The different stages of corroded steel will behave in different ways. Copied from this link :
http://www.landyzone.co.uk/lz/f38/rust-removal-using-electrolysis-63065.html1.) The outer layer of red rust is the surface we see and consists primarily of a compound with the chemical formula Fe2O3 (nH2O), known as ferric oxide. This material takes up rather more volume than the metal it replaces and as a result it tends to flake away from the surface and if it occurs in mechanical devices, can seize up moving parts. The loss of this loose layer of rust represents a total loss of metal from the component which cannot be recovered.
2.) Underneath this outer layer of red rust may be found a harder, darker layer of material having an almost purple-grey appearance when freshly exposed. This layer is often in direct contact with the surface of the metal and is able to conduct electricity. It has the chemical formula Fe3O4 which is the same chemical composition as the mineral Magnetite and is therefore sometimes referred to as such, although it is more commonly referred to simply as 'Black Rust'. As this material occupies the same volume as the original iron, surface features on the metal can sometimes be preserved in it with little distortion and as it tends to remain strongly bonded to the underlying metal, is fairly resistant to becoming detached and subsequently flaking off.
As the reaction involved in producing rust is an electrochemical one, it follows that it should be possible to somehow reverse the process using electricity, and this is indeed the case. As long as the polarity is correct, with the subject matter connected as cathode, it is possible to chemically reduce some of the black rust back to metallic iron with a good bond to the original metal. During this process, much of the red rust is converted to Fe3O4 and becomes detached from the surface, aiding its removal when the conversion is complete.
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