Corrosion protection of steel by sulfo-doped polyaniline-pigmented coating

This work presents the corrosion protection behaviour of mild steel in neutral saline conditions (salt spray/immersion) by using coatings based on chlorinated rubber and benzene-sulfonate (BS) or lignosulfonate (LS)–doped polyaniline (PANI). Standardized accelerated (salt spray) and immersion tests were performed to assess the protective performance of the coatings. Analysis and interpretation of the experimental data (i.e., EIS, potentiodynamic data and visual observations) showed that both PANI–BS and PANI–LS inhibit corrosion of painted steel exposed to a 3.5% NaCl solution. Among the tested concentrations (0.5, 1.5 and 3 wt.%), the greatest inhibition was observed at low concentration of PANI–BS (0.5%). Samples with higher PANI–BS loadings (1.5 and 3%) appear severely corroded after 560 h of salt spray exposure, which was related to the release of corrosive benzene sulfonic acid. On the contrary, the coatings with lignosulfonate doped PANI performed well both in the salt spray and immersion tests, especially at the highest PANI concentrations (1.5 or 3%). With regard to the healing properties, PANI–LS with the lowest conductivity (1 S cm−1) performed the best.

• The corrosion protection of steel by sulfo-doped polyaniline pigmented coating is studied. Both benzene-sulfonate and ligno-sulfonate doped polyaniline inhibit corrosion of paint steel exposed to a 3.5% NaCl solution.
• A concentration below 1 wt.% is preferred with benzene sulfonate while higher concentrations are preferred with ligno-sulfonate.
• Sulfonated-lignin is mainly non-grafted to polyaniline chains which opens the way to an inhibitory effect by ligno-sulfonate dopants released upon reduction of PANI–ES.