Effect of Adhesion Promoter in Corrosion Protection of 1018 Mild Steel by Using Poly(3-Hexylthiophene) Coatings in 0.5 M H2SO4 Solution

The effect of an adhesion promoter (2-[thiophen-3-yl]ethyl phosphonic acid [C6H9O3PS]) in the corrosion protection of 1018 (UNS G10180) mild steel by using poly(3-hexylthiophene) (P3HT) coatings in 0.5 M sulfuric acid (H2SO4) solution was studied. Mirror-abraded steels were pretreated with the adhesion promoter. P3HT films were prepared by the drop-casting technique on pretreated and bare steels. The corrosion resistance of P3HT-covered steel substrates was evaluated using potentiodynamic polarization curves, linear polarization resistance, and electrochemical impedance spectroscopy. Results indicate that P3HT films improved the corrosion resistance of the steel in H2SO4 solution; however, the films deposited onto non-pretreated steel showed a better corrosion protection than the ones deposited on pretreated steel, even when the P3HT adhesion was improved. Equivalent electric circuit used to simulate the experimental impedance indicated that the corrosion mechanism in these materials is by charge-transfer process and diffusion process. The corrosion protection of the films is caused not only by acting as a barrier layer against aggressive ions from the environment, but also by improving the passive film properties of the substrate. Atomic force microscopy (AFM) micrographs revealed that P3HT films deposited onto mild steel without pretreatment had a more homogeneous surface with smaller grains and a less porous one than those deposited on pretreated mild steel.