Sulfidation of Alloy C-276 Level Transmitter Diaphragm in SAGD High Pressure Steam

Abstract A comprehensive investigation was carried out into a repeatedly failed level transmitter diaphragm made of Alloy C-276, installed in a high-pressure steam separator at a SAGD plant. Laboratory analysis of the severely damaged diaphragm confirmed that the Alloy C-276 was corroded by high-temperature sulfidation. This result is unexpected considering the low concentrations of H2S and H2 in the steam (≤10 ppm). In contrast, duplex stainless steel 2507 diaphragms in the same and parallel vessels exhibited much lower corrosion rates and significantly longer service lives. Sulfidation rates predicted by ASSET, a computer program for high-temperature corrosion prediction, for a similar nickel alloy were consistent with the observed corrosion rate of the Alloy C-276 diaphragm. These findings do not support the clause in API RP 571 stating that chromium-containing nickel-based alloys have corrosion resistance similar to stainless steel provided that the chromium level is similar.