All info I have read and researched from multiple places calls for sodium carbonate. I can't speak for borax, but have used the sodium carbonate and know it works. I bought it online in a 5 lb. quantity. It's used in some cooking techniques ( pretzels, I think), also available from what I have read at swimming pool chemical stores, and in the form of "washing powder" from Arm and hammer, but I never found that product locally. All sources emphasize not to use standard baking soda (sodium bicarbonate). I googled it and found it easily and it is cheap! I agree that other chemical solutions that are conductive may also work, but I can attest to the effectiveness of Na carbonate. There's lots of info on this technique if you google "electrolytic rust removal."
It is an amazing process and is so simple . Rig a plastic drum or trash can like the one in a previous post. I used rebar connected with copper wire as my sacrificial electrodes. The rust and old paint can be just wiped off with almost no effort. The before/after pix I posted of those old hoppers, although soaked overnight for 2 days, took me less than 5 minutes of easy brushing get looking like that. And it doesn't harm chrome (unless undermined by rust). Best of all, it costs only pennies, and produces no chemical waste, just rusty water.