Oxidation of Ni-20 Cr alloy and stainless steels in the presence of chlorides

Abstract

When Ni-20 Cr alloys and stainless steels are oxidized while submerged in molten salt (NaCl or NaCl-KCl), readily oxidizable alloy components, such as chromium, and in some cases iron, migrate to the surface to form non-adherent, granular, and thus nonprotective oxides. This loss of alloy constituents causes a counter current flow of vacancies which condense into an interconnecting pore network filled with salt. Since the salt penetrates into the structure, the loss of alloying metals does not require intermetallic diffusion over long distances, but instead corrosion products are largely removed by solution of alloying metals as ions in the pore-salt network. Evidence is presented which suggests that an integral part of the corrosion process is recrystallization, occurring in the porous zone, and that in the Ni-20 Cr alloy chromium diffuses down grain boundaries of the grain network to be deposited at grain boundary-pore intersections as chromium ions. The concentration gradient of chromium ions in the salt phase forces chromium diffusion out to the bulk salt where the higher effective oxygen pressure forms Cr₂O₃ and some chromate ion.