Objective The corrosion mechanism of defected zinc rich epoxy coating was studied in simulated drinking water with different hardness, which will be valuable for the selection of the coating used in the inner wall of drinking water pipes. Methods The electrochemical behavior of the coating during the immersion was studied by electrochemical impedance spectroscopy (EIS). The morphology and composition of corrosion products were investigated by scanning electron microscope (SEM), energy dispersive spectrometer (EDS) and Raman spectra. Results The results showed that the sample immersing in the solution containing higher concentration of Ca2+ were protected by incrustation covering the defect, which delayed the process of cathodic disbonding. In the solution containing lower concentration of Ca2+, the coating blistered around the defect because of the loose accumulation of the corrosion products. Conclusion The coating had better permeability resistance in soft water while had better cathodic disbonding resistance in hard water. Some suggestion for the application of coatings in drinking water was given at last.