One crucial point of the prototype nuclear process heat is the permeation of hydrogen isotopes through the walls of the heat exchanger materials. The in situ growth of oxide layers provides an effective barrier against hydrogen permeation. Basic investigations with a large scope of heat-resistant alloys have been carried out by Kernforschungsanlage (KFA) Jülich. Further research has been done by NUKEM and Maschinenfabrik Augsburg-Nürnberg, Neue Technologie, to develop superior coatings that are effective from reactor startup. The NUKEM procedure is a simple and cost-efficient method. It comprises oxidation of the disk-shaped metal samples in excess steam (with or without addition of other gases) at elevated temperatures in a once-through manner. Adjustments of various parameters are being made to work out the conditions for optimal quality of the oxides on several alloys. Evaluation is based on permeation measurements with deuterium in the Deuperm facility of KFA. In some cases, tritium permeability was determined at the University of Münster. Backup information is provided by scanning electron microscope analysis, diffractometry, and microprobe examination. First coating experiments with steam/air yielded duplex scales consisting of an inner oxide, which is mainly Cr2O3 and an outer layer of the spinel type containing iron, nickel, and chromium, as well as some manganese throughout the scale. A compositional shift toward chromium oxide monolayers obtained with oxidation in steam correlates with improved permeation properties. Their quality depends strongly on various parameters of the procedure, which are currently being optimized. These are chiefly surface grinding, recrystallization annealing, and possibly thermocycling. For Hastelloy-X, best results are obtained with steam oxidation at 1000°C. Until now, permeability has been reduced by a factor of several hundred with disk specimens. Postoxidation by means of process gas exposure at Rheinbraun further increased the factor to over one thousand, the same as with tube specimens in Auwarm. The adherence to the metal matrix is excellent throughout; no spalling has been observed.
