Metallurgical characterization of self catalytic structural materials for sulfuric acid decomposition

Abstract

Eight heats of material with base alloy chemistries of Alloys 800 HT or 617 with platinum additions of 2, 5, 15, or 30 wt% have been characterized according to their microstructural features. The goals of characterization were to determine metallurgical stability for service as self-catalytic structural materials. The results presented herein will be useful to the development of a material for the construction of a heat exchanger designed for sulfuric acid decomposition. This type of heat exchanger is a key component to hydrogen generation by the thermochemical sulfur-iodine water-splitting process, a future technology that promises efficient hydrogen production if coupled to a Generation IV nuclear reactor heat source. Characterization of each material was carried out in the cast and wrought conditions with optical and SE microscopy, electron dispersive spectrometry, chemical composition analysis, and thermodynamic modeling. Materials have been characterized according to grain size and morphology, precipitate features, twinning characteristics, and platinum composition effects. Results indicate that platinum and carbon compositions have the greatest effect on the development of microstructural features.

(cont.) Increasing platinum compositions in both base alloy chemistries fosters the presence of annealing twins, which indicates that platinum additions reduce stacking fault energy within the alloy systems. Platinum additions appear to cause the development of larger grain structures as well as increase corrosion resistance. With the exception of the Alloy 800 HT - 30 wt% Pt system, the alloy systems characterized herein were melted with carbon contents between 1.2 - 3.6 times higher than the maximum specified compositions for the base chemistries. Excessive inter and intra-granular carbide precipitation resulted, which leads to compromised corrosion resistance and mechanical properties. Inter-granular attack due to sensitization is observed in the Alloy 800 HT - 2, 5 wt% Pt systems. SEM micrographs of the Alloy 617 - Pt systems show that these systems are less prone to inter-granular attack. The grain structures of each base alloy - Pt system are much finer than those of the respective base alloy systems included for comparison. Fine grain structures are detrimental to overall ductility and high temperature creep strength. On average, the Alloy 800 HT - Pt systems developed larger grains than the Alloy 617 - Pt systems.

(cont.) A two phase microstructure that resembles pearlite developed in the Alloy 617 - 30 wt% Pt system. This alloy system will be excluded from further characterization for self catalytic structural application due to expected poor mechanical and corrosion resistance properties. The most important microstructural improvements for the development of a self-catalytic structural material include a reduction of carbon content and an increase in grain size. Further characterization of catalytic, corrosion resistance, and mechanical properties are required for selection of the optimum platinum addition to the base chemistries of Alloys 800 HT and 617 for sulfuric acid decomposition service.