It is commonly duplex layers partially placed on a thermal sprayed coating when narrow parts are difficult to thermal spray. In a previous study, it was shown that when the duplex layer has a defect, thermal spray coating deteriorates faster than the single-layer coating. It is critical to evaluate and anticipate the occurrence and progression of duplex layer degradation and blistering in order to execute effective maintenance management for steel structures. In this investigation, specimens with duplex layers were exposed to the atmosphere and corrosion environment was monitored using ACM-type corrosion sensors and temperature-humidity sensors. Based on these findings, we proposed a method to evaluate the deterioration and blister progression of duplex layer between Al-5Mg thermal sprayed coating and heavy-duty paint coating in an environment without rain-washing action of adhered sea salt.