to be an accelerated body fluid for rapid testing of corrosion measurements of dental metallic materials prescribed by ISO 10271. EIS measurement was performed with an electrochemical measurement system. A couple of the specimen disks were prepared and embedded in cold-mounting epoxy resin. After grinding the surface, the electrodes were immediately immersed in the test solution and placed face-to-face at a constant distance. EIS measurement was started with an alternative potential of 10 mV in amplitude just 10 min after immersion. The measurement was continued for at least 48 h. For Ti-Nb-Ta, the measurement was further extended and continued for up to 28 d. The dissolution test was performed to evaluate the corrosion rate and amounts of the released metal ions during the immersion in the test solution. Twenty mL of the test solution was then put into the glass bottle, followed by the specimen. The bottle was completely sealed and kept for 7 d. The concentrations of the metal ions were measured with ICP-AES.
After the measurement of EIS, curve fittings could be performed using conventional equivalent circuit models. The corrosion rates of all specimens decreased during the immersion period. However, the declines in the corrosion rates of Co-Cr and Zr were much larger than those of Ti, Ti-Nb-Ta, Ti-Ni, and 316 SS. It means that the growth rate of the passive film of each alloy showed much different tendency in initial period. From the result of the dissolution test, it was found that the amounts of the released ions from 316L were much higher than those of another specimens. Ti-Ni released certain amount of Ni ions which are very hazardous for metal allergy. On the other hand, the amounts of the ions from Co-Cr and Zr were almost same level as detection limit of ICP-AES. It indicates that the protective performances against mass transfer via the passive films of Co-Cr and Zr are superior to those of other biomaterials. From the long-term EIS measurement of Ti-Nb-Ta, the maturation of the passive film originated from both film growth and compositional change was observed. The corrosion inhibition effects of Nb and Ta in Ti alloy was confirmed by long-term EIS measurement. Therefore, this technique provides a useful diagnostic of the long-term biosafety of metallic biomaterials, which are required to have an extremely higher level of corrosion resistance.