Hastelloy
A family of highly corrosion-resistant nickel-molybdenum alloys designed to withstand aggressive chemical environments. They are extensively used in chemical process equipment.
Hastelloy is a nickel-based material with a good amount of molybdenum in it. It's the molybdenum that makes it so corrosion-resistant. When welding, you have to be careful not to burn off or melt the molybdenum – otherwise you lose exactly the protective effect for which you use this stuff in the first place. Keep the heat input low and clean so the structure doesn't coarsen. If you get too hot, the material softens and loses hardness. Let the weld cool slowly to prevent distortion or stresses. And keep the surroundings clean so nothing diffuses in that attacks the alloy.