INVESTIGATION OF THE TRANSIENT STATE OF OXIDATION-CHLORIDATION

The transient state of the simultaneous oxidation and chloridation of Fe-2.25Cr-1 Mo at 773 K and Fe-25Cr-20Ni-Nb at 973 K have been studied in He-O2-H2O-HCl and N2-H2-H2O-HCl mixtures in short term oxidation t ests. The surface of the corroded samples were examined by SEM and AES . The simultaneous attack of O2 and Cl2 leads to the formation of meta l oxides and chlorides side by side on the alloy surfaces during the e arly stage of reaction, although the chlorides are not in stable equil ibrium with the gas phase. After longer exposure times only the thermo dynamically stable oxides were detected in the outer scale, however, a t the metal/oxide interface chloride layers are still present. The ini tial simultaneous growth of oxides and chlorides strongly affects the structure and morphology of the subsequently formed oxide scales.