Mechanism of oxide film formation on iron in simulating groundwater solutions: Raman spectroscopic studies

In the use of iron for reductive dehalogenation of chlorinated solvents in ground water, the formation of surface films may cause long-term problems by reducing the activity of the metal surfaces or by causing the clogging of pores. Normal (NRS) and enhanced (SERS) Raman spectroscopy was used to identify the surface film(s) formed during contact of iron particles with solvent-free simulated ground water solutions in column experiments. It was found that anaerobic corrosion of iron leads to the initial formation of ferrous hydroxide at the beginning of the reaction. Independent of the ground water composition, however, the final corrosion product is magnetite. The spontaneous (no current applied) formation of magnetite takes place by a dissolution/precipitation mechanism with the separation of anodic and cathodic sites across the surface film. The cathodic reaction, which takes place at the porous film/solution interface, requires the film to be electron conducting.