Smart PANI/epoxy anti-corrosive coating for protection of carbon steel in sea water

A double-layer coating composed of an inner polyaniline (PANI) and outer epoxy layer, was investigated as a potential "smart" coating for corrosion protection of carbon steel (CS) in an aqueous 3.5 wt% NaCl. The PANI coating, incorporating an anion of sodium caprylate (SC) and sodium dodecyl sulfonate (SDS), was electrochemically synthesized directly on the CS surface. A layer of commercial epoxy was then applied on top of this PANI coating. It was determined that the SC-SDS-doped coating was able to almost completely recover its anticorrosive properties, within one day upon creating the CS-surface-reaching damage (scratch) in it, sustaining the high level of corrosion protection for the remaining 29 days of constant exposure to the corrosive solution. On the other hand, the anti-corrosive properties of the damaged undoped coating deteriorated very quickly (within hours) and continued to deteriorate with time. The mechanism of "smart" protection of the doped coating was postulated to be through the release of the dopant (SC-SDS) from the PANI layer at the damaged coating site, driven by a potential difference between the reducing PANI layer and oxidizing (corroding) CS surface. This was followed by the adsorption of SC-SDS on the CS surface exposed to the electrolyte and its protection through the formation of a molecular SC-SDS layer offering a barrier for transport of corrosive species to the CS surface.