Oxidation behavior of grain boundary engineered alloy 690 in supercritical water environment

Abstract

Nickel-base alloy is an important structural material that is known for its exceptional high temperature oxidation resistance. Oxidation in this alloy at high temperatures occurs to a greater extent along the grain boundaries. Grain boundary engineering (GBE) was applied to modify the grain boundary characteristics of this alloy to affect its oxidation resistance. Specimens with both low level and high level cold works showed a high fraction of special grain boundaries, and were tested for supercritical water oxidation resistance at 500°C and 24MPa. Both GBE and as-received samples exhibited mass gain followed by mass loss during 10weeks of exposure, but the normalized mass change was small and less than 0.12mg/cm2. GBE samples showed better oxide layer retention compared to the as-received sample. XRD results indicate that nickel oxide, chromium oxide, and spinel oxide were the three main types of oxides that form on as-received and GBE alloy 690. Three distinct regions were identified on the oxidized surface: a flat region with oxide flakes aligning relatively parallel to the surface, a rough region with polygon-type oxide particles randomly distributed on the surface, and a region with aggregated oxide flakes perpendicular to the surface. The flat region of oxidation consisted of (111) orientated oxide spinel flakes formed on (111) oriented alloy 690 grains. The flat oxide region was thinner and showed better oxide adhesion compared to the rough region. Chromium oxidation was found only at random grain boundaries, leading to formation of thick Cr2O3 layer on the surface and chromium depletion underneath. None of this oxidation was found at low angle or special boundaries. The chromium oxidation was attributed to fast chromium diffusion through random boundaries and mechanically deformed regions such as scratches left after polishing. It is envisioned that the oxidation behavior of alloy 690 in supercritical water can be tailored by microstructure engineering that involves changes in grain orientation and grain boundary character distribution.