The Behaviour of Phosphate Ions as Corrosion Inhibitor for Fe(Ⅱ)―Fe(Ⅲ) Hydroxycarbonate Green Rust

摘要

The oxidation of Fe(Ⅱ)―Fe(Ⅲ) hydroxycarbonate green rust GR(CO_3~(2-)) in the presence of phosphate ions known as corrosion inhibitor is monitored by transmission Moessbauer spectroscopy. In the absence of phosphate, the first product of oxidation, previously called "amorphous active FeOOH" was identified as ferrihydrite which transformed by dissolution to goethite. The oxidation of GR(CO_3~(2-)) in the presence of phosphate ions, which were added as hydrated phosphate salt, also involved the formation of ferrihydrite but not that of goethite. The dissolution of ferrihydrite is inhibited by the adsorption of phosphate ions as confirmed by X-ray photoelectron spectroscopy. The oxidation was slowed down and interpreted as due to the suppression of the catalytic effect of iron(Ⅲ) hydroxide upon the oxidation of Fe(Ⅱ).