I don't know who said that Coca Cola contains phosphoric acid, but that's not true. If it were, Coca Cola would taste acidic all of the time, but it doesn't. In fact, it's easy to remove the acidic "bite" from the taste of Coca Cola.

Coca Cola (and all carbonated soft drinks) contain an acid called "carbonic acid". They don't actually put carbonic acid in soft drinks. They only put CO2 into the soft drink and the CO2 dissolves in the soft drink and combines with H2O to form carbonic acid: CO2 + H2O = CH2O3, which is carbonic acid.



Carbonic acid is an inherently unstable molecule and breaks down to form CO2 and H2O fairly quickly, but as long as there's lots of CO2 dissolved in water, carbonic acid will form as quickly as it breaks down, and that water will remain acidic cuz of the carbonic acid in it.

It's the formation of carbonic acid from the CO2 dissolved in the water that gives Coca Cola (and all carbonated soft drinks, and beer, and all carbonated (aka: "sparkling") wines their acidic "bite" when you taste them). To prove that, just leave any carbonated soft drink sitting in a glass over night to let all the CO2 come out of it. Then when you taste it, it'll just taste sweet like sugar water and not have any "bite" in it's taste at all. If there were acid in the soft drink to begin with it, that acid wouldn't evaporate, and it would still taste acidic, like vinegar left out in a glass over night.

It's the carbonic acid you swallow breaking down to form CO2 (gas) and H2O in your stomach that makes you burp after drinking soft drinks, beer or sparkling wine.

I kinda doubt that the amount of carbonic acid in a soft drink like Coke would be sufficiently strong to dissolve rust, so I'd stick with phosphoric acid which does work.