Anodic passivation of tin in buffered phosphate electrolyte

Abstract

The anodic behaviour of tin in buffered phosphate electrolyte (pH=3.1) has been studied by a variety of techniques. A number of anodic processes occur depending on potential and the conditions at the electrode/electrolyte interphase. On anodic polarisation the electrode, which is probably filmed with a phosphate layer, initially undergoes dissolution to form probably Sn(H2PO4·HPO4)− species. Impedance data indicate that this process has a corresponding Tafel slope of ∼0.046 V/decade. At more positive potentials three consecutive passivating processes occur. The primary passivating process involves the blocking of the electrode by Sn3(PO4)2 by a dissolution-precipitation mechanism. The formation of SnO by a slow three dimensional nucleation and growth process constitutes the second. It is formed as a result of the attainment of alkaline conditions at the electrode surface. There is also a parallel reaction path involving the formation of soluble Sn(II) species. The tertiary process consists of the oxidation of Sn to Sn(IV) species. Passivation occurs via a dissolution-precipitation mechanism when the electrode is blocked by SnO2. The relative quantities of SnO and SnO2 produced is a function of operating conditions.