I think Mrjimi has the best solution. Dip it in paint. I have steel traps that I dye with the "modern" dips. It is nothing more than oil based paint that you thin 3:1 and dip the rusty steel traps in the mixture for a few seconds to soak in. Pull them out and hang them for a week in the sun and the "dye" is dry, hard, and there is no order. It lasts for a couple of years or so, eventually chipping off under hard use. Keep in mind these traps are set under water, mud, or soil for weeks at a time. A thinned oil based paint would be plenty durable for a chain, and would last for years under normal use.