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.
Hydrogen trapping and micro-cracking of a low alloy steel during delayed fracture were studied utilizing two types of piezo-electric sensors. A rod-shaped small sensor with a 2.5mm diameter (called as a pinducer) was tapped into the near front of a notch in a CT specimen of low alloy steel. This sensor measured the change of hydrogen gas pressure in a circular cone of 7.3×10-10m3 ahead of the pinducer. Another sensor is a conventional resonant-type AE sensor with a center frequency of 0.45MHz. This sensor monitored AEs produced by micro-crack generations. The pinducer monitored only the fast hydrogen gas pressure evolutions before the crack generation. Gas pressure in a small void of 10-17m3 is estimated to reach 107MPa. After long hydrogen charging time, AEs from both the gas pressure evolution and crack generation were simultaneously monitored, suggesting a rapid gas transfer from open micro-cracks to the circular cone. Though the quantitative evaluation of hydrogen gas evolution rate in micro voids is still difficult due to the finite sensing area of the pinducer, feasibility of fast and high hydrogen gas pressure evolutions was suggested.
The variations of electric resistance of a thin iron wire (∅0.1mm) in dilute sodium sulfate and sodium chloride solutions (10-6-10-3mol dm-3) have been measured in order to show that the resistmetric technique is suitable for the monitoring of water quality of wire cut discharge machines concerning the corrosion inhibition of ferrous materials being machined. The iron wire was pretreated in various mineral acid solutions. The dissolution rate was about 100 times higher in a nitric acid solution than in other solutions. The subsequent corrosion test in a sodium sulfate solution showed that the corrosion rate of the iron wire pretreated in a nitric acid solution was smaller by one order than those pretreated in other solutions. TOF SIMS analysis showed that organic contamination became slight as the surface layer was removed by nitric acid and perchloric acid solutions but not by a hydrochloric acid solution. Consequently, the pretreatment condition was fixed as 10 minutes in 0.1mol dm-3 perchloric acid solution. Then the corrosion rate of the iron wire was measured as a function of concentration of sodium sulfate and temperature. The corrosion rate of the iron wire was dependent on the concentration of sulfate while that of iron plate was not, because the diffusion of oxygen did not limit the corrosion rate for the iron wire. Thus water quality of wire discharge machines can be evaluated adequate by a resistmetric measurement of corrosion rate of the iron wire.