Robust, polyaniline-based, multilayer composite films containing Prussian blue and hexacyanoferrate anions are considered here for the protection of stainless steel (SS) against pitting corrosion in strongly acid media. Polyaniline serves as a stable host matrix for large inorganic anions. At the interface formed by a composite film with SS, the interaction of hexacyanoferrate(II, III)(-) with iron(II, III), and some chromium(III), results in the formation of a thin insoluble metal hexacyanoferrate (mostly Prussian blue! layer. This phenomenon leads to improved passivation of the steel, and it strongly inhibits metal dissolution even in the chloride acid media. Because the metal hexacyanoferrate deposits are electronically conducting, further electropolymerization of dense and adherent composite polyaniline layers with hexacyanoferrate has been readily achieved. Due to the existence of electrostatic repulsion between negatively charged hexacyanoferrate sites and pitting-causing chloride anions, their transport through the composite film and access to the surface of SS are largely blocked. (C) 2003 The Electrochemical Society.