Overpack is a metallic container, which is one of the components of the engineered barrier system. It is expected to prevent the high level radioactive waste from contact with groundwater for 1000 years and this function will be lost by corrosion. Carbon steel, copper and titanium are chosen as the candidate materials in Japan. This paper presents the current status and future studies for long-term lifetime prediction of overpack materials. It was shown that the corrosion type of carbon steel in contact with bentonite would be general corrosion. The corrosion depth of carbon steel for 1000 years was estimated to be about 32mm by considering the contribution of oxygen and water reduction. For titanium, critical conditions of the initiation of crevice corrosion have been evaluated by repassivation method and localized corrosion can be avoided by selecting appropriate alloys that are compatible with environmental conditions of the actual repository site. The corrosion and hydrogen absorption of titanium under reducing condition have been studied and shortterm corrosion test data have just been obtained. The corrosion depth of copper was estimated on the basis of current understanding taking into account of corrosion due to oxygen and sulfide.
Corrosion has recently become a more significant factor in the reliability of electronic components, because of design requirement of higher device density and faster signal processing and expanding application areas, thus resulting in smaller devices with ever finer feature sizes and exposure to more severe uncontrolled environment. In attaining a successful corrosion preventive and controlling design of electronic components, a unique set of considerations is required for each category of corrosion factors such as environments, materials and structural configurations including packaging. This review addresses to elucidate common characteristics of corrosion damages of electronic components and to afford perspectives for prevention measures from the above three categories. Given the susceptibility of most electronic materials to corrosive attack and the functional sensitivity of most electronic structures to corrosion, some degree of isolation of the structure from the environment by the use of protective or passivity films and by packaging is required. It is emphasized that the true corrosion prevention can only be achieved with thorough considerations for materials, environments, and structural configurations.
In order to know the discoloration mechanism of Ti in atmospheric environments, the influence of surface finish and environmental factor on the growth of surface films on Ti has been examined. Commercially available Ti plates with different surface finishes, a bright-anneal (BA) and an acid-etching (2B) finish, were used as specimens. A significant amount of C (15-30at%) was found as TiC in the surface layer of the BA plates. At potentials higher than 1.5V (vs. Ag/AgCl (3.33kmol·m-3 KCl)) in neutral solutions, the growth rate of anodic oxide films on the BA plates was higher than that on the 2B plates. The high growth rate measured on the BA plates is ascribed to the fast oxidation of TiC precipitates in the surface layer of the plates. Under the illumination of UV light, a localized oxide growth was induced. The oxide growth was thought to be partly due to the formation of H2O2 by the photo-oxidation of water. In fact, the rapid growth of surface films by a dissolution-precipitation mechanism was observed on Ti and TiC exposed to a neutral solution containing H2O2. The film growth rate on the BA plates was much higher than that on the 2B plates in the solution. This is because TiC is easy to react with H2O2 to form soluble peroxo-titanium complexes. The presence of Cl- ions in a solution accelerates the initial deposition of surface films stemming from hydrolysis of the soluble titanium complexes. It has been presumed that the discoloration of Ti is accelerated when Ti containing TiC is exposed to atmospheric corrosion environments including Cl- ions under strong illumination of sunlight.
Buried steel pipelines can be effectively protected against external corrosion using high resistivity coatings together with cathodic protection (CP). Consequently, corrosion on exposed steel surface at possible coating defects can be prevented by providing protective currents. Moreover, early detection and appropriate repair of the coating defects can minimize the corrosion risk. The authors have developed a technique to detect the occurrence of a coating defect using a change in the AC impedance of a pipeline. In the present study, field tests were carried out on an existing pipeline to measure the change in the AC impedance when a simulated coating defect was connected to the tested pipeline. It has been then revealed that the AC impedance varies depending on the resistance of a coating defect to earth and the distance from AC signal input point. According to the results of the field tests, an on-line monitoring technique to detect the occurrence of a coating defect and specify its location using the change in the AC impedance of a pipeline has been established. In addition, the behavior of the AC impedance obtained in the field tests has been confirmed by a theoretical study using a distributed constant model as an equivalent circuit for the tested pipeline.
Field studies on the AC corrosion risk were conducted on a steel pipeline newly buried in proximity to an elevated AC powered rail transit system. The pipeline is coated with extruded polyethylene coatings and cathodically protected using two CP rectifiers. AC corrosion risk was assessed with respect to the cathodic protection criteria that have been developed by the authors based on AC and DC current densities measured using coupons. The average of AC current density of each coupon connected to the surveyed pipeline satisfied the criteria at all the test stations in the test section where distributed grounding system using distributed magnesium electrodes was installed for the mitigation of induced AC. However, AC current density reached at considerably high levels for short time periods during the passing of AC powered trains where the pipeline parallels the AC powered rail transit system. Then, the pipe was connected to a bare steel casing pipe through a solid-state DC decoupling device for AC mitigation. Consequently, AC current density decreased to a low level even while AC powered trains were passing and the risk of AC corrosion was completely removed.