Some say ,"Soaking in Coca Cola will safely eat off rust ". Why is that true?

If you have ever heard of a product called Naval Jelly, it is phosphoric acid in a gel. It is used as a rust remover, especially for automobiles. You brush it on, let it sit, and then wash it off with water.

Coke also has phosphoric acid. Many sodas have phosphoric acid as an ingredient.

Carbonic acid won't be effective.

I am not sure I buy into Coke having less phosphoric acid then meat. Perhaps one is referring to phosphates, and that is a totally different category.

Types of acid don't really make all that much of a difference... It's enough for it to be an acid in the first place. However, the acid is irrelevant here.

Since the rust is primarily formed of Iron Oxide, to remove it you'd need a compound that had a better oxidation potential than Iron... That's not normally going to be an acid. There'll be such a compound in coke somewhere, but I don't know what it is... Loads and loads of things have a higher oxidation potential than Iron, so that they can nick the oxygen from Iron oxide. Acid won't react with rust at all, however.

You could try the hamburger thing, but I doubt it'll work. Ignore the phosphoric acid - look up a table of redox potentials and match it to the ingredients list.