| High-Level Radioactive Waste (HLW) container is the first important artificial barrier which can separates the HLW from the disposal surroundings. The container in disposal surroundings will be faced with rigorous corrosion problem. Therefore, it is significant to selecte the corrosion resitance material to be used as the HLW container. Passive metals (stainless steels and Ti) are highly corrosion resistant due to their stable passive films spontaneously formed on surface and widely used in aerospace, deep-sea, petroleum chemistry, nuclear power industry, etc. However, passive metals are susceptible to localized corrosion in actual environment. Because of the influences of temperature, chloride concentration, hydrostatic pressure and dissolved oxygen, passive metals usually suffer the localized corrosion in service. In view of the application of passive metals in Beishan Groundwater for Geological Disposal of High-level Radioactive Nuclear Waste, this paper had investigated the localized corrosion behavior of ultra pure high chromium ferrite stainless steel and Grade-2Ti in simulated chloride environments which is very important to improve the localized corrosion and prolong their service life.The effects of temperature and Cl-concentration on the pitting corrosion of Cr26Mo1and Cr30Mo2ultra pure high chromium ferritic stainless steel had been investigated using different electrochemical techniques of potentiodynamic polarization, electrochemical impedance spectroscopy, capacitance measurements (Mott-Schottky approach) and potentiostatic polarization measurements. The results revealed that Cr26Mo1and Cr30Mo2stainless steels exhibited the passive characteristics in0.01-2M NaCl solutions at25℃, and the pitting potential all were positive. In the condition of lower temperature, the passive films could be self-repaired after pitting corrosion. When the temperature increased to45℃, the pitting corrosion resitance of Cr26Mo1stainless steel decreased evidently, and the critical temperature of Cr30Mo2stainless steel was60℃. The passive films can not be self repaired after pitting corrosion at higher temperature.In the same environment, the pitting corrosion resistance of Cr30Mo2stainless steel is better than Cr26Mo1. According to the temperature of container and the range of Cl-concentration in Beishan groundwater, Cr26Mo1stainless steel is dangerous. Cr30Mo2stainless steel will be secure when Cl-concentration is lower than0.06M, but will suffer pitting corrosion in0.06-1M when the temperature increase to60℃. Therefore, increasing temperature and Cl" concentration in groundwater all can promote the initiation and propagation of piiting corrosion of stainless steels HLW disposal container.According to the comparison of the pitting corrosion potential, the pitting corrosion resistances of Cr26Mo1and Cr30Mo2ultra pure ferritic stainless steels are better than316L austenitic stainless steel and Hastelloy C-4when the temperature is lower than45℃. Cr content plays an important role in Cr26Mo1and Cr30Mo2stainless steels. When the temperature is higher (60-90℃), the pitting corrosion resistances of Cr26Mo1and Cr30Mo2stainless steel are not as good as316L and Hastelloy C-4.This is because of the effect of Mo, especially at higher temperatures.The results showed that all the creviced specimens of Grade-2Ti exhibited the passive characteristics in the initial immersion period at25-95℃. As increasing the temperature, the galvanic current increased and the resistance of passive film decreased. With extending the immersion time, the crevice corrosion of Grade-2Ti initiated and propagated as a result of the gradual aggressive environment in the crevice. The damage caused by anodic active dissolution in the crevice mainly located near the crevice mouth. It can be speculated that the critical Cl" concentration of propagation of crevice corrosion for Grade-2Ti is0.6M in geological disposal environment at80-95℃. At70℃, the critical Cl" concentration is2M. When the temperature is lower than55℃, the effect of increasing Cl-concentration is not obvious to promte the crevice corrosion of titanium container. In addition, the results of potentiostatic polarization measurements revealed that the critical temperature of crevice corrosion of Grade-2Ti decreased with increasing Cl-concentration and the applied potential. This accelerated test method can reflect the influence of potential difference between the inside and outside of crevice on titanium. So this method is useful and feasible.In simulated crevice corrosion solutions, the effects of H+, Cl-and temperature on the active/passive characteristics of Grade-2Ti were investigated in detail. When H+concentration was0.01M, the anodic polarization curves exhibited the typical passive characteristics at95℃, and the effect of Cl" concentration was not obvious. When H+concentration was0.1M, and Cl" concentrations increased to1M, the anodic polarization curves exhibited obvious active/passive characteristics. When H+concentration was0.5M, the critical current density and passive current density increases evidently increased, Rf and Rp decreased. In1M HCl+1M NaCl solutions, the critical temperature of exhibiting obvious active/passive characteristics was75℃.It is found that the passive films on Grade-2Ti dissolved quickly in simulated crevice corrosion solutions at50-95℃, and the corrosion potential decreased to-570～-670mV (SCE) which was more negative than hydrogen evolution potential. So hydrogen absorption accompanied by the crevice corrosion of Grade-2Ti. In order to understand this problem, hydrogen absorption and mechanical performance degradation of Grade-2Titanium in simulated crevice acid solution were studied. The results showed that with the increase of temperature and cathodic potential, the velocity of H+reduction and the cathodic current density increased, and the content of hydrogen in charged specimens increased but the efficiency of absorbed hydrogen decreased. The section morphologies of Grade-2Ti after being cathodically polarized revealed that hydride accumulated on the surface of specimens. With increasing temperature or decreasing cathodic potential, the thickness of hydride layer increased and tended to be uniform. Further, the content of hydrogen decreasing with depth was approved by SIMS.Comprehensive analysis shows that:localized corrosion of passive metals container in the long-term disposal of High-level Radioactive Nuclear Waste mainly depends on the variation of environment. Under a range of temperature and Cl-concentration, ultra pure high chromium ferritic stainless steel may suffer pitting corrosion. Grade-2Ti will be safe in the environment with lower temperature or lower Cl-concentration, but may suffer crevice corrosion in the environment with higher temperature or higher Cl-concentration. Also, hydrogen absorption may occure together with crevice corrosion. In order to decrease the occurrence of crevice corrosion of titanium disposal container, it should try to avoid the gap in the process of structural design and manufacturing. |
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