Electrochemical studies on the anodic behavior of tin in citrate buffer solutions

Chronopotentiometry combined with impedance measurements are used to derive information about the structure and composition of the anodic film formed on tin in citrate buffer and to obtain properties of the anodic film itself. The results of the applied electroanalytical approach indicate that the passive film consists of a layered structure with at least three components. The inner layer is most likely SnO with a thickness of monomolecular dimensions, the thicker intermediate layer contains SnO2 and finally, the outer layer is primarily Sn(OH)4 with constant thickness. Structure and composition fully agree with a model of the passive layer on tin recently developed and based on XPS measurements under comparable working conditions. Moreover, excellent agreement was also found in relation to each layer thickness and thickness dependences on the anodization potential. Consequently, it must be emphasized that this study employs a valuable electroanalytical methodology for in situ analysis of passive films on metals.