Tin passivation in alkaline media: Formation of SnO microcrystals ashydroxyl etching product

Abstract

The mechanism of the electrochemical passivation on Tin electrodes in 0.1 M NaOH is studied at low scanrates in a wide potential range. To this aim, tin oxide layers were grown on a polycrystalline tin surfaceunder potentiostatic conditions in both the active and passive electrochemical potential ranges, andcharacterized by field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), Ramanspectroscopy and electrochemical impedance spectroscopy (EIS). The results show that the first anodicprocess in the active region corresponds to the formation of a SnO·nH2O prepassive layer that is removedupon increasing the applied potential due to surface etching occurring at the metal/oxide interface. Duringthe etching process, Sn2+ions supersaturate at the electrode vicinity thus forming a SnO crystalline phaseon top of the electrode surface in the presence of the alkaline medium. At higher anodic potentials, nearthe passive plateau, the etching process ceases and the current drops due to the formation of a n-typeSn(IV)-based oxide at the metal/SnO interface that provides an efficient electronic passivation of theelectrode.