Corrosion Protection and Corrosion Failure Analysys in Oil and Ballast Tanks under Inert Gas System of Crude Oil Tanker

摘要

Corrosion tests of mild steel and welded mild steel have been coducted to study the corrosion protection effect in the simulated inert gas environment of crude oil tankers. Morever, causes of corrosion cracking failures in inert gas environment of brass tubes for heating heavy crude oil have been invesigated. The results are as follows(1) The corrosion protection effects of mild steel in N_2 gas containing O_2 become 47 percent under O_2/N_2 =0.1, 74 percent under O_2/N_2 = 0.04, 88 percent under O_2/N_2 = 0.01. The corrosion protection effect of 74 percent at O_2/N_2 = 0.04 shows that O_2 concentration is not a corrosion factor.(2) The corrosion protection effect in inert gas environment of oil tanker becomes minus 13.5 percent to that in air.(3) The reaction of HSO_3~-+5H~+ + 4e = S + 3H_2O which occurs in low pH H_2SO_3 solution accelerates corrosion of mild steels together with O_2 and H~+ as cathodic reaction.(4) Since the inert gas environment containing 250ppm SO_2 causes the same corrosion rate as that in air, SO_2 gas effects severely corrosion in the actual environment. Accordingly, control of SO_2 gas is important for corrosion protection under inert gas system in oil tanks and ballast tanks.(5) Corrosion failures of brass tubes for heating crude oil seem to be caused due to sulfide stress corrosion cracking and sulfide corrosion fatigue.