The average age of the world's oil tanker and bulk carrier fleets is unexampled high and associated concern to ensure the structural capability of these fleets promoted the marine societies to make researches into ship life assessment and to call for enhanced survey. This paper is a part of the researches and focuses on wear by corrosion and its effects on hull strength. This first paper investigates hulls' corrosion processes. Final target of the research is to make a proposal of guidance for steel renewal. This paper describes the followings. (1) Framework of corrosion process and its effects on hull strength. (2) A theory of corrosion process. (3) An investigation of the data of plate thicknesses of actual ships and of the sea water immersion tested data of the specimens.