A model for the oxidation of mild steel in CO2 is proposed, which is an extension of the ideas of Bruckman, Romanski, and Mrowec. A single layer of magnetite forms initially by short-circuit solid-state transport of cations. Lattice vacancies are injected into the underlying metal and eventually cause loss of scale-metal adhesion in some areas. Microchannels develop in the overlying oxide and the scale continues to grow at both oxide-metal and oxide-gas interfaces. In this duplex stage of growth, inner layer oxide nodules form in the vacancy condensation volume produced by departing metal. Their growth is restricted by a build-up of CO released by the oxidation reaction, so that a microporous structure is perpetuated. Breakaway oxidation is the result of local destruction of CO when a catalyst for the Boudouard reaction eventually forms. The inner layer crystals then grow in an atmosphere of higher oxygen potential, and deposited carbon produces a very porous structure.