| In this paper, according to Experiment Procedures for Qualification of Corrosion Resistant Steel for Cargo Tanks in Crude Oil Tankers, which was set by IMO, the corrosion characteristics have been analyzed for the E36 grade conventional steel and corrosion resistant steel in simulating inner bottom conditions (Acid chloride ion medium).The effects of corrosion medium parameters (pH value and NaCI concentration) on the corrosion of the two steels were studied. The difference of corrosion behavior between traditional steel and corrosion resistant steel have been analyzed from the chemical composition design, cleanness, microstructure, rust layer characteristics and electrochemistry.As the results indicate, the corrosion rate of experiment steel is significantly affected by pH value. The corrosion rate of traditional steel and corrosion resistant steel is decreased by the increase of pH value. When the pH value is lower than 3, corrosion rate of corrosion resistant steel(NS-E36) is obviously lower than conventional steel, and the sensitivity to pH value of NS-E36 steel is lower than conventional steel; A lot of pitting corrosion is produced of Traditional steel in strong acid environment.Due to high purity, pitting corrosion resistance of corrosion resistant steel is obviously batter than conventional steel, and this is the main reason for the difference of the corrosion rate of the two. The non-metallic inclusion in the steel, especially the sulfide, are the main causes of pitting nucleation. Compared with the traditional steel, corrosion resisting steel with Cu Ni Sn alloy design, significantly improve the steel corrosion potential. On the other hand, through the rolling process, microstructure of the ferrite+pearlite is avoided, and fine and uniform microstructure is obtained. These measures can improve the uniform-corrosion resistance of corrosion resistant steel. There is no significant difference between corrosion resistant steel and conventional steel in the corrosion rate when the PH value^3.The local corrosion degree of traditional E36 steel is greatly reduced and the corrosion rate of the two kinds of steels is equal(less than/year 0.2mm). On the one hand, the acid dissolution of the steel is weakened, and the average corrosion degree of the two kinds of steels is weak, on the other hand, due to the weakening of acid, the incubation period of pitting initiation is longer and the number of localized corrosion nucleation is less. In this stage, the corrosion products are less, it is difficult to accumulate near the pit, the Fe2 is easy to exchange with the outside world, the blocking effect is reduced, the self acidification expansion of pit pits began to slow down.NaCl concentration also has a significant effect on the corrosion behavior of the test steel. Dissolved oxygen concentration, H+ activity, surface capacitance and other parameters determine the variation of corrosion rate.When the NaCl concentration is lower than 10%, corrosion rate of experiment steels was decreased sharply by the increase of NaCl concentration,and under 10%NaCl concentration, the corrosion rate of the test steel was the lowest. This stage, corrosion rate of experiment steels was decreased sharply, this is mainly because dissolved oxygen and activational area in corrosion solution are decreased; When the NaCl concentration is more than 10%, the corrosion rate of NS-E36 steel maintained at a low rate, and the corrosion rate of E36 steel increased slightly. In this phase,the activity of H+ in the solution increased with the concentration of NaCl, and the pH value of the solution decreased rapidly. Combined with the results of previous studies,the corrosion rate of E36 steel is increased slightly, this is because E36 steel on the acidic hydrogen corrosion is more sensitive. The acid corrosion resistance of NS-E36 steel is strong, so its corrosion rate is still kept in a stable state. |
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