External corrosion of pipes in chemical and petrochemical plants is developing very slowly therefore corrosion of pipes has been disregarded as maintenance objects. However, many pipes are considerably corroded on the outside surface because many plants in Japan have been operated for 30 years or more. Accidents which relate to corrosions sometimes occurred and led to serious problems and vast losses. Corrosion under insulation (CUI) is extremely difficult to be detected and quantified. The purpose of this research is to estimate CUI potential and corrosion rates by using case database and Group numbering of the continuous condition.