Corrosion of Cargo Oil Tanks of VLCC Tankers
Recently, corrosion for uncoated steel plate of cargo oil tanks, general corrosion for deck plate and pitting corrosion for bottom plate, have been reported in newly built VLCC tankers. Most of tanker owners and operators have been concerned about this accelerated corrosion, whose rate is higher than anticipated. Ship Research Panel 242 has conducted a series of studies on “causes and mechanism of the corrosion”, and obtained following results.
1) Vapor space corrosion
Mechanism of general corrosion was clarified. Flaky corrosion product layer which has laminar structure of rust and elemental S were reproduced by laboratory.
Corrosion rate was not dependent on types of hull structure and steel making process.
2) Bottom plate pitting corrosion
Mechanism of pitting initiation and its growth was clarified. It was found that pits occurred in the defect of oil coating layer on the bottom plates and grew by the formation of electrochemical cell.
Maximum pitting growth rate was not dependent on types of hull structure and steel making process.