Volatile species released during the initial stages of oxidation of a P92 ferritic steel, with and without an aluminized coating, at 650°C in Ar+20%H2O for 150h were studied. TG-MS experiments were conducted in a closed steam loop in order to obtain information about the oxyhydroxides formation as reaction between coatings and steam. From those results, the role of the different coating element could be established and optimized for the coating durability. An oxidation mechanism based on the TG-MS results is given. The morphology/composition and structure of the oxidized samples were also studied using SEM/EDS and XRD techniques.