Polyaniline-polypyrrole composite coatings were formed on low carbon steel using oxalic acid as electrolyte by a potentiostatic method under aqueous conditions. A passive layer of iron (II) oxalate is deposited on the steel surface prior to the formation of composite coatings. The electrochemical process shows three distinct regimes - formation of passive layer, dissolution of passive layer and formation of polymeric composite coatings. These three regimes have been studied in depth using spectroscopic techniques and electron microscopy. Quantitative analysis of the current-time transient (I-t) curves show that the nucleation and growth of the passive layer occur by three-dimensional (3D) instantaneous nucleation and that of the composite coatings occur by 3D progressive nucleation. It has also been shown that the morphology of the composite coatings depends upon the electrochemical deposition parameters.