Corrosions of pipes had been disregarded as a subject of maintenance because corrosions in chemical plants progress slowly. However, many chemical plants were built over 30 years ago in Japan. There are many pipes whose outside surfaces is considerably corroded. Accidents which relate to corrosions may occur and lead to serious problems and vast losses. For this reason, the maintenance is greatly needed. Although, each company takes measures to cope with corrosions, successful managerial methods are not established. The purpose of this research is to know corrosion wastages from outside surface of pipes by estimating a corrosion rate in order to select pipes to be checked. To achieve the purpose, corrosion rates based on corrosion cases are estimated by using the database modeling method.