Polypyrrole coating doped by molybdo-phosphate anions for corrosion prevention of carbon steels

Abstract

The polypyrrole coating was prepared from phosphoric acid aqueous solution containing the hetero-polyanion ${\text{PMo}}_{{{\text{12}}}} {\text{O}}^{{{\text{3 - }}}}_{{{\text{40}}}}$ and pyrrole monomer to make new coating for corrosion prevention of carbon steels. The coating thus formed in the phosphate acid solution was doped by ${\text{PMo}}_{{{\text{12}}}} {\text{O}}^{{{\text{3 - }}}}_{{{\text{40}}}}$ and by ${\text{PO}}^{{{\text{3 - }}}}_{{\text{4}}}$. The coating was flexible enough to cover the steel homogeneously without cracks, although many cracks were observed on the coating formed in a neutral aqueous solution of Na₂MoO₂. The 5.1-μm-thick polypyrrole coating makes the steel maintain the passive state for 48 h in neutral 3.5 wt% NaCl solution at pH 5.3 and for 80 h in acidic 3.5 wt% NaCl solution at pH 1.9. The coating decreased the corrosion rate of the steel by 1/200 in the neutral NaCl solution and by 1/340 in the acidic NaCl solution, if compared with the rate of the bare steel. The dissolution current of the steel during the immersion remained at the level of the typical passive current in the respective solutions.