Corrosion Monitoring for Deep Underground Environment

高放射性废物计划深埋于地下，即地质隔离。外包装是防止高放废物与地下水长期接触的金属容器，碳钢是外包装的候选材料之一。虽然外包装材料应该暴露在无氧和腐蚀性很弱的环境中，但为了长期安全保存，腐蚀监测是必要的。为了建立可靠、简便的环境腐蚀监测系统，研究了地质隔离环境中可能存在的各种离子对碳钢腐蚀速率的影响，以及电化学噪声分析评价极低腐蚀的可能性。碳酸根和碳酸氢根离子。SM 400 B显示出两个阳极峰，其高度大致为 ...更多信息 与当量碳酸氢根离子浓度成比例。另一方面，硫酸盐并不影响一般的腐蚀速率。硫酸根离子，但是，促进点蚀，而碳酸根和碳酸氢根离子抑制。硫酸根离子也被观察到减少在碳钢的腐蚀电位。因此，在无氧环境中，硫酸根离子是碳钢腐蚀的氧化剂。测量了试样间的电流和电位，并将其波动转化为频域功率谱。低频电流、电位的功率密度以及由电位和电流的波动划分得到的表观耐蚀性与极化曲线测量的腐蚀速率具有良好的相关性。腐蚀速率与3 mHz电流波动功率密度的相关系数达到0.8 ~ 0.9，具有足够的可靠性。因此，两个试样之间的电流波动的噪声功率密度可以用于在地质隔离环境中的碳钢的腐蚀监测。少

High-level radioactive waste is planned to be buried deep underground, i.e. geological isolation. Carbon steel is one of the candidate materials for an overpack, which is metallic container to prevent the high level radioactive waste from contact with groundwater for long period. Although the overpack material is supposed to be exposed to oxygen free and very week corrosive environment, corrosion monitoring is necessary for very long term safe keeping. In this work, the effect of various ionic species, which may exist in the geological isolation environment, on the corrosion rate of carbon steel, and possibility of the assessment of the very low corrosion using electrochemical noise analysis are investigated in order to establish reliable and simple corrosion monitoring system in the environment.The corrosion behavior of SM400B carbon steel was analyzed in terms of the effect of sulfate, carbonate and bi-carbonate ions. SM400B exhibited two anodic peaks the height of which were roughly … More proportional to the equivalent bi-carbonate ions concentration. On the other hand sulfate did not affect the general corrosion rate. Sulfate ions, however, promoted pitting corrosion, whereas carbonate and bi-carbonate ions suppressed. Sulfate ions were also observed to be reduced at the corrosion potential of carbon steel. Therefore, sulfate ions were proved to act as oxidant for corrosion of carbon steel in the oxygen free environment.Three coupons of SM400B were immersed in de-aerated bi-carbonate aqueous solution. The current and potential between these coupons were measured, fluctuation of them were converted into power spectrum in frequency domain. Power density at lower frequency of current, potential, and the apparent corrosion resistance, which was obtained as the division of fluctuation of potential and current, exhibited good correlation with corrosion rate which was also measured by polarization curve. The correlation coefficient between corrosion rate and power density of current fluctuation at 3 mHz attained to 0.8〜0.9, which is sufficient reliability for corrosion rate monitoring. Therefore, it is concluded that noise power density of current fluctuation between two coupons is possible to be adopted for corrosion monitoring of carbon steel in the geological isolation environment. Less