Characterization of anodic tin passive films formed in citrate buffer solutions

Voltammetry, chronoamperometry and SEM were employed to derive mechanistic information about the electroreduction processes that take place after cathodic polarization of a previously oxidized tin surface. This information proved useful to gain a deeper insight into the complex passivation process as well as into the passive layer changes generated by the anodizing procedure. Tin electrodes were anodized in citric–citrate buffers with pH 5. Three cathodic contributions were detected in the voltammetric response and they were correlated with three maxima present in the electroreduction current transients. Two electroreduction processes correspond to the electroreduction of tin surface species with different oxidation state and generated during the anodic potential sweep within the secondary passivity range. From these two processes only the electroreduction of the Sn(IV)-containing anodic film is dependent on anodizing time and hydrodynamic conditions of the electrolyte. Synchronized with the latter reaction occurs the electrodeposition of tin metal from a Sn(IV)-complex in solution.