Studies on the formation process and anti-corrosion performance of polypyrrole film deposited on the surface of Q235 steel by an electrochemical method

The polypyrrole (abbr. PPy) film was deposited on the surface of Q235 steel by potentiostatic deposition at different constant potentials (0.75 V, 0.85 V, 0.95 V, 1.05 V and 1.15 V, respectively). Through comprehensive characterizations and investigations by FT-IR, SEM, EDX, XRD and I-t curves, it is proved that the deposition potential obviously affects the composition of the deposited PPy film by synchronously adjusting the dissolution rate of ferrous oxalate passivation layer and the polymerization and deposition rate of PPy. At the deposition potential of 0.75 or 1.15 V, FeC2O4 was embedded in the deposited films because of the mismatch of the above two types of rates. While the deposition potential was designed to be 0.85, 0.95 or 1.05 V, FeC2O4 did not exist in the deposited PPy films for the good rate match of passivation layer's dissolution and PPy's polymerization and deposition. On the other hand, deposition potential also affects the morphology of the as-deposited PPy films. Among the back three samples which without FeC2O4 in the film, PPy-Q235-0.95 has the smallest aggregate size and the highest film density. The results of anti-corrosion performance study through potentiodynamic polarization curve, electrochemical impedance spectroscopy indicates that PPy-Q235 series samples have superior anti-corrosion properties than bare Q235, and PPy-Q235 samples without FeC2O4 in the film have better anti-corrosion performance than those with FeC2O4 in. Moreover, PPy-Q235-0.95 shows the best anti-corrosion behaviors (with the maximum E-corr, the minimum I-corr and CR, as well as with the maximum R-ct, R-po, R-ox and the minimum C-dl, Q(po), C-ox) in this paper, which is mainly resulted from its smallest aggregate size and highest film density.