The protection of iron against corrosion brought by polyaniline (PANI) potentiostatically polymerized in phosphoric/metanilic solution is due to the combination of the passive layer and the polymer. The passive layer is strengthened by phosphate incorporation, and sulfonated aniline is inserted in the chain to yield a copolymer (SPAN). The potentiostatic polarization is partly responsible for the heterogeneity of polymer (block-polymer). Raman and optical spectroscopies are used to characterize the modifications in the PANI composition preceding and accompanying the passivity breakdown. The breakdown is associated with the loss of PANI reoxidability, but this step is preceded by slow modifications in the polaron distributions.