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2 atmospheres. The experiments were focused mainly on common ferritic and austenitic steels such as X10, X20, 2.25Cr1Mo, AC66, Sanicro28, Esshette 1250 etc. A Scanning Electron Microscope (SEM), with Energy Dispersive Spectrometer (EDS), and X-ray diffraction (XRD) techniques were used to determine the chemical and phase composition of the corrosion products.The obtained results show that the presence of SO 2 generally suppresses the oxidation rate of ferritic materials. Suppression of the oxidation rate in an SO 2 containing atmosphere could be due to the presence of sulphides at metal/scale interfaces , which probably influences the ion transport through the oxide scale. When the oxidation reaction is surface controlled, absorbed sulphates interfere with the reaction of the oxygen on the surface. The presence of HCl in moist air at temperatures of 500 C and 600 C accelerates the oxidation rate of the studied materials, especially for the ferritic steels. The SEM/EDS studies suggest that in HCl containing atmospheres the corrosion mechanism is "active oxidation".

The results presented here are a part of the study aiming at control and optimisation of inservice performance of boiler materials and development of simulation tools for high temperature corrosion and oxidation of steels under service conditions (EU FP5 OPTICORR Project). The high temperature oxidation behaviour of three ferritic steels was studied at temperatures of 500 and 600°C under different synthetic atmospheres simulating combustion environments. Multi-sample exposure tests in combination with SEM/EDS and XRD techniques pointed out a significant influence of the temperature and gas composition on the morphology and growth rate of the scale. The results showed that oxidation kinetics and mechanism are strongly dependent on gas species such as H 2 O, HCl and SO 2 . The presence of HCl in moist air at 500°C and 600°C accelerated the oxidation process of the steels studied. In contrast, the presence of SO 2 suppressed this process. The SEM/EDS studies suggested that in HCl containing atmospheres the corrosion mechanism is active oxidation. The reduction in oxidation rate in SO 2 containing atmospheres could be due to the presence of sulphates at the metal/scale interface which probably influences ion transport through the oxide scale.

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