PetroStreet

Where exactly in oil & gas, and petrochemicals Hastelloy C-276 is selected when Inconel 625, Hastelloy C-22, or Inconel 825 can also be considered?

Hastelloy C-276 is usually selected for a fairly specific reason: severe, mixed, localized corrosion environments where its broad resistance to oxidizing + reducing media and excellent resistance to pitting/crevice corrosion outweigh its higher cost.
C-276 is used in:
FGD absorber components
Scrubber internals
Spray headers
Mist eliminator supports
Wet gas ducts
Chloride-contaminated acidic condensate environments

The chemistry can simultaneously contain chlorides + sulfur species + acidic condensate + oxidizing species.
It is kind of over and above others especially for those conditions which are unsure to cause both oxidation and reducing corrosion.

What we know from Haynes International about C-276 is that early nickel-chromium-molybdenum alloys faced a major hurdle of welding them caused carbide precipitation in the heat-affected zones, severely stripping away their corrosion resistance. Equipment manufacturers were forced to perform complex, post-weld heat treatments.

In the 1960s, advancements in Argon-Oxygen Decarburization (AOD) refining allowed manufacturers to drop carbon and silicon levels to microscopic minimums. This breakthrough resulted in Hastelloy C-276, the first wrought nickel-chromium-molybdenum alloy that resisted grain-boundary precipitation during welding. It revolutionized fabrication and remains an industry benchmark for universal corrosion resistance.

276 is good when you talk about welding due to low carbon content (<0.01%) it carries.
Carbide precipitation in grain boundaries /HAZ area would be lesser when compared to others like 625, 825 or C-22.
Excellent corrosion resistance therefore.