Corrosion of 9Cr Steel in CO2 at Intermediate Temperature II: Mechanism of Carburization

Abstract

In parallel to the formation of a duplex oxide scale, 9Cr–1Mo steel carburizes strongly under CO2 at 550 °C and this carburization accelerates with time. It is observed that an increase of the total CO2 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 CO2 and CO through the oxide layer, the Boudouard reaction and carbon diffusion through the metallic substrate is proposed.