Objective To study EIS evolution of 5083 aluminum alloy with epoxy coating under salt immersing condition, and to discuss the failure reason and mechanism of the coating. Methods The degradation process and the failure mechanism of an epoxy primer coating on 5083 aluminum alloy in the 3. 5% (mass fraction)NaCl solution were investigated by EIS, SEM, EDS and FTIR. Results Under continuous immersion condition in the 3. 5% (mass fraction)NaCl solution, the coating resistance decreased and the coating capacitance increased along with immersion time. The porosity and water absorptivity of coating also increased with time, but after a long immersion, they showed little change. With increased immersion time, the pores in coating increased, and the coating protection decreased. With increased immersion time, oxygen content in coating increased, while carbon content declined. Due to hydrolysis of epoxy groups in coating, the group —OH was produced and the C—O bond was partly broken. Conclusion The degradation of the coating under salt immersion condition was mainly due to the hydrolysis of epoxy groups in electrolyte solution. The hydrophilic groups such as —OH and NH2 - from the hydrolysis of epoxy groups, as well as the defects of coating such as micro-pores, promoted the electrolyte solution penetration and accelerated the degradation process. Moreover, corrosion of metal substrate can result in disbonding of coating / substrate interface, the accumulation of corrosion products in the coatings also can lead to increase of pores and defects and degradation of the coating.