DEVELOPMENT OF A LOCALIZED ELECTROCHEMICAL PROPERTIES TEST METHOD： A STUDY OF CoCrMo AND Ti ALLOY LOCALIZED ELECTROCHEMICAL BEHAVIOR

Corrosion of metallic biomaterials in biomedically-relevant environments depends on the local physical and chemical structure of the metal surface at the metal-solution interface, and the geometry of the electrode system. Local surface structure, including defects, inclusions, and grain boundaries may locally alter the electrode behavior, while local geometries, including crevice-like geometries or small electrode areas will give rise to specific changes in electrochemical behavior. Methods to detect local defects or surface flaws electrochemically may be possible with the appropriate combination of microelectrodes and pipette geometries. In order to have a better understanding of the localized electrochemical behavior under given conditions, a localized electrochemical impedance spectroscopy (LEIS) technique was developed and evaluated along with standard polarization testing. CoCrMo and Ti samples were tested using this technique and under varying conditions to simulate small areas, and the presence of crevice-like geometries. The reference and counter electrodes were positioned within a small-diameter pipette tube with varying cross sectional diameters and varying heights from the working electrode surface. In addition, two conditions were examined with solution both inside and outside of the pipette or with solution only on the inside. These conditions were selected to approximate different electrode conditions, surfaces and geometries relevant to medical device surfaces. Significant differences in both impedance and polarization behavior were seen with this local electrode system depending on the absence or presence of external solution, the diameter of the pipette, the distance of the pipette from the working electrode surface and the materials tested. The polarization resistance of the working electrode for both Ti and CoCrMo decreased, and the corrosion current increased with increasing pipette cross sectional area (i.e., electrode area) when no external solution was present. In the external solution condition, Ecorr of CoCrMo samples became more negative and Rp,(the polarization resistance from the Tafel fitting result) also decreased with increasing exposed area, while the Icorr was not affected; for Titanium, the value of Ecorr, Icorr, and Rp changed relatively little compared to the behavior in the no-electrolyte-covered condition. The impedance behavior of this electrode system was affected by the presence of external solution or not. In the no-external-electrolyte condition, the response behaved according to a constant phase element Randles circuit with increasing area increasing the capacitance (CPE-T) for both CoCrMo and Titanium, while the polarization resistance Rp value for both CoCrMo and Titanium sample decreased. With external solution present, the impedance behavior followed a crevice corrosion model proposed by Swaminathan and Gilbert. Increasing the internal area, caused decreases in both internal pipette resistance (Ro) and crevice resistance (Rcr) for both Titanium and CoCrMo samples, while the internal capacitance (CPEo) increased, and external capacitance (CPEoc) as well as external surface resistance (Roc) changed relatively little compared to other parameters. In the external solution case, the distance between the microelectrode and the sample surface caused the crevice resistance, Rcr, to decrease exponentially as distance increased. There is a critical distance which leads to a sudden Rcr value drop.