Even stainless steel will corrode, and salt especially will speed up the process. Solid salt is much worse than salt water, because the concentration of solid salt is 100% vs a fraction of a percent for pasta water. Regardless, leaving salt water sitting in your pot for days on end is definitely going to make corrosion occur faster, although by how much I can't say.

Although, I will say that needing to buy even one additional pot might offset any environmental benefit from reusing your pasta water. Industrial manufacturing uses a ton of water, so if your goal is to preserve it, you're likely better off just washing & drying your pot out between uses so that it lasts longer.