Effect of conducting composite polypyrrole/polyaniline coatings on the corrosion resistance of type 304 stainless steel for bipolar plates of proton-exchange membrane fuel cells

A bilayer conducting polymer coating composed of an inner layer of polypyrrole (Ppy) with large dodecylsulfate ionic groups obtained by galvanostatic deposition, and an external polyaniline (Pani) layer with small SO42− groups obtained by cyclic voltammetric deposition was prepared to protect type 304 stainless steel used for bipolar plates of a proton-exchange membrane fuel cell. The corrosion performance of the bare and coated steel in 0.3 M HCl was examined by electrochemical impedance spectroscopy, polarization and open-circuit potential measurements. The experimental results indicated that both the composite Ppy/Pani coatings and the single Ppy coatings increased the corrosion potential of the bare steel by more than 400 mV (saturated calomel electrode), and increased the pitting corrosion potential by more than 500 mV (saturated calomel electrode). The bilayer coatings could reduce the corrosion of the alloy much more effectively than the single Ppy coatings, serving as a physical barrier and providing passivity protection, with acceptable contact resistance.