The adsorption of anions on iron during anodic passivation in neutral solution

Abstract

The adsorption of phosphate and ο-iodohippurate ions on iron surfaces has been measured at varying potential during anodic passivation in near neutral solution. Considerable adsorption of both anions occurs in the active potential region, this adsorption being potential dependent. The phosphate adsorption is about three times that of ο-iodohippurate at the same potential. The adsorption of phosphate ions increases with pH in the range 7–9 and with increasing concentration in the range 4 × 10⁻⁵ to 4 × 10⁻⁴ M. When the iron surface is converted from the active to the passive state, the surface coverage with adsorbed anions is an appreciable fraction of a monolayer. The larger the anion adsorption on the active surface, the more easily does passivation occur. Considerable uptake of phosphate ions occurs into the passive film on the iron surface, indicating that passivation is associated with a three-dimensional film.