What kind of metal alloys can resist corrosion from molten salt?
George Lai's High-Temperature Corrosion and Materials Applications (excerpts available on Google Books) is a good source. Very often for molten salts, Hastelloys are used; they're special-purpose alloys developed by Haynes International. I can't find a reference, but I believe Oak Ridge National Laboratories used Hastelloys for their molten salt reactors (which had fluoride salts, very reactive).

If you're working with sodium chloride (little potassium), according to Lai, Hastelloy-188 seems to be the best bet, although many others would work as well. One reality of chemical engineering is that there will always be corrosion, it's just a matter of keeping it at an acceptable rate for the application.