The work aims to study the effects of additive hexadecyl trimethyl ammonium bromide (C19H42NBr) content on microstructure and corrosion resistance of cobalt oxide/polyaniline composite coatings during electrochemical synthesis of the coatings. The cobalt oxide/polyaniline composite coatings were synthesized in constant potential method in the electrolyte containing cobalt sulfate-sulfuric acid mixture and aniline-sulphuric acid mixture with volume ratio 1∶1. The cobalt oxide/polyaniline composite coatings of different corrosion resistance were obtained by adding different amount of C19H42NBr in the electrolyte. Corrosion resistance of the composite coatings was analyzed by combining electrochemical testing technology, scanning electron microscope (SEM), X-ray diffractometer and accelerated corrosion test. The cobalt oxide existed in the form of crystal in the cobalt oxide/polyaniline composite coatings which were synthesized in constant potential method. With the increase of addition amount C19H42NBr, microstructure of coatings was changed from irregular thin lamellar morphology into uniform dense lamellar morphology, and then into cracked overall lamellar morphology. Self-corrosion current density of the C19H42NBr-free composite coatings was 4.742×10?6 A/cm2, and self-corrosion potential was ?0.46 V. After 0.3 g/L C19H42NBr was added, self-corrosion current density of the samples was 2.622×10?7 A/cm2, corrosion potential was ?0.069 V, corrosion time of 10% HCl drop reached 483 s, and no rust was observed after neutral 56 h salt spray test. Addition of C19H42NBr has obvious effects on electrochemical synthesis of cobalt oxide/polyaniline composite coatings, and significant effects on microstructure and corrosion resistance of the composite coatings.