What kinds of kinds metals are good for a salt shaker? Are there metals which will react with the salt in them to cause either the shaker to corrode or ruin the salt (eg. cause caking etc.)
Stainless steel is usually best, as it won't react with salt or salty water. It's best to avoid other steels, although you may have some luck with aluminium as long as the shaker remains dry.
High-end salt grinders use ceramic grinding elements rather than steel ones to minimise the erosion caused by the salt.