In vitro corrosion study by EIS of an equiatomic NiTi alloy and an implant quality AISI 316 stainless steel

The electrochemical impedance spectroscopy (EIS) technique was used for the study of the electrochemical behavior of an equiatomic NiTi alloy and an implant quality AISI 316 stainless steel type ASTM F138. Experiments were carried out using four different different test solutions: phosphate buffered saline (PBS), Dulbecco minimum essential medium (MEM), MEM + fetal calf serum (FCS), and MEM + fetal calf serum + fibroblast cell (CELL). Specimens were finished to 600-grit SiC paper and were tested in conditions that did not provoke abrupt mechanical damage of the passive film. Bode-phase spectra showed the presence of two maxima and were fitted with an equivalent circuit characterized by two parallel combinations (R, resistance; CPE, constant phase element). The R₁ and CPE₁ branch was assigned to the inner compact passive film and the R₂ and CPE₂ branch to the external porous film. The resistance of the inner film R₁, roughly corresponding to the polarization resistance (R_p), which is inversely proportional to the material's corrosion rate, increased with the immersion time and was generally greater in PBS than in other media. With the exception of FCS solution, R_p for NiTi alloy is better or similar to that of ASTM F138.