Transverse Microstructure of an Oxide Scale Formed on a 20% Cr-25% Ni- Niobium Stabilized Stainless Steel

A transverse section through a scale formed during the oxidation of 20% Cr-25% Ni-niobium stabilized stainless steel, in carbon dioxide, for 4900 h at 825 ^circC, has been examined by transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM). These techniques have revealed the detailed microstructure and microchemistry of the three discrete silica, rhombohedral and spinel oxide layers comprising the scale. The variation of strain, degree of oxide order and crystallinity are consistent with the growth of the rhombohedral and spinel oxide being by outward cation movement. This is controlled by diffusion through the thin, amorphous silica layer after it has grown laterally between the rhombohedral oxide layer and the steel.