757Alloy 686 1993 to Present

This is another recent development in the C family of Ni-Cr-Mo-W alloys, which is very similar in composition to alloy C-276, but where the chromium level has been increased from 16 to 21%, while maintaining the Mo and W at similar levels. This alloy is very overalloyed with the combined Cr, Mo, and W content of around 41%. To maintain its single-phase austenitic structure, this alloy has to be solution annealed at a very high-temperature of around 1220° C followed by very rapid cooling to prevent precipitation of intermetallic phases. Its thermal behavior, as discussed later, is significantly inferior to alloy 59 and its performance in field tests in a hazardous waste incinerator at 3M Co., St. Paul, Minnesota, showed five times higher corrosion rate than alloy 59 [34]. Table 7.6 presents this data comparing alloy 59 with alloy 686 and other alloys. Alloy 59 exhibited the best performance.

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