The energetic conditions determining the active dissolution of carbon steel during electrocoagulation in sulfate media

Abstract

This study aims to investigate the active dissolution of carbon steel under rotating conditions, necessary for continuous dosing of metal cation in electrocoagulation. A reaction mechanism is proposed for its dissolution in 0.1, 0.3 and 0.5M Na2SO4, using electrochemical and chemical techniques (Raman, SEM). A continuous dissolution region was obtained from the potentiodynamic study, as a result of the competition between the rate of electrochemical reactions and the rate at which energetic conditions are imposed on the electrode. On the other hand, the dissolution of carbon steel underwent active, transition and passivation regions in potentiostatic and galvanostatic techniques, which respectively shifted to higher current densities and more positive potentials as the electrolyte concentration (Na2SO4) was increased. In general, the increase of Na2SO4 concentration promotes the iron dissolution and tightens the active region. The results revealed that the Na2SO4 concentration, the Fe(II)/Fe(III) interface concentration, and the time of perturbation influence the occurrence and evolution of the Green Rust Sulphate (NaFe6IIFe3IIIOH18SO42) intermediary, a precursor which hinders electrocoagulation through passive film formation. Scanning Electronic Microscopy and Raman spectroscopy showed that in the active dissolution region, the films grown potentiostatically are porous and contain α-FeOOH, δ-FeOOH, and Fe3O4 as the dominant species. In the transition region, these films become less porous and include α-FeOOH, Fe3O4, α-Fe2O3, γ-Fe2O3, while γ-FeOOH was identified in the passive region.