Step-polarization impedance spectroscopy of implant alloys in physiologic solutions

A novel test method is presented whereby the polarization behavior and impedance characteristics of an electrochemical interface can be determined simultaneously from potential-step current transient responses. In this test, small incremental steps in potential are applied to an electrochemical interface and the current transient response is collected digitally. Then, the data are subjected to a numerical Laplace transform technique to obtain the frequency-dependent admittance (reciprocal impedance) of the interface. From this analysis, several interesting and relevant parameters, including the high- and low-frequency resistances, interfacial capacitance, and polarization behavior, can be obtained. The mathematical basis for this technique is presented and the methodology is applied to three implant alloys (titanium, Co-Cr-Mo, and platinum). Electrochemical tests were performed in 0.9% NaCl at room temperature. Starting at an initial negative potential, the samples were stepped in 50-mV increments every 10 or 100 s up to a maximum potential and then reversed back to the starting potential. The impedances were calculated and used to evaluate the behavior. From these tests, one can determine the potential dependence of the oxide film thickness as well as the changes in the underlying electrochemical state of the interfaces with potential. This technique is inexpensive and easily applied to any electrochemical system, and yields significantly more electrochemical information than either anodic polarization or electrochemical impedance spectroscopy alone.