PAni based composite was synthesized, and the effect of n-TiO2 on the coating was analysed,
comparing its behaviour with that of the PAni coating. PAni/n-TiO2 was found to be a good
adherent and anti-corrosive coating. An increase in the adhesion and anticorrosive behaviour
was detected when the thickness coatings increased, thereby reducing the porosity of the
coating. The improvement with respect to the PAni coating resulted from increased adhesion to
the steel surface and hence the increased durability of the coating. The good adherent properties
are due to the nucleation process during the synthesis, where the n-TiO2 acts as seed for the
polymerization of PAni, decreasing the stress and increasing the adhesion when multiple layers
are deposited on the steel. The use of the PAni/n-TiO2 coating on the welded carbon steel
demonstrated that this layer is protective against corrosion on welded material
The anticorrosive behaviour of PAni-1 and PAni/n-TiO2-1 coatings were determined using
polarization curves in 3% sodium chloride medium. Figure 4a electrochemical polarization of
the coatings compared with bare steel. Both coatings present anticorrosive behaviour compared
with bare steel, being the more protective coating PAni-1, which present the lower current
density and a Ecorr close to zero. From these electrochemical meassurements the coating
porosity can be determined from relation (1), table I summarize the electrochemical parameters
and the porosity determine for each coating. As can be presented in table I, the good
anticorrosive properties of PAni coating is related with its lower porosity, indicating that PAni
coating shows a more compact structure than PAni/n-TiO2, where the polluted electrolyte can
penetrate through the coating and start the pitting corrosion process. After the electrochemical
tests, the coatings were removed, (using adhesive tape to remove the coating) and the steel surfaces were examined in order to examine the steel surface by optical microscopy (inset fig
4a). The images clearly show pitting corrosion process in the surface steel covered with PAni/n-
TiO2-1, which corroborate the higher current density determine by polarization test compared
with the steel cover with PAni-1 coating. However, the electrochemical tests performed in samples with higher coating thickness show different behaviour. Figure 4b show the
polarization curves where a decrease in anticorrosive behaviour of PAni-3, and an increase in
PAni-n-TiO2-3 can be observed. This phenomenon is associated with the defects generated in
the coating, which induce an increase in the porosity (table I). The increase of the porosity could
facilitate the ingress of chloride ions through the coating to the steel and start the pitting
corrosion process detected wen the coating is removed after the test (inset fig 4b). This effect
was associated with the presence of TiO2 in the composite that produce a reduction in the
porosity of the composite, a compactage and a better adhesion to the substrate, increasing the
protective behaviour [29]. The porosity of the film is a key parameter, and determine if the
coatings are suitable or not to prevent the corrosion of the substrate
To identify the differences between the most protective coatings (PAni-1 and PAni/n-TiO2-3),
an aggressive test of spray in a saline chamber was performed. Figure 6a shows the intense
corrosion suffered by the bare steel during the test. The images of samples covered with PAni-1
and PAni/-nTiO2-3 are presented in figures 6b and 6c, respectively. The examination of the
PAni-1 sample after the test indicates a loss of adhesion after six hours of testing, and the
coating was entirely removed during the post-cleaned test with distilled water. The steel surface
presented corrosion areas localized on the borders of the samples, which implies that the
corrosion process began in the most stressed areas of the coating and that the production of corrosion products induced a loss of adhesion, thereby reducing the durability of the coating.
However, the coating PAni/n-TiO2-3 presented good adhesion after the test, and no corrosion
products were observed after 48 h of treatment
To examine the application of PAni/ n-TiO2-3 in the corrosion protection of the welded area of
the carbon steel, measurements using the SVET technique were performed. Top of Figure 7
shows how the electrode was prepared for the SVET measurement. The SVET results showed
that after immersion, the film exhibited a cathodic behaviour (Fig. 7b), which was maintained
until 4 h of immersion (Fig. 7c). After 72 h (Fig. 7d), no anodic areas, represented by low
current were observed on the surface of the welded metal (left area). From these results, we can
conclude that the use of three layers of PAni/n-TiO2 is able to protect the metal and welded
metal areas against corrosion in a strongly corrosive medium