Experimental and ab initio investigations into the fundamentals of corrosion, in the context of supercritical water oxidation systems

Abstract

Supercritical water oxidation (SCWO) is a process which runs at 250 bar and 400- 6000 C to effect rapid and complete destruction of aqueous organics. The SCWO process streams are very corrosive and pose a materials performance challenge to even noble metals and the most advanced alloys. Corrosion in chlorinated, acidic streams at transitional temperatures (100-400° C) in a Hastelloy C-276 tube is examined through post-failure analysis and controlled exposure experiments. Dealloying and intergranular corrosion were observed. Dealloying rate was found to be strongly correlated with temperature and dielectric constant. Intergranular corrosion behavior was found to be affected by the alloy hardness. In order to understand the fundamental chemistry of localized chloride-assisted corrosion initiation, a density-functional calculation of Cr20 3 was performed and validated against experimental data. The (0001) surface of a-Cr20 3 was computed and found to experience strong relaxations in the terminal oxygens. The surface was found to be susceptible to attack by electron-rich species; this might be defeated by p-doping the oxide. Further, the chemically relevant states at the surface assumed a ferromagnetic order. Effects of a variable dielectric medium upon the chemistry of the surface were assessed for the bare surface and adsorption of H and Cl.