In situ Monitoring of pH at the Electrode|Electrolyte Interface During the Anodic Dissolution of Iron in Acidic Solutions

摘要

The anodic dissolution of iron in H 2 SO 4 and HNO 3 solutions was studied by using scanningelectrochemical microscopy (SECM) with a Pt ultramicroelectrode modified with polyaniline and by insitu monitoring of pH at the electrode|electrolyte interface. In each acidic solution, pH at the interfacechanged periodically during the observed current oscillations of iron, indicating that the oscillatorymechanism had some connection with pH. During the oscillatory processes, there were two layers of thefilm on the surface of the iron electrode: a loose outer layer (Fe(OH) 2 and the salt film) and a dense innerlayer (Fe 3 O 4 ). First, the outer layer was formed gradually, and then the inner layer was formed under theprotection of the outer layer. With the back diffusion of H + , the outer layer was dissolved gradually,indicating that the outer layer was mainly the Fe(OH) 2 film; however, the inner layer was brokenimmediately after the dissolution of the outer layer. The results showed that pH of the solution played akey role and that the anions played a minor role during the current oscillations. It was reasonable todeduce that the oscillations were mainly caused by a periodic formation and dissolution of the oxidefilm. Furthermore, the salt film played a minor role even though the acid concentrations were high.