In this paper, a coating–substrate interfacial corrosion test method was developed to simulate and study the failure processes occurring at the coating interface as a direct consequence of environment-induced degradation or corrosion. It was found that the corrosion-induced failure rate of the coating–substrate interface upon exposure to an aggressive corrosive medium was high. Microscopic pits tend to appear at the interface of the coating and the substrate. The permeation channel at the coating interface did cause the corrosive medium, primarily the chloride ions, to gradually diffuse from the sides of the sample to the inner surface of the interface thereby enabling the initiation and continued progression of “local” corrosion. The process for failure due essentially to corrosion of the coating was established, while ensuring to include the infiltration phase, the presence of “local” corrosion phases, expansion, if any, due to corrosion, and eventually culminating in failure. Based on the experimental results, a finite element simulation of the “local” corrosion occurring at the coating interface was executed. The results revealed the microscopic pits at the interface to progressively increase the “local” stress concentration on the surface of the substrate but were found to have little influence on overall stress distribution in the coating. It was also found the shape of the etch pit had an effect on failure expansion under the influence of stress. The numerical method can be used to predict structural failure caused by corrosion pits at the interface of the coating–substrate system in an aggressive environment.