Behaviour of sodium silicate and sodium phosphate (tribasic) as corrosion inhibitors for iron

Abstract

The effectiveness of sodium silicate (Na2SiO3) and trisodium phosphate (Na3PO4) as corrosion inhibitors for iron in 0.1 m NaClO4, in both aerated and deaerated solutions has been investigated. Both sodium silicate and trisodium phosphate are effective inhibitors in unbuffered solutions as a result of inhibiting the rate of the anodic iron dissolution reaction. However, the inhibition of the corrosion rate in the (final) pH range 9.6–11.6 is entirely due to the change in pH which the addition of Na2SiO3 and Na3PO4 to unbuffered solutions causes, and not to the presence of silicate/phosphate species. Where the final pH was in the range 5–7 addition of I mm sodium silicate to acidified aerated unbuffered solutions caused slight inhibition of the iron dissolution reaction when compared with a solution of the same pH. Lower concentrations of Na2SiO3 at the same final pH did not cause inhibition. Silicic acid has an approximate solubility of 1 mm and it is likely that the inhibitive behaviour in this pH range is due to the precipitation of H2SiO3 at the electrode surface. An aerated phosphate buffer of pH 7.0 inhibited both the anodic and cathodic reactions when compared with a 0.1 m sodium perchlorate solution at the same pH. This was due to the presence of phosphate species at the electrode surface. In carbonate and acetate buffers neither Na2SiO3 nor Na3PO4 caused corrosion inhibition.