| The risk of oil tanker accidents is particularly high, which has been shown in the last decades by many accidents with enormous mass of oil spillage into the sea. These oil spillage disasters endanger the natural life and caused an economical loss due to crude oil loss, ship damage or ship out of service time and due to the expenses necessary to deal with oil pollution. Traditional cargo oil tank corrosion protection is coating. The method not only increases the maintenance workload, but also reduces the tanker’s work efficiency and improves the maintenance costs of tanker, at the same time, the anti-corrosion effect can’t be guaranteed. Under the circumstances, the "Performance Standard for Alternative Means of Corrosion Protection for Cargo Oil Tanks of Crude Oil Tankers"(International Maritime Organization draft289) was adopted, and the plan has been formally effective on January1,2013. Japan’s research group has conducted the actual trial of cargo oil tank corrosion resistant steel. China research on this field is still in its infancy, only a few steel company and scientific research institutes to carry out some preliminary work. It is important practical and economic significance for cargo oil tank corrosion resistant steel industrial production and testing result of the application to break technical barriers as early as possible.Related research shows that the corrosive environment of cargo oil tanks is very complex, including the moisture corrosion on the upper deck and the solution corrosion on the bottom plate. In homemade cargo oil tank upper deck and bottom plate corrosion environment simulator, corrosion behavior and mechanism of E36grade hull structure plate steel was studied by using SEM, EBSD, EDS, XRD, IR, electrochemical and weightlessness analysis methods. The results showed that:For the upper deck, reducing carbon content d to0.035%, increasing Cr content to1%, along with other alloy elements, using TMCP process to obtain single-phase micro-structure, the corrosion rate is significantly lower. At the same time,25years extrapolation corrosion rate is1.69mm, which was reduced by82%compared with the traditional E36ship plate steel(9.35mm). It meets the requirements of the IMO standard. The corrosion form was uniform corrosion. Outer corrosion product film of E36grade ship plate steels in cargo oil tanks upper deck wet gas O2-CO2-SO2-H2S environment was mainly formed by, α-FeOOH, γ-FeOOH, element S, FeS2, Fe1-xS and FeS, and the structure was loose relatively. Inner corrosion product film was mainly formed by α-FeOOH with a compact structure. With the increasing of Cr content, Cr enrichment(13%) was turned up in the inner layer of corrosion product film, which increased the corrosion product film density and prevented the ion exchange between matrix and corrosive medium. It also improved electrochemical potential of nearly interface matrix. On the other hand, the reduction of large angle grain boundary proportion reduced the corrosion initiation to improve the corrosion resistance.For the bottom plate, it was found that pitting corrosion was inclined to occur around the place where inclusions exist. After initial corrosion, a few μm diameter areas were formed as a circinate cathode around the edge of inclusion. MnS inclusion dissolved in the simulated COT corrosion solution before steel matrix, and pitting was formed at the place where MnS dissolved. The corrosion tended to occur at the area where curvature radius of inclusion is smaller. The addition of Cu, Mo element, and the design of the low C content can obtain the optimum corrosion resistance. The corrosion rate is0.299mm/a, which was reduced by95%compared with the traditional E36steel(5.56mm). At the beginning of the corrosion, excluding the interference of inclusions, grain boundaries around inclusions in the steel matrix corrode first. Grain boundary configuration and dislocation density were the decisive factors of early corrosion stage. Single-phase micro-structure had better corrosion resistance than duplex-phase micro-structure. Corrosion rate of steel after heat treatment was annealed≈tempered<TMCP <quenched.For the welded joint, which was welded by using H08MnA welding wire meted the requirement of IMO standard and China classification society guidelines. The step of corrosion depth was46.146μm (30μm≤r≤50μm), and the step angle was14.1°<15°. Grain size of welding joint which was welded by using H08Mn2MoA welding wire was smaller, and micro structure consisted of polygonal ferrite+acicular ferrite. Corrosion rate in bottom plate environment is(0.1832mm/a) lower than matrix metal(0.299mm/a). The corrosion resistance of weld was better than the matrix metal and reverse steps were formed. Based on standards, it is recommended that the thinning synergy to the welded joint R value concept, R=CH/Cm×100%. R value between50%and200%was qualified area. Its significance lies in the fact that the qualified welding joint had a more consistent trend of thinning, and improved the effectiveness of the evaluation. |
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