Corrosion is degradation of materials' properties due to interactions with the environments, and corrosion of most metal materials is inevitable. Corrosion damage can lead to fatal environmental consequences like toxic substance leakage, as well as huge maintenance and repairing costs during the suspension of process equipment in chemical and petroleum industries. There are many pipes whose outside surfaces are considerably corroded because many chemical and petrochemical plants in Japan have been used for 30 years or more. Corrosions on the outside surface of pipes progress slowly, and corrosion under insulation (CUI) is extremely difficult to detect and quantify. The proper points of pipe inspection are hard to find. Moreover, the insulation stripping work requires a large amount of time and cost. Besides, the successful managerial methods have not been established. The purpose of this research is to estimate CUI potential and corrosion rates by using case database and Group numbering of the continuous conditions.