Corrosion protection of magnesium by electroactive polypyrrole/paint coatings

The effectiveness of the corrosion protection of an acrylic paint containing electrically conductive polypyrrole (PPy) (5, 10 and 20 parts) for a magnesium alloy (98.14% Mg and 1.64% Mn) has been investigated. Salt spray and electrochemical tests were conducted to examine the effect of PPy in acrylic paint on corrosion protection of the magnesium alloy. The salt spray test results demonstrated that the paint containing PPy coated magnesium samples showed insignificant corrosion and little blistering in a salt spray chamber after 1000 h exposure. The open circuit potential (OCP) measurements displayed large potential fluctuations in the control sample due to electrochemical activities in the defect, while the magnitude of fluctuations in the measured potential for the paint containing PPy coated sample were much smaller. The anodic potentiodynamic scans showed a displacement value of Epit−EOCP increasing from 57 mV for the control sample to 223 mV for the paint containing PPy (20 parts) coated sample. However, data obtained from the cathodic potentiodynamic scans and the electrochemical impedance spectroscopy (EIS) for the paint containing PPy coated sample showed large cathodic current densities and overall low impedance values, respectively. The high cathodic current densities were attributed to the cathodic reaction(s) with low overvoltage at the PPy particles. Such cathodic reaction(s) and the relatively high conductivity of the PPy lead to an overall decrease in the measured impedance values in the EIS scans.