The condition of this anchor shackle has degraded considerably in the ~2 weeks its been out of the ocean. Whats the best way to preserve it? I am not opposed to a long term chemical treatment if that's the best way to go.
Mistake number one: you should have kept it in a saltwater bath until you were ready to preserve it. Soak it in distilled water for about a week, changing the water daily. This will remove the salt. Then use muriatic (pool) acid to clean off the bulk of the corrosion. Soak it again, and then electrolysis.
I've tried all the above. They work. Best I can do is add to Boatlode's response. Be careful w acid.
Whichever method you chose, give it the hot wax treatment for a final stage.
Beau Ouimett, (aquachigger), on YouTube has a tutorial vid on relic preservation w hot wax.
Your big problem is the salt. It will continue to 'EAT' away at the iron until it is remover. Some good info above. It will take a long time in electrolysis to stabilize it. I have only worked with iron from the ground which is easier. Good Luck, Frank...-