Electrochemical investigation of the passive behaviour of biomaterials based on Ag-Sn and Cu-Zn-Al in carbonate buffer in the absence and presence of chloride

The electrochemical behaviour of biomaterials based on Cu-Zn-Al (cubic CU3Zn phase) and Ag-Sn (orthorhombic Ag3Sn and hexagonal Ag4Sn phases) alloys was investigated in carbonate buffer solutions (pH 9.66) in the absence and presence of chloride, using different electrochemical techniques. Analyses of the open circuit potential and the potentiodynamic polarisation curves showed that the passivation domain and the corrosion parameters depend on alloy composition and chloride concentration. Chronoamperometric studies showed that passivation kinetics and corrosion of the passive film are both well described by a linear ln(i) versus ln(t) relation. The passive film formed on the Ag-Sn alloy is less susceptible to corrosion when compared to the Cu-Zn-Al system. The impedance data obtained in the passive region for the Cu-Zn-Al alloy showed that the passive layer is compact. In contrast, the impedance data obtained for the Ag-Sn alloy showed that the passive layer is formed by a compact oxide layer covered by a porous oxide gel layer. Mott-Schottky analysis showed that the passive film formed on the Cu-Zn-Al alloy behaves as a p-type semiconductor.