Corrosion Issues of HTR Structural Metallic Materials

Within the framework of the ANTARES program, AREVA NP, EDF and the CEA have launched a joint R&D program on metallic materials for VHTR. Reference alloys for circuit and Intermediate Heat eXchanger (IHX) are nickel-based with about 22%wt. of chromium. Compatibility with the HTR primary helium appears to be a determining property for the material selection and qualification. The coolant is actually polluted by a low level of impurities that can interact with metals at high temperature. Oxidation, carburization and/or decarburization occur, in relation to atmosphere characteristics, temperature and alloy chemical composition. As these corrosion effects can notably influence the mechanical properties, they often are determining to the component service life. Since the corrosion behavior is highly sensitive to environmental conditions, material studies require dedicated facilities that shall allow for a strict control of the environment throughout the entire specimen exposure. AREVA NP, CEA, and EDF have developed experimental loops respectively under the names the Chemistry Loop, CORINTH and CORALLINE, ESTEREL; these high temperature helium flow systems are equipped with high accuracy hygrometers and gas analyzers. A benchmark was defined to cross-validate the lab devices and procedures. It is composed of two tests. The joint protocol sets the operating parameters in terms of material, specimen preparation, temperature and heating program, gas pressure and flow rate, time, gas composition. Corrosion is assessed by mass change associated to observations and analyses of the corroded coupons considering the surface scales (nature, morphology and thickness), the internal oxidation (nature, distribution and depth) and the possible carburization/decarburization (type and depth). For benchmark test 1, AREVA NP, CEA, and EDF produced similar results in terms of operation of the tests as well as about the corrosion criteria. On the other hand, benchmark test 2 showed a difference in the residual water vapor level between loops that was shown to strongly influence the specimen behavior. Discrepancies in the alloy corrosion were studied regarding gas flow rates and effective oxygen potential in helium. As a consequence, the experimental tools and procedures have been upgraded. French laboratories have now efficient corrosion facilities and methods at their disposal to study and qualify the corrosion behavior of structural materials in HTR environment.