Forschungszentrum Karlsruhe

Reduced activation ferritic-martensitic steels (e.g. EUROFER) are considered for application in future fusion technology as structural material, which is in contact with the breeding material Pb-17Li. Various corrosion experiments have been made in the past, however, evaluation of these tests, which were mostly conducted up to moderate temperatures of up to 480°C, was performed with respect to determine corrosion rates and mechanisms e.g. dissolution of some elements out of the steels and comparison of the results with earlier tested RAFM-steels of type F82H-mod. OPTIFER, and MANET. In the mean time the envisaged operation limits e.g. in temperature increased to roughly 550°C and flow regimes may change. Thus extrapolations of the RAFM-steel corrosion behavior determined in the past to the new working conditions may be problematic due to large uncertainties or reliability and, additionally, only low knowledge on transport of dissolved components in the Pb-17Li flow is present. In contrast to earlier investigations, these changes in requirements need the going over from (only) mechanism based corrosion tests to model supported tests. Furthermore, the whole loop has to be considered in the evaluation of the corrosion tests together with other occurring phenomena and mechanisms as transport effects and precipitations. Therefore, under this task the development of modeling tools for describing Pb-17Li corrosion (dissolution, material transport and precipitations) was started. The modular structured tools are based on physical, chemical and thermo-hydraulic parameters and, in the first stage, the development was focused on the dissolution of EUROFER and validation with older test results obtained at 480°C in our PICOLO loop earlier. In the second stage the new 550°C test results – obtained in the part corrosion testing of this task were used for validation at a second temperature level and transport phenomena were considered. This report consists of the two subtasks a) corrosion testing and b) modeling with the main achievements given for each part in the next paragraphs. A) Corrosion testing at 550°C The corrosion testing of bare cylindrical EUROFER samples was performed in the upgraded PICOLO loop for testing at the new blanket relevant temperature of 550°C in flowing Pb-17Li with a flow rate of about 0.22 m/s – the same flow value and configuration as used in earlier 480°C tests. The post exposure analyses of the samples from the 550°C champagne showed that the corrosion mechanisms are the same as detected at the lower exposure temperature of 480°C namely homogeneous corrosion attack and dissolution of steel components. In the test series durations up to 5,000 h were examined and a corrosion rate was evaluated of about 500 μm/a. This value is more than 5 times the rate observed in 480°C tests and represents a dramatically increase in corrosion rate by a slight increase of only 70 K in testing temperature. Under these conditions 1 m2 TBM surface would generate about. 4 kg Fe dissolved in Pb-17Li per year. The operation of PICOLO loop showed clearly the dangerous situation of loop blockages by precipitations formed in the cooler sections after short times (approx. 3,000 h) due to the high amount of dissolved and transported corrosion products. Looking in more detail the first test results obtained at 550°C with a short time base of 5,000 h will not have yet the high reliability for 4 extrapolation up to several 10,000 h known from the 480°C tests as required for TBM design. However a new test series was launched to increase reliability and also statistics with exposure times up to 10,000 h. A global valuation of the evaluated corrosion rates at 550°C in comparison with 480°C values and data coming from other laboratories using the empirical Sannier correlation showed that the new 550°C data are well positioned in the complex temperature and flow rate depending corrosion attack picture. These corrosion figures illustrate that corrosion of bare RAFM steels at high temperatures – also at reduced flow rates in real TBM’s may be a serious and challenging task in handling of blanket systems beside tritium permeation and may require corrosion protection measures by coatings. B) Development of modeling tools and their validation The developed modeling tools are based on physical and electrochemical parameters to describe the occurring mechanisms and phenomena in a nonisothermally operated loop. The developed and validated tools will simulate in the basic model configuration dissolution (corrosion attack), transportation of corrosion products and precipitation effects in dependence of the thermo hydraulic parameters (e.g. flow velocity, temperature profile) of the test loop PICOLO. The validation process showed that the selection of ‘good’ physical properties e.g. diffusion coefficients will have a critical impact on the results modeled. The validation of the two different test temperatures indicated that probably values evaluated by Feuerstein will be most reliable ones. Under these boundary conditions the developed tools reproduced the measured corrosion rates (90 μm/a at 480°C and 500 μm/a at 550°C) rather well with about 80 μm/a and 520 μm/a, respectively. The parameterization of the whole loop showed the sections where dissolution or precipitation takes place in dependence of the local temperatures. The modeling of precipitation needs more experimental input to decide whether needle like particles are formed or if precipitates grow on the surfaces. Additionally, validation with varied flow rates should be performed in future to have a tool for reliable prediction of effects in TBM’s working far away of tested loop configuration.