Materials development for process heat exchanger (PHE) in nuclear hydrogen production system

The decomposed sulfuric acid gas (SO2/SO3/H2O) channels made of super alloys (Alloy HX in this work) in process heat exchanger (PHE) are subjected to an extremely severe corrosion environment because its operation temperature is more than 900 °C. For the corrosion protection, Alloy HX was coated with SiC film in this work. The bonding between two dissimilar materials is often problematic, particularly in coating metals with ceramics protective layer. A strong bonding between SiC/Alloy HX was achieved by mixing the atoms at the interface by ion-beam: The film was not peeled-off at ≥900 °C, confirming excellent adhesion, although the thermal expansion coefficient of Alloy HX is about three times higher than that of SiC. Instead, the SiC film was cracked along the grain boundary of the substrate above 700 °C. At ≥900 °C, the film was crystallized forming islands on the substrate so that a considerable part of the substrate surface could be exposed to the corrosive environment. In order to cover the exposed areas and cracks multiple coating/IBM process has been developed. The immersion corrosion test in 80% sulfuric acid at 300 °C for 100 h showed the weight retain rate was gradually increased as increasing the number of the process time.