Characterization of implant materials in fetal bovine serum and sodium sulfate by electrochemical impedance spectroscopy. I. Mechanically polished samples

Abstract

Electrochemical impedance spectroscopy is used to monitor the long-term stability (up to 150 days) of mechanically polished commercial pure titanium, Ti6Al4V, Ti6Al7Nb, and CoCrMo alloys in 0.1M sodium sulfate and fetal bovine serum. A capacitive spectrum in the frequency range from 10(-3) to 10(5) Hz is always found and the impedance spectra can be fitted by a simple parallel RC circuit with a constant phase element. The open circuit potential observed in serum is always more cathodic and the polarization resistance (R(p)) is higher than that recorded in sodium sulfate solutions. The observed variation of the equivalent capacitance in serum bovine suggests that an adsorption layer of organic molecules develops on the electrode surface and it is responsible for both the decrease in open circuit potential and the higher R(p), because it hinders the oxygen evolution reaction and the charge transfer responsible for the passive film dissolution (or growth). Among the alloys studied, Ti6Al4V displayed the highest steady-state values of R(p) both in serum and in sodium sulfate.

Characterization of implant materials in fetal bovine serum and sodium sulfate by electrochemical impedance spectroscopy. II. Coarsely sandblasted samples

“…Hence, the total overlapping between the two time constants is entirely attributed to the decrease in the total and tips' capacitances caused by the formation of a compact adsorption layer of organic molecules onto the passive film. 7 The adsorption layer originates a capacitor in series with the film capacitance (Fig. 10) determining the decrease in the experimental values of the equivalent capacitances, as experimentally observed in Figure 7.…”

“…Mechanically polished samples show a specific polarization resistance between 8 and 20 M⍀ * cm 2 in sodium sulfate; in bovine serum values up to 40 M⍀ * cm 2 are reported 7 [ Fig. 11(a)].…”

“…In contrast to mechanically polished electrodes that were freshly prepared before immersion in solutions, 7 sandblasted samples are covered by a stable oxide film, formed during the surface treatment and aged through a long air exposure. It can be assumed that the interaction with the electrolytes does not cause changes of the inner structure and composition of the oxide film of sandblasted samples.…”

“…7 Assuming the surface of mechanically polished surface as unity (equal to the geometrical area) and that the Al 2 O 3 particles from the sandblasting process occupy a negligible fraction of the electrode surface, the effective surface area of the sandblasted samples is calculated between 5.1 cm 2 (cp titanium) and 2.8 cm…”

“…In a previous article, 7 the authors studied the longterm stability of metallic implant materials in the mechanically polished state by impedance spectroscopy in sodium sulfate and in fetal bovine serum. In this study, electrochemical impedance spectroscopy is used to monitor the corrosion resistance of coarsely sandblasted commercially pure (cp) titanium, titanium alloys, and CoCrMo alloy immersed in sodium sulfate and fetal bovine serum, to our knowledge the first time.…”