Evolution of the metal-oxide interface during the initial stage of the high temperature oxidation of ferritic stainless steels

Two grades of ferritic stainless steel, a bi-stabilised Ti, Nb (AISI 441) and a stabilised Ti (AISI 439), were oxidised at 1060 degrees C under the simulated process atmosphere for durations between 45 and 1800 s. Focused ion beam coupled with field emission gun and scanning electron microscopy was carried out to investigate the cross-section morphology of the oxide growing on ferritic stainless steels. Matrix protrusions localised at the metal - chromia interface through the silica layer are observed and the following mechanism for their formation is proposed. During the first step of oxidation, interface undulation, induced by growth stresses, in combination with silica precipitation at the metal - oxide interface lead to the formation of matrix protrusions in the chromia layer. For an increased oxidation time, due to the laterally silica growth matrix protrusions are trapped into the Cr(2)O(3) layer as matrix inclusions.