Protections of corrosion in main hull structures and offshore structures have a great influence on their life. The authors have studied on protective means of the corrosions with investigation of actual conditions of aging effects to coatings in water ballast tanks for 22 ships aged 4 to 20 years. In the report, is described, i) a tendency of secular deterioration of the coatings, ii) primary factor of infulence to the deterioration, iii) forecast method of the coating conditions with aging effects, and iv) a proposal of the best specification and a maintenance for a long life service of ships.