Role of ClO4- in breakdown of tin passivity in NaOH solutions

The anodic behaviour of a tin electrode in NaOH solutions containing differ ent concentrations of NaClO4 was studied by employing potentiodynamic, pote ntial transient under constant current density methods and complemented wit h scanning electron microscopy (SEM). In perchlorate-free NaOH solutions, t he E/i response exhibits active/passive transition. The active region invol ves two anodic peaks corresponding to the formation of Sn(II) and Sn(IV) sp ecies respectively. The permanent passive layer is duplex and consists of S nO and SnO2. Additions of NaClO4 to the alkali solution, accelerates the ac tive dissolution of tin and tends to breakdown the duplex passive layer at a certain breakdown potential. SEM examination confirms the occurrence of f ilm breakdown. The breakdown potential decreases with an increase in ClO4- concentration, but increases with increasing both OH- concentration and sca n rate. The potential-time transients display that the incubation time for pit initiation decreases with increasing both ClO4- concentration and anodi c current density. (C) 2001 Elsevier Science Ltd. All rights reserved.