Corrosion Behavior of Chemically Deposited Single and Bi-layered Conducting Polymer Coatings on Mild Steel

Abstract

The emeraldine base (EB) was synthesized by chemically oxidative polymerization using ammonium persulphate as an oxidant in hydrochloride aqueous medium. The polymer was chemically deposited on mild steel specimens using tetra methyl urea (TMU) as solvent through solvent evaporation method. The coating of polypyrrole (PPy) on carbon steel was deposited by chemical polymerization. A bi-layered polymer coating comprising of inner coat of PPy with top coat of EB (PPy/EB) was also deposited on mild steel following identical procedure. The deposited EB, PPy and PPy/EB coatings were characterized by Fourier Transform Infrared (FTIR) Spectroscopy and Scanning Electron Microscopy (SEM). The anticorrosive properties of single and bi-layered coatings was investigated in major corrosive environments such as 0.1 M HCl, 5% NaCl solution, artificial seawater, distilled water, tap water and open atmosphere by conducting various corrosion tests which include: immersion test, open circuit potential measurements, potentiodynamic polarization measurements, and atmospheric exposure test. The results of immersion tests showed that the PPy/EB coating gave best protection in all media under investigation, the protection efficiency being in the range of 72 to 79% after 30 days of immersion. The result of OCP measurements showed significant positive shift in the corrosion potential for single as well as bi-layered coatings in all corrosive medium under investigation; the bi-layered coating showing more positive corrosion potential. The potentiodynamic polarization studies also confirmed lower corrosion rates for PPy/EB coating than the single polymer coatings.