Citric acid complexes iron quite well. There are probably some other components as well, perhaps buffering agents.
To some extent oxide coatings passivate metals, and the exposed metal, once cleaned will be as subject to oxidation (rusting) as before, perhaps at a slightly faster rate than when coated with rust.
The passivation effects differ with different metals. The existence of aluminum metal is only possible because of this passivation by alumina, Al2O3, which is a kind of "rust" but is impermeable to oxygen and water at a neutral pH. In iron, the oxide coating is weakly bound to the metal surface, allowing the entry of oxygen (or electrolytes) to interact with the metal
Corrosive salts can demonstrate the same effect on iron as does citric acid, since iron forms stable chloride complexes, but it also acts as an electrolyte, causing a kind of electrochemical cell to form between oxygen in the air and the iron surface. (Oxygen reduction by an electrolyte as a kind of holy grail in battery engineering to make metal/air batteries; the effect is not rapid enough in the chloride catalyzed rusting case to generate an appreciable current however: The kinetics are way too slow. The rate limiting step is the 4 electron transfer required to generate oxide from O2 gas.) At slightly acidic pH's as may arise from the uptake of carbon dioxide, chloride solutions can behave like weak hydrochloric acid, which readily attacks iron.
I hope this helps.
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