Under CO 2 exposure at an intermediate temperature, typically 550°C, 9Cr-1Mo steel forms a duplex oxide scale made of an outer magnetite layer and an almost-as-thick inner Fe-Cr rich spinel oxide layer. It is proposed that the inner Fe-Cr spinel layer grows according to a mechanism involving void formation at the oxide/ metal interface. The driving force for pore formation is the outward magnetite growth: iron vacancies are injected at the oxide/metal interface then condense into voids. The fresh metallic surface made available is then oxidized by CO 2 , which diffuses fast through the scale. The physical aspects, the integrity and the nature of the scale are shown to be very dependent on the oxygen potential existing in the environment.

In parallel to the formation of a duplex oxide scale, 9Cr-1Mo steel carburizes strongly under CO 2 at 550°C and this carburization accelerates with time. It is observed that an increase of the total CO 2 pressure in the environment from 1 to 250 bars induces a higher carbon deposition in the inner Fe-Cr rich spinel oxide layer. In order to explain this phenomenon, modelling of the carburization process was carried out. A mechanism involving gas diffusion of CO 2 and CO through the oxide layer, the Boudouard reaction and carbon diffusion through the metallic substrate is proposed.

In the framework of a new generation of nuclear reactors, typically sodium fast reactors, supercritical carbon dioxide (CO 2 ) with a Brayton cycle (at 550°C and 250 bars) is identified as a promising energy conversion system to replace the traditional steam generators. Nevertheless, the long-term integrity of the heat exchanger structure in this environment has to be proven over at least 20 years. To this purpose, the corrosion behavior of different metallic materials under static CO 2 at 550°C and 250 bars is studied. The materials under study are one 9 wt% Cr ferrito-martensitic steel (T91) and several austenitic steels. The results about the nature of the corrosion product, morphology, and kinetics of formation were analyzed by glow discharge optical emission spectroscopy, scanning electron microscopy, wavelength-dispersive spectroscopy, and x-ray diffraction. A corrosion mechanism of the different steels is proposed and the most promising materials for heat exchanger applications are discussed.