Evolution Mechanisms of T91 Steel in Subcritical Conditions and Role of an Internal Oxidation Zone

Corrosion of a T91 steel tube, used in subcritical conditions in an oil power plant for 157,000 h, was characterized mainly through SEM, TEM, EDX and DRX analyses. Severe oxidation and carburization took place in both the outer (boiler) and inner (steam) wall sides. The nature and morphology of the oxide scale multilayer structure (hematite, magnetite and spinel) depended on the environment exposure. Specific attention was given to the internal oxidation zone at the oxide/metal interface. Diffusion of chromium during the oxidation process was determined and was proposed to be responsible for the continuous advancement of oxidation to the core of material, which eventually gave rise to the spinel. Finally, in the bulk material, coarsening of the secondary carbides (M\(_{23}\)C\(_{6}\)) was the main form of microstructural evolution.