General and Crevice Corrosion Study of the Materials for ITER Vacuum Vessel Inwall Shield

Abstract

Vacuum vessel inwall shield (IWS) (VV-IWS) which consists of block assemblies will be inserted between the inner and outer shells of the ITER vacuum vessel. The vacuum vessel will be operating at the following: 1) 100 °C and 2) 200 °C. The temperature of the vacuum vessel will be maintained by flowing water between the shells of the vessel. To achieve the temperature of the vessel, the water will be flowed at two operation phases: 1) normal operation condition (OC1), i.e., 100 °C temperature and 1.1-MPa pressure, and 2) off-normal condition, i.e., 200 °C temperature and 2.4-MPa pressure. The water flow rate is locally rather low in the VV-IWS block assemblies, and low flow rates generally promote local attacks on the stainless steel (SS). In addition, local accumulation of the aggressive compounds like chloride and other anions might be higher for ITER condition due to the different operational modes. IWS blocks are not accessible until the life of the machine after closing of the vacuum vessel. Hence, it is necessary to study the susceptibility of IWS materials to general corrosion and crevice corrosion under operations of the ITER vacuum vessel. Corrosion properties of IWS materials were studied by using the following: 1) immersion technique and 2) electrochemical polarization techniques at OC1, baking condition (OC2), and draining and drying operations of the ITER vacuum vessel. Before and after the immersion test, all the sample materials were subjected to a series of examinations, like loss/gain weight measurement, scanning electron microscopy (SEM) analysis, optical stereomicroscopy, and measurement of surface profile and hardness of materials. After the immersion test, SS 304B4 and SS 304B7 showed slight weight gains of 0.0593 and 0.0286 g, respectively, which indicate oxide layer formation on the surface of coupons. The SS 430 material showed a negligible weight loss of 0.0160 g which is equivalent to a corrosion rate of 0.00267 mm/year, indicating mild general corrosion effect. On visual observation with SEM and metallography, all materials showed pitting corrosion attack. During the electrochemical polarization test, all sample materials were subjected to a series of measurements, like open-circuit potential, cyclic polarization, pitting potential, protection potential, critical anodic current, and SEM examination. Under the OC1 condition, the ICC value (critical current density) is found lowest for SS 304B7 and highest for SS 430, and under the OC2 condition, the ICC value is found lowest for XM-19 and highest for SS 304B7 which indicates that SS 430 and SS 304B7 showed the highest general corrosion rates under the OC1 and OC2 conditions, respectively. All materials show a pitting loop in the OC2 operating condition. However, its absence in the OC1 operating condition clearly indicates the activity of chloride ion to penetrate the oxide layer on the sample surface, at higher temperature. The critical pitting temperature of all samples remains between 100 ° and 200 °C.