Recently, in Japan, Al-5Mg alloy thermal spray coating was applied as a repair method to improve the anticorrosion performance of painted steel bridges in a highly corrosive environment with chloride in various aspects such as girder ends, bearing supports, and bolted joints. However, the deterioration behavior of the overlapped layer between Al- 5Mg alloy thermal spray coating and heavy-duty paint coating in an atmospheric environment was not clarified in previous studies. In this study, atmospheric exposure tests were performed in highly corrosive atmospheric environments with and without the influence of rainfall washing-off on adhered airborne sea salt to examine the deterioration and anti-corrosion behavior. The exposure sites are located near the coastline. The test specimens were common carbon steel plates with an overlapped layer between the Al-5Mg alloy thermal spray coating and heavy-duty paint coating. To evaluate the effective area of the sacrificial anode reaction by thermal spray coating, a ribbon defect with a width of 12 mm was introduced in the overlapped layer. From the test results, the overlapped layer was deteriorated from the lower part of the thermal spray coating along with the defects in a corrosive environment without the effect of rainfall washing-off. The deterioration of the thermal spray coating layer was accelerated by the sealing treatment of the thermal spray coating layer.