Transpassive dissolution of iron in sulphuric acid solution

Abstract

Anodic oxidation characteristics of iron in 3, 10 and 12 mol dm-3 sulphuric acid solutions have been studied in the transpassive region. Dissolution current efficiency measurements have been carried out using potentiostatic current-voltage curves and solution analysis techniques. The current-voltage curves were split into metal dissolution and oxygen evolution curves assuming that the iron goes into solution as Fe3+. The current density value in the passive region increased whereas the current density in the transpassive region decreased with the increase of sulphuric acid solution concentration. In order to obtain information about the nature of the films present on the surface, potential decay curves from different anodic potentials in the transpassive region have been recorded. It seems that there is no passive film present on the specimen surface in 12 mol dm-3 sulphuric acid solution and a better surface finish is obtained after the dissolution. Depth profile analysis of oxide films by the AES technique in 3 mol dm-3 sulphuric solution reveals that the sulphur concentration is maximum at the metal/oxide interface rather than at the oxide/electrolyte solution interface as required by an ion exchange mechanism for film dissolution. © 1984 Chapman and Hall Ltd.