Uptake of Phosphate Ions by Oxide Films on Mild Steel

Abstract

phosphate ions were foud by radiotracer methods to be firmly bound by oxide films on mild steel after immersion in 0·1 M sodium phosphate over the pH range 7–13. Under inhibitive conditions, autoradiography showed.that the major part of the phosphate uptake was distributed uniformly over the surface, but localy increased uptake occurred to a variable extent at points and areas of the surface. At pH 8, approximately a monolayer of phosphate ions was taken up initially by the oxide. Thereafter the phosphate ion uptake increased linearly with the logarithm of time of immersion, this uptake being decreased by icreasing time of exposure to air before immersion. Increasing the pH decreased the amount of imtialy uptake of phosphate ions. The results are consistent with ion exchange of phosphate ions initially with the surfce layer of the air-formed oxide film and continuing during the thickening of the oxide film in the solution.