High-temperature corrosion of Inconel®Alloy 718, Haynes®282®Alloy and CoWAlloy1&2 in supercritical ammonia/ammonium chloride solution

Abstract

Corrosion of process equipment during ammonothermal synthesis at high temperature and pressure is still one of the big limitations of this process because of reduced crystal quality and safety issues. In this paper we present preliminary ammonothermal corrosion experiments with two Ni-base superalloys (Inconel®Alloy 718 and Haynes®282®Alloy) and two newly developed Co-base superalloys (CoWAlloy1&2) in a supercritical NH3/NH4Cl-solution (24 h/550 °C/105 MPa). After the experiment, the samples were analyzed by their mass change per initial surface area (specific weight change). Additionally, the morphological and elemental changes of the corrosion layers were evaluated. None of the investigated alloys developed a passivation layer and the corrosion layers were very fragile. In agreement with previous studies, the Ni-base alloys showed intergranular attack with severe general corrosive degeneration. Despite their high potential for high-pressure at high-temperature applications and in contrast to a ternary Co-Al-W-alloy, the investigated multinary Co-base alloys were strongly attacked, too. However, CoWAlloy2 presented the lowest specific weight change of the investigated alloys. Thus, it shows the best potential for application in acidic environment of the investigated alloys. Additionally, CoWAlloy2 proved good stability in a supercritical NH3/Na-solution (24 h/550 °C/93 MPa).