Coatings containing polyaniline emeraldine salt (ES) or base (EB) dispersed in organic binder were applied to Al, Fe, Zn, and Mg substrates and held in air at 93 % RH. EB coatings produce neither oxide growth nor increase in open circuit potential (E-oc). ES coatings induce oxide growth. They also increase E-oc to ca. 0.5 V vs SHE on Al and Fe, but not on Zn and Mg. Findings are consistent with metal oxides with acidic cations (pK(a1) < 5.5) being stable in contact with ES (pK(a) 5.5), while those with less acidic cations (pK(a1) >= 5.5) deprotonate ES to electroinactive EB. (c) 2005 The Electrochemical Society. All rights reserved.