Corrosion and Corrosion Fatigue of Ship Structural Steel Plates in Vapor Space of Crude Oil Tankers

Abstract

Corrosion wastage has important roles for an evaluation of strength of aged hulls. Corrosion fatigue strength is also indispensable factor in health of hull strength with aged upper deck plates. From those backgrounds, corrosion and corrosion fatigue tests were performed under an environment substituted a vapor space of crude oil tankers. The vapor space is filled up with H2S gas comes from crude oils and an inert gas for preventing an explosion of cargo oil tanks (COT).A corrosion rate, 0.2mm/y, was estimated from a continuous corrosion test in the vapor space. The rate was twice as fast as measurement data of ships. The small corrosion diminution of the upper deck plate is caused by cycles of full load and ballast load conditions.S-N curves, Seq-Nc curves, ΔK-Nc curves and crack propagation limits were compared with the data of corrosion environments in seawater and dilute sulfuric acids. Crack initiation lives, Nc, were nearly equal to the lives in dilute sulfuric acids of pH4 and pH2 in a short life range. In a long life range, the lives of Kt=1.2 decreased to the lives in dilute sulfuric acids of pH4 and pH2, and the lives of Kt=>2.0 were nearly equal to the lives in seawater. An aspect ratio, 0.6, was obtained from corrosion pits to propagate as fatigue crack. The stress intensity factor range, ΔKCF=4.70 MPa•m1/2, was the lowest corrosion fatigue strength and the boundary level whether or not the corrosion pits grow into a fatigue crack or progress as corrosion wastage.