Characterization of Sulfur-assisted Degradation in Alloy 800

Abstract Crack initiation testing was performed on Alloy 800 (Fe-33Ni-21Cr) blunt-notched tensile specimens exposed to 0.55 mol/kg sulfate solutions at 280 °C with pH280C 3. These conditions were anticipated to produce a variety of sulfur-assisted degradation phenomena in Alloy 800 for the purpose of analytical TEM characterization. Initial imaging of the notch of Alloy 800 tensile specimens revealed a combination of environmentally-assisted cracking (EAC), intergranular corrosion (IGC), and pitting corrosion occurring up to approximately 50 μm in depth. Energy dispersive x-ray spectroscopy (EDX) of EAC revealed a mixture of Ti and Cr-rich oxides, with a sulfur layer at the oxide-metal interface, either incorporated in the oxide or as a sulfide compound associated with Ti. EDX characterization of pitting corrosion indicated only minor oxide formation at the pit-metal interface, with clear identification of a nano-scale sulfur layer at the pit-metal interface, accompanied by a relatively low concentration of oxygen and major alloying elements. The sulfur layer could indicate adsorption of sulfur on the bare metal with near complete surface coverage during pitting, which effectively limits lateral oxide growth and accelerates metal dissolution in the underlying region. Further details on the mechanistic interpretation of results will be discussed.