B. G. Pound, Exponent, Menlo Park, CA
The susceptibility to localized corrosion of small implants is typically evaluated in a simulated physiological solution, such as phosphate buffered saline (PBS). For implants in contact with blood, the underlying premise is that proteins and other components in blood do not significantly change the corrosion susceptibility. This study examined the corrosion susceptibility of nitinol in bovine serum and PBS containing amino acids. Tests were performed on mechanically polished (MP) and black oxide nitinol wire using cyclic potentiodynamic polarization and electrochemical impedance spectroscopy. The impedance data were analyzed using equivalent circuit models to evaluate the capacitive and resistive components of the surface oxide with proteins and amino acids present. The cyclic polarization and impedance results were compared with previous results for MP nitinol in PBS to characterize changes in the corrosion behavior.
Summary: The susceptibility to localized corrosion of mechanically polished and black oxide nitinol was characterized in bovine serum and PBS containing different amino acids. Differences between the breakdown potential (Eb) and the corrosion potential (Ecorr) and between the protection potential (Ep) and Ecorr were used to evaluate the susceptibility to pitting corrosion and crevice corrosion, respectively. The impedance components showed considerable differences between serum and PBS, and also with the addition of amino acids to PBS. These differences were consistent with the adsorption of proteins and amino acids on the surface oxide. The impedance components exhibited a dependence on potential that differed between serum and PBS with and without the amino acids. Comparison of the cyclic polarization and impedance results with previous results for MP nitinol in PBS allowed changes in the corrosion behavior to be related to the effect of the proteins and amino acids.
