Carbon monoxide will eat away your soft clay brick over time. Being this old and contaminated it`s to late to save it. Tear it down and use a cement brick. It will be hard to match the colour but you will be able to find something close. Don`t wast your money on quick fix ideas. There will be plenty of people willing to take it. As for the salt air there is a breathable brick sealer you can apply. Don`t use a regular concrete sealer or you will blow your mortor joints wide open when the freeze comes. Should cost under $500.00 for a basic 3-4 foot chimney. Good luck!