Iron with slowly start "rusting", which is a process by which iron oxides and hydroxides form. It will not necessarily happen because of the citric acid --- most acids would start the process of rusting.

The many things that take place are very involved and somewhat complex, but the reddish-brown product (rust) is known to be mostly a compound called lepidocrocite with the formula FeO (OH).

The hydrogen ions in the acid strip the electrons off of the iron and turn into neutral hydrogen. Meanwhile, the iron corrodes because it is changing into ionic form (and probably crystallizing out some kind of salt or rust).

If you put iron in citric acid (or other acids) it will dissolve quite quickly. It may look like it is also rusting, but that is probably from the water that is usually also in the liquid (example: lemon juice has both water and citric acid).