According to Wikipedia (and Haynes International, the manufacturer of Hastelloy), it's basically corrosion-resistant nickel that's alloyed with other metals:
The predominant alloying ingredient is typically the transition metal nickel. Other alloying ingredients are added to nickel in each of the subcategories of this trademark designation and include varying percentages of the elements molybdenum, chromium, cobalt, iron, copper, manganese, titanium, zirconium, aluminum, carbon, and tungsten.
The primary function of the Hastelloy super alloys is that of effective survival under high-temperature, high-stress service in a moderately to severely corrosive, and/or erosion-prone environment where more common and less expensive iron-based alloys would fail, including the pressure vessels of some nuclear reactors, chemical reactors, distillation equipment, and pipes and valves in chemical industry. (Emphasis CJs) Although a super alloy, Hastelloy does experience degradation due to fabricating and handling. Electropolishing or passivation of Hastelloy can improve corrosion resistance.Good to know.