In this paper, the degradation mechanism of petrolatum-coated anticorrosion system and the effective evaluation method of petrolatum-coated anticorrosion materials were discussed. In addition, the degradation progress mechanism on steel under petrolatum coating was investigated and the maintenance management method considering the mechanism was proposed. The following findings were obtained.
 The oil flow of petrolatum-based anticorrosion coating occurs from the beginning of installation due to the external pressure at the time of installation of the protective cover and the internal pressure caused by the unevenness of the steel. The flow is accelerated by heat input such as sunlight, and moves and flows out along the grain of the base fabric. It is inferred that the deterioration of the corrosion protection function during service starts at the edge and butt of the protective cover, and that the former is caused by cracks and peeling of the edge seal, while the latter is caused by the oil deflection of the petrolatum anticorrosion material due to loosening of bolts and sunlight.
 Since the deterioration of the corrosion protection function of the petrolatum coating is selective and partial, it is important to observe the joints and butt joints of the protective cover, which are the starting points of the corrosion protection.
 The degradation mechanism of petrolatum-coated anticorrosion works is estimated to be as follows: the oil content of petrolatum-based anticorrosion materials fluctuates from the initial stage of installation, and the oil content fluctuation is accelerated by sunlight during the service process. Therefore, we proposed to measure the trend of the oil content in petrolatum corrosion inhibitors from the initial stage to the middle of service to confirm the trend of the oil content fluctuation, and to measure the insulation resistance after the middle of service to predict the degradation trend and the lifetime of the corrosion inhibitor. In addition, the insulation resistance of 105 Ω-m2 was proposed as an index of the insulation resistance of soundness in this study.

Key Words: Port structure, corrosion protection,petrolatum lining, accelerated test, exposure test, deterioration process