Comparison of Emeraldine Salt, Emeraldine Base, and Epoxy Coatings for Corrosion Protection of Steel During Immersion in a Saline Solution

The performance of polyaniline coatings (both emeraldine salt [ES] and emeraldine base [EB] forms) have been compared with epoxy coatings for corrosion protection of plain carbon steel in a saline solution. The corrosion rate was estimated by determining the concentration of dissolved iron in the test solution as a function of immersion time up to 30 h. While the EB and ES coatings produced higher iron dissolution rates compared with the epoxy coating, the behavior of the coatings was clearly different and short periods of reduced corrosion rate were evident for both ES and EB coatings. Visual inspection and ultraviolet visible spectroscopy studies indicated that the ES and EB were interconverted during the immersion period. Galvanic coupling of the steel to the ES resulted in reduction of the polymer to leucoemeraldine, which was subsequently re-oxidized by dissolved oxygen to EB. Conversely, the EB could be “doped” by locally high concentrations of dissolved iron to the ES. Periods of reduced corrosion rate corresponded to those times where the polymer was in the EB state, while acceleration of corrosion (compared with epoxy-coated steel) occurred when the polymer was in the ES state. The differences in corrosion rate caused by the different forms of polyaniline may be associated with their different inherent pH: the EB produces a highly alkaline environment that is conducive to passive oxide formation, while the ES produces a mildly acidic environment in which the formation of passive oxide layers is less likely.