Effect of mass transport on Si electrodissolution in aqueous hydrazine

The anodic dissolution and passivation of Si in aqueous hydrazine have been studied, with special emphasis on the effects of hydrodynamic conditions, by using a submerged impinging jet cell. Experiments, performed in the temperature range 20-50°C, show an anomalous current decrease on increasing mass-transfer rate, more pronounced at lower temperatures and observed both in the presence and in the absence of dissolved oxygen. It is shown that data may be interpreted by a mechanism similar to that previously proposed for Si dissolution in NaOH. As a result of electrode processes and of the electroneutrality constraints in a binary electrolyte, the equal surface concentrations of the two ionic species, OH- and N2H5+, increase above the bulk value; one of these species, most probably OH-, has a catalytic action on the dissolution steps and its local buildup promotes a larger current; increased mass transport, by reducing the surface concentration excess, reduces the current. The model explains the main experimental data, including the marked decrease of the mass-transport effect upon addition to the solution of significant concentrations of either NaOH or NaCl.