Corrosion Protection of Mild Steel Using Sulfonic and Phosphonic Acid-Doped Polyanilines

Growing environmental concerns regarding the use of heavy metals in coating formulations have led to a new coating strategy using inherently conducting polymers (ICP), such as polyaniline (PANI), as a key component. The principal potential advantage offered by the ICP coating technology is toleration of pinholes and minor scratches. This paper introduces the use of new PANI formulations that are doped with phosphonic acids. Salt fog exposure tests point to phosphonic acid salts of PANI being more effective for corrosion protection than traditionally used sulfonic acid salts. Scanning reference electrode technology (SRET) data support the salt fog results in that the sulfonic acid dopants exhibited an increasing galvanic activity with time while the phosphonic acid dopants showed a decrease in activity with time, indicating passivation. A qualitative model is proposed which entails passivation of the metal surface through anodization of the metal by PANI and formation of an insoluble iron-dopant salt at the metal surface.